Cannabinoid Modulation of Cutaneous Aδ Nociceptors During Inflammation

Potenzieri C, Brink TS, Pacharinsak C, Simone DA. Cannabinoid modulation of cutaneous A delta nociceptors during inflammation. J Neurophysiol 100: 2794-2806, 2008. First published September 10, 2008; doi: 10.1152/jn.90809.2008. Previous studies have demonstrated that locally administered cannabinoids attenuate allodynia and hyperalgesia through activation of peripheral cannabinoid receptors (CB1 and CB2). However, it is currently unknown if cannabinoids alter the response properties of nociceptors. In the present study, correlative behavioral and in vivo electrophysiological studies were conducted to determine if peripheral administration of the cannabinoid receptor agonists arachidonyl-2'-chloroethylamide (ACEA) or (R)-(+)-methanandamide (methAEA) could attenuate mechanical allodynia and hyperalgesia, and decrease mechanically evoked responses of A delta nociceptors. Twenty-four hours after intraplantar injection of complete Freund's adjuvant (CFA), rats exhibited allodynia (decrease in paw withdrawal threshold) and hyperalgesia (increase in paw withdrawal frequency), which were attenuated by both ACEA and methAEA. The antinociceptive effects of these cannabinoids were blocked by co-administration with the CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophen yl)-4-methyl-1-pyrazole3-carboxamide (AM251) but not with the CB2 receptor antagonist 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-y l](4-methoxyphenyl) methanone (AM630). ACEA and methAEA did not produce antinociception under control, non-inflamed conditions 24 h after intraplantar injection of saline. In parallel studies, recordings were made from cutaneous A delta nociceptors from inflamed or control, non-inflamed skin. Both ACEA and methAEA decreased responses evoked by mechanical stimulation of A delta nociceptors from inflamed skin but not from non-inflamed skin, and this decrease was blocked by administration of the CB1 receptor antagonist AM251. These results suggest that attenuation of mechanically evoked responses of A delta nociceptors contributes to the behavioral antinociception produced by activation of peripheral CB1 receptors during inflammation.