Effects of administration of somatostatin-14 and immunoneutralization of somatostatin on endocrine and growth responses in rainbow trout
Injection of somatostatin-14 (SS-14) at 5 ng g−1 body mass (BM) into rainbow trout Oncorhynchus mykiss decreased (P < 0·05, cubic, r2 = 0·54) levels of growth hormone (GH) (1·5 ± 0·9 ng ml−1v. 6·6 ± 0·6 ng ml−1) over time when compared to controls. Somatostatin-14 at 50 ng g−1 BM also decreased (P = 0·064, quadratic; r2 = 0·30) levels of GH (3·6 ± 2·1 ng ml−1v. 6·6 ± 0·6 ng ml−1) over time compared to controls. In a second study, passive immunization against SS-14 (1 : 25 dose) increased (P = 0·10, cubic, r2 = 0·12) levels of GH (11·0 ± 4·8 ng ml−1v. 5·2 ± 1·4 ng ml−1) over time. Passively immunizing against SS-14 (1 : 50 dose) increased (P < 0·05, cubic, r2 = 0·10) levels of GH (8·2 ± 2·3 ng ml−1v. 5·2 ± 1·4 ng ml−1) over time compared to controls. Overall, in the active immunization study there was no difference (P > 0·10) in specific growth rate (G) or feed conversion ratio (FCR) between the three treatment groups during the 9 weeks of the study. Only four of the fish immunized against SS-14, however, developed antibody titres against SS. Compared to controls, these fish exhibited a G of 0·89 ± 0·09 v. 0·56 ± 0·09% per 3 weeks and FCR of 0·80 ± 0·04 v. 1·20 ± 0·05 g g−1. In SS-14 immunized fish, levels of GH decreased (P < 0·05) by day 63 while levels of insulin like growth factor-I (IGF-I) increased (P < 0·05) by day 42 and 63. These results indicate the hypothalamic hormone SS-14 regulates GH secretion similarly in rainbow trout as it does in mammals. Active immunization against SS-14 could improve growth performance in rainbow trout but enhanced G and FCR is dependent upon generation of antibody titres.