Evidence that oestradiol attenuates beta-adrenoceptor function in the hypothalamus of female rats by altering receptor phosphorylation and sequestration

Activation of beta-adrenoceptors in the hypothalamus (HYP) and preoptic are a (POA) inhibits both gonadotropin release and reproductive behaviour in fe male rats. Exposure of female rats for 48 h to physiologically relevant dos es of oestrogen attenuates beta-adrenoceptor function in the HYP and POA as indicated by reduced isoproterenol (beta-adrenoceptor agonist) stimulation of adenylyl cyclase activity. Reduced beta-adrenoceptor coupling to G prot ein in the HYP-POA from oestrogen-exposed female rats correlates with atten uation of beta-adrenoceptor function. To examine potential mechanisms under lying receptor-G protein uncoupling, initial experiments tested the hypothe sis that oestrogen attenuation of beta-adrenoceptor function in the HYP and POA involves receptor phosphorylation. Activation of endogenous serine/thr eonine phosphatases with protamine restores agonist-stimulated cAMP accumul ation in HYP slices from oestrogen-exposed female rats to control levels. A dditional experiments examined whether oestrogen-induced changes in beta-ad renoceptor binding density and/or subcellular localization correlate with t he attenuation of beta-adrenoceptor function in the HYP and POA. Oestrogen treatment does not alter total beta-adrenoceptor binding density in the HYP or POA. However, oestrogen significantly reduces cell surface binding of t he hydrophilic beta-adrenoceptor antagonist [H-3] CGP 12177 to intact HYP a nd POA slices. At the same time, oestrogen decreases the fraction of beta-a drenoceptors localized in a light vesicle fraction following sucrose densit y gradient centrifugation. Therefore, oestrogen attenuates beta-adrenocepto r signalling in the HYP-POA by uncoupling the beta-adrenoceptor from G prot ein, perhaps by promoting receptor phosphorylation. Furthermore, a signific ant fraction of beta-adrenoceptors in the HYP and POA are no longer accessi ble to hydrophilic ligands, but are not internalized. Thus, physiological d oses of oestrogen may facilitate reproductive behaviour and gonadotropin re lease, in part, by stabilizing beta-adrenoceptor phosphorylation in the HYP and POA, thereby uncoupling the receptors from G protein.