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Neurosteroids and Female Reproduction: Estrogen Increases 3ß-HSD mRNA and Activity in Rat Hypothalamus

Departments of Psychology and Zoology (K.K.S.), Graduate Program in Neuroscience, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4; and Department of Neurobiology (K.S., A.L., P.E.M.), David Geffen School of Medicine at University of California, Los Angeles, and Department of Physiological Science (B.A.S.), Laboratory of Neuroendocrinology (B.A.S., P.E.M.), Brain Research Institute, University of California, Los Angeles, Los Angeles, California 90091

Address all correspondence and requests for reprints to: Paul Micevych, Department of Neurobiology, David Geffen School of Medicine at University of California, Los Angeles, California 90091-1763. E-mail: pmicevych{at}mednet.ucla.edu.

A central event in mammalian reproduction is the LH surge that induces ovulation and corpus luteum formation. Typically, the LH surge is initiated in ovariectomized rats by sequential treatment with estrogen and progesterone (PROG). The traditional explanation for this paradigm is that estrogen induces PROG receptors (PR) that are activated by exogenous PROG. Recent evidence suggests that whereas exogenous estrogen is necessary, exogenous PROG is not. In ovariectomized-adrenalectomized rats, estrogen treatment increases hypothalamic PROG levels before an LH surge. This estrogen-induced LH surge was blocked by an inhibitor of 3ß-hydroxysteroid dehydrogenase/5-4 isomerase (3ß-HSD), the proximal enzyme for PROG synthesis. These data indicate that estrogen induces de novo synthesis of PROG from cholesterol in the hypothalamus, which initiates the LH surge. The mechanism(s) by which estrogen up-regulates neuro-PROG is unknown. We investigated whether estrogen increases 1) mRNA levels for several proteins involved in PROG synthesis and/or 2) activity of 3ß-HSD in the hypothalamus. In ovariectomized-adrenalectomized rats, estrogen treatment increased 3ß-HSD mRNA in the hypothalamus, as measured by relative quantitative RT-PCR. The mRNAs for other proteins involved in steroid synthesis (sterol carrier protein 2, steroidogenic acute regulatory protein, and P450 side chain cleavage) were detectable in hypothalamus but not affected by estrogen. In a biochemical assay, estrogen treatment also increased 3ß-HSD activity. These data support the hypothesis that PROG is a neurosteroid, produced locally in the hypothalamus from cholesterol, which functions in the estrogen positive-feedback mechanism driving the LH surge.

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