Voltage-activated K+ currents of hypoglossal motoneurons in a brain stem slice preparation from the neonatal rat.

Whole cell, patch-clamp recordings were performed on motoneurons of the hypoglossus nucleus in a brain stem slice preparation from the neonatal rat brain. The aim was to investigate transient outward currents activated by membrane depolarization under voltage clamp conditions. In a Ca2+-free medium containing tetrodotoxin and Cs+, depolarizing voltage commands from a holding potential of -50 mV induced slow outward currents (Islow) with 34 +/- 6 ms (SE) onset time constant at 0 mV and minimal decline during a 1 s pulse depolarization. When the depolarizing command was preceded by a prepulse to -110 mV, the outward current became biphasic as it comprised a faster component (Ifast), which could be investigated in isolation by subtracting the two sets of records. Ifast showed rapid kinetics (9 +/- 4 ms 10-90% rise time and 70 +/- 20 ms decay time constant at 0 mV) and strong voltage-dependent inactivation (half inactivation was at -92.9 +/- 0.2 mV) from which it readily recovered with a biexponential timecourse (4.4 +/- 0.6 and 17 +/- 2 ms time constants at -110 mV membrane potential). Islow was selectively blocked by TEA (10-30 mM) while Ifast was preferentially depressed by 2-3 mM 4-aminopyridine. Analysis of tail current reversal indicated that both Islow and Ifast were predominantly due to K+ with minor permeability to Na+ (92/1 and 50/1, respectively). These results suggest that membrane depolarization activated distinct K+ conductances that, in view of their largely dissimilar kinetics, are likely to play a differential role in regulating the firing behavior of hypoglossal motoneurons.