90 



Charles H. Sawyer and M. Kawakami 



vaginal stimulation (31). When apparently estrous rabbits were treated for 

 four days with estrogen, an appreciable number of them ovulated "spon- 

 taneously" during the winter and spring months but not during the summer 

 (44) (Fig. 10). In a limited number of rabbits with chronically implanted 

 electrodes, the EEG afterreaction and arousal thresholds, and the effects of 



20 





Ci 10- 





Months 



Total No. 

 Rabbits 



Jan. 

 Feb. 



46 



Mar. 

 Apr. 



49 



May 



45 



June 



Oct. 



48 



Nov. 

 Dec. 



37 



Fig. 10. Relalive incidence of estrogen-facilitated spontaneous ovulation in the rabbit 

 during various seasons of the year. From Sawyer (44). 



steroids upon them, were assessed throughout the year (43). The most 

 dramatic difference between February and June thresholds lay in the very low 

 values of EEG afterreaction threshold in late February; even after pro- 

 gesterone the higher June threshold did not come down to the pre-progesterone 

 value in February. In some of these experiments coitus in June was not 

 followed by an afterreaction. 



EFFECTS OF HIGH DOSAGES OF 

 ESTROGEN OR TESTOSTERONE 



In experiments such as those illustrated above in which progesterone and 

 low dosages of estrogen were tested, the EEG arousal and afterreaction 

 thresholds generally changed in a quite parallel manner. Under conditions 

 in which both thresholds were minimal the pituitary activation threshold 

 was also lowest and the rabbit was in heat; the elevation of both thresholds 

 was correlated with anestrus and an elevated pituitary threshold. 



It is possible with high dosages of estrogen or testosterone to separate 

 the two thresholds in a given animal and to study the resultant changes in 

 behavior and pituitary threshold. Five daily injections of 0.5 mg estradiol 



