Taken together, the results from this study revealed a significant level of complexity with regard to the actions of abn-CBD in retinal arterioles. On the one hand, consistent with previous studies,
12–14 we found that abn-CBD is an endothelium-dependent drug and that abn-CBD–induced vasodilation in ET-1 precontracted vessels was eliminated by the putative non-CB
1 CBeR receptor antagonist O-1918. This is consistent with previous studies in isolated ET-1 precontracted retinal arterioles that reported that abn-CBD vasorelaxation, like NAGly-mediated vasodilation, is endothelium dependent and involves activation of small Ca
2+-sensitive K channels and nitric oxide.
33 However, the abn-CBD–induced contraction observed in perfused retinal arterioles with basal tone in the absence of precontraction has not been reported before and further suggests that it is the vascular contractile state and local mediators and not neural regulation that is a major determinant of the response of this CNS microvasculature to cannabinoids. Although the mechanisms underlying abn-CBD–induced contraction in retinal arterioles remains to be fully clarified, this contraction was reduced, albeit with a delayed block, by O-1918, and was also sensitive to CB
1 receptor antagonism, suggesting the potential involvement of both CB
1 and non-CB
1 (CBeR) receptors. Although only vasorelaxant responses have been reported previously for abn-CBD, both vasocontractile and vasorelaxant responses are seen in other vascular beds for cannabinoids, including THC, as well as endocannabinoids (or their metabolites). These responses were mediated by both CB
1 and non-CB
1 receptors, including prostanoid receptors.
52 It is, therefore, possible that the vasocontraction seen following abn-CBD application in perfused retinal vessels with basal tone may arise from the direct or indirect actions of abn-CBD; activation of CBeR by abn-CBD may give rise to release of endothelial mediators, including endocannabinoids and/or metabolites, that activate vascular smooth muscle CB
1 receptors providing increases in tone. Furthermore, signaling crosstalk between cannabinoid systems and other Gq-coupled systems that are coexpressed in smooth muscle has been recently reported to produce synergistic contractile effects in ocular smooth muscle, with sequential activation of Rho-kinase and PLC signaling pathways.
53 A similar cross talk in retinal arterioles with basal tone may amplify vasocontractile responses giving rise to abn-CBD induced increased tone.