The entorhinal cortex entrains fast CA1 hippocampal oscillations in the anaesthetized guinea-pig: role of the monosynaptic component of the perforant path

Analysis of hippocampal local field potentials by diffusion mapped delay coordinates

Spatial navigation through novel spaces and to known goal locations recruits multiple integrated structures in the mammalian brain. Within this extended network, the hippocampus enables formation and retrieval of cognitive spatial maps and contributes to decision making at choice points. Exploration and navigation to known goal locations produce synchronous activity of hippocampal neurons resulting in rhythmic oscillation events in local networks. Power of specific oscillatory frequencies and numbers of these events recorded in local field potentials correlate with distinct cognitive aspects of spatial navigation. Typically, oscillatory power in brain circuits is analyzed with Fourier transforms or short-time Fourier methods, which involve assumptions about the signal that are likely not true and fail to succinctly capture potentially informative features. To avoid such assumptions, we applied a method that combines manifold discovery techniques with dynamical systems theory, namely diffusion maps and Takens' time-delay embedding theory, that avoids limitations seen in traditional methods. This method, called diffusion mapped delay coordinates (DMDC), when applied to hippocampal signals recorded from juvenile rats freely navigating a Y-maze, replicates some outcomes seen with standard approaches and identifies age differences in dynamic states that traditional analyses are unable to detect. Thus, DMDC may serve as a suitable complement to more traditional analyses of LFPs recorded from behaving subjects that may enhance information yield.

Epididymal alveolar echinococcosis and tuberculosis co-infection: A case report

Alveolar echinococcosis (AE) is a common and significant public health problem caused by the larvae of the Echinococcus multilocularis. The occurrence of epididymal AE is rare and often overlooked in combination with mycobacterium tuberculosis infection. We report a case of a 34-year-old man who presented with right-sided scrotal enlargement with pain. Physical examination revealed an enlarged right scrotum with rupture. CT examination showed a blurred border and non-enhancing lesion on the right epididymis. Postoperative pathology and molecular biology identified an epididymal E. multilocularis infection. We report this rare case to emphasise the difficulty of preoperative diagnosis and the importance of complete surgical excision of the lesion.

Disrupted hippocampal synchrony following maternal immune activation in a rat model

Maternal immune activation (MIA) is a risk factor for schizophrenia and other neurodevelopmental disorders. MIA in rats models a number of the brain and behavioral changes that are observed in schizophrenia, including impaired memory. Recent studies in the MIA model have shown that the firing of the hippocampal place cells that are involved in memory processes appear relatively normal, but with abnormalities in the temporal ordering of firing. In this study, we re-analyzed data from prior hippocampal electrophysiological recordings of MIA and control animals to determine whether temporal dysfunction was evident. We find that there is a decreased ratio of slow to fast gamma power, resulting from an increase in fast gamma power and a tendency toward reduced slow gamma power in MIA rats. Moreover, we observe a robust reduction in spectral coherence between hippocampal theta and both fast and slow gamma rhythms, as well as changes in the phase of theta at which fast gamma occurs. We also find the phasic organization of place cell phase precession on the theta wave to be abnormal in MIA rats. Lastly, we observe that the local field potential of MIA rats contains more frequent sharp-wave ripple events, and that place cells were more likely to fire spikes during ripples in these animals than control. These findings provide further evidence of desynchrony in MIA animals and may point to circuit-level changes that underlie failures to integrate and encode information in schizophrenia.

A deep network-based model of hippocampal memory functions under normal and Alzheimer's disease conditions

We present a deep network-based model of the associative memory functions of the hippocampus. The proposed network architecture has two key modules: (1) an autoencoder module which represents the forward and backward projections of the cortico-hippocampal projections and (2) a module that computes familiarity of the stimulus and implements hill-climbing over the familiarity which represents the dynamics of the loops within the hippocampus. The proposed network is used in two simulation studies. In the first part of the study, the network is used to simulate image pattern completion by autoassociation under normal conditions. In the second part of the study, the proposed network is extended to a heteroassociative memory and is used to simulate picture naming task in normal and Alzheimer's disease (AD) conditions. The network is trained on pictures and names of digits from 0 to 9. The encoder layer of the network is partly damaged to simulate AD conditions. As in case of AD patients, under moderate damage condition, the network recalls superordinate words ("odd" instead of "nine"). Under severe damage conditions, the network shows a null response ("I don't know"). Neurobiological plausibility of the model is extensively discussed.

Association between abnormal brain oscillations and cognitive performance in patients with bipolar disorder; Molecular mechanisms and clinical evidence

Brain oscillations have gained great attention in neuroscience during recent decades as functional building blocks of cognitive-sensory processes. Research has shown that oscillations in "alpha," "beta," "gamma," "delta," and "theta" frequency windows are highly modified in brain pathology, including in patients with cognitive impairment like bipolar disorder (BD). The study of changes in brain oscillations can provide fundamental knowledge for exploring neurophysiological biomarkers in cognitive impairment. The present article reviews findings from the role and molecular basis of abnormal neural oscillation and synchronization in the symptoms of patients with BD. An overview of the results clearly demonstrates that, in cognitive-sensory processes, resting and evoked/event-related electroencephalogram (EEG) spectra in the delta, theta, alpha, beta, and gamma bands are abnormally changed in patients with BD showing psychotic features. Abnormal oscillations have been found to be associated with several neural dysfunctions and abnormalities contributing to BD, including abnormal GABAergic neurotransmission signaling, hippocampal cell discharge, abnormal hippocampal neurogenesis, impaired cadherin and synaptic contact-based cell adhesion processes, extended lateral ventricles, decreased prefrontal cortical gray matter, and decreased hippocampal volume. Mechanistically, impairment in calcium voltage-gated channel subunit alpha1 I, neurotrophic tyrosine receptor kinase proteins, genes involved in brain neurogenesis and synaptogenesis like WNT3 and ACTG2, genes involved in the cell adhesion process like CDH12 and DISC1, and gamma-aminobutyric acid (GABA) signaling have been reported as the main molecular contributors to the abnormalities in resting-state low-frequency oscillations in BD patients. Findings also showed the association of impaired synaptic connections and disrupted membrane potential with abnormal beta/gamma oscillatory activity in patients with BD. Of note, the synaptic GABA neurotransmitter has been found to be a fundamental requirement for the occurrence of long-distance synchronous gamma oscillations necessary for coordinating the activity of neural networks between various brain regions. This article is protected by copyright. All rights reserved.

Mutual Interactions between Brain States and Alzheimer's Disease Pathology: A Focus on Gamma and Slow Oscillations

Brain state varies from moment to moment. While brain state can be defined by ongoing neuronal population activity, such as neuronal oscillations, this is tightly coupled with certain behavioural or vigilant states. In recent decades, abnormalities in brain state have been recognised as biomarkers of various brain diseases and disorders. Intriguingly, accumulating evidence also demonstrates mutual interactions between brain states and disease pathologies: while abnormalities in brain state arise during disease progression, manipulations of brain state can modify disease pathology, suggesting a therapeutic potential. In this review, by focusing on Alzheimer's disease (AD), the most common form of dementia, we provide an overview of how brain states change in AD patients and mouse models, and how controlling brain states can modify AD pathology. Specifically, we summarise the relationship between AD and changes in gamma and slow oscillations. As pathological changes in these oscillations correlate with AD pathology, manipulations of either gamma or slow oscillations can modify AD pathology in mouse models. We argue that neuromodulation approaches to target brain states are a promising non-pharmacological intervention for neurodegenerative diseases.

The olfactory bulb modulates entorhinal cortex oscillations during spatial working memory

Cognitive functions such as working memory require integrated activity among different brain regions. Notably, entorhinal cortex (EC) activity is associated with the successful working memory task. Olfactory bulb (OB) oscillations are known as rhythms that modulate rhythmic activity in widespread brain regions during cognitive tasks. Since the OB is structurally connected to the EC, we hypothesized that OB could modulate EC activity during working memory performance. Herein, we explored OB-EC functional connectivity during spatial working memory performance by simultaneous recording local field potentials when rats performed a Y-maze task. Our results showed that the coherence of delta, theta, and gamma-band oscillations between OB and EC was increased during correct trials compared to wrong trials. Cross-frequency coupling analyses revealed that the modulatory effect of OBs low-frequency phase on EC gamma power and phase was enhanced when animals correctly performed working memory task. The influx of information from OB to EC was also increased at delta and gamma bands within correct trials. These findings indicated that the modulatory influence of OB rhythms on EC oscillations might be necessary for successful working memory performance.

Different theta frameworks coexist in the rat hippocampus and are coordinated during memory-guided and novelty tasks

Hippocampal firing is organized in theta sequences controlled by internal memory processes and by external sensory cues, but how these computations are coordinated is not fully understood. Although theta activity is commonly studied as a unique coherent oscillation, it is the result of complex interactions between different rhythm generators. Here, by separating hippocampal theta activity in three different current generators, we found epochs with variable theta frequency and phase coupling, suggesting flexible interactions between theta generators. We found that epochs of highly synchronized theta rhythmicity preferentially occurred during behavioral tasks requiring coordination between internal memory representations and incoming sensory information. In addition, we found that gamma oscillations were associated with specific theta generators and the strength of theta-gamma coupling predicted the synchronization between theta generators. We propose a mechanism for segregating or integrating hippocampal computations based on the flexible coordination of different theta frameworks to accommodate the cognitive needs.

Bottom-up gamma and bipolar disorder, clinical and neuroepigenetic implications

OBJECTIVES

This limited review examines the role of the reticular activating system (RAS), especially the pedunculopontine nucleus (PPN), one site of origin of bottom-up gamma, in the symptoms of bipolar disorder (BD).

METHODS

The expression of neuronal calcium sensor protein 1 (NCS-1) in the brains of BD patients is increased. It has recently been found that all PPN neurons manifest intrinsic membrane beta/gamma frequency oscillations mediated by high threshold calcium channels, suggesting that it is one source of bottom-up gamma. This review specifically addresses the involvement of these channels in the manifestation of BD.

RESULTS

Excess NCS-1 was found to dampen gamma band oscillations in PPN neurons. Lithium, a first line treatment for BD, was found to decrease the effects of NCS-1 on gamma band oscillations in PPN neurons. Moreover, gamma band oscillations appear to epigenetically modulate gene transcription in PPN neurons, providing a new direction for research in BD.

CONCLUSIONS

This is an area needing much additional research, especially since the dysregulation of calcium channels may help explain many of the disorders of arousal in, elicit unwanted neuroepigenetic modulation in, and point to novel therapeutic avenues for, BD.

Bottom-up gamma maintenance in various disorders

Maintained gamma band activity is a key element of higher brain function, participating in perception, executive function, and memory. The pedunculopontine nucleus (PPN), as part of the reticular activating system (RAS), is a major source of the "bottom-up" flow of gamma activity to higher regions. However, interruption of gamma band activity is associated with a number of neurological and psychiatric disorders. This review will focus on the role of the PPN in activating higher regions to induce arousal and descending pathways to modulate posture and locomotion. As such, PPN deep brain stimulation (DBS) can not only help regulate arousal and stepping, but continuous application may help maintain necessary levels of gamma band activity for a host of other brain processes. We will explore the potential future applications of PPN DBS for a number of disorders that are characterized by disturbances in gamma band maintenance.

Bottom-up Gamma: the Pedunculopontine Nucleus and Reticular Activating System

Gamma rhythms have been proposed to promote the feed forward or "bottom-up" flow of information from lower to higher regions in the brain during perception. On the other hand, beta rhythms have been proposed to represent feed back or "top-down" influence from higher regions to lower. The pedunculopontine nucleus (PPN) has been implicated in sleep-wake control and arousal, and is part of the reticular activating system (RAS). This review describes the properties of the cells in this nucleus. These properties are unique, and perhaps it is the particular characteristics of these cells that allow the PPN to be involved in a host of functions and disorders. The fact that all PPN neurons fire maximally at gamma band frequency regardless of electrophysiological or transmitter type, make this an unusual cell group. In other regions, for example in the cortex, cells with such a property represent only a sub-population. More importantly, the fact that this cell group's functions are related to the capacity to generate coherent activity at a preferred natural frequency, gamma band, speaks volumes about how the PPN functions. We propose that "bottom-up" gamma band influence arises in the RAS and contributes to the build-up of the background of activity necessary for preconscious awareness and gamma activity at cortical levels.

Do slow and fast gamma rhythms correspond to distinct functional states in the hippocampal network?

For decades, hippocampal gamma was thought to be a single type of rhythm with a continuously varying frequency. However, an increasing body of evidence supports a new hypothesis regarding hippocampal gamma. The patterns traditionally defined as hippocampal gamma may actually comprise separate gamma subtypes with distinct frequencies and unique functions. The present review discusses the evidence for and against this new viewpoint. This review will also point out key questions that remain to be answered to validate the two-gamma hypothesis. This article is part of a Special Issue entitled SI: Brain and Memory.

Pedunculopontine arousal system physiology-Implications for schizophrenia

Schizophrenia is characterized by major sleep/wake disturbances including increased vigilance and arousal, decreased slow wave sleep, and increased REM sleep drive. Other arousal-related symptoms include sensory gating deficits as exemplified by decreased habituation of the blink reflex. There is also dysregulation of gamma band activity, suggestive of disturbances in a host of arousal-related mechanisms. This review examines the role of the reticular activating system, especially the pedunculopontine nucleus, in the symptoms of the disease. Recent discoveries on the physiology of the pedunculopontine nucleus help explain many of these disorders of arousal in, and point to novel therapeutic avenues for, schizophrenia.

Spatiotemporal characteristics and pharmacological modulation of multiple gamma oscillations in the CA1 region of the hippocampus

Multiple components of "γ-oscillations" between 30-170 Hz in the CA1 region of the hippocampus have been described, based on their coherence with oscillations in other brain regions and on their cross-frequency coupling with local θ-oscillations. However, it remains unclear whether the different sub-bands are generated by a single broadband oscillator coupled to multiple external inputs, or by separate oscillators that incorporate distinct circuit elements. To distinguish between these possibilities, we used high-density linear array recording electrodes in awake behaving mice to examine the spatiotemporal characteristics of γ-oscillations and their responses to midazolam and atropine. We characterized oscillations using current source density (CSD) analysis, and measured θ-γ phase-amplitude coupling by cross frequency coupling (CFC) analysis. Prominent peaks were present in the CSD signal in the mid- and distal apical dendritic layers at all frequencies, and at stratum pyramidale for γ(slow) (30-45 Hz) and γ(mid) (50-90 Hz), but not γ(fast) (90-170 Hz) oscillations. Differences in the strength and timing of θ-γ(slow) and θ-γ(mid) cross frequency coupling, and a lack of coupling at the soma and mid-apical region for γ(fast) oscillations, indicated that separate circuit components generate the three sub-bands. Midazolam altered CSD amplitudes and cross-frequency coupling in a lamina- and frequency specific manner, providing further evidence for separate generator circuits. Atropine altered CSD amplitudes and θ-γ CFC uniformly at all locations. Simulations using a detailed compartmental model were consistent with γ(slow) and γ(mid) oscillations driven primarily by inputs at the mid-apical dendrites, and γ(fast) at the distal apical dendrite. Our results indicate that multiple distinct local circuits generate γ-oscillations in the CA1 region of the hippocampus, and provide detailed information about their spatiotemporal characteristics.

Modulation of gamma oscillations in the pedunculopontine nucleus by neuronal calcium sensor protein-1: relevance to schizophrenia and bipolar disorder

Reduced levels of gamma-band activity are present in schizophrenia and bipolar disorder patients. In the same disorders, increased neuronal calcium sensor protein-1 (NCS-1) expression was reported in a series of postmortem studies. These disorders are also characterized by sleep dysregulation, suggesting a role for the reticular activating system (RAS). The discovery of gamma-band activity in the pedunculopontine nucleus (PPN), the cholinergic arm of the RAS, revealed that such activity was mediated by high-threshold calcium channels that are regulated by NCS-1. We hypothesized that NCS-1 normally regulates gamma-band oscillations through these calcium channels and that excessive levels of NCS-1, such as would be expected with overexpression, decrease gamma-band activity. We found that PPN neurons in rat brain slices manifested gamma-band oscillations that were increased by low levels of NCS-1 but suppressed by high levels of NCS-1. Our results suggest that NCS-1 overexpression may be responsible for the decrease in gamma-band activity present in at least some schizophrenia and bipolar disorder patients.

Pedunculopontine Nucleus Gamma Band Activity-Preconscious Awareness, Waking, and REM Sleep

The pedunculopontine nucleus (PPN) is a major component of the reticular activating system (RAS) that regulates waking and REM sleep, states of high-frequency EEG activity. Recently, we described the presence of high threshold, voltage-dependent N- and P/Q-type calcium channels in RAS nuclei that subserve gamma band oscillations in the mesopontine PPN, intralaminar parafascicular nucleus (Pf), and pontine subcoeruleus nucleus dorsalis (SubCD). Cortical gamma band activity participates in sensory perception, problem solving, and memory. Rather than participating in the temporal binding of sensory events as in the cortex, gamma band activity in the RAS may participate in the processes of preconscious awareness, and provide the essential stream of information for the formulation of many of our actions. That is, the RAS may play an early permissive role in volition. Our latest results suggest that (1) the manifestation of gamma band activity during waking may employ a separate intracellular pathway compared to that during REM sleep, (2) neuronal calcium sensor (NCS-1) protein, which is over expressed in schizophrenia and bipolar disorder, modulates gamma band oscillations in the PPN in a concentration-dependent manner, (3) leptin, which undergoes resistance in obesity resulting in sleep dysregulation, decreases sodium currents in PPN neurons, accounting for its normal attenuation of waking, and (4) following our discovery of electrical coupling in the RAS, we hypothesize that there are cell clusters within the PPN that may act in concert. These results provide novel information on the mechanisms controlling high-frequency activity related to waking and REM sleep by elements of the RAS.

The role of the entorhinal cortex in epileptiform activities of the hippocampus

BACKGROUND

Temporal lobe epilepsy (TLE) is the commonest type of epilepsy in adults, and the hippocampus is indicated to have a close relationship with TLE. Recent researches also indicate that the entorhinal cortex (EC) is involved in epilepsy. To explore the essential role that the EC may play in epilepsy, a computational model of the hippocampal CA3 region was built, which consisted of pyramidal cells and two types of interneurons. By changing the input signals from the EC, the effects of EC on epileptiform activities of the hippocampus were investigated. Additionally, recent studies have found that the antiepileptic drug valproate (VPA) can block ictal discharges but cannot block interictal discharges in vitro, and the mechanism under this phenomenon is still confusing. In our model, the effects of VPA on epileptiform activities were simulated and some mechanisms were explored.

RESULTS

Interictal discharges were induced in the model without the input signals from the EC, whereas the model with the EC input produced ictal discharges when the EC input contained ictal discharges. The GABA-ergic connection strength was enhanced and the NMDA-ergic connection strength was reduced to simulate the effects of VPA, and the simulation results showed that the disappearance of ictal discharges in the model mainly due to the disappearance of ictal discharges in the input signals from the EC.

CONCLUSIONS

Simulation results showed that ictal discharges in the EC were necessary for the hippocampus to generate ictal discharges, and VPA might block the ictal discharges in the EC, which led to the disappearance of ictal discharges in the hippocampus.

Cholinergic blockade reduces theta-gamma phase amplitude coupling and speed modulation of theta frequency consistent with behavioral effects on encoding

Large-scale neural activation dynamics in the hippocampal-entorhinal circuit local field potential, observable as theta and gamma rhythms and coupling between these rhythms, is predictive of encoding success. Behavioral studies show that systemic administration of muscarinic acetylcholine receptor antagonists selectively impairs encoding, suggesting that they may also disrupt the coupling between the theta and gamma bands. Here, we tested the hypothesis that muscarinic antagonists selectively disrupt coupling between theta and gamma. Specifically, we characterized the effects of systemically administered scopolamine on movement-induced theta and gamma rhythms recorded in the superficial layers of the medial entorhinal cortex (MEC) of freely moving rats. We report the novel result that gamma power at the peak of theta was most reduced following muscarinic blockade, significantly shifting the phase of maximal gamma power to occur at later phases of theta. We also characterize the existence of multiple distinct gamma bands in the superficial layers of the MEC. Further, we observed that theta frequency was significantly less modulated by movement speed following muscarinic blockade. Finally, the slope relating speed to theta frequency, a correlate of familiarity with a testing enclosure, increased significantly less between the preinjection and recovery trials when scopolamine was administered during the intervening injection session than when saline was administered, suggesting that scopolamine reduced encoding of the testing enclosure. These data are consistent with computational models suggesting that encoding and retrieval occur during the peak and trough of theta, respectively, and support the theory that acetylcholine regulates the balance between encoding versus retrieval.

Coherence and frequency in the reticular activating system (RAS)

This review considers recent evidence showing that cells in the reticular activating system (RAS) exhibit (1) electrical coupling mainly in GABAergic cells, and (2) gamma band activity in virtually all of the cells. Specifically, cells in the mesopontine pedunculopontine nucleus (PPN), intralaminar parafascicular nucleus (Pf), and pontine dorsal subcoeruleus nucleus dorsalis (SubCD) (1) show electrical coupling, and (2) all fire in the beta/gamma band range when maximally activated, but no higher. The mechanism behind electrical coupling is important because the stimulant modafinil was shown to increase electrical coupling. We also provide recent findings demonstrating that all cells in the PPN and Pf have high threshold, voltage-dependent P/Q-type calcium channels that are essential to gamma band activity. On the other hand, all SubCD, and some PPN, cells manifested sodium-dependent subthreshold oscillations. A novel mechanism for sleep-wake control based on transmitter interactions, electrical coupling, and gamma band activity is described. We speculate that continuous sensory input will modulate coupling and induce gamma band activity in the RAS that could participate in the processes of preconscious awareness, and provide the essential stream of information for the formulation of many of our actions.

Cross-frequency phase-phase coupling between θ and γ oscillations in the hippocampus

Neuronal oscillations allow for temporal segmentation of neuronal spikes. Interdependent oscillators can integrate multiple layers of information. We examined phase-phase coupling of theta and gamma oscillators in the CA1 region of rat hippocampus during maze exploration and rapid eye movement sleep. Hippocampal theta waves were asymmetric, and estimation of the spatial position of the animal was improved by identifying the waveform-based phase of spiking, compared to traditional methods used for phase estimation. Using the waveform-based theta phase, three distinct gamma bands were identified: slow gamma(S) (gamma(S); 30-50 Hz), midfrequency gamma(M) (gamma(M); 50-90 Hz), and fast gamma(F) (gamma(F); 90-150 Hz or epsilon band). The amplitude of each sub-band was modulated by the theta phase. In addition, we found reliable phase-phase coupling between theta and both gamma(S) and gamma(M) but not gamma(F) oscillators. We suggest that cross-frequency phase coupling can support multiple time-scale control of neuronal spikes within and across structures.

Gamma band activity in the reticular activating system

This review considers recent evidence showing that cells in three regions of the reticular activating system (RAS) exhibit gamma band activity, and describes the mechanisms behind such manifestation. Specifically, we discuss how cells in the mesopontine pedunculopontine nucleus (PPN), intralaminar parafascicular nucleus (Pf), and pontine subcoeruleus nucleus dorsalis (SubCD) all fire in the beta/gamma band range when maximally activated, but no higher. The mechanisms behind this ceiling effect have been recently elucidated. We describe recent findings showing that every cell in the PPN have high-threshold, voltage-dependent P/Q-type calcium channels that are essential, while N-type calcium channels are permissive, to gamma band activity. Every cell in the Pf also showed that P/Q-type and N-type calcium channels are responsible for this activity. On the other hand, every SubCD cell exhibited sodium-dependent subthreshold oscillations. A novel mechanism for sleep–wake control based on well-known transmitter interactions, electrical coupling, and gamma band activity is described. The data presented here on inherent gamma band activity demonstrates the global nature of sleep–wake oscillation that is orchestrated by brainstem–thalamic mechanism, and questions the undue importance given to the hypothalamus for regulation of sleep–wakefulness. The discovery of gamma band activity in the RAS follows recent reports of such activity in other subcortical regions like the hippocampus and cerebellum. We hypothesize that, rather than participating in the temporal binding of sensory events as seen in the cortex, gamma band activity manifested in the RAS may help stabilize coherence related to arousal, providing a stable activation state during waking and paradoxical sleep. Most of our thoughts and actions are driven by pre-conscious processes. We speculate that continuous sensory input will induce gamma band activity in the RAS that could participate in the processes of pre-conscious awareness, and provide the essential stream of information for the formulation of many of our actions.

Cross-frequency phase-phase coupling between θ and γ oscillations in the hippocampus

Neuronal oscillations allow for temporal segmentation of neuronal spikes. Interdependent oscillators can integrate multiple layers of information. We examined phase-phase coupling of theta and gamma oscillators in the CA1 region of rat hippocampus during maze exploration and rapid eye movement sleep. Hippocampal theta waves were asymmetric, and estimation of the spatial position of the animal was improved by identifying the waveform-based phase of spiking, compared to traditional methods used for phase estimation. Using the waveform-based theta phase, three distinct gamma bands were identified: slow gamma(S) (gamma(S); 30-50 Hz), midfrequency gamma(M) (gamma(M); 50-90 Hz), and fast gamma(F) (gamma(F); 90-150 Hz or epsilon band). The amplitude of each sub-band was modulated by the theta phase. In addition, we found reliable phase-phase coupling between theta and both gamma(S) and gamma(M) but not gamma(F) oscillators. We suggest that cross-frequency phase coupling can support multiple time-scale control of neuronal spikes within and across structures.

Gamma oscillations in the hippocampus

Gamma oscillations are thought to temporally link the activity of distributed cells. We discuss mechanisms of gamma oscillations in the hippocampus and review evidence supporting a functional role for such oscillations in several key hippocampal operations, including cell grouping, dynamic routing, and memory. We propose that memory encoding and retrieval are coordinated by different frequencies of hippocampal gamma oscillations and suggest how transitions between slow and fast gamma may occur.

Impaired extinction of fear and maintained amygdala-hippocampal theta synchrony in a mouse model of temporal lobe epilepsy

PURPOSE

The relationship between epilepsy and fear has received much attention. However, seizure-modulated fear and physiologic or structural correlates have not been examined systematically, and the underlying basics of network levels remain unclear to date. Therefore, this project was set up to characterize the neurophysiologic basis of seizure-related fear and the contribution of the amygdala-hippocampus system.

METHODS

The experimental strategy was composed of the following steps: (1) use of the mouse pilocarpine model of temporal lobe epilepsy (TLE); (2) behavioral analyses of anxiety states in the elevated plus maze test, light-dark avoidance test, and Pavlovian fear conditioning; and (3) probing neurophysiologic activity patterns in amygdala-hippocampal circuits in freely behaving mice.

RESULTS

Our results displayed no significant differences in basic anxiety levels comparing mice that developed spontaneous recurrent seizures (SRS) and controls. Furthermore, conditioned fear memory retrieval was not influenced in SRS mice. However, during fear memory extinction, SRS mice showed an extended freezing behavior and a maintained amygdala-hippocampal theta frequency synchronization compared to controls.

DISCUSSION

These results indicate specific alterations in conditioned fear behavior and related neurophysiologic activities in the amygdala-hippocampal network contributing to impaired fear memory extinction in mice with TLE. Clinically, the nonextinguished fear memories may well contribute to the experience of fear in patients with TLE.

Intrinsic circuit organization and theta-gamma oscillation dynamics in the entorhinal cortex of the rat

A thorough knowledge of the intrinsic circuit properties of the entorhinal cortex (EC) and the temporal dynamics these circuits support is essential for understanding how information is exchanged between the hippocampus and neocortex. Using intracellular and extracellular recordings in the anesthetized rat and anatomical reconstruction of single cells, we found that EC5 and EC2 principal neurons form large axonal networks mainly within their layers, interconnected by the more vertically organized axon trees of EC3 pyramidal cells. Principal cells showed layer-specific unique membrane properties and contributed differentially to theta and gamma oscillations. EC2 principal cells were most strongly phase modulated by EC theta. The multiple gamma oscillators, present in the various EC layers, were temporally coordinated by the phase of theta waves. Putative interneurons in all EC layers fired relatively synchronously within the theta cycle, coinciding with the maximum power of gamma oscillation. The special wiring architecture and unique membrane properties of EC neurons may underlie their behaviorally distinct firing patterns in the waking animal.

Paradoxical sleep as a tool for understanding the hippocampal mechanisms of contextual memory

Existing data on the involvement of the hippocampus in contextual memory and the fact that contextual memory is impaired in dreams occurring during paradoxical sleep allowed us to suggest that one of the causes of this impairment consists of changes in the efficiency of synaptic transmission in the hippocampus due to increases (as compared with waking) in the concentrations of acetylcholine, dopamine, and cortisol, as well as the absence of serotonin and noradrenaline. Our previous analysis showed that in paradoxical sleep, long-term depression can be induced all components of the polysynaptic pathway through the hippocampal formation, while potentiation can occur at the inputs from the entorhinal cortex to hippocampal fields CA1 and CA3 and in the associative connections in field CA3. It is hypothesized that the correct functioning of episodic memory requires efficient transmission of signals in each component of the polysynaptic pathway through the hippocampus, allowing a neuronal representation of the context to be created within it. In the state of waking, reproduction of the context of an episode simultaneously activates the neuronal representation of the context remembered in the hippocampus and neuronal representations of the details of the episode remembered in those areas of the cortex in which they were processed. It follows from the proposed mechanism that any neurotransmitter or neuropeptide able to promote longterm potentiation in all components of the polysynaptic pathway through the hippocampus can improve episodic memory. As the consequences of the mechanism are consistent with experimental data, it can be used to seek agents improving episodic memory.

Frequency of gamma oscillations routes flow of information in the hippocampus

Gamma oscillations are thought to transiently link distributed cell assemblies that are processing related information, a function that is probably important for network processes such as perception, attentional selection and memory. This 'binding' mechanism requires that spatially distributed cells fire together with millisecond range precision; however, it is not clear how such coordinated timing is achieved given that the frequency of gamma oscillations varies substantially across space and time, from approximately 25 to almost 150 Hz. Here we show that gamma oscillations in the CA1 area of the hippocampus split into distinct fast and slow frequency components that differentially couple CA1 to inputs from the medial entorhinal cortex, an area that provides information about the animal's current position, and CA3, a hippocampal subfield essential for storage of such information. Fast gamma oscillations in CA1 were synchronized with fast gamma in medial entorhinal cortex, and slow gamma oscillations in CA1 were coherent with slow gamma in CA3. Significant proportions of cells in medial entorhinal cortex and CA3 were phase-locked to fast and slow CA1 gamma waves, respectively. The two types of gamma occurred at different phases of the CA1 theta rhythm and mostly on different theta cycles. These results point to routeing of information as a possible function of gamma frequency variations in the brain and provide a mechanism for temporal segregation of potentially interfering information from different sources.

NMDA receptor-dependent switching between different gamma rhythm-generating microcircuits in entorhinal cortex

Local circuits in the medial entorhinal cortex (mEC) and hippocampus generate gamma frequency population rhythms independently. Temporal interaction between these areas at gamma frequencies is implicated in memory-a phenomenon linked to activity of NMDA-subtype glutamate receptors. While blockade of NMDA receptors does not affect frequency of gamma rhythms in hippocampus, it exposes a second, lower frequency (25-35 Hz) gamma rhythm in mEC. In experiment and model, NMDA receptor-dependent mEC gamma rhythms were mediated by basket interneurons, but NMDA receptor-independent gamma rhythms were mediated by a novel interneuron subtype-the goblet cell. This cell was distinct from basket cells in morphology, intrinsic membrane properties and synaptic inputs. The two different gamma frequencies matched the different intrinsic frequencies in hippocampal areas CA3 and CA1, suggesting that NMDA receptor activation may control the nature of temporal interactions between mEC and hippocampus, thus influencing the pathway for information transfer between the two regions.

Coincidence detection of convergent perforant path and mossy fibre inputs by CA3 interneurons

We performed whole-cell recordings from CA3 s. radiatum (R) and s. lacunosum-moleculare (L-M) interneurons in hippocampal slices to examine the temporal aspects of summation of converging perforant path (PP) and mossy fibre (MF) inputs. PP EPSPs were evoked from the s. lacunosum-moleculare in area CA1. MF EPSPs were evoked from the medial extent of the suprapyramidal blade of the dentate gyrus. Summation was strongly supralinear when examining PP EPSP with MF EPSP in a heterosynaptic pair at the 10 ms ISI, and linear to sublinear at longer ISIs. This pattern of nonlinearities suggests that R and L-M interneurons act as coincidence detectors for input from PP and MF. Summation at all ISIs was linear in voltage clamp mode demonstrating that nonlinearities were generated by postsynaptic voltage-dependent conductances. Supralinearity was not detected when the first EPSP in the pair was replaced by a simulated EPSP injected into the soma, suggesting that the conductances underlying the EPSP boosting were located in distal dendrites. Supralinearity was selectively eliminated with either Ni2+ (30 microm), mibefradil (10 microm) or nimodipine (15 microm), but was unaffected by QX-314. This pharmacological profile indicates that supralinearity is due to recruitment of dendritic T-type Ca2+channels by the first subthreshold EPSP in the pair. Results with the hyperpolarization-activated (Ih) channel blocker ZD 7288 (50 microm) revealed that Ih restricted the time course of supralinearity for coincidently summed EPSPs, and promoted linear to sublinear summation for asynchronous EPSPs. We conclude that coincidence detection results from the counterbalanced activation of T-type Ca2+ channels and inactivation of Ih.

Gamma oscillations coordinate amygdalo-rhinal interactions during learning

The rhinal cortices contribute to memory formation by integrating and transferring neocortical information to the hippocampus. Rhinal contributions to memory are likely influenced by the amygdala because strong reciprocal connections exist between these structures. In light of previous data showing that oscillations regulate neuronal activity during memory formation and recall, we tested the possibility that coherent oscillations serve to coordinate amygdalo-rhinal activity during learning. To this end, we performed simultaneous extracellular recordings of basolateral amygdala (BLA), perirhinal, and entorhinal activity. We first tested whether there are correlated fluctuations in the power of BLA and rhinal field activity during the waking state. Correlated power fluctuations were most pronounced in the 35-45 Hz band. Within each structure, firing probability fluctuated rhythmically with the fast oscillations, indicating that they were not volume conducted. To test whether fast oscillations coordinate BLA and rhinal activity during learning, animals were trained on a trace-conditioning task in which a visual conditioned stimulus (CS) predicted a food reward after a delay. The predictive value of the CS was learned gradually over 9 d. As learning progressed, the 35-45 Hz power increased in the BLA and rhinal cortices, particularly during the late part of the CS and delay. Moreover, the firing of BLA and rhinal neurons became rhythmically entrained by BLA oscillations at that time. Thus, our data suggest that neuronal interactions are coordinated by fast oscillations in the BLA-rhinal network. By telescoping the periods of effective neuronal interactions in short recurring time windows, these fast oscillations may facilitate rhinal interactions and synaptic plasticity.

Gamma oscillations dynamically couple hippocampal CA3 and CA1 regions during memory task performance

The hippocampal formation is believed to be critical for the encoding, consolidation, and retrieval of episodic memories. Yet, how these processes are supported by the anatomically diverse hippocampal networks is still unknown. To examine this issue, we tested rats in a hippocampus-dependent delayed spatial alternation task on a modified T maze while simultaneously recording local field potentials from dendritic and somatic layers of the dentate gyrus, CA3, and CA1 regions by using high-density, 96-site silicon probes. Both the power and coherence of gamma oscillations exhibited layer-specific changes during task performance. Peak increases in the gamma power and coherence were found in the CA3-CA1 interface on the maze segment approaching the T junction, independent of motor aspects of task performance. These results show that hippocampal networks can be dynamically coupled by gamma oscillations according to specific behavioral demands. Based on these findings, we propose that gamma oscillations may serve as a physiological mechanism by which CA3 output can coordinate CA1 activity to support retrieval of hippocampus-dependent memories.

Cellular correlates of spontaneous periodic events in the medial entorhinal cortex of the in vitro isolated guinea pig brain

Periodic potentials characterized by fast oscillations superimposed on a slow complex event are typically observed in cortical structures during sleep and anaesthesia. In the entorhinal cortex (EC) similar spontaneous periodic events (SPEs) have been described both in vivo and in vitro. Simultaneous extracellular and intracellular recordings from superficial neurons of the entorhinal cortex of the isolated Hartley guinea pig brain preparation demonstrated that SPEs recur with a periodicity of 2-10 s and correlate to neuronal firing superimposed on a depolarizing plateau that lasts 0.7-1 s. During SPEs, putative interneurons in all layers discharged high frequency firing (> 100 Hz), whereas no activity was observed in principal neurons of deep entorhinal cortex layers. Linear correlation analysis demonstrated a tight relationship between the fast component of the extracellular SPE and subthreshold oscillatory activity/neuronal firing in both superficial neurons and putative interneurons; firing of deep principal cells was independent from SPEs occurrence. The present study demonstrates that recurrent spontaneous events analogous to periodic activity observed during sleep/anaesthesia are generated in the entorhinal cortex by the interactions between superficial neurons and interneurons.

Integration and Segregation of Activity in Entorhinal-Hippocampal Subregions by Neocortical Slow Oscillations

Brain systems communicate by means of neuronal oscillations at multiple temporal and spatial scales. In anesthetized rats, we find that neocortical "slow" oscillation engages neurons in prefrontal, somatosensory, entorhinal, and subicular cortices into synchronous transitions between UP and DOWN states, with a corresponding bimodal distribution of their membrane potential. The membrane potential of hippocampal granule cells and CA3 and CA1 pyramidal cells lacked bimodality, yet it was influenced by the slow oscillation in a region-specific manner. Furthermore, in both anesthetized and naturally sleeping rats, the cortical UP states resulted in increased activity of dentate and most CA1 neurons, as well as the highest probability of ripple events. Yet, the CA3-CA1 network could self-organize into gamma bursts and occasional ripples during the DOWN state. Thus, neo/paleocortical and hippocampal networks periodically reset, self-organize, and temporally coordinate their cell assemblies via the slow oscillation.

Region-specific reduction in entorhinal gamma oscillations and parvalbumin-immunoreactive neurons in animal models of psychiatric illness

Psychiatric illnesses, particularly schizophrenia, are associated with disrupted markers for interneuronal function and interneuron-mediated brain rhythms such as gamma frequency oscillations. Here we investigate a possible link between these two observations in the entorhinal cortex and hippocampus by using a genetic and an acute model of psychiatric illness. Lysophosphatidic acid 1 receptor-deficient (LPA1-deficient) mice show psychomotor-gating deficits and neurochemical changes resembling those seen in postmortem schizophrenia studies. Similar deficits are seen acutely with antagonism of the NMDA subtype of glutamate receptor. Neither model induced any change in power or frequency of gamma rhythms generated by kainate in hippocampal slices. In contrast, a dramatic decrease in the power of gamma oscillations was seen in superficial, but not deep, medial entorhinal cortex layers in both models. Immunolabeling for GABA, parvalbumin, and calretinin in medial entorhinal cortex from LPA1-deficient mice showed an approximately 40% reduction in total GABA- and parvalbumin-containing neurons, but no change in the number of calretinin-positive neurons. This deficit was specific for layer II (LII). No change in the number of neurons positive for these markers was seen in the hippocampus. Acute NMDA receptor blockade, which selectively reduces synaptic drive to LII entorhinal interneurons, also disrupted gamma rhythms in a similar manner in superficial entorhinal cortex, but not in hippocampus. These data demonstrate an area-specific deficit in gamma rhythmogenesis in animal models of psychiatric illness and suggest that loss, or reduction in function, of interneurons having a large NMDA receptor expression may underlie the network dysfunction that is seen.

Beyond Two-Cell Networks: Experimental Measurement of Neuronal Responses to Multiple Synaptic Inputs

Oscillations of large populations of neurons are thought to be important in the normal functioning of the brain. We have used phase response curve (PRC) methods to characterize the dynamics of single neurons and predict population dynamics. Our past experimental work was limited to special circumstances (e.g., 2-cell networks of periodically firing neurons). Here, we explore the feasibility of extending our methods to predict the synchronization properties of stellate cells (SCs) in the rat entorhinal cortex under broader conditions. In particular, we test the hypothesis that PRCs in SCs scale linearly with changes in synaptic amplitude, and measure how well responses to Poisson process-driven inputs can be predicted in terms of PRCs. Although we see nonlinear responses to excitatory and inhibitory inputs, we find that models based on weak coupling account for scaling and Poisson process-driven inputs reasonably accurately.

Periodic oscillatory activity in parahippocampal slices maintained in vitro

Brain slices maintained in vitro have been extensively used for studying neuronal synchronization. However, the validity of this approach may be questioned since pharmacological procedures are usually required to elicit spontaneous events similar to the EEG activity recorded in vivo. Here, we report that when superfused with control medium, rat brain slices comprising the entorhinal and perirhinal cortices along with a portion of the basolateral/lateral nuclei of the amygdala can synchronously generate periodic oscillatory activity at 5-11 Hz every 5-30 s. The periodic events: (i) correspond intracellularly to synaptic depolarizations in regularly firing neurons analyzed in the three areas; (ii) have no fixed site of onset; (iii) spread with time lags of 8-20 ms; and (iv) continue to occur asynchronously after their surgical isolation. NMDA receptor antagonism reduced the duration of the oscillatory events, while glutamatergic non-NMDA receptor antagonism abolished them. Activation of mu-opioid receptors, a procedure that hyperpolarizes interneurons thus decreasing GABA release, reversibly decreased the rate of occurrence of periodic oscillatory activity (POA). However, periodic events continued to occur during application of GABA(A) or GABA(B) receptor antagonists as well as in the presence of the cholinergic agent carbachol. We also found that POA was abolished by baclofen and irreversibly reduced by the gap junction decoupler carbenoxolone. These findings demonstrate that parahippocampal networks in a brain slice preparation can generate periodic, synchronous activity under quasi-physiological conditions. These network oscillations (i) reflect the activation of ionotropic glutamatergic and GABAergic receptors, (ii) are contributed by gap-junction interactions, and (iii) are controlled by GABA(B) receptors that are presumably located presynaptically.

Frequency bands and spatiotemporal dynamics of β burst stimulation induced afterdischarges in hippocampus in vivo

Temporal and spatial characteristics of hippocampal neuronal network activation are modified during epileptiform afterdischarges. We developed a beta burst stimulation protocol to investigate subregional variations and substrates of rhythmic population spike discharges in vivo in urethane anesthetized Wistar rat hippocampus with a 14-electrode recording array and extracellular single electrode recordings. Our 64 pulse beta burst stimulation protocol was constructed from electrical pulses delivered at intervals corresponding to beta (14-25 Hz), Delta (2 Hz), and slow (0.5 Hz) frequencies. In each experiment these interleaved pulses were all repeated four times with unchanged intervals. Stimulation of either perforant path or fimbria fornix induced a prolonged afterdischarge pattern peaking at 200 Hz fast, 20 Hz beta, and 2 Hz Delta frequencies. Analysis of variance confirmed that the response pattern of the discharges remained constant regardless of the stimulation beta frequency. Within the afterdischarge the fast frequencies were restricted to independent hippocampal subfields whereas beta and slow frequencies correlated across the subfields. Current source density (CSD) analysis revealed that the original signal propagation through subfields of the hippocampus was compromised during the beta burst stimulation induced afterdischarge. In addition, the CSD profile of the epileptiform afterdischarge was consistently similar across the different experiments. Time-frequency analysis revealed that the beta frequency afterdischarge was initiated and terminated at higher gamma (30-80 Hz) frequencies. However, the alterations in the CSD profile of the hippocampus coincided with the beta frequency dominated discharges. We propose that hippocampal epileptiform activity at fast, beta and Delta frequencies represents coupled oscillators at respectively increasing spatial scales in the hippocampal neuronal network in vivo.

Synaptic activation patterns of the perirhinal-entorhinal inter-connections

Ample neuropsychological evidence supports the role of rhinal cortices in memory. The perirhinal cortex (PRC) represents one of the main conduits for the bi-directional flow of information between the entorhinal-hippocampal network and the cortical mantle, a process essential in memory formation. However, despite anatomical evidence for a robust reciprocal connectivity between the perirhinal and entorhinal cortices, neurophysiological understanding of this circuitry is lacking. We now present the results of a series of electrophysiological experiments in rats that demonstrate robust synaptic activation patterns of the perirhinal-entorhinal inter-connections. First, using silicon multi-electrode arrays placed under visual guidance in vivo we performed current source density (CSD) analysis of lateral entorhinal cortex (LEC) responses to PRC stimulation, which demonstrated a current sink in layers II-III of the LEC with a latency consistent with monosynaptic activation. To further substantiate and extend this conclusion, we developed a PRC-LEC slice preparation where CSD analysis also revealed a current sink in superficial LEC layers in response to PRC stimulation. Importantly, intracellular recording of superficial LEC layer neurons confirmed that they receive a major monosynaptic excitatory input from the PRC. Finally, CSD analysis of the LEC to PRC projection in vivo also allowed us to document robust feedback synaptic activation of PRC neurons to deep LEC layer activation. We conclude that a clear bidirectional pattern of synaptic interactions exists between the PRC and LEC that would support a dynamic flow of information subserving memory function in the temporal lobe.

Gamma oscillations induced by kainate receptor activation in the entorhinal cortex in vitro

Gamma frequency (30-80 Hz) oscillations are recordable from human and rodent entorhinal cortex. A number of mechanisms used by neuronal networks to generate such oscillations in the hippocampus have been characterized. However, it is as yet unclear as to whether these mechanisms apply to other anatomically disparate brain regions. Here we show that the medial entorhinal cortex (mEC) in isolation in vitro generates gamma frequency oscillations in response to kainate receptor agonists. Oscillations had the same horizontal and laminar spatiotemporal distribution as seen in vivo and in the isolated whole-brain preparation. Oscillations occurred in the absence of input from the hippocampal formation and did not spread to lateral entorhinal regions. Pharmacological similarities existed between oscillations in the hippocampus and mEC in that the latter were also sensitive to GABAA receptor blockade, barbiturates, AMPA receptor blockade, and reduction in gap junctional conductance. Stellate and pyramidal neuron recordings revealed a large GABAergic input consisting of gamma frequency IPSP trains. Fast spiking interneurons in the superficial mEC generated action potentials at gamma frequencies phase locked to the local field. Stellate cells also demonstrated a subthreshold membrane potential oscillation at theta frequencies that was temporally correlated with a theta-frequency modulation in field gamma power. Disruption in this stellate theta frequency oscillation by the hyperpolarisation activated current (Ih) blocker ZD7288 also disrupted theta modulation of field gamma frequency oscillations. We propose that similar cellular and network mechanisms to those seen in the hippocampus generate and modulate persistent gamma oscillations in the entorhinal cortex.

Neuregulin-1β modulates in vivo entorhinal–hippocampal synaptic transmission in adult rats

Neuregulin-1 (NRG-1) proteins and their erbB receptors are essential for neuronal development during embryogenesis and may contribute importantly to neuronal function in the adult brain. This study tests the hypothesis that NRG-1beta acts as a modulator of synaptic activity in the adult brain, specifically at hippocampal formation synapses. Adult, male Sprague-Dawley rats were anesthetized and a recording electrode with an attached stainless steel microinjector was stereotaxically positioned to record field potentials (fEPSP) in either the dentate gyrus or the cornu ammonis (CA) 1 field of the hippocampus. The entorhinal cortex was continuously stimulated via a paired stainless steel electrode. Microinjection of NRG-1beta significantly increased the slope of the fEPSP in the dentate gyrus in a dose-dependent manner. Compared with a low dose (20 nM), a high dose (100 nM) of NRG-1beta induced a shorter latency response that was of greater magnitude. Responses to NRG-1beta were abolished by pretreatment with a selective, reversible erbB tyrosine kinase inhibitor, PD158780 (100 microM). Further, PD158780 (100 microM) itself significantly decreased the entorhinal-dentate fESPS slope by about 15%. Neither equimolar (100 nM) nor hypermolar (100 microM) sucrose or heat-inactivated NRG-1beta (100 nM) significantly altered the entorhinal-dentate fEPSP slope. In contrast to its effect at the entorhinal-dentate synapse, NRG-1beta (100 nM) depressed, and PD158780 potentiated entorhinal-CA1 synaptic transmission. Thus, in adult rats NRG-1beta potentiates transmission at the entorhinal-dentate synapse but suppresses transmission at the entorhinal-CA1 synapse. These observations indicate that NRG-1 is not only a developmental growth factor, but also modifies synaptic transmission in adult rat brain.

Slow periodic events and their transition to gamma oscillations in the entorhinal cortex of the isolated Guinea pig brain

Slow (<1 Hz) periodic activity is a distinctive discharge pattern observed in different cortical and sub-cortical structures during sleep and anesthesia. By performing field and cellular recordings, we demonstrated that slow periodic events (0.02-0.4 Hz) are spontaneously generated in the entorhinal cortex of the in vitro isolated whole brain of the guinea pig. These events were characterized by gradually developing runs of low-amplitude (50-300 microV), high-frequency (25-70 Hz) oscillations superimposed on a slow potential that lasted 1-3 s. Both slow and fast components showed a phase reversal in the superficial layers. In layer II-III entorhinal neurons, the slow periodic events correlated to a slowly developing depolarizing envelope capped by subthreshold membrane potential oscillations and action potential discharge. Slow periodic field events propagated tangentially across the entorhinal cortex and could be triggered by stimulation of superficial associative fibers, suggesting that they were generated by and propagated via network interactions in the superficial layers. Slow periodic events were reversibly abolished by muscarinic excitation elicited by carbachol (50 microM) that promoted intracellular membrane potential depolarization associated with continuous fast oscillatory activity in the gamma frequency range. These results suggest that, as proposed in vivo, activity changes in the entorhinal cortex of the in vitro isolated guinea-pig brain reflect different activation states that are under cholinergic control.

Hippocampal network patterns of activity in the mouse

Genetic engineering of the mouse brain allows investigators to address novel hypotheses in vivo. Because of the paucity of information on the network patterns of the mouse hippocampus, we investigated the electrical patterns in the behaving animal using multisite silicon probes and wire tetrodes. Theta (6-9 Hz) and gamma (40-100 Hz) oscillations were present during exploration and rapid eye movement sleep. Gamma power and theta power were comodulated and gamma power varied as a function of the theta cycle. Pyramidal cells and putative interneurons were phase-locked to theta oscillations. During immobility, consummatory behaviors and slow-wave sleep, sharp waves were present in cornu ammonis region CA1 of the hippocampus stratum radiatum associated with 140-200-Hz "ripples" in the pyramidal cell layer and population burst of CA1 neurons. In the hilus, large-amplitude "dentate spikes" occurred in association with increased discharge of hilar neurons. The amplitude of field patterns was larger in the mouse than in the rat, likely reflecting the higher neuron density in a smaller brain. We suggest that the main hippocampal network patterns are mediated by similar pathways and mechanisms in mouse and rat.

Mechanisms of gamma oscillations in the hippocampus of the behaving rat

Gamma frequency oscillations (30-100 Hz) have been suggested to underlie various cognitive and motor functions. Here, we examine the generation of gamma oscillation currents in the hippocampus, using two-dimensional, 96-site silicon probes. Two gamma generators were identified, one in the dentate gyrus and another in the CA3-CA1 regions. The coupling strength between the two oscillators varied during both theta and nontheta states. Both pyramidal cells and interneurons were phase-locked to gamma waves. Anatomical connectivity, rather than physical distance, determined the coupling strength of the oscillating neurons. CA3 pyramidal neurons discharged CA3 and CA1 interneurons at latencies indicative of monosynaptic connections. Intrahippocampal gamma oscillation emerges in the CA3 recurrent system, which entrains the CA1 region via its interneurons.

Network and pharmacological mechanisms leading to epileptiform synchronization in the limbic system in vitro

Seizures in patients presenting with mesial temporal lobe epilepsy result from the interaction among neuronal networks in limbic structures such as the hippocampus, amygdala and entorhinal cortex. Mesial temporal lobe epilepsy, one of the most common forms of partial epilepsy in adulthood, is generally accompanied by a pattern of brain damage known as mesial temporal sclerosis. Limbic seizures can be mimicked in vitro using preparations of combined hippocampus-entorhinal cortex slices perfused with artificial cerebrospinal fluid containing convulsants or nominally zero Mg(2+), in order to produce epileptiform synchronization. Here, we summarize experimental evidence obtained in such slices from rodents. These data indicate that in control animals: (i) prolonged, NMDA receptor-dependent epileptiform discharges, resembling electrographic limbic seizures, originate in the entorhinal cortex from where they propagate to the hippocampus via the perforant path-dentate gyrus route; (ii) the initiation and maintenance of these ictal discharges is paradoxically contributed by GABA (mainly type A) receptor-mediated mechanisms; and (iii) CA3 outputs, which relay a continuous pattern of interictal discharge at approximately 1Hz, control rather than sustain ictal discharge generation in entorhinal cortex. Recent work indicates that such a control is weakened in the pilocarpine model of epilepsy (presumably as a result of CA3 cell damage). In addition, in these experiments electrographic seizure activity spreads directly to the CA1-subiculum regions through the temporoammonic pathway. Studies reviewed here indicate that these changes in network interactions, along with other mechanisms of synaptic plasticity (e.g. axonal sprouting, decreased activation of interneurons, upregulation of bursting neurons) can confer to the epileptic, damaged limbic system, the ability to produce recurrent limbic seizures as seen in patients with mesial temporal lobe epilepsy.

Physiological and morphological diversity of immunocytochemically defined parvalbumin- and cholecystokinin-positive interneurones in CA1 of the adult rat hippocampus

To investigate the electrophysiological properties, synaptic connections, and anatomy of individual parvalbumin-immunoreactive (PV-IR) and cholecystokinin-immunoreactive (CCK-IR) interneurones in CA1, dual intracellular recordings using biocytin-filled microelectrodes in slices of adult rat hippocampus were combined with fluorescence labelling of PV- and CCK-containing cells. Of 36 PV-IR cells, 29 were basket cells, with most of their axonal arbours in the stratum pyramidale (SP). Six were bistratified cells with axons ramifying throughout stratum oriens (SO) and stratum radiatum (SR). One was a putative axo-axonic cell with an axonal arbour confined to half of the SP and a narrow adjacent region of the SO. Of 27 CCK-IR neurones, 13 were basket cells, with most of their axonal arbours in the SP, and included basket cells with somata in the SP (6), SO (3), and SR (2) and at the border between the stratum lacunosum-moleculare (SLM) and the SR (2). In addition, several dendrite-targeting cell classes expressed CCK-IR: 4 of 9 bistratified cells with axons ramifying in the SO and SR; all five Schaffer-associated cells whose axons ramified extensively in the SR; both cells classified as quadrilaminar because their axons ramified in the SO, SP, SR, and SLM; one SO-SO cell whose dendritic and axonal arbours were contained within the SO; and one perforant path-associated cell with axonal and dendritic arbours within the distal SR and SLM. The majority (31 of 36) of PV-IR neurones recorded were fast-spiking, and most fast-spiking cells tested (25 of 29 basket, 1 axo-axonic, and 5 of 6 bistratified cells) were PV-IR. However, 1 of 6 regular-spiking basket, 1 of 4 regular-spiking bistratified, and 3 of 5 burst-firing basket cells were also PV-IR. In contrast, the majority (17 of 27) of the CCK-IR neurones recorded were regular-spiking, 3 were burst-firing, and 7 were fast-spiking. These data confirm that the majority of PV-IR and CCK-IR axon terminals innervate proximal portions of CA1 pyramidal cells. Most PV-IR cells are fast-spiking, whereas most CCK-IR cells are regular-spiking. In both neurochemical classes basket cells predominate, but both groups included subpopulations of dendrite-targeting cells. Despite these similarities, the two populations exhibited very different somatic distributions, and each contained cellular morphologies not represented in the other.

Physiological patterns in the hippocampo-entorhinal cortex system

The anatomical connectivity and intrinsic properties of entorhinal cortical neurons give rise to ordered patterns of ensemble activity. How entorhinal ensembles form, interact, and accomplish emergent processes such as memory formation is not well-understood. We lack sufficient understanding of how neuronal ensembles in general can function transiently and distinctively from other neuronal ensembles. Ensemble interactions are bound, foremost, by anatomical connectivity and temporal constraints on neuronal discharge. We present an overview of the structure of neuronal interactions within the entorhinal cortex and the rest of the hippocampal formation. We wish to highlight two principle features of entorhinal-hippocampal interactions. First, large numbers of entorhinal neurons are organized into at least two distinct high-frequency population patterns: gamma (40-100 Hz) frequency volleys and ripple (140-200 Hz) frequency volleys. These patterns occur coincident with other well-defined electrophysiological patterns. Gamma frequency volleys are modulated by the theta cycle. Ripple frequency volleys occur on each sharp wave event. Second, these patterns occur dominantly in specific layers of the entorhinal cortex. Theta/gamma frequency volleys are the principle pattern observed in layers I-III, in the neurons that receive cortical inputs and project to the hippocampus. Ripple frequency volleys are the principle population pattern observed in layers V-VI, in the neurons that receive hippocampal output and project primarily to the neocortex. Further, we will highlight how these ensemble patterns organize interactions within distributed forebrain structures and support memory formation.

Origins and distribution of cholinergically induced beta rhythms in hippocampal slices

Regional variations and substrates of high-frequency rhythmic activity induced by cholinergic stimulation were studied in hippocampal slices with 64-electrode recording arrays. (1) Carbachol triggered beta waves (17.6 +/- 5.7 Hz) in pyramidal regions of 75% of the slices. (2) The waves had phase shifts across the cell body layers and were substantially larger in the apical dendrites than in cell body layers or basal dendrites. (3) Continuous, two-dimensional current source density analyses indicated apical sinks associated with basal sources, lasting approximately 10 msec, followed by apical sources and basal sinks, lasting approximately 20 msec, in a repeating pattern with a period in the range of 15-25 Hz. (4) Carbachol-induced beta waves in the hippocampus were accompanied by 40 Hz (gamma) oscillations in deep layers of the entorhinal cortex. (5) Cholinergically elicited beta and gamma rhythms were eliminated by antagonists of either AMPA or GABA receptors. Benzodiazepines markedly enhanced beta activity and sometimes introduced a distinct gamma frequency peak. (6) Twenty Hertz activity after orthodromic activation of field CA3 was distributed in the same manner as carbachol-induced beta waves and was generated by a current source in the apical dendrites of CA3. This source was eliminated by high concentrations of GABA(A) receptor blockers. It is concluded that cholinergically driven beta rhythms arise independently in hippocampal subfields from oscillatory circuits involving (1) bursts of pyramidal cell discharges, (2) activation of a subset of feedback interneurons that project apically, and (3) production of a GABA(A)-mediated hyperpolarization in the outer portions of the apical dendrites of pyramidal neurons.

Carbachol induces fast oscillations in the medial but not in the lateral entorhinal cortex of the isolated guinea pig brain

Fast oscillations at 25-80 Hz (gamma activity) have been proposed to play a role in attention-related mechanisms and synaptic plasticity in cortical structures. Recently, it has been demonstrated that the preservation of the entorhinal cortex is necessary to maintain gamma oscillations in the hippocampus. Because gamma activity can be reproduced in vitro by cholinergic activation, this study examined the characteristics of gamma oscillations induced by arterial perfusion or local intracortical injections of carbachol in the entorhinal cortex of the in vitro isolated guinea pig brain preparation. Shortly after carbachol administration, fast oscillatory activity at 25.2-28.2 Hz was observed in the medial but not in the lateral entorhinal cortex. Such activity was transiently associated with oscillations in the theta range that showed a variable pattern of distribution in the entorhinal cortex. No oscillatory activity was observed when carbachol was injected in the lateral entorhinal cortex. Gamma activity in the medial entorhinal cortex showed a phase reversal at 200-400 microm, had maximal amplitude at 400-500 microm depth, and was abolished by arterial perfusion of atropine (5 microM). Local carbachol application in the medial entorhinal cortex induced gamma oscillations in the hippocampus, whereas no oscillations were observed in the amygdala and in the piriform, periamygdaloid, and perirhinal cortices ipsilateral and contralateral to the carbachol injection. Hippocampal oscillations had higher frequency than the gamma activity recorded in the entorhinal cortex, suggesting the presence of independent generators in the two structures. The selective ability of the medial but not the lateral entorhinal cortex to generate gamma activity in response to cholinergic activation suggests a differential mode of signal processing in entorhinal cortex subregions.