Determination of circulating maturational gonadotropin in rainbow trout (Oncorhynchus mykiss) using a heterologous radioimmunoassay

Abstract

A radioimmunoassay (RIA) for the measurement of maturational gonadotropin (GtH II) in the plasma of rainbow trout was developed using a rabbit antiserum to b subunit of chum salmon GtH II. Intact GtH II was used as standard and radioactive competitor. RIA was performed using a double-antibody method under non-equilibrium conditions, and was used to evaluate plasma GtH II release stimulated by a gonadotropin releasing-hormone (GnRH) analogue: des-Gly-GnRH (1-9) ethylamide. The GnRH analogue was administrated during 6 days either orally or parenterally (one single ip injection or two ip injections with a 6-day interval). Plasma concentrations of GtH II in control (without hormone induction) and with oral GnRH were low (less than 2 ng/ml) in spite of a higher ovulatory rate. In contrast, both injections of GnRH (15 μg/kg body weight) induced production and secretion of GtH II (16.6 ± 9.06 ng/ml). These data suggest that GnRH injection, but not oral administration, is an appropriate method