Gonadal stage-dependent effects of gonadal steroids on gonadotropin II secretion in the Atlantic croaker (Micropogonias undulatus)

Involvement of gonadal steroids in the control of gonadotropin II (GTH II) (homologous to LH) secretion was investigated in the Atlantic croaker (Micr opogonias undulatus) using gonadectomy (Gx) and steroid replacement paradig ms. Gonadectomy in males and females during the late gonadal recrudescence phase elicited significant increases in the gonadotropin response to stimul ation by an LHRH analog (LHRHa), without altering basal GTH II secretion. S low-release silicone elastomer implants of testosterone or estradiol signif icantly inhibited LHRHa-induced GTH II secretion in gonad-intact and Cx mal es, and in Gx females, whereas 5 alpha-dihydrotestosterone, a nonaromatizab le androgen, was ineffective. Pretreatment of fish with an aromatase inhibi tor, 1,4,6-androstatrien-3,17-dione, 2 days before the administration of te stosterone implants, completely blocked the negative effect of testosterone on LHRHa-induced GTH II secretion in males, but only partially restored it in females. This suggests that the negative feedback of testosterone in ma les is primarily mediated by its conversion to estradiol at the level of th e hypothalamus and/or pituitary gland, while in females the androgen may al so exert a direct inhibitory effect on GTH II secretion, probably mediated via an androgen receptor. In addition, estradiol and testosterone exerted p ositive effects on basal and LHRHa-induced GTH II secretion during the earl y-recrudescence phase of the gonadal cycle. The steroids switched to a nega tive effect on LHRHa-induced GTH II secretion once the fish had fully devel oped gonads, possibly as a mechanism that prevents a precocious surge in GT H II secretion and final gamete maturation until gametogenesis is complete and the environmental conditions are appropriate for spawning.