DISCUSSION 



Chairman: Dr. Warren O. Nelson 



Dr. William Hansel: Among the previous speakers, Harris in particular has pointed out 

 the importance of the hypothalamus in regulating the secretion of the gonadotropins 

 necessary for follicular growth and ovulation. Sawyer has described a typical electro- 

 encephalographic (EEG) arousal pattern, which occurs after stimulation of the 

 mid-brain reticular formation and an EEG afterreaction which occurs following 

 stimulation of hypothalamic and rhinencephalic loci in rabbits. A lowered arousal 

 threshold was correlated with the induction of estrous behavior; a lowered after- 

 reaction threshold appeared to be related to the release of pituitary ovulating hormone. 

 Everett has shown that stimulation of the preoptic area consistently induces ovulation 

 in rats given appropriately-timed injections of the ovulation-blocking drugs atropine 

 and pentobarbital. 



These results inevitably raise two major questions. The first of these concerns the 

 nature of whatever humoral substances act between the hypothalamus and the adeno- 

 hypophysis. The second concerns the afferent pathways, particularly those from the 

 uterus, normally involved in activating those elements of the central nervous system 

 which affect the neurohumoral regulation of the anterior pituitary. 



The results of some of our recent studies on ovulation in the bovine are of particular 

 interest in regard to both of these questions. Impressed with the possibility that 

 oxytocin of hypothalamic origin might be involved in some way in the regulation of 

 the secretion of anterior pituitary gonadotropins. Hansel et al. (Proc. Ilird Symposium 

 on Reproduction and Infertility (Ed. Gassner), Pergamon Press, 1958) tested the effects 

 of injecting this hormone at the beginning of estrus on ovulation time in heifers. 

 Ovulation in the bovine normally occurs about 12 hr after the end of an 18-hr estrous or 

 "heat" period. In the oxytocin-treated heifers, the average time of ovulation was 

 hastened by 5 hr, a result which might have been predicted on the basis of the recent 

 finding by Sawyer and Kawakami that this hormone lowers the EEG afterreaction 

 threshold, which appears to be related to the release of pituitary ovulating hormone. 



Armstrong and Hansel {J. Dairy Sc. 42, 533-542, 1959) later studied the effects of 

 daily doses of oxytocin given at various times during the estrous cycle on ovarian 

 function and cycle length in heifers. It soon became apparent that oxytocin administered 

 on days 1 to 7 or 3 to 6 inclusive of the normal 22-day estrous cycle inhibited the 

 development of the corpus luteum and produced a precocious estrus by the 8th- 10th 

 day of the cycle. 



These results provided the first direct evidence for an effect of a neurohypophysial 

 hormone on estrous cycle regulation and ovarian function in any species and, further, 

 suggested that the administered oxytocin inhibited the production or release of 

 luteotropin. However, in a subsequent experiment, oxytocin produced precocious 

 estrus even when given concurrently with a prolactin preparation of bovine origin. 



More recent results (/. Dairy Sc, I960 (In press) have served to emphasize the 

 fundamental role played by the uterus in regulating cycle length and ovarian 

 functions in the bovine. As in several other species, corpora lutea persist and estrous 

 cycles do not occur in cattle after hysterectomy. Oxytocin injections proved incapable 

 of inducing estrus in hysterectomized heifers. 



This result led to studies of the effects of uterine dilatation and irritation on ovarian 

 function and cycle length. Dilatation of the uterus by self-retaining rubber catheters, 

 held in the uterus during the first 7 days of the cycle by small inflatable balloons, 

 caused shortened estrous cycles, often 8 to 12 days in length, in a large proportion 

 of the treated cows and heifers. In a similar experiment it was found that the infusion 



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