A, representative [Ca2+]i responses to protons (pH 5.5, left) and noxious heat (48°C, right) before and during the application of 5-HT (10 μm), and after its removal. B, summarized changes in [Ca2+]i induced by protons (left, n = 119, N = 4) and noxious heat (right, n = 77, N = 4), before (S1) and after the application of 5-HT (S2). * P < 0.05, ** P < 0.01. C, histograms of the percentage of cells–S2/S1 ratio relation obtained by stimulation with protons (left) and noxious heat (right), between IB4-positive (IB4+) and -negative neurons (IB4−) in the presence of 5-HT. D, representative currents induced by protons (pH 5.5, 15 s) at −60 mV before (S1), during the application of 5-HT (10 μm, S2) and after its removal, and summarized results (n = 25, N = 4). * P < 0.05.