Effect of mydriatic and miotic drug on rabbit eye

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BACKGROUND

The aim of the study was to examine the effect drugs on rabbit eye.

The eye is supplied both by sympathetic (radial fibres) and parasympathetic nerves (circular fibres). Stimulation of both the systems produces mydriasis and miosis respectively and their paralysis produces opposite effects.¹

Mydriatics show their action either by disrupting parasympathetic supply to the eye (atropine) or by over-activating the sympathetic system (phenylephrine).

They are used to reduce painful ciliary spasm, for diagnosing Horners syndrome, examining the retina and deeper structures, for treating chronic simple glaucoma, corneal ulcers and also used for retinoscopy particularly in children and very old patients.^2,3

Miotics constrict the pupil by contracting iris sphincter muscles (pilocarpine) or by inhibiting acetyl cholinesterase enzyme there by facilitating actions of acetylcholine at transmitted sites (physostigmine). These mechanisms increases outflow of aqueous humor and decrease the intraocular pressure. These drugs are used during cataract and anterior chamber surgeries, to treat wide angle glaucoma, xerostomia (dry mouth).^4,5

Effect of mydriatic and miotic drug on rabbit eye

REQUIREMENTS

Animal: Rabbits weighing about 2-5 kgs.

Drugs: Mydriatic drug- Atropine (1% W/V)/Phenylephrine (20% W/v), Miotic drug- Physostigmine (1% W/V)/Pilocarpine (4% W/V)

Apparatus: Rabbit holder, Droppers, measuring scale, torch, cotton wool, calculator (optional)

PROCEDURE

Carefully handle the rabbit and place it in rabbit holder box keeping the head outside. Note down the pupil size of both eyes. Observe light reflex by putting the light from side and then to front (to and fro) and corneal reflex by touching a side of the cornea with a cotton swab of tip.¹

Use pouch method for instillating of drops into the eye. Put one drop of saline to the left eye and consider it as control eye. Instill (mydriatic/miotic) in the right eye and consider it as test. After adding the drops, press the medial canthus for 10 seconds. Record the following parameters at 1 minute, 5, 10 and 15 minutes after instilling the drug and saline. Parameters to be measured:

Diameter of the pupil
Light reflex
Corneal reflex

Take 3 readings for every parameter and record observations in a tabular form. Repeat the same procedure for other drug on separate rabbits.

OBSERVATIONS

Drug	Pupillary size (in mm)	Light reflex	Corneal reflex
Saline	Normal	Present	Present
Atropine	Dilatation	Absent	Absent
Physostigmine	Constriction	Present	Present

CONCLUSION

Atropine is a mydriatic drug and causes dilatation of pupil by relaxing circular pupillary sphincter muscles and by contracting the pupillary dilator muscle. Physostigmine constricts the pupil by enhancing the actions of acetylcholine on circular pupillary sphincter muscles. Light and corneal reflexes are present.

REFERENCES

Badyal D. Practical manual of pharmacology. 1^st edition. New Delhi: Jaypee Brothers Medical Publishers; 2008
Atropine. Available at: https://en.wikipedia.org/wiki/Atropine. Accessed on 10 December 2016.
Phenylephrine. Available at: https://en.wikipedia.org/wiki/Phenylephrine. Accessed on 10 December 2016.
Pilocarpine. Available at: https://en.wikipedia.org/wiki/Pilocarpine. Accessed on 10 December 2016.
Physostigmine. Available at: https://en.wikipedia.org/wiki/Physostigmine. Accessed on 10 December 2016.

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