Effects of allopregnanolone on central reproductive functions in sheep under natural and stressful conditions

Reproductive functions may be affected by internal and external factors that are integrated in the central nervous system (CNS). Stressful stimuli induce the neuroendocrine response of the hypothalamic-pituitary-adrenal axis, as well as the synthesis of the neurosteroid allopregnanolone (AL) in the brain. This study tested the hypothesis that centrally administered AL could affect the expression of certain genes involved in reproductive functions at the hypothalamus and pituitary levels, as well as pulsatile gonadotropin secretion in sheep under both natural and stressful conditions. Luteal-phase sheep (n = 24) were subjected to a three-day (day 12-14 of the estrous cycle) series of control or AL (4 × 15 μg/60 μL/30 min, at 30 min intervals) infusions into the third ventricle. Acute stressful stimuli (isolation from other sheep and partial movement restriction) were used in the third day of infusion. Stressful stimuli reduced kisspeptin-1 mRNA levels in both the mediobasal hypothalamus (MBH) and the preoptic area (POA), while pro-dynorphin (PDYN) mRNA level only in the MBH. AL alone decreased the abundances of these transcripts in both structures. Stress increased the expression of gonadotropin-releasing hormone (GnRH) mRNA in the MBH and POA, luteinizing hormone (LH) β subunit (LHβ) mRNA in the anterior pituitary (AP) and pulsatile LH secretion. In contrast, mRNA level of follicle stimulating hormone (FSH) β subunit (FSHβ) was decreased in the AP, with no effect of stress on pulsatile FSH secretion. In stressed sheep, AL counteracted the increase in GnRH mRNA expression only in the POA, but it decreased the level of this transcript in both hypothalamic tissues when infused alone. AL prevented the stress-induced increase in LHβ mRNA expression in the AP and pulsatile LH secretion, as well as inhibited almost all aspects of FSH secretion when administered alone. The suppressive effect of AL on GnRH receptor mRNA expression was also observed in both MBH and AP. We concluded that acute stress and AL exerted multidirectional effects on hypothalamic centers that regulate reproductive functions and secretory activity of AP gonadotrophs in sheep. However, we indicated the dominant inhibitory effect of AL under natural and stressful conditions.