Influence of extracellular and intracellular pH on GABA-gated chloride conductance in crayfish muscle fibres

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.

Carbogen-Induced Respiratory Acidosis Blocks Experimental Seizures by a Direct and Specific Inhibition of NaV1.2 Channels in the Axon Initial Segment of Pyramidal Neurons

Brain pH is a critical factor for determining neuronal activity, with alkalosis increasing and acidosis reducing excitability. Acid shifts in brain pH through the breathing of carbogen (5% CO2/95% O2) reduces seizure susceptibility in animal models and patients. The molecular mechanisms underlying this seizure protection remain to be fully elucidated. Here, we demonstrate that male and female mice exposed to carbogen are fully protected from thermogenic-triggered seizures. Whole-cell patch-clamp recordings revealed that acid shifts in extracellular pH (pHo) significantly reduce action potential firing in CA1 pyramidal neurons but did not alter firing in hippocampal inhibitory interneurons. In real-time dynamic clamp experiments, acidification reduced simulated action potential firing generated in hybrid model neurons expressing the excitatory neuron predominant NaV1.2 channel. Conversely, acidification had no effect on action potential firing in hybrid model neurons expressing the interneuron predominant NaV1.1 channel. Furthermore, knockdown of Scn2a mRNA in vivo using antisense oligonucleotides reduced the protective effects of carbogen on seizure susceptibility. Both carbogen-mediated seizure protection and the reduction in CA1 pyramidal neuron action potential firing by low pHo were maintained in an Asic1a knock-out mouse ruling out this acid-sensing channel as the underlying molecular target. These data indicate that the acid-mediated reduction in excitatory neuron firing is mediated, at least in part, through the inhibition of NaV1.2 channels, whereas inhibitory neuron firing is unaffected. This reduction in pyramidal neuron excitability is the likely basis of seizure suppression caused by carbogen-mediated acidification.SIGNIFICANCE STATEMENT Brain pH has long been known to modulate neuronal excitability. Here, we confirm that brain acidification reduces seizure susceptibility in a mouse model of thermogenic seizures. Extracellular acidification reduced excitatory pyramidal neuron firing while having no effect on interneuron firing. Acidification also reduced dynamic clamp firing in cells expressing the NaV1.2 channel but not in cells expressing NaV1.1 channels. In vivo knockdown of Scn2a mRNA reduced seizure protection of acidification. In contrast, acid-mediated seizure protection was maintained in the Asic1a knock-out mouse. These data suggest NaV1.2 channel as an important target for acid-mediated seizure protection. Our results have implications on how natural variations in pH can modulate neuronal excitability and highlight potential antiseizure drug development strategies based on the NaV1.2 channel.

Blockade of TASK-1 Channel Improves the Efficacy of Levetiracetam in Chronically Epileptic Rats

Tandem of P domains in a weak inwardly rectifying K⁺ channel (TWIK)-related acid sensitive K⁺-1 channel (TASK-1) is an outwardly rectifying K⁺ channel that acts in response to extracellular pH. TASK-1 is upregulated in the astrocytes (particularly in the CA1 region) of the hippocampi of patients with temporal lobe epilepsy and chronically epilepsy rats. Since levetiracetam (LEV) is an effective inhibitor for carbonic anhydrase, which has a pivotal role in buffering of extracellular pH, it is likely that the anti-epileptic action of LEV may be relevant to TASK-1 inhibition, which remains to be elusive. In the present study, we found that LEV diminished the upregulated TASK-1 expression in the CA1 astrocytes of responders (whose seizure activities were responsive to LEV), but not non-responders (whose seizure activities were not controlled by LEV) in chronically epileptic rats. ML365 (a selective TASK-1 inhibitor) only reduced seizure duration in LEV non-responders, concomitant with astroglial TASK-1 downregulation. Furthermore, ML365 co-treatment with LEV decreased the duration, frequency and severity of spontaneous seizures in non-responders to LEV. To the best of our knowledge, our findings suggest, for the first time, that the up-regulation of TASK-1 expression in CA1 astrocytes may be involved in refractory seizures in response to LEV. This may be a potential target to improve responsiveness to LEV.

Electrophysiology of ionotropic GABA receptors

GABA_A receptors are ligand-gated chloride channels and ionotropic receptors of GABA, the main inhibitory neurotransmitter in vertebrates. In this review, we discuss the major and diverse roles GABA_A receptors play in the regulation of neuronal communication and the functioning of the brain. GABA_A receptors have complex electrophysiological properties that enable them to mediate different types of currents such as phasic and tonic inhibitory currents. Their activity is finely regulated by membrane voltage, phosphorylation and several ions. GABA_A receptors are pentameric and are assembled from a diverse set of subunits. They are subdivided into numerous subtypes, which differ widely in expression patterns, distribution and electrical activity. Substantial variations in macroscopic neural behavior can emerge from minor differences in structure and molecular activity between subtypes. Therefore, the diversity of GABA_A receptors widens the neuronal repertoire of responses to external signals and contributes to shaping the electrical activity of neurons and other cell types.

Seizures caused by brain tumors in children

PURPOSE

To review the epidemiology, clinical features, and treatment of seizures secondary to pediatric brain tumors.

METHOD

Literature review.

RESULTS

Pediatric brain tumors are the most common solid pediatric tumor and the most common cause of death in pediatric cancer. Seizures are one of the most common symptoms of pediatric brain tumors. Factors associated with increased risk of seizures include supratentorial location, gray matter involvement, low-grade, and certain histological features-especially dysembryoplastic neuroepithelial tumor, ganglioglioma, and oligodendroglioma. Leukemic infiltration of the brain, brain metastases of solid tumors, and brain injury secondary to chemotherapy or radiotherapy can also cause seizures. Mechanisms by which brain tumors cause seizures include metabolic, and neurotransmitter changes in peritumoral brain, morphologic changes - including malformation of cortical development - in peritumoral brain, and presence of peritumoral blood products, gliosis, and necrosis. As there is a high degree of uncertainty on how effective different antiepileptic drugs are for seizures caused by brain tumors, choices are often driven by the interaction and side effect profile. Classic antiepileptic drugs - phenobarbital, phenytoin, or carbamazepine - should be avoided as they may alter the metabolism of chemotherapeutic agents. Newer drugs - valproate, lamotrigine, topiramate, zonisamide, and levetiracetam - may be the preferred option in patients with tumors because of their very limited interaction with chemotherapy.

CONCLUSION

Seizures are a common presentation of pediatric brain tumors, especially in supratentorial tumors with gray matter involvement. Antiepileptic drug therapy is usually driven by the interaction and side effect profile and newer drugs with few interactions are generally preferred.

Review : Glial Neuronal Interactions: A Physiological Perspective

Throughout the nervous system, neurons are closely surrounded by glial cells, leaving only a 20-nm wide extracellular space filled with interstitial fluid. Ions, transmitters, hormones, nutrients, and waste products all share this narrow diffusion pathway. Because the interstitial space occupies only a small volume, neuronal activity can lead to appreciable changes in the extracellular concentration of ions, protons, and neurotrans mitters. These changes can affect neuronal activity and are believed to be influenced by glial cells. The proximity of glial processes to synapses and axons make glial cells ideal partners to sequester ions and transmitters released by neurons. The failure of glial cells to perform such essential homeostatic functions can have profound effects, and these homeostatic activities may constitute one way in which glial cells can influence neuronal signaling. In addition, glial cells, which, unlike most neurons, are coupled to each other through gap-junctions, communicate with each other and possibly also with adjacent neurons through prop agated intracellular Ca2+waves. The importance of such interglial signaling is not understood. Additionally, glial cells and neurons mutually modulate their expression of ion channels, most likely through factors re leased into the extracellular space. The range of responses observed in glial cells and their intimate anatomical relationship with neurons suggest a broader role for glia than is currently appreciated. It also emphasizes the importance of a better understanding of glial-neuronal interactions to an understanding of brain function. The Neuroscientist 1:328-337, 1995

Epilepsy in glioma patients: mechanisms, management, and impact of anticonvulsant therapy

Seizures are a well-recognized symptom of primary brain tumors, and anticonvulsant use is common. This paper provides an overview of epilepsy and the use of anticonvulsants in glioma patients. Overall incidence and mechanisms of epileptogenesis are reviewed. Factors to consider with the use of antiepileptic drugs (AEDs) including incidence during the disease trajectory and prophylaxis along with considerations in the selection of anticonvulsant use (ie, potential side effects, drug interactions, adverse effects, and impact on survival) are also reviewed. Finally, areas for future research and exploring the pathophysiology and use of AEDs in this population are also discussed.

Fluorescent ratiometric pH indicator SypHer2: Applications in neuroscience and regenerative biology

BACKGROUND

SypHer is a genetically encoded fluorescent pH-indicator with a ratiometric readout, suitable for measuring fast intracellular pH shifts. However, the relatively low brightness of the indicator limits its use.

METHODS

Here we designed a new version of pH-sensor called SypHer-2, which has up to three times brighter fluorescence in cultured mammalian cells compared to the SypHer.

RESULTS

Using the new indicator we registered activity-associated pH oscillations in neuronal cell culture. We observed prominent transient neuronal cytoplasm acidification that occurs in parallel with calcium entry. Furthermore, we monitored pH in presynaptic and postsynaptic termini by targeting SypHer-2 directly to these compartments and revealed marked differences in pH dynamics between synaptic boutons and dendritic spines. Finally, we were able to reveal for the first time the intracellular pH drop that occurs within an extended region of the amputated tail of the Xenopus laevis tadpole before it begins to regenerate.

CONCLUSIONS

SypHer2 is suitable for quantitative monitoring of pH in biological systems of different scales, from small cellular subcompartments to animal tissues in vivo.

GENERAL SIGNIFICANCE

The new pH-sensor will help to investigate pH-dependent processes in both in vitro and in vivo studies.

Extracellular pH modulates GABAergic neurotransmission in rat hypothalamus

Changes in extracellular pH have a modulatory effect on GABAA receptor function. It has been reported that pH sensitivity of the GABA receptor is dependent on subunit composition and GABA concentration. Most of previous investigations focused on GABA-evoked currents, which only reflect the postsynaptic receptors. The physiological relevance of pH modulation of GABAergic neurotransmission is not fully elucidated. In the present studies, we examined the influence of extracellular pH on the GABAA receptor-mediated inhibitory neurotransmission in rat hypothalamic neurons. The inhibitory postsynaptic currents (IPSCs), tonic currents, and the GABA-evoked currents were recorded with whole-cell patch techniques on the hypothalamic slices from Sprague-Dawley rats at 15-26 postnatal days. The amplitude and frequency of spontaneous GABA IPSCs were significantly increased while the external pH was changed from 7.3 to 8.4. In the acidic pH (6.4), the spontaneous GABA IPSCs were reduced in amplitude and frequency. The pH induced changes in miniature GABA IPSCs (mIPSCs) similar to that in spontaneous IPSCs. The pH effect on the postsynaptic GABA receptors was assessed with exogenously applied varying concentrations of GABA. The tonic currents and the currents evoked by sub-saturating concentration of GABA ([GABA]) (10 μM) were inhibited by acidic pH and potentiated by alkaline pH. In contrast, the currents evoked by saturating [GABA] (1mM) were not affected by pH changes. We also investigated the influence of pH buffers and buffering capacity on pH sensitivity of GABAA receptors on human recombinant α1β2γ2 GABAA receptors stably expressed in HEK 293 cells. The pH influence on GABAA receptors was similar in HEPES- and MES-buffered media, and not dependent on protonated buffers, suggesting that the observed pH effect on GABA response is a specific consequence of changes in extracellular protons. Our data suggest that the hydrogen ions suppress the GABAergic neurotransmission, which is mediated by both presynaptic and postsynaptic mechanisms.

The paradox of the paroxysm: can seizure precipitants help explain human ictogenesis?

An epileptic brain is permanently in a diseased state, but seizures occur rarely and without warning. Here we examine this paradox, common to paroxysmal diseases. We review the problem in the context of the prototypic acquired epilepsies of the medial temporal lobe. We ask how an epileptic temporal lobe differs from a healthy one and examine biological mechanisms that may explain the transition to seizure. Attempts to predict seizure timing from analyses of brain electrical activity suggest that the neurological processes involved may be initiated significantly before a seizure. Furthermore, whereas seizures are said to occur without warning, some patients say they know when a seizure is imminent. Several factors, including sleep deprivation, oscillations in hormonal levels, or withdrawal from drugs, increase the probability of a seizure. We ask whether these seizure precipitants might act through common neuronal mechanisms. Several precipitating factors seem to involve relief from a neurosteroid modulation of gamma-amino butyric acid receptor type A (GABAA) receptors. We propose tests of this hypothesis.

Neonatal exposure to high concentration of carbon dioxide produces persistent learning deficits with impaired hippocampal synaptic plasticity

Although respiratory complications with blood gas abnormalities contribute significantly to neurodevelopment in the immature brain, little is known about the mechanisms via which blood gas abnormalities, such as hypoxic hypercapnia, impair neurocognitive outcomes. To investigate the possible long-term consequences of neonatal exposure to hypoxic hypercapnia regarding learning ability, we investigated the effect of neonatal hypoxic hypercapnia on later functions in the hippocampus, which is a structure that has been implicated in many learning and memory processes. Neonatal rat pups (postnatal day 7; P7) were exposed to a high concentration of carbon dioxide (CO2; 13%) for 2 or 4h. Exposure to CO2 in P7 rat pups caused blood gas abnormalities, including hypercapnia, hypoxia, and acidosis, and disrupted later learning acquisition, as assessed in 10-week-old adult rats subjected to a Morris water maze test. Induction of long-term potentiation (LTP) in the synapses of the hippocampal CA1 area was also impaired, whereas the paired-pulse responses of population spikes exhibited a significant increase, in CO2-exposed rats, suggesting decreased recurrent inhibition in the hippocampus. Such long-lasting modifications in hippocampal synaptic plasticity may contribute to the learning impairments associated with perinatal hypoxic hypercapnia and acidosis.

Long-lasting effects of neonatal pentobarbital administration on spatial learning and hippocampal synaptic plasticity

Exposure of newborn rats to antiepileptics such as barbiturates has long-lasting detrimental effects on the hippocampus and hippocampus-dependent behavior. However, the long-term consequences of neonatal administration with barbiturates on the hippocampal synaptic plasticity remain unresolved. In this study, we investigated the long-lasting effects of a neonatal administration of pentobarbital on spatial memory, paired-pulse plasticity in the population spikes, and long-term potentiation (LTP) in the hippocampal CA1 region of rats in vivo. Eight weeks after administration of pentobarbital (10 or 20mg/kg) on the seventh postnatal day (P7), rats showed impaired induction in LTP. During paired-pulse stimulation, pentobarbital-treated rats exhibited a greater facilitation of the test pulse population spike, suggesting a disruption in the inhibitory GABAergic synaptic transmission. Spatial learning in hidden platform task of the Morris water maze was impaired in pentobarbital-treated rats. Our present findings indicate that neonatal treatment with pentobarbital causes alterations in function of the hippocampal inhibitory synaptic transmission that persist into adulthood, likely contributing to the long-lasting abnormalities in the hippocampal LTP as well as learning ability. We also demonstrated significant respiratory disturbances, i.e., severe hypoxia, hypercapnia, and extracellular acidosis, in rats treated with pentobarbital on P7. Given that extracellular acidosis can also modulate synaptic transmission in the developing hippocampus, this finding led us to speculate regarding the influence of respiratory disturbances in pentobarbital-induced long-lasting hippocampal dysfunctions.

Depolarizing effect of neocortical chandelier neurons

Chandelier (or axo-axonic) cells are one of the most distinctive types of GABAergic interneurons in the cortex. Although they have traditionally been considered inhibitory neurons, data from rat and human neocortical preparations suggest that chandelier cells have a depolarizing effect on pyramidal neurons at resting membrane potential, and could even activate synaptic chains of neurons. At the same time, recent results from rat hippocampal chandeliers indicate a predominantly inhibitory effect on their postsynaptic targets. To better understand the function of chandelier neurons, we generated Nkx2.1Cre MADM mice, a strain of genetically engineered animals that, by expressing GFP in a subset of neocortical interneurons, enable the identification and targeting of chandelier cells in living brain slices. Using these mice, we characterized the basic electrophysiological properties of a homogeneous population of chandelier neurons from upper layers of somatosensory cortical slices. These chandelier cells have characteristic axon cartridges and stereotypical electrophysiological features, distinguishable from basket cells. To investigate the effect of chandelier cells on target neurons, we performed paired recordings from chandeliers and postsynaptic pyramidal cells. In both perforated patch and cell-attached configurations, chandelier PSPs have in every case a reversal potential that is depolarized from rest. Our results support the idea that chandelier cells depolarize pyramidal neurons and could potentially have an excitatory effect on the network at rest.

Adjunctive topiramate enhances the risk of hypothermia associated with valproic acid therapy

BACKGROUND

Topiramate was approved for the treatment of epilepsy in 1999 and has since been approved for the prevention of migraine headache. It is structurally different from the majority of antiepileptic medications and is pharmacodynamically unique in its ability to inhibit the enzyme carbonic anhydrase. Postmarketing reports of topiramate-associated hypothermia have occurred but this adverse event has not been well characterized. Data mining of an adverse event database was used to assist in the identification of hypothermia.

OBJECTIVE

We sought to explore a possible association between the concomitant use of topiramate and valproic acid and the induction of hypothermia.

METHODS

This was a pharmacovigilance case series survey of spontaneous hypothermia, a reported adverse event in patients treated with topiramate and valproic acid, alone and in combination. The U.S. Food and Drug Administration's Adverse Events Reporting System (AERS) database was searched for reports of hypothermia in association with the use of topiramate. A data mining algorithm was used on the AERS to identify scores for hypothermia associated with antiepileptic drugs.

RESULTS

We identified 22 unduplicated reports of hypothermia in patients exposed to topiramate. Three of the 22 were confounded by patient overdoses with multiple drugs and not considered. Use of more than one antiepileptic drug was reported in most of the remaining 19 reports. Of these 19 reports, valproic acid was mentioned in 7. Two of the 19 reports mentioned topiramate only. Eleven of the 19 patients were men. The median age of the 19 patients was 40 years (range, 3(1/2)-82 years). Body temperatures ranged from 29.5 degrees C (moderate hypothermia) to 35 degrees C (mild hypothermia) with a median of 34 degrees C. Eleven of 18 reports of hypothermia occurred during the cooler months (one report did not indicate the time of year in which hypothermia occurred). Comorbid conditions included hypothyroidism in six reports, five in patients who received valproic acid concomitantly with topiramate and five reports of hyperammonemia in similarly treated patients. Data mining scores (empirical Bayes geometric mean) for antiepileptic drugs ranged from a high of 5.845 for phenobarbital to 2.956 for gabapentin. Hypothermia was reported 4.7 times more frequently when topiramate was used than was statistically expected.

CONCLUSION

We have found hypothermia, defined as an unintentional drop in body core temperature to <35 degrees C, to be associated with concomitant administration of topiramate (a carbonic anhydrase inhibitor) and valproic acid in patients who have tolerated either drug alone. Data mining analysis for topiramate showed a signal of hypothermia. Topiramate was reported 4.72 times more frequently in the database than would be statistically expected when considering all other drugs. Topiramate may act pharmacodynamically to potentiate the effects of valproic acid as a result of its ability to decrease blood HCO(3) (-) and increase blood ammonia levels.

Temperature and acid-base balance in the American lobster Homarus americanus

Lobsters (Homarus americanus) in the wild inhabit ocean waters where temperature can vary over a broad range (0-25 degrees C). To examine how environmental thermal variability might affect lobster physiology, we examine the effects of temperature and thermal change on the acid-base status of the lobster hemolymph. Total CO(2), pH, P(CO)2 and HCO(-)(3) were measured in hemolymph sampled from lobsters acclimated to temperature in the laboratory as well as from lobsters acclimated to seasonal temperatures in the wild. Our results demonstrate that the change in hemolymph pH as a function of temperature follows the rule of constant relative alkalinity in lobsters acclimated to temperature over a period of weeks. However, thermal change can alter lobster acid-base status over a time course of minutes. Acute increases in temperature trigger a respiratory compensated metabolic acidosis of the hemolymph. Both the strength and frequency of the lobster heartbeat in vitro are modulated by changes in pH within the physiological range measured in vivo. These observations suggest that changes in acid-base status triggered by thermal variations in the environment might modulate lobster cardiac performance in vivo.

A Novel Chloride Channel in Drosophila melanogaster Is Inhibited by Protons

A systematic analysis of the Drosophila genome data reveals the existence of pHCl, a novel member of ligand-gated ion channel subunits. pHCl shows nearly identical similarity to glutamate-, glycine-, and histamine-gated ion channels, does however not belong to any of these ion channel types. We identified three different sites, where splicing generates multiple transcripts of the pHCl mRNA. The pHCl is expressed in Drosophila embryo, larvae, pupae, and the adult fly. In embryos, in situ hybridization detected pHCl in the neural cord and the hindgut. Functional expression of the three different splice variants of pHCl in oocytes of Xenopus laevis and Sf9 cells induces a chloride current with a linear current-voltage relationship that is inhibited by extracellular protons and activated by avermectins in a pH-dependent manner. Further, currents through pHCl channels were induced by a raise in temperature. Our data give genetic and electrophysiological evidence that pHCl is a member of a new branch of ligand-gated ion channels in invertebrates with, however, a hitherto unique combination of pharmacological and biophysical properties.

Molecular Basis for Modulation of Recombinant α1β2γ2 GABAA Receptors by Protons

We have previously shown that extracellular protons inhibit recombinant and native GABAA receptors. In this report, we studied the site(s) and mechanism by which protons modulate the GABAA receptor. Whole cell GABA-activated currents were recorded from human embryonic kidney (HEK) 293 cells expressing recombinant α1β2γ2 GABAA receptors. Protons competitively inhibited the response to GABA and bicuculline. In contrast, change in pH did not influence direct gating of the channel by pentobarbital, and it did not influence spontaneous channel openings in α1(L264T)β2γ2 receptors, suggesting pH does not modulate channel activity by affecting the channel gating process directly. To test the hypothesis that protons modulate GABAA receptors at the ligand binding site, we systemically mutated N-terminal residues known to be involved in GABA binding and assessed effects of pH on these mutant receptors. Site-specific mutation of β2 Y205 to F or α1 F64 to A, both of which are known to influence GABA binding, significantly reduced pH sensitivity of the GABA response. These mutations did not affect Zn2+ sensitivity, suggesting that H+ and Zn2+ do not share a common site of action. Additional experiments further tested this possibility. Treatment with the histidine-modifying reagent diethylpyrocarbonate (DEPC) reduced Zn2+-mediated inhibition of GABAA receptors but had no effect on proton-induced inhibition of GABA currents. In addition, mutation of residues known to be involved in Zn2+ modulation had no effect on pH modulation of GABAA receptors. Our results support the hypothesis that protons inhibit GABAA receptor function by direct or allosteric interaction with the GABA binding site. In addition, the sites of action of H+ and Zn2+ in GABAA receptors are distinct.

Proton modulation of alpha 1 beta 3 delta GABAA receptor channel gating and desensitization

Alphabetagamma GABA(A) receptor currents are phasic and desensitizing, whereas alphabetadelta GABA(A) receptor currents are tonic and have no fast desensitization. alphabetagamma receptors are subsynaptic and mediate phasic inhibition, whereas alphabetadelta receptors are extra- or perisynaptic and mediate tonic inhibition. Given the different roles of these GABA(A) receptor isoforms and the fact that GABA(A) receptors are allosterically regulated by extracellular pH in a subunit-dependent manner, we compared the effects of changing pH on rat delta or gamma2L subunit-containing GABA(A) receptor currents. Human embryonic kidney cells (HEK293T) were transfected with cDNAs encoding rat alpha1, beta3, gamma2L, or delta GABA(A) receptor subunits in several binary and ternary combinations, and whole cell and single channel patch-clamp recordings were obtained. Lowering pH substantially enhanced alpha1beta3 receptor currents. This effect was significantly more pronounced for ternary alpha1beta3delta receptors, whereas ternary alpha1beta3gamma2L receptors were relatively insensitive to lowered pH. Lowering pH did not affect the extent of desensitization of alpha1beta3 and alpha1beta3gamma2L receptor currents, but significantly increased the extent of desensitization of alpha1beta3delta receptor currents. Lowering pH prolonged deactivation of alpha1beta3 and alpha1beta3delta receptor currents and enhanced the "steady-state" currents of alpha1beta3delta receptors evoked by long-duration (28 s) GABA applications. Lowering pH significantly increased mean open duration of alpha1beta3delta steady-state single channel currents due to introduction of a longer-duration open state, suggesting that low pH enhances alpha1beta3delta receptor steady-state currents by modifying GABA(A) receptor gating properties.

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.

Brain tumor and seizures: pathophysiology and its implications for treatment revisited

Seizures affect approximately 50% of patients with primary and metastatic brain tumors. Partial seizures have the highest incidence, followed by secondarily generalized, depending on histologic subtype, location, and tumor extent. The underlying pathophysiologic mechanisms of tumor-associated seizures are poorly understood and include theories of altered peritumoral amino acids, regional metabolism, pH, neuronal or glial enzyme and protein expression, as well as immunologic activity. An involvement of changed distribution and function of N-methyl-d-aspartate subclass of glutamate receptors also has been suggested. The often unpredictable responses to seizures after surgical tumor removal add substantial evidence that multiple factors are involved. The therapy of tumor-related seizures is far from perfect. Several factors contribute to these treatment difficulties, such as tumor growth and drug interactions; however, one of the main reasons for poor seizure control may result from the insufficient or even absent influence of the currently available antiepileptic drugs (AEDs) on most of the pathophysiologic mechanisms of tumor-related seizures. Studies are needed to elucidate more clearly the pathophysiologic mechanisms of tumor-related seizures and to identify and develop the optimal AEDs.

Effects of glial toxins on extracellular acidification in the hippocampal CA1 region in vivo

In the pyramidal cell layer of the CA1 region of the hippocampus in the urethane-anesthetized adult rat, there is an initial alkalinization followed by an acidification in response to synchronized seizure activity induced by stimulus trains. In this study, the role of astrocytes in these extracellular pH changes during neuronal activity was examined using local injection of two relatively selective glial toxins (fluorocitrate (FC) and fluoroacetate (FA)) into the CA1 cell layer. Both glial toxins reduced the peak level of acidification reached after 20 Hz stimulus trains to the contralateral CA3 region, without changing the lengthening of the afterdischarge, when compared to animals that had received a local injection of vehicle. After administration of either glial toxin, the peak level of acidification still correlated with the total discharge duration, but the levels of acidification were consistently lower than in control animals. Administration of either glial toxin had no effect on the peak alkalinization during the stimulus train, or on the rate of recovery from peak level of acidification. Injection of either vehicle, FA, or FC had no effect on the amplitude or frequency of the neuronal discharge during the afterdischarge. The results suggest that, in normal conditions, astrocytes contribute to the acidification of the extracellular space that occurs in response to intense neuronal activity. This acidification may contribute to feedback regulation of neuronal excitability.

Effects of antiepileptic drugs on extracellular pH regulation in the hippocampal CA1 region in vivo

Intracellular and extracellular pH are known to influence neuronal activity and may play a role in seizure termination. In the pyramidal cell layer of the CA1 region of the hippocampus in the urethane anesthetized adult rat, there is an initial alkalinization in response to stimulus trains administered to the contralateral CA3 region. This is followed by an acidification that peaks after termination of the afterdischarge. Initial experiments demonstrated that the peak level of acidification correlated with the duration of the afterdischarge, but that the peak level of alkalinization did not. The effects of several antiepileptic drugs on the initial alkalinization were determined. Systemic administration of acetazolamide (50 mg/kg, n=4) and topiramate (45 mg/kg, n=7) and local administration of benzolamide (n=3), all of which inhibit carbonic anhydrase, decreased the initial alkalinization that occurs during the stimulus train. Diazepam (3 mg/kg, n=5) and phenobarbital (60 mg/kg, n=6), agonists at the GABA(A) receptor complex, increased the initial alkalinization, while sodium channel blockers phenytoin (80 mg/kg, n=5) and carbamazepine (50 mg/kg, n=5) had no significant effect. The data suggest that the alkalinization in CA1 in vivo is predominantly regulated through activity of the GABA(A) receptor, rather than through activation of glutamatergic receptors. The change in alkalinization does not appear to be related to the mechanism of the antiepileptic effect of the drugs that were tested.

Neuronal expression of an FMRFamide-gated Na+ channel and its modulation by acid pH

The molluscan Phe-Met-Arg-Phe-amide (FMRFamide)-gated sodium channels (FaNaCs) show both structural and functional similarities to the mammalian acid-sensing ion channels (ASICs). Both channel types are related to the epithelial sodium channels and, although the neuropeptide FMRFamide directly gates the FaNaCs, it also modulates the proton-gating properties of ASICs. It is not yet known whether protons can alter the gating properties of the FaNaCs. We chose to examine this possibility at a site of FaNaC expression in the nervous system of the mollusk Lymnaea stagnalis. We cloned a putative L. stagnalis FaNaC (LsFaNaC) that exhibited a high degree of sequence identity to the Helix aspersa FaNaC (HaFaNaC, 60%), and a weaker homology to the ASICs (ASIC3, 22%). In situ hybridization was used to map the LsFaNaC expression pattern in the brain and to identify the right pedal giant1 (RPeD1) neuron as a site where the properties of the endogenous channel could be studied. In RPeD1 neurons isolated in culture, we demonstrated the presence of an FMRFamide-gated sodium current with features expected for a FaNaC: amiloride sensitivity, sodium selectivity, specificity for FMRFamide and Phe-Leu-Arg-Phe-amide (FLRFamide), and no dependency on G-protein coupling. The sodium current also exhibited rapid desensitization in response to repeated FMRFamide applications. Lowering of the pH of the bathing solution reduced the amplitude of the FMRFamide-gated inward current, while also activating an additional sustained weak inward current that was apparently not mediated by the FaNaC. Acidification also prevented the desensitization of the FMRFamide-induced inward current. The acid sensitivity of LsFaNaC is consistent with the hypothesis that FaNaCs share a common ancestry with the ASICs.

Modulation by extracellular pH of low- and high-voltage-activated calcium currents of rat thalamic relay neurons

The effects of changes in the extracellular pH (pH(o)) on low-voltage- (LVA) and high-voltage- (HVA) activated calcium currents of acutely isolated relay neurons of the ventrobasal thalamic complex (VB) were examined using the whole cell patch-clamp technique. Modest extracellular alkalinization (pH 7.3 to 7.7) reversibly enlarged LVA calcium currents by 18.6 +/- 3.2% (mean +/- SE, n = 6), whereas extracellular acidification (pH 7.3 to 6.9) decreased the current by 24.8 +/- 3.1% (n = 9). Normalized current amplitudes (I/I(7.3)) fitted as a function of pH(o) revealed an apparent pK(a) of 6.9. Both, half-maximal activation voltage and steady-state inactivation were significantly shifted to more negative voltages by 2-4 mV on extracellular alkalinization and to more positive voltages by 2-3 mV on extracellular acidification, respectively. Recovery from inactivation of LVA calcium currents was not significantly affected by changes in pH(o). In contrast, HVA calcium currents were less sensitive to changes in pH(o). Although extracellular alkalinization increased maximal HVA current by 6.0 +/- 2.0% (n = 7) and extracellular acidification decreased it by 11.9 +/- 0.02% (n = 11), both activation and steady-state inactivation were only marginally affected by the moderate changes in pH(o) used in the present study. The results show that calcium currents of thalamic relay neurons exhibit different pH(o) sensitivity. Therefore activity-related extracellular pH transients might selectively modulate certain aspects of the electrogenic behavior of thalamic relay neurons.

Influence of an extracellular acidosis on excitatory synaptic transmission and long-term potentiation in the CA1 region of rat hippocampal slices

The effects of extracellular acidification on the synaptic function and neuronal excitability were investigated on the hippocampal CA1 neurons. A decrease of extracellular pH from 7.4 to 6.7 did not alter either the resting membrane potential or the neuronal membrane input resistance. Extracellularly recorded field excitatory postsynaptic potentials (fEPSPs) and population spikes (PSs) were significantly reduced by acidosis. Additionally, the amplitude of presynaptic fiber volley was also reduced. The sensitivity of postsynaptic neurons to N-methyl-D-aspartate, but not to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, was depressed by acidosis. Lowering of extracellular pH did not significantly affect the magnitude of paired-pulse facilitation (PPF) of synaptic transmission. Acidosis also reversibly limited the sustained repetitive firing (RF) of Na(+)-dependent action potentials elicited by injection of depolarizing current pulses into the pyramidal cells. The limitation of RF by extracellular acidification was accompanied by the reduction of the maximal rate of rise (;V(max)) of the action potentials and the amplitude of afterhyperpolarization. Neither the Na (+)/H (+) antiporter blocker 5-(N -ethyl -N -isopropyl)-amiloride nor the selective adenosine A (1) receptor antagonist 1,3-dipropyl -8-cyclopentylxanthine, however, affected the acidosis -induced synaptic depression. It was also found that acidosis did not affect either the induction r maintenance of long -term potentiation (LTP) at Schaffer collateral -CA 1 synapses. These results suggest that the extracellular acidosis -induced synaptic depression is likely to result from an inhibition of presynaptic Na (+) conductance, thereby decreasing the amplitude of action potentials in individual afferent fibers or the number of afferent fiber activation to stimuli and then indirectly affecting the signaling processes contributing to trigger neurotransmitter release.

Regulation of extracellular pH in the developing hippocampus

The increased propensity of the immature brain to have seizures may be due, in part, to an inability to regulate the extracellular ionic environment. Here, we tested the regulation of extracellular pH in the developing hippocampus during and after intense neuronal activity induced by 20 Hz electrical stimulation in vivo. In CA1, stimulus trains to the CA3 region elicited an early alkaline shift associated with a later slow acid shift in all ages tested (2 weeks old, 3 weeks old and adult). In the dentate gyrus in adults, stimulation only elicited acidification. An initial alkalinization was only observed in the dentate gyrus in 2-week-old animals. The rate of recovery of the extracellular pH after termination of the stimulation was slower in the younger animals compared to adults in both CA1 and the dentate gyrus. The results indicate that the extracellular pH is regulated by mechanisms that undergo developmental changes that parallel the development of the dentate gyrus and development of the regulation of extracellular potassium.

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.

Different sensitivities of human and rat rho(1) GABA receptors to extracellular pH

We have examined the sensitivity of human and rat homo-oligomeric rho(1) GABA receptors to variations in extracellular pH (pH(o)) using the whole-cell patch clamp technique. The GABA-induced conductance mediated by the rat rho(1) receptor (rho(1)-R) decreased with a decrease in pH(o) between 9.0 to 5.4. Below pH(o) 7.4 the effect of protons on the GABA-induced conductance was apparently competitive, but above pH(o) 7.4 the inhibitory effect of extracellular protons was almost independent on the GABA concentration. Titration of the GABA-induced conductance at 3 microM GABA revealed two protonation sites on rat rho(1)-R with pKa 6.4 and pKa 8.2. At 10 microM GABA the low pKa (6.4) was shifted to a clearly lower value (5.6), but the high pKa was only slightly decreased (from 8.2 to 7.9). Zn(2+) ions were capable of relieving the proton inhibition at low pH(o) indicating that Zn(2+) interacts with the low pKa site. Unlike the rat rho(1)-R, the human rho(1)-R was sensitive only to changes in pH(o) at acidic levels. Proton inhibition of human rho(1)-R was apparently competitive, as observed on rat-rho(1) at acidic pH(o). Titration of the human rho(1)-R gave a single H(+) binding site with a pKa of 6.3, similar to the value for the low pKa on rat rho(1)-R. The pKa value of human rho(1)-R was not dependent on the GABA concentration. A chimeric receptor, consisting of the N-terminal part of the rat rho(1)-R and C-terminal part of the human rho(1)-R, displayed pH(o) sensitivity similar to that observed for rat rho(1)-R. This indicates that the high pKa of rat rho(1)-R is attributable to the 11 amino acid differences between the rat and human rho(1)-R extracellular domains.

Effect of extracellular pH on GABA-activated current in rat recombinant receptors and thin hypothalamic slices

We studied the effects of extracellular pH (pHo) on gamma-aminobutyric acid (GABA)-mediated Cl- current in rat hypothalamic neurons and recombinant type-A GABA (GABA(A)) receptors stably expressed in human embryonic kidney cells (HEK 293), using whole cell and outside-out patch-clamp recordings. In alpha3beta2gamma2s receptors, acidic pH decreased, whereas alkaline pH increased the response to GABA in a reversible and concentration-dependent manner. Acidification shifted the GABA concentration-response curve to the right, significantly increasing the EC50 for GABA without appreciably changing the slope or maximal current induced by GABA. We obtained similar effects of pH in alpha1beta2gamma2 receptors and in GABA-activated currents recorded from thin hypothalamic brain slices. In outside-out patches recorded from alpha3beta2gamma2 recombinant receptors, membrane patches were exposed to 5 microM GABA at control (7.3), acidic (6.4), or alkaline (8.4) pH. GABA activated main and subconductance states of 24 and 16 pS, respectively, in alpha3beta2gamma2 receptors. Alkaline pH(o) increased channel opening frequency and decreased the duration of the long closed state, resulting in an increase in open probability (from 0.0801 +/- 0.015 in pH 7.3 to 0.138 +/- 0.02 in pH 8.4). Exposure of the channels to acidic pH(o) had the opposite effects on open probability (decreased to 0.006 +/- 0.0001). Taken together, our results indicate that the function of GABA(A) receptors is modulated by extracellular pH. The proton effect is similar in recombinant and native receptors and is dependent on GABA concentration. In addition, the effect appears to be independent of the alpha-subunit isoform, and is due to the ability of H+ to alter the frequency of channel opening. Our findings indicate that GABAergic signaling in the CNS may be significantly altered during conditions that increase or decrease pH.

Modulation of membrane potential by extracellular pH in activated microglia in rats

Activation of cultured rat microglial cells by lipopolysaccharide (LPS) induced delayed rectifying outward K+ (I(K)) current. I(K) current was reported to have 'window current', playing a direct role in setting the membrane potential in activated microglia. We used whole-cell patch clamp method to measure the effect of extracellular pH on I(K) current. When pH was changed from 7.4 to 6.4, the activation curve of I(K) current shifted to the right by about 13 mV. Thus, extracellular acidification reduced the window current, resulting in membrane depolarization. These results suggest that extracellular pH regulate the membrane potential in activated microglia.

Interaction of H+ and Zn2+ on recombinant and native rat neuronal GABAA receptors

1. The interaction of Zn2+ and H+ ions with GABAA receptors was examined using Xenopus laevis oocytes expressing recombinant GABAA receptors composed of subunits selected from alpha1, beta1, gamma2S and delta types, and by using cultured rat cerebellar granule neurones. 2. The potency of Zn2+ as a non-competitive antagonist of GABA-activated responses on alpha1beta1 receptors was reduced by lowering the external pH from 7.4 to 5.4, increasing the Zn2+ IC50 value from 1.2 to 58.3 microM. Zinc-induced inhibition was largely unaffected by alkaline pH up to pH 9.4. 3. For alpha1beta1delta subunits, concentration-response curves for GABA were displaced laterally by Zn2+ in accordance with a novel mixed/competitive-type inhibition. The Zn2+ IC50 at pH 7.4 was 16.3 microM. Acidification of Ringer solution resulted in a reduced antagonism by Zn2+ (IC50, 49.0 microM) without affecting the type of inhibition. At pH 9.4, Zn2+ inhibition remained unaffected. 4. The addition of the gamma2S subunit to the alpha1beta1delta construct caused a marked reduction in the potency of Zn2+ (IC50, 615 microM), comparable to that observed with alpha1beta1gamma2S receptors (IC50 639 microM). GABA concentration-response curves were depressed in a mixed/non-competitive fashion. 5. In cultured cerebellar granule neurones, Zn2+ inhibited responses to GABA in a concentration-dependent manner. Lowering external pH from 7.4 to 6.4 increased the IC50 from 139 to 253 microM. 6. The type of inhibition exhibited by Zn2+ on cerebellar granule neurones, previously grown in high K+-containing culture media, was complex, with the GABA concentration-response curves shifting laterally with reduced slopes and similar maxima. The Zn2+-induced shift in the GABA EC50 values was reduced by lowering the external pH from 7.4 to 6.4. 7. The interaction of H+ and Zn2+ ions on GABAA receptors suggests that they share either a common regulatory pathway or coincident binding sites on the receptor protein. The apparent competitive mode of block induced by Zn2+ on alpha1beta1delta receptors is shared by GABAA receptors on cerebellar granule neurones, which are known to express delta-subunit-containing receptors. This novel mechanism is masked when a gamma2 subunit is incorporated into the receptor complex, revealing further diversity in the response of native GABAA receptors to endogenous cations.

Regulation of acetylcholine release by intracellular acidification of developing motoneurons in Xenopus cell cultures

1. The effects of intracellular pH changes on the acetylcholine (ACh) release and cytoplasmic Ca2+ concentration at developing neuromuscular synapses were studied in Xenopus nerve-muscle co-cultures. 2. Spontaneous and evoked ACh release of motoneurons was monitored by using whole-cell voltage-clamped myocytes. Intracellular alkalinization with 15 mM NH4Cl slightly reduced the frequency of spontaneous synaptic currents (SSCs). However, cytosolic acidification following withdrawal of extracellular NH4Cl caused a marked and transient increase in spontaneous ACh release. 3. Another method of cytosolic acidification was used in which NaCl in Ringer solution was replaced with weak organic acids. The increase in spontaneous ACh release paralleled the level of intracellular acidification resulting from addition of these organic acids. Acetate and propionate but not isethionate, methylsulphate and glucuronate, caused an increase in intracellular pH and a marked increase in spontaneous ACh release. 4. Impulse-evoked ACh release was slightly augmented by intracellular alkalinization and inhibited by cytosolic acidification. 5. Cytosolic acidification was accompanied by an elevation in the cytoplasmic Ca2+ concentration ([Ca2+]i), resulting from both external Ca2+ influx and intracellular Ca2+ mobilization. In contrast, the increase in [Ca2+]i induced by high K+ was inhibited by cytosolic acidification. 6. We conclude that cytosolic acidification regulates spontaneous and evoked ACh release differentially in Xenopus motoneurons, increasing spontaneous ACh release but inhibiting evoked ACh release.

Site-specific effects of local pH changes in the substantia nigra pars reticulata on flurothyl-induced seizures

Local cerebral changes of acid-base balance may interfere with neuronal communication. Acidosis enhances and alkalosis suppresses GABAA receptor neurotransmission while there are opposite effects on NMDA receptor transmission. In this study, we determined site-specific effects of acidified solutions of Na-HEPES-artificial cerebrospinal fluid infused into the anterior or posterior area of the substantia nigra pars reticulata (SNR) in rats. Two levels of pH were compared: 6.7 and 7.4. Rats were challenged with flurothyl and the threshold for clonic and tonic-clonic seizures was determined. In the anterior SNR, there were no differences between the effects of the solution with pH 6.7 and 7.4 on flurothyl seizures. In contrast in the posterior SNR, microinfusions with pH 6.7 had proconvulsant effects. The results suggest that local pH changes may have site-specific effects on seizure susceptibility in vivo.

Different sensitivities to pH of ATP-induced currents at four cloned P2X receptors

The effect of changing extracellular pH was studied on the currents induced by ATP or alphabeta-methylene-ATP in HEK293 cells transfected with different P2X receptor subunits. In cells expressing P2X1, P2X3, or P2X4 receptors, the effect of ATP was decreased by acidification. In cells expressing P2X2 receptors, acidification increased the ATP-induced current; this effect was also seen in cells expressing heteromeric P2X2 and P2X3 receptors. At P2X2 receptors, acidification caused a leftward shift in the ATP concentration-response curve, without change in maximum; the pKa for this effect was 7.3. At P2X4 receptors, acidification caused a rightward shift in the ATP concentration-response curve, without change in the maximum; the pKa for this effect was 6.8. The pH dependence of the action of ATP should be taken into account in studies of synaptic transmission, and it may provide a further tool to assign molecular identity to P2X receptors expressed by brain neurons.

Differential sensitivity to intracellular pH among high- and low-threshold Ca2+ currents in isolated rat CA1 neurons

The effects of intracellular pH (pHi) on high-threshold (HVA) and low-threshold (LVA) calcium currents were examined in acutely dissociated rat hippocampal Ca1 neurons with the use of the whole cell patch-clamp technique (21-23 degrees C). Internal pH was manipulated by external exposure to the weak base NH4Cl or in some cases to the weak acid Na-acetate (20 mM) at constant extracellular pH (7.4). Confocal fluorescence measurements using the pH-sensitive dye SNARF-1 in both dialyzed and intact cells confirmed that NH4Cl caused a reversible alkaline shift. However, the external TEA-Cl concentration used during ICa recording was sufficient to abolish cellular acidification upon NH4Cl wash out. With 10 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) in the pipette, NH4Cl exposure reversibly enhanced HVA currents by 29%, whereas exposure to Na-acetate markedly and reversibly depressed HVA Ca currents by 62%. The degree to which NH4Cl enhanced HVA currents was inversely related to the internal HEPES concentration but was unaffected when internal ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) was replaced by equimolar bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA). When depolarizing test pulses were applied shortly after break-in (Vh = -100 mV), NH4Cl caused a proportionally greater increase in the sustained current relative to the peak. The dihydropyridine Ca channel antagonist nifedipine (5 microM) blocked nearly all of this sustained current. A slowly inactivating nifedipine-sensitive (L-type) HVA current could be evoked from a depolarized holding potential of -50 mV; NH4Cl enhanced this current by 40 +/- 3% (mean +/- SE) and reversibly shifted the tail-current activation curve by +6-8 mV. L-type currents exhibited more rapid rundown than N-type currents; HVA currents remaining after prolonged cell dialysis, or in the presence of nifedipine, inactivated rapidly and were depressed by omega-conotoxin (GVIA). NH4Cl enhanced these N-type currents by 76 +/- 9%. LVA Ca currents were observed in 32% of the cells and exhibited little if any rundown. These amiloride-sensitive currents activated at voltages negative to -50 mV, were enhanced by extracellular alkalosis and depressed by extracellular acidosis, but were unaffected by exposure to either NH4Cl or NaAC. These results demonstrate that HVA Ca currents in hippocampal CA1 neurons are bidirectionally modulated by internal pH shifts, and that N-type currents are more sensitive to alkaline shifts than are L- or T-type (N > L > T). Our findings strengthen the idea that distinct cellular processes governed by different Ca channels may be subject to selective modulation by uniform shifts in cytosolic pH.

Salutary and deleterious effects of acidity on an indirect measure of metabolic rate and ATP concentrations in CNS cultures

Acidosis has traditionally been considered to mediate certain types of hypoxic-ischemic injury to the brain. However, the recent demonstration that moderate acidosis will reduce NMDA-mediated currents suggested that acidity could actually protect against types of ischemia and excitotoxicity, and in vitro studies now support this idea. Prompted by this, we have utilized the silicon microphysiometer, a recently-developed instrument that allows for indirect real-time measurement of metabolic rate by detecting proton efflux from small numbers of cultured cells, to determine whether acidity has protective effects upon cellular metabolism. Reducing extracellular pH from 7.4 to as low as 6.0 caused prompt, step-wise, and reversible inhibition of proton efflux rate in cortical and hippocampal cultures both normally and restricted to either glycolysis or oxidative metabolism. Approximately half of the inhibition was due to acidotic effects of NMDA-mediated currents, as demonstrated with NMDA receptor antagonists. Such an inhibition of this indirect metabolic measure could be associated with constant or increased ATP concentrations and represent a beneficial decrease in energy demands upon a neuron. Alternatively, an inhibition of proton efflux rate could be associated with ATP depletion and reflect impaired energy production. We observed a complex interplay between these opposing patterns. Reducing pH to 6.7 for 20 min caused significantly increased ATP concentrations, and prevented excitotoxin-induced ATP depletion. These effects of acidosis involved both NMDA-dependent and- independent actions. More severe (less than pH 6.7) acidosis did not cause ATP concentrations to rise, and if sustained for more than an hour caused a significant decline in ATP concentrations. Thus, despite the recent emphasis on the surprising neuroprotective potential of acidosis, a drop in pH is still likely to have complex and mixed consequences for brain tissue.

Wiring and volume transmission in the central nervous system: the concept of closed and open synapses

During the past two decades, several revisions of the concepts underlying interneuronal communication in the central nervous system (CNS) have been advanced. Our group has proposed to classify intercellular communication in the CNS under two general frames: 'wiring' (WT) and 'volume' transmission (VT). WT is characterized by a single 'transmission channel' made by cellular (neuronal or glial) structures and with a region of discontinuity not larger than a synaptic cleft. VT is characterized by the diffusion from a cell source (neuronal or glial) of chemical and electrical signals in the extracellular fluid (ECF) for a distance larger than the synaptic cleft Based on morphological and functional characteristics, and in light of the distinction proposed, six main modes of intercellular communication can be recognized in the CNS: gap-junction, membrane juxtaposition, and closed synapse (which represent WT-type modes of communication); open synapse, paracrine transmission and endocrine-like transmission (which represent VT-type modes of communication). Closed and open synapses are distinguished on the basis of the sealing of the signal within or the leakage of the signal outside the synapse Intra-synaptic restriction or extra-synaptic diffusion of transmitters are insured by a number of anatomical arrangements (e.g. glial ensheathment of synapse, size of the synaptic cleft) and functional mechanisms (e.g. density and location of transmitter re-uptake sites and metabolic enzymes). Some central synapses can switch from closed to open state and vice versa, e.g. by changing the amount of transmitter released. Finally, a synapse containing several transmitters can work as an open synapse for one transmitter and as a closed synapse for another.

Effects of extracellular pH on voltage-gated Na+, K+ and Ca2+ currents in isolated rat CA1 neurons

1. The effects of extracellular H+ (pHo) in the pathophysiological range (pH 6-8) on voltage-gated sodium, potassium, and calcium currents were examined in acutely dissociated rat hippocampal CA1 neurons using the whole-cell patch clamp technique. All experiments were conducted in Hepes-buffered solutions and were performed at room temperature (21-23 degrees C). 2. TTX-sensitive sodium currents, evoked by both step and ramp depolarization, were reversibly depressed by moderate acidosis and enhanced slightly by alkaline exposure. Changes in current amplitude were coincident with small reversible shifts (+/- 3 mV) in the voltage dependence of activation. In contrast, sodium current activation and decay kinetics as well as steady-state inactivation were unaffected by acidosis. 3. Outward potassium currents could be separated into a transient, rapidly inactivating current (IA) and a sustained, slowly inactivating component (IK). Steady-state activation of both currents was unaffected by an increase or decrease in pHo. Similarly, IK activation and IA decay kinetics remained stable during pHo exchange. In contrast, the steady-state inactivation (h infinity) of both potassium currents was reversibly shifted by approximately +10 mV during acid exposure, but remained unchanged during alkaline treatment. 4. Calcium currents were found to be predominantly of the high-voltage-activated (HVA) type, which could be carried by Ba2+ and inhibited completely by cadmium. Moderate acidosis (pH 6.9-6.0) reversibly depressed HVA Ca2+ current amplitude and caused a positive shift in its voltage dependence. For both of these parameters, alkaline treatment (pH 8.0) had the opposite effect. The depression of HVA Ca2+ currents by low pHo was unaffected by raising the internal Hepes concentration from 10 to 50 mM in the patch pipette. A Hill plot of the effect of pH on Ca2+ current amplitude revealed a pK value (defined as the mid-point of the titration curve) of 7.1 and a slope of 0.6. 5. The rate of Ca2+ current activation was unaffected by pHo at positive potentials, but below 0 mV the activation rate increased at low pH and decreased at high pH, becoming significant at -20 mV. At this membrane voltage, a second HVA current was revealed during acid exposure as the whole-cell HVA current was depressed. Ca2+ current decay was described by two time constants, both of which were significantly reduced at pH 6.4 and slightly enhanced at pH 8.0. Steady-state Ca2+ current inactivation reached 50% near -50 mV and was not affected at either pH extreme. 6. These results demonstrate that extracellular pH shifts within the pathophysiological range are capable of modulating both the conductance and gating properties of voltage-gated ion channels in hippocampal CA1 neurons. The effects we describe are consistent with the wellknown effects of pHo on neuronal excitability and strengthen the idea that endogenous pHo shifts may help regulate cell activity in situ.

Proton sensitivity of the GABA(A) receptor is associated with the receptor subunit composition

1. Modulation of GABA(A) receptors by external H(+) was examined in cultured rat sympathetic neurones, and in Xenopus laevis oocytes and human embryonic kidney (HEK) cells expressing recombinant GABA(A) receptors composed of combinations of alpha 1, beta 1, beta 2, gamma 2S and delta subunits. 2. Changing the external pH from 7.4 reduced GABA-activated currents in sympathetic neurones. pH titration of the GABA-induced current was fitted with a pH model which predicted that H(+) interact with two sites (PK(a) values of 6.4 and 7.2). 3. For alpha 1 beta 1 GABA(A) receptors, low external pH (< 7.4) enhanced responses to GABA. pH titration predicted the existence of two sites with PK(a) values of 6.6 and 7.5. The GABA concentration-response curve was shifted to the left by low pH and non-competitively inhibited at high pH (> 7.4). 4. alpha 1 beta 1 gamma 2S receptor constructs were not affected by external pH, whereas exchanging the beta 1 subunit for beta 2 conferred a sensitivity to pH, with predicted PK(a) values of 5.16 and 9.44. 5. Low pH enhanced the responses to GABA on alpha 1 beta 1 delta subunits, whilst high pH caused an inhibition (PK(a) values of 6.6 and 9.9). The GABA concentration-response curves were enhanced (pH 5.4) or reduced (pH 9.4) with no changes in the GABA EC(50). 6. Immunoprecipitation with subunit and epitope-specific antisera to alpha 1, beta 1 and delta subunits demonstrated that these subunits could co-assemble in cell membranes. 7. Expression of alpha 1 beta 1 gamma 2S delta constructs resulted in a 'bell-shaped' pH titration relationship. Increasing or decreasing external pH inhibited the responses to GABA. 8. The pH sensitivity of recombinant GABA(A) receptors expressed in HEK cells was generally in accordance with data accrued from Xenopus oocytes. However, rapid application of GABA to alpha 1 beta 1 constructs at high pH (> 7.4) caused an increased peak and reduced steady-state current, with a correspondingly increased rate of desensitization. 9. Modulation of GABA(A) receptor function was apparently unaffected by the internal pH. Moreover, pH values between 5 and 9.5 did not significantly affect the charge distribution on the zwitterionic GABA molecules. 10. In conclusion, this study demonstrates that external pH can either enhance, have little effect, or reduce GABA-activated responses, and this is apparently dependent on the receptor subunit composition. The potential importance of H(+) sensitivity of GABA(A) receptors is discussed.

pH regulation and proton signalling by glial cells

The regulation of H+ in nervous systems is a function of several processes, including H+ buffering, intracellular H+ sequestering, CO2 diffusion, carbonic anhydrase activity and membrane transport of acid/base equivalents across the cell membrane. Glial cells participate in all these processes and therefore play a prominent role in shaping acid/base shifts in nervous systems. Apart from a homeostatic function of H(+)-regulating mechanisms, pH transients occur in all three compartments of nervous tissue, neurones, glial cells and extracellular spaces (ECS), in response to neuronal stimulation, to neurotransmitters and hormones as well as secondary to metabolic activity and ionic membrane transport. A pivotal role for H+ regulation and shaping these pH transients must be assigned to the electrogenic and reversible Na(+)-HCO3-membrane cotransport, which appears to be unique to glial cells in nervous systems. Activation of this cotransporter results in the release and uptake of base equivalents by glial cells, processes which are dependent on the glial membrane potential. Na+/H+ and Cl-/HCO3-exchange, and possibly other membrane carriers, accomplish the set of tools in both glial cells and neurones to regulate their intracellular pH. Due to the pH dependence of a great variety of processes, including ion channel gating and conductances, synaptic transmission, intercellular communication via gap junctions, metabolite exchange and neuronal excitability, rapid and local pH transients may have signalling character for the information processing in nervous tissue. The impact of H+ signalling under both physiological and pathophysiological conditions will be discussed for a variety of nervous system functions.

Effects of CO2 on excitatory transmission apparently caused by changes in intracellular pH in the rat hippocampal slice

It is generally known that hyperventilation produces an increase in neuronal excitability. However, the mechanism whereby a change in CO2 partial pressure (PCO2) leads to changes in neural excitability is not known. We have studied this phenomenon in rat hippocampal slices using double-barrelled microelectrodes for simultaneous recording of field excitatory postsynaptic potentials (EPSPs) and extracellular pH in stratum radiatum of area CA1. A drop in PCO2 from the control level, 36 mmHg to 7 mmHg, produced an increase in extracellular pH of 0.4-0.6 pH units and a transient increase in EPSP slope by about 20-30%. Despite the stable extracellular alkalosis, the EPSP reverted back to its original level within 10 min. Switching back to 36 mmHg PCO2 restored the original extracellular pH and caused a transient decrease in the EPSP slope. Pharmacological blockade of NMDA receptor and/or GABAA receptor had no influence on the effects of CO2. An increase in PCO2 to 145 mmHg led to a stable fall in extracellular pH by 0.6 units and to a transient 30-50% decrease in EPSP slope. The above results indicate that the CO2-induced changes in neuronal excitability were not caused by changes in extracellular pH but they might have been mediated by changes in intracellular pH. Indeed, exposing the slices to the permeant weak base, trimethylamine (20 mM), which is known to produce a rise in intracellular pH, increased the EPSP slope by 50-70%. Application of 20 mM propionate (a permeant weak acid) decreased the EPSP slope by 40-60%. We conclude that the transient changes in the EPSP seen in response to changes in PCO2 are mediated by in intracellular pH.

Proton modulation of functionally distinct GABAA receptors in acutely isolated pyramidal neurons of rat hippocampus

We have studied the effect of extracellular pH (pHo) on the GABAA receptor-mediated chloride conductance in acutely isolated pyramidal neurons from area CA1 of the rat hippocampus under whole-cell voltage clamp in bicarbonate-free solutions. The conductance evoked by saturating or near-saturating concentrations (200-1000 microM) of GABA showed a marked sensitivity to variations of pHo around 7.4. A decrease in pHo between 8.4 and 6.4 increased the GABAA receptor-mediated chloride conductance by about two-fold per pH unit. In contrast, when evoked by a low agonist concentration (1-10 microM) the conductance showed an equally marked decrease upon a decrease in pHo. The half-time for desensitization of the conductance induced by 500 microM GABA was around 900 ms at pHo 6.4 and 7.4, but decreased to 650 ms at pHo 8.4. A fall in pHo decreased the amount of desensitization of the conductance evoked by a 5 s application of 5 microM, but not of 500 microM, GABA. The concentration-response relationship of the GABA-induced conductance showed a local plateau between 50 and 100 microM of GABA, which was particularly evident at high pHo. Assuming two receptor populations with a high and a low affinity for GABA, the effect of H+ on the GABAA receptors could be explained as an increase in the EC50 of the high affinity receptor, and an apparently non-competitive potentiation of both the high and the low affinity receptors. The GABAA receptor-mediated conductance was markedly inhibited by 20-50 microM Zn2+. In addition, Zn2+ reverted the down-modulation by H+ observed at low GABA concentrations to up-modulation. Diazepam (1-10 microM) had only a marginal effect on the GABA-gated conductance. Taken together, the results suggest the coexistence in individual hippocampal neurons of two distinct GABAA receptor populations having differential sensitivities to H+. In the light of the inhibitory action of Zn2+ and the virtual absence of an effect of diazepam it is probable that a significant fraction of the GABAA receptors lack the gamma 2 subunit. The observation that an elevated pH has a strong suppressing effect on the conductance evoked by high concentrations of GABA may at least partly explain why an extracellular alkalosis leads to neuronal hyperexcitability.

The effect of hyperventilation on motor cortical inhibition in humans: a study of the electromyographic silent period evoked by transcranial brain stimulation

We studied the effects of hyperventilation under control of the end-tidal PCO2, on the electromyographic silent period evoked by transcranial magnetic brain stimulation and by peripheral nerve stimulation. We also studied the effects of hyperventilation on the threshold, latency and amplitude of motor potentials. Hyperventilation significantly reduced the duration of the cortical silent period, but did not affect the length of the peripheral silent period. Neither did it alter the latency, amplitude or threshold of the motor potentials. These findings suggest that hyperventilation selectively depresses motor cortical inhibition in humans.

Extracellular H+ iontophoresis modifies responses to gamma-aminobutyric acid and cyanide of reticulospinal vasomotor neurons in rats

Responses of reticulospinal vasomotor neurons, recorded in the rostral ventrolateral reticular nucleus of the medulla oblongata, to gamma-aminobutyric acid (GABA) and cyanide microiontophoreses were examined during H+ iontophoresis in anesthetized rats. Extracellular H+ iontophoresis attenuated GABA-evoked decreases and enhanced cyanide-induced increases in the neuronal activity, but had no effect on the neuronal activity when applied alone. Opposite responses were produced during OH- iontophoresis. Similar effects were also observed on the glycine-evoked inhibition of these neurons during H+ and OH- iontophoreses, suggesting that H+ modulation of the GABA-evoked inhibition may not result from a specific action at the GABA receptor-channel complex. It is concluded that extracellular H+ ions exert a modulatory action on responses of the reticulospinal vasomotor neurons to other neuro-active substances and may significantly contribute to hypoxic-ischemic cardiovascular regulation.

The role of bicarbonate in GABAA receptor-mediated IPSPs of rat neocortical neurones

1. The ionic mechanism underlying the fast, GABAA receptor-mediated inhibitory postsynaptic potential (IPSPA) was examined in rat neocortical neurones using intracellular recording techniques. Synaptic responses were evoked by orthodromic stimulation applied to the subcortical white matter or to the pial surface. All experiments were carried out at a constant extracellular Cl- concentration. 2. The resting membrane potential was -76.2 +/- 1.0 mV (mean +/- S.E.M., n = 32) and in most cells IPSPA was depolarizing. The reversal potential of IPSPA (EIPSP-A) was -70.2 +/- 0.9 mV (n = 32) and that of a more slowly developing hyperpolarizing response (IPSPB) was -91.4 +/- 1.3 mV (n = 28). 3. An examination of the temporal relationships between excitatory postsynaptic potentials (EPSPs) and IPSPAs in different cells suggested that, despite partial overlap of these responses, EPSPs had little influence on the measured values of EIPSP-A. 4. Application of 20 mM trimethylamine (TriMA), a membrane-permeant weak base which is expected to produce a rise in pHi (and hence in intracellular HCO3-), induced a reversible positive shift in EIPSP-A of up to +9.0 mV (mean + 4.2 mV) at an extracellular pH (pHo) of 7.4. In some experiments, the shift in reversal potential was associated with a change in the polarity of IPSPA from hyperpolarizing to depolarizing. 5. Application of 20 mM lactate (a membrane-permeant weak acid which is expected to produce a fall in pHi and hence in intracellular HCO3-) at pHo 7.0 produced a hyperpolarizing shift in EIPS-A of up to -7.5 mV (mean -5.6 mV). In some experiments, exposure to lactate changed the polarity of IPSPA from depolarizing to hyperpolarizing. 6. Changes in pHo from 7.4 to 7.0 reduced the effect of TriMA and augmented that of lactate on EIPSP-A, as could be expected on the basis of the pHo-dependent change in the fraction of membrane permeable non-charged weak base or acid. 7. Under control conditions, a change in pHo from 7.4 to 7.0 produced a slight positive shift (< +2 mV) in EIPSP-A. In the presence of TriMA, a similar change in pHo gave rise to a negative shift (-1.8 to -2.7 mV). 8. The results obtained indicate that HCO3- ions contribute significantly to the IPSPA, thereby making EIPSP-A more positive than the Cl- equilibrium potential.(ABSTRACT TRUNCATED AT 400 WORDS)