5-HT reduces the amplitude of dorsal root-evoked monosynaptic EPSPs. A, Compound monosynaptic EPSPs were elicited in giant interneurons (G-IN) by stimulation of an ipsilateral dorsal root (DR). B, The amplitude of the chemical component of dorsal root-evoked EPSP was depressed by 5-HT (5 μm), whereas the electrical component was not significantly affected. Traces are averages of 30 sweeps. Inset, No change was observed in the input resistance of the postsynaptic neuron during application of 5-HT.C, The effect of 5-HT showed a fast onset and required a long washout before recovery. D, Average effect of 5-HT on dorsal root-evoked EPSPs in six different giant interneurons. Theasterisks indicate statistical significance (p < 0.0001).