Intracellular ion activities and equilibrium potentials in motoneurones and glia cells of the frog spinal cord

Changes in Astroglial K+ upon Brief Periods of Energy Deprivation in the Mouse Neocortex

Malfunction of astrocytic K⁺ regulation contributes to the breakdown of extracellular K⁺ homeostasis during ischemia and spreading depolarization events. Studying astroglial K⁺ changes is, however, hampered by a lack of suitable techniques. Here, we combined results from fluorescence imaging, ion-selective microelectrodes, and patch-clamp recordings in murine neocortical slices with the calculation of astrocytic [K⁺]. Brief chemical ischemia caused a reversible ATP reduction and a transient depolarization of astrocytes. Moreover, astrocytic [Na⁺] increased by 24 mM and extracellular [Na⁺] decreased. Extracellular [K⁺] increased, followed by an undershoot during recovery. Feeding these data into the Goldman-Hodgkin-Katz equation revealed a baseline astroglial [K⁺] of 146 mM, an initial K⁺ loss by 43 mM upon chemical ischemia, and a transient K⁺ overshoot of 16 mM during recovery. It also disclosed a biphasic mismatch in astrocytic Na⁺/K⁺ balance, which was initially ameliorated, but later aggravated by accompanying changes in pH and bicarbonate, respectively. Altogether, our study predicts a loss of K⁺ from astrocytes upon chemical ischemia followed by a net gain. The overshooting K⁺ uptake will promote low extracellular K⁺ during recovery, likely exerting a neuroprotective effect. The resulting late cation/anion imbalance requires additional efflux of cations and/or influx of anions, the latter eventually driving delayed astrocyte swelling.

Physiological impacts of elevated carbon dioxide and ocean acidification on fish

Most fish studied to date efficiently compensate for a hypercapnic acid-base disturbance; however, many recent studies examining the effects of ocean acidification on fish have documented impacts at CO2 levels predicted to occur before the end of this century. Notable impacts on neurosensory and behavioral endpoints, otolith growth, mitochondrial function, and metabolic rate demonstrate an unexpected sensitivity to current-day and near-future CO2 levels. Most explanations for these effects seem to center on increases in Pco2 and HCO3− that occur in the body during pH compensation for acid-base balance; however, few studies have measured these parameters at environmentally relevant CO2 levels or directly related them to reported negative endpoints. This compensatory response is well documented, but noted variation in dynamic regulation of acid-base transport pathways across species, exposure levels, and exposure duration suggests that multiple strategies may be utilized to cope with hypercapnia. Understanding this regulation and changes in ion gradients in extracellular and intracellular compartments during CO2 exposure could provide a basis for predicting sensitivity and explaining interspecies variation. Based on analysis of the existing literature, the present review presents a clear message that ocean acidification may cause significant effects on fish across multiple physiological systems, suggesting that pH compensation does not necessarily confer tolerance as downstream consequences and tradeoffs occur. It remains difficult to assess if acclimation responses during abrupt CO2 exposures will translate to fitness impacts over longer timescales. Nonetheless, identifying mechanisms and processes that may be subject to selective pressure could be one of many important components of assessing adaptive capacity.

Water compartmentalization and spread of ischemic injury in thick-slice ischemia model

Water compartmentalization was studied in a thick-slice (1000 microm) model of ischemia by combining water-content measurements with extracellular diffusion analysis. Thick slices bathed in artificial cerebrospinal fluid continually gained water. Total tissue water content was increased by 67% after 6 hours of the incubation. Diffusion measurements using the tetramethylammonium method showed that the extracellular space, typically occupying 20% of brain tissue in vivo, was decreased to 10% at 30 minutes and 15% at 6 hours in both deep and superficial layers of thick slices. Quantification of water compartmentalization revealed that water moved initially from the extracellular space into the cells. Later, however, both compartments gained water. The initial cell swelling was accompanied by dramatic shifts in potassium. An initial rise of extracellular potassium to about 50 mmol/L was measured with a potassium-selective microelectrode positioned in the center of the thick slice; the concentration decreased slowly afterwards. Potassium content analysis revealed a 63% loss of tissue potassium within two hours of the incubation. In thick slices, ionic shifts, water redistribution, and a loss of synaptic transmission occur in both deep and superficial layers, indicating the spread of ischemic conditions even to areas with an unrestricted supply of nutrients.

Membrane currents and morphological properties of neurons and glial cells in the spinal cord and filum terminale of the frog

Using the patch-clamp technique in the whole-cell configuration combined with intracellular dialysis of the fluorescent dye Lucifer yellow (LY), the membrane properties of cells in slices of the lumbar portion of the frog spinal cord (n=64) and the filum terminale (FT, n=48) have been characterized and correlated with their morphology. Four types of cells were found in lumbar spinal cord and FT with membrane and morphological properties similar to those of cells that were previously identified in the rat spinal cord (Chvátal, A., Pastor, A., Mauch, M., Syková, E., Kettenmann, H., 1995. Distinct populations of identified glial cells in the developing rat spinal cord: Ion channel properties and cell morphology. Eur. J. Neurosci. 7, 129-142). Neurons, in response to a series of symmetrical voltage steps, displayed large repetitive voltage-dependent Na(+) inward currents and K(+) delayed rectifying outward currents. Three distinct types of non-neuronal cells were found. First, cells that exhibited passive symmetrical non-decaying currents were identified as astrocytes. These cells immunostained for GFAP and typically had at least one thick process and a number of fine processes. Second, cells with the characteristic properties of rat spinal cord oligodendrocytes, with passive symmetrical decaying currents and large tail currents after the end of the voltage step. These cells exhibited either long parallel or short hairy processes. Third, cells that expressed small brief inward currents in response to depolarizing steps, delayed rectifier outward currents and small sustained inward currents identical to rat glial precursor cells. Morphologically, they were characterized by round cell bodies with a number of finely branched processes. LY dye-coupling in the frog spinal cord gray matter and FT was observed in neurons and in all glial populations. All four cell types were found in both the spinal cord gray matter and FT. The glia/neuron ratio in the spinal cord was 0.78, while in FT it was 2.0. Moreover, the overall cell density was less in the FT than in the spinal cord. The present study shows that the membrane and morphological properties of glial cells in the frog and rat spinal cords are similar. Such striking phylogenetic similarity suggests a significant contribution from distinct glial cell populations to various spinal cord functions, particularly ionic and volume homeostasis in both mammals and amphibians.

Role of calcium conductances on spike afterpotentials in rat trigeminal motoneurons

Intracellular recordings were obtained from rat trigeminal motoneurons in slice preparations to investigate the role of calcium conductances in the depolarizing and hyperpolarizing spike afterpotential (ADP and mAHP, respectively). The mAHP was suppressed by bath application of 1 microM apamin, 2 mM Mn2+, and 2 mM Co2+, and also by intracellular injection of ethylene glycol-bis(b-aminoethylenether)-N,N,N',N'-tetraacetic acid (EGTA), suggesting that the potassium conductance generating the mAHP is activated by Ca2+ influx. Mn2+ (2 mM) or Cd2+ (500 microM) reduced the ADP, whereas the ADP amplitude was increased by raising extracellular Ca2+ concentration from 2 to 8 mM by bath application of Ba2+ (0.5-5 mM) and by intracellular injection of EGTA. This would suggest that Ca2+ itself is likely to be the charge carrier generating the ADP. Focal application of omega-conotoxin GVIA (10-30 microM) suppressed the mAHP and enhanced the ADP, whereas focal application of omega-agatoxin IVA (10-100 microM) reduced the ADP amplitude without apparent effects on the mAHP. We conclude that Ca2+ influx through omega-agatoxin IVA-sensitive calcium channels is at least in part responsible for the generation of the ADP and that Ca2+ influx through omega-conotoxin GVIA-sensitive calcium channels contributes to the generation of the mAHP. Because of the selective suppression of the ADP and mAHP by omega-agatoxin IVA and omega-conotoxin GVIA, respectively, it is suggested that both calcium channels are separated geometrically in rat trigeminal motoneurons.

Potential-dependent block of human delayed rectifier K+ channels by internal Na+

The delayed rectifier K+ currents in differentiated human SH-SY5Y neuroblastoma cells were characterized with tight-seal recording techniques. Activation and inactivation parameters were measured. At high positive potentials, the current showed a marked rectification, causing a region of negative slope conductance in the current vs. potential curve. The rectification depended markedly on the pipette Na+ concentration. Without Na+, no rectification was observed, whereas with high Na+ (20-60 mM), a marked rectification was always observed. Tail current measurements showed a fast ( < 400 microseconds) block of K+ currents in the presence of internal Na+. With 60 mM Na+ in the pipette 8% of the K+ current was blocked at 0 mV, 27% at +20 mV, and 82% at +100 mV. Similar degrees of block were often seen with 30 mM Na+ in the pipette. The submembrane Na+ concentration in intact cells was estimated, on the basis of the reversal of Na+ current, to be approximately 15 mM. Single-channel K+ currents, in the cell-attached configuration, showed a conductance of approximately 20 pS at 40-60 mV above rest but showed rectification at high potentials.

Intracellular sodium homeostasis in rat hippocampal astrocytes

1. We determined the intracellular Na+ concentration ([Na+]i) and mechanisms of its regulation in cultured rat hippocampal astrocytes using fluorescence ratio imaging of the Na+ indicator SBFI-AM (acetoxymethylester of sodium-binding benzofuran isophthalate, 10 microM). Dye signal calibration within the astrocytes showed that the ratiometric dye signal changed monotonically with changes in [Na+]i from 0 to 140 nM. The K+ sensitivity of the dye was negligible; intracellular pH changes, however, slightly affected the 'Na+' signal. 2. Baseline [Na+]i was 14.6 +/- 4.9 mM (mean +/- S.D.) in CO2/HCO3(-)-containing saline with 3 mM K+. Removal of extracellular Na+ decreased [Na+]i in two phases: a rapid phase of [Na+]i reduction (0.58 +/- 0.32 mM min-1) followed by a slower phase (0.15 +/- 0.09 mM min-1). 3. Changing from CO2/HCO3(-)-free to CO2/HCO3(-)-buffered saline resulted in a transient increase in [Na+]i of approximately 5 mM, suggesting activation of inward Na(+)-HCO3- cotransport by CO2/HCO3-. During furosemide (frusemide, 1 mM) or bumetanide (50 microM) application, a slow decrease in [Na+]i of approximately 2 mM was observed, indicating a steady inward transport of Na+ via Na(+)-K(+)-2Cl- cotransport under control conditions. Tetrodotoxin (100 microM) did not influence [Na+]i in the majority of cells (85%), suggesting that influx of Na+ through voltage-gated Na+ channels contributed to baseline [Na+]i in only a small subpopulation of hippocampal astrocytes. 4. Blocking Na+, K(+)-ATPase activity with cardiac glycosides (ouabain or strophanthidin, 1 mM) or removal of extracellular K+ led to an increase in [Na+]i of about 2 and 4 mM min-1, respectively. This indicated that Na+, K(+)-ATPase activity was critical in maintaining low [Na+]i in the face of a steep electrochemical gradient, which would favour a much higher [Na+]i. 5. Elevation of extracellular K+ concentration ([K+]o) by as little as 1 mM (from 3 to 4 mM) resulted in a rapid and reversible decrease in [Na+]i. Both the slope and the amplitude of the [K+]o-induced reductions in [Na+]i were sensitive to bumetanide. A reduction of [K+]o by 1 mM increased [Na+]i by 3.0 +/- 2.3 mM. In contrast, changing extracellular Na+ concentration by 20 mM resulted in changes in [Na+]i of less than 3 mM. 6. These results implied that in hippocampal astrocytes low baseline [Na+]i is determined by the action of Na(+)-HCO3- cotransport, Na(+)-K(+)-2Cl- cotransport and Na+, K(+)-ATPase, and that both Na+, K(+)-ATPase and inward Na(+)-K(+)-2Cl cotransport are activated by small, physiologically relevant increases in [K+]o. These mechanisms are well suited to help buffer increases in [K+]o associated with neural activity.

Na(+)-activated K+ channels localized in the nodal region of myelinated axons of Xenopus

1. A potassium channel activated by internal Na+ ions (K+Na channel) was identified in peripheral myelinated axons of Xenopus laevis using the cell-attached and excised configurations of the patch clamp technique. 2. The single-channel conductance for the main open state was 88 pS with [K+]o = 105 mM and pS with [K+]o = 2.5 mM ([K+]i = 105 mM). The channel was selectively permeable to K+ over Na+ ions. A characteristic feature of the K+Na channel was the frequent occurrence of subconductance states. 3. The open probability of the channel was strongly dependent on the concentration of Na+ ions at the inner side of the membrane. The half-maximal activating Na+ concentration and the Hill coefficient were 33 mM and 2.9, respectively. The open probability of the channel showed only weak potential dependence. 4. The K+Na channel was relatively insensitive to external tetraethylammonium (TEA+) in comparison with voltage-dependent axonal K+ channels; the half-maximal inhibitory concentration (IC50) was 21.3 mM (at -90 mV). In contrast, the channel was blocked by low concentrations of external Ba2+ and Cs+ ions, with IC50 values of 0.7 and 1.1 mM, respectively (at -90 mV). The block by Ba2+ and Cs+ was more pronounced at negative than at positive membrane potentials. 5. A comparison of the number of K+Na channels in nodal and paranodal patches from the same axon revealed that the channel density was about 10-fold higher at the node of Ranvier than at the paranode. Moreover, a correlation between the number of K+Na channels and voltage-dependent Na+ channels in the same patches was found, suggesting co-localization of both channel types. 6. As weakly potential-dependent ('leakage') channels, axonal K+Na channels may be involved in setting the resting potential of vertebrate axons. Simulations of Na+ ion diffusion suggest two possible mechanisms of activation of K+Na channels: the local increase of Na+ concentration in a cluster of Na+ channels during a single action potential or the accumulation in the intracellular axonal compartment during a train of action potentials.

Sodium-activated potassium current in sensory neurons: a comparison of cell-attached and cell-free single-channel activities

Single-channel currents from Na(+)-dependent K+ channels (KNa) were recorded from cell-attached and inside-out membrane patches of cultured avian trigeminal ganglion neurons by means of the patch-clamp technique. Single-channel properties, such as the high elementary conductance and the occurrence of sub-conductance levels, were unchanged after the patches had been excised from the cells, indicating that they are not under the control of soluble cytoplasmic factors. In cell-attached recordings at the cell resting potential the degree of KNa activity, measured as the probability of the channel being open, Po, was low in most cases (around 0.01) and similar to that observed in the inside-out configuration when the bath solution contained concentrations of Na+ around 30 mM and of K+ close to the physiological intracellular levels. However, in some cell-attached patches Po was high (around 0.2) and comparable to the values measured in cell-free recordings with high Na+ concentrations in the bath (100 mM). The excision of a high-activity patch in the presence of 30 mM Na+ resulted in a fall of Po in about 20 s, which is consistent with the wash-out of a soluble cytoplasmic molecule. After the excision, all KNa displayed a similar Na+ sensitivity, irrespective of the degree of activation observed in the cell-attached mode. In inside-out patches the Po values observed in the presence of either low or high concentrations of Na+ in bath solutions were not modified by internal Ca2+ (0.8-8.5 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Cl- and Na+ homeostasis during anoxia in rat hypoglossal neurons: intracellular and extracellular in vitro studies

1. To understand the mechanisms which lead to acute neuronal swelling during anoxia, we studied the ionic movements of Cl- and Na+ during O2 deprivation in the hypoglossal (XII) neurons of rat brain slices using double-barrelled ion-selective microelectrodes. 2. Baseline extracellular Cl- and Na+ activities ([Cl-]o, [Na+]o) were 128.3 +/- 7.4 and 150.0 +/- 3.4 mM respectively (n = 12) in the adult. Similar baseline values were obtained from neonatal brain slices. 3. During a period of anoxia (4 min), [Na+]o decreased by about 40 mM in adult slices while [Na+]o did not show any significant change in the neonate (n = 12). Although anoxia induced a significant decrease of [Cl-]o in both adult and neonate, [Cl-]o dropped 7 times more in the adult than in the neonate (n = 12). 4. Intracellular Cl- activity ([Cl-]i) was studied in twenty-seven adult hypoglossal cells. All showed an increase in [Cl-]i) was studied in twenty-seven adult hypoglossal cells. All showed an increase in [Cl-]i with O2 deprivation. Detailed analysis carried out on ten hypoglossal neurons showed a baseline [Cl-]i of 11.4 +/- 4.5 mM and an increase in [Cl-]i by 20.6 +/- 7.2 mM during O2 limitation. 5. Baseline [Cl-]i in neonatal XII neurons was similar to that of the adult. Anoxia, however, produced an increase in [Cl-]i by only 4.5 +/- 2.4 mM (n = 7). This increase in [Cl-]i was significantly less than that in the adult (P less than 0.001). Prolonged anoxia (6-12 min) in the neonate led to a more substantial increase in [Cl-]i, an observation consistent with the decrease in [Cl-]o after prolonged O2 deprivation. 7. We conclude that during anoxia: (1) intracellular [Cl-] increases in the adult and this most likely occurs because of entry of extracellular Cl- into the cytosol and (2) there is a major maturational difference in mechanisms regulating Cl- and Na+ homeostasis between newborn and adult brain tissue. We speculate that these mechanisms may account, at least partially, for the relative tolerance to anoxia in the newly born.

High conductance anion channel in Schwann cell vesicles from rat spinal roots

Potassium uptake, possibly together with chloride, is one of the presumed functions of Schwann cells in the peripheral nervous system. However, the presence of chloride channels has not been demonstrated in adult Schwann cells. We present here a new method which allows single channel recordings to be made from Schwann cells in situ without enzymatic treatment. Isolated rat spinal roots were split mechanically into several bundles. Within about 30 min after this procedure small bleb-like vesicles (approximately 20-30 microns in diameter) with a clean surface appeared at the edges of the fibre bundles. Immunofluorescence microscopy with a surface marker for Schwann cell membranes (monoclonal antibody O4) revealed that the vesicles originate from Schwann cells. In standard patch clamp recordings with symmetrical bath and pipette solutions (excised inside-out configuration) an anion channel with the following characteristics was mainly observed: 1) single channel slope conductance of 337 +/- 5 pS in 125 mM KCl and 209 +/- 6 pS in 125 mM K+ methylsulphate; 2) ion permeability ratio: PCl/PK/Pgluconate = 1/0.12/0.06; 3) linear current-voltage relationship (range +/- 60 mV); and 4) voltage- and time-dependent inactivation (the channel was most active at potentials +/- 20 mV). Pharmacologically, the channel was completely blocked with zinc (1 mM) and barium (10 mM). A similar anion channel, showing characteristics 1-4), has been described in cultured Schwann cells of newborn rats (Gray et al., 1984). We now demonstrate that this channel is also present in adult Schwann cells in situ.

Evidence that action potentials activate an internodal potassium conductance in lizard myelinated axons

1. We have studied action potentials and after-potentials evoked in the internodal region of visualized lizard intramuscular nerve fibres by stimulation of the proximal nerve trunk. Voltage recordings were obtained using microelectrodes inserted into the axon (intra-axonal) or into the layers of myelin (peri-internodal), with the goal of studying conditions required to activate internodal K+ currents. 2. Peri-internodal recordings made using K2SO4-, KCl- or NaCl-filled electrodes exhibited a negligible resting potential (less than 2 mV), but showed action potentials with peak amplitudes of up to 78 mV and a duration less than or equal to that of the intra-axonally recorded action potential. 3. Following ionophoretic application of potassium from a peri-internodal microelectrode, the peri-internodal action potential was followed by a prolonged (hundreds of milliseconds) negative plateau. This plateau was not seen following peri-internodal ionophoresis of sodium. The prolonged negative potential (PNP) was confined to the K(+)-injected internode: it could be recorded by a second peri-internodal microelectrode inserted into the same internode, but not into an adjacent internode. 4. The peri-internodally recorded PNP was accompanied by an equally prolonged intra-axonal depolarizing after-potential, and by an increase in the conductance of the internodal axolemma. However, the K+ ionophoresis that produced the PNP had little or no detectable effect on the intra-axonally or peri-internodally recorded resting potential or action potential. These findings suggest that the PNP is generated by an inward current across the axolemma of the K(+)-injected internode, through channels opened following the action potential. 5. Following peri-internodal K+ ionophoresis a PNP could also be evoked by passage of depolarizing current pulses through an intra-axonal electrode or by passage of negative current pulses through an electrode in the K(+)-filled peri-internodal region. The threshold for evoking a PNP was less than the threshold for evoking an action potential, and the PNP persisted in 10 microM-tetrodotoxin. Thus the PNP is evoked by depolarization of the axolemma rather than by Na+ influx. 6. The PNP was reversibly blocked by tetraethylammonium (TEA, 2-10 mM), but was not blocked by 100 microM-3,4-diaminopyridine or 5 mM-4-aminopyridine.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of potassium on the anion and cation contents of primary cultures of mouse astrocytes and neurons

In astrocytes, as [K+]o was increased from 1.2 to 10 mM, [K+]i and [Cl-]i were increased, whereas [Na+]i was decreased. As [K+]o was increased from 10 to 60 mM, intracellular concentration of these three ions showed no significant change. When [K+]o was increased from 60 to 122 mM, an increase in [K+]i and [Cl-]i and a decrease in [Na+]i were observed. In neurons, as [K+]o was increased from 1.2 to 2.8 mM, [Na+]i and [Cl-]i were decreased, whereas [K+]i was increased. As [K+]o was increased from 2.8 to 30 mM, [K+]i, [Na+]i and [Cl-]i showed no significant change. When [K+]o was increased from 30 to 122 mM, [K+]i and [Cl-]i were increased, whereas [Na+]i was decreased. In astrocytes, pHi increased when [K+]o was increased. In neurons, there was a biphasic change in pHi. In lower [K+]o (1.2-2.8 mM) pHi decreased as [K+]o increased, whereas in higher [K+]o (2.8-122 mM) pHi was directly related to [K+]o. In both astrocytes and neurons, changes in [K+]o did not affect the extracellular water content, whereas the intracellular water content increased as the [K+]o increased. Transmembrane potential (Em) as measured with Tl-204 was inversely related to [K+]o between 1.2 and 90 mM, a ten-fold increase in [K+]o depolarized the astrocytes by about 56 mV and the neurons about 52 mV. The Em values measured with Tl-204 were close to the potassium equilibrium potential (Ek) except those in neurons at lower [K+]o. However, they were not equal to the chloride equilibrium potential (ECl) at [K+]o lower than 30 mM in both astrocytes and neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Potassium current activated by intracellular sodium in quail trigeminal ganglion neurons

Whole-cell voltage clamp and single-channel recordings were performed on cultured trigeminal ganglion neurons from quail embryos in order to study a sodium-activated potassium current (KNa). When KNa was activated by a step depolarization in voltage clamp, there was a proportionality between KNa and INa at all voltages between the threshold of INa and ENa. Single-channel recordings indicated that KNa could be activated already by 12 mM intracellular sodium and was almost fully activated at 50 mM sodium. 100 mM lithium, 100 mM choline, or 5 microM calcium did not activate KNa. The relationship between the probability for the channel to be open (Po) vs. the sodium concentration and the relationship of KNa open time-distributions vs. the sodium concentration suggest that two to three sodium ions bind cooperatively before KNa channels open. KNa channels were sensitive to depolarization; at 12 mM sodium, a 42-mV depolarization caused an e-fold increase in Po. Under physiological conditions, the conductance of the KNa channel was 50 pS. This conductance increased to 174 pS when the intra- and extracellular potassium concentrations were 75 and 150 mM, respectively.

Intracellular chloride activity in glial cells of the leech central nervous system

1. Chloride-sensitive double-barrelled microelectrodes were used to measure the intracellular Cl- activity (aicl) and membrane potential (Em) in neuropile glial cells of the leech, Hirudo medicinalis. 2. A close relation between the equilibrium potential for Cl- (ECl = -66.1 +/- 4.9 mV; mean +/- S.D.) and the resting potential (Em = -67.8 +/- 5.2 mV; n = 19) was observed in nominally CO2-HCO3(-)-free, HEPES-buffered solutions. A saline buffered with 2% CO2, 11 mM-HCO3- elicited a membrane hyperpolarization and a concomitant decrease of aCl. 3. Changes in ECl followed these of Em with a lag of less than 30 s in response to various extracellular K+ concentrations [( K+]o) or due to bath-application of carbachol or serotonin. 4. Introduction of a Cl(-)-free solution resulted in a transient depolarization indicating a substantial Cl- conductance and a rapid decrease of aiCl to an apparent value of 0.5-0.9 mM. 5. The loop diuretics furosemide (1 mM) and bumetanide (0.2 mM) did not affect the K(+)-induced changes of aiCl. 6. The results indicate a passive Cl- distribution across the membrane of leech neuropile glial cells as a result of a high Cl- conductance.

Evidence for the uptake of neuronally derived choline by glial cells in the leech central nervous system

1. With ion-sensitive microelectrodes based on the Corning exchanger 477317, the accumulation of an unidentified interfering substance was monitored in leech neuropile glial cells but not in neurons after a 10-fold increase in extracellular K+ concentration. Evidence is presented which shows that this substance may be choline. 2. The accumulation of interfering ions was not observed in Ca2(+)-free saline and was substantially reduced in the presence of eserine (a blocker of acetylcholinesterase). 3. In neuropile (and also packet) glial cells, extracellularly applied choline (10(-4) M) caused a steady increase in ion signal. This increase was not affected by removal of extracellular calcium, by hemicholinium-3 (a blocker of high-affinity choline uptake) or eserine. Shortly after the removal of choline from the saline the increase in ion signal stopped and the ion signal then decreased slowly to its original level. 4. Extracellular acetylcholine (10(-4) M) caused a similar increase in intracellular ion signal of neuropile glial cells to that caused by choline. This increase was blocked by eserine. 5. Extracellular choline caused a comparatively small increase in ion signal of Retzius neurones which was blocked by hemicholinium-3. In pressure neurones, choline or hemicholinium-3 had no effect on intracellular ion signal. 6. Autoradiographic analysis of [3H]choline uptake showed that most of the choline was taken up by glial cells in a time- and dose-dependent manner. Small but significant amounts of choline were taken up by neurones and connective tissue. 7. It is concluded that the neuropile and packet glial cells possess an effective choline uptake system which is activated by exogenous choline but also by choline that stems from enzymatic inactivation of acetylcholine released by neurones.

The dependence of motoneurone membrane potential on extracellular ion concentrations studied in isolated rat spinal cord

1. Intracellular recordings from ninety-nine motoneurones have been made in an in vitro hemisected spinal cord preparation. Their mean resting membrane potential in normal artificial cerebrospinal fluid (CSF) was -71 +/- 0.5 mV (+/- S.E.M.). The mean amplitude of the action potential was 84.0 +/- 1.4 mV (n = 50), and the mean input conductance was 101 +/- 7 nS (n = 49). 2. Both membrane potential and input conductance were sensitive to changes in [K+]o, [Na+]o, [Cl-]o and [Ca2+]o. 3. Replacement of extracellular Ca2+ by Mn2+ resulted in less than 1 mV hyperpolarization and a decrease in input conductance from 102 +/- 7 to 93 +/- 6 nS (n = 15). 4. At high [K+]o (greater than 10 mM) the membrane potential followed the potential predicted by the Nernst equation for K+ ions with a slope of 58 mV per 10-fold change in [K+]o. At low [K+]o (less than 10 mM) there was significant deviation from K+ equilibrium potential (EK). 5. [K+]i was found to be 106 mM when estimated from the reversal potential of the after-hyperpolarization of the antidromic action potential. 6. The reversal potential of the recurrent inhibitory postsynaptic potential (IPSP) in normal CSF was used to calculate [Cl-]i. This was 6.6 mM, which is less than would be expected if Cl- was passively distributed, indicating the presence of an outwardly directed Cl- pump. 7. Decreasing [Cl-]o from control (134 mM) to 4 mM resulted in a depolarization of 6.9 +/- 0.9 mV and a decrease in input conductance from 102 +/- 5 to 90 +/- 5 nS (n = 14) in 3 mM [K+]o. 8. Decreasing [Na+]o from 156 to 26 mM by substitution with choline resulted in a 6.2 +/- 0.5 mV hyperpolarization and a decrease in input conductance from from 102 +/- 4 to 76 +/- 4 nS (n = 5) in 3 mM [K+]o. 9. The input conductances for Na+, Cl- and K+ at the resting potential were calculated. After allowing for a microelectrode leak conductance, the relative input conductances were gNa/gK = 0.13 and gCl/gK = 0.25.

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.

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+.

Changes in free calcium ion concentration recorded inside hippocampal pyramidal cells in situ

In rats under urethane or pentobarbitone anesthesia, Ca2+ -sensitive microelectrodes were inserted into CA3 and CA1 hippocampal cells. In 23 neurons with a mean resting membrane potential (Vm) of -56.9 mV, the Ca potential (VCa) fell below Vm by an average of -22.1 mV (S.D. +/- 19.1 mV), indicating a mean intracellular free Ca2+ concentration ([Ca]i) of 9.7 microM (S.D. 14.9 microM). In spite of their better and more stable Vm (mean -67.1 mV), unresponsive cells (probably neuroglia) had a higher and more variable [Ca]i (mean 37.0 +/- 51.2 microM). In 21 of the neurons, repetitive stimulation of the fimbria--at 5-20 Hz for 30s, which is sufficient to elicit bursts of population spikes--evoked substantial increases in [Ca]i: the mean increase observed during or just after 29 such tetani was +27.1 +/- 54.5 microM. Typically [Ca]i reached a peak near the end of the tetanus and then decayed with a half-time of 5-10 s, though not necessarily to the initial level. In 7 cells, a large increase in [Ca] (mean +239 +/- 367 microM) appeared as a late event, 20-30 s after the end of the tetanus. In 5 cells, [Ca]i could thus be raised transiently to 10(-4) M or higher. All these increases in [Ca]i are far greater than can be evoked by tetanic activation in spinal motoneurons; their possible significance for long term potentiation or cell necrosis in the hippocampus is discussed.

Possible involvement of K+-conductance in the action of gamma-aminobutyric acid in the guinea-pig hippocampus

The mechanism underlying the action of gamma-aminobutyric acid (GABA) in the hippocampus was investigated using guinea-pig brain slices. GABA either superfused or applied directly by microiontophoresis produced a biphasic response in pyramidal cells, comprising hyperpolarizing and depolarizing components. When different concentrations of GABA were applied to the same neurone, the lower concentrations generally produced a hyperpolarization-predominant response, while higher concentrations resulted in a depolarization-predominant response. The depolarizing component of the response to GABA was augmented in a medium containing a low concentration of Cl-, relatively unaffected by a change in external K+ concentration, and blocked by picrotoxin (2 X 10(-5) M). The depolarizing response to GABA persisted in a Ca2+-free medium in which the concentration of Na+ was reduced to 13 mM. Combined application of low doses of picrotoxin and bicuculline eliminated the major part of the depolarizing component of the biphasic response to GABA and produced a relatively pure hyperpolarizing response. The reversal potential of this pharmacologically 'isolated' hyperpolarizing response to GABA was estimated, from the current-voltage relationships, to be about -90 mV and was the same as that of the hyperpolarization induced by baclofen. When the membrane was successively hyperpolarized by inward direct current (d.c.) injections, the reversal point of the 'pharmacologically isolated' hyperpolarizing response to GABA coincided with that of the post-burst hyperpolarization. Low concentrations of Cl- in the bathing medium had no noticeable effect on the hyperpolarizing component of the response to GABA, whereas it markedly increased the amplitude of the depolarizing component. These results suggest that the action of GABA in the hippocampus may involve an activation of K+ conductance.

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.

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.

The ionic mechanism of postsynaptic inhibition in motoneurones of the frog spinal cord

The ionic mechanism of postsynaptic inhibition in frog spinal motoneurones was studied with conventional and with ion-sensitive microelectrodes. In these neurones the inhibitory postsynaptic potential was depolarizing, its reversal potential being 15 mV less negative than the resting membrane potential. During the inhibitory postsynaptic potential the input resistance of the motoneurones was reduced to 20% of the resting value, indicating a strong increase of membrane conductance. The Cl- equilibrium potential calculated from intra- and extracellular Cl- activity measurements coincided with the reversal potential of the inhibitory postsynaptic potential to within a few millivolts. During repetitive inhibitory postsynaptic activity the intracellular Cl- activity decreased markedly, while the extracellular Cl- activity increased slightly. These changes of intra- and extracellular Cl- activities were no longer found after suppression of the inhibitory postsynaptic potential by strychnine. Blockade of an active, inward-going Cl- transport system in motoneurones by NH+4 led to a shift of the Cl- equilibrium potential and the reversal potential of the inhibitory postsynaptic potential towards the resting membrane potential. After prolonged action of NH+4, the Cl- equilibrium potential approached the membrane potential to within 5 mV, while the reversal potential of the inhibitory postsynaptic potential and resting membrane potential coincided. The difference between Cl- equilibrium potential and membrane potential after blockade of the Cl- pump is traced back to interfering intracellular ions, such as HCO-3 or SO42-, leading to an overestimation of intracellular Cl- activity and to the calculation of an erroneous Cl- equilibrium potential. Inhibitory amino acids like gamma-aminobutyrate or beta-alanine evoked depolarizations with reversal potentials similar to that of the inhibitory postsynaptic potential. These depolarizations were associated with a marked decrease of neuronal input resistance during inhibition. During the actions of these compounds a decrease of intracellular and a small increase of extracellular Cl- activity were found. The activities of other ions (K+, Ca2+ and Na+) did not change significantly, with the exception of extracellular K+ activity, which was slightly increased. Evidence is presented that the inhibitory postsynaptic potential, as well as the depolarizing action of inhibitory amino acids in motoneurones, is the result of an increase in membrane Cl- permeability and an efflux of Cl- from these cells, while other ions do not seem to be involved.

Changes in intracellular free Ca ion concentration evoked by electrical activity in cat spinal neurons in situ

In cats under allobarbitone anaesthesia, Ca2+-sensitive microelectrodes were inserted into the lumbosacral spinal neurons to measure intracellular free Ca2+ concentration [Ca]i. In 72 resting motoneurons, the global mean [Ca]i was 7.9 microM (SD +/- 25.9). In the 36 "best" cells (with resting and action potentials better than 60 mV), mean [Ca]i was 1.6 microM (SD +/- 1.64). Activation of motoneurons by antidromic or direct stimulation evoked mean increases in [Ca]i of about 90 nM when stimulating for 30 s at 10 Hz, and 170 nM at 20 Hz. The mean time to half-recovery was 23 s (SD +/- 14.5). Orthodromic stimulation consistently produced smaller increases in [Ca]i. Measurements in motor axons showed a comparable resting level of [Ca]i, but only minimal changes during stimulation, even at 100 Hz. Sensory axons (also recorded within the spinal cord) similarly failed to show any increase in [Ca]i during high frequency stimulation. In some interneurons, however, particularly large and rapid increases in [Ca]i could be evoked by dorsal root stimulation at 1-5 Hz. Unresponsive cells (presumably neuroglia), with a typically high and stable resting potential, had a variable [Ca]i giving a mean of 32 microM (SD +/- 63.0). A tentative theoretical analysis of the magnitude and time course of delta [Ca]i evoked in motoneurons by tetanic stimulation is consistent with remarkably slow apparent diffusion of intracellular Ca2+ (1/250 of rate of diffusion in water), such as might be expected in the presence of very efficient mechanisms of Ca2+ sequestration.

Depolarization of cultured oligodendrocytes by glutamate and GABA

Subpopulations of cultured oligodendrocytes from mouse spinal cord respond with depolarization to glutamate and GABA. Heterogeneity in the oligodendrocyte population was indicated by the observation that some cells respond to both GABA and glutamate, while others respond to only one and some are not responsive to either. Depolarizations are not mediated by an increase of [K+]o released from neurons. The response to GABA was blocked in Na+-free solution and is not accompanied by a change in input resistance. Several other neurotransmitters did not induce changes in membrane potential.

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.

Intracellular ion changes of astrocytes in response to extracellular potassium

Intracellular changes of K+, Na+, and Cl- were investigated by the aid of radiotracers in primary cultures of astrocytes when extracellular K+ was (1) increased from 3 to 12 mM and subsequently again decreased to 3 mM; and (2) increased from 5.4 to 54 mM with subsequent decrease to 5.4 mM. In both situations the K+ content increased by 50% within seconds, and it doubled within 1-2 min. The increase must be carrier mediated, because keeping the K X Cl product (Donnan equilibrium) constant did not lower the K+ accumulation rates. The Na+ content decreased when K+ was increased to 12 mM, but the decrease corresponded only to 10% of the accumulated K+. When K+ was increased to 54 mM, the Na+ content increased transiently. Cl- increased by about 15-25% of the accumulated K+. Return of extracellular K+ to original levels evoked a very fast K+ release, reversing all ion changes. The Na+ content increased transiently during the release process. For an interpretation of these observations, it is necessary to postulate endogenous production of an anion and of H+, which in turn is partly exchanged with Na+.