THE EFFECTS OF ESTRADIOL ON THE PROLACTIN AND GROWTH-HORMONE CONTENT OF THE PITUITARY OF THE TILAPIA, OREOCHROMIS-MOSSAMBICUS, WITH OBSERVATIONS ON THE INCIDENCE OF BLACK-MALES

In a preliminary experiment, male Oreochromis mossambicus which receiv ed silastic implants of oestradiol (E-2; 10, 50 or 125 mu g/g body wei ght) 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 maintai ned in this medium for the subsequent 10 days, or they were transferre d back to freshwater on the day after implantation and maintained in t he latter for the remaining nine days. There was evidence for a dose-i ndependent increase in the proportion of black males in E-2-treated gr oups, regardless of salinity, implying increased territorial aggressio n. Subsequent to the 10-day exposure, polyacrylamide gel electrophores is 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 greate r in fish maintained for most of the post-operative period in freshwat er. There was a dose-dependent increase in the pituitary content of al l three hormones in fish receiving implants of E-2; this was most mark ed for the smaller of the two forms of PRL in fish adapted to freshwat er.