The influence of clonidine on intraocular pressure after topical application to the eyes of anesthetized cats

Abstract

Clonidine was topically applied to the right eye of anesthetized cats. Unilateral administration induced a fall in intraocular pressure (IOP) in both eyes. This effect is not secondary to the slight fall in arterial blood pressure. Distribution experiments with 14C-clonidine revealed that only minor or negligible amounts of clonidine could be demonstrated in the contralateral eye. The concentrations of the labeled drug determined in the brain after topical application are sufficient to explain the bilateral decrease in IOP by a central mechanism. The IOP-lowering effect of clonidine upon its topical application to the eye is probably of central nervous origin and, therefore, similar to the centrally induced effect on IOP after systemic administration of the drug.

Short-term Effects of Unilateral 1% Apraclonidine Therapy

“…This fellow eye effect may be due to a central neural action similar to that observed with topical clonidine. [6][7][8][9] This contralateral effect may make it more difficult to determine the effica¬ cy of IOP lowering by apraclonidine if a one-eye therapeutic trial is used in eyes with glaucoma.14 Topical 1% apraclonidine hydro¬ chloride caused no significant change in the facility of aqueous outflow. These tonographic results are similar to those involving topical clonidine.8 Apraclonidine decreases aqueous hu¬ mor flow approximately 30% in fluorophotometric studies according to R. F. Brubaker, MD (written commu¬ nication, Aug 31, 1987).…”

Ocular α2-Receptor Subclasses and Antiglaucoma Efficacy

“…The effect of selective ct2-receptor agonists on intraocular pressure in the conscious rabbit was initially reported by Innemee et al (20,21). Stimulation of the 0.2-subtype with B-HT 920 or 933, caused a marked and dose-dependent ocular hypotensive response.…”

“…This effect was blocked by the ot2-adrenergic antagonist, yohimbine. Since this initial observation, several other selective cx2-adrenergic agonists have been shown to reduce IOP in rabbits (6,7), cats (18,20) and monkeys (6), and a few promising compounds have entered the clinical development phase, apraclonidine and brimonidine (UK 14,304). These agents have demonstrated to safely and effectiviely lower IOP in normotensive volunteers as well as in glaucoma patients (22,23).…”

Does B-HT 920 Lower Intraocular Pressure in Cats by Interacting with α2- and/or DA2 Adrenoceptors?

“…It has been postulated that this inhibitory activity can be initiated at sites within the brain (5,6) and/or on peripheral noradrenergic neurons (5,7). Recently, it was suggested that dopaminergic agonists may produce depression of sympathetic neuronal function by an action on prejunctional a2-adrenoceptors (8), although results indicate the functional presence of DA2 receptors in the eyes of cats, rabbits, and monkeys (2,4,9) as well as humans (10,11).…”