Na+ channels of Müller (glial) cells isolated from retinae of various mammalian species including man

Sodium channels enable fast electrical signaling and regulate phagocytosis in the retinal pigment epithelium

BACKGROUND

Voltage-gated sodium (Nav) channels have traditionally been considered a trademark of excitable cells. However, recent studies have shown the presence of Nav channels in several non-excitable cells, such as astrocytes and macrophages, demonstrating that the roles of these channels are more diverse than was previously thought. Despite the earlier discoveries, the presence of Nav channel-mediated currents in the cells of retinal pigment epithelium (RPE) has been dismissed as a cell culture artifact. We challenge this notion by investigating the presence and possible role of Nav channels in RPE both ex vivo and in vitro.

RESULTS

Our work demonstrates that several subtypes of Nav channels are found in human embryonic stem cell (hESC)-derived and mouse RPE, most prominently subtypes Nav1.4, Nav1.6, and Nav1.8. Whole cell patch clamp recordings from the hESC-derived RPE monolayers showed that the current was inhibited by TTX and QX-314 and was sensitive to the selective blockers of the main Nav subtypes. Importantly, we show that the Nav channels are involved in photoreceptor outer segment phagocytosis since blocking their activity significantly reduces the efficiency of particle internalization. Consistent with this role, our electron microscopy results and immunocytochemical analysis show that Nav1.4 and Nav1.8 accumulate on phagosomes and that pharmacological inhibition of Nav channels as well as silencing the expression of Nav1.4 with shRNA impairs the phagocytosis process.

CONCLUSIONS

Taken together, our study shows that Nav channels are present in RPE, giving this tissue the capacity of fast electrical signaling. The channels are critical for the physiology of RPE with an important role in photoreceptor outer segment phagocytosis.

Comparative electrophysiology of retinal Müller glial cells-A survey on vertebrate species

Müller cells are the dominant macroglial cells in the retina of all vertebrates. They fulfill a variety of functions important for retinal physiology, among them spatial buffering of K⁺ ions and uptake of glutamate and other neurotransmitters. To this end, Müller cells express inwardly rectifying K⁺ channels and electrogenic glutamate transporters. Moreover, a lot of voltage- and ligand-gated ion channels, aquaporin water channels, and electrogenic transporters are expressed in Müller cells, some of them in a species-specific manner. For example, voltage-dependent Na⁺ channels are found exclusively in some but not all mammalian species. Whereas a lot of data exist from amphibians and mammals, the results from other vertebrates are sparse. It is the aim of this review to present a survey on Müller cell electrophysiology covering all classes of vertebrates. The focus is on functional studies, mainly performed using the whole-cell patch-clamp technique. However, data about the expression of membrane channels and transporters from immunohistochemistry are also included. Possible functional roles of membrane channels and transporters are discussed. Obviously, electrophysiological properties involved in the main functions of Müller cells developed early in vertebrate evolution. GLIA 2017;65:533-568.

REVIEW ■ : Regulation of Extracellular K and pH by Polarized Ion Fluxes in Glial Cells: The Retinal Müller Cell

Müller cells, the principal glial cells of the retina, exhibit a high degree of functional and morphological polarization. An inward rectifying K+ channel, the dominant ion channel in Müller cells, is localized preferentially to cell endfeet, which terminate on the vitreal surface of the retina and on blood vessels. Two acid/ base transport systems, an Na+/HCO3-cotransporter and a CI-/HCO3-anion exchanger, also are localized preferentially to the endfeet. These functional specializations facilitate the ability of Müller cells to regulate extracellular ion levels in the retina. Müller cells regulate extracellular K+levels by transporting K+away from the neural retina to the vitreous humor and the subretinal space. Müller cells may also regulate retinal CO2and pH by the combined action of cell carbonic anhydrase and acid/base transporters localized to the endfeet, and they may control blood flow by the depolarization-induced release of potassium and protons from cell endfeet onto blood vessels. The physiology of ion transport in CNS astrocytes is not understood as well as it is in Müller cells. The presence of inward rectifying K+channels and acid/base transporters in astrocytes, however, suggests that these cells may utilize mechanisms similar to those of Müller cells in regulating the extracellular microenvironment and in controlling blood flow. The Neuroscientist 2:109-117, 1996

Sodium channels in astroglia and microglia

Voltage-gated sodium channels are required for electrogenesis in excitable cells. Their activation, triggered by membrane depolarization, generates transient sodium currents that initiate action potentials in neurons, cardiac, and skeletal muscle cells. Cells that have not traditionally been considered to be excitable (nonexcitable cells), including glial cells, also express sodium channels in physiological conditions as well as in pathological conditions. These channels contribute to multiple functional roles that are seemingly unrelated to the generation of action potentials. Here, we discuss the dynamics of sodium channel expression in astrocytes and microglia, and review evidence for noncanonical roles in effector functions of these cells including phagocytosis, migration, proliferation, ionic homeostasis, and secretion of chemokines/cytokines. We also examine possible mechanisms by which sodium channels contribute to the activity of glial cells, with an eye toward therapeutic implications for central nervous system disease. GLIA 2016;64:1628-1645.

Two-Photon Autofluorescence Imaging Reveals Cellular Structures Throughout the Retina of the Living Primate Eye

PURPOSE

Although extrinsic fluorophores can be introduced to label specific cell types in the retina, endogenous fluorophores, such as NAD(P)H, FAD, collagen, and others, are present in all retinal layers. These molecules are a potential source of optical contrast and can enable noninvasive visualization of all cellular layers. We used a two-photon fluorescence adaptive optics scanning light ophthalmoscope (TPF-AOSLO) to explore the native autofluorescence of various cell classes spanning several layers in the unlabeled retina of a living primate eye.

METHODS

Three macaques were imaged on separate occasions using a custom TPF-AOSLO. Two-photon fluorescence was evoked by pulsed light at 730 and 920 nm excitation wavelengths, while fluorescence emission was collected in the visible range from several retinal layers and different locations. Backscattered light was recorded simultaneously in confocal modality and images were postprocessed to remove eye motion.

RESULTS

All retinal layers yielded two-photon signals and the heterogeneous distribution of fluorophores provided optical contrast. Several structural features were observed, such as autofluorescence from vessel walls, Müller cell processes in the nerve fibers, mosaics of cells in the ganglion cell and other nuclear layers of the inner retina, as well as photoreceptor and RPE layers in the outer retina.

CONCLUSIONS

This in vivo survey of two-photon autofluorescence throughout the primate retina demonstrates a wider variety of structural detail in the living eye than is available through conventional imaging methods, and broadens the use of two-photon imaging of normal and diseased eyes.

New functions of Müller cells

Müller cells, the major type of glial cells in the retina, are responsible for the homeostatic and metabolic support of retinal neurons. By mediating transcellular ion, water, and bicarbonate transport, Müller cells control the composition of the extracellular space fluid. Müller cells provide trophic and anti-oxidative support of photoreceptors and neurons and regulate the tightness of the blood-retinal barrier. By the uptake of glutamate, Müller cells are more directly involved in the regulation of the synaptic activity in the inner retina. This review gives a survey of recently discoved new functions of Müller cells. Müller cells are living optical fibers that guide light through the inner retinal tissue. Thereby they enhance the signal/noise ratio by minimizing intraretinal light scattering and conserve the spatial distribution of light patterns in the propagating image. Müller cells act as soft, compliant embedding for neurons, protecting them in case of mechanical trauma, and also as soft substrate required for neurite growth and neuronal plasticity. Müller cells release neuroactive signaling molecules which modulate neuronal activity, are implicated in the mediation of neurovascular coupling, and mediate the homeostasis of the extracellular space volume under hypoosmotic conditions which are a characteristic of intense neuronal activity. Under pathological conditions, a subset of Müller cells may differentiate to neural progenitor/stem cells which regenerate lost photoreceptors and neurons. Increasing knowledge of Müller cell function and responses in the normal and diseased retina will have great impact for the development of new therapeutic approaches for retinal diseases.

Alterations in protein expression and membrane properties during Müller cell gliosis in a murine model of transient retinal ischemia

Retinal Müller glial cells are involved in K+ ion homeostasis of the tissue. Inwardly rectifying K(+) (Kir) channels play a decisive role in the process of spatial K+ buffering. It has been demonstrated that Kir-mediated currents of Müller cells are downregulated in various cases of retinal neurodegeneration. However, this has not yet been verified for any murine animal model. The aim of the present study was to investigate Müller cells after transient retinal ischemia in mice. High intraocular pressure was applied for 1h; the retina was analysed 1 week later. We studied protein expression in the tissue by immunohistochemistry, and membrane currents of isolated cells by patch-clamp experiments. We found the typical indicators of reactive gliosis such as upregulation of glial fibrillary acidic protein. Moreover, the membrane capacitance of isolated Müller cells was increased and the amplitudes of Kir-mediated currents were slightly, but significantly decreased. This murine high intraocular pressure model of transient retinal ischemia is proposed as a versatile tool for further studies on Müller cell functions in retinal degeneration.

Interaction between chromaffin and sustentacular cells in adrenal medulla of viscacha (Lagostomus maximus maximus)

New evidence provides valuable information about the participation of sustentacular cells in chromaffin cell catecholamine secretion. In this process, calcium ions play an important role. It has been shown that there is an intense ionic traffic between both types of cells. Moreover, sustentacular cells take an active part in calcium metabolism, regulating levels of the ion and indirectly, the synthesis and release of catecholamines. This background information encouraged us to study the sustentacular population of Lagostomus adrenal medulla and its morphologic relationship with the chromaffin population. The animals were captured, transported to the animal facilities, anaesthetized and killed. The adrenal gland was processed by immunohistochemistry using antiserum against S-100 (subunit alpha and beta), a specific marker. Through the morphological and immunohistochemical study, it was found that there are sustentacular cells in deferent regions of adrenal medulla, mainly in the basal zone of chromaffin cells, which constitute the glomerular structure around blood capillaries. Cytoplasmic extentions of sustentacular cells penetrate into chromaffin cells and make contact with the basal membrane of the capillary endothelium. The relationship among chromaffin cells, capillaries and sustentacular cells suggests that they may intervene actively in the adrenal medulla metabolism.

Atypical gliosis in Müller cells of the slowly degenerating rds mutant mouse retina

Retinal Müller glial cells are known to undergo reactive changes (gliosis) in various retinal diseases. In virtually all cases studied, an upregulation of glial fibrillary acidic protein (GFAP) and a hypertrophy can be observed. Physiological alterations, such as a strong downregulation of inwardly rectifying K+ (Kir) currents, were found after retinal detachment (man, rabbit) and after ischemia/reperfusion (rat) but not in more slowly progressing retinal degenerations (Borna Disease Virus-infected rats, RCS rats). This led us to hypothesize that Müller cells respond with 'typical' reactive gliosis only to rapid but not to slow retinal degeneration. To test this hypothesis, we studied Müller cells from rds mutant mice (PrphRd2), which show a retinal degeneration of early onset and slow progression, resulting in a complete loss of photoreceptors after 9-12 months. In Müller cells of rds mice, we found immunoreactivity for GFAP, a marker of gliosis in Müller cells, from postnatal day 21 on, accompanied by a moderately increased membrane capacitance (taken as an indicator of hypertrophy), whereas no change in the expression of the Kir4.1 protein occurred in adult rds mice. We failed to observe significant changes in the membrane resistance and the membrane potential of cells from rds mice from first week after birth until 1 year of age. Current densities were decreased in cells from 3- and 5-week old rds mice. Furthermore, as in control cells from wildtype animals, these cells displayed dominant Kir currents, voltage-dependent Na+ currents, and glutamate uptake currents. These data support the idea that in mice as well as previously shown in rats, slow retinal degeneration induces an atypical gliosis of Müller cells.

GABA(A) receptors in Müller glial cells of the human retina

The present study was aimed at characterizing the GABA(A) receptor-mediated currents in acutely isolated glial (Müller) cells of the human retina and investigating their subcellular localization across the Müller cell membrane. Extracellular application of GABA evoked two current responses in human Müller cells: a fast transient GABA(A) receptor-mediated current that inactivated within 10 s and that was independent of extracellular Na(+), and a sustained current that was dependent on extracellular Na(+) and that was mediated by high-affinity GABA transporters. The receptor current was half-maximally activated at a GABA concentration of 32 microM, while the transporter current showed an affinity constant of 7.9 microM GABA. The receptor currents were blocked by bicuculline and picrotoxin and were also activated by muscimol or by other amino acids. The receptor currents are Cl(-) currents, as indicated by the close relationship between the reversal potential of these currents and the Cl(-) equilibrium potential. Using perforated-patch recordings, a mean intracellular Cl(-) concentration of 37 +/- 12 mM was determined in human Müller cells. Using electrophysiological and fluorescence imaging methods, it was revealed that GABA(A) receptors are unevenly distributed across the Müller cell membrane, with higher densities at the endfoot, at the soma, and at the distal sclerad end of the cells. It is concluded that GABA(A) receptor expression may allow a sensing of retinal GABAergic neuronal signal transmission by Müller cells.

Ion conductances in supporting cells isolated from the mouse vomeronasal organ

The vomeronasal organ (VNO) is a chemosensory structure involved in the detection of pheromones in most mammals. The VNO sensory epithelium contains both neurons and supporting cells. Data suggest that vomeronasal neurons represent the pheromonal transduction sites, whereas scarce information is available on the functional properties of supporting cells. To begin to understand their role in VNO physiology, we have characterized with patch-clamp recording techniques the electrophysiological properties of supporting cells isolated from the neuroepithelium of the mouse VNO. Supporting cells were distinguished from neurons by their typical morphology and by the lack of immunoreactivity for Ggamma8 and OMP, two specific markers for vomeronasal neurons. Unlike glial cells in other tissues, VNO supporting cells exhibited a depolarized resting potential (about -29 mV). A Goldman-Hodgkin-Katz analysis for resting ion permeabilities revealed indeed an unique ratio of P(K):P(Na):P(Cl) = 1:0.23:1.4. Supporting cells also possessed voltage-dependent K(+) and Na(+) conductances that differed significantly in their biophysical and pharmacological properties from those expressed by VNO neurons. Thus glial membranes in the VNO can sustain significant fluxes of K(+) and Na(+), as well as Cl(-). This functional property might allow supporting cells to mop-up and redistribute the excess of KCl and NaCl that often occurs in certain pheromone-delivering fluids, like urine, and that could blunt the sensitivity of VNO neurons to pheromones. Therefore vomeronasal supporting cells could affect chemosensory transduction in the VNO by regulating the ionic strength of the pheromone-containing medium.

Molecular characterization of voltage-gated sodium channels in human gliomas

Voltage-sensitive sodium channels appear to be an electrophysiological hallmark of gliomas. However, the expression of channel subtypes is unclear in these tumors. In this study different gliomas were investigated for the expression of sodium channel subtypes Na(v)1.1, Na(v)1.2, Na(v)1.3, Na(v)1.4, Na(v)1.6, and Na(x)(Na(v)2.1) using RT-PCR. At least one subtype of channels could be detected in each tumor. High-grade gliomas expressed fewer sodium channel subtypes and these at weaker levels than low-grade tumors. Expression of Na(v)1.6, the most abundant isoform in the CNS, was almost absent in the gliomas except the pilocytic variant. Our study gives clear evidence for a differential expression of sodium channel subtypes in gliomas and indicates a predominant expression of channels related to malignancy grades.

High-affinity GABA uptake in retinal glial (Müller) cells of the guinea pig: electrophysiological characterization, immunohistochemical localization, and modeling of efficiency

Glial cells may act as important modulators of neuronal information processing, in particular, via fast uptake of neuronally released transmitters. Here, we characterize the electrogenic gamma-aminobutyric acid (GABA) transporters present in the plasma membranes of Müller (glial) cells of the guinea pig retina and present an estimate of their functional efficiency. The GABA-evoked whole-cell currents are voltage-dependent, with increasing amplitudes and decreasing affinity constants at more negative membrane potentials. The transmembranal GABA transport is concentration-dependent, with near-maximal currents at 100 microM GABA, and is dependent on extracellular sodium and chloride ions; the stoichiometry is 1 GABA/2 Na(+)/1 Cl(-). Immunohistochemical labeling and whole-cell voltage-clamp records reveal that Müller cells express both GAT-1 and GAT-3 (but not GAT-2), and that the transporter proteins are expressed predominantly at plasma membrane sites that, in situ, are localized in the outer retina where GABA uptake is performed exclusively by Müller cells. When extracellular GABA enters the cell interior, it evokes, via activation of the GABA transaminase, an NAD(P)H fluorescence signal selectively in the distal region of the Müller cells where their mitochondria are located. Using our experimental data, we simulated the GABA clearance from the extracellular space surrounding one Müller cell; these estimates show that a pulse of 100 microM extracellular GABA is fully cleared after 70 ms. It is suggested that Müller cells may be involved in the regulation of GABAergic transmission within the retina by providing a fast termination of GABAergic signaling via their highly efficient GABA uptake.

Electrophysiological characterization of retinal Müller glial cells from mouse during postnatal development: comparison with rabbit cells

The electrophysiology of murine Müller cells, and of their precursors, during postnatal development was investigated by using the whole-cell patch-clamp technique. Membrane potential, membrane capacitance, and expression of voltage-gated Na+ currents increased during the first 3 postnatal weeks. During the same period, the membrane resistance decreased due to an upregulation of both inward and outward K+ currents. Glutamate transporter-mediated currents increased during the first postnatal weeks, as well. In Müller (precursor) cells from rabbits, these transporter currents already achieved adult levels at postnatal day 6, the earliest stage studied. Together with the developmental time course of K+ currents, this indicates a delay in the maturation of murine Müller cells, as compared with cells from rabbit.

Developmental regulation of calcium channel-mediated currents in retinal glial (Müller) cells

Whole cell voltage-clamp recordings of freshly isolated cells were used to study changes in the currents through voltage-gated Ca(2+) channels during the postnatal development of immature radial glial cells into Müller cells of the rabbit retina. Using Ba(2+) or Ca(2+) ions as charge carriers, currents through transient low-voltage-activated (LVA) Ca(2+) channels were recorded in cells from early postnatal stages, with an activation threshold at -60 mV and a peak current at -25 mV. To increase the amplitude of currents through Ca(2+) channels, Na(+) ions were used as the main charge carriers, and currents were recorded in divalent cation-free bath solutions. Currents through transient LVA Ca(2+) channels were found in all radial glial cells from retinae between postnatal days 2 and 37. The currents activated at potentials positive to -80 mV and displayed a maximum at -40 mV. The amplitude of LVA currents increased during the first postnatal week; after postnatal day 6, the amplitude remained virtually constant. The density of LVA currents was highest at early postnatal days (days 2-5: 13 pA/pF) and decreased to a stable, moderate level within the first three postnatal weeks (3 pA/pF). A significant expression of currents through sustained, high-voltage-activated Ca(2+) channels was found after the third postnatal week in approximately 25% of the investigated cells. The early and sole expression of transient currents at high-density may suggest that LVA Ca(2+) channels are involved in early developmental processes of rabbit Müller cells.

Farnesol modulates membrane currents in human retinal glial cells

Farnesol, a C(15) natural isoprenoid, exerts complex modulating effects on the membrane permeability of human retinal glial (Müller) cells. Several glial cationic currents were examined. At low micromolar concentrations, farnesol reduced the amplitudes of all fast and depolarization-activated membrane currents expressed by Müller cells, that is, currents through 1) transient low-voltage-activated (LVA; IC(50) = 2.2 microM), 2) sustained high-voltage-activated Ca(2+) channels (HVA; IC(50) = 1.2 microM), 3) fast Na(+) channels (IC(50) = 9.0 microM), and 4) transient (A-type) K(+) channels (IC(50) = 4.7 microM). Furthermore, farnesol shifted the activation of LVA and HVA currents to more depolarized potentials by 21.3 +/- 7.4 mV and 8.3 +/- 4.5 mV, respectively. On the other hand, neither inwardly rectifying nor iberiotoxin-sensitive calcium-activated K(+) currents were affected by farnesol. Therefore, farnesol is assumed to be a biologically active substance that regulates ion channel activity in the glial cell membrane. Depressing rapid changes of the membrane potential and supporting a stable hyperpolarized status of the glial cells may enhance the efficiency of crucial glial functions such as extracellular K(+) clearance and neurotransmitter uptake.

Excitable properties in astrocytes derived from human embryonic CNS stem cells

Although it is widely believed that astrocytes lack excitability in adult tissue, primitive action potential-like responses have been elicited from holding potentials negative to -80 mV, in cultured and injury-induced gliotic rodent astrocytes and in human glia under pathological conditions such as glioblastomas and temporal lobe epilepsy. The present study was designed to investigate the properties of astrocytes (identified by immunoreactivity for glial fibrillary acidic protein) derived from multipotent human embryonic CNS stem cells and cultured for 12-25 days in differentiating conditions. We describe here for the first time that brief (1 ms) current pulses elicit spikes from a resting potential (VREST) of approximately -37 mV and, more interestingly, that spontaneous firing can be occasionally recorded in human astrocytes. A voltage-clamp study revealed that in these cells: (i) the half-inactivation of the tetrodotoxin (TTX)-sensitive Na+ channels is around VREST; (ii) the delayed rectifier K+ current is very small; (iii) the ever-present transient outward A-type K+ channels are paradoxically capable of inhibiting the action potentials elicited from a negative membrane potential (-55 to -60 mV); and (iv) inwardly rectifying currents are not present. The responses predicted from a simulation model are in agreement with the experiments. As suggested by recent studies, the decrease of Na+ channel expression and the changes of the electrophysiological properties during the postnatal maturation of the CNS seem to exclude the possibility that astrocytes may play an excitable role in adult tissue. Our data show that excitability and firing should be considered an intrinsic attribute of human astrocytes during CNS development. This is likely to have physiological importance because the role of astrocytes during development is different from the [K+]o-buffering role played in adult CNS, namely the glutamate release and/or the guiding of migrating neurons.

Spatial distribution of spermine/spermidine content and K(+)-current rectification in frog retinal glial (Müller) cells

Previous studies in retinal glial (Müller) cells have suggested that (1) the dominant membrane currents are mediated by K(+) inward-rectifier (Kir) channels (Newman and Reichenbach, Trends Neurosci 19:307-312, 1996), and (2) rectification of these Kir channels is due largely to a block of outward currents by endogenous polyamines such as spermine/spermidine (SPM/SPD) (Lopatin et al., Nature 372:366-369, 1994). In frog Müller cells, the degree of rectification of Kir-mediated currents is significantly higher in the endfoot than in the somatic membrane (Skatchkov et al., Glia 27:171-181, 1999). This article shows that in these cells there is a topographical correlation between the local cytoplasmic SPM/SPD immunoreactivity and the ratio of inward to outward K(+) currents through the surrounding membrane area. Throughout the retina, Müller cell endfeet display a high SPM/SPD immunolabel (assessed by densitometry) and a large inward rectification of K(+) currents, as measured by the ratio of inward to outward current produced by step changes in [K(+)](o). In the retinal periphery, Müller cell somata are characterized by roughly one-half of the SPM/SPD immunoreactivity and K(+)-current rectification as the corresponding endfeet. In the retinal center, Müller cell somata are virtually devoid of both SPM/SPD immunolabel and K(+)-current inward rectification. Comparing one region of the retina with another, we find an exponential correlation between the local K(+) rectification and the local SPM/SPD content. This finding suggests that the degree of inward rectification in a given membrane area is determined by the local cytoplasmic polyamine concentration.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

A function of delayed rectifier potassium channels in glial cells: maintenance of an auxiliary membrane potential under pathological conditions

Müller glial cells from human and guinea-pig retinae were investigated using the whole-cell patch-clamp technique. Human Müller cells from eyes with different diseases were characterized by diminished inwardly-rectifying K(+) currents. A comparable reduction of these currents was achieved in guinea pig Müller cells by treatment with iodoacetate to generate ischemia-like conditions. Consequently, the membrane potentials were reduced significantly in both diseased human and iodoacetate-treated guinea-pig Müller cells as compared to normal controls. However, the potentials were still clearly negative. Delayed rectifier currents could still be recorded under these conditions. Application of quinine blocked the delayed rectifier K(+) channels, and resulted in a total breakdown of the membrane potentials. Thus, it becomes apparent that the glial delayed rectifier K(+) channels are necessary to maintain an 'auxiliary' membrane potential under certain pathological conditions that are characterized by an almost total loss of inward rectifier conductance. Therefore, the delayed rectifier K(+) channels of glial cells may become crucial for the support of basic glial functions.

Ion channels in glial cells

Functional and molecular analysis of glial voltage- and ligand-gated ion channels underwent tremendous boost over the last 15 years. The traditional image of the glial cell as a passive, structural element of the nervous system was transformed into the concept of a plastic cell, capable of expressing a large variety of ion channels and neurotransmitter receptors. These molecules might enable glial cells to sense neuronal activity and to integrate it within glial networks, e.g., by means of spreading calcium waves. In this review we shall give a comprehensive summary of the main functional properties of ion channels and ionotropic receptors expressed by macroglial cells, i.e., by astrocytes, oligodendrocytes and Schwann cells. In particular we will discuss in detail glial sodium, potassium and anion channels, as well as glutamate, GABA and ATP activated ionotropic receptors. A majority of available data was obtained from primary cell culture, these results have been compared with corresponding studies that used acute tissue slices or freshly isolated cells. In view of these data, an active glial participation in information processing seems increasingly likely and a physiological role for some of the glial channels and receptors is gradually emerging.

Ca(2+) channel-mediated currents in retinal glial (Müller) cells of the toad (Bufo marinus)

Whole-cell voltage-clamp recordings were used to detect voltage-gated Ca(2+) channels in freshly isolated retinal glial (Müller) cells of the toad (Bufo marinus). Using Ca(2+) ions (2 mM) as charge carriers (in the presence of 1 mM Mg(2+)), no inwardly directed currents could be observed during the application of depolarizing voltage steps. However, after omitting the divalent cations from the bath solution, large-amplitude inwardly directed currents were evoked that were carried by Na(+) ions, and were mediated by at least two different kinds of Ca(2+) channels, transient low voltage-activated (LVA) channels and sustained high voltage-activated (HVA) channels. While the LVA currents activated at potentials positive to -90 mV and peaked at -40 mV, the HVA currents activated positive to -60 mV and peaked at -20 mV. It is concluded that Müller glial cells of the toad express distinct types of voltage-gated Ca(2+) channels that may be activated, under certain conditions, close to physiological membrane potentials.

Role of glial K(+) channels in ontogeny and gliosis: a hypothesis based upon studies on Müller cells

The electrophysiological properties of Müller cells, the principal glial cells of the retina, are determined by several types of K(+) conductances. Both the absolute and the relative activities of the individual types of K(+) channels undergo important changes in the course of ontogenetic development and during gliosis. Although immature Müller cells express inwardly rectifying K(+) (K(IR)) currents at a very low density, the membrane of normal mature Müller cells is predominated by the K(IR) conductance. The K(IR) channels mediate spatial buffering K(+) currents and maintain a stable hyperpolarized membrane potential necessary for various glial-neuronal interactions. During "conservative" (i.e., non-proliferative) reactive gliosis, the K(IR) conductance of Müller cells is moderately reduced and the cell membrane is slightly depolarized; however, when gliotic Müller cells become proliferative, their K(IR) conductances are dramatically down-regulated; this is accompanied by an increased activity of Ca(2+)-activated K(+) channels and by a conspicuous unstability of their membrane potential. The resultant variations of the membrane potential may increase the activity of depolarization-activated K(+), Na(+) and Ca(2+) channels. It is concluded that in respect to their K(+) current pattern, mature Müller cells pass through a process of dedifferentiation before proliferative activity is initiated.

Electrical activity regulates dendritic reorganization in ganglion cells after neonatal retinal lesion in the cat

During the first month of postnatal life, the dendritic arborizations of cat retinal ganglion cells continue to develop and undergo a substantial remodeling. Mechanical and pharmacological interferences with the normal development induce, during this period of time, substantial modifications in ganglion cell morphology. Specifically, the degeneration of those neurons whose axons were severed by a neonatal retinal lesion leads to a zone depleted of ganglion cells. Neurons at the border of the depleted area develop an abnormal elongation of the dendritic trees toward the empty space. In the present paper, we report data showing that this dendritic reorganization can be prevented by blocking the electrical activity with repeated tetrodotoxin injections into the eye during the whole critical period. Our analysis was performed on neurons filled with horseradish peroxidase.

Alterations of potassium channel activity in retinal Müller glial cells induced by arachidonic acid

Arachidonic acid, which is thought to be involved in pathogenetic mechanisms of the central nervous system, has been shown previously to modulate neuronal ion channels and the glutamate uptake carrier of retinal glial (Müller) cells. We have used various configurations of the patch-clamp technique to determine the effects of arachidonic acid on the K+ currents of freshly isolated Müller glial cells from rabbit and human. Arachidonic acid reduced the peak amplitude of the transient (A-type) outward K+ currents in a dose-dependent and reversible manner, with a 50% reduction achieved by 4.1 microM arachidonic acid. The inward rectifier-mediated currents remained unchanged after arachidonic acid application. The amplitude of the Ca(2+)-activated K+ outward currents (KCa), which were blocked by 1 mM tetraethylammonium chloride and 40 nM iberiotoxin, respectively, was dose-dependently elevated by bath application of arachidonic acid. The activation curve of the KCa currents shifted towards more negative membrane potentials. Furthermore, arachidonic acid was found to suppress inwardly directed Na+ currents. In cell-attached recordings with 3 mM K+ in the bath and 130 mM K+ in the pipette, the KCa channels of rabbit Müller cells displayed a linear current-voltage relation, with a mean slope conductance of 102 pS. In excised patches, the slope conductance was 220 pS (150 mM K+i/130 mM K+o). The opening probability of the KCa channels increased during membrane depolarization and during elevation of the free Ca2+ concentration at the intracellular face of the membrane patches. Bath application of arachidonic acid caused a reversible increase of the single-channel opening probability, as well as an increase of the number of open channels. Arachidonic acid did not affect the single-channel conductance. Since arachidonic acid also stimulates the KCa channel activity in excised patches, the action of arachidonic acid is assumed to be independent of changes of the intracellular calcium concentration. Our results demonstrate that arachidonic acid exerts specific effects on distinct types of K+ channels in retinal glial, cells. In pathological cases, elevated arachidonic acid levels may contribute to prolonged Müller cell depolarizations, and to the initiation of reactive glial cell proliferation.

Electrophysiological characterization of a putative supporting cell isolated from the frog taste disk

Chemosensory cells in vertebrate taste organs have two obvious specializations: an apical membrane with access to the tastants occurring in food, and synapses with sensory axons. In many species, however, certain differentiated taste cells have access to the tastants but lack any synaptic contacts with axons, and a supportive rather than chemosensory function has been attributed to them. Until now, no functional data are available for these taste cells. To begin to understand their role in taste organ physiology, we have characterized with patch-clamp recording techniques the electrophysiological properties of a putative supporting cell-the so-called wing cell-isolated from frog taste disks. Wing cells were distinguished from chemosensory elements by the presence of a typical, sheet-like apical process. Their resting potential was approximately -52 mV, and the average input resistance was 4.8 GOmega. Wing cells possessed voltage-gated Na+ currents sensitive to TTX, and an inactivating, voltage-gated K+ current sensitive to TEA. Current injections elicited single action potentials but not repetitive firing. We found no evidence for voltage-gated Ca2+ currents under various experimental conditions. Amiloride-sensitive Na+ channels, thought to be involved in Na+ chemotransduction, were present in wing cells. Many of the membrane properties of wing cells have been also reported for chemosensory taste cells. The presence of ion channels in wing cells might be suggestive of a role in controlling the microenvironment inside the taste organs or the functioning of chemosensory cells or both. In addition, they might participate directly in the sensory transduction events by allowing loop currents to flow inside the taste organs during chemostimulation.

Electrophysiological properties of human astrocytic tumor cells In situ: enigma of spiking glial cells

To better understand physiological changes that accompany the neoplastic transition of astrocytes to become astrocytoma cells, we studied biopsies of low-grade, pilocytic astrocytomas. This group of tumors is most prevalent in children and the tumor cells maintain most antigenic features typical of astrocytes. Astrocytoma cells were studied with the use of whole cell patch-clamp recordings in acute biopsy slices from 4-mo- to 14-yr-old pediatric patients. Recordings from 53 cells in six cases of low-grade astrocytomas were compared to either noncancerous peritumoral astrocytes or astrocytes obtained from other surgeries. Astrocytoma cells almost exclusively displayed slowly activating, sustained, tetraethylammonium (TEA)-sensitive outward potassium currents (delayed rectifying potassium currents; IDR) and transient, tetrodotoxin (TTX)-sensitive sodium currents (INa). By contrast, comparison glial cells from peritumoral regions or other surgeries showed IDR and INa, but in addition these cells also expressed transient "A"-type K+ currents and inwardly rectifying K+ currents (IIR), both of which were absent in astrocytoma cells. IIR constituted the predominant conductance in comparison astrocytes and was responsible for a high-resting K+ conductance in these cells. Voltage-activated Na+ currents were observed in 37 of 53 astrocytoma cells. Na+ current densities in astrocytoma cells, on average, were three- to fivefold larger than in comparison astrocytes. Astrocytoma cells expressing INa could be induced to generate slow action potential-like responses (spikes) by current injections. The threshold for generating such spikes was -34 mV (from a holding potential of -70 mV). The spike amplitude and time width were 52.5 mV and 12 ms, respectively. No spikes could be elicited in comparison astrocytes, although some of them expressed Na+ currents of similar size. Comparison of astrocytes to astrocytoma cells suggests that the apparent lack of IIR, which leads to high-input resistance (>500 MOmega), allows glioma cells to be sufficiently depolarized to generate Na+ spikes, whereas the high resting K+ conductance in astrocytes prevents their depolarization and thus generation of spikes. Consistent with this notion, Na+ spikes could be induced in spinal cord astrocytes in culture when IIR was experimentally blocked by 10 microM Ba2+, suggesting that the absence of IIR in astrocytoma cells is primarily responsible for the unusual spiking behavior seen in these glial tumor cells. It is unlikely that such glial spikes ever occur in vivo.

Differential expression of sodium channel genes in retinal ganglion cells

Action potential electrogenesis in the axons of retinal ganglion cells is supported by voltage-gated sodium channels, and a tetrodotoxin (TTX)-inhibitable sodium conductance participates in anoxic injury of these axons within the optic nerve. However, the subtypes of sodium channels expressed in retinal ganglion cells have not been identified. In this study, we used reverse transcription-polymerase chain reaction (RT-PCR) and restriction enzyme mapping, together with in situ hybridization, to examine the expression of transcripts for sodium channel alpha-subunits I, II, III, NaG, Na6, hNE/PN1 and SNS, and beta-subunits 1 and 2, in the retina of the adult rat. RT-PCR yielded high levels of amplification of I, II, III, Na6, beta1 and beta2 transcripts. In situ hybridization demonstrated the presence of all these mRNAs in the cell bodies of retinal ganglion cells. Retinal ganglion cells thus express multiple sodium channel mRNAs, suggesting that they deploy several different types of sodium channels.

Postnatal development of ionic currents in rat hippocampal astrocytes in situ

Developmental changes in ion channel expression and cell morphology were studied in glial cells with the use of whole cell patch-clamp recordings in rat [postnatal day (P)5-P50] hippocampal slices. Recordings were obtained from 234 cells, presumed to be glia, in stratum radiatum and stratum lacunosum-moleculare of the CA1 region. Of 66 recorded cells filled with Lucifer yellow, 48 stained positive for glial fibrillary acidic protein, which identified them as astrocytes. All glial cells studied were of a stellate morphology, and developmental changes primarily comprised an increase in the length and number of cell processes associated with an overall increase in cell size and membrane capacitance. Two distinct outward potassium currents could be identified: a transient 4-aminopyridine-sensitive current (Ia) and a persistent outward current sensitive to tetraethylammonium (Id). Ia activated at -40 mV, and steady-state activation and inactivation midpoints were -16 and -74 mV, respectively. Decay time constants ranged from 7 ms at -30 mV to 19 ms at +80 mV. Id activated at -30 mV. A third K+ current sensitive to cesium activated with hyperpolarizing command voltages and showed strong inward rectification. Transient, voltage-activated sodium currents (I(Na)) were tetrodotoxin sensitive (100 nM) and activated at about -40 mV, peaked at about -10 mV, and reversed at +63 mV. I(Na) was half-inactivated at -49 mV and half-activated at -19 mV. During the first 2 wk of postnatal development, the percentage of cells showing inwardly rectifying K+ current (Ir), Ia, and I(Na) increased significantly from 40% (at P5) to 90% (at P20-P50). By contrast, almost all cells independent of age expressed Id. Specific conductances for Ir (g(ir)) and Ia increased significantly between P5 and P20, concomitant with a decrease in input resistance. By contrast, specific conductance of the outwardly rectifying K+ current (g(d)) decreased threefold between P5 and P20. Specific Na+ conductance was always <1/4 of the total potassium conductance. These results indicate that CA1 hippocampal astrocytes are characterized by expression of voltage-activated Na+ channels and three types of K+ channels showing changes in their relative expression during early postnatal development: 1) the number of cells expressing Ia, Ir, and I(Na) increases significantly and 2) their specific conductance changes such that g(d), predominant at P5-P20, is gradually replaced by g(ir), the predominant conductance in adult astrocytes. Adult morphological and electrophysiological phenotypes are established at about P20. These data suggest that previous studies in which cultured or acutely isolated cells from immature or embryonic rats were used were not adequately reflecting the properties of hippocampal astrocytes in situ.

Sodium current amplitude increases dramatically in human retinal glial cells during diseases of the eye

Müller cells, the main macroglial cells of the retina, express several types of voltage and ligand-activated ion channels, including Na+ channels. Using the whole-cell voltage-clamp technique, we studied the expression of Na+ currents in acutely isolated, non-cultivated human Müller cells from retinas of healthy organ donors and patients suffering from different eye diseases. In both types of retinas transient Na+ currents could be recorded from Müller cells. The tetrodotoxin-resistant Na+ currents, which were not completely blocked even at a concentration of 10 microM tetrodotoxin, had a mean current density of 3.0 +/- 3.0 pA/pF (mean +/- SD, n = 10) in Müller cells from donor retinas and of 12.2 +/- 9.6 pA/pF (n = 74) in Müller cells from patient retinas. Only 33.3% of healthy but 88.4% of pathological Müller cells depicted such currents. The GNa+/GK+ ratio was very high in several Müller cells from patient retinas, such that action potential-like activity could be generated after prehyperpolarizing current injection in some of these cells. Apparently, the Na+ channels, due to their negative steady-state inactivation curve (Vh = -84.5 mV), do not influence the lowered membrane potential of the pathological cells, since they are inactivated at these voltages. Currently, we do not have an explanation for the increase in amplitude and frequency of Na+ currents in human Müller cells under pathological conditions. However, the up-regulation of Na+ channels may mirror a basic glial response to pathological conditions, since it has also been found previously in human hippocampal astrocytes from epileptic foci and in rat cortex stab wounds lined by an astrocytic scar.

The Müller cell: a functional element of the retina

Müller cells are the principal glial cells of the retina, assuming many of the functions carried out by astrocytes, oligodendrocytes and ependymal cells in other CNS regions. Müller cells express numerous voltage-gated channels and neurotransmitter receptors, which recognize a variety of neuronal signals and trigger cell depolarization and intracellular Ca2+ waves. In turn, Müller cells modulate neuronal activity by regulating the extracellular concentration of neuroactive substances, including: (1) K+, which is transported via Müller-cell spatial-buffering currents; (2) glutamate and GABA, which are taken up by Müller-cell high-affinity carriers; and (3) H+, which is controlled by the action of Müller-cell Na(+)-HCO3- co-transport and carbonic anhydrase. The two-way communication between Müller cells and retinal neurons indicates that Müller cells play an active role in retinal function.

Voltage-gated Na+ channels in glia: properties and possible functions

Glial cells are nervous-system cells that have classically been considered to be inexcitable. Despite their lack of electrical excitability, they can express voltage-activated Na+ channels with properties similar to the Na+ channels used by excitable cells to generate action potentials. The functional role that these voltage-activated Na+ channels play in glia is unclear. Three functions have been proposed: (1) glial cells might synthesize Na+ channels and donate them to adjacent neurons, thereby reducing the biosynthetic load of neurons; (2) Na+ channels might endow glial cells with the ability to sense electric activity of neighboring neurons, and might thus play a role in neuro-glial communication; and (3) Na+ influx through voltage-gated Na+ channels could be important to fuel the glial (Na+,K+)-ATPase, thereby facilitating and possibly modulating K+ uptake from the extracellular space.

Müller glial cells of the tree shrew retina

The tree shrew is one of the few mammalian species whose retinae are strongly cone dominated, which is usually the case in reptilian and avian retinae. Müller cells of the tree shrew (Tupaia belangeri) retina were studied by transmission electron microscopy of tissue sections and freeze-fracture replicas, by immunolabeling of the intermediate filament protein vimentin in radial paraffin sections and in whole retinae, as well as by intracellular dye injection in slices of retinae. In addition, enzymatically isolated cells were stained by Pappenheim's panoptic staining method. The cells showed an ultrastructure that is similar to other mammalian Müller cells with two exceptions: Due to the extensive lateral fins of cone inner segments, the apical microvilli of Müller cells are arranged in peculiar palisades, and the basket-like Müller cell sheaths around neuronal somata in both nuclear layers consist of unusual multilayered membrane lamellae. Unlike Müller cells in other mammalian species studied thus far, but similar to reptilian and avian Müller cells, those of tree shrews commonly have two or more vitread processes rather than one main trunk. Müller cell densities range between some 13,000 mm-2 in the periphery and about 20,000 mm-2 in the retinal center. Neuron:(Müller)glial cell ratios were estimated to be 7.9:1 in the center and 6.2:1 in the periphery. For each Müller cell, about 1.5 (cone) photoreceptor cells, four or five interneurons of the inner nuclear layer, and about one cell of the ganglion cell layer were counted. This is a much lower number of neurons per Müller cell than in most other mammals studied.

Potassium currents in endfeet of isolated Müller cells from the frog retina

Voltage dependent potassium currents were recorded using the whole-cell mode of the patch-clamp technique for the first time from endfeet of Müller cells dissociated from the frog retina. Recordings from intact cells and isolated endfeet indicate that the inward rectifier potassium channel is the dominant ion channel in these cells and that the density of these channels is highest in the endfoot as has been previously reported for several other species. The present study uses rapid changes in [K+]o to understand the behavior of these channels in buffering [K+]o in the retina. With rapid changes in [K+]o, it was found that, at a membrane potential of -90mV, which is close to EK, increasing [K+]o from 3 to 10 mM produced an inward K+ current 5.48 +/- 0.89 SD (n = 9) times larger than outward current induced by decreasing [K+]o from 3 to 1 mM. The outward current was maximal at a holding potential of about -80mV and exhibited inactivation at more positive potentials. At -40 mV both the inward and outward currents are markedly reduced. The current voltage curve for the inward current was linear at holding potentials from -50 mV to -140 mV. Using 20 mV voltage steps, it was found that the voltage dependent K+ currents were unaffected by the addition of 2 mM Cd2+, a blocker of Ca(2+)-activated potassium currents, decreasing [Cl-]o from 120 mM to 5 mM or the substitution of 30 mM Na+ by TEA. The addition of 5 mM [Cs+]o blocked only the inward current. Both the outward and the inward currents disappeared in the absence of intracellular and extracellular K+; 0.3 mM [Ba2+]o blocked the inward current completely and strongly inhibited the outward current in a time and voltage dependent manner. We conclude that at physiological [K+]o and membrane potential, the K+ channels in the Müller cell endfoot are well suited to carry K+ both inward and outward across the membrane as required for spatial buffering.