Cannabinoid CB1 Receptor Expression in Rat Spinal Cord

While evidence implicates the endogenous cannabinoid system as a novel analgesic target at a spinal level, detailed analysis of the distribution of the cannabinoid receptor CB1 in spinal cord has not been reported. Here, immunocytochemical studies were used to characterize the CB1 receptor expression in rat spinal cord. Staining was found in the dorsolateral funiculus, the superficial dorsal horn (a double band of CB1 immunoreactivity (ir) in laminae I and II inner/III transition), and lamina X. Although CB1-ir was present in the same laminae as primary afferent nociceptor markers, there was limited colocalization at an axonal level. Interruption of both primary afferent input by dorsal root rhizotomy and descending input by rostral spinal cord hemisection produced minor changes in CB1-ir. This and colocalization of CB1-ir with interneurons expressing protein kinase C subunit γ-ir suggest that the majority of CB1 expression is on spinal interneurons. These data provide a framework and implicate novel analgesic mechanisms for spinal actions of cannabinoids at the CB1 receptor.