Ex-vivo imaging of fatty acid amide hydrolase (FAAH) activity and its inhibition in the mouse brain

There is recent behavioral evidence that FAAH inhibitors produce a sub-set 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 [H]anandamide administration, brain FAAH activity generates [H]arachidonic acid, which is promptly trapped in membrane phospholipids. As a result, WT brains accumulate tritium in a regionally specific manner that is dependent upon regional FAAH activity, while brains from FAAH KO mice show a uniform [H]anandamide distribution. Increasing doses of anandamide+[H]anandamide fail to alter regional tritium accumulation, suggesting insensitivity towards this process by anandamideinduced changes in regional cerebral blood flow. Regional tritiated metabolite levels in WT brains were highest in the somatosensory and visual cortex, and thalamus. Treatment with methylarachidonyl fluorophosphonate (MAFP) (1 mg/kg, i.p.), reduced regional tritium accumulation in the somatosensory and visual cortex (p<0.01), and at higher doses, the thalamus (p<0.05). The selective FAAH inhibitor CAY10435, while having similar efficacy as MAFP in reducing tritium in the thalamus, somatosensory and visual cortex, also reduces caudate putamen and cerebellum (p<0.01) activity. These data indicate FAAH activity generates heterogeneous regional accumulation of [H]anandamide and metabolites, and suggest the modulation of endocannabinoid tone by FAAH inhibitors depends upon not only the dose and compound used, but also 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. This article has not been copyedited and formatted. The final version may differ from this version. JPET Fast Forward. Published on November 8, 2005 as DOI: 10.1124/jpet.105.094748 at A PE T Jornals on N ovem er 7, 2016 jpet.asjournals.org D ow nladed from