Three kinetically distinct Ca2+-independent depolarization-activated K+ currents in callosal-projecting rat visual cortical neurons - PubMed (original) (raw)

Three kinetically distinct Ca2+-independent depolarization-activated K+ currents in callosal-projecting rat visual cortical neurons

R E Locke et al. J Neurophysiol. 1997 Nov.

Abstract

Three kinetically distinct Ca2+-independent depolarization-activated K+ currents in callosal-projecting rat visual cortical neurons. J. Neurophysiol. 78: 2309-2320, 1997. Whole cell, Ca2+-independent, depolarization-activated K+ currents were characterized in identified callosal-projecting (CP) neurons isolated from postnatal day 7-16 rat primary visual cortex. CP neurons were identified in vitro after 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 (>/=100 mu M) concentrations of 4-AP (IA) activates and inactivates rapidly; the current blocked completely by low (</=50 mu M) 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 also were 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 also have 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 +/- SD activation taus are 0. 83 +/- 0.24 ms for IA, 1.74 +/- 0.49 ms for ID, and 14.7 +/- 4.0 ms for IK. Mean +/- SD inactivation taus for IA and ID are 26 +/- 7 ms and 569 +/- 143 ms, respectively. Inactivation of IK is biexponential with mean +/- SD inactivation time constants of 475 +/- 232 ms and 3,128 +/- 1,328 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 +/- SD recovery time constants of 38 +/- 7 ms and 79 +/- 26 ms, respectively; ID recovers an order of magnitude more slowly (588 +/- 274 ms). The properties of IA, ID, and IK in CP neurons are compared with those of similar currents described previously in other mammalian central neurons and, in the accompanying paper, the roles of these conductances in regulating the firing properties of CP neurons are explored.

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Figures

Fig. 1

Waveforms of Ca2+-independent, depolarization-activated K+ currents in callosal-projecting (CP) visual cortical neurons vary among cells. Whole cell K+ currents displayed in this and in subsequent figures were evoked as described in METHODS, unless stated otherwise. Currents plotted in A–C were recorded from 3 different CP cells isolated at postnatal day 9 ∼10 h after plating; the cells in A and C were in the same preparation.

Fig. 2

Increasing concentrations of 4-aminopyridine (4-AP) block increasing amounts of the peak, but not of the plateau, current. Currents were recorded in control bath solution and in the presence of varying concentrations of 4-AP. Current records displayed in A were obtained by digital subtraction of the current waveforms recorded in 4-AP from the control records in the same cell. Peak and plateau current amplitudes in individual cells were measured at 3 and 150 ms, respectively, after the onset of depolarizing voltages to +30 mV, and the percentages of the peak and plateau currents blocked by 4-AP were determined. A: representative waveforms of the currents blocked by 50 _μ_M (1), 100 _μ_M (2), 500 _μ_M (3), and 5 mM 4-AP (4). Note that the records in 1 and 4 are from the same cell; the records in 2 and 3 are from 2 other cells. B: mean ± SD percentages of the peak and plateau currents blocked by increasing concentrations of 4-AP. n values refer to the number of cells studied.

Fig. 3

Separation of 2 distinct 4-AP–sensitive current components from the total Ca2+ -independent, outward currents. After control currents (A) were recorded, the cell was 1st exposed to 50 _μ_M and then to 5 mM 4-AP. Currents recorded in the presence of 50 _μ_M and 5 mM 4-AP are displayed in B and C, respectively. Currents blocked by 50 _μ_M 4-AP (D) were obtained by subtraction of records in B from those in A. The 5 mM, but not 50 _μ_M, 4-AP–sensitive currents (E) were obtained by subtraction of records in C from those in B. Note that the currents blocked by low concentrations of 4-AP inactivate much more slowly than the currents sensitive only to the high concentration of 4-AP. Similar results were obtained on 13 cells.

Fig. 4

_I_A also can be isolated using a conditioning prepulse protocol. Currents were recorded during 160-ms voltage steps to potentials of 0, +20, +40, and +60 mV either directly from a holding potential of −70 mV (A) or after a 50-ms prepulse to 0 mV (B); the protocol is illustrated below the records. Current inactivated by the 50-ms prepulse to 0 mV (C) was obtained by subtraction of the records in B from those in A. Similar results were obtained in experiments conducted on 8 other cells.

Fig. 5

Rapidly activating, slowly inactivating K+ current (_I_D) also is blocked by _α_-dendrotoxin (_α_-DTX). Currents were recorded before (A) and after superfusion of 100 nM _α_-DTX (B). DTX-sensitive currents (C), obtained by subtraction of the records in B from those in A, activate rapidly and inactivate slowly, and are similar, therefore, to the currents blocked by 50 _μ_M 4-AP (Fig. 3D). Similar results were obtained on 10 other cells.

Fig. 6

Tetraethylammonium (TEA) blocks a slowly activating current (_I_K) in CP neurons. Currents were recorded under control conditions (A) and after superfusion of 30 mM TEA (B). Current blocked by TEA (C) was obtained by subtraction of records in B from those in A. Similar results were obtained on 7 other cells.

Fig. 7

Kinetics of activation (A and B) and inactivation (C and D) of _I_A, _I_D, and _I_K are distinct. A and B: time constants (τ) of activation for _I_A and _I_D were determined from single exponential fits to the rising phases of the currents in subtracted records, such as those in Fig. 3, D and E, respectively. For _I_K, activation t were determined from fits to the rising phases of currents remaining in 5 mM 4-AP in records such as those in Fig. 3C. Typical fits for _I_A, _I_D, and _I_K are illustrated in B. Mean ± SD activation _τ_s for _I_A, _I_D, and _I_K are plotted as a function of test potential (note that the _y_-axis scale is different for the _I_K data). Continuous lines are best fits to the averaged data using τ(V) = a [exp(_V_m/b)] + c (see text). C: time constants (_τ_s) of inactivation were determined from exponential fits to the decay phases of currents recorded during 12-s voltage steps to 0, 10, 20, and 30 mV from a holding potential of −70 mV in control bath solution and in the presence of 8 mM 4-AP (see text). Mean ± SD inactivation _τ_s for _I_A (n = 7), _I_D (n = 7), and _I_K (n = 7) are plotted as a function of test potential. Typical fits are shown in D.

Fig. 8

_I_A, _I_D, and _I_K display similar voltage dependences of activation (A) and steady state inactivation (B). To examine the voltage dependences of current activation, _I_A, _I_D, and _I_K conductances at each test potential in individual cells were calculated from records such as those in Fig. 3, C–E, using the experimentally determined reversal potentials. Conductances at each test potential were normalized to their respective conductance values at +60 mV (in the same cell). Mean normalized conductances are plotted as a function of test potential and the Boltzmann fits to the averaged data are shown (—). B: to examine the voltage dependences of steady state inactivation, _I_A, _I_D, and _I_K amplitudes, evoked at +30 mV after 15-to 20-s conditioning prepulses to potentials between −90 and +20 mV were measured; inset: protocol is illustrated. Using the experimentally determined reversal potentials, conductances then were calculated and normalized to their respective conductance values after the −90 mV conditioning prepulse. Mean normalized _I_A (n = 9), _I_D (n = 5), and _I_K (n = 5) conductances are plotted as a function of conditioning potential, and the Boltzmann fits to the averaged data are shown (—).

Fig. 9

Rates of recovery of _I_A, _I_D, and _I_K are distinct. After inactivating the currents by a 12-s prepulse to 0 mV, the cell was hyperpolarized to −70 mV for times ranging from 0 to 5,000 ms before a test depolarization to +30 mV. Typical current waveforms recorded during the +30 mV depolarization after variable (0–5,000 ms) recovery periods are displayed in A; protocol is beneath the records. Note that the current recorded after a 10 ms recovery time at −70 mV (•) activates and inactivates rapidly (see text). B: amplitudes of the peak and plateau currents evoked at +30 mV after each recovery period were measured, normalized to their respective (peak and plateau; ▪) amplitudes after a 5,000-ms recovery period and plotted as a function of time. Recovery of the peak and plateau currents was best described by the sum of 2 exponentials (—). Inset: initial phase of recovery of both the peak and the plateau currents is shown on an expanded time scale. Similar results were obtained on 7 cells.

References

1. Albert JA, Nerbonne JM. Calcium-independent depolarization-activated potassium currents in superior colliculus-projecting rat visual cortical neurons. J Neurophysiol. 1995;73:2163–2178. - PubMed
1. Barry DM, Nerbonne JM. Myocardial potassium channels: Electrophysiological and molecular diversity. Annu Rev Physiol. 1996;56:363–394. - PubMed
1. Beck H, Ficker E, Heinemann U. Properties of two voltage-activated potassium currents in acutely isolated juvenile rat dentate gyrus granule cells. J Neurophysiol. 1992;68:2086–2099. - PubMed
1. Budde T, Mager R, Pape H. Different types of potassium outward current in relay neurons acutely isolated from the rat lateral geniculate nucleus. Eur J Neurosci. 1992;4:708–722. - PubMed
1. Connors BW, Gutnick MJ. Intrinsic firing patterns of diverse neocortical neurons. Trends Neurosci. 1990;13:99–104. - PubMed

Three kinetically distinct Ca2+-independent depolarization-activated K+ currents in callosal-projecting rat visual cortical neurons - PubMed (original) (raw)