Project Summary The development of therapeutic drugs to cure obesity has not been successful due to unwanted side effects and limited efficacy. My long term research goal is to delineate neural pathways responsible for body weight homeostasis, and provide a framework for effective and specific therapeutics against obesity. Despite exciting progress has been made in understanding feeding behavior regulated by agouti-related protein (AgRP) neurons, these neurons only represent a small percent of Arc GABAergic neurons and the role of the majority of other hypothalamic neurons in feeding regulation is unknown. Recent studies suggest that activation of the lateral hypothalamus (LH) glutamatergic neurons inhibit feeding, which is in stark contrast to the hypophagia phenotype induced by LH lesion, suggesting a potential role of other LH neurons, including GABAergic neurons in feeding regulation. We previously demonstrated that, when channelrhodopsin2 (ChR2) is expressed in LH Pdx1-Cre neurons, photo-stimulation of ChR2-expressing fibers located in the PVH induced voracious feeding, which depends on GABA release. In addition, disruption of GABA release from LH neurons reduces feeding and body weight, suggesting physiological relevance for LH GABAergic neurons in feeding regulation. Our preliminary data showed that LH Pdx1-Cre neurons send monosynaptic excitatory and inhibitory inputs to PVH neurons and that activation of LH→Pdx1-Cre fibers lacking GABA release in the PVH inhibited fast-refeeding. These results, in combination with the previous results that PVH neuron activity levels dictate feeding promotion versus inhibition, prompt us to hypothesize that competing inhibitory and excitatory monosynaptic LH→PVH projections regulate feeding through controlling PVH MC4R neuron activity. Aim 1 will determine whether 1) selective activation of inhibitory LH→PVH projections in feeding promotion is mediated by PVH; and 2) whether inhibition of LH→PVH GABAergic fibers is sufficient to reduce feeding behavior. Aim 2 will test 1) whether selective activation of excitatory LH→PVH projections inhibits fast-refeeding; 2) whether glutamate release from LH Pdx1-Cre neurons is required for feeding inhibition; 3) whether feeding inhibition elicited by LH→PVH glutamatergic projections is mediated by PVH. Aim 3 will determine whether LH→PVH GABAergic and glutamatergic monosynaptic projections target and modulate the activity of PVH MC4R neurons to exert effects on feeding promotion or inhibition, respectively. The results will establish, for first time, competing and parallel glutamatergic and GABAergic LH→PVH projections that play opposite roles in feeding by controlling PVH MC4R neuron activity and will represent a significant step in understanding the neural basis for feeding regulation.