Cannabinoid and lipid-mediated vasorelaxation in retinal microvasculature

The endocannabinoid system plays a role in regulation of vasoactivity in the peripheral vasculature; however, little is known about its role in regulation of the CNS microvasculature. This study investigated the pharmacology of cannabinoids and cannabimimetic lipids in the retinal microvasculature, a CNS vascular bed that is autoregulated. Vessel diameter (edge detector) and calcium transients (fura-2) were recorded from segments of retinal microvasculature isolated from adult, male Fischer 344 rats. Results showed that abnormal cannabidiol (Abn-CBD), an agonist at the putative endothelial cannabinoid receptor, CB_e, inhibited endothelin 1 (ET-1) induced vasoconstriction in retinal arterioles. These actions of Abn-CBD were independent of CB₁/CB₂ receptors and were not mediated by agonists for GPR55 or affected by nitric oxide synthase (NOS) inhibition. However, the vasorelaxant effects of Abn-CBD were abolished when the endothelium was removed and were inhibited by the small Ca²⁺-sensitive K channel (SK_Ca) blocker, apamin. The effects of the endogenous endocannabinoid metabolite, N-arachidonyl glycine (NAGly), a putative agonist for GPR18, were virtually identical to those of Abn-CBD. GPR18 mRNA and protein were present in the retina, and immunohistochemistry demonstrated that GPR18 was localized to the endothelium of retinal vessels. These findings demonstrate that Abn-CBD and NAGly inhibit ET-1 induced vasoconstriction in retinal arterioles by an endothelium-dependent signaling mechanism that involves SK _Ca channels. The endothelial localization of GPR18 suggests that GPR18 could contribute to cannabinoid and lipid-mediated retinal vasoactivity.