Feedback control of reproduction in Atlantic salmon, Salmo salar, male parr

Sammanfattning: In fish, as in other vertebrates, the gonads are stimulated by two pituitary gonadotropic hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH). LH and FSH secretion are regulated by the gonadal steroids acting either directly on the pituitary or indirectly via the hypothalamus. Both positive and negative gonadal feedback mechanisms are present in fishes. Plasma and pituitary LH and FSH levels are higher in Atlantic salmon parr males that have been sham-operated in spring than in castrated fish, when sampled in the periods of gonadal growth and breeding, indicated the existence of a physiological positive feedback involved in the control of LH and FSH secretion. However, for FSH also a physiological negative feedback controlling FSH secretion was found in the early phase of sexual maturation in early summer.The involvement of different steroids and the role of aromatization in feedback systems controlling reproduction has also been studied by administration of different androgens and aromatase inhibitors in vivo. Testosterone (T) stimulates LH by an aromatase-dependent positive feedback. Also 11-androgens exert a positive effect on LH, though weaker than T. FSH appears to be controlled both by a negative aromatase dependent feedback and a negative non-aromatase dependent T effect, as well as possibly a positive aromatase-dependent feedback. FSH is also controlled by both a negative and a positive feedback of 11-androgens. In addition, treatment with different aromatase inhibitors increased the proportion of fish maturing.Positive gonadal feedback eventually separate gonadotropic functions between rematuring high level and non-rematuring low level fish. However, FSH secretion appears not to be the only responsible factor for the onset of maturation in Atlantic salmon male parr, since immunoassayable plasma FSH levels are equally high in non-rematuring and rematuring fish during the onset of gonadal growth in early summer.The decline of androgens at the beginning of breeding season is not due to a suppressive action of elevated 17,20P at either the pituitary or testicular level in salmon.Moreover, administration of T at the end of spawning season diminished the postbreeding decline of 17,20P, testes weights, milt production and sperm motility, possibly via a positive feedback on LH secretion.