1 10 John W. Everett 



the central nervous system. That factor manifests itself in the elevated 

 preoptic threshold of the diestrous rat. With fair certainty the prediction 

 can be made that when put to the test progesterone or estrogen supplements 

 will bring the diestrus threshold to the procstrus level. 



The 60-sec stimulations in Series V, even better than the 5-min stimulations 

 of Series IV, display a triggering effect. Closely allied with this may be the 

 all-or-none eflect observed at threshold levels of stimulation in Series IV. 

 Heretofore it had been thought that in the rat, quite unlike the rabbit, the 

 neural mechanisms that provoke release of ovulating hormone operate 

 continuously in an obligatory way during the half-hour or so occupied in 

 the discharge of the hormone (10, 14). That view was based on results of 

 atropine injection at various times during the "critical period". Numerous 

 cases of only partial interference with ovulation were encountered, as well 

 as the cases of complete blockade or complete ovulation. The proportion 

 of partial effects was strikingly like that observed in a parallel series of partial 

 hypophysectomies during the critical period. On the other hand, the adminis- 

 tration of a threshold dose of atropine before the critical period in another 

 group of rats resulted in an essentially all-or-none response. Thus, it seemed 

 that the partial effects obtained during the critical period either by the full 

 dose of atropine or by hypophysectomy must have been the result of 

 interruption of the discharge of ovulating hormone already in progress. 

 The frequency of the partial effects indicated that the atropine-sensitive 

 component must act for at least 10 min, and more likely about half an hour. 

 Although circumstantial evidence led to the conclusion that the site of action 

 of atropine lies in the central nervous system, there was no direct proof that 

 this applied to this species. It was necessary to fall back on the proof furnished 

 by experiments in the rabbit (33). There remained a shade of doubt, therefore. 

 Because of species differences and the pronounced differences in dosage and 

 route of administration, it was still possible that in rats there might be a 

 direct blocking action on the hypophysial cells. The doubt has now been 

 dispelled. 



At the same time, however, the new experiments present us with a paradox. 

 It is now evident that a triggering stimulus of 60 sec or less can be effective 

 although it operates upon a part of the system less remote from the median 

 eminence than the blocking sites of pentobarbital and atropine. It thus 

 becomes necessary to inquire what happens within the hypothalamus in 

 response to the trigger. Does it set up some prolonged electrical activity? 

 This should be readily subject to direct test. 



Finally, attention must be called to observations by Christian (5) who 

 noted ovulation in 5 estrous rabbits that ovulated after electrical stimulation 

 of the preoptic region, in spite of the fact that they had been pretreated 

 with atropine in amounts adequate to block the ovulation reflex. Saul and 

 Sawyer (29) reported negative results from similar experiments, stating 



