Figure 6. Enterochromaffin cells modulate 5HT₃R-expressing afferent nerves.

A. Representative recordings from single mucosal afferent nerve fibers innervating intact colonic epithelium in an ex vivo preparation. Norepinephrine (NE, 1µM) applied to the epithelium elicited chemosensory responses that were blocked by the TRPC4 inhibitor ML204 (10µM) or the 5HT₃R antagonist alosetron (10µM). Isovalerate (IVL, 200µM) also evoked alosetron-sensitive afferent activity. Scale bars: 500µV, 50s. Representative of n=8–9 per condition. p<0.0001 for number of action potentials measured in response to NE (321.5±55.4 spikes, 4/8 responsive fibers) versus NE+ML204 (0 spikes, 0/8 responsive fibers) or NE+alosetron (0 spikes, 0/9 responsive fibers), one-way ANOVA with post-hoc Bonferroni test. p < 0.01 for number of action potentials measured in response to IVL (648.7±339.3 spikes, 3/8 responsive fibers) versus IVL+alosetron (0 spikes, 0/8 responsive fibers).

B. The 5HT₃R agonist mCPBG (10µM), but not NE (1µM) or isovalerate (IVL, 200µM), evoked representative Ca²⁺ responses in retrogradely-labeled colonic sensory neurons isolated from lumbosacral dorsal root ganglia. All neurons quantified responded to high extracellular K⁺. Black traces represent an average of all cells in the field shown in grey. Scale bar: 0.1 Fura-2 ratio, 60s. Responsive neurons: n=0/16 for NE, n=0/62 for IVL, n=16/35 for mCPBG.