Ophthalmic arachidonylethanolamide decreases intraocular pressure in normotensive rabbits

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Abstract
Arachidonylethanolamide (AEA) was the first anandamide to be identified as an endogenous ligand for the cannabinoid receptor of porcine brain. Since cannabinoids have shown some value in the reduction of ocular hypertension, the title compound was evaluated in normotensive rabbits as a possible topically applied agent for reducing intraocular pressure. AEA was dissolved in an aqueous solution of 2-hydroxy-propyl-β-cyclodextrin. Single eyedrops (25 μl) containing 3.13, 6.25, 31.25, 62.5 or 125.0 μg of AEA were instilled unilaterally into eyes of normotensive albino and pigmented rabbits. The intraocular pressures (IOPs) of these rabbits were then measured at fixed time intervals. The effect of AEA on IOP in treated and untreated (contralateral) eyes was similar in both types of rabbits. Administration of 31.25 μg of AEA caused an immediate IOP reduction in the treated eyes. AEA doses of 62.5 μg caused an initial increase and subsequent decrease of IOP in the treated eyes. In the untreated eyes, a marginal ocular hypotensive response of limited duration occurred immediately after administration of AEA at doses 31.25 or 62.5 μg. A significant increase (without subsequent decrease below baseline) in IOP occurred in treated eyes after a dose of 125.0 μg. The lowest dose (3.13 μg) did not have an effect on IOP. This study constitutes the first published demonstration that topical, unilateral administration of AEA significantly decreases IOP in normotensive albino and pigmented rabbits. Although the mechanism of action by which this compound produces its hypotensive effect in the eye is not known, the results suggest that AEA may prove useful in the investigation of glaucoma therapy.