Effect of R–(+)–methanandamide on intraocular pressure and aqueous humor dynamics in rabbits and on prostaglandin E2 production in cultured human ciliary muscle cells.

Abstract

Abstract: : Purpose: To evaluate the effect of methanandamide, a cannabinoid (CB₁) agonist, on intraocular pressure (IOP) and aqueous humor dynamics (AHDs) in conscious rabbits, and on the production of prostaglandin (PG) E₂ in cultured human ciliary muscle cells. Methods:Twenty–five µl of R–(+)–methanandamide (0.125% or 0.25%) was topically applied to one eye and vehicle to the contralateral control eye of each New Zealand albino rabbit. IOPs were measured by peumatonometry and AHDs were evaluated by fluorophotometry. Cultured human ciliary muscle cells with serum free media were treated with methanandamide (13.8 µM) or vehicle for 24 hours. The supernatant was collected and measured for PGE₂ by enzyme immunoassay. Results: Topical application of methanandamide significantly reduced IOP. The maximal IOP reduction occurred at two hours, and was 12.7+2.0 vs 17.8+1.4 mmHg for 0.125%, and 15.0+1.3 vs 19.0+1.9 mmHg for 0.25% methanandamide in the treated vs control eyes, respectively (means+SEMs, P<0.05, n=6 for each concentration). AM251 0.25%, a selective CB₁ receptor antagonist, and diclofenac 0.1%, a cyclooxygenase inhibitor, inconsistently inhibited the methanandamide–induced reduction of IOP. Aqueous humor flow was significantly reduced in methanandamide treated eyes compared to contralateral control eyes (1.77+0.12 vs 2.30+0.14 µl/min., P<0.05, n=6). The levels of PGE₂ were significantly (P<0.02) lower in the supernatants of human ciliary muscle cells treated with methanandamide (13.3+3.5 ng/ml) compared with either vehicle treated (29.3+3.5 ng/ml) or untreated control cells (33.3+2.7 ng/ml) (n=3 for each treatment group). Conclusions:Topical application of methanandamide lowered IOP in rabbits primarily by decreasing aqueous humor flow. Effects on CB₁ and/or prostanoid receptors are possible. Further experiments are needed to verify the decrease in PGE₂ in the supernatant of human ciliary muscle cells treated with methanandamide and its implications.

Keywords: aqueous • ciliary muscle • pharmacology