Cholinergic transmission

CHOLINERGIC TRANSMISSION
SATYAJIT GHOSH
B. PHARM 4TH
SEMESTER

 Introduction:
Acetylcholine is the important neurotransmitter in the cholinergic transmission. The neurochemical events that underlie cholinergic neurotransmission are as:
 Synthesis and Storage and Destruction of ACh:
Two enzymes, choline acetyltransferase and AChE, are involved in ACh synthesis and degradation, respectively.
A- Choline Acetyltransferase:
i. Choline acetyltransferase catalyses the synthesis of ACh—the acetylation of choline with acetyl CoA. Choline acetyltransferase is synthesized within
the perikaryon and then is transported along the length of the axon to its terminal.
ii. Axonal terminals contain a large number of mitochondria, where acetyl CoA is synthesized. Choline is taken up from the extracellular fluid into the
axoplasm by active transport. The final step in the synthesis occurs within the cytoplasm, following which most of the ACh is sequestered within
synaptic vesicles.
Choline and Choline Transport:
- Choline must be derived primarily from the diet or, secondarily, from recycling of choline. Once ACh is released from cholinergic neurons, ACh is
hydrolysed by acetylcholinesterase (AChE) to acetate and choline.
- Much of the choline is taken up actively at cholinergic nerve terminals by Na+
: Choline cotransporter and reused for ACh synthesis. Under many
circumstances, this reuptake and availability of choline appear to be rate limiting in ACh synthesis.
- There are three mammalian transport systems for choline; all three are transmembrane proteins with multiple TM segments; all are inhibited by
hemicholinium but at distinct concentrations in the same order as their affinities for choline:

 The high-affinity (4-μM) choline transporter CHT1 (SLC5A7) present on presynaptic membranes of cholinergic neurons. This transporter is a
member of the SLC5 family of solute carrier proteins that includes Na+
-glucose cotransporters and shares about 25% homology with those
transporters. Choline transport by CHT1 is Na+
and Cl−
dependent. This system provides choline for ACh synthesis and is the high-affinity
hemicholinium-binding protein (Ki = 0.05 μM).
 A low-affinity (40-μM), Na+
-independent transporter, CTL1 (SLC44A), which is widely distributed and appears to supply choline for
phospholipid synthesis (e.g., phosphatidyl choline, sphingomyelin).
 A lower-affinity (100-μM) Na+
-independent transporter, OCT2 (SLC22A2), a nonspecific organic cation secretory transporter found in renal
proximal tubule, hepatocytes, the choroid plexus, the luminal membrane of brain endothelium.
Storage of ACh:
- ACh is transported into synaptic vesicles by the vesicular ACh transporter VAChT (a solute carrier protein, SLC18A3) using the potential energy of a
proton electrochemical gradient that a vacuolar ATPase establishes, such that the transport of protons out of the vesicle is coupled to uptake of ACh
into the vesicle and against a concentration gradient.
- The process is inhibited by the non-competitive and reversible inhibitor vesamicol, which does not affect the vesicular ATPase. There are to be two
types of vesicles in cholinergic terminals: electron-lucent vesicles (40–50 nm in diameter) and dense-cored vesicles (80–150 nm).
- The core of the vesicles contains both ACh and ATP, at a ratio of about 11:1, which are dissolved in the fluid phase with metal ions (Ca2+
and Mg2+
) and
a proteoglycan called vesiculin. Vesiculin, negatively charged and thought to sequester the Ca2+
or ACh, is bound within the vesicle, with the protein
moiety anchoring it to the vesicular membrane.
- In some cholinergic terminals, there are peptides, such as vasoactive intestinal polypeptide (VIP), that act as cotransmitters. The peptides usually are
located in the dense-cored vesicles. Estimates of the ACh content of synaptic vesicles ranges from 1000 to over 50,000 molecules per vesicle, with a
single motor nerve terminal containing 300,000 or more vesicles.

Release of ACh:
- Exocytotic release of ACh and cotransmitters (e.g., ATP, VIP) occurs on depolarization of the nerve terminals. Depolarization of the terminals allows
the entry of Ca2+
through voltage-gated Ca2+
channels and promotes fusion of the vesicular membrane with the plasma membrane, allowing exocytosis
to occur.
- Two pools of ACh appear to exist. One pool, the “depot” or “readily releasable” pool, consists of vesicles located near the plasma membrane of the
nerve terminals; these vesicles contain newly synthesized transmitter. Depolarization of the terminals causes these vesicles to release ACh rapidly or
readily.
- The other pool, the “reserve pool,” seems to replenish the readily releasable pool and may be required to sustain ACh release during periods of
prolonged or intense nerve stimulation.
- Botulinum toxin blocks ACh release by interfering with the machinery of transmitter release. The active fragments of botulinum toxins are
endopeptidases; the SNARE proteins are their substrates. Botulinum toxin A and B are highly potent exotoxins produced by Clostridium botulinum
that are responsible for botulism (a type of food poisoning). These neurotoxic proteins cause long- lasting loss of cholinergic transmission by
interacting with axonal proteins involved in exocytotic release of ACh.
Localised injection of minute quantity of botulinum toxin A (BOTOX) or its haemagglutinin complex (DYSPORT) can be used in the treatment of a
number of spastic and other neurological conditions like blepharospasm, spastic cerebral palsy, strabismus. spasmodic torticollis. nystagmus,
hemifacial spasm, post stroke spasticity, spasmodic dysphonia. axillary hyperhydrosis. etc. which are due to overactivity of cholinergic nerves.
- There are eight isotypes of botulinum toxin, each cleaving a specific site on SNARE proteins. Tetanus toxins act similarly, but in the CNS. The active
fragments of these toxins cleave synaptobrevin and block exocytosis in specific sets of neurons (inhibitory neurons in the CNS for tetanus, the NMJ for
botulinum).

- Black widow spider toxin induces massive release of acetyl choline. The venom from the black widow spider, containing the neurotoxin α-latrotoxin
(α-LTX), is deadly. This high molecular weight toxin binds to a specific 'receptor' at the surface membrane of synapses and neurosecretory PC12 cells,
activating cation influx and neurotransmitter release.
- α-LTX in its tetrameric form interacts with receptors (neurexins and latrophilins) on the neuronal membrane, which causes insertion of α-LTX into
the membrane.
- Once the tetramer is inserted into the cell membrane, two mechanism of actions can occur. First, insertion may lead to pore formation and possibly
other effects, and second, the receptor may be activated, which leads to intracellular signalling.
- The pores formed by α-LTX in the membrane are permeable to Ca2+
and therefore allow an influx of Ca2+
into the cell. This influx into an excitable cell
stimulates exocytosis directly and efficiently.
- The cation influx is proportional to the number of pores and hence the number of involved receptors expressed on the cell membrane. Also, Ca2+
strongly facilitates the forming of the tetramers and so its pore formation. The pore is also permeable to neurotransmitters, which causes massive
leakage of the neurotransmitter pool in the cytosol.
B- Acetylcholinesterase:
i. At the NMJ, immediate hydrolysis of ACh by AChE into choline and acetate reduces lateral diffusion of the transmitter and activation of adjacent
receptors.
ii. The time required for hydrolysis of ACh at the NMJ is less than a millisecond. AChE is found in cholinergic neurons and is highly concentrated at the
postsynaptic end plate of the NMJ.
iii. BuChE (butyrylcholinesterase, also called pseudocholinesterase) is virtually absent in neuronal elements of the central and peripheral nervous
systems. BuChE is synthesized primarily in the liver and is found in liver and plasma; its likely physiological function is the hydrolysis of ingested
esters from plant sources.

iv. AChE and BuChE typically are distinguished by the relative rates of ACh and butyrylcholine hydrolysis and by effects of selective inhibitors. Almost
all pharmacological effects of the anti-ChE agents are due to the inhibition of AChE, with the consequent accumulation of endogenous ACh in the
vicinity of the nerve terminal.
Acetylcholinesterase Butyrylcholinesterase
Distribution All cholinergic sites, RBC, gray matter Plasma, liver, intestine, white matter
Hydrolysis
ACh
Methacholine
Benzoyl choline
Butyrylcholine
Very fast (in μs)
Slower than ACh
Not hydrolysed
Not hydrolysed
Slow
Not hydrolysed
Hydrolysed
Hydrolysed
Function Termination of ACh action Hydrolysis of ingested esters

 Cholinoreceptors:
- Two classes of receptors for ACh are recognised-muscarinic and nicotinic; the former is a G protein coupled receptor, while the latter is a ligand gated
cation channel.
- Nicotinic receptors are ligand-gated ion channels whose activation always causes a rapid (millisecond) increase in cellular permeability to Na+
andCa2+
,
depolarization, and excitation. Muscarinic receptors are GPCRs. Responses to muscarinic agonists are slower; they may be either excitatory or inhibitory,
and they are not necessarily linked to changes in ion permeability.
 Subtypes of nAChRs:
i. The nAChRs exist at the skeletal NMJ, autonomic ganglia, adrenal medulla, and CNS and in nonneuronal tissues. The nAChRs are composed of five
homologous subunits organized around a central pore. In general, the nAChRs are further divided into two groups:
a- Muscle type (Nm), found in the vertebrate skeletal muscle, where they mediate transmission at the NMJ
b- Neuronal type (Nn), found mainly throughout the peripheral nervous system, CNS, and nonneuronal tissues
ii. At pre- and peri-synaptic sites, nAChRs appear to act as autoreceptors or heteroreceptors to regulate the release of several neurotransmitters (ACh, DA,
NE, glutamate, and 5HT) at diverse sites in the brain.
Muscle-Type nAChRs:
- In foetal muscle the nAChRs subunit stoichiometry is α2 β γ δ, whereas in adult muscle the γ subunit is replaced by ε to give the α2 β ε δ stoichiometry.
- The γ/ε and δ subunits are involved together with the α subunits in forming the ligand-binding sites and in the maintenance of cooperative interactions
between the α subunit.
- Different affinities to the two binding sites are conferred by the presence of different non-α subunits. Binding of ACh to the αγ and αδ sites is thought to
induce a conformational change predominantly in the α1 subunits that interacts with the transmembrane region to cause channel opening.

Neuronal-Type nAChRs:
- Neuronal nAChRs are widely expressed in peripheral ganglia, the adrenal medulla, numerous areas of the brain, and nonneuronal cells, such as epithelial
cells and cells of the immune system.
- To date, nine α (α2–α10) and three β (β2–β4) subunit genes have been cloned. The α7–α10 subunits are found either as homo pentamers (of five α7, α8, and
α9 subunits) or as hetero pentamers of α7, α8, and α9/α10.
- By contrast, the α2–α6 and β2–β4 subunits form hetero pentamers usually with (αx)2(βy)3 stoichiometry. The α5 and β3 subunits do not appear to be able
to form functional receptors when expressed alone or in paired combinations with α or β subunits, respectively.
- Neuronal nAChRs are widely distributed in the CNS and are found at presynaptic, peri-synaptic, and postsynaptic sites. At pre- and peri-synaptic sites,
nAChRs appear to act as autoreceptors or heteroreceptors to regulate the release of several neurotransmitters (ACh, DA, NE, glutamate, and 5HT) at
sites throughout the brain.
- The synaptic release of a particular neurotransmitter can be regulated by different neuronal-type nAChR subtypes in different CNS regions. For instance,
DA release from striatal and thalamic DA neurons can be controlled by the α4β2 subtype or both α4β2and α6β2β3 subtypes, respectively. In contrast,
glutamatergic neurotransmission is regulated everywhere by α7 nAChRs.
Receptor
(primary receptor
Subtype)
Main synaptic
Location
Membrane response Molecular
Mechanism
Agonists Antagonists
Skeletal Muscle (Nm)
(α1)2β1εδ adult
(α1)2 β1γδ foetal
Skeletal
neuromuscular
junction
(postjunctional)
Excitatory; end-plate
depolarization; skeletal
muscle contraction
Increased cation
permeability (Na+; K+)
ACh, Nicotine
Succinylcholine
Atracurium,
Vecuronium,
d-Tubocurarine,
Pancuronium,
α-Conotoxin
α-Bungarotoxin
Peripheral neuronal
(Nn)
(α3)2(β4)3
Autonomic ganglia;
adrenal medulla
Excitatory;
depolarization; firing of
post-ganglion neuron;
depolarization
and secretion of
catecholamines
Increased cation
ACh, Nicotine, Epibatidine
Dimethylphenylpiperazinium
Trimethaphan
Mecamylamine

CNS neuronal
(α4)2(β4)3
(α-BTX-insensitive)
(α7)5
(α-BTX-sensitive)
CNS; pre- and
Postjunctional
CNS; pre- and
postsynaptic
Pre- and postsynaptic
excitation; prejunctional
control of transmitter
release
Increased cation
Increased permeability
(Ca2+)
Cytosine, epibatidine,
Anatoxin A
Anatoxin A
Mecamylamine,
DHbE
Erysodine,
Lophotoxin
Methyl lycaconitine
α-Bungarotoxin
(BTX)
α-Conotoxin 1mI
 Subtypes of Muscarinic Receptors:
i. In mammals, there are five distinct subtypes of mAChRs (M1→M5), each produced by a different gene. The mAChRs are GPCRs, present in virtually all
organs, tissues, and cell types. Most cell types have multiple mAChR subtypes, but certain subtypes often predominate in specific sites. For example, the
M2 receptor is the predominant subtype in the heart and in CNS neurons is mostly located presynaptically, whereas the M3 receptor is the predominant
subtype in the detrusor muscle of the bladder.
ii. The functions of mAChRs are mediated by interactions with G proteins. The M1, M3, and M5 subtypes couple through Gq/11 to stimulate the PLC-
IP3/DAG-Ca2+
pathway, leading to activation of PKC and Ca2+
-sensitive enzymes.
iii. Activation of M1, M3, and M5 receptors can also cause the activation of PLA2, leading to the release of arachidonic acid and consequent eicosanoid
synthesis; these effects of M1, M3, and M5 mAChRs are generally secondary to elevation of intracellular Ca2+
.
iv. Stimulated M2 and M4 cholinergic receptors couple to Gi and Go, with resulting inhibition of adenylyl cyclase, leading to a decrease in cellular cAMP,
activation of inwardly rectifying K+
channels, and inhibition of voltage-gated Ca2+
channels. The functional consequences of these effects are
hyperpolarization and inhibition of excitable membranes.
Receptor Cellular and tissue location Cellular response Functional response
Disease
relevance
M1
CNS; most abundant in cerebral
cortex, hippocampus, striatum, and
thalamus.
Autonomic ganglia
Glands (gastric and salivary)
Enteric nerves
Couples by Gq/11 to activatePLC-
IP3-Ca2+-PKC-pathway
Depolarization and excitation
(↑sEPSP) Activation of PLD2, PLA2;
↑AA
Increased cognitive function (learning and
memory)
Increased seizure activity
Decrease in dopamine release and locomotion
Increase in depolarization of autonomic ganglia
Increase in secretions
Alzheimer disease
Cognitive
dysfunction
Schizophrenia

M2
Widely expressed in CNS, hindbrain,
thalamus, cerebral cortex,
hippocampus, striatum, heart,
smooth muscle, autonomic nerve
terminals
Couples by Gi/Go (PTX sensitive)
Inhibition of AC, ↓cAMP
Activation of inwardly rectifying
K+ channels
Inhibition of voltage-gated
Ca2+ channels Hyperpolarization
and inhibition
Heart:
SA node: slowed spontaneous depolarization;
hyperpolarization, ↓HR
AV node: decrease in conduction velocity
Atrium: ↓refractory period, ↓contraction
Ventricle: slight ↓contraction
Smooth muscle: ↑Contraction
Peripheral nerves:
Neural inhibition via autoreceptors and
heteroreceptor ↓Ganglionic transmission.
CNS:
Neural inhibition, ↑Tremors; hypothermia;
analgesia
Alzheimer
disease, Cognitive
dysfunction, Pain
M3
Widely expressed in CNS (<other
mAChRs), cerebral
cortex, hippocampus
Abundant in smooth muscle and
glands
Heart
(↑sEPSP)
Activation of PLD2, PLA2; ↑AA
Smooth muscle:
↑Contraction (predominant in some, e.g., bladder)
Glands:
↑Secretion (predominant in salivary gland)
Increases food intake, body weight, fat deposits
Inhibition of DA release
Synthesis of NO
Chronic
obstructive
Pulmonary
disease
(COPD)
Urinary
incontinence
Irritable bowel
disease
M4
Preferentially expressed in CNS,
particularly forebrain, also striatum,
cerebral cortex, hippocampus
Couples by Gi/Go (PTX sensitive)
Inhibition of AC, ↓cAMP
Activation of inwardly rectifying
K+ channels
Inhibition of voltage-gated
Ca2+ channels Hyperpolarization
and inhibition
Autoreceptor- and heteroreceptor mediated
inhibition of transmitter release in CNS and
periphery
Analgesia; cataleptic activity
Facilitation of DA release
Parkinson disease
Schizophrenia
Neuropathic pain
M5
Substantia nigra
Expressed in low levels in CNS and
periphery Predominant mAchR in
neurons in VTA and substantia nigra
(↑sEPSP) Activation of PLD2, PLA2;
↑AA
Mediator of dilation in cerebral arteries
and arterioles
Facilitates DA release
Augmentation of drug-seeking behaviour and
reward (e.g., opiates, cocaine)
Drug dependence
Parkinson disease
Schizophrenia
 Parasympathomimetic or Cholinomimetic or Cholinergic drugs:
i. These are the drugs which produce actions similar to that of ACh, either by directly acting cholinergic receptors or by increasing the availability of ACh at
these sites.
ii. These are classified as follows:

 Direct acting cholinomimetic drugs:
 Pharmacokinetics (Absorption, Distribution, and Metabolism):
- Choline esters are poorly absorbed and poorly distributed into the central nervous system because they are hydrophilic. They all are hydrolysed in the
gastrointestinal tract and less active by the oral route.
Cholinoreceptor stimulant
Direct acting drugs
Alkaloids
Muscarine
Pilocarpine
Arecoline
Nicotine
Lobeline
Choline esters
Acetylcholine
Methacholine
Bethacholine
Carbachol
Indirect acting drugs
(Anticholinesterases)
Reversible
Carbamates
Physostigmine
Neostigmine
Pyridostigmine
Rivastigmine
Noncarbamates
Edrophonium
Tacrine
Donepezil
Galantamine
Irreversible
Carbamates
Carbaryl (Sevin)
Propoxur (Baygon)
Organophosphates
Dyflos (DFP)
Echothiophate
Malathione
Diazinon (TIK-20)
Tabun
Sarin
Soman
Parathion

- Acetylcholine is very rapidly hydrolysed; large amounts must be infused intravenously to achieve concentrations sufficient to produce detectable
effects. A large intravenous bolus injection has a brief effect, typically 5–20 seconds, whereas intramuscular and subcutaneous injections produce only
local effects.
- Methacholine is more resistant to hydrolysis, and the carbamic acid esters carbachol and bethanechol are still more resistant to hydrolysis by
cholinesterase and have correspondingly longer durations of action. The β-methyl group (methacholine, bethanechol) reduces the potency of these
drugs at nicotinic receptors.
- The tertiary natural cholinomimetic alkaloids (pilocarpine, nicotine, lobeline) are well absorbed from most sites of administration. Nicotine, a liquid,
is sufficiently lipid-soluble to be absorbed across the skin.
- Muscarine, a quaternary amine, is less completely absorbed from the gastrointestinal tract than the tertiary amines but is nevertheless toxic when
ingested—e.g., in certain mushrooms—and it even enters the brain.
- Lobeline is a plant derivative similar to nicotine. These amines are excreted chiefly by the kidneys. Acidification of the urine accelerates clearance of
the tertiary amines.
 Pharmacodynamics:
A- Mechanism of Action:
- Activation of the parasympathetic nervous system modifies organ function by two major mechanisms. First, acetylcholine released from
parasympathetic nerves activates muscarinic receptors on effector cells to alter organ function directly. Second, acetylcholine released from
parasympathetic nerves interacts with muscarinic receptors on nerve terminals to inhibit the release of their neurotransmitter.
- By this mechanism, acetylcholine release and circulating muscarinic agonists indirectly alter organ function by modulating the effects of the
parasympathetic and sympathetic nervous systems and perhaps non-adrenergic, noncholinergic (NANC) systems.

- Several cellular events occur when muscarinic receptors are activated, one or more of which might serve as second messengers for muscarinic
activation. All muscarinic receptors appear to be of the G protein coupled type.
- Muscarinic agonist binding to M1, M3, and M5 receptors activates the inositol trisphosphate (IP3), diacylglycerol (DAG) cascade. Some evidence
implicates DAG in the opening of smooth muscle calcium channels; IP3 releases calcium from endoplasmic and sarcoplasmic reticulum.
- Muscarinic agonists also increase cellular cGMP concentrations. Activation of muscarinic receptors also increases potassium flux across cardiac cell
membranes and decreases it in ganglion and smooth muscle cells. This effect is mediated by the binding of an activated G protein βγ subunit
directly to the channel.
- Finally, activation of M2 and M4 muscarinic receptors inhibits adenylyl cyclase activity in tissues (e.g., heart, intestine). Moreover, muscarinic
agonists decrease the activation of adenylyl cyclase and modulate the increase in cAMP levels induced by hormones such as catecholamines. These
muscarinic effects on cAMP generation reduce the physiologic response of the organ to stimulatory hormones.
- The mechanism of nicotinic receptor activation has been studied in great detail, taking advantage of three factors:
1. The receptor is present in extremely high concentration in the membranes of the electric organs of electric fish;
2. Α-bungarotoxin, a component of certain snake venoms, binds tightly to the receptors and is readily labelled as a marker for isolation
procedures; and
3. receptor activation results in easily measured electrical and ionic changes in the cells involved.
- The nicotinic receptor in muscle tissues is a pentamer of four types of glycoprotein subunits (one monomer occurs twice) with a total molecular
weight of about 250,000.
- Agonist binding to the receptor sites causes a conformational change in the protein (channel opening) that allows sodium and potassium ions to
diffuse rapidly down their concentration gradients (calcium ions may also carry charge through the nicotinic receptor ion channel).

- Nicotinic receptor activation causes depolarization of the nerve cell or neuromuscular end plate membrane. In skeletal muscle, the depolarization
initiates an action potential that propagates across the muscle membrane and causes contraction.
- Prolonged agonist occupancy of the nicotinic receptor abolishes the effector response; that is, the postganglionic neuron stops firing (ganglionic
effect), and the skeletal muscle cell relaxes (neuromuscular end plate effect).
B- Organ System Effects:
1. Eye: -
- Muscarinic agonists instilled into the conjunctival sac cause contraction of the smooth muscle of the iris sphincter (resulting in miosis) and of
the ciliary muscle (resulting in accommodation).
- As a result, the iris is pulled away from the angle of the anterior chamber, and the trabecular meshwork at the base of the ciliary muscle is
opened. Both effects facilitate aqueous humour outflow into the canal of Schlemm, which drains the anterior chamber.
2. Cardiovascular system: -
- The primary cardiovascular effects of muscarinic agonists are reduction in peripheral vascular resistance and changes in heart rate. Intravenous
infusions of minimally effective doses of acetylcholine in humans (e.g., 20–50 mcg/min) cause vasodilation, resulting in a reduction in blood
pressure, often accompanied by a reflex increase in heart rate.
- Larger doses of acetylcholine produce bradycardia and decrease atrioventricular node conduction velocity in addition to causing hypotension.
The direct cardiac actions of muscarinic stimulants include the following:
a- an increase in a potassium current (IK(ACh)) in the cells of the sinoatrial and atrioventricular nodes, in Purkinje cells, and also in atrial and
ventricular muscle cells;
b- a decrease in the slow inward calcium current (ICa) in heart cells; and
c- a reduction in the hyperpolarization-activated current (If) that underlies diastolic depolarization.

- All these actions are mediated by M2 receptors and contribute to slowing the pacemaker rate. Effects (a) and (b) cause hyperpolarization,
reduce action potential duration, and decrease the contractility of atrial and ventricular cells.
- The knockout of M2 receptors eliminates the bradycardic effect of vagal stimulation and the negative chronotropic effect of carbachol on
sinoatrial rate. The direct slowing of sinoatrial rate and atrioventricular conduction that is produced by muscarinic agonists is often opposed by
reflex sympathetic discharge. Therefore, the net effect on heart rate depends on local concentrations of the agonist in the heart and in the
vessels and on the level of reflex responsiveness.
- Muscarinic receptors that are present on postganglionic parasympathetic nerve terminals allow neutrally released acetylcholine to inhibit its
own secretion. The neuronal muscarinic receptors need not be the same subtype as found on effector cells.
- Intravascular injection of muscarinic agonists produces vasodilation by M3 receptors. Muscarinic agonists release endothelium-derived relaxing
factor (EDRF), identified as nitric oxide (NO), from the endothelial cells.
- The NO diffuses to adjacent vascular smooth muscle, where it activates guanylyl cyclase and increases cGMP, resulting in relaxation. The
relaxing effect of acetylcholine was maximal at 3 × 10−7
M. This effect was eliminated in the absence of endothelium, and acetylcholine, at
concentrations greater than 10−7
M, then caused contraction. This results from a direct effect of acetylcholine on vascular smooth muscle in
which activation of M3 receptors stimulates IP3 production and releases intracellular calcium.
- Parasympathetic nerves can regulate arteriolar tone in vascular beds in thoracic and abdominal visceral organs. Acetylcholine released from
postganglionic parasympathetic nerves relaxes coronary arteriolar smooth muscle via the NO/cGMP pathway. Damage to the endothelium, as
occurs with atherosclerosis, eliminates this action, and acetylcholine is then able to contract arterial smooth muscle and produce
vasoconstriction.

- The cardiovascular effects of all the choline esters are similar to those of acetylcholine—the main difference being in their potency and duration
of action. Because of the resistance of Methacholine, Carbachol, and Bethanechol to acetylcholinesterase, lower doses given intravenously are
sufficient to produce effects similar to those of acetylcholine, and the duration of action of these synthetic choline esters is longer.
- The cardiovascular effects of most of the cholinomimetic natural alkaloids and the synthetic analogs are also generally similar to those of
acetylcholine. Pilocarpine is an interesting exception to the above statement. If given intravenously (an experimental exercise), it may produce
hypertension after a brief initial hypotensive response.
3. Respiratory system: -
- Muscarinic stimulants contract the smooth muscle of the bronchial tree. In addition, the glands of the tracheobronchial mucosa are stimulated
to secrete. This combination of effects can occasionally cause symptoms, especially in individuals with asthma. The bronchoconstriction caused
by muscarinic agonists is eliminated in knockout animals in which the M3 receptor has been mutated.
4. Gastrointestinal tract: -
- Administration of muscarinic agonists, as in parasympathetic nervous system stimulation, increases the secretory and motor activity of the gut.
The salivary and gastric glands are strongly stimulated; the pancreas and small intestinal glands are stimulated less so.
- Peristaltic activity is increased throughout the gut, and most sphincters are relaxed. Stimulation of contraction in this organ system involves
depolarization of the smooth muscle cell membrane and increased calcium influx.
- The M3 receptor is required for direct activation of smooth muscle contraction, whereas the M2 receptor reduces cAMP formation and
relaxation caused by sympathomimetic drugs.

5. Genitourinary tract: -
- Muscarinic agonists stimulate the detrusor muscle and relax the trigone and sphincter muscles of the bladder, thus promoting voiding. The
function of M2 and M3 receptors in the urinary bladder appears to be the same as in intestinal smooth muscle. The human uterus is not notably
sensitive to muscarinic agonists.
6. Central nervous system: -
- CNS functions of ACh include modulation of sleep, wakefulness, learning, and memory; suppression of pain at the spinal cord level; and
essential roles in neural plasticity, early neural development, immunosuppression, and epilepsy.
- Both nicotinic and muscarinic receptors are expressed in central neurons. Nicotinic receptors are primarily involved as presynaptic
heteroreceptors that modulate the release of other neurotransmitters, such as glutamate, whereas muscarinic presynaptic receptors are
primarily autoreceptors that modulate the release of ACh.
- As part of the ascending reticular activating system, cholinergic neurons play an important role in arousal and attention. Levels of ACh
throughout the brain increase during wakefulness and REM sleep and decrease during inattentive states and non-REM/slow-wave sleep
(SWS).
- While systemically administered ACh has limited CNS penetration, muscarinic agonists that can cross the blood-brain barrier evoke a
characteristic cortical arousal or activation response similar to that produced by injection of cholinesterase inhibitors or by electrical
stimulation of the brainstem reticular formation.
- All five muscarinic receptor subtypes are expressed in the brain, and recent studies suggest that muscarinic receptor–regulated pathways may
have an important role in cognitive function, motor control, appetite regulation, nociception, and other processes.

7. Neuromuscular junction: -
- The nicotinic receptors on the neuromuscular end plate apparatus are similar but not identical to the receptors in the autonomic ganglia. Both
types respond to acetylcholine and nicotine.
- When a nicotinic agonist is applied directly (by iontophoresis or by intra-arterial injection), an immediate depolarization of the end plate
results, caused by an increase in permeability to sodium and potassium ions.
- The contractile response varies from disorganized fasciculations of independent motor units to a strong contraction of the entire muscle
depending on the synchronization of depolarization of endplates throughout the muscle.
- Depolarizing nicotinic agents that are not rapidly hydrolysed cause rapid development of depolarization blockade; transmission blockade
persists even when the membrane has repolarized.
 Choline esters:
A- Acetylcholine:
- Although rarely given systemically, ACh is used topically for the induction of miosis during ophthalmologic surgery, instilled into the eye as a 1%
solution.
B- Methacholine:
- Methacholine is administered by inhalation for the diagnosis of bronchial airway hyperreactivity in patients who do not have clinically apparent
asthma. It is available as a powder that is diluted with 0.9% NaCl and administered via a nebulizer.
- While muscarinic agonists can cause bronchoconstriction and increased tracheobronchial secretions in all individuals, asthmatic patients respond
with intense bronchoconstriction and a reduction in vital capacity.
- The response to methacholine may be exaggerated or prolonged in patients taking β adrenergic receptor antagonists. Contraindications to
methacholine testing include severe airflow limitation, recent myocardial infarction or stroke, uncontrolled hypertension, or pregnancy.

- Emergency resuscitation equipment, oxygen, and medications to treat severe bronchospasm (e.g., β2 adrenergic receptor agonists for inhalation)
should be available during testing.
C- Bethanechol:
- Bethanechol primarily affects the urinary and GI tracts. In the urinary tract, bethanechol has utility in treating urinary retention and inadequate
emptying of the bladder when organic obstruction is absent, as in postoperative urinary retention, diabetic autonomic neuropathy, and certain
cases of chronic hypotonic, myogenic, or neurogenic bladder; catheterization can thus be avoided.
- When used chronically, 10–50 mg of the drug is given orally three to four times daily; the drug should be administered on an empty stomach (i.e., 1
h before or 2 h after a meal) to minimize nausea and vomiting.
- In the GI tract, bethanechol stimulates peristalsis, increases motility, and increases resting lower oesophageal sphincter pressure. Bethanechol
formerly was used to treat postoperative abdominal distention, gastric atony, gastroparesis, adynamic ileus, and gastroesophageal reflux.
D- Carbachol:
- Carbachol is used topically in ophthalmology for the treatment of glaucoma and the induction of miosis during surgery; it is instilled into the eye as
a 0.01%–3% solution.
 Cholinomimetic Alkaloids:
A- Pilocarpine:
- It is obtained from the leaves of Pilocarpus microphyllus and other species. It has prominent muscarinic actions and also stimulates ganglia.
Pilocarpine causes marked sweating, salivation and increase in other secretions.
- Small doses generally cause fall in BP (muscarinic), but higher doses cause rise in BP and tachycardia which is probably due to ganglionic
stimulation (through ganglionic muscarinic receptors).

- Applied to the eye, it penetrates cornea and promptly causes miosis, ciliary muscle contraction and fall in intraocular tension lasting 4-8 hours.
Pilocarpine is used only in the eye as 0.5-4% drops. It is a third-line drug in open angle glaucoma.
- Other uses as a miotic are- to counteract mydriatics after they have been used for testing refraction and to prevent/break adhesions of iris with lens
or cornea by alternating it with mydriatics.
Mechanism of action:
- It acts on a subtype of muscarinic receptor (M3) found on the iris sphincter muscle, causing the muscle to contract - resulting in pupil
constriction (miosis).
- Pilocarpine also acts on the ciliary muscle and causes it to contract. When the ciliary muscle contracts, it opens the trabecular meshwork
through increased tension on the scleral spur. This action facilitates the rate that aqueous humour leaves the eye to decrease intraocular
pressure.
- Paradoxically, when pilocarpine induces this ciliary muscle contraction (known as an accommodative spasm) it causes the eye's lens to thicken
and move forward within the eye. This movement causes the iris (which is located immediately in front of the lens) to also move forward,
narrowing the Anterior chamber angle. Narrowing of the anterior chamber angle increases the risk of increased intraocular pressure.
Medical Uses:
- Pilocarpine stimulates the secretion of large amounts of saliva and sweat. It is used to treat dry mouth, particularly in Sjogren syndrome, but
also as a side effect of radiation therapy for head and neck cancer.
- It may be used to help differentiate Adie syndrome from other causes of unequal pupil size. It may be used to treat a form of dry eye called
aqueous deficient dry eye (ADDE).

a- Surgery: -
Pilocarpine is sometimes used immediately before certain types of corneal grafts and cataract surgery. In ophthalmology, pilocarpine is also
used to reduce symptomatic glare at night from lights when the patient has undergone implantation of phakic intraocular lenses; the use of
pilocarpine would reduce the size of the pupils, partially relieving these symptoms.
The most common concentration for this use is pilocarpine 1%.
b- Other: -
Pilocarpine is used to stimulate sweat glands in a sweat test to measure the concentration of chloride and sodium that is excreted in sweat.
It is used to diagnose cystic fibrosis.
B- Muscarine:
- Muscarine, L- (+)-muscarine, or muscarin is a natural product found in certain mushrooms, particularly in Inocybe and Clitocybe species, such as
the deadly C. dealbata.
- Muscarine has been found in harmless trace amounts in Boletus, Hygrocybe, Lactarius and Russula. Trace concentrations of muscarine are also
found in Amanita muscaria, though the pharmacologically more relevant compound from this mushroom is the Z-drug-like alkaloid muscimol.
- A. muscaria fruitbodies contain a variable dose of muscarine, usually around 0.0003% fresh weight. This is very low and toxicity symptoms occur
very rarely. Inocybe and Clitocybe contain muscarine concentrations up to 1.6%.
Pharmacodynamics: -
- Muscarine mimics the action of the neurotransmitter acetylcholine by agonising muscarinic acetylcholine receptors. These receptors were
named after muscarine, to differentiate them from the other acetylcholine receptors (nicotinic receptors), which are comparatively
unresponsive to muscarine.

- There are 5 different types of muscarinic receptors; M1, M2, M3, M4 and M5. Most tissues express a mixture of subtypes. The M2 and M3
subtypes mediate muscarinic responses at peripheral autonomic tissues.
- M1 and M4 subtypes are more abundant in brain and autonomic ganglia. The odd numbered receptors, M1, M3 and M5, interact with Gq proteins
to stimulate phosphoinositide hydrolysis and the release of intracellular calcium. Conversely, the even numbered receptors, M2 and M4, interact
with Gi proteins to inhibit adenylyl cyclase, which results in a decrease of intracellular concentration of cyclic adenosine monophosphate
(cAMP).
Metabolism: -
- This compound is not metabolized by humans. Though there has been extensive research in the field of acetylcholine metabolism by
acetylcholinesterase, muscarine is not metabolized by this enzyme, partly explaining the compound's potential toxicity.
- Muscarine is readily soluble in water. The most likely way for muscarine to leave the blood is via renal clearance; it will eventually leave the
body in urine.
C- Arecoline: -
- Arecoline is a nicotinic acid-based mild parasympathomimetic stimulant alkaloid found in the areca nut, the fruit of the areca palm (Areca
catechu). It is an odourless oily liquid.
- Arecoline has been compared to nicotine; however, nicotine acts primarily on the nicotinic acetylcholine receptor. Arecoline is known to be a
partial agonist of muscarinic acetylcholine M1, M2, M3 receptors and M4, which is believed to be the primary cause of its parasympathetic effects.
Arecoline also acts as an agonist on the nicotinic receptor.

Effects on nervous system: -
- Arecoline promotes excitation and decreases sleeping time. It also enhances learning and memory. Intraperitoneal administration of arecoline
decreases locomotor activity dose dependently. Arecoline reversed scopolamine induced memory loss. It could also decrease symptoms of
depression and schizophrenia.
Effects on cardiovascular system: -
- AN (Areca Nut) is a vasodilator mainly due to presence of arecoline. It also has anti-thrombosis and anti-atherogenic effects by increasing
plasma nitric oxide, and mRNA expression and decreasing IL-8 along with other downregulations.
Effects on endocrine system: -
- It increases level of testosterone by stimulating Leydig's cells as well as levels of FSH and LH. It also activates HPA axis and stimulates CRH
release. It prevents dysfunction of B cells of pancreas from high fructose intake.
Effects on digestive system: -
- Arecoline has the ability to stimulate digestive system through activation os muscarinic receptors. Areca nut water extract could increase the
contractions of gastric smooth muscle and muscle strips of duodenum, ileum and colon significantly. This activity could be caused my arecoline.
 Indirect acting cholinomimetic drugs or anticholinesterases (AChEs):
 Chemistry and Structural activity relationships: -
- There are three chemical groups of cholinesterase inhibitors:
A- Simple alcohols bearing a quaternary ammonium group, e.g., edrophonium;
B- Carbamic acid esters of alcohols having quaternary or tertiary ammonium groups (carbamates, e.g., neostigmine);
C- Organic derivatives of phosphoric acid (organophosphates, e.g., echothiophate).

- The general formulae of carbamates and organophosphates are as follows:
- The R1 in carbamates may have a nonpolar tertiary amino, e.g. in physostigmine, rendering the compound lipid soluble. In others, e.g. neostigmine, R1
has a quaternary – rendering it lipid insoluble. All organophosphates are highly lipid soluble except echothiophate which is water soluble.
 Absorption, Distribution, and Metabolism: -
i. Absorption of the quaternary carbamates from the conjunctiva, skin, gut, and lungs is predictably poor, since their permanent charge renders them
relatively insoluble in lipids. Thus, much larger doses are required for oral administration than for parenteral injection.
ii. Distribution into the central nervous system is negligible. Physostigmine, in contrast, is well absorbed from all sites and can be used topically in
the eye. It is distributed into the central nervous system and is more toxic than the more polar quaternary carbamates.
iii. The carbamates are relatively stable in aqueous solution but can be metabolized by nonspecific esterases in the body as well as by cholinesterase.
However, the duration of their effect is determined chiefly by the stability of the inhibitor-enzyme complex, not by metabolism or excretion.
iv. The organophosphate cholinesterase inhibitors (except for echothiophate) are well absorbed from the skin, lung, gut, and conjunctiva—thereby
making them dangerous to humans and highly effective as insecticides. They are relatively less stable than the carbamates when dissolved in water.

v. Echothiophate is highly polar and more stable than most other organophosphates. The thiophosphate insecticides (parathion, malathion, and
related compounds) are quite lipid-soluble and are rapidly absorbed by all routes. They must be activated in the body by conversion to the oxygen
analogs, a process that occurs rapidly in both insects and vertebrates.
vi. Malathion is rapidly metabolized by other pathways to inactive products in birds and mammals; these agents are therefore considered safe enough
for sale to the general public. Unfortunately, fish cannot detoxify malathion, and significant numbers of fish have died from the heavy use of this
agent on and near waterways.
vii. Parathion is not detoxified effectively in vertebrates; thus, it is considerably more dangerous than malathion to humans and livestock and is not
available for general public use in the USA.
viii. All the organophosphates except echothiophate are distributed to all parts of the body, including the central nervous system. Therefore, central
nervous system toxicity is an important component of poisoning with these agents.
 Molecular Mechanism of Action of AChE Inhibitors: -
i. Three distinct domains on AChE constitute binding sites for inhibitory ligands and form the basis for specificity differences between AChE and
butyrylcholinesterase:
a- the acyl pocket of the active centre;
b- the choline subsite of the active centre
c- the peripheral anionic site.
ii. Reversible inhibitors, such as edrophonium and tacrine, bind to the choline subsite in the vicinity of Trp86 and Glu202. Edrophonium has a brief
duration of action because its quaternary structure facilitates renal elimination, and it binds reversibly to the AChE active centre.

iii. Additional reversible inhibitors, such as donepezil, bind with higher affinity to the active centre gorge. Other reversible inhibitors, such as
propidium and the snake peptidic toxin fasciculin, bind to the peripheral anionic site on AChE. This site resides at the rim of the gorge and is
defined by Try286 and Tyr72 andTyr124.
iv. Drugs that have a carbamoyl ester linkage, such as physostigmine and neostigmine, are hydrolysed by AChE, but much more slowly than is ACh.
The quaternary amine neostigmine and the tertiary amine physostigmine exist as cations at physiological pH.
v. By serving as alternate substrates to ACh, their reaction with the active centre serine progressively generates the carbamoylated enzyme. The
conjugated carbamoyl moiety resides in the acyl pocket outlined by Phe295 and Phe297. In contrast to the acetyl enzyme, methyl carbamoyl AChE
and dimethyl carbamoyl AChE are far more stable (the t1/2 for hydrolysis of the dimethyl carbamoyl enzyme is 15–30 min). Sequestration of the
enzyme in its carbamoylated form thus precludes the enzyme-catalysed hydrolysis of ACh for extended periods of time.
vi. The organophosphate inhibitors, such as DFP, serve as true hemi substrates; the resultant conjugate with the active centre serine phosphorylated
or phosphonylated is extremely stable. The organophosphorus inhibitors are tetrahedral in configuration, a configuration that resembles the
transition state formed in carboxyl ester hydrolysis.
vii. The phosphoryl oxygen binds within the oxyanion hole of the active centre. If the alkyl groups in the phosphorylated enzyme are ethyl or methyl,
spontaneous regeneration of active enzyme requires several hours.
viii. Secondary (as in DFP) or tertiary alkyl groups further enhance the stability of the phosphorylated enzyme, and significant regeneration of active
enzyme usually is not observed. The stability of the phosphorylated enzyme is enhanced through “aging,” which results from the loss of one of the
alkyl groups.
ix. Hence, the return of AChE activity depends on biosynthesis of new AChE protein. Thus, the terms reversible and irreversible as applied to the
carbamoyl ester and organophosphate anti-ChE agents are relative terms, reflecting only quantitative differences in rates of de-carbamoylation or

dephosphorylation of the conjugated enzyme. Both chemical classes react covalently with the active centre serine in essentially the same manner as
does ACh in forming the transient acetyl enzyme.
 Pharmacological actions: -
- The sites of action of anti-ChE agents of therapeutic importance are the CNS, eye, intestine, and neuromuscular junction of skeletal muscle; other
actions are of toxicological consequence.
A- Eye: -
i. When applied locally to the conjunctiva, anti-ChE agents cause conjunctival hyperaemia and constriction of the pupillary sphincter muscle
around the pupillary margin of the iris (miosis) and the ciliary muscle (block of accommodation reflex with resultant focusing to near vision).
ii. Although the pupil may be “pinpoint” in size, it generally contracts further when exposed to light. Intraocular pressure, when elevated,
usually falls as the result of facilitation of outflow of the aqueous humour.
B- GI Tract: -
i. In humans, neostigmine enhances gastric contractions and increases the secretion of gastric acid. After bilateral vagotomy, the effects of
neostigmine on gastric motility are greatly reduced.
ii. The lower portion of the esophagus is stimulated by neostigmine; in patients with marked achalasia and dilation of the esophagus, the drug
can cause a salutary increase intone and peristalsis.
iii. Neostigmine also increases motor activity of the small and large bowel; the colon is particularly stimulated. The total effect of anti-ChE
agents on intestinal motility probably represents a combination of actions at the ganglion cells of the Auerbach plexus and at the smooth
muscle fibres as a result of the preservation of ACh released by the cholinergic preganglionic and postganglionic fibres, respectively.

C- Neuromuscular Junction: -
i. The cholinesterase inhibitors have important therapeutic and toxic effects at the skeletal muscle neuromuscular junction. Low (therapeutic)
concentrations moderately prolong and intensify the actions of physiologically released acetylcholine. This increases the strength of
contraction, especially in muscles weakened by curare-like neuromuscular blocking agents or by myasthenia gravis.
ii. At higher concentrations, the accumulation of acetylcholine may result in fibrillation of muscle fibres. Antidromic firing of the motor neuron
may also occur, resulting in fasciculations that involve an entire motor unit.
iii. With marked inhibition of acetylcholinesterase, depolarizing neuromuscular blockade occurs and that may be followed by a phase of
nondepolarizing blockade as seen with succinylcholine.
iv. Some quaternary carbamate cholinesterase inhibitors, e.g., neostigmine and pyridostigmine, have an additional direct nicotinic agonist effect
at the neuromuscular junction. This may contribute to the effectiveness of these agents as therapy for myasthenia.
D- Cardiopulmonary System: -
i. The predominant effect on the heart of accumulated ACh is bradycardia, resulting in a fall in cardiac output. Higher doses usually enhance
the fall in blood pressure, as a consequence of effects of anti-ChE agents on the medullary vasomotor centres of the CNS.
ii. Anti-ChE agents increase vagal influences on the heart. This shortens the effective refractory period of atrial muscle fibres and increases the
refractory period and conduction time at the sinoatrial and atrioventricular nodes.
iii. At the ganglionic level, accumulating ACh initially is excitatory on nicotinic receptors, but at higher concentrations, ganglionic blockade
occurs as a result of persistent depolarization of the postsynaptic nerve.
iv. The excitatory action on the parasympathetic ganglion cells would diminish cardiac output, whereas the opposite sequence results from the
action of ACh on sympathetic ganglion cells.

v. Excitation followed by inhibition also is elicited by ACh at the central medullary vasomotor and cardiac centres. All of these effects are
complicated further by the hypoxemia resulting from the bronchoconstrictor and secretory actions of increased ACh on the respiratory
system; hypoxemia, in turn, can reinforce both sympathetic tone and ACh-induced discharge of epinephrine from the adrenal medulla.
vi. Hence, it is not surprising that an increase in heart rate is seen with severe ChE inhibitor poisoning. Hypoxemia probably is a major factor in
the CNS depression that appears after large doses of anti-ChE agents.
E- Actions at Other Sites: -
i. Secretory glands that are innervated by postganglionic cholinergic fibres include the bronchial, lacrimal, sweat, salivary, gastric (antral G
cells and parietal cells), intestinal, and pancreatic acinar glands.
ii. Low doses of anti-ChE agents increases secretory responses to nerve stimulation, and higher doses actually produce an increase in the resting
rate of secretion. Anti-ChE agents increase contraction of smooth muscle fibres of the bronchioles and ureters, and the ureters may show
increased peristaltic activity.
 Therapeutic uses of anti-ChE:
- Current use of anti-ChE agents is limited to four conditions in the periphery:
a- atony of the smooth muscle of the intestinal tract and urinary bladder
b- glaucoma
c- myasthenia gravis
d- reversal of the paralysis of competitive neuromuscular blocking drugs
A- Paralytic Ileus and Atony of the Urinary Bladder:
i. In the treatment of both paralytic ileus and urinary bladder atony, neostigmine generally is preferred among the anti-ChE agents. Directly acting
muscarinic agonists are employed for the same purposes.

ii. Neostigmine is used for the relief of abdominal distension and acute colonic pseudo-obstruction from a variety of medical and surgical causes.
The usual subcutaneous dose of neostigmine methyl sulphate for postoperative paralytic ileus is 0.5 mg, given as needed.
iii. Peristaltic activity commences 10–30 min after parenteral administration, whereas 2–4 h are required after oral administration of neostigmine
bromide (15–30 mg). It may be necessary to assist evacuation with a small low enema or gas with a rectal tube.
iv. When neostigmine is used for the treatment of atony of the detrusor muscle of the urinary bladder, postoperative dysuria is relieved. The drug is
used in a similar dose and manner as in the management of paralytic ileus.
v. Neostigmine should not be used when the intestine or urinary bladder is obstructed, when peritonitis is present, when the viability of the bowel
is doubtful, or when bowel dysfunction results from inflammatory bowel disease.
B- Glaucoma and Other Ophthalmologic Indications: -
i. Glaucoma is a complex disease characterized by an increase in intraocular pressure that, if sufficiently high and persistent, will damage the optic
disc at the juncture of the optic nerve and the retina; irreversible blindness can result.
ii. Of the three types of glaucoma—primary, secondary, and congenital—anti-AChE agents are of value in the management of the primary as well as
of certain categories of the secondary type (e.g., aphakic glaucoma, following cataract extraction); congenital glaucoma rarely responds to any
therapy other than surgery.
iii. Primary glaucoma is subdivided into narrow-angle (acute congestive) and wide-angle (chronic simple) types, based on the configuration of the
angle of the anterior chamber where the aqueous humour is reabsorbed.
iv. Narrow-angle glaucoma is nearly always a medical emergency in which drugs are essential in controlling the acute attack, but the long-range
management is often surgical (e.g., peripheral or complete iridectomy).
v. Wide-angle glaucoma, on the other hand, has a gradual, insidious onset and is not generally amenable to surgical improvement; in this type,
control of intraocular pressure usually is dependent on continuous drug therapy.

vi. Because the cholinergic agonists and ChE inhibitors also block accommodation and induce myopia, these agents produce transient blurring of far
vision, limited visual acuity in low light, and loss of vision at the margin when instilled in the eye.
vii. With long-term administration of the cholinergic agonists and anti-ChE agents, the compromise of vision diminishes. Topical treatment with
long-acting ChE inhibitors such as echothiophate give rise to symptoms characteristic of systemic ChE inhibition.
viii. Pilocarpine is preferred as miotic. The action is rapid and short lasting (4-6 hr); 6-8 hourly instillation is required. Diminution of vision,
especially in dim light (due to constricted pupil), spasm of accommodation and brow pain are frequent side effects. Systemic effects-nausea,
diarrhoea, sweating and bronchospasm may occur with higher concentration eye drops.
ix. Physostigmine (0.1 %) is used only to supplement pilocarpine. Miotics are now 3rd choice drugs, used only as add on therapy in advanced cases.
Pilocarpine (along with other drugs) is used in angle closure glaucoma as well.
C- Myasthenia Gravis: -
i. Myasthenia gravis is a neuromuscular disease characterized by exacerbations and remissions of weakness and marked fatigability of skeletal
muscle.
ii. Anti-receptor antibodies are detectable in sera of 90% of patients with the disease. Myasthenia gravis is caused by an autoimmune response
primarily to the ACh receptor at the postjunctional end plate.
iii. These antibodies reduce the number of receptors detectable either by snake α-neurotoxin–binding assays or by electrophysiological
measurements of ACh sensitivity. Immune complexes along with marked ultrastructural abnormalities appear in the synaptic cleft and enhance
receptor degradation through complement-mediated lysis in the end plate.
iv. In a subset of about 10% of patients presenting with a myasthenic syndrome, muscle weakness has a congenital rather than an autoimmune basis.
Characterization of biochemical and genetic bases of the congenital condition has demonstrated mutations in the ACh receptor that affect ligand-
binding, channel-opening kinetics and durations; receptor biosynthesis; and synaptic location of receptors.

Diagnosis: -
i. Diagnosis of autoimmune myasthenia gravis usually can be made from the history, signs, and symptoms. However, in autoimmune
myasthenia gravis, the aforementioned deficiencies and enhancement of muscle strength can be improved dramatically by anti-ChE
medication. The edrophonium test for initial diagnosis relies on these responses.
ii. The edrophonium test is performed by rapid intravenous injection of 2 mg of edrophonium chloride, followed 45 sec later by an additional 8
mg if the first dose is without effect. A positive response consists of brief improvement in strength, unaccompanied by lingual fasciculation
(which generally occurs in nonmyasthenic patients).
iii. An excessive dose of an anti-ChE drug results in a cholinergic crisis. The condition is characterized by weakness resulting from generalized
depolarization of the motor end plate; other features result from overstimulation of muscarinic receptors.
iv. Detection of anti-receptor antibodies in muscle biopsies or plasma is now widely employed to establish the diagnosis.
Treatment of Myasthenia Gravis: -
i. Pyridostigmine, neostigmine, and ambenonium are the standard anti-ChE drugs used in the symptomatic treatment of myasthenia gravis. All
can increase the response of myasthenic muscle to repetitive nerve impulses, primarily by the preservation of endogenous ACh.
ii. Following AChE inhibition, receptors over a greater cross-sectional area of the endplate are exposed to concentrations of ACh that are
sufficient for channel opening and production of a postsynaptic end-plate potential.
iii. Pyridostigmine is available in sustained-release tablets containing a total of 180 mg, of which 60 mg are released immediately and 120 mg are
released over several hours; this preparation is of value in maintaining patients for 6- to 8-h periods but should be limited to use at bedtime.
iv. Muscarinic cardiovascular and GI side effects of anti-ChE agents generally can be controlled by atropine or other anticholinergic drugs.
However, these anticholinergic drugs mask many side effects of an excessive dose of an anti-ChE agent.

v. Other therapeutic measures are essential elements in the management of this disease. Glucocorticoids promote clinical improvement in a
high percentage of patients. Initiation of steroid treatment increases muscle weakness; however, as the patient improves with continued
administration of steroids, doses of anti-ChE drugs can be reduced.
vi. Other immunosuppressive agents, such as azathioprine and cyclosporine and high-dose cyclophosphamide, have also been beneficial in more
refractory cases. Thymectomy should be considered in myasthenia associated with a thymoma or when the disease is not controlled
adequately by anti-ChE agents and steroids.
D- Alzheimer Disease: -
i. Alzheimer disease is the progressive destruction of memory and other important mental function. A deficiency of intact cholinergic neurons,
particularly those extending from subcortical areas such as the nucleus basalis, has been observed in patients with progressive dementia of the
Alzheimer type.
ii. In 1993, the FDA approved tacrine (tetrahydroaminoacridine) for use in mild-to-moderate Alzheimer disease, but a high incidence of enhanced
alanine aminotransferase and hepatotoxicity limited the utility of this drug.
iii. Subsequently, donepezil was approved for clinical use and has emerged as the primary agent for treatment in multiple countries. Initially, 5-mg
doses are administered daily, and if tolerated, doses are increased to 10 mg for mild-to-moderate conditions. Recent clinical trials in moderate-
to-severe Alzheimer disease have confirmed benefits for a 23-mg/d sustained release form. Adverse side effects have been attributed to excessive
peripheral cholinergic stimulation and include nasopharyngitis, diarrhoea, nausea, and vomiting. Rhabdomyolysis reportedly occurs, requiring
discontinuation of the drug. Cotreatment with memantine did not result in significant improvement over the higher-dose donepezil treatment.
iv. Rivastigmine, a more lipid soluble, longer-acting carbamylating inhibitor, is approved for use in the U.S. and Europe in both oral and skin patch
forms. While having similar side effects to other cholinesterase inhibitors, rivastigmine have a higher incidence of fatalities than other

cholinesterase inhibitors used in Alzheimer dementias. It has not been determined whether the increase relates to misuse of the transdermal form
of administration.
v. Galantamine is another FDA-approved agent for Alzheimer dementias, acting as a reversible AChE inhibitor with a side-effect profile similar to
that of donepezil. These three cholinesterase inhibitors, which have the requisite affinity and hydrophobicity to cross the blood-brain barrier and
exhibit a prolonged duration of action, along with an excitatory amino acid transmitter mimic, memantine, constitute current modes of therapy.
E- Cobra bite: -
i. Cobra venom has a curare like neurotoxin. Though specific antivenom serum is the primary treatment, neostigmine + atropine prevent
respiratory paralysis.
F- Belladonna poisoning: -
i. Physostigmine 0.5- 2 mg i.v. repeated as required is the specific antidote for poisoning with belladonna or other anticholinergics. It penetrates
blood-brain barrier and antagonizes both central and peripheral actions.
ii. However, physostigmine often itself induces hypotension, arrhythmias and undesirable central effects. It is therefore employed only as a last
resort. Neostigmine does not block the central effect, but is less risky.
 Anticholinesterase poisoning:
i. Anticholinesterases are used as agricultural and household insecticides; accidental as well as suicidal and homicidal poisoning is common. Local
muscarinic manifestations at the site of exposure occur immediately and are followed by complex systemic effects due to muscarinic, nicotinic and
central actions.
ii. They are-
a- Irritation of eye, lacrimation, salivation, sweating, copious tracheo-bronchial secretions, miosis, blurring of vision, bronchospasm, breathlessness,
colic, involuntary defecation and urination.

b- Fall in BP, bradycardia or tachycardia, cardiac arrhythmias, vascular collapse.
c- Muscular fasciculations, weakness, respiratory paralysis (central as well as peripheral).
d- Irritability, disorientation, unsteadiness, tremor, ataxia, convulsions, coma and death.
Treatment: -
a- Termination of further exposure to the poison-fresh air, wash the skin and mucous membranes with soap and water, gastric lavage according to need.
b- Maintain patent airway, positive pressure respiration if it is failing.
c- Supportive measures- maintain BP, hydration, control of convulsions with judicious use of diazepam.
d- Specific antidotes-
Atropine: -
It is highly effective in counteracting the muscarinic symptoms, but higher doses are required to antagonize the central effects. It does not reverse
peripheral muscular paralysis which is a nicotinic action.
All cases of anti-ChE poisoning must be promptly given atropine 2 mg i.v. repeated every 10 min till dryness of mouth or other signs of atropinisation
appear (upto 200 mg has been administered in a day). Continued treatment with maintenance doses may be required for 1- 2 weeks.
Cholinesterase reactivators: -
Oximes are used to restore neuromuscular transmission only in case of organophosphate anti-ChE poisoning. The phosphorylated ChE reacts very
slowly or not at all with water. However, if more reactive OH groups in the form of oximes (generic formula R-CH =N−OH) are provided,
reactivation occurs more than a million times faster.
Pralidoxime (2-PAM) has a positively charged quaternary nitrogen: attaches to the anionic site of the enzyme which remains unoccupied in the
presence of organophosphate inhibitors. Its oxime end reacts with the phosphorus atom attached to the esteratic site: the oxime-phosphonate so
formed diffuses away leaving the reactivated ChE.

Pralidoxime is ineffective as an antidote to carbamate anti-ChEs (physostigmine, neostigmine, carbaryl, propoxur) in which case the anionic site of
the enzyme is not free to provide attachment to it. It is rather contraindicated in carbamate poisoning, because not only it does not reactivate
carbamylated enzyme, it has weak anti-ChE activity or its own.
Pralidoxime (NEOPAM, PAM-A INJ. 500 mg/20 ml infusion, LYPHE I g/vial for inj.) is injected i.v. slowly in a dose of 1-2 g (children 20-
40mg/kg). Another regimen is 30 mg/kg i.v. loading dose, followed by 8- 10 mg/kg/hour continuous infusion till recovery.
Pralidoxime causes more marked reactivation or skeletal muscle ChE than at autonomic sites and not at all in the CNS, because it does not penetrate
into brain. Treatment should be started as early as possible (within few hours), before the phosphorylated enzyme has undergone ‘aging’ and
become resistant to hydrolysis.
 Individual compounds: -
A- Physostigmine: -
Physostigmine is a highly toxic parasympathomimetic alkaloid, specifically, a reversible cholinesterase inhibitor. It occurs naturally in the Calabar
bean and the Manchineel tree.
Medical uses: -
i. Physostigmine is used to treat glaucoma and delayed gastric emptying. Because it enhances the transmission of acetylcholine signals in the brain
and can cross the blood–brain barrier, physostigmine salicylate is used to treat anticholinergic poisoning (that is, poisoning by substances that
interfere with the transmission of acetylcholine signalling, such as atropine, scopolamine, and other anticholinergic drug overdoses).
ii. It is also used to reverse neuromuscular blocking. Physostigmine is the antidote of choice for Datura stramonium poisoning. It is also an antidote
for Atropa belladonna poisoning, the same as for atropine.
iii. It has been also used as an antidote for poisoning with γ-Hydroxybutyric acid (GHB), but is poorly effective and often causes additional toxicity,
so is not a recommended treatment.

iv. It improves long term memory, and was once explored as a therapy for Alzheimer’s disease.
v. Recently, physostigmine has been proposed as an antidote for intoxication with gamma hydroxybutyrate (GHB, a potent sedative-hypnotic agent
that can cause loss of consciousness, loss of muscle control, and death). Physostigmine may counteract GHB by producing a nonspecific state of
arousal.
Pharmacology: -
i. Physostigmine acts by interfering with the metabolism of acetylcholine. It is a covalent (reversible – bond hydrolysed and released) inhibitor of
acetylcholinesterase. It indirectly stimulates both nicotinic and muscarinic acetylcholine receptors.
ii. Physostigmine has an LD50 of 3 mg/kg in mice.
iii. Combination of acetylcholine and physostigmine is an example of supra-additive phenomenon.
iv. Physostigmine functions as an acetylcholinesterase inhibitor. Its mechanism is to prevent the hydrolysis of acetylcholine by acetylcholinesterase
at the transmitted sites of acetylcholine. This inhibition enhances the effect of acetylcholine, making it useful for the treatment of cholinergic
disorders and myasthenia gravis.
v. More recently, physostigmine has been used to improve the memory of Alzheimer’s patients due to its potent anticholinesterase activity.
However, its drug form, physostigmine salicylate, has poor bioavailability.
vi. Physostigmine also has a miotic function, causing pupillary constriction. It is useful in treating mydriasis. Physostigmine also increases outflow of
the aqueous humour in the eye, making it useful in the treatment of glaucoma.
Side effects: -
i. An overdose can cause cholinergic syndrome. Other side effects may include nausea, vomiting, diarrhoea, anorexia, dizziness, headache, stomach
pain, sweating, dyspepsia, and seizures.

ii. The carbamate functional group readily hydrolyses in water, and in bodily conditions. The metabolite thus formed from physostigmine and some
other alkaloids (e.g. cymserine) is eseroline, which research has suggested may be neurotoxic to humans.
iii. Death can occur rapidly following overdose as a result of respiratory arrest and paralysis of the heart.
B- Neostigmine: -
It is given by injection either into a vein, muscle, or under the skin. After injection effects are generally greatest within 30 minutes and last up to 4
hours.
Medical uses: -
i. Neostigmine is a medication used to treat myasthenia gravis, Ogilvie syndrome, and urinary retention without the presence of a blockage. It is
also used together with atropine to end the effects of neuromuscular blocking medication of the non-depolarizing type.
ii. Hospitals sometimes administer a solution containing neostigmine intravenously to delay the effects of envenomation through snakebite. Some
promising research results have also been reported for administering the drug nasally as a snakebite treatment.
Pharmacology: -
i. By interfering with the breakdown of acetylcholine, neostigmine indirectly stimulates both nicotinic and muscarinic receptors.
ii. Neostigmine has a quaternary nitrogen; hence, it is more polar and does not cross the blood–brain barrier and enter the CNS, but it does cross
the placenta.
iii. Neostigmine has moderate duration of action – usually two to four hours. Neostigmine binds to the anionic and ester site of cholinesterase. The
drug blocks the active site of acetylcholinesterase so the enzyme can no longer break down the acetylcholine molecules before they reach the
postsynaptic membrane receptors. This allows for the threshold to be reached so a new impulse can be triggered in the next neuron.
iv. In myasthenia gravis there are too few acetylcholine receptors so with the acetylcholinesterase blocked, acetylcholine can bind to the few
receptors and trigger a muscular contraction.

Side effects: -
i. Common side effects include nausea, increased saliva, crampy abdominal pain, and slow heart rate. More severe side effects include low blood
pressure, weakness, and allergic reactions.
ii. Neostigmine can induce generic ocular side effects including: headache, brow pain, blurred vision, phacodonesis, pericorneal injection,
congestive iritis, various allergic reactions, and rarely, retinal detachment.
iii. Neostigmine will cause slowing of the heart rate (bradycardia); for this reason, it is usually given along with a parasympatholytic drug such as
atropine or glycopyrrolate. Gastrointestinal symptoms occur earliest after ingestion and include anorexia, nausea, vomiting, abdominal cramps,
and diarrhoea.
C- Pyridostigmine: -
Medical uses
i. Pyridostigmine is used to treat muscle weakness in people with myasthenia gravis or forms of congenital myasthenic syndrome and to combat the
effects of curariform drug toxicity.
ii. Pyridostigmine bromide has been FDA approved for military use during combat situations as an agent to be given prior to exposure to the nerve
agent Soman in order to increase survival.
iii. Pyridostigmine sometimes is used to treat orthostatic hypotension. It may also be of benefit in chronic axonal polyneuropathy.
iv. It is also being prescribed 'off-label' for the postural tachycardia syndrome as well as complications resulting from Ehlers–Danlos syndrome.
Mechanism of action: -
i. Pyridostigmine inhibits acetylcholinesterase in the synaptic cleft, thus slowing down the hydrolysis of acetylcholine. It is a quaternary carbamate
inhibitor of cholinesterase that does not cross the blood–brain barrier which carbamylates about 30% of peripheral cholinesterase enzyme. The
carbamylated enzyme eventually regenerates by natural hydrolysis and excess ACh levels revert to normal.

ii. The ACh diffuses across the synaptic cleft and binds to receptors on the post synaptic membrane, causing an influx of Na+, resulting in
depolarization.
Side effects
Common side effects include:
Sweating, Diarrhoea, Nausea, Vomiting, Abdominal cramps, Increased salivation, Tearing, Increased bronchial secretions, Constricted pupils, Facial
flushing due to vasodilation, Erectile dysfunction.
D- Edrophonium: -
The drug has a brief duration of action, about 10–30 mins. Dose: - 2-10mg i.v.
Clinical uses: -
i. Edrophonium is used to differentiate myasthenia gravis from cholinergic crisis and Lambert-Eaton. In myasthenia gravis, the body produces
autoantibodies which block, inhibit or destroy nicotinic acetylcholine receptors in the neuromuscular junction.
ii. Edrophonium—an effective acetylcholinesterase inhibitor—will reduce the muscle weakness by blocking the enzymatic effect of
acetylcholinesterase enzymes, prolonging the presence of acetylcholine in the synaptic cleft. It binds to a Serine-103 allosteric site, while
pyridostigmine and neostigmine bind to the AchE active site for their inhibitory effects.
iii. In a cholinergic crisis, where a person has too much neuromuscular stimulation, edrophonium will make the muscle weakness worse by inducing
a depolarizing block. In practice, the edrophonium test has been replaced by testing for autoantibodies, including acetylcholine receptor (AchR)
autoantibodies and muscle specific tyrosine kinase (MuSK) autoantibodies.
iv. Lambert-Eaton myasthenic syndrome (LEMS), is similar to myasthenia gravis in that it is an autoimmune disease. However, in LEMS the
neuron is unable to release enough acetylcholine for normal muscle function due to autoantibodies attacking P/Q-type calcium channel that are
necessary for acetylcholine release. This means there is insufficient calcium ion influx into presynaptic terminal resulting in reduced exocytosis of

acetylcholine containing vesicles. Consequently, there will be not as much increase in muscle strength observed after edrophonium injection, if
any with LEMS.
E- Rivastigmine: -
Rivastigmine (sold under the trade name Exelon) is a cholinesterase inhibitor used for the treatment of mild to moderate Alzheimer's disease and
Parkinson's.
Pharmacokinetics: -
i. When given orally, rivastigmine is well absorbed, with a bioavailability of about 40% in the 3-mg dose. Pharmacokinetics are linear up to 3 mg
BID, but nonlinear at higher doses.
ii. Elimination is through the urine. Peak plasma concentrations are seen in about one hour, with peak cerebrospinal fluid concentrations at 1.4–3.8
hours. When given by once-daily transdermal patch, the pharmacokinetic profile of rivastigmine is much smoother, compared with capsules, with
lower peak plasma concentrations and reduced fluctuations.
iii. The compound does cross the blood–brain barrier. Plasma protein binding is 40%. The major route of metabolism is by its target enzymes via
cholinesterase-mediated hydrolysis.
Pharmacodynamics: -
i. Rivastigmine, a cholinesterase inhibitor, inhibits both butyrylcholinesterase and acetylcholinesterase. It works by inhibiting these cholinesterase
enzymes, which would otherwise break down the brain neurotransmitter acetylcholine.
Medical uses: -
i. Rivastigmine capsules, liquid solution and patches are used for the treatment of mild to moderate dementia of the Alzheimer's type and
Parkinson's disease.

ii. Rivastigmine has effects on the cognitive (thinking and memory), functional (activities of daily living) and behavioural problems commonly
associated with Alzheimer's and Parkinson's disease dementias.
Side effects: -
i. Side effects may include nausea and vomiting, decreased appetite and weight loss.
ii. For the patch and oral formulations, skin rashes can occur. For the patch, patients and caregivers should monitor for any intense itching, redness,
swelling or blistering at the patch location. If this occurs, remove the patch, rinse the area and call the doctor immediately.
F- Donepezil: -
Medical uses: -
i. This is a centrally acting anti-ChE that has produced cognitive and behavioural improvement in AD & Lewy body dementia.
ii. Traumatic brain injury: Some research suggests an improvement in memory dysfunction in patients with traumatic brain injury with donepezil
use.
iii. Vascular dementia: Studies have shown that donepezil may improve cognition in patients with vascular dementia but not overall global
functioning.
iv. Dementia associated with Parkinson disease: Some evidence suggests that donepezil can improve cognition, executive function, and global status
in Parkinson disease dementia.
Mechanism of action
i. Donepezil binds and reversibly inactivates the cholinesterase, thus inhibiting hydrolysis of acetylcholine. This increases acetylcholine
concentrations at cholinergic synapses.
ii. Certainly, Alzheimer's disease involves a substantial loss of the elements of the cholinergic system and it is generally accepted that the symptoms
of Alzheimer's disease are related to this cholinergic deficit, particularly in the cerebral cortex and other areas of the brain.

iii. It is noted that the hippocampal formation plays an important role in the processes of control of attention, memory and learning. Just the severity
of the loss of cholinergic neurons of the central nervous system (CNS) has been found to correlate with the severity of cognitive impairment.
iv. Donepezil upregulates the nicotinic receptors in the cortical neurons, adding to neuroprotective property. It inhibits voltage-activated sodium
currents reversibly and delays rectifier potassium currents and fast transient potassium currents.
Adverse effects
i. In clinical trials the most common adverse events leading to discontinuation were nausea, diarrhoea, and vomiting. Other side effects included
difficulty sleeping, muscle cramps and loss of appetite.
ii. Most side effects were observed in patients taking the 23 mg dose compared to 10 mg or lower doses.

 Anticholinergic drugs or Parasympatholytic or Muscarinic antagonists or Atropinic drugs: -
i. The term 'anticholinergic drugs' is restricted to those which block actions of ACh on autonomic effectors and in the CNS exerted through muscarinic
receptors.
ii. Though nicotinic receptor antagonists also block certain actions of ACh, they are generally referred to as ‘ganglion blockers' and ‘neuromuscular blockers.’
iii. Atropine the prototype drug of this class, is highly selective for muscarinic receptors, but some of its synthetic substitutes do possess significant nicotinic
blocking property in addition.
 Classification of anticholinergic drugs: -
Parasympatholytics
Natural
Alkaloids
Atropine
Hyoscine
(Scopolamine)
Semisynthetic
Derivatives
Atropine
methonitrate
Homatropine
Hyoscine butyl
bromide
Ipratropium
bromide
Tiotropium
bromide
Synthetic Compounds
Antisecretory-Antispasmodics
Quaternary comps.
Propantheline
Oxyphenonium
Clidinium
Cimetropium bromide
Isopropamide
Glycopyrrolate
Tertiary amines
Dicyclomine
Valethamate
Pirenzepine
Mydriatics
Cyclopentolate
Tropicamide
Vasicoselective
Oxybutynin
Flavoxate
Tolterodine
Darifenacin
Solifenacin
Antiparkinsonian
Trihexyphenidyl
(Benzhexol)
Procyclidine
Biperiden

 Chemistry & Pharmacokinetics: -
A- Source and Chemistry: -
i. Atropine and its naturally occurring congeners are tertiary amine alkaloid esters of tropic acid. Atropine (hyoscyamine) is found in the plant
Atropa belladonna, or deadly nightshade, and in Datura stramonium, also known as jimson-weed (Jamestown weed), sacred Datura, or thorn
apple.
ii. Scopolamine (hyoscine) occurs in Hyoscyamus niger, or henbane, as the l (−) stereoisomer.
iii. Naturally occurring atropine is l (−)-hyoscyamine, but the compound readily racemizes, so the commercial material is racemic d, l-hyoscyamine.
The l- (−) isomers of both alkaloids are at least100 times more potent than the d (+) isomers.
B- Absorption: -
i. Natural alkaloids and most tertiary antimuscarinic drugs are well absorbed from the gut and conjunctival membranes. When applied in a suitable
vehicle, some (e.g., scopolamine) are even absorbed across the skin (transdermal route).
ii. In contrast, only 10–30% of a dose of a quaternary antimuscarinic drug is absorbed after oral administration, reflecting the decreased lipid
solubility of the charged molecule.
C- Distribution: -
i. Atropine and the other tertiary agents are widely distributed in the body. Significant levels are achieved in the CNS within 30 minutes to 1 hour.
Scopolamine is rapidly and fully distributed into the CNS where it has greater effects than other antimuscarinic drugs.
ii. In contrast, the quaternary derivatives are poorly taken up by the brain and therefore are relatively free—at low doses—of CNS effects.
D- Metabolism and Excretion: -
i. After administration, the elimination of atropine from the blood occurs in two phases: the half-life (t1/2) of the rapid phase is 2 hours and that of
the slow phase is approximately 13 hours.

ii. About 50% of the dose is excreted unchanged in the urine. Most of the rest appears in the urine as hydrolysis and conjugation products.
iii. The drug’s effect on parasympathetic function declines rapidly in all organs except the eye. Effects on the iris and ciliary muscle persist for ≥ 72
hours.
 Pharmacodynamics: -
A- Mechanism of Action: -
i. Atropine and related compounds compete with ACh and other muscarinic agonists for the orthosteric ACh site on the muscarinic receptor. The
antagonism by atropine is competitive; thus, it is surmountable by ACh if the concentration of ACh at muscarinic receptors is increased
sufficiently.
ii. Muscarinic receptor antagonists inhibit responses to postganglionic cholinergic nerve stimulation less effectively than they inhibit responses to
injected choline esters.
iii. The difference may be explained by the fact that release of ACh by cholinergic nerve terminals occurs in close proximity to the receptors,
resulting in very high concentrations of the transmitter at the receptors.
iv. When atropine binds to the muscarinic receptor, it prevents actions such as the release of inositol trisphosphate (IP3) and the inhibition of
adenylyl cyclase that are caused by muscarinic agonists.
v. Most drugs that block the actions of acetylcholine are inverse agonists that shift the equilibrium to the inactive state of the receptor.
Muscarinic blocking drugs that are inverse agonists include atropine, pirenzepine, trihexyphenidyl, AF-DX 116, 4-DAMP, ipratropium,
glycopyrrolate, and a methyl derivative of scopolamine.
vi. The effectiveness of antimuscarinic drugs varies with the tissue and with the source of agonist. Tissues most sensitive to atropine are the
salivary, bronchial, and sweat glands.

vii. Secretion of acid by the gastric parietal cells is the least sensitive. In most tissues, antimuscarinic agents block exogenously administered
cholinoreceptor agonists more effectively than endogenously released acetylcholine.
B- Pharmacological Effects of Muscarinic Antagonists: -
 Cardiovascular System: -
 Heart: -
i. The main effect of atropine on the heart is to alter the rate. Although the dominant response is tachycardia, there is often a transient
bradycardia with average clinical doses (0.4–0.6 mg).
The slowing is modest, occurs with no accompanying changes in blood pressure or cardiac output, and is usually absent after rapid
intravenous injection.
This unexpected effect has been attributed to the block of presynaptic M1 muscarinic receptors on parasympathetic postganglionic
nerve terminals in the SA node, which normally inhibit ACh release.
ii. Larger doses of atropine cause progressive tachycardia by blocking M2 receptors on the SA nodal pacemaker cells, thereby antagonizing
parasympathetic (vagal) tone to the heart.
iii. The resting heart rate is increased by about 35–40 beats per min in young men given 2 mg of atropine intramuscularly.
iv. Atropine can abolish many types of reflex vagal cardiac slowing, such as that occurring from inhalation of irritant vapours, stimulation of
the carotid sinus, pressure on the eyeballs, peritoneal stimulation, or injection of contrast dye during cardiac catheterization.
v. Atropine also prevents or abruptly abolishes bradycardia or asystole caused by choline esters, acetylcholinesterase inhibitors, or other
parasympathomimetic drugs, as well as cardiac arrest from electrical stimulation of the vagus.

vi. The removal of vagal tone to the heart by atropine may facilitate AV conduction. Atropine shortens the functional refractory period of
the AV node and can increase the ventricular rate in patients who have atrial fibrillation. In certain cases of second-degree AV block
(e.g., Wenckebach AV block) in which vagal activity is an etiological factor, atropine may lessen the degree of block.
 Circulation: -
i. Atropine has little effect on blood pressure because most vessels lack significant cholinergic innervation. However, in clinical doses,
atropine completely counteracts the peripheral vasodilation and sharp fall in blood pressure caused by choline esters.
ii. In toxic and therapeutic doses, atropine can dilate cutaneous blood vessels, especially those in the blush area (atropine flush).
 Respiratory System: -
i. Although atropine can cause some bronchodilation and decrease in tracheobronchial secretion in normal individuals by blocking
parasympathetic (vagal) tone to the lungs, its effects on the respiratory system are most significant in patients with respiratory disease.
ii. Atropine can inhibit the bronchoconstriction caused by histamine, bradykinin, and the eicosanoids.
iii. Muscarinic antagonists have an important role in the treatment of chronic obstructive pulmonary disease. Atropine inhibits the secretions
of the nose, mouth, pharynx, and bronchi and thus dries the mucous membranes of the respiratory tract.
iv. Muscarinic antagonists are used to decrease the rhinorrhea (“runny nose”) associated with the common cold or with allergic and
nonallergic rhinitis.
v. The quaternary ammonium compounds ipratropium, tiotropium, aclidinium, and umeclidinium are used exclusively for their effects on the
respiratory tract.
vi. A therapeutically important property of ipratropium and tiotropium is their minimal inhibitory effect on muco-ciliary clearance relative to
atropine.

 Eye: -
i. Muscarinic receptor antagonists block the cholinergic responses of the pupillary sphincter muscle of the iris and the ciliary muscle
controlling lens curvature.
ii. Thus, these agents dilate the pupil (mydriasis) and paralyze accommodation (cycloplegia). The wide pupillary dilation results in
photophobia; the lens is fixed for far vision, near objects are blurred, and objects may appear smaller than they are.
iii. The normal pupillary reflex constriction to light or on convergence of the eyes is abolished. These effects are most evident when the agent is
instilled into the eye but can also occur after systemic administration of the alkaloids.
iv. Conventional systemic doses of atropine (0.6 mg) have little ocular effect, in contrast to equal doses of scopolamine, which cause evident
mydriasis and loss of accommodation.
v. Locally applied atropine produces ocular effects of considerable duration; accommodation and pupillary reflexes may not fully recover for 7–
12 days.
 GI Tract: -
 Motility: -
i. Parasympathetic nerves enhance GI tone and motility and relax sphincters, thereby favouring the passage of gastrointestinal contents.
ii. In normal subjects and in patients with GI disease, muscarinic antagonists produce prolonged inhibitory effects on the motor activity of
the stomach, duodenum, jejunum, ileum, and colon, characterized by a reduction in tone and in amplitude and frequency of peristaltic
contractions.
iii. Although atropine can completely abolish the effects of exogenous muscarinic agonists on GI motility and secretion, it does not
completely inhibit the GI responses to vagal stimulation.
 Gastric Acid Secretion: -

i. Atropine partially inhibits the gastric acid secretory responses to vagal activity because vagal stimulation of gastrin secretion is
mediated not by ACh but by peptidergic neurons in the vagal trunk that release gastrin-releasing peptide (GRP).
ii. GRP stimulates gastrin release from G cells; gastrin promotes acid secretion by parietal cells and to stimulate histamine release from
enterochromaffin-like (ECL) cells.
iii. Parietal cells respond to at least three agonists: gastrin, histamine, and ACh. Atropine will inhibit only the components of acid secretion
that result from muscarinic stimulation of parietal cells and from muscarinic stimulation of ECL cells that secrete histamine.
 Secretions: -
i. Salivary secretion is particularly sensitive to inhibition by muscarinic receptor antagonists, which can completely abolish the copious,
watery secretion induced by parasympathetic stimulation.
ii. The mouth becomes dry, and swallowing and talking may become difficult. The gastric cells that secrete mucin and proteolytic enzymes
are more directly under vagal influence than are the acid-secreting cells, and atropine selectively decreases their secretory function.
iii. Although atropine can reduce gastric secretion, the doses required also affect salivary secretion, ocular accommodation, micturition,
and GI motility.
iv. In contrast to most muscarinic receptor antagonists, pirenzepine, which shows some degree of selectivity for M1 receptors, inhibits
gastric acid secretion.
 Other Smooth Muscle: -
 Urinary Tract: -
Muscarinic antagonists decrease the normal tone and amplitude of contractions of the ureter and bladder and often eliminate drug-induced
enhancement of ureteral tone.
However, this effect is usually accompanied by reduced salivation and lacrimation and blurred vision.

 Biliary Tract: -
Atropine exerts mild antispasmodic action on the gallbladder and bile ducts in humans.
 Sweat Glands and Temperature: -
Small doses of atropine inhibit the activity of sweat glands innervated by sympathetic cholinergic fibres, and the skin becomes hot and dry.
 CNS: -
i. Atropine has an overall CNS stimulant action. However, these effects are not appreciable at low doses which produce only peripheral
effects because of restricted entry into the brain.
ii. Hyoscine produces central effects (depressant) even at low doses.
 Atropine stimulates many medullary centres vagal, respiratory, vasomotor.
 It depresses vestibular excitation and has anti-motion sickness property.
 By blocking the relative cholinergic overactivity in basal ganglia, it suppresses tremor and rigidity of parkinsonism.
 High doses cause cortical excitation, restlessness, hallucinations and delirium followed by respiratory depression and coma.
 Therapeutic uses of parasympatholytic: -
 As antisecretory: -
 Preanesthetic medication: -
 Prior to administration of irritant general anaesthetics (ether), anticholinergics (atropine, hyoscine, glycopyrrolate) were used to check
increased salivary and tracheobronchial secretions.
 The use of non-irritating anaesthetics (halothane, etc.) the requirement has decreased, though atropine may still be employed because
halothane sensitizes the heart to NA mediated ventricular arrhythmias.
 Atropinic drugs also prevent laryngospasm, by reducing respiratory secretions.

 Pulmonary embolism: -
 Atropine benefits by reducing pulmonary secretions evoked reflexly by embolism.
 As antispasmodic: -
 Symptoms of intestinal and renal colic, abdominal cramps could be relief, if there is no mechanical obstruction. In renal colic parenteral opioids
and SAIDs provide greater pain relief than atropine. Atropine is less effective in biliary colic.
 Nervous, functional and drug induced diarrhoea may be controlled to some extent, but anticholinergics are not useful in infective diarrhoea.
 In spastic constipation modest symptomatic relief in abdominal discomfort and irregular bowel evacuation may be obtained.
 To relieve urinary frequency and urgency, enuresis in children, vasicoselective M3 antimuscarinics like oxybutynin, tolterodine, flavoxate and
darifenacin are used. They may also increase bladder capacity.
 Bronchial asthma, asthmatic bronchitis, COPD: -
 Orally administered atropinic drugs are bronchodilators, but less effective than adrenergic drugs. They dry up secretion in the respiratory tract,
may lead to its inspissation and plugging of bronchioles resulting in alveolar collapse and predisposition to infection. The muco-ciliary clearance
is also impaired.
 Inhaled ipratropium bromide has been found effective in asthmatic bronchitis and COPD. Given by aerosol, it neither decreases respiratory
secretions nor impairs muco-ciliary clearance, and there are few systemic side effects. Thus, it is given in the management of COPD.
 Tiotropium bromide is an equally effective and longer acting alternative to ipratropium bromide, good for once daily maintenance therapy.

 As mydriatic and cycloplegic: -
 Diagnostic: -
 For testing error of refraction, both mydriasis and cycloplegia are needed. Tropicamide in combination with phenylephrine is used for this
purpose.
 Atropine ointment (1 %) applied 24 hours and 2 hours before for children below 5 years. Cyclopentolate drops are a more rapidly acting
alternative to atropine.
 To facilitate fundoscopy only mydriasis is needed; a short acting antimuscarinic may be used, but phenylephrine is preferred, especially in the
elderly, for fear of precipitating or aggravating glaucoma.
 Therapeutic: -
 Because of its long lasting mydriatic-cycloplegic and local pain-relieving action on cornea, atropine is very valuable in the treatment of iritis,
iridocyclitis, choroiditis, keratitis and corneal ulcer. It gives rest to the intraocular muscles and cuts down their painful spasm.
 For central action: -
 Parkinsonism: -
 Central anticholinergics are less effective than levodopa. They are used in mild cases, in drug-induced extrapyramidal syndromes and as
adjuvant to levodopa.
 Motion sickness: -
 Hyoscine is the most effective drug for motion sickness. It is particularly valuable in highly susceptible individuals and for vigorous motions.
The drug should be given prophylactically (0.2 mg oral), because administration after symptoms have set in is less effective; action lasts4-6
hours.

 A transdermal preparation applied behind the pinna 4 hours before journey has been shown to protect for 3 days. Side effects with low oral
doses and transdermal medication are few, but dry mouth and sedation can occur.
 Dicyclomine is another anticholinergic used for motion sickness. These drugs are not effective in other types of vomiting.
 Cholinergic Poisoning: -
Severe cholinergic excess is a medical emergency, and following methods are used for treating acute poisoning:
 Antimuscarinic therapy—
 There is no effective method for directly blocking the nicotinic effects of cholinesterase inhibition, because nicotinic agonists and antagonists
cause blockade of transmission.
 To reverse the muscarinic effects, a tertiary amine drug is used (preferably atropine) to treat the CNS effects as well as the peripheral effects
of the organophosphate inhibitors.
 Large doses of atropine may be needed to oppose the muscarinic effects of extremely potent agents like parathion and chemical warfare nerve
gases: 1–2 mg of atropine sulphate maybe given intravenously every 5–15 minutes until signs of effect (dry mouth, reversal of miosis) appear.
 Cholinesterase regenerator compounds—
 A second class of compounds, composed of substituted oximes capable of regenerating active enzyme from the organophosphorus-
cholinesterase complex, is also available to treat organophosphorus poisoning. These oxime agents include pralidoxime (PAM), diacetyl
monoxime (DAM), obidoxime, and others.
 Organophosphates cause phosphorylation of the serine OH group at the active site of cholinesterase. The oxime group (=NOH) has a very
high affinity for the phosphorus atom, for which it competes with serine OH. These oximes can hydrolyse the phosphorylated enzyme and
regenerate active enzyme from the organophosphorus-cholinesterase complex if the complex has not “aged”.
 Pralidoxime is most effective in regenerating the cholinesterase associated with skeletal muscle neuromuscular junctions.

 Pralidoxime and obidoxime are ineffective in reversing the central effects of organophosphate poisoning because each has positively charged
quaternary ammonium groups that prevent entry into the CNS.
 Diacetyl monoxime, on the other hand, crosses the blood-brain barrier and, in experimental animals, can regenerate some of the CNS
cholinesterase.
 Pralidoxime is administered by intravenous infusion, 1–2 g given over 15–30 minutes. In excessive doses, pralidoxime can induce
neuromuscular weakness and other adverse effects.
 Individual agents: -
 Atropine: -
Atropine is found in many members of the family Solanaceae. The most commonly found sources are Atropa belladonna (the deadly nightshade),
Datura innoxia, D. metel, and D. stramonium. Other sources include members of the genera Brugmansia (angel's trumpets) and Hyoscyamus.
Medical uses: -
a- Eyes: -
i. Topical atropine is used as a cycloplegic, to temporarily paralyze the accommodation reflex, and as a mydriatic, to dilate the pupils. Atropine
degrades slowly, typically wearing off in 7 to 14 days, so it is generally used as a therapeutic mydriatic, whereas tropicamide (a shorter-acting
cholinergic antagonist) or phenylephrine (an α-adrenergic agonist) is preferred as an aid to ophthalmic examination.
ii. In refractive and accommodative amblyopia, when occlusion is not appropriate sometimes atropine is given to induce blur in the good eye.
b- Heart: -
i. Injections of atropine are used in the treatment of bradycardia (a heart rate < 60 beats per minute).
ii. Atropine was previously included in international resuscitation guidelines for use in cardiac arrest associated with asystole and PEA.

iii. For symptomatic bradycardia, the usual dosage is 0.5 to 1 mg IV push, may repeat every 3 to 5 minutes up to a total dose of 3 mg (maximum
0.04 mg/kg).
iv. Atropine is also useful in treating second-degree heart block Mobitz type 1 (Wenckebach block), and also third-degree heart block with a high
purkinje or AV-nodal escape rhythm.
v. Atropine has also been used in an effort to prevent a low heart rate during intubation of children.
c- Secretions: -
i. Atropine's actions on the parasympathetic nervous system inhibit salivary and mucus glands. The drug may also inhibit sweating via the
sympathetic nervous system. This can be useful in treating hyperhidrosis.
Pharmacology: -
i. Atropine is a competitive antagonist of the muscarinic acetylcholine receptor types M1, M2, M3, M4 and M5.
ii. In cardiac uses, it works as a nonselective muscarinic acetyl cholinergic antagonist, increasing firing of the sinoatrial node (SA) and conduction
through the atrioventricular node (AV) of the heart, opposes the actions of the vagus nerve, blocks acetylcholine receptor sites, and decreases
bronchial secretions.
iii. In the eye, atropine induces mydriasis by blocking contraction of the circular pupillary sphincter muscle, thereby allowing the radial iris dilator
muscle to contract and dilate the pupil. Atropine induces cycloplegia by paralyzing the ciliary muscles, whose action inhibits accommodation to
allow accurate refraction in children, helps to relieve pain associated with iridocyclitis, and treats ciliary block (malignant) glaucoma.
 Hyoscine or Scopolamine: -
The name "scopolamine" is derived from one type of nightshade known as Scopolia, while the name "hyoscine" is derived from another type known as
Hyoscyamus niger.

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