Influence of sexual hormones on the differentiation of neural centers.

Abstract

The influence of hormones on the differentiation of various neural functions is reviewed. Whether a cyclic feminine or an acyclic masculine pattern of gonadotropin secretion throughout reproductive life is established depends on the presence or absence of androgenic hormones during the appropriate period of fetal development. Androgen influence, either natural in the male sex or experimental in females, leads invariably to permanent destruction of cyclicity. The absence of androgen during a well-defined phase of fetal development allows the cyclic feminine pattern of gonadotropin secretion to continue throughout life, regardless of the genetic sex. This has been revealed through a variety of experimental approaches including castration and gonad transplantation, and androgen and antiandrogen treatment during the phase of brain differentiation. This critical period occurs before birth in most species but after birth in some others, e.g., rats and mice. Similar experiments have revealed that the pattern of prolactin secretion is also determined by early hormonal influence on neural structures. In males or experimentally androgenized females, there is less estrogenic inhibition of a hypothalamic prolactin inhibiting factor (PIF) which exerts a chronic inhibition of prolactin secretion from the pituitary. These animals respond with less prolactin release than normal females to estrogen treatment. Behavioral traits of the adult animal are also predetermined by hormonal influence during neural differentiation. If the developing brain is deprived of androgens during a critical period, the adult animal's sexual behavior will be feminine. Pertinent experiments on genetic males who were castrated or treated with an antiandrogen are described. Experimental evidence exists indicating that the higher aggressiveness of males is also predisposed by early androgen influence on the brain. The conclusion is drawn that the feminine differentiation of the brain does not require any hormonal impulses but that androgens are essential for the impression of masculinity.