Fatty acid amides (FAAs) represent a class of neuromodulatory
lipids that includes the
endocannabinoid anandamide and the sleep-inducing substance
oleamide. Both
anandamide and
oleamide produce behavioral effects indicative of
cannabinoid activity, but only
anandamide binds the
cannabinoid (
CB1) receptor in vitro. Accordingly,
oleamide has been proposed to induce its behavioral effects by serving as a competitive substrate for the brain
enzyme fatty acid amide hydrolase (FAAH) and inhibiting the degradation of endogenous
anandamide. To test the role that FAAH plays as a mediator of
oleamide activity in vivo, we have compared the behavioral effects of this FAA in FAAH(+/+) and (-/-) mice. In both genotypes,
oleamide produced hypomotility,
hypothermia, and ptosis, all of which were enhanced in FAAH(-/-) mice, were unaffected by the CB1 antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-di-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (
SR141716A) and occurred in CB1(-/-) mice. Additionally,
oleamide displayed negligible binding to the
CB1 receptor in brain extracts from either FAAH(+/+) or (-/-) mice. In contrast,
anandamide exhibited a 15-fold increase in apparent affinity for the
CB1 receptor in brains from FAAH(-/-) mice, consistent with its pronounced CB1-dependent behavioral effects in these animals. Contrary to both
oleamide and
anandamide,
monoacylglycerol lipids exhibited equivalent hydrolytic stability and pharmacological activity in FAAH(+/+) and (-/-) mice. Collectively, these results indicate that FAAH is a key regulator, but not mediator of FAA activity in vivo. More generally, these findings suggest that FAAs represent a family of signaling
lipids that, despite sharing similar chemical structures and a common pathway for catabolism, produce their behavioral effects through distinct receptor systems in vivo.