Area-specific hormonal regulation of brain aromatase

In vitro experiments were conducted to compare the steroid regulation of aromatase in preoptic and posterior hypothalamic areas. Kinetic analysis of aromatase activity in the preoptic area (POA) and posterior hypothalamus (AHP) of castrated and intact doves indicate that both areas have a similar, high substrate affinity (apparent Km less than 15 nM), but castration decreases the Vmax to a greater extent in POA than AHP. This differential effect was confirmed using a single substrate (10 nM) concentration representing the Km of the enzyme. Comparison of the effects of non-aromatisable androgens, methyltrienolone (R1881) and 5 alpha-dihydrotestosterone (DHT), on aromatase activity in 20-day castrates showed that neither R1881 nor DHT induced aromatase activity in POA. This was confirmed in 40-day castrates which also showed a reduced inductive effect of testosterone (T) on the brain enzyme activity. R1881 specifically increased aromatase activity in AHP, but DHT did not affect either area. The non-aromatisable androgens influenced androgen-dependent vocal behaviour to the same extent. Oestradiol (E2) increased aromatase activity in both POA and AHP. We conclude that non-aromatisable androgens affect hypothalamic, but not preoptic oestrogen formation. Aromatase activity in the male preoptic area associated with behaviour is specifically sensitive to the aromatisable androgen, T and E2. The results suggest that hormonal regulation of the aromatase differs locally within androgen target areas of the brain.