Ovarian steroids block the isoproterenol-induced elevation of pineal melatonin production in the female rat.

Changes in pineal indole metabolism during the estrous cycle, as well as in response to estrogen administration, were studied in female rats. Tyrosine hydroxylase (TH) activity, norepinephrine (NE) and indoleamine levels were determined by high-performance liquid chromatography (HPLC). Pineal melatonin was measured by radioimmunoassay (RIA). Both 5-hydroxytryptamine (5-HT) and melatonin levels, but not TH activity or NE content, were reduced during proestrus. Ovarian hormones blocked the isoproterenol-induced elevation of pineal melatonin, and reduced 5-HT levels in ovariectomized rats. These results suggest that an estrogen-induced inhibition of the pineal response to adrenergic stimulation, could be responsible for the reduction in pineal melatonin production occurring during the proestrus stage of the rat estrous cycle.