The Effects of Testosterone on Plasma and Platelet Monoamine Oxidase (MAO) and Plasma Dopamine-β-Hydroxylase (DBH) Activities in the Male Rhesus Monkey

Dopamine-beta-hydroxylase (DBH) and monoamine oxidase (MAO) in plasma and platelets were studied in male rhesus monkeys to evaluate the effects of changes in endogenous testosterone on the activity of these enzymes. High plasma testosterone concentrations attained at the end of the mating season were associated with relatively low levels of platelet MAO activity. When testosterone levels fell, platelet MAO increased significantly. Mean differences in platelet MAO activity between a group of castrated and control males were in the direction expected, with the highest MAO activity in the castrated monkeys, but these differences did not attain statistical significance. No significant mating season or castration effects were seen in plasma DBH or MAO enzyme activities. These data support the hypothesis of a link between testosterone and some monoamine systems, and possibly the monoamine-related behaviors linked to testosterone in primates.