Fig. 4. Serotonin (5-HT) potentiates both IJP amplitude and the size of neurally evoked relaxations. (A) Intracellular recordings of IJPs (left) evoked at 2°C by a stimulus train (7 Hz, 700 ms) delivered to the inhibitory (OI) motoneuron in the absence (control) and in the presence of 100 nmol l^–1 5-HT. 5-HT depolarized the resting potential of this muscle fiber at 2°C from –66 mV to –60 mV. Corresponding relaxations of muscle tension are shown at right. Both intracellular and tension traces are averaged from five stimulus trials. (B) Plot of IJP amplitude (squares) and neurally evoked relaxations (triangles) as a function of temperature in the absence (open symbols) and presence (solid symbols) of 5-HT. Data in B are from the same experiment as in A. The reversal potential of the IJPs in this experiment was 4.3°C under control conditions and 9°C in the presence of 5-HT. (C) Intracellular recording of IJPs from the same muscle fiber at 6°C under control conditions (upper trace) and in the presence of 5-HT; data are from the same experiment as in A and B. (D) Data pooled from four experiments; symbols represent the amplitude of relaxations evoked by stimulus trains delivered to the inhibitory (OI) motoneuron in the absence (open symbols) and the presence (solid symbols) of 5-HT. At each temperature N=4 except as follows: in control data N=3 at 10°C and N=1 at 12°C; in 5-HT data N=2 for 8°C, 10°C and 12°C. Asterisks indicate values that are significantly different from controls (paired t-test, P<0.05).