Fig. 2. 5-HT₄ receptor is required for potentiation of DSFs and OA by 5-HT.

For electrophysiological recordings, cells were pre-treated for 5–15 min with 1 μM of the 5-HTR antagonist or vehicle control before cells were patched in the presence of the antagonist plus 100–300 nM 5-HT. (A) Representative recordings at −45 mV in different neurons exposed to vehicle/5-HT or GR113808/5-HT. Note the two APs (OA) and large DSFs in the left panel. MP = membrane potential. (B) 5-HTR antagonist effects on the incidence of OA during exposure to 5-HT. OA was measured at RMP for ≥60 s and then during depolarization to −45 mV for 30 s. *** p < 0.00025 (after Bonferroni correction for 4 comparisons). (C-E) 5-HTR antagonist effects on the frequency of medium- (3–5 mV) and large-amplitude (> 5 mV) DSFs during 30-s depolarization to −45 mV (C), AP threshold (D), and rheobase (E) in the presence of 5-HT. Mean ± SEM. * Dunn’s adjusted p value < 0.05. ** p < 0.01. (F) PKA activation measured by PKA-pRII staining in rat DRG neurons. Different concentrations of antagonists were applied 30 min prior to 5-min co-application of the antagonist and 100 nM 5-HT. Each data point represents the mean ± SEM of 3–5 separate experiments. **** Dunnett’s adjusted p value < 0.0001. (G) Representative traces of Fura-2 ratio changes used to indicate increases in [Ca²⁺]_i during superfusion of 300 nM or 10 μM 5-HT in single capsaicin-sensitive neurons. (H) Incidence of 5-HT-associated increases in [Ca²⁺]_i as measured with Fura-2. Cap-sens = capsaicin sensitive. Cap-insens = capsaicin-insensitive. Lo 5-HT = 300 nM 5-HT. Hi 5-HT = 10 μM 5-HT. Cap = 500 nM capsaicin. Hi KCl = 60 mM KCl.