The incidence of obesity and related co-morbidities is rapidly growing, and the need for novel efficacious treatments is pressing. Nociceptin/Orphanin FQ, an endogenous orexigenic peptide for the G protein-coupled receptor, NOP (previously known as opioid-like receptor 1 (OLR-1)) has been recently identified as a therapeutic target for obesity. Antagonists of the NOP receptor have been shown to selectively block N/OFQ-induced food intake (for review please see [1]). The primary objective of our research is to delineate discrete chemically defined NOP receptor expressing neuronal populations critically affecting food intake and body weight. N/OFQ and its receptor, NOP, are diffusely expressed in the dorsal raphe nucleus (DRN), a brain region containing a critical population of serotonin (5-HT) neurons, as well as other brain regions associated with satiety. The 5-HT system is well established in food intake regulation. We hypothesize that N/OFQ is an upstream modulator of 5-HT bioavailability in DRN, a neurotransmitter system targeted for the treatment of obesity. The anatomical distribution of NOP receptor in DRN lends circumstantial support to a role for this neuropeptide in the CNS regulation of ingestion. We hypothesize that N/OFQ and 5-HT are not co-expressed in the DRN, but rather that N/OFQ is released locally within the DRN and inhibits the action of 5-HT neurons via NOP receptors. To examine co-expression of N/OFQ and 5-HT in the DRN, we performed dual-immunohistochemical labelling using a rabbit anti-N/OFQ antibody (Phoenix, 1:1000) and a goat anti-5-HT antibody (Chemicon, 1:1000) using rat brain tissue. Our results show very few neurons co-expressing N/OFQ and 5-HT. Additionally, under conditions of food deprivation/hunger, the NOP receptor has been shown to be bilaterally and significantly down-regulated in the paraventricular nucleus of hypothalamus (PVH) and central nucleus of the amygdala (CeA). To test our hypothesis that NOP receptors expressed in DRN are also involved in regulation of feeding, we investigated whether the NOP receptor is nutritionally regulated in the DRN. By performing in situ hybridization histochemistry we observed, like in the amygdala and hypothalamus, a significant down-regulation of NOP receptors in the dorsal raphe nucleus in response to food deprivation/hunger. In future experiments, we will investigate the effect of basal and N/OFQ-induced feeding in selective DRN NOP receptor null rats. We will compare the DRN NOP receptor null phenotype with that obtained following global brain NOP receptor ablation and site specific NOP receptor deletion in classic regions associated with satiety. Data generated in this proposal will help elucidate critical NOP receptor expressing populations affecting ingestive behavior and add to our understanding of the complex neural networks modulating appetite.