Receptor phosphorylation mediates estradiol reduction of alpha2-adrenoceptor coupling to G protein in the hypothalamus of female rats

Estrogen increases evoked norepinephrine release in the hypothalamus of female rodents, in part by reducing the ability of α₂-adrenoceptors to act as negative feedback inhibitors of norepinephrine release. Estrogen enhancement of norepinephrine release in the hypothalamus correlates with decreased coupling of the α₂-adrenoceptor to G protein. To determine the mechanism by which estrogen uncouples α₂-adrenoceptors from G protein, we tested the hypothesis that estrogen increases α₂-adrenoceptor phosphorylation. Short-term activation of endogenous serine/threonine phosphatases with protamine or treatment with exogenous phosphatase restored α₂-adrenoceptor coupling to G protein to control levels in hypothalami from estrogen-exposed female rats. Additional experiments examined whether estrogen alters G protein-coupled receptor kinase expression or activity or serine/threonine phosphatase activity. These proteins are involved in G protein-coupled receptor phosphorylation, internalization, and recycling. Estrogen exposure reduced G protein-coupled receptor kinase mRNA, protein, and activity in the hypothalamus. Furthermore, estrogen treatment reduced serine/threonine phosphatase activity in the hypothalamus. Analysis of ligand binding in subcellular fraction demonstrated that estrogen decreases the fraction of internalized α₂-adrenoceptors in the hypothalamus. Therefore, estrogen promotes norepinephrine release in the hypothalamus by stabilizing α₂-adrenoceptor phosphorylation, uncoupling the receptor from G protein. Estrogen may stabilize α₂-adrenoceptor phosphorylation by inhibiting receptor internalization and dephosphorylation.