152 Brain Research, 118 (1976) 152- 15~ ,(~ Elsevier/North-Holland Biomedical Press, Amsterdam -- Printed in The Netherlands Distr~ution of catechol-O-methyltransferase, histamine N-methyltraneferase and monoamine oxidase in, specific areas of the rat brain JUAN M. SAAVEDRA, MICHAEL J. BROWNSTEIN and MIKLOS PALKOVITS* Section on Pharmacology, Laboratory of Clinical Science, NIMH, Bethesda, Md. 20014 (U.S.A.) (Accepted September 3rd, 1976) The biogenic amines norepinephrine, dopamine and serotonin have been shown to be highly localized in specific neurones within the mammalian brain s,29. Detailed quantitative studies on the distribution of these neurotransmitters, as well as their corresponding synthesizing enzymes, and also of histamine have recently been under- taken in the rat brain6,7,19-21, ~a-2e. These studies have shown that the biogenic amines and also their synthesizing enzymes are unevenly distributed in the different brain nuclei, adding evidence to the hypothesis that each amine must play a different role in the central nervous system. We wish to report the regional distribution of the main catabolic enzymes for the biogenic amines, catechol-O-methyltransferase (COMT, EC 2.1.1.6), histamine N- methyltransferase (HMT, EC 2.1.1.8) and monoamine oxidase (MAO, EC 1.4.3.4). Adult male Sprague-Dawley rats (Zivic-Mitler), 200 g in body weight, were used. The animals were killed at 9 a.m. by decapitation and their brains were immedi- ately removed and frozen on dry ice. The brains were cut into sections (300/zm thick) in a cryostat, and the different nuclei were removed as previously described 19-21. Catechol-O-methyltransferase was measured by a modification of the method of Axelrod and Cohn 2 using L-norepinephrine as a substrate. Histamine N-methyl- transferase activity was quantitated by a modification of the method of Taylor and Snyder za. For both methyltransferase assays, two important modifications were introduced. A methyl donor of high specific activity ([aH]S-adenosyl-L-methionine, specific activity 5 mCi//~mole, New England Nuclear Corporation, Boston, Mass.) was used at a 1.6 × 10-6 M final concentration in the incubation mixture. The use of such a high specific activity methyl donor resulted in a 50-100-fold increase in the sensitivity of the assay, so that as little as 50 fmoles of product could easily be detected. In both cases, internal standards of partially purified COMT la and HMT 5 were added to tissue aliquots from each of the different areas studied and carried through the assay. The use of internal standards allowed for correction of possible interferences by * Present address: The First Department of Anatomy, Semmelweis Medical University, Tuzolto-U 58, Hungary.