Functional expression of voltage dependent sodium channels in Xenopus oocytes injected with mRNA from neonatal or adult rat brain

The two electrode voltage clamp technique was used to study voltage-dependent sodium currents (INa) in Xenopus laevis oocytes previously injected with mRNA extracted from adult (A) or neonatal (N, < 5 days old) rat brains. In the presence of niflumic acid (300 μM) to block endogenous Ca2+-activated Cl- currents, depolarizing voltage steps from a holding potential of -100 mV to various voltages elicited in both groups of oocytes fast inward sodium currents which peaked at ∼ 0 mV and then slowly declined to ∼ 75% of the maximum current at +40 mV. At the peak, A INa was significantly larger than N INa (296 ± 59 nA vs. 147 ± 32 nA). Inactivation kinetics of N INa was best fit with one exponential component whereas A INa with two exponential components. A significant difference in the voltage dependence of inactivation was found between A INa or N INa. The values of Vh were -53 ± 0.9 mV or -59.8 ± 0.7 mV for A INa or N INa respectively. The recovery from inactivation was fitted in both groups with two exponential functions (τf and τs) whose values were not significantly different. However the ratio between τf and τs was significantly higher for N INa comparing to A INa (5.7 vs. 2.1). TTX reversibly blocked INa. The IC50 value was 58.2 ± 6.3 nM for A INa and 20.4 ± 2.2 nM for N INa. These results suggest that different isoforms of TTX-sensitive, voltage-dependent sodium channel subunits are functionally expressed, may be in different proportions in oocytes injected with A or N mRNA. © 1995.