Comparison of two types of Na+ currents with low-voltage-activated T-type Ca2+ current in newborn rat dorsal root ganglia.

Na+ currents and the low-voltage-activated T-type Ca2+ current (T-ICa) were recorded from neurons of rat dorsal root ganglia under similar ionic environments using the whole-cell patch-clamp technique. Two types of Na+ currents were identified on the basis of their sensitivity to tetrodotoxin (TTX) and channel kinetics. One type was blocked by 1 nM TTX and had a faster activation and inactivation time courses (F-INa), while the other type was insensitive to 100 microM TTX and had a much slower channel kinetics (S-INa). Activation thresholds were -60, -40 and -70 mV for F-INa, S-INa and T-ICa, respectively. Peak amplitudes were obtained in respective current/voltage curves at -30 mV (F-INa), 0 mV (S-INa) and -50 mV (T-ICa). The time to peak and the decay time constant measured at potential levels giving peak amplitudes were 0.5 and 1.5 ms for F-INa, 1.4 and 2.9 ms for S-INa and 8.1 and 17 ms for T-ICa, respectively. Cd2+ in a concentration of 50 microM totally blocked T-ICa, whereas it had no effect on either type of Na+ current. T-INa was found in 18 out of 25 cells which possessed F-INs, whereas it was found in only 2 cells among 15 which lacked F-INa. These three types of inward currents having different kinetic and pharmacological properties may mediate diverse functional roles in processing sensory signals.