Receptor phosphorylation mediates estradiol reduction of α2-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 alpha (2)-adrenoceptors to act as negative feedback inhibitors of norepinephrine release. Estrogen enhancement of norepinephrine release in the hypothalamus correlates with decreased coupling of the alpha (2)-adrenoceptor to G protein. To determine the mechanism by which estrogen uncouples alpha (2)-adrenoceptors from C protein, we tested the hypothesis that estrogen increases alpha (2)-adrenoceptor phosphorylation, Shortterm activation of endogenous serine/threonine phosphatases with protamine or treatment with exogenous phosphatase restored alpha (2)-adrenoceptor coupling to C protein to control levels in hypothalami from estrogen-exposed female rats, Additional experiments examined whether estrogen alters C 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 C 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 fractions demonstrated that estrogen decreases the fraction of internalized alpha (2)-adrenoceptors in the hypothalamus. Therefore, estrogen promotes norepinephrine release in the hypothalamus by stabilizing alpha (2)-adrenoceptor phosphorylation, uncoupling the receptor from C protein, Estrogen may stabilize alpha (2)-adrenoceptor phosphorylation by inhibiting receptor internalization and dephosphorylation.