Peak current recovery as a function of extracellular K⁺ concentrations. (A) Original trace of a representative experiment using a K⁺-free external solution containing 35 mM Na⁺. Insets: 5-HT–induced peak currents 20 s (left) and 80 s (right) after application of 30 µM PAL-1045. The black bars indicate the 200-ms 5-HT application. (B) Original trace of a representative experiment using an external solution containing 35 mM Na⁺ and 70 mM K⁺. K⁺ was applied for 15 s after the PAL-1045 pulse. The external Na⁺ concentration (35 mM) was held constant throughout the recording. Inset: 5-HT–induced peak current after application of 70 mM K⁺ for 15 s and a wash period of 5 s with 35 mM Na⁺. Note the outward current component, which was not seen in the absence of prior K⁺. (C) Peak current traces taken from five experiments per experimental condition (the five experimental conditions are indicated on the left). The black traces represent the means of individual traces shown in gray. The black bars indicate the application of 5-HT for 200 ms. The left- and right-hand traces are 5-HT–induced currents 20 s and 80 s after PAL-1045 application, respectively.