CHARACTERISTICS OF A FAST TRANSIENT OUTWARD CURRENT IN GUINEA-PIG TRIGEMINAL MOTONEURONS

A fast transient voltage dependent outward current (TOC) in trigeminal motoneurons (TMNs) was studied in guinea pig brainstem slices by use of sharp electrodes in combination with single electrode voltage clamp techniques. In solutions containing TTX, low Ca2+/Mn2+ and 20 mM TEA this current activated around -55 to -60 mV from holding potentials ne gative to resting potential, obtained its peak amplitude within 5 ms a nd decayed as a single exponential with a time constant of 6-8 ms. Hal f maximal values for inactivation and activation were -72 and -37 mV, respectively. Bath application of 5 mM 4-AP suppressed this current by approximately 90% and eliminated the early depolarizing transient mem brane rectification observed in response to a constant depolarizing cu rrent pulse, prolonged the action potential duration, and reduced the threshold voltage and delay to onset of the action potential. It is su ggested that this current resembles the typical A-current observed in many CNS neurons and, as a result of its voltage and time dependent pr operties, could contribute to control of motoneuronal discharge and ti ming of burst onset during rhythmical jaw movements. Therefore, any ce llular models of masticatory activity should include the properties of this current.