The Effect of Pilocarpine on a Rabbit's Eye

Pilocarpine is a drug that stimulates the parasympathetic nervous system in mammals such as rabbits and human beings. This portion of the autonomic nervous system works in similar ways in all mammals. In particular, stimulation of the parasympathetic nervous system in the eye will cause pupil constriction, eye focusing spasm and a reduction of eye pressure. The pilocarpine eye drop can cause eye irritation and increased tear production. As soon as the eye drop has been instilled, it begins to drain from the eye via the nasolacrimal drainage system.

  1. Pupil Constriction

    • The National Institutes of Health cites a study that shows how the instillation of pilocarpine into the iris of a rabbit eye causes an increase in the parasympathetic function of the iris and a decrease in the sympathetic function of the iris. The parasympathetic nervous system controls the iris constrictor muscle. Excitation of this muscle causes the pupil to become smaller. The sympathetic nervous system controls the iris dilator muscle. Inhibition of this muscle makes it even easier for the pupil to become constricted.

    Eye Focusing Spasm

    • Drugs.com reports that the instillation of pilocarpine has a stimulating effect on the ciliary muscle of the eyeball. The ciliary muscle is responsible for helping the eye to focus. The parasympathetic nervous system stimulates the ciliary muscle causing the eye to focus for near vision. The sympathetic nervous system has little to no effect on the ciliary muscle. Too much parasympathetic nervous system stimulation from pilocarpine can cause the ciliary muscle to go into a focusing spasm.

    Reduced Eye Pressure

    • Pilocarpine's activation of the iris constrictor muscle causes the pupil to constrict. In addition to that, the action of the constrictor muscle pulls the body of the iris toward the center of the eyeball. This stretches the trabecular meshwork, which is the apparatus that allows the internal eye fluid to drain. The stretching of the trabecular meshwork increases the rate at which the eye fluid, or aqueous humor, is drained from the eye. The eye fluid is generally being produced and drained at a constant rate. If one increases the rate of drainage and the rate of production is not increased, the pressure of the fluid in the eyeball will decrease. This is why pilocarpine eye drops can be used to treat glaucoma.

    Increased Tear Production

    • As Drugs.com states, the instillation of pilocarpine eye drops can cause the eyes to become irritated. When the eyes become irritated, they increase tear production, otherwise known as watery eyes. This is an adaptive mechanism that allows the eyes to wash away any irritating chemical that comes into contact with the eye. If the pilocarpine causes the eye to become irritated, the eye will increase its tear production.

    Drainage of Pilocarpine from the Eye

    • According to the Journal of Pharmaceutical Sciences, the main way that pilocarpine eye drops are moved from the surface of the eye is drainage through the nasolacrimal drainage system. This system begins with a small opening in the inner corners of the upper and lower eyelids, known as the puncta. The tears and any eye drops will drain through the puncta and into a network of tubes that drain the tears and eye drops into the nasal cavity and the throat. Some of the eye drop also is absorbed into the eyeball. Despite that fact, nearly all of it is drained through the nasolacrimal drainage system.