Some properties of single potassium channels in cultured oligodendrocytes

How big is the myelinating orchestra? Cellular diversity within the oligodendrocyte lineage: facts and hypotheses

Since monumental studies from scientists like His, Ramón y Cajal, Lorente de Nó and many others have put down roots for modern neuroscience, the scientific community has spent a considerable amount of time, and money, investigating any possible aspect of the evolution, development and function of neurons. Today, the complexity and diversity of myriads of neuronal populations, and their progenitors, is still focus of extensive studies in hundreds of laboratories around the world. However, our prevalent neuron-centric perspective has dampened the efforts in understanding glial cells, even though their active participation in the brain physiology and pathophysiology has been increasingly recognized over the years. Among all glial cells of the central nervous system (CNS), oligodendrocytes (OLs) are a particularly specialized type of cells that provide fundamental support to neuronal activity by producing the myelin sheath. Despite their functional relevance, the developmental mechanisms regulating the generation of OLs are still poorly understood. In particular, it is still not known whether these cells share the same degree of heterogeneity of their neuronal companions and whether multiple subtypes exist within the lineage. Here, we will review and discuss current knowledge about OL development and function in the brain and spinal cord. We will try to address some specific questions: do multiple OL subtypes exist in the CNS? What is the evidence for their existence and those against them? What are the functional features that define an oligodendrocyte? We will end our journey by reviewing recent advances in human pluripotent stem cell differentiation towards OLs. This exciting field is still at its earliest days, but it is quickly evolving with improved protocols to generate functional OLs from different spatial origins. As stem cells constitute now an unprecedented source of human OLs, we believe that they will become an increasingly valuable tool for deciphering the complexity of human OL identity.

Patching the glia reveals the functional organisation of the brain

The neuroglia was initially conceived by Rudolf Virchow as a non-cellular connective tissue holding neurones together. In 1894, Carl Ludwig Schleich proposed a hypothesis of fully integrated and interconnected neuronal-glial circuits as a substrate for brain function. This hypothesis received direct experimental support only hundred years later, after several physiological techniques, and most notably the patch-clamp method, were applied to glial cells. These experiments have demonstrated the existence of active and bi-directional neuronal-glial communications, integrating neuronal networks and glial syncytium into one functional circuit. The data accumulated during last 15 years prompt rethinking of the neuronal doctrine towards more inclusive concept, which regards both neurones and glia as equally responsible for information processing in the brain.

Single Channel Recording from Glial Cells on the Untreated Surface of the Frog Optic Nerve

The patch clamp technique has been used to record single channel currents from the untreated surface of the intact frog optic nerve after the meninges and basal lamina have been mechanically removed. Cells filled via dialysis with Lucifer yellow (LY) from the patch pipette had a typical astrocyte morphology and were dye-coupled to adjacent astrocytes. This is consistent with the electron-microscopic observation that all the cells on the surface of this nerve are astrocytes. Two types of ion channels were studied in detached patches. One, identified as a K+ channel, had a conductance of 88 +/- 4 (S.E.) n=9 pS and an equilibrium potential of -59 +/- 8 mV in physiological K+ solutions. The steady-state open probability was not significantly altered by changing the membrane potential. A second channel had a large conductance of 300 - 1200 pS, a reversal potential of approximately 0 mV in symmetrical and non-symmetrical solutions, and was open only in the voltage range of +/-20 mV. These are the characteristics of a large anionic channel described in other preparations including cultured astrocytes.

Heterogeneous expression of voltage-gated potassium channels of the shaker family (Kv1) in oligodendrocyte progenitors

Outwardly rectifying K(+) channels determine the membrane conductance and influence the proliferation rate of glial progenitor cells. To analyze the molecular identity and the functional role of K(+) channels in glial progenitors of mouse brain, expression of shaker-type Kv1 genes was studied at three levels: (1) presence of Kv1 mRNAs, (2) biosynthesis of channel proteins and (3) electrophysiological and pharmacological properties of K(+) currents. mRNA expression of Kv1.1 to Kv1.6 genes was studied by single-cell reverse transcription-mediated polymerase chain reaction (RT-PCR) using degenerate primers to amplify the six Kv1 transcripts. Most cells expressed several mRNA combinations simultaneously. In more than half of the cells, messages for Kv1.2, Kv1.5 and Kv1.6 were found, while Kv1.1, Kv1.3 and Kv1.4 were detected in only a minority of cells. In contrast, at the level of protein expression - employing immunocytochemistry with subtype-specific antibodies - Kv1. 2 and Kv1.3 were undetectable (<2%), while almost all cells expressed Kv1.4 (85%), Kv1.5 (99%) and Kv1.6 (99%). Kv1.1 was present in a minor cell population (10%). Functional contribution of Kv1 proteins to progenitor membrane conductance was determined by analyzing the voltage-dependence of K(+) current activation and inactivation as well as their current sensitivities to the subtype-preferring blockers and toxins tetraethylammonium (TEA), 4-aminopyridine (4-AP), charybdotoxin (CTX), alpha-dendrotoxin (DTX) and mast-cell degranulating peptide (MCDP). From these results, it is concluded: first, glial progenitor cells can express all transcripts of the six Kv1 genes, but do not express all proteins; second, Kv1.4, Kv1.5 and Kv1.6 proteins are most abundant and were found in the majority of cells; and third, K(+) currents flow predominantly either through heteromeric channel complexes or through homomeric Kv1.5 ion pores, but not through homomeric Kv1.4 or Kv1.6 channels.

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

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.

Potassium currents in cultured glia of the frog optic nerve

The processes that participate in clearing increases in [K+]o produced by active neurons include KCl uptake, Na pump stimulation, and spatial buffering. The latter process requires glial cells to carry: 1) inward K+ currents in regions where K+ is elevated at a glial membrane potential more negative than EK; and 2) outward K+ currents at normal K+ and glial membrane potential more positive than EK (Orkand et al: J Neurophysiol 29:788, 1966). Techniques for isolation and culturing glial cells brought new possibilities for studying ionic channels involved in spatial buffering. However, they raised the question of the extent to which the properties of ionic channels are changed due to the process of culturing when glial cells are exposed to an artificial environment and deprived of direct interaction with neurons. We studied potassium currents in glial cells from the frog optic nerve that were cultured for 1-8 days. At 24-48 h, cells exhibited an inwardly rectifying Cs+ blocked current (IK(IN)) that increased in amplitude and shifted its threshold of activation to EK when [K+]o was increased from 3 to 6 or 10 mM. IK(IN), diminished after 3 days in culture and virtually disappeared after 5 days. At 24-48 h, a potassium delayed rectifier current (IKD) was relatively small but became large at 3 days, and was practically the only current present after 5 days. IKD was activated at -8.5 +/- 0.58 mV(SE, n = 48) and 58 +/- 2.2% (SE, n = 48) blocked by 20 mM tetraethylammonium. The results of this study support the idea that the inward rectifying potassium channels (Kir) are responsible for carrying K+ into glial cells whenever [K+]o increases. However, the delayed rectifier potassium channels (KD) cannot provide the pathway for outward K+ current during spatial buffering, and another mechanism must be involved in this process. Our study provides further evidence that culture conditions can greatly influence functional expression of ionic channels in glial cells.

Control of myelination, axonal growth, and synapse formation in spinal cord explants by ion channels and electrical activity

The involvement of axonal electrical activity and ion channels as mediators of neuron-glial communication during myelin formation has been tested in explant culture. Transverse slices of embryonic mouse spinal cord were maintained under conditions normally leading to extensive myelination. Axonal conduction was measured optically through the use of a voltage-sensitive dye. Glial development was at a very early stage at the time of plating, and oligodendrocyte precursor cells had not yet appeared. Spontaneous electrical activity was blocked either by tetrodotoxin or by elevation of external K+ concentrations. Myelin development was unaffected by tetrodotoxin and was also present, though quantitatively reduced, in elevated K+. Tetraethylammonium ion (TEA+), a blocker of many K+ channels, almost entirely eliminated myelination at a concentration of 1 mM, but axonal growth and conduction were unaffected. Synapse formation was followed both morphologically and functionally, and was altered neither by conduction block nor by 1 mM TEA+. It is concluded that in the spinal cord oligodendrocyte development and myelination can proceed in the absence of axonal action potentials, but ion channels, possibly in glial membranes, play an important role in these events.

Voltage-gated ionic currents in mature oligodendrocytes isolated from rat cerebellum

Mature rat cerebellar oligodendrocytes were isolated in culture using a serum free medium and identified using anti-galactocerebroside (GalC) and OL-1 antibodies. The morphology of such oligodendrocytes changes with time in culture from a multipolar shape at about 4 days in vitro (DIV) to a monopolar shape at about 8 DIV, a transition that has been previously described only in situ. Voltage-gated ionic currents were characterized at both oligodendrocyte developmental stages using the whole cell configuration of the patch-clamp technique. No differences between these two stages were detected, both types expressed a large K+ inward rectifying current similar to that described for oligodendrocytes from other vertebrate neuronal structures. This current could play an important role in the control of oligodendrocyte resting membrane potential. Our culture system provides a valuable model, close to the situation encountered in situ, not only to study the process of oligodendrocyte maturation, but also to identify the possible interactions between oligodendrocytes and cerebellar neurons such as granular and Purkinje cells during development.

Identification of G protein-regulated inwardly rectifying K+ channels in rat brain oligodendrocytes

Rat brain oligodendroglia in culture express a dominant inwardly rectifying K+ current IKIR which can be inhibited through G. proteins by the activation of glial G protein-coupled receptors. Electrophysiologically we have isolated in these cells several conductances of K+ inward rectifiers (KIR) between 12 and 175 pS. Experiments on the single channel level with preloading or bath-application of GTP gamma S revealed the selective suppression of an 18 pS and maybe other KIR conductances, possibly via a direct membrane-delimited mechanism. mRNA amplification from single oligodendrocytes together with polymerase chain reaction resulted in the isolation of IRK-type K+ channels which may correspond to the channel species negatively controlled by G proteins.

Fast inhibition of inwardly rectifying K+ channels by multiple neurotransmitter receptors in oligodendroglia

An essential function of myelinating oligodendroglia in the mammalian central nervous system is the regulation of extracellular potassium levels by means of a prominent inwardly rectifying K+ current. Cardiac and neuronal K+ inward rectifiers are either activated by hyperpolarizing voltages or controlled by neurotransmitters through the action of receptor-activated G proteins. Neuromodulation of inward rectifiers has not previously been considered as a way to regulate oligodendrocyte function. Here we report the expression of serotonin, somatostatin and muscarinic acetylcholine G protein-coupled receptors in rat brain oligodendrocytes. Activation of these receptors leads to pertussis toxin-sensitive inhibition of inwardly rectifying K+ channels within < 1 s. By contrast, in the heart and in neurons, similar pathways activate an inwardly rectifying conductance. Thus, transmitter-mediated blockade of inward rectifiers appears to be an oligodendrocyte-specific variation of a common motif for convergent signalling pathways. In vivo, expression of this mechanism, which may be dependent on neuron-glia signalling, may have a regulatory role in K+ homeostasis during neuron activity in the central nervous system.

Demonstration of voltage-dependent and TTX-sensitive Na(+)-channels in human melanocytes

The electrophysiological properties of cultured human melanocytes were investigated using the whole-cell configuration of the patch-clamp technique. Depolarizations to membrane potentials more positive than -30 mV resulted in the rapid development ( < 1 ms to peak) of an inward current. The maximum peak current was observed at +10 mV and reached an average amplitude of about 270 pA. During the depolarizations, the current inactivated with a time constant of about 2 ms. The current was abolished by the addition of 0.3 microM tetrodotoxin, a blocker of voltage-gated Na(+)-channels, and disappeared when Na+ was omitted from the extracellular medium. In addition, the melanocytes contain at least two types of outward K(+)-current. The first type, observed in every cell, was highly sensitive (Ki 1 mM) to the K(+)-channel blocker TEA, required depolarizations beyond zero to be activated and did not inactivate. The second type was less regularly observed (10% of the cells). This current activated at more negative voltages (-20 mV), was resistant to TEA (20 mM) but was blocked by 2 mM 4-aminopyridine and inactivated rapidly during depolarizations. We conclude that human melanocytes are equipped with voltage-dependent Na(+)-channels, a delayed rectifying K(+)-current and a K(+)-current similar to the A-current in neurones.

Single-channel and whole-cell recordings from on-neurone glial cells in Helix pomatia ganglia

A procedure is described for performing patch-clamp recordings on satellite glial cells kept in place within the nervous ganglia in the mollusc Helix. Glial cell properties were deduced from whole-cell and cell-attached recordings. The glial membrane was found to contain densely packed inwardly rectifying K+ channels. Activation of the neurones, under either current-clamp or voltage-clamp conditions, depolarized the glial cell layer wrapped around the neurones and induced a delayed persistent increase in the K+ channel opening probability. These results suggest that the glial channels opened in response to a signal emanating from the active neurones. This preparation provides a useful means of detecting and analysing neurone-glial interactions at the cell and unitary channel levels.

Physiological properties of oligodendrocytes during development

The electrical properties of oligodendrocytes during their development in cell culture were analyzed by combining two techniques: cell identification with cell-type and stage-specific antibodies and the patch-clamp technique. The transition from the bipotential precursor cell, which can still develop into astrocytes and oligodendrocytes, into an oligodendrocyte results in a marked change in the ion channel pattern. During this developmental transition, voltage-activated Na+ and several types of K+ currents disappear, whereas a comparatively passive, inwardly rectifying K+ current becomes dominant. GABAA receptor-mediated Cl- currents and a pH-activated Na+ current are down-regulated at this transition but are still present at all developmental stages. In contrast, electrical coupling develops only in oligodendrocytes. This change in the channel repertoire could reflect the transition of a cell in a mobile, mitotic, plastic state (the glial precursor) to a more differentiated specialized state (the oligodendrocyte).

Expression of plasmolipin in oligodendrocytes

Plasmolipin is a plasma membrane proteolipid which has recently been described as a component of myelin (Cochary et al.: Journal of Neurochemistry 55:602-610, 1990). The present study reports the expression and localization of plasmolipin in primary glial cultures and secondary oligodendrocyte cultures. Double-label immunofluorescence showed that plasmolipin was expressed by galactocerebroside (GC)-positive oligodendrocytes, but was absent from astrocytes, characterized by their positive staining for glial fibrillary acidic protein (GFAP). At 1 week in culture plasmolipin staining was relatively weak in the cell body of some of the GC-positive cells. During the following 3 weeks in culture plasmolipin staining of oligodendrocytes gradually increased and was present in the cell body, its plasma membrane, and all the processes. However, the plasmolipin antibodies did not stain regions of the flat membrane sheets. Western blot analysis of homogenates from primary glial cultures showed that plasmolipin levels gradually increased during the first 5 weeks in culture. We conclude that the presence of plasmolipin in myelin is a result of its expression by oligodendrocytes.

Potassium channels in crustacean glial cells

Unitary currents through single ion channels in the glial cells, which ensheath the abdominal stretch receptor neurons of the crayfish, were characterized with respect to their basic kinetic properties. In cell-attached and excised patches two types of Ca(++)-independent K+ channels were observed with slope conductances of 57 pS and 96 pS in symmetrical K+ solution. The 57 pS K+ channel was weakly voltage-dependent with a slope of the Po vs. membrane potential relationship of +95 mV for an e-fold change in Po. In addition to the main conductance level, the channel displayed conductance levels of 80 and 109 pS. In excised patches, channel activity of this "subconductance" K+ channel showed "rundown" that could be prevented with 2 mM ATP-Mg on the cytoplasmic side of the membrane. The 96 pS K+ channel was strongly voltage-dependent with a slope of +12 mV for an e-fold change in Po. Averaged single-channel currents elicited by voltage jumps proved the channel to be of the delayed rectifying type. Channel activity persisted in excised patches with minimal salt solution and in virtually Ca(++)-free saline. Because of its dependence on intracellular ATP-Mg, the subconductance K+ channel is discussed as a target of modulation by transmitters or peptides via phosphorylation of the channel.

Channel-mediated and carrier-mediated uptake of K+ into cultured ovine oligodendrocytes

Uptake of radioactive K+ by mature ovine oligodendrocytes (OLGs) maintained in primary culture was measured under steady-state conditions, i.e., in cells maintained in a normal tissue culture medium (5.4 mM K+), and in cells after depletion of intracellular K+ to less than 15% of its normal value by pre-incubation in K(+)-free medium. The latter value is dominated by an active, carrier-mediated uptake (although it may include some diffusional uptake), whereas the former, in addition to active uptake, also reflects passive K+ diffusion through ion selective channels and possible self-exchange between extracellular and intracellular K+, which may be carrier-mediated. The total uptake rate was 144 +/- 10 nmol/min/mg protein, and the uptake after K+ depletion was 60 +/- 2 nmol/min/mg protein, much lower rates than previously observed in astrocytes. The uptake into K(+)-depleted cells was inhibited by about 80% in the presence of ouabain (1 mM) and about 30% in the presence of furosemide (2 mM). Activators of protein kinase C (phorbol esters) and cAMP-dependent protein kinase (forskolin) have been shown to alter the myelinogenic metabolism as well as outward K+ current in cultured OLGs. The present study demonstrates that K+ homeostasis in OLGs is modulated through similar second messenger pathways. Active uptake was inhibited by about 60% in the presence of active phorbol esters (100 nM) but was not affected by forskolin (100 nM). Forskolin likewise had no effect on total uptake, whereas phorbol esters caused a much larger inhibition than expected from their effect on carrier-mediated uptake alone, suggesting that channel-mediated uptake was also reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage-dependent potassium channels in cultured mammalian Schwann cells

Voltage-dependent ionic currents were recorded from cultured mammalian Schwann cells (rabbits, mice, rats and humans), by a whole-cell voltage-clamp technique to clarify the properties of voltage-dependent K+ currents in Schwann cells of various species. Voltage-dependent K+ channels were classified into 3 groups according to the degree of inactivation and sensitivity to blockade by tetraethylammonium (TEA): (1) little inactivation and TEA sensitivity (rabbit); (2) rapid inactivation and TEA resistance (rat and human); and (3) intermediate degree of both inactivation and sensitivity to TEA (mouse). In rabbit Schwann cells TEA blocked K+ channels predominantly from outside of the cells, while in the other species K+ channels were blocked by TEA inside the Schwann cells. The voltage-dependent K+ channels in different species had different electrophysiological and pharmacological properties, nevertheless the K+ channels in mammalian Schwann cells were similar to those observed in human or murine T-lymphocytes which are very sensitive to blockade by quinine. Although the culture conditions of human Schwann cells were different from those of other species, K+ channels in human Schwann cells were more similar to rat K+ channels than to those of rabbits or mice.

Existence of inward potassium currents in adult human oligodendrocytes

We report the first measurements of single channel currents in cultured adult human oligodendrocytes, obtained postmortem. The channels selectively pass potassium inwardly and possess properties which would be suitable for a physiological role for oligodendrocytes in the human brain as part of a regulatory mechanism for maintenance of extracellular potassium concentration near active neurons. These properties include a long mean open time near normal resting potential and a marked dependence of membrane depolarization to increase the channel open time which could be an important factor when the driving force for inward potassium movement was small. Furthermore, the properties of these potassium channels are remarkably similar to those of channels previously observed in adult bovine oligodendrocytes, which supports the use of animal data to describe function in the human brain.

Properties of single potassium channels in cultured primary astrocytes

The patch-clamp technique was used to characterize the single channel ion currents in primary cultures of rat astrocytes. The most dominant channel type, which was found in over half of the inside-out membrane patches, was a potassium channel. The measured reversal potential was -67 mV, which is close to the calculated Nernst potential for potassium ions (-80 mV). These potassium channels activated with bursts of very brief openings. Once activated the channels did not inactivate. The measured probabilities of the channels to be closed showed at least 3 different modes of channel behaviour: one voltage-independent and two voltage-dependent modes. During each activity-mode a 'main' conductance level plus two other conductance levels were observed. In some recordings a pronounced outward rectification could also be seen.

Single channel potassium currents in cultured adult bovine oligodendrocytes

These studies have enabled the first characterization of the properties of ion channels in adult oligodendrocytes. Cell-attached recordings from cultured adult bovine cells showed channel activity with 140 mM KCl in the patch pipette; the amplitude of the currents was increased with increasing membrane hyperpolarization. This channel, with a conductance of 29 pS, was selective for inward K+ current; little or no outward current was measured for large depolarizing voltage steps. The channel open time was strongly dependent upon membrane potential, with membrane hyperpolarization decreasing the mean open time 100-fold over a range of 80 mV; at the resting potential of the cell the mean open time of the channel was in excess of 50 ms. Decreasing the concentration of K+ in the pipette diminished the channel conductance with no significant effect to alter the channel open time dependence on potential. The rectification and kinetic properties of the channel would be consistent with a physiological role for the channel in the regulation of external K+ near active neurons; in particular the effect of membrane depolarization to cause maintained channel open duration could be important when the driving force for inward potassium movement through oligodendrocyte membrane was low. Channels selective for outward potassium movement were seen with inside-out excised patch recordings with symmetrical potassium concentrations across the patch; the density of these channels in the bovine membrane was low.

Heterogeneity of potassium currents in cultured oligodendrocytes

In the present study we have analyzed the membrane currents of mature oligodendrocytes in cultures from dissociated fetal mouse cerebral hemispheres and explant cultures from fetal mouse spinal cord. Both types of oligodendrocytes showed large voltage-dependent, but time-independent inward and outward currents that were partially blocked by Ba2+. In addition, time- and voltage-dependent inward and outward currents were observed in a minority of oligodendrocytes from spinal cord. All voltage-dependent currents were completely blocked by Ba2+, and inward currents were completely blocked by Cs+, suggesting that they are mediated by K+ channels. Current-voltage curves of mouse spinal cord oligodendrocytes varied from being linear to outwardly or inwardly rectifying. In contrast, oligodendrocytes cultured from mouse brain always showed an inward rectification of the current voltage relation and a lack of time-dependent currents. It thus appears that mature oligodendrocytes in explant cultures of mouse spinal cord, in contrast to oligodendrocytes from dissociated brain, consist of different cell populations that are distinguished by their expression or active state of K+ channels.

Ion channel expression by white matter glia: I. Type 2 astrocytes and oligodendrocytes

White matter is a compact structure consisting primarily of neuronal axons and glial cells. As in other parts of the nervous system, the function of glial cells in white matter is poorly understood. We have explored the electrophysiological properties of two types of glial cells found predominantly in white matter: type 2 astrocytes and oligodendrocytes. Whole-cells and single-channel patch-clamp techniques were used to study these cell types in postnatal rat optic nerve cultures prepared according to the procedures of Raff et al. (Nature, 303:390-396, 1983b). Type 2 astrocytes in culture exhibit a "neuronal" channel phenotype, expressing at least six distinct ion channel types. With whole-cell recording we observed three inward currents: a voltage-sensitive sodium current qualitatively similar to that found in neurons and both transient and sustained calcium currents. In addition, type 2 astrocytes had two components of outward current: a delayed potassium current which activated at 0 mV and an inactivating calcium-dependent potassium current which activated at -30 mV. Type 2 astrocytes in culture could be induced to fire single regenerative potentials in response to injections of depolarizing current. Single-channel recording demonstrated the presence of an outwardly rectifying chloride channel in both type 2 astrocytes and oligodendrocytes, but this channel could only be observed in excised patches. Oligodendrocytes expressed only one other current: an inwardly rectifying potassium current that is mediated by 30- and 120-pS channels. Because these channels preferentially conduct potassium from outside to inside the cell, and because they are open at the resting potential of the cell, they would be appropriate for removing potassium from the extracellular space; thus it is proposed that oligodendrocytes, besides myelinating axons, play an important role in potassium regulation in white matter. The conductances present in oligodendrocytes suggest a "modulated Boyle and Conway mechanism" of potassium accumulation.

Intracellular recordings from isolated rabbit retinal Müller (glial) cells

Müller (glial) cells were isolated from rabbit retinae by papaine and mechanical dissociation. The cells were fixed on a gelatine-covered glass slide by means of concanavalin A, and the slide was mounted in a perfusion chamber under a light microscope with modified optics. Besides the recording microelectrode, two other micropipettes could be adjusted with their tips near the cell. These micropipettes were used for application of test solutions into the environment of the cells. On application of high K+ solutions, the cell depolarized strongly but during prolonged application there was a marked repolarization. After the end of high K+ application the cells showed a hyperpolarization which was enhanced in both amplitude and duration with prolongation of the K+ exposure. Both repolarization and afterhyperpolarization disappeared under ouabain. Ouabain application itself caused a small reversible depolarization. Na+ free solution caused hyperpolarization. The results suggest the existence of an active membrane pump mechanism in our cells. This pump seems to be electrogenic under our experimental conditions and seems to be activated even in the absence of sodium. The cell membrane is demonstrated to contain a significant Na+ conductance.

Calcium activated potassium channels in cultured astrocytes

The patch clamp technique was used to analyze single channel currents in intact and excised patches of glial cell membranes grown in primary cultures from newborn rat brain. Glial cells were morphologically identified by immunohistochemical staining for glial fibrillary acidic protein. Outward currents due to single channels were observed in recordings from both intact and excised patches obtained from the cell body region. The channel responsible for these currents was preferentially permeable to K+ because the reversal potential for this current was correlated with changes in the potassium equilibrium potential, when experimentally altered. The single channel conductance was 25 pS when measured between -20 and +20 mV in solutions with physiological K+ concentrations (10 degrees C). Channel gating was dependent on both the internal Ca2+ concentration and the membrane potential. Either depolarization of the membrane patch, or the addition of increasing Ca2+ concentrations to the internal surface, increased the probability of channel opening. Tetraethylammonium reversibly blocked the channel whereas 4-aminopyridine had no effect. The characteristics exhibited by this channel indicate that a Ca2+-activated K+ channel is present in the membrane of astrocytes grown in culture. These results, combined with previous evidence for a voltage dependent Ca2+ channel, suggest a dynamic role for glial cells in controlling excitability in the central nervous system. Influx of Ca2+ upon depolarization would increase the membrane permeability to K+ and could increase the "buffering" capacity of glial cells for extracellular K+.

A reversible decrease in electrical coupling of cultured mouse glial cells induced by superfusion from a micropipette

Cultured mouse oligodendrocytes were superfused by pressure application to the rear-end of a 3-20 microns micropipette filled with normal bathing fluid. Input resistance was determined during superfusion with two separate electrodes. The input resistance increased reversibly by 90% in 26 cells tested and was unaffected in 39 cells. When pairs of oligodendrocytes were electrically coupled, coupling decreased in a reversible manner during superfusion. Therefore, the flow from a micropipette can uncouple cells, and studies involving application of substances by pressure ejection from microelectrodes must be evaluated with care.

Electrical properties of oligodendrocytes in culture

The electrical properties of immunocytologically identified oligondendrocytes from embryonic mouse spinal cord maintained in culture for 3 to 6 weeks studied by passing current and recording potential changes with two separate intracellular electrodes. The average input resistance was 3.3 M omega and ranged from 0.7 to 16 M omega (n = 35). The input resistance increased by 19% with depolarization and decreased by 9% with hyperpolarization of 25 mV. The membrane time constant determined from the slope of the late exponential tail was 3.45 +/- 2.5 ms SD (n = 15). The specific membrane resistance of three cells was determined by a simplified square pulse analysis combined with measurement of membrane area. Membrane area was estimated from photomicrographs of cells injected with Lucifer Yellow CH and stained with the cell surface-reactive antibody 04 and from electron micrographs. An average specific membrane resistance of 1.3 X 10(3) omega cm2 and specific capacitance of 1.7 mu F/cm2 were calculated. Increasing [K+]o depolarized the cells and decreased the input resistance and the time constant.