CB₂ cannabinoid receptor activation produces antinociception by stimulating peripheral release of endogenous opioids

CB₂ cannabinoid receptor-selective agonists are promising candidates for the treatment of pain. CB₂ receptor activation inhibits acute, inflammatory, and neuropathic pain responses but does not cause central nervous system (CNS) effects, consistent with the lack of CB₂ receptors in the normal CNS. To date, there has been virtually no information regarding the mechanism of CB₂ receptor-mediated inhibition of pain responses. Here, we test the hypothesis that CB₂ receptor activation stimulates release from keratinocytes of the endogenous opioid β-endorphin, which then acts at opioid receptors on primary afferent neurons to inhibit nociception. The antinociceptive effects of the CB₂ receptor-selective agonist AM1241 were prevented in rats when naloxone or antiserum to β-endorphin was injected in the hindpaw where the noxious thermal stimulus was applied, suggesting that β-endorphin is necessary for CB₂ receptor-mediated antinociception. Further, AM1241 did not inhibit nociception in μ-opioid receptor-deficient mice. Hindpaw injection of β-endorphin was sufficient to produce antinociception. AM1241 stimulated β-endorphin release from rat skin tissue and from cultured human keratinocytes. This stimulation was prevented by AM630, a CB₂ cannabinoid receptor-selective antagonist and was not observed in skin from CB₂ cannabinoid receptor-deficient mice. These data suggest that CB₂ receptor activation stimulates release from keratinocytes of β-endorphin, which acts at local neuronal μ-opioid receptors to inhibit nociception. Supporting this possibility, CB₂ immunolabeling was detected on β-endorphin-containing keratinocytes in stratum granulosum throughout the epidermis of the hindpaw. This mechanism allows for the local release of β-endorphin, where CB₂ receptors are present, leading to anatomical specificity of opioid effects.