Electrophysiological properties of sodium current subtypes in small cells from adult rat dorsal roof ganglia

1. Whole-cell and single-channel Na+ currents were recorded from small (ca. 20 mu m diameter) cells isolated from adult rat dorsal root ganglia (DRG). Currents were classified by their sensitivity to 0.3 mu M tetrodotoxin (TTX), electrophysiological properties and single-channel amplitude. Cells were classified according to the types of current recorded from them. 2. Type A cells expressed essentially pure TTX-sensitive (TTX-S) currents. Availability experiments with prepulse durations between 50 ms and 1 s gave a half-available voltage (V-h) of around -65 mV but the availability curves often had a complex shape, consistent with multiple inactivation processes. Measured inactivation time constants ranged from less than 1 ms to over 100 s, depending on the protocol used. 3. Cell types B and C each had, in addition to TTX-S currents, substantial and different TTX-resistant (TTX-R) currents that we have designated TTX-R1 and TTX-R2, respectively. TTX-R1 currents had a 1 s V-h of -29 mV, showed little 1 Hz use dependence at -67 mV and recovered from the inactivation induced by a 60 ms depolarizing pulse with time constants of 1.6 ms (91 %) and 908 ms. They also exhibited slow inactivation processes with component time constants around 10 and 100 s. TTX-R2 currents activated and inactivated at more negative potentials (1 s V-h = -46 mV), showed substantial 1 Hz use dependence and had inactivation (60 ms pulse) recovery time constants at -67 mV of 3.3 ms (58 %) and 902 ms. 4. Type D cells had little or no current in 0.3 mu M TTX at a holding potential of -67 mV. Current amplitude increased on changing the holding potential to -107 mV. Type D cell currents had more hyperpolarized availability and I-V curves than even TTX-R2 currents and suggest the existence of TTX-R3 channels. 5. In outside-out patches with 250 mM external NaCl, the single-channel conductance (gamma) of TTX-S channels was 19.5 pS and the potential for half-maximal activation (V-a) was -45 mV. One population of TTX-R channels had a gamma of 9.2 pS and a V-a of -27 mV. A second population had a gamma of 16.5 pS and a more negative V-a of -42 mV. The latter population may underlie the type D cell current. 6. Small DRG cells express multiple Na+ currents with varied time constants and voltage dependences of activation and inactivation. Nociceptive cells still fire when chronically depolarized by an increased external K+ concentration. TTX-R1 and TTX-R2 Na+ channels may support that firing, while the range of inactivation time constants described here would increase the repertoire of DRG cell burst firing behaviour generally.