Interactions between the Central Nervous System and Hormones 95 



coital behavior witii a responsive pituitary-gonad axis. Gonadotropins, 

 testosterone, the 19-nor gestagens and high dosages of estrogen affect the 

 thresholds, and perhaps the receptors, differentially, thus permitting estrous 

 behavior without ovulation and ovulation without estrous behavior. 



It would appear that both the facilitory and the inhibitory influences of 

 the sex steroids on the adenohypophysis are attributable to, or mediated by, 

 their actions on the nervous system. There may be multiple receptors within 

 the brain which are influenced in a coordinated manner by endogenous 

 hormones or systemically administered exogenous hormones. Bodily needs, 

 according to Dell (51), demand a nonspecific excitatory state for appetitive 

 behavior; this condition is induced by the effects of elements in the internal 

 environment on the brainstem reticular system. Consummatory behavior 

 then produces changes which depress the generalized reticular activity and 

 vigilance. The present results are consistent with this scheme, and they point 

 to sex steroids as crucial elements of the internal environment responsible 

 for the vigilant appetitive phase. Consummatory depression of the arousal 

 system is effected by, or coordinated with, unusual activation of a rhin- 

 encephalic-hypothalamic circuit, probably involving actions of pituitary 

 hormones on the nervous system. 



SUMMARY 

 The hormonal feed-back circuit in the rabbit by which ovarian steroids 

 alter pituitary susceptibility for ovulation and coordinate this condition with 

 the estrous state has been shown to include the action of the steroids on two 

 thresholds of activity in the brain. The EEG arousal threshold appears to be 

 concerned with estrous behavior while the EEG afterreaction threshold 

 parallels the threshold of pituitary activation. The natural EEG afterreaction, 

 which is a common sequel to coitus in the rabbit, seems to represent an effect 

 of the released pituitary hormones on the nervous system, perhaps in the 

 nature of a negative feed-back to stop further release of gonadotropin. 

 Certain steroids appear to be excellent antifertility agents by virtue of a 

 differential elevation of the EEG afterreaction threshold and the absence 

 of an effect on the EEG arousal threshold. The results are consistent with 

 the concept that hypothalamic "centers" controlling sex behavior and 

 gonadotropic secretion may represent important neuroendocrine receptors 

 of hormonal influence on brain function. 



Acknowledgments — The authors wish to thank the Schering Corporation 

 for the estradiol benzoate (Progynon B), Dr. J. D. Fisher of Armour 

 Pharmaceutical Corporation for the pituitary hormones. Dr. E. C. Reifenstein 

 of E. R. Squibb and Sons for Delalutin, Dr. D. A. McGinty of Parke, Davis 

 and Co. for Norlutin and Dr. F. J. Saunders of G. D. Searle and Co. for 

 norethynodrel and Nilevar employed in the experiments. The figures were 

 drawn by Charles Bridgman and photographed by Timothy Dodge. 



