THREE KINETICALLY DISTINCT CA++-INDEPENDENT DEPOLARIZATION-ACTIVATED K+CURRENTS IN CALLOSAL-PROJECTING RAT VISUAL CORTICAL NEURONS Rachel E. Locke and Jeanne M. Nerbonne From.
Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St. Louis, MO 63110 USA APStracts 4:118N, 1997.
ABSTRACT
Whole-cell, Ca++-independent, depolarization-activated K+ currents were characterized in identified callosal-projecting (CP) neurons isolated from postnatal day 7 to 16 rat primary visual cortex. CP neurons were identified in vitro following in vivo retrograde labeling with fluorescently tagged latex microbeads. During brief (160 ms) depolarizing voltage steps to potentials between -50 and +60 mV, outward K+ currents in these cells activate rapidly and inactivate to varying degrees. Three distinct K+ currents were separated based on differential sensitivity to 4-aminopyridine (4-AP); these are referred to here as IA, ID and IK, because their properties are similar (but not identical) K+ currents termed IA, ID and IK in other cells. The current sensitive to high (ò 100 æM) concentrations of 4-AP (IA) activates and inactivates rapidly; the current blocked completely by low (ó 50 æM) 4-AP (ID) activates rapidly and inactivates slowly. A slowly activating, slowly inactivating current (IK) remains in the presence of 5 mM 4-AP. IA, ID and IK were also separated and characterized in experiments that did not rely on the use of 4-AP. All CP cells express all three K+ current types, although the relative densities of IA, ID and IK vary among cells. The experiments here have also revealed that IA, ID and IK display similar voltage dependences of activation and steady-state inactivation, whereas the kinetic properties of the currents are distinct. At +30 mV, for example, mean ñ SD activation taus are 0.83 ñ 0.24 ms for IA, 1.74 ñ 0.49 ms for ID, and 14.7 ñ 4.0 ms for IK. Mean ñ SD inactivation taus for IA and ID are 26 ñ 7 ms and 569 ñ 143 ms, respectively. Inactivation of IK is biexponential with mean ñ SD inactivation time constants of 475 ñ 232 ms and 3128 ñ 1328 ms; approximately 20 % of the 4-AP-insensitive current is noninactivating. For all three components, activation is voltage-dependent, increasing with increasing depolarization, whereas inactivation is voltage-independent. Both IA and IK recover rapidly from steady-state inactivation with mean ñ SD recovery time constants of 38 ñ 7 ms and 79 ñ 26 ms, respectively; ID recovers an order of magnitude more slowly (588 ñ 274 ms). The properties of IA, ID and IK in CP neurons are compared to those of similar currents described previously in other mammalian central neurons and, in the accompanying manuscript (Locke & Nerbonne, 1997), the roles of these conductances in regulating the firing properties of CP neurons are explored. 

Received 3 September 1996; accepted in final form 30 June 1997.
APS Manuscript Number J705-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 24 July 1997