Plasma ghrelin levels in rainbow trout in response to fasting, feeding and food composition, and effects of ghrelin on voluntary food intake Elisabeth Jönsson a, ⁎ , Antti Forsman b , Ingibjörg E. Einarsdottir a , Hiroyuki Kaiya c , Kari Ruohonen b , Björn Thrandur Björnsson a a Fish Endocrinology Laboratory, Department of Zoology/Zoophysiology, Göteborg University, Box 463, S-405 30 Göteborg, Sweden b Finnish Game and Fisheries Research Institute, Turku Game and Fisheries Research, Itäinen, Pitkäkatu 3, 20520 Turku, Finland c Department of Biochemistry, National Cardiovascular Center Research Institute, Osaka 565-8565, Japan Received 12 December 2006; received in revised form 23 March 2007; accepted 24 March 2007 Available online 30 March 2007 Abstract Ghrelin, a peptide hormone which stimulates growth hormone (GH) release, appetite and adiposity in mammals, was recently identified in fish. In this study, the roles of ghrelin in regulating food intake and the growth hormone (GH)–insulin-like growth factor I (IGF-I) system of rainbow trout (Oncorhynchus mykiss) were investigated in three experiments: 1) Pre- and postprandial plasma levels of ghrelin were measured in relation to dietary composition and food intake through dietary inclusion of radio-dense lead-glass beads, 2) the effect of a single intraperitoneal (i.p.) injection with rainbow trout ghrelin on short-term voluntary food intake was examined and 3) the effect of one to three weeks fasting on circulating ghrelin levels and the correlation with plasma GH and IGF-I levels, growth and lipid content in the liver and muscle was studied. There was no postprandial change in plasma ghrelin levels. Fish fed a normal-protein/high-lipid (31.4%) diet tended to have higher plasma ghrelin levels than those fed a high-protein/low-lipid (14.1%) diet. Plasma ghrelin levels decreased during fasting and correlated positively with specific growth rates, condition factor, liver and muscle lipid content, and negatively with plasma GH and IGF-I levels. An i.p. ghrelin injection did not affect food intake during 12-hours post-injection. It is concluded that ghrelin release in rainbow trout may be influenced by long-term energy status, and possibly by diet composition. Further, in rainbow trout, ghrelin seems to be linked to growth and metabolism, but does not seem to stimulate short- term appetite through a peripheral action. © 2007 Elsevier Inc. All rights reserved. Keywords: Ghrelin; Oncorhynchus mykiss; Feeding; Fasting; Appetite; Postprandial; Energy balance; Lipid 1. Introduction In mammals, ghrelin was originally discovered as a growth hormone (GH) secretagogue, acting through the GH secreta- gogue receptor (GHS-R), which is present in the pituitary, hypothalamus, and several peripheral tissues (Howard et al., 1996; Kojima et al., 1999; Shuto et al., 2001; Gnanapavan et al., 2002). Subsequent studies demonstrated that ghrelin has a cen- tral role in energy balance regulation in mammals (Tschöp et al., 2000; Nakazato et al., 2001; Wren et al., 2000, 2001). Ghrelin was recently cloned in rainbow trout (Oncorhynchus mykiss) (Kaiya et al., 2003). Similar to mammals, ghrelin in fish is predominantly produced in endocrine cells in the stomach (Kaiya et al., 2003; Sakata et al., 2004). Ghrelin promotes body weight gain and adiposity by stimulating appetite and reducing fat utilization in rats and mice (Tschöp et al., 2000; Nakazato et al., 2001; Wren et al., 2000, 2001). Likewise, long-term treatment with ghrelin implants increases food intake, adiposity, and body weight in tilapia (Oreochromis mossambicus)(Riley et al., 2005). Ghrelin also increases short-term food intake in goldfish (Carassius auratus)(Unniappan et al., 2002, 2004; Matsuda et al., 2006), in rainbow trout using rat ghrelin (Shepherd et al., 2007), and Japanese quail (Coturnix coturnix japonica)(Shousha et al., 2005). Exceptions to the orexigenic action of ghrelin have been Comparative Biochemistry and Physiology, Part A 147 (2007) 1116 – 1124 www.elsevier.com/locate/cbpa ⁎ Corresponding author. E-mail address: elisabeth.jonsson@zool.gu.se (E. Jönsson). 1095-6433/$ - see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpa.2007.03.024