DOPAMINE-DEGRADING ACTIVITY OF MONOAMINE-OXIDASE IS NOT DETECTED BY HISTOCHEMISTRY IN NEURONS OF THE SUBSTANTIA-NIGRA PARS COMPACTA OF THE RAT

Monoamine oxidase (MAO) activity was examined in neurons of the substa ntia nigra pars compacta (SNC) of the rat using a histochemical method , and compared to MAO activity in neurons of the locus coeruleus (LC) and dorsal raphe nucleus (DR). Using dopamine as a substrate, dopamine -degrading MAO activity was not detected in any SNC neurons, although LC and DR neurons were intensely stained for this activity. We further examined MAO activity in these neurons using other substrates, includ ing serotonin (an MAO type A preferential substrate), beta-phenylethyl amine (an MAO type B preferential substrate), and tyramine (a substrat e common to both MAO types A and B). As for dopamine, no SNC neurons w ere stained for MAO activity using any of these other substrates. In c ontrast, LC neurons were intensely stained when either serotonin or ty ramine was used, and DR neurons were darkly stained when either P-phen ylethylamine or tyramine was used. The lack of evidence of MAO activit y in the SNC is surprising given that there are densely packed tyrosin e hydroxylase (TH)-immunoreactive neurons in the SNC (i.e., dopaminerg ic neurons). By comparison, in the LC and DR the distribution patterns of the MAO-stained neurons were similar to those of TH-immunolabeled neurons (i.e., noradrenergic neurons) and serotonin-immunoreactive neu rons, respectively. Our results suggest that dopamine-degrading MAO ac tivity and MAO types A and B activities in SNC dopamine neurons are ve ry low compared to MAO activity in LC noradrenaline neurons and in DR serotonin neurons. (C) 1998 Elsevier Science B.V. All rights reserved.