Hippocampal EEG Research Articles

Oseltamivir reduces hippocampal abnormal EEG activities after a virus infection (influenza) in isoflurane-anesthetized rats.

BackgroundOseltamivir phosphate (OP, Tamiflu®) is a widely used drug in the treatment of influenza with fever. However, case reports have associated OP intake with sudden abnormal behaviors. In rats infected by the influenza A virus (IAV), the electroencephalogram (EEG) displayed abnormal high-voltage amplitudes with spikes and theta oscillations at a core temperature of 39.9°C to 41°C. Until now, there has been no information describing the effect of OP on intact brain hippocampal activity of IAV-infected animals during hyperthermia.ObjectiveThe aim of the present study was to examine the effect of OP on abnormal EEG activities in the hippocampus using the rat model of influenza-associated encephalopathy.MethodsMale Wistar rats aged 3 to 4 weeks were used for the study. Influenza A/WSN/33 strain (1 × 105 plaque forming unit in PBS, 60 µL) was applied intranasally to the rats. To characterize OP effects on the IAV-infected rats, EEG activity was studied more particularly in isoflurane-anesthetized IAV-infected rats during hyperthermia.ResultsWe found that the hippocampal EEG of the OP-administered (10 mg/kg) IAV-infected rats showed significant reduction of the high-voltage amplitudes and spikes, but the theta oscillations, which had been observed only at >40°C in OP non-administered rats, appeared at 38°C core temperature. Atropine (30 mg/kg) blocked the theta oscillations.ConclusionOur data suggest that OP efficiently reduces the abnormal EEG activities after IAV infection during hyperthermia. However, OP administration may stimulate ACh release in rats at normal core temperature.

Role of M-Type Potassium Channels Glutamatergic Synaptic Transmission and Status Epilepticus (IN5-1.003)

Objective: Whether M-currents modulate glutamatergic synaptic transmission and whether the drugs that open these channels could terminate seizures induced by muscarinic stimulation. Background Muscarinic activation by pilocarpine and anologs result in seizures presumably by the inhibition of M-currents. Muscarinic stimulation modulates glutamatergic synaptic transmission but the mechanisms remain uncertain. Design/Methods: Whole-cell patch clamp recordings from CA3 and CA1 hippocampal pyramidal neurons and video EEG recoprdings from rats. Results: In voltage clamp recordings, XE-991 an M channel blocker increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs), but had no effect on their amplitude. XE-991 and its anolog linopordine also increased the frequency of miniature excitatory postsynaptic currents (mEPSCs) but had no effect on their amplitude. M1 receptor agonist NcN-A-343 also increased the frequency of mEPSCs. The M-channel opener flupirtine had the effect opposite of XE-991. The XE-991 enhancement of mEPSCs frequency was not observed in calcium free external ACSF. When P/Q- or N-type calcium channel was blocked, the effect of XE-991 on mEPSCs was partially prevented. These data suggest that M-channel blockage increases presynaptic calcium dependent glutamate release in CA1 neurons. Current clamp recordings performed in CA3 neurons revealed that XE-991 depolarized membrane potential and increased burst firing. McN-A-343 had similar effect on CA3 neurons as XE-991. We then tested whether M-channel opener flupirtine could terminate status epilepticus (SE). Administration of flupirtine (50 mg/kg) and diazepam (10 mg/kg) individually 10 minutes after continuous EEG activity of first grade seizure was observed did not terminate SE; however, co-administration both drugs significantly reduced the length of SE, with 80% (4/5) rats returning to normal EEG within 90 minutes of treatment. Conclusions: Cholinergic activation causes depolarization of presynaptic CA3 neurons via M-channels, which activates calcium channels to increase glutamate release. Cholinergic inhibition of M-channel contributes to seizure generation.Drugs targeting M currents can terminate status epilepticus. Supported by: Congessionaly Directed Medical Research Program of the Department of Defense. NIH. Disclosure: Dr. Kapur has received (royalty or license fee or contractual rights) payments from a patent. Dr. Sun has nothing to disclose.

State dependent modulation of breathing in rats during urethane anesthesia

Breathing is an automatic process that is highly influenced by brain state, with abnormalities occurring more frequently during sleep. Although rodents are commonly used for respiratory neurobiology studies, they show irregular sleep patterns and cycles, which make studies of sleep‐dependent breathing challenging. The objective of this study was to determine if sleep‐like brain state alternations apparent under urethane anesthesia are correlated to changes in breathing parameters and respiratory muscle modulation, and to determine if they mimic changes occurring during natural sleep.Rats were anesthetized and respiratory parameters (rate, minute ventilation and EMG activity in diaphragm, genioglossus, abdominal and facial muscles) were recorded across different brain states as assessed by neocortical and hippocampal EEG. Results were compared with recordings in natural sleep. Our data show that brain state alternations under urethane anesthesia are correlated with changes in breathing rate, variability and respiratory muscle modulation and closely reproduce changes observed in natural sleep. Of particular interest is the present demonstration of prominent state dependent modulation of both genioglossus and abdominal muscle activity, which is a robust feature of sleep and sleep‐like brain state transitions.Supported by CIHR (JJG) and NSERC (CTD). SP is a Parker B. Francis fellow.

Hippocampal Sleep Features: Relations to Human Memory Function

The recent spread of intracranial electroencephalographic (EEG) recording techniques for presurgical evaluation of drug-resistant epileptic patients is providing new information on the activity of different brain structures during both wakefulness and sleep. The interest has been mainly focused on the medial temporal lobe, and in particular the hippocampal formation, whose peculiar local sleep features have been recently described, providing support to the idea that sleep is not a spatially global phenomenon. The study of the hippocampal sleep electrophysiology is particularly interesting because of its central role in the declarative memory formation. Recent data indicate that sleep contributes to memory formation. Therefore, it is relevant to understand whether specific patterns of activity taking place during sleep are related to memory consolidation processes. Fascinating similarities between different states of consciousness (wakefulness, REM sleep, non-REM sleep) in some electrophysiological mechanisms underlying cognitive processes have been reported. For instance, large-scale synchrony in gamma activity is important for waking memory and perception processes, and its changes during sleep may be the neurophysiological substrate of sleep-related deficits of declarative memory. Hippocampal activity seems to specifically support memory consolidation during sleep, through specific coordinated neurophysiological events (slow waves, spindles, ripples) that would facilitate the integration of new information into the pre-existing cortical networks. A few studies indeed provided direct evidence that rhinal ripples as well as slow hippocampal oscillations are correlated with memory consolidation in humans. More detailed electrophysiological investigations assessing the specific relations between different types of memory consolidation and hippocampal EEG features are in order. These studies will add an important piece of knowledge to the elucidation of the ultimate sleep function.

General pharmacological profile of the novel muscarinic receptor agonist SNI-2011, a drug for xerostomia in Sjögren's syndrome. 1st communication: effects on general behavior and central nervous system.

A novel muscarinic receptor agonist, SNI-2011 ((+/-)-cis-2-methylspiro[1,3-oxathiolane-5,3'-quinuclidine] monohydrochloride hemihydrate, cevimeline, CAS 153504-70-2), is a candidate therapeutic drug for xerostomia in Sjögren's syndrome. The general pharmacological properties of this drug on general behavior and the central nervous system were investigated in mice, rats and cats. 1. General behavior: When SNI-2011 was administered orally to mice at 100 mg/kg, mydriasis, a decrease of spontaneous motor activity, tremor, convulsions, salivation, abnormal posture, abnormal gait, reduced grip strength and reduced response against external stimulating were observed, and 2 out of 6 animals died. At 10 mg/kg or lower, no particular sign was observed except mydriasis, which appeared to be caused via the peripheral muscarinic acetylcholine receptors. 2. Central nervous system: SNI-2011 had no effect on the motor coordination in mice. Hypothermia was observed in rats and reduced spontaneous motor activity, analgesia and enhanced maximum electroshock-induced convulsions were observed in mice after oral administration of 30 mg/kg SNI-2011. Slight increase in the rate of theta-wave band in the hippocampal EEG of rats and spinal multisynaptic reflexes in cats were observed after intravenous injection of 10 mg/kg SNI-2011. At an oral dose of 10 mg/kg, prolongation of thiopental-induced sleeping time in mice was observed. The prolongation of sleeping time was inhibited by a peripheral muscarinic antagonist. These results suggest that SNI-2011 has muscarinic effects on general behavior and the central nervous system at the doses approximately 10-fold higher than the effective doses needed for saliva secretion.

Slow EEG rhythms and inter-hemispheric synchronization across sleep and wakefulness in the human hippocampus

Converging data that attribute a central role to sleep in memory consolidation have increased the interest to understand the characteristics of the hippocampal sleep and their relations with the processing of new information. Neural synchronization between different brain regions is thought to be implicated in long-term memory consolidation by facilitating neural communication and by promoting neural plasticity. However, the majority of studies have focused their interest on intra-hippocampal, rhinal-hippocampal or cortico-hippocampal synchronization, while inter-hemispheric synchronization has been so far neglected. To clarify the features of spontaneous human hippocampal activity and to investigate inter-hemispheric hippocampal synchronization across vigilance states, pre-sleep wakefulness and nighttime sleep were recorded from right and left homologous hippocampal loci using stereo-EEG techniques. Hence, quantitative and inter-hemispheric coherence analyses of hippocampal activity across sleep and waking states were carried out. The results showed the presence of delta activity in human hippocampal spontaneous EEG also during wakefulness. The activity in the delta range exhibited a peculiar bimodal distribution, namely a low frequency non-oscillatory activity (up to 2 Hz) synchronized between hemispheres mainly during wake and REM sleep, and a faster oscillatory rhythm (2-4 Hz). The latter was less synchronized between the hippocampi and seemed reminiscent of animal RSA (rhythmic slow activity). Notably, the low-delta activity showed high inter-hemispheric hippocampal coherence during REM sleep and, to a lesser extent, during wakefulness, paralleled by a (unexpected) decrease of coherence during NREM sleep. Therefore, low-delta hippocampal state-dependent synchronization starkly contrasts with neocortical behavior in the same frequency range. Further studies might shed light on the role of these low frequency rhythms in the encoding processes during wakefulness and in the consolidation processes during subsequent sleep.

Influences of Hippocampal Field CA1 on Theta Activity in Urethane-Anesthetized Rats

We report results from studies of the influences of hippocampal field CA1 on the generation of theta activity in urethane-anesthetized rats in relation to the efficiency of its synaptic inputs. Long-term post-tetanic potentiation of the projections of Schaffer collaterals increased the theta power peak in the spectrum of the hippocampal EEG, without altering the theta frequency. A strong correlations between increases in the amplitude of focal potentials and increases in the expression of theta activity was seen in long-term posttetanic potentiation. The mechanisms and possible functional significance of this relationship between potentiation and theta activity are discussed.

Early suppression of intracranial EEG signals predicts ischemic outcome in adult mice following hypoxia–ischemia

The objective of this study is to determine whether early alterations in intracranial EEG activity predict overall outcome in non-anesthetized adult mice following hypoxia–ischemia (HI). Adult C57BL/6 mice received surgical implantation of bilateral intracranial EEG electrodes in the hippocampus and cerebral cortex. Animals were subjected to a hypoxic–ischemic (HI) episode consisting of permanent occlusion of the right common carotid artery and subsequent systemic hypoxia (8% O 2 for 30 min). EEG activities were sorted based on the observance of motor seizures, poor physical outcome, brain injury, and mortality. EEG signals were quantified as amplitude, variance, and root mean square, and early alterations in these parameters were compared. Animals with poor-HI outcome exhibited longer and more profound suppression of EEG signals in the hippocampus ipsilateral to the carotid artery occlusion during HI. Of the parameters chosen to quantify EEG activity, root mean square demonstrated the greatest sensitivity in predicting subsequent outcome. Thus, ipsilateral hippocampal EEG signals are a reliable early marker for assessing HI outcome in adult mice, and further characterization of ischemic EEG signals may aid in the development of novel quantitative variables for use in animal models of experimental cerebral ischemia.

Comparative analysis of the neurophysiological profile of group II metabotropic glutamate receptor activators and diazepam: Effects on hippocampal and cortical EEG patterns in rats

Selective activation of the Group II metabotropic glutamate receptors 2/3 (mGlu2/3) by either full agonists or positive allosteric modulators (PAMs) show anxiolytic activity. In the present study the anxiolytic profile of mGlu2/3 receptor agonists LY-354740 and LY-404039 and the mGlu2 receptor PAM 1-methyl-2-((cis-3-methyl-4-(4-trifluoromethyl-2-methoxy)-phenyl)piperidin-1-yl)-1H-imidazo[4,5-b]pyridine (MTFIP) were evaluated using neurophysiology-based assays. Activation of mGlu2/3 receptors by these compounds, as well as the positive control diazepam, significantly decreased the frequency of hippocampal theta oscillation elicited by stimulation of the brainstem nucleus pontis oralis (nPO), a characteristic action of anxiolytic compounds. Since the nPO is a critical region involved in regulation of rapid eye movement sleep, mGlu2/3 receptor activators were also tested on sleep parameters, as well as on cortical and hippocampal encephalography (EEG) activity. Both mGlu2/3 agonists and the mGlu2 PAM significantly prolonged REM sleep latency and reduced total REM sleep duration while during the active awake state all compounds lowered hippocampal peak theta frequency. However, diazepam and mGlu2/3 agonists/PAM elicited opposite changes in cortical EEG delta and beta bands. Delta power significantly increased after any of the mGlu2/3 compounds but decreased after diazepam. In the beta band, mGlu2/3 receptor agonists dose-dependently decreased beta power in contrast to the well-known beta activation by diazepam. These effects lasted 3–4h and could not be explained by modest, transient changes (<1h) in waking and slow wave sleep. The current observations support the role of mGlu2/3 receptor activators as potential anxiolytic compounds, but indicate a distinct action on cortical EEG activity which is different from the effects of GABAA PAMs.This article is part of a Special Issue entitled ‘Anxiety and Depression’.

Activation of immobility‐related hippocampal theta by cholinergic septohippocampal neurons during vestibular stimulation

The vestibular system has been suggested to participate in spatial navigation, a function ascribed to the hippocampus. Vestibular stimulation during spatial navigation activates a hippocampal theta rhythm (4-10 Hz), which may enhance spatial processing and motor response. We hypothesize that a cholinergic, atropine-sensitive theta is generated during passive whole-body rotation in freely behaving rats. Hippocampal EEGs were recorded by implanted electrodes in CA1 while rats were rotated on a vertical axis, for a minute or longer, at different angular velocities. Rotation induced a continuous hippocampal theta rhythm while the rat was immobile, in both light and dark conditions. Theta peak frequency showed a significant increase during high (50-70 rpm) as compared with a lower (20-49 rpm) rotational velocity. Rotation-induced theta was abolished by muscarinic receptor antagonist atropine sulfate (50 mg/kg i.p.) but not by atropine methyl nitrate (50 mg/kg i.p.), which did not pass the blood-brain barrier. Theta was attenuated in rats in which cholinergic neurons in the medial septum (MS) were lesioned with 192 IgG-saporin (0.14 μg in 0.4 μl), as confirmed by depletion of MS cells immunoreactive to choline acetyltransferase and an absence of acetylcholinesterase staining in the hippocampus. Bilateral lesion of the vestibular receptors by sodium arsanilate (30 mg in 0.1 ml, intratympanically) also attenuated the rotation-induced theta rhythm. In intact rats, field excitatory postsynaptic potentials (fEPSPs) in CA1 evoked by commissural stimulation were smaller during walking or rotation as compared with during immobility. Modulation of fEPSP was absent following atropine sulfate in intact rats and in 192 IgG-saporin lesion rats. In summary, this is the first report of a continuous atropine-sensitive hippocampal theta in the rat induced by vestibular stimulation during rotation, and accompanied by cholinergic modulation of hippocampal synaptic transmission. Vestibular-activated septohippocampal cholinergic activity could be an important component in sensorimotor processing and spatial memory.

Activation of the Glutamatergic System of the Medial Septal Area Accelerates Epileptogenesis

The effects of injection of L-glutamate into the medial septal area on epileptogenesis associated with stimulation of the perforant pathway were studied in conscious guinea pigs. EEG field potentials were recorded in the hippocampus and medial septal area in the initial state and on formation of epileptic foci in animals given physiological saline (controls) or L-glutamate (glutamate group) into the medial septal area. Administration of glutamate enhanced the theta rhythm in both structures. During kindling in animals given glutamate, there was significantly earlier onset of the convulsive state and secondary epileptiform discharges than in controls. As the epileptic focus formed, sharp weakening of theta oscillations was seen in all animals, though onset was much earlier in the glutamate group. The correlation between the hippocampal and septal EEG decreased sharply in animals given glutamate, while changes in controls were minor. These results are of value in deciphering the mechanisms of temporal epilepsy.

Control of hippocampal theta rhythm by serotonin: Role of 5-HT2c receptors

The hippocampus plays an important role in learning and memory and has been implicated in a number of diseases, including epilepsy, anxiety and schizophrenia. A prominent feature of the hippocampal network is the capability to generate rhythmic oscillations. Serotonergic modulation is known to play an important role in the regulation of theta rhythm. 5-HT2c receptors represent a specific target of psychopharmacology and, in particular, the behavioral effects of the 5-HT2c receptor agonist mCPP have been thoroughly tested. The present study used this compound and the selective 5-HT2c receptor antagonist SB-242084 to elucidate the role of 5-HT2c receptors in the generation of hippocampal oscillations. Hippocampal EEG was recorded and the power in the theta frequency range was monitored in different behaviors in freely-moving rats and after brainstem stimulation in anesthetized animals. We found that in freely-moving rats, mCPP suppressed hippocampal theta rhythm and the effect was stronger during REM sleep than during waking theta states. Under urethane anesthesia, mCPP decreased the power for both spontaneous and elicited theta rhythm in a dose-dependent manner and the 5-HT2c antagonist reversed this effect. The results of this study demonstrate that 5-HT2c receptors are important element of the serotonergic modulation of hippocampal theta oscillations and thus pharmacological interactions with these receptors can modulate physiological and pathological processes associated with limbic theta activity.SB-242084=6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy)pyridin-3-yl carbamoyl]indoline mCPP=1-(3-chlorophenyl)piperazine dihydrochloride

Hippocampal serotonin depletion facilitates place learning concurrent with an increase in CA1 high frequency theta activity expression in the rat

Acetylcholine- and serotonin-dependent theta activities have been long proposed to exist. However, several studies have shown that serotonin tends to desynchronise hippocampal EEG activity. Theta activity has been related to the processing of hippocampal place learning. Since the serotonergic system can influence hippocampal theta activity, it could function as a modulator of spatial learning. For these reasons, we investigated the possible role of hippocampal serotonin in the regulation of theta activity during the acquisition of map-based spatial information. Following 5-HT hippocampal depletion through 5,7-dihydroxytriptamine-induced lesions to the fimbria, fornix and cingulate bundle of adult rats, CA1 hippocampal theta activity was recorded during place learning training. Only rats with reduction higher than 90% from controls, verified post-mortem by HPLC were studied. A facilitation of place learning after hippocampal serotonin depletion occurred, and was associated with earlier expression of dominant high frequency theta activity (6.5–9.5 Hz). Therefore, theta activity was related to the accuracy of behavioural performance through 5-HT modulation in a place learning test.

Hippocampal theta rhythm after serotonergic activation of the pedunculopontine tegmental nucleus in anesthetized rats

The pedunculopontine tegmental nucleus (PPN), as a part of reticular formation activating system, is thought to be involved in the sleep/wake cycle regulation, and plays an important role in the generation and regulation of hippocampal rhythmical slow activity. The activity of PPN can be modulated by serotonergic system, mainly through multiple projections from raphe nuclei, which can influence PPN neurons through different classes of 5-HT receptors. In the present study, the effect of intra-PPN injection of two serotonin agonists: 8-OH-DPAT and 5-CT, on hippocampal formation EEG activity was examined in urethane-anesthetized rats. The study found that the microinjections induced prolonged spontaneous theta rhythm in both hippocampi with a short latency. The results obtained suggest that local inhibition of presumably cholinergic neurons in the PPN acts as a trigger for hippocampal theta activity.

Selective lesions of medial septal GABAergic neurons disrupt Hippocampal Theta Rhythm Amplitude

Septal inputs critically modulate hippocampal function. However, the role of Medial Septal/Diagonal Band (MS/DB) neuronal populations in hippocampal theta rhythm generation is still under debate. MS/DB GABAergic neurons selectively innervate inhibitory hippocampal interneurons and through this action exert a powerful control on hippocampal excitability. To further investigate the role of the MS/DB GABAergic neurons in hippocampal theta rhythm generation, the antibody Gamma amino butyric acid Transporter 1 (GAT-1) conjugated to saporin (SAP, 3µL at 325ng/µL) was injected in the MS/DB of adults Sprague Dawley rats to selectively destroy GABAergic neurons. Population estimates of MS/DB GABAergic neurons were analyzed using StereoInvestigator 9. A significant reduction (75%, p<0.0001) in the MS/DB GABAergic population was observed, while the cholinergic and glutamatergic neuronal population were spared. The effects of this GABAergic neuronal loss in hippocampal function were investigated by hippocampal EEG recordings. EEG recordings of freely moving rats were acquired using the Harmonie Stellate monitoring system. EEGs were recorded before and after the intraseptal injection of anti-GAT-1-SAP. Two second duration trace, representative of theta I and theta II were transferred off line to ClampFit 9 software to measure amplitude and produce a power spectrum. After analysis of the theta traces the rat brains were sliced and stereology was once again performed to determine the relationship between the destruction of MS/DB GABAergic neurons and the hippocampal theta rhythm.

Dual Coding with STDP in a Spiking Recurrent Neural Network Model of the Hippocampus

The firing rate of single neurons in the mammalian hippocampus has been demonstrated to encode for a range of spatial and non-spatial stimuli. It has also been demonstrated that phase of firing, with respect to the theta oscillation that dominates the hippocampal EEG during stereotype learning behaviour, correlates with an animal's spatial location. These findings have led to the hypothesis that the hippocampus operates using a dual (rate and temporal) coding system. To investigate the phenomenon of dual coding in the hippocampus, we examine a spiking recurrent network model with theta coded neural dynamics and an STDP rule that mediates rate-coded Hebbian learning when pre- and post-synaptic firing is stochastic. We demonstrate that this plasticity rule can generate both symmetric and asymmetric connections between neurons that fire at concurrent or successive theta phase, respectively, and subsequently produce both pattern completion and sequence prediction from partial cues. This unifies previously disparate auto- and hetero-associative network models of hippocampal function and provides them with a firmer basis in modern neurobiology. Furthermore, the encoding and reactivation of activity in mutually exciting Hebbian cell assemblies demonstrated here is believed to represent a fundamental mechanism of cognitive processing in the brain.

Effects of Factors Inducing Diffuse Damage to Brain Tissue on Sleep Structure in Laboratory Rats

Chronic experiments on laboratory rats with implanted electrodes for recording neocortical and hippocampal EEG and cervical muscle electromyogram traces were performed to study the effects of strong treatments inducing diffuse damage to brain tissue on subsequent sleep. Four different experimental models were used: one "chronic" (generalized cerebral ischemia induced by permanent occlusion of one of the common carotid arteries) and three "acute" (hypoxic hypoxia, hypoglycemia, and "penicillin" epilepsy). Sleep recordings were made in freely moving animals either day-round (in the "chronic" model) or daily for 3 h ("acute" models). In all models, traces showed significant increases in the mean total duration of paradoxical sleep, reaching a peak 1-3 days after treatment. The subsequent dynamics depended on the treatment used: in the "acute" models, the duration of paradoxical sleep returned to control levels in 5-6 days, while in the "chronic" model, this occurred at 40-45 days after the beginning of treatment. The sharp increases in the durations of paradoxical sleep after use of strong treatments inducing damage to brain tissue can be regarded as supporting the suggestion that there is an increase in neuronal recovery processes during paradoxical sleep.

An integrated system for video and telemetric electroencephalographic recording to measure behavioural and physiological parameters

Introduction The combined evaluation of physiology and behaviour allows a complete and more comprehensive pre-clinical assessment of central nervous system (CNS) function. An integrated video-telemetric electroencephalography (Video-tEEG) system, which allows the simultaneous and continuous recording of EEG and video images for long periods, was developed. Methods This study focuses on the refinement of the surgical methodology for the combined recording of cortical, hippocampal EEG and electromyogram (EMG) waveforms in freely moving rats. The post-operative recovery of animals was monitored by recording EEGs by telemetry and the general activity by video, on days 1, 6 and 15 after surgery, for approximately 24 h, on each occasion. Results The results suggested that the applied surgical technique for the implantation of the telemetric transmitter, allowed for a gradual recovery of the animals within 15 days. During the recovery period the behavioural and locomotor parameters measured, indicated that there were no changes to the light-dark circadian cycle, and these parameters gradually tended to reach background levels within a 15-day period. Using a mechanical connection between the deep and the telemetric electrodes, 15 days after surgical implantation the recording system was able to acquire cortical and hippocampal EEG traces of good quality. Discussion This present study is concerned with the refinement of the surgical technique, as well as the integration and synchronisation of the commercially available Dataquest telemetry system and the Noldus video system, in order to study cortical, hippocampal EEG waveforms, in combination with behaviour and locomotion. The application of this integrated Video-tEEG system could provide advantages in the ethical use of animals in different pre-clinical research areas.

Identification of the current generator underlying cholinergically induced gamma frequency field potential oscillations in the hippocampal CA3 region

Gamma frequency oscillations (30-100 Hz) are prominent in the hippocampal EEG signal during active network states. An intrahippocampal gamma generator has been identified in the CA3 region. To understand the mechanism of oscillation generation, both the rhythm and the current generators must be identified. While earlier work has elucidated mechanisms of rhythm generation, little attention has been given to identifying the CA3 gamma current generator. Here, we aimed to identify a current generator underlying cholinergically induced gamma frequency oscillations in vitro. To this end, we analysed the instantaneous fluctuations in the wavelet amplitude of the field potential oscillation recorded in the stratum pyramidale, and concomitantly recorded action potentials and synaptic input in individual, anatomically identified neurons. The data revealed that perisomatic inhibitory currents in pyramidal cells generated the majority of the field potential. Pyramidal cell action currents also contributed to the field. In contrast, we found no evidence that excitatory currents contribute significantly to the field oscillations in this model. The moment-by-moment analysis of the dynamics of the field potential presented here provides insight into the distinct contributions of synaptic and action currents to the EEG signal and sheds light on the changing balance of excitation and inhibition during cholinergically induced gamma frequency oscillations.

Effects of adolescent ethanol exposure on event-related oscillations (EROs) in the hippocampus of adult rats

Electrophysiological studies have shown that adolescent ethanol (EtOH) exposure can produce long-term changes in hippocampal EEG and ERP activity. Recently, evidence has emerged suggesting that event-related oscillations (EROs) may be good indices of alcoholism risk in humans, however, have not been evaluated for their ability to index the effects of EtOH exposure. The objective of the present study was to characterize EROs generated in hippocampus in adult rats exposed to EtOH during adolescence. Adolescent male Sprague–Dawley rats were exposed to EtOH vapor for 12 h/d for 10 days. A time–frequency representation method was used to determine delta, theta, alpha and beta ERO energy and the degree of phase variation in the hippocampus of adult rats exposed to EtOH and age-matched controls. The present results suggest that the decrease in P3 amplitudes, previously observed in adult rats exposed to EtOH during adolescence, is associated with increases in evoked theta ERO energy. These studies suggest that EROs are suitable for characterizing the long-term effects of adolescent EtOH exposure. Further studies are needed to determine the relationship between the mechanisms that regulate these neurophysiological endophenotypes and the consequences of adolescent EtOH exposure.