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Nutritional and neuroendocrine control of appetite in transgenic coho salmon Raven, Peter Andrew


The increased feeding motivation and enhanced growth found in growth hormone (GH) transgenic coho salmon, Oncorhynchus kisutch, raises questions regarding the hormonal changes regulating growth and feed intake. These fish exhibit increased circulating GH and insulin-like growth factor I (IGF-I) and GH mRNA expression resulting from the GH transgene, but the role of the other hormones in the GH-axis and appetite control are unknown. Transgenic and non-transgenic coho salmon pre-smolts were fed diets with 15, 17, 19 and 21 MJ of digestible energy/kg for 84 days and they were analyzed for growth, feed intake and protein utilization. Gene mRNA expression of the major regulators in the GH-axis: growth hormone, growth hormone receptor (GHR), insulin-like growth factor I, growth hormone-releasing hormone (GHRH) and somatostatin (SS) as well as the appetite hormone, cholecystokinin (CCK), were analyzed with quantitative PCR in the brain of fish from this experiment and in six brain regions (telencephalon, hypothalamus, pituitary, optic tectum, midbrain, cerebellum) and muscle and liver of size-matched (55g) full-ration transgenic, restricted-ration transgenic (pair-fed to nontransgenic fish) and full-ration non-transgenic fish. Transgenic fish had greater feed intake and growth which were respectively negatively and positively correlated with the digestible energy content of the diet. Protein use was more efficient in transgenic fish when fed the 15 and 17 MJ/kg diets only and diet had no effect on gene expression. Plasma IGF-I was greater in transgenic fish but did not correlate with weight as in the non-transgenic coho salmon. In the size-matched study, GH expression was highest in transgenic fish, except in the pituitary where there were no differences. GHR expression generally followed that of GH but was tissue specific. Transgenic fish has higher IGF-I expression but hepatic IGF-I levels in ration-restricted transgenic fish were reduced to that of non-transgenic fish. GHRH, SS or CCK expression did not differ between groups. It is concluded that transgenic fish, with enhanced growth, feed intake and protein utilization, ingested feed to meet a caloric demand that was possibly set by increased GH, GHR and IGF-I whereas the other GH regulating hormones and the CCK satiety signal showed little change.

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