ARTICLESAnandamide, the naturally occurring amide of arachidonic acid with ethanolamine, meets all key criteria of an endogenous cannabinoid substance 1 : it is released on demand by stimulated neurons 2,3 ; it activates cannabinoid receptors with high affinity 1 ; and it is rapidly eliminated through a two-step process consisting of carrier-mediated transport followed by intracellular hydrolysis 2,4 . Anandamide hydrolysis is catalyzed by the enzyme fatty acid amide hydrolase (FAAH), a membrane-bound serine hydrolase 5,6 that also cleaves other bioactive fatty acid ethanolamides such as oleoylethanolamide 7 and palmitoylethanolamide 8 . Mutant mice lacking the gene encoding FAAH (Faah) cannot metabolize anandamide 9 and, although fertile and generally normal, show signs of enhanced anandamide activity at cannabinoid receptors such as reduced pain sensation 9 . This is suggestive that drugs targeting FAAH may heighten the tonic actions of anandamide, while possibly avoiding the multiple and often unwanted effects produced by ∆ 9 -tetrahydrocannabinol (∆ 9 -THC) and other direct-acting cannabinoid agonists 10,11 . To test this hypothesis, potent, selective and systemically active inhibitors of intracellular FAAH activity are needed. However, most current inhibitors of this enzyme lack the target selectivity and biological availability required for in vivo studies [12][13][14] , whereas newer compounds, though promising, have not yet been characterized 15,16 . Thus, the therapeutic potential of FAAH inhibition remains essentially unexplored. Lead identification and optimizationDespite its unusual catalytic mechanism 6 , FAAH is blocked by a variety of serine hydrolase inhibitors, including compounds with activated carbonyls 16 . Therefore we examined whether esters of carbamic acid such as the anti-cholinesterase agent carbaryl (compound 1; Table 1) might inhibit FAAH activity in rat brain membranes. Although compound 1 was ineffective, its positional isomer 2 produced a weak inhibition of FAAH (half-maximal inhibitory concentration (IC 50 ) = 18.6 ± 0.7 µM; mean ± s.e.m., n = 3), which was enhanced by replacing the N-methyl substituent with a cyclohexyl group (compound 3; IC 50 = 324 ± 31 nM). The aryl ester 4, the benzyloxyphenyl group of which can be regarded as an elongated bioisosteric variant of the naphthyl moiety of compound 2, inhibited the activity of FAAH with a potency (IC 50 = 396 ± 63 nM) equivalent to that of compound 3. A conformational analysis of compound 4 revealed families of accessible conformers differing mainly in the torsion angle around the O-CH 2 bond, with substituents in anti or gauche conformations (data not shown). As the latter conformations more closely resembled the shape of the naphthyl derivative 3, we hypothesized that they might be responsible for the interac-

Oleylethanolamide (OEA) is a naturally occurring lipid that regulates satiety and body weight. Although structurally related to the endogenous cannabinoid anandamide, OEA does not bind to cannabinoid receptors and its molecular targets have not been defined. Here we show that OEA binds with high affinity to the peroxisome-proliferator-activated receptor-alpha (PPAR-alpha), a nuclear receptor that regulates several aspects of lipid metabolism. Administration of OEA produces satiety and reduces body weight gain in wild-type mice, but not in mice deficient in PPAR-alpha. Two distinct PPAR-alpha agonists have similar effects that are also contingent on PPAR-alpha expression, whereas potent and selective agonists for PPAR-gamma and PPAR-beta/delta are ineffective. In the small intestine of wild-type but not PPAR-alpha-null mice, OEA regulates the expression of several PPAR-alpha target genes: it initiates the transcription of proteins involved in lipid metabolism and represses inducible nitric oxide synthase, an enzyme that may contribute to feeding stimulation. Our results, which show that OEA induces satiety by activating PPAR-alpha, identify an unexpected role for this nuclear receptor in regulating behaviour, and raise possibilities for the treatment of eating disorders.

Acute stress suppresses pain by activating brain pathways that engage opioid or non-opioid mechanisms. Here we show that an opioid-independent form of this phenomenon, termed stressinduced analgesia 1 , is mediated by the release of endogenous marijuana-like (cannabinoid) compounds in the brain. Blockade of cannabinoid CB 1 receptors in the periaqueductal grey matter of the midbrain prevents non-opioid stress-induced analgesia. In this region, stress elicits the rapid formation of two endogenous cannabinoids, the lipids 2-arachidonoylglycerol 2 (2-AG) and anandamide 3 . A newly developed inhibitor of the 2-AG-deactivating enzyme, monoacylglycerol lipase 4,5 , selectively increases 2-AG concentrations and, when injected into the periaqueductal grey matter, enhances stress-induced analgesia in a CB 1 -dependent manner. Inhibitors of the anandamide-deactivating enzyme fatty-acid amide hydrolase 6 , which selectively elevate anandamide concentrations, exert similar effects. Our results indicate that the coordinated release of 2-AG and anandamide in the periaqueductal grey matter might mediate opioid-independent stress-induced analgesia. These studies also identify monoacylglycerol lipase as a previously unrecognized therapeutic target.Stress activates neural systems that inhibit pain sensation. This adaptive response, referred to as stress-induced analgesia (SIA), depends on the recruitment of brain pathways that project from the amygdala to the midbrain periaqueductal grey matter (PAG) and descend to the brainstem rostroventromedial medulla and dorsal horn of the spinal cord 7 . Endogenous opioid peptides have key functions in this process 1,8 , but other as yet unidentified neurotransmitters are also known to be involved 1 . We proposed that endocannabinoids might be implicated in stress analgesia for two reasons. First, agonists of CB 1 receptors-the predominant cannabinoid receptor subtype present in the brain 9,10 -exert profound antinociceptive effects 7 and suppress activity in nociceptive neurons 11-14 . Second, CB 1 antagonists increase the activity of nociceptive rostroventromedial medulla neurons 14 and enhance sensitivity to noxious stimuli 15 , indicating that an intrinsic endocannabinoid tone might regulate descending antinociceptive pathways 7 . To study non-opioid SIA we delivered brief, continuous electric foot shock to rats and quantified their sensitivity to pain after stress by using the tail-flick test. As demonstrated previously 1,16 , this stimulation protocol caused a profound antinociceptive effect that was not affected by intraperitoneal (i.p.) injection of the opiate antagonist naltrexone (14 mg kg 21 ) (Fig. 1a). However, the response was almost abolished by administration of the CB 1 antagonist rimonabant (SR141617A, 5 mg kg

Although anecdotal reports suggest that cannabis may be used to alleviate symptoms of depression, the psychotropic effects and abuse liability of this drug prevent its therapeutic application. The active constituent of cannabis, Delta(9)-tetrahydrocannabinol, acts by binding to brain CB, cannabinoid receptors, but an alternative approach might be to develop agents that amplify the actions of endogenous cannabinoids by blocking their deactivation. Here, we show that URB597, a selective inhibitor of the enzyme fatty-acid amide hydrolase, which catalyzes the intracellular hydrolysis of the endocannabinoid anandamide, exerts potent antidepressant-like effects in the mouse tail-suspension test and the rat forced-swim test. Moreover, URB597 increases firing activity of serotonergic neurons in the dorsal raphe nucleus and noradrenergic neurons in the nucleus locus ceruleus. These actions are prevented by the CB, antagonist rimonabant, are accompanied by increased brain anandamide levels, and are maintained upon repeated URB597 administration. Unlike direct CB, agonists, URB597 does not exert rewarding effects in the conditioned place preference test or produce generalization to the discriminative effects of Delta(9)-tetrahydrocannabinol in rats. The findings support a role for anandamide in mood regulation and point to fatty-acid amide hydrolase as a previously uncharacterized target for antidepressant drugs