Involvement of Cannabinoid Receptors in the Intraocular Pressure-Lowering Effects of WIN55212-2 ZHAO-HUI SONG and CAROLE-ANNE SLOWEY

It is known that marijuana smoking and administration of natural cannabinoids reduce intraocular pressure. However, it has not been established whether the intraocular pressure-lowering effects of cannabinoids are mediated by cannabinoid receptors. Aminoalkylindoles are a new class of cannabimimetics with structures entirely different from those of natural cannabinoids. WIN55212-2, a prototypic aminoalkylindole, has been shown to bind cannabinoid receptors and to exhibit cannabinoid-like activities. The objective of this study was to determine whether aminoalkylindoles lower intraocular pressure and whether the effects of aminoalkylindoles are mediated by ocular cannabinoid receptors. The intraocular pressure of New Zealand White rabbits was measured with the use of applanation pneumatonography. After the measurement of baseline intraocular pressure, drugs were applied topically and the intraocular pressure was monitored. The topical application of WIN55212-2 significantly reduced intraocular pressure in the treated eyes. The intraocular pressure-lowering effects of WIN55212-2 were time and dose dependent, and the maximal reduction was 4.7 6 0.5 mm Hg at a dose of 100 mg. In contrast to treated eyes, the intraocular pressure on the contralateral eyes was not significantly affected. The topical application of WIN55212-3, the enantiomer of WIN55212-2, had no effect on intraocular pressure. Furthermore, the intraocular pressurelowering effects of WIN55212-2 were significantly reduced by topically administered SR141716A, a selective antagonist for the CB1 cannabinoid receptor. The dose-response curve of WIN55212-2 is shifted parallel to the right by SR141716A. These data demonstrate that like natural cannabinoids, WIN55212-2 also reduces intraocular pressure, and the effects of WIN55212-2 are mediated at least in part by the CB1 cannabinoid receptors in the eye. Marijuana (Cannabis sativa) is one of the oldest and most widely abused drugs. The primary psychoactive active constituent of marijuana is D-tetrahydrocannabinol (D-THC; Gaoni and Mechoulam, 1964). In addition to psychotropic activity, D-THC and other cannabinoids produce a variety of effects with therapeutic potentials, such as analgesia, antinausea, immunosuppression, and intraocular pressure (IOP) decrease (Hollister, 1986). To date, cannabinoids have been found to act through G protein-coupled receptors (Devane et al., 1988). Several cDNAs and genes encoding cannabinoid receptors have been cloned, including CB1 and CB2 (Matsuda et al., 1990; Munro et al., 1993). The endogenous cannabinoid ligand anandamide has been isolated from the brain (Devane et al., 1992). Marijuana smoking was first reported to reduce IOP in 1971 (Hepler and Frank, 1971). After this initial observation, many studies have been conducted on human subjects and animal models that confirmed the IOP-lowering properties of marijuana, D-THC, and classic cannabinoid derivatives (Flom et al., 1975; Purnell and Gregg, 1975; Green, 1984; Colasanti, 1986). Recently, anandamide, an endogenous cannabinoid agonist, also was found to lower IOP after topical administration to the rabbit eye (Pate et al., 1995). It has been suggested that compounds from the classic cannabinoid family may lower IOP by reducing aqueous humor formation and by enhancing aqueous humor outflow in the anterior chamber of the eye (Colasanti, 1986). However, the precise mechanisms for the IOP-lowering effects of cannabinoids have not been elucidated. It is not clear whether cannabinoid receptors are involved in the IOP-lowering effects of canna-