VPT 321: Veterinary Neuropharmacology (2+1) copy 1

AUTONOMIC PHARMACOLOGY OF THE EYE

• The sphincter muscles (constrictor pupillae) are innervated by the parasympathetic system.
• Stimulation of these muscles results in contraction of the muscle and in turn pupillary constriction (miosis).
• Radial muscles are innervated by the sympathetic system.
• Stimulation of these muscles results in contraction of dilator muscles and in turn pupillary dilatation (mydriasis). However, the activity of parasympathetic system predominates.
• The ciliary muscles receive modest sympathetic innervation. Parasympathetic activity of ciliary muscle leads to contraction of the ciliary muscle, relaxation of the ligaments to the lens and the lens becomes more globular with accommodation to near vision.

Atropine

• causes relaxation of the iris sphincter muscle pupillary diatation.
• Atropine has a very long duration of action on the iris.
• Atropine causes relaxation of the ciliary muscle and paralysis of accommodation (cycloplegia). This effect is also long lasting.

Pilocarpine

• causes contraction of the iris sphincter and miosis.
• Pilocarpine can counter the mydriasis caused by atropine.
• On the cilicary muscle pilocarpine and cholinesterase inhibitors cause contraction and in turn accommodation to near vision.
• Shorter acting muscarinic antagonists like tropicamide have been developed for use in ophthalmology.
• Ganglionic blockade also causes cycloplegia.

Glaucoma 

• In glaucoma parasympathomimetic and anticholinesterase drugs produce a fall in intraocular pressure mainly by lowering the resistance to outflow of aqueous humor.
• In narrow angle glaucoma, which is acute congestive, drugs are used to reduce pressure at the time of attack and until surgery (iridectomy).
• The outflow of aqueous humor is facilitated, by freeing the entrance to the trabecular space at the canal of Schlemm from blockade by iris tissue.
• This is accomplished by inducing miosis and contraction of the ciliary muscle. Drugs useful in narrow angle glaucoma include parasympathomimetic drugs like physostigmine and pilocarpine, carbonic anhydrase inhibitors like acetazolamide (they inhibit the formation of aqueous humor) and osmotic diuretics like mannitol and glycerine (they produce intraocular dehydration).
• Atropine either applied locally for ophthalmology or given parenterally prior to anaesthesia and other sympathomimetics and antihistaminics are contraindicated in narrow angle glaucoma as they can precipitate an attack of acute congestive glaucoma.
• In wide angle glaucoma, which is chronic and simple surgical correction is not possible. Long term correction with drugs like pilocarpine, physostigmine or carbachol is necessary.
• Diisopropylfluorophosphate or echothiophate can be used to treat glaucoma. Their major advantage is frequent ocular administration is not required. Other drugs used include timolol, epinephrine, phenylephrine, prazosin and acetazolamide are also useful in wide angle glaucoma.