Serotonin neurons integrate GABA and dopamine inputs to regulate meal initiation.

Obesity is a growing global health epidemic with limited orally administered therapeutics. Serotonin (5-HT) is one neurotransmitter which remains an excellent target for new weight-loss therapies, but a gap remains in understanding the mechanisms involved in 5-HT produced in the dorsal Raphe nucleus (DRN) and its involvement in meal initiation. Using an optogenetic feeding paradigm, we showed that the 5-HT➔arcuate nucleus (ARH) circuit plays a role in meal initiation. Incorporating electrophysiology and ChannelRhodopsin-2-Assisted Circuit Mapping, we demonstrated that 5-HT neurons receive inhibitory input partially from GABAergic neurons in the DRN, and the 5-HT response can be enhanced by hunger. Additionally, deletion of the GABA receptor subunit in 5-HT neurons inhibits meal initiation with no effect on the satiation process. Finally, we identified the role of dopaminergic inputs via dopamine receptor D2 in enhancing the response to GABA-induced feeding. Thus, our results indicate that 5-HT neurons are inhibited by synergistic inhibitory actions of GABA and dopamine, for the initiation of a meal.