Müller cells express the cannabinoid CB2 receptor in the vervet monkey retina

The presence of the cannabinoid receptor type 1 (CB1R) has been largely documented in the rodent and primate retinae in recent years. There is, however, some controversy concerning the presence of the CB2 receptor (CB2R) within the central nervous system. Only recently, CB2R has been found in the rodent retina, but its presence in the primate retina has not yet been demonstrated. The aim of this study was twofold: 1) to characterize the distribution patterns of CB2R in the monkey retina and compare this distribution with that previously reported for CB1R and 2) to resolve the controversy on the presence of CB2R in the neural component of the retina. We therefore thoroughly examined the cellular localization of CB2R in the vervet monkey (Chlorocebus sabeus) retina, using confocal microscopy. Our results demonstrate that CB2R, like CB1R, is present throughout the retinal layers, but with striking dissimilarities. Double labeling of CB2R and glutamine synthetase shows that CB2R is restricted to Müller cell processes, extending from the internal limiting membrane, with very low staining, to the external limiting membrane, with heavy labeling. We conclude that CB2R is indeed present in the retina but exclusively in the retinal glia, whereas CB1R is expressed only in the neuroretina. These results extend our knowledge on the expression and distribution of cannabinoid receptors in the monkey retina, although further experiments are still needed to clarify their role in retinal functions.