Fatty acid amide hydrolase is located preferentially in large neurons in the rat central nervous system as revealed by immunohistochemistry Kang Tsou a, *, M. Isabel Nogueron c , Shanmugam Muthian c , M. Clara San ˜udo-Pen ˜a a , Cecilia J. Hillard c , Dale G. Deutsch d , J. Michael Walker a,b a Schrier Research Laboratory, Department of Psychology, Brown University, Providence, RI 02912, USA b Department of Neuroscience, Brown University, Providence, RI 02912, USA c Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, WI 53226, USA d Department of Biochemistry and Cell Biology, State University of New York at Stony Brook, Stony Brook, NY 11794, USA Received 18 June 1998; received in revised form 10 August 1998; accepted 10 August 1998 Abstract The distribution in the rat brain of fatty acid amide hydrolase (FAAH) an enzyme that catalyzes the hydrolysis of the endo- genous cannabinoid anandamide was studied by immunohistochemistry. An immunopurified, polyclonal antibody to the C terminal region of FAAH was used in these studies. The large principal neurons, such as pyramidal cells in the cerebral cortex, the pyramidal cells the hippocampus, Purkinje cells in the cerebellar cortex and the mitral cells in the olfactory bulb, showed the strongest FAAH immunoreactivity. These FAAH-containing principal neurons except the mitral cells in the olfactory bulb are in close proximity with cannabinoid CB1 receptors as revealed by our previous immunohistochemical study. Moderately or lightly stained FAAH-containing neurons were also found in the amygdala, the basal ganglia, the deep cerebellar nuclei, the ventral posterior nuclei of the thalamus, the optic layer and the intermediate white layer of the superior colliculus and the red nucleus in the midbrain, and motor neurons of the spinal cord. These data demonstrate that FAAH is heterogeneously distributed and this distribution exhibits considerable, although not complete, overlap with the distribution of cannabinoid CB1 receptors in rat brain.  1998 Elsevier Science Ireland Ltd. All rights reserved Keywords: Fatty acid amide hydrolase; N-arachidonylethanolamine; Cannabinoids; Amidohydrolase; 2-Arachidonylglycerol The putative endogenous cannabinoid receptor ligand, N- arachidonylethanolamine (anandamide) is hydrolyzed to free arachidonic acid and ethanolamine by fatty acid amide hydrolase (FAAH) [5,12,13]. Alternate substrates of the enzyme include other long chain, unsaturated N-acyl etha- nolamines [4], primary amides of fatty acids such as olea- mide [3] and 2-arachidonylglycerol [7]. Brain FAAH is membrane-associated [9] and is inhibited by serine protease inhibitors such as phenylmethylsulfonyl fluoride (PMSF) [5]. FAAH has been purified from liver, sequenced and cloned [2]. The sequence information reveals a likely trans- membrane domain at the N terminal which is consistent with the presence of enzymatic activity in membrane fractions. It has been shown that FAAH can also catalyze the synthesis of anandamide in membrane preparations in the presence of very high concentrations of ethanolamine [5,6,11]; however, FAAH has not been shown to participate in the synthesis of anandamide in intact cells. The distribution of FAAH activity in the brain is not homogeneous; regions with high neuronal cannabinoid receptor (CB1) density including the cerebel- lum, cortex and hippocampus [8] exhibit high amidohydro- lase specific activity [9]. In a recent in situ hybridization study carried out using adult rat brain, FAAH mRNA was observed in neuronal cells with the most prominent signals in the neocortex, hippocampal formation, amygdala and cere- bellum [14]. The purpose of the studies outlined here is to study the distribution of FAAH protein in the brain on a cellular level using immunohistochemistry. A peptide corresponding to residues 561–579 of the C- terminus of FAAH [2] was synthesized at the Protein and Nucleic Acid Shared Facility of the Medical College of Neuroscience Letters 254 (1998) 137–140 0304-3940/98/$19.00  1998 Elsevier Science Ireland Ltd. All rights reserved PII S0304-3940(98)00700-9 * Corresponding author. P.O. Box 1853. Tel.: +1 401 8632605; fax: +1 401 8631300; e-mail: tk@fizzy.psych.brown.edu