Testicular feed back on the hypothalamo - pituitary axis in rainbow trout ( Salmo gairdneri R . )

A testicular feed back on the hypothalamo-pituitary system was studied in male rainbow trout (Salmo gairdneri) after bilateral castration and steroid or testicular supplementation. Experiments were carried out at various periods of the spermatogenetic cycle ; in March-April (resting stage), June (initiation of spermatogenesis), October (end of spermatogenesis and before spermiation), and December (running stage). In March and April, plasma t-GTH is low in intact males (< 2 ng/mi) and castration induced a threefold rise of GTH. Testosterone (T) given by a silastic capsule inserted in the body cavity of the castrate did not change plasma t-GTH in comparison with the intact control. When T is given in the same manner in intact males GTH is significantly increased. In June, a significant t-GTH rise was observed after castration (P < 0.05), this rise was prevented by injection of T and estradiol (E 2). When E2 and T were given to intact males plasma t-GTH was significantly decreased. In October plasma t-GTH was high (13 ng/ml) and was not modified by castration ; injected T or E2 had no effects on plasma t-GTH either in intacts or castrates. In December, a sevenfold increase of t-GTH is observed after castration. This was partly suppressed by intraperitoneal injection of T and E2 and declined after stereotaxic implantation of 11-ketotestosterone into the pituitary. It was not significantly suppressed by intraperitoneal injection of testicular extracts. It is suggested that at this period steroids are the main gonadal factors involved in the feed back at the hypothalamo-pituitary system level.