In the present study, 11KA had a strong stimulatory effect on FSH. In most previous studies, however, 11-ketoan-drogens have been found to have no or weak feedback effects on the hypothalamus-pituitary-gonadal axis; some exceptions have been observed, but these all concern LH. Although 11KT is well known to have pronounced effects on many aspects of teleost reproduction, genomic steroid receptors for 11KT have not been found. Specific binding of 11KT has recently been shown in the membrane-organelle fraction of stickleback kidneys; the stickleback kidney is a target organ for 11KT. Specific binding sites of 11KT are also present in the membrane-organ-elle fraction of salmon brains (unpublished results). This specific binding may represent 11KT receptors through which feedback actions may be exerted. birth control pills
In early-maturing male salmon parr, the testes start to grow rapidly in the summer, whereas in nonmaturing males the testes remain threadlike. The proportion of males that mature varies from year to year, between rivers and families, and can be influenced by food availability. However, each individual male either matures fully or not at all; thus sexual maturation can be regarded as an all-or-nothing response. Whether or not a male matures in a certain year is a recurrent phenomenon. A male that has matured one year may not mature the next. At the salmon hatchery where the present study was conducted, almost all mature fish rema-ture the next year, but at other hatcheries this is far from being the case. Feedback mechanisms may be involved in controlling whether an individual fish will mature or not. In this respect it is tempting to speculate that if gonads are stimulated above a certain threshold, then a positive feedback can give rise to full maturation, whereas fish that do not reach the threshold remain immature. In the present study, positive feedback control was demonstrated for both FSH and LH. Of the two, it is more likely that FSH rather than LH, whose plasma levels peak much later, could be involved in controlling the initiation of maturation.