Neuroendocrine Factors and the Ontogeny of Behavior

Abstract

During recent years there has been a growing body of evidence which indicates the important influence of endocrine processes upon the ontogeny of physiological and behavioral systems. Although the major evidence of the influence of hormones during very sensitive periods in development upon later behavior comes primarily from manipulations of gonadal hormones early in development, there is now new evidence which also indicates that manipulation of the pituitary-adrenal system pre- and postnatally may also have profound effects upon later neuroendocrine processes. The evidence on the influence of gonadal hormones on determination of sexuality, both behaviorally and physiologically, is now well documented. The basic hypothesis developed by Professor Harris (1) and the late Professor W. C. Young (2) states that hormones acting on the central nervous system during fetal and neonatal life organize the sexually undifferentiated brain with regard to patterns of gonadotropin secretion in sexual behavior. Specifically, this hypothesis states first that androgens acting on the central nervous system during critical periods in development are responsible for the programming of male patterns of gonadotropin secretion and sex behavior in much the same way that they determine the development of anatomical sexual characteristics. Second, during adult life gonadal hormones activate the sexually differentiated brain and elicit the responses that were programmed earlier. Third, an additional component of the process of sexual differentiation is to render the tissues which are responsive to gonadal hormones differentially sensitive in the male and the female. Illustrative of this latter aspect of sexual differentiation is a phenomenon observed in normal adult rats. If a normal adult male is castrated and given large replacement doses of estrogen, rudimentary and partial female lordosis behavior will be facilitated, but in the normal castrate female, minute quantities of estrogen will facilitate complete sexual behavior. Further, if the male which is showing rudimentary sexual behavior is given doses of progesterone, no further elaboration of sexual behavior is observed. However, the female given estrogen alone and then given progesterone will show further facilitation of sexual behavior. Thus the capacity of both estrogen and estrogen plus progesterone to elicit lordosis behavior is much greater in the female than in the normal adult castrate male.