Appetite regulation in Arctic fish; how to fast without being hungry

In mammals, appetite is regulated through a hypothalamic integration of stimulatory (orexigenic) and inhibitory (anorexigenic) factors. Although major gaps exist in our understanding of the control of food intake in fish, the regulation seems to involve the same signalling molecules (hormones, neuropeptides) as in mammals. Food intake and energy balance are influenced by several long-term neuroendocrine signals, including such that reflect the size of energy stores. In common with mammals, body fatness seems to have an inhibitory effect on food intake in fish; feed intake is negatively related to adiposity, and feed intake after a period of feed restriction targets restoration of fat reserves. It is therefore likely that the “lipostatic model” for appetite regulation in mammals also relate to fish. Underpinning this mechanism, the hormone leptin in mammals provides the endocrine link between peripheral fat stores and the brain centers that control appetite and energy balance. Leptin, together with orexigenic (neuropeptide Y) and anorexigenic (pro-opiomelanocortin) neuropetides has now been cloned in Arctic charr and tools for transcriptomics for these effectors established.

The anadromous life strategy of the Arctic charr implies that most of the annual growth and resource accumulation occurs during the short summer seawater residence. Hence this species possess an extreme feeding strategy, with months of fasting and emaciation during winter and intense feeding and energy deposition during a few summer weeks. The seasonal change in appetite and growth in this species seems to be a strictly regulated process; captive offspring of anadromous Arctic charr exhibit pronounced seasonal changes in appetite and growth when held at constant temperature and given food in excess. The anadromous Arctic charr is an excellent model for studying mechanisms underlying long-term regulation of appetite and energy homeostasis, in particular within a context of a lipostatic model, because of its extreme (and voluntary) seasonal cycles in body fattening and emaciation.Within a broad context, our aim is to investigate the role of anorexigenic and orexigenic factors in the regulation of appetite in fish, and how leptin interact with photoperiod, melatonin and adiposity in the regulation of the somatotropic axis. In particular, we are interested in how the anadromous Arctic charr can maintain an anorexic state throughout winter despite loss of fat reserves and in a long-term view, reveal if there may be targets for the development of therapeutic agents for use in tackling problems related to disturbed energy homeostasis and human obesity.

References:

• Frøiland, E., Murashita, K., Jørgensen, E.H. and Kurokawa, T. 2010. Leptin and ghrelin in anadromous Arctic charr: cloning and change in expressions during a seasonal feeding cycle. Gen. Comp. Endocrinol. 165, 136-143.
• Frøiland,E., Jobling, M., Björnsson, BT, Kling, P, Jørgensen, E.H. LeptinA2 is not a signal of adiposity, but may be a metabolic regulator in Arctic charr (Salvelinus alpinus). Gen. Comp. Endocrinol. In revision.