Effects of the fatty acid amide hydrolase inhibitor URB597 on pain-stimulated and pain-depressed behavior in rats

Cannabinoid receptor (CBR) agonists produce antinociception in conventional preclinical assays of pain-stimulated behavior but are not effective in preclinical assays of pain-depressed behavior. Fatty acid amide hydrolase (FAAH) inhibitors increase physiological levels of the endocannabinoid anandamide, which may confer improved efficacy and safety relative to direct CBR agonists. To further evaluate FAAH inhibitors as candidate analgesics, this study assessed the effects of the FAAH inhibitor URB597 in assays of acute pain-stimulated and pain-depressed behavior in male Sprague-Dawley rats. Intraperitoneal injection of dilute lactic acid served as a noxious stimulus to stimulate a stretching response or depress positively reinforced operant behavior (intracranial self-stimulation), and URB597 was tested 1 and 4 h after administration. Consistent with FAAH inhibitor effects in other assays of pain-stimulated behavior, URB597 (1-10 mg/kg intraperitoneally) produced dose-related and CB1R-mediated decreases in acid-stimulated stretching. Conversely, in the assay of acid-depressed intracranial self-stimulation, URB597 produced a delayed, partial and non-CBR-mediated antinociceptive effect. The antinociceptive dose of URB597 (10 mg/kg) increased plasma and brain anandamide levels. These results suggest that URB597 produces antinociception in these models of 'pain stimulated' and 'pain depressed' behavior, but with different rates of onset and differential involvement of CBRs.