Influence of selective, reversible inhibitors of monoamine oxidase on the prolonged depletion of striatal dopamine by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) hydrochloride injected s.c. at 20 mg/kg once daily for four days resulted in marked depletion of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in mouse striatum one week after the last dose. Pretreatment with MD 240928, (R)-4-((3-chlorophenyl)-methoxy)phenyl]-5-(methylamino)methyl]-2- oxazolidinone methanesulfonate, prevented the depletion of striatal dopamine, DOPAC and HVA, whereas pretreatment with harmaline did not. MD 240928 selectively inhibited type B not type A monoamine oxidase (MAO), whereas harmaline selectively inhibited type A MAO in mouse striatum. Acutely after injection of harmaline, DOPAC and HVA concentrations were decreased in mouse striatum; these changes were not produced by MD 240928. The acute changes in dopamine metabolites reveal that MAO-A not MAO-B is responsible for the oxidation of dopamine in mouse striatum. Protection against the neurotoxic effects of MPTP by MD 240928 but not by harmaline indicates that prevention of dopamine oxidation is not the mechanism of the protective effect; instead the protection probably is due to prevention of MPTP metabolism by MAO-B, this metabolism having been shown to occur by other workers. The results with these reversible, competitive inhibitors of the two types of MAO are in agreement with previously reported results from studies using irreversible inhibitors of MAO.