Contribution of persistent sodium currents to the excitability of tonic firing substantia gelatinosa neurons of the rat

The roles of persistent Na⁺ currents (I_NaP) in intrinsic membrane properties were examined in rat substantia gelatinosa (SG) neurons of the trigeminal subnucleus caudalis using a conventional whole-cell patch clamp technique. In a voltage-clamp mode, riluzole inhibited the slow voltage ramp-induced I_NaP but had little effect on the peak amplitude of transient Na⁺ currents in SG neurons. In a current-clamp mode, most SG neurons exhibited spontaneous action potentials and tonic firing pattern. Riluzole reduced both spontaneous and elicited action potentials in a concentration-dependent manner. The present results suggest that the riluzole-sensitive I_NaP plays an important role in the excitability of SG neurons and are thus, likely to contribute to the modulation of nociceptive transmission from the orofacial tissues.