Role of voltage-gated calcium channels in the generation of activity-induced extracellular pH transients in the rat hippocampal slice

TRP Channels as Molecular Targets to Relieve Cancer Pain

Transient receptor potential (TRP) channels are critical receptors in the transduction of nociceptive stimuli. The microenvironment of diverse types of cancer releases substances, including growth factors, neurotransmitters, and inflammatory mediators, which modulate the activity of TRPs through the regulation of intracellular signaling pathways. The modulation of TRP channels is associated with the peripheral sensitization observed in patients with cancer, which results in mild noxious sensory stimuli being perceived as hyperalgesia and allodynia. Secondary metabolites derived from plant extracts can induce the activation, blocking, and desensitization of TRP channels. Thus, these compounds could act as potential therapeutic agents, as their antinociceptive properties could be beneficial in relieving cancer-derived pain. In this review, we will summarize the role of TRPV1 and TRPA1 in pain associated with cancer and discuss molecules that have been reported to modulate these channels, focusing particularly on the mechanisms of channel activation associated with molecules released in the tumor microenvironment.

Computational modeling predicts ephemeral acidic microdomains in the glutamatergic synaptic cleft

At chemical synapses, synaptic vesicles release their acidic contents into the cleft, leading to the expectation that the cleft should acidify. However, fluorescent pH probes targeted to the cleft of conventional glutamatergic synapses in both fruit flies and mice reveal cleft alkalinization rather than acidification. Here, using a reaction-diffusion scheme, we modeled pH dynamics at the Drosophila neuromuscular junction as glutamate, ATP, and protons (H⁺) were released into the cleft. The model incorporates bicarbonate and phosphate buffering systems as well as plasma membrane calcium-ATPase activity and predicts substantial cleft acidification but only for fractions of a millisecond after neurotransmitter release. Thereafter, the cleft rapidly alkalinizes and remains alkaline for over 100 ms because the plasma membrane calcium-ATPase removes H⁺ from the cleft in exchange for calcium ions from adjacent pre- and postsynaptic compartments, thus recapitulating the empirical data. The extent of synaptic vesicle loading and time course of exocytosis have little influence on the magnitude of acidification. Phosphate but not bicarbonate buffering is effective at suppressing the magnitude and time course of the acid spike, whereas both buffering systems are effective at suppressing cleft alkalinization. The small volume of the cleft levies a powerful influence on the magnitude of alkalinization and its time course. Structural features that open the cleft to adjacent spaces appear to be essential for alleviating the extent of pH transients accompanying neurotransmission.

Novel hybrid action of GABA mediates inhibitory feedback in the mammalian retina

The stream of visual information sent from photoreceptors to second-order bipolar cells is intercepted by laterally interacting horizontal cells that generate feedback to optimize and improve the efficiency of signal transmission. The mechanisms underlying the regulation of graded photoreceptor synaptic output in this nonspiking network have remained elusive. Here, we analyze with patch clamp recording the novel mechanisms by which horizontal cells control pH in the synaptic cleft to modulate photoreceptor neurotransmitter release. First, we show that mammalian horizontal cells respond to their own GABA release and that the results of this autaptic action affect cone voltage-gated Ca2+ channel (CaV channel) gating through changes in pH. As a proof-of-principle, we demonstrate that chemogenetic manipulation of horizontal cells with exogenous anion channel expression mimics GABA-mediated cone CaV channel inhibition. Activation of these GABA receptor anion channels can depolarize horizontal cells and increase cleft acidity via Na+/H+ exchanger (NHE) proton extrusion, which results in inhibition of cone CaV channels. This action is effectively counteracted when horizontal cells are sufficiently hyperpolarized by increased GABA receptor (GABAR)-mediated HCO3- efflux, alkalinizing the cleft and disinhibiting cone CaV channels. This demonstrates how hybrid actions of GABA operate in parallel to effect voltage-dependent pH changes, a novel mechanism for regulating synaptic output.

Carbonic anhydrases and brain pH in the control of neuronal excitability

H(+) ions are remarkably efficient modulators of neuronal excitability. This renders brain functions highly sensitive to small changes in pH which are generated "extrinsically" via mechanisms that regulate the acid-base status of the whole organism; and "intrinsically", by activity-induced transmembrane fluxes and de novo generation of acid-base equivalents. The effects of pH changes on neuronal excitability are mediated by diverse, largely synergistically-acting mechanisms operating at the level of voltage- and ligand-gated ion channels and gap junctions. In general, alkaline shifts induce an increase in excitability which is often intense enough to trigger epileptiform activity, while acidosis has the opposite effect. Brain pH changes show a wide variability in their spatiotemporal properties, ranging from long-lasting global shifts to fast and highly localized transients that take place in subcellular microdomains. Thirteen catalytically-active mammalian carbonic anhydrase isoforms have been identified, whereof 11 are expressed in the brain. Distinct CA isoforms which have their catalytic sites within brain cells and the interstitial fluid exert a remarkably strong influence on the dynamics of pH shifts and, consequently, on neuronal functions. In this review, we will discuss the various roles of H(+) as an intra- and extracellular signaling factor in the brain, focusing on the effects mediated by CAs. Special attention is paid on the developmental expression patterns and actions of the neuronal isoform, CA VII. Studies on the various functions of CAs will shed light on fundamental mechanisms underlying neuronal development, signaling and plasticity; on pathophysiological mechanisms associated with epilepsy and related diseases; and on the modes of action of CA inhibitors used as CNS-targeting drugs.

The Na(+)/H (+) exchanger NHE5 is sorted to discrete intracellular vesicles in the central and peripheral nervous systems

The pH milieu of the central and peripheral nervous systems is an important determinant of neuronal excitability, function, and survival. In mammals, neural acid-base homeostasis is coordinately regulated by ion transporters belonging to the Na(+)/H(+) exchanger (NHE) and bicarbonate transporter gene families. However, the relative contributions of individual isoforms within the respective families are not fully understood. This report focuses on the NHE family, specifically the plasma membrane-type NHE5 which is preferentially transcribed in brain, but the distribution of the native protein has not been extensively characterized. To this end, we generated a rabbit polyclonal antibody that specifically recognizes NHE5. In both central (cortex, hippocampus) and peripheral (superior cervical ganglia, SCG) nervous tissue of mice, NHE5 immunostaining was punctate and highly concentrated in the somas and to lesser amounts in the dendrites of neurons. Very little signal was detected in axons. Similarly, in primary cultures of differentiated SCG neurons, NHE5 localized predominantly to vesicles in the somatodendritic compartment, though some immunostaining was also evident in punctate vesicles along the axons. NHE5 was also detected predominantly in intracellular vesicles of cultured SCG glial cells. Dual immunolabeling of SCG neurons showed that NHE5 did not colocalize with markers for early endosomes (EEA1) or synaptic vesicles (synaptophysin), but did partially colocalize with the transferrin receptor, a marker of recycling endosomes. Collectively, these data suggest that NHE5 partitions into a unique vesicular pool in neurons that shares some characteristics of recycling endosomes where it may serve as an important regulated store of functional transporters required to maintain cytoplasmic pH homeostasis.

Rapid rise of extracellular pH evoked by neural activity is generated by the plasma membrane calcium ATPase

In hippocampus, synchronous activation of CA1 pyramidal neurons causes a rapid, extracellular, population alkaline transient (PAT). It has been suggested that the plasma membrane Ca(2+)-ATPase (PMCA) is the source of this alkalinization, because it exchanges cytosolic Ca(2+) for external H(+). Evidence supporting this hypothesis, however, has thus far been inconclusive. We addressed this long-standing problem by measuring surface alkaline transients (SATs) from voltage-clamped CA1 pyramidal neurons in juvenile mouse hippocampal slices, using concentric (high-speed, low-noise) pH microelectrodes placed against the somata. In saline containing benzolamide (a poorly permeant carbonic anhydrase blocker), a 2-s step from -60 to 0 mV caused a mean SAT of 0.02 unit pH. Addition of 5 mM HEPES to the artificial cerebrospinal fluid diminished the SAT by 91%. Nifedipine reduced the SAT by 53%. Removal of Ca(2+) from the saline abolished the SAT, and addition of BAPTA to the patch pipette reduced it by 79%. The inclusion of carboxyeosin (a PMCA inhibitor) in the pipette abolished the SAT, whether it was induced by a depolarizing step, or by simulated, repetitive, antidromic firing. The peak amplitude of the "antidromic" SAT of a single cell averaged 11% of the PAT elicited by comparable real antidromic activation of the CA1 neuronal population. Caloxin 2A1, an extracellular PMCA peptide inhibitor, blocked both the SAT and PAT by 42%. These results provide the first direct evidence that the PMCA can explain the extracellular alkaline shift elicited by synchronous firing.

The plasma membrane calcium ATPase (PMCA) of neurones is electroneutral and exchanges 2 H+ for each Ca2+ or Ba2+ ion extruded

The coupling between Ca2+ extrusion and H+ uptake by the ubiquitous plasma membrane calcium ATPase (PMCA) has not been measured in any neurone. I have investigated this with Ca2+- and pH-sensitive microelectrodes in large voltage-clamped snail neurones, which have no Na+-Ca2+ exchangers. The recovery of [Ca2+]i and surface pH after a brief depolarization or Ca2+ injection was not slowed by hyperpolarization to -90 mV from a holding potential of -50 mV, consistent with a 1 Ca2+ : 2 H+ coupling ratio. Since Ca2+ injections proved difficult to quantify, and Ca2+ currents through channels were obscured by K+ currents, Ba2+ was used as a substitute. When the cell was bathed in Ca2+-free Ba2+ Ringer solution, the K+ currents were blocked and large inward currents were revealed on depolarization. The Ca2+-sensitive microelectrodes were sensitive to intracellular Ba2+ as well as Ca2+. With equal depolarizations Ba2+ entry appeared larger than Ca2+ entry and generated similar but slower pH changes. Ba2+ extrusion was insensitive to hyperpolarization, blocked by eosin or high pH, and about 5 times slower than Ca2+ extrusion. The ratio of the pH change caused by the extrusion of unit charge of Ba2+ influx to that caused by unit charge of H+ injection was 0.85 +/- 0.08 (s.e.m., n = 8), corresponding to a Ba2+ : H+ ratio of 1 : 1.7. Both this ratio and the electroneutrality of the PMCA suggest that the Ca2+ : H+ ratio is 1 : 2, ensuring that after a Ca2+ influx [Ca2+]i recovery is not influenced by the membrane potential and maximizes the conversion of Ca2+ influxes into possible pH signals.

Kinetics of activity-evoked pH transients and extracellular pH buffering in rat hippocampal slices

The kinetics of activity-dependent, extracellular alkaline transients, and the buffering of extracellular pH (pH(e)), were studied in rat hippocampal slices using a fluorescein-dextran probe. Orthodromic stimuli generated alkaline transients < or = 0.05 pH units that peaked in 273 +/- 26 ms and decayed with a half-time of 508 +/- 43 ms. Inhibition of extracellular carbonic anhydrase (ECA) with benzolamide increased the rate of rise by 25%, doubled peak amplitude, and prolonged the decay three- to fourfold. The slow decay in benzolamide allowed marked temporal summation, resulting in a severalfold increase in amplitude during long stimulus trains. Addition of exogenous carbonic anhydrase reduced the rate of rise, halved the peak amplitude, but had no effect on the normalized decay. A simulation of extracellular buffering kinetics generated recoveries from a base load consistent with the observed decay of the alkaline transient in the presence of benzolamide. Under control conditions, the model approximated the observed decays with an acceleration of the CO2 hydration-dehydration reactions by a factor of 2.5. These data suggest low endogenous ECA activity, insufficient to maintain equilibrium during the alkaline transients. Disequilibrium implies a time-dependent buffering capacity, with a CO2/HCO3- contribution that is small shortly after a base load. It is suggested that within 100 ms, extracellular buffering capacity is about 1% of the value at equilibrium and is provided mainly by phosphate. Accordingly, in the time frame of synaptic transmission, small base loads would generate relatively large changes in interstitial pH.

TRPV1 acts as proton channel to induce acidification in nociceptive neurons

The low extracellular pH of inflamed or ischemic tissues enhances painful sensations by sensitizing and activating the vanilloid receptor 1 (TRPV1). We report here that activation of TRPV1 results in a marked intracellular acidification in nociceptive dorsal root ganglion neurons and in a heterologous expression system. A characterization of the underlying mechanisms revealed a Ca(2+)-dependent intracellular acidification operating at neutral pH and an additional as yet unrecognized direct proton conductance through the poorly selective TRPV1 pore operating in acidic extracellular media. Large organic cations permeate through the activated TRPV1 pore even in the presence of physiological concentrations of Na(+), Mg(2+), and Ca(2+). The wide pore and the unexpectedly high proton permeability of TRPV1 point to a proton hopping permeation mechanism along the water-filled channel pore. In acidic media, the high relative proton permeability through TRPV1 defines a novel proton entry mechanism in nociceptive neurons.

Regulation and modulation of pH in the brain

The regulation of pH is a vital homeostatic function shared by all tissues. Mechanisms that govern H+ in the intracellular and extracellular fluid are especially important in the brain, because electrical activity can elicit rapid pH changes in both compartments. These acid-base transients may in turn influence neural activity by affecting a variety of ion channels. The mechanisms responsible for the regulation of intracellular pH in brain are similar to those of other tissues and are comprised principally of forms of Na+/H+ exchange, Na+-driven Cl-/HCO3- exchange, Na+-HCO3- cotransport, and passive Cl-/HCO3- exchange. Differences in the expression or efficacy of these mechanisms have been noted among the functionally and morphologically diverse neurons and glial cells that have been studied. Molecular identification of transporter isoforms has revealed heterogeneity among brain regions and cell types. Neural activity gives rise to an assortment of extracellular and intracellular pH shifts that originate from a variety of mechanisms. Intracellular pH shifts in neurons and glia have been linked to Ca2+ transport, activation of acid extrusion systems, and the accumulation of metabolic products. Extracellular pH shifts can occur within milliseconds of neural activity, arise from an assortment of mechanisms, and are governed by the activity of extracellular carbonic anhydrase. The functional significance of these compartmental, activity-dependent pH shifts is discussed.

Statistical morphological analysis of hippocampal principal neurons indicates cell-specific repulsion of dendrites from their own cell

Traditionally, the sources of guidance cues for dendritic outgrowth are mainly associated with external bodies (A) rather than with the same neuron from which dendrites originate (B). To quantify the relationship between factors A and B as determinants of the adult dendritic shape, the morphology of 83 intracellularly characterized, stained, completely reconstructed, and digitized principal neurons of the rat hippocampus was statistically analyzed using Bayesian optimization. It was found that the dominant directional preference (tropism) manifested in dendritic turns is to grow away from the soma rather than toward the incoming fibers or in any other fixed direction; therefore, B is predominant. Results are robust and consistent for all examined morphological classes (dentate gyrus granule cells, basal and apical trees of CA3 and CA1 pyramidal cells). In addition, computer remodeling of neurons based on the measured parameters produced virtual structures consistent with real morphologies, as confirmed by measurement of several global emergent parameters. Thus, the simple description of dendritic shape based on dendrites' tendency to grow straight, away from their own soma, and with additional random deflections, proves remarkably accurate and complete. Although based on adult neurons, these results suggest that dendritic guidance during development may be associated primarily with the host cell. This possibility challenges the traditional concept of dendritic guidance: in that hippocampal cells are densely packed and have highly overlapping dendritic fields, the somatodendritic repulsion must be cell specific. Plausible mechanisms involving extracellular effects of spikes are discussed, together with feasible experimental tests and predicted results.

Voltage-gated proton channels in microglia

Microglia, macrophages that reside in the brain, can express at least 12 different ion channels, including voltage-gated proton channels. The properties of H+ currents in microglia are similar to those in other phagocytes. Proton currents are elicited by depolarizing the membrane potential, but activation also depends strongly on both intracellular pH (pH(i)) and extracellular pH (pH(o)). Increasing pH(o) or lowering pH(i) promotes H+ channel opening by shifting the activation threshold to more negative potentials. H+ channels in microglia open only when the pH gradient is outward, so they carry only outward current in the steady state. Time-dependent activation of H+ currents is slow, with a time constant roughly 1 s at room temperature. Microglial H+ currents are inhibited by inorganic polyvalent cations, which reduce H+ current amplitude and shift the voltage dependence of activation to more positive potentials. Cytoskeletal disruptive agents modulate H+ currents in microglia. Cytochalasin D and colchicine decrease the current density and slow the activation of H+ currents. Similar changes of H+ currents, possibly due to cytoskeletal reorganization, occur in microglia during the transformation from ameboid to ramified morphology. Phagocytes, including microglia, undergo a respiratory burst, in which NADPH oxidase releases bactericidal superoxide anions into the phagosome and stoichiometrically releases protons into the cell, tending to depolarize and acidify the cell. H+ currents may help regulate both the membrane potential and pH(i) during the respiratory burst. By compensating for the efflux of electrons and counteracting intracellular acidification, H+ channels help maintain superoxide anion production.

Extracellular pH responses in CA1 and the dentate gyrus during electrical stimulation, seizure discharges, and spreading depression

Since neuronal excitability is sensitive to changes in extracellular pH and there is regional diversity in the changes in extracellular pH during neuronal activity, we examined the activity-dependent extracellular pH changes in the CA1 region and the dentate gyrus. In vivo, in the CA1 region, recurrent epileptiform activity induced by stimulus trains, bicuculline, and kainic acid resulted in biphasic pH shifts, consisting of an initial extracellular alkalinization followed by a slower acidification. In vitro, stimulus trains also evoked biphasic pH shifts in the CA1 region. However, in CA1, seizure activity in vitro induced in the absence of synaptic transmission, by perfusing with 0 Ca(2+)/5 mM K(+) medium, was only associated with extracellular acidification. In the dentate gyrus in vivo, seizure activity induced by stimulation to the angular bundle or by injection of either bicuculline or kainic acid was only associated with extracellular acidification. In vitro, stimulus trains evoked only acidification. In the dentate gyrus in vitro, recurrent epileptiform activity induced in the absence of synaptic transmission by perfusion with 0 Ca(2+)/8 mM K(+) medium was associated with extracellular acidification. To test whether glial cell depolarization plays a role in the regulation of the extracellular pH, slices were perfused with 1 mM barium. Barium increased the amplitude of the initial alkalinization in CA1 and caused the appearance of alkalinization in the dentate gyrus. In both CA1 and the dentate gyrus in vitro, spreading depression was associated with biphasic pH shifts. These results demonstrate that activity-dependent extracellular pH shifts differ between CA1 and dentate gyrus both in vivo and in vitro. The differences in pH fluctuations with neuronal activity might be a marker for the basis of the regional differences in seizure susceptibility between CA1 and the dentate gyrus.

An electrogenic Na(+)-HCO(-)(3) cotransporter (NBC) with a novel COOH-terminus, cloned from rat brain

We screened rat brain cDNA libraries and used 5' rapid amplification of cDNA ends to clone two electrogenic Na(+)-HCO(-)(3) cotransporter (NBC) isoforms from rat brain (rb1NBC and rb2NBC). At the amino acid level, one clone (rb1NBC) is 96% identical to human pancreas NBC. The other clone (rb2NBC) is identical to rb1NBC except for 61 unique COOH-terminal amino acids, the result of a 97-bp deletion near the 3' end of the open-reading frame. Using RT-PCR, we confirmed that mRNA from rat brain contains this 97-bp deletion. Furthermore, we generated rabbit polyclonal antibodies that distinguish between the unique COOH-termini of rb1NBC (alpharb1NBC) and rb2NBC (alpharb2NBC). alpharb1NBC labels an approximately 130-kDa protein predominantly from kidney, and alpharb2NBC labels an approximately 130-kDa protein predominantly from brain. alpharb2NBC labels a protein that is more highly expressed in cortical neurons than astrocytes cultured from rat brain; alpharb1NBC exhibits the opposite pattern. In expression studies, applying 1.5% CO(2)/10 mM HCO(-)(3) to Xenopus oocytes injected with rb2NBC cRNA causes 1) pH(i) to recover from the initial CO(2)-induced acidification and 2) the cell to hyperpolarize. Subsequently, removing external Na(+) reverses the pH(i) increase and elicits a rapid depolarization. In the presence of 450 microM DIDS, removing external Na(+) has no effect on pH(i) and elicits a small hyperpolarization. The rate of the pH(i) decrease elicited by removing Na(+) is insensitive to removing external Cl(-). Thus rb2NBC is a DIDS-sensitive, electrogenic NBC that is predominantly expressed in brain of at least rat.

Extracellular pH changes and accompanying cation shifts during ouabain-induced spreading depression

Interstitial ionic shifts that accompany ouabain-induced spreading depression (SD) were studied in rat hippocampal and cortical slices in the presence and absence of extracellular Ca(2+). A double-barreled ion-selective microelectrode specific for H(+), K(+), Na(+), or Ca(2+) was placed in the CA1 stratum radiatum or midcortical layer. Superfusion of 100 microM ouabain caused a rapid, negative, interstitial voltage shift (2-10 mV) after 3-5 min. The negativity was accompanied by a rapid alkaline transient followed by prolonged acidosis. In media containing 3 mM Ca(2+), the alkalosis induced by ouabain averaged 0.07 +/- 0.01 unit pH. In media with no added Ca(2+) and 2 mM EGTA, the alkaline shift was not significantly different (0.09 +/- 0.02 unit pH). The alkaline transient was unaffected by inhibiting Na(+)-H(+) exchange with ethylisopropylamiloride (EIPA) or by blocking endoplasmic reticulum Ca(2+) uptake with thapsigargin or cyclopiazonic acid. Alkaline transients were also observed in Ca(2+)-free media when SD was induced by microinjecting high K(+). The late acidification accompanying ouabain-induced SD was significantly reduced in Ca(2+)-free media and in solutions containing EIPA. The ouabain-induced SD was associated with a rapid but relatively modest increase in [K(+)](o). In the presence of 3 mM external Ca(2+), the mean peak elevation of [K(+)](o) was 12 +/- 0.62 mM. In Ca(2+)-free media, the elevation of [K(+)](o) had a more gradual onset and reached a significantly larger peak value, which averaged 22 +/- 1.1 mM. The decrease in [Na(+)](o) that accompanied ouabain-induced SD was somewhat greater. The [Na(+)](o) decreased by averages of 40 +/- 7 and 33 +/- 3 mM in Ca(2+) and Ca(2+)-free media, respectively. In media containing 1.2 mM Ca(2+), ouabain-induced SD was associated with a substantial decrease in [Ca(2+)](o) that averaged 0.73 +/- 0. 07 mM. These data demonstrate that in comparison with conventional SD, ouabain-induced SD exhibits ion shifts that are qualitatively similar but quantitatively diminished. The presence of external Ca(2+) can modulate the phenomenon but is irrelevant to the generation of the SD and its accompanying alkaline pH transient. Significance of these results is discussed in reference to the propagation of SD and the generation of interstitial pH changes.

Activity-related changes in intracellular pH in rat thalamic relay neurons

Activity-related shifts in intracellular pH (pHi) can exert potent neuromodulatory actions. Different states of neuronal activity of thalamocortical neurons were found to differentially modulate pHi. Tonic activity evoked by injection of depolarizing current led to a reversible rise in [H+]i which was nearly abolished in the presence of TTX. Block of voltage-gated calcium channels with I mM Ni2+ reduced the [H+]i transients related to tonic activity. Rhythmic activation of burst discharges caused changes of [H+]i which were decreased by TTX, whereas I mM Ni2+ almost abolished the [H+]i transients. The present results show that different forms of neuronal activity can lead to intracellular acidification caused by different mechanisms, i.e. Na+ and Ca2+ influx through sodium and Ca2+ channels, respectively, and the subsequent activation of a Ca2+/H+ pump. The resulting acidosis is suggested to reduce further Ca2+ influx and prevent excessive neuronal excitation.

Effect of divalent cations on AMPA-evoked extracellular alkaline shifts in rat hippocampal slices

The generation of activity-evoked extracellular alkaline shifts has been linked to the presence of external Ca(2+) or Ba(2+). We further investigated this dependence using pH- and Ca(2+)-selective microelectrodes in the CA1 area of juvenile, rat hippocampal slices. In HEPES-buffered media, alkaline transients evoked by pressure ejection of RS-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) averaged approximately 0.07 unit pH and were calculated to arise from the equivalent net addition of approximately 1 mM strong base to the interstitial space. These alkaline responses were correlated with a mean decrease in [Ca(2+)](o) of approximately 300 microM. The alkalinizations were abolished reversibly in zero-Ca(2+) media, becoming indiscernible at a [Ca(2+)](o) of 117+/-29 microM. Addition of as little as 30-50 microM Ba(2+) caused the reappearance of an alkaline response. In approximately one-fourth of slices, a persistent alkaline shift of approximately 0.03 unit pH was observed in zero-Ca(2+) saline containing EGTA. In HEPES media, addition of 300 microM Cd(2+), 100 microM Ni(2+), or 100 microM nimodipine inhibited the alkaline shifts by roughly one-half, one-third, and one-third, respectively, whereas Cd(+) and Ni(2+) in combination fully blocked the response. In bicarbonate media, by contrast, Cd(+) and Ni(2+) blocked only two-thirds of the response. In the presence of bicarbonate, Ni(2+) caused an unexpected enhancement of the alkalinization by approximately 150%. However, when the extracellular carbonic anhydrase was blocked by benzolamide, addition of Ni(2+) reduced the alkaline shift. These results suggested that Ni(2+) partially inhibited the interstitial carbonic anhydrase and thereby increased the alkaline responses. These data indicate that an activity-dependent alkaline shift is largely dependent on the entry of Ca(2+) or Ba(2+) via voltage-gated calcium channels. However, sizable alkaline transients still can be generated with little or no external presence of these ions. Implications for the mechanism of the activity-dependent alkaline shift are discussed.

Novel role of the Ca(2+)-ATPase in NMDA-induced intracellular acidification

The mechanism involved in N-methyl-D-glucamine (NMDA)-induced Ca(2+)-dependent intracellular acidosis is not clear. In this study, we investigated in detail several possible mechanisms using cultured rat cerebellar granule cells and microfluorometry [fura 2-AM or 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-AM]. When 100 microM NMDA or 40 mM KCl was added, a marked increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) and a decrease in the intracellular pH were seen. Acidosis was completely prevented by the use of Ca(2+)-free medium or 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid-AM, suggesting that it resulted from an influx of extracellular Ca(2+). The following four mechanisms that could conceivably have been involved were excluded: 1) Ca(2+) displacement of intracellular H(+) from common binding sites; 2) activation of an acid loader or inhibition of acid extruders; 3) overproduction of CO(2) or lactate; and 4) collapse of the mitochondrial membrane potential due to Ca(2+) uptake, resulting in inhibition of cytosolic H(+) uptake. However, NMDA/KCl-induced acidosis was largely prevented by glycolytic inhibitors (iodoacetate or deoxyglucose in glucose-free medium) or by inhibitors of the Ca(2+)-ATPase (i.e., Ca(2+)/H(+) exchanger), including La(3+), orthovanadate, eosin B, or an extracellular pH of 8.5. Our results therefore suggest that Ca(2+)-ATPase is involved in NMDA-induced intracellular acidosis in granule cells. We also provide new evidence that NMDA-evoked intracellular acidosis probably serves as a negative feedback signal, probably with the acidification itself inhibiting the NMDA-induced [Ca(2+)](i) increase.

Synaptic activation of GABAA receptors induces neuronal uptake of Ca2+ in adult rat hippocampal slices

Synaptically evoked transmembrane movements of Ca2+ in the adult CNS have almost exclusively been attributed to activation of glutamate receptor channels and the consequent triggering of voltage-gated calcium channels (VGCCs). Using microelectrodes for measuring free extracellular Ca2+ ([Ca2+]o) and extracellular space (ECS) volume, we show here for the first time that synaptic stimulation of gamma-aminobutyric acid-A (GABAA) receptors can result in a decrease in [Ca2+]o in adult rat hippocampal slices. High-frequency stimulation (100-200 Hz, 0.4-0.5 s) applied in stratum radiatum close (</=0.5 mm) to the recording site induced a 0.1- to 0.3-mM transient fall in [Ca2+]o from a baseline level of 1.6 mM. Concomitantly, a 30-40% decrease in the ECS volume was seen. Exposure of drug-naïve slices to the GABAA receptor antagonist picrotoxin (100 microM) first attenuated and only thereafter augmented the Ca2+ shifts. Application of ionotropic glutamate receptor antagonists resulted in a monotonic reduction of the Ca2+ response, but a large Ca2+ shift persisted (60-70% of the original), which was attenuated by a subsequent application of picrotoxin or bicuculline. In the absence of ionotropic glutamatergic transmission, pentobarbital sodium (100 microM), an up-modulator of the GABAA receptor, strongly enhanced the activity-evoked changes in [Ca2+]o. We suggest that the underlying mechanism of GABA-induced Ca2+ transients is the activation of VGCCs by bicarbonate-dependent GABA-mediated depolarizing postsynaptic potentials. Accordingly, stimulation-evoked Ca2+ shifts were inhibited by the membrane-permeant inhibitor of carbonic anhydrase, ethoxyzolamide (50 microM) or in N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES)-buffered HCO3-free solution. Neuronal Ca2+ uptake caused by intense synaptic activation of GABAA receptors may prove to be an important mechanism in the modulation of activity-dependent neuronal plasticity, epileptogenesis, and cell survival in the adult brain.

NMDA induces a biphasic change in intracellular pH in rat hippocampal slices

As alterations in intracellular pH (pH(i)) tend to exert a profound effect on the properties of cells, this study was undertaken to examine NMDA-induced changes in pH(i) in rat hippocampal slices using the BCECF fluorescent technique. The 'resting' pH(i) in the CA1 pyramidal cell layers was 6.93 +/- 0.07 (mean +/- S.D., n = 72 slices) in 25 mM HCO3-/5% CO2-buffered solution at 37 degrees C. Exposure of hippocampal slices to NMDA in the range of 10-1000 microM produced a biphasic change in pH(i): an initial transient alkaline shift was followed by a long-lasting acid shift. Dizocilpine (10 microM) but not CNQX (40 microM) blocked the NMDA-induced changes in pH(i). In 0 Ca medium (0 mM Ca2+ supplemented 1 mM EGTA, referred to as 0 Ca), pH(i) acid shift caused by NMDA (20 microM) declined by about 11%, whereas the initial alkaline shift almost completely disappeared. In an independent experiment, the NMDA-induced increase in intracellular Ca2+ ([Ca2+]i) was reduced by more than 80% in 0 Ca medium. Glucose substitution using equimolar pyruvate (as an energy-yielding substrate) suppressed this NMDA-induced pH(i) acid shift by two-thirds, while the NMDA-induced pH(i) alkaline shift was enhanced. Fluoride (10 mM), a glycolytic inhibitor, abolished NMDA-induced pH(i) acid shift. Furthermore, the lactate content of hippocampal slices was markedly increased following exposure to NMDA. In conclusion, activation of NMDA receptors in rat hippocampal slices evokes a biphasic change in pH(i). The initial alkaline shift is suggested to be associated with calcium influx, and the following acid shift may be caused by an increase in lactate production through the acceleration of glycolysis, as well as the increased [Ca2+]i. The pH(i) acid shift produced by the increased lactate may contribute to proton modulation of the NMDA receptor and NMDA-induced cell injury or death.

GABA- and glycine-mediated fall of intracellular pH in rat medullary neurons in situ

In the region of the ventral respiratory group in brain stem slices from neonatal rats, intracellular pH (pHi) and membrane currents (I(m)) or potentials were measured in neurons dialyzed with the pH-sensitive dye 2',7'-bis-carboxyethyl-5(6)-carboxyfluorescein. Currents and increases in membrane conductance (g(m)) during bath application of 0.1 or 1 mM gamma-aminobutyric acid (GABA) were accompanied by a delayed mean fall of pHi by 0.17 and 0.25 pH units, respectively, from a pHi baseline of 7.33. These effects were reversibly suppressed by 50-100 microM bicuculline. Similar effects on I(m), g(m), and pHi were revealed on administration of 0.1 or 1 mM glycine. These responses were abolished by 10-100 microM strychnine. Dialysis of the cells with 15-30 microM carbonic anhydrase led to an acceleration of the kinetics and a potentiation of the GABA-induced pHi decrease. GABA- and glycine-evoked pHi decreases were very similar during recordings with either high- or low-Cl- patch electrodes, although the reversal potential of the accompanying currents differed by approximately 60 mV. The GABA-induced pHi decrease, but not the accompanying I(m) and g(m) responses, was suppressed in CO2/HCO3(-)-free, N-2-hydroxy-ethylpiperazine-N'-2-ethane sulphonic acid pH-buffered solution. Depolarization from -60 to +30 mV resulted in a sustained fall of pHi by maximally 0.5 pH units. In this situation, the GABA-induced fall of pHi turned into an intracellular alkalosis of 0.09-0.15 pH units. The results confirm and extend previous findings obtained in vivo that GABA- or glycine-induced intracellular acidosis of respiratory neurons is due to efflux of HCO3- via the receptor-coupled Cl- channel.