Project Summary The sympathetic activation of brown adipose tissue (BAT) increases the metabolism of fatty acids within this tissue. Due to the presence of uncoupling protein-1 in the mitochondria of BAT the electrochemical gradient generated by the electron transport chain is dissipated in the absence of the production of ATP. This process which is unique to BAT (as well as inducible forms of BAT, “beige” adipose tissue) in essence metabolizes fat to produce heat. The obvious implications of this “fat burning” process for energy balance and body weight regulation have led to intense interest in the biological mechanisms governing this process. The activity of the sympathetic nerves innervating BAT is the principal regulator of this process. Our research has defined the fundamental neural pathways through which thermal and febrile stimuli elicit changes in the sympathetic outflow to BAT. However, relatively little is known about the neural circuits involved in the metabolic influences on BAT and how dietary components (such as the fat content of the diet) influence these regulatory circuits. In the proposed research project, we will perform an extensive series of in vivo and in vitro electrophysiological, anatomical, neuropharmacological, and Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) experiments to address specific aims that will provide new insights into the neural mechanisms responsible for the impairment of BAT activation during high fat diet (HFD) and the specific contribution of this impairment to HFD- induced weight gain. The first aim will determine the role of transient receptor potential vanilloid type 1 (TRPV1) in NTS in the impairment of BAT activation during maintenance on a high fat diet. The second aim will define the downstream projection target of the NTS that is responsible for inhibition of sympathetic output to BAT during HFD. The third aim will define the role of preprodynorphin neurons in the lateral parabrachial nucleus and kappa opioid receptor activation in the preoptic area in the impairment of BAT activation during HFD. The forth aim will define the output projection target and neurophysiological characteristics of kappa opioid receptor containing neurons of the preoptic area.