Modification of potassium movement through the retina of the drone (Apis mellifera male) by glial uptake

Sensory pathways and motoneurons of the proboscis reflex in the suboesophageal ganglion of the honey bee

A morphological description is given for the motoneurons underlying the proboscis extension reflex in the honey bee (Apis mellifera). Sensory neurons from the mandibles and labium were examined and their projection areas were related to those of the motoneurons. Motoneurons are located in the ventral part of the suboesophageal ganglion, have their somata in the hemiganglion ipsilateral to the filled nerve, and send prominent branches through identified ventral commissures into the contralateral hemiganglion. Motoneurons to the same muscle have parallel-running projections into the contralateral hemiganglion, often parallel-running neurites and show vast areas of dendritic overlap. Arborizations of motoneurons are not restricted to their neuromere of origin but invade adjacent neuromeres, resulting in regions of dendritic overlap of motoneurons to different muscles. Sensory fibers from the mouthparts terminate in medio- and mediolateral parts of the suboesophageal ganglion, where their arborizations overlap with projections from motoneurons. A few sensory fibers descend in ventral parts of the cervical connective. Although sensory neurons from the mandible are restricted to the ipsilateral hemiganglion, those of the labium also show contralateral branches. These observations are discussed in the context of the suboesophageal ganglion as a ganglion composed of the embryonic fusion of three single neuromeres.

Increase in glial intracellular K+ in drone retina caused by photostimulation but not mediated by an increase in extracellular K+

The predominant glial cells of the drone retina (outer pigment cells) respond to an increase in extracellular [K+] (Ko) by a net uptake of K+; thus, they contribute to bringing Ko back toward its baseline value. The authors report herein that there is also a different mechanism by which light stimulation of the retina causes an increase in intracellular free [K+] in the glial cells. In superfused retinal slices, after 5-10 minutes of continuous illumination at physiological intensities, extracellular [K+] often fell back to below its original level in the dark. This fall can be explained by increased activity of the Na/K pump in the photoreceptors and diffusion of K+ down their axons. Despite the absence of raised Ko, K+-selective microelectrodes in glial cells recorded a small increase in intracellular [K+] that was maintained for the duration of the illumination; i.e. a change occurred in the glia that was not mediated by an increase in Ko. The increase in intracellular [K+] is not mediated by illumination of the screening pigment in the glia. Unless the increase is caused by illumination of some other, unknown, pigment in the glia, the results show that some unidentified signal (that is not K+) passes from the photoreceptors to the glia.

Do glia contribute to behaviour? A neuromodulatory review

1. The links between behavioural state, gross electrophysiology and the activity of neurons and astrocytes are reviewed to stimulate interest in the contributions that glia make to behaviour. 2. Behavioural arousal in which neuronal responsivity ("sensitivity") is elevated is also associated with a sustained (0.5-10 sec) potential shift (SPS). 3. There is powerful and accumulating evidence that the SPS is primarily of glial origin. 4. In epilepsy neurons are hyperactive and there is a massive SPS during seizures. In seizure free periods, epileptic animals frequently have elevated arousal responses and increased neuronal sensitivity, indicating that seizures may be due to elevation of the activity of a normally adaptive sensitizing mechanism. 5. The common finding of an astrocytic pathology in epilepsy and the links between arousal, neuronal sensitization, SPSs and seizures implicates a modulatory role for astrocytes in both health and disease. 6. Glia, especially astrocytes, may modulate neuronal responsiveness by regulation of the microenvironment. 7. At the current state of knowledge, regulation of extracellular ionic K+, Ca2+ and neurotransmitter glutamate and GABA seem to be the most important candidates for modulating neuronal sensitivity in arousal and abnormally for seizure genesis. 8. Both in phylogeny and in ontogeny, glia and neurons have intimate associations. 9. The functional astrocytic syncitium is in a prime position to control the ecology of neuronal populations and thereby their activity. 10. The physiology and biochemistry of glia-neuronal interactions offers exciting new prospects for developments in behavioural neuroscience.

Glia:neuron index: review and hypothesis to account for different values in various mammals

The present paper proposes a hypothesis to account for different values of the glia:neuron index in comparable central nervous system tissues of various mammals. This hypothesis assumes that K+ ions released by active neurons are a mitogenic signal for glial cells. The thicker the tissue (for example, the brain wall), the more difficult is efficient K+ clearance, and more perinatal glial cell proliferation should occur. Thus, this hypothesis accounts for higher glia:neuron indices in mammals with thicker brain walls.

Functions of glial cells in the retina of the honeybee drone

In the retina of the honey bee drone, Apis mellifera male, physiological interactions between glial cells and neurons (the photoreceptors) are exceptionally clear-cut and amenable to investigation. The principal glia (outer pigment cells) contribute to the homeostasis of extracellular [K+] and [Na+] by 1) spatial buffering of K+ and 2) net uptake of K+ and Cl-. The glia supply carbohydrate metabolic substrate to the neurons; only the glia take up and phosphorylate glucose. Neuronal activity 1) modifies glycogen metabolism in the glia, and 2) can be signalled to the glia in the absence of elevated extracellular [K+].

Stimulus-related potassium changes in the organ of Corti of guinea-pig

1. Potassium concentration was measured with double-barrelled K+-selective microelectrodes within the organ of Corti in the first turn of the guinea-pig cochlea. 2. Penetration of the electrode from scala tympani through the basilar membrane was accompanied by an increase in K+ resting level from 3.0 mmol/l in perilymph to 3.4 mmol/l in cortilymph (n = 8). K+ resting level was not significantly different in various extracellular regions of the organ of Corti. On penetration of the cuticular plate, the K+ level reached 140 mmol/l simultaneously with the occurrence of a +80 mV endocochlear potential. Impalement of hair cells and supporting cells was accompanied by an increase in K+ level, but intracellular K+ level was not systematically measured. 3. Stimulation with pure tones over the frequency range 500 Hz to 25 kHz produced changes in the K+ level in the organ of Corti. The magnitude of these changes was dependent on stimulus frequency and intensity. At high sound intensities the K+ level in the tunnel of Corti could increase by typically 1 mmol/l, while a maximum increase of 3 mmol/l with respect to the resting level was observed immediately adjacent to inner hair cells. 4. During brief exposures to moderate intensity, pure tone acoustic stimulation (10 s, less than 80 dB SPL (sound pressure level] of frequency 4 kHz or greater the K+ level in the extracellular fluid of the organ of Corti rose monotonically to a steady peak level. On cessation of the stimulus the K+ level fell monotonically with a time constant of about 2 s to a level close to the pre-stimulus level. In some cases this level was slightly above the pre-stimulus level. 5. For brief exposures to moderate intensity sound (10 s, less than 80 dB SPL) the extracellular potential in the organ of Corti became more positive. The amplitude of this sound-evoked change adapted during stimulation to a level approximately one-fifth of its initial value. Upon cessation of the stimulus the potential fell transiently below its pre-stimulus level, before recovering to that level. The time constant of these changes was between 2 and 3 s. 6. Iso-response tuning curves for the sound-evoked elevation in K+ level in the organ of Corti in animals in good condition were similar to iso-rate tuning curves for primary afferent fibres reported previously.(ABSTRACT TRUNCATED AT 400 WORDS)

Relations between slow extracellular potential changes, glial potassium buffering, and electrolyte and cellular volume changes during neuronal hyperactivity in cat brain

The aim of this investigation is to estimate the contribution of spatial glial K+ buffer currents to extracellular K+ homeostasis during enhanced neuronal activity. Neuronal hyperactivity was induced by electrical stimulation of the cortical surface or the ventrobasal thalamic nuclei of cats (5-50 Hz, 0.1-0.2 ms, two to three times threshold stimulation intensity, 5-20 s). The accompanying slow field potential changes were recorded simultaneously across the grey matter with vertical assemblies of eight micropipettes glued 300 microns apart. Using the Poisson equation, the amplitudes of the underlying current sources and sinks were calculated. The current source densities depended on the depth of recording, frequency, strength, and duration of the stimulation. Current sinks, corresponding to a removal of 0.1-0.5 mmoles of monovalent cations per liter of brain tissue and second from the extracellular space, were observed in middle cortical layers, whereas sources appeared at superficial and deeper sites. These sinks and sources might represent K+ moved across glial membranes by spatial buffer currents. The consequences of glial buffer currents of this magnitude were investigated with model calculations. It turned out that measurements of electrolyte and volume changes of the extracellular space (Dietzel et al. Exp. Brain Res. 40:432-439, 1980; Exp. Brain Res. 46:73-84, 1982) could only partially be explained by spatial buffer currents of this magnitude. Comparison of the calculated values with intracellular measurements in neurons and glial cells (Coles et al. Ann. N.Y. Acad. Sci. 481:303-317, 1986; Ballanyi et al. J. Physiol. 382:159-174, 1987) suggests that spatial buffering combines with an approximately equimolar KCl transport and, depending on the preparation, also K+/Na+-exchange across glial membranes.

Chloride enters glial cells and photoreceptors in response to light stimulation in the retina of the honey bee drone

Double-barrelled ion-selective microelectrodes were used to measure free [Cl-] in photoreceptors, extracellular space, and glial cells in superfused slices of drone retina. Tests indicated that with normal superfusate the intracellular electrode signal was due essentially to Cl- and not to some other interfering anion. The results indicate that Cl- is more concentrated in both photoreceptors and glial cells than would be predicted for a passive electrochemical distribution. When the photoreceptors were stimulated by a standard train of 20 ms flashes, 1/s for 90 s, their intracellular free [Cl-] (Cli) rose by 8 +/- 1 mM. At the end of stimulation Cli usually continued to rise for up to a further 2 min and then returned toward the baseline over about 10 min. During light stimulation Cli in the glia rose. The magnitude of the increase was 5.1 +/- 0.4 mM, about half the increase in Ki. In some extracellular recording sites, light stimulation caused [Cl-] to increase and in others to decrease. The mean change was -0.7 mM, SD 6.5 mM. The Cl- that entered the photoreceptors and the glia was presumably made available by the shrinking of the extracellular space. When the cells were depolarized by increasing [K+] in the superfusate from 7.5 mM to 18 mM, Cli increased. The half-time of the change in Cli was longer than the half-time of the depolarization by 10-30 s in the glia and 50-250s in the photoreceptors. During superfusion with 0 Cl- Ringer's solution, the light-induced rise in extracellular [K+] was greater by a factor of 1.4-2.7, and the clearance after the end of the stimulation was slower. The rate of increase in glial Ki during light stimulation fell; the rate of increase of glial Ki caused by superfusion with raised [K+] (in the absence of Cl-) fell more. We conclude that when extracellular [K+] is increased, entry of Cl- into the glia is necessary for part, but not all, of the net uptake of K+. During light stimulation, the observed movement of CL- into glia contributes to homeostasis of extracellular [K+], and the cell swelling associated with movement of Cl- into both glia and photoreceptors contributes to homeostasis of extracellular [Na+].

Effects of carbon dioxide on extracellular potassium accumulation and volume in isolated frog spinal cord

A 6-10-fold increase in pCO2 in the superfusing Ringer solution increased the volume of the extracellular space (ECS) and changed the spatial distribution and amplitude of the extracellular K+ accumulation which resulted from dorsal root stimulation. Using the increase in tetraethylammonium concentration [( TEA+]) resulting from iontophoretic injection of that ion in the extracellular fluid as an indication of the volume of the ECS, it was found that in high pCO2 the ECS volume in spinal dorsal horn increased by more than 60%. In addition, in the presence of raised pCO2 we also observed the following: (1) The rate of diffusion of TEA+ into the dorsal horn increased. (2) The accumulation of K+ evoked by single or tetanic stimulation of the dorsal root was less. (3) The clearance of K+ was slowed down. (4) The regions where K+ accumulated were more restricted. (5) The K+ evoked depolarization of the primary afferent fibres decreased. (6) In contrast to TEA+, the rate of diffusion of K+ into the dorsal horn decreased. The effects of an increase in pCO2 on K+ accumulation and clearance appear to result from an increase in ECS volume and a possible decrease in glial electrical coupling which interferes with glial spatial buffering of K+.

Studies of axon-glial cell interactions and periaxonal K+ homeostasis--III. The effect of anisosmotic media and potassium on the relationship between the resistance in series with the axon membrane and glial cell volume

The effect of anisosmotic physiological solutions and [K+]o on the resistance in series with the axon membrane were studied in medial giant axons of the crayfish, Procambarus clarkii, to determine if changes in series resistance are correlated with changes in glial cell volume and volume regulatory responses. Series resistance was estimated from computer analysed voltage waveforms generated by constant current and space clamp techniques using piggy-back axial wire current passing and glass pipette recording electrodes. Axons subjected to anisosmotic physiological solution in the range of 23 to 175% of isosmolar solution demonstrated that the series resistance of axons changes in a manner similar to that expected for a volume change in isolated cells. In hyperosmotic solution the series resistance changes biphasically, initially decreasing followed by a recovery of the series resistance, similar to the regulatory volume increase described for glial cells in culture. The increase in series resistance following the initial decrease is inhibited by bumetanide (0.1 mM). Ouabain (1 mM), an inhibitor of the volume decreasing Na-K pump, causes the series resistance to increase significantly above that seen for the no-drug control. Bumetanide, an inhibitor of the volume increasing Na-K-Cl cotransporter, inhibits the volume regulatory response to anisosmotic media. Treating the axon with three times normal external [K+] causes the series resistance to decrease approximately 15% while five times normal [K+] leads to a 15% increase in series resistance. Both ouabain and d-tubocurare (10(-p8) M) prevent the three-fold [K+]-induced decrease in series resistance while carbachol (10(-7) M) and bumetanide have little effect. On the other hand, ouabain enhances the five-fold [K+]-induced increase in series resistance while carbachol and bumetanide cause the five-fold [K+] response to be in a decreasing direction. d-Tubocurare has little effect on the five-fold [K+]-induced increase in series resistance. The study demonstrates that under the conditions of these experiments changes in series resistance are a reflection of changes in cell volume modulated by ouabain- and bumetanide-sensitive K+ uptake mechanisms. The effects of carbachol and d-tubocurare on the series resistance suggest that their effects are modulated through their actions on the glial cell membrane potential and the electrochemical gradient for K+, which in turn controls the amount of K+ that appears in the periaxonal space.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies of axon-glial cell interactions and periaxonal K- homeostasis--I. The influence of Na+, K+, Cl- and cholinergic agents on the membrane potential of the adaxonal glia of the crayfish medial giant axon

The ionic basis for the low (-40 mV) resting membrane potential of glial cells surrounding the giant axons of the crayfish and their hyperpolarization by cholinergic agents (to -55 mV) was studied using standard electrophysiological techniques, ionic substitutions and pharmacological agents. The resting membrane potential of the glial cell was depolarized by increasing [K+]o, but the response was not Nernstian. Na+ depletion caused a small depolarization of the glial resting membrane potential, whereas Cl- depletion resulted in a hyperpolarization comparable to that seen with carbachol at various [K+]o. Both furosemide (1 mM) and bumetanide (0.1 mM) produced an 8-10 mV hyperpolarization as compared to 15-17 mV seen with Cl- depletion or carbachol. Carbachol has no further effect on the potential following furosemide treatment or Cl- depletion. After carbachol administration or Cl- depletion the resting membrane potential of the glial cell responded to [K+]o in a more Nernstian manner. The data indicate that the low resting membrane potential of glial cells is due to a combination of a low [K+]i and an outwardly-directed (depolarizing) Cl- electrochemical gradient. Carbachol acts to decrease Cl- conductance, resulting in the hyperpolarization of the glial cell membrane and a decrease in the outwardly-directed K+ electrochemical gradient by approximately two-thirds. We hypothesize that this mechanism for modulation of the glial cell membrane potential and the K+ electrochemical gradient serves to enhance the uptake of K+ by the glial cell transport system.

Bias current modifies the selectivity of liquid membrane ion-selective microelectrodes

A negative bias potential of up to -80 mV applied to the back of a liquid membrane ion-selective microelectrode containing classical "K+" ion-exchanger was found to make it more selective for millimolar concentrations of K+ over micromolar concentrations of choline, tetramethylammonium, tetraethylammonium and 5-hydroxytryptamine. Conversely, positive bias potential increased severalfold the sensitivity to micromolar concentrations of these ions while decreasing the sensitivity to K+. An increase in response amplitude for millimolar changes of ion concentration was also observed in neutral carrier electrodes for Na+, K+ and Ca2+ with negative bias potential. The various ions caused the resistances of the electrodes to change; these resistance changes contributed to the changes in response amplitude, but there were additional, unexplained, factors. The phenomenon was used to test if the signal from a K+ ion-exchanger microelectrode in extracellular space in bee retina was contaminated by substances other than K+.

Electrical coupling between astrocytes and between oligodendrocytes studied in mammalian cell cultures

The characteristics of electrical coupling between astrocytes and between oligodendrocytes were analyzed in cell cultures derived from rodent central nervous system. Experiments were carried out by impaling one member of a glial pair with separate voltage recording and current passing electrodes (cell 1) and the other cell, a measured distance from the first, with a voltage-recording electrode (cell 2). Astrocyte pairs within 300 microns of one another were always coupled. The coupling ratio was determined for 23 astrocytic pairs various distances apart, and decreased with distance in a roughly exponential manner. The average coupling ratio of astrocytes within 100 microns of each other was 0.44 +/- 0.32. Oligodendrocytes were less strongly coupled to each other than astrocytes. Even cells immediately adjacent to one another were often uncoupled. Among coupled oligodendrocytes within 100 microns of each other, the average coupling ratio was 0.11 +/- 0.1. Current passage between pairs of astrocytes and pairs of oligodendrocytes was nonrectifying. Application of 0.5 mM BaCl2 or 44.6 mM CsCl (substituted for NaCl) depolarized and increased the input resistance of astrocytes and oligodendrocytes. These ions also increased the coupling ratio in astrocyte pairs and oligodendrocyte pairs; this effect was rapid in onset and completely reversible. Ba++ and Cs+ appear to block resting K+ conductance in glia and probably increase the coupling ratio by increasing the effective length constant of the glial membrane without any direct effect on junctional resistance. In three cases, oligodendrocyte pairs that appear uncoupled in normal solution exhibited coupling in the presence of BaCl2 or CsCl. This suggests that oligodendrocytes may be widely coupled by junctions that provide only weak electrical interaction; such junctions might be important for the exchange of small metabolically active molecules. The strong electrical coupling among astrocytes, in concert with their K+-selective membrane conductance, would provide for an electrical syncytium well designed to transport K+ away from areas of focal extracellular accumulation (i.e., the spatial buffer mechanism), and these cells, more than oligodendrocytes, may provide this function.

Quartz micropipettes for intracellular voltage microelectrodes and ion-selective microelectrodes

We find that quartz is better than borosilicate glass as a material for micropipettes for certain microelectrode applications, including ion-selective microelectrodes of the liquid membrane type. Details are given of a graphite heating element that can be mounted on a standard microelectrode puller and used for making quartz micropipettes. A novel method of silanizing micropipettes is described. Experience with a novel miniature beveller that is fitted on a microscope stage is reported.

Extracellular potassium changes in the rat neurohypophysis during activation of the magnocellular neurosecretory system

1. Potassium-sensitive microelectrodes were used to measure extracellular [K+] in the isolated rat neurohypophysis maintained in vitro. Electrical stimulation of the neurohypophysial stalk (20 Hz 5 s) increased the inferred extracellular [K+] by 9.2 +/- 0.4 mM (mean +/- S.E. of mean; n = 21). 2. Veratridine (10 microM) enhanced the response to stalk stimulation, and at a higher concentration (50 microM) increased extracellular [K+] in the absence of stimulation. By contrast, tetrodotoxin (1 microM) blocked the [K+] increase completely and reversibly in each of five experiments, indicating that the increase was a consequence of action potential generation. 3. At the end of brief periods of stimulation, the raised extracellular [K+] returned to pre-stimulation levels within 30 s. In the presence of ouabain (100 microM), the recovery was slower: the half-decay time was extended by 150-300% in each of three experiments. 4. Replacement of calcium in the medium with cobalt, cadmium or magnesium reduced the amplitude of the [K+] increase by 26-30%, indicating that the [K+] increase was largely independent of events subsequent to evoked release of hormone and/or transmitters. 5. Potassium-sensitive microelectrodes were placed in the neurohypophysis of rats anaesthetized with urethane. Electrical stimulation of the pituitary stalk (50 Hz, 5 s) produced transient voltage increases of 7.6 +/- 0.9 mV (mean +/- S.E. of mean of seven experiments). These voltage increases were similar in magnitude to the response of the electrodes to the addition of 7.6 +/- 1.0 mM-K+ to rat plasma. 6. In seven lactating rats, the suckling of a litter of hungry pups evoked periodic reflex milk ejections, as detected by increases in intramammary pressure. Potassium-sensitive microelectrodes in the neurohypophysis recorded transient voltage increases prior to each milk ejection (0.4-5.5 mV). Each increase preceded an increase in intramammary pressure by 12-30 s. 7. Thus synchronized high-frequency activation of magnocellular neurones can produce large changes in extracellular [K+]. The implications of these findings for stimulus-secretion coupling in the neurohypophysis are discussed in the light of previous reports that hormone release from the neurohypophysis is highly dependent on the frequency and pattern of electrical stimulation.

Glial potassium uptake following depletion by intracellular ionophoresis

The K+ uptake processes of immunologically identified oligodendrocytes from embryonic mouse spinal cord were studied in primary culture by injecting ions and recording membrane potential changes and, in some experiments, K+ ion activity with intracellular electrodes. When Na+ was injected [K+]i decreased. Immediately before and after current injection the membrane potential was close to the K+ equilibrium potential (EK) and this finding was used to study K+ uptake following its depletion by intracellular ionophoresis. The uptake of K+ following Na+ injection was blocked by ouabain and unaffected by removal of extracellular Cl- or Cl- transport blockers. This suggests that recovery comes about mostly through the activity of the Na+/K+ -ATPase stimulated by either the increase in [Na+]i or the decrease in [K+]i. Pump current could be determined by clamping at different membrane potentials and was found to increase in proportion to the depolarization of the cell resulting from [K+]i depletion. The time course of recovery of membrane potential following either Li+ or tetramethylammonium (TMA+) injection was similar to that after Na+ injection, indicating that injection of these ions to produce a comparable decrease in [K+]i leads to a similar stimulation of the Na+/K+ -ATPase. In addition, the recovery of membrane potential following injection of TMA+, but not of Na+ or Li+, was blocked when the external Na+ was removed. Internal Na+ or Li+ appears necessary for Na+/K+ -ATPase-activity, but under conditions of normal or low [Na+]i the rate of Na+/K+ -ATPase activity seems to be sensitive to [K+]i and/or membrane potential.

Effects of bicarbonate on glial cell membrane potential in Necturus optic nerve

Intracellular electrodes were used to continuously monitor the membrane potential of glial cells in the isolated Necturus optic nerve. Addition of up to 10 mM extracellular bicarbonate (with CO2), at constant pH, produced a hyperpolarization of up to 10 mV (with a time course almost as fast as that of a K+ depolarization) that returned toward baseline during the following 2-15 min. Upon bicarbonate withdrawal, the potential transiently became more positive. The bicarbonate effects were magnified when the K+ conductance was decreased and the cell depolarized by the addition of barium. Similar bicarbonate effects were observed in Cl- free solutions. These results suggest to us that: glial cells have a bicarbonate permeability of the same order as that to K+ and glial cells buffer transient changes in acid base balance in the neuronal microenvironment at the expense of their internal pH.

Ion activities and potassium uptake mechanisms of glial cells in guinea-pig olfactory cortex slices

1. Double-barrelled ion-sensitive micro-electrodes were used to measure changes in the intracellular activities of K+, Na+ and Cl- (aiK, aiNa, aiCl) in glial cells of slices from guinea-pig olfactory cortex during repetitive stimulation of the lateral olfactory tract. 2. Base-line levels of aiK, aiNa and aiCl were about 66, 25 and 6 mM, respectively, for cells with resting potentials higher than -80 mV. During stimulation, intraglial aiK and aiCl increased, whereas aiNa decreased. Within about 2 min after stimulation the ion activities returned to their base-line levels. 3. The Cl- equilibrium potential was found to be close to the membrane potential (Em). There was also a strong correlation between changes of Em and aiCl. These observations indicate a high Cl- conductance of the glial cell membrane. 4. In the presence of Ba2+, the usual depolarizing response of the glial cells to a rise of the extracellular K+ activity (aeK) reversed into a membrane hyperpolarization. Furthermore, Ba2+ strongly reduced the stimulus-related rise of intraglial aiK. An additional application of ouabain blocked both the membrane hyperpolarization as well as the remaining rise of aiK. 5. In conclusion, our data show that glial cells in guinea-pig olfactory cortex slices possess at least two mechanisms of K+ accumulation. One mechanism is sensitive to the K+ channel blocker Ba2+ and might be a passive KCl influx. The other appears to be the electrogenic Na+/K+ pump, which can be activated by excess extracellular K+.

Dynamic variations of the brain cell microenvironment in relation to neuronal hyperactivity

Neuronal hyperactivity has been shown to cause transient changes in ionic concentrations as well as in the volume of the extracellular space (ECS). During enhanced neuronal activity in the sensorimotor cortex of the cat, increases in the extracellular K+ concentration to a ceiling value of 10 mM have been observed concomitant with decreases in the Na+ concentration of similar magnitude. Simultaneously, the Cl- concentration rose steadily during the enhanced neuronal activity, and the extracellular space decreased by about 30%. A mathematical model allowed the interpretation of these concentration and volume changes as results of K+ release by active neurons in exchange for Na+, removal of K+ from the ECS by spatial glial buffering and movements of KCl into glial cells, as well as cell swelling due to metabolically induced transient increases in cellular osmolarity. Similar mechanisms have also been found to operate in gliotic scar tissue, where glial cells appeared to function the same way as in normal cortex, in rat hippocampal slices, and in preparations studied by other investigators.

Brain extracellular space: developmental studies in rat optic nerve

Analysis of neural activity-dependent fluctuations in K+, H+, and ECS dimensions in the developing RON has revealed major changes during the first two to three postnatal weeks. The emergence of the adult ceiling level for evoked extracellular K+ (10 to 12 mM) and significant ECS shrinkage are roughly correlated in time with the proliferation and maturation of glial cells in this structure. This observation and others have led to the hypothesis that ECS shrinkage depends upon electrolyte and water transport into glial cells with subsequent swelling. Development of the adult K+ ceiling level may also depend upon glial cells, but it is likely that other factors contribute to this homeostatic mechanism. Marked alterations in activity-dependent pHo shifts were seen with development and may be related to changes in the activity of carbonic anhydrase in this structure. The technological means are at hand to pursue these questions vigorously in an effort to provide further insight into the mechanisms of ionic and fluid homeostasis of brain ECS, and the developing RON appears to be a useful model system in this regard.

Intracellular recordings from isolated rabbit retinal Müller (glial) cells

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

pO2-dependent distribution of potassium in hippocampal slices of the guinea pig

The distribution of extracellular K+-concentration (cK+s) in 200-1000-micron thick hippocampal slices was studied with ion-selective microelectrodes. In ca. 500-micron thick slices cK+s increased from the surface to the innermost layers by ca. 2 mmol/liter if the pO2 of the bath (pBO2) ranged from 300-600 mm Hg and if the temperature was 28 degrees C. In thicker slices and lowered pO2-values further elevations of cK+s were observed. In vital slices thinner than 500 micron cK+s-values exceeded the potassium-concentration of the bath (cK+B) only when pBO2 was markedly lowered. When pBO2 was reincreased in such thin slices, cK+s rapidly declined and often decreased transiently below ck+B. Similar undershoots of cK+s were observed when cK+B was lowered from high to normal levels. The rapid decline was blocked by hypoxia, ouabain, antimycine and a temperature of 18 degrees C. A stepwise rise of cK+B also caused rapid changes of cK+s in vital thin slices. The rates of changes, however, were hardly affected e.g. by a transient hypoxia. Diffusion did not contribute significantly to these steep changes of cK+s. These rapid distribution modes were widely missing in slices thicker than 500 micron. Therefore in such preparations, the extracellular microenvironment of neurons may markedly differ from the ionic concentrations in the bath.

Light induced sodium dependent accumulation of calcium and potassium in the extracellular space of bee retina

Intense illumination of long duration induced a large transient increase in extracellular calcium (delta[Ca2+]o) and potassium (delta[K+]o) during and after light in bee retina when measured with ion-selective microelectrodes. Whenever a large delta[Ca2+]o appeared, it was accompanied by a transient afterdepolarization (TA). Both the increase in [Ca2+]o, [K+]o and the TA were reduced or abolished when sodium was replaced by arginine, choline or lithium (Li+) ions. At 0-Na conditions a Na independent decrease in [Ca2+]o was observed during illumination only. A pronounced transient depolarization of the photoreceptor in the dark due to transient anoxia did not result in a significant change in [Ca2+]o. In some retinae the elevated level of [K+]o after light was absent, however a small Na-dependent TA was still observed. The above findings suggest that intense long illumination induces a large Ca2+ influx into the photoreceptors which is followed by Na-dependent Ca2+ efflux due to Na-Ca exchange. The light-induced afterdepolarization arises mainly from K+ accumulation in the extracellular space but partially from the electrogenicity of Na-Ca exchange.

A simple method for making ion-selective microelectrodes suitable for intracellular recording in vertebrate cells

A simple procedure for manufacturing Cl-, K+, and pH liquid membrane ion-sensitive microelectrodes is presented in detail. Electrodes suitable for recording from the specimen of interest are back-filled with a small amount of silane solution and heated for 5 min on a hot plate at a temperature between 400 and 500 degrees C, after which they are injected with the ion-sensitive resin. The procedure is adaptable to many different glass stocks, e.g., single-barreled, double-barreled, or theta glass, and can be used to produce electrodes having a wide range of tip sizes for recording either extracellular or intracellular ion activities. Another advantage of the method is speed; up to 10 electrodes can be prepared simultaneously, permitting over 40 functional electrodes to be made per hour.

Carrier-mediated KCl accumulation accompanied by water movements is involved in the control of physiological K+ levels by astrocytes

Potassium accumulation and water transport into mouse astrocytes in primary cultures were investigated when external potassium was increased from 3 to 12 mM. The intracellular potassium content increased by 63% within 50 s of such a change. The increase consisted of a ouabain- and furosemide-sensitive component, both contributing in about the same amounts. Experiments with altered ion composition revealed that the furosemide-sensitive component consisted of a KCl accumulation. Water moved into the astrocytes without delay after such an external K+ increase and increased the cell water by 27%. This water increase was abolished in solutions with reduced Cl- and during application of furosemide. Thus, these results on a KCl uptake accompanied by water movements into astrocytes suggest a potential mechanism by which glial cells in situ can regulate external K+ levels.

Differences in cation transport properties of primary astrocyte cultures from mouse and rat brain

42K and 22Na contents and unidirectional fluxes, as well as net accumulation of 42K in response to elevated extracellular K+, were investigated in primary cultures of astrocytes prepared from neonatal rat and mouse brain. The major difference between both species affected the unidirectional K+ influx which was up to 75 times higher in mouse as compared to rat cultures. The flux rates in mouse astrocytes were doubled by measuring uptake in salt solution instead of growth medium, while 42K influx in rat astrocytes was unaffected by such treatment. 22Na transport was very similar in astrocytes from both species. The length of culture period and treatment with DBcAMP (2',3'-dibutyryl cyclic adenyl monophosphate) modified K+ transport but not Na+ transport. Both types of cultures showed the same accumulation of 42K in response to raised medium K+. Amiloride inhibited 42K influx by 41% and 13% in mouse and rat cultures, respectively. In contrast, furosemide inhibited 42K uptake in rat astrocytes cultures by 50% but had no effect on mouse astrocyte cultures. 50 microM barium chloride markedly inhibited 42K uptake in mouse cultures by 96% (or 1491 nmol X mg-1 X min-1), but inhibited 42K uptake in rat cultures by only 23% (or 9 nmol X mg-1 X min-1). Ouabain was similarly effective in both types of astrocyte cultures. We conclude that Na+ transport as well as net K+ accumulation and Cl- transport (based on previous studies) properties are reasonably stable and reproduced in primary cultures from both mouse and rat brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in sodium activity during light stimulation in photoreceptors, glia and extracellular space in drone retina

Ion-selective micro-electrodes were used to measure Na+ activity, aNa, in the two types of cell, photoreceptors and glial cells, and in the extracellular space, in superfused slices of the retina of the honey-bee drone, Apis mellifera male. Movements of Na+ were induced by light stimulation, or by increasing [K+] in the superfusate. In the dark, aNa in the photoreceptors was 10 mM (S.E. of the mean = 1 mM); in the glial cells it was higher: 37 +/- 2 mM. We estimate that in this preparation about 2/3 of the free Na+ in the tissue is in the glial cells. Stimulation with a train of light flashes, 1 s-1 for 90 s caused aNa in the photoreceptors to increase by 16 +/- 2 mM. K+ activity, aK, decreased by 21 +/- 3 mM. During the standard train of light flashes, aNa in glial cells decreased by only 1.5 +/- 0.3 mM, much less than the increase in aK (7 +/- 2 mM). One possible interpretation of this result is that most of the increase in aK is due to K+ uptake by a mechanism other than Na+-K+ exchange. In extracellular fluid, stimulation caused aNa to fall to a relatively steady value in about 10 s. Unlike aK, there was no tendency for aNa to return to the base line during the remainder of the 90 s stimulation. The fall in aNa was 14 +/- 1 mM: a greater fall is prevented by extracellular electric currents and a decrease in extracellular volume. When [K+] in the superfusate was increased from 7.5 to 18 mM, aNa decreased in the glial cells but not in the photoreceptors. In this tissue, stimulation causes changes in aNa in the neurones that might be large enough to modify the biochemistry of the cells. But in the glia, the fractional changes are small.

Coupling and uncoupling of amphibian neuroglia

Glial cells in the optic nerve of Necturus are coupled to each other by low resistance pathways which also permit the diffusion of the fluorescent dye Lucifer Yellow CH among the cells. The spread of dye is readily observed as nuclei of cells distant from the site of intracellular injection are stained. By contrast, horseradish peroxidase does not traverse the intercellular pathways. This protein remains in the injected cell. The addition of weak acids (carbonic or propionic) to the bathing medium reversibly uncouples the glia; it blocks the spread of ionic current and Lucifer Yellow among the cells. A block of ionic coupling will block the spatial buffering of potassium by the glial syncytium.

Barium-induced inhibition of K+ transport mechanisms in cortical astrocytes--its possible contribution to the large Ba2+-evoked extracellular K+ signal in brain

Homogenous mouse astrocytes in primary cultures were used to investigate the action of different Ba2+ concentrations on 42K transport, membrane potential and Na+,K+-adenosine triphosphatase activity. Five millimolar Ba2+ reduced total K+ influx and efflux (each by 83%) and ouabain-sensitive net K+ uptake (by 80%); it decreased the K+ content, depolarized the membrane potential reversibly and completely inhibited the Na+,K+-adenosine triphosphatase activity. The concentration dependence of these effects was biphasic. Concentrations between 2 and 20 microM affected only the passive K+ fluxes (IC50: 6 microM). Concentrations between 50 microM and 5 mM inhibited the Na+,K+-adenosine triphosphatase and had no further effect on passive fluxes, but inhibited the ouabain-sensitive net uptake of K+ (IC50: 3.1-0.6 mM). It is suggested that the large evoked extracellular K+ increase in the brain observed in Ba2+-treated preparations in vivo or in brain slices to a large extent is due to the impairment of passive and active K+ clearance by glial cells.

Reaccumulation of [K+]o in the toad retina during maintained illumination

Using K+-selective microelectrodes, [K+]o was measured in the subretinal space of the isolated retina of the toad, Bufo marinus. During maintained illumination, [K+]o fell to a minimum and then recovered to a steady level that was approximately 0.1 mM below its dark level. Spatial buffering of [K+]o by Müller (glial) cells could contribute to this reaccumulation of K+. However, superfusion with substances that might be expected to block glial transport of K+ had no significant effect upon the reaccumulation of K+. These substances included blockers of gK (TEA+, Cs+, Rb+, 4-AP) and a gliotoxin (alpha AAA). Progressive slowing of the rods' Na+/K+ pump (perhaps caused by a light-evoked decrease in [Na+]i) also could contribute to this reaccumulation of K+ by reducing the uptake of K+ from the subretinal space. As evidence for a major contribution by this mechanism, treatments designed to prevent such slowing of the pump reversibly blocked reaccumulation. These treatments included superfusion with 2 microM ouabain, or lowering [K+]o, PO2, or temperature. It is likely that such treatments inhibit the pump, increase [Na+]i, and attenuate any light-evoked decrease in [Na+]i. The results are consistent with the following hypothesis. At light onset, the decrease in rod gNa will reduce the Na+ influx and the resulting rod hyperpolarization will reduce the K+ efflux. In combination with these reduced passive fluxes, the continuing active fluxes will lower both [K+]o and [Na+]i, which in turn will inhibit the pump. In support of this hypothesis, the solutions to a pair of coupled differential equations that model changes in both [K+]o and [Na+]i match quantitatively the time course of the observed changes in [K+]o during and after maintained illumination for all stimuli examined.

Electrical properties of oligodendrocytes in culture

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

Intense furosemide-sensitive potassium accumulation in astrocytes in the presence of pathologically high extracellular potassium levels

An intense K+ accumulation in primary cultures of astrocytes, occurring when external K+ was increased from 5.4 to 54 mM, was investigated. This increase resulted in a doubling of the K+ content within 10 s. Thirty percent of the accumulation was inhibited by furosemide (2 mM). This drug had no effect on the unidirectional influx of K+ at 5.4 mM K+, but when the extracellular K+ concentration was increased, there appeared to be a furosemide-sensitive component of the influx. This component increased with increasing external K+ levels, reaching 44% of the total influx at 72 mM. These results show that astrocytes exhibit an intense furosemide-sensitive K+ accumulation which is activated by K+ levels resembling those occurring in the extracellular compartment during pathological events. Previous studies on a furosemide-sensitive Cl- pump in cultured astrocytes suggest that this accumulation might be via KCl cotransport, which in other systems is involved in volume control.

Light-induced changes in extracellular volume in the retina of the drone, Apis mellifera

Slices of drone retina were superfused with a Ringer solution containing 1 mM tetraethylammonium (TEA), and the concentration of this ion in the extracellular space [( TEA]0) was measured with ion-sensitive microelectrodes. A train of light flashes for 90 s caused [TEA] to increase by 48 +/- 4% (S.E.), n = 12. Since water crosses cell membranes more readily than TEA does this indicates a volume decrease of at least 32%. Measurements of Ca2+ activity under similar conditions showed an increase of 32 +/- 4% (S.E.), n = 14. Since this is less than the increase in [TEA]0 it suggests that the total amount of Ca2+ in the extracellular space actually decreased.

Some properties of single potassium channels in cultured oligodendrocytes

K+ channels were studied in oligodendrocytes in cultures of mouse spinal cord. Single channel currents were measured using the gigaseal technique. The conductance of the channels varied greatly i.e. from 6 to 125 pS (38 +/- 28 SD, N = 21). In some patches there were up to three current levels of the same size. At -70 mV the open state probability was 0.51 +/- 0.17 and the average duration of an opening 70 +/- 20 ms for 4 channels with conductance from 16-57 pS. These analyses exclude brief flickering (less than 2 ms) or long closed periods (seconds to minutes). These times were not markedly affected by pulling the patch off the cell or by superfusing the isolated patch with media containing 10 mmol X 1(-1) TEA or EGTA without Ca2+. At membrane potentials between -90 and -30 mV there was a small but consistent effect of depolarization to increase the open state probability. Large positive or negative voltage steps decreased the open state probability. Current voltage measurements on intact cells showed a striking decrease in membrane conductance at these large membrane potentials. The leakage conductance of the patch also exhibited some K+ selectivity. The oligodendrocyte membrane appears to contain about one K+ channel per 5 micron 2. The known electrical properties of cultured oligodendrocytes can essentially be explained by the distribution and properties of these K+ channels.

Astrocytes in primary cultures: membrane potential characteristics reveal exclusive potassium conductance and potassium accumulator properties

Mouse astrocytes in homogenous primary cultures were used to study the membrane potential response, measured with intracellular microelectrodes, to alterations of external ion composition and to certain drugs, and the observations were correlated with radiotracer measurements of equilibrated K+ content. The membrane potential was--92 mV at 3 mM K+, and reduction of external Na+ and Cl-concentration, as well as addition of furosemide, which acts on Cl- and Na+ fluxes, had no effect, showing that there is no other ion than K+ which significantly contributes to membrane conductance. Addition of ouabain showed that there is no electrogenic component of the membrane potential at either 'resting' conditions or during stimulation. Nevertheless, the behavior of the membrane potential was found to deviate from a Nernst potential for K+ when extracellular K+ was changed (the slope was linear from 1.5 to 100 mM K+ with 51 mV/10-fold change compared with 61 mV for a Nernstian behavior at 37 degrees C). The radioisotope measurements at different external K+ showed that this was due to accumulation of intracellular K+. We conclude that these astrocytes have spatial buffer and active accumulator properties towards the K+ ion.

Sodium transport in astrocytes

Sodium transport in astrocytes in homogeneous primary cultures from mouse brain cortex were investigated with radiotracer (22Na) and electrophysiological methods. The equilibrated Na+ content was 190 nmol X mg-1 protein and the influx and efflux rates were identical at about 560 nmol X mg-1 X min-1. No significant change was observed in Na+ efflux or influx when external K+ was raised from 5.4 to 12 or 54 mM, but the Na+ content decreased. Intracellular Na+ loading, evoked by previous exposure to ice-cold K+-free medium, double the Na+ efflux. Ouabain, a Na+-K+ exchange inhibitor, exerted a small, nonsignificant inhibition of Na+ efflux at both 5.4 and 12 mM K+ and caused a large increase in Na+ content. At 5.4 mM K+, amiloride, a Na+-H+ exchange inhibitor, decreased both influx and efflux of Na+ and caused an increase in Na+ content. Furosemide, an inhibitor of a cation-Cl- carrier, decreased both content and influx of Na+ slightly but had no significant effect on Na+ efflux. The effects of amiloride or furosemide on Na+ influx were abolished at elevated (12 and 54 mM) K+. Attempts to stimulate the Na+-K+ pump with elevated external K+ or internal Na+ produced no electrogenic component of the membrane potential, probably owing to the high K+ permeability. Based on the present results and earlier experiments on K+ influx, it is concluded that 1) the Na+-K+ pump of astrocytes under normal conditions transports more K+ than Na+; 2) intracellular Na+ loading increases Na+ efflux; 3) some Na+-H+ exchange and cotransport of Na+ and Cl- seem to occur at 5.4 mM K+; and 4) neither of the latter two transport mechanisms is enhanced at elevated K+ concentrations.

Comparison between fluxes of potassium and of chloride in astrocytes in primary cultures

Transport processes operating in astrocytes were examined by measuring unidirectional fluxes of 42K and 36Cl in primary cultures of mouse astrocytes, at steady-state with respect to ion composition. The total K+ uptake rate was 2025 nmol X mg-1 protein X min-1. This rate was not influenced by furosemide (2 mM), an inhibitor of Cl- uptake and K+-K+ exchange, or acetazolamide (0.1 mM), a carbonic anhydrase inhibitor. Ouabain (1 mM) inhibited the uptake rate by 541 nmol X mg-1 X min-1. The equilibrated K+ content was determined to be 696 nmol X mg-1. The rate constant for efflux was 2.76 min-1. This equals an efflux rate of 1921 nmol X mg-1 X min-1, i.e. a similar value as the influx. Furosemide and ouabain did not inhibit the efflux. The equilibrated Cl- content was found to be 167 nmol X mg-1 and it decreased in furosemide-treated cells to 68.1 nmol X mg-1. The total Cl- uptake was 35 nmol X mg-1 X min-1 and it was inhibited by furosemide or bumetanide by 27 nmol X mg-1 X min-1. The mean resting membrane potential was found to be -77.4 mV. From these data we conclude: (1) that the K+ uptake rates are high, as can be expected from estimates based on literature data for K+ conductance in mammalian glial cells in situ; and (2) that the cells possess a very low relative Cl- permeability.

Silanization of glass in the making of ion-sensitive microelectrodes

The silanization of glass, particularly Pyrex, was studied using reaction conditions that might be applied in the fabrication of ion-sensitive microelectrodes of the liquid-membrane type. The efficacy was tested by measuring the hydrophobicity (contact angle) or electrical resistivity of the treated surface. Aminosilanes, such as trimethyl-(dimethylamino)-silane are better than chlorosilanes, the optimum temperature is 250-330 degrees C, and the reaction comes near to completion in 5 min. Silanization of glass that is newly exposed (as in the pulling of a micropipette) is greatly improved if the surface is treated with acid. There is considerable variation from one kind of glass to another. A recipe for making double-barrelled ion-sensitive microelectrodes is given.