Drugs A, B, C, and D were applied to the eyes of rabbits to demonstrate their effects. Drug A caused pupil dilation, while Drugs B and C caused constriction. When Drug A was applied after Drug B or C, it reversed the initial constriction and caused dilation instead. This demonstrated antagonism between the drugs - one reversed the other's effects. The experiment showed how different drugs can interact on the eyes in antagonistic ways.

1. Title: Effects of drugs on the eye and to demonstrate antagonism
Aim: To demonstrate the effects of drugs on the eye and to demonstrate antagonism.
INTRODUCTION
Acetylcholine is an organic molecule that acts as a neurotransmitter in many organisms,
including humans. It is an ester of acetic acid and choline, with chemical formula CH
3COO(CH 2)2N+(CH 3)3 and systematic name 2-acetoxy-N,N,N-trimethylethanaminium.
Acetylcholine is one of many neurotransmitters in the autonomic nervous system (ANS). It
acts on both the peripheral nervous system (PNS) and central nervous system (CNS) and is
the only neurotransmitter used in the motor division of the somatic nervous system.
Acetylcholine is also the principal neurotransmitter in all autonomic ganglia.
There are two main classes of acetylcholine receptor (AChR), nicotinic acetylcholine
receptors (nAChR) and muscarinic acetylcholine receptors (mAChR). They are named for the
ligands used to activate the receptors.
Nicotinic AChRs are ionotropic receptors permeable to sodium, potassium, and calcium ions.
They are stimulated by nicotine and acetylcholine. They are of two main types, muscle-type
and neuronal-type. The former can be selectively blocked by curare and the latter by
hexamethonium. The main location of nicotinic AChRs is on muscle end plates, on
autonomic ganglia (both sympathetic and parasympathetic), and in the CNS.
Muscarinic receptors are metabotropic, and affect neurons over a longer time frame. They
are stimulated by muscarine and acetylcholine. Muscarinic receptors are found in both the
central nervous system and in the peripheral nervous system of the heart, lungs, upper GI
tract, and sweat glands. ACh is sometimes used during cataract surgery to produce rapid
constriction of the pupil. Atropine, occurring in the plant Deadly nightshade produces the
opposite effect (anticholinergic) by blocking of the muscarinic AChRs and thereby increasing
pupil size (dilation). This gives the plant both its common name (for atropine-caused heart
attacks make it deadly) and its scientific name, Atropa belladonna (for women used to dilate
their pupils with this plant for cosmetic purposes, "bella donna" is Italian for "beautiful
lady"). It must be administered intraocularly because corneal cholinesterase metabolizes
topically administered ACh before it can diffuse into the eye. Similar drugs are used to
induce mydriasis (dilation of the pupil), in cardiopulmonary resuscitation and many other
situations.
Epinephrine (also known as adrenaline, adrenalin, or 4,5-β-trihydroxy-N-methylphenethylamine)
is a hormone and a neurotransmitter. The investigation of the
pharmacology of epinephrine made a major contribution to the understanding of the
autonomic system and the function of the sympathetic system. Epinephrine ophthalmic
reduces congestion in the eye by constricting blood vessels in the eye. It also enlarges the
pupil, and it reduces the amount of fluid in the eye in two ways: It reduces the production of

2. fluid inside the eye, and it increases the amount of fluid that drains from the eye. Adrenaline
mainly causes the muscle of the iris to contract thereby making the pupil (opening) to
become bigger (dilated). This allows more light to enter into the eye and brighter images
formed that helps to detect dangers in the surroundings.
There are two main groups of adrenergic receptors, α and β, with several subtypes. α
receptors have the subtypes α1 (a Gq coupled receptor) and α2 (a Gi coupled receptor).
Phenylephrine is a selective agonist of the alpha receptor. Alpha receptors are mainly
dilators of the eys. β receptors have the subtypes β1, β2 and β3. All three are linked to Gs
proteins (although β2 also couples to Gi), which in turn are linked to adenylate cyclase.
Agonist binding thus causes a rise in the intracellular concentration of the second messenger
cAMP. Downstream effectors of cAMP include cAMP-dependent protein kinase (PKA), which
mediates some of the intracellular events following hormone binding. Isoprenaline is a non-selective
agonist. Beta2 receptors cause Secretory epithelium of the ciliary body to produce
aqueous humour. Alpha1 agonists will cause mydriasis of the eye.
Epinephrine (adrenaline) reacts with both α- and β-adrenoreceptors, causing
vasoconstriction and vasodilation, respectively. Although α receptors are less sensitive to
epinephrine, when activated, they override the vasodilation mediated by β-adrenoreceptors
because there are more peripheral α1 receptors than β-adrenoreceptors. The result is that
high levels of circulating epinephrine cause vasoconstriction. At lower levels of circulating
epinephrine, β-adrenoreceptor stimulation dominates, producing vasodilation followed by
decrease of peripheral vascular resistance.
Neostigmine is a reversible, medium-acting anticholinesterase drug (therapeutic effect up to
4 hours). Anticholinesterases inhibit the action of acetylcholinesterase (AChE), which
destroys the substance acetylcholine at nerve endings. By interfering with the breakdown of
acetylcholine, neostigmine indirectly (via action of ACh) stimulates both nicotinic and
muscarinic receptors. Because neostigmine increases the effective concentration of
acetylcholine, it causes such body changes as contraction of the pupils, increased activity of
intestinal muscles, and increased secretion by the salivary and sweat glands. Neostigmine is
a parasympathomimetic, ie it mimics the effects of stimulation of the parasympathetic
nervous system. Neostigmine causes decreased fluid pressure in the eye. It is useful in
treating certain types of glaucoma (e-Drug: Malawi).
Atropine is a naturally occurring tropane alkaloid extracted from deadly nightshade (Atropa
belladonna), Jimson weed (Datura stramonium), mandrake (Mandragora officinarum) and
other plants of the family Solanaceae. It is a secondary metabolite of these plants and serves
as a drug with a wide variety of effects. In general, atropine counters the "rest and digest"
activity of glands regulated by the parasympathetic nervous system. This occurs because
atropine is a competitive antagonist of the muscarinic acetylcholine receptors (acetylcholine
being the main neurotransmitter used by the parasympathetic nervous system). Atropine
dilates the pupils, increases heart rate, and reduces salivation and other secretions.

3. Atropine induces mydriasis by blocking contraction of the circular pupillary sphincter muscle,
which is normally stimulated by acetylcholine release, thereby allowing the radial pupillary
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. Atropine is contraindicated in patients pre-disposed to narrow angle
glaucoma. Atropine can be given to patients who have direct globe trauma.
MATERIALS AND METHODS
Materials
 3 live rabbits per group
 Droppers
 Drugs labelled A, B, C and D
 Stop watch
Procedure
1. The rabbits were marked as Red, Blue and Unmarked.
2. On the Red rabbit, three drops of Drug A were applied to the right conjunctival sac
and the effects were noted.
3. Blue rabbit
a) Two drops of drug B was applied to the right conjunctival sac and the effects
were noted.
b) After five minutes on the stop watch, three drops of drug A was put into each
of the conjunctival sac and the effects were noted as well.
4. Unmarked rabbit
a) One drop of drug C was put into right conjunctival sac and effects were noted.
b) Then one drop of drug D was put into left conjunctival sac of the same rabbit
and effect were noted.
c) Ten (10) minutes later, three drops of drug A were put in each conjunctival sac
of the rabbit and the effects were recorded.

4. RESULTS
1st drug effects 2nd Drug Effects 3rd Drug effects
Red rabbit A Dilation of
pupil
- none - none
Blue rabbit B Constriction A Dilation - none
unmarked C Constriction
of the pupil
D Constriction
(myosis)
A Dilation
(mydriasis)