A two-step model for sexual differentiation

Prenatal nicotine exposure selectively affects perinatal forebrain aromatase activity and fetal adrenal function in male rats

Previous studies have revealed effects of prenatal nicotine treatment on fetal plasma testosterone and perinatal sexual brain differentiation in the rat. In an attempt to further elucidate the processes underlying this action of nicotine, we studied the effect of the drug on brain steroid aromatase which converts androgens to estrogens and is known to be important in sexual brain differentiation. Aromatase activity (AA) was measured by the conversion of [1 beta-3H]-androstenedione to estrone in a brain region comprising preoptic, hypothalamic and amygdaloid areas. In untreated animals, the development of AA between gestational day (GD) 18 and postnatal day (PN) 15 was similar in both sexes, except for a significant drop of AA in female brain at PN6, i.e., during the later part of the critical period for sexual brain differentiation. When time-pregnant rats were treated with nicotine delivered by an osmotic minipump for either one week (2 mg/kg/d or 6 mg/kg/d from GD12) or two weeks (6 mg/kg/d from GD8), their male offspring showed a decrease of AA to female levels at PN6, the sex difference existing at this stage thus being abolished. AA of offspring from dams bearing tartaric acid-containing minipumps or sham-operated at GD8 or GD12 was identical to that of untreated controls. No drug effect was seen in female fetuses and offspring. Sex differences in the developmental effect of nicotine may thus involve brain aromatase. An additional sex-dependent effect of nicotine was observed in the male fetal adrenal axis at GD18. Whether the drug effects on the two steroid hormone systems are interrelated, remains to be elucidated.

Sexual differentiation in quail: critical period and hormonal specificity

There is a discrepancy between results showing that male quail are demasculinized by exogenous estrogens only if the treatment is given before Day 12 of egg incubation and results showing that ovariectomy of females after hatching still affects their sexual differentiation which leads to the conclusion that female demasculinization by ovarian estrogens is a continuing process extending into posthatching life. The first experiment was performed to test different models which have been proposed to reconcile these apparently contradictory results. Male and female quail were treated with 0, 5, or 25 micrograms of estradiol benzoate (EB) on either Day 9 or Day 14 of embryonic life. Birds were castrated at the age of 4 days to avoid the confounding effects of postnatal gonadal hormones and were treated as adults with testosterone (T). Whereas EB-treatment demasculizined sexual behavior and cloacal gland growth of males when administered on Day 9, it was without effect on Day 14. This result confirms the presence of a "critical period" for sexual differentiation of behavior in embryonic life. However, the time course of sexual differentiation and the sensitivity to the demasculinizing actions of estrogens were not the same for different behavioral and morphological characteristics. Some dependent variables such as plasma levels of luteinizing hormone and crowing were still affected by the EB treatment on Day 14. These results show that the whole process of demasculinization is not retricted to the "critical period" ending on Day 12 of incubation. A second experiment was performed to determine if 5 beta-dihydrotestosterone (5 beta-DHT), a metabolite of testosterone, also exerts demasculinizing effects during embryonic life. A large dose of 5 beta-DHT (2 mg/egg) had no effects on behavior and morphology in males if administered on Day 9 of egg incubation. This suggests that 5 beta-DHT, which is a steroid devoid of behavioral effects in the adult bird, is also an inactive compound as far as sexual differentiation of the quail is concerned. The high 5 beta-reductase activity which was previously identified in the hypothalamus of the embryonic quail thus probably plays a protective role. By transforming testosterone into inactive nonaromatizable androgens, it prevents male embryos from being demasculinized by their endogenous testosterone acting through aromatization.

Testosterone induces hypothalamic aromatase during early development in quail

The effects of testosterone on androgen metabolizing enzymes were examined in the developing hypothalamus of male and female quail using an in vitro radiometric assay which measures metabolite formation in individual brain samples. Testosterone (T) administered by subcutaneous silastic implants to gonadectomized 4-day-old chicks increased formation of estradiol-17 beta (E2) in both preoptic area + anterior hypothalamus (PA) and posterior hypothalamus + tuberal area (HT) to adult levels. The T-induced increase in E2 formation occurred to the same degree in both sexes. The increase was very small in control non-target areas, neostriatum intermediale + hyperstriatum ventrale (VN), of either sex. Testosterone had no effect on formation of 5 alpha-dihydrotestosterone (5 alpha-DHT), 5 beta-dihydrotestosterone (5 beta-DHT) and 5 beta-androstane 3 alpha, 17 beta-diol (5 beta, 3 alpha-diol) in PA. Kinetic analysis of the rate of E2 production by hypothalamic tissue from castrated chicks (CX-chicks) and castrates treated with T (CX + T-chicks) indicates that the increase in hypothalamic aromatase activity by T corresponds to induction of the enzyme: the Vmax (maximum velocity) was increased by T (CX-chicks, 21; CX + T-chicks, 91 fmol/mg FW/h), whereas the Km was unaffected (CX-chicks, 5.5; CX + T-chicks, 4.7 X 10(-8) M). Testosterone treatment, effective for inducing PA and HT aromatase activity, also activated crowing and caused cloacal gland development; neither of these effects were sexually dimorphic. Our results indicate that: (1) T induces aromatase specifically in the hypothalamus during early post-hatching development, other pathways of T metabolism are not affected; and (2) the inducible aromatase is not sexually dimorphic in the developing brain. Since there are sex differences in adult brain aromatase, we conclude that capacity for induction of the hypothalamic aromatase becomes sexually differentiated after the post-hatching period.