Regulation of excitability in tonic firing substantia gelatinosa neurons of the spinal cord by small-conductance Ca(2+)-activated K(+) channels.

The excitability of substantia gelatinosa (SG) neurons in the spinal dorsal horn determines the processing of nociceptive information from the periphery to the central nervous system. Small conductance Ca(2+)-activated K(+) (SK) channels on neurons supply strong negative feedback control on neuronal excitability by affecting afterhyperpolarization (AHP). However, the role of SK channels in regulating tonic-firing SG neuron excitability remains elusive. In the present study, whole-cell recordings were conducted in SG neurons from acute spinal cord slices of adult rats. The SK channel opener 1-ethyl-2-benzimidazolinone (1-EBIO) attenuated spike discharges and increased AHP amplitudes; this effect was mimicked by a high Ca(2+) external solution. Systemic administration of 1-EBIO attenuated the thermal-induced nociception behavior. Conversely, the inhibition of SK channels with apamin, a specific SK channel inhibitor, increased neuronal excitability and decreased the AHP amplitudes; this effect was mimicked by a Ca(2+)-free external solution. Apamin increased excitatory synaptic transmission by increasing the amplitudes of evoked excitatory postsynaptic potentials (eEPSPs). This facilitation depended on N-methyl-d-aspartate (NMDA) receptors, extracellular Mg(2+) and intracellular Ca(2+). Voltage-gated Ca(2+) channels (VGCCs) were also involved in the apamin-induced effects. Strikingly, 1-EBIO action on decreasing excitability persisted in the presence of apamin, indicating that 1-EBIO manipulates SK channels via a pathway rather than via apamin-sensitive SK channels. The data reveal a previously uncharacterized mechanism for manipulating SG neuronal excitability by Ca(2+) conductances via both apamin-sensitive and apamin-insensitive pathways. Because SG neurons in the dorsal horn are involved in regulating nociception, manipulating neuronal excitability via SK channels indicates a potential therapeutic target.