Estrogen regulation of alpha-2-adrenoceptor function

Abstract

Estradiol (E{dollar}\sb2{dollar}) facilitation of reproductive behavior is strongly correlated with enhanced hypothalamic norepinephrine release. The {dollar}\alpha\sb2{dollar}-adrenoceptor is a potent inhibitor of norepinephrine release in central and peripheral sites. We hypothesized that estrogen elevates hypothalamic norepinephrine release by attenuating {dollar}\alpha\sb2{dollar}-adrenoceptor-mediated inhibition of norepinephrine release.;The {dollar}\alpha\sb2{dollar} antagonists idazoxan (IDA) and RX821002 (RX) applied to hypothalamic slices from control female rats increased depolarization-evoked release of norepinephrine by 80% relative to drug-free release. Therefore, {dollar}\alpha\sb2{dollar}-adrenoceptors actively inhibit norepinephrine release in the hypothalamus of control female rats. In contrast, IDA and RX had little or no effect on norepinephrine release in slices from E{dollar}\sb2{dollar}-treated animals, suggesting that {dollar}\alpha\sb2{dollar}-adrenergic inhibitory mechanisms are attenuated by E{dollar}\sb2.{dollar} Binding studies identified the {dollar}\alpha\sb{lcub}\rm 2D{rcub}{dollar}-adrenoceptor as the estrogen-regulated subtype.;We examined whether E{dollar}\sb2{dollar} attenuates {dollar}\alpha\sb2{dollar}-adrenoceptor function by receptor downregulation or reduction in ligand binding affinity. Paradoxically E{dollar}\sb2{dollar} treatment, produced a modest increase rather than a decrease in {dollar}\alpha\sb2{dollar}-adrenoceptor density in the hypothalamus and ligand-binding affinity was not reduced. Interestingly, the entropic contribution to both agonist and antagonist binding was markedly increased, and enthalpy changes were reduced by E{dollar}\sb2{dollar} treatment. Since the thermodynamics of ligand-receptor binding are strongly correlated with efficacy, these data suggest that E{dollar}\sb2{dollar} may act by stabilizing a receptor conformation with reduced intrinsic efficacy.;E{dollar}\sb2{dollar} also inhibits {dollar}\alpha\sb2{dollar}-adrenoceptor-G-protein coupling. Norepinephrine displacement of {dollar}\sp3{dollar}H-RX in hypothalamic membranes from control animals was monophasic and characterized by high affinity. In contrast, norepinephrine competition for {dollar}\sp3{dollar}H-RX binding sites in the hypothalamus from rats exposed to E{dollar}\sb2{dollar} was biphasic, and 82% of norepinephrine binding was to a low affinity site. Furthermore, binding of the {dollar}\alpha\sb2{dollar} agonist {dollar}\sp3{dollar}H-UK-14,304 to the agonist high affinity state of {dollar}\alpha\sb2{dollar}-adrenoceptors was dramatically decreased by E{dollar}\sb2{dollar} treatment.;In summary, E{dollar}\sb2{dollar} treatment attenuates {dollar}\alpha\sb{lcub}\rm 2D{rcub}{dollar}-adrenoceptor-mediated inhibition of norepinephrine release. E{dollar}\sb2{dollar} may mediate this attenuation by interfering with {dollar}\alpha\sb2{dollar}-adrenoceptor-G-protein coupling and by reducing the efficacy of ligand-receptor binding to activate signal transduction.