Energy homeostasis involves extensive interactions between hypothalamic neuronal populations, and their communication with both central and peripheral targets. The adipoctye-derived hormone leptin, acts in the hypothalamus to control food intake and energy expenditure. It is widely assumed that the major site of leptin’s effects within the hypothalamus is the arcuate nucleus. However, recently Dhillon et al. (2006) demonstrated clearly the functional importance of another hypothalamic leptin target, the ventromedial nucleus (VMN). The VMN has important functions in controlling feeding, energy expenditure and blood glucose levels. Even so, there is still little known about the VMN cell types or transmitters involved. We have evidence that pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide that is strongly expressed in the VMN, may have a significant role in leptin’s central actions. Both leptin and PACAP dose-dependently decrease fast-induced re-feeding in outbred CD1 mice when administered to freely behaving animals via an ICV cannula inserted under isoflurane 1 week earlier. PACAP also has robust effects on metabolism, increasing oxygen consumption and core-body temperature, and induces c-Fos protein expression in discrete brain regions associated with thermogenesis and appetite suppression. The effects of PACAP appear to be mediated by PAC₁ receptors rather than VPAC receptors (that also bind vasoactive intestinal peptide), since they are blocked by the partially-selective PAC₁ receptor antagonist, PACAP_6-38. Moreover, our data indicate that leptin-induced effects on feeding and body temperature are significantly attenuated by pre-treatment with the PAC₁ antagonist. Semi-quantitative in situ hybridisation histology reveals that PACAP mRNA expression in the VMN, but not elsewhere in the forebrain, is regulated according to metabolic state. Either fasting or genetic leptin deficiency, both states of hunger and hypometabolism, are accompanied by a significant reduction in the relative expression of VMN PACAP compared with controls. Conversely, both high-energy diet and exogenous leptin treatment increase PACAP mRNA. Furthermore, mice with genetic VMN-specific deletion of leptin receptor also show reduced PACAP in the VMN, along with a modestly obese phenotype. To further investigate the direct action of leptin on VMN PACAP neurons we are examining endogenous PACAP and leptin receptor expression, and c-Fos induction using dual in situ hybridisation histological techniques. Together, these data reveal a highly plastic system which may be essential for maintaining normal energy homeostasis and modulating the central actions of leptin.