Spatial buffering of light-evoked potassium increases by retinal Müller (glial) cells

Aquaporin-4 water channel protein in the rat retina and optic nerve: polarized expression in Müller cells and fibrous astrocytes

The water permeability of cell membranes differs by orders of magnitude, and most of this variability reflects the differential expression of aquaporin water channels. We have recently found that the CNS contains a member of the aquaporin family, aquaporin-4 (AQP4). As a prerequisite for understanding the cellular handling of water during neuronal activity, we have investigated the cellular and subcellular expression of AQP4 in the retina and optic nerve where activity-dependent ion fluxes have been studied in detail. In situ hybridization with digoxigenin-labeled riboprobes and immunogold labeling by a sensitive postembedding procedure demonstrated that AQP4 and AQP4 mRNA were restricted to glial cells, including MHller cells in the retina and fibrous astrocytes in the optic nerve. A quantitative immunogold analysis of the MHller cells showed that these cells exhibited three distinct membrane compartments with regard to AQP4 expression. End feet membranes (facing the vitreous body or blood vessels) were 10-15 times more intensely labeled than non-end feet membranes, whereas microvilli were devoid of AQP4. These data suggest that MHller cells play a prominent role in the water handling in the retina and that they direct osmotically driven water flux to the vitreous body and vessels rather than to the subretinal space. Fibrous astrocytes in the optic nerve similarly displayed a differential compartmentation of AQP4. The highest expression of AQP4 occurred in end feet membranes, whereas the membrane domain facing the nodal axolemma was associated with a lower level of immunoreactivity than the rest of the membrane. This arrangement may allow transcellular water redistribution to occur without inducing inappropriate volume changes in the perinodal extracellular space.

Extracellular K+ activates a K(+)- and H(+)-permeable conductance in frog taste receptor cells

1. The effect of extracellular K+ on membrane currents of bull frog (Rana catesbeiana) taste receptor cells (TRCs) was investigated by the patch clamp and fast perfusion techniques. Extracellular K+ (2.5-90 mM) increased a TRC resting conductance and enhanced both inward and outward whole-cell currents. 2. To isolate the inward current activated by external potassium (PA current), TRCs were dialysed with 110 mM NMGCl while extracellular NaCl was replaced with NMGCl. Under these conditions, the PA current displayed an S-shaped current-voltage (I-V) curve in the -100 to 100 mV range. Extracellular Rb+ and NH4+, but not Li+, Na+ or Cs+, evoked similar currents. 3. The PA current reversal potential (Vr) did not follow the equilibrium K+ potential under experimental conditions. Therefore, K+ ions were not the only current carriers. The influence of other ions on the PA current Vr indicated that the channels involved are permeable to K+ and H+ and much less so to Na+, Ca2+ and Mg2+. Relative permeabilities were estimated on the basis of the Goldman-Hodgkin-Katz equation as follows: PH:PK:PNa = 4000:1:0.04. 4. All I-V curves of the PA current were nearly linear at low negative potentials. The slope conductance at these voltages was used to characterize the dependence of the PA current on external K+ and H+. The slope conductance versus K+ concentration was fitted by the Hill equation. The data yielded a half-maximal concentration, K1/2 = 19 +/- 3 mM and a Hill coefficient, nH = 1.53 +/- 0.36 (means +/- S.E.M.). 5. The dependence of the mean PA current and the current variance on the K+ concentration indicated a rise in the open probability of the corresponding channels as extracellular K+ was increased. With 110 mM KCl in the bath, the single channel conductance was estimated at about 6 pS. Taken together, the data suggest that extracellular K+ may serve as a ligand to activate specific small-conductance cation channels (PA channels). The mean number of the PA channels per TRC was estimated as at least 2000. 6. Extracellular Ba2+, Cd2+, Co2+, Ni2+ and Cs+ blocked the PA current in a potential-dependent manner. The PA current was blocked by Cs+ as quickly as the blocker could be applied (approximately 15 ms). The time course of the divalent cation block was well fitted by a single exponential function. The time constants were estimated at 26.5 +/- 1.9, 41.7 +/- 3.1, 56.1 +/- 4.2 and 370 +/- 18 ms at 1 mM Cd2+, Co2+, Ni2+ and Ba2+, respectively. The blocker efficiency at negative voltages followed the sequence: Cs+ > Cd2+ > Ba2+ > Ni2+ > Co2+. 7. The data indicate that protons and divalent blockers act within the PA channel pore and that H+ and the divalent ions probably act via similar mechanisms to affect the PA current. These observations and the strong pH dependence of the PA current Vr suggest that H+ occupation of the PA channel pore leading to interruption of K+ flux is the main mechanism of the pH dependence of the PA current. 8. Extracellular K+ enhanced the sensitivity of isolated TRCs to bath solution acidification due to activation of the PA channels. With 10 mM K+ in the bath, half-maximal depolarization of the TRCs was observed at pH values of 6.4-6.8. The possible role of the PA channels in sour transduction is discussed.

Potassium homeostasis and glial energy metabolism

Since capillaries appear not to contribute significantly to rapid removal of K+ from brain tissue, the K+ released into extracellular clefts by neurons at the onset of electrical activity is presumably removed either by redistribution in the clefts or by uptake into cells. What appear to be the three major processes require no energy from the glial cells. These are diffusion through the extracellular clefts, spatial buffering by glial cells, and net uptake of K+ into glial cells through glial K+ channels associated with uptake of Cl- through an independent Cl- conductance. There is a relatively slow uptake by the Na+/K+-ATPase, which directly consumes ATP. In addition, some glial cells take up K+ on the Na+/K+/2Cl- cotransporter, which leads indirectly to energy consumption when the Na+ is subsequently pumped out. Currently available data suggest that the glial energy metabolism devoted to K+ homeostasis is less than a tenth of the total tissue energy metabolism, even under conditions of pathologically high extracellular [K+]. Hence, in situ, it is possible that glial cells could function with much less ATP than neurons do. All the various routes of muffling of changes in extracellular [K+] can be modulated, directly or indirectly, by transmitters liberated by neurons. A consequence of this could be regulation of the entry of Na+ into glial cells such that the Na+/K+-ATPase is activated. The degree of activation might be adjusted so that the resulting activation of the glial glycolytic pathway is appropriate to the provision of the quantity of metabolic substrates required by the neurons.

Relevance of Na,K-ATPase to local extracellular potassium homeostasis and modulation of synaptic transmission

The ion gradients generated by the Na,K-ATPase are essential for Na+-coupled transport systems, osmoregulation and restoration of ion concentrations in excitable tissues. Indirectly, the sodium pump controls intracellular Ca2+ concentration through the Na/Ca exchanger. In the nervous system various neurotransmitters can modulate Na,K-ATPase activity. The great diversity of Na,K-ATPase subunit isoforms, their complex spatial and temporal regulation of expression and their cellular localisation imply a functional role of the sodium pump in different regulatory pathways. Among these, potassium homeostasis and modulation of synaptic transmission are discussed here.

Loss of inwardly rectifying potassium currents by human retinal glial cells in diseases of the eye

We compared the inward K+ currents of Müller glial cells from healthy and pathologically changed human retinas. To this purpose, the whole-cell voltage-clamp technique was performed on noncultured Müller cells acutely isolated from human retinas. Cells originated from retinas of four healthy organ donors and of 24 patients suffering from different vitreoretinal and chorioretinal diseases. Müller cells from organ donors displayed inward K+ currents in the whole-cell mode similar to those found in other species. In contrast, this pattern was clearly changed in the Müller cells from patient retinas. In whole-cell recordings many Müller cells had strongly decreased inward K+ current amplitudes or lost these currents completely. Thus, the mean input resistance of Müller cells from patients was significantly increased to 1,129 +/- 812 M omega, compared to 279 +/- 174 M omega in Müller cells from healthy organ donor retinas. Accordingly, since the membrane potential is mainly determined by the K+ inward conductance in healthy Müller cells, a large amount of Müller cells from patient retinas had a membrane potential which was significantly lower than that of Müller cells from control eyes. The mean membrane potentials were -37 +/- 24 mV and -63 +/- 25 mV for patient and donor Müller cells, respectively. The newly described membrane characteristic changes of Müller cells from patient eyes are assumed to interfere severely with normal retinal function: (1) the retinal K+ homeostasis, which is partly regulated by the Müller cell-mediated spatial buffering, should be disturbed, and (2) the diminished membrane potential should influence voltage-dependent transporter systems of the Müller cells, e.g., the Na(+)-dependent glutamate uptake.

Intracellular ATP activates inwardly rectifying K+ channels in human and monkey retinal Müller (glial) cells

1. In the vertebrate retina, the inwardly rectifying K+ (KIR) channels of the Müller (glial) cells are pathways for the redistribution of excess extracellular K+. Due to this role in K+ homeostasis, the activity of Müller cell KIR channels is likely to have significant functional consequences for the retina. In this study we asked whether intracellular ATP regulates the function of KIR channels expressed by Müller cells, the principal glia of the retina. 2. Freshly dissociated Müller cells from the human and monkey (Macaca fascicularis) retina were studied with various configurations of the patch-clamp technique. 3. Whole-cell recordings from Müller cells revealed that a run-down of the inwardly rectifying K+ current (IK(IR)) was prevented if the pipette solution contained Mg-ATP. Chemical ischaemia induced by inhibitors of glycolysis and oxidative phosphorylation caused a nearly 10-fold reduction in the IK(IR)) that was fully restored when metabolically inhibited Müller cells were internally perfused with ATP. 4. In recordings from membrane patches of fresh primate Müller cells, we found that inward-rectifying channels with a conductance of 20 pS in 100 mM Ko+ were the predominant type of KIR channel. In excised patches these 20 pS KIR channels were activated when Mg-ATP was at the cytoplasmic surface. Experiments with inside-out patches indicated that the activity of the 20 pS KIR channels can be maintained by ATP synthesized at sites located close to the channel. 5. The inability of the non-hydrolysable ATP analogue 5'-adenylylimidodiphosphate (AMP-PNP) to prevent the run-down of IK(IR))and the Mg2+ dependence of the ATP effect on KIR channels are consistent with a mechanism of activation requiring the hydrolysis of ATP. 6. These observations suggest that the metabolic state of a Müller cell regulates the activity of its 20 pS KIR channels and thus influences the function of the glial cell in maintaining K+ homeostasis in the retina.

cGMP-mediated effects on the physiology of bovine and human retinal Müller (glial) cells

1. Whole-cell currents of freshly dissociated or cultured Müller cells from human and bovine retinas were studied using the perforated-patch and standard whole-cell recording techniques. 2. We found that internal perfusion of cGMP or external exposure to 8-bromo-cGMP activated a calcium permeable, non-selective cation current in Müller cells, the principal glial cells of the retina. In addition, the activity of calcium-activated potassium channels increased markedly. These currents were minimally affected by cAMP. 3. Molecular studies using the reverse transcription-polymerase chain reaction demonstrated that human müller cells in culture contain transcripts closely related to the rod cyclic nucleotide-gated (CNG) channel. 4. Since guanylate cyclase is a known target for nitric oxide (NO), we tested the effect of NO donors on Müller cell currents. These agents induced currents that were qualitatively similar to those activated by cGMP. 5. Our experiments support the idea that the NO-cGMP pathway regulates the physiology of Müller cells and may play a role in integrating neuron-glia interactions in the retina.

Manipulation of the delayed rectifier Kv1.5 potassium channel in glial cells by antisense oligodeoxynucleotides

Glial cells have been shown to express several biophysically and pharmacology distinct potassium channel types. However, the molecular identity of most glial K+ channels is unknown. We have developed an antibody specific for the Shaker type potassium channel Kv1.5 protein, and demonstrate by immunohistochemistry the presence of this channel in glial cells of adult rat hippocampal and cerebellar slices, as well as in cultured spinal cord astrocytes. Immunoreactivity was particularly intense in the endfoot processes of astrocytes surrounding the microvasculature of the hippocampus. The specific contribution of this channel protein to the delayed rectifying K+ current of spinal cord astrocytes was determined by incubating these cells with antisense oligodeoxynucleotides complementary to the mRNA coding for Kv1.5 protein. Such treatment reduced delayed rectifier current density and shifted the potassium current steadystate inactivation, without altering current activation, cell capacitance, or cell resting potential. The tetraethylammonium acetate (TEA) sensitivity of astrocytic delayed rectifier current was enhanced following antisense oligodeoxynucleotide treatment, suggesting that Kv1.5 channel protein may provide a significant component of the TEA-insensitive current in this preparation. Our results suggest that Kv1.5 is widely expressed in glial cells of brain and spinal cord and that delayed rectifying K+ currents in astrocytes are largely mediated by Kv1.5 channel protein.

Chemically mediated cross-excitation in rat dorsal root ganglia

Primary afferent neurons in mammalian dorsal root ganglia (DRGs) are anatomically isolated from one another and are not synaptically interconnected. As such, they are classically thought to function as independent sensory communication elements. However, it has recently been shown that most DRG neurons are transiently depolarized when axons of neighboring neurons of the same ganglion are stimulated repetitively. Here we further characterize this functional coupling. In electrophysiological recordings made from excised rat DRGs, we found that DRG "cross-depolarization" is excitatory in that it is accompanied by an increase in the probability of spiking in response to otherwise subthreshold test pulses delivered intracellularly. Cross-depolarization contributes to this mutual cross-excitation. However, at least as important a contribution comes from a net increase in the neurons' input resistance (Rin) triggered by the stimulation of neighboring neurons. This change in Rin occurs even when cross-depolarization is absent or is balanced out. The amplitude of cross-depolaration was found to be voltage-dependent, with a reversal potential at approximately -23mV. Reversibility and the change in Rin both indicated that activity of neighboring neurons causes a membrane conductance change that is chemically mediated. Thus, far from being isolated, most DRG neurons participate in ongoing mutual interactions in which neuronal excitability is continuously modulated by afferent spike activity. This intraganglionic dialog appears to be mediated, at least in part, by an activity-dependent diffusable substance(s) released from neuronal somata and/or adjacent axons, and detected by neighboring cell somata and/or axons.

Glial potassium channels activated by neuronal firing or intracellular cyclic AMP in Helix

1. Cell-attached and whole cell patch clamp experiments were performed on satellite glial cells adhering to the cell body of neurones in situ within the nervous system of the snail Helix pomatia. The underlying neurone was under current or voltage-clamp control. 2. Neuronal firing induced a delayed (20-30 s) persistent (3-4 min) increase in the opening probability of glial K+ channels. The channels were also activated by perfusing the ganglion with a depolarizing high-K+ saline, except when the underlying neurone was prevented from depolarizing under voltage-clamp conditions. 3. Two K(+)-selective channels were detected in the glial membrane. The channel responding to neuronal firing was present in 95% of the patches (n = 393). It had a unitary conductance of 56 pS, a Na+ :K+ permeability ratio < 0.02 and displayed slight inward rectification in symmetrical [K+] conditions. It was sensitive to TEA, Ba2+ and Cs+. The following results refer to this channel as studied in the cell-attached configuration. 4. The glial K+ channel was activated by bath application of the membrane-permeant cyclic AMP derivatives 8-bromo-cAMP and dibutyryl-cAMP, the adenylyl cyclase activator forskolin and the diesterase inhibitors IBMX, theophylline and caffeine. It was insensitive to cyclic GMP activators and to conditions that might alter the intracellular [Ca2+] (ionomycin, low-Ca2+ saline and Ca2+ channel blockers). 5. The forskolin-induced changes in channel behaviour (open and closed time distributions, burst duration, short and long gaps within bursts) could be accounted for by a four-state model (3 closed states, 1 open state) by simply changing one of the six rate parameters. 6. The present results suggest that the signal sent by an active neurone to satellite glial cells is confined to the glial cells round that neurone. The effect of this signal on the class of glial K+ channels studied can be mimicked by an increase in glial cAMP concentration. The subsequent delayed opening of the glial K+ channels does not appear to play a role in siphoning the excess K+ released by active neurones. It is hypothesized that the cAMP-gated glial K+ channels may be involved in the control of glial cell proliferation.

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.

Chemically mediated cross-excitation in rat dorsal root ganglia

Primary afferent neurons in mammalian dorsal root ganglia (DRGs) are anatomically isolated from one another and are not synaptically interconnected. As such, they are classically thought to function as independent sensory communication elements. However, it has recently been shown that most DRG neurons are transiently depolarized when axons of neighboring neurons of the same ganglion are stimulated repetitively. Here we further characterize this functional coupling. In electrophysiological recordings made from excised rat DRGs, we found that DRG "cross-depolarization" is excitatory in that it is accompanied by an increase in the probability of spiking in response to otherwise subthreshold test pulses delivered intracellularly. Cross-depolarization contributes to this mutual cross-excitation. However, at least as important a contribution comes from a net increase in the neurons' input resistance (Rin) triggered by the stimulation of neighboring neurons. This change in Rin occurs even when cross-depolarization is absent or is balanced out. The amplitude of cross-depolaration was found to be voltage-dependent, with a reversal potential at approximately -23mV. Reversibility and the change in Rin both indicated that activity of neighboring neurons causes a membrane conductance change that is chemically mediated. Thus, far from being isolated, most DRG neurons participate in ongoing mutual interactions in which neuronal excitability is continuously modulated by afferent spike activity. This intraganglionic dialog appears to be mediated, at least in part, by an activity-dependent diffusable substance(s) released from neuronal somata and/or adjacent axons, and detected by neighboring cell somata and/or axons.

Structural specializations of human retinal glial cells

Electron microscopy and immunohistochemistry have been used to study the structural specializations of astrocyte and Müller glia cells in human retinas. The astrocytes and Müller cells contribute to the formation of the internal limiting membrane, the retina, the blood vessel glial limiting membranes and the glial sheaths of the ganglion cells. Two types of junctions were observed among retinal glial cells. Adherent junctions were found between astrocytes and Müller cells, and between adjacent astrocytes. Gap junctions were only observed between astrocyte processes. These similarities suggest that astrocytes and Müller cells can perform the same functions in human retinas. Finally, the "perivascular astrocyte of Wolter", related only to the blood vessels, was not found. All the retinal astrocytes have the same ultrastructural characteristics, confirming the absence of these astroglial cells in human retinas observed by immunohistochemical techniques.

Potassium-evoked directionally selective responses from rabbit retinal ganglion cells

Previous studies have shown that directionally selective (DS) retinal ganglion cells cannot only discriminate the direction of a moving object but they can also discriminate the sequence of two flashes of light at neighboring locations in the visual field: that is, the cells elicit a DS response to both real and apparent motion. This study examines whether a DS response can be elicited in DS ganglion cells by simply stimulating two neighboring areas of the retina with high external K+. Extracellular recordings were made from ON-OFF DS ganglion cells in superfused rabbit retinas, and the responses of these cells to focal applications of 100 mM KCl to the vitreal surface of the retina were measured. All cells produced a burst of spikes (typically lasting 50-200 ms) when a short pulse (10-50 ms duration) of KCl was ejected from the tip of a micropipette that was placed within the cell's receptive field. When KCl was ejected successively from the tips of two micropipettes that were aligned along the preferred-null axis of a cell, sequence-dependent responses were observed. The response to the second micropipette was suppressed when mimicking motion in the cell's null direction, whereas an enhancement during apparent motion in the opposite direction frequently occurred. Sequence discrimination in these cells was eliminated by the GABA antagonist picrotoxin and by the Ca(2+)-channel blocker omega-conotoxin MVIIC, two drugs that are known to abolish directional selectivity in these ganglion cells. The spatiotemporal properties of the K(+)-evoked sequence-dependent responses are described and compared with previous findings on apparent motion responses of ON-OFF DS ganglion cells.

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.

Effect of cutting the optic nerve on K+ currents in endfeet of Muller cells isolated from frog retina

Membrane currents were recorded from Muller cells isolated from normal retinas and from retinas whose ganglion cell axons had been cut in the optic nerve 30-60 days previously. The surgical procedure did not block the retinal blood supply and did not allow the axons to regenerate. The principal finding was that after severing the optic nerve there was less inward rectification in response to voltage commands. That is, the maintained inward current (I K(IN)) produced in response to a hyperpolarizing voltage command decreased leading to a decrease in the ratio I K(IN)/I K(OUT) In 98 mM [K+]O, this ratio was 2.86 +/- 0.21 (mean +/- SE; n = 24) in controls and 1.13 +/- 0.13 (n = 21) in Muller cells from denervated retinae. Barium, a blocker of the potassium inward rectifier (I (KIR)), eliminated this difference. Moreover, severing the optic nerve also decreased the resting potentials of Muller cells in 2.5 mM [K+]O from -83 +/- 7 mV to -63 +/- 9 mV. The results suggest that the voltage-dependent behavior and selectivity of K+ inward rectifying channels (K (ir)) in the endfeet depends on the integrity of the closely apposed ganglion cells.

Nerve impulses increase glial intercellular permeability

Coordinating the activity of neurons and their satellite glial cells requires mechanisms by which glial cells detect neuronal activity and change their properties as a result. This study monitors the intercellular diffusion of the fluorescent dye Lucifer Yellow (LY), following its injection into glial cells of the frog optic nerve, and demonstrates that nerve impulses increase the permeability of interglial gap junctions. Consequently, the spatial buffer capacity of the neuroglial cell syncytium for potassium, other ions, and small molecules will be enhanced; this may facilitate glial function in maintaining homeostasis of the neuronal microenvironment.

Activation of NMDA receptor-channels in human retinal Müller glial cells inhibits inward-rectifying potassium currents

Although it is well known that neurotransmitters mediate neuron-to-neuron communication, it is becoming clear that neurotransmitters also affect glial cells. However, knowledge of neuron-to-glial signalling is limited. In this study, we examined the effects of the glutamate agonist N-methyl-D-aspartate (NMDA) on Müller cells, the predominant glia of the retina. Our immunocytochemical studies and immunodetection by Western blotting with monoclonal antibodies specific for the NMDAR1 subunit provided evidence for the expression by human Müller cells of this essential component of NMDA receptor-channels. Under conditions in which potassium currents were blocked, NMDA-induced currents could be detected in perforated-patch recordings from cultured and freshly dissociated human Müller cells. These currents were inhibited by competitive and non-competitive blockers of NMDA receptor-channels. Extracellular magnesium reduced the NMDA-activated currents in a voltage-dependent manner. However, despite a partial block by magnesium, Müller cells remained responsive to NMDA at the resting membrane potential. Under assay conditions not blocking K+ currents, exposure of Müller cells to NMDA was associated with an MK-801 sensitive inhibition of the inward-rectifying K+ current (IK(IR)), the largest current of these glia. This inhibitory effect of NMDA appears to be mediated by an influx of calcium since the inhibition of IK(IR) was significantly reduced when calcium was removed from the bathing solution or when the Müller cells contained the calcium chelator, BAPTA. Inhibition of the Müller cell KIR channels by the neurotransmitter glutamate is likely to have significant functional consequences for the retina since these ion channels are involved in K+ homeostasis, which in turn influences neuronal excitability.

Current-source density analysis of the electroretinogram of the frog: methodological issues and origin of components

The technique of current-source density (CSD) analysis for extracellular potentials is reviewed, along with some methodological features that are important for performing CSD analysis of the electroretinogram. In addition, three formulas for computing CSD's are examined on model circuits of resistors and current generators. Finally, CSD results from frog retina that bear on the origins of the b, d, and M waves, along with slow PIII, are presented. It is concluded that the b and d waves are generated primarily and directly by bipolar cells, whereas the M wave and the slow PIII are generated by Müller (glial) cells through the K+ spatial buffer mechanism.

High potassium promotes differentiation of retinal neurons but does not favor rod differentiation

Neural retinal cells of newborn rats were cultured under dissociated culture conditions. Differentiation of several types of retinal cells was confirmed by immunohistochemical detection of type-specific neural phenotypes. We used Thy-1.1 antigen as a ganglion cell marker, HPC-1 or GABA as an amacrine cell marker and rhodopsin as a rod cell marker. With a high concentration of potassium (38 mM), expression of the respective neural phenotypes were differentially affected. High K+ increased the number of Thy-1.1 positive cells 6 to 8 fold, and drastically promoted their neurite extension. The same culture conditions, however, reduced considerably the number of rhodopsin positive cells, possibly due to the unique membrane properties of photoreceptors. A high K+ concentration also promoted differentiation of HPC-1 positive and GABA positive cells, but to a lesser extent than the Thy-1.1 positive cells. Several possibilities were examined to understand the effect of a high K+ concentration on retinal neural cells. The total cell number in cultures with a high K+ concentration was approximately half of that in control cultures at day 3 and slightly smaller at day 11, suggesting that high K+ did not have a positive general effect on the proliferation or survival of retinal cells. Naturally occurring neuronal death (apoptosis) is a well-known phenomenon during retinal development. A histochemical method for detecting DNA fragmentation, a step preceding apoptosis, showed that high K+ had no preventive effect. BrdU (bromodeoxyuridine) immunohistochemistry showed that high K+ did not seem to enhance proliferation of neural precursor cells. These results indicate that a high K+ concentration promotes the expression of neuronal phenotypes but is not a favorable condition for rod differentiation. Since a high K+ concentration is considered to induce depolarization of nerve cells, the present results suggest an anterograde influence from surrounding neuronal cells, through chronic depolarization by elevated K+, is essential for the differentiation and maturation of retinal cells.

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.

Thrombin-induced inhibition of potassium currents in human retinal glial (Müller) cells

1. Glial cells are known to play a role in regulating the microenvironment of the nervous system. While earlier considerations of glial function assumed a passive, static physiology for these cells, this is not likely to be the case. In this study, we begin to examine how the physiology of Müller glial cells changes in response to molecules in the microenvironment. 2. Perforated-path recordings and intracellular calcium measurements were performed on human retinal Müller cells in vitro. 3. Analysis of whole-cell currents revealed that the human Müller glial cells have an inwardly rectifying K+ current (IK(IR) which is active near the resting membrane potential. This IK(IR) is significantly inhibited when the Müller cell is exposed to thrombin, a molecule that is likely to enter the retina with a breakdown of the blood-retinal barrier and may be endogenous to the nervous system. 4. A variety of experiments point to a role for Ca2+ as a second messenger mediating the inhibitory effect of thrombin on the IK(IR) of Müller cells. Specifically, thrombin evokes an increase in intracellular [Ca2+] in the Müller cells; the Ca2+ chelator BAPTA blocks the effects of thrombin on both the inhibition of IK(IR) and the rise in intracellular [Ca2+]; exposure to ionomycin, a calcium ionophore, induces a reduction in the IK(IR) of Müller cells. 5. A thrombin- induced inhibition in the IK(IR) of Müller cells is likely to have significant functional consequences for the retina since these ion channels are involved in K+ homeostasis. 6. Our experiments support the idea that the physiology of Müller glial cells is dynamic and can be markedly affected by molecules in the microenvironment.

The late positive retinal potential in dogs

The late positive potential of the mammalian electroretinogram has been called the 'PI' or the 'c-wave' potential. It is unusual among retinal potentials because its peak implicit time increases in response to increasing stimulus intensity and because it cannot be demonstrated consistently in small samples of normal humans or normal dogs. We recorded wideband (DC-1 kHz) responses from 34 normal Beagles or dogs of similar size. Of the 34, 11 produced a late positive potential set that satisfied the criteria for c-waves. Multiple aspartate injections always increased c-wave amplitude and stimulus-response linearity in all 'producers'. Non-producers were never converted to producer status by aspartate blocking of the inner retina. Interaction of late positive and negative potentials and the possible influence of normal individual variations in the trans-epithelial potential are discussed. Individual mammal c-wave production is controlled by outer retinal phenomena which vary between individuals.

The c-wave of the electroretinogram possesses a third component from the proximal retina

ERG and light-induced extracellular potassium ([K+]o) changes have been measured in isolated retinas of both Rana esculenta and Rana temporaria. The conditions of the preparations have been varied. Isolated frog retinas kept receptor side-upward in a moist chamber without perfusion showed the well-known slow PIII in the ERG. Retinas superfused from the receptor side, with O2 enrichment at their vitreal surface, however, exhibit a slow cornea-positive potential in the ERG. The slow ERG-potentials relate to different light-induced potassium changes in the proximal retina. There was a long lasting and larger proximal potassium increase in adequately maintained retinas but a smaller and shorter one in preparations lacking superfusion and oxygen. There was no significant difference between the size of potassium decrease around receptors of retinas superfused from their vitreal side and those superfused from receptor side. A reduction of slow PIII should therefore not be responsible for the slow cornea-positive potential. The long lasting and larger (by 59%) potassium increase in the proximal retina may counteract the potential in the Müller cells caused by the potassium decrease around receptors and thereby cancel slow PIII and generate a third component of the electroretinogram c-wave.

A tetraethylammonium-insensitive inward rectifier K+ channel in Müller cells of the turtle (Pseudemys scripta elegans) retina

Ion channels present in isolated glial (Müller) cells from the retina of the turtle (Pseudemys scripta elegans) were studied with the patch clamp technique. The predominant conductance in these cells was due to an inward rectifying potassium current. The whole-cell conductance of the inward rectifier was 20.2 +/- 1.9 nS (n = 7 cells) in a standard extracellular saline solution (3 mM extracellular potassium). This conductance was dependent on the extracellular potassium concentration, with a 2.88-fold change in conductance per tenfold shift in concentration. The relative permeability sequence to potassium of the inward rectifier was found to be: potassium (1.0) > rubidium (0.7) > ammonium (0.2) > lithium (0.1) = sodium (0.1), which corresponded to the Eisenman sequence IV or V for a strong-field-strength potassium binding site on the channel. The single channel conductance measured in cell-attached patches with potassium chloride (150 mM) in the pipette was 68.5 +/- 6.0 pS (n = 3 patches). The inward rectifier current was not blocked by extracellular tetraethylammonium (TEA+, 20 mM), but was blocked by extracellular barium (5 mM) or cesium (5 mM). The TEA+ insensitivity of the inward rectifier potassium channel in Müller cells is unusual, given that this type of channel in most excitable cells is sensitive to micromolar concentrations of this compound, and may be a characteristic of inward rectifier potassium channels that are primarily involved with extracellular potassium regulation.

A physiological measure of carbonic anhydrase in Müller cells

Carbonic anhydrase activity was characterized in freshly dissociated Müller cells of the salamander retina. Intracellular pH was monitored using ratio imaging of the indicator dye BCECF as extracellular PCO2 was varied. The extracellular solution was switched rapidly (141 ms rise time) from a HEPES buffered to a CO2-HCO3- buffered solution (both pH 7.4). Introduction of CO2-HCO3- produced a rapid cell acidification. Cell pH dropped from a steady-state pH of 7.02 in HEPES solution to pH 6.81 in CO2-HCO3-. Methazolamide, a carbonic anhydrase inhibitor, dramatically reduced the initial rate of acidification, demonstrating that the acidification was produced by the carbonic anhydrase-catalyzed hydration of CO2. The initial rate of acidification, 52.6 pH units per min (0.88 pH units per s), was reduced approximately 150-fold to 0.36 pH units per min by 10(-3) M methazolamide. Half-maximal inhibition occurred at a methazolamide concentration of 5.6.10(-7) M. The carbonic anhydrase inhibitor acetazolamide (10(-3) M) also greatly reduced the rate of cell acidification. The latency to the onset of carbonic anhydrase inhibition was 660 ms for methazolamide and 7.5 s for acetazolamide. The carbonic anhydrase inhibitor benzolamide (10(-4) M, 4 min exposure), which is poorly membrane permeant, had little effect on the rate of cell acidification, indicating that the site of carbonic anhydrase action was intracellular. The activity of Müller cell carbonic anhydrase may help to buffer extracellular CO2 variations in the retina.

pHi-dependent Cl-HCO3 exchange at the basolateral membrane of frog retinal pigment epithelium

Intracellular pH (pHi) measurements in frog retinal pigment epithelium using the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein demonstrate that the basolateral membrane contains a pHi-sensitive Cl-HCO3 exchanger. In control Ringer solution, the removal of Cl from the basal bath alkalinized the cells by 0.07 +/- 0.03 (SD) pH units (n = 39) with an initial rate of 0.022 +/- 0.0013 pH units/min. This effect was blocked by 0.5 mM basal 4,4'-diisothiocyanostilbene-2,2'- disulfonic acid or the removal of HCO3 from both the apical and basal baths. The rate of the exchange is reduced by acidification and increased by alkalinization. Increasing apical bath K concentration ([K]o) from 2 to 5 mM approximates the [K]o change in the subretinal space of the intact eye following a transition from light to dark. This [K]o change alkalinized the cells by increasing the rate of the apical membrane Na-HCO3 cotransporter. In 5 mM apical [K]o, the initial rate of the 0 Cl-induced alkalinization was significantly increased to 304 +/- 13% (n = 4) of control (2 mM [K]o). These mechanisms regulate pHi and could also buffer changes in subretinal pH.

Glial contributions to excitatory neurotransmission in cultured hippocampal cells

Although many glial cells possess neurotransmitter receptors and transporters, little is known about glial participation in neurotransmission. To explore this issue, we recorded neuronal autaptic and glial responses from cultured hippocampal single-neuron micro-islands. Excitatory synaptic events activate rapid electrogenic glial glutamate transporter currents similar to those elicited by exogenous glutamate in other preparations. We show here that glial transporter responses may be used to sense changes in presynaptic efficacy and that glial uptake helps to remove synaptically released glutamate, thereby contributing to the termination of excitatory synaptic currents under certain conditions. These observations provide a framework for understanding the role of glia in both normal and pathological processes.

Three distinct types of voltage-dependent K+ channels are expressed by Müller (glial) cells of the rabbit retina

There is ample evidence that retinal radial glial (Müller) cells play a crucial role in retinal ion homeostasis. Nevertheless, data on the particular types of ion channels mediating this function are very rare and incomplete; this holds especially for mammalian Müller cells. Thus, the whole-cell variation of the patch-clamp technique was used to study voltage-dependent currents in Müller cells from adult rabbit retinae. The membrane of Müller cells was almost exclusively permeable to K+ ions, as no significant currents could be evoked in K(+)-free internal and external solutions, external Ba2+ (1 mM) reversibly blocked most membrane currents, and external Cs+ ions (5 mM) blocked all inward currents. All cells expressed inwardly rectifying channels that showed inactivation at strong hyperpolarizing voltages (> or = -120 mV), and the conductance of which varied with the square root of extracellular K+ concentration ([K+]e). Most cells responded to depolarizing voltages (> or = -30 mV) with slowly activating outward currents through delayed rectifier channels. These currents were reversibly blocked by external application of 4-aminopyridine (4-AP, 0.5 mM) or tetraethylammonium (TEA, > 20 mM). Additionally, almost all cells showed rapidly inactivating currents in response to depolarizing (> or = -60 mV) voltage steps. The currents were blocked by Ba2+ (1 mM), and their amplitude increased with the [K+]e. Obviously, these currents belonged to the A-type family of K+ channels. Some of the observed types of K+ channels may contribute to retinal K+ clearance but at least some of them may also be involved in regulation of proliferative activity of the cells.

Patch-clamp recording from Müller (glial) cell endfeet in the intact isolated retina and acutely isolated Müller cells of mouse and guinea-pig

Müller cells span through the entire retina and terminate with the formation of endfeet at the vitreous body. These endfeet are thought to be specialized for maintaining the K+ homeostasis in the retina based on the assumption that voltage signals can passively spread from the cell body to the endfeet. We employed the patch-clamp technique to study the physiological properties of these endfeet in a retinal wholemount preparation from guinea-pig or mouse. After assessing one endfoot with the patch pipette and establishing the whole cell recording configuration, a membrane area which approximately matched the size of one endfoot and proximal process could be voltage-clamped. This morphological correlation could be established by filling the cytoplasm with the fluorescent dye Lucifer Yellow via the patch-pipette. The morphological, immunocytochemical and ultrastructural inspection of the recorded cells revealed that mouse Müller cell endfeet were connected by only a thin stalk to the proximal process. In contrast, guinea-pig endfeet were connected by thick stalks. The endfoot current in the mouse was dominated by a voltage and time-independent K+ conductance. In contrast, in some of the recordings from guinea-pig, delayed and inwardly rectifying K+ currents were observed. These voltage-gated currents were more frequently observed or were facilitated when the membrane area under voltage clamp was increased, blocking the passive K+ currents by Ba2+ in both, mouse and guinea-pig. We thus assume that the voltage-gated currents were not in the endfeet membrane, but rather in the proximal process and could thus be better activated in the guinea-pig with its thicker stalk or after increasing the membrane area under voltage clamp control. Similar results were obtained in freshly isolated Müller cells; in contrast to the cells from the wholemount the voltage-gated currents were more frequently observed. These studies demonstrate that the Müller cell endfoot of the mouse with its vascularized retina is an electrically isolated unit and that voltage signals do not spread to the proximal process. Such a property would, however, be required for the redistribution of K+ via spatial buffer currents. In contrast, guinea-pig Müller glial cells with their stout morphological connection between endfoot and proximal process are better suited to fulfil this task.

What do retinal müller (glial) cells do for their neuronal 'small siblings'?

Müller (radial glial) cells are the predominant glia of the vertebrate retina. They arise, together with rod photoreceptor cells, bipolar cells, and a subset of amacrine cells, from common precursor cells during a late proliferative phase. One Müller cell and a species-specific number of such neurons seem to form a columnar unit within the retinal tissue. In contrast, 'extracolumnar neurons' (ganglion cells, cone photoreceptor cells, horizontal cells, and another subset of amacrine cells) are born and start differentiation before most Müller cells are generated. It may be essential for such neurons to develop metabolic capacities sufficient to support their own survival, whereas late-born ('columnar') neurons seem to depend on a nursing function of their 'sisterly' Müller cell. Thus, out of the cell types within a retinal column it is exclusively the Müller cell that possesses the enzymes for glycogen metabolism. We present evidence that Müller cells express functional insulin receptors. Furthermore, isolated Müller cells rapidly hydrolyse glycogen when they are exposed to an elevated extracellular K+ ion concentration, a signal that is involved in the regulation of neuronal-glial metabolic cooperation in the brain. Müller cells are also thought to be essential for rapid and effective retinal K+ homeostasis. We present patch-clamp measurements on Müller cells of various vertebrate species that all demonstrate inwardly rectifying K+ channels; this type of channel is well-suited to mediate spatial buffering currents. A mathematical model is presented that allows estimation of Müller cell-mediated K+ currents. A simulation analysis shows that these currents greatly limit lateral spread of excitation beyond the borders of light-stimulated retinal columns, and thus help to maintain visual acuity.

Voltage-dependent K+ currents in guinea pig Müller (glia) cells show different sensitivities to blockade by Ba2+

The effect of externally applied Ba2+ and Na+ on K+ currents was investigated by means of whole-cell patch-clamp in isolated and in situ Müller cells from guinea pig retina. Müller cells express a typical set of K+ currents, i.e. an ohmic current, an inactivating inward current (IK(IR)), a delayed rectifier (IK(DR)) and an inactivating outward current (IK(A)). Inactivation of the inward current did not occur when extracellular Na+ was replaced by choline. When administered in increasing concentrations, Ba2+ blocked these K+ currents in a typical sequence: the ohmic current and IK(A) were most sensitive, followed by IK(IR), whereas IK(DR) was not completely blocked even in 1 mM Ba2+. The differential sensitivity of Müller cell K+ currents to external Ba2+ may be a tool which can be used to improve our understanding of the Müller cell response to physiological stimulation of the retina.

The beta subunit modulates potassium activation of the Na-K pump

We recently cloned the alpha 1 and the beta 1 and beta 3 subunits of the Na,K-ATPase of the toad Bufo marinus. To investigate possible functional differences between beta 1 and beta 3, we studied the potassium activation of Na-K pumps expressed in the oocyte of Xenopus laevis. Na-K pump activity was measured as K(+)-induced current in voltage-clamped oocytes. We could take advantage of the relative resistance to ouabain conferred by the Bufo alpha subunit to study specifically the exogenously expressed Na-K pumps after inhibition of the ouabain-sensitive endogenous Xenopus Na-K pumps. Coinjection of Bufo alpha 1 subunit cRNA with either beta 1 or beta 3 cRNAs results in the expression of functional Na-K pumps that share similar low ouabain sensitivity but differ in their K+ half activation constant (K1/2). Similar results were obtained with Xenopus alpha 1 and beta 1 or beta 3 subunits and with Bufo/Xenopus heterodimers. We conclude that some specific sequence of the beta subunit can influence the activation of the Na,K pump by extracellular K+ ions.

Spatial buffering of extracellular potassium by Müller (glial) cells in the toad retina

We examined the role of Müller (glial) cells in buffering light-evoked changes in extracellular K+ concentration, [K+]o, in the isolated retina of the toad, Bufo marinus. We found evidence for two opposing Müller cell current loops that are generated by a light-evoked increase in [K+]o in the inner plexiform layer. These current loops, which are involved in the generation of the M-wave of the electroretinogram (ERG), prevent the accumulation of K+ in the inner plexiform layer by transporting K+ both to vitreous and to distal retina. In addition, under dark-adapted conditions, we found evidence for a Müller cell current loop that is generated by a light-evoked decrease in [K+]o in the receptor layer. This current loop, which is involved in the generation of the slow PIII component of the ERG, helps to buffer the light-evoked decrease in [K+]o throughout distal retina by transporting K+ from vitreous. The spatial buffering fluxes of K+ can be abolished by blocking Müller cell K+ conductance with 200 microM Ba2+. The separate contributions of the M-wave and slow PIII currents to Müller cell spatial buffering were isolated by various pharmacological treatments that were designed to enhance or suppress light-evoked activity in specific retinal neurons. Our results show that Müller cell K+ currents not only buffer light-evoked increases in [K+]o, but also buffer light-evoked decreases in [K+]o, and thereby diminish any deleterious effects upon neuronal function that could arise in response to large changes in [K+]o in the plexiform layers. Moreover, our results emphasize that spatial buffering currents generate many components of the electroretinogram.

Neuronal activity triggers calcium waves in hippocampal astrocyte networks

The recent discovery that the neurotransmitter glutamate can trigger actively propagating Ca2+ waves in the cytoplasm of cultured astrocytes suggests the possibility that synaptically released glutamate may trigger similar Ca2+ waves in brain astrocytes in situ. To explore this possibility, we used confocal microscopy and the Ca2+ indicator fluo-3 to study organotypically cultured slices of rat hippocampus, where astrocytic and neuronal networks are intermingled in their normal tissue relationships. We find that astrocytic Ca2+ waves are present under these circumstances and that these waves can be triggered by the firing of glutamatergic neuronal afferents with latencies as short as 2 s. The Ca2+ waves closely resemble those previously observed in cultured astrocytes: they propagate both within and between astrocytes at velocities of 7-27 microns/s at 21 degrees C. The ability of tissue astrocyte networks to respond to neuronal network activity suggests that astrocytes may have a much more dynamic and active role in brain function than has been generally recognized.

The effect of K(+)-conductance-blocking substances on the occurrence of retinal spreading depression

Retinal spreading depression (SD) has been thought to be generated by an abnormal increase in [K+]o in the inner plexiform layer. When a retina isolated from bullfrog was immersed in a medium conditioned for SD (low Cl- Ringer's solution), spontaneous SDs periodically occurred at a fixed interval in the dark. The effect of K(+)-conductance blocking substances (Ba2+, Cs+, TEA and 4-AP) on the occurrence of SD was studied using the concomitant changes in the field potential (spreading depression potential; SDP) as an index. These substances increased the frequency of occurrence and decreased the amplitude of the potential. According to the Müller cell theory of SDP, an increase in [K+]o depolarizes the Müller cells and the resulting extracellular current generates the potential change. Since the small amplitude of the SDP reflects the decreased K+ current, it may be said that the K(+)-buffering capacity of the Müller cells was partially reduced by the substances and that the frequency of the SD occurrence consequently increased in order to clear accumulating [K+]o from neurons with the decreased buffering capacity. The present results lend a good support to the current Müller cell theory for the occurrence of SDP.

Extracellular K+ in the supraoptic nucleus of the rat during reflex bursting activity by oxytocin neurones

1. We have investigated changes in extracellular potassium concentration [K+]o in the supraoptic nucleus of lactating rats and in particular those that occur during the intense burst of firing by the oxytocin neurones involved in the milk ejection reflex. 2. Double-barrelled K(+)-selective microelectrodes containing a highly selective sensor based on valinomycin were lowered through the exposed cortex towards the supraoptic nucleus (SON) of female rats anaesthetized with urethane. The mean resting [K+]o in the hypothalami of five rats was 2.4 mM, S.D. = 0.3 mM. 3. Where the reference barrel recorded extracellular action potentials from an oxytocin cell, the reflex burst of firing (4 s, typical maximum 50 Hz) was accompanied by a mean increase in [K+]o (delta[K+]o) of 0.22 mM (S.E.M. = 0.02 mM, fifty-seven bursts in eight cells in seven rats). The rise in [K+]o did not begin more than 0.1 s before the onset of the burst, and began to fall from its maximum during the burst. Slow field potentials, indicative of spatial buffering of K+, were undetectable (less than 50 microV). When the electrode was advanced in steps, the amplitudes of both delta[K+]o and the action potential declined steeply to about 10% over a distance of 20 microns: K+ from oxytocin cells appears to be prevented from dispersing freely through the extracellular space of the SON. 4. When the electrode recorded action potentials from a vasopressin cell, delta[K+]o during an oxytocin cell burst was very small: 0.021 mM (S.E.M. = 0.005 mM). At other sites in the SON, where antidromic stimulation evoked a field potential but no action potential, delta[K+]o was 0.047 +/- 0.005 mM. We conclude that the reason oxytocin bursts do not affect vasopressin cells is that [K+]o rises very little around vasopressin cells. A fortiori, since the increases in [K+]o were very small except where action potentials from oxytocin cells were recorded, they can make no significant contribution to synchronizing the onsets of bursts in oxytocin cells that are not contiguous. 5. A standard antidromic stimulation from the pituitary stalk, at 40 Hz for 4 s, which stimulated both oxytocin neurones and vasopressin neurones, caused a delta[K+]o of 0.17-1.8 mM, the variation being mainly from rat to rat. The larger delta[K+]o values were accompanied by slow negative potentials of up to 1.5 mV, there was a gradient in delta[K+]o decreasing towards the pia at the inferior limit of the SON, and there was a slow increase in [K+] in the subarachnoid space.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of activity-dependent potassium accumulation in mammalian peripheral nerve in vitro

Ion-sensitive microelectrodes were used to study the behavior of extracellular ions in rat sciatic nerve during and following activity. Nerve stimulation produced increases in [K+]o that were dependent upon the frequency and duration of stimulation; no change in extracellular pH occurred with stimulation. Increases in [K+]o depended on axonal discharge since they were blocked by inhibiting sodium channels with tetrodotoxin. At 22 degrees C, stimulation could induce increases in [K+]o of several mM; at 36 degrees C, stimulation rarely produced increases in [K+]o greater than 1 mM. Stimulated increases in [K+]o dissipated very slowly (i.e. t 1/2 = 50-100 s) and the rate of dissipation was not significantly affected by anoxia, changes in temperature, changes in extracellular pH, or the application of a blocker of Na+, K(+)-ATPase (ouabain) or a K+ channel blocker (Ba2+). In comparison to the central nervous system, neural activity in rat sciatic nerve produced smaller increases in [K+]o and these increases dissipated much more slowly. The primary mechanism of K+ dissipation appeared to be diffusion, probably facilitated by the larger extracellular space in peripheral nerve compared to the central nervous system, but impeded by diffusion barriers imposed by the blood-nerve barrier.

Ca2+ waves in astrocytes

The glial cell is the most numerous cell type in the central nervous system and is believed to play an important role in guiding brain development and in supporting adult brain function. One type of glial cell, the astrocyte also may be an integral computational element in the brain since it undergoes neurotransmitter-triggered signalling. Here we review the role of the astrocyte in the central nervous system, emphasizing receptor-mediated Ca2+ physiology. One focus is the recent discovery that the neurotransmitter glutamate induces a variety of intracellular Ca2+ changes in astrocytes. Simple Ca2+ spikes or intracellular Ca2+ oscillations often appear spatially uniform. However, in many instances, the Ca2+ rise has a significant spatial dimension, beginning in one part of the cell it spreads through the rest of the cell in the form of a wave. With high enough agonist concentration an astrocyte syncitium supports intercellular waves which propagate from cell to cell over relatively long distances. We present results of experiments using more specific pharmacological glutamate receptor agonists. In addition to describing the intercellular Ca2+ wave we present evidence for another form of intercellular signalling. Some possible functions of a long-range glial signalling system are also discussed.

Functions and distribution of voltage-gated sodium and potassium channels in mammalian Schwann cells

Recent patch-clamp studies on freshly isolated mammalian Schwann cells suggest that voltage-gated sodium and potassium channels, first demonstrated in cells under culture conditions, are present in vivo. The expression of these channels, at least at the cell body region, appears to be dependent on the myelinogenic and proliferative states of the Schwann cell. Specifically, myelin elaboration is accompanied by a down regulation of functional potassium channel density at the cell body. One possibility to account for this is a progressive regionalization of ion channels on a Schwann cell during myelin formation. In adult myelinating Schwann cells, voltage-gated potassium channels appear to be localized at the paranodal region. Theoretical calculations have been made of activity-dependent potassium accumulations in various compartments of a mature myelinated nerve fibre; the largest potassium accumulation occurs not at the nodal gap but rather at the adjacent 2-4 microns length of periaxonal space at the paranodal junction. Schwann cell potassium channels at the paranode may contribute to ionic regulation during nerve activities.

Voltage-gated potassium currents in myelinating Schwann cells in the mouse

1. The whole-cell variation of the patch-clamp technique was used to record ionic currents in Schwann cells obtained from enzyme-treated mouse sciatic nerves before and after the onset of myelination. 2. Only outward currents were evoked in embryonic Schwann cells, which had no myelin, at membrane potentials more positive than -40 mV. Neonatal myelinating cells developed depolarization-activated outward currents and hyperpolarization-activated inward currents. For large hyperpolarizations below -160 mV, inward currents exhibited a sag following a peak which appeared to be mainly due to Na+ blockade. 3. Membrane potentials of neonatal myelinating cells were more negative than those of embryonic cells. The depolarization of the membrane potentials per 10-fold increase in external K+ concentrations in neonatal myelinating cells was 57 mV which fits the Nernst equation for a K+ electrode. 4. Quinine (0.5-2 mM) blocked the outward currents in embryonic cells and Ba2+ (2 mM) blocked both outward and inward currents in neonatal myelinating cells leaving quinine-sensitive outward currents of the embryonic type. External Cs+ (5 mM) blocked mainly inward currents and internal Cs+ blocked outward currents. 5. Developmental changes of these voltage-gated K+ currents in myelinating cells showed that Ba2(+)-sensitive K+ currents disappeared rapidly during the first week of life in association with the membrane potential becoming more positive. In contrast, quinine-sensitive outward K+ currents of the embryonic type disappeared slowly during the first 3-4 weeks after birth. 6. It is concluded that neonatal myelinating Schwann cells developed new voltage-gated K+ channels, which are Ba2(+)-sensitive and set a new membrane potential, in addition to the voltage-gated K+ channels of embryonic type. The Ba2(+)-sensitive K+ channels in myelinating cells were suggested to play an important role in siphoning K+ ions accumulated in periaxonal space during nerve activities.

Ion channel expression by white matter glia: the type-1 astrocyte

We describe the electrophysiological properties of acutely isolated type-1 astrocytes using a new "tissue print" dissociation procedure. Because the enzymes used did not destroy or modify the ion channels, and the cells retained many processes, the properties may reflect those in vivo. The types of ion channels in type-1 astrocytes changed rapidly during the first 10 postnatal days, when they attained their adult phenotype. This change was dependent on the presence of neurons. In culture, most of these channel types were not expressed, but a phenotype more typical of that in vivo could be induced by co-culture with neurons. The electrophysiological properties of astrocytes make some existing hypotheses of astrocyte function less likely.

K(+)-evoked Müller cell depolarization generates b-wave of electroretinogram in toad retina

We tested the hypothesis that a light-evoked increase in [K+]o produces a depolarization of the Müller cell membrane, which in turn generates the electroretinogram b-wave current. Using Bufo marinus isolated retinas and K(+)-selective microelectrodes, we recorded two distinct light-evoked increases in extracellular K+ concentration: one in the inner plexiform layer and the other near the outer plexiform layer; the "distal" K+ increase was found over only 10-microns depth and had a maximum amplitude of 0.3 mM. We also recorded the electroretinogram and the light-evoked responses of rods and Müller cells. After correction for the response time of the K(+)-selective microelectrode, the waveforms of all three of these responses were almost exactly as predicted by an earlier computer simulation of the K+/Müller cell hypothesis of the b-wave by Newman and Odette [Newman, E.A. & Odette, L.L. (1984) J. Neurophysiol. 51, 164-182]. The distal K+ increase and the b-wave varied in a similar manner as a function of stimulus irradiance. Superfusion with 0.2 mM Ba2+ attenuated both the Müller cell depolarization and the b-wave by approximately 65% but had no significant effect upon the distal K+ increase. Because Ba2+ reduces K+ conductance of Müller cells, these results are very strong support of the K+/Müller cell hypothesis of the origin of the electroretinogram b-wave; the light-evoked increase in extracellular potassium concentration still is present during superfusion with Ba2+, but the K(+)-evoked Müller cell depolarization and the b-wave are decreased in amplitude because Müller cell K+ conductance is reduced.

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)

Potassium conductance block by barium in amphibian Müller cells

The effect of barium on Müller cell K+ conductance was evaluated in the tiger salamander using enzymatically dissociated cells and cells in situ (retinal slice and isolated retina). Barium effects were similar in both cases. In dissociated cells, 50 microM Ba2+ depolarized cells 14.7 mV and raised cell input resistance from a control value of 16.0 to 133 M omega. For cells in situ, 50 microM Ba2+ depolarized cells 6.9 mV and raised cell resistance from 12.5 to 50.4 M omega. At corresponding Ba2+ concentrations, the resistance of cells in situ was somewhat lower than was the resistance of dissociated cells, a phenomenon that may be due to the small degree of electrical coupling present between Müller cells in situ. There was a similar positive correlation between the magnitude of Ba2+-induced depolarization and input resistance in both dissociated cells and in situ cells. The magnitude of depolarizations generated by localized K+ ejections onto Müller cells was reduced substantially by Ba2+. These observations indicate that Ba2+ is an effective K+ channel blocker in Müller cells in situ as well as in enzymatically dissociated cells.