Increased pituitary content of a hormone could be due to either increased synthesis or diminished release of the hormone. In the present study, plasma levels of LH in sham-operated fish were, though low, higher than in castrated males in autumn. This indicates that there is indeed a physiological positive feedback from the gonads to the pituitary in the regulation of LH secretion in salmon. This positive feedback could play a role in the temporal shift from predominantly androgen production to 17,20P production by the gonads over the reproductive cycle. Androgens, particularly T, eventually stimulate the secretion of LH, which in turn stimulates the production of 17,20P. In the present study, pituitary and plasma levels of FSH were also higher in sham-operated fish than in castrated ones. buy ortho tri-cyclen
The regulation of FSH secretion has been studied to a far lesser extent than the regulation of LH. Positive feedback effects on FSH have not been reported previously; on the contrary, the only previous study on effects of various steroids on plasma and pituitary FSH and LH in juvenile salmonids showed positive effects on LH and no effects on FSH in coho salmon. This difference may be attributable to a species effect, but another possible explanation is that GTH control in juveniles may not work as does GTH control in maturing fish and that only in the latter model is it possible to ascertain what the gonads are really doing in relation to GTH control at maturation. In the present study, the feedback effects on FSH appeared to be quite different from that on LH. In relation to the latter, there was a consistent positive effect of T on pituitary LH content, whereas 11KA displayed a similar, but much weaker, effect. In contrast, three different effects of steroid treatments on pituitary and plasma FSH levels were observed: a negative effect of T in July, a positive effect of T in November, and a positive effect of 11KA at all three sampling dates (though for plasma levels, particularly so in July).