Further Studies on Sexual Differentiation of the Brain: Response to Electrical Stimulation in Gonadectomized and Estrogen Primed Rats

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Sexual differentiation of the brain was investigated in normal females, neonatally androgenized females and males from the viewpoints of the effects of steroid feedback and the responsiveness to electrical stimulation. Pituitary and serum gonadotropins and prolactin were measured by radioimmunoassay. 1) Pituitary LH in the male was as high as or slightly higher than that in the normal female, but pituitary FSH was much higher in the male than that in the female and pituitary prolactin was much lower in the male than the female. In androgenized females pituitary LH was as low as or slightly lower than that in the normal female, but pituitary prolactin was higher than those in normal cycling females and males. No sex differences were observed in the serum LH and prolactin levels, and serum LH in androgenized females was the same levels in male and female rats. Serum prolactin in androgenized females was higher than that in both normal females and males. Serum FSH in males was higher than that in normal females or in androgenized females. 2) Gonadectomy increased in pituitary and serum LH and FSH in all of normal females, androgenized females and males. Gonadectomy, however, decreased both pituitary and serum prolactin in normal females, but did not change their concentrations in males. In androgenized females pituitary prolactin was reduced by ovariectomy, but not serum prolactin. Estrogen injection into all of gonadectomized animals reduced serum LH and FSH and increased serum prolactin. 3) Electrical stimulation of the medial preoptic area consistently increased serum LH in orchidectomized males and ovariectomized-androgenized females under the estrogen influences. However, stimulation of the medial amygdala did not increase serum LH in orchidectomized males and ovariectomized-androgenized females under the estrogen influences, by contrast, it increased serum LH in the ovariectomized normal females under the same conditions. Hippocampal stimulation increased serum LH slightly in gonadectomized males and androgenized females; by contrast, it decreased serum LH in ovariectomized normal females. 4) In the light-induced constant estrous rats electrical stimulation of both medial preoptic area and medial amygdala increased serum LH, while stimulation of the hippocampus neither increased nor decreased it.
Thus, there exists the sexual differentiation of the brain in control of gonadotropin secretion including regulation by the negative and positive feedback influence with gonadal steroids. However, the sexual differentiation of the brain capacity to be involved in the ovulatory release of gonadotropins responding to electrical stimulation appears to be induced by neonatal androgenization in both sexes.