Effects of nutritional state on ghrelin, somatolactin, and the growth hormone and insulin-like growth factor axis in the tilapia, Oreochromis mossambicus

Mozambique tilapia (Oreochromis mossambicus) is an important aquaculture species that is cultured in tropical climates worldwide. Similar to terrestrial vertebrates, a functional growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis has recently been identified in the tilapia which responds to nutritional status. Tilapia also possess two forms of the stomach-derived hormone ghrelin, which stimulates GH release, appetite, and fat deposition. Somatolactin (SL) is a fish-specific pituitary-derived member of the GH/prolactin (PRL) family of peptide hormones which has been shown to be involved in energy metabolism in some fish species. The primary objective of this dissertation was to investigate the physiological effects of tilapia ghrelin, and characterize modulations in ghrelin levels and the GH/IGF-I axis in response to nutrient restriction. In the present study, ghrelin treatment stimulated GH release from primary tilapia pituitary cells, and a ghrelin receptor antagonist blocked this stimulatory effect. Similarly, peripheral injection of homologous tilapia ghrelin increased plasma GH and IGF-I levels. These results suggest that a functional ghrelin-GHS-R system exists, and that ghrelin's actions in the pituitary are conserved between the tilapia and terrestrial species. Furthermore, prolonged fasting (2 and 4 weeks) brought about a significant elevation in plasma ghrelin levels, suggesting a possible role for ghrelin in long-term energy partitioning and homeostasis in the tilapia. Fasting for 4 weeks in both seawater and fresh water resulted in elevated or unchanged plasma GH levels, with consistently reduced plasma IGF-I and liver IGF-I mRNA levels suggesting GH resistance. Changes in hepatic expression of IGF-I, GH-R, and SL-R mRNA levels were minor compared with those detected in muscle tissue suggesting a significant contribution by muscle to metabolic recovery during both the catabolic state of fasting and during the nutrient surplus that follows re-feeding. Additionally, pituitary expression of SL mRNA levels were reduced during re-feeding suggesting a role for this hormone in the partitioning and deposition of substrates during energy surplus. The findings resulting from the studies described herein suggest that ghrelin, along with the GH/IGF-I axis play important roles in coordinating metabolism according to the nutritional state of tilapia.