The Effects of Oestradiol on the Prolactin and Growth Hormone Content of the Pituitary of the Tilapia, Oreochromis mossambicus, with Observations on the Incidence of Black Males

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In a preliminary experiment, male Oreochromis mossambicus which received silastic implants of oestradiol (E_2 ; 10, 50 or 125 μg/g body weight) had elevated serum E_2 levels 14 days later, compared with those receiving blank implants. In two subsequent experiments, groups of 15 males received either blank implants or one of these three doses of E_2, and were then transferred to 33% seawater ; they were either maintained in this medium for the subsequent 10 days, or they were transferred back to freshwater on the day after implantation and maintained in the latter for the remaining nine days. There was evidence for a dose-independent increase in the proportion of black males in E_2-treated groups, regardless of salinity, implying increased territorial aggression. Subsequent to the 10-day exposure, polyacrylamide gel electrophoresis of their pituitaries was used to quantify levels of growth hormone (GH) and the large and small forms of prolactin (PRL). When comparing groups receiving blank pellets, levels of GH and both PRLs were greater in fish maintained for most of the post-operative period in freshwater. There was a dose-dependent increase in the pituitary content of all three hormones in fish receiving implants of E_2 ; this was most marked for the smaller of the two forms of PRL in fish adapted to freshwater.
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