Effects of the maturation-inducing steroid on LH secretion and the GnRH system at different stages of the gonadal cycle in Atlantic croaker

Abstract

The effects of treatment with the maturation-inducing steroid 17,20β,21-trihydroxy-4-pregnen-3-one (20β-S) on luteinizing hormone-releasing hormone analog (LHRHa)-induced LH secretion were examined during several phases of the gonadal cycle in Atlantic croaker, Micropogonias undulatus. 20β-S (1 and 5 μg/ g of body wt) was administered by intraperitoneal (ip) injection, 24 h prior to injection with LHRHa (10–50 ng/g of body wt) and fish were bled 1 h after LHRHa injection. Treatment with both doses of 20β-S resulted in plasma concentrations of the steroid within the normal physiological range for this species during final oocyte maturation and ovulation. The 20β-S treatments altered the LH response to LHRHa throughout the reproductive cycle in both sexes, but the direction and magnitude of the response varied. 20β-S treatment decreased the LH response to LHRHa in fish with recrudescing and fully recrudesced gonads and in females with regressed gonads. On the other hand, 20β-S treatment significantly increased the LH response to LHRHa in males with regressing or regressed gonads. 20β-S treatment also altered preoptic anterior hypothalamic (POAH) and pituitary seabream gonadotropin-releasing hormone (sbGnRH) contents, and the patterns of these changes were similar to those observed in LH secretion. The finding that moderate increases in plasma 20β-S concentrations, similar to those occurring during final oocyte maturation, significantly inhibit the LH response to LHRHa at the end of the reproductive cycle suggests that this action of 20β-S is of physiological importance during the periovulatory period. Moreover, the fact that concurrent changes occur in POAH and pituitary sbGnRH contents suggests that the actions of 20β-S on LH secretion are at least partly mediated via the GnRH system.