Layer-specific pyramidal cell oscillations evoked by tetanic stimulation in the rat hippocampal area CA1 in vitro and in vivo

Multifocal transcranial stimulation in chronic ischemic stroke: A phase 1/2a randomized trial

BACKGROUND AND PURPOSE

Repetitive transcranial magnetic stimulation (rTMS) may promote recovery of motor function after stroke by inducing functional reorganization of cortical circuits. The objective of this study was to examine whether multifocal cortical stimulation using a new wearable transcranial rotating permanent magnet stimulator (TRPMS) can promote recovery of motor function after stroke by inducing functional reorganization of cortical circuits.

METHODS

Thirty³⁰ patients with chronic ischemic stroke and stable unilateral weakness were enrolled in a Phase 1/2a randomized double-blind sham-controlled clinical trial to evaluate safety and preliminary efficacy. Bilateral hemispheric stimulation was administered for 20 sessions 40 min each over 4 weeks. The primary efficacy endpoint was the change in functional MRI BOLD activation immediately after end of treatment. Secondary efficacy endpoints were clinical scales of motor function, including the Fugl-Meyer motor arm score, ARAT, grip strength, pinch strength, gait velocity, and NIHSS.

RESULTS

TRPMS treatment was well-tolerated with no device-related adverse effects. Active treatment produced a significantly greater increase in the number of active voxels on fMRI than sham treatment (median +48.5 vs -30, p = 0.038). The median active voxel number after active treatment was 8.8-fold greater than after sham (227.5 vs 26, p = 0.016). Although the statistical power was inadequate to establish clinical endpoint benefits, numerical improvements were demonstrated in 5 of 6 clinical scales of motor function. The treatment effects persisted over a 3-month duration of follow-up.

CONCLUSIONS

Multifocal bilateral TRPMS was safe and showed significant fMRI changes suggestive of functional reorganization of cortical circuits in patients with chronic ischemic stroke. A larger randomized clinical trial is warranted to verify recovery of motor function.

Astroglial Isopotentiality and Calcium-Associated Biomagnetic Field Effects on Cortical Neuronal Coupling

Synaptic neurotransmission is necessary but does not sufficiently explain superior cognitive faculties. Growing evidence has shown that neuron-astroglial chemical crosstalk plays a critical role in the processing of information, computation, and memory. In addition to chemical and electrical communication among neurons and between neurons and astrocytes, other nonsynaptic mechanisms called ephaptic interactions can contribute to the neuronal synchronization from different brain regions involved in the processing of information. New research on brain astrocytes has clearly shown that the membrane potential of these cells remains very stable among neighboring and distant astrocytes due to the marked bioelectric coupling between them through gap junctions. This finding raises the possibility that the neocortical astroglial network exerts a guiding template modulating the excitability and synchronization of trillions of neurons by astroglial Ca²⁺-associated bioelectromagnetic interactions. We propose that bioelectric and biomagnetic fields of the astroglial network equalize extracellular local field potentials (LFPs) and associated local magnetic field potentials (LMFPs) in the cortical layers of the brain areas involved in the processing of information, contributing to the adequate and coherent integration of external and internal signals. This article reviews the current knowledge of ephaptic interactions in the cerebral cortex and proposes that the isopotentiality of cortical astrocytes is a prerequisite for the maintenance of the bioelectromagnetic crosstalk between neurons and astrocytes in the neocortex.

Ephaptic Coupling of Cortical Neurons: Possible Contribution of Astroglial Magnetic Fields?

The close anatomical and functional relationship between neuronal circuits and the astroglial network in the neocortex has been demonstrated at several organization levels supporting the idea that neuron-astroglial crosstalk can play a key role in information processing. In addition to chemical and electrical neurotransmission, other non-synaptic mechanisms called ephaptic interactions seem to be important to understand neuronal coupling and cognitive functions. Recent interest in this issue comes from the fact that extra-cranial electric and magnetic field stimulations have shown therapeutic actions in the clinical practice. The present paper reviews the current knowledge regarding the ephaptic effects in mammalian neocortex and proposes that astroglial bio-magnetic fields associated with Ca²⁺ transients could be implicated in the ephaptic coupling of neurons by a direct magnetic modulation of the intercellular local field potentials.

Optogenetic approaches addressing extracellular modulation of neural excitability

The extracellular ionic environment in neural tissue has the capacity to influence, and be influenced by, natural bouts of neural activity. We employed optogenetic approaches to control and investigate these interactions within and between cells, and across spatial scales. We began by developing a temporally precise means to study microdomain-scale interactions between extracellular protons and acid-sensing ion channels (ASICs). By coupling single-component proton-transporting optogenetic tools to ASICs to create two-component optogenetic constructs (TCOs), we found that acidification of the local extracellular membrane surface by a light-activated proton pump recruited a slow inward ASIC current, which required molecular proximity of the two components on the membrane. To elicit more global effects of activity modulation on ‘bystander’ neurons not under direct control, we used densely-expressed depolarizing (ChR2) or hyperpolarizing (eArch3.0, eNpHR3.0) tools to create a slow non-synaptic membrane current in bystander neurons, which matched the current direction seen in the directly modulated neurons. Extracellular protons played contributory role but were insufficient to explain the entire bystander effect, suggesting the recruitment of other mechanisms. Together, these findings present a new approach to the engineering of multicomponent optogenetic tools to manipulate ionic microdomains, and probe the complex neuronal-extracellular space interactions that regulate neural excitability.

Endogenous field feedback promotes the detectability for exogenous electric signal in the hybrid coupled population

This paper presents the endogenous electric field in chemical or electrical synaptic coupled networks, aiming to study the role of endogenous field feedback in the signal propagation in neural systems. It shows that the feedback of endogenous fields to network activities can reduce the required energy of the noise and enhance the transmission of input signals in hybrid coupled populations. As a common and important nonsynaptic interactive method among neurons, particularly, the endogenous filed feedback can not only promote the detectability of exogenous weak signal in hybrid coupled neural population but also enhance the robustness of the detectability against noise. Furthermore, with the increasing of field coupling strengths, the endogenous field feedback is conductive to the stochastic resonance by facilitating the transition of cluster activities from the no spiking to spiking regions. Distinct from synaptic coupling, the endogenous field feedback can play a role as internal driving force to boost the population activities, which is similar to the noise. Thus, it can help to transmit exogenous weak signals within the network in the absence of noise drive via the stochastic-like resonance.

Transition between fast and slow gamma modes in rat hippocampus area CA1 in vitro is modulated by slow CA3 gamma oscillations

Hippocampal gamma oscillations have been associated with cognitive functions including navigation and memory encoding/retrieval. Gamma oscillations in area CA1 are thought to depend on the oscillatory drive from CA3 (slow gamma) or the entorhinal cortex (fast gamma). Here we show that the local CA1 network can generate its own fast gamma that can be suppressed by slow gamma-paced inputs from CA3. Moderate acetylcholine receptor activation induces fast (45 ± 1 Hz) gamma in rat CA1 minislices and slow (33 ± 1 Hz) gamma in CA3 minislices in vitro. Using pharmacological tools, current-source density analysis and intracellular recordings from pyramidal cells and fast-spiking stratum pyramidale interneurons, we demonstrate that fast gamma in CA1 is of the pyramidal-interneuron network gamma (PING) type, with the firing of principal cells paced by recurrent perisomal IPSCs. The oscillation frequency was only weakly dependent on IPSC amplitude, and decreased to that of CA3 slow gamma by reducing IPSC decay rate or reducing interneuron activation through tonic inhibition of interneurons. Fast gamma in CA1 was replaced by slow CA3-driven gamma in unlesioned slices, which could be mimicked in CA1 minislices by sub-threshold 35 Hz Schaffer collateral stimulation that activated fast-spiking interneurons but hyperpolarised pyramidal cells, suggesting that slow gamma frequency CA3 outputs can suppress the CA1 fast gamma-generating network by feed-forward inhibition and replaces it with a slower gamma oscillation driven by feed-forward inhibition. The transition between the two gamma oscillation modes in CA1 might allow it to alternate between effective communication with the medial entorhinal cortex and CA3, which have different roles in encoding and recall of memory.

GluN2B antagonism affects interneurons and leads to immediate and persistent changes in synaptic plasticity, oscillations, and behavior

Although antagonists to GluN2B-containing N-methyl-D-aspartate receptors (NMDARs) have been widely considered to be neuroprotective under certain pathological conditions, their immediate and lasting impacts on synaptic, circuit, and cognitive functions are poorly understood. In hippocampal slices, we found that the GluN2B-selective antagonist Ro25-6981 (Ro25) reduced synaptic NMDAR responses and consequently neuronal output in a subpopulation of GABAergic interneurons, but not pyramidal neurons. Consistent with these effects, Ro25 reduced GABAergic responses in pyramidal neurons and hence could affect circuit functions by altering the excitation/inhibition balance in the brain. In slices from Ts65Dn mice, a Down syndrome model with excess inhibition and cognitive impairment, acutely applied Ro25-rescued long-term potentiation (LTP) and gamma oscillation deficits, whereas prolonged dosing induced persistent rescue of LTP. In contrast, Ro25 did not impact LTP in wild-type (wt) mice but reduced gamma oscillations both acutely and following prolonged treatment. Although acute Ro25 treatment impaired memory performance in wt mice, memory deficits in Ts65Dn mice were unchanged. Thus, GluN2B-NMDARs contribute to the excitation/inhibition balance via impacts on interneurons, and blocking GluN2B-NMDARs can alter functions that depend on this balance, including synaptic plasticity, gamma oscillations, and memory. That prolonged GluN2B antagonism leads to persistent changes in synaptic and circuit functions, and that the influence of GluN2B antagonism differs between wt and disease model mice, provide critical insight into the therapeutic potential and possible liabilities of GluN2B antagonists.

Computational modeling reveals dendritic origins of GABA(A)-mediated excitation in CA1 pyramidal neurons

GABA is the key inhibitory neurotransmitter in the adult central nervous system, but in some circumstances can lead to a paradoxical excitation that has been causally implicated in diverse pathologies from endocrine stress responses to diseases of excitability including neuropathic pain and temporal lobe epilepsy. We undertook a computational modeling approach to determine plausible ionic mechanisms of GABA(A)-dependent excitation in isolated post-synaptic CA1 hippocampal neurons because it may constitute a trigger for pathological synchronous epileptiform discharge. In particular, the interplay intracellular chloride accumulation via the GABA(A) receptor and extracellular potassium accumulation via the K/Cl co-transporter KCC2 in promoting GABA(A)-mediated excitation is complex. Experimentally it is difficult to determine the ionic mechanisms of depolarizing current since potassium transients are challenging to isolate pharmacologically and much GABA signaling occurs in small, difficult to measure, dendritic compartments. To address this problem and determine plausible ionic mechanisms of GABA(A)-mediated excitation, we built a detailed biophysically realistic model of the CA1 pyramidal neuron that includes processes critical for ion homeostasis. Our results suggest that in dendritic compartments, but not in the somatic compartments, chloride buildup is sufficient to cause dramatic depolarization of the GABA(A) reversal potential and dominating bicarbonate currents that provide a substantial current source to drive whole-cell depolarization. The model simulations predict that extracellular K(+) transients can augment GABA(A)-mediated excitation, but not cause it. Our model also suggests the potential for GABA(A)-mediated excitation to promote network synchrony depending on interneuron synapse location - excitatory positive-feedback can occur when interneurons synapse onto distal dendritic compartments, while interneurons projecting to the perisomatic region will cause inhibition.

Chronic stimulation of cultured neuronal networks boosts low-frequency oscillatory activity at theta and gamma with spikes phase-locked to gamma frequencies

Slow wave oscillations in the brain are essential for coordinated network activity but have not been shown to self-organize in vitro. Here, the development of dissociated hippocampal neurons into an active network with oscillations on multi-electrode arrays was evaluated in the absence and presence of chronic external stimulation. Significant changes in signal power were observed in the range of 1-400 Hz with an increase in amplitude during bursts. Stimulation increased oscillatory activity primarily in the theta (4-11 Hz) and slow gamma (30-55 Hz) bands. Spikes were most prominently phase-locked to the slow gamma waves. Notably, the dissociated network self-organized to exhibit sustained delta, theta, beta and gamma oscillations without input from cortex, thalamus or organized pyramidal cell layers.

GABAergic synchronization in the limbic system and its role in the generation of epileptiform activity

GABA is the main inhibitory neurotransmitter in the adult forebrain, where it activates ionotropic type A and metabotropic type B receptors. Early studies have shown that GABA(A) receptor-mediated inhibition controls neuronal excitability and thus the occurrence of seizures. However, more complex, and at times unexpected, mechanisms of GABAergic signaling have been identified during epileptiform discharges over the last few years. Here, we will review experimental data that point at the paradoxical role played by GABA(A) receptor-mediated mechanisms in synchronizing neuronal networks, and in particular those of limbic structures such as the hippocampus, the entorhinal and perirhinal cortices, or the amygdala. After having summarized the fundamental characteristics of GABA(A) receptor-mediated mechanisms, we will analyze their role in the generation of network oscillations and their contribution to epileptiform synchronization. Whether and how GABA(A) receptors influence the interaction between limbic networks leading to ictogenesis will be also reviewed. Finally, we will consider the role of altered inhibition in the human epileptic brain along with the ability of GABA(A) receptor-mediated conductances to generate synchronous depolarizing events that may lead to ictogenesis in human epileptic disorders as well.

Coordination of size and number of excitatory and inhibitory synapses results in a balanced structural plasticity along mature hippocampal CA1 dendrites during LTP

Enlargement of dendritic spines and synapses correlates with enhanced synaptic strength during long-term potentiation (LTP), especially in immature hippocampal neurons. Less clear is the nature of this structural synaptic plasticity on mature hippocampal neurons, and nothing is known about the structural plasticity of inhibitory synapses during LTP. Here the timing and extent of structural synaptic plasticity and changes in local protein synthesis evidenced by polyribosomes were systematically evaluated at both excitatory and inhibitory synapses on CA1 dendrites from mature rats following induction of LTP with theta-burst stimulation (TBS). Recent work suggests dendritic segments can act as functional units of plasticity. To test whether structural synaptic plasticity is similarly coordinated, we reconstructed from serial section transmission electron microscopy all of the spines and synapses along representative dendritic segments receiving control stimulation or TBS-LTP. At 5 min after TBS, polyribosomes were elevated in large spines suggesting an initial burst of local protein synthesis, and by 2 h only those spines with further enlarged synapses contained polyribosomes. Rapid induction of synaptogenesis was evidenced by an elevation in asymmetric shaft synapses and stubby spines at 5 min and more nonsynaptic filopodia at 30 min. By 2 h, the smallest synaptic spines were markedly reduced in number. This synapse loss was perfectly counterbalanced by enlargement of the remaining excitatory synapses such that the summed synaptic surface area per length of dendritic segment was constant across time and conditions. Remarkably, the inhibitory synapses showed a parallel synaptic plasticity, also demonstrating a decrease in number perfectly counterbalanced by an increase in synaptic surface area. Thus, TBS-LTP triggered spinogenesis followed by loss of small excitatory and inhibitory synapses and a subsequent enlargement of the remaining synapses by 2 h. These data suggest that dendritic segments coordinate structural plasticity across multiple synapses and maintain a homeostatic balance of excitatory and inhibitory inputs through local protein-synthesis and selective capture or redistribution of dendritic resources.

Dynamic modulation of short-term synaptic plasticity in the auditory cortex: the role of norepinephrine

Norepinephrine (NE) is an important modulator of neuronal activity in the auditory cortex. Using patch-clamp recording and a pair pulse protocol on an auditory cortex slice preparation we recently demonstrated that NE affects cortical inhibition in a layer-specific manner, by decreasing apical but increasing basal inhibition onto layer II/III pyramidal cell dendrites. In the present study we used a similar protocol to investigate the dependence of noradrenergic modulation of inhibition on stimulus frequency, using 1s-long train pulses at 5, 10, and 20 Hz. The study was conducted using pharmacologically isolated inhibitory postsynaptic currents (IPSCs) evoked by electrical stimulation of axons either in layer I (LI-eIPSCs) or in layer II/III (LII/III-eIPSCs). We found that: 1) LI-eIPSC display less synaptic depression than LII/III-eIPSCs at all the frequencies tested, 2) in both type of synapses depression had a presynaptic component which could be altered manipulating [Ca²+]₀, 3) NE modestly altered short-term synaptic plasticity at low or intermediate (5-10 Hz) frequencies, but selectively enhanced synaptic facilitation in LI-eIPSCs while increasing synaptic depression of LII/III-eIPSCs in the latest (>250 ms) part of the response, at high stimulation frequency (20 Hz). We speculate that these mechanisms may limit the temporal window for top-down synaptic integration as well as the duration and intensity of stimulus-evoked gamma-oscillations triggered by complex auditory stimuli during alertness.

Activity-dependent intracellular chloride accumulation and diffusion controls GABA(A) receptor-mediated synaptic transmission

In the CNS, prolonged activation of GABA(A) receptors (GABA(A)Rs) has been shown to evoke biphasic postsynaptic responses, consisting of an initial hyperpolarization followed by a depolarization. A potential mechanism underlying the depolarization is an acute chloride (Cl(-)) accumulation resulting in a shift of the GABA(A) reversal potential (E(GABA)). The amount of GABA-evoked Cl(-) accumulation and accompanying depolarization depends on presynaptic and postsynaptic properties of GABAergic transmission, as well as on cellular morphology and regulation of Cl(-) intracellular concentration ([Cl(-)](i)). To analyze the influence of these factors on the Cl(-) and voltage behavior, we studied spatiotemporal dynamics of activity-dependent [Cl(-)](i) changes in multicompartmental models of hippocampal cells based on realistic morphological data. Simulated Cl(-) influx through GABA(A) Rs was able to exceed physiological Cl(-) extrusion rates thereby evoking HCO(3)(-) -dependent E(GABA) shift and depolarizing responses. Depolarizations were observed in spite of GABA(A) receptor desensitization. The amplitude of the depolarization was frequency-dependent and determined by intracellular Cl(-) accumulation. Changes in the dendritic diameter and in the speed of GABA clearance in the synaptic cleft were significant sources of depolarization variability. In morphologically reconstructed granule cells subjected to an intense GABAergic background activity, dendritic inhibition was more affected by accumulation of intracellular Cl(-) than somatic inhibition. Interestingly, E(GABA) changes induced by activation of a single dendritic synapse propagated beyond the site of Cl(-) influx and affected neighboring synapses. The simulations suggest that E(GABA) may differ even along a single dendrite supporting the idea that it is necessary to assign E(GABA) to a given GABAergic input and not to a given neuron.

Field effects in the CNS play functional roles

An endogenous electrical field effect, i.e., ephaptic transmission, occurs when an electric field associated with activity occurring in one neuron polarizes the membrane of another neuron. It is well established that field effects occur during pathological conditions, such as epilepsy, but less clear if they play a functional role in the healthy brain. Here, we describe the principles of field effect interactions, discuss identified field effects in diverse brain structures from the teleost Mauthner cell to the mammalian cortex, and speculate on the function of these interactions. Recent evidence supports that relatively weak endogenous and exogenous field effects in laminar structures reach significance because they are amplified by network interactions. Such interactions may be important in rhythmogenesis for the cortical slow wave and hippocampal sharp wave-ripple, and also during transcranial stimulation.

High frequency stimulation can block axonal conduction

High frequency stimulation (HFS) is used to control abnormal neuronal activity associated with movement, seizure, and psychiatric disorders. Yet, the mechanisms of its therapeutic action are not known. Although experimental results have shown that HFS suppresses somatic activity, other data has suggested that HFS could generate excitation of axons. Moreover it is unclear what effect the stimulation has on tissue surrounding the stimulation electrode. Electrophysiological and computational modeling literature suggests that HFS can drive axons at the stimulus frequency. Therefore, we tested the hypothesis that unlike cell bodies, axons are driven by pulse train HFS. This hypothesis was tested in fibers of the hippocampus both in-vivo and in-vitro. Our results indicate that although electrical stimulation could activate and drive axons at low frequencies (0.5-25 Hz), as the stimulus frequency increased, electrical stimulation failed to continuously excite axonal activity. Fiber tracts were unable to follow extracellular pulse trains above 50 Hz in-vitro and above 125 Hz in-vivo. The number of cycles required for failure was frequency dependent but independent of stimulus amplitude. A novel in-vitro preparation was developed, in which, the alveus was isolated from the remainder of the hippocampus slice. The isolated fiber tract was unable to follow pulse trains above 75 Hz. Reversible conduction block occurred at much higher stimulus amplitudes, with pulse train HFS (>150 Hz) preventing propagation through the site of stimulation. This study shows that pulse train HFS affects axonal activity by: (1) disrupting HFS evoked excitation leading to partial conduction block of activity through the site of HFS; and (2) generating complete conduction block of secondary evoked activity, as HFS amplitude is increased. These results are relevant for the interpretation of the effects of HFS for the control of abnormal neural activity such as epilepsy and Parkinson's disease.

Monoamines block kainate- and carbachol-induced gamma-oscillations but augment stimulus-induced gamma-oscillations in rat hippocampus in vitro

Monoamines are implicated in a cognitive processes in a variety of brain regions, including the hippocampal formation, where storage and retrieval of information are facilitated by synchronous network activities. We have investigated the effects of norepinephrine, serotonin, and dopamine on carbachol-, kainate-, and stimulus-induced hippocampal gamma-oscillations employing combined extra- and intracellular recordings. Monoamines dose-dependently and reversibly suppressed kainate- and carbachol-induced gamma-oscillations while increasing the frequency. The effect of serotonin was mimicked by fenfluramine, which releases serotonin from presynaptic terminals. Forskolin also suppressed kainate- and carbachol-induced gamma-oscillations. This effect was mimicked by 8-Br-cAMP and isoproterenol, an agonist of noradrenergic beta-receptor suggesting that the monoamines-mediated suppression of these oscillations could involve intracellular cyclic adenosine 3',5'-cyclic monophosphate (AMP). By contrast, stimulus-induced gamma-oscillations were dose-dependently augmented in power and duration after monoamines application. Intracellular recordings from pyramidal cells revealed that monoamines prolonged the stimulus-induced depolarization and membrane potential oscillations. Stimulus-induced gamma-oscillations were also suppressed by isoproterenol, the D1 agonist SKF-38393 forskolin, and 8-Br-cAMP. This suggests that the augmentation of stimulus-induced gamma-oscillations by monoamines involves--at least in part-different classes of cells than in case of carbachol- and kainate-induced gamma-oscillations.

Comparison between spontaneous and kainate-induced gamma oscillations in the mouse hippocampus in vitro

Neuronal synchronization at gamma frequency, implicated in cognition, can be evoked in hippocampal slices by pharmacological activation. We characterized spontaneous small-amplitude gamma oscillations (SgammaO) recorded in area CA3 of mouse hippocampal slices and compared it with kainate-induced gamma oscillations (KgammaO). SgammaO had a lower peak frequency, a more sinusoidal waveform and was spatially less coherent than KgammaO, irrespective of oscillation amplitude. CA3a had the smallest oscillation power, phase-led CA3c by approximately 4 ms and had the highest SgammaO frequency in isolated subslices. During SgammaO CA3c neurons fired at the rebound of inhibitory postsynaptic potentials (IPSPs) that were associated with a current source in stratum lucidum, whereas CA3a neurons often fired from spikelets, 3-4 ms earlier in the cycle, and had smaller IPSPs. Kainate induced faster/larger IPSPs that were associated with an earlier current source in stratum pyramidale. SgammaO and KgammaO power were dependent on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, gap junctions and gamma-aminobutyric acid (GABA)(A) receptors. SgammaO was suppressed by elevating extracellular KCl, blocking N-methyl-d-aspartate (NMDA) receptors or muscarinic receptors, or activating GluR5-containing kainate receptors. SgammaO was not affected by blocking metabotropic glutamate receptors or hyperpolarization-activated currents. The adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dimethoxyxanthine (8-CPT) and the CB1 cannabinoid receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) increased SgammaO power, indicating that endogenous adenosine and/or endocannabinoids suppress or prevent SgammaO in vitro. SgammaO emerges from a similar basic network as KgammaO, but differs in involvement of somatically projecting interneurons and pharmacological modulation profile. These observations advocate the use of SgammaO as a natural model for hippocampal gamma oscillations, particularly during less activated behavioural states.

Epileptiform activities in slices of hippocampus from mice after intra-hippocampal injection of kainic acid

Intra-hippocampal kainate injection induces the emergence of recurrent seizures after a delay of 3-4 weeks. We examined the cellular and synaptic basis of this activity in vitro using extracellular and intracellular records from longitudinal hippocampal slices. These slices permitted recordings from the dentate gyrus, the CA3 and CA1 regions and the subiculum of both the injected and the contralateral non-injected hippocampus. A sclerotic zone was evident in dorsal regions of slices from the injected hippocampus, while ventral regions and tissue from the contralateral hippocampus were not sclerotic. Interictal field potentials of duration 50-200 ms were generated spontaneously in both ipsilateral and contralateral hippocampal slices, but not in the sclerotic region, at 3-12 months after injection. They were initiated in the CA1 and CA3 regions and the subiculum. They were blocked by antagonists at glutamatergic receptors and were transformed into prolonged epileptiform events by GABAergic receptor antagonists. The membrane potential and the reversal potential of GABAergic synaptic events were more depolarized in CA1 pyramidal cells from kainate-treated animals than in control animals. Ictal-like events of duration 8-80 s were induced by tetanic stimulation (50 Hz, 0.2-1 s) preferentially in dorsal contralateral and ventral ipsilateral slices. Similar events were initiated by focal application of a combination of high K(+) and GABA. These data show that both interictal and ictal-like activities can be induced in slices of both ipsilateral and contralateral hippocampus from kainate-treated animals and suggest that changes in cellular excitability and inhibitory synaptic signalling may contribute to their generation.

Neurophysiological endophenotypes across bipolar and schizophrenia psychosis

The search for liability genes of the world's 2 major psychotic disorders, schizophrenia and bipolar disorder I (BP-I), has been extremely difficult even though evidence suggests that both are highly heritable. This difficulty is due to the complex and multifactorial nature of these disorders. They encompass several intermediate phenotypes, some overlapping across the 2 psychotic disorders that jointly and/or interactively produce the clinical manifestations. Research of the past few decades has identified several neurophysiological deficits in schizophrenia that frequently occur before the onset of psychosis. These include abnormalities in smooth pursuit eye movements, P50 sensory gating, prepulse inhibition, P300, mismatch negativity, and neural synchrony. Evidence suggests that many of these physiological deficits are distinct from each other. They are stable, mostly independent of symptom state and medications (with some exceptions) and are also observed in non-ill relatives. This suggests a familial and perhaps genetic nature. Some deficits are also observed in the BP-I probands and to a lesser extent their relatives. These deficits in physiological measures may represent the intermediate phenotypes that index small effects of genes (and/or environmental factors). The use of these measures in genetic studies may help the hunt for psychosis liability genes and clarify the extent to which the 2 major psychotic disorders share etio-pathophysiology. In spite of the rich body of work describing these neurophysiological measures in psychotic disorders, challenges remain: Many of the neurophysiological phenotypes are still relatively complex and are associated with low heritability estimates. Further refinement of these physiological phenotypes is needed that could identify specific underlying physiological deficits and thereby improve their heritability estimates. The extent to which these neurophysiological deficits are unique or overlap across BP-I and schizophrenia is unclear. And finally, the clinical and functional consequences of the neurophysiological deficits both in the probands and their relatives are not well described.

From baseline to epileptiform activity: a path to synchronized rhythmicity in large-scale neural networks

In large-scale neural networks in the brain the emergence of global behavioral patterns, manifested by electroencephalographic activity, is driven by the self-organization of local neuronal groups into synchronously functioning ensembles. However, the laws governing such macrobehavior and its disturbances, in particular epileptic seizures, are poorly understood. Here we use a mean-field population network model to describe a state of baseline physiological activity and the transition from the baseline state to rhythmic epileptiform activity. We describe principles which explain how this rhythmic activity arises in the form of spatially uniform self-sustained synchronous oscillations. In addition, we show how the rate of migration of the leading edge of the synchronous oscillations can be theoretically predicted, and compare the accuracy of this prediction with that measured experimentally using multichannel electrocorticographic recordings obtained from a human subject experiencing epileptic seizures. The comparison shows that the experimentally measured rate of migration of the leading edge of synchronous oscillations is within the theoretically predicted range of values. Computer simulations have been performed to investigate the interactions between different regions of the brain and to show how organization in one spatial region can promote or inhibit organization in another. Our theoretical predictions are also consistent with the results of functional magnetic resonance imaging (fMRI), in particular with observations that lower-frequency electroencephalographic (EEG) rhythms entrain larger areas of the brain than higher-frequency rhythms. These findings advance the understanding of functional behavior of interconnected populations and might have implications for the analysis of diverse classes of networks.

Short-latency median-nerve somatosensory-evoked potentials and induced gamma-oscillations in humans

Recent studies have suggested that cortical gamma-oscillations are tightly linked with various forms of physiological activity. In the present study, the dynamic changes of intracranially recorded median-nerve somatosensory-evoked potentials (SEPs) and somatosensory-induced gamma-oscillations were animated on a three-dimensional MR image, and the temporal and spatial characteristics of these activities were analysed in 10 children being evaluated for epilepsy surgery. Visual and quantitative assessments revealed that short-latency SEPs and somatosensory-induced gamma-oscillations predominantly involved the post-central gyrus and less intensely involved the pre-central gyrus and the anterior parietal lobule. Formation of a dipole of N20 peak with opposite polarities across the central sulcus was well delineated in animation movies. High-frequency (100-250 Hz) somatosensory-induced gamma-oscillations emerged in the post-central gyrus at 13.6-17.5 ms after median-nerve stimulation, gradually slowed down in frequency around and below 100 Hz, and progressively involved the neighbouring areas. A substantial proportion of somatosensory-induced gamma-oscillations was initially phase-locked and the proportion of a non-phase-locked component gradually increased over time. The primary motor hand areas proven by cortical stimulation frequently coincided with the sites showing the largest N20 peak and the largest somatosensory-induced gamma oscillations. In vivo animation of SEPs and somatosensory-induced gamma oscillations both may be utilized to localize the primary sensory-motor hand area in pre-surgical evaluation. The dipole on SEPs is consistent with the previously accepted notion that the cortices along the central sulcus are activated. The high-frequency somatosensory-induced gamma-oscillations in the post-central gyrus may represent the initial neural processing for external somatosensory stimuli, whereas the subsequent lower-frequency oscillations might represent the reafferent cortical activity occurring in larger cortical networks.

Hippocampal network activity is transiently altered by induction of long-term potentiation in the dentate gyrus of freely behaving rats

A role for oscillatory activity in hippocampal neuronal networks has been proposed in sensory encoding, cognitive functions and synaptic plasticity. In the hippocampus, theta (5-10 Hz) and gamma (30-100 Hz) oscillations may provide a mechanism for temporal encoding of information, and the basis for formation and retrieval of memory traces. Long-term potentiation (LTP) of synaptic transmission, a candidate cellular model of synaptic information storage, is typically induced by high-frequency tetanisation (HFT) of afferent pathways. Taking into account the role of oscillatory activity in the processing of information, dynamic changes may occur in hippocampal network activity in the period during HFT and/or soon after it. These changes in rhythmic activity may determine or, at least, contribute to successful potentiation and, in general, to formation of memory. We have found that short-term potentiation (STP) and LTP as well LTP-failure are characterised with different profiles of changes in theta and gamma frequencies. Potentiation of synaptic transmission was associated with a significant increase in the relative theta power and mean amplitude of theta cycles in the period encompassing 300 seconds after HFT. Where LTP or STP, but not failure of potentiation, occurred, this facilitation of theta was accompanied by transient increases in gamma power and in the mean amplitude of gamma oscillations within a single theta cycle. Our data support that specific, correlated changes in these parameters are associated with successful synaptic potentiation. These findings suggest that changes in theta-gamma activity associated with induction of LTP may enable synaptic information storage in the hippocampus.

Schizophrenia endophenotypes as treatment targets

Schizophrenia is a complex disorder that encompasses several clinical symptom domains and functional impairments. Existing treatments are meager, effective only against positive symptoms without benefiting negative symptoms and functional impairments. The drug discovery process has focused mostly on targeting D2 dopamine receptors. This followed the serendipitous discovery of the antipsychotic effects of chlorpromazine in the 1950s and, more recently, clozapine. There is a need to identify novel mechanisms in order to discover novel drugs that are effective against each of the symptom clusters and functional impairments associated with the illness. Neurophysiological studies in schizophrenia over the past 3 decades have identified several brain deficits that are stable, using valid animal models that are related to the etiology of the disorder. Many of these deficits are distinct and heritable; these are called endophenotypes. Many have well-characterized neurobiology and may therefore provide molecular targets for drug development. In addition, these endophenotypes help reduce the heterogeneity by identifying homogeneous subgroups of patients with similar pathophysiology, symptoms and functional deficits. Clinical trials of drugs, whose development is based on an endophenotype, will have enhanced statistical power when the trial is carried out in an appropriate cohort of subjects using outcome measures related to the corresponding endophenotype. Furthermore, genes that are associated with these endophenotypes are beginning to be identified. These findings will identify novel molecular targets for drug development with treatment implications for clinical symptom complex and functional deficits marked by the endophenotype. As endophenotypes are present during childhood and adolescence, novel drugs that are developed on the basis of this subgroup could have implications for preventive strategies in schizophrenia.

Distinct types of ionic modulation of GABA actions in pyramidal cells and interneurons during electrical induction of hippocampal seizure-like network activity

It has recently been shown that electrical stimulation in normal extracellular fluid induces seizure-like afterdischarge activity that is always preceded by GABA-dependent slow depolarization. These afterdischarge responses are synchronous among mature hippocampal neurons and driven by excitatory GABAergic input. However, the differences in the mechanisms whereby the GABAergic signals in pyramidal cells and interneurons are transiently converted from hyperpolarizing to depolarizing (and even excitatory) have remained unclear. To clarify the network mechanisms underlying this rapid GABA conversion that induces afterdischarges, we examined the temporal changes in GABAergic responses in pyramidal cells and/or interneurons of the rat hippocampal CA1 area in vitro. The extents of slow depolarization and GABA conversion were much larger in the pyramidal cell group than in any group of interneurons. Besides GABA(A) receptor activation, neuronal excitation by ionotropic glutamate receptors enhanced GABA conversion in the pyramidal cells and consequent induction of afterdischarge. The slow depolarization was confirmed to consist of two distinct phases; an early phase that depended primarily on GABA(A)-mediated postsynaptic Cl- accumulation, and a late phase that depended on extracellular K+ accumulation, both of which were enhanced by glutamatergic neuron excitation. Moreover, extracellular K+ accumulation augmented each oscillatory response of the afterdischarge, probably by further Cl- accumulation through K+-coupled Cl- transporters. Our findings suggest that the GABA reversal potential may be elevated above their spike threshold predominantly in the pyramidal cells by biphasic Cl- intrusion during the slow depolarization in GABA- and glutamate-dependent fashion, leading to the initiation of seizure-like epileptiform activity.

The adult rat hippocampal slice revisited with multi-electrode arrays

The multi-electrode arrays (MEA) technology for the recording of brain slices is available for more than 10 years. However, despite its relative simplicity, this recording technique is not widely used in academic or pharmaceutical research laboratories. We illustrate here that MEA provide multiple possibilities to investigate some network physiological properties as well as to evaluate the pharmacological effects of compounds. We first document that MEA allow to trigger and to record conventional FP which are inhibited by the block of action potential propagation (with 500 nM TTX). FP recorded with MEA are sensitive to ionic substitutions, to ionotropic glutamate receptor antagonists (CNQX or NBQX) and to energetic failure. Second, we illustrate that different "classical" protocols (paired-pulse, LTP, chemical LTD), revealing synaptic plasticity mechanisms, could be performed. Third, we document that MEA allow spatial and temporal discriminations for the effects of known pharmacological compounds such as competitive antagonist (gabazine, bicuculline) and allosteric modulators (steroids) of GABA(A) receptors. In conclusion, we illustrate that MEA recordings of adult rat hippocampal slices constitute a powerful and sensitive system to evaluate the effect of molecules on basic synaptic propagation/transmission and on synaptic plasticity processes.

Comparable GABAergic Mechanisms of Hippocampal Seizurelike Activity in Posttetanic and Low-Mg2+ Conditions

It is known that GABA is a major inhibitory neurotransmitter in mature mammalian brains, but the effect of this substance is sometimes converted into depolarizing or even excitatory when the postsynaptic Cl– concentration becomes high. Recently we have shown that seizurelike afterdischarge induced by tetanic stimulation in normal extracellular fluid (posttetanic afterdischarge) is mediated through GABAergic excitation in mature hippocampal CA1 pyramidal cells. In this study, we examined the possible contribution of similar depolarizing/excitatory GABAergic input to the CA1 pyramidal cells to the seizurelike afterdischarge induced in a low extracellular Mg2+ condition, another experimental model of epileptic seizure activity (low-Mg2+ afterdischarge). Perfusion of the GABAA antagonist bicuculline abolished the low-Mg2+ afterdischarge, but not the interictal-like activity, in most cases. Each oscillatory response during the low-Mg2+ afterdischarge was dependent on Cl– conductance and contained an F–-insensitive depolarizing component in the pyramidal cells, thus indicating that the afterdischarge response may be mediated through both GABAergic and nonGABAergic transmissions. In addition, local GABA application to the recorded cells revealed that GABA responses were indeed depolarizing during the low-Mg2+ afterdischarge. Furthermore, the GABAergic interneurons located in the strata pyramidale and oriens fired in oscillatory cycles more actively than those in other layers of the CA1 region. These results suggest that the depolarizing GABAergic input may facilitate oscillatory synchronization among the hippocampal CA1 pyramidal cells during the low-Mg2+ afterdischarge in a manner similar to the expression of the posttetanic afterdischarge.

Region-specific modulation of electrically induced synchronous oscillations in the rat hippocampus and cerebral cortex

Strong tetanization induces synchronous membrane potential oscillations (seizure-like afterdischarge) in mature pyramidal cells of the hippocampal CA1 region. To investigate whether local networks in other brain regions can generate such an afterdischarge independently, we studied the inducibility of afterdischarge in individual 'isolated slices' of the rat hippocampal CA1 and CA3 regions, dentate gyrus (DG), entorhinal cortex (EC), and temporal cortex (TC) using intracellular and extracellular recordings. The strong tetanization constantly induced afterdischarges in the CA1 and CA3 pyramidal cells as well as in the EC and TC superficial principal cells. However, parameters of the afterdischarges, such as the frequency and duration of afterdischarges, varied among the regions. A mixture of N-methyl-D-aspartate (NMDA) and non-NMDA receptor antagonists or a GABA(A) receptor antagonist completely blocked the afterdischarges. Local GABA application during the afterdischarge elicited depolarization, rather than hyperpolarization. Moreover, reversal potentials of the afterdischarge were around -40 mV. In contrast, the tetanization resulted in occasional afterdischarge-like activities in DG slices, which were blocked by the non-NMDA or GABA(A) receptor antagonist. These findings suggest that the afterdischarges mediated through the excitatory GABAergic and glutamatergic transmissions might be common to, but be modulated differently by individual local networks in the hippocampus and cortex.