Identification of 3β,5β-tetrahydroprogesterone, a progesterone metabolite, and its stimulatory action on preoptic neurons in the avian brain

We have demonstrated recently that the quail brain possesses the cholesterol side-chain cleavage enzyme (cytochrome P450scc) and 3β-hydroxysteroid dehydrogenase/Δ⁵-Δ⁴- isomerase (3β-HSD) and produces pregnenolone, pregnenolone sulfate and progesterone from cholesterol. The present study was therefore conducted to investigate progesterone metabolism in the brain of adult male quails. Employing biochemical techniques combined with HPLC and TLC analyses, the conversion of progesterone to 3β,5β-tetrahydroprogesterone (3β,5β-THP) via 5β-dihydroprogesterone (5β-DHP) was found in the brain. There was a clear regional difference in progesterone metabolism. The formation of 3β,5β-THP was high in the diencephalon and cerebrum and low in the cerebellum. Based on such a region-dependent formation of 3β,5β-THP, the action of this progesterone metabolite on preoptic neurons in the diencephalon was then investigated electrophysiologically using a brain slice preparation of the adult male. 3β,5β-THP significantly increased, in a dose-related way, the spontaneous firing activity of subsets of preoptic neurons. The stimulatory effect of 3β,5β-THP was greater than that of progesterone and its threshold concentration ranged between 10 ^-6 and 3×10^-6 M. In 33% of cells in the preoptic area, however, 3β,5β-THP did not change the spontaneous firing activity even at the high concentration, 10^-5 M. Because preoptic neurons are considered to be involved in the regulation of a variety of male reproductive behaviors, 3β,5β-THP may regulate some reproductive behavior through the mechanism that provokes such a stimulation.