Cannabinoid CB1 receptor expression in rat spinal cord

While evidence implicates the endogenous cannabinoid system as a novel anal gesic target at a spinal level, detailed analysis of the distribution of th e cannabinoid receptor CB1 in spinal cord has not been reported. Here, immu nocytochemical studies were used to characterize the CB1 receptor expressio n in rat spinal cord. Staining was found in the dorsolateral funiculus, the superficial dorsal horn (a double band of CB1 immunoreactivity (ir) in lam inae I and II inner/III transition), and lamina X. Although CB1-ir was pres ent in the same laminae as primary afferent nociceptor markers, there was l imited colocalization at an axonal level. Interruption of both primary affe rent 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 gamma-ir su ggest that the majority of CB1, expression is on spinal interneurons. These data provide a framework and implicate novel analgesic mechanisms for spin al actions of cannabinoids at the CB1 receptor.