Contact Heat in Magnetoencephalography: A Systematic Review

BACKGROUND

Contact heat is commonly used in experimental research to evoke brain activity, most frequently acquired with electroencephalography (EEG). Although magnetoencephalography (MEG) improves spatial resolution, using some contact heat stimulators with MEG can present methodological challenges. This systematic review assesses studies that utilise contact heat in MEG, their findings and possible directions for further research.

METHODS

Eight electronic databases were searched for relevant studies, in addition to the selected papers' reference lists, citations and ConnectedPapers maps. Best practice recommendations for systematic reviews were followed. Papers met inclusion criteria if they used MEG to record brain activity in conjunction with contact heat, regardless of stimulator equipment or paradigm.

RESULTS

Of 646 search results, seven studies met the inclusion criteria. Studies demonstrated effective electromagnetic artefact removal from MEG data, the ability to elicit affective anticipation and differences in deep brain stimulation responders. We identify contact heat stimulus parameters that should be reported in publications to ensure comparisons between data outcomes are consistent.

CONCLUSIONS

Contact heat is a viable alternative to laser or electrical stimulation in experimental research, and methods exist to successfully mitigate any electromagnetic noise generated by PATHWAY CHEPS equipment - though there is a dearth of literature exploring the post-stimulus time window.

Oligomeric β-Amyloid Suppresses Hippocampal γ-Oscillations through Activation of the mTOR/S6K1 Pathway

Neuronal synchronization at gamma frequency (30-100 Hz: γ) is impaired in early-stage Alzheimer's disease (AD) patients and AD models. Oligomeric Aβ_1-42 caused a concentration-dependent reduction of γ-oscillation strength and regularity while increasing its frequency. The mTOR1 inhibitor rapamycin prevented the Aβ_1-42-induced suppression of γ-oscillations, whereas the mTOR activator leucine mimicked the Aβ_1-42-induced suppression. Activation of the downstream kinase S6K1, but not inhibition of eIF4E, was required for the Aβ_1-42-induced suppression. The involvement of the mTOR/S6K1 signaling in the Aβ_1-42-induced suppression was confirmed in Aβ-overexpressing APP/PS1 mice, where inhibiting mTOR or S6K1 restored degraded γ-oscillations. To assess the network changes that may underlie the mTOR/S6K1 mediated γ-oscillation impairment in AD, we tested the effect of Aβ_1-42 on IPSCs and EPSCs recorded in pyramidal neurons. Aβ_1-42 reduced EPSC amplitude and frequency and IPSC frequency, which could be prevented by inhibiting mTOR or S6K1. These experiments indicate that in early AD, oligomer Aβ_1-42 impairs γ-oscillations by reducing inhibitory interneuron activity by activating the mTOR/S6K1 signaling pathway, which may contribute to early cognitive decline and provides new therapeutic targets.

Activation of Dopamine 4 Receptor Subtype Enhances Gamma Oscillations in Hippocampal Slices of Aged Mice

Aim

Neural network oscillation at gamma frequency band (γ oscillation, 30–80 Hz) is synchronized synaptic potentials important for higher brain processes and altered in normal aging. Recent studies indicate that activation of dopamine 4 receptor (DR4) enhanced hippocampal γ oscillation of young mice and fully recovered the impaired hippocampal synaptic plasticity of aged mice, we determined whether this receptor is involved in aging-related modulation of hippocampal γ oscillation.

Methods

We recorded γ oscillations in the hippocampal CA3 region from young and aged C57bl6 mice and investigated the effects of dopamine and the selective dopamine receptor (DR) agonists on γ oscillation.

Results

We first found that γ oscillation power (γ power) was reduced in aged mice compared to young mice, which was restored by exogenous application of dopamine (DA). Second, the selective agonists for different D1- and D2-type dopamine receptors increased γ power in young mice but had little or small effect in aged mice. Third, the D4 receptor (D4R) agonist PD168077 caused a large increase of γ power in aged mice but a small increase in young mice, and its effect is blocked by the highly specific D4R antagonist L-745,870 or largely reduced by a NMDAR antagonist. Fourth, D3R agonist had no effect on γ power of either young or aged mice.

Conclusion

This study reveals DR subtype-mediated hippocampal γ oscillations is aging-related and DR4 activation restores the impaired γ oscillations in aged brain, and suggests that D4R is the potential target for the improvement of cognitive deficits related to the aging and aging-related diseases.

Improvement in Dysphagia Outcomes Following Clinical Target Volume Reduction in the De-ESCALaTE Study

AIMS

The De-ESCALaTE study showed an overall survival advantage for the administration of synchronous cisplatin chemotherapy with radiotherapy in low-risk oropharyngeal cancer when compared with synchronous cetuximab. During the trial, a radiotherapy quality assurance protocol amendment permitted centres to swap from the original radiotherapy contouring protocol (incorporating the whole oropharynx into the high-dose clinical target volume (CTV); anatomical protocol) to a protocol that incorporated the gross tumour volume with a 10 mm margin into the CTV (volumetric protocol). The purpose of this study was to examine both toxicity and tumour control related to this protocol amendment.

MATERIALS AND METHODS

Overall survival and recurrence at 2 years were used to compare tumour control in the two contouring cohorts. For toxicity, the cohorts were compared by both the number of severe (grades 3-5) and all grades acute and late toxicities. In addition, quality of life and swallowing were compared using EORTC-C30 and MD Anderson Dysphagia Inventory, respectively.

RESULTS

Of 327 patients included in this study, 185 were contoured according to the anatomical protocol and 142 by the volumetric protocol. The two cohorts were well balanced, with the exception of significantly more patients in the anatomical cohort undergoing prophylactic feeding tube insertion (P < 0.001). With a minimum of 2 years of follow-up there was no significant difference in overall survival or recurrence between the two contouring protocols. Similarly, there was no significant difference in the rate of reported severe or all grades acute or late toxicity and no sustained significant difference in quality of life. However, there was a significant difference in favour of volumetric contouring in several domains of the MD Anderson Dysphagia Inventory questionnaire at 1 year, which persisted to 2 years in the dysphagia functional (P = 0.002), dysphagia physical (P = 0.009) and dysphagia overall function (P = 0.008) domains.

CONCLUSION

In the context of the unplanned post-hoc analysis of a randomised trial, measurable improvement in long-term dysphagia has been shown following a reduction in the CTV. Further reductions in the CTV should be subject to similar scrutiny within the confines of a prospective study.

Hypofractionated chemoradiation for head and cancer: Data from the PET NECK trial

There has been increased interest in hypofractionated accelerated chemoradiation for head and neck cancer during the recent first peak of the COVID-19 pandemic. Prospective data regarding this approach from randomised trials is lacking. In the PET NECK study, 564 patients with squamous cell carcinoma of the head and neck receiving definitive chemoradiation were randomised to either planned neck dissection or PET CT scan guided surveillance. In this surgical trial, three radiotherapy fractionation schedules delivered over 7, 6 or 4 weeks were permitted with synchronous chemotherapy. The purpose of this study was to determine efficacy and quality of life outcomes associated with the use of these schedules. Primary local control and overall survival in addition to quality of life measures at immediately post treatment and 6, 12 and 24 months post-treatment were compared between the three fractionation cohorts. In the 525 patients where fractionation data was available, 181 (34%), 288 (55%) and 56 (11%) patients received 68-70 Gy in 34-35 fractions (#), 60-66 Gy in 30# and 55 Gy in 20# respectively. At a minimum follow up of two years following treatment there was no significant difference between the three fractionation schemes in local control, overall survival or any quality of life measure. Despite the obvious limitations of this study, some data is provided to support the use of hypofractionated accelerated chemoradiation to avoid delays in cancer treatment and reduce hospital visits during the peak of a pandemic. Data from on-going randomised trials examining hypofractionated chemoradiation may be useful for selecting fractionation schedules during future pandemics.

Modulation of Hippocampal Gamma Oscillations by Dopamine in Heterozygous Reeler Mice in vitro

The reelin haploinsufficient heterozygous reeler mice (HRM), an animal model of schizophrenia, have altered mesolimbic dopaminergic pathways and share similar neurochemical and behavioral properties with patients with schizophrenia. Dysfunctional neural circuitry with impaired gamma (γ) oscillation (30–80 Hz) has been implicated in abnormal cognition in patients with schizophrenia. However, the function of neural circuitry in terms of γ oscillation and its modulation by dopamine (DA) has not been reported in HRM. In this study, first, we recorded γ oscillations in CA3 from wild-type mice (WTM) and HRM hippocampal slices, and we studied the effects of DA on γ oscillations. We found that there was no difference in γ power between WTM and HRM and that DA increased γ power of WTM but not HRM, suggesting that DA modulations of network oscillations in HRM are impaired. Second, we found that N-methyl-D-aspartate receptor (NMDAR) antagonist MK-801 itself increased γ power and occluded DA-mediated enhancement of γ power in WTM but partially restored DA modulation of γ oscillations in HRM. Third, inhibition of phosphatidylinositol 3-kinase (PI3K), a downstream molecule of NMDAR, increased γ power and blocked the effects of DA on γ oscillation in WTM and had no significant effect on γ power but largely restored DA modulation of γ oscillations in HRM. Our results reveal that impaired DA function in HRM is associated with dysregulated NMDAR–PI3K signaling, a mechanism that may be relevant in the pathology of schizophrenia.

Near-Death High-Frequency Hyper-Synchronization in the Rat Hippocampus

Near-death experiences (NDE) are episodes of enhanced perception with impending death, which have been associated with increased high-frequency (13-100 Hz) synchronization of neuronal activity, which is implicated in cognitive processes like perception, attention and memory. To test whether the NDE-associated high-frequency oscillations surge is related to cardiac arrest, recordings were made from the hippocampus of anesthetized rats dying from an overdose of the sedative chloral hydrate (CH). At a lethal dose, CH caused a surge in beta band power in CA3 and CA1 and a surge in gamma band power in CA1. CH increased the inter-regional coherence of high-frequency oscillations within and between hippocampi. Whereas the surge in beta power developed at non-lethal chloral hydrate doses, the surge in gamma power was specific for impending death. In contrast, CH strongly suppressed theta band power in both CA1 and CA3 and reduced inter-regional coherence in the theta band. The simultaneously recorded electrocardiogram showed a small decrease in heart rate but no change in waveform during the high-frequency oscillation surge, with cardiac arrest only developing after the cessation of breathing and collapse of all oscillatory activity. These results demonstrate that the high-frequency oscillation surge just before death is not limited to cardiac arrest and that especially the increase in gamma synchronization in CA1 may contribute to NDE observed both with and without cardiac arrest.

The Modulation of Gamma Oscillations by Methamphetamine in Rat Hippocampal Slices

Gamma frequency oscillations (γ, 30–100 Hz) have been suggested to underlie various cognitive and motor functions. The psychotomimetic drug methamphetamine (MA) enhances brain γ oscillations associated with changes in psychomotor state. Little is known about the cellular mechanisms of MA modulation on γ oscillations. We explored the effects of multiple intracellular kinases on MA modulation of γ induced by kainate in area CA3 of rat ventral hippocampal slices. We found that dopamine receptor type 1 and 2 (DR1 and DR2) antagonists, the serine/threonine kinase PKB/Akt inhibitor and N-methyl-D-aspartate receptor (NMDAR) antagonists prevented the enhancing effect of MA on γ oscillations, whereas none of them affected baseline γ strength. Protein kinase A, phosphoinositide 3-kinase and extracellular signal-related kinases inhibitors had no effect on MA. We propose that the DR1/DR2-Akt-NMDAR pathway plays a critical role for the MA enhancement of γ oscillations. Our study provides an new insight into the mechanisms of acute MA on MA-induced psychosis.

μ-Opioid receptor activation modulates CA3-to-CA1 gamma oscillation phase-coupling

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CA3 gamma oscillation (γ) drives CA1 gamma and suppresses CA1 intrinsic fast γ.

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μ-opioid receptor (MOR) activation reduces γ frequency in CA3 and CA1.

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MOR activation in CA1 phase-uncouples CA1 γ from CA3 γ.

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Uncoupling is not due to CA3 γ deceleration by MOR activation.

In the intact brain, hippocampal area CA1 alternates between low-frequency gamma oscillations (γ), phase-locked to low-frequency γ in CA3, and high-frequency γ, phase-locked to γ in the medial entorhinal cortex. In hippocampal slices, γ in CA1 is phase-locked to CA3 low-frequency γ. However, when Schaffer collaterals are cut, CA1 can generate its own high-frequency γ. Here we test whether (un)coupling of CA1 γ from CA3 γ can be caused by μ-opioid receptor (MOR) modulation.

In CA1 minislices isolated from rat ventral hippocampus slices, MOR activation by DAMGO reduced the dominant frequency of intrinsic fast γ, induced by carbachol. In intact slices, DAMGO strongly reduced the dominant frequency of CA3 slow γ, but did not affect γ power consistently. DAMGO suppressed the phase coupling of CA1 γ to CA3 slow γ and increased the power of CA1 intrinsic fast γ, but not in the presence of the MOR antagonist CTAP. The benzodiazepine zolpidem and local application of DAMGO to CA3 both mimicked the reduction in dominant frequency of CA3 slow γ, but did not reduce the phase coupling. Local application of DAMGO to CA1 reduced phase coupling.

These results suggest that MOR-expressing CA1 interneurons, feed-forwardly activated by Schaffer collaterals, are responsible for the phase coupling between CA3 γ and CA1 γ. Modulating their activity may switch the CA1 network between low-frequency γ and high-frequency γ, controlling the information flow between CA1 and CA3 or medial entorhinal cortex respectively.

Dopamine D4 receptor activation restores CA1 LTP in hippocampal slices from aged mice

Normal aging is characterized with a decline in hippocampal memory functions that is associated with changes in long-term potentiation (LTP) of the CA3-to-CA1 synapse. Age-related deficit of the dopaminergic system may contribute to impairment of CA1 LTP. Here we assessed how the modulation of CA1 LTP by dopamine is affected by aging and how it is dependent on the Ca²⁺ source. In slices from adult mice, the initial slope of the field potential showed strong LTP, but in slices from aged mice LTP was impaired. Dopamine did not affect LTP in adult slices, but enhanced LTP in aged slices. The dopamine D1/D5 receptor (D1R/D5R) agonist SKF-81297 did not affect LTP in adult but caused a relative small increase in LTP in aged slices; however, although there was no difference in dopamine D4 receptor (D4R) expression, the D4R agonist PD168077 increased LTP in aged slices to a magnitude similar to that in adult slices. The N-Methyl-D-aspartate receptor antagonist D-AP5 reduced LTP in adult slices, but not in aged slices. However, in the presence of D-AP5, PD168077 completely blocked LTP in aged slices. The voltage-dependent calcium channel (VDCC) blocker nifedipine reduced LTP in adult slices, but surprisingly enhanced LTP in aged slices. Furthermore, in the presence of nifedipine, PD168077 caused a strong enhancement of LTP in aged slices to a magnitude exceeding LTP in adult slices. Our results indicate that the full rescue of impaired LTP in aging by the selective D4R activation and that a large potentiation role on LTP by co-application of D4R agonist and VDCC blocker may provide novel strategies for the intervention of cognitive decline of aging and age-related diseases.

Multiple Kinases Involved in the Nicotinic Modulation of Gamma Oscillations in the Rat Hippocampal CA3 Area

Neuronal synchronization at gamma band frequency (20–80 Hz, γ oscillations) is closely associated with higher brain function, such as learning, memory and attention. Nicotinic acetylcholine receptors (nAChRs) are highly expressed in the hippocampus, and modulate hippocampal γ oscillations, but the intracellular mechanism underlying such modulation remains elusive. We explored multiple kinases by which nicotine can modulate γ oscillations induced by kainate in rat hippocampal area CA3 in vitro. We found that inhibitors of cyclic AMP dependent kinase (protein kinase A, PKA), protein kinase C (PKC), N-methyl-D-aspartate receptor (NMDA) receptors, Phosphoinositide 3-kinase (PI3K) and extracellular signal-related kinases (ERK), each individually could prevent the γ oscillation-enhancing effect of 1 μM nicotine, whereas none of them affected baseline γ oscillation strength. Inhibition of the serine/threonine kinase Akt increased baseline γ oscillations and partially blocked its nicotinic enhancement. We propose that the PKA-NMDAR-PI3K-ERK pathway modifies cellular properties required for the nicotinic enhancement of γ oscillations, dependent on a PKC-ERK mediated pathway. These signaling pathways provide clues for restoring γ oscillations in pathological conditions affecting cognition. The suppression of γ oscillations at 100 μM nicotine was only dependent on PKA-NMDAR activation and may be due to very high intracellular calcium levels.

Magnetic Shape Memory Micropump for Submicroliter Intracranial Drug Delivery in Rats

Point-of-care diagnostic devices, micrototal analysis (μTAS) systems, lab-on-a-chip development, and biomedical research rely heavily upon microfluidic management and innovative micropump design. Here, we describe the design and prototype deployment of a magnetic shape memory (MSM) micropump capable of submicroliter per minute flow rates. The pump contains no valves or moving parts in the fluid channel and is capable of bidirectional fluid transport. This pump was employed as the mechanism to deliver small intracranial dosages of ketamine and tetrodotoxin (TTX) at 0.33 μl/min during in vivo electrophysiological recordings in anesthetized rats, performing to required specifications.

Acute Ethanol Inhibition of γ Oscillations Is Mediated by Akt and GSK3β

Hippocampal network oscillations at gamma band frequency (γ, 30-80 Hz) are closely associated with higher brain functions such as learning and memory. Acute ethanol exposure at intoxicating concentrations (≥50 mM) impairs cognitive function. This study aimed to determine the effects and the mechanisms of acute ethanol exposure on γ oscillations in an in vitro model. Ethanol (25-100 mM) suppressed kainate-induced γ oscillations in CA3 area of the rat hippocampal slices, in a concentration-dependent, reversible manner. The ethanol-induced suppression was reduced by the D1R antagonist SCH23390 or the PKA inhibitor H89, was prevented by the Akt inhibitor triciribine or the GSk3β inhibitor SB415286, was enhanced by the NMDA receptor antagonist D-AP5, but was not affected by the MAPK inhibitor U0126 or PI3K inhibitor wortmanin. Our results indicate that the intracellular kinases Akt and GSk3β play a critical role in the ethanol-induced suppression of γ oscillations and reveal new cellular pathways involved in the ethanol-induced cognitive impairment.

Atorvastatin enhances kainate-induced gamma oscillations in rat hippocampal slices

Atorvastatin has been shown to affect cognitive functions in rodents and humans. However, the underlying mechanism is not fully understood. Because hippocampal gamma oscillations (γ, 20-80 Hz) are associated with cognitive functions, we studied the effect of atorvastatin on persistent kainate-induced γ oscillation in the CA3 area of rat hippocampal slices. The involvement of NMDA receptors and multiple kinases was tested before and after administration of atorvastatin. Whole-cell current-clamp and voltage-clamp recordings were made from CA3 pyramidal neurons and interneurons before and after atorvastatin application. Atorvastatin increased γ power by ~ 50% in a concentration-dependent manner, without affecting dominant frequency. Whereas atorvastatin did not affect intrinsic properties of both pyramidal neurons and interneurons, it increased the firing frequency of interneurons but not that of pyramidal neurons. Furthermore, whereas atorvastatin did not affect synaptic current amplitude, it increased the frequency of spontaneous inhibitory post-synaptic currents, but did not affect the frequency of spontaneous excitatory post-synaptic currents. The atorvastatin-induced enhancement of γ oscillations was prevented by pretreatment with the PKA inhibitor H89, the ERK inhibitor U0126, or the PI3K inhibitor wortmanin, but not by the NMDA receptor antagonist D-AP5. Taken together, these results demonstrate that atorvastatin enhanced the kainate-induced γ oscillation by increasing interneuron excitability, with an involvement of multiple intracellular kinase pathways. Our study suggests that the classical cholesterol-lowering agent atorvastatin may improve cognitive functions compromised in disease, via the enhancement of hippocampal γ oscillations.

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.

NMDA receptor antagonists distort visual grouping in rats performing a modified two-choice visual discrimination task

RATIONALE

Visual perception is impaired during pathological psychosis, which can be mimicked by NMDA receptor antagonists. However, the underlying mechanisms are poorly understood, partly due to limits of current rodent models for visual integration.

OBJECTIVES

The objectives of the study are (1) to develop a rodent task that can differentiate between effects on perception and nonspecific effects on task performance and (2) to test whether NMDA receptor antagonists affect visual perception in rats.

METHODS

We used an adaptation of Glass patterns to assess visual grouping in rats using a two-choice visual discrimination task in an infrared touch screen conditioning chamber. After rats learned to discriminate between a radial and a concentric bipole pattern, the ability to discriminate between these patterns was tested at various levels of distortion and a psychometric function was fit to obtain the maximum task performance and signal level needed for half-maximum performance.

RESULTS

NMDA receptor antagonists ketamine and phencyclidine at low doses increased the signal quality needed to discriminate between the visual patterns, without affecting the ability to discriminate between undistorted images. At higher doses, the ability to perform the task even with undistorted images was impaired, which was associated with stereotypic behaviour and increased impulsivity.

CONCLUSIONS

The Glass pattern-based visual grouping task is able to differentiate the effect of psychotomimetic NMDA receptor antagonists on visual perception from the effects on motor and memory functions. The half-maximum performance signal level allows quantification of cognitive psychosis in rodents, which can be translated to human psychometric functions and can be used in the development of more effective treatments.

The effect of aging-associated impaired mitochondrial status on kainate-evoked hippocampal gamma oscillations

Oscillations in hippocampal neuronal networks in the gamma frequency band have been implicated in various cognitive tasks and we showed previously that aging reduces the power of such oscillations. Here, using submerged hippocampal slices allowing simultaneous electrophysiological recordings and imaging, we studied the correlation between the kainate-evoked gamma oscillation and mitochondrial activity, as monitored by rhodamine 123. We show that the initiation of kainate-evoked gamma oscillations induces mitochondrial depolarization, indicating a metabolic response. Aging had an opposite effect on these parameters: while depressing the gamma oscillation strength, it increases mitochondrial depolarization. Also, in the aged neurons, kainate induced significantly larger Ca2+ signals. In younger slices, acute mitochondrial depolarization induced by low concentrations of mitochondrial protonophores strongly, but reversibly, inhibits gamma oscillations. These data indicating that the complex network activity required by the maintenance of gamma activity is susceptible to changes and modulations in mitochondrial status.

Temperature- and concentration-dependence of kainate-induced γ oscillation in rat hippocampal slices under submerged condition

AIM

Fast neuronal network oscillation at the γ frequency band (γ oscillation: 30-80 Hz) has been studied extensively in hippocampal slices under interface recording condition. The aim of this study is to establish a method for recording γ oscillation in submerged hippocampal slices that allows simultaneously monitoring γ oscillation and the oscillation-related intracellular events, such as intracellular Ca(2+) concentration or mitochondrial membrane potentials.

METHODS

Horizontal hippocampal slices (thickness: 300 μm) of adult rats were prepared and placed in a submerged or an interface chamber. Extracellular field recordings were made in the CA3c pyramidal layer of the slices. Kainate, an AMPA/kainate receptor agonist, was applied via perfusion. Data analysis was performed off-line.

RESULTS

Addition of kainate (25-1000 nmol/L) induced γ oscillation in both the submerged and interface slices. Kainate increased the γ power in a concentration-dependent manner, but the duration of steady state oscillation was reduced at higher concentrations of kainate. Long-lasting γ oscillation was maintained at the concentrations of 100-300 nmol/L. Under submerged condition, γ oscillation was temperature-dependent, with the maximum power achieved at 29 °C. The induction of γ oscillation under submerged condition also required a fast rate of perfusion (5-7 mL/min) and showed a fast dynamic during development and after the washout.

CONCLUSION

The kainite-induced γ oscillation recorded in submerged rat hippocampal slices is useful for studying the intracellular events related to neuronal network activities and may represent a model to reveal the mechanisms underlying the normal neuronal synchronizations and diseased conditions.

Threshold for epileptiform activity is elevated in prion knockout mice

Prion protein (PrP) is abundant in the nervous system, but its role remains uncertain. Prion diseases depend on an aggregation of the protein that is likely to interfere with its normal function. Loss of function does not in itself cause neurodegeneration, but whether it contributes to the clinical features of the disease remains an open question. Patients with classical Creutzfeldt-Jakob disease (CJD) have a higher than expected incidence of epilepsy. To study the mechanisms by which loss of PrP function may underlie changes in vulnerability to epilepsy in disease, we used several acute epilepsy models: we applied a variety of convulsant treatments (zero-magnesium, bicuculline, and pentylenetetrazol) to slices in vitro from PrP knockout (Prnp0/0) and control mice. In all three epilepsy models, we found that longer delays and/or higher concentrations of convulsants were necessary to generate spontaneous epileptiform activity in Prnp0/0 mice. These results together indicate an increased seizure threshold in Prnp0/0 mice, suggesting that loss of PrP function cannot explain a predisposition to seizures initiation in CJD.

NMDA receptor hypofunction phase couples independent γ-oscillations in the rat visual cortex

Hallucinations, a hallmark of psychosis, can be induced by the psychotomimetic N-methyl-D-aspartic acid (NMDA) receptor antagonists, ketamine and phencyclidine (PCP), and are associated with hypersynchronization in the γ-frequency band, but it is unknown how reduced interneuron activation associated with NMDA receptor hypofunction can cause hypersynchronization or distorted perception. Low-frequency γ-oscillations (LFγ) and high-frequency γ-oscillations (HFγ) serve different aspects of perception. In this study, we test whether ketamine and PCP affect the interactions between HFγ and LFγ in the rat visual cortex in vitro. In slices of the rat visual cortex, kainate and carbachol induced LFγ (∼ 34 Hz at 32°C) in layer V and HFγ (∼ 54 Hz) in layer III of the same cortical column. In controls, HFγ and LFγ were independent, and pyramidal neurons recorded in layer III were entrained by HFγ, but not by LFγ. Sub-anesthetic concentrations of ketamine selectively decelerated HFγ by 22 Hz (EC(50)=2.7 μM), to match the frequency of LFγ in layer V. This caused phase coupling of the two γ-oscillations, increased spatial coherence in layer III, and entrained the firing of layer III pyramidal neurons by LFγ in layer V. PCP similarly decelerated HFγ by 22 Hz (EC(50)=0.16 μM), causing cross-layer phase coupling of γ-oscillations. Selective NMDA receptor antagonism, selective NR2B subunit-containing receptor antagonism, and reduced D-serine levels caused a similar selective deceleration of HFγ, whereas increasing NMDA receptor activation through exogenous NMDA, D-serine, or mGluR group 1 agonism selectively accelerated HFγ. The NMDA receptor hypofunction-induced phase coupling of the normally independent γ-generating networks is likely to cause abnormal cross-layer interactions, which may distort perceptions due to aberrant matching of top-down information with bottom-up information. If decelerated HFγ and subsequent cross-layer synchronization also underlie pathological psychosis, acceleration of HFγ could be the target for improved antipsychotic therapy.

In vitro hippocampal gamma oscillation power as an index of in vivo CA3 gamma oscillation strength and spatial reference memory

Neuronal synchronisation at gamma frequencies (30-100 Hz) has been implicated in cognition and memory. Gamma oscillations can be studied in various in vitro models, but their in vivo validity and their relationship with reference memory remains to be proven. By using the natural variation of wild type C57bl/6J mice, we assessed the relationships between reference memory and gamma oscillations recorded in hippocampal area CA3 in vivo and in vitro. Local field potentials (LFPs) were recorded from area CA3 in behaviourally-characterised freely moving mice, after which hippocampal slices were prepared for recordings in vitro of spontaneous gamma oscillations and kainate-induced gamma oscillations in CA3. The gamma-band power of spontaneous oscillations in vitro correlated with that of CA3 LFP oscillations during inactive behavioural states. The gamma-band power of kainate-induced oscillations correlated with the activity-dependent increase in CA3 LFP gamma-band power in vivo. Kainate-induced gamma-band power correlated with Barnes circular platform performance and object location recognition, but not with object novelty recognition. Kainate-induced gamma-band power was larger in mice that recognised the aversive context, but did not correlate with passive avoidance delay. The correlations between behavioural and electrophysiological measures obtained from the same animals show that the gamma-generating capacity of the CA3 network in vitro is a useful index of in vivo gamma strength and supports an important role of CA3 gamma oscillations in spatial reference memory.

High-frequency gamma oscillations coexist with low-frequency gamma oscillations in the rat visual cortex in vitro

Synchronization of neuronal activity in the visual cortex at low (30-70 Hz) and high gamma band frequencies (> 70 Hz) has been associated with distinct visual processes, but mechanisms underlying high-frequency gamma oscillations remain unknown. In rat visual cortex slices, kainate and carbachol induce high-frequency gamma oscillations (fast-gamma; peak frequency approximately 80 Hz at 37 degrees C) that can coexist with low-frequency gamma oscillations (slow-gamma; peak frequency approximately 50 Hz at 37 degrees C) in the same column. Current-source density analysis showed that fast-gamma was associated with rhythmic current sink-source sequences in layer III and slow-gamma with rhythmic current sink-source sequences in layer V. Fast-gamma and slow-gamma were not phase-locked. Slow-gamma power fluctuations were unrelated to fast-gamma power fluctuations, but were modulated by the phase of theta (3-8 Hz) oscillations generated in the deep layers. Fast-gamma was spatially less coherent than slow-gamma. Fast-gamma and slow-gamma were dependent on gamma-aminobutyric acid (GABA)(A) receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and gap-junctions, their frequencies were reduced by thiopental and were weakly dependent on cycle amplitude. Fast-gamma and slow-gamma power were differentially modulated by thiopental and adenosine A(1) receptor blockade, and their frequencies were differentially modulated by N-methyl-D-aspartate (NMDA) receptors, GluK1 subunit-containing receptors and persistent sodium currents. Our data indicate that fast-gamma and slow-gamma both depend on and are paced by recurrent inhibition, but have distinct pharmacological modulation profiles. The independent co-existence of fast-gamma and slow-gamma allows parallel processing of distinct aspects of vision and visual perception. The visual cortex slice provides a novel in vitro model to study cortical high-frequency gamma oscillations.

Calcium and normal brain ageing

Normal brain ageing is associated with a varying degree of cognitive impairment. Although ageing is a complex, multifactorial process, and no single process could explain the ageing phenotype, a number of processes and homeostatic systems, due to their central roles in cellular physiology, have been identified as playing important roles in the process of normal ageing. In this review we revisit the basic tenets of the Ca2+ hypothesis of neuronal ageing and stress the major conceptual changes that occurred between the time of its original proposal and now, in particular in respect to the extent of neuronal loss in normal ageing. We provide a general overview of the most important ageing-associated changes in neuronal Ca2+ homeostasis and then discuss in some detail how such homeostatic changes are affecting basic neuronal properties, such as intrinsic excitability and how, by extension, such changes could lead to significant perturbations in the activity of whole neuronal network ensembles. Since some of these network activities, such as the synchronisation of neuronal activity in the gamma frequency range, have been linked to learning and cognition, understanding the metabolic substrates and homeostatic dysregulation that underpin them could provide a novel basis for attempts at counteracting the cognitive decline of older individuals.

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.

ERK activation causes epilepsy by stimulating NMDA receptor activity

The ERK MAPK signalling pathway is a highly conserved kinase cascade linking transmembrane receptors to downstream effector mechanisms. To investigate the function of ERK in neurons, a constitutively active form of MEK1 (caMEK1) was conditionally expressed in the murine brain, which resulted in ERK activation and caused spontaneous epileptic seizures. ERK activation stimulated phosphorylation of eukaryotic translation initiation factor 4E (eIF4E) and augmented NMDA receptor 2B (NR2B) protein levels. Pharmacological inhibition of NR2B function impaired synaptic facilitation in area cornus ammonicus region 3 (CA3) in acute hippocampal slices derived from caMEK1-expressing mice and abrogated epilepsy in vivo. In addition, expression of caMEK1 caused phosphorylation of the transcription factor, cAMP response element-binding protein (CREB) and increased transcription of ephrinB2. EphrinB2 overexpression resulted in increased NR2B tyrosine phosphorylation, which was essential for caMEK1-induced epilepsy in vivo, since conditional inactivation of ephrinB2 greatly reduced seizure frequency in caMEK1 transgenic mice. Therefore, our study identifies a mechanism of epileptogenesis that links MAP kinase to Eph/Ephrin and NMDA receptor signalling.

Age-dependent reduction of gamma oscillations in the mouse hippocampus in vitro

Normal brain ageing is associated with a decline in hippocampal memory functions. Neuronal oscillations in the gamma frequency band have been implicated in various cognitive tasks. In this study we test the effect of normal brain ageing on gamma oscillations in the mouse hippocampus in vitro. gamma Oscillations were evoked by either 10 microM carbachol or 100 nM kainate in ventral hippocampus slices from young (>5 month) and aged (>22 month) C57Bl/J6 mice. In slices from young mice carbachol-induced gamma oscillations were more regular and more coherent than those induced by kainate. Compared with young, the power in the 20-80 Hz frequency range in area CA3 of slices from aged mice was reduced to 14% for kainate-induced oscillations and to 7% for carbachol-induced oscillations, whereas waveform, dominant frequency and coherence of the oscillation were unchanged. Local network properties were assessed by paired-pulse stimulation of Schaffer collateral/commissural fibers. The excitatory synaptic response in stratum radiatum of CA3 was reduced, in correlation with the antidromic population spike, but functional inhibition in CA3 and CA1 was unaffected. Changes in local network properties could not explain the reduced gamma oscillation strength. Since oscillations driven by two different pathways are similarly affected with age, an age-dependent effect on tonic depolarizing drive of principal cells is unlikely to explain the current results. Other mechanisms, including a change with age in the use-dependent modulation of synaptic strength, should account for the impaired gamma oscillations in the aged hippocampus that may contribute to age-dependent memory impairment.

The role of extracellular potassium in the epileptogenic transformation of recurrent GABAergic inhibition

PURPOSE

Epileptiform burst-firing can occur in hippocampal area CA1 where recurrent excitation is relatively weak and recurrent inhibition strong. Recent observations suggest that recurrent inhibition can transform into recurrent excitation because of collapse of the chloride gradient. Here we assess the role of potassium in this epileptogenic transformation.

METHODS

Extracellular field potential recordings, combined with either intracellular recordings from pyramidal neurons or extracellular potassium concentration recordings, were made in vitro from isolated CA1 minislices cut from the rat hippocampus and in vivo from area CA1 in urethane-anesthetized rats. Burst responses were evoked by 5-Hz alveus stimulation.

RESULTS

The 5-Hz alveus stimulation in vitro caused a transient period of burst responses that was associated with a transient increase in synaptic input in stratum oriens and a transient shift of the reversal potential of the synaptic potential. These changes were related to the transient increase in extracellular potassium concentration in stratum oriens. Observations in vivo confirmed the relation between bursting and extracellular potassium concentration in stratum oriens.

CONCLUSIONS

Use-dependent increase of extracellular potassium concentration in stratum oriens facilitates the collapse of the chloride gradient in the basal dendrites and transforms gamma-aminobutyric acid (GABA)ergic inhibition into GABAergic excitation, giving rise to burst firing. Improvement of intracellular chloride homeostasis or extracellular potassium homeostasis could reduce epileptogenicity.

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

Tetanic stimulation of axons terminating in the CA1 region of the hippocampus induces oscillations in the gamma-to-beta frequency band (13-100 Hz) and can induce long-term potentiation (LTP). The rapid pyramidal cell discharge is driven by a mainly GABA(A)-receptor-mediated slow depolarization and entrained mainly through ephaptic interactions. This study tests whether cellular compartmentalization can explain how cells, despite severely reduced input resistance, can still fire briskly and have IPSPs superimposed on the slow GABAergic depolarization, and whether this behaviour occurs in vivo. Oscillations induced in CA1 in vitro by tetanic stimulation of the stratum radiatum or oriens were analysed using intracellular and multichannel field potentials along the cell axis. Layer-specific effects of focal application of bicuculline indicate that the GABAergic depolarization is concentrated on tetanized dendrites. Current-source density analysis and characteristics of partial spikes indicate that early action potentials are initiated in the proximal nontetanized dendrite but cannot invade the tetanized dendrite, where recurrent EPSPs and evoked IPSPs were largely suppressed. As the oscillation progresses, IPSPs recover and slow the neuronal firing to beta frequencies, with a small subpopulation of neurons continuing to fire at gamma frequency. Carbonic anhydrase dependence, threshold intensity, frequency, field strength and spike initiation/propagation of tetanus-evoked oscillations in urethane-anaesthetized rats, validate our observations in vitro, and show that these mechanisms operate in healthy tissue. However, the disrupted electrophysiology of the tetanized dendrites will disable normal information processing, has implications for LTP induction and is likely to play a role in pathological synchronization as found during epileptic discharges.

Parvalbumin deficiency affects network properties resulting in increased susceptibility to epileptic seizures

Networks of GABAergic interneurons are of utmost importance in generating and promoting synchronous activity and are involved in producing coherent oscillations. These neurons are characterized by their fast-spiking rate and by the expression of the Ca(2+)-binding protein parvalbumin (PV). Alteration of their inhibitory activity has been proposed as a major mechanism leading to epileptic seizures and thus the role of PV in maintaining the stability of neuronal networks was assessed in knockout (PV-/-) mice. Pentylenetetrazole induced generalized tonic-clonic seizures in all genotypes, but the severity of seizures was significantly greater in PV-/- than in PV+/+ animals. Extracellular single-unit activity recorded from over 1000 neurons in vivo in the temporal cortex revealed an increase of units firing regularly and a decrease of cells firing in bursts. In the hippocampus, PV deficiency facilitated the GABA(A)ergic current reversal induced by high-frequency stimulation, a mechanism implied in the generation of epileptic activity. We postulate that PV plays a key role in the regulation of local inhibitory effects exerted by GABAergic interneurons on pyramidal neurons. Through an increase in inhibition, the absence of PV facilitates synchronous activity in the cortex and facilitates hypersynchrony through the depolarizing action of GABA in the hippocampus.

Persistent sodium current in subicular neurons isolated from patients with temporal lobe epilepsy

The persistent sodium current is a common target of anti-epileptic drugs and contributes to burst firing. Intrinsically burst firing subicular neurons are involved in the generation and spread of epileptic activity. We measured whole-cell sodium currents in pyramidal neurons isolated from the subiculum resected in drug-resistant epileptic patients and in rats. In half of the cells from both patients and rats, the sodium current inactivated within 500 ms at -30 mV. Others displayed a tetrodotoxin-sensitive slowly or non-inactivating sodium current of up to 53% of the total sodium current amplitude. Compared with the transient sodium current in the same cells, this persistent sodium current activated with normal kinetics but its voltage-dependent activation occurred 7 mV more hyperpolarized. Depolarizing voltage steps that lasted 10 s completely inactivated the persistent sodium current. Its voltage dependence did not differ from that of the transient sodium current but its slope was less steep. The voltage dependence and kinetics of the persistent sodium current in cells from patients were not different from that in subicular cells from rats. The current density and the relative amplitude contribution were 3-4 times greater in neurons from drug-resistant epilepsy patients. The abundant presence of persistent sodium current in half of the subicular neurons could lead to a larger number of neurons with intrinsic burst firing. The extraordinarily large amplitude of the persistent sodium current in this subset of subicular neurons might explain why these patients are susceptible to seizures and hard to treat pharmacologically.

Parvalbumin-deficiency facilitates repetitive IPSCs and gamma oscillations in the hippocampus

In the hippocampus, the calcium-binding protein parvalbumin (PV) is expressed in interneurons that innervate perisomatic regions. PV in GABAergic synaptic terminals was proposed to limit repetitive GABA release by buffering of "residual calcium." We assessed the role of presynaptic PV in Ca(2+)-dependent GABA release in the hippocampus of PV-deficient (PV-/-) mice and wild-type (PV+/+) littermates. Pharmacologically isolated inhibitory postsynaptic currents (IPSCs) were evoked by low-intensity stimulation of the stratum pyramidale and recorded from voltage-clamped CA1 pyramidal neurons. The amplitude and decay time constant of single IPSCs were similar for both genotypes. Under our experimental conditions of reduced release probability and minimal presynaptic suppression, paired-pulse facilitation of IPSCs occurred at intervals from 2 to 50 ms, irrespective of the presence of PV. The facilitation of IPSCs induced by trains of 10 stimuli at frequencies >20 Hz was enhanced in cells from PV-/- mice, the largest difference between PV-/- and PV+/+ animals (220%) being observed at 33 Hz. The effect of IPSC facilitation at sustained gamma frequencies was assessed on kainate-induced rhythmic IPSC-paced neuronal oscillations at gamma frequencies, recorded with dual field potential recordings in area CA3. The maximum power of the oscillation was 138 microV(2) at 36 Hz in slices from PV+/+ mice and was trebled in slices from PV-/- mice. PV deficiency caused a similar increase in gamma power under conditions used to study IPSC facilitation and can be explained by an increased facilitation of GABA release at sustained high frequencies. The dominant frequency and coherence were not affected by PV deficiency. These observations suggest that PV deficiency, due to an increased short-term facilitation of GABA release, enhances inhibition by high-frequency burst-firing PV-expressing interneurons and may affect the higher cognitive functions associated with gamma oscillations.

Quinine suppresses extracellular potassium transients and ictal epileptiform activity without decreasing neuronal excitability in vitro

The effect of quinine on pyramidal cell intrinsic properties, extracellular potassium transients, and epileptiform activity was studied in vitro using the rat hippocampal slice preparation. Quinine enhanced excitatory post-synaptic potentials and decreased fast- and slow-inhibitory post-synaptic potentials. Quinine reduced the peak potassium rise following tetanic stimulation but did not affect the potassium clearance rate. Epileptiform activity induced by either low-Ca(2+) or high-K(+) artificial cerebrospinal fluid (ACSF) was suppressed by quinine. The frequency of spontaneous inter-ictal bursting induced by picrotoxin, high-K(+), or 4-aminopyridine was significantly increased. In normal ACSF, quinine did not affect CA1 pyramidal cell resting membrane potential, input resistance, threshold for action potentials triggered by intracellular or extracellular stimulation, or the orthodromic and antidromic evoked population spike amplitude. The main effects of quinine on intrinsic cell properties were to increase action potential duration and to reduce firing frequency during sustained membrane depolarizations, but not at normal resting membrane potentials. This attenuation was enhanced at increasingly depolarized membrane potentials. These results suggest that quinine suppresses extracellular potassium transients and ictal activity and modulates inter-ictal activity by limiting the firing rate of cells in a voltage-dependent manner. Because quinine does not affect 'normal' neuronal function, it may merit consideration as an anticonvulsant.

Tetanus toxin induces long-term changes in excitation and inhibition in the rat hippocampal CA1 area

Intrahippocampal tetanus toxin induces a period of chronic recurrent limbic seizures in adult rats, associated with a failure of inhibition in the hippocampus. The rats normally gain remission from their seizures after 6-8 weeks, but show persistent cognitive impairment. In this study we assessed which changes in cellular and network properties could account for the enduring changes in this model, using intracellular and extracellular field recordings in hippocampal slices from rats injected with tetanus toxin or vehicle, 5 months previously. In CA1 pyramidal neurones from toxin-injected rats, the slope of the action potential upstroke was reduced by 32%, the fast afterhyperpolarisation by 32% and the slow afterhyperpolarisation by 54%, suggesting changes in voltage-dependent conductances. The excitatory postsynaptic potential slope was reduced by 60% and the population synaptic potential slope was reduced at all stimulus intensities, suggesting a reduced afferent input in CA1. Paired-pulse stimulation showed an increase of the excitability ratio and an increase of cellular excitability only for the second pulse, suggesting a reduced inhibition. The polysynaptic inhibitory postsynaptic potential was reduced by 34%, whereas neither the inhibitory postsynaptic potential at subthreshold stimulus intensities,nor the pharmacologically isolated monosynaptic inhibitory postsynaptic potential were different in toxin-injected rats, suggesting a reduced synaptic excitation of interneurones. Stratum radiatum stimuli in toxin-injected rats, and not in controls, evoked antidromic activation of CA1 neurones, demonstrating axonal sprouting into areas normally devoid of CA1 pyramidal cell axons.We conclude that this combination of enduring changes in cellular and network properties, both pro-epileptic (increased recurrent excitatory connectivity, reduced recurrent inhibition and reduced afterhyperpolarisations) and anti-epileptic (impaired firing and reduced excitation), reaches a balance that allows remission of seizures, perhaps at the price of persistent cognitive impairment.

Prolonged epileptiform bursting induced by 0-Mg(2+) in rat hippocampal slices depends on gap junctional coupling

The transition from brief interictal to prolonged seizure, or 'ictal', activity is a crucial event in epilepsy. In vitro slice models can mimic many phenomena observed in the electroencephalogram of patients, including transition from interictal to ictaform or seizure-like activity. In field potential recordings, three discharge types can be distinguished: (1) primary discharges making up the typical interictal burst, (2) secondary bursts, lasting several hundred milliseconds, and (3) tertiary discharges lasting for seconds, constituting the ictal series of bursts. The roles of chemical synapses in these classes of burst have been explored in detail. Here we test the hypothesis that gap junctions are necessary for the generation of secondary bursts. In rat hippocampal slices, epileptiform activity was induced by exposure to 0-Mg(2+). Epileptiform discharges started in the CA3 subfield, and generally consisted of primary discharges followed by 4-13 secondary bursts. Three drugs that block gap junctions, halothane (5-10 mM), carbenoxolone (100 microM) and octanol (0.2-1.0 mM), abolished the secondary discharges, but left the primary bursts intact. The gap junction opener trimethylamine (10 mM) reversibly induced secondary and tertiary discharges. None of these agents altered intrinsic or synaptic properties of CA3 pyramidal cells at the doses used. Surgically isolating the CA3 subfield made secondary discharges disappear, and trimethylamine under these conditions was able to restore them.We conclude that gap junctions can contribute to the prolongation of epileptiform discharges.

Dynamic modulation of excitation and inhibition during stimulation at gamma and beta frequencies in the CA1 hippocampal region

Fast oscillations at gamma and beta frequency are relevant to cognition. During this activity, excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) are generated rhythmically and synchronously and are thought to play an essential role in pacing the oscillations. The dynamic changes occurring to excitatory and inhibitory synaptic events during repetitive activation of synapses are therefore relevant to fast oscillations. To cast light on this issue in the CA1 region of the hippocampal slice, we used a train of stimuli, to the pyramidal layer, comprising 1 s at 40 Hz followed by 2--3 s at 10 Hz, to mimic the frequency pattern observed during fast oscillations. Whole cell current-clamp recordings from CA1 pyramidal neurons revealed that individual stimuli at 40 Hz produced EPSPs riding on a slow biphasic hyperpolarizing-depolarizing waveform. EPSP amplitude initially increased; it then decreased concomitantly with the slow depolarization and with a large reduction in membrane resistance. During the subsequent 10-Hz train: the cells repolarized, EPSP amplitude and duration increased to above control, and no IPSPs were detected. In the presence of GABA(A) receptor antagonists, the slow depolarization was blocked, and EPSPs of constant amplitude were generated by 10-Hz stimuli. Altering pyramidal cell membrane potential affected the time course of the slow depolarization, with the peak being reached earlier at more negative potentials. Glial recordings revealed that the trains were associated with extracellular potassium accumulation, but the time course of this event was slower than the neuronal depolarization. Numerical simulations showed that intracellular chloride accumulation (due to massive GABAergic activation) can account for these observations. We conclude that synchronous activation of inhibitory synapses at gamma frequency causes a rapid chloride accumulation in pyramidal neurons, decreasing the efficacy of inhibitory potentials. The resulting transient disinhibition of the local network leads to a short-lasting facilitation of polysynaptic EPSPs. These results set constraints on the role that synchronous, rhythmic IPSPs may play in pacing oscillations at gamma frequency in the CA1 hippocampal region.

Ictal epileptiform activity is facilitated by hippocampal GABAA receptor-mediated oscillations

The cellular and network mechanisms of the transition of brief interictal discharges to prolonged seizures are a crucial issue in epilepsy. Here we used hippocampal slices exposed to ACSF containing 0 Mg(2+) to explore mechanisms for the transition to prolonged (3-42 sec) seizure-like ("ictal") discharges. Epileptiform activity, evoked by Shaffer collateral stimulation, triggered prolonged bursts in CA1, in 50-60% of slices, from both adult and young (postnatal day 13-21) rats. In these cases the first component of the CA1 epileptiform burst was followed by a train of population spikes at frequencies in the gamma band and above (30-120 Hz, reminiscent of tetanically evoked gamma oscillations). The gamma burst in turn could be followed by slower repetitive "tertiary" bursts. Intracellular recordings from CA1 during the gamma phase revealed long depolarizations, action potentials rising from brief apparent hyperpolarizations, and a drop of input resistance. The CA1 gamma rhythm was completely blocked by bicuculline (10-50 microm), by ethoxyzolamide (100 microm), and strongly attenuated in hyperosmolar perfusate (50 mm sucrose). Subsequent tertiary bursts were also blocked by bicuculline, ethoxyzolamide, and in hyperosmolar perfusate. In all these cases intracellular recordings from CA3 revealed only short depolarizations. We conclude that under epileptogenic conditions, gamma band oscillations arise from GABA(A)ergic depolarizations and that this activity may lead to the generation of ictal discharges.

Modulation of sodium currents in rat CA1 neurons by carbamazepine and valproate after kindling epileptogenesis

PURPOSE

To determine the modulation of sodium currents in hippocampal CA1 neurons by carbamazepine (CBZ) and valproate (VPA), before and after kindling epileptogenesis.

METHODS

Voltage-dependent sodium current was measured in isolated hippocampal CA1 neurons, by using the whole-cell voltage-clamp technique. CBZ (15-100 microM) or VPA (0.5-5 mM) was applied by bath perfusion. Cells from fully kindled rats were compared with controls, 1 day and 5 weeks after the tenth generalized seizure.

RESULTS

CBZ did not affect sodium current activation but selectively shifted the voltage dependence of steady-state inactivation to more hyperpolarized potentials. One day after the last kindled generalized seizure, the shift induced by 15 microM CBZ was 2.1+/-0.5 mV (mean +/- SEM; n = 20) compared with 4.3+/-0.3 mV (n = 16; p<0.001) in matched controls. The EC50 of the concentration-effect relation was 57+/-6 microM compared with 34+/-2 microM (p<0.01) in controls. Five weeks after kindling, these values had recovered to a level not different from control. VPA induces at a relatively high concentration a similar but smaller shift in voltage dependence of inactivation than does CBZ. After kindling, the shift induced by 2 mM VPA (2.8+/-0.6 mV; n = 19) was not different from controls (3.0+/-0.5 mV; n = 22). The EC50 for VPA was 2.6+/-0.3 mM compared with 2.5+/-0.4 mM in controls.

CONCLUSIONS

Both CBZ and VPA selectively modulate the voltage dependence of sodium current steady-state inactivation and as a consequence reduce cellular excitability. The effect of CBZ was reduced immediately after kindling epileptogenesis, apparently by a reduced affinity of its receptor. In contrast, the shift induced by VPA was not different at any stage after kindling epileptogenesis. The change in CBZ sensitivity after kindling is related to epileptic activity rather than to the epileptic state, because it almost completely recovers in a period without seizures.

On the synchronizing mechanisms of tetanically induced hippocampal oscillations

gamma (30-100 Hz) and beta (10-30 Hz) oscillations follow tetanic stimulation in the CA1 region of the rat hippocampal slice. Pyramidal neurons undergo a slow depolarization after the tetanus and generate synchronous action potentials. The slow depolarization was previously attributed to metabotropic glutamate receptor (mGluR) activation. However, we found that this event was mediated by GABA(A) receptors, being blocked by bicuculline (50 microM) and accompanied by a dramatic drop in input resistance. Experiments with NMDA and non-NMDA glutamate receptor antagonists revealed that fast synaptic excitation was not necessary for oscillations. IPSPs were strongly depressed during the oscillations. Instead, synchronization was caused by field effects, as shown by: (1) Action potentials of pyramidal neurons proximal (<200 micrometer) to the stimulation site were often preceded by negative deflections of the intracellular potential that masked a net transmembrane depolarization caused by the population spike. (2) Pyramidal neurons located on the surface of the slice, where field effects are weak, fired repetitively but were not synchronized to the network activity. (3) A moderate decrease (50 mOsm) in artificial CSF (ACSF) osmolality did not affect the slow depolarization but increased oscillation amplitude and duration and recruited previously silent neurons into oscillations. (4) 50 mOsm increase in ACSF osmolality dramatically reduced, or abolished, post-tetanic oscillations. Phasic IPSPs, not detectable in proximal neurons, were present, late in the oscillation, in cells located 200-400 micrometer from the stimulation site and possibly contributed to slowing the rhythm during the gamma to beta transition.

Sodium currents in isolated rat CA1 neurons after kindling epileptogenesis

Cellular excitability of CA1 neurons from a kindled focus in the rat hippocampus is persistently increased. The changes in the underlying voltage-dependent sodium current were characterized under whole-cell voltage-clamp conditions. We compared sodium currents in acutely isolated CA1 neurons from kindled rats with those in matched controls, one day and five weeks after cessation of kindling stimulations. The sodium current in CA1 neurons was tetrodotoxin sensitive and inactivated completely with two time-constants. In 97 cells from control rats, the current evoked at -20 mV consisted of a fast-inactivating component of 3.8 +/- 0.2 nA which decayed with a time-constant of 1.0 +/- 0.1 ms, and a slow-inactivating component of 1.2 +/- 0.1 nA with a time-constant of 3.6 +/- 0.1 ms. The potential of half-maximal inactivation was -72.2 +/- 1.0 mV for the fast-inactivating component and -63.2 +/- 1.0 mV for the slow-inactivating component. The time-constant of recovery at -80 mV was 14.1 +/- 0.4 ms for the fast-inactivating component and 9.3 +/- 0.4 ms for the slow-inactivating component. One day after kindling, the voltage dependence of inactivation of the slow-inactivating and the fast-inactivating component was shifted in the depolarizing direction (3.2 +/- 1.3 and 3.0 +/- 1.3 mV, respectively). The voltage dependence of recovery from inactivation was shifted in the same direction. Five weeks after kindling, the shift in voltage dependence of inactivation was (3.3 +/- 1.2 and 2.9 +/- 1.2 mV, respectively) and was accompanied by a 20% increase in sodium current amplitude. The voltage-dependent activation was not different after kindling. The changes in sodium current inactivation will increase the number of channels available for activation and may enhance the maximum firing rate. This implies that the changes in sodium current inactivation will contribute to the enhanced excitability of pyramidal neurons observed after kindling.

Effect of valproic acid on sodium currents in cortical neurons from patients with pharmaco-resistant temporal lobe epilepsy

In a selected group of temporal lobe epilepsy patients with seizures refractory to pharmacological treatment, pharmacological seizure control can be attained by surgical resection of the epileptic zone. We investigated to what extent pharmaco-resistance is reflected in a reduced response at the cellular level, in neurons acutely isolated from the temporal cortex resected in 20 patients. We studied the effect of valproic acid (VPA) on the transient sodium current, measured under whole-cell voltage-clamp conditions. We compared neurons from patients with temporal lobe sclerosis (S) with neurons from patients without hippocampal sclerosis (nS) and compared hippocampal CA1 neurons (CA) with neocortical neurons (NC). We could not detect differences in the voltage dependence and kinetics of sodium current activation and inactivation in any of the group comparisons. VPA shifted the voltage dependence of steady-state inactivation (expressed as V(h,i) in a Boltzmann fit) to more hyperpolarized levels. The shift induced by 2 mM VPA was -5.1 +/- 0.7 mV in CA-S (n = 13), -5.1 +/- 0.7 mV in CA-nS (n = 25), -4.3 +/- 0.5 mV in NC-S (n = 17) and -4.9 +/- 0.5 mV in NC-nS (n = 16) The relation between concentration and voltage shift had an EC50 of 1.4 +/- 0.2 mM VPA (n = 16) and a maximal shift of 9.6 +/- 0.9 mV. We conclude that pharmaco-resistance in these patients is not associated with a changed modulation of the sodium current by VPA. Results are discussed in the light of a reduced sodium current modulation by carbamazepine in CA1 neurons of patients with hippocampal sclerosis and of similar observations in the kindling model of epileptogenesis.

Anticonvulsant effect of polyunsaturated fatty acids in rats, using the cortical stimulation model

Recent studies have shown that long-chain polyunsaturated fatty acids can prevent cardiac arrhythmias, attributed to the reduction in excitability of cardiomyocytes, owing mainly to a shift in hyperpolarizing direction of the inactivation curves of both Na+ and Ca2+ currents and to a slowed recovery from inactivation. Qualitatively similar effects of polyunsaturated fatty acids on inactivation parameters have been observed in freshly isolated hippocampal neurons. Since the same effects are presumed to underlie the action of some established anticonvulsant drugs, polyunsaturated fatty acids might have an anticonvulsant action as well. We have investigated this for eicosapentaenoic acid, docosahexaenoic acid, linoleic acid and oleic acid, employing cortical stimulation in rats, a seizure model allowing the determination of the full anticonvulsant effect-time profile in freely moving, individual animals. I.v. infusion of 40 micromol of eicosapentaenoic acid or docosahexaenoic acid over a period of 30 min, modestly increased the threshold for localized seizure activity after 6 h by 73 +/- 13 microA (mean +/- S.E.M.; n = 7) and 77 +/- 17 microA (n = 7), respectively, and the threshold for generalized seizure activity by 125 +/- 20 and 130 +/- 19 microA, respectively (P < 0.001). The thresholds remained elevated for 6 h after infusion, but returned to baseline the next day. Free plasma concentrations in rats treated with eicosapentaenoic acid or docosahexaenoic acid, averaged 5.7 +/- 1.6 microM (n = 4) for eicosapentaenoic acid and 12.9 +/- 1.8 microM (n = 5) for docosahexaenoic acid at the end of infusion, but declined to undetectable levels within 3 h. Linoleic acid and oleic acid were less effective. Possible mechanisms for the modest anticonvulsant effect but of long duration with the polyunsaturated fatty acids are discussed.

Calcium currents in pyramidal CA1 neurons in vitro after kindling epileptogenesis in the hippocampus of the rat

Calcium is an important second messenger which plays a role in the regulation of neuronal excitability and in many forms of synaptic plasticity. In kindling epileptogenesis, a model of focal epilepsy, calcium plays an important role. The in situ patch-clamp technique was used to record calcium currents in slices obtained from kindled rats and controls. We found that low-voltage-activated calcium currents, probably of dendritic origin, were larger after kindling (80%). The transient high-voltage-activated calcium currents were also enhanced after kindling (50% higher). The increase of the current is accompanied by a decrease in the time constant of inactivation. The change was still present six weeks after the kindling stimulations were stopped. These data demonstrate that low-voltage-activated calcium currents are involved in epileptogenesis. Their enhancement in the dendrites will boost synaptic depolarization and result in enhanced calcium influx, which is critically dependent on the specific activation pattern.

Polyunsaturated fatty acids modulate sodium and calcium currents in CA1 neurons

Recent evidence indicates that long-chain polyunsaturated fatty acids (PUFAs) can prevent cardiac arrhythmias by a reduction of cardiomyocyte excitability. This was shown to be due to a modulation of the voltage-dependent inactivation of both sodium (INa) and calcium (ICa) currents. To establish whether PUFAs also regulate neuronal excitability, the effects of PUFAs on INa and ICa were assessed in CA1 neurons freshly isolated from the rat hippocampus. Extracellular application of PUFAs produced a concentration-dependent shift of the voltage dependence of inactivation of both INa and ICa to more hyperpolarized potentials. Consequently, they accelerated the inactivation and retarded the recovery from inactivation. The EC50 for the shift of the INa steady-state inactivation curve was 2.1 +/- 0.4 microM for docosahexaenoic acid (DHA) and 4 +/- 0.4 microM for eicosapentaenoic acid (EPA). The EC50 for the shift on the ICa inactivation curve was 2.1 +/- 0.4 for DHA and > 15 microM for EPA. Additionally, DHA and EPA suppressed both INa and ICa amplitude at concentrations > 10 microM. PUFAs did not affect the voltage dependence of activation. The monounsaturated oleic acid and the saturated palmitic acid were virtually ineffective. The combined effects of the PUFAs on INa and ICa may reduce neuronal excitability and may exert anticonvulsive effects in vivo.

Potassium currents in isolated CA1 neurons of the rat after kindling epileptogenesis

Daily tetanic stimulation of the Schaffer collaterals generates an epileptogenic focus in area CA1 of the rat hippocampus, ultimately leading to generalized tonic-clonic convulsions (kindling). Potassium currents were measured under voltage-clamp conditions in pyramidal neurons, acutely dissociated from the focus of fully kindled rats, one day and six weeks after the last generalized seizure. Their amplitude, kinetics, voltage dependence and calcium dependence were compared with controls. With Ca2+ influx blocked by 0.5 mM Ni2+, the sustained current (delayed rectifier) and the transient current (A-current) were not different after kindling. Calcium influx evoked an additional fast transient current component. This transient calcium-dependent current component was increased by 154%, but only immediately after the seizure. A second, slow calcium-dependent potassium current component was dependent on the intracellular calcium level, set by the pipette as well as on calcium influx. The peak amplitude of this slow calcium-dependent current was under optimal calcium conditions not different after kindling, but we found indications that either calcium homeostasis or the calcium sensitivity of the potassium channels was affected by the kindling process. In contrast to the previously described enhancement of calcium current, kindling epileptogenesis did not change the total potassium current amplitude. The minor changes that were observed can be related either to changes in calcium current or to changes in intracellular calcium homeostasis.

An experimental model of progressive epilepsy: the development of kindling of the hippocampus of the rat

Kindling epileptogenesis was induced by periodic electrical stimulation of the Schaffer collateral/commissural pathway in the CA1 area of the rat hippocampus. The progressive nature of hippocampal kindling is demonstrated by a detailed description of the behavioral signs and the progressive increase of the after-discharge duration in the course of kindling acquisition. Furthermore, the evolution of the alterations in the paired-pulse local evoked field potentials and the modifications of the GABAA receptor binding and of the expression of mRNAs encoding for the subunits of the GABAA and glutamate receptors are considered. Evidence is presented that during kindling opposite changes occur in the CA1 and the fascia dentata in terms of the balance between excitation and inhibition due to contrasting changes in GABA-mediated inhibitory function.