The effect of ghrelin on glucose metabolism and the growth hormone/insulin-like growth factor-I (GH/IGF-I) axis in the tilapia, Oreochromis mossambicus
The peptide hormone ghrelin has been shown to stimulate growth hormone (GH) release, appetite, and fat deposition in vertebrates. More recently, ghrelin has been shown to play a role in glucose metabolism in mammals. Ghrelin acts through the growth hormone secretagogue receptor (GHS-R). The GHS-R codes for two gene transcripts, GHS-R1a and GHS-R1b. In tilapia, two forms of ghrelin have been identified, ghrelin-C8 and ghrelin-C10. This study measured the effect of ghrelin on the hormonal regulators involved in glucose metabolism, appetite, and growth (GH/IGF-I axis) in the tilapia, Oreochromis mossambicus. Fish were injected with two doses of ghrelin-C8 and ghrelin-C10. Blood and tissue samples were collected at 2, 4, and 8 h post-injection. The low dose of ghrelin-C8 elevated blood glucose levels at 4 h post-injection. Both ghrelin-C8 and -C10 reduced plasma IGF-I levels and ghrelin-C10 elevated IGF-I mRNA levels in the liver. Ghrelin-C8 and -C10 elevated liver GHR2 mRNA levels, however GHR1 mRNA levels remained unaffected. In the brain, ghrelin-C8 and -C10 treatment elevated both GHS-R1a and GHS-R1b mRNA levels. NPY mRNA levels in the brain, a regulator of appetite, were elevated by ghrelin-C10. These data confirm ghrelin�s role in appetite and glucose metabolism and show that ghrelin up-regulates certain components of the GH/IGF-I axis in tilapia.