R. Fuller, S. Hemrick-Luecke, K. Perry
Sep 1, 1981
Citations
1
Influential Citations
6
Citations
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Journal
Journal of Pharmacy and Pharmacology
Abstract
Harmaline is a competitive, reversible inhibitor of monoamine oxidase (MAO; EC 1.4.3.4) (Udenfriend et al 1958). Early studies with harmaline revealed greater inhibition when 5-hydroxytryptamine (5-HT) was substrate than when other substrates like phenethylamine or tyramine were used to assay M A 0 (Long 1962; Fuller 1968a). Subsequent studies of MA0 led to the definition of two forms, type A which is particularly susceptible to inhibition by clorgyline and prefers 5-HT as its substrate, and type B which is relatively resistant to inhibition by clorgyline and prefers phenethylamine or benzylamine as substrate (Johnston 1968; Neff & Yang 1974). Harmaline was recognized to be a selective inhibitor of type A MAO. In recent years attempts have been made to elucidate which M A 0 form is responsible for the in vivo oxidation of dopamine in rat brain, since dopamine can be oxidized by both forms of M A 0 in vitro (Neff & Yang 1974). Results from these studies. which have generally involved the use of irreversible inhibitors of MAO, have mostly agreed that M A 0 type A is primarily involved. We now describe a study showing that no inhibition of M A 0 type B occurs in vivo after harmaline injection into rats at a 30 mg kg-' dose, whereas virtually complete inhibition of MA0 type A occurs for several h, though enzyme activity has returned essentially to the untreated level within 24 h. The concentration of two dopamine metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), is decreased markedly over a time course that parallels the inhibition of M A 0 type A. These results strengthen the belief that DOPAC and HVA formation in rat brain occur predominantly through the action of type A MAO. Male Wistar rats (13G150 g) from Harlan Industries, Cumberland, Indiana were housed in groups of 5 with food and water freely available. Harmaline hydrochloride (Sigma) was injected at 30 mg kg-' i.p., and rats were decapitated at 2, 4, 8 or 24 h thereafter. Whole brains were quickly removed, frozen on dry ice. and stored at -15 "C before analysis. The tissue was homogenized in 0.1 M sucrose containing 0.5 mg cysteine ml-'. An aliquot of the homogenate was used for radiometric M A 0 assay (Wurtman & Axelrod 1965; Fuller 1968b) with 1 0 0 ~ ~ [IT]-5-hydroxytryptamine or 12.5 PM ['4C]phenethylamine (isotopes from New England Nuclear) as substrates for type A and type B MAO, respectively. This concentration of 5-HT has been found to be optimum for deamination by rat