Changes in glial K+ currents with decreased extracellular volume in developing rat white matter

Abstract

Whole cell patch-clamp recordings of K+ currents from oligodendrocyte precursors in 10-day-old rats (P10) and, following myelination, in mature oligodendrocytes from 20-day-old rats (P20) were correlated with extracellular space (ECS) diffusion parameters measured by the local diffusion of iontophoretically injected tetramethylammonium ions (TMA+). The aim of this study was to find an explanation for the changes in glial currents that occur with myelination. Oligodendrocyte precursors (P10) in slices from corpus callosum were characterized by the presence of A-type K+ currents, delayed and inward rectifier currents, and lack of tail currents after the offset of a voltage jump. Mature oligodendrocytes in corpus callosum slices from P20 rats were characterized by passive, decaying currents and large tail currents after the offset of a voltage jump. Measurements of the reversal potential for the tail currents indicate that they result from increases in [K+]e by an average of 32 mM during a 20 msec 100 mV voltage step. Concomitant with the change in oligodendrocyte electrophysiological behavior after myelination there is a decrease in the ECS of the corpus callosum. ECS volume decreases from 36% (P9-10) to 25% (P20-21) of total tissue volume. ECS tortuosity lambda = (D/ADC)0.5, where D is the free diffusion coefficient and ADC is the apparent diffusion coefficient of TMA+ in the brain, increases as measured perpendicular to the axons from 1.53 +/- 0.02 (n = 6, mean +/- SEM) to 1.70 +/- 0.02 (n = 6). TMA+ non-specific uptake (k') was significantly larger at P20 (5.2 +/- 0.6 x 10(-3) s(-1), n = 6) than at P10 (3.5 +/- 0.4 x 10(-3) s(-1), n = 6). It can be concluded that membrane potential changes in mature oligodendrocytes are accompanied by rapid changes in the K+ gradient resulting from K+ fluxes across the glial membrane. As a result of the reduced extracellular volume and increased tortuosity, the membrane fluxes produce larger changes in [K+]e in the more mature myelinated corpus callosum than before myelination. These conclusions also account for differences between membrane currents in cells in slices compared to those in tissue culture where the ECS is essentially infinite. The size and geometry of the ECS influence the membrane current patterns of glial cells and may have consequences for the role of glial cells in spatial buffering.

Diffusion in Brain Extracellular Space

“…On the other hand, the electrophysiological properties of oligodendrocytes, i.e., the K + movement indicated by the presence of tail-currents, show that this type of cell may more efficiently contribute to the regulation of K + changes arising from neuronal activity. Further confirmation was obtained in studies of the oligodendrocytes that are responsible for myelinating the axons of the corpus callosum in both mice (33) and rats (67). Myelination leads to changes in membrane currents consistent with a reduction in the space around the oligodendrocytes as well as an overall reduction in α (67).…”

“…The volume of the immediate ECS surrounding glial cells in the CNS affects membrane currents generated in response to voltage steps applied though a patch-clamp because of K + accumulation just outside the cell (33, 66, 67). In the gray matter of rat spinal cord slices at different ages, which contains neurons as well as mature astrocytes and oligodendrocytes and their respective precursors (68), the occurrence of large tail-currents in patch-clamp studies is observed in oligodendrocytes but not in other cell types (66).…”

Glial Cells, Glutamate & the Hippocampus

“…In situ electrophysiological studies in oligodendrocytes revealed decaying passive currents evoked by de-or hyperpolarizing voltage steps and large tail currents (Itau) after the offset of the voltage jump. In our previous study in the rat corpus callosum (Chvatal et al 1997), reversal potential (V r ev ) of I tail was shifted to more positive values in mature oligodendrocytes at postnatal day 20 (P20) than in oligodendrocyte precursors at PlO. These changes in glial K+ currents coincided with a decrease in extracellular volume 15P during myelination .…”

Analysis of K+ accumulation reveals privileged extracellular region in the vicinity of glial cells in situ

“…The application of Ba 2+ almost completely blocked the decaying component of the inward and outward currents, as well as tail currents after depolarizing and hyperpolarizing voltage steps, indicating that these currents are carried predominantly by K + (Berger et al, 1991; Chvátal et al, 1995). Further analysis of oligodendrocyte currents in brain slice preparations (Chvátal et al, 1997, 1999; Vargová et al, 2001) revealed that I tail observed after the offset of a depolarizing pulse are caused by the reversed shift of K + across the cell membrane from the ECS into the cell. Because the glial membrane is highly permeable for K + , the membrane potential (V m ) is close to the reversal potential (V rev ) determined by the gradient of K + outside and inside the cell according to the Nernst equation.…”

“…In oligodendrocyte‐like cells in hippocampal slices, a membrane depolarization up to +20 mV evoked a shift of V rev of I tail to −19 mV (Steinhäuser et al, 1992), which corresponds to 61 mM [K + ] e . In a rat corpus callosum slice preparation, a depolarizing prepulse evoked an increase in [K + ] e up to 12 mM in oligodendrocyte precursors, whereas, in mature oligodendrocytes, [K + ] e reached 37 mM (Chvátal et al, 1997). In rat spinal cord slices, depolarization evoked an increase in [K + ] e up to 47 mM in oligodendrocytes, whereas, in astrocytes [K + ] e reached 12 mM, in astrocyte precursors 15 mM, and in oligodendrocyte precursors 22 mM (Chvátal et al, 1999).…”