Hepler and Frank (1971) studied the spectrum of physiologic ocular changes produced by smoking marihuana. Among their early findings was a consistent, dose-related, clinically significant reduction of intraocular pressure (IOP) in normal subjects. The IOP was also reduced with doses of oral ?-9-THC, ? -8-THC and, to a much lesser degree, with cannabinol and cannabidiol. Publications by Shapiro (1974), Purnell and Gregg (1975), and Green and Podos (1974) have confirmed the IOP-reducing effect of marihuana and the tetrahydrocannabinols. Flan et aL. (1975) suggested that the lowering of IOP was apparently secondary to a relaxing and euphoriant effect. However, when Hepler gave subjects full doses of diazepam (Valium) blind, he found that the IOP reduction was not significantly different from the effect of a placebo.
Twelve open angle glaucoma patients were studied by the UCLA group (Hepler et al., 1976). In 10 of the 12, impressive reductions in ocular hypertension were achieved. The reduction averaged 30 percent and lasted 4-5 hours. In two instances, however, the smoked marihuana or ingested ?-9-THC failed to induce a pressure reduction. The IOP-reducing effects of cannabis appear to be additive to the conventional glaucoma medications. In a preliminary study of a topically applied eye drop preparation of a ?-9-THC in sesame oil, 12 rabbits showed a 40 percent IOP reduction when compared to those treated with sesame oil alone.
Green (1975) and his associates (Green & Kim, 1976, 1973; Green et al., 1976) demonstrated a decrease in the IOP of rabbits given intravenous ?-9-THC. They postulated that ?-9-THC interacts with the adrenergic innervation system of the eye; in other words, that ?-adrenergic blockade would dampen the??-9-THC effect. Apparently, ?-adrenergic blockade also partly inhibits the ?-9-THC effect. The end result of the adrenergic stimulation by ?-9-THC appears to be a dilation of the efferent blood vessels, modulated by an inhibition of prostaglandin synthetase. Green and Kim (1973) have concluded that the outflow facility may be regulated by adrenergic receptors with the ?-9-THC acting as a vasodilator or the outflow blood vessels of the anterior uvea. There is also the possibility that cannabis acts to constrict afferent episcleral plexus vessels.