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, r ² = 0.54) levels of growth hormone (GH) (1.5 ± 0.9 ng ml ^-1 v. 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 ^-1 v. 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^-1 v. 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^-1 v. 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.