A PROGESTERONE METABOLITE STIMULATES THE RELEASE OF GONADOTROPIN-RELEASING-HORMONE FROM GT1-1 HYPOTHALAMIC NEURONS VIA THE GAMMA-AMINOBUTYRIC-ACID TYPE-A RECEPTOR

The reduced progesterone metabolite tetrahydroprogesterone (3 alpha-hydroxy-5 alpha-pregnan-20-one; 3 alpha,5 alpha-THP) is a positive modulator of the gamma-aminobutyric acid type A (GABA(A)) receptor. Experiments performed in vitro with hypothalamic fragments have previously shown that GABA could modulate the release of gonadotropin releasing hormone (GnRH). Using GT1-1 immortalized GnRH neurons, we investigated the role of GABA(A) receptor ligands, including 3 alpha,5 alpha-THP, on the release of GnRH. We first characterized the GABA(A) receptors expressed by these neurons. [H-3]Muscimol, but not [H-3] flunitrazepam, bound with high affinity to GT1-1 cell membranes (K-d = 10.9 +/- 0.3 nM; B-max = 979 +/- 12 fmol/mg of protein), and [H-3]muscimol binding was enhanced by 3 alpha,5 alpha-THP. mRNAs encoding the alpha(1) and beta(3) subunits of the GABA(A) receptor were detected by the reverse transcriptase polymerase chain reaction. In agreement with binding data, the benzodiazepine-binding gamma subunit mRNA was absent. GnRH release studies showed a dose-related stimulating action of muscimol. 3 alpha,5 alpha-THP not only modulated muscimol-induced secretion but also stimulated GnRH release when administered alone. Bicuculline and picrotoxin blocked the effects of 3 alpha,5 alpha-THP and muscimol. Finally, we observed that GT1-1 neurons convert progesterone to 3 alpha,5 alpha-THP. We propose that progesterone may increase the release of GnRH by a membrane mechanism, via its reduced metabolite 3 alpha,5 alpha-THP acting at the GABA(A) receptor.