FUNCTIONAL EXPRESSION OF VOLTAGE-DEPENDENT SODIUM-CHANNELS IN XENOPUSOOCYTES INJECTED WITH MESSENGER-RNA FROM NEONATAL OR ADULT-RAT BRAIN

The two electrode voltage clamp technique was used to study voltage-de pendent sodium currents (I-Na) in Xenopus laevis oocytes previously in jected with mRNA extracted from adult (A) or neonatal (N, < 5 days old ) rat brains. In the presence of niflumic acid (300 mu M) to block end ogenous Ca2+-activated Cl- currents, depolarizing voltage steps from a holding potential of -100 mV to various voltages elicited in both gro ups of oocytes fast inward sodium currents which peaked at similar to 0 mV and then slowly declined to similar to 75% of the maximum current at +40 mV. At the peak, A I-Na was significantly larger than N I-Na ( 296 +/- 59 nA vs. 147 +/- 32 nA). Inactivation kinetics of N I-Na was best fit with one exponential component whereas A I-Na with two expone ntial components. A significant difference in the voltage dependence o f inactivation was found between A I-Na or N I-Na. The values of V-h w ere -53 +/- 0.9 mV or -59.8 +/- 0.7 mV for A I-Na or N I-Na respective ly. The recovery from inactivation was fitted in both groups with two exponential functions (tau(f) and tau(s),) whose values were not signi ficantly different. However the ratio between tau(f), and tau(s), was significantly higher for N I-Na comparing to A I-Na (5.7 vs. 2.1). TTX reversibly blocked I-Na. The IC50 value was 58.2 +/- 6.3 nM for A I-N a and 20.4 +/- 2.2 nM for N I-Na These results suggest that different isoforms of TTX-sensitive, voltage-dependent sodium channel subunits a re functionally expressed, may be in different proportions in oocytes injected with A or N mRNA.