Figure 4: Imaging the gut-brain axis.

a, We imaged calcium responses in vagal sensory neurons expressing the fluorescent calcium indicator GCaMP6s while stimulating the intestines. b, Heat maps depicting z-score normalized fluorescence traces^33,47 from vagal neurons identified as glucose responders. Each row represents the average activity of a single cell to 3 trials. Stimulus window shown by dashed white lines. The left panels are responses of n = 206 vagal neurons to a 60 s intestinal infusion of 500 mM glucose; note lack of responses to 30 mM AceK. The right heatmaps depict n = 133 vagal neurons that responded to 60 s 500 mM glucose, and tested for their responses to 500 mM MDG; heatmaps were normalized across stimuli; responses to glucose and MDG were similar (two-tailed paired t-test, p = 0.06). c, Sample traces of vagal neuron responses to intestinal stimulation with 60 s pulses of 30 mM AceK and 500 mM glucose from 3 mice (upper traces), or to 10 s pulses of 500 mM glucose and 500 mM MDG (lower traces). Note the reliability and rapid onset of responses to the 10 s stimulus (Extended Data Fig. 6c). When using a 10 s stimulus, to minimize potential osmolarity responses (Extended Data Fig. 8), approximately 5% of imaged neurons show statistically significant responses to glucose (Extended Data Fig. 6d). We compared imaging sessions with both the right and left ganglia²⁵ and did not observe any meaningful difference in the proportion of glucose-responding neurons (Extended Data Fig. 6e). d, Vagal neuron responses to 3-OMG (top traces) and galactose (lower traces), n = 3 independent experiments each. These agonists activate vagal neurons in a similar manner to glucose (Extended Data Fig. 2b and 10a–b). e, The monosaccharides fructose and mannose, which are not substrates for SGLT-1, do not activate glucose-responsive neurons. Shown are heat maps of 46 glucose-responding neurons to 500 mM fructose and 500 mM mannose (n = 5 ganglia). Fewer than 10% of glucose responders were activated by either fructose or mannose. f-g, Summary of responses to a 10 s stimulus of 500 mM glucose for 33 neurons before and after intestinal application 8 mM Phlorizin for 5 min (n = 4 mice). Responses are severely diminished after blocker application (see Extended Data Fig. 10d–e and Methods).