Fig. 3. Serotonin-mediated depolarization of a nonspiking interneurone (ns int). (A,B) Effect of current injected into a nonspiking interneurone upon the reductor motor neurone (red mn). The passage of a 2 nA depolarizing current into the interneurone decreased the tonic spike frequency of the motor neurone (A) while the injection of a 1 nA hyperpolarizing current had little effect upon the motor neurone (B). (C) Bath application of serotonin (5-HT; 100 µmol l^-1) for 5 min (indicated by thick bar) decreased the number of tonically occurring spikes of the motor neurone (impulses s^-1; ) and elicited a depolarization of the nonspiking interneurone ([UNK]). (D) Temporal relationship between the serotonin-mediated depolarization of the interneurone and the effect of hyperpolarizing current injected into the interneurone upon the motor neurone. The passage of a 1 nA hyperpolarizing current into the interneurone increased the tonic spike level of the motor neurones only during serotonin-mediated depolarization. The time indicated on each trace (Di—iii) shows the elapsed time after serotonin application. The dashed line indicates the resting membrane potential level.