Gonadal Stage-Dependent Effects of Gonadal Steroids on Gonadotropin II Secretion in the Atlantic Croaker (Micropogonias undulatus)1

Abstract

Involvement of gonadal steroids in the control of gonadotropin II (GTH II) (homologous to LH) secretion was investigated in the Atlantic croaker (Micropogonias undulatus) using gonadectomy (Gx) and steroid replacement paradigms. Gonadectomy in males and females during the late gonadal recrudescence phase elicited significant increases in the gonadotropin response to stimulation by an LHRH analog (LHRHa), without altering basal GTH II secretion. Slow-release silicone elastomer implants of testosterone or estradiol significantly inhibited LHRHa-induced GTH II secretion in gonad-intact and Gx males, and in Gx females, whereas 5alpha-dihydrotestosterone, a nonaromatizable androgen, was ineffective. Pretreatment of fish with an aromatase inhibitor, 1,4, 6-androstatrien-3,17-dione, 2 days before the administration of testosterone 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 males is primarily mediated by its conversion to estradiol at the level of the hypothalamus and/or pituitary gland, while in females the androgen may also exert a direct inhibitory effect on GTH II secretion, probably mediated via an androgen receptor. In addition, estradiol and testosterone exerted positive effects on basal and LHRHa-induced GTH II secretion during the early-recrudescence phase of the gonadal cycle. The steroids switched to a negative effect on LHRHa-induced GTH II secretion once the fish had fully developed gonads, possibly as a mechanism that prevents a precocious surge in GTH II secretion and final gamete maturation until gametogenesis is complete and the environmental conditions are appropriate for spawning.

Biochemical Characterization of a Membrane Androgen Receptor in the Ovary of the Atlantic Croaker (Micropogonias undulatus)1

“…Evidence suggests that the androgen, 5␣-androstane-3␣,17␤-diol, inhibits sexual behavior via this mechanism in rats [38]. In croaker, both GABA and androgens decrease LH secretion in fully mature females [39,40]. However, to our knowledge, competition studies to determine whether androgens and GABA bind to the same receptor in croaker brain have not been conducted.…”

Effects of the thermal threshold and the timing of temperature reduction on the initiation and course of oocyte development in cultured female of Eurasian perch Perca fluviatilis

“…For instance, a very cold temperature delays vitellogenesis, the growth of oocytes as well as the spawning in the rainbow trout Oncorhynchus mykiss (Mackay and Lazier, 1993). However, the effect of high temperature is highly variable and depends on species and reproductive stage (Kagawa et al, 1983;Khan et al, 1999) as well as on different regulatory factors according to developmental stage (Van Der Kraak and Pankhurst, 1997). For instance, King et al (2003) showed that a high temperature decreases the plasma levels of oestradiol and vitellogenin in Atlantic salmon.…”

Sex Steroids Are Involved in the Regulation of Gonadotropin-Releasing Hormone and Dopamine D2 Receptors in Female Tilapia Pituitary1

“…Other studies have indicated that the responses to GnRH vary in conjunction with the season, sexual maturity, gender, and age [31][32][33][34][35][36][37]. This would indicate that the changes in GnRH and dopamine levels to which the gonadotrophs are exposed are not sufficient for modulating the gonadotropin response.…”