Seasonal dynamics in gonadotropin secretion and E(2)-binding in the catfish Heteropneustes fossilis.

In the present investigation, significant annual/seasonal variations were noticed in plasma and pituitary gonadotropin (GTH) which were correlated with gonado-somatic index, plasma estradiol-17beta, and nuclear E(2) receptor (NE(2)R) in the pituitary, hypothalamus and telencephalon. The NE(2)R concentrations and dissociation constant (k (d)) values showed significant seasonal variations with high values in the late preparatory phase and low values in the postspawning phase. The NE(2)R levels were the highest in the pituitary, followed by the hypothalamus and telencephalon in all the seasons. In the prespawning phase, ovariectomy (OVX) elicited a strong negative feedback on GTH secretion with a bimodal pattern of release and elevated the NE(2)R levels and k (d) values, without producing any significant change in the resting phase suggesting that E(2) appears to exert differential feedbacks on GTH secretion.