Whereas effects of season and feedback on plasma and pituitary LH are largely parallel, this is not consistently so for FSH. For instance, plasma FSH levels in castrated llKA-treated fish are higher than in sham-operated fish in July, whereas the opposite is the case for pituitary content. Considering that pituitary content is controlled by synthesis and release, and plasma levels by release and clearance, it is not surprising that their patterns are not always the same. Negative feedback on FSH has previously been shown in adult coho salmon: gonadectomy caused rapid increases in plasma levels in maturing male and female fish.
It is not clear why there are positive feedback effects on GTH secretion in some studies on salmonids and negative feedback effects in others. In both coho salmon and rainbow trout, positive feedback effects have been found in juvenile fish and negative effects in already mature adult fish. This suggests that whether the feedback effects are positive or negative is perhaps more a question of reproductive stage than species. Buy Asthma Inhalers Online
Positive effects of steroids on LH are exerted on the pituitary level, though positive effects via the hypothalamus may also occur. For the feedback effects on FSH, it cannot at present be determined whether they are exerted at the pituitary or hypothalamus level. T might exert its effects by itself or after metabolizing into other compounds; aromatization to estradiol is an important, but not the only, possibility. Implantation of Silastic capsules with aromatase inhibitors increased the proportion of young maturing male parr, which could possibly suggest that the negative effect of T on FSH is aromatase dependent.