Pain-modulating effects of peripheral (CB2) cannabinoid receptors

Abstract

Cannabinoid receptor agonists diminish responses to painful stimuli. Extensive evidence implicates the CB1, receptor in the production of antinociception, inflammatory hyperalgesia, and peripheral nerve injury-induced sensory hypersensitivity. In previous work included in my masters thesis, our laboratory has demonstrated the capacity of CB2 receptors located outside the central nervous system (CNS) to inhibit acute nociception and inflammatory hyperalgesia. In this thesis, I use AM1241, a CB2 receptor-selective agonist to test the hypothesis that CB2 receptor activation reverses the tactile and thermal hypersensitivity characteristic of neuropathic pain in L5/L6 spinal nerve ligation model. The CB2 receptor-mediated nature of these effects was demonstrated using receptor-selective antagonists, as well as mice deficient in the genes coding for CB1 or CB2 receptors. Experiments using site-specific injections suggest AM1241 acts peripherally at the site of nerve injury and the site of application of the sensory stimulus. The peripheral nature of the effects of AM1241 is consistent with the peripheral distribution of CB 2 receptors. Given the peripheral actions of AM1241, I hypothesized and demonstrated that topical application of AM1241 modulates pain responses. Additionally, I began to examine the mechanisms by which CB2 receptor activation modulates pain responses. The effects of AM1241 were reversed by the opioid receptor antagonist, naloxone and by a sequestering antiserum to beta-endorphin. In addition, the effects of AM1241 were not observed in beta-opioid receptor knockout mice. These results suggest that the endogenous opioid peptide, mu-endorphin plays an essential role in CB2 receptor mediated pain inhibition. Further, AM1241 stimulated release of beta-endorphin from rat skin tissue and cultured human keratinocytes. The stimulation of beta-endorphin release by AM1241 was inhibited by the CB2 receptor-selective antagonist, AM630, and was not observed in skin from CB2 receptor knockout mice, demonstrating that it is mediated by CB2 receptor. These results suggest that CB2 receptor activation produces antinociception by stimulating the release of beta-endorphin from local cells and that beta-endorphin released acts at beta-opioid receptors to inhibit the responsiveness of primary afferent neurons.