Gonadal regulation of gonadotropin subunit expression and pituitary Lh protein content in female hybrid striped bass

Abstract

To assess the response of the GtH system to the removal of gonadal feedback, effects of bilateral gonadectomy and 17β-estradiol (E₂) replacement on gonadotropin (GtH) subunit expression, as well as LH protein levels, were monitored in the pituitary. Adult female hybrid striped bass undergoing mid-vitellogenesis were divided into one sham and two gonadectomized (gdx) groups. One gdx group received E₂ (2 mg kg⁻¹) via microspheric delivery systems, the other was given a vehicle injection. Groups were sampled on days 3 and 14 post-surgery (PS). Expression of all GtH subunits was significantly increased in the gdx group on both days compared to the shams, whereas E₂ replacement in gdx fish restored alpha and LHβ mRNA levels to those of the sham fish and dramatically reduced FSHβ mRNA levels. Gdx had no effect on the pituitary concentration of LH. E₂ replacement, however, reduced protein levels significantly on day 3, but not day 14. In conclusion, during mid-vitellogenesis expression of both GtHβ subunits is negatively controlled by gonadal feedback, which is also evident by restoration of gdx subunit mRNA levels to those of the shams after E₂ replacement.