Dorsal raphe serotonergic neurons suppress feeding through redundant forebrain circuits

Serotonin (5HT) is a well-known anorexigenic molecule, and 5HT neurons of dorsal raphe nucleus (DRN) have been implicated in suppression of feeding; however, the downstream circuitry is poorly understood. Here we explored major projections of DRN5HT neurons for their capacity to modulate feeding. We used optogenetics to selectively activate DRN5HT axonal projections in hypothalamic and extrahypothalamic areas and monitored food intake. We next used fiber photometry to image the activity dynamics of DRN5HT axons and 5HT levels in projection areas in response feeding and metabolic hormones. Finally, we used electrophysiology to determine how DRN5HT axons affect downstream neuron activity. We found that selective activation of DRN5HT axons in (DRN5HT → LH) and (DRN5HT → BNST) suppresses feeding whereas activating medial hypothalamic projections has no effect. Using in vivo imaging, we found that food access and satiety hormones activate DRN5HT projections to LH where they also rapidly increase extracellular 5HT levels. Optogenetic mapping revealed that DRN5HT → LHvGAT and DRN5HT → LHvGlut2 connections are primarily inhibitory and excitatory respectively. Further, in addition to its direct action on LH neurons, we found that 5HT suppresses GABA release from presynaptic terminals arriving from AgRP neurons. These findings define functionally redundant forebrain circuits through which DRN5HT neurons suppress feeding and reveal that these projections can be modulated by metabolic hormones. •DRN5HT neurons suppress feeding through projections to LH and BNST.•5HT levels in LH respond to food and satiety hormones but not to the hunger hormone ghrelin.•5HT from DRN axons inhibit GABA release from AgRP presynaptic terminals onto LH neurons.•DRN axons activate and inhibit glutamatergic and GABAergic LH neurons respectively.