Ex Vivo Imaging of Fatty Acid Amide Hydrolase Activity and Its Inhibition in the Mouse Brain

Abstract

There is recent behavioral evidence that fatty acid amide hydrolase (FAAH) inhibitors produce a subset of cannabinoid receptor agonist effects, suggesting both anandamide-specific behavioral functions and possible regional differences in FAAH inhibitory effects. Here, we introduce a novel imaging method to quantify regional differences in brain FAAH activity. Upon intravenous [3H]anandamide administration, brain FAAH activity generates [3H]arachidonic acid, which is promptly trapped in membrane phospholipids. As a result, wild-type (WT) brains accumulate tritium in a regionally specific manner that is dependent upon regional FAAH activity, whereas brains from FAAH knockout (KO) mice show a uniform [3H]anandamide distribution. Increasing doses of anandamide + [3H]anandamide fail to alter regional tritium accumulation, suggesting insensitivity toward this process by anandamide-induced changes in regional cerebral blood flow. Regional tritiated metabolite levels in WT brains were highest in the somatosensory and visual cortices and the thalamus. Treatment with methylarachidonyl fluorophosphonate (MAFP) (1 mg/kg i.p.) reduced regional tritium accumulation in the somatosensory and visual cortices (p < 0.01), and at higher doses, the thalamus (p < 0.05). The selective FAAH inhibitor 1-oxazolo[4,5-b]pyridin-2-yl-1-dodecanone (CAY10435), although having similar efficacy as MAFP in reducing tritium in the thalamus and somatosensory and visual cortices, also reduces caudate putamen and cerebellum (p < 0.01) activity. These data indicate FAAH activity generates heterogeneous regional accumulation of [3H]anandamide and metabolites, and they suggest the modulation of endocannabinoid tone by FAAH inhibitors depends upon not only the dose and compound used but also on the degree of FAAH expression in the brain regions examined. This imaging method determines regionally specific FAAH inhibition and can elucidate the in vivo effects of pharmacological agents targeting anandamide inactivation.