OBJECTIVE: Glucagon-like peptide-1 (GLP-1) and 5-HT are potent regulators of food intake within the brain. GLP-1 is expressed by preproglucagon (PPG) neurons in the nucleus tractus solitarius (NTS). We have previously shown that PPG neurons innervate 5-HT neurons in the ventral brainstem. Here, we investigate whether PPG neurons receive serotonergic input and respond to 5-HT. METHODS: We employed immunohistochemistry to reveal serotonergic innervation of PPG neurons. We investigated the responsiveness of PPG neurons to 5-HT using in vitro Ca²⁺ imaging in brainstem slices from transgenic mice expressing the Ca²⁺ indicator, GCaMP3, in PPG neurons, and cell-attached patch-clamp recordings.

RESULTS: Close appositions from 5-HT-immunoreactive axons occurred on many PPG neurons. Application of 20 μM 5-HT produced robust Ca²⁺ responses in NTS PPG dendrites but little change in somata. Dendritic Ca²⁺ spikes were concentration-dependent (2, 20, and 200 μM) and unaffected by blockade of glutamatergic transmission, suggesting 5-HT receptors on PPG neurons. Neither activation nor blockade of 5-HT₃ receptors affected [Ca²⁺]i. In contrast, inhibition of 5-HT₃ receptors attenuated increases in intracellular Ca²⁺ and 5-HT₂c receptor activation produced Ca²⁺ spikes. Patch-clamp recordings revealed that 44% of cells decreased their firing rate under 5-HT, an effect blocked by 5-HT₁ᴀ receptor antagonism.

CONCLUSIONS: PPG neurons respond directly to 5-HT with a 5-HT₂c receptor-dependent increase in dendritic [Ca²⁺]i. Electrical responses to 5-HT revealed additional inhibitory effects due to somatic 5-HT₁ᴀ receptors. Reciprocal innervation between 5-HT and PPG neurons suggests that the coordinated activity of these brainstem neurons may play a role in the regulation of food intake.