The brain-pituitary-gonadal axis in the rainbow trout, Salmo gairdneri. III. Absence of an inhibiting action of testosterone on gonadotrophin release in juveniles.

In juvenile rainbow trout the effects of exogenous testosterone and of a synthetic gonadotrophin-releasing hormone (GnRH) on the secretion of gonadotrophin (GTH) were investigated. Treatment with implanted testosterone resulted in an accumulation of GTH in the pituitary, but did not affect the concentration of GTH in the plasma. After the testosterone implants were removed, the levels of testosterone in the circulation dropped to undetectable or control values, but the concentration of GTH in the plasma did not increase. These results indicate that testosterone stimulated the synthesis and storage of GTH, and did not prevent the release of this hormone. The synthetic GnRH, des-Gly10[D-Ala6]LH-RH ethylamide (LH-RHa) stimulated the release of GTH when injected into testosterone-pretreated fish, indicating that accumulated GTH is present in a releasable pool.