Differentiation of the sexually dimorphic nucleus in the preoptic area of the rat brain is inhibited by postnatal treatment with an estrogen antagonist.

The volume of the sexually dimorphic nucleus in the preoptic area (SDN-POA) of the rat brain is several fold larger in adult male rats that in adult females. This sex difference in brain structure was previously shown to develop under the influence of androgenic and estrogenic hormones during the perinatal period. We here report that treatment of newborn male and female rats with the estrogen antagonist tamoxifen significantly inhibited growth and differentiation of the SDN-POA in both sexes and it resulted in permanent anovulatory sterility in females. The findings suggest (a) that testicular androgens exert their growth promoting activity on SDN-POA development only after being converted into estrogens, and (b) that also in the female rat structural and possibly functional brain differentiation is under estrogenic control.