Estrogen regulation of noradrenergic signalling in hypothalamus

Abstract

To investigate presynaptic actions of estradiol on noradrenergic signalling, the activity of tyrosine hydroxylase, the rate limiting enzyme of norepinephrine biosynthesis, was assayed in hypothalamus and preoptic area of ovariectomized female rats receiving vehicle or estradiol. Estradiol did not increase enzyme activity above control at any time tested from 0-48 h. Significant decreases in enzyme activity were found at 12 and 18 h after hormone injection. Stimulation of tyrosine hydroxylase by calcium and cyclic AMP was also tested. Neither estradiol nor progesterone modified enzyme stimulation by these activators. These studies suggest that estradiol does not produce global increases in norepinephrine biosynthesis in brain regions that regulate reproduction.;Postsynaptic actions of estradiol on the {dollar}\beta{dollar}-adrenergic receptor/effector system were then examined. The {dollar}\beta{dollar}-agonist, isoproterenol, produced a concentration-dependent activation of adenylyl cyclase in membranes from ovariectomized rat hypothalamus-preoptic area. This response was abolished in membranes from estradiol-treated rats. The effect of hormone was demonstrated 48 but not 24 h after injection. Estradiol had no measurable effect on adenylyl cyclase activation by either GTP or forskolin or on the quantity of the guanine nucleotide regulatory proteins, Gs or Gi/Go. Binding studies with the {dollar}\beta{dollar}-adrenergic antagonist ({dollar}\sp{lcub}125{rcub}{dollar}I) iodocyanopindolol showed that estradiol had no effect on either radioligand binding affinity or receptor density in hypothalamus. However, estradiol reduced agonist binding affinity as assessed by isoproterenol displacement of ({dollar}\sp{lcub}125{rcub}{dollar}I) iodocyanopindolol. In hypothalamic membranes from ovariectomized controls, high affinity agonist binding was apparent and was abolished by guanine nucleotides, indicating that most {dollar}\beta{dollar}-adrenergic receptors are coupled to Gs. In membranes from estradiol-treated rats, only low affinity agonist binding was measurable, and it was unaffected by guanine nucleotides. These studies suggest that estradiol suppression of {dollar}\beta{dollar}-adrenergic activation of adenylyl cyclase is not due to receptor downregulation but rather to disruption of receptor coupling to Gs.;Estradiol targets the {dollar}\beta{dollar}-adrenergic receptor/effector system, but not tyrosine hydroxylase, to regulate responsiveness of cells in preoptic area and hypothalamus to norepinephrine. More specifically, the experiments demonstrate a novel, estradiol-induced form of receptor desensitization involving stable uncoupling of the {dollar}\beta{dollar}-adrenergic receptor from Gs.