Links between the appetite regulating systems and the neuroendocrine hypothalamus: Lessons from the sheep

The hypothalamus is integral to the regulation of energy homeostasis and the secretion of hormones from the pituitary gland. Consequently, hypothalamic systems may have a dual purpose in regulating both neuroendocrine function and appetite. To date, most studies investigating the interface between appetite and hormone secretion have been performed in rats or mice that have been acutely fasted or baring a genetic abnormality causing either obesity or aphagia. By contrast, various physiological models, including chronic food-restriction or photoperiodically driven changes in voluntary food intake, add further perspective to the issue. In this regard, sheep provide an innovative model whereby long-term changes in body weight or extended feeding rhythms can be investigated. This review compares and contrasts data obtained in different species with regard to the neuroendocrinology of appetite, and discusses the benefits and knowledge gained from using various nonrodent models with a particular emphasis on a ruminant species.