KINETICS OF SLOW INACTIVATION OF PERSISTENT SODIUM CURRENT IN LAYER V NEURONS OF MOUSE NEOCORTICAL SLICES. Fleidervish, I. A. and M. J. Gutnick. Department of Physiology, Faculty of Health Sciences, and Zlotowski Center for Neuroscience, Ben-Gurion University of the Negev, Beersheva 84105, Israel.
APStracts 3:0105N, 1996.
1. In whole cell recordings from Layer V neurons in slices of mouse somatosensory neocortex, TTX-sensitive persistent Na + current (I NaP ) was studied by blocking K + currents with intracellular Cs + and Ca ++ currents with extracellular Cd ++ . During slow voltage ramps, I NaP began to activate at around -60 mV, and attained a peak at around -25 mV. The peak amplitude of I NaP varied widely from cell to cell (range from 60 to 3160 pA; median 308 pA, n=77). At potentials more positive than -35 mV, I NaP in all cells was superimposed on a large, TTX-resistant outward current. 2. In hybrid clamp experiments, I NaP was significantly reduced by a preceding high frequency train of spikes. 3. I NaP underwent pronounced slow inactivation, which was revealed by systematically varying the ramp speed between 233 and 2.33 mV/s, or varying the duration of a depolarizing prepulse between 0.1 and 10 s. 4. Onset of slow inactivation at +20 mV was monoexponential with _=2.06 s (n=17 cells). Recovery from slow inactivation was voltage dependent. It followed a monoexponential time course with _=2.31 s (n=6) at -70 mV and _=1.10 s (n=4) at -90 mV. These values are not significantly different than values previously reported for slow inactivation of fast-inactivating I Na . 5. Slow inactivation of neocortical I NaP will influence all neuronal functions in which this current plays a role, including spike threshold determination, synaptic integration and active propagation in dendrites. The kinetics of slow inactivation suggest that it may be a factor not only during extremely intense spiking, but also during periods of "spontaneous" activity.

Received 25 March 1996; accepted in final form 21 May 1996.
APS Manuscript Number J248-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 June 96