Intracellular microelectrode recordings were made from the circular muscle anal of the site of distension (Upper panel). A, Balloon distension evokes a burst of IJPs/hyperpolarization in the circular muscle of Tph1+/+. B, Balloon distension evokes a smaller response after ondansetron (3 μM) in Tph1+/+. C, Response to balloon distension in Tph1−/− colon. D, Response to balloon distension in Tph1−/− colon after ondansetron (3 μM). E, Plot summarizing area, peak amplitude and duration of the hyperpolarization activated by balloon distension in Tph1+/+ and Tph1−/− colon, before and after ondansetron and following the further addition of hexamethonium (100 μM). Note that ondansetron and hexamethonium also reduced the duration of the inhibitory response in Tph1−/− colon. **P < 0.01, ***P < 0.001.