Progesterone regulates gamma-aminobutyric acid B (GABA B) receptors in the neocortex of female rats

Abstract

After discovering that binding to GABA B receptors in rat neocortex varied as a function of the estrous cycle of the rat, we asked whether either or both of the major ovarian steroids could affect binding to GABA B receptors in the same way, namely, by regulating the apparent density ( B max) of GABA B receptors. We report here that in ovariectomized rats, subcutaneous injection of progesterone alone, without the necessity of estrogen priming, increased the Borax of baclofen binding to GABA B receptors in the neocortex. Radioimmunoassay of plasma progesterone before and after progesterone injections revealed that plasma progesterone levels similar to those reached during the progesterone surge in proestrus were associated with increased baclofen binding. The effect of progesterone upon baclofen binding was evident 4 h but not 1 h following progesterone treatment. There was some specificity with respect to the cortical receptors affected by progesterone in that under our conditions, progesterone did not increase agonist binding to 5-HT 1A or GABA A receptors. We interpret our results to indicate that progesterone variation during the estrous cycle could be responsible for a component of the regulation of GABA B receptors that occurs in neocortex during the estrous cycle of the rat.