Background Inhibition of endocannabinoid catabolic enzymes fatty acid amide hydrolase (FAAH) and/or monoacylglycerol lipase (MAGL) reduces somatic morphine withdrawal signs but its effects on aversive aspects of withdrawal are unknown. CPA in morphine-pelleted but not placebo-pelleted mice. Morphine pretreatment prevented the occurrence of withdrawal CPA and withdrawal jumping while clonidine (an α2 adrenergic receptor agonist) only blocked withdrawal CPA. THC JZL184 and SA-57 significantly reduced the percentage of mice that jumped during the conditioning session but did not affect acquisition of withdrawal CPA. PF-3845 did not reduce morphine withdrawal CPA or jumping. Finally neither THC nor the endocannabinoid catabolic enzyme inhibitors in non-dependent mice elicited a conditioned place preference or aversion. Conclusions These findings suggest that inhibiting endocannabinoid catabolic enzymes reduces somatic morphine withdrawal signs but not aversive aspects as inferred in the CPA paradigm. The observation that non-dependent mice administered inhibitors of endocannabinoid degradation did not display place preferences is consistent with the idea that that endocannabinoid catabolic enzymes might be targeted therapeutically with reduced risk of abuse. Keywords: opioid morphine dependence withdrawal cannabinoid fatty acid amide hydrolase (FAAH) monoacylglycerol lipase cannabinoid THC conditioned place aversion (CPA) conditioned place preference (CPP) 1 INTRODUCTION Opioid abuse and dependence continue to present a serious threat BIIE 0246 to public health (Johnston et al. 2010 Fear of withdrawal symptoms that include diarrhea emesis body aches anxiety dysphoria (Farrell 1994 Gossop 1988 Jasinski 1981 Wesson and Ling 2003 are thought to contribute to the maintenance of drug-taking in opioid dependent individuals. Likewise continued opioid use alleviates the withdrawal state thus serving as a negative reinforcer (Koob and Le Moal 2005 Current available treatments for opioid dependence such as methadone and buprenorphine possess their BIIE 0246 own abuse liability (Cicero and Inciardi 2005 and are not fully effective at alleviating withdrawal (Dyer et al. 1999 Kuhlman et al. 1998 Thus new pharmacotherapies BIIE 0246 that lack abuse potential are needed to alleviate opioid withdrawal. Extracts from cannabis and the primary constituent of marijuana Δ9-tetrahydrocannabinol (THC) have long been known to ameliorate somatic morphine withdrawal signs (Birch 1889 Hine et al. JAGL1 1975 THC produces the bulk of its pharmacological effects through two known G-protein coupled receptors cannabinoid type-1 (CB1; Matsuda et al. 1990 and type-2 (CB2; Munro et al. 1993 These receptors as well as the endogenous cannabinoids (endocannabinoids) 2-arachidonoylglycerol (2-AG; Mechoulam et al. 1995 Sugiura et al. 1995 and N-arachidonoylethanolamine (anandamide AEA; Devane et al. 1992 comprise the endogenous cannabinoid system. These endocannabinoids are rapidly degraded by the respective enzymes fatty acid amide hydrolase (FAAH; Cravatt et al. 2001 and monoacylglycerol lipase (MAGL; Dinh et al. 2002 Selective inhibitors of these endocannabinoid degradative enzymes reduce somatic signs of opioid withdrawal (e.g. jumping paw fluttering head/body shaking weight loss diarrhea; Ramesh et al. 2013 2011 However it is unknown whether the anti-withdrawal effects extend to the affective components of morphine withdrawal. Opioid-dependent individuals undergoing withdrawal experience aversive subjective effects a process that is modeled in the Pavlovian conditioned place avoidance (CPA) paradigm. In this assay morphine-dependent rats (Gracy et al. 2001 Hand et al. 1988 Parker and Rennie 1992 Schnur et al. 1992 Stinus et al. 2000 1990 Watanabe et al. 2003 or mice (Broseta et al. 2005 Maldonado et al. 2003 Olson et al. 2006 Sato et al. 2005 Shoblock and Maidment 2005 undergo conditioning trials in which BIIE 0246 naloxone precipitates an aversive interoceptive stimulus that is paired with a distinct chamber. Following subsequent placement into the test apparatus the subjects spend less time in the BIIE 0246 conditioning chamber than in the control chamber (i.e. CPA). In this assay lower doses of naloxone produce CPA than those doses necessary to elicit somatic withdrawal signs (Caillé et al. 1999 Frenois et al. 2002 Furthermore the α2 adrenergic agonist clonidine which is known to reduce opioid withdrawal in humans (Gold et al. 1978 Gossop 1988 attenuates opioid withdrawal CPA (Kosten 1994.