Simultaneous Field Potential Recordings Research Articles

Noradrenergic stimulation of α1 adrenoceptors in the medial prefrontal cortex mediates acute stress-induced facilitation of seizures in mice

Stress is one of the critical facilitators for seizure induction in patients with epilepsy. However, the neural mechanisms underlying this facilitation remain poorly understood. Here, we investigated whether noradrenaline (NA) transmission enhanced by stress exposure facilitates the induction of medial prefrontal cortex (mPFC)-originated seizures. In mPFC slices, whole-cell current-clamp recordings revealed that bath application of picrotoxin induced sporadic epileptiform activities (EAs), which consisted of depolarization with bursts of action potentials in layer 5 pyramidal cells. Addition of NA dramatically shortened the latency and increased the number of EAs. Simultaneous whole-cell and field potential recordings revealed that the EAs are synchronous in the mPFC local circuit. Terazosin, but not atipamezole or timolol, inhibited EA facilitation, indicating the involvement of α1 adrenoceptors. Intra-mPFC picrotoxin infusion induced seizures in mice in vivo. Addition of NA substantially shortened the seizure latency, while co-infusion of terazosin into the mPFC inhibited the effect of NA. Finally, acute restraint stress shortened the latency of intra-mPFC picrotoxin infusion-induced seizures, whereas prior infusion of terazosin reversed this stress-induced shortening of seizure latency. Our findings suggest that stress facilitates the induction of mPFC-originated seizures via NA stimulation of α1 adrenoceptors.

Involvement of GABAergic Interneuron Subtypes in 4-Aminopyridine-Induced Seizure-Like Events in Mouse Entorhinal Cortex in Vitro.

Single-unit recordings performed in temporal lobe epilepsy patients and in models of temporal lobe seizures have shown that interneurons are active at focal seizure onset. We performed simultaneous patch-clamp and field potential recordings in entorhinal cortex slices of GAD65 and GAD67 C57BL/6J male mice that express green fluorescent protein in GABAergic neurons to analyze the activity of specific interneuron (IN) subpopulations during acute seizure-like events (SLEs) induced by 4-aminopyridine (4-AP; 100 μm). IN subtypes were identified as parvalbuminergic (INPV, n = 17), cholecystokinergic (INCCK), n = 13], and somatostatinergic (INSOM, n = 15), according to neurophysiological features and single-cell digital PCR. INPV and INCCK discharged at the start of 4-AP-induced SLEs characterized by either low-voltage fast or hyper-synchronous onset pattern. In both SLE onset types, INSOM fired earliest before SLEs, followed by INPV and INCCK discharges. Pyramidal neurons became active with variable delays after SLE onset. Depolarizing block was observed in ∼50% of cells in each INs subgroup, and it was longer in IN (∼4 s) than in pyramidal neurons (<1 s). As SLE evolved, all IN subtypes generated action potential bursts synchronous with the field potential events leading to SLE termination. High-frequency firing throughout the SLE occurred in one-third of INPV and INSOM We conclude that entorhinal cortex INs are very active at the onset and during the progression of SLEs induced by 4-AP. These results support earlier in vivo and in vivo evidence and suggest that INs have a preferential role in focal seizure initiation and development.SIGNIFICANCE STATEMENT Focal seizures are believed to result from enhanced excitation. Nevertheless, we and others demonstrated that cortical GABAergic networks may initiate focal seizures. Here, we analyzed for the first time the role of different IN subtypes in seizures generated by 4-aminopyridine in the mouse entorhinal cortex slices. We found that in this in vitro focal seizure model, all IN types contribute to seizure initiation and that INs precede firing of principal cells. This evidence is in agreement with the active role of GABAergic networks in seizure generation.

GABAB receptor outward currents are transiently disclosed by the convulsant 4-aminopyridine in vitro

The K+ channel blocker 4-aminopyridine (4AP) has been extensively used to investigate the mechanisms underlying neuronal network synchronization in both in vitro and in vivo animal models of focal epilepsy. 4AP-induced effects are paralleled by an increase in both excitatory and inhibitory neurotransmitter release, but the mechanisms of action of 4AP on neuronal networks remain unclear. By employing simultaneous whole-cell patch clamp and field potential recordings from hippocampal CA3/4 pyramidal layer of acute brain slices obtained from mice (n = 30), we found that the appearance of epileptiform discharges induced by 4AP (100 μM) is consistently preceded by the transient recurrence of presumptive GABAB outward currents, which are not mirrored by any field activity. These GABAB outward currents still occurred during application of ionotropic glutamatergic antagonists (n = 12 cells) but were blocked by the GABAB receptor antagonist CGP55845 (n = 7). Our findings show that the transient occurrence of distinct GABAB outward currents precedes the appearance of 4AP-induced neuronal network synchronization leading to epileptiform activity in the rodent hippocampus in vitro.

Mecamylamine inhibits seizure-like activity in CA1-CA3 hippocampus through antagonism to nicotinic receptors.

Cholinergic modulation of hippocampal network function is implicated in multiple behavioral and cognitive states. Activation of nicotinic and muscarinic acetylcholine receptors affects neuronal excitability, synaptic transmission and rhythmic oscillations in the hippocampus. In this work, we studied the ability of the cholinergic system to sustain hippocampal epileptiform activity independently from glutamate and GABA transmission. Simultaneous CA3 and CA1 field potential recordings were obtained during the perfusion of hippocampal slices with the aCSF containing AMPA, NMDA and GABA receptor antagonists. Under these conditions, spontaneous epileptiform discharges synchronous between CA3 and CA1 were recorded. Epileptiform discharges were blocked by addition of the calcium-channel blocker Cd2+ and disappeared in CA1 after a surgical cut between CA3 and CA1. Cholinergic antagonist mecamylamine abolished CA3-CA1 synchronous epileptiform discharges, while antagonists of α7 and α4β2 nAChRs, MLA and DhβE, had no effect. Our results suggest that activation of nicotinic acetylcholine receptors can sustain CA3-CA1 synchronous epileptiform activity independently from AMPA, NMDA and GABA transmission. In addition, mecamylamine, but not α7 and α4β2 nAChRs antagonists, reduced bicuculline-induced seizure-like activity. The ability of mecamylamine to decrease hippocampal network synchronization might be associated with its therapeutic effects in a wide variety of CNS disorders including addiction, depression and anxiety.

Analysis of electro-mechanical relationship in human iPS cell-derived cardiomyocytes sheets under proarrhythmic condition assessed by simultaneous field potential and motion vector recordings

We investigated the electro-mechanical relationship of human iPS cell-derived cardiomyocyte sheets under arrhythmic condition, which was induced by digitalis intoxication along with the electrical train stimulation (n = 4). We adopted motion vector analysis by high-speed video microscopy and extracellular field potential recording by 64-microelectrode array system. The motion vector analysis uncovered local contractile events at resting phase, at which the field potential analysis showed no deflection in any cell sheet. Thus, motion vector analysis may provide supplemental information over field potential recording in detecting Ca2+-triggered arrhythmias, which may become a new strategy for assessing arrhythmic liability of test articles.

Carbachol-induced network oscillations in an in vitro limbic system brain slice

We employed simultaneous field potential recordings from CA3, subiculum and entorhinal cortex in an in vitro brain slice preparation to understand the involvement of these limbic areas in the generation of the field potential oscillations that are induced by bath application of the muscarinic receptor agonist carbachol. Regularly spaced oscillations that mainly presented at theta frequency range (5–12Hz) occurred synchronously in all three structures in the presence of carbachol. These oscillations, which disappeared when slices were perfused with pirenzepine or with glutamatergic receptor antagonists, were categorized as short (<4s) and long (>4s) with short events oscillating at higher frequencies than long events. Field oscillations were highly synchronized between regions and latency analysis revealed that they often initiated in the entorhinal cortex later than in the other two structures. Blocking GABAA receptors modified the activity patterns of both short and long oscillations and decreased their coherence in the theta frequency range. Finally, blocking KCC2 activity disclosed a pattern of recurrent short oscillations. Our results suggest that in the presence of carbachol both subiculum and CA3 most often drive theta generators in the entorhinal cortex and that these oscillations are influenced but not abolished by altering GABAA receptor signaling.

Kainic acid-induced albumin leak across the blood-brain barrier facilitates epileptiform hyperexcitability in limbic regions.

Systemic administration of kainic acid (KA) is a widely used procedure utilized to develop a model of temporal lobe epilepsy (TLE). Despite its ability to induce status epilepticus (SE) in vivo, KA applied to in vitro preparations induces only interictal-like activity and/or isolated ictal discharges. The possibility that extravasation of the serum protein albumin from the vascular compartment enhances KA-induced brain excitability is investigated here. Epileptiform activity was induced by arterial perfusion of 6 μm KA in the in vitro isolated guinea pig brain preparation. Simultaneous field potential recordings were carried out bilaterally from limbic (CA1, dentate gyrus [DG], and entorhinal cortex) and extralimbic regions (piriform cortex and neocortex). Blood-brain barrier (BBB) breakdown associated with KA-induced epileptiform activity was assessed by parenchymal leakage of intravascular fluorescein-isothiocyanate albumin. Seizure-induced brain inflammation was evaluated by western blot analysis of interleukin (IL)-1β expression in brain tissue. KA infusion caused synchronized activity at 15-30 Hz in limbic (but not extralimbic) cortical areas, associated with a brief, single seizure-like event. A second bolus of KA, 60 min after the induction of the first ictal event, did not further enhance excitability. Perfusion of serum albumin between the two administrations of KA enhanced epileptiform discharges and allowed a recurrent ictal event during the second KA infusion. Our data show that arterial KA administration selectively alters the synchronization of limbic networks. However, KA is not sufficient to generate recurrent seizures unless serum albumin is co-perfused during KA administration. These findings suggest a role of serum albumin in facilitating acute seizure generation.

Nonlinear dendritic integration in CA1 pyramidal neurons during locomotion.

Event Abstract Back to Event Nonlinear dendritic integration in CA1 pyramidal neurons during locomotion. Jeff Magee1* and Katie Bittner1 1 Howard Hugues Medical Institute, Janelia Farm, United States Most neuronal circuits receive at least two functionally distinct input types (intrinsic vs. extrinsic; sensory vs. motor; etc). In many pyramidal neuron based microcircuits integration of these two input signals can proceed nonlinearly through the production of active dendritic voltage signals. For example, appropriately timed, perisomatically located, hippocampal (SC) and distal dendrite targeting entorhinal (EC3) input produces a distal dendritic Ca2+ plateau potential that drives burst firing output from CA1 pyramidal neurons in vitro. Related signals have been observed in neocortical pyramidal neurons. Until recently it was unknown whether these events occurred in vivo and, if so, during what behavioral states. Here we used simultaneous whole-cell patch and field potential recordings in head-fixed mice running on a linear track treadmill to study this dendritic plateau driven burst firing (plateaus) in CA1 neurons. We find that during locomotion dendritic plateau potentials occur within the neuron’s place field with initiation probability peaking near the peak of the firing field. Plateaus produce a large (32±4mV; n=12), slow (duration; 51±7ms) somatic depolarization that appears similar to that measured in vitro. Interestingly, plateaus exhibit a dramatic level of theta-phase modulation (~97%) that peaks late in the theta cycle (~330°). This late phase peak in plateau potential initiation is near the theta-phase preference of EC3 inputs, suggesting a theta-phase dependent interaction of SC and EC3 inputs. We tested this idea by manipulating the phase of SC inputs by injecting phase adjusted theta frequency currents into CA1 somas. Biasing AP firing earlier in the theta phase decreased plateau probability to ~48% of control whereas biasing AP firing later in phase increased plateau probability 276%. We next directly examined the role of EC3 inputs by inactivating the EC3 axons in CA1 via local light activation of axons expressing archaerhodopsin. This manipulation reduced the probability of plateau initiation by ~40% and decreased the duration of the plateaus ~60%. These data indicate that CA1 pyramidal cells compare theta-phase relations of SC and EC3 inputs with backpropagating action potentials acting as a link between the two isolated subcellular input integration compartments. Keywords: Ca spikes, Hippocampus, in vivo, Entorhinal Cortex, supervised learning Conference: 4th NAMASEN Training Workshop - Dendrites 2014, Heraklion, Greece, 1 Jul - 4 Jul, 2014. Presentation Type: Poster presentation Topic: dendritic integration and compartmentalization Citation: Magee J and Bittner K (2014). Nonlinear dendritic integration in CA1 pyramidal neurons during locomotion.. Front. Syst. Neurosci. Conference Abstract: 4th NAMASEN Training Workshop - Dendrites 2014. doi: 10.3389/conf.fnsys.2014.05.00020 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 11 Apr 2014; Published Online: 12 Jun 2014. * Correspondence: Dr. Jeff Magee, Howard Hugues Medical Institute, Janelia Farm, Ashburn, VA, United States, mageej@janelia.hhmi.org Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Jeff Magee Katie Bittner Google Jeff Magee Katie Bittner Google Scholar Jeff Magee Katie Bittner PubMed Jeff Magee Katie Bittner Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Neuronal and astroglial correlates underlying spatiotemporal intrinsic optical signal in the rat hippocampal slice.

Widely used for mapping afferent activated brain areas in vivo, the label-free intrinsic optical signal (IOS) is mainly ascribed to blood volume changes subsequent to glial glutamate uptake. By contrast, IOS imaged in vitro is generally attributed to neuronal and glial cell swelling, however the relative contribution of different cell types and molecular players remained largely unknown. We characterized IOS to Schaffer collateral stimulation in the rat hippocampal slice using a 464-element photodiode-array device that enables IOS monitoring at 0.6 ms time-resolution in combination with simultaneous field potential recordings. We used brief half-maximal stimuli by applying a medium intensity 50 Volt-stimulus train within 50 ms (20 Hz). IOS was primarily observed in the str. pyramidale and proximal region of the str. radiatum of the hippocampus. It was eliminated by tetrodotoxin blockade of voltage-gated Na+ channels and was significantly enhanced by suppressing inhibitory signaling with gamma-aminobutyric acid(A) receptor antagonist picrotoxin. We found that IOS was predominantly initiated by postsynaptic Glu receptor activation and progressed by the activation of astroglial Glu transporters and Mg2+-independent astroglial N-methyl-D-aspartate receptors. Under control conditions, role for neuronal K+/Cl− cotransporter KCC2, but not for glial Na+/K+/Cl− cotransporter NKCC1 was observed. Slight enhancement and inhibition of IOS through non-specific Cl− and volume-regulated anion channels, respectively, were also depicted. High-frequency IOS imaging, evoked by brief afferent stimulation in brain slices provide a new paradigm for studying mechanisms underlying IOS genesis. Major players disclosed this way imply that spatiotemporal IOS reflects glutamatergic neuronal activation and astroglial response, as observed within the hippocampus. Our model may help to better interpret in vivo IOS and support diagnosis in the future.

A Multimodal Micro-Optrode Combining Field and Single Unit Recording, Multispectral Detection and Photolabeling Capabilities

Microelectrodes have been very instrumental and minimally invasive for in vivo functional studies from deep brain structures. However they are limited in the amount of information they provide. Here, we describe a, aluminum-coated, fibre optic-based glass microprobe with multiple electrical and optical detection capabilities while retaining tip dimensions that enable single cell measurements (diameter ≤10 µm). The probe enables optical separation from individual cells in transgenic mice expressing multiple fluorescent proteins in distinct populations of neurons within the same deep brain nucleus. It also enables color conversion of photoswitchable fluorescent proteins, which can be used for post-hoc identification of the recorded cells. While metal coating did not significantly improve the optical separation capabilities of the microprobe, the combination of metal on the outside of the probe and of a hollow core within the fiber yields a microelectrode enabling simultaneous single unit and population field potential recordings. The extended range of functionalities provided by the same microprobe thus opens several avenues for multidimensional structural and functional interrogation of single cells and their surrounding deep within the intact nervous system.

Associative Properties of the Perirhinal Network

The perirhinal area is a rostrocaudally oriented cortical region involved in recognition and associative memory. It receives topographically organized transverse projections from high-order neocortical areas and is endowed with intrinsic longitudinal connections that distribute neocortical inputs rostrocaudally. Earlier work has revealed that neocortical inputs strongly recruit perirhinal interneurons located at the same transverse level, limiting the depolarization of principal cells. In contrast, at a distance, neocortical stimuli only evoke excitation because longitudinal perirhinal pathways do not engage interneurons. This raises the possibility that the perirhinal network allows for Hebbian-like associative interactions between coincident and spatially distributed inputs. To test this, we analyzed the effects of theta-frequency neocortical stimulation using simultaneous field potential recordings and optical imaging in the whole guinea pig brain in vitro. Theta-frequency stimulation (TFS) at one neocortical site resulted in a prolonged input-specific response depression at all perirhinal levels. In contrast, paired TFS of 2 distant neocortical sites resulted in a prolonged response potentiation to the paired inputs, suggesting that longitudinal perirhinal connections can support associative interactions between coincident but spatially distributed inputs. Moreover, we found that induction of these 2 forms of plasticity depended on the competing influence of glutamate group I metabotropic and NMDA receptors, respectively.

Direct detection of a single evoked action potential with MRS in Lumbricus terrestris

Functional MRI (fMRI) measures neural activity indirectly by detecting the signal change associated with the hemodynamic response following brain activation. In order to alleviate the temporal and spatial specificity problems associated with fMRI, a number of attempts have been made to detect neural magnetic fields (NMFs) with MRI directly, but have thus far provided conflicting results. In this study, we used MR to detect axonal NMFs in the median giant fiber of the earthworm, Lumbricus terrestris, by examining the free induction decay (FID) with a sampling interval of 0.32 ms. The earthworm nerve cords were isolated from the vasculature and stimulated at the threshold of action potential generation. FIDs were acquired shortly after the stimulation, and simultaneous field potential recordings identified the presence or absence of single evoked action potentials. FIDs acquired when the stimulus did not evoke an action potential were summed as background. The phase of the background‐subtracted FID exhibited a systematic change, with a peak phase difference of (−1.2 ± 0.3) × 10–5 radians occurring at a time corresponding to the timing of the action potential. In addition, we calculated the possible changes in the FID magnitude and phase caused by a simulated action potential using a volume conductor model. The measured phase difference matched the theoretical prediction well in both amplitude and temporal characteristics. This study provides the first evidence for the direct detection of a magnetic field from an evoked action potential using MR. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Glutamate uptake shapes low-[Mg 2+] induced epileptiform activity in juvenile rat hippocampal slices

A wide range of data support a role for ambient glutamate (Glu) in epilepsy, although temporal patterns associated with the cellular uptake of Glu have not been addressed in detail. We report on the effects of Glu uptake inhibitors on recurrent seizure-like events (SLEs) evoked by low-[Mg 2+] condition in juvenile rat hippocampal slices. Effects were compared for inhibitors such as l-trans-pyrrolidine-2,4-dicarboxylate ( tPDC), dl-threo-β-benzyloxyaspartate ( dl-TBOA) and dihydrokainic acid (DHK), representing different transporter specificity and transportability profiles. Latency to the first SLE after drug application was shortened by the inhibitors (in % of control: 500 μM tPDC: 54 ± 7, 15 μM dl-TBOA: 74 ± 5, 50 μM dl-TBOA: 70 ± 6, 100 μM DHK: 69 ± 4, 300 μM DHK: 71 ± 7). Further SLEs were frequently aborted by higher inhibitor concentrations applied (500 μM tPDC: 2/6, 50 μM TBOA: 5/5, 100 μM DHK: 6/8, 300 μM DHK: 3/3). Simultaneous field potential and whole-cell voltage recordings showed depolarization-induced inactivation of CA3 pyramidal neurons during inhibitor application. In the presence of inhibitors, the amplitude of forthcoming SLE was also decreased (in % of control: 500 μM tPDC: 66 ± 9, 15 μM dl-TBOA: 88 ± 5, 50 μM dl-TBOA: 59 ± 6, 100 μM DHK: 67 ± 4, 300 μM DHK: 68 ± 1). Dependent on type and concentration of the inhibitor, the duration of the first SLE of drug application either increased (100 μM DHK: 375 ± 90 %; 100 μM tPDC: 137 ± 13 %) or decreased (50 μM TBOA: 62 ± 13 %; 300 μM DHK: 60 ± 15 %) reflecting differences in subtype-specificity or mechanism of action of the inhibitors. Our findings suggest a role for ambient Glu in the genesis and maintenance of recurrent epileptiform discharges.

Synaptic contributions to in vitro hippocampal sharp-wave ripples

Introduction In freely behaving rodents, the hippocampus displays prominent deflections in the local field potential during immobility and sleep. These are called sharp waves, last for about 40 to 60 ms and are jagged with a fast oscillation, or ripple, of about 200 Hz. Sharp waves have been shown in rats to co-occur with multi-unit replay and preplay patterns following and preceding a learned spatial experience [1-3]. Patterns are compressed in order to fit within the tight temporal frame offered by the sharp-wave ripple complexes. On a cellular level, it is known that both interneurons and pyramidal populations are significantly phase-locked to the ripple phenomenon. Our focus is to understand the coordinated cellular activity that triggers, shapes and terminates the sharp-wave ripple complexes. To this end, we resort to in vitro simultaneous field potential and single-cell voltage clamp recordings on submerged mouse hippocampal slices, where the phenomenon appears with characteristics known from the in vivo situation [4]. Recent pharmacological investigations with this setup (Maier 2009, in preparation) suggest that the intracellular high frequency oscillations are mainly supported by excitatory currents (see also [5]). To test this hypothesis, we have compared the patterns of synaptic input to pyramidal cells during sharp waves under two voltage clamp conditions. The first, at a holding potential of around -72 mV is representative of a bias towards excitatory input only, since it is close to the reversal potential for inhibition and the other, around -50 mV is dominated by outward, inhibitory currents while still containing some excitatory activity. Figure 1 shows the cumulative distribution functions of relative ripple powers for excitatory (blue) and inhibitory bias (green). Excitatory synaptic inputs show consistently higher relative power in the ripple band when compared with a mix of excitation and inhibition. To investigate the timing of individual post synaptic currents (PSCs) we estimated putative PSC time via a deconvolution approach. We find that ripple-locked excitation arrives predominantly at the beginning of the sharp wave, whereas inhibition arrives late in the sharp wave. Inhibitory currents are less coupled to the ripples than excitatory ones.

Fast excitation during sharp-wave ripples

Event Abstract Back to Event Fast excitation during sharp-wave ripples Álvaro Tejero-Cantero1*, Nikolaus Maier2, Jochen Winterer2, Genela Morris2, Christian Leibold3 and Dietmar Schmitz2 1 Rutgers University, Newark, Division of Neurobiology, Department of Biology II, Germany 2 University of Munich, Neurowissenschaftliches Forschungszentrum, Germany 3 Bernstein Center for Computational Neuroscience, Germany In freely behaving rodents, the local field potential measured in the hippocampus displays prominent deflections during immobility and sleep. These are called sharp waves, last for about 40 to 60 ms and are jagged with a fast oscillations, or ripples, of about 200Hz. Sharp waves have been shown in rats to co-occur with multi-unit replay and preplay patterns following and preceding a learned spatial experience [1-3]. Patterns are compressed in order to fit within the tight temporal frame offered by the sharp-wave ripple complexes. On a cellular level, it is known that both interneurons and pyramidal cells are significantly phase-locked to the ripple phenomenon.We aim at understanding the coordinated cellular activity that during sharp-wave ripple complexes. To this end, we resort to in vitro simultaneous field potential and single-cell voltage clamp recordings on submerged mouse hippocampal slices, where the phenomenon appears with characteristics known from the in vivo situation [4]. Our results stem from the first direct analysis of sharp-wave associated post synaptic currents (PSCs). These were recorded at different holding potentials representative of different excitation/inhibition mixes (−60 mV vs around −50 mV) as well as under intracellular block of inhibition. The following evidence suggests that the intracellular high frequency oscillations are supported by strong excitatory currents (see also [5]) and complements the present view that high-frequency oscillations during sharp-waves in vivo are mainly mediated by inhibitory interneurons: 1. The kinetics of sharp-wave associated currents were consistent with fast EPSCs.2. The shape of sharp-wave associated PSCs is not compatible with inhibition riding on tonic excitation.3. Intracellular block of inhibition did not affect the absolute ripple-band power nor the frequency of the sharp-wave associated fast PSCs.4. Putative EPSCs showed strong locking to the extracellular ripple and lead the sharp wave peak. Acknowledgements: This work was supported by the Bundensministerium für Bildung und Forschung (BMBF, grant numbers 01GQ0440 and 01GQ0410) and the Deutsche Forschungsgemeinschaft (DFG, grant number LE 2250/2-1).

Glutamate and dopamine receptors contribute to the lateral spread of epileptiform discharges in rat neocortical slices

The effects of AMPA-type glutamate receptor as well as dopamine D1 and D2 receptors on the lateral propagation of epileptiform field potentials (EFP) were studied across adjacent areas of rat neocortical tissues. Epileptiform burst discharges were induced by superfusion of Mg(2+)-free artificial cerebrospinal fluid. Simultaneous field potential recordings of EFP were obtained from four microelectrodes placed 2-3 mm apart across coronal slices in the third layer of the neocortex. The effects of AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), dopamine D1 receptor agonist SKF 81297, and dopamine D2 receptor agonist quinpirole on lateral propagation of burst discharges were investigated. CNQX, applied focally between recording sites, blocked rapid propagation across treated areas and resulted in the emergence of spatially separate, independent pacemakers. Focal application of SKF 81297 between recording sites increased the repetition rate of EFP, but reduced the amplitude as well as the duration of epileptic discharges. However, addition of SKF 81297 to the bath medium abolished EFP. Both local and systemic applications of quinpirole irreversibly enhanced repetition rate of epileptiform burst discharges. The results indicate the prerequisite of AMPA synaptic transmission for synchronized lateral propagation of Mg(2+)-free ACSF-induced epileptic activity and the modulatory effects of dopamine D1 and D2 receptors on both EFP initiation and propagation in epileptic tissues.

Neocortical and Thalamic Spread of Amygdala Kindled Seizures

Amygdala kindling is an epilepsy model involving long-term network plasticity in the nervous system. In this model, repeated weak stimulation of the amygdala eventually leads to severe motor seizures. The mechanisms for worsening behavioral seizures, and the possible role of enhanced connectivity between the amygdala and other structures have not been thoroughly investigated. We performed simultaneous field potential recordings from the amygdala, frontal cortex, and medial thalamus during kindling in rats. Seizures were analyzed for signal power compared with baseline and for correlation between recording sites. Interictal signals were analyzed for changes in coherence between electrode contacts in kindled animals compared with sham kindled controls. We found that increased behavioral severity of seizures was related to increased seizure duration and to increased signal power in the frontal cortex and medial thalamus. Kindling was associated with increased connectivity between the amygdala and frontal cortex, based on increased amygdala-frontal signal correlation during seizures. In addition, during the interictal period, increased coherence was noted between amygdala and frontal contacts in kindled animals compared with controls. We found evidence for increased connectivity between the amygdala and frontal cortex both during seizures and in the interictal period, as a result of kindling. Enhanced connections between limbic and neocortical circuits may be important for the development of epilepsy, as well as for normal long-range network plasticity in the nervous system.

Ipsilateral shoulder pain after thoracotomy surgery

Analgesics and anaesthetics have diverse synaptic actions that nonetheless have a common net inhibitory action on neuronal discharge. It is puzzling, therefore, that these two classes of compounds have fundamentally different affects, one blocking pain and the other consciousness. Indeed, beyond the isolated synapse, little is known of the larger scale mechanisms that mediate actual function, for example, transient neuronal assemblies. It was hypothesized that the two classes of drugs might have, respectively, differential effects on transient activation of these assemblies of neurons working together. Hippocampal tissue from juvenile Wistar rats was used for in vitro optical imaging with voltage-sensitive dyes and simultaneous field potential recordings. The response to paired pulse stimulation of the hippocampus was recorded in the presence and absence of two types of analgesic (morphine and gabapentin) and two types of anaesthetic (thiopental and propofol). Optical imaging and electrophysiology used in parallel yield quite different results. Most consistently, the imaging technique was able to detect an enhanced period of activation following anaesthetic, but not analgesic application. This effect was not readily seen from electrophysiology field potential recordings. These findings suggest that, irrespective of the effects of the two drug classes at a synaptic level, the dynamics of transient neuronal assemblies are modified selectively by anaesthetics and not analgesics.

Initiation of electrographic seizures by neuronal networks in entorhinal and perirhinal cortices in vitro

The hippocampus is often considered to play a major role in the pathophysiology of mesial temporal lobe epilepsy. However, emerging clinical and experimental evidence suggests that parahippocampal areas may contribute to a greater extent to limbic seizure initiation, and perhaps epileptogenesis. To date, little is known about the participation of entorhinal and perirhinal networks to epileptiform synchronization. Here, we addressed this issue by using simultaneous field potential recordings in horizontal rat brain slices containing interconnected limbic structures that included the hippocampus proper. Epileptiform discharges were disclosed by bath applying the convulsant drug 4-aminopyridine (50 μM) or by superfusing Mg 2+-free medium. In the presence of 4-aminopyridine, slow interictal- (duration=2.34±0.29 s; interval of occurrence=25.75±2.11 s, n=16) and ictal-like (duration=31.25±3.34 s; interval of occurrence=196.96±21.56 s, n=17) discharges were recorded in entorhinal and perirhinal cortices after abating the propagation of CA3-driven interictal activity to these areas following extended hippocampal knife cuts. Simultaneous recordings obtained from the medial and lateral entorhinal cortex, and from the perirhinal cortex revealed that interictal and ictal discharges could initiate from any of these areas and propagate to the neighboring structure with delays of 8–66 ms. However, slow interictal- and ictal-like events more often originated in the medial entorhinal cortex and perirhinal cortex, respectively. Cutting the connections between entorhinal and perirhinal cortices ( n=10), or functional inactivation of cortical areas by local application of a glutamatergic receptor antagonist ( n=11) made independent epileptiform activity occur in all areas. These procedures also shortened ictal discharge duration in the entorhinal cortices, but not in the perirhinal area. Similar results could be obtained by applying Mg 2+-free medium ( n=7). These findings indicate that parahippocampal networks provide independent epileptiform synchronization sufficient to sustain limbic seizures as well as that the perirhinal cortex plays a preferential role in in vitro ictogenesis.

Two reentrant pathways in the hippocampal-entorhinal system.

The entorhinal cortex has long been recognized as an important interface between the hippocampal formation and the neocortex. The notion of bidirectional connections between the entorhinal cortex and the hippocampal formation have led to the suggestion that hippocampal output originating in CA1 and subiculum may reenter hippocampal subfields via the entorhinal cortex. To investigate this, we used simultaneous multi-site field potential recordings and current source density analysis in the entorhinal cortex and hippocampal formation of the rat in vivo. Under ketamine/xylazine anesthesia, we found that repetitive stimulation of subiculum or Schaffer collaterals facilitated entorhinal responses, such that a population spike appeared in layer III. In addition, a current sink in stratum lacunosum-moleculare of area CA1 was found, that followed responses in the entorhinal cortex, indicating reentrance into this area. Responses indicating reentrance in the dentate gyrus were not found under ketamine/xylazine anesthesia, but were readily evoked under urethane anesthesia. Reentrance into CA1 was also encountered under urethane anesthesia. These results suggest that parallel, but possibly functionally distinct, connections are present between the output of the hippocampal formation and cells in layers III and II of the entorhinal cortex that project to area CA1 and the dentate gyrus, respectively.