Low-Threshold, Persistent Sodium Current in Rat Large Dorsal Root Ganglion Neurons in Culture

Abstract

Baker, Mark D. and Hugh Bostock. Low-threshold, persistent sodium current in rat large dorsal root ganglion neurons in culture. J. Neurophysiol. 77: 1503–1513, 1997. Dorsal root ganglion neurons from adult rats (≥200 g) were maintained in culture for between 1 and 3 days. Membrane currents generated by large neurons (50–75 μm apparent diameter) were recorded with the whole cell patch-clamp technique. Large neurons generated transient Na⁺ currents and at least two types of inward current that persisted throughout 200-ms voltage-clamp steps to +20 mV. One persistent current activated close to −35 mV (high threshold), whereas in about half of the cells another persistent current began to activate negative to −70 mV (low threshold). The high-threshold persistent current was identified as a Ca²⁺ current, as previously described in these neurons. The low-threshold current was reversibly suppressed either by replacing external Na⁺ with tetramethylammonium ions or by reducing external Na⁺ concentration ([Na⁺]) and simultaneously raising external [Ca²⁺]. It was blocked by tetrodotoxin (TTX) with an apparent equilibrium dissociation constant in the single nanomolar range. We conclude that the low-threshold current is a TTX-sensitive, persistent Na⁺ current. The persistent TTX-sensitive current contributed to steady-state membrane current from at least −70 mV to 0 mV, a wider potential range than predicted by activation-inactivation gating overlap for transient Na⁺ current. Because of its low threshold and fast activation kinetics, the persistent Na⁺ current is expected to play an important role in determining membrane excitability.