A progesterone metabolite stimulates the release of gonadotropin-releasing hormone from GT1-1 hypothalamic neurons via the γ-aminobutyric acid type A receptor

Abstract

The reduced progesterone metabolite tetrahydroprogesterone (3α-hydroxy- 5α-pregnan-20-one; 3α,5α-THP) is a positive modulator of the γ- 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 3α,5α-THP, on the release of GnRH. We first characterized the GABA(A) receptors expressed by these neurons. [3H]Muscimol, but not [3H]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 [3H]muscimol binding was enhanced by 3α,5α-THP. mRNAs encoding the α1 and β3 subunits of the GABA(A) receptor were detected by the reverse transcriptase polymerase chain reaction. In agreement with binding data, the benzodiazepine-binding γ subunit mRNA was absent. GnRH release studies showed a dose-related stimulating action of muscimol. 3α,5α-THP not only modulated muscimol-induced secretion but also stimulated GnRH release when administered alone. Bicuculline and picrotoxin blocked the effects of 3α,5α-THP and muscimol. Finally, we observed that GT1-1 neurons convert progesterone to 3α,5α-THP. We propose that progesterone may increase the release of GnRH by a membrane mechanism, via its reduced metabolite 3α,5α-THP acting at the GABA(A) receptor.