Monoamine oxidase in Teleosts

The main aim of this paper is to provide an overview of the characteristics, function and distribution of monoamine oxidase activity in fish. Studies on MAO performed in different classes of terrestrial vertebrates have demonstrated that MAO is represented by two isoenzymes called MAO A and MAO B. By contrast, in fish, monoamine oxidase does not fit into the classical MAO A/MAO B binary classification. Most of the research carried out so far on fish has concerned Teleosts. In all the species examined it has been found that MAO activity is due to a single form of MAO. In several species, this form has been classified as MAO A. In other species it has been considered as a novel form, distinct from both MAO A and MAO B. This latter view has been confirmed in trout by cloning MAO cDNA from liver. The second part of the present review deals with the literature data on the hormonal effects on brain MAO activity in fish. In particular, significant effects on hypothalamic MAO activity were caused by estradiol. Lastly, a summary is provided of studies on the effects induced by heavy metals and organic compounds on fish brain MAO activity. These pollutants can alter brain MAO activity to different degrees and such effects could interfere indirectly with gonad maturation and thus with reproductive capacity.