Role for Kisspeptin and Neurokinin B in Regulation of Luteinizing Hormone and Testosterone Secretion in the Fetal Sheep.

Evidence suggests that the hypothalamic-pituitary-gonadal (HPG) axis is active during the critical period for sexual differentiation of the ovine sexually dimorphic nucleus, which occurs between gestational day (GD) 60 and 90. Two possible neuropeptides that could activate the fetal HPG axis are kisspeptin and neurokinin B (NKB). We used GD85 fetal lambs to determine whether intravenous administration of kisspeptin-10 (KP-10) or senktide (NKB agonist) could elicit luteinizing hormone (LH) release. Immunohistochemistry and fluorescent in situ hybridization (FISH) were employed to localize these peptides in brains of GD60 and GD85 lamb fetuses. In anesthetized fetuses, KP-10 elicited robust release of LH that was accompanied by a delayed rise in serum testosterone in males. Pretreatment with the GnRH receptor antagonist (acyline) abolished the LH response to KP-10, confirming a hypothalamic site of action. In unanesthetized fetuses, senktide, as well as KP-10, elicited LH release. The senktide response of females was greater than that of males, indicating a difference in NKB sensitivity between sexes. Gonadotropin-releasing hormone also induced a greater LH discharge in females than in males, indicating that testosterone negative feedback is mediated through pituitary gonadotrophs. Kisspeptin and NKB immunoreactive cells in the arcuate nucleus were more abundant in females than in males. Greater than 85% of arcuate kisspeptin cells costained for NKB. FISH revealed that the majority of these were kisspeptin/NKB/dynorphin (KNDy) neurons. These results support the hypothesis that kisspeptin-GnRH signaling regulates the reproductive axis of the ovine fetus during the prenatal critical period acting to maintain a stable androgen milieu necessary for brain masculinization.

Excess Testosterone Exposure Alters Hypothalamic-Pituitary-Testicular Axis Dynamics and Gene Expression in Sheep Fetuses.

Prenatal exposure to excess androgen may result in impaired adult fertility in a variety of mammalian species. However, little is known about what feedback mechanisms regulate gonadotropin secretion during early gestation and how they respond to excess T exposure. The objective of this study was to determine the effect of exogenous exposure to T on key genes that regulate gonadotropin and GnRH secretion in fetal male lambs as compared with female cohorts. We found that biweekly maternal testosterone propionate (100 mg) treatment administered from day 30 to day 58 of gestation acutely decreased (P < .05) serum LH concentrations and reduced the expression of gonadotropin subunit mRNA in both sexes and the levels of GnRH receptor mRNA in males. These results are consistent with enhanced negative feedback at the level of the pituitary and were accompanied by reduced mRNA levels for testicular steroidogenic enzymes, suggesting that Leydig cell function was also suppressed. The expression of kisspeptin 1 mRNA, a key regulator of GnRH neurons, was significantly greater (P < .01) in control females than in males and reduced (P < .001) in females by T exposure, indicating that hypothalamic regulation of gonadotropin secretion was also affected by androgen exposure. Although endocrine homeostasis was reestablished 2 weeks after maternal testosterone propionate treatment ceased, additional differences in the gene expression of GnRH, estrogen receptor-ß, and kisspeptin receptor (G protein coupled receptor 54) emerged between the treatment cohorts. These changes suggest the normal trajectory of hypothalamic-pituitary axis development was disrupted, which may, in turn, contribute to negative effects on fertility later in life.