Estradiol-17β Stimulates Gonadotropin II Expression and Release in the Protandrous Male Black Porgy Acanthopagrus schlegeli Bleeker: A Possible Role in Sex Change

Abstract

The objective of the present study was to investigate the in vivo effects of sex steroids (estradiol-17β, E2; testosterone, T) and the nonaromatizable androgen 5α-dihydrotestosterone (DHT) on the levels of gonadotropin II (GTH II) in plasma and pituitary and on aromatase activity in 2-year-old male black porgy, Acanthopagrus schlegeli, during the prereproductive season. Black porgy GTH II and GTH II β subunits were purified and anti-GTH II β serum was induced. A specific radioimmunoassay for black porgy GTH II was developed. cDNA GTH II β was also cloned from a black porgy pituitary cDNA library for use as a probe for Northern analysis. Male fish were divided into eight groups (n = 64): control; E2 (3 doses, 2.4 ng, 72 ng, and 2.2 μg/g body weight); T (2 doses, 72 ng and 2.2 μg/g body weight); and DHT (2 doses, 72 ng and 2.2 μg/g body weight). Fish were injected with the respective vehicle or different doses of material on days 0, 8, and 16. Plasma was collected at 4-day intervals from days 4 to 20. Plasma GTH II concentrations were significantly increased (up to 45-fold) in the E2 group from days 4 to 20 in a dose-dependent manner. In a further experiment during the late reproductive season, plasma GTH II levels increased at 4 h and on days 1 and 2 following a single injection of 1.0 μg E2/g body weight (on day 0). Androgens (T or DHT) had little or no effect on plasma GTH II. Pituitary GTH II contents on day 20 were significantly lower in the 72-ng E2 and 2.2-μg E2 groups but not in the 2.4-ng E2 group compared with the control group. Pituitary GTH II β mRNA levels were significantly stimulated in the 72-ng and 2.2-μg E2 groups on day 20. Gonadal aromatase activity was not significantly changed in any of the treated or control groups. It is concluded that GTH II secretion in black porgy is regulated by an estrogen-specific effect. Increased plasma GTH II levels or other factors in addition to E2 might be involved in the regulation of gonadal aromatase activity and sex change in protandrous black porgy.