Positive feedback control by the gonads on gonadotropin (GTH) and gonadoliberin (GnRH) levels in experimentally matured female silver eels,Anguilla anguilla

Treatment of sham-operated female silver eels with carp pituitary extract stimulated ovarian development and induced increases in pituitary gonadotropin (GTH) and gonadoliberin (GnRH) contents. Both effects of carp pituitary extract were abolished in ovariectomized eels, indicating the involvement of the gonads. Endogenous sexual steroids, the secretion of which was increased during sexual maturation, should be responsible for the stimulation of GTH and GnRH levels. Ovariectomy itself had no significant effect on pituitary GTH and GnRH contents, reflecting the fact that, at the silver stage, sexual steroid levels are too low to exert any significant effect on pituitary GTH and GnRH. The positive feedback control exerted by the gonads on GTH and GnRH levels during sexual maturation, in the eel as well as in some other teleosts, would produce an amplification of the pubertal stimulation of the hypothalamo-pituitary-gonadal axis.