Purinergic modulation of synaptic input to Purkinje neurons in rat cerebellar brain slices

Adenosine triphosphate (ATP) is a cotransmitter and an extracellular neuromodulator in nervous systems, and it influences neural circuits and synaptic strength. We have studied a stimulating effect of ATP (100 micro m) on the synaptic input of Purkinje neurons in acute cerebellar brain slices of juvenile rats (p14-19). Bath application of ATP increased the frequency of spontaneous postsynaptic currents (sPSCs) almost twofold, and increased their amplitude. These effects were fully suppressed by the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2'4'-disulphonic acid (PPADS; 10 microm), or after blocking action potentials with tetrodotoxin (TTX; 0.5 microm), but were not impaired by inhibiting ionotropic, non-NMDA glutamate receptors with 2,3-dioxo-6-nitro-1,2,3,4,-tetrahydrobenzo[f]quinoxaline-7-sulphonamide (NBQX; 5 microm). The frequency of sPSCs was reduced by 35% by PPADS and increased by 50% after inhibiting ectonucleotidase with ARL67156 (50 microm), suggesting intrinsic, 'tonic', stimulation of synaptic activity via P2 receptors. The pharmacological profile indicated that the ATP effect was mediated by both P2X and P2Y receptors, most probably of the P2X5- and P2Y(2,4)-like subtypes. The action potential frequency in the inhibitory basket cells was increased by 65%, and decreased in Purkinje neurons by 25%, in the presence of ATP. Our results suggest that ATP continuously modulates the cerebellar circuit by increasing the activity of inhibitory input to Purkinje neurons, and thus decreasing the main cerebellar output activity, which contributes to locomotor coordination.

Astroglial glutamine transport by system N is upregulated by glutamate

Release of glutamine from astrocytes is an essential step of the glutamate-glutamine cycle, and hence for the maintenance of neuronal glutamate and gamma-aminobutyric acid (GABA) pools. The glutamine transporter SNAT3 (SN1) has recently been identified as one of the major mediators of glutamine efflux from astrocytes. We investigated the regulation of SNAT3 mediated glutamine transport in cultured astrocytes. Incubation of primary astrocyte cultures with physiological concentrations of glutamate resulted in a rapid, about twofold, upregulation of SNAT3-mediated transport activity. The effect was not mediated by glutamate receptors but required uptake of glutamate into astrocytes. Both net uptake and net efflux increased after treatment of cells with glutamate, excluding an acceleration of the transport by way of an exchange mechanism. Elevated intracellular glutamate most likely reduces the K(m) of SNAT3 for its substrate glutamine. The results suggest that astrocytes respond actively to the release of glutamate by increasing glutamine release and thereby may modulate glutamatergic neurotransmission.

Second messenger cascade of glial responses evoked by interneuron activity and by a myomodulin peptide in the leech central nervous system

The giant glial cell in the neuropil of segmental ganglia of the leech Hirudo medicinalis responds to the activity of the Leydig interneuron and to a peptide of the myomodulin family, the presumed transmitter mediating the Leydig neuron-to-giant glial cell transmission, with a membrane hyperpolarization due to an increased membrane K+ conductance [Britz et al. (2002) Glia, 38, 215-227]. We have now studied the second messenger cascade initiated by Leydig neuron stimulation and by the endogenous myomodulin (MMHir) in the voltage-clamped giant glial cell. Glial responses to both stimuli are mediated by a G-protein-coupled receptor linked to adenylyl cyclase by the following criteria: (i) injection of GDP-beta-S, but not GDP, resulted in an irreversible decrease of the glial responses to both stimuli; (ii) the responses to both stimuli were reversibly inhibited by the adenylyl cyclase inhibitor SQ22,536; and (3) bath-applied di-butyryl-cyclic AMP, but not di-butyryl-cyclic GMP, elicited an outward current, which reduced the responses elicited by neuronal stimulation or myomodulin. A cocktail of protein kinase (PK) inhibitors (H-8, KT5720), the PKA antagonist Rp-cAMPS, or presumed inhibitors of cyclic nucleotide channels, LY83583 and l-cis-diltiazem, had no effect on the glial responses. Our results suggest that Leydig neuron stimulation and MMHir activate a cAMP-mediated K+ conductance in the glial cell, which appeared neither to be due to the activation of PKA nor of known cyclic nucleotide-gated channels directly.

Calcium release from intracellular stores in rodent astrocytes and neurons in situ

Endoplasmic reticular Ca(2+) stores, instrumental for intra- and intercellular calcium signalling, can be depleted by different receptor agonists. In the present study, the functional status of ER Ca(2+) stores was probed by cyclopiazonic acid (CPA, 10-30 microM, inhibitor of SERCA-dependent ER Ca(2+) uptake) and/or caffeine (20 mM, ryanodine receptor activator) in astrocytes and neurons of rat and mouse acute hippocampal brain slices (Stratum radiatum, Stratum moleculare), and in cultured astrocytes, using confocal microscopy and conventional Ca(2+) imaging. Astrocytes and neurons in situ, identified by their Ca(2+) response in K(+)-free saline (Dallwig and Deitmer [J. Neurosci. Methods 116 (2002) 77]), had a resting cytosolic Ca(2+) level of 105 and 157 nM, respectively (P<0.05). CPA evoked a Ca(2+) transient, which was faster and larger in neurons than in astrocytes, indicating larger Ca(2+) leak of neuronal Ca(2+) stores. Caffeine evoked a Ca(2+) rise in most neurons (>80%), but only in less than 40% of astrocytes. The glial Ca(2+) transients in the presence of caffeine had a large and variable delay (>50 s), as compared to those in neurons (< or =10 s), and appeared to be spontaneous and/or secondary to the neuronal Ca(2+) response, leading to release of neuronal transmitters. Astrocytes in culture responded to CPA, but never to caffeine with a Ca(2+) rise. Our results indicate that astrocytes, in contrast to neurons, lack caffeine-sensitive Ca(2+) stores, and have a relatively smaller leak from CPA-sensitive Ca(2+) stores than neurons.

The loop between helix 4 and helix 5 in the monocarboxylate transporter MCT1 is important for substrate selection and protein stability

Transport of lactate, pyruvate and the ketone bodies acetoacetate and beta-hydroxybutyrate, is mediated in most mammalian cells by members of the monocarboxylate transporter family (SLC16). A conserved signature sequence has been identified in this family, which is located in the loop between helix 4 and helix 5 and extends into helix 5. We have mutated residues in this signature sequence in the rat monocarboxylate transporter (MCT1) to elucidate the significance of this region for monocarboxylate transport. Mutation of R143 and G153 resulted in complete inactivation of the transporter. For the MCT1(G153V) mutant this was explained by a failure to reach the plasma membrane. The lack of transport activity of MCT1(R143Q) could be partially rescued by the conservative exchange R143H. The resulting mutant transporter displayed reduced stability, a decreased V (max) of lactate transport but not of acetate transport, and an increased stereoselectivity. Mutation of K137, K141 and K142 indicated that only K142 played a significant role in the transport mechanism. Mutation of K142 to glutamine resulted in an increase of the K (m) for lactate from 5 mM to 12 mM. In contrast with MCT1(R143H), MCT1(K142Q) was less stereoselective than the wild-type. A mechanism is proposed that includes all critical residues.

Glutamine efflux from astrocytes is mediated by multiple pathways

The neurotransmitter glutamate, once released into the synaptic cleft, is largely recycled by the glutamate-glutamine cycle, which involves uptake into astrocytes, conversion into glutamine and subsequent release of glutamine from astrocytes as a precursor for neuroneal glutamate synthesis. We analysed glutamine efflux from cultured astrocytes by pre-loading cells with labelled glutamine for 30 min and subsequently measured glutamine efflux for 30 min. Efflux of pre-loaded glutamine was rapid and almost complete after 30 min with a first order rate of 0.11 +/- 0.01/min. Efflux was 50% reduced when cells were depleted of intracellular Na+. Increasing intracellular Na+ concentration had a small stimulatory effect on glutamine efflux, indicating the participation of a Na+-dependent transport mechanism. About 50% of the basal efflux could not be inhibited by depletion of the intracellular Na+, suggesting the presence of an additional Na+-independent transport mechanism. Glutamine efflux was stimulated two- to threefold by addition of extracellular neutral amino acids, such as alanine or leucine. The stimulatory effects of alanine and leucine had a Na+-dependent and a Na+-independent component, suggesting the presence of two antiport mechanisms one involving Na+. When compared to the expression of glutamine transporter mRNAs in cultured astrocytes it appeared likely that glutamine efflux was mediated by SN1, LAT2, ASCT2 and an additional, yet unidentified, transporter that mediates about 40% of the basal efflux.

Membrane responses of the leech giant glial cell to the peptide transmitter myomodulin

A myomodulin peptide has been suggested to mediate the response of the giant glial cells to stimulation of the Leydig interneuron in the central nervous system of the leech Hirudo medicinalis [Eur. J. Neurosci. 11 (1999) 3125]. We have now studied the glial response to the endogenous leech MM peptide (GMGALRL-NH(2), MMHir). The peptide evokes a membrane outward current (EC(50) approximately 2 microM), which neither desensitizes nor shows any sign of run-down, and elicits a K(+) conductance increase of the glial cell membrane. The peptidase inhibitor phenylmethylsulfonyl fluoride (PMSF) enhances the glial current response, suggesting the presence of endogenous extracellular peptidases.

Long-lasting modulation of synaptic input to Purkinje neurons by Bergmann glia stimulation in rat brain slices

Information processing in the nervous system is achieved primarily at chemical synapses between neurons. Recent evidence suggests that glia-neuron interactions contribute in multiple ways to the synaptic process. In the present study we used the frequency of spontaneous postsynaptic currents (sPSC) in Purkinje neurons in acute cerebellar brain slices from juvenile rats (13-19 days old) as a measure of synaptic activity. Following 50 depolarizing pulses to an adjacent Bergmann glial cell (50 mV; duration 0.5 s; 1 Hz) the sPSC frequency of the Purkinje neuron was reduced to 65 +/- 7 % of control values within 10 min after glial stimulation and remained depressed for at least 40 min. Depolarizing pulses to 0 mV had a comparable effect (70 +/- 5 % of control). The frequency of miniature PSCs, as recorded in 300 nM TTX, was not modulated after glial stimulation. Blockade of ionotropic glutamate receptors (iGluRs) with kynurenic acid (1 mM) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5 microM) suppressed the reduction of neuronal activity induced by glial depolarization, whereas the glial modulation of synaptic activity was not inhibited by a block of N-methyl-D-aspartate iGluRs, metabotropic glutamate receptors, cannabinoid receptors or GABA(B) receptors. Fluorometric measurements of the intraglial Ca(2+) concentration revealed no glial Ca(2+) transients during the depolarization series, and glial cell stimulation reduced the neuronal sPSC frequency even after loading the glial cell with 20 mM of the Ca(2+) chelator BAPTA. Our results indicate a glia-induced long-lasting depression of neuronal communication mediated by iGluRs.

Characterization of a synaptiform transmission between a neuron and a glial cell in the leech central nervous system

The cross-talk between neurons and glial cells is receiving increased attention because of its potential role in information processing in nervous systems. Stimulation of a single identifiable neuron, the neurosecretory Leydig interneuron in segmental ganglia of the leech Hirudo medicinalis, which modulates specific behaviors in the leech, evokes membrane hyperpolarization directly in the giant glial cell (Schmidt and Deitmer. Eur J Neurosci 11:3125-3133, 1999). We have studied the neuron-to-glia signal transmission in the voltage-clamped giant glial cell to determine whether this interaction exhibits properties of a chemical synapse. The glial response had a mean latency of 4.9 s and was dependent on the action potential frequency; the glial cell responded to as few as five Leydig neuron action potentials in 50% of the trials. The glial current was sustained for minutes during repetitive Leydig neuron activity without any sign of desensitization. The current was sensitive to tetraethylammonium, and its reversal potential of -78 mV shifted with the external K+ concentration. The glial response increased with the duration of the neuronal action potentials and was sensitive to the external Ca2+/Mg2+ concentration ratio. The results suggest that Leydig neuron activity leads to a Ca2+-dependent release of transmitter from the neuronal dendrites, evoking an K+ outward current in the giant glial cell, implying a synapse-like transmission between a neuron and a glial cell.

Cell-type specific calcium responses in acute rat hippocampal slices

The identification of glial cells and neurons in brain slices is often difficult or uncertain. We have previously found that cultured rat cerebellar astrocytes and presumed astrocytes in acute brain slices, but not neurons, respond with cytosolic Ca(2+) transients following Ca(2+) influx in low external K+ concentrations (<1 mM; Cell Calcium 28 (2000) 247). We have now studied the possibility whether this Ca(2+) response can be employed to identify astrocytes during calcium imaging experiments. The Ca(2+) responses to low and high (50 mM) K+ were investigated in cells in culture and in hippocampal slices. In the stratum radiatum of hippocampal slices, S-100B-positive cells, presumed to be astrocytes, preferentially accumulated Fluo-4, while pyramidal neurons, identified by neuron-specific enolase, showed much lower Fluo-4 fluorescence, fixed with ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDAC). 81% of the cells with prominent Fluo-4 fluorescence showed responses to low K+, and 86% of these cells were S-100B-positive. Our results suggest that the responsiveness to low K+ can help to identify astrocytes in acute brain slices.

A role for CO(2) and bicarbonate transporters in metabolic exchanges in the brain

Acid-base transporters are linked to the energy metabolism via the end product of oxidative metabolism, carbon dioxide, together with carbonic anhydrase activity. In a tissue such as the brain, where some cells (neurones) are high-energy consumers when active, and other cells (astroglial cells) are destined for homeostatic and trophic tasks, transport systems may complement each other and cooperate in order to maintain physiological functions. Here, some aspects of the coupling of metabolite shuttling and acid/base-dependent transport in neurones and glial cells are discussed.

Calcium influx into dendrites of the leech Retzius neuron evoked by 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) is a ubiquitous neurotransmitter and neuromodulator that affects neural circuits and behaviours in vertebrates and invertebrates. In the present study, we have investigated 5-HT-induced Ca(2+) transients in subcellular compartments of Retzius neurons in the leech central nervous system using confocal laser scanning microscopy, and studied the effect of 5-HT on the electrical coupling between the Retzius neurons. Bath application of 5-HT (50mM) induced a Ca(2+) transient in axon, dendrites and cell body of the Retzius neuron. This Ca(2+) transient was significantly faster and larger in dendrites than in axon and cell body, and was half-maximal at a 5-HT concentration of 5-12mM. The Ca(2+) transient was suppressed in the absence of extracellular Ca(2+) and by methysergide (100mM), a non-specific antagonist of metabotropic 5-HT receptors, and was strongly reduced by bath application of the Ca(2+) channel blocker Co(2+) (2mM). Injection of the non-hydrolysable GTP analogue GTPgammaS increased and prolonged the dendritic 5-HT-induced Ca(2+) transient. The non-selective protein kinase inhibitor H7 (100mM) and the adenylate cyclase inhibitor SQ22536 (500 mM) did not affect the Ca(2+) transient, and the membrane-permeable cAMP analogue dibutyryl-cAMP (500 mM) did not mimic the effect of 5-HT application. 5-HT reduced the apparent electrical coupling between the two Retzius neurons, whereas suppression of the Ca(2+) influx by removal of external Ca(2+) improved the transmission of action potentials at the electrical synapses which are located between the dendrites of the adjacent Retzius neurons. The results indicate that 5-HT induces a Ca(2+) influx through calcium channels located primarily in the dendrites, and presumably activated by a G protein-coupled 5-HT receptor. The dendritic Ca(2+) increase appears to modulate the excitability of, and the synchronization between, the two Retzius neurons.

Activity-dependent accumulation of Ca2+ in axon and dendrites of the leech Leydig neuron

We have investigated Ca2+ changes evoked by single action potentials (APs) in axon and dendrites of leech Leydig neurons. Dendritic Ca2+ transients induced by an AP were twice as large as in the axon, and Ca2+ recovery was significantly faster in the dendrites as compared to the axon. The AP-induced Ca2+ transients were blocked by Co2+ and suppressed in Ca2+-free saline, indicating Ca2+ influx through voltage-activated channels. During a train of APs, Ca2+ accumulated significantly more in the axon than in the dendrites. Suppression of the Ca2+ influx changed the shape of the action potential and increased the firing frequency. The results suggest a functional role of Ca2+ influx and Ca2+ accumulation during electrical activity in different neuronal subcompartments.

Strategies for metabolic exchange between glial cells and neurons

The brain is a major energy consumer and dependent on carbohydrate and oxygen supply. Electrical and synaptic activity of neurons can only be sustained given sufficient availability of ATP. Glial cells, which have long been assigned trophic functions, seem to play a pivotal role in meeting the energy requirements of active neurons. Under conditions of high neuronal activity, a number of glial functions, such as the maintenance of ion homeostasis, neurotransmitter clearance from synaptic domains, the supply of energetic compounds and calcium signalling, are challenged. In the vertebrate brain, astrocytes may increase glucose utilization and release lactate, which is taken up and consumed by neurons to generate ATP by oxidative metabolism. The CO(2) produced is processed primarily in astrocytes, which display the major activity of carboanhydrase in the brain. Protons and bicarbonate in turn may contribute to drive acid/base-coupled transporters. In the present article a scenario is discussed which couples the transfer of energy and the conversion of CO(2) with the high-affinity glutamate uptake and other transport processes at glial and neuronal cell membranes. The transporters can be linked to glial signalling and may cooperate with each other at the cellular level. This could save energy, and would render energy exchange processes between glial cells and neurons more effective. Functions implications and physiological responses, in particular in chemosensitive brain areas, are discussed.

Calcium transients in subcompartments of the leech Retzius neuron as induced by single action potentials

Regional Ca(2+) influx into neurons plays an essential role for fast signal processing, yet it is little understood. We have investigated intracellular Ca(2+) transients induced by a single action potential (AP) in Retzius neurons in situ of isolated ganglia of the leech Hirudo medicinalis using confocal laser scanning microscopy in the cell body, in different axonal branches, and in dendrites. In the cell body, a single AP induced a Ca(2+) transient in submembrane regions, while in central regions no fluorescence change was detected. Burst activity evoked a much larger Ca(2+) influx, which elicited Ca(2+) signals in central somatic regions, including the cell nucleus. A single AP induced a Ca(2+) transient in distal branches of the axon and in dendrites that was significantly larger than in the proximal axon and in the cell body (p <.05), and the recovery of the Ca(2+) transient was significantly faster in axonal branches than in dendrites (p <.01). The AP-induced Ca(2+) transient was inhibited by Co(2+) (2 mM). The P/Q-type Ca(2+) channel blocker omega-agatoxin TK (500 nM) and the L-type Ca(2+) channel blocker nifedipine (20 microM) had no effect on the Ca(2+) transient, whereas the L-type Ca(2+) channel blocker methoxyverapamil (D600, 0.5-1 mM) irreversibly reduced the Ca(2+) transient by 37% in axons and by 42% in dendrites. Depletion of intracellular Ca(2+) stores following inhibition of endoplasmic Ca(2+)-ATPases by cyclopiazonic acid (10 microM) decreased the AP-induced Ca(2+) transient in the dendrites by 21% (p <.01), but not in axons, and increased the Ca(2+) recovery time constant (tau) in the axonal branches by 129% (p <.01), but not in dendrites. The results indicate that an AP evokes a voltage-gated Ca(2+) influx into all subcompartments of the Retzius neuron, where it produces a Ca(2+) signal of different size and/or kinetics. This may contribute to the modulation of electrical excitation and propagation of APs, and to different modes of synaptic and nonsynaptic processes.

Bursting activity in leech Retzius neurons induced by low external chloride

We have investigated the bursting activity of Retzius neurons in the central nervous system of the leech Hirudo medicinalis as induced in Cl(-)-free saline by measuring membrane potential, membrane current and the intracellular calcium concentration ([Ca2+]i), using fura-2 or Oregon-Green488-Bapta-1. The Retzius neurons changed their low tonic firing to rhythmical bursting activity when the extracellular Cl- concentration ([Cl-]o) was lowered to 1 mM or less. In Cl(-)-free saline (Cl- exchanged by gluconate), bursting was accompanied by a rise in intracellular Ca2+ in both cell body and axon, which oscillated in synchrony with the bursts. The Ca2+ transients depended on the amplitude and duration of the depolarization underlying the burst, and were presumably due to Ca2+ influx through voltage-dependent Ca2+ channels. In Ca(2+)-free, EGTA-buffered saline or in the presence of Ca2+ channel blockers verapamil (1 mM) or diltiazem (500 microM) the depolarizations underlying the bursts in Cl(-)-free saline were enhanced in amplitude and duration. Bursting was not affected by depleting the intracellular Ca2+ stores with cyclopiazonic acid. The depolarization in Cl(-)- and Ca(2+)-free saline did not evoke intracellular Ca2+ changes. The burst-underlying membrane depolarization induced by Cl- removal was found to be due to a Na(+)-dependent persistent inward current and could be inhibited by saxitoxin (25-50 microM). The results suggest that a persistent Na+ current is generated in Cl(-)-free saline and induces the depolarization underlying rhythmic activity, and that presumably Ca(2+)-induced K+ currents modulate the bursting behaviour.

Developmental downregulation of ATP-sensitive potassium conductance in astrocytes in situ

In many neural and non-neural cells, ATP-sensitive potassium (K(ATP)) channels couple the membrane potential to energy metabolism. We investigated the activation of K(ATP) currents in astrocytes of different brain regions (hippocampus, cerebellum, dorsal vagal nucleus) by recording whole-cell currents with the patch-clamp technique in acute rat brain slices. Pharmacological tools, hypoglycemia and specific compounds in the pipette solution (cAMP, UDP), were used to modulate putative K(ATP) currents. The highest rate of K(ATP) specific currents was observed with a pipette solution containing cAMP and external stimulation with diazoxide (0.3 mM). The diazoxide-activated current had a reversal potential negative to -80 mV and was inhibited by tolbutamide (0.2 mM). We found that not all cells activated a K(ATP) current, and that the portion of cells with functional K(ATP) channel expression was developmentally downregulated. Whereas diazoxide activated K(ATP) currents in 57% of the astrocytes in rats aged 8-11 days (n = 21), the rate decreased to 38% at 12-15 days (n = 29) and to 8% at 16-19 days (n = 12). No significant difference was observed for the three brain regions. In recordings without cAMP in the internal solution, only 21% (12-15 days; n = 19) or none (16-19 days; n = 7), respectively, showed a potassium current upon diazoxide application. This metabolically regulated potassium conductance may be of importance, particularly in immature astrocytes with a complex current pattern, which have a relatively high input resistance: K(ATP) currents activated by energy depletion may hyperpolarize the cells, or stabilize a negative resting potential during depolarizing stimuli mediated, e.g., by glutamate receptors and/or uptake carriers.

A novel barium-sensitive calcium influx into rat astrocytes at low external potassium

Cultured rat cerebellar astrocytes, loaded with the Ca2+-sensitive fluorescent dyes Fura-2 or Fluo-3, responded with cytoplasmic Ca2+ transients, when the external K+ concentration was reduced from 5 mM to below 1 mM. Ca2+ transients were generated after changing to a saline containing 0.2 mM K+ in 82% of the cells (n =303) with a delay of up to 4 min. Cultured rat cortical neurones, which responded in high-K+ saline (50 mM) with Ca2+ transients, showed no Ca2+ responses in low K+ (n =22). In acute rat hippocampal brain slices, presumed glial cells responded with Ca2+ transients in low K+ similar to astrocytes in culture (88%, n =17). The Ca2+ transients were observed both in somatic and dendritic regions of cultured astrocytes, as examined with confocal laser scanning microscopy. Patch-clamped astrocytes hyperpolarized in 0.2 mM K+ from an average resting potential of -65 +/- 4 mV to -98 +/- 20 mV (n =15). The Ca2+ transients in low K+ were suppressed in Ca2+-free saline, buffered with 0.5 mM EGTA, but not after depletion of intracellular Ca2+ stores by thapsigargin, cyclopiazonic acid or by Ruthenium Red, indicating that they were due to Ca2+ influx into the cells, and not caused by intracellular Ca2+ release. The addition of different divalent cations revealed that Ba2+, but not Ni2+, Cd2+, Sr2+ or Mg2+, reversibly blocked the Ca2+ transients in low K+. There was a significant reduction of the Ca2+ responses at micromolar Ba2+ concentrations (Ki = 3.8 microM). The application of different K+ channel blockers, tetraethylammonium, dequalinium, tolbutamide, clotrimazole, or quinidine had no effect on the Ca2+ responses. Removal of external Na+, or intracellular acidification by the addition of 40 mM propionate to the saline, had also no influence on the generation of the Ca2+ transients. The results suggest that reducing the external K+ concentration elicits a Ca2+ influx into rat astrocytes which is highly sensitive to Ba2+. It is discussed that this Ca2+ influx might occur through K+ inward rectifier channels, which become Ca2+-permeable when the extracellular K+ concentration decreases to 1 mM or below.

Histamine-induced calcium entry in rat cerebellar astrocytes: evidence for capacitative and non-capacitative mechanisms

We have investigated the effects of histamine on the intracellular calcium concentration ([Ca2+]i) of cultured rat cerebellar astrocytes using fura-2-based Ca2+ imaging microscopy. Most of the cells responded to the application of histamine with an increase in [Ca2+]i which was antagonized by the H1 receptor blocker mepyramine. When histamine was applied for several minutes, the majority of the cells displayed a biphasic Ca2+ response consisting of an initial transient peak and a sustained component. In contrast to the initial transient [Ca2+]i response, the sustained, receptor-activated increase in [Ca2+]i was rapidly abolished by chelation of extracellular Ca2+ or addition of Ni2+, Mn2+, Co2+ and Zn2+, but was unaffected by nifedipine, an antagonist of L-type voltage-activated Ca2+ channels. These data indicate that the sustained increase in [Ca2+]i was dependent on Ca2+ influx. When intracellular Ca2+ stores were emptied by prolonged application of histamine in Ca2+-free conditions, Ca2+ re-addition after removal of the agonist did not lead to an 'overshoot' of [Ca2+]i indicative of store-operated Ca2+ influx. However, Ca2+ stores were refilled despite the absence of any substantial change in the fura-2 signal. Depletion of intracellular Ca2+ stores using cyclopiazonic acid in Ca2+-free saline and subsequent re-addition of Ca2+ to the saline resulted in an increase in [Ca2+]i that was significantly enhanced in the presence of histamine. The results suggest that besides capacitative mechanisms, a non-capacitative, voltage-independent pathway is involved in histamine-induced Ca2+ entry into cultured rat cerebellar astrocytes.

The low-affinity monocarboxylate transporter MCT4 is adapted to the export of lactate in highly glycolytic cells

Transport of lactate and other monocarboxylates in mammalian cells is mediated by a family of transporters, designated monocarboxylate transporters (MCTs). The MCT4 member of this family has recently been identified as the major isoform of white muscle cells, mediating lactate efflux out of glycolytically active myocytes [Wilson, Jackson, Heddle, Price, Pilegaard, Juel, Bonen, Montgomery, Hutter and Halestrap (1998) J. Biol. Chem. 273, 15920-15926]. To analyse the functional properties of this transporter, rat MCT4 was expressed in Xenopus laevis oocytes and transport activity was monitored by flux measurements with radioactive tracers and by changes of the cytosolic pH using pH-sensitive microelectrodes. Similar to other members of this family, monocarboxylate transport via MCT4 is accompanied by the transport of H(+) across the plasma membrane. Uptake of lactate strongly increased with decreasing extracellular pH, which resulted from a concomitant drop in the K(m) value. MCT4 could be distinguished from the other isoforms mainly in two respects. First, MCT4 is a low-affinity MCT: for L-lactate K(m) values of 17+/-3 mM (pH-electrode) and 34+/-5 mM (flux measurements with L-[U-(14)C]lactate) were determined. Secondly, lactate is the preferred substrate of MCT4. K(m) values of other monocarboxylates were either similar to the K(m) value for lactate (pyruvate, 2-oxoisohexanoate, 2-oxoisopentanoate, acetoacetate) or displayed much lower affinity for the transporter (beta-hydroxybutyrate and short-chain fatty acids). Under physiological conditions, rat MCT will therefore preferentially transport lactate. Monocarboxylate transport via MCT4 could be competitively inhibited by alpha-cyano-4-hydroxycinnamate, phloretin and partly by 4, 4'-di-isothiocyanostilbene-2,2'-disulphonic acid. Similar to MCT1, monocarboxylate transport via MCT4 was sensitive to inhibition by the thiol reagent p-chloromercuribenzoesulphonic acid.

Glial strategy for metabolic shuttling and neuronal function

Glial cells serve a variety of functions in nervous systems, some of which are activated by neurotransmitters released from neurons. Glial cells respond to these neurotransmitters via receptors, but also take up some of the transmitters to help terminate the synaptic process. Among these, glutamate uptake by glial cells is pivotal to avoid transmitter-mediated excitotoxicity. Here, a new model is proposed in which glutamate uptake via the excitatory amino acid transporter (EAAT) is functionally coupled to other glial transporters, in particular the sodium-bicarbonate cotransporter (NBC) and the monocarboxylate transporter (MCT), as well as other glial functions, such as calcium signalling, a high potassium conductance and CO(2) consumption. The central issue of this hypothesis is that the shuttling of sodium ions and acid/base equivalents, which drive the metabolite transport across the glial membrane, co-operate with each other, and hence save energy. As a result, glutamate removal from synaptic domains and lactate secretion serving the energy supply to neurons would be facilitated and could operate with greater capacity.

Enhancement of glutamate uptake transport by CO(2)/bicarbonate in the leech giant glial cell

Glutamate uptake into glial cells via the excitatory amino acid transporter (EAAT) is accompanied by an influx of sodium and acid equivalents into the cells. The sodium-bicarbonate cotransport (NBC) in glial cells moves sodium and base equivalents across the glial membrane in both directions. We have studied possible interactions between these two electrogenic transporters in the giant glial cell of isolated ganglia of the leech Hirudo medicinalis. Changes in membrane potential, membrane current, intracellular sodium, and intracellular pH evoked by aspartate (1 mM), an EAAT agonist, were measured both in the absence and in the presence of CO(2)/bicarbonate. When 5% CO(2) and 24 mM bicarbonate was added to the saline (at constant pH 7.4), the aspartate-induced membrane current was increased, while the change in intracellular sodium was decreased. The acid influx evoked by aspartate was enhanced by CO(2)/bicarbonate but, because of the increased intracellular CO(2)/bicarbonate-dependent buffering power, the change in intracellular pH was decreased. 4,4'-Diisothiocyanatostilbene-2, 2'-disulfonic acid (DIDS, 0.5 mM), which inhibits the NBC, reversed the effects of CO(2)/bicarbonate on the aspartate-induced current and pH change. Our results suggest that the NBC helps counteract dissipation of the sodium and the acid-base gradients induced by the EAAT, enhancing the rate and capacity of glutamate uptake by glial cells.