Obesity and overweight affect most Americans as we struggle with the current obesigenic environment. Yet some resist obesity, just as some animals resist obesity. Animal and human studies indicate that spontaneous physical activity (SPA), which generates non-exercise activity thermogenesis (NEAT), is an important defense against weight gain. We have demonstrated that obesity resistant (OR) rats have greater SPA than obesity prone (OP) rats. OP and OR rats, as polygenic strains differing in sensitivity to obesity, are excellent models for polygenic human obesity. Lean humans with elevated SPA resist obesity during caloric challenge, while human obesity is characterized by dramatically reduced SPA, which, by lowering expended energy has a substantial influence on body weight. Brain mechanisms control the level of SPA, and the NEAT so generated. Orexin neurons appear to play a key role as they project to brain areas involved in arousal and activity and loss of brain orexin activity increases body mass index. Sensitivity to orexin A (OxA) SPA promotion is inherent to OR rats and may be critically important in resisting obesity. Recent evidence and our preliminary data suggest that orexin action in the dorsal raphe nucleus (DR) and the locus coeruleus (LC), may mediate SPA and NEAT. We hypothesize that 1) orexin action in DR and LC are key mechanisms mediating SPA and NEAT; 2) viral- mediated overexpression of orexin receptor(s) in obese rodents will reduce body weight; 3) intranasal administration of orexin A, by enhancing orexin levels in brains of rat, will produce enhancement of NEAT. We will test these ideas with three specific aims: Aim 1 will define the role of DR and LC orexin in regulating SPA and NEAT in obesity prone and resistant rats. We will first verify that orexin receptor gene expression in DR and LC is increased in obesity resistant rats (1A); determine sensitivity to DR and LC orexin A infusion in obesity prone and obesity resistant rats (1B), test whether reduced DR or LC orexin receptor function in obesity resistant rats will promote weight gain (1C); determine if orexin A stimulation of DR and LC reduces weight gain in obesity prone rats (1D). In Aim 2 we plan the use of virus-mediated overexpression of orexin receptors in LC or DR to enhance NEAT. Sub-aims will test whether single (2A and 2B) and combined (2C) overexpression of Ox1R and Ox2R in LC or DR of obesity prone rats will reduce weight gain. Finally, in Aim 3 we will perform intranasal (IN) delivery of OxA to determine its effectiveness at increasing NEAT. Sub-aims will determine what effective dose and timing will increase LC and DRN orexin A levels, and whether acute IN (3B) or chronic IN (3C) administration into obese rats increases SPA and NEAT. These studies will fill the gap in knowledge of brain mechanisms underlying SPA, which is important to obesity resistance, and will provide information necessary to developing new obesity therapies for Veterans and the population as a whole.