1 While cannabinoid receptor agonists have analgesic activity in chronic pain states, they produce a spectrum of central CB 1 receptor-mediated motor and psychotropic side effects. The actions of endocannabinoids, such as anandamide are terminated by removal from the extracellular space, then subsequent enzymatic degradation by fatty-acid amide hydrolase (FAAH). In the present study, we compared the effect of a selective FAAH inhibitor, URB597, to that of a pan-cannabinoid receptor agonist HU210 in rat models of chronic inflammatory and neuropathic pain. 2 Systemic administration of URB597 (0.3 mg kg À1 ) and HU210 (0.03 mg kg À1 ) both reduced the mechanical allodynia and thermal hyperalgesia in the CFA model of inflammatory pain. In contrast, HU210, but not URB597, reduced mechanical allodynia in the partial sciatic nerve-ligation model of neuropathic pain. HU210, but not URB597, produced a reduction in motor performance in unoperated rats. 3 The effects of URB597 in the CFA model were dose dependent and were reduced by coadministration with the cannabinoid CB 1 antagonist AM251 (1 mg kg À1 ), or the CB 2 and SR144528 (1 mg kg À1 ). Coadministration with AM251 plus SR144528 completely reversed the effects of URB597. 4 These findings suggest that the FAAH inhibitor URB597 produces cannabinoid CB 1 and CB 2 receptor-mediated analgesia in inflammatory pain states, without causing the undesirable side effects associated with cannabinoid receptor activation.

1. Although cannabinoid receptor agonists have analgesic activity in chronic pain states, they produce a spectrum of central cannabinoid CB(1) receptor-mediated motor and psychotropic side-effects. The actions of endocannabinoids, such as anandamide, are terminated by uptake and subsequent intracellular enzymatic degradation. In the present study, we examined the effect of acute administration of the anandamide transport inhibitor AM404 in rat models of chronic neuropathic and inflammatory pain. 2. Systemic administration of AM404 (10 mg/kg) reduced mechanical allodynia in the partial sciatic nerve ligation (PNL) model of neuropathic pain, but not in the complete Freund's adjuvant (CFA) model of inflammatory pain. 3. The effect of AM404 in the PNL model was abolished by coapplication with the selective cannabinoid CB(1) receptor antagonist AM251 (1 mg/kg). AM404 did not produce a reduction in motor performance in either the PNL or CFA models. 4. These findings suggest that acute administration of AM404 reduces allodynia in a neuropathic pain model via cannabinoid CB(1) receptor activation, without causing the undesirable motor disruption associated with cannabinoid receptor agonists.