Single voltage‐activated Na+ and K+ channels in the somata of rat motoneurones.

Abstract

1. 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 Nae 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 halfmaximal activation (E50) of -38-9 mV and steepness factor (k) of 6-1 mV.3. Kinetics of Nae 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 (E,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 E 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 +P14 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 Eho50 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. The channel conductance in Ringer solution was 10-2 pS and in symmetrical high-K+ solutions was 31P1 and 22-5 pS for inward and outward currents, respectively. The activation of the channels took place at -60 to 0 mV with an E50 of -43-8 mV and k of 8-5 mV. In external high-K+ solution DR-channels showed a slow deactivation with a time constant of 5-9 ms at -110 mV and 60-0 ms at -60 mV. 8. Tetraethylammonium suppressed both A-and DR-components of whole-cell K+ conductance and reduced the amplitudes of the single-channel currents. The concentration giving 50% inhibition (IC50) values was 14 8 and 0 8 mm, respectively.9. ITIX-sensitive Nae channels, together with A-and DR-types of K+ channels, form the basis of voltage-activated conductance i...

Properties and functions of Na(+)‐activated K+ channels in the soma of rat motoneurones.

“…Activation of a KNa conductance was the simplest explanation for such an sAHP, since it disappeared together with trains of APs in the presence of TTX. In contrast to motoneurones (Safronov & Vogel, 1995), these two cell types were shown to possess persistent non-inactivating voltage-gated Na+ currents (Stafstrom, Schwindt, Chubb & Crill, 1985;Kubota & Saito, 1991), which could considerably increase an influx of Nae into the cell. Unfortunately, the reported sAHP could result not only from activation of KNa channels, but it could be alternatively explained by a regenerative process in dendrites (Dryer, 1994).…”

“…The preparation of spinal cord slices and the methods of motoneurone identification have been previously described (Edwards, Konnerth, Sakmann & Takahashi, 1989;Takahashi, 1990;Safronov & Vogel, 1995). In brief, 2-to 6-day-old rats were rapidly decapitated and the spinal cords cut out.…”

“…The mean lengths of the long and short diameters of the cells investigated here were 23-5 + 0*3 1izm (mean ±S.E.M., 52 cells) and the mean resting potential was -67'8 1P0 mV (mean + S.E.M., 43 cells). Experiments were performed by means of the patch-clamp technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981 Inside-out patches were studied in an additional small (0'2 ml) chamber as described elsewhere (Safronov & Vogel, 1995 (Wang, Kimitsuki & Noma, 1991). The channel PO did not depend on the holding potential in the range between -80 and +10 mV.…”

Na+‐activated K+ channels in small dorsal root ganglion neurones of rat

“…Investigation of inside-out patches was performed in an additional small 0·2 ml bath as described previously (Safronov & Vogel, 1995). This procedure allowed us to obtain several inside-out patches from the same slice without its destruction due to perfusion with internal solutions containing high concentrations of K¤ ions.…”

“…Single KNa channels were investigated in inside-out membrane patches in a small additional bath (Safronov & Vogel, 1995). The pipettes contained high-Kº or Ca¥-free o external solutions and the bath was perfused with internal solutions containing different Na¤ concentrations.…”