Effects of changes in extracellular potassium, magnesium and calcium concentration on synaptic transmission in area CA1 and the dentate gyrus of rat hippocampal slices

A Photoelectrochemical Sensor for Real-Time Monitoring of Neurochemical Signals in the Brain of Awake Animals

Metal ion homeostasis is imperative for normal functioning of the brain. Considering the close association between brain metal ions and various pathological processes in brain diseases, it becomes essential to track their dynamics in awake animals for accurate physiological insights. Although ion-selective microelectrodes (ISMEs) have demonstrated great advantage in recording ion signals in awake animals, their intrinsic potential drift impairs their accuracy in long-term in vivo analysis. This study addresses the challenge by integrating ISMEs with photoelectrochemical (PEC) sensing, presenting an excitation-detection separated PEC platform based on potential regulation of ISMEs. A flexible indium tin oxide (Flex-ITO) electrode, modified with MoS2 nanosheets and Au NPs, serves as the photoelectrode and is integrated with a micro-LED. The integrated photoelectrode is placed on the rat skull to remain unaffected by animal activity. The potential of ISME dependent on the concentration of target K+ serves as the modulator of the photocurrent signal of the photoelectrode. The proposed design allows deep brain detection while minimizing interference with neurons, thus enabling real-time monitoring of neurochemical signals in awake animals. It successfully monitors changes in extracellular K+ levels in the rat brain after exposure to PM2.5, presenting a valuable analytical tool for understanding the impact of environmental factors on the nervous system.

Interdependence of cellular and network properties in respiratory rhythmogenesis

How breathing is generated by the preBötzinger Complex (preBötC) remains divided between two ideological frameworks, and the persistent sodium current (INaP) lies at the heart of this debate. Although INaP is widely expressed, the pacemaker hypothesis considers it essential because it endows a small subset of neurons with intrinsic bursting or "pacemaker" activity. In contrast, burstlet theory considers INaP dispensable because rhythm emerges from "pre-inspiratory" spiking activity driven by feed-forward network interactions. Using computational modeling, we discover that changes in spike shape can dissociate INaP from intrinsic bursting. Consistent with many experimental benchmarks, conditional effects on spike shape during simulated changes in oxygenation, development, extracellular potassium, and temperature alter the prevalence of intrinsic bursting and pre-inspiratory spiking without altering the role of INaP. Our results support a unifying hypothesis where INaP and excitatory network interactions, but not intrinsic bursting or pre-inspiratory spiking, are critical interdependent features of preBötC rhythmogenesis.

Interstitial ions: A key regulator of state-dependent neural activity?

Throughout the nervous system, ion gradients drive fundamental processes. Yet, the roles of interstitial ions in brain functioning is largely forgotten. Emerging literature is now revitalizing this area of neuroscience by showing that interstitial cations (K+, Ca2+ and Mg2+) are not static quantities but change dynamically across states such as sleep and locomotion. In turn, these state-dependent changes are capable of sculpting neuronal activity; for example, changing the local interstitial ion composition in the cortex is sufficient for modulating the prevalence of slow-frequency neuronal oscillations, or potentiating the gain of visually evoked responses. Disturbances in interstitial ionic homeostasis may also play a central role in the pathogenesis of central nervous system diseases. For example, impairments in K+ buffering occur in a number of neurodegenerative diseases, and abnormalities in neuronal activity in disease models disappear when interstitial K+ is normalized. Here we provide an overview of the roles of interstitial ions in physiology and pathology. We propose the brain uses interstitial ion signaling as a global mechanism to coordinate its complex activity patterns, and ion homeostasis failure contributes to central nervous system diseases affecting cognitive functions and behavior.

Brain extracellular space, hyaluronan, and the prevention of epileptic seizures

Mutant mice deficient in hyaluronan (HA) have an epileptic phenotype. HA is one of the major constituents of the brain extracellular matrix. HA has a remarkable hydration capacity, and a lack of HA causes reduced extracellular space (ECS) volume in the brain. Reducing ECS volume can initiate or exacerbate epileptiform activity in many in vitro models of epilepsy. There is both in vitro and in vivo evidence of a positive feedback loop between reduced ECS volume and synchronous neuronal activity. Reduced ECS volume promotes epileptiform activity primarily via enhanced ephaptic interactions and increased extracellular potassium concentration; however, the epileptiform activity in many models, including the brain slices from HA synthase-3 knockout mice, may still require glutamate-mediated synaptic activity. In brain slice epilepsy models, hyperosmotic solution can effectively shrink cells and thus increase ECS volume and block epileptiform activity. However, in vivo, the intravenous administration of hyperosmotic solution shrinks both brain cells and brain ECS volume. Instead, manipulations that increase the synthesis of high-molecular-weight HA or decrease its breakdown may be used in the future to increase brain ECS volume and prevent seizures in patients with epilepsy. The prevention of epileptogenesis is also a future target of HA manipulation. Head trauma, ischemic stroke, and other brain insults that initiate epileptogenesis are known to be associated with an early decrease in high-molecular-weight HA, and preventing that decrease in HA may prevent the epileptogenesis.

Astrocytic gap junction blockade markedly increases extracellular potassium without causing seizures in the mouse neocortex

Extracellular potassium concentration, [K⁺]_o, is a major determinant of neuronal excitability. In the healthy brain, [K⁺]_o levels are tightly controlled. During seizures, [K⁺]_o increases up to 15mM and is thought to cause seizures due to its depolarizing effect. Although astrocytes have been suggested to play a key role in the redistribution (or spatial buffering) of excess K⁺ through Connexin-43 (Cx43)-based Gap Junctions (GJs), the relation between this dynamic regulatory process and seizure generation remains unknown. Here we contrasted the role of astrocytic GJs and hemichannels by studying the effect of GJ and hemichannel blockers on [K⁺]_o regulation in vivo. [K⁺]_o was measured by K⁺-sensitive microelectrodes. Neuronal excitability was estimated by local field potential (LFP) responses to forepaw stimulation and changes in the power of resting state activity. Starting at the baseline [K⁺]_o level of 1.61±0.3mM, cortical microinjection of CBX, a broad spectrum connexin channel blocker, increased [K⁺]_o to 11±3mM, Cx43 GJ/hemichannel blocker Gap27 increased it from 1.9±0.7 to 9±1mM. At these [K⁺]_o levels, no seizures were observed. Cx43 hemichannel blockade with TAT-Gap19 increased [K⁺]_o by only ~1mM. Microinjection of 4-aminopyridine, a known convulsant, increased [K⁺]_o to ~10mM and induced spontaneously recurring seizures, whereas direct application of K⁺ did not trigger seizure activity. These findings are the first in vivo demonstration that astrocytic GJs are major determinants for the spatial buffering of [K⁺]_o and that an increase in [K⁺]_o alone does not trigger seizures in the neocortex.

REVIEW ■ : Regulation of Extracellular K and pH by Polarized Ion Fluxes in Glial Cells: The Retinal Müller Cell

Müller cells, the principal glial cells of the retina, exhibit a high degree of functional and morphological polarization. An inward rectifying K+ channel, the dominant ion channel in Müller cells, is localized preferentially to cell endfeet, which terminate on the vitreal surface of the retina and on blood vessels. Two acid/ base transport systems, an Na+/HCO3-cotransporter and a CI-/HCO3-anion exchanger, also are localized preferentially to the endfeet. These functional specializations facilitate the ability of Müller cells to regulate extracellular ion levels in the retina. Müller cells regulate extracellular K+levels by transporting K+away from the neural retina to the vitreous humor and the subretinal space. Müller cells may also regulate retinal CO2and pH by the combined action of cell carbonic anhydrase and acid/base transporters localized to the endfeet, and they may control blood flow by the depolarization-induced release of potassium and protons from cell endfeet onto blood vessels. The physiology of ion transport in CNS astrocytes is not understood as well as it is in Müller cells. The presence of inward rectifying K+channels and acid/base transporters in astrocytes, however, suggests that these cells may utilize mechanisms similar to those of Müller cells in regulating the extracellular microenvironment and in controlling blood flow. The Neuroscientist 2:109-117, 1996

TRPC3 mediates hyperexcitability and epileptiform activity in immature cortex and experimental cortical dysplasia

Neuronal hyperexcitability plays an important role in epileptogenesis. Conditions of low extracellular calcium (Ca) or magnesium (Mg) can induce hyperexcitability and epileptiform activity with unclear mechanisms. Transient receptor potential canonical type 3 (TRPC3) channels play a pivotal role in neuronal excitability and are activated in low-Ca and/or low-Mg conditions to depolarize neurons. TRPC3 staining was highly enriched in immature, but very weak in mature, control cortex, whereas it was strong in dysplastic cortex at all ages. Depolarization and susceptibility to epileptiform activity increased with decreasing Ca and Mg. Combinations of low Ca and low Mg induced larger depolarization in pyramidal neurons and greater susceptibility to epileptiform activity in immature and dysplastic cortex than in mature and control cortex, respectively. Intracellular application of anti-TRPC3 antibody to block TRPC3 channels and bath application of the selective TRPC3 inhibitor Pyr3 greatly diminished depolarization in immature control and both immature and mature dysplastic cortex with strong TRPC3 expression. Epileptiform activity was initiated in low Ca and low Mg when synaptic activity was blocked, and Pyr3 completely suppressed this activity. In conclusion, TRPC3 primarily mediates low Ca- and low Mg-induced depolarization and epileptiform activity, and the enhanced expression of TRPC3 could make dysplastic and immature cortex more hyperexcitable and more susceptible to epileptiform activity.

Peri-infarct blood-brain barrier dysfunction facilitates induction of spreading depolarization associated with epileptiform discharges

Recent studies showed that spreading depolarizations (SDs) occurs abundantly in patients following ischemic stroke and experimental evidence suggests that SDs recruit tissue at risk into necrosis. We hypothesized that BBB opening with consequent alterations of the extracellular electrolyte composition and extravasation of albumin facilitates generation of SDs since albumin mediates an astrocyte transcriptional response with consequent disturbance of potassium and glutamate homeostasis. Here we show extravasation of Evans blue-albumin complex into the hippocampus following cortical photothrombotic stroke in the neighboring neocortex. Using extracellular field potential recordings and exposure to serum electrolytes we observed spontaneous SDs in 80% of hippocampal slices obtained from rats 24 h after cortical photothrombosis. Hippocampal exposure to albumin for 24 h through intraventricular application together with serum electrolytes lowered the threshold for the induction of SDs in most slices irrespective of the pathway of stimulation. Exposing acute slices from naive animals to albumin led also to a reduced SD threshold. In albumin-exposed slices the onset of SDs was usually associated with larger stimulus-induced accumulation of extracellular potassium, and preceded by epileptiform activity, which was also observed during the recovery phase of SDs. Application of ifenprodil (3 μM), an NMDA-receptor type 2 B antagonist, blocked stimulus dependent epileptiform discharges and generation of SDs in slices from animals treated with albumin in-vivo. We suggest that BBB opening facilitates the induction of peri-infarct SDs through impaired homeostasis of K+.

NIPA2 located in 15q11.2 is mutated in patients with childhood absence epilepsy

While pathogenic copy number variations (CNVs) in 15q11.2 were recently identified in Caucasian patients with idiopathic generalized epilepsies (IGEs), the epilepsy-associated gene(s) in this region is/are still unknown. Our study investigated whether the CNVs in 15q11.2 are associated with childhood absence epilepsy (CAE) in Chinese patients and whether the selective magnesium transporter NIPA2 gene affected by 15q11.2 microdeletions is a susceptive gene for CAE. We assessed IGE-related CNVs by Affymetrix SNP 5.0 microarrays in 198 patients with CAE and 198 controls from northern China, and verified the identified CNVs by high-density oligonucleotide-based CGH microarrays. The coding region and exon-intron boundaries of NIPA2 were sequenced in all 380 patients with CAE and 400 controls. 15q11.2 microdeletions were detected in 3 of 198 (1.5%) patients and in no controls. Furthermore, we identified point mutations or indel in a heterozygous state of the NIPA2 gene in 3 out of 380 patients, whereas they were absent in 700 controls (P = 0.043). These mutations included two novel missense mutations (c.532A>T, p.I178F; c.731A>G, p.N244S) and one small novel insertion (c.1002_1003insGAT, p.N334_335EinsD). No NIPA2 mutation was found in 400 normal controls. We first identified that NIPA2, encoding a selective magnesium transporter, is a susceptible gene of CAE, and 15q11.2 microdeletions are important pathogenic CNVs for CAE with higher frequency in Chinese populations than that previously reported in Caucasians. The haploinsufficiency of NIPA2 may be a mechanism underlying the neurological phenotypes of 15q11.2 microdeletions.

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.

Morphology and properties of astrocytes

Astrocytes were identified about 150 years ago, and, for the longest time, were considered to be supporting cells in the brain providing trophic, metabolic, and structural support for neural networks. Research in the last 2 decades has uncovered many novel molecules in astrocytes and the finding that astrocytes communicate with neurons via Ca2+ signaling, which leads to release of chemical transmitters, termed gliotransmitters, has led to renewed interest in their biology. This chapter will briefly review the unique morphology and molecular properties of astrocytes. The reader will be introduced to the role of astrocytes in blood-brain barrier (BBB) maintenance, in Ca2+ signaling, in synaptic transmission, in CNS synaptogenesis, and as neural progenitor cells. Mention is also made of the diseases in which astrocyte dysfunction has a role.

Extracellular potassium regulates the chloride reversal potential in cultured hippocampal neurons

Inhibitory GABAergic synaptic transmission in the mammalian hippocampus depends upon a hyperpolarized reversal potential for Cl(-) (ECl). To examine the regulation of ECl hyperpolarization we cultured hippocampal neurons for two weeks in either a low- or a high-concentration of KCl (2.6 or 18.7 mM, respectively). Neurons were then recorded from standard extracellular solution containing 3 mM K+, using the dual perforated patch clamp technique. Low-KCl cultured neurons fired spontaneous action potentials (APs; 0.33+/-0.11 Hz), while high-KCl cultured neurons were quiescent, resulting in a significant difference in AP activity (p=0.042). This high-KCl-induced decrease in activity was accompanied by depolarizations of both the AP threshold (p<0.001) and ECl (p<0.001), and a decrease in input resistance (IR, p<0.001), when compared with low-KCl cultured neurons. Blocking AP firing of low-KCl neurons during culturing with 1 muM tetrodotoxin did not alter ECl hyperpolarization, when compared with drug-free cultured low-KCl neurons (p=0.627); thus AP firing is not required for ECl hyperpolarization. Acute perfusion of a high-KCl extracellular solution onto low- or high-KCl cultured neurons demonstrated that high-KCl significantly depolarized the resting membrane potential (RMP). The KCl-induced change in ECl did not correspond with alterations in the expression of the cation chloride cotransporters KCC2 and NKCC1, as determined by western blotting (p=0.736). These findings suggest that: (1) extracellular K+ regulates ECl hyperpolarization; and, (2) the use of high-KCl during neuronal culture produces biophysically abnormal parameters, and thus should be discouraged.

Role of potassium lateral diffusion in non-synaptic epilepsy: A computational study

An increase of extracellular potassium ion concentration can result in neuronal hyperexcitability, and thus contribute to non-synaptic epileptiform activity. It has been shown that potassium lateral diffusion alone is sufficient for synchronization in the low-calcium epilepsy in-vitro model. However, it is not yet known whether the lateral diffusion can, by itself, induce seizure activity. We hypothesize that spontaneous sustained neuronal activity can be generated by potassium coupling between neurons. To test this hypothesis, neuronal simulations with 2-cell or 4-cell models were used. Each model neuron was embedded in a bath of K+ and surrounded by interstitial space. Interstitial potassium concentration was regulated by both K+-pump and glial buffer mechanisms. Simulations performed with two coupled neurons with parameter values within physiological range show that, without chemical and electrical synapses, potassium lateral diffusion alone can generate and synchronize zero-Ca2+ non-synaptic epileptiform activity. Simulations performed with a network of four zero-Ca2+ CA1 pyramidal neurons modeled in zero-calcium conditions also show that spontaneous sustained activity can propagate by potassium lateral diffusion alone with a velocity of approximately 0.93 mm/sec. This diffusion model used for the simulations is based on physiological parameters, is robust for various kinetics, and is able to reproduce both the spontaneous triplet bursting of non-synaptic activity and speed of propagation in low-Ca2+ non-synaptic epilepsy experiments. These simulations suggest that potassium lateral diffusion can play an important role in the synchronization and generation on non-synaptic epilepsy.

Glial modulation of synaptic transmission in the retina

Glial modulation of synaptic transmission and neuronal excitability in the mammalian retina is mediated by several mechanisms. Stimulation of glial cells evokes Ca(2+) waves, which propagate through the network of retinal astrocytes and Müller cells and result in the modulation of the activity of neighboring ganglion cells. Light-evoked spiking is enhanced in some ganglion cells and depressed in others. A facilitation or depression of light-evoked excitatory postsynaptic currents is also seen in ganglion cells following glial stimulation. In addition, stimulation of glial cells evokes a sustained hyperpolarizing current in ganglion cells which is mediated by ATP release from Müller cells and activation of neuronal A(1) adenosine receptors. Recent studies reveal that light-evoked activity in retinal neurons results in an increase in the frequency of Ca(2+) transients in Müller cells. Thus, there is two-way communication between neurons and glial cells, suggesting that glia contribute to information processing in the retina.

New roles for astrocytes: Regulation of synaptic transmission

Abstract Although glia often envelop synapses, they have traditionally been viewed as passive participants in synaptic function. Recent evidence has demonstrated, however, that there is a dynamic two-way communication between glia and neurons at the synapse. Neurotransmitters released from presynaptic neurons evoke Ca2+ concentration increases in adjacent glia. Activated glia, in turn, release transmitters, including glutamate and ATP. These gliotransmitters feed back onto the presynaptic terminal either to enhance or to depress further release of neurotransmitter. Transmitters released from glia can also directly stimulate postsynaptic neurons, producing either excitatory or inhibitory responses. Based on these new findings, glia should be considered an active partner at the synapse, dynamically regulating synaptic transmission.

Ion regulation in the brain: implications for pathophysiology

Ions in the brain are regulated independently from plasma levels by active transport across choroid plexus epithelium and cerebral capillary endothelium, assisted by astrocytes. In "resting" brain tissue, extracellular potassium ([K+]o) is lower and [H]o is higher (i.e., pHo is lower) than elsewhere in the body. This difference probably helps to maintain the stability of cerebral function because both high [K]o and low [H+]o enhance neuron excitability. Decrease in osmolarity enhances synaptic transmission and neuronal excitability whereas increased osmolarity has the opposite effect. Iso-osmotic low Na+ concentration also enhances voltage-dependent Ca2+ currents and synaptic transmission. Hypertonicity is the main cause of diabetic coma. In normally functioning brain tissue, the fluctuations in ion levels are limited, but intense neuronal excitation causes [K+]o to rise and [Na+]o, [Ca2+]o to fall. When excessive excitation, defective inhibition, energy failure, mechanical trauma, or blood-brain barrier defects drive ion levels beyond normal limits, positive feedback can develop as abnormal ion distributions influence neuron function, which in turn aggravates ion maldistribution. Computer simulation confirmed that elevation of [K+]o can lead to such a vicious circle and ignite seizures, spreading depression (SD), or hypoxic SD-like depolarization (anoxic depolarization).

In vivo electrophysiological evidences for cortical neuron-glia interactions during slow (<1 Hz) and paroxysmal sleep oscillations

The cortical activity results from complex interactions within networks of neurons and glial cells. The dialogue signals consist of neurotransmitters and various ions, which cross through the extracellular space. Slow (<1 Hz) sleep oscillations were first disclosed and investigated at the neuronal level where they consist of an alternation of the membrane potential between a depolarized and a hyperpolarized state. However, neuronal properties alone could not account for the mechanisms underlying the oscillatory nature of the sleeping cortex. Here I will show the behavior of glial cells during the slow sleep oscillation and its relationship with the variation of the neuronal membrane potential (pairs of neurons and glia recorded simultaneously and intracellularly) suggesting that, in contrast with previous assumptions, glial cells are not idle followers of neuronal activity. I will equally present measurements of the extracellular concentration of K(+) and Ca(2+), ions known to modulate the neuronal excitability. They are also part of the ionic flux that is spatially buffered by glial cells. The timing of the spatial buffering during the slow oscillation suggests that, during normal oscillatory activity, K(+) ions are cleared from active spots and released in the near vicinity, where they modulate the excitability of the neuronal membrane and contribute to maintain the depolarizing phase of the oscillation. Ca(2+) ions undergo a periodic variation of their extracellular concentration, which modulates the synaptic efficacy. The depolarizing phase of the slow oscillation is associated with a gradual depletion of the extracellular Ca(2+) promoting a progressive disfacilitation in the network. This functional synaptic neuronal disconnection is responsible for the ending of the depolarizing phase of the slow oscillation and the onset of a phasic hyperpolarization during which the neuronal network is silent and the intra- and extracellular ionic concentrations return to normal values. Spike-wave seizures often develop during sleep from the slow oscillation. Here I will show how the increased gap junction communication substantiates the facility of the glial syncytium to spatially buffer K(+) ions that were uptaken during the spike-wave seizures, and therefore contributing to the long-range recruitment of cortical territories. Similar mechanisms as those described during the slow oscillation promote the periodic (2-3 Hz) recurrence of spike-wave complexes.

Spatial buffering during slow and paroxysmal sleep oscillations in cortical networks of glial cells in vivo

The ability of neuroglia to buffer local increases of extracellular K(+) has been known from in vitro studies. This property may confer on these cells an active role in the modulation and spreading of cortical oscillatory activities. We addressed the question of the spatial buffering in vivo by performing single and double intraglial recordings, together with measures of the extracellular K(+) and Ca(2+) concentrations ([K(+)](out) and [Ca(2+)](out)) in the cerebral cortex of cats under ketamine and xylazine anesthesia during patterns of slow sleep oscillations and spike-wave seizures. In addition, we estimated the fluctuations of intraglial K(+) concentrations ([K(+)](in)). Measurements obtained during the slow oscillation indicated that glial cells phasically take up part of the extracellular K(+) extruded by neurons during the depolarizing phase of the slow oscillation. During this condition, the redistribution of K(+) appeared to be local. Large steady increases of [K(+)](out) and phasic potassium accumulations were measured during spike-wave seizures. In this condition, [K(+)](in) rose before [K(+)](out) if the glial cells were located at some distance from the epileptic focus, suggesting faster K(+) diffusion through the interglial syncytium. The simultaneously recorded [Ca(2+)](out) dropped steadily during the seizures to levels incompatible with efficient synaptic transmission, but also displayed periodic oscillations, in phase with the intraseizure spike-wave complexes. In view of this fact, and considering the capability of K(+) to modulate neuronal excitability both at the presynaptic and postsynaptic levels, we suggest that the K(+) long-range spatial buffering operated by glia is a parallel synchronizing and/or spreading mechanism during paroxysmal oscillations.

Ca2+ signalling and changes of mitochondrial function during low-Mg2+-induced epileptiform activity in organotypic hippocampal slice cultures

Several lines of evidence indicate that augmented neuronal activity is associated with increased mitochondrial function, however, the mechanisms of coupling are still unclear. In this study we used a low extracellular Mg2+ concentration and short stimulus trains to evoke neuronal hyperactivity in the form of seizure-like events (SLE) in hippocampal slice cultures. Simultaneous microfluorimetric and electrophysiological techniques were applied to gain insight into changes of Ca2+ concentration in different compartments and into mitochondrial function. SLEs were associated with a large decrease of the extracellular Ca2+ concentration ([Ca2+]e), a spiking increase of the cytoplasmic and a smoothed elevation of the mitochondrial Ca2+ concentration (cytoplasmic concentration [Ca2+]i; intramitrochondrial concentration [Ca2+]m). Following an initial apparent decline in the mitochondrial membrane potential (DeltaPsi) and NAD(P)H autofluorescence, mitochondria depolarized and NADH production was augmented. Furthermore, SLEs were associated with increased oxidation of dihydroethidine (HEt). Our data suggest that intramitochondrial Ca2+ accumulation stimulates NADH production and production of radical oxygen species (ROS). Interestingly, mitochondrial depolarization followed [Ca2+]i and [Ca2+]m changes with a delay implying that electrogenic extrusion of Ca2+ from the mitochondrial matrix might be responsible for the depolarization of the mitochondrial membrane.

Astrocytic regulation of the recovery of extracellular potassium after seizures in vivo

Glial cells are believed to play a major role in the regulation of the extracellular potassium concentration ([K+]o), particularly when the [K+]o is increased. Using ion-selective electrodes, we compared the [K+]o changes in the dentate gyrus of urethane-anaesthetized adult rats in the presence of reactive astrocytes and after reduction of glial function. The regulation of [K+]o in the dentate gyrus was determined by measuring the ceiling level of [K+]o and the half-time of recovery of [K+]o during and after seizures produced by 20 Hz trains of stimulation to the angular bundle. Reactive astrocytes were induced by repeated seizures and their presence was confirmed by a qualitative increase in glial fibrillary acidic protein (GFAP) and vimentin immunoreactivity. To inhibit glial function, fluorocitrate (FC), a reversible metabolic inhibitor, or alpha-aminoadipate (alpha-AA), an irreversible toxin, was injected into the dentate gyrus region, and the regulation of [K+]o was monitored for 8 h or 2 days later, respectively. After alpha-aminoadipate, loss of astrocytes in the dentate gyrus was demonstrated by loss of staining for GFAP. In the presence of reactive astrocytes there was no significant change in the peak [K+]o during seizures or the half-time of recovery of [K+]o after seizures compared to control animals. alpha-Aminoadipate significantly slowed the rate of recovery of [K+]o, but did not change the ceiling [K+]o. Fluorocitrate reversibly decreased the ceiling [K+]o, but also slowed the rate of recovery of [K+]o. Overall our results suggest that normal glial function is required for the recovery of elevated [K+]o after seizures in vivo.

Interaction between superficial layers of the entorhinal cortex and the hippocampus in normal and epileptic temporal lobe

The entorhinal cortex (EC) is a major gateway for sensory information into the hippocampal formation. The information flow from layer II and III of the medial EC to the hippocampus is regulated in a frequency dependent manner. Spread of low Mg2+-induced epileptiform activity from EC to hippocampus differs in slices obtained from normal and kindled rats, and in adult versus juvenile rats. In slices from normal rats, low Mg2+-induced epileptiform activity in the EC had only moderate effects on the areas CA3 and CA1, apparently gated by powerful inhibition in the dentate gyrus. In slices from kindled rats, and from juvenile rats, there is facilitated propagation of the seizure-like events and late recurrent discharges through the EC-hippocampal slice. Temporal lobe epilepsy is associated with selective lesions in layer III of the medial EC. Such loss of layer III cells of the medial EC during epilepsy may contribute to the disturbance of frequency dependent information flow from the EC to the hippocampus, and, therefore, to the cognitive impairments associated with these disorders.

Polyamines and cerebral ischemia

It has been well established that alterations in polyamine metabolism are associated with animal models of global ischemia. Recently, this has been extended to include models of focal ischemia and traumatic brain injury. There is much evidence to support the idea that polyamines may play a multifaceted detrimental role following ischemia reperfusion. Due to the deficit of knowledge about their physiology in the CNS, the link between ischemia-induced alterations in polyamine metabolism and neuronal injury remains to be substantiated. With the recent revelation that polyamines are major intracellular modulators of inward rectifier potassium channels and certain types of NMDA and AMPA receptors, the long wait for the physiologic relevance of these ubiquitous compounds may be in sight. Therefore, it is now conceivable that the alterations in polyamines could have major effects on ion homeostasis in the CNS, especially potassium, and thus account for the observed injury after cerebral ischemia.

Serotonergic modulation of hippocampal pyramidal cells in euthermic, cold-acclimated, and hibernating hamsters

Serotonergic fibers project to the hippocampus, a brain area previously shown to have distinctive changes in electroencephalograph (EEG) activity during entrance into and arousal from hibernation. The EEG activity is generated by pyramidal cells in both hibernating and nonhibernating species. Using the brain slice preparation, we characterized serotonergic responses of these CA1 pyramidal cells in euthermic, cold-acclimated, and hibernating Syrian hamsters. Stimulation of Shaffer-collateral/commissural fibers evoked fast synaptic excitation of CA1 pyramidal cells, a response monitored by recording population spikes (the synchronous generation of action potentials). Neuromodulation by serotonin (5-HT) decreased population spike amplitude by 54% in cold-acclimated animals, 80% in hibernating hamsters, and 63% in euthermic animals. The depression was significantly greater in slices from hibernators than from cold-acclimated animals. In slices from euthermic animals, changes in extracellular K+ concentration between 2.5 and 5.0 mM did not significantly alter serotonergic responses. The 5-HT1A agonist 8-hydroxy-2(di-n-propylamino)tetralin mimicked serotonergic inhibition in euthermic hamsters. Results show that 5-HT is a robust neuromodulator not only in euthermic animals but also in cold-acclimated and hibernating hamsters.

Regulation of intracellular sodium in cultured rat hippocampal neurones

1. We studied regulation of intracellular Na+ concentration ([Na+]i) in cultured rat hippocampal neurones using fluorescence ratio imaging of the Na+ indicator dye SBFI (sodium-binding benzofuran isophthalate). 2. In standard CO2/HCO3(-)-buffered saline with 3 mM K+, neurones had a baseline [Na+]i of 8.9 +/- 3.8 mM (mean +/- S.D.). Spontaneous, transient [Na+]i increases of 5 mM were observed in neurones on 27% of the coverslips studied. These [Na+]i increases were often synchronized among nearby neurones and were blocked reversibly by 1 microM tetrodotoxin (TTX) or by saline containing 10 mM Mg2+, suggesting that they were caused by periodic bursting activity of synaptically coupled cells. Opening of voltage-gated Na+ channels by application of 50 microM veratridine caused a TTX-sensitive [Na+]i increase of 25 mM. 3. Removing extracellular Na+ caused an exponential decline in [Na+]i to values close to zero within 10 min. Inhibition of Na+,K(+)-ATPase by removal of extracellular K+ or ouabain application evoked a [Na+]i increase of 5 mM min-1. Baseline [Na+]i was similar in the presence or absence of CO2/HCO3-; switching from CO2/HCO3(-)-free to CO2/HCO3(-)-buffered saline, however, increased [Na+]i transiently by 3 mM, indicating activation of Na(+)-dependent Cl(-)-HCO3- exchange. Inhibition of Na(+)-K(+)-2Cl- cotransport by bumetanide had no effect on [Na+]i. 4. Brief, small changes in extracellular K+ concentration ([K+]o) influenced neuronal [Na+]i only weakly. Virtually no change in [Na+]i was observed with elevation or reduction of [K+]o by 1 mM. Only 30% of cells reacted to 3 min [K+]o elevations of up to 5 mM. In contrast, long [K+]o alterations (> or = 10 min) to 6 mM or greater slowly changed steady-state [Na+]i in the majority of cells. 5. Our results indicate several differences between [Na+]i regulation in cultured hippocampal neurones and astrocytes. Baseline [Na+]i is lower in neurones compared with astrocytes and is mainly determined by Na+,K(+)-ATPase, whereas Na(+)-dependent Cl(-)-HCO3- exchange, Na(+)-HCO3- cotransport or Na(+)-K(+)-2Cl- cotransport do not play a significant role. In contrast to glial cells, [Na+]i of neurones changes only weakly with small alterations in bath [K+]o, suggesting that activity-induced [K+]o changes in the brain might not significantly influence neuronal Na+,K(+)-ATPase activity.

Extracellular Ca2+ changes during transmitter application in the leech central nervous system

Changes in extracellular Ca2+ concentration ([Ca2+]e) evoked by transmitters and transmitter agonists, respectively, and by elevation of bath K+ concentration were recorded in isolated segmental ganglia of the leech Hirudo medicinalis using Ca(2+)-selective microelectrodes. A 1-min bath application of kainate (10 microM), glutamate (1 mM), aspartate (1 mM), or carbachol (200 microM) decreased [Ca2+]e by up to 1 mM, whereas the inhibitory transmitters gamma-amino butyric acid (GABA, 100 microM) and serotonin (5-HT, 100 microM) did not change [Ca2+]e. The amplitude of the kainate-induced changes in [Ca2+]e increased with repetitive applications, and changes were blocked by 6-cyano-7-dinitroquinozaline-2,3-dione (CNQX). Elevation of bath K+ concentration from 4 to 40 mM led to a Ni(2+)-sensitive decrease in [Ca2+]e by 0.9 mM. Our results suggest that excitatory transmission in the leech central nervous system might be accompanied by substantial decreases in [Ca2+]e.

Synaptic transmission and paired-pulse behaviour of CA1 pyramidal cells in hippocampal slices from a hibernator at low temperature: importance of ionic environment

To investigate the effects of ionic changes possibly associated with hibernation, hippocampal slices prepared from golden hamsters were studied in artificial cerebrospinal fluid (ACSF) of variable composition (K+ 3-5 mM, Ca2+ 2-4 mM, Mg2+ 2-4 mM, pH 7.0-7.7) at temperatures of 15-20 degrees C, just above the temperature below which synaptic transmission is blocked. Population action potentials (population spikes, PSs) of CA1 pyramidal cells were evoked by stimulation of the Schaffer collaterals/commissural fibers with paired pulses (interpulse interval 50 ms, interval between pairs 30 s). The responses evoked at given temperatures were investigated as a function of extracellular ion concentrations. In ACSF containing 3 mM K+, 2 mM Ca2+ and 2 mM Mg2+, PSs could be evoked at temperatures of > approximately 16 degrees C whereas at lower temperatures synaptic transmission was blocked. The threshold temperature was slightly higher for the first (PS1) than for the second PS (PS2) evoked by paired-pulse stimulation. The slices displayed paired-pulse facilitation (PPF) at all temperatures. Elevation of [K+]o from 3 to 5 mM depressed the amplitudes of both PS1 and PS2, with a stronger effect on PS2. PPF was reduced and, at near-threshold temperatures, turned into paired-pulse depression (PPD). Elevation of [Ca2+]o from 2 to 4 mM increased the amplitude of PS1. The amplitude of PS2, in contrast, was reduced at near-threshold temperatures. PPF turned into PPD. Elevation of [Mg2+]o from 2 to 4 mM reduced the amplitudes of both PS1 and PS2, with a stronger effect on PS1. Accordingly, PPF was increased. Acidification by 0.3 pH units strongly depressed the amplitudes of PS1 as well as PS2 and increased PPF. Alkalization by 0.4 pH units had only weak effects in the opposite direction. Changes in the ionic composition comparable to those investigated in the present study presumably occur in the brain interstitium of hamsters during entrance into hibernation. According to our results, such changes depress synaptic transmission at low temperatures in the hamster hippocampus in vitro. This modulation may be important for the regulation of neuronal activity during entrance into hibernation.

Developmental changes in the number, size, and orientation of GFAP-positive cells in the CA1 region of rat hippocampus

Changes in extracellular potassium concentration as measured with ion-selective microelectrodes revealed abnormally large accumulations in the hippocampus during postnatal development. While rises in [K+]o during stimulation of the Schaffer collaterals were limited to about 12 mM in adult animals, identical stimulations elicited rises to levels as large as 18 mM in juveniles. Since astrocytes are believed to play an important role in K+ homeostasis, we studied the postnatal development of astrocytes in the CA1 region of rat hippocampus in four age groups using a polyclonal antibody against glial fibrillary acidic protein (GFAP). The main proliferation of GFAP-positive cells (GFAPpc) occurred in all laminae between postnatal days 8 and 16. The number of GFAP-positive astrocytes per unit area was reached in stratum lacunosum-moleculare and stratum oriens at about 2 weeks and in stratum radiatum at about 3 weeks of age. During further development--at the age of 24 days--the orientation of individual astrocytes in stratum radiatum became polar with an orientation almost perpendicular to stratum pyramidale. This was revealed by an analysis based on determination of the quotients between the angular orientation of the processes of single individual GFAP-positive cells. When the crossing points of all glial processes over vertical and horizontal grid lines were determined and respective quotients evaluated, the same development towards a perpendicular orientation of astrocytes was noted in stratum radiatum. The same approach revealed a transient orientation parallel to the fissure in stratum lacunosum-moleculare around day 24. Camera lucida drawings of GFAPpc in stratum radiatum revealed that astrocytes became larger during the first three postnatal weeks, followed by a reduction of various parameters (e.g., cell extension, branching pattern) until adulthood. The observed developmental changes of astroglial cells may contribute to the known delayed maturation of potassium regulation in rat hippocampus.

Long lasting functional alterations in the rat dentate gyrus following entorhinal cortex lesion: a current source density analysis

The functional consequences of lesions of the entorhinal cortex of rats were studied by analysing laminar distributions of stimulus induced field potentials in the dentate gyrus with a subsequent current source density analysis. Stimulation of the inner molecular layer elicits large excitatory postsynaptic potentials with small if any population spikes in the stratum granulare both in normal and lesioned animals. In lesioned animals middle molecular layer stimulation causes large excitatory sinks in the stratum moleculare without generation of population spikes in stratum granulare, while the same stimulation in slices from normal animals readily induces population spikes. The current source density analysis revealed a shift of current sinks induced by stimulation of either the inner or the middle molecular layer to common site. The N-methyl-D-aspartate receptor contribution to the current sink and source was found to be more prominent after middle molecular layer stimulation in comparison to inner molecular layers stimulation in the control group, while such a distinction could not be made in the lesioned group. Activation of mossy fibers did not reveal any significant differences between normal and lesioned animals. Following entorhinal cortex lesion sprouting of remaining afferents (e.g. commissural fibers) into the termination zones of the degenerated perforant path has been reported suggesting a compensatory replacement of excitatory synaptic input. However, persistent transneuronal dendritic alterations of neurons in the dentate gyrus have been observed which might result in altered dentate gyrus function. Our findings suggest that the reorganization process after entorhinal cortex lesion does not lead to full functional compensation of the lost perforant path input, resulting in an altered balance between excitation and inhibition.

Potassium-induced changes in excitability in the hippocampal CA1 region of immature and adult rats

Orthodromic and spontaneous population spike activity was measured in vitro in the CA1 region of rat hippocampal slices to determine maturational differences in excitability and susceptibility to K(+)-induced seizures. Several indices of excitability in the CA1 region changed in a non-monotonic fashion during maturation, in response to step-wise increases in bath [K+]. Slices from rats 18-22 days old, showed a greater probability of both spontaneous epileptiform activity and episodes of seizure-like activity followed by spreading depression, and more prolonged durations of evoked seizure-like events. Elevation of [K+] in the bathing medium increased these indices in a similar manner in older rats but not to the same degree as in 18- to 22-day-old rats. However, the threshold level of bath [K+] resulting in evoked bursts of population spikes was lower in adult and 28- to 32-day-old rats than in 18- to 22-day-old rats, suggesting that excitability is not uniformly greater at any given age. In 10- to 15-day-old rats, elevation of bath [K+] either produced persistent blockade of population responses, or increased the amplitude of the initial population spike, without producing bursts. Basal levels of [K+] in the interstitium of the slices corresponded to the various levels of [K+] placed in the bathing medium and there were no differences among age groups. Therefore, differences in basal [K+]o cannot account for the maturational changes in excitability and seizure activity. The period from 18-22 days of age in the rat is a useful focal point for investigating mechanisms underlying maturational changes in propensity to develop seizures.

Pharmacological and electrographic properties of epileptiform activity induced by elevated K+ and lowered Ca2+ and Mg2+ concentration in rat hippocampal slices

We studied some of the physiological and pharmacological properties of an in vitro model of epileptic seizures induced by elevation of [K+]0 (to 8 mM and 10 mM) in combination with lowering of [Mg2+]0 (to 1.4 mM and 1.6 mM) and [Ca2+]0 (to 0.7 mM and 1 mM) in rat hippocampal slices. These concentrations correspond to the ionic constitution of the extracellular microenvironment during seizures in vivo. The resulting activity was rather variable in appearance. In area CA3 recurrent discharges were observed which resulted in seizure-like events with either clonic-like or tonic-clonic-like ictaform events in area CA1. With ion-sensitive electrodes, we measured the field potential and the changes in extracellular ion concentrations which accompany this activity. The recurrent discharges in area CA3 were accompanied by small fluctuations in [K+]0 and [Ca2+]0. The grouped clonic-like discharges in area CA1 were associated with moderate increases in [K+]0 and small decreases in [Ca2+]0 in the order of 2 mM and 0.2 mM, respectively. Large, negative field-potential shifts and increases in [K+]0 to 13 mM, as well as decreases in [Ca2+]0 by up to 0.4 mM, accompanied the tonic phase of ictaform events. The ictaform events were not blocked by D-2-aminophosphonovalerate (2-APV) but were sensitive to 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) alone and in combination with 2-APV and ketamine. In order to determine the pharmacological characteristics of the ictaform events we bath-applied most clinically employed anticonvulsants (carbamazepine, phenytoin, valproate, phenobarbital, ethosuximide, trimethadione) and some experimental anticonvulsants (losigamone, vinpocetine, and apovincaminic acid). Carbamazepine, phenytoin, valproate, and phenobarbital were effective at clinically relevant doses. The data suggest that the high-K+ model of epileptiform activity is a good model of focal convulsant activity.

Comparison of the effects of increased potassium and of adenosine on hippocampal neurons from normal and genetically epileptic tg/tg mice

Tottering mice are an experimental model of genetically determined generalized epilepsy of the absence type. We investigated possible mechanisms underlying epileptogenic hyperexcitability in these mice by studying input/output (I/O) curves of the extracellular response of CA1 neurons to stratum radiatum stimulation in hippocampal slices maintained in vitro. Increases in extracellular potassium are considered to contribute to epileptogenesis, whereas adenosine has been proposed to be an endogenous antiepileptic agent. Moderate elevations (+2 mM) of extracellular K+ concentrations induced a significantly smaller increase of this response (leftward shift of the input/output curves) in slices from epileptic mice as compared with controls. Perfusion of slices with adenosine 10 microM decreased excitability in both groups of slices, especially with regard to response threshold. Adenosine more effectively decreased the responses elicited by low-intensity stimulation than those elicited by high intensity. No significant difference between the groups of slices was observed. On the basis of the present data, it is unlikely that the previously observed hyperexcitability of hippocampal neurons of tottering mice results from a genetically altered sensitivity to moderate increases in [K+]o or to adenosine.

The effect of omega-conotoxin GVIA on synaptic transmission within the nucleus accumbens and hippocampus of the rat in vitro

1. The actions of two calcium channel antagonists, the N-channel blocker omega-conotoxin GVIA (omega-CgTx) and the L-channel antagonist nisoldipine, on synaptic transmission were investigated in the hippocampus and nucleus accumbens of the rat in vitro. 2. omega-CgTx (100 nM for 10 min) produced a marked and irreversible reduction of focally evoked population spikes and intracellularly recorded excitatory postsynaptic potentials (e.p.s.ps) in the nucleus accumbens, which could not be overcome by increasing the stimulus strength. 3. Nisoldipine (10 microM for 10 min) had no effect on population spikes in the nucleus accumbens or the CA1 of the hippocampus. 4. In the hippocampus, population spikes were not irreversibly reduced by omega-CgTx (100 nM for 10 min) but rather, multiple population spikes were produced along with spontaneous synchronous discharges. This indicated that inhibitory synaptic transmission was being preferentially reduced. 5. Intracellular recordings demonstrated that omega-CgTx powerfully reduced inhibitory synaptic transmission in an irreversible manner and that excitatory transmission was also reduced but to a lesser extent. Unlike excitatory transmission in the nucleus accumbens and inhibitory transmission in the hippocampus, increasing the stimulus strength overcame the reduction of hippocampal excitatory transmission. 6. It is concluded that omega-CgTx-sensitive calcium channels are involved in the calcium entry that precedes the synaptic transmission in all these synapses. The apparent lower sensitivity of the hippocampal excitatory fibres to omega-CgTx may indicate that calcium entry that promotes transmitter release at central synapses may be mediated by pharmacologically distinct calcium channels.

Effects of the tetronic acid derivatives AO33 (losigamone) and AO78 on epileptiform activity and on stimulus-induced calcium concentration changes in rat hippocampal slices

The effects of members of a new class of anticonvulsants, the tetronic acid derivatives, were studied in 3 in vitro models of epileptogenesis in rat hippocampal slices; the picrotoxin, the low magnesium and the low calcium model. The effects of AO33 (losigamone) and AO78 on stimulus-induced decreases in extracellular calcium concentration were also investigated. In all 3 models of epileptogenesis, both drugs blocked spontaneous and reduced stimulus-induced epileptiform discharges dose dependently and reversibly. Stimulus-induced changes in [Ca2+]0 were markedly diminished by these agents. The fact that the tetronic acid derivatives block the low Ca seizure-like events which develop independently from chemical synaptic transmission suggests that these agents have non-synaptic or direct membrane actions with subsequently reduced cellular excitability.