Modulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor desensitization by extracellular protons

Mechanisms underlying modulation of human GlyRα3 receptors by Zn2+ and pH

Glycine receptors (GlyRs) regulate motor control and pain processing in the central nervous system through inhibitory synaptic signaling. The subtype GlyRα3 expressed in nociceptive sensory neurons of the spinal dorsal horn is a key regulator of physiological pain perception. Disruption of spinal glycinergic inhibition is associated with chronic inflammatory pain states, making GlyRα3 an attractive target for pain treatment. GlyRα3 activity is modulated by numerous endogenous and exogenous ligands that consequently affect pain sensitization. To understand the mechanism of two such endogenous modulators, Zn2+ and protons, we have used cryo-electron microscopy to determine structures of full-length human GlyRα3 in various functional states. Whereas acidic pH reduces peak glycine response, Zn2+ displays biphasic modulation in a concentration-dependent manner. Our findings reveal the effector sites and also capture intermediate conformations in the gating cycle. Combined with molecular dynamics simulations and electrophysiology, this work provides important insights into GlyRα3 activation and regulation.

Proton-triggered rearrangement of the AMPA receptor N-terminal domains impacts receptor kinetics and synaptic localization

AMPA glutamate receptors (AMPARs) are ion channel tetramers that mediate the majority of fast excitatory synaptic transmission. They are composed of four subunits (GluA1-GluA4); the GluA2 subunit dominates AMPAR function throughout the forebrain. Its extracellular N-terminal domain (NTD) determines receptor localization at the synapse, ensuring reliable synaptic transmission and plasticity. This synaptic anchoring function requires a compact NTD tier, stabilized by a GluA2-specific NTD interface. Here we show that low pH conditions, which accompany synaptic activity, rupture this interface. All-atom molecular dynamics simulations reveal that protonation of an interfacial histidine residue (H208) centrally contributes to NTD rearrangement. Moreover, in stark contrast to their canonical compact arrangement at neutral pH, GluA2 cryo-electron microscopy structures exhibit a wide spectrum of NTD conformations under acidic conditions. We show that the consequences of this pH-dependent conformational control are twofold: rupture of the NTD tier slows recovery from desensitized states and increases receptor mobility at mouse hippocampal synapses. Therefore, a proton-triggered NTD switch will shape both AMPAR location and kinetics, thereby impacting synaptic signal transmission.

Possible Compensatory Role of ASICs in Glutamatergic Synapses

Proton-gated channels of the ASIC family are widely distributed in central neurons, suggesting their role in common neurophysiological functions. They are involved in glutamatergic neurotransmission and synaptic plasticity; however, the exact function of these channels remains unclear. One problem is that acidification of the synaptic cleft due to the acidic content of synaptic vesicles has opposite effects on ionotropic glutamate receptors and ASICs. Thus, the pH values required to activate ASICs strongly inhibit AMPA receptors and almost completely inhibit NMDA receptors. This, in turn, suggests that ASICs can provide compensation for post-synaptic responses in the case of significant acidifications. We tested this hypothesis by patch-clamp recordings of rat brain neuron responses to acidifications and glutamate receptor agonists at different pH values. Hippocampal pyramidal neurons have much lower ASICs than glutamate receptor responses, whereas striatal interneurons show the opposite ratio. Cortical pyramidal neurons and hippocampal interneurons show similar amplitudes in their responses to acidification and glutamate. Consequently, the total response to glutamate agonists at different pH levels remains rather stable up to pH 6.2. Besides these pH effects, the relationship between the responses mediated by glutamate receptors and ASICs depends on the presence of Mg2+ and the membrane voltage. Together, these factors create a complex picture that provides a framework for understanding the role of ASICs in synaptic transmission and synaptic plasticity.

Deregulation of Astroglial TASK-1 K+ Channel Decreases the Responsiveness to Perampanel-Induced AMPA Receptor Inhibition in Chronic Epilepsy Rats

Tandem of P domains in a weak inwardly rectifying K+ channel (TWIK)-related acid sensitive K+-1 channel (TASK-1) is activated under extracellular alkaline conditions (pH 7.2–8.2), which are upregulated in astrocytes (particularly in the CA1 region) of the hippocampi of patients with temporal lobe epilepsy and chronic epilepsy rats. Perampanel (PER) is a non-competitive α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) antagonist used for the treatment of focal seizures and primary generalized tonic–clonic seizures. Since AMPAR activation leads to extracellular alkaline shifts, it is likely that the responsiveness to PER in the epileptic hippocampus may be relevant to astroglial TASK-1 regulation, which has been unreported. In the present study, we found that PER ameliorated astroglial TASK-1 upregulation in responders (whose seizure activities were responsive to PER), but not non-responders (whose seizure activities were not responsive to PER), in chronic epilepsy rats. ML365 (a selective TASK-1 inhibitor) diminished astroglial TASK-1 expression and seizure duration in non-responders to PER. ML365 co-treatment with PER decreased spontaneous seizure activities in non-responders to PER. These findings suggest that deregulation of astroglial TASK-1 upregulation may participate in the responsiveness to PER, and that this may be a potential target to improve the efficacies of PER.

Functional impairment of cortical AMPA receptors in schizophrenia

Clinical and preclinical studies suggest that some of the behavioral alterations observed in schizophrenia (SZ) may be mechanistically linked to synaptic dysfunction of glutamatergic signaling. Recent genetic and proteomic studies suggest alterations of cortical glutamate receptors of the AMPA-type (AMPARs), which are the predominant ligand-gated ionic channels of fast transmission at excitatory synapses. The impact of gene and protein alterations on the electrophysiological activity of AMPARs is not known in SZ. In this proof of principle work, using human postmortem brain synaptic membranes isolated from the dorsolateral prefrontal cortex (DLPFC), we combined electrophysiological analysis from microtransplanted synaptic membranes (MSM) with transcriptomic (RNA-Seq) and label-free proteomics data in 10 control and 10 subjects diagnosed with SZ. We observed in SZ a reduction in the amplitude of AMPARs currents elicited by kainate, an agonist of AMPARs that blocks the desensitization of the receptor. This reduction was not associated with protein abundance but with a reduction in kainate's potency to activate AMPARs. Electrophysiologically-anchored dataset analysis (EDA) was used to identify synaptosomal proteins that linearly correlate with the amplitude of the AMPARs responses, gene ontology functional annotations were then used to determine protein-protein interactions. Protein modules associated with positive AMPARs current increases were downregulated in SZ, while protein modules that were upregulated in SZ were associated with decreased AMPARs currents. Our results indicate that transcriptomic and proteomic alterations, frequently observed in the DLPFC in SZ, converge at the synaptic level producing a functional electrophysiological impairment of AMPARs.

Tandem pore domain acid-sensitive K channel 3 (TASK-3) regulates visual sensitivity in healthy and aging retina

Retinal ganglion cells (RGCs) not only collect but also integrate visual signals and send them from the retina to the brain. The mechanisms underlying the RGC integration of synaptic activity within retinal circuits have not been fully explored. Here, we identified a pronounced expression of tandem pore domain acid-sensitive potassium channel 3 (TASK-3), a two-pore domain potassium channel (K2P), in RGCs. By using a specific antagonist and TASK-3 knockout mice, we found that TASK-3 regulates the intrinsic excitability and the light sensitivity of RGCs by sensing neuronal activity-dependent extracellular acidification. In vivo, the blockade or loss of TASK-3 dampened pupillary light reflex, visual acuity, and contrast sensitivity. Furthermore, overexpressing TASK-3 specifically in RGCs using an adeno-associated virus approach restored the visual function of TASK-3 knockout mice and aged mice where the expression and function of TASK-3 were reduced. Thus, our results provide evidence that implicates a critical role of K2P in visual processing in the retina.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Cadmium activates AMPA and NMDA receptors with M3 helix cysteine substitutions

AMPA and NMDA receptors are ligand-gated ion channels that depolarize postsynaptic neurons when activated by the neurotransmitter L-glutamate. Changes in the distribution and activity of these receptors underlie learning and memory, but excessive change is associated with an array of neurological disorders, including cognitive impairment, developmental delay, and epilepsy. All of the ionotropic glutamate receptors (iGluRs) exhibit similar tetrameric architecture, transmembrane topology, and basic framework for activation; conformational changes induced by extracellular agonist binding deform and splay open the inner helix bundle crossing that occludes ion flux through the channel. NMDA receptors require agonist binding to all four subunits, whereas AMPA and closely related kainate receptors can open with less than complete occupancy. In addition to conventional activation by agonist binding, we recently identified two locations along the inner helix of the GluK2 kainate receptor subunit where cysteine (Cys) substitution yields channels that are opened by exposure to cadmium ions, independent of agonist site occupancy. Here, we generate AMPA and NMDA receptor subunits with homologous Cys substitutions and demonstrate similar activation of the mutant receptors by Cd. Coexpression of the auxiliary subunit stargazin enhanced Cd potency for activation of Cys-substituted GluA1 and altered occlusion upon treatment with sulfhydryl-reactive MTS reagents. Mutant NMDA receptors displayed voltage-dependent Mg block of currents activated by agonist and/or Cd as well as asymmetry between Cd effects on Cys-substituted GluN1 versus GluN2 subunits. In addition, Cd activation of each Cys-substituted iGluR was inhibited by protons. These results, together with our earlier work on GluK2, reveal a novel mechanism shared among the three different iGluR subtypes for prying open the gate that controls ion entry into the pore.

Protons as Messengers of Intercellular Communication in the Nervous System

In this review, evidence demonstrating that protons (H⁺) constitute a complex, regulated intercellular signaling mechanisms are presented. Given that pH is a strictly regulated variable in multicellular organisms, localized extracellular pH changes may constitute significant signals of cellular processes that occur in a cell or a group of cells. Several studies have demonstrated that the low pH of synaptic vesicles implies that neurotransmitter release is always accompanied by the co-release of H⁺ into the synaptic cleft, leading to transient extracellular pH shifts. Also, evidence has accumulated indicating that extracellular H⁺ concentration regulation is complex and implies a source of protons in a network of transporters, ion exchangers, and buffer capacity of the media that may finally establish the extracellular proton concentration. The activation of membrane transporters, increased production of CO₂ and of metabolites, such as lactate, produce significant extracellular pH shifts in nano- and micro-domains in the central nervous system (CNS), constituting a reliable signal for intercellular communication. The acid sensing ion channels (ASIC) function as specific signal sensors of proton signaling mechanism, detecting subtle variations of extracellular H⁺ in a range varying from pH 5 to 8. The main question in relation to this signaling system is whether it is only synaptically restricted, or a volume modulator of neuron excitability. This signaling system may have evolved from a metabolic activity detection mechanism to a highly localized extracellular proton dependent communication mechanism. In this study, evidence showing the mechanisms of regulation of extracellular pH shifts and of the ASICs and its function in modulating the excitability in various systems is reviewed, including data and its role in synaptic neurotransmission, volume transmission and even segregated neurotransmission, leading to a reliable extracellular signaling mechanism.

The influence of acidic media on the effect of beta-amyloid peptide on the function of glycine receptor in hippocampal neurons

We have previously shown that application of beta-amyloid peptide 1-42 (Aβ) at picomolar/nanomolar concentrations caused a decrease in the peak amplitude and acceleration of desensitization of the glycine-activated chloride current (I_Gly) in hippocampal pyramidal neurons (Bukanova et al., 2016). The aim of this work was to study the effect of Aβ on I_Gly in an acidified medium. The relevance of this work is determined by the fact that the pathogenic effects of Aβ in Alzheimer's disease are usually accompanied by inflammatory processes and acidosis. The I_Gly was induced by 600 ms application of 100 μM (nearly EC₅₀) or 500 μM (nearly saturating) glycine on isolated rat hippocampal neurons. The solution of glycine was neutral (pH 7.4) or acidic over a pH range of 5.0-7.0. It was found that 600 ms application of protons rapidly, reversibly and in dose-dependent manner decreased the peak amplitude and accelerated the desensitization of I_Gly. The effect of H⁺ on I_Gly desensitization did not depend on glycine concentration and may be considered noncompetitive, while the effect on I_Gly peak disappeared at saturating glycine concentration and can be regarded as a competitive. These characteristics of the proton effects on I_Gly coincide with the characteristics of the Aβ effects on I_Gly. Experiments with joint application of Aβ and H⁺ showed interdependence of their effects. Addition of Aβ to perfusing solution reduced H⁺ effects on I_Gly while long pretreatment of Aβ with acid solution prevented the effects of the peptide on I_Gly. Our results suggest the existence of common sites for Aβ and H⁺ on the GlyR and indicate a mutual weakening of the inhibitory action of these molecules on I_Gly.

Deactivation kinetics of acid-sensing ion channel 1a are strongly pH-sensitive

Acid-sensing ion channels (ASICs) are trimeric cation-selective ion channels activated by protons in the physiological range. Recent reports have revealed that postsynaptically localized ASICs contribute to the excitatory postsynaptic current by responding to the transient acidification of the synaptic cleft that accompanies neurotransmission. In response to such brief acidic transients, both recombinant and native ASICs show extremely rapid deactivation in outside-out patches when jumping from a pH 5 stimulus to a single resting pH of 8. Given that the resting pH of the synaptic cleft is highly dynamic and depends on recent synaptic activity, we explored the kinetics of ASIC1a and 1a/2a heteromers to such brief pH transients over a wider [H⁺] range to approximate neuronal conditions better. Surprisingly, the deactivation of ASICs was steeply dependent on the pH, spanning nearly three orders of magnitude from extremely fast (<1 ms) at pH 8 to very slow (>300 ms) at pH 7. This study provides an example of a ligand-gated ion channel whose deactivation is sensitive to agonist concentrations that do not directly activate the receptor. Kinetic simulations and further mutagenesis provide evidence that ASICs show such steeply agonist-dependent deactivation because of strong cooperativity in proton binding. This capacity to signal across such a large synaptically relevant bandwidth enhances the response to small-amplitude acidifications likely to occur at the cleft and may provide ASICs with the ability to shape activity in response to the recent history of the synapse.

Acid-Sensing Ion Channels Activated by Evoked Released Protons Modulate Synaptic Transmission at the Mouse Calyx of Held Synapse

Acid-sensing ion channels (ASICs) regulate synaptic activities and play important roles in neurodegenerative diseases. We found that these channels can be activated in neurons of the medial nucleus of the trapezoid body (MNTB) of the auditory system in the CNS. A drop in extracellular pH induces transient inward ASIC currents (I_ASICs) in postsynaptic MNTB neurons from wild-type mice. The inhibition of I_ASICs by psalmotoxin-1 (PcTx1) and the absence of these currents in knock-out mice for ASIC-1a subunit (ASIC1a^-/-) suggest that homomeric ASIC-1as are mediating these currents in MNTB neurons. Furthermore, we detect ASIC1a-dependent currents during synaptic transmission, suggesting an acidification of the synaptic cleft due to the corelease of neurotransmitter and H⁺ from synaptic vesicles. These currents are capable of eliciting action potentials in the absence of glutamatergic currents. A significant characteristic of these homomeric ASIC-1as is their permeability to Ca²⁺ Activation of ASIC-1a in MNTB neurons by exogenous H⁺ induces an increase in intracellular Ca²⁺ Furthermore, the activation of postsynaptic ASIC-1as during high-frequency stimulation (HFS) of the presynaptic nerve terminal leads to a PcTx1-sensitive increase in intracellular Ca²⁺ in MNTB neurons, which is independent of glutamate receptors and is absent in neurons from ASIC1a^-/- mice. During HFS, the lack of functional ASICs in synaptic transmission results in an enhanced short-term depression of glutamatergic EPSCs. These results strongly support the hypothesis of protons as neurotransmitters and demonstrate that presynaptic released protons modulate synaptic transmission by activating ASIC-1as at the calyx of Held-MNTB synapse.SIGNIFICANCE STATEMENT The manuscript demonstrates that postsynaptic neurons of the medial nucleus of the trapezoid body at the mouse calyx of Held synapse express functional homomeric Acid-sensing ion channel-1a (ASIC-1as) that can be activated by protons (coreleased with neurotransmitter from acidified synaptic vesicles). These ASIC-1as contribute to the generation of postsynaptic currents and, more relevant, to calcium influx, which could be involved in the modulation of presynaptic transmitter release. Inhibition or deletion of ASIC-1a leads to enhanced short-term depression, demonstrating that they are concerned with short-term plasticity of the synapse. ASICs represent a widespread communication system with unique properties. We expect that our experiments will have an impact in the neurobiology field and will spread in areas related to neuronal plasticity.

Endosomal pH in neuronal signaling and synaptic transmission: role of Na(+)/H(+) exchanger NHE5

Neuronal precursor cells extend multiple neurites during development, one of which extends to form an axon whereas others develop into dendrites. Chemical stimulation of N-methyl D-aspartate (NMDA) receptor in fully-differentiated neurons induces projection of dendritic spines, small spikes protruding from dendrites, thereby establishing another layer of polarity within the dendrite. Neuron-enriched Na⁺/H⁺ exchanger NHE5 contributes to both neurite growth and dendritic spine formation. In resting neurons and neuro-endocrine cells, neuron-enriched NHE5 is predominantly associated with recycling endosomes where it colocalizes with nerve growth factor (NGF) receptor TrkA. NHE5 potently acidifies the lumen of TrkA-positive recycling endosomes and regulates cell-surface targeting of TrkA, whereas chemical stimulation of NMDA receptors rapidly recruits NHE5 to dendritic spines, alkalinizes dendrites and down-regulates the dendritic spine formation. Possible roles of NHE5 in neuronal signaling via proton movement in subcellular compartments are discussed.

Cytosolic zinc accumulation contributes to excitotoxic oligodendroglial death

Dyshomeostasis of cytosolic Zn(2+) is a critical mediator of neuronal damage during excitotoxicity. However, the role of this cation in oligodendrocyte pathophysiology is not well understood. The current study examined the contribution of Zn(2+) deregulation to oligodendrocyte injury mediated by AMPA receptors. Oligodendrocytes loaded with the Zn(2+)-selective indicator FluoZin-3 responded to mild stimulation of AMPA receptors with fast cytosolic Zn(2+) rises that resulted from intracellular release, as they were not blocked by the extracellular Zn(2+) chelator Ca-EDTA. Pharmacological experiments suggested that AMPA-induced Zn(2+) mobilization depends on cytosolic Ca(2+) accumulation, arises from mitochondria and protein-bound pools, and is triggered by mechanisms that do not involve the generation of reactive oxygen species. Moreover, intracellular Zn(2+) rises resulting from AMPA receptor activation seem to be promoted by Ca(2+)-dependent cytosolic acidification. Addition of the cell-permeable Zn(2+) chelator TPEN significantly reduced mitochondrial membrane depolarization, reactive oxygen species production, and cell death by sub-maximal activation of AMPA receptors both in vitro and in situ, suggesting that Zn(2+) deregulation is an important mediator of oligodendrocyte excitotoxicity. These data provide evidence that strategies aimed at maintaining Zn(2+) homeostasis may be useful for the treatment of disorders in which excitotoxicity is an important trigger of oligodendroglial death.

Acidification of the synaptic cleft of cone photoreceptor terminal controls the amount of transmitter release, thereby forming the receptive field surround in the vertebrate retina

In the vertebrate retina, feedback from horizontal cells (HCs) to cone photoreceptors plays a key role in the formation of the center-surround receptive field of retinal cells, which induces contrast enhancement of visual images. The mechanism underlying surround inhibition is not fully understood. In this review, we discuss this issue, focusing on our recent hypothesis that acidification of the synaptic cleft of the cone photoreceptor terminal causes this inhibition by modulating the Ca channel of the terminals. We present evidence that the acidification is caused by proton excretion from HCs by a vacuolar type H(+) pump. Recent publications supporting or opposing our hypothesis are discussed.

Extracellular protons reduce quantal content and prolong synaptic currents at the Drosophila larval neuromuscular junction

Fluctuations in extracellular pH occur in the nervous system in response to a number of physiological and pathological processes, such as ischemia, hypercapnea, and high-frequency activity. Using the Drosophila larval neuromuscular junction, the author has examined acute effects of low and high pH on excitability and synaptic transmission. Acidification rapidly and reversibly reduces the size of electrically evoked excitatory junctional currents (EJCs) in a concentration-dependent manner, with transmission nearly abolished at pH 5.0. Conversely, raising pH to 7.8 increases EJC amplitude significantly. Further elevation to pH 8.5 causes an initial increase in amplitude, followed by profound, long-lasting depression of the synapse. Amplitudes of spontaneous miniature EJCs (mEJCs) are modestly, but significantly reduced at pH 5.0. It is therefore the number of quanta released per action potential, rather than the size of individual quanta, that is most strongly affected. Decay times of both EJCs and mEJCs are dramatically lengthened at low pH, suggesting that glutamate remains in the synaptic cleft for much longer than normal. Presynaptic excitability is also reduced, as indicated by increased latency between nerve shock and EJC onset. The response to low pH was not altered by mutations in genes encoding Transient Receptor Potential, Mucolipin subfamily (TRPML) and Slowpoke ion channels, which had previously been implicated as possible targets of extracellular protons. The author concludes that extracellular protons have strong effects on the release of glutamate and the time course of synaptic currents. These phenotypes can be exploited to study the mechanisms of acid-mediated changes in neuronal function, and to pursue the way in which pH modulates synaptic function in normal and pathophysiological conditions.

Regulation of dendritic spine growth through activity-dependent recruitment of the brain-enriched Na⁺/H⁺ exchanger NHE5

pH homeostasis in neurons plays crucial roles in normal synaptic functions. It is found that the Na⁺/H⁺ exchanger NHE5 is targeted to the synapse on neuronal activation, regulates the synaptic pH, and controls the morphology of dendritic spines.

Subtle changes in cellular and extracellular pH within the physiological range have profound impacts on synaptic activities. However, the molecular mechanisms underlying local pH regulation at synapses and their influence on synaptic structures have not been elucidated. Dendritic spines undergo dynamic structural changes in response to neuronal activation, which contributes to induction and long-term maintenance of synaptic plasticity. Although previous studies have indicated the importance of cytoskeletal rearrangement, vesicular trafficking, cell signaling, and adhesion in this process, much less is known about the involvement of ion transporters. In this study we demonstrate that N-methyl-d-aspartate (NMDA) receptor activation causes recruitment of the brain-enriched Na⁺/H⁺ exchanger NHE5 from endosomes to the plasma membrane. Concomitantly, real-time imaging of green fluorescent protein–tagged NHE5 revealed that NMDA receptor activation triggers redistribution of NHE5 to the spine head. We further show that neuronal activation causes alkalinization of dendritic spines following the initial acidification, and suppression of NHE5 significantly retards the activity-induced alkalinization. Perturbation of NHE5 function induces spontaneous spine growth, which is reversed by inhibition of NMDA receptors. In contrast, overexpression of NHE5 inhibits spine growth in response to neuronal activity. We propose that NHE5 constrains activity-dependent dendritic spine growth via a novel, pH-based negative-feedback mechanism.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Control of assembly and function of glutamate receptors by the amino-terminal domain

The extracellular amino-terminal domains (ATDs) of the ionotropic glutamate receptor subunits form a semiautonomous component of all glutamate receptors that resides distal to the membrane and controls a surprisingly diverse set of receptor functions. These functions include subunit assembly, receptor trafficking, channel gating, agonist potency, and allosteric modulation. The many divergent features of the different ionotropic glutamate receptor classes and different subunits within a class may stem from differential regulation by the amino-terminal domains. The emerging knowledge of the structure and function of the amino-terminal domains reviewed here may enable targeting of this region for the therapeutic modulation of glutamatergic signaling. Toward this end, NMDA receptor antagonists that interact with the GluN2B ATD show promise in animal models of ischemia, neuropathic pain, and Parkinson's disease.

Mitochondrial fragmentation leads to intracellular acidification in Caenorhabditis elegans and mammalian cells

It is unclear how lesions in genes that regulate mitochondrial structural dynamics lead to disease. Here, tissue-specific biosensors expressed in the genetic model organism Caenorhabditis elegans are used to demonstrate that robust cellular acidification occurs when mitochondria are fragmented and may contribute to the etiology of morphology diseases in mammals.

Mitochondrial structural dynamics are regulated through the opposing processes of membrane fission and fusion, which are conserved from yeast to man. The chronic inhibition of mitochondrial fusion as a result of genetic mutation is the cause of human autosomal dominant optic atrophy (ADOA) and Charcot-Marie-Tooth syndrome type 2A (CMT-2A). Here, we demonstrate that genetic fragmentation of the mitochondrial network in Caenorhabditis elegans induces cellular acidification in a broad range of tissues from the intestine, to body wall muscles, and neurons. Genetic epistasis analyses demonstrate that fragmentation itself, and not the loss of a particular protein, leads to acidosis, and the worm's fitness matches the extent of acidification. We suggest that fragmentation may cause acidification through two distinct processes: oxidative signaling after the loss of the ability of the mitochondrial inner membrane to undergo fusion and lactic acidosis after the loss of outer membrane fusion. Finally, experiments in cultured mammalian cells demonstrate a conserved link between mitochondrial morphology and cell pH homeostasis. Taken together these data reveal a potential role for acidosis in the differing etiology of diseases associated with mitochondrial morphology defects such as ADOA and CMT-2A.

Arresting a Seizure by Dropping a Little Acid

Seizure Termination by Acidosis Depends on ASIC1a. Ziemann AE, Schnizler MK, Albert GW, Severson MA, Howard MA 3rd, Welsh MJ, Wemmie JA. Nat Neurosci 2008;11(7):816–822. Most seizures stop spontaneously; however, the molecular mechanisms that terminate seizures remain unknown. Observations that seizures reduced brain pH and that acidosis inhibited seizures indicate that acidosis halts epileptic activity. Because acid-sensing ion channel 1a (ASIC1a) is exquisitely sensitive to extracellular pH and regulates neuron excitability, we hypothesized that acidosis might activate ASIC1a, which would terminate seizures. Disrupting mouse ASIC1a increased the severity of chemoconvulsant-induced seizures, whereas overexpressing ASIC1a had the opposite effect. ASIC1a did not affect seizure threshold or onset, but shortened seizure duration and prevented seizure progression. CO2 inhalation, long known to lower brain pH and inhibit seizures, required ASIC1a to interrupt tonic-clonic seizures. Acidosis activated inhibitory interneurons through ASIC1a, suggesting that ASIC1a might limit seizures by increasing inhibitory tone. Our results identify ASIC1a as an important element in seizure termination when brain pH falls and suggest both a molecular mechanism for how the brain stops seizures and new therapeutic strategies.

Neuroprotective actions of PIKE-L by inhibition of SET proteolytic degradation by asparagine endopeptidase

Ischemia and seizure cause excessive neuronal excitation that is associated with brain acidosis and neuronal cell death. However, the molecular mechanism of acidification-triggered neuronal injury is incompletely understood. Here, we show that asparagine endopeptidase (AEP) is activated under acidic condition, cuts SET, an inhibitor of DNase, and triggers DNA damage in brain, which is inhibited by PIKE-L. SET, a substrate of caspases, was cleaved by acidic cytosolic extract independent of caspase activation. Fractionation of the acidic cellular extract yielded AEP that is required for SET cleavage. We found that kainate provoked AEP activation and SET cleavage at N175, triggering DNA nicking in wild-type, but not AEP null, mice. PIKE-L strongly bound SET and prevented its degradation by AEP, leading to resistance of neuronal cell death. Moreover, AEP also mediated stroke-provoked SET cleavage and cell death in brain. Thus, AEP might be one of the proteinases activated by acidosis triggering neuronal injury during neuroexcitotoxicity or ischemia.

Structural rearrangements of NR1/NR2A NMDA receptors during allosteric inhibition

Ionotropic glutamate receptor (iGluR) subunits contain a large N-terminal domain (NTD) that precedes the agonist-binding domain (ABD) and participates in subunit oligomerization. In NMDA receptors (NMDARs), the NTDs of NR2A and NR2B subunits also form binding sites for the endogenous inhibitor Zn(2+) ion. Although these allosteric sites have been characterized in detail, the molecular mechanisms by which the NTDs communicate with the rest of the receptor to promote its inhibition remain unknown. Here, we identify the ABD dimer interface as a major structural determinant that permits coupling between the NTDs and the channel gate. The strength of this interface also controls proton inhibition, another form of allosteric modulation of NMDARs. Conformational rearrangements at the ABD dimer interface thus appear to be a key mechanism conserved in all iGluR subfamilies, but have evolved to fulfill different functions: fast desensitization at AMPA and kainate receptors, allosteric inhibition at NMDARs.

Local extracellular acidification caused by Ca2+-dependent exocytosis in PC12 cells

Exocytosis of acidic synaptic vesicles may produce local extracellular acidification, but this effect has not been measured directly and its magnitude may depend on the geometry and pH-buffering capacity of both the vesicles and the extracellular space. Here we have used SNARF dye immobilized by conjugation to dextran to measure the release of protons from PC12 cells. The PC12 cells were stimulated by exposure to depolarizing K(+)-rich solution and activation was verified by fluorescence measurement of intracellular Ca(2+) and the release kinetics of GFP-labeled vesicles. Confocal imaging of the pH-dependent fluorescence from the immobile extracellular SNARF dye showed transient acidification around the cell bodies and neurites of activated PC12 cells. The local acidification was abolished when extracellular solution was devoid of Ca(2+) or strong pH-buffering was imposed with 10mM of HEPES. We conclude that the release of secretory vesicles induces local rises in proton concentrations that are co-released from synaptic vesicles with the primary neurotransmitter, and propose that the co-released protons may modulate the signaling in confined micro-domains of synapses.

Role of the plasma membrane calcium adenosine triphosphatase on domoate-induced intracellular acidification in primary cultures of cerebelar granule cells

Changes in intracellular pH (pH(i)) and cytosolic calcium concentration ([Ca(2+)](c)) caused by the glutamate agonist domoate (DOM) were studied in single cultured mouse cerebellar granule cells (CGC) by using the fluorescent probes 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester (BCECF-AM) and simultaneous evaluation of cytosolic calcium concentration with the fluorescent dye Fura-2 acetoxymethyl ester (Fura-2 AM). DOM caused a concentration-dependent increase in [Ca(2+)](c) and a concentration-dependent intracellular acidification of CGC. DOM-induced intracellular acidification was completely abolished by the use of Ca(2+)-free medium, suggesting that it was due mostly to an influx of extracellular calcium. The pH(i) decrease caused by DOM was also completely blocked in the presence of the AMPA/kainate receptor antagonist CNQX, indicating that the DOM-induced intracellular acidification was caused by DOM activation of the AMPA/kainate subtype of glutamate receptors. Different mechanisms that could be involved in DOM-induced pH(i) decrease, such as displacement of H(+) by Ca(2+) from a common intracellular binding site, DOM-induced alteration of pH(i) regulation mechanisms, and a possible acidification caused by DOM-induced increase of mitochondrial Ca(2+) uptake, were excluded. DOM-induced intracellular acidification was completely prevented by inhibitors of the plasma membrane calcium adenosine triphosphatase (ATPase) (PMCA), including orthovanadate, lanthanum extracellular pH of 8.5, and the specific PMCA inhibitor caloxin 2A1. Our results therefore indicate that PMCA is involved in DOM-induced intracellular acidification in primary cultures of CGC. Simultaneous recording of [Ca(2+)](c) and pH(i) indicates that the increase in intracellular calcium evoked by DOM will activate the calcium extrusion mechanisms through the calcium pump, which, in turn, will decrease intracellular pH by countertransport of H(+) ions.

Acid-sensing ionic channels in the rat vestibular endorgans and ganglia

Acid-sensing ionic channels (ASICs) are members of the epithelial Na+ channel/degenerin (ENaC/DEG) superfamily. ASICs are widely distributed in the central and peripheral nervous system. They have been implicated in synaptic transmission, pain perception, and the mechanoreception in peripheral tissues. Our objective was to characterize proton-gated currents mediated by ASICs and to determine their immunolocation in the rat vestibular periphery. Voltage clamp of cultured afferent neurons from P7 to P10 rats showed a proton-gated current with rapid activation and complete desensitization, which was carried almost exclusively by sodium ions. The current response to protons (H+) has a pH0.5 of 6.2. This current was reversibly decreased by amiloride, gadolinium, lead, acetylsalicylic acid, and enhanced by FMRFamide and zinc, and negatively modulated by raising the extracellular calcium concentration. Functional expression of the current was correlated with smaller-capacitance neurons. Acidification of the extracellular pH generated action potentials in vestibular neurons, suggesting a functional role of ASICs in their excitability. Immunoreactivity to ASIC1a and ASIC2a subunits was found in small vestibular ganglion neurons and afferent fibers that run throughout the macula utricle and crista stroma. ASIC2b, ASIC3, and ASIC4 were expressed to a lesser degree in vestibular ganglion neurons. The ASIC1b subunit was not detected in the vestibular endorgans. No acid-pH-sensitive currents or ASIC immunoreactivity was found in hair cells. Our results indicate that proton-gated current is carried through ASICs and that ionic current activated by H+ contributes to shape the vestibular afferent neurons' response to its synaptic input.

Stargazin modulates native AMPA receptor functional properties by two distinct mechanisms

AMPA receptors play a central role in basal excitatory synaptic transmission as well as synaptic maturation and plasticity. The transmembrane AMPA receptor regulatory protein (TARP) stargazin (gamma2) serves multiple roles in trafficking and stabilizing synaptic AMPA receptors and may be incorporated as an auxiliary subunit. We wanted to determine whether stargazin altered channel function of neuronal AMPA receptors. Transfection of cultured hippocampal neurons with stargazin produced two distinct effects on AMPA receptor functional properties: a sixfold reduction in glutamate-evoked desensitization and a twofold increase in the relative size of responses to the partial agonist kainate. Kinetic and dose-response analyses suggest that the effect of stargazin on glutamate desensitization results from an allosteric interaction that destabilizes the desensitized state of the receptor and that potentiation of kainate responses reflects increased efficacy rather than a change in affinity. These functional effects were also observed in human embryonic kidney 293 cells transfected with various heteromeric and homomeric AMPA receptors, with distinct subunit-dependent effects on glutamate desensitization, kainate efficacy, and trafficking. Two regions of stargazin mediate its functional effects: the C-terminal intracellular domain seems to be more important for effects on glutamate-evoked desensitization and receptor trafficking, whereas the first extracellular domain makes a larger contribution to effects on kainate efficacy. These data indicate that TARPs are involved both in trafficking and direct modulation of channel function and, as auxiliary subunits of neuronal AMPA receptors, must be considered in the functional heterogeneity of neuronal AMPA receptors.

Extracellular protons differentially potentiate the responses of native AMPA receptor subtypes regulating neurotransmitter release

1. The effects of pH changes on the basal and evoked release of [(3)H]noradrenaline ([(3)H]NA) and [(3)H]5-hydrohytryptamine ([(3)H]5-HT) from hippocampal synaptosomes and of [(3)H]dopamine ([(3)H]DA) and [(3)H]acetylcholine ([(3)H]ACh) from striatal and cortical synaptosomes were investigated in rat brain. 2. Changing pH between 6.4 and 8.0 did not affect the spontaneous release of the four [(3)H]neurotransmitters; alkalinization to pH 8.8 significantly enhanced release. Acidification to pH 6.4 augmented the AMPA-evoked overflows of [(3)H]NA, [(3)H]5-HT and [(3)H]DA, but not that of [(3)H]ACh. In contrast, lowering pH to 6.4 decreased the K(+)-evoked overflows of [(3)H]NA, [(3)H]5-HT, [(3)H]DA and [(3)H]ACh. 3. AMPA released transmitters in a Ca(2+)-dependent, exocytotic manner since its effects, at pH 7.4 or 6.4, were abolished by omitting external Ca(2+) or by depleting vesicular transmitter stores with bafilomycin A1. AMPA did not evoke carrier-mediated release because the uptake blockers nisoxetine, 6-nitroquipazine, GBR12909 and hemicholinium-3 could not inhibit the AMPA-induced release of [(3)H]NA, [(3)H]5-HT, [(3)H]DA and [(3)H]ACh. 4. Extraterminal acidification to pH 6.4 prevented the potentiating effect of cyclothiazide on the AMPA-evoked release of [(3)H]NA, [(3)H]DA and [(3)H]5-HT, whereas the proton-insensitive AMPA-evoked release of [(3)H]ACh, previously found to be cyclothiazide-insensitive at pH 7.4 was cyclothiazide-resistant also at pH 6.4. 5. To conclude, the cyclothiazide-sensitive AMPA receptors mediating release of NA, 5-HT and DA, but not the cyclothiazide-insensitive AMPA receptors mediating the release of ACh, become more responsive when external pH is lowered to pathophysiologically relevant values. The results with cyclothiazide suggest that H(+) ions may prevent desensitization of some AMPA receptor subtypes.

Calcium and pH homeostasis in neurons during hypoxia and ischemia

One of the important events during hypoxia or ischemia in the brain (or other organs for that matter, including the myocardium) is the accumulation of Ca2+ ions intracellularly. Although various studies have shown various sources of and routes for Ca2+ entry and accumulation, it is clear now that it is likely that there is a multitude rather than a single mechanism for this accumulation. In this review, we highlight this Ca2+ accumulation during low O2 states and discuss some of the mechanisms leading to accumulation for two main reasons: (a) an accumulation of Ca2+ in the cytosol has been proven to be deleterious for cell function although this accumulation of Ca2+ and consequences represent only a limited view of events that can lead to cell injury during such stress and (b) developing therapeutic strategies involving the reduction or elimination of this accumulation depends, by and large, on the mechanism of entry. In addition to reviewing some of these Ca2+ events, we will also review the relation between pH (H+) and Ca2+ since these two ions and their regulation are tied to each other in a major way. For example, extracellular acidosis, which can occur during ischemia, has a remarkable effect on the function of some of the Ca2+ entry routes.

Ephaptic interactions within a chemical synapse: hemichannel-mediated ephaptic inhibition in the retina

The two best-known types of cell-cell communication are chemical synapses and electrical synapses, which are formed by gap junctions. A third, less well known, form of communication is ephaptic transmission, in which electric fields generated by a specific neuron alter the excitability of neighboring neurons as a result of their anatomical and electrical proximity. Ephaptic communication can be present in a variety of forms, each with their specific features and functional implications. One of these is ephaptic modulation within a chemical synapse. This type of communication has recently been proposed for the cone-horizontal cell synapse in the vertebrate retina. Evidence indicates that the extracellular potential in the synaptic terminal of photoreceptors is modulated by current flowing through connexin hemichannels at the tips of the horizontal cell dendrites, mediating negative feedback from horizontal cells to cones. This example can be added to the growing list of cases of ephaptic communication in the central nervous system.

Lactate induced excitotoxicity in hippocampal slice cultures

During the initial minutes of cerebral ischemia, lactic acid accumulates and acidifies brain pH to 6.0-6.7. Glutamate is also released during ischemia that activates glutamate receptors and induces excitotoxicity. While glutamate excitotoxicity is well established to induce ischemic injury, a role of lactic acidosis in ischemic brain damage is poorly understood. This study analyzes acidosis neurotoxicity in hippocampal slice cultures in the presence or absence of lactate. At pH 6.7, neuronal loss was similar whether or not lactate was present. At pH 6.4, neuronal loss was significantly greater in the presence of lactate suggesting that lactate potentiates the acidosis toxicity. At pH 6.4 in the presence of lactate, NMDA or non-NMDA receptor antagonists reduced neuronal loss, while in the absence of lactate, NMDA or non-NMDA receptor antagonists had little effect. [3H]-Glutamate uptake was inhibited by acidic pH, and the amount of inhibition was significantly greater in the presence of lactate. These findings suggest that lactate plays a role in acidosis neurotoxicity by inducing excitotoxicity. Lactic acidosis and excitotoxicity have been previously thought to be independent events during ischemia. This study suggests that during ischemia, lactic acidosis contributes to excitotoxic neuronal loss.

Synaptic cleft acidification and modulation of short-term depression by exocytosed protons in retinal bipolar cells

The release of vesicular protons during exocytosis causes a feedback inhibition of Ca2+ channels in photoreceptor terminals; however, the effect of this inhibition on subsequent exocytosis has not been studied. Here we show that a similar L-type Ca2+ channel inhibition occurs in bipolar cell terminals in slices of goldfish retina, and we investigate the effect that this has on subsequent exocytosis with membrane capacitance measurements. We find that transient Ca2+ current inhibition is correlated with exocytosis and modulated by the concentration of extracellular pH buffer. Ca2+ current inhibition is negligible in acutely dissociated terminals, demonstrating the importance of an intact synaptic cleft. The sensitivity of bipolar cell Ca2+ currents to extracellular pH was assessed: channel conductance is reduced and activation is shifted to more positive potentials by acidification. The effect of Ca2+ current inhibition on subsequent exocytosis was investigated by measuring paired-pulse depression. Under conditions in which there is a large amount of inhibition of Ca2+ influx, the degree of paired-pulse depression is significantly reduced. Finally, we show that under physiological (bicarbonate) buffering conditions, pronounced Ca2+ current inhibition occurs after exocytosis ( approximately 60% peak inhibition), which can decrease subsequent exocytosis during single depolarizations. We estimate that exocytosis is accompanied by a transient change in synaptic cleft pH from 7.5 to approximately 6.9. We suggest that this effect serves as an activity-dependent modulator of exocytosis at ribbon-type synapses where a large and compact coterie of vesicles can fuse at each active zone.

pH changes in the invaginating synaptic cleft mediate feedback from horizontal cells to cone photoreceptors by modulating Ca2+ channels

Feedback from horizontal cells (HCs) to cone photoreceptors plays a key role in the center-surround–receptive field organization of retinal neurons. Recordings from cone photoreceptors in newt retinal slices were obtained by the whole-cell patch-clamp technique, using a superfusate containing a GABA antagonist (100 μM picrotoxin). Surround illumination of the receptive field increased the voltage-dependent calcium current (I_Ca) in the cones, and shifted the activation voltage of I_Ca to negative voltages. External alkalinization also increased cone I_Ca and shifted its activation voltage toward negative voltages. Enrichment of the pH buffering capacity of the extracellular solution increased cone I_Ca, and blocked any additional increase in cone I_Ca by surround illumination. Hyperpolarization of the HCs by a glutamate receptor antagonist-augmented cone I_Ca, whereas depolarization of the HCs by kainate suppressed cone I_Ca. From these results, we propose the hypothesis that pH changes in the synaptic clefts, which are intimately related to the membrane voltage of the HCs, mediate the feedback from the HCs to cone photoreceptors. The feedback mediated by pH changes in the synaptic cleft may serve as an additional mechanism for the center-surround organization of the receptive field in the outer retina.

Mechanisms of H+ modulation of glycinergic response in rat sacral dorsal commissural neurons

Many ionotropic receptors are modulated by extracellular H+. So far, few studies have directly addressed the role of such modulation at synapses. In the present study, we investigated the effects of changes in extracellular pH on glycinergic miniature inhibitory postsynaptic currents (mIPSCs) as well as glycine-evoked currents (IGly) in mechanically dissociated spinal neurons with native synaptic boutons preserved. H+ modulated both the mIPSCs and IGly biphasically, although it activated an amiloride-sensitive inward current by itself. Decreasing extracellular pH reversibly inhibited the amplitude of the mIPSCs and IGly, while increasing external pH reversibly potentiated these parameters. Blockade of acid-sensing ion channels (ASICs) with amiloride, the selective antagonist of ASICs, or decreasing intracellular pH did not alter the modulatory effect of H+ on either mIPSCs or IGly. H+ shifted the EC50 of the glycine concentration-response curve from 49.3 +/- 5.7 microM at external pH 7.4 to 131.5 +/- 8.1 microM at pH 5.5, without altering the Cl- selectivity of the glycine receptor (GlyR), the Hill coefficient and the maximal IGly, suggesting a competitive inhibition of IGly by H+. Both Zn2+ and H+ inhibited IGly. However, H+ induced no further inhibition of IGly in the presence of a saturating concentration of Zn2+. In addition, H+ significantly affected the kinetics of glycinergic mIPSCs and IGly. It is proposed that H+ and/or Zn2+ compete with glycine binding and inhibit the amplitude of glycinergic mIPSCs and IGly. Moreover, binding of H+ induces a global conformational change in GlyR, which closes the GlyR Cl- channel and results in the acceleration of the seeming desensitization of IGly as well as speeding up the decay time constant of glycinergic mIPSCs. However, the deprotonation rate is faster than the unbinding rate of glycine from the GlyR, leading to reactivation of the undesensitized GlyR after washout of agonist and the appearance of a rebound IGly. H+ also modulated the glycine cotransmitter, GABA-activated current (IGABA). Taken together, the results support a "conformational coupling" model for H+ modulation of the GlyR and suggest that H+ may act as a novel modulator for inhibitory neurotransmission in the mammalian spinal cord.

Desensitization of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors facilitates use-dependent inhibition by pentobarbital

Although the mechanisms underlying the use-dependent inhibition of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) by barbiturates are not well understood, it has generally been assumed to involve open channel block. We examined the properties of the inhibition of AMPARs by the barbiturate pentobarbital (PB) in acutely isolated and cultured hippocampal neurons. PB caused a use- and concentration-dependent inhibition (IC50 = 20.7 microM) of AMPAR-mediated currents evoked by kainate. Contrary to the properties of an open channel blocker, the inhibition by PB developed with double exponential kinetics was reduced under conditions that favor the open channel state of AMPARs and was independent of membrane voltage. In addition, the inhibition was reduced at basic pH, indicating that the uncharged form of PB is active at AMPARs. Preventing AMPAR desensitization with cyclothiazide reduced the potency of inhibition by PB and prevented its trapping after the removal of agonist. PB preferentially reduced the steady-state (IC50 = 92.8 microM), rather than peak (IC50 > 1 mM) component of responses evoked by glutamate and accelerated the onset of desensitization in a concentration-dependent manner. Miniature excitatory postsynaptic currents recorded from cultured hippocampal neurons, the time course of which is minimally influenced by desensitization, are not inhibited by PB. The sensitivity of AMPAR-mediated synaptic responses to inhibition by PB therefore depends on the contribution of desensitization to these events. Our results suggest that PB does not act as an open channel blocker of AMPARs. Rather, the sensitivity, use dependence, and trapping of inhibition by PB are determined by AMPARs desensitization.

Bimodal action of protons on ATP currents of rat PC12 cells

The mode of action of extracellular protons on ATP-gated P2X2 receptors remains controversial as either enhancement or depression of ATP-mediated currents has been reported. By investigating, at different pH, the electrophysiological effect of ATP on P2X2 receptors and complementing it with receptor modelling, the present study suggests a unified mechanism for both potentiation and inactivation of ATP receptors by protons. Our experiments on patch-clamped PC12 cells showed that, on the same cell, mild acidification potentiated currents induced by low ATP concentrations (<0.1 mM) and attenuated responses to high ATP concentrations (>1 mM) with emergence of current fading and rebound. To clarify the nature of the ATP/H+ interaction, we used the Ding and Sachs's "loop" receptor model which best describes the behavior of such receptors with two open states linked via one inactivated state. No effects by protons could be ascribed to H+-mediated open channel block. However, by assuming that protons facilitated binding of ATP to resting as well as open receptors, the model could closely replicate H+-induced potentiation of currents evoked by low ATP doses plus fading and rebound induced by high ATP doses. The latter phenomenon was due to receptor transition to the inactive state. The present data suggest that the high concentration of protons released with ATP (and catecholamines) from secretory vesicles may allow a dual action of H+ on P2X2 receptors. This condition might also occur on P2X2 receptors of central neurons exposed to low pH during ischemia.

Subunit-dependent modulation of kainate receptors by extracellular protons and polyamines

Synaptic activity causes significant fluctuations in proton concentrations in the brain. Changes in pH can affect neuronal excitability by acting on ligand-gated channels, including those gated by glutamate. We show here a subunit-dependent regulation of native and recombinant kainate receptors by physiologically relevant proton concentrations. The effect of protons on kainate receptors is voltage-independent and subunit dependent, with GluR5(Q), GluR6(Q), GluR6(R), and GluR6(R)/KA2 receptors being inhibited and GluR6(R)/KA1 receptors being potentiated. Mutation of two acidic residues (E396 and E397) to neutral amino acids significantly reduces the proton sensitivity of the GluR6(Q) receptor, suggesting that these residues influence proton inhibition. The endogenous polyamine spermine potentiated GluR6(R) kainate currents in a pH-dependent manner, producing an acidic shift in the IC(50) for proton inhibition. Spermine potentiation of GluR6(R) is voltage independent, does not affect receptor desensitization, and only slightly shifts the agonist affinity of the receptor. These results suggest that, similar to its action on NMDA receptors, spermine potentiates kainate receptors by relieving proton inhibition of the receptor. Furthermore, they suggest that fluctuations in brain pH during both normal and pathological processes could regulate synaptic transmission and plasticity mediated by kainate receptors.

External anions and cations distinguish between AMPA and kainate receptor gating mechanisms

Experiments were designed to examine if ion-flow through alpha-amino-3-hydroxy-5-methyl-isoxazolepropionic acid (AMPA) or kainate receptors interferes with protein structures associated with the gating machinery. Gating was studied using ultra-fast drug perfusion of outside-out patches containing rat GluR-A or GluR6 subunits excised from transfected human embryonic kidney cells. Deactivation rates of GluR6 kainate receptors observed following brief L-glutamate (10 mM Glu, 1 ms) applications differed by two to threefold in high (405 mM symmetrical Na(+), tau(decay) = 2.7 ms at -100 mV) and low ionic strength (55 mM, tau(decay) = 1.1 ms) solutions. In comparison, GluR-A AMPA receptors were much less sensitive. Ion effects on GluR6 receptors did not reflect surface potential screening or ion-agonist competition at the agonist-binding site since deactivation rates were slower in high ionic strength solutions. Moreover, the apparent agonist affinity did not decrease with increasing ionic strength (e.g. 55 mM, EC(50) = 110 microM vs. 405 mM, EC(50) = 61 microM). GluR6 responses were strongly dependent on ions present on the external, but not the internal, side of the plasma membrane. Decay kinetics was regulated by the type of ion present suggesting that the chemical nature of the solution, not its ionic strength, governed channel behaviour. Both external anions and cations modulated the amplitude and decay kinetics of GluR6 responses in a concomitant manner. AMPA receptor responses recorded in identical ionic conditions did not exhibit this behaviour. These results identify a novel mechanism that distinguishes AMPA and kainate receptors. External ions regulate the gating machinery of kainate receptors through an allosteric mechanism that involves both anions and cations.

Expression of AMPA/kainate receptors during development of chick embryo retina cells: in vitro versus in vivo studies

The activity and the subunit expression of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate ionotropic glutamate receptors were studied in retina cells developing in chick embryos and in retina cells cultured as retinospheroids, at the same stages of development. In the retinospheroids, the activity of the AMPA/kainate receptors was monitored by following the changes in the intracellular free calcium concentration ([Ca(2+)](i)), in response to AMPA, kainate or to L-glutamate, and the expression of the receptor subunits GluR1, GluR2/3, GluR4 and GluR6/7 was determined in the retinospheroids and in chick retinas by immunodetection using polyclonal antibodies. The changes in [Ca(2+)](i) in response to 400 microM kainate increased from 5h in vitro to 3 days, and remained constant until day 14, whereas the [Ca(2+)](i) in response to 500 microM L-glutamate or 400 microM AMPA increased from 5h in vitro to 3 days, and thereafter decreased slightly until day 14. The [Ca(2+)](i) responses to kainate are mainly due to AMPA receptor stimulation, since the signals were abolished by LY303070, the AMPA receptor antagonist, and were not affected by MK-801, the NMDA receptor antagonist. In retinospheroids, the levels of expression of GluR1 subunit increased from 5h in vitro until day 7, then decreased until day 14. The levels of expression of GluR2/3 and GluR4 subunits increased from 5h in vitro until day 10, and remained constant until day 14. The levels of kainate receptor subunits GluR6/7 increased from 5h in vitro until day 3, and thereafter decreased slightly until day 14. In the retinas, the expression of GluR1 and GluR6/7 subunits increased from day 8 until day 15, and then decreased until day 22 (post-natal 1). The subunits GluR2/3 and GluR4 increased from day 8 until day 18, and remained constant until day 22. The results suggest that AMPA/kainate receptors are expressed at early embryonic stages, although at low levels and before synapse formation (E12). However, the AMPA receptors are not completely functional at the first stage studied since they do not respond to the agonist AMPA. Also, the patterns of AMPA/kainate receptor subunit expression in retinospheroids of chick embryo retina cells cultured in vitro and in retina cells developing in the embryo (in vivo) were similar, indicating that the AMPA/kainate receptor subunits expression in these primary cultures mimics their expression in the developing chick retina.

Exocytosed protons feedback to suppress the Ca2+ current in mammalian cone photoreceptors

A proton pump acidifies synaptic vesicles and provides the electrochemical gradient for transmitter uptake. Although external protons can modulate membrane voltage- and ligand-gated conductances, the fate of the protons released when vesicles fuse with the plasma membrane is unclear. In the dark, the glutamate-laden vesicles of cone photoreceptors fuse continuously with the plasma membrane. I now show that vesicular protons feed back to block the nearby calcium channels that mediate release. This local proton-mediated feedback is a novel mechanism through which neurons may regulate the release of transmitter.