Novel aspects of dopamine oxidative metabolism (confounding outcomes take place of certainties)

Understanding dopamine (DA) oxidative metabolism allows to get a deeper insight into neurologic and psychiatric disorders featured by an altered DA neurotransmission as well as developing appropriate therapeutic strategies. Oxidative DA deamination is carried out by two highly conserved isoenzymes: monoamine oxidase (MAO) A and B; these isoenzymes both metabolize DA to dihydroxyphenylacetaldehyde (DOPALD), which, in turn, is converted to dihydroxyphenylacetic acid (DOPAC). In the past twenty years most studies on MAO activity were performed using brain dialysis in freely moving rats and measuring DA and DOPAC levels after administration of specific MAO inhibitors. This led to concepts on DA metabolism grounded on a single brain area (striatum) investigated, almost exclusively, in a single animal species (rat). These experiments were based on measurement of striatal levels of DOPAC which represents the indirect product of MAO activity. At present, the specific role of each MAO isoform appears to differ significantly depending on varying experimental conditions such as measuring the direct product of DA metabolism. In particular, recent studies allowed the estimate of the first metabolite (DOPALD) formed by MAO, showing that DOPAC levels do not necessarily reflect MAO activity. Again, the relative contribution of the two MAO iso forms in sustaining DA metabolism varies considerably, depending on the animal species and the specific brain area (either striatum or substantia nigra) under investigation. In this article we will briefly review these concepts in light of new evidence derived from innovative approaches: improved in vivo analysis of direct MAO metabolic products, measurement of oxidative metabolism in different parts of the DA nigrostriatal pathway; measurement of MAO activity in various animal species including MAO knockout mice.