Fig. 4. Post-serotonin enhancement of the depolarizing response of a nonspiking interneurone (ns int) that showed a serotonin-mediated hyperpolarization during sensory stimulation. Bath application of 100 µmol l^-1 serotonin for 3 min mediated a hyperpolarization of the membrane potential of the interneurone shown in Fig. 2A. The amplitude of the depolarizing postsynaptic potentials in the interneurone elicited by sensory stimulation at 20 Hz of 11 stimuli (indicated by arrows) increased after serotonin perfusion. The time indicated on each trace (A—D) shows the elapsed time after serotonin application. The dashed line indicates the resting membrane potential level; red mn, reductor motor neurone.