pH regulation in single CA1 neurons acutely isolated from the hippocampi of immature and mature rats

Topiramate Decelerates Bicarbonate-Driven Acid-Elimination of Human Neocortical Neurons: Strategic Significance for its Antiepileptic, Antimigraine and Neuroprotective Properties

BACKGROUND

Mammalian central neurons regulate their intracellular pH (pHi) strongly and even slight pHi-fluctuations can influence inter-/intracellular signaling, synaptic plasticity and excitability.

OBJECTIVE

For the first time, we investigated topiramate´s (TPM) influence on pHi-behavior of human central neurons representing a promising target for anticonvulsants and antimigraine drugs.

METHODS

In slice-preparations of tissue resected from the middle temporal gyrus of five adults with intractable temporal lobe epilepsy, BCECF-AM-loaded neocortical pyramidal-cells were investigated by fluorometry. The pHi-regulation was estimated by using the recovery-slope from intracellular acidification after an Ammonium-Prepulse (APP).

RESULTS

Among 17 pyramidal neurons exposed to 50 μM TPM, seven (41.24%) responded with an altered resting-pHi (7.02±0.12), i.e., acidification of 0.01-0.03 pH-units. The more alkaline the neurons, the greater the TPM-related acidifications (r=0.7, p=0.001, n=17). The recovery from APPacidification was significantly slowed under TPM (p<0.001, n=5). Further experiments using nominal bicarbonate-free (n=2) and chloride-free (n=2) conditions pointed to a modulation of the HCO3 -- driven pHi-regulation by TPM, favoring a stimulation of the passive Cl-/HCO3 --antiporter (CBT) - an acid-loader predominantly in more alkaline neurons.

CONCLUSION

TPM modulated the bicarbonate-driven pHi-regulation, just as previously described in adult guinea-pig hippocampal neurons. We discussed the significance of the resulting subtle acidifications for beneficial antiepileptic, antimigraine and neuroprotective effects as well as for unwanted cognitive deficits.

Levetiracetam mediates subtle pH-shifts in adult human neocortical pyramidal cells via an inhibition of the bicarbonate-driven neuronal pH-regulation - Implications for excitability and plasticity modulation

The intracellular pH (pHi) of mammalian central neurons is tightly regulated and small pHi-fluctuations can fine-tune inter-/intracellular signaling, excitability, and synaptic plasticity. The research-gap about the pHi-regulation of human brain neurons is addressed here by testing possible influences of the anticonvulsant levetiracetam (LEV). BCECF-AM-loaded neocortical pyramidal cells were fluorometrically investigated in slice-preparations of tissue resected from the middle temporal gyrus of five adults with intractable temporal-lobe epilepsy. Recovery-slope from intracellular acidification following an ammonium prepulse (APP) was used to measure the pHi-regulation. Among twenty pyramidal cells exposed to 50 μM LEV, the resting pHi (7.09 ± 0.14) was lowered in eight (40%) neurons, on average by 0.02 ± 0.011 pH-units. In three (15%) and nine (45%) neurons, a minimal alkaline shift (0.017 ± 0.004 pH-units) and no pHi-shift occurred, respectively. The LEV-induced pHi-shifts were positively correlated with the resting pHi (r = 0.6, p = 0.006, n = 20). In five neurons, which all had responded on LEV with an acidification before, the recovery from APP-acidification was significantly delayed during LEV (p < 0.001). This inhibitory LEV-effect on pHi-regulation i) was similar to that of 200 μM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (n = 2) and ii) did not occur under nominal bicarbonate-free conditions (n = 2). Thus, LEV lowered the pHi of human neocortical pyramidal cells most likely by a weakening of the transmembrane HCO3(-)-mediated acid-extrusion. This might contribute to LEV's anticonvulsive potency. Neurons with more acidic resting pHi-values showed a minimal alkalization upon LEV providing a mechanism for paradoxical proconvulsive LEV-effects rarely observed in epilepsy patients. The significance of these subtle pHi-shifts for cortical excitability and plasticity is discussed.

Bactridine 2 effect in DRG neurons. Identification of NHE as a second target

Bactridine 2 (Bact-2) is an antibacterial toxin from Tityus discrepans venom which modifies isoforms 1.2, 1.4 and 1.6 voltage-dependent sodium (Na_v) channels. Bactridine-induced Na⁺ outflow in Yersinia enterocolitica was blocked by amiloride, suggesting that Bact-2 effect was mediated by an amiloride sensitive sodium channel. In this study we show that Bact-2 increases also an outward rectifying current in rat dorsal root ganglia (DRG) sensory neurons; therefore, the nature of the outward rectifying currents was characterized and then the effect of Bact-2 on these currents was studied. These currents are enhanced by amiloride, are decreased by Na⁺ when an outward pH gradient is present and its reversal potential coincides with that of a Cl^-/H⁺ exchanger, suggesting that rectifying currents are produced by the electrogenic Cl^-/H⁺ exchanger modulated by the Na⁺/H⁺ antiporter. Bact-2 also leads to an increase of the outward currents in a similar way to the produced by the inhibition of the Na⁺/H⁺ exchanger. Additionally, the subsequent application of Bact-2 after blocking the Na⁺/H⁺ exchanger does not produce any further effect, suggesting that Bact-2 modifies the outward current by modulating the activity of the Na⁺/H⁺ exchanger. The effect of Bact-2 on pH_i regulation was determined using the pH indicator BCECF. The results show that the Na⁺/H⁺ exchanger is blocked by amiloride and Na⁺ free solutions and is modulated by Bact-2 in a similar way as cariporide. This study validates that besides Na_v channels, Bact-2 modulates the activity of the Na⁺/H⁺ exchanger.

Simultaneous two-photon imaging of intracellular chloride concentration and pH in mouse pyramidal neurons in vivo

Intracellular chloride ([Cl^-]_i) and pH (pH_i) are fundamental regulators of neuronal excitability. They exert wide-ranging effects on synaptic signaling and plasticity and on development and disorders of the brain. The ideal technique to elucidate the underlying ionic mechanisms is quantitative and combined two-photon imaging of [Cl^-]_i and pH_i, but this has never been performed at the cellular level in vivo. Here, by using a genetically encoded fluorescent sensor that includes a spectroscopic reference (an element insensitive to Cl^- and pH), we show that ratiometric imaging is strongly affected by the optical properties of the brain. We have designed a method that fully corrects for this source of error. Parallel measurements of [Cl^-]_i and pH_i at the single-cell level in the mouse cortex showed the in vivo presence of the widely discussed developmental fall in [Cl^-]_i and the role of the K-Cl cotransporter KCC2 in this process. Then, we introduce a dynamic two-photon excitation protocol to simultaneously determine the changes of pH_i and [Cl^-]_i in response to hypercapnia and seizure activity.

Silencing Neurons: Tools, Applications, and Experimental Constraints

Reversible silencing of neuronal activity is a powerful approach for isolating the roles of specific neuronal populations in circuit dynamics and behavior. In contrast with neuronal excitation, for which the majority of studies have used a limited number of optogenetic and chemogenetic tools, the number of genetically encoded tools used for inhibition of neuronal activity has vastly expanded. Silencing strategies vary widely in their mechanism of action and in their spatial and temporal scales. Although such manipulations are commonly applied, the design and interpretation of neuronal silencing experiments present unique challenges, both technically and conceptually. Here, we review the most commonly used tools for silencing neuronal activity and provide an in-depth analysis of their mechanism of action and utility for particular experimental applications. We further discuss the considerations that need to be given to experimental design, analysis, and interpretation of collected data. Finally, we discuss future directions for the development of new silencing approaches in neuroscience.

Role of Cl- -HCO3- exchanger AE3 in intracellular pH homeostasis in cultured murine hippocampal neurons, and in crosstalk to adjacent astrocytes

KEY POINTS

A polymorphism of human AE3 is associated with idiopathic generalized epilepsy. Knockout of AE3 in mice lowers the threshold for triggering epileptic seizures. The explanations for these effects are elusive. Comparisons of cells from wild-type vs. AE3^-/- mice show that AE3 (present in hippocampal neurons, not astrocytes; mediates HCO₃^- efflux) enhances intracellular pH (pH_i ) recovery (decrease) from alkali loads in neurons and, surprisingly, adjacent astrocytes. During metabolic acidosis (MAc), AE3 speeds initial acidification, but limits the extent of pH_i decrease in neurons and astrocytes. AE3 speeds re-alkalization after removal of MAc in neurons and astrocytes, and speeds neuronal pH_i recovery from an ammonium prepulse-induced acid load. We propose that neuronal AE3 indirectly increases acid extrusion in (a) neurons via Cl^- loading, and (b) astrocytes by somehow enhancing NBCe1 (major acid extruder). The latter would enhance depolarization-induced alkalinization of astrocytes, and extracellular acidification, and thereby reduce susceptibility to epileptic seizures.

ABSTRACT

The anion exchanger AE3, expressed in hippocampal (HC) neurons but not astrocytes, contributes to intracellular pH (pH_i ) regulation by facilitating the exchange of extracellular Cl^- for intracellular HCO₃^- . The human AE3 polymorphism A867D is associated with idiopathic generalized epilepsy. Moreover, AE3 knockout (AE3^-/- ) mice are more susceptible to epileptic seizure. The mechanism of these effects has been unclear because the starting pH_i in AE3^-/- and wild-type neurons is indistinguishable. The purpose of the present study was to use AE3^-/- mice to investigate the role of AE3 in pH_i homeostasis in HC neurons, co-cultured with astrocytes. We find that the presence of AE3 increases the acidification rate constant during pH_i recovery from intracellular alkaline loads imposed by reducing [CO₂ ]. The presence of AE3 also speeds intracellular acidification during the early phase of metabolic acidosis (MAc), not just in neurons but, surprisingly, in adjacent astrocytes. Additionally, AE3 contributes to braking the decrease in pH_i later during MAc in both neurons and astrocytes. Paradoxically, AE3 enhances intracellular re-alkalization after MAc removal in neurons and astrocytes, and pH_i recovery from an ammonium prepulse-induced acid load in neurons. The effects of AE3 knockout on astrocytic pH_i homeostasis in MAc-related assays require the presence of neurons, and are consistent with the hypothesis that the AE3 knockout reduces functional expression of astrocytic NBCe1. These findings suggest a new type of neuron-astrocyte communication, based on the expression of AE3 in neurons, which could explain how AE3 reduces seizure susceptibility.

Activation of AMP-activated protein kinase regulates hippocampal neuronal pH by recruiting Na(+)/H(+) exchanger NHE5 to the cell surface

Strict regulation of intra- and extracellular pH is an important determinant of nervous system function as many voltage-, ligand-, and H(+)-gated cationic channels are exquisitely sensitive to transient fluctuations in pH elicited by neural activity and pathophysiologic events such as hypoxia-ischemia and seizures. Multiple Na(+)/H(+) exchangers (NHEs) are implicated in maintenance of neural pH homeostasis. However, aside from the ubiquitous NHE1 isoform, their relative contributions are poorly understood. NHE5 is of particular interest as it is preferentially expressed in brain relative to other tissues. In hippocampal neurons, NHE5 regulates steady-state cytoplasmic pH, but intriguingly the bulk of the transporter is stored in intracellular vesicles. Here, we show that NHE5 is a direct target for phosphorylation by the AMP-activated protein kinase (AMPK), a key sensor and regulator of cellular energy homeostasis in response to metabolic stresses. In NHE5-transfected non-neuronal cells, activation of AMPK by the AMP mimetic AICAR or by antimycin A, which blocks aerobic respiration and causes acidification, increased cell surface accumulation and activity of NHE5, and elevated intracellular pH. These effects were effectively blocked by the AMPK antagonist compound C, the NHE inhibitor HOE694, and mutation of a predicted AMPK recognition motif in the NHE5 C terminus. This regulatory pathway was also functional in primary hippocampal neurons, where AMPK activation of NHE5 protected the cells from sustained antimycin A-induced acidification. These data reveal a unique role for AMPK and NHE5 in regulating the pH homeostasis of hippocampal neurons during metabolic stress.

Effects of metabolic acidosis on intracellular pH responses in multiple cell types

Metabolic acidosis (MAc), a decrease in extracellular pH (pHo) caused by a decrease in [HCO3 (-)]o at a fixed [CO2]o, is a common clinical condition and causes intracellular pH (pHi) to fall. Although previous work has suggested that MAc-induced decreases in pHi (ΔpHi) differ among cell types, what is not clear is the extent to which these differences are the result of the wide variety of methodologies employed by various investigators. In the present study, we evaluated the effects of two sequential MAc challenges (MAc1 and MAc2) on pHi in 10 cell types/lines: primary-cultured hippocampal (HCN) neurons and astrocytes (HCA), primary-cultured medullary raphé (MRN) neurons, and astrocytes (MRA), CT26 colon cancer, the C2C12 skeletal muscles, primary-cultured bone marrow-derived macrophages (BMDM) and dendritic cells (BMDC), Ink4a/ARF-null melanocytes, and XB-2 keratinocytes. We monitor pHi using ratiometric fluorescence imaging of 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein while imposing MAc: lowering (pHo) from 7.4 to 7.2 by decreasing [HCO3 (-)]o from 22 to 14 mM at 5% CO2 for 7 min. After MAc1, we return cells to the control solution for 10 min and impose MAc2. Using our definition of MAc resistance [(ΔpHi/ΔpHo) ≤ 40%], during MAc1, ∼70% of CT26 and ∼50% of C2C12 are MAc-resistant, whereas the other cell types are predominantly MAc-sensitive. During MAc2, some cells adapt [(ΔpHi/ΔpHo)2 < (ΔpHi/ΔpHo)1], particularly HCA, C2C12, and BMDC. Most maintain consistent responses [(ΔpHi/ΔpHo)2 ≅ (ΔpHi/ΔpHo)1], and a few decompensate [(ΔpHi/ΔpHo)2>(ΔpHi/ΔpHo)1], particularly HCN, C2C12, and XB-2. Thus, responses to twin MAc challenges depend both on the individual cell and cell type.

Intracellular pH regulation by acid-base transporters in mammalian neurons

Intracellular pH (pH_i) regulation in the brain is important in both physiological and physiopathological conditions because changes in pH_i generally result in altered neuronal excitability. In this review, we will cover 4 major areas: (1) The effect of pH_i on cellular processes in the brain, including channel activity and neuronal excitability. (2) pH_i homeostasis and how it is determined by the balance between rates of acid loading (J_L) and extrusion (J_E). The balance between J_E and J_L determine steady-state pH_i, as well as the ability of the cell to defend pH_i in the face of extracellular acid-base disturbances (e.g., metabolic acidosis). (3) The properties and importance of members of the SLC4 and SLC9 families of acid-base transporters expressed in the brain that contribute to J_L (namely the Cl-HCO₃ exchanger AE3) and J_E (the Na-H exchangers NHE1, NHE3, and NHE5 as well as the Na⁺- coupled HCO₃⁻ transporters NBCe1, NBCn1, NDCBE, and NBCn2). (4) The effect of acid-base disturbances on neuronal function and the roles of acid-base transporters in defending neuronal pH_i under physiopathologic conditions.

Carbonic anhydrases and brain pH in the control of neuronal excitability

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

Minireview: pH and synaptic transmission

As a general rule a rise in pH increases neuronal activity, whereas it is dampened by a fall of pH. Neuronal activity per se also challenges pH homeostasis by the increase of metabolic acid equivalents. Moreover, the negative membrane potential of neurons promotes the intracellular accumulation of protons. Synaptic key players such as glutamate receptors or voltage-gated calcium channels show strong pH dependence and effects of pH gradients on synaptic processes are well known. However, the processes and mechanisms that allow controlling the pH in synaptic structures and how these mechanisms contribute to normal synaptic function are only beginning to be resolved.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

Immunocytochemical identification of electroneutral Na⁺-coupled HCO₃⁻ transporters in freshly dissociated mouse medullary raphé neurons

The medullary raphé (MR) of the medulla oblongata contains chemosensitive neurons that respond to increases in arterial [CO₂], by altering firing rate, with increases being associated with serotonergic (5-hydroxytryptamine [5HT]) neurons and decreases, with GABAergic neurons. Both types of neurons contribute to increased alveolar ventilation. Decreases in intracellular pH are thought to link the rise in [CO₂] to increased ventilation. Because electroneutral Na(+)-coupled HCO₃(-) transporters (nNCBTs), which help protect cells from intracellular acidosis, are expressed robustly in the neurons of the central nervous system, a key question is whether these transporters are present in chemosensitive neurons. Therefore, we used an immunocytochemistry approach to identify neurons (using a microtubule associated protein-2 monoclonal antibody) and specifically 5HT neurons (TPH monoclonal antibody) or GABAergic neurons (GAD2 monoclonal antibody) in freshly dissociated cells from the mouse MR. We also co-labeled with polyclonal antibodies against the three nNCBTs: NBCn1, NDCBE, and NBCn2. We exploited ePet-EYFP (enhanced yellow fluorescent protein) mice (with EYFP-labeled 5HT neurons) as well as mice genetically deficient in each of the three nNCBTs. Quantitative image analysis distinguished positively stained cells from background signals. We found that >80% of GAD2(+) cells also were positive for NDCBE, and >90% of the TPH(+) and GAD2(+) cells were positive for the other nNCBTs. Assuming that the transporters are independently distributed among neurons, we can conclude that virtually all chemosensitive MR neurons contain at least one nNCBT.

Acid extrusion via blood-brain barrier causes brain alkalosis and seizures after neonatal asphyxia

Birth asphyxia is often associated with a high seizure burden that is predictive of poor neurodevelopmental outcome. The mechanisms underlying birth asphyxia seizures are unknown. Using an animal model of birth asphyxia based on 6-day-old rat pups, we have recently shown that the seizure burden is linked to an increase in brain extracellular pH that consists of the recovery from the asphyxia-induced acidosis, and of a subsequent plateau level well above normal extracellular pH. In the present study, two-photon imaging of intracellular pH in neocortical neurons in vivo showed that pH changes also underwent a biphasic acid–alkaline response, resulting in an alkaline plateau level. The mean alkaline overshoot was strongly suppressed by a graded restoration of normocapnia after asphyxia. The parallel post-asphyxia increase in extra- and intracellular pH levels indicated a net loss of acid equivalents from brain tissue that was not attributable to a disruption of the blood–brain barrier, as demonstrated by a lack of increased sodium fluorescein extravasation into the brain, and by the electrophysiological characteristics of the blood–brain barrier. Indeed, electrode recordings of pH in the brain and trunk demonstrated a net efflux of acid equivalents from the brain across the blood–brain barrier, which was abolished by the Na/H exchange inhibitor, N-methyl-isobutyl amiloride. Pharmacological inhibition of Na/H exchange also suppressed the seizure activity associated with the brain-specific alkalosis. Our findings show that the post-asphyxia seizures are attributable to an enhanced Na/H exchange-dependent net extrusion of acid equivalents across the blood–brain barrier and to consequent brain alkalosis. These results suggest targeting of blood–brain barrier-mediated pH regulation as a novel approach in the prevention and therapy of neonatal seizures.

Preemptive regulation of intracellular pH in hippocampal neurons by a dual mechanism of depolarization-induced alkalinization

Numerous studies have documented the mechanisms that regulate intracellular pH (pH(i)) in hippocampal neurons in response to an acid load. Here, we studied the response of pH(i) to depolarization in cultured hippocampal neurons. Elevation of external K+ (6-30 mm) elicited an acid transient followed by a large net alkaline shift. Similar responses were observed in acutely dissociated hippocampal neurons. In Ca2+ -free media, the acid response was curtailed and the alkaline shift enhanced. DIDS blocked the alkaline response and revealed a prolonged underlying acidification that was highly dependent on Ca2+ entry. Similar alkaline responses could be elicited by AMPA, indicating that this rise in pH(i) was a depolarization-induced alkalinization (DIA). The DIA was found to consist of Cl- -dependent and Cl- -independent components, each accounting for approximately one-half of the peak amplitude. The Cl- -independent component was postulated to arise from operation of the electrogenic Na+ -HCO3- cotransporter NBCe1. Quantitative PCR and single-cell multiplex reverse transcription-PCR demonstrated message for NBCe1 in our hippocampal neurons. In neurons cultured from Slc4a4 knock-out (KO) mice, the DIA was reduced by approximately one-half compared with wild type, suggesting that NBCe1 was responsible for the Cl- -independent DIA. In Slc4a4 KO neurons, the remaining DIA was virtually abolished in Cl- -free media. These data demonstrate that DIA of hippocampal neurons occurs via NBCe1, and a parallel DIDS-sensitive, Cl- -dependent mechanism. Our results indicate that, by activating net acid extrusion in response to depolarization, hippocampal neurons can preempt a large, prolonged, Ca2+ -dependent acidosis.

Synaptic glutamate release is modulated by the Na+ -driven Cl-/HCO₃⁻ exchanger Slc4a8

On the one hand, neuronal activity can cause changes in pH; on the other hand, changes in pH can modulate neuronal activity. Consequently, the pH of the brain is regulated at various levels. Here we show that steady-state pH and acid extrusion were diminished in cultured hippocampal neurons of mice with a targeted disruption of the Na(+)-driven Cl(-)/HCO(3)(-) exchanger Slc4a8. Because Slc4a8 was found to predominantly localize to presynaptic nerve endings, we hypothesize that Slc4a8 is a key regulator of presynaptic pH. Supporting this hypothesis, spontaneous glutamate release in the CA1 pyramidal layer was reduced but could be rescued by increasing the intracellular pH. The reduced excitability in vitro correlated with an increased seizure threshold in vivo. Together with the altered kinetics of stimulated synaptic vesicle release, these data suggest that Slc4a8 modulates glutamate release in a pH-dependent manner.

Lactate Effectively Covers Energy Demands during Neuronal Network Activity in Neonatal Hippocampal Slices

Although numerous experimental data indicate that lactate is efficiently used for energy by the mature brain, the direct measurements of energy metabolism parameters during neuronal network activity in early postnatal development have not been performed. Therefore, the role of lactate in the energy metabolism of neurons at this age remains unclear. In this study, we monitored field potentials and contents of oxygen and NAD(P)H in correlation with oxidative metabolism during intense network activity in the CA1 hippocampal region of neonatal brain slices. We show that in the presence of glucose, lactate is effectively utilized as an energy substrate, causing an augmentation of oxidative metabolism. Moreover, in the absence of glucose lactate is fully capable of maintaining synaptic function. Therefore, during network activity in neonatal slices, lactate can be an efficient energy substrate capable of sustaining and enhancing aerobic energy metabolism.

High-performance genetically targetable optical neural silencing by light-driven proton pumps

The ability to silence the activity of genetically specified neurons in a temporally precise fashion would open up the ability to investigate the causal role of specific cell classes in neural computations, behaviors, and pathologies. Here we show that members of the class of light-driven outward proton pumps can mediate very powerful, safe, multiple-color silencing of neural activity. The gene archaerhodopsin-31 (Arch) from Halorubrum sodomense enables near-100% silencing of neurons in the awake brain when virally expressed in mouse cortex and illuminated with yellow light. Arch mediates currents of several hundred picoamps at low light powers, and supports neural silencing currents approaching 900 pA at light powers easily achievable in vivo. In addition, Arch spontaneously recovers from light-dependent inactivation, unlike light-driven chloride pumps that enter long-lasting inactive states in response to light. These properties of Arch are appropriate to mediate the optical silencing of significant brain volumes over behaviourally-relevant timescales. Arch function in neurons is well tolerated because pH excursions created by Arch illumination are minimized by self-limiting mechanisms to levels comparable to those mediated by channelrhodopsins2,3 or natural spike firing. To highlight how proton pump ecological and genomic diversity may support new innovation, we show that the blue-green light-drivable proton pump from the fungus Leptosphaeria maculans4 (Mac) can, when expressed in neurons, enable neural silencing by blue light, thus enabling alongside other developed reagents the potential for independent silencing of two neural populations by blue vs. red light. Light-driven proton pumps thus represent a high-performance and extremely versatile class of “optogenetic” voltage and ion modulator, which will broadly empower new neuroscientific, biological, neurological, and psychiatric investigations.

pH regulating transporters in neurons from various chemosensitive brainstem regions in neonatal rats

We studied the membrane transporters that mediate intracellular pH (pH(i)) recovery from acidification in brainstem neurons from chemosensitive regions of neonatal rats. Individual neurons within brainstem slices from the retrotrapezoid nucleus (RTN), the nucleus tractus solitarii (NTS), and the locus coeruleus (LC) were studied using a pH-sensitive fluorescent dye and fluorescence imaging microscopy. The rate of pH(i) recovery from an NH(4)Cl-induced acidification was measured, and the effects of inhibitors of various pH-regulating transporters determined. Hypercapnia (15% CO(2)) resulted in a maintained acidification in neurons from all three regions. Recovery in RTN neurons was nearly entirely eliminated by amiloride, an inhibitor of Na(+)/H(+) exchange (NHE). Recovery in RTN neurons was blocked approximately 50% by inhibitors of isoform 1 of NHE (NHE-1) but very little by an inhibitor of NHE-3 or by DIDS (an inhibitor of HCO(3)-dependent transport). In NTS neurons, amiloride blocked over 80% of the recovery, which was also blocked approximately 65% by inhibitors of NHE-1 and 26% blocked by an inhibitor of NHE-3. Recovery in LC neurons, in contrast, was unaffected by amiloride or blockers of NHE isoforms but was dependent on Na(+) and increased by external HCO(3)(-). On the basis of these findings, pH(i) recovery from acidification appears to be largely mediated by NHE-1 in RTN neurons, by NHE-1 and NHE-3 in NTS neurons, and by a Na- and HCO(3)-dependent transporter in LC neurons. Thus, pH(i) recovery is mediated by different pH-regulating transporters in neurons from different chemosensitive regions, but recovery is suppressed by hypercapnia in all of the neurons.

Imaging synaptic inhibition in transgenic mice expressing the chloride indicator, Clomeleon

We describe here a molecular genetic approach for imaging synaptic inhibition. The thy-1 promoter was used to express high levels of Clomeleon, a ratiometric fluorescent indicator for chloride ions, in discrete populations of neurons in the brains of transgenic mice. Clomeleon was functional after chronic expression and provided non-invasive readouts of intracellular chloride concentration ([Cl(-)](i)) in brain slices, allowing us to quantify age-dependent declines in resting [Cl(-)](i) during neuronal development. Activation of hippocampal interneurons caused [Cl(-)](i) to rise transiently in individual postsynaptic pyramidal neurons. [Cl(-)](i) increased in direct proportion to the amount of inhibitory transmission, with peak changes as large as 4 mM. Integrating responses over populations of pyramidal neurons allowed sensitive detection of synaptic inhibition. Thus, Clomeleon imaging permits non-invasive, spatiotemporally resolved recordings of [Cl(-)](i) in a large variety of neurons, opening up new opportunities for imaging synaptic inhibition and other forms of chloride signaling.

Expression and localization of Na-driven Cl-HCO(3)(-) exchanger (SLC4A8) in rodent CNS

The Na(+)-driven Cl-HCO(3) exchanger (NDCBE or SLC4A8) is a member of the solute carrier 4 (SLC4) family of HCO(3)(-) transporters, which includes products of 10 genes with similar sequences. Most SLC4 members play important roles in regulating intracellular pH (pH(i)). Physiological studies suggest that NDCBE is a major pH(i) regulator in at least hippocampal (HC) pyramidal neurons. We generated a polyclonal rabbit antibody directed against the first 18 residues of the cytoplasmic N terminus (Nt) of human NDCBE. By Western blotting, the antibody distinguishes NDCBE-as a purified Nt peptide or a full-length transporter (expressed in Xenopus oocytes)-from other Na(+)-coupled HCO(3)(-) transporters. By Western blotting, the antiserum recognizes an approximately 135-kDa band in several brain regions of adult mice: the cerebral cortex (CX), subcortex (SCX), cerebellum (CB), and HC. In CX, PNGase F treatment reduces the molecular weight to approximately 116 kDa. By immunocytochemistry, affinity-purified (AP) NDCBE antibody stains the plasma membrane of neuron cell bodies and processes of rat HC neurons in primary culture as well as freshly dissociated mouse HC neurons. The AP antibody does not detect substantial NDCBE levels in freshly dissociated HC astrocytes, or astrocytes in HC or CB sections. By immunohistochemistry, the AP antibody recognizes high levels of NDCBE in neurons of CX, HC (including pyramidal neurons in Cornu Ammonis (CA)1-3 and dentate gyrus), substantial nigra, medulla, cerebellum (especially Purkinje and granular cells), and the basolateral membrane of fetal choroid plexus. Thus, NDCBE is in a position to contribute substantially to pH(i) regulation in multiple CNS neurons.

Potential contribution of a voltage-activated proton conductance to acid extrusion from rat hippocampal neurons

We examined the potential contribution of a voltage-gated proton conductance (gH+) to acid extrusion from cultured postnatal rat hippocampal neurons. In neurons loaded with Ca2+- and/or pH-sensitive fluorophores, transient exposures to 25-139.5 mM external K+ (K+o) or 20 microM veratridine in the presence of 2 mM Ca2+o (extracellular pH (pHo) constant at 7.35) caused reversible increases and decreases in intracellular free calcium concentration ([Ca2+]i) and intracellular pH (pHi), respectively. In contrast, under external Ca2+-free conditions, the same stimuli failed to affect [Ca2+]i but caused an increase in pHi, the magnitude of which was related to the [K+]o applied and the change in membrane potential. Consistent with the properties of gH+s in other cell types, the magnitude of the rise in pHi observed in the absence of external Ca2+ was not affected by the removal of external Na+ but was sensitive to external Zn2+ and temperature and was dependent on the measured transmembrane pH gradient (DeltapHmemb). Increasing DeltapH(memb) by pretreatment with carbonylcyanide-p-trifluoromethoxyphenylhydrazone augmented both the high-[K+]o-evoked rise in pHi and the Zn2+-sensitive component of the rise in pHi, suggestive of increased acid extrusion via a gH+. The inhibitory effect of Zn2+ at a given DeltapHmemb was further enhanced by increasing pHo from 7.35-7.8, consistent with a pHo-dependent inhibition of the putative gH+ by Zn2+. Under conditions designed to isolate H+ currents, a voltage-dependent outward current was recorded from whole-cell patch-clamped neurons. Although the outward current appeared to show some selectivity for protons, it was not sensitive to Zn2+ or temperature and the H+-selective component could not be separated from a larger conductance of unknown selectivity. Nonetheless, taken together, the results suggest that a Zn2+-sensitive proton conductive pathway is present in rat hippocampal neurons and contributes to H+ efflux under depolarizing conditions.

Use of a new polyclonal antibody to study the distribution and glycosylation of the sodium-coupled bicarbonate transporter NCBE in rodent brain

NCBE (SLC4A10) is a member of the SLC4 family of bicarbonate transporters, several of which play important roles in intracellular-pH regulation and transepithelial HCO(3)(-) transport. Here we characterize a new antibody that was generated in rabbit against a fusion protein consisting of maltose-binding protein and the first 135 amino acids (aa) of the N-terminus of human NCBE. Western blotting--both of purified peptides representing the initial approximately 120 aa of the transporters and of full-length transporters expressed in Xenopus oocytes--demonstrated that the antibody is specific for NCBE versus the two most closely related proteins, NDCBE (SLC4A8) and NBCn1 (SLC4A7). Western blotting of tissue in four regions of adult mouse brain indicates that NCBE is expressed most abundantly in cerebral cortex (CX), cerebellum (CB) and hippocampus (HC), and less so in subcortex (SCX). NCBE protein was present in CX, CB, and HC microdissected to avoid choroid plexus. Immunocytochemistry shows that NCBE is present at the basolateral membrane of embryonic day 18 (E18) fetal and adult choroid plexus. NCBE protein is present by Western blot and immunocytochemistry in cultured and freshly dissociated HC neurons but not astrocytes. By Western blot, nearly all NCBE in mouse and rat brain is highly N-glycosylated (approximately 150 kDa). PNGase F reduces the molecular weight (MW) of natural NCBE in mouse brain or human NCBE expressed in oocytes to approximately the predicted MW of the unglycosylated protein. In oocytes, mutating any one of the three consensus N-glycosylation sites reduces glycosylation of the other two, and the triple mutant exhibits negligible functional expression.

Effect of chronic elevated carbon dioxide on the expression of acid-base transporters in the neonatal and adult mouse

Several pulmonary and neurological conditions, both in the newborn and adult, result in hypercapnia. This leads to disturbances in normal pH homeostasis. Most mammalian cells maintain tight control of intracellular pH (pHi) using a group of transmembrane proteins that specialize in acid-base transport. These acid-base transporters are important in adjusting pHiduring acidosis arising from hypoventilation. We hypothesized that exposure to chronic hypercapnia induces changes in the expression of acid-base transporters. Neonatal and adult CD-1 mice were exposed to either 8% or 12% CO2for 2 wk. We used Western blot analysis of membrane protein fractions from heart, kidney, and various brain regions to study the response of specific acid-base transporters to CO2. Chronic CO2increased the expression of the sodium hydrogen exchanger 1 (NHE1) and electroneutral sodium bicarbonate cotransporter (NBCn1) in the cerebral cortex, heart, and kidney of neonatal but not adult mice. CO2increased the expression of electrogenic NBC (NBCe1) in the neonatal but not the adult mouse heart and kidney. Hypercapnia decreased the expression of anion exchanger 3 (AE3) in both the neonatal and adult brain but increased AE3 expression in the neonatal heart. We conclude that: 1) chronic hypercapnia increases the expression of the acid extruders NHE1, NBCe1 and NBCn1 and decreases the expression of the acid loader AE3, possibly improving the capacity of the cell to maintain pHiin the face of acidosis; and 2) the heterogeneous response of tissues to hypercapnia depends on the level of CO2and development.

Novel functional interaction between Na+/H+exchanger 1 and tyrosine phosphatase SHP-2

Besides being a intracellular pH (pHi) regulator, Na+/H+exchanger (NHE)1 has recently been postulated as a membrane scaffold that assembles protein complexes and coordinates various signaling pathways. The aim of the present study was to uncover NHE1 interactive partners and study their functional implications. NHE1 interactive partners were screened in the mouse brain with a signal transduction AntibodyArray. Ten of 400 tested proteins appeared to be potentially associated with NHE1. These partners have been shown to be involved in either cell proliferative or apoptotic pathways. The interactions between NHE1 and Src homology 2 domain-containing protein tyrosine phosphatase (SHP-2), Bin1, and heat shock protein (HSP)70 were reciprocally confirmed by coimmunoprecipitation. Moreover, in vitro binding data have shown that NHE1 COOH terminus interacts directly with SHP-2. The functional significance of the association between NHE1 and SHP-2 was further investigated by measuring pHi, cell proliferation, and cell death with the fluorescent dye BCECF, [3H]thymidine incorporation, and medium lactate dehydrogenase activity, respectively. Our results revealed that cells with SHP-2 overexpression exhibited a higher steady-state pHiand a faster, NHE1-dependent pHirecovery rate from acid load in HEPES buffer. In addition, SHP-2 overexpression diminished the HOE-642-induced inhibition of cell proliferation and protected cells from hypoxic injury, especially in the presence of HOE-642. Together, our findings demonstrate that SHP-2 not only is physically associated with NHE1 but also modulates NHE1 functions such as pHiregulation, cell proliferation, and cell death under hypoxia.

Photophysics of Clomeleon by FLIM: discriminating excited state reactions along neuronal development

In this work, fluorescence lifetime imaging microscopy in the time domain was used to study the fluorescence dynamics of ECFP and of the ratiometric chloride sensor Clomeleon along neuronal development. The multiexponential analysis of fluorophores combined with the study of the contributions of the individual lifetimes (decay-associated spectra) was used to discriminate the presence of energy transfer from other excited state reactions. A characteristic change of sign of the pre-exponential factors of lifetimes from positive to negative near the acceptor emission maxima was observed in presence of energy transfer. By fluorescence lifetime imaging microscopy, we could show that the individual conformations of CFP display differential quenching properties depending on their microenvironment. Suitability of Clomeleon as an optical indicator to obtain a direct readout of the intracellular chloride concentrations in living cells was verified by steady-state and time-resolved spectroscopy. The simultaneous study of the photophysical properties of Clomeleon, the calcium indicator Cameleon, and ECFP with neuronal development provided a kinetic model for the mechanism when competitive quenching effects as well as energy transfer occur in the same molecule. Simultaneous analysis of donor and acceptor kinetics was necessary to discriminate Försters resonance energy transfer along neuronal development due to the different cellular effects involved.

The Na+/H+exchanger is a major pH regulator in GABAergic presynaptic nerve terminals synapsing onto rat CA3 pyramidal neurons

The effects of pH(i) on GABAergic miniature inhibitory postsynaptic currents (mIPSCs) were studied in mechanically dissociated CA3 pyramidal neurons, by use of ammonium prepulse and whole-cell patch-clamp techniques, under the voltage-clamp condition. NH(4)Cl itself, which is expected to alkalinize pH(i), increased GABAergic mIPSC frequency in a concentration-dependent manner. In contrast, NH(4)Cl decreased mIPSC frequency, either in the presence of 200 microm Cd(2+) or in Ca(2+)-free external solution, suggesting that intraterminal alkalosis decreased GABAergic mIPSC frequency while [NH4(+)] itself may activate Ca(2+) channels by depolarizing the terminal. On the other hand, GABAergic mIPSC frequency was greatly increased immediately after NH(4)Cl removal, a condition expected to acidify pH(i), and recovered to the control level within 2 min after NH(4)Cl removal. This explosive increase in mIPSC frequency observed after NH(4)Cl removal was completely eliminated after depletion of Ca(2+) stores with 1 microm thapsigargin in the Ca(2+)-free external solution, suggesting that acidification increases in intraterminal Ca(2+) concentration via both extracellular Ca(2+) influx and Ca(2+) release from the stores. However, the acidification-induced increase in mIPSC frequency had not recovered by 10 min after NH(4)Cl removal either in the Na(+)-free external solution or in the presence of 10 microm 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), a specific Na(+)/H(+) exchanger (NHE) blocker. The present results suggest that NHEs are major intraterminal pH regulators on GABAergic presynaptic nerve terminals, and that the NHE-mediated regulation of pH(i) under normal physiological or pathological conditions might play an important role in the neuronal excitability by increasing inhibitory tones.

Relationships Between Calcium and pH in the Regulation of the Slow Afterhyperpolarization in Cultured Rat Hippocampal Neurons

The Ca2+-dependent slow afterhyperpolarization (AHP) is an important determinant of neuronal excitability. Although it is established that modest changes in extracellular pH (pHo) modulate the slow AHP, the relative contributions of changes in the priming Ca2+ signal and intracellular pH (pHi) to this effect remain poorly defined. To gain a better understanding of the modulation of the slow AHP by changes in pHo, we performed simultaneous recordings of intracellular free calcium concentration ([Ca2+]i), pHi, and the slow AHP in cultured rat hippocampal neurons coloaded with the Ca2+- and pH-sensitive fluorophores fura-2 and SNARF-5F, respectively, and whole cell patch-clamped using the perforated patch technique. Decreasing pHo from 7.2 to 6.5 lowered pHi, reduced the magnitude of depolarization-evoked [Ca2+]i transients, and inhibited the subsequent slow AHP; opposite effects were observed when pHo was increased from 7.2 to 7.5. Although decreases and increases in pHi (at a constant pHo) reduced and augmented, respectively, the slow AHP in the absence of marked changes in preceding [Ca2+]i transients, the inhibition of the slow AHP by decreases in pHo was correlated with low pHo-dependent reductions in [Ca2+]i transients rather than the decreases in pHi that accompanied the decreases in pHo. In contrast, high pHo-induced increases in the slow AHP were correlated with the accompanying increases in pHi rather than high pHo-dependent increases in [Ca2+]i transients. The results indicate that changes in pHo modulate the slow AHP in a manner that depends on the direction of the pHo change and substantiate a role for changes in pHi in modulating the slow AHP during changes in pHo.

Potassium homeostasis in the ischemic brain

Extracellular [K+] can range within 2.5-3.5 mM under normal conditions to 50-80 mM under ischemic and spreading depression events. Sustained exposure to elevated [K+]o has been shown to cause significant neuronal death even under conditions of abundant glucose supply. Astrocytes are well equipped to buffer this initial insult of elevated [K] through extensive gap junctional coupling, Na+/K+ pump activity (with associated glycogen and glycolytic potential), and endfoot siphoning capability. Their abundant energy availability and alkalinizing mechanisms help sustain Na+/K+ ATPase activity under ischemic conditions. Furthermore, passive K+ uptake mechanisms and water flux mediated through aquaporin-4 channels in endfoot processes are important energy-independent mechanisms. Unfortunately, as the length of ischemic episode is prolonged, these mechanisms increase to a point where they begin to have repercussions on other important cellular functions. Alkalinizing mechanisms induce an elevation of [Na+]i, increasing the energy demand of Na+/K+ ATPase and leading to eventual detrimental reversal of the Na+/glutamate- cotransporter and excitotoxic damage. Prolonged ischemia also results in cell swelling and activates volume regulatory processes that release excessive excitatory amino acids, further exacerbating excitotoxic injury. In the days following ischemic injury, reactive astrocytes demonstrate increased cell size and process thickness, leading to improved spatial buffering capacity in regions outside the lesion core where there is better neuronal survival. There is a substantial heterogeneity among reactive astrocytes, with some close to the lesion showing decreased buffering capacity. However, it appears that both Na+/K+ ATPase activity (along with energy production processes) as well as passive K+ uptake mechanisms are upregulated in gliotic tissue outside the lesion to enhance the above-mentioned homeostatic mechanisms.

Effect of extracellular acid-base disturbances on the intracellular pH of neurones cultured from rat medullary raphe or hippocampus

Previous reports suggest that an important characteristic of chemosensitive neurones is an unusually large change of steady-state intracellular pH in response to a change in extracellular pH (DeltapH(i)/DeltapH(o)). To determine whether such a correlation exists between neurones from the medullary raphe (a chemosensitive brain region) and hippocampus (a non-chemosensitive region), we used BCECF to monitor pH(i) in cultured neurones subjected to extracellular acid-base disturbances. In medullary raphe neurones, respiratory acidosis (5%--> 9% CO(2)) caused a rapid fall in pH(i) (DeltapH(i) approximately 0.2) with no recovery and a large DeltapH(i)/DeltapH(o) of 0.71. Hippocampal neurones had a similar response, but with a slightly lower DeltapH(i)/DeltapH(o) (0.59). We further investigated a possible link between pH(i) regulation and chemosensitivity by following the pH(i) measurements on medullary raphe neurones with an immunocytochemistry for tryptophan hydroxylase (a marker of serotonergic neurones). We found that the DeltapH(i)/DeltapH(o) of 0.69 for serotonergic neurones (which are stimulated by acidosis) was not different from either the DeltapH(i)/DeltapH(o) of 0.75 for non-serotonergic neurones (most of which are not chemosensitive), or from the DeltapH(i)/DeltapH(o) of hippocampal neurones. For both respiratory alkalosis (5%--> 3% CO(2)) and metabolic alkalosis (22 mm--> 35 mm HCO(3)(-)), DeltapH(i)/DeltapH(o) was 0.42-0.53 for all groups of neurones studied. The only notable difference between medullary raphe and hippocampal neurones was in response to metabolic acidosis (22 mm--> 14 mm HCO(3)(-)), which caused a large pH(i) decrease in approximately 80% of medullary raphe neurones (DeltapH(i)/DeltapH(o)= 0.71), but relatively little pH(i) decrease in 70% of the hippocampal neurones (DeltapH(i)/DeltapH(o)= 0.09). Our comparison of medullary raphe and hippocampal neurones indicates that, except in response to metabolic acidosis, the neurones from the chemosensitive region do not have a uniquely high DeltapH(i)/DeltapH(o). Moreover, regardless of whether neurones were cultured from the chemosensitive or the non-chemosensitive region, pH(i) did not recover during any of the acid-base stresses.

Reduced contribution from Na+/H+ exchange to acid extrusion during anoxia in adult rat hippocampal CA1 neurons

The effect of anoxia on Na+/H+ exchange activity was examined in acutely isolated adult rat hippocampal CA1 neurons loaded with the H+-sensitive fluorophore, BCECF. Five-minute anoxia imposed under nominally HCO3-/CO2-free conditions induced a fall in pHi, the magnitude of which was smaller following prolonged exposure to medium in which N-methyl-D-glucamine (NMDG+) was employed as an extracellular Na+ (Na(+)(o)) substitute. Also consistent with the possibility that Na+/H+ exchange becomes inhibited soon after the induction of anoxia, rates of Na(+)(o)-dependent pHi recovery from internal acid loads imposed during anoxia were slowed, compared to rates of Na(+)(o)-dependent pHi recovery observed prior to anoxia. At the time at which rates of pHi recovery were reduced during anoxia, cellular adenosine triphosphate (ATP) levels had fallen to 35% of preanoxic levels, suggesting that ATP depletion might contribute to the observed inhibition of Na+/H+ exchange. In support, incubation of neurons with 2-deoxyglucose and antimycin A under normoxic conditions induced a fall in cellular ATP levels that was also associated with reduced Na(+)(o)-dependent rates of pHi recovery from imposed acid loads; conversely, pre-treatment with 10 mm creatine attenuated the effects of anoxia to reduce both ATP levels and Na(+)(o)-dependent rates of pHi recovery from internal acid loads. Taken together, the results are consistent with the possibility that functional Na+/H+ exchange activity in adult rat CA1 neurons declines soon after the onset of anoxia, possibly as a result of anoxia-induced falls in intracellular ATP.

Regulation and modulation of pH in the brain

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

Expression of Na+/H+ and HCO3−-dependent transporters in Na+/H+ exchanger isoform 1 null mutant mouse brain

Acid-base transporters, such as the sodium-hydrogen exchangers (NHEs) and bicarbonate-dependent transporters, play an important role in the regulation of intracellular pH (pH(i)) in the CNS. Previous studies from our laboratory have shown that the absence of the major NHE isoform 1 (NHE1) reduced the steady-state pH(i) and recovery rate from an acid load in the hippocampal neurons not only in HEPES but also in HCO(3)(-) solutions (Yao et al., 1999). The purpose of the current study was to determine whether the NHE1 null mutation affects the expression of pH-regulatory transporters in the mouse CNS. Immunoblotting and semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) were performed to examine the protein and mRNA levels of NHE1-4, electrogenic sodium-bicarbonate cotransporter 1 variants (NBCe1), and brain-specific anion exchanger 3 (AE3) in four brain regions (cerebral cortex, hippocampus, cerebellum and brainstem-diencephalon). NHE1 null mutant mice were compared with their wild type controls at the average age of approximately 4 weeks. Our results revealed that the NHE1 null mutation caused a significant increase in NHE3 in the cerebellum (84% for protein, 105% for mRNA), an increase in NBCe1 expression in the brainstem-diencephalon (approximately 40-50% for protein, 9-15% for mRNA), as well as a decrease in AE3 in the hippocampus (approximately 60% for protein, 24% for mRNA). We conclude that the NHE1 null mutation does alter the expression of other membrane transporters at both protein and mRNA levels. The alteration is region-specific. An increase in acid extruders (e.g. NHE3) and a decrease in acid loaders (e.g. AE3) suggest that there are some compensatory mechanisms that occur in NHE1 null mutant mice.

Active sites of thioredoxin reductases: why selenoproteins?

Selenium, an essential trace element for mammals, is incorporated into a selected class of selenoproteins as selenocysteine. All known isoenzymes of mammalian thioredoxin (Trx) reductases (TrxRs) employ selenium in the C-terminal redox center -Gly-Cys-Sec-Gly-COOH for reduction of Trx and other substrates, whereas the corresponding sequence in Drosophila melanogaster TrxR is -Ser-Cys-Cys-Ser-COOH. Surprisingly, the catalytic competence of these orthologous enzymes is similar, whereas direct Sec-to-Cys substitution of mammalian TrxR, or other selenoenzymes, yields almost inactive enzyme. TrxRs are therefore ideal for studying the biology of selenocysteine by comparative enzymology. Here we show that the serine residues flanking the C-terminal Cys residues of Drosophila TrxRs are responsible for activating the cysteines to match the catalytic efficiency of a selenocysteine-cysteine pair as in mammalian TrxR, obviating the need for selenium. This finding suggests that the occurrence of selenoenzymes, which implies that the organism is selenium-dependent, is not necessarily associated with improved enzyme efficiency. Our data suggest that the selective advantage of selenoenzymes is a broader range of substrates and a broader range of microenvironmental conditions in which enzyme activity is possible.

The role of HCO3(-)-dependent mechanisms in pHi regulation during O2 deprivation

We have reported in our previous work that, in the absence of HCO(3)(-), Na(+)/H(+) exchanger is responsible for an anoxia-induced alkalinization in hippocampal CA1 neurons. HCO(3)(-)-dependent mechanisms have been reported to play a key role in pH(i) regulation in nerve cells, but how their function is affected by O(2) deprivation has not been well studied. In this work, pH(i) measurements (obtained from dissociated neurons loaded with carboxy-seminaphthorhodafluor-1 and using confocal microscopy) and whole-cell patch clamp recording techniques were used to investigate the role of HCO(3)(-)-dependent membrane exchangers on CA1 neurons during O(2) deprivation. Anoxia (5 min) induced a small acidification in neurons in the presence of HCO(3)(-) and this acidification was changed to a significant alkalinization when neurons were bathed with Hepes buffer or when 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid was applied in a HCO(3)(-) solution, indicating that HCO(3)(-)-dependent mechanisms were involved. A marked anoxia-induced acidification (0.33+/-0.11 pH unit) was seen when the Na(+)/H(+) exchange was blocked with 3-(methylsulfonyl-4-piperidino-benzoyl)-guanidine methanesulfonate in the presence of HCO(3)(-), but the same anoxia did not cause a significant pH(i) change in a Na(+) free, HCO(3)(-) solution, suggesting that the anoxia-induced acidification in the presence of 3-(methylsulfonyl-4-piperidino-benzoyl)-guanidine methanesulfonate is dependent on both Na(+) and HCO(3)(-). Furthermore, anoxia did not cause a significant pH(i) change when both 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and 3-(methylsulfonyl-4-piperidino-benzoyl)-guanidine methanesulfonate were present. Current clamp recordings showed a significant membrane depolarization following anoxia in HCO(3)(-) solution but not in Hepes buffer. Our data suggest that, in hippocampal neurons: a) pH(i) regulation during O(2) deprivation is affected not only by metabolism but also by membrane exchangers, and b) besides the activation of Na(+)/H(+) exchange, anoxia activates a 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive, Na(+)-dependent acid loader (possibly electrogenic).

Regulation of Cl--HCO3- exchangers by cAMP-dependent protein kinase in adult rat hippocampal CA1 neurons

The contributions of HCO(3)(-)-dependent, DIDS-sensitive mechanisms to the maintenance of steady-state pH(i), and the regulation of their activities by cAMP-dependent protein kinase (PKA), were investigated in CA1 neurons with the H(+)-sensitive fluorophore, BCECF. The addition of HCO(3)(-)/CO(2) to neurons with "low" (pH(i) < or = 7.20) and "high" (pH(i) > 7.20) initial pH(i) values under Hepes-buffered conditions, increased and decreased steady-state pH(i), respectively. Conversely, under HCO(3)(-)/CO(2)-buffered conditions, DIDS caused pH(i) to decrease and increase in neurons with low and high initial pH(i) values, respectively. In the presence, but not the absence, of HCO(3)(-), the PKA inhibitor Rp-adenosine-3',5'-cyclic monophosphorothioate (Rp-cAMPS; 50 microM) evoked DIDS-sensitive increases and decreases in pH(i) in neurons with low and high initial pH(i) values, respectively. In contrast, in neurons with low initial pH(i) values, activation of PKA with the Sp isomer of cAMPS (Sp-cAMPS; 25 microM) elicited increases in pH(i) that were smaller in the presence than in the absence of HCO(3)(-), whereas in neurons with high initial pH(i) values, Sp-cAMPS-evoked rises in pH(i) were larger in the presence than in the absence of HCO(3)(-); the differences between the effects of Sp-cAMPS on pH(i) under the different buffering conditions were attenuated by DIDS. Consistent with the possibility that changes in the activities of HCO(3)(-)-dependent, DIDS-sensitive mechanisms contribute to the steady-state pH(i) changes evoked by the PKA modulators, in neurons with initial pH(i) values < or = 7.20, Rp-cAMPS concurrently inhibited Na(+)-independent Cl(-)-HCO(3)(-) exchange and stimulated Na(+)-dependent Cl(-)-HCO(3)(-) exchange; in contrast, Sp-cAMPS concurrently stimulated Na(+)-independent Cl(-)-HCO(3)(-) exchange and inhibited Na(+)-dependent Cl(-)-HCO(3)(-) exchange. Data from a limited number of neurons with initial pH(i) values > 7.20 suggested that the directions of the reciprocal changes in anion exchange activities (inhibition or stimulation) evoked by Rp- and Sp-cAMPS may be opposite in cells with low vs. high resting pH(i) values. Taken together, the results indicate that the effects of modulating PKA activity on steady-state pH(i) in rat CA1 neurons under HCO(3)(-)/CO(2)-buffered conditions reflect not only changes in Na(+)-H(+) exchange activity but also changes in Na(+)-dependent and Na(+)-independent Cl(-)-HCO(3)(-) exchange activity that, in turn, may be dependent upon the initial pH(i).

Intracellular pH response to anoxia in acutely dissociated adult rat hippocampal CA1 neurons

The effects of anoxia on intracellular pH (pH(i)) were examined in acutely isolated adult rat hippocampal CA1 neurons loaded with the H(+)-sensitive fluorophore, 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein. During perfusion with HCO/CO(2)- or HEPES-buffered media (pH 7.35) at 37 degrees C, 5- or 10-min anoxic insults were typified by an intracellular acidification on the induction of anoxia, a subsequent rise in pH(i) in the continued absence of O(2), and a further internal alkalinization on the return to normoxia. The steady-state pH(i) changes were not consequent on changes in [Ca(2+)](i) and, examined in the presence of HCO, were not significantly affected by (DIDS). In the absence of HCO, the magnitude of the postanoxic alkalinization was attenuated when external Na(+) was reduced by substitution with N-methyl-D-glucamine (NMDG(+)), but not Li(+), suggesting that increased Na(+)/H(+) exchange activity contributes to this phase of the pH(i) response. In contrast, 100-500 microM Zn(2+), a known blocker of H(+)-conductive pathways, reduced the magnitudes of the internal alkalinizations that occurred both during and following anoxia. The effects of NMDG(+)-substituted medium and Zn(2+) to reduce the increase in pH(i) that occurred after anoxia were additive. Consistent with the steady-state pH(i) changes, rates of pH(i) recovery from internal acid loads imposed immediately after anoxia were increased, and the application of Zn(2+) and/or perfusion with NMDG(+)-substituted medium slowed pH(i) recovery. Reducing extracellular pH from 7.35 to 6.60, or reducing ambient temperature from 37 degrees C to room temperature, also attenuated the increases in steady-state pH(i) observed during and after anoxia and reduced rates of pH(i) recovery from acid loads imposed in the immediate postanoxic period. Finally, inhibition of the cAMP/protein kinase A second-messenger system reduced the magnitude of the rise in pH(i) after anoxia in a manner that was dependent on external Na(+); conversely, activation of the system with isoproterenol increased the postanoxic alkalinization, an effect that was attenuated by pretreatment with propranolol, Rp-cAMPS, or when NMDG(+) (but not Li(+)) was employed as an external Na(+) substitute. The results suggest that a Zn(2+)-sensitive acid efflux mechanism, possibly a H(+)-conductive pathway activated by membrane depolarization, contributes to the internal alkalinization observed during anoxia in adult rat CA1 neurons. The rise in pH(i) after anoxia reflects acid extrusion via the H(+)-conductive pathway and also Na(+)/H(+) exchange, activation of the latter being mediated, at least in part, through a cAMP-dependent signaling pathway.

Intracellular pH regulation of neurons in chemosensitive and nonchemosensitive areas of brain slices

The role of changes of intracellular pH (pH(i)) as the proximal signal in central chemosensitive neurons has been studied. pH(i) recovery from acidification is mediated by Na(+)/H(+) exchange in all medullary neurons and pH(i) recovery from alkalinization is mediated by Cl(-)/HCO(3)(-) exchange in most medullary neurons. These exchangers are more sensitive to inhibition by changes in extracellular pH (pH(o)) in neurons from chemosensitive regions compared to those from nonchemosensitive regions. Thus, neurons from chemosensitive regions exhibit a maintained intracellular acidification in response to hypercapnic acidosis but they show pH(i) recovery in response to isohydric hypercapnia. A similar pattern of pH(i) response is seen in other CO(2)/H(+)-responsive cells, including glomus cells, sour taste receptor cells, and chemosensitive neurons from snails, suggesting that a maintained fall of pH(i) is a common feature of the proximal signal in all CO(2)/H(+)-sensitive cells. To further evaluate the potential role of pH(i) changes as proximal signals for chemosensitive neurons, studies must be done to: determine why a lack of pH(i) recovery from hypercapnic acidosis is seen in some nonchemosensitive neurons; establish a correlation between hypercapnia-induced changes of pH(i) and membrane potential (V(m)); compare the hypercapnia-induced pH(i) changes seen in neuronal cell bodies with those in dendritic processes; understand why the V(m) response to hypercapnia of many chemosensitive neurons is washed out when using whole cell patch pipettes; and employ knock out mice to investigate the role of certain proteins in the CO(2)/H(+) response of chemosensitive neurons.

Role of Na(+)/H(+) exchanger during O(2) deprivation in mouse CA1 neurons

To determine the role of membrane transporters in intracellular pH (pH(i)) regulation under conditions of low microenvironmental O(2), we monitored pH(i) in isolated single CA1 neurons using the fluorescent indicator carboxyseminaphthorhodafluor-1 and confocal microscopy. After total O(2) deprivation or anoxia (PO(2) approximately equal to 0 Torr), a large increase in pH(i) was seen in CA1 neurons in HEPES buffer, but a drop in pH(i), albeit small, was observed in the presence of HCO(3)(-). Ionic substitution and pharmacological experiments showed that the large anoxia-induced pH(i) increase in HEPES buffer was totally Na(+) dependent and was blocked by HOE-694, strongly suggesting the activation of the Na(+)/H(+) exchanger (NHE). Also, this pH(i) increase in HEPES buffer was significantly smaller in Na(+)/H(+) exchanger isoform 1 (NHE1) null mutant CA1 neurons than in wild-type neurons, demonstrating that NHE1 is responsible for part of the pH(i) increase following anoxia. Both chelerythrine and H-89 partly blocked, and H-7 totally eliminated, this anoxia-induced pH(i) increase in the absence of HCO. We conclude that 1) O(2) deprivation activates Na(+)/H(+) exchange by enhancing protein kinase activity and 2) membrane proteins, such as NHE, actively participate in regulating pH(i) during low-O(2) states in neurons.

Acute inhibition of brain-specific Na(+)/H(+) exchanger isoform 5 by protein kinases A and C and cell shrinkage

Little is known of the functional properties of the mammalian, brain-specific Na(+)/H(+) exchanger isoform 5 (NHE5). Rat NHE5 was stably expressed in NHE-deficient PS120 cells, and its activity was characterized using the fluorescent pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. NHE5 was insensitive to ethylisopropyl amiloride. The transport kinetics displayed a simple Michaelis-Menten relationship for extracellular Na(+) (apparent K(Na) = 27 +/- 5 mM) and a Hill coefficient near 3 for the intracellular proton concentration with a half-maximal activity at an intracellular pH of 6.93 +/- 0.03. NHE5 activity was inhibited by acute exposure to 8-bromo-cAMP or forskolin (which increases intracellular cAMP by activating adenylate cyclase). The kinase inhibitor H-89 reversed this inhibition, suggesting that regulation by cAMP involves a protein kinase A (PKA)-dependent process. In contrast, 8-bromo-cGMP did not have a significant effect on activity. The protein kinase C (PKC) activator phorbol 12-myristrate 13-acetate inhibited NHE5, and the PKC antagonist chelerythrine chloride blunted this effect. Activity was also inhibited by hyperosmotic-induced cell shrinkage but was unaffected by a hyposmotic challenge. These results demonstrate that rat brain NHE5 is downregulated by activation of PKA and PKC and by cell shrinkage, important regulators of neuronal cell function.

Increased neuronal excitability and seizures in the Na(+)/H(+) exchanger null mutant mouse

Mice lacking the Na(+)/H(+) exchanger isoform 1 (NHE1) manifest neurological diseases that include ataxia, motor deficits, and a seizure disorder. The molecular basis for the phenotype has not been clear, and it has not been determined how the lack of NHE1 leads, in particular, to the seizure disorder. We have shown in this work that hippocampal CA1 neurons in mutant mice have a much higher excitability than in wild-type mice. This higher excitability is partly based on an upregulation of the Na(+) current density (608.2 +/- 123.2 pA/pF in NHE1 mutant vs. 334.7 +/- 63.7 pA/pF in wild type in HCO/CO(2)). Alterations in Na(+) channel characteristics, including steady-state inactivation (shift of 18 mV in the depolarization direction in the mutant), recovery from inactivation (tau(h) = 5.22 +/- 0.49 ms in wild-type neurons and 2.20 +/- 0.20 ms in mutant neurons), and deactivation (at -100 mV, tau(d) = 1.75 +/- 0.53 ms in mutant and 0.21 +/- 0.05 ms in wild-type neurons) further enhance the differences in excitability between mutant and wild-type mice. Our investigation demonstrates the existence of an important functional interaction between the NHE1 protein and the voltage-sensitive Na(+) channel. We hypothesize that the increased neuronal excitability and possibly the seizure disorder in mice lacking the NHE1 is due, at least in part, to changes in Na(+) channel expression and/or regulation.

Sodium-hydrogen exchangers and sodium-bicarbonate co-transporters: ontogeny of protein expression in the rat brain

We used western blotting to examine the developmental profiles (at embryonic day 16 and postnatal days 1, 13, 23, 33 and 105) of protein expression for three sodium-hydrogen exchanger isoforms (1, 2 and 4) and for a sodium-bicarbonate co-transporter in three CNS regions (cortex, cerebellum and brainstem-diencephalon). In microsomal preparations, sodium-hydrogen exchanger isoform 1 and sodium-bicarbonate co-transporter protein expression in the CNS increases gradually from embryonic day 16 (25-40% of the adult level) to postnatal day 105. In contrast, sodium-hydrogen exchanger isoform 2 and 4 expression reaches a maximum (three to 20 times the adult level) at around three to four weeks of age. There is significant regional heterogeneity in the expression of sodium-hydrogen exchanger and sodium-bicarbonate co-transporter proteins in the rat CNS. Sodium-hydrogen exchanger isoform 1 was highly expressed in the brainstem-diencephalon, whereas the sodium-bicarbonate co-transporter was robustly expressed in the cerebellum and brainstem-diencephalon. These data indicate that the expression of sodium-hydrogen exchanger and sodium-bicarbonate co-transporter proteins varies as a function of both development and specific brain region.

Effect of extracellular HCO(3)(-) on Na(+) channel characteristics in hippocampal CA1 neurons

The effect of HCO(3)(-)/CO(2) on membrane properties of isolated hippocampal CA1 neurons was studied with the use of the whole cell configuration of the patch-clamp technique. Neurons were acutely dissociated from 21- to 30-day-old mice. In the current-clamp mode, HCO(3)(-)/CO(2) significantly hyperpolarized CA1 neurons by more than 10 mV and decreased their input resistance. In addition, the overall excitability of these neurons was lower in the presence of HCO(3)(-)/CO(2) than in HEPES. Spontaneous and evoked action potential firing frequency was lower in the presence of HCO(3)(-)/CO(2) than in its absence. In the voltage-clamp mode, both activation and steady-state inactivation of a fast Na(+) current were shifted in the hyperpolarized direction in such a way that the window currents were smaller in HCO(3)(-)/CO(2) than in HEPES. Recovery from inactivation and deactivation from the open state of the fast Na(+) current was slower in HCO(3)(-)/CO(2) than in HEPES. We conclude that HCO(3)(-)/CO(2) decreases the intrinsic excitability of CA1 neurons by altering not only the passive properties of the neuronal membranes but also by changing several characteristics of the fast Na(+) current, including activation and inactivation kinetics as well as the recovery from inactivation and deactivation.

31P-MRS-based determination of brain intracellular and interstitial pH: its application to in vivo H+ compartmentation and cellular regulation during hypoxic/ischemic conditions

In the last decade, significant progress has been made in the characterization of pH regulation in nervous tissue in vitro. However, little work has been directed at understanding how pH regulatory mechanisms function in vivo. We are interested in how ischemic acidosis can effect pH regulation and modulate the extent of post-ischemic brain damage. We used 31P-MRS to determine normal in vivo pH(i) and pH(e) simultaneously in both the isolated canine brain and the intact rat brain. We observed that the 31P(i) peak in the 31P-MRS spectrum is heterogeneous and can be deconvoluted into a number of discrete constituent peaks. In a series of experiments, we identified these peaks as arising from either extracellular or intracellular sources. In particular, we identified the peak representing the neurons and astrocytes and showed that they maintain different basal pH (6.95 and 7.05, respectively) and behave differently during hypoxic/ischemic episodes.

A genetically encoded ratiometric indicator for chloride: capturing chloride transients in cultured hippocampal neurons

We constructed a novel optical indicator for chloride ions by fusing the chloride-sensitive yellow fluorescent protein with the chloride-insensitive cyan fluorescent protein. The ratio of FRET-dependent emission of these fluorophores varied in proportion to the concentration of Cl and was used to measure intracellular chloride concentration ([Cl-]i) in cultured hippocampal neurons. [Cl-]i decreased during neuronal development, consistent with the shift from excitation to inhibition during maturation of GABAergic synapses. Focal activation of GABAA receptors caused large changes in [Cl-]i that could underlie use-dependent depression of GABA-dependent synaptic transmission. GABA-induced changes in somatic [Cl-]i spread into dendrites, suggesting that [Cl-]i can signal the location of synaptic activity. This genetically encoded indicator will permit new approaches ranging from high-throughput drug screening to direct recordings of synaptic Cl- signals in vivo.

Effects of chloride transport inhibition and chloride substitution on neuron function and on hypoxic spreading-depression-like depolarization in rat hippocampal slices

Chloride fluxes play a crucial role in synaptic inhibition, cell pH regulation, as well as in cell volume control. In many neuropathological processes, cell swelling is a pivotal parameter, since cell volume changes and the dimension of the interstitial space critically modulate synchronized neuronal activity as well as the tissue's susceptibility to seizures or spreading depression. This study therefore focuses on the effects of different Cl(-) transport inhibitors and Cl(-) substitution on neuronal function and hypoxia-induced changes in rat hippocampal tissue slices. Orthodromically evoked focal excitatory postsynaptic potentials were depressed by furosemide (2mM), 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid (1mM) and Cl(-) substitution by methylsulfate, but were enhanced by 4,4'-dinitrostilbene-2,2'-disulfonic acid (1mM). All four treatments induced multiple population spike firing in response to single orthodromic volleys, suggesting reduced synaptic inhibition. Antidromic population spikes increased following Cl(-) withdrawal, were unaffected in the presence of furosemide and 4, 4'-dinitrostilbene-2,2'-disulfonic acid, but were abolished by 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid. The amplitude of the hypoxic spreading-depression-like extracellular potential shift was reduced by furosemide, 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid and Cl(-) withdrawal, i.e. by the same treatments that depressed orthodromically evoked postsynaptic potentials. Furosemide prolonged the time to onset and the duration of the spreading-depression-like extracellular potential shift, while 4, 4'-dinitrostilbene-2,2'-disulfonic acid shortened the time to onset. Spreading-depression-related cell swelling was recorded as the shrinkage of relative interstitial space, which was measured as tetramethylammonium-chloride space. Neither the Cl(-) transport inhibitors nor Cl(-) withdrawal had any detectable effect on spreading-depression-related cell swelling. CA1 pyramidal neurons usually hyperpolarized during drug application and their input resistance decreased. Cl(-) withdrawal increased their input resistance and caused spontaneous burst firing. Hypoxia caused the expected spreading-depression-like rapid, near complete depolarization of single pyramidal neurons and drastically reduced their input resistance. The three Cl(-) transport inhibitors and Cl(-) withdrawal delayed the onset of the hypoxic depolarization. In low Cl(-) solutions, the apparent threshold potential at which spreading depression was triggered shifted to more positive membrane potentials. The final voltage of the hypoxic depolarization was, however, not affected. It appears from these results that the reduction in the hypoxic spreading-depression-like extracellular potential shifts by Cl(-) transport inhibitors is at least partially attributable to desynchronization of depolarization, not to decreased depolarization in individual cells. Other contributing factors could be changes in recording conditions, depression of swelling-induced amino acid release from glial cells and unspecific side-effects of the applied drugs. Desynchronization could also account for the delayed spreading-depression onset. It is concluded that Cl(-) fluxes play a role in the triggering of spreading depression, but the spreading-depression-like depolarization itself or its self-regenerative character is not mediated by Cl(-).

Intracellular pH regulation in ventral horn neurones cultured from embryonic rat spinal cord

Intracellular pH was measured with the pH-sensitive fluorescent probe BCECF in spinal cord neurones cultured from rat embryos. At an external pH of 7.3, the average steady-state pHi was 7.18 +/- 0.03 (SEM, n = 97) and 7.02 +/- 0.01 (n = 221) in HEPES-buffered and in bicarbonate-buffered medium, respectively. In both external media, pHi was strongly dependent on external pH (pHe). In HEPES-buffered medium, pHi recovery following an acid load induced by transient application of ammonium required external Na+ and was inhibited by amiloride, indicating the presence of a Na+/H+ exchange. Na(+)- and HCO3(-)-dependent, DIDS-sensitive alkalinizing mechanisms also contributed to pHi regulation in CO2/bicarbonate-buffered medium. The presence of an electrogenic Na(+)-HCO3- cotransporter was confirmed by the alkalinizing effect of KCl application. The fact that pHi is lower in CO2/bicarbonate- than in HEPES-buffered medium and the alkalinization observed upon suppression of external Cl- suggest that the acidifying Cl-/HCO3- transporter plays an important role in defining pHi.

Intracellular pH modulates spontaneous and epileptiform bioelectric activity of hippocampal CA3-neurones

A growing body of evidence hints at intracellular free protons to be involved in the modulation of electric activity of cortical neurones. In this study we demonstrate that application of the weak acid propionate (2.5-20 mM) transiently lowers intracellular pH (pH(i)) of BCECF-AM loaded CA3-neurones in hippocampal slices. The predictability of this acidification prompted us to use propionate as a tool to investigate effects of pH(i) on spontaneous bioelectric activity (SBA) and epileptiform activity (EA, induced by bicuculline, caffeine or low magnesium) of CA3 neurones: SBA and EA were transiently suppressed by 2-20 mM propionate - coinciding with the transient neuronal acidification. As activation of Na(+)/H(+)-exchangers (NHE) is involved in the recovery from neuronal acidosis and NHE-inhibition alone is known to increase the activity of intracellular free protons of hippocampal neurones, we tested the effect of the NHE-blockers amiloride (0.5-1 mM) or HOE642 (200 microM) on SBA and EA of CA3-neurones. Long-term application of NHE-inhibitors alone continuously suppressed SBA and EA, which recovered during additional exposure to the weak base trimethylamine (5-10 mM). Simultaneous administration of propionate and NHE-blockers intensified the inhibition of neuronal activity. Together, these results indicate that intracellular acidification inhibits bioelectric activity of hippocampal CA3-neurones. This supports the hypothesis that pH(i) contributes to the control of cortical excitability.