Alteration of the Bovine Estrous Cycle with Oxytocin

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Abstract
Experiments were performed using post-puberal non-lactating dairy heifers in an attempt to demonstrate a possible regulatory function of the neurohypophyseal hormone, oxytocin, on anterior pituitary gonadotrophin secretion. Daily injection of various oxytocin preparations, natural or synthetic, during the 1st week of the estrous cycle resulted in a marked shortening of the diestrual period; the next heat occurring 8 to 12 days after the previous estrus, instead of the usual 21 days. These precocious heat periods appeared normal in every respect, being followed by ovulation at the expected time. The subsequent diestrual periods were of normal lengths. The corpora lutea formed during the oxytocin treatments failed to attain the expected size, and contained a decreased proportion of normal luteal cells. Concurrent injections of prolactin failed to overcome the diestrum-shortening effect of oxytocin. Daily progesterone injections prevented the oxytocin-treated heifers from returning to estrus until 4 to 6 days after cessation of the progesterone injections. It was concluded that the observed effects of oxytocin were caused by inhibition of normal corpus luteum function, possibly by interfering with the secretion by the anterior pituitary of a luteotrophic hormone distinct from prolactin. Oxytocin was less effective in causing these effects when either atropine or reserpine was administered concurrently, and was completely ineffective in inducing precocious estrus in hysterectomized, but not in sham-operated heifers.