Ovarian steroid regulation of signal transduction by noradrenergic receptors

Abstract

These experiments test the hypothesis that the ovarian hormones estradiol (E{dollar}\sb2{dollar}) and progesterone (P) regulate noradrenergic receptor responsiveness in neuronal populations which control reproduction, the hypothalamus and preoptic area (POA). Noradrenergic receptor function was assessed by analyzing the activation of the adenylyl cyclase/cAMP generating system.;Receptor subtype selective ligands were employed to determine which noradrenergic receptors mediate norepinephrine (NE)-stimulated cAMP accumulation in hypothalamic and POA slices. Activation of {dollar}\beta{dollar} receptors stimulates cAMP formation, and activation of {dollar}\alpha\sb1{dollar} receptors potentiates this response. The enhancement of cAMP synthesis by {dollar}\alpha\sb1{dollar} receptors in rat hypothalamus and POA slices appears to involve phospholipase C activation of protein kinase C and to be mediated by the {dollar}\alpha\sb{lcub}\rm 1B{rcub}{dollar} receptor subtype.;In vivo administration of E{dollar}\sb2{dollar} to ovariectomized rats attenuates {dollar}\beta{dollar} receptor-stimulated cAMP formation in hypothalamic and POA slices, apparently without receptor downregulation. E{dollar}\sb2{dollar} also enhances {dollar}\alpha\sb1{dollar} agonist augmentation of {dollar}\beta{dollar}-stimulated cAMP synthesis, and this effect is correlated with significant increases in {dollar}\alpha\sb1{dollar} receptor number in hypothalamic and POA membranes. Binding studies utilizing agents that distinguish {dollar}\alpha\sb1{dollar} receptor subtypes revealed that E{dollar}\sb2{dollar} selectively increases {dollar}\alpha\sb{lcub}\rm 1B{rcub}{dollar} receptor number.;In vivo administration of P to E{dollar}\sb2{dollar}-primed rats suppresses NE-stimulated cAMP formation in hypothalamus and POA slices. This effect of P is estrogen-dependent. P inhibits the cAMP response to NE by abolishing {dollar}\alpha\sb1{dollar} receptor-mediated augmentation of agonist-stimulated adenylyl cyclase activity. However, P does not reduce total {dollar}\alpha\sb1{dollar} or {dollar}\alpha\sb{lcub}\rm 1B{rcub}{dollar} receptor number in hypothalamus and POA membranes. Additional studies demonstrated that preincubation of slices from E{dollar}\sb2{dollar}-primed rats with 20 nM P in vitro for as little as 5 minutes significantly suppresses NE-stimulated cAMP formation. Furthermore, {dollar}\alpha\sb1{dollar} receptor augmentation of {dollar}\beta{dollar} receptor-stimulated cAMP accumulation is eliminated in slices incubated in vitro for 5 minutes with P or a membrane impermeable analog of P.;The data presented in this thesis suggest that in brain regions which regulate reproductive function, ovarian steroids regulate the coupling of both {dollar}\alpha\sb1{dollar}- and {dollar}\beta{dollar}-noradrenergic receptors to the membrane effector systems which generate intracellular cAMP. The possible relevance of these observations to female reproductive physiology are discussed.