Voltage-activated Na+ and K+ channels were investigated in the soma membrane of motoneurones using the patch-clamp technique applied to thin slices of neonatal rat spinal cord. 2. One type of TTX-sensitive Na+ channel, with a conductance of 14.0 pS, was found to underlie the macroscopic Na+ conductance in the somata of motoneurones. These channels activated within a potential range between -60 and -20 mV with a potential of half-maximal activation (E50) of -38.9 mV and steepness factor (k) of 6.1 mV. 3. Kinetics of Na+ channel inactivation could be fitted with a single exponential function at all potentials investigated. The curve of the steady-state inactivation had the following parameters: a half-maximal potential (Eh,50) of -81.6 mV and k of -10.2 mV. 4. Kinetics of recovery of Na+ channels from inactivation at a potential of -80 mV were double exponential with fast and slow components of 16.2 (76%) and 153.7 ms (24%), respectively. It is suggested that the recovery of Na+ channels from inactivation plays a major role in defining the limiting firing frequency of action potentials in motoneurones. 5. Whole-cell K+ currents consisted of transient (A)- and delayed-rectifier (DR)-components. The A-component activated between -60 and +20 mV with an E50 of -33.3 mV and k of 15.7 mV. The curve of steady-state inactivation was best fitted with an Eh,50 of -82.5 mV and k of -10.2 mV. The DR-component of K+ current activated smoothly at more positive potentials. E50 and k for DR-currents were +1.4 and 16.9 mV, respectively. 6. The most frequent single K+ channel found in the somata of motoneurones was the fast inactivating A-channel with a conductance of 19.2 pS in external Ringer solution. In symmetrical high-K+ solutions the conductance was 50.9 and 39.6 pS for inward and outward currents, respectively. The channel activation took place between -60 and +20 mV. The curve of steady-state inactivation of single A-channels had an Eh,50 of -87.1 mV and k of -12.8 mV. In high-Ko+ solution A-channels demonstrated a rapid deactivation at potentials between -110 and -60 mV. The time constant of the channel deactivation depended on the membrane potential and changed from 1.5 ms at -110 mV to 6.3 ms at -60 mV. 7. Delayed-rectifier K+ channels were found in the soma membrane at a moderate density.(ABSTRACT TRUNCATED AT 250 WORDS)
