Prenatal Influence of an Androgen Agonist and Antagonist on the Differentiation of the Ovine Sexually Dimorphic Nucleus in Male and Female Lamb Fetuses

Abstract

The ovine sexually dimorphic nucleus (oSDN) is 2 times larger in rams than in ewes. Sexual differentiation of the oSDN is produced by testosterone exposure during the critical period occurring between gestational day (GD)60 and GD90 (term, 147 d). We tested the hypothesis that testosterone acts through the androgen receptor to control development of the male-typical oSDN. In experiment 1, pregnant ewes received injections of vehicle, androgen receptor antagonist flutamide, or nonaromatizable androgen dihydrotestosterone (DHT) propionate during the critical period. Fetuses were delivered at GD135. Both antagonist and agonist treatments significantly reduced mean oSDN volume in males but had no effects in females. Experiment 2, we analyzed the effect of treatments on the fetal hypothalamic-pituitary-gonadal axis to determine whether compensatory changes in hormone secretion occurred that could explain the effect of DHT. Pregnant ewes were injected with vehicle, flutamide, or DHT propionate from GD60 to GD84, and fetuses were delivered on GD85. Flutamide significantly increased LH and testosterone in males, whereas DHT significantly decreased both hormones. In females, LH was unaffected by flutamide but significantly reduced by DHT exposure. DHT significantly decreased pituitary gonadotropin and hypothalamic kisspeptin mRNA expression in males and females. These results suggest that androgen receptor mediates the effect of testosterone on oSDN masculinization, because this process was blocked by the androgen receptor antagonist flutamide in eugonadal males. In contrast, the reduction of oSDN volume observed after DHT exposure appears to be mediated by a negative feedback mechanism exerted on the hypothalamus to reduce LH and testosterone secretion. The reduced androgen exposure most likely accounted for the decreased oSDN volume. We conclude that, during the critical period, the male reproductive axis in long gestation species, such as sheep, is sufficiently developed to react to perturbations in serum androgens and mitigate disruptions in brain masculinization.

Effects of Long‐Term Flutamide Treatment During Development on Sexual Behaviour and Hormone Responsiveness in Rams

“…We chose to use the anti‐androgen Flu that is rapidly metabolised to its primary active form hydroxyflutamide, which competes with testosterone for AR binding. We showed previously that the dose of Flu used in the present study crosses the placenta and produces therapeutic levels of hydroxyflutamide in the foetal circulation . The anti‐androgen effects of Flu were clearly apparent and, as expected, the external genitalia of prenatally exposed males were feminised.…”

“…Prenatal exposure to the aromatase inhibitor ATD does not change adult male sexual preferences or disrupt masculinisation of the oSDN , which fails to support a role for oestrogens. However, we recently reported that the mean oSDN volume of Flu ram foetuses is intermediate between that of control male and females, indicating that ARs mediate the effect of testosterone on oSDN masculinisation . In this earlier experiment, maternal Flu treatment was used to study oSDN development during foetal gestation and did not assess the effect of the treatment on adult behaviour.…”

Prenatal androgen exposure causes hypertension and gut microbiota dysbiosis

“…High levels of T throughout the critical period will masculinize basal LH release during adulthood and eliminate the ability of the GnRH circuitry to generate a preovulatory surge in response to estradiol. T exposure within a narrower window around the normal T peak will masculinize sexual behavior and the brain regions that control it 94 , 95 In contrast, low doses of T throughout this critical window will have no effect on genitalia, behavior, or adult basal LH release, but will partially impair LH surges. 96 …”

Neurobiology of gender identity and sexual orientation

“…13 However, males appear to be resistant to suppression of the action of androgen during gestation because the foetal hypothalamic-pituitary-axis is active in the second trimester (term pregnancy approximately 150 days) and mitigates against changes in circulating testosterone that could disrupt brain masculinisation. 64 These data suggest that, in sheep, brain sexual differ-…”

“…Appropriately timed experimental exposure of female lamb foetuses to testosterone can alter oSDN size independently of genetic and phenotypic sex . However, males appear to be resistant to suppression of the action of androgen during gestation because the foetal hypothalamic‐pituitary‐axis is active in the second trimester (term pregnancy approximately 150 days) and mitigates against changes in circulating testosterone that could disrupt brain masculinisation . These data suggest that, in sheep, brain sexual differentiation is initiated during gestation by central mechanisms acting through gonadotrophin‐releasing hormone neurones to stimulate and maintain the foetal testicular testosterone synthesis needed to masculinise the oSDN and behaviour.…”