Influence of the gonads and/or of LHRH analogue on gonadotropic function in testosterone-treated or untreated juvenile rainbow trout

International audience; The influence of the gonads and/or of LHRH on gonadotropic function, and possibly on gonadal development, was studied in juvenile rainbow trout treated with low doses of testosterone or untreated. Whatever the treatment, all fish of both sexes remained sexually immature, although large individual variations in plasma GTH were observed in all experimental and control groups. This dispersion in the individual values of plasma GTH seemed to be the first sign of an increase in the previously low gonadotropic activity and might announce the beginning of a pubertal period several months before the onset of meiosis in males and of vitellogenesis in females. Castration did not change the plasma and pituitary GTH values, suggesting that control of the gonadotropic function did not depend mainly on sexual steroids during this period. In our experimental conditions, LHRH administration did not lead to any change in plasma or pituitary levels of GTH. Only the testosterone treatment (20 μg) had an effect, causing a higher pituitary GTH load in some fish which was not modified either by castration or by LHRH and decreased with time ; neither of these treatments changed the plasma GTH levels. Thus, the fact that the required amount of GTH was not released to initiate early onset of gametogenesis did not seem to be due to a repressive control by the gonads or to the absence of stimulation by an LHRH-like hypothalamic factor. The quantity of GTH accumulated in the pituitary gland and the dose of steroid used have been discussed.