Monoamine Oxidase and Cerebral Uptake of Dopaminergic Drugs

The brain uptake of amines that do not enter the brain or enter it poorly was promoted by noncompetitive inhibitors of monoamine oxidase, as shown by behavioral and chemical criteria. Mice pretreated with water or enzyme inhibitors other than those mentioned were placid after receiving dopamine (3,4-dihydroxyphenethylamine). Mice pretreated with monoamine oxidase inhibitors (nialamide or iproniazid) showed upon treatment with dopamine the brisk motor responses characteristic of treatment with its precursor, L-dopa (3,4-dihydroxyphenylalanine). After receiving dopamine, intact nialamide-pretreated mice showed marked increases of brain dopamine, in contrast to water-pretreated test mice or water-treated controls. In unilaterally caudectomized, nialamide-pretreated mice, dopamine induced marked lateral curving of the body toward the lesion followed by running in that direction. Noradrenaline or adrenaline induced curving in caudectomized mice, whereas intact ones remained placid.These catecholamines are bound and inactivated by monoamine oxidase. The cerebral uptakes of chemicals that are bound but not inactivated by monoamine oxidase were thereafter tested. Nialamide induced increased behavioral responses to apomorphine and to N-propyl noraporphine, increased cerebral concentrations of both, and a deep coloration of the brain from methylene blue (bound by monoamine oxidase) but not Evans blue (bound by albumin). Even large doses of nialamide, however, failed to affect the behavioral responses to oxotremorine, which has cholinergic rather than adrenergic or dopaminergic properties. Mitochondrial monoamine oxidase seems therefore to play a specific regulatory role in the transport of substances that it binds, either to inactivate or to release them.