Ia. Khan et al., Gonadal stage-dependent effects of gonadal steroids on gonadotropin II secretion in the Atlantic croaker (Micropogonias undulatus), BIOL REPROD, 61(3), 1999, pp. 834-841
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.