Localization of rat striatal monoamine oxidase activities towards dopamine, serotonin and kynuramine by gradient centrifugation and nigro-striatal lesions.

Subfractionation of the crude synaptosomal-mitochondrial fraction of rat striatum in a continuous sucrose gradient in a zonal rotor led to the following results. The distribution pattern of monoamine oxidase (MAO) activity towards dopamine (DA) was very similar to the pattern of MAO activity towards serotonin (5HT), but differed from the pattern of MAO activity towards kynuramine (KYN). As 5HT is specifically deaminated by MAO-A while KYN is a common MAO substrate, this supports earlier suggestions that in rat striatal preparations DA is deaminated preferentially by MAO-A. The patterns of the MAO activities towards DA and 5HT were clearly dissimilar, despite considerable overlap, to the patterns of tyrosine hydroxylase (TH) and DOPA decarboxylase (DD) activity, both marking the presence of striatal dopaminergic synaptosomes. The peak activities were separated and all patterns were symmetrical without showing a shoulder. This indicates that rat striatal MAO activity towards DA and 5HT is not specifically or for the greater part localized in dopaminergic terminals. We also investigated the effects of electrolytic and 6-hydroxydopamine lesions of the substantia nigra, both causing extensive degeneration of striatal dopaminergic terminals as appeared from the large decrease of striatal TH and DD activity. However, neither type of lesion induced a reduction of the MAO activity towards any of the substrates used. It is concluded that the amount of MAO activity towards DA and 5HT (probably MAO-A activity) present in dopaminergic terminals is very low compared with the total activity of this enzyme in rat striatal tissue.