Lowering of extracellular pH suppresses low-Mg(2+)-induces seizures in combined entorhinal cortex-hippocampal slices

Seizure control via pH manipulation: a phase II double-blind randomised controlled trial of inhaled carbogen as adjunctive treatment of paediatric convulsive status epilepticus (Carbogen for Status Epilepticus in Children Trial (CRESCENT))

BACKGROUND

Paediatric convulsive status epilepticus is the most common neurological emergency presenting to emergency departments. Risks of resultant neurological morbidity and mortality increase with seizure duration. If the seizure fails to stop within defined time-windows, standard care follows an algorithm of stepwise escalation to more intensive treatments, ultimately resorting to induction of general anaesthesia and ventilation. Additionally, ventilatory support may also be required to treat respiratory depression, a common unwanted effect of treatment. There is strong pre-clinical evidence that pH (acid-base balance) is an important determinant of seizure commencement and cessation, with seizures tending to start under alkaline conditions and terminate under acidic conditions. These mechanisms may be particularly important in febrile status epilepticus: prolonged fever-related seizures which predominantly affect very young children. This trial will assess whether imposition of mild respiratory acidosis by manipulation of inhaled medical gas improves response rates to first-line medical treatment.

METHODS

A double-blind, placebo-controlled trial of pH manipulation as an adjunct to standard medical treatment of convulsive status epilepticus in children. The control arm receives standard medical management whilst inhaling 100% oxygen; the active arm receives standard medical management whilst inhaling a commercially available mixture of 95% oxygen, 5% carbon dioxide known as 'carbogen'. Due to the urgent need to treat the seizure, deferred consent is used. The primary outcome is success of first-line treatment in seizure cessation. Planned subgroup analyses will be undertaken for febrile and non-febrile seizures. Secondary outcomes include rates of induction of general anaesthesia, admission to intensive care, adverse events, and 30-day mortality.

DISCUSSION

If safe and effective 95% oxygen, 5% carbon dioxide may be an important adjunct in the management of convulsive status epilepticus with potential for pre-hospital use by paramedics, families, and school staff.

TRIAL REGISTRATION

EudraCT: 2021-005367-49. CTA: 17136/0300/001.

ISRCTN

52731862. Registered on July 2022.

Neurovascular coupling is optimized to compensate for the increase in proton production from nonoxidative glycolysis and glycogenolysis during brain activation and maintain homeostasis of pH, pCO2, and pO2

During transient brain activation cerebral blood flow (CBF) increases substantially more than cerebral metabolic rate of oxygen consumption (CMRO2) resulting in blood hyperoxygenation, the basis of BOLD-fMRI contrast. Explanations for the high CBF versus CMRO2 slope, termed neurovascular coupling (NVC) constant, focused on maintenance of tissue oxygenation to support mitochondrial ATP production. However, paradoxically the brain has a 3-fold lower oxygen extraction fraction (OEF) than other organs with high energy requirements, like heart and muscle during exercise. Here, we hypothesize that the NVC constant and the capillary oxygen mass transfer coefficient (which in combination determine OEF) are co-regulated during activation to maintain simultaneous homeostasis of pH and partial pressure of CO2 and O2 (pCO2 and pO2). To test our hypothesis, we developed an arteriovenous flux balance model for calculating blood and brain pH, pCO2, and pO2 as a function of baseline OEF (OEF0), CBF, CMRO2, and proton production by nonoxidative metabolism coupled to ATP hydrolysis. Our model was validated against published brain arteriovenous difference studies and then used to calculate pH, pCO2, and pO2 in activated human cortex from published calibrated fMRI and PET measurements. In agreement with our hypothesis, calculated pH, pCO2, and pO2 remained close to constant independently of CMRO2 in correspondence to experimental measurements of NVC and OEF0. We also found that the optimum values of the NVC constant and OEF0 that ensure simultaneous homeostasis of pH, pCO2, and pO2 were remarkably similar to their experimental values. Thus, the high NVC constant is overall determined by proton removal by CBF due to increases in nonoxidative glycolysis and glycogenolysis. These findings resolve the paradox of the brain's high CBF yet low OEF during activation, and may contribute to explaining the vulnerability of brain function to reductions in blood flow and capillary density with aging and neurovascular disease.

Mechanisms of infantile epileptic spasms syndrome: What have we learned from animal models?

The devastating developmental and epileptic encephalopathy of infantile epileptic spasms syndrome (IESS) has numerous causes, including, but not limited to, brain injury, metabolic, and genetic conditions. Given the stereotyped electrophysiologic, age-dependent, and clinical findings, there likely exists one or more final common pathways in the development of IESS. The identity of this final common pathway is unknown, but it may represent a novel therapeutic target for infantile spasms. Previous research on IESS has focused largely on identifying the neuroanatomic substrate using specialized neuroimaging techniques and cerebrospinal fluid analysis in human patients. Over the past three decades, several animal models of IESS were created with an aim to interrogate the underlying pathogenesis of IESS, to identify novel therapeutic targets, and to test various treatments. Each of these models have been successful at recapitulating multiple aspects of the human IESS condition. These animal models have implicated several different molecular pathways in the development of infantile spasms. In this review we outline the progress that has been made thus far using these animal models and discuss future directions to help researchers identify novel treatments for drug-resistant IESS.

Impact of Hypo- and Hyper-capnia on Spreading Depolarizations in Rat Cerebral Cortex

Patients with traumatic brain injury are typically maintained at low-normal levels of arterial partial pressure of carbon dioxide (PaCO2) to counteract the risk of elevated intracranial pressure during intensive care. However, several studies suggest that management at hypercarbic levels may have therapeutic benefit. Here we examined the impact of CO2 levels on spreading depolarizations (SD), a mechanism and marker of acute lesion development in stroke and brain trauma. In an acute preparation of mechanically ventilated (30/70 O2/N2) female rats, SDs were evoked by cortical KCl application and monitored by electrophysiology and laser doppler flowmetry; CO2 levels were adjusted by ventilator settings and supplemental CO2. During 90 min of KCl application, rats were maintained at hypocapnia (end-tidal CO2 22 ± 2 mmHg) or hypercapnia (57 ± 4 mmHg) but did not differ significantly in arterial pH (7.31 ± 0.10 vs. 7.22 ± 0.08, p = 0.31) or other variables. Surprisingly, there was no difference between groups in the number of SDs recorded (10.7 ± 4.2 vs. 11.7 ± 3.1; n = 3 rats/group; p = 0.75) nor in SD durations (64 ± 27 vs. 69 ± 37 sec, p = 0.54). In separate experiments (n = 3), hypoxia was induced by decreasing inhaled O2 to 10% and single SDs were induced under interleaved conditions of hypo-, normo-, and hypercapnia. No differences in SD duration were observed. In both normoxia and hypoxia experiments, however, mean arterial pressures were negatively correlated with SD durations (normoxia R2 = -0.29; hypoxia R2 = -0.61, p's < 0.001). Our results suggest that any therapeutic benefit of elevated CO2 therapy may be dependent on an acidic shift in pH or may only be observed in conditions of focal brain injury.

Rapid volume pulsation of the extracellular space coincides with epileptiform activity in mice and depends on the NBCe1 transporter

KEY POINTS

Extracellular space (ECS) rapid volume pulsation (RVP) accompanying epileptiform activity is described for the first time. Such RVP occurs robustly in several in vitro and in vivo mouse models of epileptiform activity. In the in vitro 4-aminopyridine model of epileptiform activity, RVP depends on the activity of the electrogenic Na⁺ /HCO₃ ^- cotransporter (NBCe1). NBCe1 pharmacological inhibition suppresses RVP and epileptiform activity. Inhibition of changes in ECS volume may be a useful target in epilepsy patients who are resistant to current treatments. ​ ABSTRACT: The extracellular space (ECS) of the brain shrinks persistently by approximately 35% during epileptic seizures. Here we report the discovery of rapid volume pulsation (RVP), further transient drops in ECS volume which accompany events of epileptiform activity. These transient ECS contractions were observed in multiple mouse models of epileptiform activity both in vivo (bicuculline methiodide model) and in vitro (hyaluronan synthase 3 knock-out, picrotoxin, bicuculline and 4-aminopyridine models). By using the probe transients quantification (PTQ) method we show that individual pulses of RVP shrank the ECS by almost 15% in vivo. In the 4-aminopyridine in vitro model, the individual pulses of RVP shrank the ECS by more than 4%, and these transient changes were superimposed on a persistent ECS shrinkage of 36% measured with the real-time iontophoretic method. In this in vitro model, we investigated several channels and transporters that may be required for the generation of RVP and epileptiform activity. Pharmacological blockages of Na⁺ /K⁺ /2Cl^- cotransporter type 1 (NKCC1), K⁺ /Cl^- cotransporter (KCC2), the water channel aquaporin-4 (AQP4) and inwardly rectifying potassium channel 4.1 (Kir4.1) were ineffective in halting the RVP and the epileptiform activity. In contrast, pharmacological blockade of the electrogenic Na⁺ /HCO₃ ^- cotransporter (NBCe1) by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) eliminated both the RVP and the persistent ECS shrinkage. Importantly, this blocker also stopped the epileptiform activity. These results demonstrate that RVP is closely associated with epileptiform activity across several models of epileptiform activity and therefore the underlying mechanism could potentially represent a novel target for epilepsy management and treatment.

The Contribution of Astrocyte and Neuronal Panx1 to Seizures Is Model and Brain Region Dependent

Pannexin1 (Panx1) is an ATP release channel expressed in neurons and astrocytes that plays important roles in CNS physiology and pathology. Evidence for the involvement of Panx1 in seizures includes the reduction of epileptiform activity and ictal discharges following Panx1 channel blockade or deletion. However, very little is known about the relative contribution of astrocyte and neuronal Panx1 channels to hyperexcitability. To this end, mice with global and cell type specific deletion of Panx1 were used in one in vivo and two in vitro seizure models. In the low-Mg²⁺ in vitro model, global deletion but not cell-type specific deletion of Panx1 reduced the frequency of epileptiform discharges. This reduced frequency of discharges did not impact the overall power spectra obtained from local field potentials. In the in vitro KA model, in contrast, global or cell type specific deletion of Panx1 did not affect the frequency of discharges, but reduced the overall power spectra. EEG recordings following KA-injection in vivo revealed that although global deletion of Panx1 did not affect the onset of status epilepticus (SE), SE onset was delayed in mice lacking neuronal Panx1 and accelerated in mice lacking astrocyte Panx1. EEG power spectral analysis disclosed a Panx1-dependent cortical region effect; while in the occipital region, overall spectral power was reduced in all three Panx1 genotypes; in the frontal cortex, the overall power was not affected by deletion of Panx1. Together, our results show that the contribution of Panx1 to ictal activity is model, cell-type and brain region dependent.

Discovery of Potent Carbonic Anhydrase Inhibitors as Effective Anticonvulsant Agents: Drug Design, Synthesis, and In Vitro and In Vivo Investigations

Two sets of benzenesulfonamide-based effective human carbonic anhydrase (hCA) inhibitors have been developed using the tail approach. The inhibitory action of these novel molecules was examined against four isoforms: hCA I, hCA II, hCA VII, and hCA XII. Most of the molecules disclosed low to medium nanomolar range inhibition against all tested isoforms. Some of the synthesized derivatives selectively inhibited the epilepsy-involved isoforms hCA II and hCA VII, showing low nanomolar affinity. The anticonvulsant activity of selected sulfonamides was assessed using the maximal electroshock seizure (MES) and subcutaneous pentylenetetrazole (sc-PTZ) in vivo models of epilepsy. These potent CA inhibitors effectively inhibited seizures in both epilepsy models. The most effective compounds showed long duration of action and abolished MES-induced seizures up to 6 h after drug administration. These sulfonamides were found to be orally active anticonvulsants, being nontoxic in neuronal cell lines and in animal models.

Synaptic GluN2A-Containing NMDA Receptors: From Physiology to Pathological Synaptic Plasticity

N-Methyl-d-Aspartate Receptors (NMDARs) are ionotropic glutamate-gated receptors. NMDARs are tetramers composed by several homologous subunits of GluN1-, GluN2-, or GluN3-type, leading to the existence in the central nervous system of a high variety of receptor subtypes with different pharmacological and signaling properties. NMDAR subunit composition is strictly regulated during development and by activity-dependent synaptic plasticity. Given the differences between GluN2 regulatory subunits of NMDAR in several functions, here we will focus on the synaptic pool of NMDARs containing the GluN2A subunit, addressing its role in both physiology and pathological synaptic plasticity as well as the contribution in these events of different types of GluN2A-interacting proteins.

Discovery of Benzenesulfonamide Derivatives as Carbonic Anhydrase Inhibitors with Effective Anticonvulsant Action: Design, Synthesis, and Pharmacological Evaluation

Two series of novel benzenesulfonamide derivatives were synthesized and evaluated for their human carbonic anhydrase (CA, EC 4.2.1.1) inhibitory activity against four isoforms, hCA I, hCA II, hCA VII, and hCA IX. It was found that compounds of both series showed low to medium nanomolar inhibitory potential against all isoforms. Some of these derivatives displayed selective inhibition against the epileptogenesis related isoforms hCA II and VII, within the nanomolar range. These potent hCA II and VII inhibitors were evaluated as anticonvulsant agents against MES and sc-PTZ induced convulsions. These sulfonamides effectively abolished induced seizures in both models. Furthermore, time dependent seizure protection capability of the most potent compound was also evaluated. A long duration of action was displayed, with efficacy up to 6 h after drug administration. The compound appeared as an orally active anticonvulsant agent without showing neurotoxicity in a rotarod test, a nontoxic chemical profile being observed in subacute toxicity study.

Contribution of Intrinsic Lactate to Maintenance of Seizure Activity in Neocortical Slices from Patients with Temporal Lobe Epilepsy and in Rat Entorhinal Cortex

Neuronal lactate uptake supports energy metabolism associated with synaptic signaling and recovery of extracellular ion gradients following neuronal activation. Altered expression of the monocarboxylate transporters (MCT) in temporal lobe epilepsy (TLE) hampers lactate removal into the bloodstream. The resulting increase in parenchymal lactate levels might exert both, anti- and pro-ictogen effects, by causing acidosis and by supplementing energy metabolism, respectively. Hence, we assessed the contribution of lactate to the maintenance of transmembrane potassium gradients, synaptic signaling and pathological network activity in chronic epileptic human tissue. Stimulus induced and spontaneous field potentials and extracellular potassium concentration changes (∆[K⁺]_O) were recorded in parallel with tissue pO₂ and pH in slices from TLE patients while blocking MCTs by α-cyano-4-hydroxycinnamic acid (4-CIN) or d-lactate. Intrinsic lactate contributed to the oxidative energy metabolism in chronic epileptic tissue as revealed by the changes in pO₂ following blockade of lactate uptake. However, unlike the results in rat hippocampus, ∆[K⁺]_O recovery kinetics and field potential amplitude did not depend on the presence of lactate. Remarkably, inhibition of lactate uptake exerted pH-independent anti-seizure effects both in healthy rat and chronic epileptic tissue and this effect was partly mediated via adenosine 1 receptor activation following decreased oxidative metabolism.

Discovery of Benzenesulfonamides with Potent Human Carbonic Anhydrase Inhibitory and Effective Anticonvulsant Action: Design, Synthesis, and Pharmacological Assessment

We report two series of novel benzenesulfonamide derivatives acting as effective carbonic anhydrase (CA, EC 4.2.1.1) inhibitors. The synthesized compounds were tested against human (h) isoforms hCA I, hCA II, hCA VII, and hCA XII. The first series of compounds, 4-(3-(2-(4-substitued piperazin-1-yl)ethyl)ureido)benzenesulfonamides, showed low nanomolar inhibitory action against hCA II, being less effective against the other isoforms. The second series, 2-(4-substitued piperazin-1-yl)-N-(4-sulfamoylphenyl)acetamide derivatives, showed low nanomolar inhibitory activity against hCA II and hCA VII, isoforms involved in epileptogenesis. Some of these derivatives were evaluated for their anticonvulsant activity and displayed effective seizure protection against MES and scPTZ induced seizures in Swiss Albino mice. These sulfonamides were also found effective upon oral administration to Wistar rats and inhibited MES induced seizure episodes in this animal model of the disease. Some of the new compounds showed a long duration of action in the performed time course anticonvulsant studies, being nontoxic in subacute toxicity studies.

Chronic metformin treatment facilitates seizure termination

The AMP-activated protein kinase (AMPK) is a key energy sensor. Its activator metformin could suppress epileptogenesis in the pentylenetetrazol (PTZ) kindling model. However, the effect of metformin on the acute and chronic seizures has not been studied. We first detected the expression of AMPK in the brain tissue of human and mice with chronic seizures, as well as in mice with acute seizures. Second, using behavioral assay and local filed potentials (LFPs) recording, we investigated the effect of chronic metformin treatment on seizures in a acute seizure model and a chronic seizure model. Our results showed that AMPK was expressed in neurons in the epileptic brain. The expression level was decreased in the brain tissue that experienced chronic and acute seizures. In PTZ-induced acute seizures model, behavioral assay showed that chronic metformin treatment decreased the mortality, and LFPs recording showed that chronic metformin treatment shortened the duration of generalized tonic-clonic seizures and prolonged the duration of postictal depression. Moreover, in kainic acid-induced chronic seizures model, LFPs recording showed that chronic metformin treatment shortened the duration of epileptic activity. Our study suggests that chronic metformin treatment could facilitate seizure termination.

Emerging roles of Na⁺/H⁺ exchangers in epilepsy and developmental brain disorders

Epilepsy is a common central nervous system (CNS) disease characterized by recurrent transient neurological events occurring due to abnormally excessive or synchronous neuronal activity in the brain. The CNS is affected by systemic acid-base disorders, and epileptic seizures are sensitive indicators of underlying imbalances in cellular pH regulation. Na(+)/H(+) exchangers (NHEs) are a family of membrane transporter proteins actively involved in regulating intracellular and organellar pH by extruding H(+) in exchange for Na(+) influx. Altering NHE function significantly influences neuronal excitability and plays a role in epilepsy. This review gives an overview of pH regulatory mechanisms in the brain with a special focus on the NHE family and the relationship between epilepsy and dysfunction of NHE isoforms. We first discuss how cells translocate acids and bases across the membrane and establish pH homeostasis as a result of the concerted effort of enzymes and ion transporters. We focus on the specific roles of the NHE family by detailing how the loss of NHE1 in two NHE mutant mice results in enhanced neuronal excitability in these animals. Furthermore, we highlight new findings on the link between mutations of NHE6 and NHE9 and developmental brain disorders including epilepsy, autism, and attention deficit hyperactivity disorder (ADHD). These studies demonstrate the importance of NHE proteins in maintaining H(+) homeostasis and their intricate roles in the regulation of neuronal function. A better understanding of the mechanisms underlying NHE1, 6, and 9 dysfunctions in epilepsy formation may advance the development of new epilepsy treatment strategies.

Acid-Sensing Ion Channels as Potential Pharmacological Targets in Peripheral and Central Nervous System Diseases

Acid-sensing ion channels (ASICs) are widely expressed in the body and represent good sensors for detecting protons. The pH drop in the nervous system is equivalent to ischemia and acidosis, and ASICs are very good detectors in discriminating slight changes in acidity. ASICs are important pharmacological targets being involved in a variety of pathophysiological processes affecting both the peripheral nervous system (e.g., peripheral pain, diabetic neuropathy) and the central nervous system (e.g., stroke, epilepsy, migraine, anxiety, fear, depression, neurodegenerative diseases, etc.). This review discusses the role played by ASICs in different pathologies and the pharmacological agents acting on ASICs that might represent promising drugs. As the majority of above-mentioned pathologies involve not only neuronal dysfunctions but also microvascular alterations, in the next future, ASICs may be also considered as potential pharmacological targets at the vasculature level. Perspectives and limitations in the use of ASICs antagonists and modulators as pharmaceutical agents are also discussed.

Synchronization of action potentials during low-magnesium-induced bursting

The relationship between mono- and polysynaptic strength and action potential synchronization was explored using a reduced external Mg(2+) model. Single and dual whole cell patch-clamp recordings were performed in hippocampal cultures in three concentrations of external Mg(2+). In decreased Mg(2+) medium, the individual cells transitioned to spontaneous bursting behavior. In lowered Mg(2+) media the larger excitatory synaptic events were observed more frequently and fewer transmission failures occurred, suggesting strengthened synaptic transmission. The event synchronization was calculated for the neural action potentials of the cell pairs, and it increased in media where Mg(2+) concentration was lowered. Analysis of surrogate data where bursting was present, but no direct or indirect connections existed between the neurons, showed minimal action potential synchronization. This suggests the synchronization of action potentials is a product of the strengthening synaptic connections within neuronal networks.

Elevated Expression of Acid-Sensing Ion Channel 3 Inhibits Epilepsy via Activation of Interneurons

Recent studies have indicated that acid-sensing ion channels may play a significant role in the termination of epilepsy. In particular, acid-sensing ion channel 3 (ASIC3) is expressed in the central nervous system and is most sensitive to extracellular pH. However, whether ASIC3 plays a role in epilepsy is unknown. In this study, qRT-PCR, Western blot, immunohistochemistry, double immunofluorescence labeling, and slice recordings were used. We first detected elevated ASIC3 expression patterns in the brains of temporal lobe epilepsy patients and epileptic rats. ASIC3 was expressed in neurons and glia in both humans and in an experimental model of epilepsy, and ASIC3 was colocalized with inhibitory GABAergic interneurons. By blocking ASIC3 with its antagonist APETx2, we observed that injected APETx2 shortened the latency to seizure and increased the incidence of generalized tonic clonic seizure compared to the control group in models of both pilocarpine- and pentylenetetrazole (PTZ)-induced seizures. Additionally, blocking ASIC3 significantly decreased the frequency of action potential (AP) firing in interneurons. Moreover, APETx2 significantly reduced the amplitudes and frequencies of miniature inhibitory postsynaptic currents (mIPSCs) while showed no differences with the APETx2 + bicuculline group and the bicuculline group. These findings suggest that elevated levels of ASIC3 may serve as an anti-epileptic mechanism via postsynaptic mechanisms in interneurons. It could represent a novel therapeutic strategy for epilepsy treatment.

Glycolysis in energy metabolism during seizures

Studies have shown that glycolysis increases during seizures, and that the glycolytic metabolite lactic acid can be used as an energy source. However, how lactic acid provides energy for seizures and how it can participate in the termination of seizures remains unclear. We reviewed possible mechanisms of glycolysis involved in seizure onset. Results showed that lactic acid was involved in seizure onset and provided energy at early stages. As seizures progress, lactic acid reduces the pH of tissue and induces metabolic acidosis, which terminates the seizure. The specific mechanism of lactic acid-induced acidosis involves several aspects, which include lactic acid-induced inhibition of the glycolytic enzyme 6-diphosphate kinase-1, inhibition of the N-methyl-D-aspartate receptor, activation of the acid-sensitive 1A ion channel, strengthening of the receptive mechanism of the inhibitory neurotransmitter γ-minobutyric acid, and changes in the intra- and extracellular environment.

Simultaneous enhancement of excitation and postburst inhibition at the end of focal seizures

OBJECTIVE

Comprehension of the events that lead to seizure termination contributes to the development of strategies to confine propagation of ictal discharges. It is commonly assumed that the inhibitory control fails during seizures and recovers after the end of the ictal event. We examine the possibility that a progressive increase of inhibition that counters an increase in the strength of excitation contributes to terminating a focal seizure.

METHODS

We analyzed seizures acutely induced by pharmacological manipulations (bicuculline and 4-aminopyridine) in the entorhinal cortex and in the hippocampus of the in vitro isolated guinea pig brain.

RESULTS

As seizures ended, extracellular and intracellular recordings showed periodic bursting that progressively decreased in frequency. During the late bursting phase, the duration, number, and rate of occurrence of spikes within single bursts remained constant, whereas cumulative spike amplitude (index of excitation during a burst) and interburst interval (index of inhibition between bursts) progressively increased. The increment of average/cumulative burst excitation and interburst interval toward seizure end was confirmed in human focal seizures recorded with intracerebral electrodes in patients with drug-resistant partial epilepsies. A postburst refractory period of circa 2 seconds that increases with time toward the end of the seizure was confirmed in the experimental model by probing interburst epochs in the CA1 region with local dentate gyrus stimulation just suprathreshold for burst generation.

INTERPRETATION

Our findings support the concept that focal seizures are terminated by the simultaneous and opposing enhancement of excitation (burst activity) in addition to postburst inhibition. We hypothesize that a seizure stops when postburst inhibition becomes large enough to prevent reactivation of excitation.

Blood-brain barrier dysfunction can contribute to pharmacoresistance of seizures

OBJECTIVE

We tested the hypothesis that interstitial albumin can contribute to pharmacoresistance, which is common among patients with focal epilepsies. These patients often present with an open blood-brain barrier (BBB), resulting in diffusion of drug-binding albumin into the brain interstitial space.

METHODS

Seizure-like events (SLEs) induced by 100 μm 4-aminopyridine (4-AP) were monitored using extracellular field potential recordings from acute rat entorhinal cortex-hippocampus slices. Effects of standard antiepileptic drugs (phenytoin, valproic acid, carbamazepine, and phenobarbital) were studied in the presence of albumin applied acutely or by intraventricular injection. Unbound antiepileptic drugs (AEDs) were detected by ultrafiltration and high-performance liquid chromatography (HPLC).

RESULTS

Contrary to the absence of albumin, conventional AEDs failed to suppress SLEs in the rat entorhinal cortex in the presence of albumin. This effect was partially caused by buffering of phenytoin and carbamazepine (CBZ) by albumin. Increasing CBZ concentration from 50 μm to 100 μm resulted in block of SLEs. In slices obtained from animals that were pretreated with intraventricular albumin application 24 h prior to experiment, CBZ suppressed SLEs similar to control slices. We also found that application of serum-like electrolytes transformed SLEs into late recurrent discharges (LRDs), which were no longer responding to CBZ.

SIGNIFICANCE

A dysfunctional BBB with acute extravasation of serum albumin into the brain's interstitial space could contribute to pharmacoresistance. In such instances, choice of an AED with low albumin binding affinity may help in seizure control.

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

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

Acid-sensing ion channels (ASICs): therapeutic targets for neurological diseases and their regulation

Extracellular acidification occurs not only in pathological conditions such as inflammation and brain ischemia, but also in normal physiological conditions such as synaptic transmission. Acid-sensing ion channels (ASICs) can detect a broad range of physiological pH changes during pathological and synaptic cellular activities. ASICs are voltage-independent, proton-gated cation channels widely expressed throughout the central and peripheral nervous system. Activation of ASICs is involved in pain perception, synaptic plasticity, learning and memory, fear, ischemic neuronal injury, seizure termination, neuronal degeneration, and mechanosensation. Therefore, ASICs emerge as potential therapeutic targets for manipulating pain and neurological diseases. The activity of these channels can be regulated by many factors such as lactate, Zn²⁺, and Phe-Met-Arg-Phe amide (FMRFamide)-like neuropeptides by interacting with the channel’s large extracellular loop. ASICs are also modulated by G protein-coupled receptors such as CB₁ cannabinoid receptors and 5-HT₂. This review focuses on the physiological roles of ASICs and the molecular mechanisms by which these channels are regulated. [BMB Reports 2013; 46(6): 295-304]

Neuronal carbonic anhydrase VII provides GABAergic excitatory drive to exacerbate febrile seizures

Brain carbonic anhydrases (CAs) are known to modulate neuronal signalling. Using a novel CA VII (Car7) knockout (KO) mouse as well as a CA II (Car2) KO and a CA II/VII double KO, we show that mature hippocampal pyramidal neurons are endowed with two cytosolic isoforms. CA VII is predominantly expressed by neurons starting around postnatal day 10 (P10). The ubiquitous isoform II is expressed in neurons at P20. Both isoforms enhance bicarbonate-driven GABAergic excitation during intense GABAA-receptor activation. P13-14 CA VII KO mice show behavioural manifestations atypical of experimental febrile seizures (eFS) and a complete absence of electrographic seizures. A low dose of diazepam promotes eFS in P13-P14 rat pups, whereas seizures are blocked at higher concentrations that suppress breathing. Thus, the respiratory alkalosis-dependent eFS are exacerbated by GABAergic excitation. We found that CA VII mRNA is expressed in the human cerebral cortex before the age when febrile seizures (FS) occur in children. Our data indicate that CA VII is a key molecule in age-dependent neuronal pH regulation with consequent effects on generation of FS.

Network activity in hippocampal slice cultures revealed by long-term in vitro recordings

Organotypic hippocampal slice cultures (OHSCs) are widely used for anatomical, molecular and electrophysiological studies of the development of neuronal networks. Electrophysiological recordings are usually limited to a single time point during development, and recording conditions differ greatly based on culture conditions. Consequently, little is known about the maturation of neuronal network activity in vitro. Here, we describe a simple method that allows long-term electrophysiological recordings during culture maintenance in a CO2 incubator. We compared the occurrence of spontaneous network activity, including epileptiform activity, in OHSCs (maintained in Neurobasal/B27 serum-free medium) prepared at different postnatal days and investigated the effects of changes in osmolality and pH. Recordings over 48 h revealed spontaneous network activity culminating in seizure-like events (SLEs) in 65.4% of the OHSCs (n=78). SLE incidence peaked during the first 6h following implantation of the microelectrodes and a secondary increase in SLE-incidence began after 9h of recording and averaged 2.65SLEs/h. The initial peak was likely initiated by transient alkalosis induced by the low pCO2 during the positioning of the electrodes, whereas successive changes in the composition of the culture medium might explain the secondary increase in SLE incidence. Notably, changes in osmolality had no effect on SLE induction. In conclusion, long-term recordings in OHSCs will help to reveal changes in spontaneous network activity during maturation. The extent to which the axonal reorganization known to occur in OHSCs contributes to the susceptibility to epileptogenesis remains to be determined.

Surface charge impact in low-magnesium model of seizure in rat hippocampus

Putative mechanisms of induction and maintenance of seizure-like activity (SLA) in the low Mg(2+) model of seizures are: facilitation of NMDA receptors and decreased surface charge screening near voltage-gated channels. We have estimated the role of such screening in the early stages of SLA development at both physiological and room temperatures. External Ca(2+) and Mg(2+) promote a depolarization shift of the sodium channel voltage sensitivity; when examined in hippocampal pyramidal neurons, the effect of Ca(2+) was 1.4 times stronger than of Mg(2+). Removing Mg(2+) from the extracellular solution containing 2 mM Ca(2+) induced recurrent SLA in hippocampal CA1 pyramidal layer in 67% of slices. Reduction of [Ca(2+)](o) to 1 mM resulted in 100% appearance of recurrent SLA or continuous SLA. Both delay before seizure activity and the inter-SLA time were significantly reduced. Characteristics of seizures evoked in low Mg(2+)/1 mM Ca(2+)/3.5 K(+) were similar to those obtained in low Mg(2+)/2 Ca(2+)/5mM K(+), suggesting that reduction of [Ca(2+)](o) to 1 mM is identical to the increase in [K(+)](o) to 5 mM in terms of changes in cellular excitability and seizure threshold. An increase of [Ca(2+)](o) to 3 mM completely abolished SLA generation even in the presence of 5 mM [K(+)](o). A large variation in the ability of [Ca(2+)](o) to stop epileptic discharges in initial stage of SLA was found. Our results indicate that surface charge of the neuronal membrane plays a crucial role in the initiation of low Mg(2+)-induced seizures. Furthermore, our study suggests that Ca(2+) and Mg(2+), through screening of surface charge, have important anti-seizure and antiepileptic properties.

Five percent CO₂ is a potent, fast-acting inhalation anticonvulsant

PURPOSE

CO₂ has been long recognized for its anticonvulsant properties. We aimed to determine whether inhaling 5% CO₂ can be used to suppress seizures in epilepsy patients. The effect of CO₂ on cortical epileptic activity accompanying behavioral seizures was studied in rats and nonhuman primates, and based on these data, preliminary tests were carried out in humans.

METHODS

  In freely moving rats, cortical afterdischarges paralleled by myoclonic convulsions were evoked by sensorimotor cortex stimulation. Five percent CO₂ was applied for 5 min, 3 min before stimulation. In macaque monkeys, hypercarbia was induced by hypoventilation while seizure activity was electrically or chemically evoked in the sensorimotor cortex. Seven patients with drug-resistant partial epilepsy were examined with video-EEG (electroencephalography) and received 5% CO₂ in medical carbogen shortly after electrographic seizure onset.

RESULTS

In rats, 5% CO₂ strongly suppressed cortical afterdischarges, by approximately 75%, whereas responses to single-pulse stimulation were reduced by about 15% only. In macaques, increasing pCO₂) from 37 to 44-45 mm Hg (corresponding to inhalation of 5% CO₂ or less) suppressed stimulation-induced cortical afterdischarges by about 70% and single, bicuculline-induced epileptiform spikes by approximately 25%. In a pilot trial carried out in seven patients, a rapid termination of electrographic seizures was seen despite the fact that the application of 5% CO₂ was started after seizure generalization.

CONCLUSIONS

Five percent CO₂ has a fast and potent anticonvulsant action. The present data suggest that medical carbogen with 5% CO₂ can be used for acute treatment to suppress seizures in epilepsy patients.

Functional, metabolic, and synaptic changes after seizures as potential targets for antiepileptic therapy

Little is known about how the brain limits seizure duration and terminates seizures. Depending on severity and duration, a single seizure is followed by various functional, metabolic, and synaptic changes that may form targets for novel therapeutic strategies. It is long known that most seizures are followed by a period of postictal refractoriness during which the threshold for induction of additional seizures is increased. The endogenous anticonvulsant mechanisms involved in this phenomenon may be relevant for both spontaneous seizure arrest and increase of seizure threshold after seizure arrest. Postictal refractoriness has been extensively studied in various seizure and epilepsy models, including electrically and chemically induced seizures, kindling, and genetic animal models of epilepsy. During kindling development, two antagonistic processes occur simultaneously, one responsible for kindling-like events and the other for terminating ictus and postictal refractoriness. Frequently occurring seizures may lead to an accumulation of postictal refractoriness that may last weeks. The mechanisms involved in seizure termination and postictal refractoriness include changes in ionic microenvironment, in pH, and in various endogenous neuromodulators such as adenosine and neuropeptides. In animal models, the anticonvulsant efficacy of several antiepileptic drugs (AEDs) is increased during postictal refractoriness, which is a logical consequence of the interaction between endogenous anticonvulsant processes and the mechanism of AEDs. As discussed in this review, enhanced understanding of these endogenous processes may lead to novel targets for AED development.

Na-coupled bicarbonate transporters of the solute carrier 4 family in the nervous system: function, localization, and relevance to neurologic function

Many cellular processes including neuronal activity are sensitive to changes in intracellular and/or extracellular pH-both of which are regulated by acid-base transporter activity. HCO(3)(-)-dependent transporters are particularly potent regulators of intracellular pH in neurons and astrocytes, and also contribute to the composition of the cerebrospinal fluid (CSF). The molecular physiology of HCO(3)(-) transporters has advanced considerably over the past ∼14 years as investigators have cloned and characterized the function and localization of many Na-Coupled Bicarbonate Transporters of the solute carrier 4 (Slc4) family (NCBTs). In this review, we provide an updated overview of the function and localization of NCBTs in the nervous system. Multiple NCBTs are expressed in neurons and astrocytes in various brain regions, as well as in epithelial cells of the choroid plexus. Characteristics of human patients with SLC4 gene mutations/deletions and results from recent studies on mice with Slc4 gene disruptions highlight the functional importance of NCBTs in neuronal activity, somatosensory function, and CSF production. Furthermore, energy-deficient states (e.g., hypoxia and ischemia) lead to altered expression and activity of NCBTs. Thus, recent studies expand our understanding of the role of NCBTs in regulating the pH and ionic composition of the nervous system that can modulate neuronal activity.

Evaluation of intense physical effort in subjects with temporal lobe epilepsy

People with epilepsy have been discouraged from participating in physical activity due to the fear that it will exacerbate seizures. Although the beneficial effect of aerobic exercise in people with epilepsy, little objective evidence regarding the intensity of exercise has been reported. We investigated the effect of incremental physical exercise to exhaustion in people with epilepsy. Seventeen persons with temporal lobe epilepsy and twenty one control healthy subjects participated in this study. Both groups were submitted to echocolordoppler and electrocardiogram at rest and during physical effort. None of patients reported seizures during physical effort or in the recovery period of ergometric test. Both groups presented physiological heart rate and blood pressure responses during the different stages of the ergometric test. Only few patients presented electrocardiography or echocardiography alterations at rest or during effort. In conclusion, this work suggests that physical effort to exhaustion is not a seizure-induced component.

Physical exercise in epilepsy: what kind of stressor is it?

Stress has been considered the most frequently self-reported precipitant of seizures in people with epilepsy. The literature documents that physical stress, that is, physical exercise, can have beneficial effects in people with epilepsy. In view of evidence indicating that sensitivity to stress is reduced after a physical exercise program, physical activity could be a potential candidate for stress reduction in people with epilepsy. This review considers how physical exercise could contribute to reduce seizure susceptibility and, hence, seizure frequency. Possible mechanisms by which exercise can be beneficial for people with epilepsy are highlighted. Hypothalamic-pituitary-adrenal axis adaptation, neurotransmitter system modulation, and metabolic and neuroendocrine changes may interfere with seizure susceptibility. The psychological stress of different sports activities is an important concern that must also be taken into account. Overall, among stress reduction therapies for the treatment of seizures, exercise might be a potential candidate.

Seizure termination by acidosis depends on ASIC1a

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

How do seizures stop?

Although often overshadowed by factors influencing seizure initiation, seizure termination is a critical step in the return to the interictal state. Understanding the mechanisms contributing to seizure termination could potentially identify novel targets for anticonvulsant drug development and may also highlight the pathophysiological processes contributing to seizure initiation. In this article, we review known physiological mechanisms contributing to seizure termination and discuss additional mechanisms that are likely to be relevant even though specific data are not yet available. This review is organized according to successively increasing "size scales"-from membranes to synapses to networks to circuits. We first discuss mechanisms of seizure termination acting at the shortest distances and affecting the excitable membranes of neurons in the seizure onset zone. Next we consider the contributions of ensembles of neurons and glia interacting at intermediate distances within the region of the seizure onset zone. Lastly, we consider the contribution of brain nuclei, such as the substantia nigra pars reticulata (SNR), that are capable of modulating seizures and exert their influence over the seizure onset zone (and neighboring areas) from a relatively great-in neuroanatomical terms-distance. It is our hope that the attention to the mechanisms contributing to seizure termination will stimulate novel avenues of epilepsy research and will contribute to improved patient care.

Differential effects of low glucose concentrations on seizures and epileptiform activityin vivoandin vitro

In vivo, severe hypoglycemia is frequently associated with seizures. The hippocampus is a structure prone to develop seizures and seizure-induced damage. Patients with repeated hypoglycemic episodes have frequent memory problems, suggesting impaired hippocampal function. Here we studied the effects of moderate hypoglycemia on primarily generalized flurothyl-induced seizures in vivo and, using EEG recordings, we determined involvement of the hippocampus in hypoglycemic seizures. Moderate systemic hypoglycemia had proconvulsant effects on flurothyl-induced clonic (forebrain) seizures. During hypoglycemic seizures, seizure discharges were recorded in the hippocampus. Thus, we continued the studies in combined entorhinal cortex-hippocampus slices in vitro. However, in vitro, decreases in extracellular glucose from baseline 10 mM to 2 or 1 mM did not induce any epileptiform discharges. In fact, low glucose (2 and 1 mM) attenuated preexisting low-Mg2+-induced epileptiform activity in the entorhinal cortex and hippocampal CA1 region. Osmolarity compensation in low-glucose solution using mannitol impaired slice recovery. Additionally, using paired-pulse stimuli we determined that there was no impairment of GABAA inhibition in the dentate gyrus during glucopenia. The data strongly indicate that, although forebrain susceptibility to seizures is increased during moderate in vivo hypoglycemia and the hippocampus is involved during hypoglycemic seizures, glucose depletion in vitro contributes to an arrest of epileptiform activity in the system of the entorhinal cortex-hippocampus network and there is no impairment of net GABAA inhibition during glucopenia.

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.

Desynchronisation of spontaneously recurrent experimental seizures proceeds with a single rhythm

Here we investigate the temporal properties of recurrent seizure-like events (SLEs) in a low-[Mg(2+)] model of experimental epilepsy. Simultaneous intra- and extracellular electric signals were recorded in the CA3 region of rat hippocampal slices whereby cytosolic [Ca(2+)] transients were imaged by fluorescence detection. Recurrence pattern analysis was applied to give a measure of synchrony of simultaneously recorded intra- and extracellular electric signals and the SLE frequencies were extracted by complex wavelet analysis. Slices from the juvenile, but not the young adult rats, displayed several high-amplitude triplets of electric and [Ca(2+)] transients, termed paroxysmal spikes, followed by an SLE. Occurrence of the full-blown SLE was associated with decreased synaptic activity between the paroxysmal spikes that were seen as incomplete SLE starting sequences. The time series of recurrent SLEs provide evidence for a single SLE rhythm as continuously declining from about 200 Hz to below 1 Hz at the onset and termination of SLE, respectively, with an intermediate spectral discontinuity, tentatively identified with the tonic/clonic transition. All other frequency components were the harmonics of the fundamental rhythm, whereby the gamma and the theta band oscillations were not detected as separate activities. Recurrence showed decreasing temporal synchrony of intra- and extracellular signals during the SLE, suggesting that coincidence is destroyed by the SLE. Blockade of gap junctions with 200 microM carbenoxolone ceased recurrent SLEs. Release of gap junction blockade shortened both SLEs and their tonic phase indicating that persistent changes occurred via an altered gap junction coupling. We conclude that the initially precise temporal synchrony is gradually destroyed during ictal events with a single rhythm of continuously decreasing frequency. Blockade of gap junction coupling might prevent epileptic synchronisation.

Synaptic and nonsynaptic ictogenesis occurs at different temperatures in submerged and interface rat brain slices

To investigate the temperature sensitivity of low-Ca2+-induced nonsynaptic and low-Mg2+-induced synaptic ictogenesis under submerged and interface conditions, we compared changes of extracellular field potential and extracellular potassium concentration at room temperature (23 +/- 1 degrees C; mean +/- SD) and at 35 +/- 1 degrees C in hippocampal-entorhinal cortex slices. The induction of spontaneous epileptiform activity under interface conditions occurred at 35 +/- 1 degrees C in both models. In contrast, under submerged conditions, spontaneous epileptiform activity in low-Mg2+ artificial cerebrospinal fluid (ACSF) was observed at 35 +/- 1 degrees C, whereas epileptiform discharges induced by low-Ca2+ ACSF occurred only at room temperature. To investigate the different temperature effects under submerged and interface conditions, measurements of extra- and intracellular pH and extracellular space volume were performed. Lowering the temperature from 35 +/- 1 degrees C to room temperature effected a reduction in extracellular pH under submerged and interface conditions. Under submerged conditions, temperature changes had no significant influence on the intracellular pH in presence of either normal or low-Mg2+ ACSF. In contrast, application of low-Ca2+ ACSF effected a significant increase in intracellular pH at room temperature but not at 35 +/- 1 degrees C under submerged conditions. Therefore increasing intracellular pH by lowering the temperature in low-Ca2+ ACSF may push slices to spontaneous epileptiform activity by opening gap junctions. Finally, extracellular space volume significantly decreased by switching from submerged to interface conditions. The reduced extracellular space volume under interface conditions may lead to an enlarged ephaptic transmission and therefore promotes low-Mg2+- and low-Ca2+-induced spontaneous epileptiform activity. The results of the study indicate that gas-liquid interface and total-liquid submerged slice states impart distinct physiological parameters on brain tissue.

Magnesium deficiency differentially affects the retina and visual cortex of intact rats

To determine the influence of magnesium (Mg) on the visual system, electroretinograms (ERG) and visual evoked potentials (VEP) were recorded under dark-(DA) and light-adapted (LA) conditions in intact rats. Weanling rats were fed either a Mg-deficient (Mg-D) or a control diet for 17 d before the tests, and ERG, VEP and immunohistopathological analyses of retinae and cortices were made. In the Mg-D rats, ear congestion, hair loss and loss of body weight were observed, and serum Mg concentration was approximately 25% of that in the control rats (P < 0.01). The amplitudes of the DA a-wave and the second positive peak of the oscillatory potentials (OP2) of the ERG, and the negative component of the VEP (N1) in Mg-D rats were significantly greater than those of control rats. However, the amplitudes of the DA b-wave, LA 2 Hz b-wave, the 20 Hz flicker responses and the implicit times of all response components did not differ between the two groups. The immunohistopathologic results also were not altered in the Mg-D rats. We suggest that the functional abnormalities induced by Mg deficiency may depend not only on the hyperactivity of the N-methyl-D-aspartate (NMDA) receptor, but also on the behavior of the Ca(2+) and Mg(2+) ions in the intact eye.

Regulation of extracellular pH in the developing hippocampus

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

pH Sensitivity of non-synaptic field bursts in the dentate gyrus

Under conditions of low [Ca(2+)](o) and high [K(+)](o), the rat dentate granule cell layer in vitro develops recurrent spontaneous prolonged field bursts that resemble an in vivo phenomenon called maximal dentate activation. To understand how pH changes in vivo might affect this phenomenon, the slices were exposed to different extracellular pH environments in vitro. The field bursts were highly sensitive to extracellular pH over the range 7.0-7.6 and were suppressed at low pH and enhanced at high pH. Granule cell resting membrane potential, action potentials, and postsynaptic potentials were not significantly altered by pH changes within the range that suppressed the bursts. The pH sensitivity of the bursts was not altered by pharmacologic blockade of N-methyl-D-aspartate (NMDA), non-NMDA, and GABA(A) receptors at concentrations of these agents sufficient to eliminate both spontaneous and evoked synaptic potentials. Gap junction patency is known to be sensitive to pH, and agents that block gap junctions, including octanol, oleamide, and carbenoxolone, blocked the prolonged field bursts in a manner similar to low pH. Perfusion with gap junction blockers or acidic pH suppressed field bursts but did not block spontaneous firing of single and multiple units, including burst firing. These data suggest that the pH sensitivity of seizures and epileptiform phenomena in vivo may be mediated in large part through mechanisms other than suppression of NMDA-mediated or other excitatory synaptic transmission. Alterations in electrotonic coupling via gap junctions, affecting field synchronization, may be one such process.

Prenatal morphine exposure enhances seizure susceptibility but suppresses long-term potentiation in the limbic system of adult male rats

The present study examined the effects of prenatal morphine exposure on NMDA-dependent seizure susceptibility in the entorhinal cortex (EC), and on activity-dependent synaptic plasticity at Schaffer collateral and perforant path synapses in the hippocampus. During perfusion with Mg(2+)-free ACSF, an enhancement of epileptiform discharges was found in the EC of slices from prenatally morphine-exposed male rats. A submaximal tetanic stimulation (2x50 Hz/1 s) in control slices elicited LTP at the Schaffer collateral-CA1 synapses, but neither LTP nor LTD was evoked at the perforant path-DG synapses. In slices from prenatally morphine-exposed adult male rats, long-term potentiation of synaptic transmission was not observed at Schaffer collateral-CA1 synapses, while the submaximal tetanus now elicited frank LTD of synaptic EPSPs at perforant path synapses. These data suggest that prenatal morphine exposure enhances the susceptibility of entorhinal cortex to the induction of epileptiform activity, but shifts long-term plasticity of hippocampal synapses in favor of LTD.

Activity-dependent intracellular acidification correlates with the duration of seizure activity

Synchronized neuronal activity (seizures) can appear in the presence or absence of synaptic transmission. Mechanisms of seizure initiation in each of these conditions have been studied, but relatively few studies have addressed seizure termination. In particular, how are seizures terminated in the absence of synaptic activity where there is no loss of excitatory drive or augmentation of inhibitory inputs? We have studied dynamic activity-dependent changes of intracellular pH in the absence of synaptic transmission using the fluorescent pH indicator carboxylseminaphthorhodafluo-1. During epileptiform activity we observed intracellular acidification, whereas between seizures the intracellular pH recovered. Experimental conditions that shortened the epileptiform discharge correlated with more rapid intracellular acidification. On the other hand, experimental manipulation of intracellular pH altered the duration of the seizure discharge, with acidification resulting in early termination of the epileptiform activity. These data show a direct relationship between the level of intracellular acidification and the duration of the seizures, suggesting that an intracellular pH-dependent process can terminate nonsynaptic neuronal synchronization.

Anticonvulsant actions of furosemide in vitro

Anticonvulsant properties of furosemide have been suggested to reduce neuronal synchronization via its inhibitory effect on the Na+/K+/2Cl- co-transport system. We have studied effects of furosemide on spontaneous epileptiform activity and analysed effects of furosemide on amplitudes of stimulus-induced population-spikes, on stimulus-induced K+ changes, on extracellular pH changes at rest and during stimulation, and on changes in the extracellular space-volume. We used three different in vitro models of epilepsy in the combined hippocampal-entorhinal cortex slice preparation. Furosemide reversibly suppressed low Ca2+-induced epileptiform activity in hippocampus proper and blocked or significantly reduced different types of epileptiform discharges in the low Mg2+ model and the 4-aminopyridine model. Amplitudes of evoked field potentials underwent an initial slight increase followed by a significant reduction after prolonged treatment with furosemide. Stimulus-induced increases in extracellular potassium were also significantly reduced. Furosemide caused an alkaline shift at rest. Stimulus-induced pH transients changed from a biphasic alkalotic-acidotic sequence to a monophasic alkalotic shift. Stimulation-induced shrinkage of extracellular space-volume was reduced by furosemide, whereas no effect on baseline extracellular space-volume was seen. We conclude, that furosemide possesses strong anticonvulsive effects in various in vitro models of epilepsy. The anticonvulsive properties of furosemide cannot be explained by its effects on extracellular pH changes but appear in part to be mediated via a reduced excitability with consequent reduction of activity-induced potassium rises. Finally, partial inhibition of activity-induced extracellular space shrinkage may contribute to its anticonvulsant properties.