Gonadal Influences on the Sexual Differentiation of Monoamine Oxidase Type A and B Activities in the Rat Brain

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Abstract
The sex-dependent differentiation of monoamine oxidase (MAO) in the hypothalamus of 60-day-old, Charles River rats involved only the type A (MAO-A) and not the type B (MAO-B) enzyme. In vivo inhibition of type A by clorgyline, and tyep B by (-)deprenyl decreased the specific activity of both types of MAO to a smaller extent in the female than in the male hypothalamus. When masculinization was prevented by neonatal administration of estradiol (E) to males, hypothalamic MAO-A and MAO-B activities increased in both control and MAO-inhibited rats. Androgenization of females had little effect on the MAO activity. The effects of neonatal estrogenization were attributable neither to a direct influence of E nor to a sexal difference in the peripheral clearance of the MAO-inhibitor used, single, high doses of steroids to adult, but not to newborn rats, did acutely affect the kinetics of MAO-A. The activity of MAO-A was decreased by high concentrations of E or TS [testosterone enanthate] in vitro. The imprinting for patterns of hypothalamic MAO-A and MAO-B in the 2 sexes results, probably, from genetic predetermination. Neonatal changes in the homeostasis of gonadal hormones may result in type-MAO nonspecific effects in adulthood; the short-term effects of high concentrations of steroids may be selective for the A form.