Fig. 3. (A—D) The time course of the transepithelial potential (TEP) during injection of 50 nmol of serotonin. The sections indicated in A are shown on an enlarged time scale in B—D. The TEP oscillation before drug injection was asymmetrical, exhibiting a shoulder (arrowhead) after the positive peak (B). After the injection, the oscillation was transiently suppressed and then gradually recovered to approximately two-thirds of the original peak-to-peak amplitude. After recovery, the oscillation was more regular than before, and the shoulder was absent (C). Five hours after the injection, the peak-to-peak amplitude had fully recovered, but the shoulder was not as prominent as before (D). The animal exhibited flight activity only transiently immediately after the drug injection.