In vivo whole-cell voltage-clamp recordings were made from substantia gelatinosa (SG, lamina II of the spinal cord) neurons to analyze modality-dependent excitatory and inhibitory synaptic responses and their receptive fields. All SG neurons examined exhibited spontaneous excitatory postsynaptic currents (EPSCs) at a holding potential of −70 mV. Pinch and touch stimuli applied to the skin of the hind limb elicited a barrage of EPSCs. The spontaneous and evoked EPSCs were blocked by superfusing CNQX, a non-NMDA receptor antagonist. At a holding potential of 0 mV, all SG neurons exhibited spontaneous inhibitory postsynaptic currents (IPSCs) which were inhibited by either bicuculline, a GABAA receptor-antagonist, or strychnine, a glycine-receptor antagonist. In most neurons, touch stimuli produced a barrage of IPSCs. These evoked IPSCs were inhibited either by superfusing strychnine or bicuculline. When EPSCs and IPSCs were recorded from the same neurons, the size of receptive fields of evoked IPSCs was larger than that of evoked EPSCs. These results suggest that innocuous mechanical stimulation applied to the skin adjacent to a painful point inhibits mechanical noxious transmission in the SG.
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