Cylical Fluctuations in Reflexive Oxytocin Release During the Estrous Cycle of the Goat

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Abstract
Elevations in the circulating level of oxytocin resulting from stimulation of the vagina were determined repeatedly in each of 23 estrous cycles. To delineate the course of each cycle, the day of behavioral estrus was determined and radioimmunoassayable serum progesterone was measured every 2 days. Vaginal stimulation induced greater elevations in the level of oxytocin near the time of estrus and during the early luteal phase than during periods characterized by maximal concentrations of progesterone. Elevations averaged 25.2 ± 8.8 µU/ml plasma in the early luteal phase and then declined progressively, reaching a minimum of 4.2 ± 1.7 µU/ml shortly before progesterone levels fell prior to estrus. This cyclic variation in the oxytocin-elevating effect of vaginal stimulation, viewed in the light of previous studies that have shown strong effects of exogenous ovarian steroids on oxytocin release, suggests that fluctuations in endogenous ovarian hormones during the estrous cycle may influence the oxyotocin secretory mechanism.

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