Relative effects of estradiol-17β (E2), catecholestrogens and clomiphene citrate on in vitro oocyte maturation in the catfish Heteropneustes fossilis (Bloch) and E2 inhibition of 2-hydroxyestradiol-induced maturation

In vitro effects of estradiol-17β (E2), the catecholestrogens 2-hydroxyE2 (2-OHE2) and 2-methoxyE2, and the nonsteroidal antiestrogen clomiphene citrate (clomid) on oocyte maturation were investigated in the catfish Heteropneustes fossilis. Incubation of postvitellogenic follicles with 2-OHE2 induced germinal vesicle breakdown (GVBD; 86% at 5 μM for 30 h) and progression of meiosis up to metaphase II, as evident from the presence of Hoechst stained metaphase chromosomes and anti-α-tubulin-positive bipolar spindles. The response was both concentration (1, 2.5, 5, 10, and 20 μM)- and duration (0, 3, 6, 12, 24, and 30 h)-dependent. The diameter of the follicles increased and about 20% follicles elicited ovulation. Incubation of the follicles with clomid (20 μM) induced only about 29–35% GVBD at 30 h. This might be due to the dual properties of clomid with estrogenic (cis-isomer) and antiestrogenic (trans-isomer) actions or due to estrogen receptor binding dynamics. Incubations of the follicles with E2 or 2-methoxyE2 did not induce oocyte maturation. The higher concentrations of 2-methoxyE2 caused degenerative changes in the follicles. In competition studies, E2 inhibited the GVBD response of 2-OHE2 (5 μM) significantly in a concentration (1, 5, 10, and 20 μM) or duration (2, 4, and 6 h)-dependent manner after pre-incubation with 20 μM E2 (P < 0.001, one-way ANOVA, P < 0.05, Newman–Keuls’ test). The results show that 2-OHE2 induces maturational activity while the parent estrogen is a strong inhibitor, alone or in combination with 2-OHE2.