Large-conductance calcium-activated potassium channels (BK channels) have been suggested to play a substantial role in synaptic transmission in the spinal cord dorsal horn. In the present experiments, we attempted to clarify the physiological significance of BK channels in the modulation of synaptic transmission in the superficial dorsal horn where nociceptive information is processed. Spontaneously occurring excitatory postsynaptic currents (sEPSCs) were recorded from the neurons located in the superficial dorsal horn of a mouse spinal cord slice, and the effects of iberiotoxin, a BK channel blocker, on sEPSCs were analyzed. The frequency of sEPSCs was significantly higher in the peripheral nerve-ligated neuropathic mice than in the sham-operated control mice, but the amplitude of sEPSCs was equivalent between the two groups. Iberiotoxin increased the frequency of sEPSCs in the control mice to the same level as that in the neuropathic mice without affecting the amplitude of sEPSCs. In contrast, iberiotoxin did not show any significant effects on the sEPSCs in the neuropathic mice. These findings suggest that the BK channels that are located in presynaptic terminals control synaptic transmission in the superficial dorsal horn, and that functional downregulation of BK channels accompanies the neuropathic pain induced by peripheral nerve injury. This downregulation was confirmed by real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis of the BK channel alpha subunit. Taken together, our present results indicate that BK channels play crucial roles in the synaptic transmission of nociceptive information in the superficial dorsal horn.