Physiological characteristics of NG2-expressing glial cells

Antibodies against the chondroitin sulfate proteoglycan NG2 label a subpopulation of glial cells within the CNS, which have a small cell body and thin radiating processes. Physiological recordings from these small cells in acute brain slices have revealed that they possess unique properties, suggesting that they may comprise a class of glial cells distinct from astrocytes, oligodendrocytes, or microglia. NG2-expressing glial cells (abbreviated as NG2 cells here) have a moderate input resistance and are not dye- or tracer-coupled to adjacent cells. They express voltage-gated Na⁺, K⁺and Ca²⁺conductances, though they do not exhibit regenerative Na⁺or Ca²⁺action potentials due to the much larger K⁺conductances present. In addition to voltage-gated conductances, they express receptors for various neurotransmitters. In the hippocampus, AMPA and GABA_Areceptors on these cells are activated by release of transmitter from neurons at defined synaptic junctions that are formed with CA3 pyramidal neurons and GABAergic interneurons. These rapid forms of neuron-glial communication may regulate the proliferation rate of NG2 cells or their development into mature oligodendrocytes. These depolarizing inputs may also trigger the release of neuroactive substances from NG2 cells, providing feedback regulation of signaling at neuronal synapses. Although the presence of Ca²⁺permeable AMPA receptors provides a pathway to link neuronal activity to Ca²⁺dependent processes within the NG2 cells, these receptors also put these cells at risk for glutamate-associated excitotoxicity. This vulnerability to the sustained elevation of glutamate may underlie ischemic induced damage to white matter tracts and contribute to cerebral palsy in premature infants.     

Membrane ionic currents and properties of freshly dissociated rat brainstem neurons

It is well known that neuronal firing properties are determined by synaptic inputs and inherent membrane functions such as specific ionic currents. To characterize the ionic currents of brainstem cardio-respiratory neurons, cells from the hypoglossal (XII) nucleus and the dorsal motor nucleus of the vagus (DMX) were freshly dissociated and membrane ionic currents were studied under whole-cell voltage and current clamp. Both of these neurons showed a TTX-sensitive Na⁺ current with a much larger current density in XII than DMX neurons. This Na⁺ current had two (fast and slow) distinct inactivation decay components. The ratio of the magnitudes of the fast to slow component was roughly two-fold greater in DMX than in XII cells. Both DMX and XII neurons also showed a high voltage-activated Ca²⁺ current, but this current density was significantly greater (three-fold) in DMX than XII neurons. A relatively small amount of low-voltage activated Ca²⁺ current was also observed in DMX neurons, but not in the majority of XII cells. A transient and a sustained outward current components were observed in DMX cells, but only sustained currents were present in XII neurons. These outward currents had a reversal potential of about − 70 mV with 3 mM external K⁺ and −30 mV with 25 mM K⁺, and substitution of K⁺ with cesium and tetraethylammonium suppressed more than 90% the outward currents, indicating that most outward currents were carried by K⁺. The transient outward current consisted of two components with onesensitive to 4-aminopyridine and the other to intracellular Ca²⁺. In XII neurons, BRL 38227 (lemakalim), an ATP-sensitive K⁺ (K_ATP) channel activator, increased the sustained K⁺ currents by 10% of control, and glibenclamide, a K_ATP channel blocker, decreased the sustained K⁺ currents by 20%. Evidence for the presence of an inward rectifier K⁺ current was also obtained from both XII and DMX neurons. These results on XII and DMX neurons indicate that (1) the methods used to dissociate neurons provide a useful means to overcome voltage clamp technical difficulties; (2) ion channel characteristics such as density and biophysical properties of DMX neurons are very different from those of XII neurons; and (3) several newly discovered membrane ionic currents are present in these cells.     

High extracellular K+ evokes changes in voltage-dependent K+ and Na+ currents and volume regulation in astrocytes

[K⁺]_e increase accompanies many pathological states in the CNS and evokes changes in astrocyte morphology and glial fibrillary acidic protein expression, leading to astrogliosis. Changes in the electrophysiological properties and volume regulation of astrocytes during the early stages of astrocytic activation were studied using the patch-clamp technique in spinal cords from 10-day-old rats after incubation in 50 mM K⁺. In complex astrocytes, incubation in high K⁺ caused depolarization, an input resistance increase, a decrease in membrane capacitance, and an increase in the current densities (CDs) of voltage-dependent K⁺ and Na⁺ currents. In passive astrocytes, the reversal potential shifted to more positive values and CDs decreased. No changes were observed in astrocyte precursors. Under hypotonic stress, astrocytes in spinal cords pre-exposed to high K⁺ revealed a decreased K⁺ accumulation around the cell membrane after a depolarizing prepulse, suggesting altered volume regulation. 3D confocal morphometry and the direct visualization of astrocytes in enhanced green fluorescent protein/glial fibrillary acidic protein mice showed a smaller degree of cell swelling in spinal cords pre-exposed to high K⁺ compared to controls. We conclude that exposure to high K⁺, an early event leading to astrogliosis, caused not only morphological changes in astrocytes but also changes in their membrane properties and cell volume regulation.     

Membrane ionic currents and properties of freshly dissociated rat brainstem neurons

It is well known that neuronal firing properties are determined by synaptic inputs and inherent membrane functions such as specific ionic currents. To characterize the ionic currents of brainstem cardio-respiratory neurons, cells from the hypoglossal (XII) nucleus and the dorsal motor nucleus of the vagus (DMX) were freshly dissociated and membrane ionic currents were studied under whole-cell voltage and current clamp. Both of these neurons showed a TTX-sensitive Na⁺ current with a much larger current density in XII than DMX neurons. This Na⁺ current had two (fast and slow) distinct inactivation decay components. The ratio of the magnitudes of the fast to slow component was roughly two-fold greater in DMX than in XII cells. Both DMX and XII neurons also showed a high voltage-activated Ca²⁺ current, but this current density was significantly greater (three-fold) in DMX than XII neurons. A relatively small amount of low-voltage activated Ca²⁺ current was also observed in DMX neurons, but not in the majority of XII cells. A transient and a sustained outward current components were observed in DMX cells, but only sustained currents were present in XII neurons. These outward currents had a reversal potential of about -70 mV with 3 mM external K⁺ and -30mV with 25 mM K⁺, and substitution of K⁺ with cesium and tetraethylammonium suppressed more than 90% the outward currents, indicating that most outward currents were carried by K⁺. The transient outward current consisted of two components with one sensitive to 4-aminopyridine and the other to intracellular Ca²⁺. In XII neurons, BRL 38227 (lemakalim), an ATP-sensitive K⁺ (K_ATP) channel activator, increased the sustained K⁺ currents by 10% of control, and glibenclamide, a K_ATP channel blocker, decreased the sustained K⁺ currents by 20%. Evidence for the presence of an inward rectifier K⁺ current was also obtained from both XII and DMX neurons. These results on XII and DMX neurons indicate that (1) the methods used to dissociate neurons provide a useful means to overcome voltage clamp technical difficulties; (2) ion channel characteristics such as density and biophysical properties of DMX neurons are very different from those of XII neurons; and (3) several newly discovered membrane ionic currents are present in these cells.     

Epidermal growth factor stimulates Ca2+ uptake of human erythrocytes

To examine the functional significance of epidermal growth factor (EGF) binding sites present on the human erythrocyte membrane [Engelmann et al. (1992) Am J Hematol 39:239–241], the effect of EGF on ⁴⁵Ca²⁺ uptake and on ²²Na⁺ efflux from these cells has been studied. In all cases media contained 1.25 mM Ca²⁺, whereas Na⁺ and K⁺ were varied. In 140 mM Na⁺/5 mM K⁺ medium EGF (250 ng/ml) stimulated ⁴⁵Ca²⁺ uptake by 50%–90% in quin-2-loaded cells, and by up to threefold in untreated cells. Increasing extracellular K⁺ up to 75 mM at the expense of extracellular Na²⁺ stimulated the EGF-induced ⁴⁵Ca²⁺ uptake by about twofold compared to 145 mM Na⁺ medium both in quin-2-loaded and in untreated cells. In 145 mM K⁺ medium, however, no EGF-induced ⁴⁵Ca²⁺ uptake was detectable in quin-2-loaded cells, while in untreated cells Ca²⁺ entry was stimulated twofold by EGF. After increasing intracellular Na⁺ from 6 mmol/l cells to 18 mmol/l cells in untreated cells suspended in 145 mM K⁺ medium, ⁴⁵Ca²⁺ uptake induced by EGF gradually increased. In contrast, in 140 mM Na⁺/5 mM K⁺ as well as in 70 mM Na⁺/75 mM K⁺ medium, ⁴⁵Ca²⁺ uptake accelerated by EGF was largely unaffected by a modified red cell Na⁺ content. When ²²Na-loaded untreated red cells were suspended in 145 mM K⁺ medium EGF stimulated red cell ²²Na⁺ efflux by more than threefold. In 140 mM Na⁺/5 mM K⁺ as well as in 70 mM Na⁺/75 mM K⁺ medium, no ²²Na⁺ efflux induced by the growth factor was evident. The results are consistent with the idea that EGF stimulates (at least) two components of ⁴⁵Ca²⁺ uptake in human erythrocytes. One of the two is unmasked in 145 mM K⁺ medium, inhibited by quin-2 loading, accelerated by intracellular Na⁺ and appears to involve reversed Na⁺/Ca²⁺ exchange.     

Effects induced by the antiepileptic drug valproic acid upon the ionic currents recorded in rat neocortical neurons in cell culture

Summary Rat neocortical neurons in culture were subjected to the whole cell mode of voltage clamping under experimental conditions designed to study Na⁺, Ca{2su+} and K⁺ currents in isolation. Following pharmacological blockade of most of the Ca²⁺ and K⁺ channels, depolarizing commands which brought the membrane potential from — 80 to +10 mV elicited an inward current. This current was sensitive to tetrodotoxin (TTX) and was therefore caused by the opening of voltage-dependent channels permeable to Na⁺. Extracellular application of the antiepileptic drug valproic acid (VPA, 0.2–2mM) reduced in a dose-related, reversible way this Na⁺ current. VPA also evoked an increase of the voltage-dependent inward current recorded in the presence of TTX and thus presumably carried by Ca²⁺; this effect was seen in the presence of doses of VPA larger than 0.5 mM and was not reversible. Two types of outward K⁺ currents evoked by depolarizing steps in the presence of Na⁺ and Ca²⁺ channels blockers were not affected by VPA (up to 5 mM). Our data indicate that doses of VPA that are within the range present when it is used as an anticonvulsant, can influence inward currents generated by rat neocortical cells in culture. The reduction of the Na⁺, inward current is in line with findings obtained in mouse neurons by using standard intracellular recording techniques. This effect might represent an important mechanism of action for VPA in neocortex.Supported by operating grants from the Consiglio Nazionale della Ricerca of Italy (CZ88.00691.04) and the Medical Research Council of Canada (MA-8109).     

The calcium channel antagonist diltiazem effectively blocks two types of potassium channels in the neuronal membranes

Effect of the Ca²⁺-channel antagonist diltiazem on potential-operated Ca²⁺ and K⁺ currents was studied on isolated edible snail neurons by a two-microelectrode patch-clamp technique. Diltiazem in a concentration of 0.1 mM inhibits Ca²⁺ current, high-threshold Ca²⁺-dependent K⁺ current, and Ca²⁺-independent K⁺ current and has no effect on low-threshold K⁺ current and leakage current. It is suggested that therapeutic effect of diltiazem is mediated through blockade of Ca²⁺ and K⁺ channels. Tranlated fromByulleten' Eksperimental'noi biologii i Meditsiny, Vol. 124, No. 9, pp. 271–274. September, 1997     

Multiple actions of zinc on transmitter release at mouse end-plates

In mouse diaphragm, the increase in frequency of mini end-plate potentials (f _mepp), by Ca²⁺ or Ba²⁺ in 20 mM K⁺, was reversibly inhibited by Zn²⁺ in a manner consistent with competition between Zn²⁺ and Ca²⁺ at a site which interacts with only one atom of Zn with an apparent dissociation constant (K_i′) of about 0.015 mM. Between 0.5 mM and 2 mM, Zn²⁺ caused a rapid and reversible dose-dependent increasef _mepp in 20 mM K⁺/0 Ca²⁺. Prolonged or repeated exposure to Zn²⁺ produced a slow increase inf _mepp followed by a decline, which once started, was not modified by of Zn²⁺. The time course was prolonged in raised Mg²⁺, bekanamycin, or in 5 mM K⁺ solution, and graded with Zn²⁺ concentration, but total numbers of MEPPs induced by 0.1 mM, 1 mM or 4 mM Zn²⁺ were not significantly different. Whenf _mepp fell it became insensitive to Ca²⁺, Ba²⁺, La³⁺ (in 20 mM K⁺), ethanol and raised osmotic pressure. Before complete block of responses to Ca²⁺, the Ca²⁺/f_mepp dose/response curve in 20 mM K⁺ was shifted to the right. These results indicate that Zn²⁺ enters the terminal via voltage-gated Ca²⁺ channels that interact in a complex way with these ions and then acts (a) as a partial agonist at sites where Ca²⁺ normally governs transmitter release, and (b) to produce irreversible changes in the nerve terminal, associated with a rise and subsequent fall off _mepp and loss of sensitivity of the release mechanism to Ca²⁺ and other agents.     

Single calcium-activated potassium channel in cultured mammary epithelial cells

The properties of Ca²⁺-activated K⁺ channels in mouse mammary epithelial cells in primary culture were studied by the patch-clamp technique. In cell-attached patches, spontaneous channel openings were sometimes observed; the slope conductance of the currents was about served; the slope conductance of the currents was about 12 pS at negative membrane potentials with a physiological solution (152 mM Na⁺, 5.4 mM K⁺) in the pipette. External application of A23187, a calcium ionophore, activated this channel. In excised inside-out patches, the channel was activated by increasing the internal Ca²⁺ concentration (10^–7 to 10^–6 M). No voltage dependence of the channel activity was observed. Internal Na⁺ blocked the outward K⁺ current in a voltage dependent manner and this block led to the non-linear I–V relationship at positive membrane potentials. The channel was blocked by internal Ba²⁺ (0.1 mM) and tetracthylammonium (TEA⁺, 20–50 mM). Ba²⁺ reduced the open probability but not the single channel conductance, whereas TEA⁺ reduced the single channel conductance. The single channel conductance of this channel, measured from the inward current with a high-K⁺ solution (150 mM K⁺) in the pipette, was large (about 40 pS), and showed inward rectification. These results suggest that this channel is different from the usual small conductance Ca²⁺-activated K⁺ channels observed in many other cells.     

Efflux of choline from neurons and glia in culture

The efflux of radioactive choline from exclusively neuronal or glial cell cultures was dependent upon the concentrations of choline present in the cells and in the incubation medium, suggesting the possible presence of a homoexchange phenomenon between influx and efflux. The ionic dependence of the outward movement of choline from these cells showed that it could be stimulated by high K⁺ concentrations and by the absence of Ca²⁺. In glial cells, however, the efflux of choline was increased with a much lower concentration of K⁺ compared to neurons. The result may suggest that during nerve stimulation the release of K⁺ from neurons could stimulate, from glia, the efflux of choline which would then be taken up in neurons.     

Sodium-sensitive, calcium-independent potassium conductance in the leech giant glial cell

 We have measured membrane current, membrane potential and intracellular Na⁺ and Ca²⁺ concentrations, [Na⁺]_i and [Ca²⁺]_i, of the giant glial cell in the nervous system of the leech Hirudo medicinalis using conventional microelectrodes and the fluorescent dyes sodium-binding benzofuran isophthalate (SBFI) and fura-2. When the Na⁺ was removed from the saline, the membrane conductance increased twofold from 1.29±0.1 μS to 2.57±0.18 μS (mean ± SEM; n=27). The rise in membrane conductance was accompanied by a current, which reversed around –74 mV, and the amplitude of K⁺-induced depolarizations or currents increased during Na⁺ removal, suggesting an increase in the K⁺ conductance of the glial membrane. We also monitored [Ca²⁺]_i when removing external Na⁺ in the presence and absence of external Ca²⁺, and during injection of the Ca²⁺-chelator BAPTA into the cells. Our results indicate that Na⁺ modulates a K⁺ conductance of these glial cells, independent of intra- and extracellular Ca²⁺. Received: 1 April 1998 / Received after revision and accepted: 22 May 1998     

Silencing by raised extracellular Ca of pre-Bötzinger complex neurons in newborn rat brainstem slices without change of membrane potential or input resistance

Breathing is controlled by inspiratory pre-Bötzinger complex (preBötC) networks that remain active in transversal brainstem slices from perinatal rodents. In 600 μm thick preBötC slices, inspiratory-related bursting in physiological (3 mM) [K⁺] is depressed by <1 mM elevation of superfusate [Ca²⁺]. Here, we studied underlying cellular mechanisms in whole-cell-recorded neurons of 400 μm thin newborn rat slices with the <200 μm thin preBötC in the middle (“m-preBötC[400]” slices). Extracellular activity in the ventrolateral slice area in 3 mM K⁺ and a most common physiological Ca²⁺ range (1–1.2 mM) stopped spontaneously within 2 h (“in vitro apnea”). Contrary, rhythm was stable for >3 h at 6–8 bursts/min in 7 mM K⁺ and 1.2 mM Ca²⁺ solution. In non-pacemaker preBötC inspiratory cells and neighboring inspiratory or tonically active neurons, block or frequency depression by >90% of rhythm in the latter solution by 2–3 mM Ca²⁺ changed neither resting potential nor input resistance. High Ca²⁺ silenced inspiratory neurons and depressed tonic discharge of non-respiratory neurons. However, in both cell types current injection evoked normal action potentials with unchanged threshold potential. The findings show that m-preBötC[400] slices represent a good compromise between long term viability of rhythmogenic preBötC neurons and minimal modulation of these cells by adjacent tissue, but need to be studied in elevated K⁺. The lack of postsynaptic K⁺ channel-mediated hyperpolarization suggests that saturation of surface charges, presynaptic block of transmission and/or inhibition of postsynaptic burst-promoting conductances such as Ca²⁺ activated non-selective cation channels are involved in inspiratory depression by high Ca²⁺.     

Depolarization of cultured astrocytes by glutamate and aspartate

The excitatory transmitter substances glutamate and aspartate are known to have a depolarizing action on cultured CNS neurones, the depolarization being associated with an increase in membrane conductance. When the effects of these amino acids (at a concentration of 10^?4 M) were studied on the membrane potential and resistance of cultured glial cells, they also caused a depolarization of many astrocytes but without producing significant changes in membrane resistance. The majority of glial cells depolarized by glutamate and aspartate were lying in the vicinity of neurones in the dense zone of the cultures, whereas isolated astrocytes in the outgrowth zone were usually not affected by the amino acids. 4-Aminopyridine (5 mM), a substance known to block K⁺-conductance in various excitable membranes, reversibly reduced or abolished the depolarization caused by glutamate and aspartate on glial cells, but had no or only a small effect on the depolarization of neurones caused by these amino acids.These results suggest that the depolarization of glial cells by glutamate and aspartate is caused by an increase in the concentration of extracellular K⁺ which is released from neighbouring neurones during their activation by the amino acids.     

Evaluation of glutamate as a neurotransmitter of cerebellar parallel fibers.

Glutamate release, in response to excess K⁺, from synaptosomal preparations from rat cerebellum was studied in relationship to the characteristics of stimulus-secretion coupling processes used by known transmitters and compared to those of γ-aminobutyrate (GABA), a known cerebellar neurotransmitter. The properties of glutamate release were indistinguishable from those of GABA: the release of both substances is dependent on Ca²⁺, antagonized by Mg²⁺ and stimulated by K⁺ depolarization and therefore mimics the essential characteristics for the release of a neurotransmitter.Evidence that parallel fiber boutons are a source of evoked glutamate release is supported by the fact that synaptosomal preparations from the cerebellar molecular layer released twice as much glutamate per tissue content as did similar preparations from whole cerebellum. Furthermore, neonatal X-irradiation of the cerebellum, which decreases the granule cell and the parallel fiber populations, results in a reduction of Ca²⁺-dependent glutamate release indicating that Ca²⁺-dependent release originates in part from parallel fiber boutons. Under these conditions neither glutamate levels nor high affinity glutamate uptake are significantly changed. These data indicate that glutamate is either a neurotransmitter of the parallel fibers or a molecule released by the stimulus-secretion coupling process along with the neurotransmitter.Depolarizing K⁺ concentrations in the absence of external Ca²⁺ also induced an increase in the efflux of glutamate and [³H]GABA. This effect was much more prominent for exogenously loaded amino acids than for endogenous glutamate. Accumulated glutamate appears to be stored in at least two separate compartments, which differed in their storage capacity and sensitivity to K⁺ and Ca²⁺. One pool, highly sensitive to K⁺ in the absence of Ca²⁺, showed a poor retention of glutamate. The second pool, sensitive to Ca²⁺ in the presence of a depolarizing agent, stored glutamate more stably. Since the K⁺-induced efflux of glutamate independent of external Ca²⁺ decreased after loss of the granule cells, it is likely that both glutamate compartments are present, at least in part, in neurons.     

Pacemaker currents in mouse locus coeruleus neurons

We have characterized the currents that flow during the interspike interval in mouse locus coeruleus (LC) neurons, by application of depolarizing ramps and pulses, and compared our results with information available for rats. A tetrodotoxin (TTX)-sensitive current was the only inward conductance active during the interspike interval; no TTX-insensitive Na⁺ or oscillatory currents were detected. Ca²⁺-free and Ba²⁺-containing solutions failed to demonstrate a Ca²⁺ current during the interspike interval, although a Ca²⁺ current was activated at membrane potentials positive to −40 mV. A high- tetraethylammonium chloride (TEA) (15 mM) sensitive current accounted for almost all the K⁺ conductance during the interspike interval. Ca²⁺-activated K⁺, inward rectifier and low-TEA (10 μM) sensitive currents were not detected within the interspike interval. Comparison of these findings to those reported for neonatal rat LC neurons indicates that the pacemaker currents are similar, but not identical, in the two species with mice lacking a persistent Ca²⁺ current during the interspike interval. The net pacemaking current determined by differentiating the interspike interval from averaged action potential recordings closely matched the net ramp-induced currents obtained either under voltage clamp or after reconstructing this current from pharmacologically isolated currents. In summary, our results suggest the interspike interval pacemaker mechanism in mouse LC neurons involves a combination of a TTX-sensitive Na⁺ current and a high TEA-sensitive K⁺ current. In contrast with rats, a persistent Ca²⁺ current is not involved.     

Patch-clamp study of rubidium and potassium conductances in single cation channels from mammalian exocrine acini

Single-channel current recordings were carried out on excised inside-out patches of baso-lateral plasma membrane from exocrine acinar cells. The mouse pancreas and submandibular gland as well as the pig pancreas were investigated.In the mouse pancreas the voltage-insensitive Ca²⁺-activated cation channel was studied. Single-channel current-voltage (i/v) relationships were studied in symmetrical Rb⁺-rich solutions and in asymmetrical Rb⁺/Na⁺ and Na⁺/Rb⁺ solutions. In all cases the i/v relations were linear and had the same slope representing a single-channel conductance of about 33 pS which is identical to that previously obtained with symmetrical Na⁺ solutions or asymmetrical Na⁺/K⁺ solutions.In the mouse submandibular gland and the pig pancreas the voltage and Ca²⁺-activated K⁺ channel was studied. The outward currents observed after depolarization in the presence of quasi-physiological Na⁺/K⁺ gradients were immediately abolished when all the K⁺ in the bath fluid was replaced by Rb⁺ (bath fluid in contact with inside of plasma membrane). This effect was immediately and fully reversible upon return to the high K⁺ solution.The voltage and Ca²⁺-activated K⁺ channel was also studied in asymmetrical K⁺/Rb⁺ and Rb⁺/K⁺ solutions. In the first case inward (K⁺) currents could be observed but not outward (Rb⁺) currents, while in the other case inward (Rb⁺) currents could not be seen whereas outward (K⁺) currents were measured. The current-voltage relationships were approximately linear and the null potential was close to 0 mV in both situations. In contrast the null potential for current through the K⁺ channel in the presence of asymmetrical Na⁺/K⁺ or Li⁺/K⁺ solutions was about –70 mV and with reversed gradients about +60 mV.Outward K⁺ currents of reduced size (through the voltage and Ca²⁺-activated K⁺ channel) could be observed when the bath fluid contained 75 mM K⁺ and 75 mM Rb⁺, but not (in the same membrane patches) when 150 mM Rb⁺ and no K⁺ was present.It is concluded that the large voltage- and Ca²⁺-activated K⁺ channel has an extremely low Rb⁺ conductance. It is possible, however, that the permeability for Rb⁺ may be about the same as for K⁺. The voltage-insensitive Ca²⁺-activated cation channel does not discriminate between K⁺ and Rb⁺.     

Na+, K+-ATPase activity in neurons and glial cells of the olfactory cortex of the rat brain during the development of long-term potentiation

Na⁺, K⁺-ATPase and Mg²⁺-ATPase activities were studied in neurons and glial cells of the olfactory cortex of the rat by quantitative cytophotometry in conditions of long-term potentiation (LTP), and significant changes in direction and extent were found. Na⁺, K⁺-ATPase activity decreased in neurons in the first 15 min after LTP, with subsequent elevation by 30 min. Mg²⁺-ATPase activity remained unchanged in these conditions. Glial cells showed significant increases in Na⁺, K⁺-ATPase activity in the initial period after LTP, with return to control by 30 min. Again, there were no significant changes in Mg²⁺-ATPase activity. The formation and persistence of LTP in neurons and glial cells was accompanied by significant changes in Na⁺, K⁺-ATPase activity, which were reciprocal in nature. Functional Neurochemistry Laboratory (Director N. A. Emel'yanov), I. P. Pavlov Institute of Physiology, Russian Academy of Sciences, St. Petersburg. Translated from Fiziologicheskii Zhurnal im. I. M. Sechenova, Vol. 81, No. 3, pp. 16–20, March, 1995.     

Ischemic injury decreases parvalbumin expression in a middle cerebral artery occlusion animal model and glutamate-exposed HT22 cells

Measles virus (MV) infection may lead to severe chronic CNS disease processes, including MV-induced encephalitis. Because the intracellular Ca²⁺ concentration ([Ca²⁺]_i) is a major determinant of the (patho-)physiological state in all cells we asked whether important Ca²⁺ conducting pathways are affected by MV infection in cultured cortical rat neurons. Patch-clamp measurements revealed a decrease in voltage-gated Ca²⁺ currents during MV-infection, while voltage-gated K⁺ currents and NMDA-evoked currents were unaffected. Calcium-imaging experiments using 50 mM extracellular KCl showed reduced [Ca²⁺]_i increases in MV-infected neurons, confirming a decreased activity of voltage-gated Ca²⁺ channels. In contrast, the group-I metabotropic glutamate receptor (mGluR) agonist DHPG evoked changes in [Ca²⁺]_i that were increased in MV-infected cells. Our results show that MV infection conversely regulates Ca²⁺ signals induced by group-I mGluRs and by voltage-gated Ca²⁺ channels, suggesting that these physiological impairments may contribute to an altered function of cortical neurons during MV-induced encephalitis.     

Efflux of potassium from neurones excited by glutamate and aspartate causes a depolarization of cultured glial cells

The time course of the depolarization of cultured astrocytes and neurones by glutamate and aspartate corresponds well with the increase of the extracellular K⁺ concentration ([K⁺]₀) measured with an ion-sensitive microelectrode placed in the close vicinity of the cells tested. It is concluded that the depolarization of glial cells is caused by an efflux of K⁺ from neighbouring neurones during their excitation by the amino acids.     

Stimulation-induced calcium signalling and ion transport in rat pancreatic acini

In the present study we have characterized receptor-mediated Ca²⁺ signalling patterns as well as Ca²⁺-mediated ion transport mechanisms in collagenase isolated rat pancreatic acini. Measurements of the initial Ca²⁺ response to maximal carbachol stimulation revealed a rapid increase in [Ca²⁺]_i, which, in general, occurred synchronously throughout the cells. Less frequently, not all cells in the acinus responded to carbachol, but did respond to subsequent stimulation with bombesin, indicating that not all cells possess receptors for all the applied agonists. In view of the heterogeneity in the agonist-evoked Ca²⁺ responses, ionomycin was used to assess the role of Ca²⁺ in activating K⁺, Na⁺ and Cl^- transport mechanisms. Ionomycin induced a rise in [Ca²⁺]_i, thereby increasing Cl^- permeability as well as stimulating K⁺ efflux, probably through non-specific cation channels. However, the resting K⁺ efflux was insensitive to blockers of non-specific cation channels, indicating the existence of a selective resting K⁺ conductance. Ionomycin also stimulated influx of Na⁺, which in part was mediated by non-specific cation channels. The changes in ion fluxes measured in the present study revealed that when [Ca²⁺]_i is raised in rat pancreatic acini, they gain Na⁺ and Cl^- and lose K⁺, with non-specific cation channels being essential for this process.