120G 



HORMONAL REGULATION 



on which they act, and (2) the period during 

 which the chemical integrity of the injected 

 hormones is maintained (Fee, Marrian and 

 Parkes, 1929; Kochakian, 1939; Davis and 

 Plotz, 1957). Inquiries into the mechanism 

 of hormonal action should be directed to- 

 ward the events taking place during this in- 

 terval. 



References to relationships specific for the 

 female have been made by Young (1941) 

 and de Alba and Asdell (1946). The former 

 noted that estrogens stimulate responses ap- 

 proximating those of the true heat, but the 

 responses fall short of those displayed when 

 progesterone is given. At the time, the na- 

 ture of this "facilitating action" was not 

 known ; it was only known that the chain of 

 events initiated by the estrogen reciuires 

 progesterone for its completion and that the 

 latter hormone acts to terminate heat as 

 well, at least in the rabbit, ferret, sheep, and 

 goat (Makepeace, Weinstein and Friedman, 

 1937; Marshall and Hammond, Jr., 1945; 

 Phillips, Fraps and Frank, 1946; and more 

 recently, Sawyer and Everett, 1959). De 

 Alba and Asdell postulated the development 

 of a refractory condition and central nerv- 

 ous block after the threshold to estrogen has 

 been reached in the cow, but if such a state 

 develops, the means are not known. Such a 

 block was not observed by Melampy and 

 Rakes (1958). 



More precise information with respect to 

 the neural responses of estrogen-primed rab- 

 bits to progesterone is contained in the study 

 of Kawakami and Sawyer (1959a). Sawyer 

 and Everett (1959) had previously shown 

 that progesterone at first facilitates and sub- 

 sequently inhibits the release of pituitary 

 ovulating hormone and that these effects are 

 paralleled by a heightened degree of estrus 

 and later by a depression of heat to the 

 anestrous state. In the investigation by Ka- 

 wakami and Sawyer, alterations in two types 

 of neural thresholds accompanying these 

 changes were followed: (1 ) the EEG arousal 

 threshold for direct stimulation of the mid- 

 brain reticular formation, and (2) the EEG 

 after-reaction threshold for low frequency 

 stimulation of hypothalamic or rhinenceph- 

 alic nuclei. Presumptive evidence is pre- 

 sented for believing that the former is more 

 closely linked to sexual behavior and the 



latter to pituitary activation. Briefly, a low- 

 ering of the arousal threshold soon after the 

 administration of progesterone and its ele- 

 vation later correlated with the behavioral 

 changes and were taken as evidence for the 

 action of progesterone on central nervous 

 system tissues. Presumably because of the 

 authors' o]Hnion that it was not possible to 

 do so, neither the significance of the lowered 

 threshold for the change in behavior was in- 

 dicated, nor was any clue given w'ith respect 

 to the nature of the initial preparatory or 

 ]iriming action of the estrogen. 



Identification of the neural tissues in- 

 volved in the disjilay of sexual behavior is 

 a part of the problem of tlie mechanism of 

 action of the hormones in bringing such be- 

 havior to expression. In the case of female 

 lower mammals, this behavior, whether it 

 be appetitive or consummatory, is thought 

 to be associated with subcortical tracts and 

 nuclei rather than with the cortex (Beach, 

 1958b). This belief is consistent with the 

 opinion that a sexual center or centers might 

 be found and much effort has been directed 

 toward this objective. In their brief review 

 of the early investigations, Harris, Michael 

 and Scott (1958) stated that, largely from 

 negative evidence from ablation experiments 

 the neural area on which the integrity of 

 the mechanism for the full expression of 

 sexual behavior depends lies somewhere in 

 the upper mesencephalon, hypothalamus, or 

 preoptic region. Confirmation of this hy- 

 pothesis was thought to be given by Kent 

 and Liberman (1949) wlio placed progester- 

 one directly into the third ventricle of the 

 hamster brain and observed typical mating 

 behavior wuthin an hour. The results ob- 

 tained following refinements of this proce- 

 dure on female cats have not only added sup- 

 port to this view, but would seem to have 

 excluded the involvement of the cerebellum, 

 preoptic region, frontal white matter, cau- 

 date nucleus, thalamus, and amygdaloid 

 complex, although in 4 of 11 members of tlie 

 latter group (preoptic region to amygdaloid 

 complex) some components of the sexual re- 

 sponse were seen (Harris, Michael and 

 Scott, 1958). 



The direction of attention to these effects 

 of steroid hormones when they are placed 

 in contact with the hypothalamus must not 



