Monoamine oxidases are key molecules for personality and behaviour

MAOs are present in almost all cells in the body, however, naturally, it is the activities of MAOs in the brain that are of major interest in relation to personality and behaviour.

In association with a complete lack of MAO-A, as in mice in which the MAO-A gene has been knocked out, and in a Dutch family with a dysfunction of this gene, aggressiveness has been reported (Brunner, 1996). In the Dutch family aggressiveness was also combined with arson and cases of rape indicating a lack of impulse control. Extensive search for more families with this abnormality has, however, been negative.

Measurements of MAO activities in brain tissue, either post-mortem or using various imaging techniques, are, however, tedious and not suitable for larger samples of individuals. Furthermore, unless combined with functional tasks, as in functional Magnetic Resonance Imaging (fMRI), findings in relation to behaviour have usually not been particularly rewarding. This could possibly be explained by the fact that the majority of the enzyme is localised in glial cells, and that the population of enzyme molecules of immediate interest for monoamine inactivation is difficult to measure.

One of the MAO enzymes – the MAO-B – is localised in blood platelets and is therefore accessible for activity measurements in larger samples of individuals. MAO-B enzyme activity is highly inheritable, as shown in twin studies, and is stable in the individual during lifetime. Interestingly, decades ago low platelet MAO-B activity was shown to be associated with personality traits such as impulsiveness, monotony avoidance and aggressiveness, and, as a consequence, vulnerability for the type of alcoholism characterised by strong heritability and antisocial behaviour (type 2). While low platelet MAO-B activity thus involves a risk for the individual, it also might be associated, at the other end of the spectrum, with positive outcomes of impulsiveness and ´sensation seeking´ such as creativity and success.

By European College of Neuropsychopharmacology