The present study examined whether enhancement of endogenous cannabinoid levels by administration of the fatty acid amide hydrolase inhibitor URB597 could modulate joint nociception in 2 rodent models of osteoarthritis (OA). OA-like changes were induced in male Wistar rats by intra-articular injection of monoiodoacetate, while Dunkin-Hartley guinea pigs (age 9-12 months) develop OA naturally and were used as a model of spontaneous OA. Joint nociception was measured by recording electrophysiologically from knee joint primary afferents in response to noxious hyper-rotation of the joint before and after close intra-arterial injection of URB597 (0.03 mg; 0.1 mL bolus); the CB(1) receptor antagonist AM251 (1 mg/kg intraperitoneally) or the CB(2) receptor antagonist AM630 (1 mg/kg intraperitoneally). The effect of systemic URB597 administration (5 mg/kg) on joint pain perception in the monoiodoacetate model was determined by hindlimb incapacitance. Peripheral injection of URB597 caused afferent firing rate to be significantly reduced by up to 56% in the rat OA model and by up to 69% in the guinea pig OA model. Systemic co-administration of AM251, but not AM630, abolished the antinociceptive effect of URB597 in both models. URB597 had no effect in saline-injected control rat joints or in nonarthritic guinea pigs. Systemic URB597 administration significantly reduced hindlimb incapacitance in monoiodoacetate joints and co-administration of the CB(1) antagonist abolished this effect. Local injection of URB597 into OA knee joints reduces mechanonociception and pain, and this response is mediated by CB(1) receptors. Targeting endocannabinoid-metabolizing enzymes in the peripheral nervous system could offer novel therapeutic approaches for the treatment of OA pain.
Copyright © 2010 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.