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Brain Res. 1997 Aug 1;764(1-2):158-66.

Synthesis, metabolism and levels of the neuroactive steroid, 3alpha-hydroxy-4-pregnen-20-one (3alphaHP), in rat pituitaries.

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  • 1Department of Zoology, University of Western Ontario, London, Canada.


The neuroactive steroid, 3a-hydroxy-4-pregnen-20-one (3alphaHP), is a metabolite of progesterone and a precursor of 3alpha-hydroxy-5alpha-pregnan-20-one (5alphaP3alpha; allopregnanolone). In addition to analgesic and anxiolytic effects by interaction with the GABA(A) receptor complex, 3alphaHP regulates pituitary FSH secretion by rapid non-genomic interaction with the Ca2+-driven cell signaling mechanisms. Since gonadectomy and adrenalectomy do not result in elimination of 3alphaHP, and since there is the possibility of paracrine and/or autocrine regulation of FSH release, the capacity of pituitary cells to regulate levels (by synthesis, metabolism, and storage) of 3alphaHP was examined. Anterior pituitaries from random cycling female rats were incubated, either as fragments or as cultured cells, for 1, 4 or 8 h with 3H- or 14C-labeled progesterone. The steroid metabolites were identified by thin-layer chromatography, autoradiography, high pressure liquid chromatography (HPLC), derivatization and GC/MS. Pituitary cells actively converted progesterone to 3alphaHP along with 5alphaP3alpha, 5alpha-pregnane-3,20-dione, 20alpha-hydroxy-5alpha-pregnan-3-one, 3beta-hydroxy-5alpha-pregnan-20-one, 5alpha-pregnane-3alpha(beta), 20alpha-diols, 20alpha-hydroxy-4-pregnen-3-one, and 4-pregnene-3alpha(beta), 20alpha-diols. The results indicate the presence of the following steroidogenic enzymes in anterior pituitary cells: 3alpha-hydroxysteroid oxidoreductase (3alpha-HSO), 20alpha-HSO, 3beta-HSO, and 5alpha-reductase. The activities of 5alpha-reductase and 3alpha-HSO were approximately equal and greatly exceeded those of the other enzymes. After 8 h of incubation with 100 ng progesterone per pituitary, about 20% of the progesterone was metabolized and 3.18 ng of 3alphaHP had been formed. The accumulation of 3alphaHP increased approximately linearly with the time of incubation. Metabolism studies using [1,2,6,7-(3)H]3alphaHP showed that pituitary cells convert about 29% and 8% of the 3alphaHP to progesterone and 5alphaP3alpha, respectively, in 2 h. Specific radioimmunoassays determined 11.6 and 7.5 ng of 3alphaHP per pituitary, respectively, in 25- and 40-day-old non-cycling female rats; these concentrations of 3alphaHP were about 2-3-fold greater than those of progesterone in the same pituitaries. In older (80-100 days old) cycling rats, the levels of 3alphaHP were about 9.4 and 18.6 ng/pituitary at 13.00 h and 22.00 h, respectively, on the day of proestrus, while the concomitant circulating levels were 13.7 and 5.4 ng/ml. The results indicate a marked capacity of rat pituitary cells to synthesize the neuroactive and FSH regulating steroid, 3alphaHP, from progesterone, and in turn to metabolize 3alphaHP to the neurosteroid, allopregnanolone, and to progesterone. The studies suggest cyclic biosynthetic and metabolic pathways for 3alphaHP and other steroids in the pituitary. They also indicate that the regulation of FSH secretion by 3alphaHP may be (in part, or in whole) via paracrine or autocrine mechanisms.

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