2. OBJECTIVES
Explain the structure and function of neurons and the nervous system
1. Explain the role of neurotransmitters in the nervous system and the process
of neurotransmission
2. Differentiate inhibitory and excitatory neurotransmitters
3. Different types of neurotransmitters
4. Synthesis and catabolism of neurotransmitters.
2
3. ANATOMY OF A NEURON
A specialized cell used for
communication
Consists of dendrites, a cell body and
an axon
4. DENDRITES
Short extensions of the neuron cell
body
Have receptors for chemical
messengers
Transmit electrical signals received
from another neuron
7. NEUROTRANSMITTER CRITERIA
Must be produced & stored in the neuron
Must be released when the neuron is stimulated
Must bind to postsynaptic receptors & have a biological effect
Must be inactivated by degradation, uptake and metabolism by an adjacent
cell, or reuptake by the presynaptic neuron
Must mimic endogenous activity by exogenous application to neurons
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10. Classification of NTs based on their function
A. Based on their function
1. Excitatory NTs:
Create excitatory postsynaptic potentials
Stimulate neuron production of an action potential
e.g. Epinephrine, Norephinephrine, Glutamate, Aspartate etc
2. Inhibitory NTs:
Create inhibitory postsynaptic potentials
Reduce probability that neuron will show an action potential
E.g. Glycine, GABA, Dopamine, Serotonin
10
11. 1. ACETYLCHOLINE
It synthesized from acetyl CoA & choline by the enzyme choline
acetyltransferase .
It is stored in vesicles & released by Ca+2 mediated exocytosis.
Choline is taken up by the presynaptic terminal from the blood
Choline derived from phosphatidylcholine.
Membrane lipids are a storage site for choline.
11
12. Choline is a common component of the diet but also can be synthesized in
human as synthesis of phospholipids.
Choline synthesized via the sequential addition of 3 methyl groups from
SAM to the ethanolamine portion of phosphatidylethanolamine to form
phosphatidylcholine.
Phosphatidylcholine is subsequently hydrolyzed to release choline or
phosphocholine.
Conversion of phosphatidylethanolamine to phosphatidylcholine occurs
mainly in liver & brain.
This conversion requires vitamin B6 & B12
12
14. Inactivation of Ach
Ach is the major NT at the NMJs
Ach is inactivated by acetylcholinesterase.
This rapid removal enables the nerves to transmit more than 100 signals per
second.
The enzyme is inhibited by a wide range of compounds
Inability to inactivate Ach leads to constant activation of the nerve muscle
synapses that leads to paralysis.
14
15. 2. CATECHOLAMINE SYNTHESIS
1. Hydroxylation
The reaction involves the conversion of tyrosine, O2 and tetrahydrobiopterin
to dopa & dihydrobiopterin.
It is irreversible rate limiting step
This reaction is catalyzed by tyrosine hydroxylase
Present in adrenal medulla, brain, and all sympathetically innervated tissues
Rate-limiting enzyme & activated by phosphorylation
Converts tyrosine into DOPA
15
17. 2. Decarboxylation
Dopa decaboxylase catalyze the decarboxylation of dopa to produce
dopamine.
Deficiency of this enzyme causes Parkinson’s disease
It is irreversible reaction.
PLP is the cofactor for this reaction
In Dopaminergic neurons stop synthesis at this point, because these neurons
do not synthesize the enzymes required for the subsequent steps
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18. 3. Hydroxylation
• This is an irreversible reaction.
• It is catalyzed by dopamine β- hydroxylase.
• It change dopamine to norepinephrine
• The reactants include dopamine, O2 and ascorbate.
• Ascorbic acid serves as the electron donor and is oxidized in the reaction
• The products are NE, water & dehydroascorbate.
4. Methylation
• This reaction is catalyzed by phenylethanolamine N-methyltransferase.
• NE & S-adenosyl methionin form epinephrine & S-adenosyl homocysteine
• Epinephrine synthesis is dependent on the presence of adequate levels of
B12 and folate (B9) 18
19. Storage & Release of Catecholamines
• Events begin when a message is transmitted from one neuron to the next by
NTs.
• The message is initiated by calcium ions.
• When the concentration in a neuron reaches a certain level (more than 0.1 mM),
the vesicles containing Ach fuse with the presynaptic membrane of the nerve
cells.
• Then they empty the NTs into the synapse.
• The messenger molecules travel across the synapse and are adsorbed onto
specific receptor sites.
19
20. Inactivation of Catecholamines
Catecholamines are inactivated by oxidative deamination catalyzed my
monoamine oxidase (MAO) & by O-methylation carried out by catechol-o-
methyl-transferase (COMT).
MAO is present on the outer mitochondrial membrane
MAO oxidizes the carbon containing amino group to an aldehyde, thereby
releasing ammonium ion
In the presynaptic terminal, MAO inactivates catecholamines that are not
protected in storage vesicles
20
23. DOPAMINE
Dopamine is a monoamine NT that upon binding to a dopamine receptor (G-
protein coupled) releases a variety of downstream signals.
Dopamine is mainly synthesized in areas of the CNS and PNS, such as in the
hypothalamus
23
24. There are two isoforms of MAO
MAO-A preferentially deaminates NE & serotonin, whereas MAO-B acts on
phenylethylamines.
MAO in the liver and other sites protects against the ingestion of dietary
biogenic amines.
COMT can metabolize both intra- or extracellularly & it is also found in many
cells, like; erythrocyte.
It works on a broad spectrum of extraneuronal catechols and those that have
diffused away from the synapse.
24
25. COMT transfers a methyl group from SAM to a hydroxyl group of
catecholamine in the presence of Mg2+ , vitamins B12 and folate.
The metabolic products of MAO and COMT are excreted in the urine as
vanillylmandelic acid, metanephrine & normetanephrine.
Cerebrospinal homovanillylmandelic acid is an indicator of dopamine
degradation.
Its concentration is decreased in Parkinson’s disease
Functions of Dopamine
Dopamine plays a significant role in the CVS, renal, hormonal, and CNS.
It is thought to control processes as diverse as movement to drug addiction.
Dopamine dendrites extend into various regions of the brain, controlling
different functions through the stimulation of α and β adrenergic and
dopaminergic receptors 25
26. Function depends on type of receptor
• Involved in pleasure (cocaine blocks its reuptake)
• Involved in motor control of muscles in body.
• Assists in normal brain function (thoughts)
• Induces emotions.
26
27. Dopamine-Related Diseases
Dopamine deficiency in the striatum or substantia nigra results in
Parkinson’s-like symptoms.
In this case, movement becomes slow and rigid, accompanied by muscle
tremor.
An excessive amount of dopamine is affiliated with schizophrenia,
characterized by altered behavior.
A deficiency of dopamine is a leading candidate for the etiology of certain
symptoms of depression.
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28. Drugs Stimulate Dopamine Receptors
• Some drugs are known as dopamine agonists.
• These drugs bind to dopamine receptors in place of dopamine and directly
stimulate those receptors.
• They can stimulate dopamine receptors even in someone without dopamine
neurons.
• Antagonists are drugs that bind but don't stimulate dopamine receptors.
• Antagonists can prevent or reverse the actions of dopamine.
• They prevent dopamine from attaching to receptors.
• Dopamine plays a major role in addiction.
• The activation & deactivation of dopamine receptors can lead to activation
of the brain center responsible for pleasure.
• Dopamine is a key element in the reward system - the expectation of
reward can change behavior.
28
29. Norepinephrine & Epinephrine
Outside the NS, NE, and E act as regulators of CHO & lipid
metabolism.
NE & E are released from storage vesicles in the adrenal medulla in
response to fear, exercise, cold and low levels of blood glucose.
They increase the degradation of glycogen and TAG as well as
increase blood pressure and the output of the heart.
29
30. Norepinephrine (NE)
NE recognized as a secretory product of the adrenal medulla and a
major NT of the postganglionic sympathetic nerves
Local NT in the peripheral nerves
Acts locally and reaches general circulation only when intense
activation
The synthsesis of catecholamines in the CNS, sympathetic
postganglionic neurons and chromaffin tissue
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32. Fat Metabolism
Catecholamines cause lipolysis (β3 receptor)
The products are used as energy sources
Lipolysis increase → plasma FFA↑↑ → FFA serves as energy source and
source for glucose formation
A reduced production of NE results in obesity
32
33. PROTEIN METABOLISM
Protein degradation↓, plasma AA levels decreased
Epinephrine acting on β-adreno receptors decreases the release of AAs from
muscle via proteolysis inhibition
This increases the energy available
33
34. 3. SEROTONIN
Serotonin had multiple physiological roles.
90% of all serotonin in human body in the GIT, 8% in blood platelets & 2%
in CNS.
Neurons in brain make their own; none from body crosses Blood Brain
Barrier (BBB).
Serotonin is synthesized from the amino acid Tryptophan
The synthesis of serotonin involve two reactions
34
35. SEROTONIN SYNTHESIS
Tryptophan hydroxylase
High serotonin levels within neuron do not inhibit enzyme synthesis -
serotonin just builds up.
Rate of enzyme activity can be modulated by second messengers involving
cAMP.
Also, can be modulated by Oxygen levels in blood; more oxygen, more
synthesis of serotonin.
5-hydroxytryptophan( 5HTP) Decarboxylase
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37. INACTIVATION & BREAKDOWN
Action terminated by active reuptake process into neurons and ganglia.
Then broken down by MAO.
MAO breaks down 5HT into several things.
5-hydroxyindoleacetic acid (5HIAA) is a metabolite that is often used to
index activity in system
The NT melatonin is also synthesized from tryptophan.
Melatonin is produced in the pineal gland in response to the light–dark
cycle, its level in the blood rising in a dark environment.
It is probably through melatonin that the pineal gland conveys information
about light–dark cycles to the body, organizing seasonal and circadian
rhythms.
Melatonin also may be involved in regulating reproductive functions.
37
38. 4. HISTAMINE
Histamine is derived from the decarboxylation of the AA histidine by mast
cells & by certain neuronal fibers.
Histamine is stored within the nerve terminal vesicle.
Depolarization of nerve terminals activates the exocytotic release of
histamine by voltage-dependent as well as a calcium-dependent mechanism.
The first step in the inactivation of histamine in the brain is methylation.
The enzyme histamine methyltransferase transfers a methyl group from
SAM to a ring nitrogen of histamine to form methylhistamine.
38
39. The second step is oxidation by MAO-B, followed by an additional
oxidation step.
In peripheral tissues, histamine undergoes deamination by diamine oxidase,
followed by oxidation to a carboxylic acid
Biogenic amine regulating physiological function in the gut & acting as a
NT
Histamine causes several allergic symptoms.
1.It contributes to an inflammatory response.
2.It causes constriction of smooth muscle.
3.Is cause second type of allergic response
39
41. Blockers of Histamine (Antihistaminics)
Blockers of H1 receptors: The anaphylactic reaction can be minimised by
pharmacological agents, e.g. Promethazine and Mepyramine which block
H1 receptors.
Blockers of H2 receptors: ‘Cimetidine’ is used to reduce the gastric acidity
in peptic ulcer patients, it is blocker of H2 receptor.
Metabolism of Histamine
41
42. CLINICAL ASPECT
In patients with antigen-induced bronchial asthma.
Also formed in injured tissues. Excessive liberation of histamine may be
related to traumatic shock.
Histamine markedly depresses blood pressure ↓ and large doses may cause
extreme vascular collapse.
After challenge by specific antigens in patients with ‘atopy’, histamine
demonstrated in nasal lavage fluid and skin blister fluid.
42
43. 5. GLUTAMATE
It is an excitatory NT within the CNS
Glutamate is primarily synthesized from the TCA cycle intermediate -
ketoglutarate.
Synthesized from glutamine in neurons by glutaminase.
Taken up by neurons and glutaminase in mitochondria convert it to
glutamate
Most important NT for brain function
High levels of extracellular glutamate are toxic to neurons
Released from neurons after trauma & cannot cross BBB
43
44. SYNTHESIS OF GLUTAMATE
Glutamate dehydrogenase, which reduces α-ketoglutarate to glutamate, by
incorporating free NH3 into the carbon backbone.
The second route is through transamination reactions in which an amino
group is transferred from other AAs to α-ketoglutarate to form glutamate.
Glutamate is stored in vesicles, and its release is Ca2 dependent.
It is removed from the synaptic cleft by high-affinity uptake systems present
in nerve terminals and glial cells.
44
45. 6. Γ-AMINOBUTYRIC ACID (GABA)
It is a major inhibitory NT in the CNS
It is synthesized by the decarboxylation of glutamate by the enzyme
glutamic acid decarboxylase (GAD).
It plays functional role in many neurologic and psychiatric disorders.
GABA is recycled in the CNS by a series of reactions called the GABA
shunt, which conserves glutamate & GABA.
Much of the uptake of GABA occurs in glial cells.
The GABA shunt in glial cells produces glutamate, which is converted to
glutamine and transported out of the glial cells to neurons, where it is
converted back to glutamate.
Glutamine thus serves as a transporter of glutamate between cells in the
CNS.
Glial cells lack GAD and cannot synthesize GABA.
45
47. Increased NH3 concentration enhances amination of α-ketoglutarate, an
intermediate in TCA cycle to form Glutamate in brain.
This reduces mitochondrial pool of α-ketoglutarate ↓ consequently
depressing the TCA cycle, affecting the cellular respiration.
Increased NH3 concentration enhances glutamine formation from glutamate
and thus reduces ‘braincell’ pool of glutamic acid.
Hence there is decreased formation of inhibitory neurotransmitter GABA.
Rise in brain glutamine level enhances the outflow of glutamine from brain
cells. Glutamine is carried ‘out’ by the same “transporter” which allows the
entry of ‘tryptophan’ into brain cells. Hence ‘tryptophan’ concentration in
brain cells increases which leads to abnormal increases in synthesis of
“serotonin”, a neurotransmitter.
47
48. GABA SHUNT
GABA by its conversion to succinic acid can form a “bypass” in TCA cycle
and this is called as GABA-shunt
48
49. METABOLISM OF GABA
GABA is metabolised by deamination to form succinic semialdehyde.
The deamination is accomplished by a Pyridoxal-P dependant enzyme and
the NH3 removed is transaminated to α-ketoglutarate forming more
glutamate.
Succinic semialdehyde thus formed has two fates, it is oxidised to
succinate, the reaction is catalysed by the enzyme Succinic semialdehyde
dehydrogenase using NAD+ as H-acceptor, or
It is reduced to γ-OH butyrate by the enzyme lactate dehydrogenase (LDH)
using NADH as H-donor.
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50. CLINICAL ASPECT
Vit B6 deficiency in children may be responsible for some of the cases of
infantile convulsions.
B6-deficiency causes less formation of GABA leading to neuronal
hyperexcitability and convulsions.
50
51. 7. ASPARTATE
Aspartate is an excitatory NT
It is synthesized from the TCA cycle intermediate Oxaloacetate via
transamination reactions.
Like glutamate synthesis, aspartate synthesis uses Oxaloacetate that must be
replaced through anaplerotic reactions.
Aspartate cannot pass through the BBB
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52. 8. GLYCINE
Glycine is the major inhibitory NT in the spinal cord.
Glycine in neurons is synthesized from serine by the enzyme
serine hydroxy methyltransferase in the presence of folic acid and B6.
Serine, in turn, is synthesized from the intermediate 3-phosphoglycerate in
the glycolytic pathway.
The action of glycine is probably terminated via uptake by a high-affinity
transporter.
52
53. 9. NITRIC OXIDE (NO)
• NO is synthesized from L- arginine by NO synthase
• It is produced by vascular endothelium and smooth muscle, cardiac muscle,
macrophage & other cell types.
• NO is a gas and cannot be stored in the tissue
• It needs molecular oxygen and NADPH
Functions of NO
• NO important for vasodilation & neural transmission
• NO activates a soluble guanylate cyclase
• Inhibition of platelet adhesion to the vascular endothelium (anti-thrombotic)
• It act as anti-inflammatory
• NO responsible for the relaxation of smooth muscle and the subsequent dilation
of vessels
• Scavenging superoxide anion
53
54. NITRIC OXIDE SYNTHESIS
NO is synthesized by nitric oxide synthases (NOS)
These enzymes convert arginine into citrulline, & NO
O2 and NADPH are necessary co-factors.
There are three isoforms of NOS named according to their activity
or the tissue.
Neuronal NOS (nNOS or NOS1)
Endothelial NOS (eNOS or NOS2)
Inducible NOS (iNOS or NOS3)
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55. nNOS& eNOS are synthesise NO in response to increases in intracellular
calcium levels.
Increases in cellular calcium lead to increases in levels of calmodulin and
the increased binding of calmodulin to eNOS and nNOS leads to a transient
increase in NO production by these enzymes.
iNOS synthesise NO independent of the level of calcium in the cell.
iNOS is able to bind tightly to calmodulin even at very low cellular
concentration of calcium
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56. LIPID SYNTHESIS IN BRAIN & PNS
The BBB significantly inhibits the entry of certain FAs & lipids into the
CNS, virtually all lipids found there must be synthesized within the CNS.
Cholesterol, glycerol, and sphingolipids, glycosphingolipids, and
cerebrosides are all synthesized using pathways
VLCFAs are synthesized in the brain, where they play a major role in
myelin formation.
Peroxisomal FA oxidation is important in the brain because the brain
contains very LCFAs and phytanic acid
Both of which are oxidized in the peroxisomes by β-oxidation.
Thus, disorders that affect peroxisome biogenesis severely affect brain cells
because of the inability to metabolize both branched-chain and very-LCFAs
56
57. MYELIN SYNTHESIS
It is important for rapid nerve transmission
In the PNS, the Schwann cell is responsible for myelinating one
portion of an axon of one nerve cell.
In the CNS, the oligodendrocyte is responsible for myelination.
Oligodendrocytes can myelinate portions of numerous axons (up to
40)
To maintain the myelin structure, the oligodendrocyte synthesizes 4
times its own weight in lipids per day.
57
58. METABOLIC ENCEPHALOPATHY'S AND
NEUROPATHIES
A. Hypoglycemic Encephalopathy
During the progression of hypoglycemic encephalopathy, as blood
glucose falls below 2.5 mM (45 mg/dL), the brain attempts to use
internal substrates such as glutamate and TCA cycle intermediates as
fuels.
Because the pool size of these substrates is quite small, they are
quickly depleted.
If blood glucose levels continue to fall below 1 mM (18 mg/dL), ATP
levels become depleted.
As the blood glucose drops from 2.5 to 2.0 mM (45 to 36 mg/dL), the
symptoms appear to arise from decreased synthesis of
neurotransmitters in particular regions of the brain rather than a global
energy deficit. 58
59. Glucose metabolism leading to the synthesis of the neurotransmitters glycine,
aspartate, glutamate, and GABA. As blood glucose levels drop and brain glucose
levels diminish, synthesis of these neurotransmitters may be compromised.
59
60. B. Hypoxic Encephalopathy
Experimental studies with human volunteers show that cerebral energy
metabolism remains normal when mild to moderate hypoxia (partial
pressure of oxygen, or PaO2 = 25–40 mm Hg) results in severe
cognitive dysfunction.
The diminished cognitive function is believed to result from impaired
neurotransmitter synthesis.
In mild hypoxia, cerebral blood flow increases to maintain oxygen
delivery to the brain. In addition, anaerobic glycolysis is accelerated,
resulting in maintenance of ATP levels.
This occurs, however, at the expense of an increased lactate production
and a fall of pH. Acute hypoxia (PaO2 - 20 mm Hg) generally results in
a coma. 60
61. Hypoxia can result from insufficient oxygen reaching the blood (e.g., at high
altitudes), severe anemia (e.g., iron deficiency), or a direct insult to the
oxygen-utilizing capacity of the brain (e.g., cyanide poisoning).
All forms of hypoxia result in diminished neurotransmitter synthesis.
Glutamate and GABA synthesis, which depend on a functioning TCA cycle.
NADH levels are increased when oxygen is unavailable to accept electrons
from the electron transport chain and NADH cannot be converted back into
NAD.
Even the synthesis of catecholamine neurotransmitters may be decreased
because the hydroxylase reactions require O2.
61
62. METHODS TO DETERMINE
NEUROTRANSMITTER IMBALANCES
Serum, Plasma (Blood)
Cerebral Spinal Fluid (CSF)
Urine
1. Serum & Plasma
No established target ranges
Influenced by venipuncture
Rapidly degraded
Invasive
62
63. 2. Cerebral Spinal Fluid
Invasive
No established target ranges
NT levels influenced by lumbar puncture
Possible complications
3. Urine
One of oldest forms of medical testing
Non-invasive
Low associated stress
63
65. OPTIMAL RANGES FOR URINARY
NEUROTRANSMITTERS
• Spot urine collected 2-3 hours after rising.
• Ranges are reported in µg/gCr
Epi 8-12
NE 30-55
Dopa 125-175
Sero 175-225
Glycine 200-400
Taurine 150-300
GABA 1.5-4.0
Glutamine 150-400
Glutamate 10-25
Aspartic Acid 20-40
PEA 175-350
Histamine 10-25
Agmatine 1-2
65
66. URINARY NEUROTRANSMITTER
TESTING USES
Identify imbalances that may contribute to a clinical condition
Guide treatment selection
Monitor treatment effectiveness
66
67. SUMMARY
Neurotransmitters are the chemical messengers
Maintenance of the proper balance of neurotransmitters is necessary
for good health
Neurotransmitter imbalances have been implicated in disease
Imbalances may result from stress, poor diet, neurotoxins, &
genetics
Nervous system function can be assessed via urinary
neurotransmitter testing
Restoring balance can lead to improvement in symptoms
67
