Molecular cloning of FSH and LH β subunits and their regulation by estrogen in Atlantic croaker

The cDNAs of FSH and LH β subunits were isolated from Atlantic croaker pituitary. The isolated genes in croaker showed 35–75% and 60–75% identities with FSH and LH β of other teleosts, and 41% and 45% with human FSH and LH β, respectively. The homology models of croaker FSH and LH β were constructed using the currently known X-ray crystallography structures of human FSH β and chorionic gonadotropin as templates. Real-time quantitative RT-PCR protocols were developed and validated to measure FSH and LH mRNAs. The FSH mRNA was higher in the pituitaries of early-pubertal croaker than that in late-maturing/mature individuals, whereas LH mRNA showed an opposite trend with substantially higher expression in late-maturing/mature fish. Administration of 17β-estradiol (E2; 1 and 5 μg/g body weight) in early-pubertal fish significantly decreased FSH but increased LH mRNA expression in the pituitary. In late-maturing/mature females, gonadectomy significantly increased FSH mRNA while E2 replacement suppressed the elevated expression. On the other hand, E2 or gonadectomy with E2 replacement did not significantly alter LH mRNA in the same experiment consistent with similar lack of effect on circulating LH levels in croaker described previously. This finding together with the existing evidence for estrogen negative feedback on GnRH-induced LH secretion in late-maturing/mature croaker suggests that the negative feedback mechanism does not involve inhibition of LH mRNA or protein and may be limited to the blockage of GnRH-induced LH release.