5-HT NEURONS AND MEAL REGULATION

PROJECT SUMMARY Despite the rapidly growing rate in obesity and tremendous efforts in weight loss research, the development of safe and effective therapeutics has progressed significantly, yet is still limited, largely due to insufficient understanding of how the brain manages body weight and feeding. Lorcaserin, a 5-HT (serotonin) receptor agonist, showed great promise for effectively reducing weight, but was withdrawn due to increased risk for cancer development. 5-HT remains an excellent potential target for an anti-obesity therapeutic, but more work needs to be completed to determine the most effective way for targeting 5-HT. Previous work in this area has shown that GABA and dopamine inhibition of 5-HT neurons is critical for mediating food intake in mice. In fact, my preliminary work has revealed that 2 (GABA receptor subunit) deletion in TPH2 (rate limiting enzyme for 5-HT synthesis) neurons causes a decreased meal number and increased meal interval compared to wild-type mice, indicating a potential dysfunction in meal initiation. Furthermore, DRD2 (a dopamine receptor) deletion reduced miniature inhibitory post synaptic current (mIPSC) in TPH2 neurons, indicating that DRD2 plays an essential role in mediating 5-HT neuronal response to GABA inhibitory input. Finally, I found that in hungry mice, the selective activation of 5-HT neurons in the Dorsal Raphe Nucleus (DRN) which project to the Arcuate Nucleus (ARH) in the Hypothalamus during food approach significantly reduces food intake compared to control mice. Taken together, these preliminary data led me to the hypothesis that GABA inputs to 5-HTDRN neurons are regulated, in part, by DRD2 and may contribute to the regulation of meal initiation. The focus of this proposal is to 1) determine whether the 5-HTDRN→ARH circuit regulates meal initiation and 2) to determine whether DA influences GABA signaling to ARH-projecting 5-HTDRN neurons for meal initiation. To achieve these goals, I will examine (1) whether photostimulation or photoinhibition of the 5-HTDRN→ARH circuit during food approach alters meal initiation and food intake, and (2) the dynamic activity of ARH-projecting 5-HTDRN neurons before or during food approach, at meal initiation, eating and meal termination using fiber photometry. Further, I will generate mice with a selective deletion of either the GABAA γ2 subunit or DA receptor DRD2 in ARH-projecting 5-HTDRN neurons and perform metabolic and meal pattern studies, and (2) examine the electrophysiological properties (mIPSC, resting membrane potential, action potential firing, and their responses to GABA or DRD2 agonists) of ARH- projecting 5-HTDRN neurons in which either the GABAA γ2 subunit or DA receptor DRD2 is deleted. With support from this fellowship, I will utilize the extensive resources at Baylor College of Medicine to expand my expertise so that I can advance in my career and learn more novel and comprehensive approaches to studying the mechanisms underlying meal initiation. This opportunity will provide me with the necessary tools to enhance my training and move toward my career goal of becoming an independent research scientist at an academic institution.