ANDROGENS REGULATE AROMATASE CYTOCHROME-P450 MESSENGER-RIBONUCLEIC-ACID IN RAT-BRAIN

The conversion of androgens to estrogens by aromatase cytochrome P450 (P450(AROM)) is an important step in the mechanism of androgen action in the brain. In adult rats, P450(AROM) activity (AA) is regulated by androgens in the preoptic area and medial basal hypothalamus, but is c onstitutive in the amygdala. This study was undertaken to determine th e distribution of P450(AROM) messenger RNA (mRNA) and AA in adult rat brain and examine the effects of steroid treatments on their concentra tions in various brain regions. AA was determined by a sensitive assay that measures the production of (H2O)-H-3 during the conversion of [1 beta-H-3]androstenedione to estrone. P450(AROM) mRNA was measured by a ribonuclease protection assay using a RNA probe complementary to the 5'-coding region of rat P450(AROM) mRNA. The P-32-labeled P450(AROM) 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 c orresponded in size to the full-length protected complementary RNA, wh ereas the shorter protected RNA fragment was 300 nt long. Brain tissue s that did not exhibit AA contained either the smaller protected RNA f ragment (cingulate and parietal cortex) or no protected RNA (cerebellu m). These results suggest that the 430-nt protected RNA fragment repre sents mRNA that encodes the functional P450(AROM) enzyme. In agreement with this conclusion, we found that immature rat ovaries that were st imulated with PMSG to synthesize estrogen contained only the 430-nt pr otected fragment. The levels of the 430-nt protected RNA fragment diff ered significantly between brain regions (amygdala >> preoptic area gr eater than or equal to medial basal hypothalamus greater than or equal to hippocampus) and were significantly correlated with AA (r = 0.994; P < 0.001). After castration, the concentrations of P450(AROM) mRNA a nd AA decreased significantly in the preoptic area and medial basal hy pothalamus (P < 0.05), but not in the amygdala. Treatments with testos terone or dihydrotestosterone maintained P450(AROM) mRNA and AA at lev els approximating those found in intact males. Although 17 beta-estrad iol treatment increased AA in the preoptic area, it did not affect the P450(AROM) mRNA content. These results suggest that the increase in A A observed after exposure to androgens results from regulation of the transcription and/or stability of P450(AROM) mRNA. In contrast, estrad iol appears to exert an effect on AA at the posttranscriptional level. Our results also indicate that the levels of P450(AROM) mRNA and AA a re heterogeneously distributed in different regions of the brain and a ppear to be under different modes of regulation in the amygdala compar ed to the preoptic area and medial basal hypothalamus. The identity of the 300-nt RNA fragment is not yet known, but it may represent an alt ernately spliced form of P450(AROM) mRNA. Its presence in rat brain, w hich is not associated with AA, suggests that caution is warranted whe n measuring P450(AROM) mRNA by methods unable to distinguish between t hese different RNA transcripts.