DISTRIBUTION AND STEROID-HORMONE REGULATION OF AROMATASE MESSENGER-RNA EXPRESSION IN THE FOREBRAIN OF ADULT MALE AND FEMALE RATS - A CELLULAR-LEVEL ANALYSIS USING IN-SITU HYBRIDIZATION

Many of the effects of gonadal steroid hormones in the male brain are due to the actions of the testosterone metabolite estradiol, which is synthesized by the actions of the P450 enzyme aromatase. Aromatase act ivity is present in regions of the preoptic area, hypothalamus, and li mbic system. Levels of aromatase activity in the brain are highly depe ndent on gonadal steroid hormones in many brain regions, but not all. We examined the distribution of aromatase mRNA in adult male and femal e rat brains as well as the regulation of the levels of aromatase mRNA in the brains of males by gonadal steroid hormones using in situ hybr idization. This method was performed using a S-35-labelled cRNA probe, transcribed in vitro from the rat ovarian aromatase cDNA. In the adul t male, many heavily labelled cells were found in the encapsulated bed nucleus of the stria terminalis (BNST), the medial preoptic nucleus ( MPN), the ventromedial nucleus (VMN), the medial amygdala (mAMY), and the cortical amygdala (CoAMY). The regional distribution of aromatase mRNA was similar in females, but females tended to have a lower number of aromatase mRNA expressing cells in each region compared to males. Aromatase mRNA levels in the BNST, MPN, VMN, and mAMY tended to be low er in castrated males than in intact males, whereas aromatase mRNA lev els were unaltered by castration in the CoAMY. The degree of reduction in mean levels of aromatase mRNA following castration does not simply account for the large changes measured in activity following castrati on. Examination of the entire population of individual cells expressin g aromatase mRNA in castrated males suggests that aromatase mRNA may b e regulated by steroid hormones differentially in specific populations of neurons within regions where activity is known to decrease followi ng castration. (C) 1996 Wiley-Liss, Inc.