In the present study, plasma FSH levels in sham-operated fish displayed their highest levels in July, followed by a decline in autumn. Plasma and pituitary LH, on the other hand, increased from July to September and further in November. These seasonal changes are in general agreement with previous findings on other salmonids. In mature male Atlantic salmon parr in northern Sweden, plasma levels of T and 11KT peak in September-October, and the levels of 17,20P peak in November. Since plasma levels of FSH decreased over the breeding season also in castrated animals, this decline per se does not appear to be dependent on gonadal feedback mechanisms.
Pituitary LH contents were much lower in castrated than in sham-operated males in the present study. The pituitary LH content increased after treatment with T or 11KA; of the two treatments, the former was far more effective. This is consistent with previous studies showing a stimulatory effect of T, and to a lesser extent of 11KT, on pituitary “maturational” GTH (now regarded as LH) accumulation. It has also been observed that T treatment of juvenile salmonids can result in increased plasma GTH levels, though higher doses and a longer period of administration are needed for this effect than for stimulating accumulation in the pituitary. ventolin inhaler
Evidence of negative feedback has been reported for the rainbow trout, in which plasma GTH levels increase after castration of males that have already matured (e.g., ). Negative effects of high-dose androgen treatment on testes development have been found in Atlantic salmon mature male parr, suggesting the presence of negative feedback effects also in intact fish. In juvenile male and female rainbow trout, castration alone had no significant effect on plasma and pituitary GTH. However, GTH injections led to a strong increase in pituitary GTH in sham-operated but not in castrated males. This demonstrates that under GTH stimulation, the juvenile testes are capable of stimulating pituitary GTH accumulation. Castration had, however, no effects on plasma GTH levels in male landlocked Atlantic salmon while pituitary GTH content was higher in sham-operated than in castrated fish in autumn, although the differences were smaller than in the present study.