Estradiol attenuates the forskolin-induced increase in hypothalamic tyrosine hydroxylase activity.

The purpose of this study was to evaluate interactions between estradiol and the 3',5' cyclic adenosine monophosphate (cAMP) signaling pathway to regulate tyrosine hydroxylase (TH) activity in hypothalamic dopaminergic neurons. The first experiment examined the ability of forskolin to activate TH in the tuberoinfundibular dopaminergic neurons of adult ovariectomized rats with or without estradiol treatment. Estradiol treatment reduced both basal and forskolin-stimulated TH activity in the median eminence. The second group of experiments examined the effect of estradiol on the forskolin-induced activation of TH in fetal hypothalamic cells cultures. Estradiol decreased basal TH activity in the hypothalamic cell cultures to 80% of control levels. Forskolin treatment for 1 h increased TH activity in a concentration-dependent manner in control and estradiol-treated cells, but estradiol attenuated the stimulatory response to 0.01-10 microM forskolin. The suppressive effect of estradiol on cAMP-dependent activation of TH was evident with 1-12 h of forskolin treatment. The responses to other activators of the cAMP- protein kinase A pathway, including dibutyryl cAMP and 8-bromo-cAMP, and to a depolarizing stimulus were blunted in estradiol-treated cultures. Forskolin treatment for 1 h increased radiolabeled phosphate incorporation into TH protein in control but not estradiol-treated cells, suggesting that estradiol interferes with the ability of the cAMP pathway to phosphorylate TH. Forskolin caused a time-dependent increase in TH mRNA signal levels in control cultures. The magnitude of the forskolin-induced increase in TH mRNA levels was less in the estradiol-treated cells after 6 h of forskolin treatment, indicating that estradiol hinders cAMP-regulated TH gene expression. These data indicate that estradiol attenuates the ability of hypothalamic dopaminergic neurons to respond to cAMP-dependent stimulation by interfering with phosphorylation mechanisms in the short term and control of TH mRNA levels in the long term.