CCK expressing neurons in the NTS are directly activated by CCK-sensitive C-type vagal afferents.

Vagal sensory afferents carrying information from the gastrointestinal tract (GI) terminate in the nucleus of the solitary tract (NTS). Different subpopulations of NTS neurons then relay this information throughout the brain. Cholecystokinin (CCK) is a satiety peptide that activates vagal afferents in the GI. However, CCK is also expressed by neurons in the NTS, and activation of these neurons decreases food intake. What is less clear is how these NTS CCK neurons are activated by vagal afferents and what type of information they integrate about meal size and content. To address this, we identified NTS-CCK neurons by crossing CCK-IRES-Cre mice with floxed-Rosa-tdtomato mice and made a horizontal brain slice containing vagal afferents in the solitary tract (ST). Voltage clamp recordings of NTS-CCK neurons show that activation of the ST evokes excitatory post-synaptic currents (EPSCs) mediated by both AMPA and NMDA receptors. Analysis of these EPSCs revealed that 80% of NTS-CCK neurons receive direct, monosynaptic inputs, with many also receiving indirect, or polysynaptic, inputs. NTS-CCK neurons are sensitive to the TRPV1 agonist capsaicin, suggesting they are downstream of C-fibers. In addition, both CCK and a 5-HT3R agonist increased sEPSC frequency in NTS-CCK neurons, with 69% of NTS-CCK neurons sensitive to CCK and 42% to 5-HT3 receptor agonists, as well as 45% sensitive to both and 10% to neither. Taken together with previous studies, this suggests that NTS-CCK neurons are driven primarily by vagal afferents that are sensitive to CCK and are only weakly driven by those sensitive to 5-HT.