Fig. 6. Post-serotonin enhancement of the hyperpolarizing postsynaptic potentials of a nonspiking interneurone (ns int) during sensory stimulation. (A) Bath application of 100 µmol l^-1 serotonin (5-HT) for 3 min mediated a hyperpolarization of the interneurone. After serotonin perfusion, the amplitude of hyperpolarizing postsynaptic potentials of the interneurone during sensory stimulation at 20 Hz of 11 stimuli (indicated by arrows) initially decreased in association with a serotonin-mediated hyperpolarization of the membrane. After approximately 10 min of washing, the amplitude of the evoked hyperpolarizing postsynaptic potentials increased, even though the membrane was still hyperpolarized. The time indicated on each trace (Ai—v) shows the elapsed time after serotonin application. The dashed line indicates the resting membrane potential level. (B) The amplitude of hyperpolarizing postsynaptic potentials of the interneurone during sensory stimulation at 20 Hz of 11 stimuli (indicated by arrows) increased after serotonin-mediated depolarization. red mn, reductor motor neurone.