Fig. 8.

Responses of submucosal neurons to mucosal applications of 5-HT are not inhibited by antagonism of 5-HT₃ receptors but are antagonized by high concentrations of tropisetron. Responses of four cells (A–D) from different animals are illustrated. A, Fast EPSPs and an associated burst of action potentials are evoked by mucosal application of 5-HT (↑). These responses are not inhibited by 0.1 μm tropisetron (T).B, In another cell, fast EPSPs and associated action potentials are evoked by mucosal application of 5-HT (↑). These responses are not inhibited by 1 μm ondansetron (O), but they are abolished by tropisetron (10 μm). C, A slow EPSP is evoked in a submucosal neuron by mucosal 5-HT (↑). This response is blocked by 10 μm tropisetron. D, Mucosal 5-HT (↑) elicits a prolonged slow EPSP superimposed on which are fast EPSPs and associated action potentials. Neither the fast nor the slow EPSPs are inhibited by ondansetron (O), but they are reversibly inhibited by a high concentration of tropisetron (3 μm). After the washout of tropisetron (W), both fast and slow EPSPs return. Fast but not slow EPSPs are abolished by hexamethonium (C6).