Effect of estrogen treatment on plasma oxytocin and vasopressin in ovariectomized rats.

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Abstract
Effects of estrogen treatment (estradiol benzoate 20μg per day for 2days) on plasma concentration of oxytocin and vasopressin were studied in ovariectomized female rats under urethane-anesthesia (1.2g/kg). Plasma oxytocin was measured by bioassay, while plasma vasopressin was determined by both bioassay and radioimmunoassay. Plasma oxytocin levels (46.2-232.2μU /ml) in the jugular blood of estrogentreated rats were found to be significantly (pl) of the ovariectomized control rats. The mean plasma vasopressin levels determined by bioassay were 172±66 (mean±SEM)%mu;U/ml in control rats and 176±32μU/ml in estrogen-treated rats. The mean concentrations of immunoreactive plasma vasopressin were60.1±19.6μU/ml in the control and75.5±26.8μU/ml in estrogen treated rats. Thus no significant change in plasma vasopressin due to the estrogen treatment was found, whatever method was employed to determine its concentrations. Anterior or posterior hypothalamic deafferentations were made in estrogen-treated ovariectomized rats. The facilitatory effect of estrogen on oxytocin release was not found in the rats with anterior deafferentation. The plasma oxytocin level in the estrogentreated rats with the anterior deafferentation was significantly (p<0.05) lower than that in the estrogen-treated rats without the hypothalamic deafferentation. On the other hand, the plasma oxytocin level in the estrogen-treated rats with posterior deafferentation did not show any significant difference from either that of ovariectomized control rats or that of estrogen-treated ovariectomized rats with intact brain. The present data indicate that estrogen stimulates oxytocin release in the female rat and that such an action of estrogen is exerted mainly by activating the anterior neural input to the neurosecretory cells in the hypothalamo-neurohypophysial system. Whether or not estrogen gives influence on the posterior neural input to the neurosecretory cells is equivocal in the present experiment. An additional experiment was made to determine the effect of urethane on plasma vasopressin. Urethane-anesthetized rats showed a significantly (p<0.01) higher plasma vasopressin level than conscious or pentobarbitone-anesthetized rats.