ESTRADIOL REDUCTION OF THE AGONIST HIGH-AFFINITY FORM OF THE ALPHA(2)-ADRENOCEPTOR IN THE HYPOTHALAMUS OF FEMALE RATS - IDENTIFICATION AS THE ALPHA(2D) SUBTYPE

These studies examined which alpha2-adrenoceptor subtype is expressed in the hypothalamus and preoptic area and the influence of estradiol a dministration on alpha2-adrenoceptors in the hypothalamus of female ra ts. The alpha2-adrenoceptor antagonist [H-3] RX821002 bound to a singl e site in hypothalamus, preoptic area, and cortex membranes, with high affinity and low nonspecific binding, as determined by Scatchard and kinetic binding analyses. Competition for [H-3]RX821002 binding in the hypothalamus and preoptic area by various noradrenergic agonists and antagonists revealed a unique pharmacological specificity with a high degree of similarity to that of the alpha2D-adrenoceptor. Norepinephri ne displacement of [H-3]RX821002 binding in hypothalamic membranes fro m ovariectomized animals was monophasic and characterized by high affi nity. In contrast, norepinephrine competition for [H-3]RX821002 bindin g sites in the hypothalamus from rats exposed to estradiol for 48 hr w as biphasic, and norepinephrine bound to both a high (18%) and a low ( 82%) affinity site in these membranes. Thus, the formation of agonist high affinity alpha2D-adrenoceptor complexes was inhibited by prior ex posure to estrogen. In both control and estradiol-exposed hypothalamic membranes, 100 muM 5'-guanylylimidodiphosphate [Gpp(NH)p] converted t he norepinephrine competition curves to ones characterized by monophas ic, low affinity binding. In addition, binding of the full alpha2-adre noceptor agonist [H-3]UK-14,304 in the hypothalamus and preoptic area of female rats was concentration-dependently diminished by Gpp(NH)p tr eatment. Complete loss of [H-3]UK-14,304 binding was effected by 100 m uM Gpp(NH)p. This suggests that [H-3]UK-14,304 may be useful in labeli ng the agonist high affinity state of alpha2-adrenoceptors. Decreasing the incubation temperature in saturation studies from 250 to 00 incre ased, [H-3]UK-14,304 binding in hypothalamic membranes of control rats but not in membranes from estradiol-treated rats. Estradiol treatment for 48 hr decreased [H-3]UK14,304 binding in hypothalamic membranes b y 34% (0-degrees) to 60% (250), without changing the K(d). These resul ts suggest that the alpha2D-adrenoceptor is the predominant subtype in the hypothalamus and preoptic area of female rats and that estradiol treatment markedly reduces the number of alpha2D-adrenoceptors in the agonist high affinity state.