Glial depolarization evokes a larger potassium accumulation around oligodendrocytes than around astrocytes in gray matter of rat spinal cord slices

Abstract

The cell membrane of astrocytes and oligodendrocytes is almost exclusively permeable for K+. Depolarizing and hyperpolarizing voltage steps produce in oligodendrocytes, but not in astrocytes, decaying passive currents followed by large tail currents (Itail) after the offset of a voltage jump. The aim of the present study was to characterize the properties of Itail in astrocytes, oligodendrocytes, and their respective precursors in the gray matter of spinal cord slices. Studies were carried out on 5- to 11-day-old rats, using the whole-cell patch clamp technique. The reversal potential (Vrev) of Itail evoked by membrane depolarization was significantly more positive in oligodendrocytes (-31.7+/-2.58 mV, n = 53) than in astrocytes (-57.9+/-2.43 mV, n = 21), oligodendrocyte precursors (-41.2+/-3.44 mV, n = 36), or astrocyte precursors (-52.1+/-1.32 mV, n = 43). Analysis of the Itail (using a variable amplitude and duration of the de- and hyperpolarizing prepulses as well as an analysis of the time constant of the membrane currents during voltage steps) showed that the Itail in oligodendrocytes arise from a larger shift of K+ across their membrane than in other cell types. As calculated from the Nernst equation, changes in Vrev revealed significantly larger accumulation of the extracellular K+ concentration ([K+]e) around oligodendrocytes than around astrocytes. The application of 50 mM K+ or hypotonic solution, used to study the effect of cell swelling on the changes in [K+]e evoked by a depolarizing prepulse, produced in astrocytes an increase in [K+]e of 201% and 239%, respectively. In oligodendrocytes, such increases (22% and 29%) were not found. We conclude that K+ tail currents, evoked by a larger accumulation of K+ in the vicinity of the oligodendrocyte membrane, could result from a smaller extracellular space (ECS) volume around oligodendrocytes than around astrocytes. Thus, in addition to the clearance of K+ from the ECS performed by astrocytes, the presence of the K+ tail currents in oligodendrocytes indicates that they might also contribute to efficient K+ homeostasis.

High extracellular K+ evokes changes in voltage-dependent K+ and Na+ currents and volume regulation in astrocytes

“…In the control group, ΔK increased to 142.1±19.0% (n=25) of the initial values (taken to be 100%) after 20 min of perfusion with hypotonic solution, whereas in astrocytes preincubated in ACF50, ΔK decreased to 75.5±17.5% (n=12) of the initial values. We conclude that under hypotonic stress, preincubation in high K + leads to a decrease in K + accumulation in the vicinity of the astrocyte membrane after a depolarizing prepulse, which might suggest an astrocyte volume decrease [8,44].…”

“…The reversal potential of the tail current (V rev tail ) was determined from the I/V relationship measured 5 ms after the offset of a 20 ms depolarizing (+20 mV) prepulse followed by a series of 20-ms de-and hyperpolarizing pulses ranging from −130 to +20 mV. The K + accumulation in the vicinity of the cell membrane after a depolarizing prepulse (ΔK + ) was determined by subtracting the values of the extracellular K + concentration before ( K þ m  à e ) and after the depolarizing prepulse ( K þ rev  à e ) [7,8]. Because the astrocytic membrane is almost exclusively permeable to K + , K þ m  à e and K þ rev  à e can be calculated using the Nernst equation from the corresponding values of V m and V rev tail , respectively, i.e.,…”

“…The following parameters were analyzed: resting membrane potential (V m ), reversal potential of the tail current (V rev tail ), and K + accumulation in the vicinity of the astrocyte membrane after a depolarizing prepulse (ΔK). Only passive astrocytes were considered, as these cells possess predominantly high passive K + conductance and the values of V rev tail and ΔK in complex astrocytes or in astrocyte precursors are affected by the activation of K DR and K A during the depolarizing prepulse [8].…”

“…Astrocyte swelling evoked by hypotonic stress-volume changes Significant differences in K + accumulation around astrocytes after a depolarizing prepulse during perfusion with hypotonic solution might indicate changes in astrocyte volume regulation [8,44]. Thus, 3D confocal morphometry (see "Materials and methods") was used to examine the Fig.…”

“…The exposure of astrocytes to elevated [K + ] e results in reversible membrane depolarization, the accumulation of intracellular K + , and rapid cell swelling [30,47]. Astrocyte swelling, which can be induced in vitro by hypotonic solution or by an isotonic solution with an increased [K + ] [2,8,17], evokes a large accumulation of K + in the vicinity of the cell membrane after a depolarizing prepulse, resulting from an extracellular space (ECS) volume decrease around swollen astrocytes [7,8,44]. Sykova et al [43] showed that incubating rat spinal cords in 50 mM K + evokes cell swelling, resulting in a decrease in the ECS volume fraction and astrocyte activation [manifested as an increase in glial fibrillary acidic protein (GFAP) immunoreactivity].…”