Interactions between the Central Nervous System and Hormones 89 



EFFECTS OF ESTROGEN, LOW DOSAGE 



In the progesterone experiments described above tiic ovariectomized 

 rabbits were primed for two days witii estradiol bcnzoate (0.08-0.1 mg s.c, 

 in oil) daily. This treatment usually neither lowered the thresholds appreciably 

 nor brought the rabbits into heat. However, in the intact ancstrous rabbit, 

 possibly through synergism with endogenous progesterone, treatment with 

 exogenous estrogen often lowered both thresholds. An example of this is 



4 \ TO 4 V. 



• EEG AROUSAL 



4 EEG AFTERREACTION 



4 NO AFTERREACTION 



- 025 o S 



HOURS AFTER PROGESTERONE 



I I LATENCY 



E V C V C E EVC V CEVE C V 



-ELECTRIC STIM, V-VAGINAL STIM., C-COITUS 



Fig. 9. Thresholds and EEG afterreaction data following estrogen and subsequently 

 progesterone treatment of an intact (non-castrate) female rabbit. From Kawakami and 



Sawyer (43). 



seen in Fig. 9. On the day after the first injection of estrogen, the still 

 anestrous rabbit reveals a lowered EEG afterreaction threshold to electrical 

 stimulation. On the day following the second estrogen injection the EEG 

 afterreaction threshold is depressed still further and the arousal threshold 

 somewhat lowered. At this time, prior to treatment with progesterone, 

 the now estrous rabbit copulates and reveals an EEG afterreaction which 

 is not, however, fully evocable by vaginal stimulation. Progesterone lowers 

 the thresholds still further and during the next few hours even vaginal 

 stimulation induces the EEG afterreaction. Between six and eight hours after 

 progesterone administration the afterreaction threshold rises sharply while 

 the arousal threshold slopes upward gradually. 



Treatment of estrous rabbits with exogenous estrogen for two days or 

 anestrous rabbits for four days lowered the threshold of pituitary activation 

 to such an extent that ovulation could be induced in 40-50 % of the cases by 



