Prenatal androgens defeminize activation of GnRH neurons in response to estradiol stimulation.

To determine if prenatal androgens prevent activation of GnRH neurons in response to estradiol stimulation, Fos colocalization with GnRH was compared in the brains of normal female lambs, normal males, and androgenized females in response to a surge-inducing dose of estradiol. Blood samples were collected every 1-2 h for 6 h before estradiol treatment up to the time of sacrifice at 17-19 h post-treatment. Following perfusion, 60 micrograms coronal brain sections were immunostained for Fos (1:1000, Santa Cruz Biochemicals) and GnRH (1:40,000, LR-1) using NiCl-enhanced and unenhanced DAB, respectively. Although LH secretion increased in females before sacrifice, no increase was observed in males or androgenized females. Despite differences in LH secretion, the number and distribution of GnRH neurons was not sexually dimorphic. Moreover, Fos immunostaining was visible throughout steroid-responsive limbic regions in all three groups of lambs. However, the colocalization of Fos with GnRH was highly sexually dimorphic. In females perfused after the peak of the LH surge, 65.7% of GnRH neurons in the preoptic area, anterior hypothalamus, and mediobasal hypothalamus expressed Fos, whereas only 1.7% of GnRH neurons were Fos-positive in males and androgenized females. These findings indicate that sex differences in the activation of GnRH neurons in response to estradiol are determined prenatally through the actions of testosterone.