1. The whole-cell patch-clamp technique was used to investigate the characteristics of two types of sodium current (I(Na)) recorded at room temperature from small diameter (13-25 muM) dorsal root ganglion (DRG) cells, isolated from adult rats and maintained overnight in culture. 2. Sodium currents were isolated pharmacologically. Internal Cs+ and external tetraethylammonium (TEA) ions were used to suppress potassium currents. A combination of internal EGTA, internal F-, a low (10 muM) concentration of external Ca2+ and a relatively high (5 mm) concentration of internal and external Mg2+ was used to block calcium channels. The remaining voltage-dependent currents reversed direction at the calculated sodium equilibrium potential. Both the reversal potential and magnitude of the currents exhibited the expected dependence on the external sodium concentration. 3. I(Na) subtypes were characterized initially in terms of their sensitivity to tetrodotoxin (TTX). TTX-sensitive (TTX(s)) currents were at least 97 % suppressed by 0.1 muM TTX. TTX-resistant (TTX(r)) I(Na) were recorded in the presence of 0.3 muM TTX and appeared to be reduced in amplitude by less than 50 % in 75 muM TTX (n = 1). 4. As in earlier studies, the peak of the current-voltage relationship, the mid-point of the normalized conductance curve and the potential (V(h)) at which the steady-state inactivation parameter (h(infinity)) was 0.5 were found to be significantly more depolarized for the TTX(r) I(Na) (by ca 10, 14 and 37 mV respectively). There was little difference in the slope at the mid-point of the normalized conductance curves (the mean slope factors were 5.1 mV for the TTX(s) I(Na) and 4.9 mV for the TTX(r) current) but the h(infinity) curves for TTX(r) currents were significantly steeper than those for TTX(s) currents (mean slope factors of 3.8 and 11.5 mV respectively). Both the time to peak and the decay time constant of the peak current recorded from a holding potential of - 67 mV were more than a factor of three slower for the TTX(r) I(Na) than for the TTX(s) current. 5. However, in direct contrast to the difference in activation and decay kinetics, 'slow' TTX(r) I(Na) recovered from inactivation at - 67 mV, or reprimed, more than a factor of ten faster than 'fast' TTX(s) I(Na). 6. The differences apparent in both the repriming kinetics of TTX(s) and TTX(r) I(Na) at - 67 mV and the kinetics of the decay phase of the peak I(Na) are shown to be explicable largely in terms of the voltage dependence of their respective inactivation systems. 7. The possible fundamental importance to DRG cells of such large differences in the voltage dependence of the inactivation systems (both resting inactivation and repriming kinetics) of the two sodium channel subtypes will be discussed.
