Effect of gonadal steroid hormones on plasma growth hormone concentrations in sexually immature rainbow trout, Oncorhynchus mykiss.

In the present study we investigated the effects of gonadal steroid hormones on plasma growth hormone (GH), triiodothyronine (T3), and thyroxine (T4) levels in steroid-primed sexually immature rainbow trout, Oncorhynchus mykiss, in order to determine whether changes in plasma GH and thyroid hormone levels during sexual maturation are a result of elevated gonadal steroid levels or are an altered condition associated with reproduction (as proposed by Sumpter et al., 1991b). We found no significant correlation between plasma GH and condition factor in either of two cohorts of rainbow trout (1+ and 2+ year old fish) examined. To evaluate the effects of gonadal steroids on plasma GH and thyroid hormone levels, fed and fasted (4 weeks fasted) fish were given a slow-release implant of testosterone (T), 17beta-estradiol (E2), 17alpha-methyltestosterone (17alphaMT), or 5alpha-dihydrotestosterone (5alphaDHT) dissolved in coconut oil, or coconut oil alone (control). Blood samples were taken after 2 weeks and analyzed for GH, T3, and T4. Neither 17alphaMT nor 5alphaDHT had any effect on plasma GH levels in fed or fasted fish. However, plasma GH levels were elevated after E2 treatment in both fed and fasted (P </= 0.01) fish, and after T treatment in fasted but not fed fish (P < 0.01). The effects of gonadal steroid hormones on thyroid hormone levels were less clear. T3 levels were depressed in fed E2-primed fish (P < 0.05) and elevated in fasted T- and 17alphaMT-primed fish (P < 0.05). T4 levels were depressed in T- and 5alphaDHT-primed fed fish (P < 0.01) and elevated in fasted E2-primed fish (P < 0.01). These elevated GH levels associated with sexual maturation may be more clearly linked to elevated gonadal steroid levels rather than to changes in nutritional status. The effects of gonadal steroids on thyroid hormone levels are less clear.