Several key functions of ghrelin are well conserved through vertebrate phylogeny. However, some of ghrelin's effects are contradictory and among teleosts only a limited number of species have been used in functional studies on food intake and foraging-related behaviors. Herewe investigated the long-term effects of ghrelin on food intake, growth, swimming activity and aggressive contest behavior in one year old wild brown trout (Salmo trutta) using intraperitoneal implants. Food intake and swimming activity were individually recorded starting from day 1, and aggressive behavior was tested at day 11, after ghrelin implantation. Body weight and growth rate were measured from the beginning to the end of the experiment. Triglycerides and lipase activity in muscle and liver; monoaminergic activity in the telencephalon and brainstem; and neuropeptide Y (NPY) mRNA levels in the hypothalamus were analyzed. Ghrelin treatmentwas found to increase food intake and growth withoutmodifying lipid deposition or lipidmetabolism in liver andmuscle. Ghrelin treatment led to an increased foraging activity and a trend towards a higher swimming activity. Moreover, ghrelin-treated fish showed a tendency to initiate more conflicts, but this motivation was not reflected in a higher ability to win the conflicts. No changes were observed in monoaminergic activity and NPYmRNA levels in the brain. Ghrelin is therefore suggested to act as an orexigenic hormone regulating behavior in juvenile wild brown trout. These actions are accompanied with an increased growth without the alteration of liver andmuscle lipidmetabolism and they do not seem to be mediated by changes in brain monoaminergic activity or hypothalamic expression of NPY.
