Effects of energy status and metabolic hormones on pubertal development in Pacific salmon

Experiments were conducted to identify proximate mechanisms mediating the previously described relationship between pubertal age and energy status in Pacific salmon. To determine which parameters of the reproductive endocrine axis of salmon are sensitive to energy balance, the effects of fasting in 2-year coho salmon (Oncorhynchus kisutch) were examined in two consecutive trials, in early autumn and in late autumn through early winter. Fasting strongly suppressed the marked increases in pituitary content of follicle-stimulating hormone (FSH) occurring through autumn in fed fish. Fasting completely prevented increases in testicular mass, but only moderately suppressed increases in ovarian mass. In males, fasting reduced 11-ketotesterone levels in the early trial, but not in the later trial. Fasting suppressed circulating levels of insulin-like growth factor I (IGF-I) in both trials, and suppressed insulin levels in the early autumn trial. The effects of fasting in early autumn were also assessed in two groups of yearling male chinook salmon (O. tshawytscha ) differing in mean size and lipid content prior to fasting. The effects of fasting on reproductive parameters were similar in both groups, and similar to those observed in coho salmon during early autumn. To determine whether metabolic hormones may serve as signals of metabolic state to gonadotropes, the effects of IGF-I and insulin on FSH content and release in vitro were tested. IGF-I strongly increased intracellular content and gonadotropen-releasing hormone (GnRH)-stimulated release of FSH in coho salmon pituitary cell culture, whereas insulin was much less potent. To test for evidence that leptin may serve as a signal of energy stores to the reproductive axis, as has been described in mammals, the effects of recombinant human leptin in juvenile coho salmon in vivo were tested. Despite long-term exposure to high levels of leptin, no effects of the hormone on a variety of metabolic and reproductive parameters were detected. In coho salmon pituitary cell culture, leptin mildly, but inconsistently, increased GnRH-stimulated FSH release. These studies demonstrate the sensitivity of the salmonid reproductive axis to energy status, and provide evidence that IGF-I may serve as a proximate metabolic cue to promote pubertal development.