The activity of platelet MAO seems to be under strong genetic control. The hereditability has been estimated at 70% to 80%, according to twin studies (16). Within the individual, the activity of the enzyme seems to be stable over time, and thus it seems possible to regard this enzyme activity as a genetic marker (11). Low activity of platelet MAO has consistently been related to certain person­ality traits as sensation seeking, monotony avoidance, and extraversion (11,24). It was also demonstrated early on that subjects with low platelet MAO activity seemed to have an increased vulnerability with more psychopathology, more antisocial behavior, more experimentation with drugs, and more attempted sui­cides in their relatives (4). In patients with affective disorders, the results have been conflicting. In patients with unipolar or bipolar affective disorders, high, normal, and low platelet MAO activity has been reported (Table (10). In our own series, we tend to find


normal or high platelet MAO activity in patients with unipolar or bipolar affective disorders, although we find low platelet MAO activity in patients with neurotic-reactive depression (10). Morbidity Risk for Psychiatric Disorders in First-Degree Relatives of Patients with Affective Disorders Subdivided According to Their Platelet MAO Activity If patients with affective disorders are subdivided according to their platelet MAO activity, patients with low platelet MAO activity seem to have an increased morbidity risk for neurotic-reactive depression and alcoholism in their first-degree relatives, whereas relatives of patients with high platelet MAO activity tend to have a higher morbidity risk for bipolar affective disorders (Table 9) (9). Thus, it is possible that there exists a subgroup of patients with nonpsychotic depressive disorders, and with low platelet MAO activity as a biological marker. These patients would have an increased tendency to react with adjustment dis­orders under the influence of stressful life events, since we have been able to demonstrate that low platelet MAO activity subjects experienced more life events negatively and needed fewer life events to react with a nervous breakdown than patients with high platelet MAO activity (14). CONCLUSIONS The most important problem in the field of nonpsychotic depressive disorders seems to be the difficulty in finding diagnostic criteria for clinically homogeneous

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