Alteration of the Bovine Estrous Cycle with Oxytocin

Oxytocin was less effective in causing diestrual shortening when either atropine or reserpine was administered concurrently, and was completely ineffective in inducing precocious estrus in hysterectomized, but not in sham-operated, heifers.

Abstract

The daily administration of natural or synthetic oxytocin to post-puberal nonlactating dairy heifers, during the first week of the estrous cycle, resulted in marked shortening of the diestrual period, the next heat occurring from eight to 12 days after the previous estrus, instead of the usual 21 days.These precocious heat periods appeared normai in every respect, being followed in all eases by ovulation at the expected time.In all eases, the subsequent diestrual periods were of normal lengths.The corpora htea formed during the oxytocin treatments failed to attain the expected size and contained a decreased number of normal luteal cells and an increased proportion of connective tissue elements.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 frost four to six days after cessation of the progesterone injections.It was concluded that the observed effects of oxytocia were caused by inhibition of normal corpus lutemn function, possibly by interfering with the secretion of a luteotrophie hormone from the anterior pituitary.0xytocin was less effective in causing diestrual shortening when either atropine or reserpine was administered concurrently, and was completely ineffective in inducing precocious estrus in hysterectomized, but not in sham-operated, heifers.The importance of the hypothalamus in the regulation of anterior pituitary hormone secretion is well established ( 12), although the mechanisms by which this regulation is brought about are less well understood.The secretion of gonadotrophie hormones has been suggested by Benoit and Assenmacher (2) to be under the control of neuroseeretory material, believed to be elaborated in hypothalamic nuclei and transported to the anterior pituitary by means of the hypophyseal portal blood vessels.The belief that this neurosecretory material contains the nenrohypophyseal hormones, oxytoein and vasopressin (20), has led to the investigation of possible physiological roles which these hormones may play in the regulation of anterior pituitary hormone secretion.Shibusawa et al. (21) have reported pitoein-induced alterations in urinary 17-ketosteroid excretion which they have interpreted as indicating that oxytoein induces gonadotrophie hormone secretion.Evidence for a close association between endogenous oxytoein release and gonadotrophin section has been reported in cattle.VanDemark and Hays (13,24) have reported increased uterine motility and increased intramammary pressure, suggesting oxytocin output, in cows which received sexual stimulation in the form of manipulation of the