Cannabinoid modulation of nociception and nociceptor activity during inflammation.

Abstract

Previous studies have demonstrated that peripherally-administered cannabinoids at the site of injury produce antinociception in animal models of acute and persistent pain. Peripheral cannabinoid one (CB1) receptor-mediated antinociception has been attributed to CB1 receptors located on nociceptive DRG neurons and their peripheral nerve terminals. Although these studies suggest that activation of peripheral CB1 receptors located on nociceptive nerve terminals produces antinociception, how cannabinoids modulate nociceptor activity is not known. The overall aim of this thesis was to relate the behavioral antinociceptive effects of locally-administered cannabinoids with changes in the response properties of nociceptors during non-inflamed and inflamed conditions. It was hypothesized that activation of peripheral CB1 receptors attenuated nociception and nociceptor activity only during inflammation. In behavioral studies, intraplantar administration of complete Freund's adjuvant (CFA), but not saline, produced mechanical allodynia, mechanical hyperalgesia, and heat hyperalgesia. Activation of peripheral CB1 receptors produced antiallodynia and antihyperalgesia following inflammation, but did not alter nociception during non-inflamed conditions. In electrophysiological studies, only cutaneous nociceptors (Adelta and C) from inflamed skin were sensitized, and not Abeta mechanoreceptors. Local administration of CB1 receptor agonists attenuated mechanically-evoked responses of Adelta nociceptors from inflamed skin, but did not alter the evoked responses of Adelta nociceptors from non-inflamed skin. The responses of C nociceptors and Abeta mechanoreceptors from either non-inflamed or inflamed skin were not altered following local administration of cannabinoids. Our results demonstrated that peripherally-mediated cannabinoid antinociception through CB1 receptors is mediated, at least in part, by attenuation of Adelta nociceptor activity. The results from the present studies suggest that peripherally-acting CB1 receptor agonists could be administered alone or co-administered with other analgesic drugs to treat acute and persistent pain in humans and animals.