Endocrine regulation of cytochrome P-450 in the rat brain and pituitary gland

In an effort to understand the physiological functions of cytochrome P-450 in the central nervous system and pituitary gland, we evaluated changes in the level of the enzyme as a function of the endocrine status of rats and the ability of these tissues to synthesize or degrade steroids. The P-450 content of microsomes prepared from the hypothalamic preoptic area (HPOA), the olfactory lobes and the cerebrum was 0.040 ± 0.009 and in the pituitary gland 2.2 ± 0.6 (S.D.) nmol/g tissue. The P-450 content of the HPOA and olfactory lobes, but not of the rest of the cerebrum, was influenced by the endocrine status of the rats. In microsomes it increased five- to tenfold over control levels during late pregnancy in the olfactory lobes and during lactation in the HPOA, and in both brain regions treatment of rats with 5α-dihydrotestosterone (DHT) caused an eight- to tenfold increase in the P-450 content. Androstenedione was not a good substrate for brain P-450. The level of androstenedione 19-hydroxylase in the olfactory lobe microsomal fraction was 0.50 ± 0.06 nmol 19-hydroxyandrostenedione formed/g tissue per h. This activity was tenfold lower in other brain areas and was not detectable in the pituitary gland. The rate of aromatization of androstenedione to oestradiol in the HPOA and olfactory lobe of lactating rats was 0.46 ± 0.14 and 0.38 ± 0.05 pmol/oestradiol formed/g tissue per h respectively. 5α-Androstane-3β,17β-diol (A-5α-3β,17β-diol) was a much better substrate for P-450 throughout the brain and pituitary gland. Catalytic activity was 125 ± 46 and 307 ± 108 nmol triols formed/g tissue per h in the brain and pituitary gland respectively. The P-450 responsible for this catalytic activity was isolated and its substrate specificity examined. In addition to A-5α-3β,17β-diol, 5-androstene-3β,17β-diol, dehydroepiandrosterone and DHT were also substrates, with turnover numbers of 27, 8, 12 and 1 mol product/mol P-450 per min respectively. None of these catalytic activities was induced in the rat brain during pregnancy, lactation or DHT treatment. The enzyme was also present in the brains of mice but not guinea-pigs. The yield of P-450 from the mitochondrial fraction of the HPOA and olfactory lobes in control rats was 0.01-0.02 nmol/g tissue. This increased tenfold during pregnancy. Immunological evidence for the presence of the cholesterol side-chain cleavage enzyme P-450 SCC was found in the HPOA and olfactory lobes of pregnant but not of control rats. However, no SCC catalytic activity was detectable in these brain mitochondrial P-450 fractions. From these studies we conclude that there is a major influence of the endocrine system on the content and quality of P-450 in the brain. However, the function and substrate specificities of these P-450s as well as of those in the pituitary gland remain to be characterized.