Previous research has shown that cannabinoids exert their action through CB1 and CB2 receptors which are members of the G-proteincoupled receptor superfamily, and that the G-protein coupled receptor superfamily is one of the controlling factors of the volume-sensitive Cl channels which regulate part of the aqueous humor production. This experiment proposed that CB1 receptors could be localized in the ciliary tissue and the retina of the rat eye, and thus could be linked to a reduction in the intraocular pressure. As intraocular pressure is one of the major risk factors for glaucoma, it was hypothesized that ingestion of THC could then help lower the chance of damage to the optic nerve.
Recent findings on animal models, including in Dr. Orlando Hung’s laboratory, show that the active component of marijuana, ∆9 THC and other cannabinoid agonists decrease IOP. The present study investigated the cellular components that may be responsible for the IOP-lowering effects of cannabinoids. The data obtained suggests that the cannabinoids could represent a new class of anti-glaucoma agents, which would be extremely beneficial to the subpopulation of glaucoma patients which suffer severe side effects from current medicinal treatments.
Three methods were used to evaluate the presence of cannabinoid receptors in the rat retina and ciliary tissue: Immunohistochemistry, RT-PCR and Western Blotting. These techniques confirmed the presence of CB1 receptors in the ciliary body epithelium and the retinal tissue in the rat.
A rich immunohisochemical staining for CB1 receptors was discovered in the ciliary body epithelium, and was localized to non-pigmented ciliary epithelial cells. This finding suggests that cannabinoid compounds may exert their IOP lowering effects through the reduction of aqueous humor formation which results from the activation of CB1 receptors. The expression of CB1 receptors on radial muscles implies that cannabinoid compounds may also play a role in the regulation of the outflow of aqueous humor. It is possible that stimulation of CB1 receptor agonists may lead to the constriction of the pupil by radial muscles and increased area for the outflow of aqueous humor.
The RT-PCR amplification, which was performed with a primer pair specific for human CB1 receptor cDNA, detected a PCR product with a predicted size of 520 base pairs. The PCR product obtained for the hCB1 receptor was then cloned and sequenced. The oligonucleotide sequence was found to be identical to that of hCB1 receptor cDNA reported in GenBank (GenBank#: XM 004350). This also indicates the presence of CB1 receptors in the retina and ciliary body epithelium in the rat eye.
The Western Blotting technique, which recognizes the specific protein receptor, depends upon the use of the specific antibody in the correct concentration. Although the Western Blotting technique in this experiment indicated the presence of some CB1 receptors in the ciliary tissue and the retina, further trials would have to be performed to obtain conclusive results.
Although current research of cannabinoid application in the treatment of glaucoma remains controversial, the understanding of the physiology of IOP control is important, particularly as 9% - 12% of the cases of blindness in the United States are due to glaucoma. With greater understanding of the mechanisms which control the production and the outflow of aqueous humor, it is possible that a more definite cure could be found. For now, however, the use of medicinal marijuana remains a definite option as an alternative treatment for those patients for whom conventional methods of treatment have failed.