The conversion of androgens to estrogens by aromatase cytochrome P450 (P450AROM) is an important step in the mechanism of androgen action in the brain. In adult rats, P450AROM activity (AA) is regulated by androgens in the preoptic area and medial basal hypothalamus, but is constitutive in the amygdala. This study was undertaken to determine the distribution of P450AROM messenger RNA (mRNA) and AA in adult rat brain and examine the effects of steroid treatments on their concentrations in various brain regions. AA was determined by a sensitive assay that measures the production of 3H2O during the conversion of [1 beta-3H]androstenedione to estrone. P450AROM mRNA was measured by a ribonuclease protection assay using a RNA probe complementary to the 5'-coding region of rat P450AROM mRNA. The 32P-labeled P450AROM probe protected two mRNA fragments in brain tissues that expressed AA (preoptic area, medial basal hypothalamus, amygdala, and hippocampus). The larger protected RNA fragment was 430 nucleotides (nt) long and corresponded in size to the full-length protected complementary RNA, whereas the shorter protected RNA fragment was 300 nt long. Brain tissues that did not exhibit AA contained either the smaller protected RNA fragment (cingulate and parietal cortex) or no protected RNA (cerebellum). These results suggest that the 430-nt protected RNA fragment represents mRNA that encodes the functional P450AROM enzyme. In agreement with this conclusion, we found that immature rat ovaries that were stimulated with PMSG to synthesize estrogen contained only the 430-nt protected fragment. The levels of the 430-nt protected RNA fragment differed significantly between brain regions (amygdala > > preoptic area > medial basal hypothalamus > or = hippocampus) and were significantly correlated with AA (r = 0.994; P < 0.001). After castration, the concentrations of P450AROM mRNA and AA decreased significantly in the preoptic area and medial basal hypothalamus (P < 0.05), but not in the amygdala. Treatments with testosterone or dihydrotestosterone maintained P450AROM mRNA and AA at levels approximating those found in intact males. Although 17 beta-estradiol treatment increased AA in the preoptic area, it did not affect the P450AROM mRNA content. These results suggest that the increase in AA observed after exposure to androgens results from regulation of the transcription and/or stability of P450AROM mRNA. In contrast, estradiol appears to exert an effect on AA at the posttranscriptional level.(ABSTRACT TRUNCATED AT 400 WORDS)