The conversion of androgens to active metabolites by neural tissue is believed to be an essential component in the cellular mechanism of androgen-induced neuroendocrine responses. In this study, we measured the in vitro aromatization and 5 a-reduction of androgens in incubations of microdissected brain regions from four intact and five castrated (6 weeks) adult male rhesus monkeys. Individual nuclei were microdissected from 600-Mm frozen brain sections and homogenized in a potassium phosphate buffer. Aromatase activity was measured by a radiometric assay that uses the incorporation of tritium from [1Î²-3H]androstenedione into 3H2O as an index of estrogen formation. We estimated 5a-reductase activity by isolating 5Î±- dihydrotestosterone on two different chromatography systems and measuring the amount of this product formed from [1a,2a- 3H]testosterone. We acidified a portion of each homogenate and determined LHRH content by RIA. Between brain nuclei, aromatase activity varied 1500-fold, whereas 5a-reductase activity varied only 3-fold. Both enzyme activities were highest in amygdaloid, medial preoptic, and medial diencephalic nuclei and lowest in the caudate nucleus. Aromatase activities in the supraoptic nucleus, periventricular area, medial preoptic areaanterior hypothalamus, and lateral hypothalamus were significantly (P < 0.05) lower in castrated males. Castration did not significantly affect 5a-reductase activity, except for an increase in the basolateral amygdala. The highest concentrations of LHRH were in the infundibular nucleus-median eminence and were 30 times greater than amounts measured in preoptic and medial hypothalamic nuclei. The LHRH contents of the infundibular nucleus-median eminence, ventral medial nucleus, and lateral hypothalamus were significantly lower in castrated males (P < 0.05). In addition, we observed a significant correlation between aromatase activity and LHRH content in the basal hypothalamus of intact males (r = 0.947; P < 0.05; n = 8), but not in the preoptic-anterior hypothalamus (r = 0.068; P < 0.05; n = 10). No correlation was observed between 5a-reductase activity and LHRH content in either area. These data indicate that castration selectively affects androgen metabolism and LHRH content in discrete regions of the brain of male monkeys and suggest that aromatase and 5a-reductase are regulated differentially in the primate brain.
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