Cortical inhibition, pH and cell excitability in epilepsy: what are optimal targets for antiepileptic interventions?

Screening tools to evaluate the neurotoxic potential of botanicals: building a strategy to assess safety

AREAS COVERED

This paper outlines the selection of NAMs, including in vitro assays using primary rat cortical neurons, zebrafish embryos, and Caenorhabditis elegans. These assays aim to assess neurotoxic endpoints such as neuronal activity and behavioral responses. Microelectrode array recordings of rat cortical neurons provide insights into the impact of botanical extracts on neuronal function, while the zebrafish embryos and C. elegans assays evaluate neurobehavioral responses. The paper also provides an account of the selection of botanical case studies based on expert judgment and existing neuroactivity/toxicity information. The proposed battery of assays will be tested with these case studies to evaluate their utility for neurotoxicity screening.

EXPERT OPINION

The complexity of botanicals necessitates the use of multiple NAMs for effective neurotoxicity screening. This paper discusses the evaluation of methodologies to develop a robust framework for evaluating botanical safety, including complex neuronal models and key neurodevelopmental process assays. It aims to establish a comprehensive screening framework.

pH regulating mechanisms of astrocytes: A critical component in physiology and disease of the brain

Strict homeostatic control of pH in both intra- and extracellular compartments of the brain is fundamentally important, primarily due to the profound impact of free protons ([H+]) on neuronal activity and overall brain function. Astrocytes, crucial players in the homeostasis of various ions in the brain, actively regulate their intracellular [H+] (pHi) through multiple membrane transporters and carbonic anhydrases. The activation of astroglial pHi regulating mechanisms also leads to corresponding alterations in the acid-base status of the extracellular fluid. Notably, astrocyte pH regulators are modulated by various neuronal signals, suggesting their pivotal role in regulating brain acid-base balance in both health and disease. This review presents the mechanisms involved in pH regulation in astrocytes and discusses their potential impact on extracellular pH under physiological conditions and in brain disorders. Targeting astrocytic pH regulatory mechanisms represents a promising therapeutic approach for modulating brain acid-base balance in diseases, offering a potential critical contribution to neuroprotection.

Extracellular glutamate and GABA transients at the transition from interictal spiking to seizures

Focal epilepsy is associated with intermittent brief population discharges (interictal spikes), which resemble sentinel spikes that often occur at the onset of seizures. Why interictal spikes self-terminate whilst seizures persist and propagate is incompletely understood. We used fluorescent glutamate and GABA sensors in an awake rodent model of neocortical seizures to resolve the spatiotemporal evolution of both neurotransmitters in the extracellular space. Interictal spikes were accompanied by brief glutamate transients which were maximal at the initiation site and rapidly propagated centrifugally. GABA transients lasted longer than glutamate transients and were maximal ∼1.5 mm from the focus where they propagated centripetally. Prior to seizure initiation GABA transients were attenuated, whilst glutamate transients increased, consistent with a progressive failure of local inhibitory restraint. As seizures increased in frequency, there was a gradual increase in the spatial extent of spike-associated glutamate transients associated with interictal spikes. Neurotransmitter imaging thus reveals a progressive collapse of an annulus of feed-forward GABA release, allowing seizures to escape from local inhibitory restraint.

Dynamic electrical synapses rewire brain networks for persistent oscillations and epileptogenesis

One of the very fundamental attributes for telencephalic neural computation in mammals involves network activities oscillating beyond the initial trigger. The continuing and automated processing of transient inputs shall constitute the basis of cognition and intelligence but may lead to neuropsychiatric disorders such as epileptic seizures if carried so far as to engross part of or the whole telencephalic system. From a conventional view of the basic design of the telencephalic local circuitry, the GABAergic interneurons (INs) and glutamatergic pyramidal neurons (PNs) make negative feedback loops which would regulate the neural activities back to the original state. The drive for the most intriguing self-perpetuating telencephalic activities, then, has not been posed and characterized. We found activity-dependent deployment and delineated functional consequences of the electrical synapses directly linking INs and PNs in the amygdala, a prototypical telencephalic circuitry. These electrical synapses endow INs dual (a faster excitatory and a slower inhibitory) actions on PNs, providing a network-intrinsic excitatory drive that fuels the IN-PN interconnected circuitries and enables persistent oscillations with preservation of GABAergic negative feedback. Moreover, the entities of electrical synapses between INs and PNs are engaged in and disengaged from functioning in a highly dynamic way according to neural activities, which then determine the spatiotemporal scale of recruited oscillating networks. This study uncovers a special wide-range and context-dependent plasticity for wiring/rewiring of brain networks. Epileptogenesis or a wide spectrum of clinical disorders may ensue, however, from different scales of pathological extension of this unique form of telencephalic plasticity.

Effects of carbonic anhydrase activity on the excitability of hippocampal axons during high-frequency firing

Epileptic activity is known to cause a lowering of intraneuronal pH, which has been suggested to serve as a feedback signal to terminate seizures. The mechanism of such signaling is unclear, but likely involves an altered function of several types of ligand- and voltage-gated channels in postsynaptic membranes caused by increasing cytosolic and extracellular [H+]. In addition, axonal conduction properties may be altered by endogenous pH signals, but this has not been investigated. In the present study, we have recorded the axonal compound action potential (fiber volley) in hippocampal slices in the presence of glutamatergic and GABAergic antagonists. During high-frequency stimulation (HFS) of the Schaffer collaterals, the fiber volley was depressed and its latency from stimulus to peak increased. In the CA1 stratum radiatum these changes were enhanced when the carbonic anhydrase inhibitor acetazolamide (1 mM) was co-perfused. The enhancing effect of acetazolamide was absent after lowering of [Ca2+] in the perfusion medium. Acetazolamide had no detectable effect on HFS-evoked fiber volleys recorded from a more proximal site along the Schaffer collaterals (at the CA2-CA3 border) or from axons in the alveus of CA1. Intracellular acidification imposed by washout of NH4Cl (5 mM) had qualitatively similar effects on fiber volleys evoked at low frequency as those observed with acetazolamide during HFS in CA1 stratum radiatum. The results suggest that carbonic anhydrase-dependent pH regulation counteracts activity-induced reduction of the excitability of Schaffer collateral axons in CA1. A possible influence from local synaptic terminals on this effect is discussed.

Daily rhythm in cortical chloride homeostasis underpins functional changes in visual cortex excitability

Cortical activity patterns are strongly modulated by fast synaptic inhibition mediated through ionotropic, chloride-conducting receptors. Consequently, chloride homeostasis is ideally placed to regulate activity. We therefore investigated the stability of baseline [Cl-]i in adult mouse neocortex, using in vivo two-photon imaging. We found a two-fold increase in baseline [Cl-]i in layer 2/3 pyramidal neurons, from day to night, with marked effects upon both physiological cortical processing and seizure susceptibility. Importantly, the night-time activity can be converted to the day-time pattern by local inhibition of NKCC1, while inhibition of KCC2 converts day-time [Cl-]i towards night-time levels. Changes in the surface expression and phosphorylation of the cation-chloride cotransporters, NKCC1 and KCC2, matched these pharmacological effects. When we extended the dark period by 4 h, mice remained active, but [Cl-]i was modulated as for animals in normal light cycles. Our data thus demonstrate a daily [Cl-]i modulation with complex effects on cortical excitability.

Optogenetic stimulation reveals a latent tipping point in cortical networks during ictogenesis

Brain-state transitions are readily apparent from changes in brain rhythms,1 but are difficult to predict, suggestive that the underlying cause is latent to passive recording methods. Among the most important transitions, clinically, are the starts of seizures. We here show that an 'active probing' approach may have several important benefits for epileptic management, including by helping predict these transitions. We used mice expressing the optogenetic actuator, channelrhodopsin, in pyramidal cells, allowing this population to be stimulated in isolation. Intermittent stimulation at frequencies as low as 0.033 Hz (period = 30 s) delayed the onset of seizure-like events in an acute brain slice model of ictogenesis, but the effect was lost if stimulation was delivered at even lower frequencies (1/min). Notably, active probing additionally provides advance indication of when seizure-like activity is imminent, revealed by monitoring the postsynaptic response to stimulation. The postsynaptic response, recorded extracellularly, showed an all-or-nothing change in both amplitude and duration, a few hundred seconds before seizure-like activity began-a sufficient length of time to provide a helpful warning of an impending seizure. The change in the postsynaptic response then persisted for the remainder of the recording, indicative of a state change from a pre-epileptic to a pro-epileptic network. This occurred in parallel with a large increase in the stimulation-triggered Ca2+ entry into pyramidal dendrites, and a step increase in the number of evoked postsynaptic action potentials, both consistent with a reduction in the threshold for dendritic action potentials. In 0 Mg2+ bathing media, the reduced threshold was not associated with changes in glutamatergic synaptic function, nor of GABAergic release from either parvalbumin or somatostatin interneurons, but simulations indicate that the step change in the optogenetic response can instead arise from incremental increases in intracellular [Cl-]. The change in the response to stimulation was replicated by artificially raising intracellular [Cl-], using the optogenetic chloride pump, halorhodopsin. By contrast, increases in extracellular [K+] cannot account for the firing patterns in the response to stimulation, although this, and other cellular changes, may contribute to ictal initiation in other circumstances. We describe how these various cellular changes form a synergistic network of positive feedback mechanisms, which may explain the precipitous nature of seizure onset. This model of seizure initiation draws together several major lines of epilepsy research as well as providing an important proof-of-principle regarding the utility of open-loop brain stimulation for clinical management of the condition.

Inhibition of NHE1 transport activity and gene transcription in DRG neurons in oxaliplatin-induced painful peripheral neurotoxicity

Oxaliplatin (OHP)-induced peripheral neurotoxicity (OIPN), one of the major dose-limiting side effects of colorectal cancer treatment, is characterized by both acute and chronic syndromes. Acute exposure to low dose OHP on dorsal root ganglion (DRG) neurons is able to induce an increase in intracellular calcium and proton concentration, thus influencing ion channels activity and neuronal excitability. The Na⁺/H⁺ exchanger isoform-1 (NHE1) is a plasma membrane protein that plays a pivotal role in intracellular pH (pH_i) homeostasis in many cell types, including nociceptors. Here we show that OHP has early effects on NHE1 activity in cultured mouse DRG neurons: the mean rate of pH_i recovery was strongly reduced compared to vehicle-treated controls, reaching levels similar to those obtained in the presence of cariporide (Car), a specific NHE1 antagonist. The effect of OHP on NHE1 activity was sensitive to FK506, a specific calcineurin (CaN) inhibitor. Lastly, molecular analyses revealed transcriptional downregulation of NHE1 both in vitro, in mouse primary DRG neurons, and in vivo, in an OIPN rat model. Altogether, these data suggest that OHP-induced intracellular acidification of DRG neurons largely depends on CaN-mediated NHE1 inhibition, revealing new mechanisms that OHP could exert to alter neuronal excitability, and providing novel druggable targets.

Synergistic Positive Feedback Mechanisms Underlying Seizure Initiation

Investigations into seizure initiation, in recent years, have focused almost entirely upon alterations of interneuronal function, chloride homeostasis, and extracellular potassium levels. In contrast, little attention has been directed toward a possible role of dendritic plateau potentials in the actual ictogenic transition, despite a substantial literature dating back 40 years regarding its importance generally in epilepsy. Here, we argue that an increase in dendritic excitability, coordinated across the population of pyramidal cells, is a key stage in ictogenesis.

Genetically encoded sensors for Chloride concentration

Insights into chloride regulation in neurons have come slowly, but they are likely to be critical for our understanding of how the brain works. The reason is that the intracellular Cl^- level ([Cl^-]_i) is the key determinant of synaptic inhibitory function, and this in turn dictates all manner of neuronal network function. The true impact on the network will only be apparent, however, if Cl^- is measured at many locations at once (multiple neurons, and also across the subcellular compartments of single neurons), which realistically, can only be achieved using imaging. The development of genetically-encoded anion biosensors (GABs) brings the additional benefit that Cl^- imaging may be done in identified cell-classes and hopefully in subcellular compartments. Here, we describe the historical background and motivation behind the development of these sensors and how they have been used so far. There are, however, still major limitations for their use, the most important being the fact that all GABs are sensitive to both pH and Cl^-. Disambiguating the two signals has proved a major challenge, but there are potential solutions; notable among these is ClopHensor, which has now been developed for in vivo measurements of both ion species. We also speculate on how these biosensors may yet be improved, and how this could advance our understanding of Cl^- regulation and its impact on brain function.

Carbonic anhydrase seven bundles filamentous actin and regulates dendritic spine morphology and density

Intracellular pH is a potent modulator of neuronal functions. By catalyzing (de)hydration of CO2 , intracellular carbonic anhydrase (CAi ) isoforms CA2 and CA7 contribute to neuronal pH buffering and dynamics. The presence of two highly active isoforms in neurons suggests that they may serve isozyme-specific functions unrelated to CO2 -(de)hydration. Here, we show that CA7, unlike CA2, binds to filamentous actin, and its overexpression induces formation of thick actin bundles and membrane protrusions in fibroblasts. In CA7-overexpressing neurons, CA7 is enriched in dendritic spines, which leads to aberrant spine morphology. We identified amino acids unique to CA7 that are required for direct actin interactions, promoting actin filament bundling and spine targeting. Disruption of CA7 expression in neocortical neurons leads to higher spine density due to increased proportion of small spines. Thus, our work demonstrates highly distinct subcellular expression patterns of CA7 and CA2, and a novel, structural role of CA7.

High-performance extended-gate ion-sensitive field-effect transistors with multi-gate structure for transparent, flexible, and wearable biosensors

In this study, we developed a high-performance extended-gate ion-sensitive field-effect transistor (EG-ISFET) sensor on a flexible polyethylene naphthalate (PEN) substrate. The EG-ISFET sensor comprises a tin dioxide (SnO₂) extended gate, which acts as a detector, and an amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) for a transducer. In order to self-amplify the sensitivity of the pH sensors, we designed a uniquely-structured a-IGZO TFT transducer with a high-k engineered top gate insulator consisting of a silicon dioxide/tantalum pentoxide (SiO₂/Ta₂O₅) stack, a floating layer under the channel, and a control gate coplanar with the channel. The SiO₂/Ta₂O₅ stacked top gate insulator and in-plane control gate significantly contribute to capacitive coupling, enabling the amplification of sensitivity to be enlarged compared to conventional dual-gate transducers. For a pH sensing method suitable for EG-ISFET sensors, we propose an in-plane control gate (IG) sensing mode, instead of conventional single-gate (SG) or dual-gate (DG) sensing modes. As a result, a pH sensitivity of 2364 mV/pH was achieved at room temperature - this is significantly superior to the Nernstian limit (59.15 mV/pH at room temperature). In addition, we found that non-ideal behavior was improved in hysteresis and drift measurements. Therefore, the proposed transparent EGISFFET sensor with an IG sensing mode is expected to become a promising platform for flexible and wearable biosensing applications.

Extracellular pH modulation of excitatory synaptic transmission in hippocampal CA3 neurons

In this study, the effect of extracellular pH on glutamatergic synaptic transmission was examined in mechanically dissociated rat hippocampal CA3 pyramidal neurons using a whole-cell patch-clamp technique under voltage-clamp conditions. Native synaptic boutons were isolated without using any enzymes, using a so-called "synapse bouton preparation," and preserved for the electrical stimulation of single boutons. Both the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) were found to decrease and increase in response to modest acidic (~pH 6.5) and basic (~pH 8.5) solutions, respectively. These changes in sEPSC frequency were not affected by the addition of TTX but completely disappeared by successive addition of Cd²⁺. However, changes in sEPSC amplitude induced by acidic and basic extracellular solutions were not affected by the addition of neither TTX nor Cd²⁺. The glutamate-induced whole-cell currents were decreased and increased by acidic and basic solutions, respectively. Acidic pH also decreased the amplitude and increased the failure rate (R_f) and paired-pulse rate (PPR) of glutamatergic electrically evoked excitatory postsynaptic currents (eEPSCs), while a basic pH increased the amplitude and decreased both the R_f and PPR of eEPSCs. The kinetics of the currents were not affected by changes in pH. Acidic and basic solutions decreased and increased voltage-gated Ca²⁺ but not Na⁺ channel currents in the dentate gyrus granule cell bodies. Our results indicate that extracellular pH modulates excitatory transmission via both pre- and postsynaptic sites, with the presynaptic modulation correlated to changes in voltage-gated Ca²⁺ channel currents.NEW & NOTEWORTHY The effects of external pH changes on spontaneous, miniature, and evoked excitatory synaptic transmission in CA3 hippocampal synapses were examined using the isolated nerve bouton preparation, which allowed for the accurate regulation of extracellular pH at the synapses. Acidification generally reduced transmission, partly via effects on presynaptic Ca²⁺ channel currents, while alkalization generally enhanced transmission. Both pre- and postsynaptic sites contributed to these effects.

Ultrasound Neuromodulation Inhibits Seizures in Acute Epileptic Monkeys

Ultrasound stimulation has recently emerged as a non-invasive method for modulating brain activity in animal and human studies with healthy subjects. Whether brain diseases such as Alzheimer's disease, epilepsy, and depression can be treated using ultrasound stimulation still needs to be explored. Recent studies have reported that ultrasound stimulation suppressed epileptic seizures in a rodent model of epilepsy. These findings raise the crucial question of whether ultrasound stimulation can inhibit seizures in non-human primates with epilepsy. Here, we addressed this critical question. We confirmed that ultrasound stimulation significantly reduced the frequency of seizures in acute epileptic monkeys. Furthermore, the results showed that the number and duration of seizures were reduced, whereas the inter-seizure interval was increased after ultrasound stimulation. Besides, no significant brain tissue damage was observed by T2-weighted MR imaging. Our results are of great importance for future clinical applications of ultrasound neuromodulation in patients with epilepsy.

Seizure initiation in infantile spasms vs. focal seizures: proposed common cellular mechanisms

Infantile spasms (IS) and seizures with focal onset have different clinical expressions, even when electroencephalography (EEG) associated with IS has some degree of focality. Oddly, identical pathology (with, however, age-dependent expression) can lead to IS in one patient vs. focal seizures in another or even in the same, albeit older, patient. We therefore investigated whether the cellular mechanisms underlying seizure initiation are similar in the two instances: spasms vs. focal. We noted that in-common EEG features can include (i) a background of waves at alpha to delta frequencies; (ii) a period of flattening, lasting about a second or more - the electrodecrement (ED); and (iii) often an interval of very fast oscillations (VFO; ~70 Hz or faster) preceding, or at the beginning of, the ED. With IS, VFO temporally coincides with the motor spasm. What is different between the two conditions is this: with IS, the ED reverts to recurring slow waves, as occurring before the ED, whereas with focal seizures the ED instead evolves into an electrographic seizure, containing high-amplitude synchronized bursts, having superimposed VFO. We used in vitro data to help understand these patterns, as such data suggest cellular mechanisms for delta waves, for VFO, for seizure-related burst complexes containing VFO, and, more recently, for the ED. We propose a unifying mechanistic hypothesis - emphasizing the importance of brain pH - to explain the commonalities and differences of EEG signals in IS versus focal seizures.

Infantile Spasms: An Update on Pre-Clinical Models and EEG Mechanisms

Infantile spasms (IS) is an epileptic encephalopathy with unique clinical and electrographic features, which affects children in the middle of the first year of life. The pathophysiology of IS remains incompletely understood, despite the heterogeneity of IS etiologies, more than 200 of which are known. In particular, the neurobiological basis of why multiple etiologies converge to a relatively similar clinical presentation has defied explanation. Treatment options for this form of epilepsy, which has been described as “catastrophic” because of the poor cognitive, developmental, and epileptic prognosis, are limited and not fully effective. Until the pathophysiology of IS is better clarified, novel treatments will not be forthcoming, and preclinical (animal) models are essential for advancing this knowledge. Here, we review preclinical IS models, update information regarding already existing models, describe some novel models, and discuss exciting new data that promises to advance understanding of the cellular mechanisms underlying the specific EEG changes seen in IS—interictal hypsarrhythmia and ictal electrodecrement.

Selective modulation of neuronal firing by pulse stimulations with different frequencies in rat hippocampus

Background

Deep brain stimulation (DBS) has a good prospect for treating many brain diseases. Recent studies have shown that axonal activation induced by pulse stimulations may play an important role in DBS therapies through wide projections of axonal fibers. However, it is undetermined whether the downstream neurons are inhibited or excited by axonal stimulation. The present study addressed the question in rat hippocampus by in vivo experiments.

Methods

Pulse stimulations with different frequencies (10–400 Hz) were applied to the Schaffer collateral, the afferent fiber of hippocampal CA1 region in anaesthetized rats. Single-unit spikes of interneurons and pyramidal cells in the downstream region of stimulation were recorded and evaluated.

Results

Stimulations with a lower frequency (10 or 20 Hz) did not change the firing rates of interneurons but decreased the firing rates of pyramidal cells (the principal neurons) significantly. The phase-locked firing of interneurons during these stimulations might increase the efficacy of GABAergic inhibitions on the principal neurons. However, stimulations with a higher frequency (100–400 Hz) increased the firing rates of both types of the neurons significantly. In addition, the increases of interneurons’ firing were greater than the increases of pyramidal cells. Presumably, increase of direct excitation from afferent impulses together with failure of GABAergic inhibition might result in the increase of pyramidal cells’ firing by a higher stimulation frequency. Furthermore, silent periods appeared immediately following the cessation of stimulations, indicating a full control of the neuronal firing by the stimulation pulses during axonal stimulation. Furthermore longer silent periods were associated with higher stimulation frequencies.

Conclusions

Low-frequency (10–20 Hz) and high-frequency (100–400 Hz) stimulations of afferent axonal fibers exerted opposite effects on principal neurons in rat hippocampus CA1. These results provide new information for advancing deep brain stimulation to treat different brain disorders.

KCC2 overexpression prevents the paradoxical seizure-promoting action of somatic inhibition

Although cortical interneurons are apparently well-placed to suppress seizures, several recent reports have highlighted a paradoxical role of perisomatic-targeting parvalbumin-positive (PV+) interneurons in ictogenesis. Here, we use an acute in vivo model of focal cortical seizures in awake behaving mice, together with closed-loop optogenetic manipulation of PV+ interneurons, to investigate their function during seizures. We show that photo-depolarization of PV+ interneurons rapidly switches from an anti-ictal to a pro-ictal effect within a few seconds of seizure initiation. The pro-ictal effect of delayed photostimulation of PV+ interneurons was not shared with dendrite-targeting somatostatin-positive (SOM+) interneurons. We also show that this switch can be prevented by overexpression of the neuronal potassium-chloride co-transporter KCC2 in principal cortical neurons. These results suggest that strategies aimed at improving the ability of principal neurons to maintain a trans-membrane chloride gradient in the face of excessive network activity can prevent interneurons from contributing to seizure perpetuation.

Region-specific differences and areal interactions underlying transitions in epileptiform activity

Key points

Local neocortical and hippocampal territories show different and sterotypical patterns of acutely evolving, epileptiform activity.

Neocortical and entorhinal networks show tonic–clonic‐like events, but the main hippocampal territories do not, unless it is relayed from the other areas.

Transitions in the pattern of locally recorded epileptiform activity can be indicative of a shift in the source of pathological activity, and may spread through both synaptic and non‐synaptic means.

Hippocampal epileptiform activity is promoted by 4‐aminopyridine and inhibited by GABA_B receptor agonists, and appears far more sensitive to these drugs than neocortical activity.

These signature features of local epileptiform activity can provide useful insight into the primary source of ictal activity, aiding both experimental and clinical investigation.

Abstract

Understanding the nature of epileptic state transitions remains a major goal for epilepsy research. Simple in vitro models offer unique experimental opportunities that we exploit to show that such transitions can arise from shifts in the ictal source of the activity. These transitions reflect the fact that cortical territories differ both in the type of epileptiform activity they can sustain and in their susceptibility to drug manipulation. In the zero‐Mg²⁺ model, the earliest epileptiform activity is restricted to neocortical and entorhinal networks. Hippocampal bursting only starts much later, and triggers a marked transition in neo‐/entorhinal cortical activity. Thereafter, the hippocampal activity acts as a pacemaker, entraining the other territories to their discharge pattern. This entrainment persists following transection of the major axonal pathways between hippocampus and cortex, indicating that it can be mediated through a non‐synaptic route. Neuronal discharges are associated with large rises in extracellular [K⁺], but we show that these are very localized, and therefore are not the means of entraining distant cortical areas. We conclude instead that the entrainment occurs through weak field effects distant from the pacemaker, but which are highly effective at recruiting other brain territories that are already hyperexcitable. The hippocampal epileptiform activity appears unusually susceptible to drugs that impact on K⁺ conductances. These findings demonstrate that the local circuitry gives rise to stereotypical epileptic activity patterns, but these are also influenced by both synaptic and non‐synaptic long‐range effects. Our results have important implications for our understanding of epileptic propagation and anti‐epileptic drug action.

Local neocortical and hippocampal territories show different and sterotypical patterns of acutely evolving, epileptiform activity.

Neocortical and entorhinal networks show tonic–clonic‐like events, but the main hippocampal territories do not, unless it is relayed from the other areas.

Transitions in the pattern of locally recorded epileptiform activity can be indicative of a shift in the source of pathological activity, and may spread through both synaptic and non‐synaptic means.

Hippocampal epileptiform activity is promoted by 4‐aminopyridine and inhibited by GABA_B receptor agonists, and appears far more sensitive to these drugs than neocortical activity.

These signature features of local epileptiform activity can provide useful insight into the primary source of ictal activity, aiding both experimental and clinical investigation.

Silicon Nanoribbon pH Sensors Protected by a Barrier Membrane with Carbon Nanotube Porins

Limited biocompatibility and fouling propensity can restrict real-world applications of a large variety of biosensors. Biological systems are adept at protecting and separating vital components of biological machinery with semipermeable membranes that often contain defined pores and gates to restrict transmembrane transport only to specific species. Here we use a similar approach for creating fouling-resistant pH sensors. We integrate silicon nanoribbon transistor sensors with an antifouling lipid bilayer coating that contains proton-permeable carbon nanotube porin (CNTP) channels and demonstrate robust pH detection in a variety of complex biological fluids.

GABAergic Interneurons in Seizures: Investigating Causality With Optogenetics

Seizures are complex pathological network events characterized by excessive and hypersynchronized activity of neurons, including a highly diverse population of GABAergic interneurons. Although the primary function of inhibitory interneurons under normal conditions is to restrain excitation in the brain, this system appears to fail intermittently, allowing runaway excitation. Recent developments in optogenetics, combined with genetic tools and advanced electrophysiological and imaging techniques, allow us for the first time to assess the causal roles of identified cell-types in network dynamics. While these methods have greatly increased our understanding of cortical microcircuits in epilepsy, the roles played by individual GABAergic cell-types in controlling ictogenesis remain incompletely resolved. Indeed, the ability of interneurons to suppress epileptic discharges varies across different subtypes, and an accumulating body of evidence paradoxically implicates some interneuron subtypes in the initiation and maintenance of epileptiform activity. Here, we bring together findings from this growing field and discuss what can be inferred regarding the causal role of different GABAergic cell-types in seizures.

Bioenergetic Mechanisms of Seizure Control

Epilepsy is characterized by the regular occurrence of seizures, which follow a stereotypical sequence of alterations in the electroencephalogram. Seizures are typically a self limiting phenomenon, concluding finally in the cessation of hypersynchronous activity and followed by a state of decreased neuronal excitability which might underlie the cognitive and psychological symptoms the patients experience in the wake of seizures. Many efforts have been devoted to understand how seizures spontaneously stop in hope to exploit this knowledge in anticonvulsant or neuroprotective therapies. Besides the alterations in ion-channels, transmitters and neuromodulators, the successive build up of disturbances in energy metabolism have been suggested as a mechanism for seizure termination. Energy metabolism and substrate supply of the brain are tightly regulated by different mechanisms called neurometabolic and neurovascular coupling. Here we summarize the current knowledge whether these mechanisms are sufficient to cover the energy demand of hypersynchronous activity and whether a mismatch between energy need and supply could contribute to seizure control.

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.

Depolarized GABAergic Signaling in Subicular Microcircuits Mediates Generalized Seizure in Temporal Lobe Epilepsy

Secondary generalized seizure (sGS) is a major source of disability in temporal lobe epilepsy (TLE) with unclear cellular/circuit mechanisms. Here we found that clinical TLE patients with sGS showed reduced volume specifically in the subiculum compared with those without sGS. Further, using optogenetics and extracellular electrophysiological recording in mouse models, we found that photoactivation of subicular GABAergic neurons retarded sGS acquisition by inhibiting the firing of pyramidal neurons. Once sGS had been stably acquired, photoactivation of GABAergic neurons aggravated sGS expression via depolarized GABAergic signaling. Subicular parvalbumin, but not somatostatin subtype GABAergic, neurons were easily depolarized in sGS expression. Finally, photostimulation of subicular pyramidal neurons genetically targeted with proton pump Arch, rather than chloride pump NpHR3.0, alleviated sGS expression. These results demonstrated that depolarized GABAergic signaling in subicular microcircuit mediates sGS in TLE. This may be of therapeutic interest in understanding the pathological neuronal circuitry underlying sGS. VIDEO ABSTRACT.

Development of a Robust Mammalian Cell-based Assay for Studying Recombinant α4 β1/3 δ GABAA Receptor Subtypes

δ-Containing GABA_A receptors are located extrasynaptically and mediate tonic inhibition. Their involvement in brain physiology positions them as interesting drug targets. There is thus a continued interest in establishing reliable recombinant expression systems for δ-containing GABA_A receptors. Inconveniently, the recombinant expression of especially α₄ β_1/3 δ receptors has been found to be notoriously difficult, resulting in mixed receptor populations and/or stoichiometries and differential pharmacology depending on the expression system used. With the aim of developing a facile and robust 96-well format cell-based assay for extrasynaptic α₄ β_1/3 δ receptors, we have engineered and validated a HEK293 Flp-In™ cell line stably expressing the human GABA_A δ-subunit. Upon co-transfection of α₄ and β_1/3 subunits, at optimized ratios, we have established a well-defined system for expressing α₄ β_1/3 δ receptors and used the fluorescence-based FLIPR Membrane Potential (FMP) assay to evaluate their pharmacology. Using the known reference compounds GABA and THIP, ternary α₄ β_1/3 δ and binary α₄ β_1/3 receptors could be distinguished based on potency and kinetic profiles but not efficacy. As expected, DS2 was able to potentiate only δ-containing receptors, whereas Zn²⁺ had an inhibitory effect only at binary receptors. By contrast, the hitherto reported δ-selective compounds, AA29504 and 3-OH-2'MeO6MF, were non-selective. The expression system was further validated using patch clamp electrophysiology, in which the superagonism of THIP was confirmed. The established FMP assay set-up, based on transient expression of human α₄ and β_1/3 subunits into a δ-subunit stable HEK293 Flp-In™ cell line, portrays a simple 96-well format assay as a useful supplement to electrophysiological recordings on δ-containing GABA_A receptors.

Behavioral disinhibition and antiepileptic treatment in childhood epilepsy: A retrospective cohort study

OBJECTIVE

To test whether specific classes of antiepileptic drugs increase the risk for behavioral disinhibition, a frequent complication of treatment of childhood epilepsy.

METHODS

In a sample of children with active epilepsy and antiepileptic drug (AED) treatment (n = 146, age 4-17 years), we performed a retrospective chart analysis of the occurrence of symptoms indicating reduced behavioral disinhibition following AED treatment. We used a risk-set approach to analyze whether the presence or recent addition of AED categories defined by their mechanism of action were associated with enhanced risk for behavioral disinhibition symptoms.

RESULTS

Mean duration of follow-up was 2,343 days (range 218-6,292, standard deviation [SD] 1,437). Episodes of behavioral disinhibition were reported in 51 (34.9%) children, with variable latencies between latest change and occurrence of behavioral disinhibition symptoms (mean 67 days, range 2-367). Current use of AEDs targeting gamma-aminobutyric acid (GABA) (odds ratio [OR] 1.8, 95% confidence interval [CI] 1.02-3.29, p = 0.04) and SV2A-mediated neurotransmitter release (SV2A)-mediated (2.0, 1.13-3.60, p = 0.02) neurotransmitter release was associated with increased risk for behavioral disinhibition. Restricting the analysis to the 90 days before behavioral disinhibition episode occurrence revealed that only addition of GABAergic AEDs (OR = 26.88, 95% CI = 6.71-107.76, p < 0.001) was associated with behavioral disinhibition. In contrast to our expectations, seizure control was reported to have improved parallel to most behavioral disinhibition episodes.

SIGNIFICANCE

This exploration of behavioral disinhibition in relation to antiepileptic drug treatment indicates that GABA potentiating drugs are specifically associated with behavioral problems during treatment of childhood epilepsy. Behavioral disinhibition episodes often occurred while seizure control improved, which may have reduced alertness for the consequences of AEDs on interictal symptoms. Our findings may be related to the increasing evidence for a role for excitatory actions of GABA in childhood epilepsy.

Oscillating Magnet Array-Based Nanomagnetic Gene Transfection: A Valuable Tool for Molecular Neurobiology Studies

To develop treatments for neurodegenerative disorders, it is critical to understand the biology and function of neurons in both normal and diseased states. Molecular studies of neurons involve the delivery of small biomolecules into cultured neurons via transfection to study genetic variants. However, as cultured primary neurons are sensitive to temperature change, stress, and shifts in pH, these factors make biomolecule delivery difficult, particularly non-viral delivery. Herein we used oscillating nanomagnetic gene transfection to successfully transfect SH-SY5Y cells as well as primary hippocampal and cortical neurons on different days in vitro. This novel technique has been used to effectively deliver genetic material into various cell types, resulting in high transfection efficiency and viability. From these observations and other related studies, we suggest that oscillating nanomagnetic gene transfection is an effective method for gene delivery into hard-to-transfect neuronal cell types.

Optogenetic study of networks in epilepsy

Currently, approximately 30% of patients with epilepsy do not have adequate seizure control. A greater understanding of the underlying mechanisms by which seizures start or propagate could lead to new therapeutic strategies. The recent development of optogenetics, because of its unprecedented precision for controlling activity within distinct neuronal populations, has revolutionized neuroscience, including epilepsy research. This Review discusses recent breakthroughs made with optogenetics in epilepsy research. These breakthroughs include new insights into the key roles that different cell types play in mediating seizures as well as in the development of epilepsy. Subsequently, we discuss how targeting different brain regions and cell populations has opened up the possibility of highly specific therapies that can stop seizures on demand. Finally, we illustrate how combining newly available neuroscience tools with whole-brain imaging techniques will allow researchers to understand better the spread of seizures on a network level. © 2016 The Authors. Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Seizing Control: From Current Treatments to Optogenetic Interventions in Epilepsy

The unpredictability and severity of seizures contribute to the debilitating nature of epilepsy. These factors also render the condition particularly challenging to treat, as an ideal treatment would need to detect and halt the pathological bursts of hyperactivity without disrupting normal brain activity. Optogenetic techniques offer promising tools to study and perhaps eventually treat this episodic disorder by controlling specific brain circuits in epileptic animals with great temporal precision. Here, we briefly review the current treatment options for patients with epilepsy. We then describe the many ways optogenetics has allowed us to untangle the microcircuits involved in seizure activity, and how it has, in some cases, changed our perception of previous theories of seizure generation. Control of seizures with light is no longer a dream, and has been achieved in numerous different animal models of epilepsy. Beyond its application as a seizure suppressor, we highlight another facet of optogenetics in epilepsy, namely the ability to create "on-demand" seizures, as a tool to systematically probe the dynamics of networks during seizure initiation and propagation. Finally, we look into the future to discuss the possibilities and challenges of translating optogenetic techniques to clinical use.

GABAergic networks jump-start focal seizures

Abnormally enhanced glutamatergic excitation is commonly believed to mark the onset of a focal seizure. This notion, however, is not supported by firm evidence, and it will be challenged here. A general reduction of unit firing has been indeed observed in association with low-voltage fast activity at the onset of seizures recorded during presurgical intracranial monitoring in patients with focal, drug-resistant epilepsies. Moreover, focal seizures in animal models start with increased γ-aminobutyric acid (GABA)ergic interneuronal activity that silences principal cells. In vitro studies have shown that synchronous activation of GABAA receptors occurs at seizure onset and causes sizeable elevations in extracellular potassium, thus facilitating neuronal recruitment and seizure progression. A paradoxical involvement of GABAergic networks is required for the initiation of focal seizures characterized by low-voltage fast activity, which represents the most common seizure-onset pattern in focal epilepsies.

GABAergic Synchronization in Epilepsy

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the cerebral cortex. GABAergic inhibition enables synchronization of activity in cortical networks, and contributes to generation of variety of brain activity patterns. In relation to epilepsy, GABAergic inhibition has been traditionally viewed as the main mechanism counterbalancing glutamatergic excitation and preventing hypersynchronous neuronal discharges. Indeed, deficits in GABAergic functions most commonly result in a hyperexcitable epileptic state, and many of the currently used antiepileptic drugs act through enhancement of GABAergic functions. However, a number of observations show that some epileptiform activity patterns involve synchronization by GABAergic mechanisms. These include two main categories that will be reviewed here: (1) synchronization of epileptiform oscillations based on GABAergic inhibition, and (2) epileptiform events driven by depolarizing and excitatory GABA. The conclusion is reached that GABAergic control of spike timing, either through inhibition or excitation under certain conditions, may work as a powerful synchronizing mechanism during epilepsy.

Proton transfer unlocks inactivation in cyclic nucleotide-gated A1 channels

KEY POINTS

Desensitization and inactivation provide a form of short-term memory controlling the firing patterns of excitable cells and adaptation in sensory systems. Unlike many of their cousin K(+) channels, cyclic nucleotide-gated (CNG) channels are thought not to desensitize or inactivate. Here we report that CNG channels do inactivate and that inactivation is controlled by extracellular protons. Titration of a glutamate residue within the selectivity filter destabilizes the pore architecture, which collapses towards a non-conductive, inactivated state in a process reminiscent of the usual C-type inactivation observed in many K(+) channels. These results indicate that inactivation in CNG channels represents a regulatory mechanism that has been neglected thus far, with possible implications in several physiological processes ranging from signal transduction to growth cone navigation.

ABSTRACT

Ion channels control ionic fluxes across biological membranes by residing in any of three functionally distinct states: deactivated (closed), activated (open) or inactivated (closed). Unlike many of their cousin K(+) channels, cyclic nucleotide-gated (CNG) channels do not desensitize or inactivate. Using patch recording techniques, we show that when extracellular pH (pHo ) is decreased from 7.4 to 6 or lower, wild-type CNGA1 channels inactivate in a voltage-dependent manner. pHo titration experiments show that at pHo  < 7 the I-V relationships are outwardly rectifying and that inactivation is coupled to current rectification. Single-channel recordings indicate that a fast mechanism of proton blockage underlines current rectification while inactivation arises from conformational changes downstream from protonation. Furthermore, mutagenesis and ionic substitution experiments highlight the role of the selectivity filter in current decline, suggesting analogies with the C-type inactivation observed in K(+) channels. Analysis with Markovian models indicates that the non-independent binding of two protons within the transmembrane electrical field explains both the voltage-dependent blockage and the inactivation. Low pH, by inhibiting the CNGA1 channels in a state-dependent manner, may represent an unrecognized endogenous signal regulating CNG physiological functions in diverse tissues.

Depolarizing GABA and developmental epilepsies

Early in development, GABA, which is the main inhibitory neurotransmitter in adult brain, depolarizes immature neurons and exerts dual--excitatory and shunting/inhibitory--effects in the developing neuronal networks. The present review discusses some general questions, including the properties of excitation at depolarizing GABAergic synapse and shunting inhibition by depolarizing GABA; technical issues in exploration of depolarizing GABA using various techniques and preparations, including the developmental aspects of traumatic injury and what is known (or rather unknown) on the actions of GABA in vivo; complex roles of depolarizing GABA in developmental epilepsies, including a contribution of depolarizing GABA to enhanced excitability in the immature networks, caused by repetitive seizures accumulation of intracellular chloride concentration that increases excitatory GABA power and its synchronizing proconvulsive effects, and correction of chloride homeostasis as a potential strategy to treat neonatal seizures.

Cation-chloride cotransporters in neuronal development, plasticity and disease

Electrical activity in neurons requires a seamless functional coupling between plasmalemmal ion channels and ion transporters. Although ion channels have been studied intensively for several decades, research on ion transporters is in its infancy. In recent years, it has become evident that one family of ion transporters, cation-chloride cotransporters (CCCs), and in particular K(+)-Cl(-) cotransporter 2 (KCC2), have seminal roles in shaping GABAergic signalling and neuronal connectivity. Studying the functions of these transporters may lead to major paradigm shifts in our understanding of the mechanisms underlying brain development and plasticity in health and disease.

Neuronal networks and energy bursts in epilepsy

Epilepsy can be defined as the abnormal activities of neurons. The occurrence, propagation and termination of epileptic seizures rely on the networks of neuronal cells that are connected through both synaptic- and non-synaptic interactions. These complicated interactions contain the modified functions of normal neurons and glias as well as the mediation of excitatory and inhibitory mechanisms with feedback homeostasis. Numerous spread patterns are detected in disparate networks of ictal activities. The cortical-thalamic-cortical loop is present during a general spike wave seizure. The thalamic reticular nucleus (nRT) is the major inhibitory input traversing the region, and the dentate gyrus (DG) controls CA3 excitability. The imbalance between γ-aminobutyric acid (GABA)-ergic inhibition and glutamatergic excitation is the main disorder in epilepsy. Adjustable negative feedback that mediates both inhibitory and excitatory components affects neuronal networks through neurotransmission fluctuation, receptor and transmitter signaling, and through concomitant influences on ion concentrations and field effects. Within a limited dynamic range, neurons slowly adapt to input levels and have a high sensitivity to synaptic changes. The stability of the adapting network depends on the ratio of the adaptation rates of both the excitatory and inhibitory populations. Thus, therapeutic strategies with multiple effects on seizures are required for the treatment of epilepsy, and the therapeutic functions on networks are reviewed here. Based on the high-energy burst theory of epileptic activity, we propose a potential antiepileptic therapeutic strategy to transfer the high energy and extra electricity out of the foci.

Intracellular bicarbonate regulates action potential generation via KCNQ channel modulation

Bicarbonate (HCO3(-)) is an abundant anion that regulates extracellular and intracellular pH. Here, we use patch-clamp techniques to assess regulation of hippocampal CA3 pyramidal cell excitability by HCO3(-) in acute brain slices from C57BL/6 mice. We found that increasing HCO3(-) levels enhances action potential (AP) generation in both the soma and axon initial segment (AIS) by reducing Kv7/KCNQ channel activity, independent of pH (i.e., at a constant pH of 7.3). Conversely, decreasing intracellular HCO3(-) leads to attenuation of AP firing. We show that HCO3(-) interferes with Kv7/KCNQ channel activation by phosphatidylinositol-4,5-biphosphate. Consequently, we propose that, even in the presence of a local depolarizing Cl(-) gradient, HCO3(-) efflux through GABAA receptors may ensure the inhibitory effect of axoaxonic cells at the AIS due to activation of Kv7/KCNQ channels.

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.

How do we make models that are useful in understanding partial epilepsies?

The goals of constructing epilepsy models are (1) to develop approaches to prophylaxis of epileptogenesis following cortical injury; (2) to devise selective treatments for established epilepsies based on underlying pathophysiological mechanisms; and (3) use of a disease (epilepsy) model to explore brain molecular, cellular and circuit properties. Modeling a particular epilepsy syndrome requires detailed knowledge of key clinical phenomenology and results of human experiments that can be addressed in critically designed laboratory protocols. Contributions to understanding mechanisms and treatment of neurological disorders has often come from research not focused on a specific disease-relevant issue. Much of the foundation for current research in epilepsy falls into this category. Too strict a definition of the relevance of an experimental model to progress in preventing or curing epilepsy may, in the long run, slow progress. Inadequate exploration of the experimental target and basic laboratory results in a given model can lead to a failed effort and false negative or positive results. Models should be chosen based on the specific issues to be addressed rather than on convenience of use. Multiple variables including maturational age, species and strain, lesion type, severity and location, latency from injury to experiment and genetic background will affect results. A number of key issues in clinical and basic research in partial epilepsies remain to be addressed including the mechanisms active during the latent period following injury, susceptibility factors that predispose to epileptogenesis, injury - induced adaptive versus maladaptive changes, mechanisms of pharmaco-resistance and strategies to deal with multiple pathophysiological processes occurring in parallel.

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.

Modulation of neuronal activity by phosphorylation of the K-Cl cotransporter KCC2

The K-Cl cotransporter KCC2 establishes the low intraneuronal Cl- levels required for the hyperpolarizing inhibitory postsynaptic potentials mediated by ionotropic γ-aminobutyric acid receptors (GABAARs) and glycine receptors (GlyRs). Decreased KCC2-mediated Cl- extrusion and impaired hyperpolarizing GABAAR- and/or GlyR-mediated currents have been implicated in epilepsy, neuropathic pain, and spasticity. Recent evidence suggests that the intrinsic ion transport rate, cell surface stability, and plasmalemmal trafficking of KCC2 are rapidly and reversibly modulated by the (de)phosphorylation of critical serine, threonine, and tyrosine residues in the C terminus of this protein. Alterations in KCC2 phosphorylation have been associated with impaired KCC2 function in several neurological diseases. Targeting KCC2 phosphorylation directly or indirectly via upstream regulatory kinases might be a novel strategy to modulate GABA- and/or glycinergic signaling for therapeutic benefit.

Single-unit activities during epileptic discharges in the human hippocampal formation

Between seizures the brain of patients with epilepsy generates pathological patterns of synchronous activity, designated as interictal epileptiform discharges (ID). Using microelectrodes in the hippocampal formations of 8 patients with drug-resistant temporal lobe epilepsy, we studied ID by simultaneously analyzing action potentials from individual neurons and the local field potentials (LFPs) generated by the surrounding neuronal network. We found that ~30% of the units increased their firing rate during ID and 40% showed a decrease during the post-ID period. Surprisingly, 30% of units showed either an increase or decrease in firing rates several hundred of milliseconds before the ID. In 4 patients, this pre-ID neuronal firing was correlated with field high-frequency oscillations at 40-120 Hz. Finally, we observed that only a very small subset of cells showed significant coincident firing before or during ID. Taken together, we suggested that, in contrast to traditional views, ID are generated by a sparse neuronal network and followed a heterogeneous synchronization process initiated over several hundreds of milliseconds before the paroxysmal discharges.

Effect of acute acid-base disturbances on ErbB1/2 tyrosine phosphorylation in rabbit renal proximal tubules

The renal proximal tubule (PT) is a major site for maintaining whole body pH homeostasis and is responsible for reabsorbing ∼80% of filtered HCO3(-), the major plasma buffer, into the blood. The PT adapts its rate of HCO3(-) reabsorption (JHCO3(-)) in response to acute acid-base disturbances. Our laboratory previously showed that single isolated perfused PTs adapt JHCO3(-) in response to isolated changes in basolateral (i.e., blood side) CO2 and HCO3(-) concentrations but, surprisingly, not to pH. The response to CO2 concentration can be blocked by the ErbB family tyrosine kinase inhibitor PD-168393. In the present study, we exposed enriched rabbit PT suspensions to five acute acid-base disturbances for 5 and 20 min using a panel of phosphotyrosine (pY)-specific antibodies to determine the influence of each disturbance on pan-pY, ErbB1-specific pY (four sites), and ErbB2-specific pY (two sites). We found that each acid-base treatment generated a distinct temporal pY pattern. For example, the summated responses of the individual ErbB1/2-pY sites to each disturbance showed that metabolic acidosis (normal CO2 concentration and reduced HCO3(-) concentration) produced a transient summated pY decrease (5 vs. 20 min), whereas metabolic alkalosis produced a transient increase. Respiratory acidosis (normal HCO3(-) concentration and elevated CO2 concentration) had little effect on summated pY at 5 min but produced an elevation at 20 min, whereas respiratory alkalosis produced a reduction at 20 min. Our data show that ErbB1 and ErbB2 in the PT respond to acute acid-base disturbances, consistent with the hypothesis that they are part of the signaling cascade.

Fractalkine/CX3CL1 modulates GABAA currents in human temporal lobe epilepsy

PURPOSE

The chemokine fractalkine/CX3CL1 and its receptor CX3CR1 are widely expressed in the central nervous system (CNS). Recent evidence showed that CX3CL1 participates in inflammatory responses that are common features of CNS disorders, such as epilepsy. Mesial temporal lobe epilepsy (MTLE) is the prevalent form of focal epilepsy in adults, and hippocampal sclerosis (HS) represents the most common underlying pathologic abnormality, as demonstrated at autopsy and postresection studies. Relevant features of MTLE are a characteristic pattern of neuronal loss, as are astrogliosis and microglia activation. Several factors affect epileptogenesis in patients with MTLE, including a lack of γ-aminobutyric acid (GABA)ergic inhibitory efficacy. Therefore, experiments were designed to investigate whether, in MTLE brain tissues, CX3CL1 may influence GABAA receptor (GABAA R) mediated transmission, with a particular focus on the action of CX3CL1 on the use-dependent decrease (rundown) of the GABA-evoked currents (IGABA ), a feature underlying the reduction of GABAergic function in epileptic tissue.

METHODS

Patch-clamp recordings were obtained from cortical pyramidal neurons in slices from six MTLE patients after surgery. Alternatively, the cell membranes from epileptic brain tissues of 17 MTLE patients or from surgical samples and autopsies of nonepileptic patients were microtransplanted into Xenopus oocytes, and IGABA were recorded using the standard two-microelectrode voltage-clamp technique. Immunohistochemical staining and double-labeling studies were carried out on the same brain tissues to analyze CX3CR1 expression.

KEY FINDINGS

In native pyramidal neurons from cortical slices of patients with MTLE, CX3CL1 reduced IGABA rundown and affected the recovery of IGABA amplitude from rundown. These same effects were confirmed in oocytes injected with cortical and hippocampal MTLE membranes, whereas CX3CL1 did not influence IGABA in oocytes injected with nonepileptic tissues. Consistent with a specific effect of CX3CL1 on tissues from patients with MTLE, CX3CR1 immunoreactivity was higher in MTLE sclerotic hippocampi than in control tissues, with a prominent expression in activated microglial cells.

SIGNIFICANCE

These findings indicate a role for CX3CL1 in MTLE, supporting recent evidence on the relevance of brain inflammation in human epilepsies. Our data demonstrate that in MTLE tissues the reduced GABAergic function can be modulated by CX3CL1. The increased CX3CR1 expression in microglia and the modulation by CX3CL1 of GABAergic currents in human epileptic brain suggests new therapeutic approaches for drug-resistant epilepsies based on the evidence that the propagation of seizures can be influenced by inflammatory processes.