Monoamine Oxidase Variation during Sexual Differentiation

Monoamine oxidase (MAO) activity was measured in the anterior and posterior hypothalami of male and female newborn intact rats and in animals whose hypothalami were experimentally sexually differentiated. In intact rats, MAO activity was significantly higher in the anterior hypothalamus of male rats at day 12, which was comparable to the females at days 1, 4 and 8. When comparing the anterior to the posterior hypothalamus, an increase in MAO activity in only the anterior hypothalamus was observed in the male rats at day 12, but not before then; in females, MAO activity was always comparable at the same age. When sex was experimentally differentiated through either castration of the newborn male or testosterone treatment in the female, the MAO activity in the posterior hypothalamus remained unchanged. The anterior hypothalamus changed, however, such that a higher level of activity was observed in the female treated with TP and a lesser level in the newborn castrated male. These results support the idea that MAO is involved in the process of sexual differentiation of the brain and that this process basically takes place in the anterior hypothalamus.