A progesterone metabolite enhances the activity of the GABAA receptor complex at the pituitary level

The interaction of 5 alpha-pregnane-3 alpha-ol-20-one (5 alpha 3 alpha P), a progesterone metabolite, with the GABAA receptor chloride channel complex was investigated at the pituitary level. In nanomolar concentrations this steroid potentiated the inhibitory effect of muscimol (a GABAA agonist) on prolactin release from rat pituitary cells in culture. In micromolar concentrations 5 alpha 3 alpha P had a direct inhibitory effect, similar to that of muscimol, with an IC50 value of 370 nM. This effect was antagonized by bicuculline, a GABAA antagonist, and by picrotoxin, a chloride ion channel blocker. Its reduced isomer, 5 alpha 3 beta P, and progesterone (Pg) were devoid of activity. Using [35S]t-butylbicyclophosphorothionate ([35S]TBPS) as a ligand, we demonstrated, for the first time, specific barbiturate sites on pituitary membranes, similar to those of the central nervous system, with a Kd value of 25 nM and a Bmax value of 62 fmol/mg protein. 5 alpha 3 alpha P inhibited the binding of [35S]TBPS. In contrast, its 3 beta isomer was inactive. These data show that 5 alpha 3 alpha P enhanced the activity of the GABAA receptor complex at the pituitary level and suggest that its inhibitory effect on prolactin release might be mediated by the barbiturate site or by a closely related site.