Development of resurgent and persistent sodium currents in mesencephalic trigeminal neurons

Sodium channels play multiple roles in the formation of neural membrane properties in mesencephalic trigeminal (Mes V) neurons and in other neural systems. Mes V neurons exhibit conditional robust high-frequency spike discharges. As previously reported, resurgent and persistent sodium currents (I-NaR and I-NaP, respectively) may carry small currents at subthreshold voltages that contribute to generation of spike firing. These currents play an important role in maintaining and allowing high-frequency spike discharge during a burst. In the present study, we investigated the developmental changes in tetrodotoxin-sensitive I-NaR and I-NaP underlying high-frequency spike discharges in Mes V neurons. Whole-cell patch-clamp recordings showed that both current densities increased one and a half times from postnatal day (P) 0-6 neurons to P7-14 neurons. Although these neurons do not exhibit subthreshold oscillations or burst discharges with high-frequency firing, I-NaR and I-NaP do exist in Mes V neurons at P0-6. When the spike frequency at rheobase was examined in firing Mes V neurons, the developmental change in firing frequency among P7-14 neurons was significant. I-NaR and I-NaP density at -40 mV also increased significantly among P7-14 neurons. The change to an increase in excitability in the P7-14 group could result from this quantitative change in I-NaP. In neurons older than P7 that exhibit repetitive firing, quantitative increases in I-NaR and I-NaP density may be major factors that facilitate and promote high-frequency firing as a function of age in Mes V neurons.