Sodium Currents in Dissociated Bull-frog Sympathetic Neurones

1. Sodium currents were recorded from the cell bodies of single dissociated sympathetic neurones of bull-frogs, using patch electrodes in the whole-cell configuration, in Cs+-loaded cells, using external Mn2+ to block calcium currents. 2. A discontinuous single-electrode voltage-clamp method was used. Switching frequencies of 40-50 kHz were possible with 0.5-2 M omega electrodes, giving clamp settling times of approximately 0.2 ms and adequate clamp of currents up to 30 nA. 3. The sodium currents required unusually positive voltages for both activation and inactivation, with half of the maximal observed conductance activating at +2 mV, and half-maximal steady-state inactivation at -35 mV. Both fast (in the order of milliseconds) and slow (in the order of seconds) inactivation processes occurred. 4. Two pharmacologically and kinetically distinct sodium currents were observed. The larger current was blocked by tetrodotoxin (TTX) and saxitoxin (STX) with I50 values (i.e. the concentration which results in 50% inhibition) of 10 nM or lower, and activated and inactivated relatively rapidly. 5. A smaller current (approximately 25% of peak current) was blocked by 0.1-1 microM-STX but not by 1-10 microM-TTX. It also activated rapidly, but inactivated approximately 3-fold more slowly than the larger current. The slower current was blocked 75-90% by Cd2+ (50-200 microM).