title: 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.
abstract: These studies examined which alpha 2-adrenoceptor subtype is expressed in the hypothalamus and preoptic area and the influence of estradiol administration on alpha 2-adrenoceptors in the hypothalamus of female rats. The alpha 2-adrenoceptor antagonist [3H] RX821002 bound to a single 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 [3H]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 alpha 2D-adrenoceptor. Norepinephrine displacement of [3H]RX821002 binding in hypothalamic membranes from ovariectomized animals was monophasic and characterized by high affinity. In contrast, norepinephrine competition for [3H]RX821002 binding sites in the hypothalamus from rats exposed to estradiol for 48 hr was 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 alpha 2D-adrenoceptor complexes was inhibited by prior exposure to estrogen. In both control and estradiol-exposed hypothalamic membranes, 100 microM 5'-guanylylimidodiphosphate [Gpp(NH)p] converted the norepinephrine competition curves to ones characterized by monophasic, low affinity binding. In addition, binding of the full alpha 2-adrenoceptor agonist [3H]UK-14,304 in the hypothalamus and preoptic area of female rats was concentration-dependently diminished by Gpp(NH)p treatment. Complete loss of [3H]UK-14,304 binding was effected by 100 microM Gpp(NH)p. This suggests that [3H]UK-14,304 may be useful in labeling the agonist high affinity state of alpha 2-adrenoceptors. Decreasing the incubation temperature in saturation studies from 25 degrees to 0 degrees increased [3H]UK-14,304 binding in hypothalamic membranes of control rats but not in membranes from estradiol-treated rats. Estradiol treatment for 48 hr decreased [3H]UK-14,304 binding in hypothalamic membranes by 34% (0 degrees) to 60% (25 degrees), without changing the Kd. These results suggest that the alpha 2D-adrenoceptor is the predominant subtype in the hypothalamus and preoptic area of female rats and that estradiol treatment markedly reduces the number of alpha 2D-adrenoceptors in the agonist high affinity state.
authors.affiliation: Department of Neuroscience, Albert Einstein College of Medicine, Bronx, New York 10461.
authors.forename: G B
authors.initials: GB
authors.lastname: Karkanias
authors.forename: A M
authors.initials: AM
authors.lastname: Etgen
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000378:metabolism; D000317:Adrenergic alpha-Antagonists / Q000378:metabolism; D000818:Animals; D001667:Binding, Competitive; D000068438:Brimonidine Tartrate; D004146:Dioxanes / Q000378:metabolism; D004958:Estradiol / Q000494:pharmacology*; D005260:Female; D007031:Hypothalamus / Q000378:metabolism*; D019329:Idazoxan / Q000031:analogs & derivatives; D007700:Kinetics; D011301:Preoptic Area / Q000378:metabolism; D011810:Quinoxalines / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects / Q000378:metabolism*
chemical_list: D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D004146:Dioxanes; D011810:Quinoxalines; D018341:Receptors, Adrenergic, alpha-2; D000068438:Brimonidine Tartrate; D004958:Estradiol; C058525:2-methoxyidazoxan; D019329:Idazoxan