Multisite recording of brain field potentials and unit activity in freely moving rats

Intracortical brain-computer interfaces for improved motor function: a systematic review

In this systematic review, we address the status of intracortical brain-computer interfaces (iBCIs) applied to the motor cortex to improve function in patients with impaired motor ability. This study followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 Guidelines for Systematic Reviews. Risk Of Bias In Non-randomized Studies - of Interventions (ROBINS-I) and the Effective Public Health Practice Project (EPHPP) were used to assess bias and quality. Advances in iBCIs in the last two decades demonstrated the use of iBCI to activate limbs for functional tasks, achieve neural typing for communication, and other applications. However, the inconsistency of performance metrics employed by these studies suggests the need for standardization. Each study was a pilot clinical trial consisting of 1-4, majority male (64.28 %) participants, with most trials featuring participants treated for more than 12 months (55.55 %). The systems treated patients with various conditions: amyotrophic lateral sclerosis, stroke, spinocerebellar degeneration without cerebellar involvement, and spinal cord injury. All participants presented with tetraplegia at implantation and were implanted with microelectrode arrays via pneumatic insertion, with nearly all electrode locations solely at the precentral gyrus of the motor cortex (88.88 %). The development of iBCI devices using neural signals from the motor cortex to improve motor-impaired patients has enhanced the ability of these systems to return ability to their users. However, many milestones remain before these devices can prove their feasibility for recovery. This review summarizes the achievements and shortfalls of these systems and their respective trials.

3D Printed Skull Cap and Benchtop Fabricated Microwire-Based Microelectrode Array for Custom Rat Brain Recordings

Microwire microelectrode arrays (MEAs) have been a popular low-cost tool for chronic electrophysiological recordings and are an inexpensive means to record the electrical dynamics crucial to brain function. However, both the fabrication and implantation procedures for multi-MEAs on a single rodent are time-consuming and the accuracy and quality are highly manual skill-dependent. To address the fabrication and implantation challenges for microwire MEAs, (1) a computer-aided designed and 3D printed skull cap for the pre-determined implantation locations of each MEA and (2) a benchtop fabrication approach for low-cost custom microwire MEAs were developed. A proof-of-concept design of a 32-channel 4-MEA (8-wire each) recording system was prototyped and tested through Sprague Dawley rat recordings. The skull cap design, based on the CT-scan of a single rat conforms well with multiple Sprague Dawley rats of various sizes, ages, and weight with a minimal bregma alignment error (A/P axis standard error of the mean = 0.25 mm, M/L axis standard error of the mean = 0.07 mm, n = 6). The prototyped 32-channel system was able to record the spiking activities over five months. The developed benchtop fabrication method and the 3D printed skull cap implantation platform would enable neuroscience groups to conduct in-house design, fabrication, and implantation of customizable microwire MEAs at a lower cost than the current commercial options and experience a shorter lead time for the design modifications and iterations.

"I Felt the Ball"-The Future of Spine Injury Recovery

Spinal cord injury (SCI) has no cure and individuals with SCI become dependent on others for life. After injury, the signals below the lesion are disrupted, but the brain still produces motor commands. Researchers have bypassed this obstacle, which has given rise to the brain-machine interface (BMI). BMI devices are implanted in the brain or spinal cord, where they decode and send signals beyond the injured segment. Experiments were initially conducted on animals, with favorable results. BMIs are classified according to their type, function, signal generation, and so on. Because of invasiveness, their long-term use is questionable, because of infections and complications. The use of an implantable epidural array in patients with chronic SCI showed that participants were able to walk with the help of a stimulator, and after months of training, they were able to walk with the stimulator turned off. Another innovation is a robotic suit for paraplegics and tetraplegics that supports the movement of limbs. The research on stem cells has not shown favorable results. In future, one of these cutting-edge technologies will prevail over the other, but BMI seems to have the upper hand. The future of BMI with fusion of robotics and artificial intelligence is promising for patients with chronic SCI. These modern devices need to be less invasive, biocompatible, easily programmable, portable, and cost-effective. After these hurdles are overcome, the devices may become the mainstay of potential rehabilitation therapy for partial recovery. The time may come when all patients with severe SCI are told "You will walk again."

Optogenetic Entrainment of Hippocampal Theta Oscillations in Behaving Mice

Extensive data on relationships of neural network oscillations to behavior and organization of neuronal discharge across brain regions call for new tools to selectively manipulate brain rhythms. Here we describe an approach combining projection-specific optogenetics with extracellular electrophysiology for high-fidelity control of hippocampal theta oscillations (5-10 Hz) in behaving mice. The specificity of the optogenetic entrainment is achieved by targeting channelrhodopsin-2 (ChR2) to the GABAergic population of medial septal cells, crucially involved in the generation of hippocampal theta oscillations, and a local synchronized activation of a subset of inhibitory septal afferents in the hippocampus. The efficacy of the optogenetic rhythm control is verified by a simultaneous monitoring of the local field potential (LFP) across lamina of the CA1 area and/or of neuronal discharge. Using this readily implementable preparation we show efficacy of various optogenetic stimulation protocols for induction of theta oscillations and for the manipulation of their frequency and regularity. Finally, a combination of the theta rhythm control with projection-specific inhibition addresses the readout of particular aspects of the hippocampal synchronization by efferent regions.

Brain-Machine Interfaces: From Basic Science to Neuroprostheses and Neurorehabilitation

Brain-machine interfaces (BMIs) combine methods, approaches, and concepts derived from neurophysiology, computer science, and engineering in an effort to establish real-time bidirectional links between living brains and artificial actuators. Although theoretical propositions and some proof of concept experiments on directly linking the brains with machines date back to the early 1960s, BMI research only took off in earnest at the end of the 1990s, when this approach became intimately linked to new neurophysiological methods for sampling large-scale brain activity. The classic goals of BMIs are 1) to unveil and utilize principles of operation and plastic properties of the distributed and dynamic circuits of the brain and 2) to create new therapies to restore mobility and sensations to severely disabled patients. Over the past decade, a wide range of BMI applications have emerged, which considerably expanded these original goals. BMI studies have shown neural control over the movements of robotic and virtual actuators that enact both upper and lower limb functions. Furthermore, BMIs have also incorporated ways to deliver sensory feedback, generated from external actuators, back to the brain. BMI research has been at the forefront of many neurophysiological discoveries, including the demonstration that, through continuous use, artificial tools can be assimilated by the primate brain's body schema. Work on BMIs has also led to the introduction of novel neurorehabilitation strategies. As a result of these efforts, long-term continuous BMI use has been recently implicated with the induction of partial neurological recovery in spinal cord injury patients.

CNS Adverse Effects: From Functional Observation Battery/Irwin Tests to Electrophysiology

This chapter describes various approaches for the preclinical assessment of drug-induced central nervous system (CNS) adverse effects. Traditionally, methods to evaluate CNS effects have consisted of observing and scoring behavioral responses of animals after drug is administered. Among several behavioral testing paradigms, the Irwin and the functional observational battery (FOB) are the most commonly used assays for the assessment of CNS effects. The Irwin and FOB are considered good first-tier assays to satisfy the ICH S7A guidance for the preclinical evaluation of new chemical entities (NCE) intended for humans. However, experts have expressed concern about the subjectivity and lack of quantitation that is derived from behavioral testing. More importantly, it is difficult to gain insight into potential mechanisms of toxicity by assessing behavioral outcomes. As a complement to behavioral testing, we propose using electrophysiology-based assays, both in vivo and in vitro, such as electroencephalograms and brain slice field-potential recordings. To better illustrate these approaches, we discuss the implementation of electrophysiology-based techniques in drug-induced assessment of seizure risk, sleep disruption, and cognitive impairment.

Mice and rats achieve similar levels of performance in an adaptive decision-making task

Two opposing constraints exist when choosing a model organism for studying the neural basis of adaptive decision-making: (1) experimental access and (2) behavioral complexity. Available molecular and genetic approaches for studying neural circuits in the mouse fulfill the first requirement. In contrast, it is still under debate if mice can perform cognitive tasks of sufficient complexity. Here we compare learning and performance of mice and rats, the preferred behavioral rodent model, during an acoustic flexible categorization two-alternative choice task. The task required animals to switch between two categorization definitions several times within a behavioral session. We found that both species achieved similarly high performance levels. On average, rats learned the task faster than mice, although some mice were as fast as the average rat. No major differences in subjective categorization boundaries or the speed of adaptation between the two species were found. Our results demonstrate that mice are an appropriate model for the study of the neural mechanisms underlying adaptive decision-making, and suggest they might be suitable for other cognitive tasks as well.

Functional connectivity in the brain estimated by analysis of gamma events

It is known that gamma activity is generated by local networks. In this paper we introduced a new approach for estimation of functional connectivity between neuronal networks by measuring temporal relations between peaks of gamma event amplitudes. We have shown in freely moving rats that gamma events recorded between electrodes 1.5 mm apart in the majority of cases, are generated by different neuronal modules interfering with each other. The map of functional connectivity between brain areas during the resting state, created based on gamma event temporal relationships is in agreement with anatomical connections and with maps described by fMRI methods during the resting state. The transition from the resting state to exploratory activity is accompanied by decreased functional connectivity between most brain areas. Our data suggest that functional connectivity between interhemispheric areas depends on GABAergic transmission, while intrahemispheric functional connectivity is kainate receptor dependent. This approach presents opportunities for merging electrographic and fMRI data on brain functional connectivity in normal and pathological conditions.

Oscillatory correlates of memory in non-human primates

The ability to navigate through our environment, explore with our senses, track the passage of time, and integrate these various components to form the experiences which make up our lives is shared among humans and animals. The use of animal models to study memory, coupled with electrophysiological techniques that permit the direct measurement of neural activity as memories are formed and retrieved, has provided a wealth of knowledge about these mechanisms. Here, we discuss current knowledge regarding the specific role of neural oscillations in memory, with particular emphasis on findings derived from non-human primates. Some of these findings provide evidence for the existence in the primate brain of mechanisms previously identified only in rodents and other lower mammals, while other findings suggest parallels between memory-related activity and processes observed in other cognitive modalities, including attention and sensory perception. Taken together, these results provide insight into how network activity may be organized to promote memory formation, and suggest that key aspects of this activity are similar across species, providing important information about the organization of human memory.

Recombinase-driver rat lines: tools, techniques, and optogenetic application to dopamine-mediated reinforcement

Currently there is no general approach for achieving specific optogenetic control of genetically defined cell types in rats, which provide a powerful experimental system for numerous established neurophysiological and behavioral paradigms. To overcome this challenge we have generated genetically restricted recombinase-driver rat lines suitable for driving gene expression in specific cell types, expressing Cre recombinase under the control of large genomic regulatory regions (200-300 kb). Multiple tyrosine hydroxylase (Th)::Cre and choline acetyltransferase (Chat)::Cre lines were produced that exhibited specific opsin expression in targeted cell types. We additionally developed methods for utilizing optogenetic tools in freely moving rats and leveraged these technologies to clarify the causal relationship between dopamine (DA) neuron firing and positive reinforcement, observing that optical stimulation of DA neurons in the ventral tegmental area (VTA) of Th::Cre rats is sufficient to support vigorous intracranial self-stimulation (ICSS). These studies complement existing targeting approaches by extending the generalizability of optogenetics to traditionally non-genetically-tractable but vital animal models.

High-fidelity evoked potential for mapping the rat tail in thalamus

The technique of field potentials (FPs) provides a macroscopic view for exploring brain function, and is supplementary to single-unit recording, a microscopic view that investigates each neuron in great detail. Mapping the rat tail in the ventroposterolateral (VPL) nucleus of the thalamus was carried out by analyzing the current source density (CSD) of the evoked FPs. The results showed a clear somatotopic organization of the tail in the VPL nucleus. Also, to obtain high-fidelity FPs, two recording parameters were determined. Based upon cross-correlation coefficient (rho), the cycles of FPs needed to be averaged should not be less than 50 and the distance between the two recording sites should be no longer than 50 mum in each direction (mediolateral, anteroposterior and ventrodorsal). Under these conditions, the representation (or reproducibility) of an FP can be >95%. The procedures used to determine these parameters can serve as a guide to obtain FPs with high signal-to-noise ratio and without spatial aliasing error.

Neural signal sampling via the low power wireless pico system

This paper presents a powerful new low power wireless system for sampling multiple channels of neural activity based on Texas Instruments MSP430 microprocessors and Nordic Semiconductor's ultra low power high bandwidth RF transmitters and receivers. The system's development process, component selection, features and test methodology are presented.

Quantification of motor cortex activity and full-body biomechanics during unconstrained locomotion

Recent progress in the understanding of motor cortex function has been achieved primarily by simultaneously recording motor cortex neuron activity and the movement kinematics of the corresponding limb. We have expanded this approach by combining high-quality cortical single-unit activity recordings with synchronized recordings of full-body kinematics and kinetics in the freely behaving cat. The method is illustrated by selected results obtained from two cats tested while walking on a flat surface. Using this method, the activity of 43 pyramidal tract neurons (PTNs) was recorded, averaged over 10 bins of a locomotion cycle, and compared with full-body mechanics by means of principal component and multivariate linear regression analyses. Patterns of 24 PTNs (56%) and 219 biomechanical variables (73%) were classified into just four groups of inter-correlated variables that accounted for 91% of the total variance, indicating that many of the recorded variables had similar patterns. The ensemble activity of different groups of two to eight PTNs accurately predicted the 10-bin patterns of all biomechanical variables (neural decoding) and vice versa; different small groups of mechanical variables accurately predicted the 10-bin pattern of each PTN (neural encoding). We conclude that comparison of motor cortex activity with full-body biomechanics may be a useful tool in further elucidating the function of the motor cortex.

A laser micromachined probe for recording multiple field potentials in the thalamus

Multichannel recording provides integral information about electrical brain activities at one instant in time. In this study, multielectrode probes were fabricated to record the thalamic field potentials (FPs) responding to the electrical stimulation of nerve at the rat tail. At first, the number of sweeps used to form the evoked FP average and the spatial sampling density were determined by using cross-correlation functions, which were then statistically analyzed. The difference was significant at P < 0.05, if the number of sweeps for averaging was more than 50 and the spatial interval between two consecutive recording sites was less than 50 microm in the anteroposterior, mediolateral and ventrodorsal directions. The responsive area was distributed vertically in the thalamus (ventral posterior lateral (VPL) nucleus); therefore, the recording sites were arranged in one linear array. Sixteen recording sites, which were 50 microm apart from each other, were distributed in the ventrodorsal direction. A 16-channel silicon probe was fabricated by using a standard photolithography process and laser micromachining techniques. The probe provides capabilities to record multiple thalamic evoked FPs and multiunit activities simultaneously.

Complexity and temporal dynamics of frequency coding in the awake rat auditory cortex

Auditory cortical neurons are elements of a neuronal network that decomposes sounds into spectral and temporal information. In particular, their frequency selectivity has been investigated in great detail. Most studies used anaesthetized preparations and found mainly simple V-shaped tuning. The few data available from awake animals indicate that more complex forms of spectral receptive fields, i.e. frequency response areas, can be found there. We investigated frequency response areas in the awake rat primary auditory cortex using statistical evaluation and found complex forms of frequency response areas with several separate subregions in many neurons, besides classical V-shaped tuning. Response areas, as determined with narrow band noise, were very similar to those measured with pure tones. Their width was well correlated to the response strength to white noise stimulation. These results suggest that the excitatory subregions of frequency response areas were the neurons' predominant characteristic, relevant also for the processing of more complex types of stimuli. Investigating the spectrotemporal dynamics of frequency response areas revealed that approximately one-third of the neurons showed long-lasting excitatory or inhibitory components in addition to the typical ON-response. Inhibition was usually of longer duration and occurred mainly in frequency ranges outside the range of initial excitatory responses. These results indicate that auditory cortical neurons in awake animals can represent spectrotemporal information of rather different complexity.

A simple and effective process for noise reduction of multichannel cortical field potential recordings in freely moving rats

Simple and useful steps, i.e. placing a grounded plate under the recording chamber as well as using multiple reference electrodes, are introduced here for obtaining reliable low-noise recordings of brain activity in freely moving rats. A general circuit model was built to analyze the electrical interference of both single-grounded and two-reference ground-free recording configurations. In both simulated and realistic conditions under two recording states, 60-Hz magnitude was in the microvolt range. Moreover, the noise was significantly reduced by shortening the distance between the subject and the grounded plate under the recording chamber. Furthermore, in chronically implanted rats, average 60-Hz interference of multichannel electroencephalograms of two-reference ground-free recordings (3.74 +/- 0.18 microV) was significantly smaller than that of the single-grounded condition (9.03 +/- 1.98 microV). Thus, we demonstrated that a lower-noise recording can be achieved by a two-reference configuration and a closely-placed metal grounded plate in an open-field circumstance. As compared to the use of a Faraday cage, this simple procedure is of benefit for long-term behavioral tracking with a video camera and for pharmacological experiments.

Multineuronal firing patterns in the signal from eye to brain

Population codes in the brain have generally been characterized by recording responses from one neuron at a time. This approach will miss codes that rely on concerted patterns of action potentials from many cells. Here we analyze visual signaling in populations of ganglion cells recorded from the isolated salamander retina. These neurons tend to fire synchronously far more frequently than expected by chance. We present an efficient algorithm to identify what groups of cells cooperate in this way. Such groups can include up to seven or more neurons and may account for more than 50% of all the spikes recorded from the retina. These firing patterns represent specific messages about the visual stimulus that differ significantly from what one would derive by single-cell analysis.

Noradrenergic Control of Thalamic Oscillation: the Role of alpha-2 Receptors

The effects of alpha-adrenergic drugs on neocortical high voltage spike and wave spindles (HVS), reflecting thalamic oscillation, was investigated in freely moving rats. HVS occurred spontaneously in the awake but immobile animal. Peripheral administration of the alpha-1 antagonist, prazosin and alpha-2 agonists, xylazine and clonidine increased the incidence and duration of HVS in a dose-dependent manner. The alpha-2 antagonist, yohimbine and the tricyclic antidepressants, desipramine and amitriptyline, significantly decreased the incidence of the neocortical HVS. Bilateral microinjections of the alpha-2 agonists into the nucleus ventralis lateralis area of the thalamus, but not into the hippocampus or corpus callosum, was as effective as peripheral injection of these drugs. Xylazine was most effective in Fischer 344 rats that display high spontaneous rate of HVS and less effective in the Sprague - Dawley and Buffalo strains. The HVS-promoting effect of clonidine was antagonized by prior intrathalamic injection of the alpha-2 antagonist, yohimbine. The amplitude of the HVS was increased by picomole amounts of unilaterally-injected clonidine. Neurotoxic destruction of the thalamopetal noradrenergic afferents by intracisternal or intrathalamic injection of 6-hydroxydopamine, but not by peripheral administration of DSP-4, increased the incidence of HVS. Importantly, intrathalamic administration of xylazine continued to induce HVS after destroying the thalamic noradrenergic terminals. Following downregulation of the alpha-2 adrenoceptors by chronic administration (3 weeks) of amitriptylene the incidence of HVS decreased and the effectiveness of intrathalamic xylazine on the induction of HVS was significantly reduced. Based on these findings, we suggest that a major action of alpha-2 adrenergic drugs on thalamic oscillation may be mediated by postsynaptic alpha-2 adrenoceptors located on the thalamocortical neurons. We hypothesize that noradrenaline in the thalamus has a dual effect on the relay cells: blocking and promoting thalamic oscillation via alpha-1 and alpha-2 receptors, respectively. The final physiological effect is assumed to be a function of the relative density and affinity of these adrenergic receptor subtypes.

The application of elastomeric connector for multi-channel electrophysiological recordings

Interest in recording multi-channel electrophysiological data from behaving animals is rapidly growing, and many laboratories tend to record a large number of EEG and/or multi-unit channels, despite the limitation of the size of the headpiece that a small behaving animal can carry. A common drawback of these experiments, therefore, is the relatively large size of even the smallest, commercially available, high-density micro-connectors for the headset. To overcome this problem, we suggest the application of elastomeric or silicone inter-rubber connectors, that are widely used in electronics. The elastomeric or "zebra" connector consists of alternating thin strips of layered electrically conductive and non-conductive materials. The conductive strips provide electrical connections between uninsulated contact surfaces of printed circuit boards such as the connector plate of the micro-drive, that holds the brain electrode wires, and the preamplifier board of the recording system. In the present paper, we provide technical details of the design of this type of connector-sets and discuss common issues arising from their use. By comparing the applicability of two designs, we aim to demonstrate the simplicity, reliability and durability of the elastomeric inter-rubber connectors in electrophysiological experiments on freely moving laboratory animals.

Anesthesia changes frequency tuning of neurons in the rat primary auditory cortex

The vast majority of investigations on central auditory processing so far were conducted under the influence of an anesthetic agent. It remains unclear, however, to what extend even basic response properties of central auditory neurons are influenced by this experimental manipulation. We used a combination of chronic recording in unrestrained animals, computer-controlled randomized acoustic stimulation, and statistical evaluation of responses to directly compare the response characteristics of single neurons in the awake and anesthetized state. Thereby we were able to quantify the effects of pentobarbital/chloral hydrate anesthesia (Equithesin) on rat auditory cortical neurons. During Equithesin anesthesia, only a portion of central neurons were active and some of their basic response properties were changed. Only 29% of the neurons still had a frequency response area. Their tuning sharpness was increased under anesthesia. Most changes are consistent with an enhancement of inhibitory influences during Equithesin anesthesia. Thus when describing response properties of central auditory neurons, the animal's anesthetic state has to be taken into account.

Multiple microelectrode-recording system for human intracortical applications

The human brain is dominated by the neocortex, a large folded surface, whose cellular and synaptic elements are arranged in layers. Since cortical structure is relatively constant across its surface, local information processing can be inferred from multiple laminar recordings of its electrical activity along a line perpendicular to its surface. Such recordings need to be spaced at least as close together as the cortical layers, and need to be wideband in order to sample both low frequency synaptic currents as well as high-frequency action potentials. Finally, any device used in the human brain must comply with strict safety standards. The current paper presents details of a system meeting these criteria, together with sample results obtained from epileptic subjects undergoing acute or chronic intracranial monitoring for definition of the epileptogenic region.

Multiple site silicon-based probes for chronic recordings in freely moving rats: implantation, recording and histological verification

This paper describes the procedure of assembling a miniature microdrive and silicon probe system for surgical implantation into the adult rat brain. Successful recordings of single and multiunit activity with parallel depth profiles of spontaneous and evoked field potentials are shown. The procedure for histological verification of the position of the silicon probe is described.

Rabbit and monkey visual cortex: more than a year of recording with up to 64 microelectrodes

In the visual cortex of rabbits and a marmoset monkey, 32 and 64 microwires, respectively, were chronically implanted by an indirect insertion method so that the cortex was penetrated from the white matter. For more than 1 year recordings of action potentials of good quality were obtained at most electrodes. Recording stability was judged by spike shape, spike train autocorrelograms, and spike rates: within recording sessions, stability was essentially perfect. Periods in which the signals of several electrodes were stable could last for several days. A method of in vivo reconstruction of the electrode locations by micro-X-rays and subsequent stereophotogrammetry is presented. The aspect of animal welfare is considered.

Singing-related neural activity in a dorsal forebrain-basal ganglia circuit of adult zebra finches

The anterior forebrain pathway (AFP) of songbirds, a specialized dorsal forebrain-basal ganglia circuit, is crucial for song learning but has a less clear function in adults. We report here that neurons in two nuclei of the AFP, the lateral magnocellular nucleus of the anterior neostriatum (LMAN) and Area X, show marked changes in neurophysiological activity before and during singing in adult zebra finches. The presence of modulation before song output suggests that singing-related AFP activity originates, at least in part, in motor control nuclei. Some neurons in LMAN of awake birds also responded selectively to playback of the bird's own song, but neural activity during singing did not completely depend on auditory feedback in the short term, because neither the level nor the pattern of this activity was strongly affected by deafening. The singing-related activity of neurons in AFP nuclei of songbirds is consistent with a role of the AFP in adult singing or song maintenance, possibly related to the function of this circuit during initial song learning.

Electrophysiologic analysis of a chronic seizure model after unilateral hippocampal KA injection

PURPOSE

Unilateral intrahippocampal injections of kainic acid (KA) in rats produce spontaneous recurrent limbic seizures and morphologic changes in hippocampus that resemble hippocampal sclerosis in patients with medically refractory mesial temporal lobe epilepsy (MTLE), that form of temporal lobe epilepsy (TLE) associated with hippocampal sclerosis. Interictal in vivo electrophysiologic studies have revealed high-frequency (250-500 Hz) oscillations, termed fast ripples (FRs). These oscillations may uniquely occur in or adjacent to the site of hippocampal KA injection, in areas that generate spontaneous seizures. Similar field potentials also have been demonstrated in the epileptogenic region of patients with TLE. We have now characterized ictal electrographic patterns in this rat model for comparison with those in human TLE and begun to evaluate the role of FRs in the transition to ictus in the KA-treated rat.

METHODS

Rats received unilateral intrahippocampal injections of KA and, after the development of spontaneous seizures, were implanted with multiple fixed and moveable microelectrodes for single unit, field potential, and EEG recording. They were then monitored by using video-EEG telemetry for several weeks to capture and evaluate electrographic and behavioral seizure types. Results were correlated with Timm's stain demonstration of mossy fiber sprouting.

RESULTS

Low-voltage fast (LVF) and hypersynchronous electrographic ictal-onset patterns were seen in the KA-treated rat that resembled similar ictal-onset patterns in patients with TLE. Hypersynchronous, but not LVF, ictal discharges were associated with recurrent FRs. As in the human, hypersynchronous ictal onsets originated predominantly in hippocampus, whereas LVF ictal onsets more often involved extrahippocampal structures. LVF ictal onsets occurred during wakefulness or paradoxical sleep and were usually associated with motor behavior, whereas hypersynchronous ictal onsets occurred during slow-wave sleep or periods of immobility and were not associated with motor behavior unless there was transition to another ictal electrographic pattern. Mossy fiber sprouting did not correlate with the frequency of ictal EEG discharges exhibited by each rat but was greater in those rats that demonstrated frequent behavioral seizures.

CONCLUSIONS

The electrographic features of spontaneous seizures in the KA-treated rat resemble those of patients with medically refractory TLE with respect to EEG pattern and localization. Our data suggest that hypersynchronous ictal onsets represent epileptogenic disturbances in hippocampal circuits, whereas LVF ictal onsets may involve extrahippocampal areas having more direct connections to the motor system. Hypersynchronous seizures may involve the same neuronal mechanisms that generate interictal FRs.

Interdependence of multiple theta generators in the hippocampus: a partial coherence analysis

The extracellularly recorded theta oscillation reflects a dynamic interaction of various synaptic and cellular mechanisms. Because the spatially overlapping dipoles responsible for the generation of theta field oscillation may represent different mechanisms, their separation might provide clues with regard to their origin and significance. We used a novel approach, partial coherence analysis, to reveal the various components of the theta rhythm and the relationship among its generators. Hippocampal field activity was recorded by a 16-site silicon probe in the CA1-dentate gyrus axis of the awake rat. Field patterns, recorded from various intrahippocampal or entorhinal cortex sites, were used to remove activity caused by a common source by the partialization procedure. The findings revealed highly coherent coupling between theta signals recorded (1) from the hippocampal fissure and stratum (str.) oriens of the CA1 region and (2) between CA1 stratum radiatum and the dentate molecular layer. The results of partial coherence analysis indicated that rhythmic input from the entorhinal cortex explained theta coherence between signals recorded from the hippocampal fissure and str. oriens but not the coherence between signals derived from str. radiatum and the dentate molecular layer. After bilateral lesions of the entorhinal cortex, all signals recorded from both below and above the CA1 hippocampal pyramidal cell layer became highly coherent. These observations indicate the presence of two, relatively independent, theta generators in the hippocampus, which are mediated by the entorhinal cortex and the CA3-mossy cell recurrent circuitry, respectively. The CA3-mossy cell theta generator is partially suppressed by the dentate gyrus interneuronal output in the intact brain. We suggest that timing of the action potentials of pyramidal cells during the theta cycle is determined by the cooperation between the active CA3 neurons and the entorhinal input.

Termination of epileptic afterdischarge in the hippocampus

The mechanism of afterdischarge termination in the various hippocampal regions was examined in the rat. Stimulation of the perforant path or the commissural system was used to elicit afterdischarges. Combination of multiple site recordings with silicon probes, current source density analysis, and unit recordings in the awake animal allowed for a high spatial resolution of the field events. Interpretation of the field observations was aided by intracellular recordings from anesthetized rats. Irrespective of the evoking conditions, afterdischarges always terminated first in the CA1 region. Termination of the afterdischarge was heralded by a large DC shift initiated in dendritic layers associated with a low amplitude "afterdischarge termination oscillation" (ATO) at 40 to 80 Hz in the cell body layer. ATOs were also observed in the CA3 region and the dentate gyrus. The DC shift spread at the same velocity (0. 1-0.2 mm/sec) in all directions and could cross the hippocampal fissure. All but 1 of the 25 putative interneurons in the CA1 and dentate regions ceased to fire before the onset of ATO. Intracellularly, ATO and the emerging DC potential were associated with fast depolarizing potentials and firing of pyramidal cells and depolarization block of spike initiation, respectively. Both field ATO and the intracellular depolarization shift were replicated by focal microinjection of potassium. We hypothesize that [K+]o lost by the intensely discharging neurons during the afterdischarge triggers propagating waves of depolarization in the astrocytic network. In turn, astrocytes release potassium, which induces a depolarization block of spike generation in neurons, resulting in "postictal depression" of the EEG.

Hippocampal and entorhinal cortex high-frequency oscillations (100--500 Hz) in human epileptic brain and in kainic acid--treated rats with chronic seizures

PURPOSE

Properties of oscillations with frequencies >100 Hz were studied in kainic acid (KA)-treated rats and compared with those recorded in normal and kindled rats as well as in patients with epilepsy to determine differences associated with epilepsy.

METHODS

Prolonged in vivo wideband recordings of electrical activity were made in hippocampus and entorhinal cortex (EC) of (a) normal rats, (b) kindled rats, (c) rats having chronic recurrent spontaneous seizures after intrahippocampal KA injections, and (d) patients with epilepsy undergoing depth electrode evaluation in preparation for surgical treatment.

RESULTS

Intermittent oscillatory activity ranging from 100 to 200 Hz in frequency and 50-150 ms in duration was recorded in CA1 and EC of all three animal groups, and in epileptic human hippocampus and EC. This activity had the same characteristics in all groups, resembled previously observed "ripples" described by Buzsáki et al., and appeared to represent field potentials of inhibitory postsynaptic potentials (IPSPs) on principal cells. Unexpectedly, higher frequency intermittent oscillatory activity ranging from 200 to 500 Hz and 10-100 ms in duration was encountered only in KA-treated rats and patients with epilepsy. These oscillations, termed fast ripples (FRs), were found only adjacent to the epileptogenic lesion in hippocampus, EC, and dentate gyrus, and appeared to represent field potential population spikes. Their local origin was indicated by correspondence with the negative phase of burst discharges of putative pyramidal cells.

CONCLUSIONS

The persistence of normal-appearing ripples in epileptic brain support the view that inhibitory processes are preserved. FRs appear to be field potentials reflecting hyper-synchronous bursting of excitatory neurons and provide an opportunity to study the role of this pathophysiologic phenomenon in epilepsy and seizure initiation. Furthermore, if FR activity is unique to brain areas capable of generating spontaneous seizures, its identification could be a powerful functional indicator of the epileptic region in patients evaluated for surgical treatment.

Sustained activation of hippocampal pyramidal cells by 'space clamping' in a running wheel

In contrast to sensory cortical areas of the brain, the relevant physiological inputs to the hippocampus, leading to selective activation of pyramidal cells, are largely unknown. Pyramidal cells are thought to be phasically activated by spatial cues and a variety of sensory and motor stimuli. Here, we used a behavioural 'space clamp' method, which involved the confinement of the actively running animal in a defined position in space (running wheel) and kept sensory inputs constant. Twelve percent of the recorded CA1 pyramidal cells were selectively active while the rat was running in the wheel. Cell firing was specific to the direction of running and disappeared after rotating the recording apparatus. The discharge frequency of pyramidal cells and interneurons was sustained as long as the rat ran continuously in the wheel. Furthermore, the discharge frequency of pyramidal cells and interneurons increased with increasing running velocity, even though the frequency of hippocampal theta waves remained constant. The discharge frequency of some 'wheel-related' pyramidal cells could increase more than 10-fold between 10 and 100 cm/s, whereas the firing rate of 'non-wheel' cells remained constantly low. We hypothesize that: (i) a necessary condition for place-specific discharge of hippocampal pyramidal cells is the presence of theta oscillation; and (ii) relevant stimuli can tonically and selectively activate hippocampal pyramidal cells as long as theta activity is present.

Cellular-synaptic generation of sleep spindles, spike-and-wave discharges, and evoked thalamocortical responses in the neocortex of the rat

Thalamocortical neuronal oscillations underlie various field potentials that are expressed in the neocortex, including sleep spindles and high voltage spike-and-wave patterns (HVSs). The mechanism of extracellular current generation in the neocortex was studied in the anesthetized and awake rat. Field potentials and unit activity were recorded simultaneously along trajectories perpendicular to the cortical layers at spatial intervals of 100 microm by multiple-site recording silicon probes. Current source density (CSD) analysis revealed that the spatial positions of sinks in layers IV, V-VI, and II-III and of the accompanying sources were similar during sleep spindles, HVSs, and thalamic-evoked responses, although their relative strengths and timings differed. The magnitude and relative timing of the multiple pairs of sinks and sources determined the amplitude variability of HVSs and sleep spindles. The presence of temporally shifted dipoles was also supported by the time distribution of unit discharges in different layers. Putative interneurons discharged with repetitive bursts of 300-500 Hz. The spike component of HVSs was associated with fast field oscillations (400-600 Hz "ripples"). Discharges of pyramidal cells were phase-locked to the ripples. These findings indicate that the major extracellular currents underlying sleep spindles, HVSs, and evoked responses result from activation of intracortical circuitries. We hypothesize that the fast field ripples reflect summed IPSPs in pyramidal cells resulting from the high frequency barrage of interneurons.

High-frequency oscillations in the output networks of the hippocampal-entorhinal axis of the freely behaving rat

Population bursts of the CA3 network, which occur during eating, drinking, awake immobility, and slow-wave sleep, produce a large field excitatory postsynaptic potential throughout stratum radiatum of the CA1 field (sharp wave). The CA3 burst sets into motion a short-lived, dynamic interaction between CA1 pyramidal cells and interneurons, the product of which is a 200 Hz oscillatory field potential (ripple) and phase-related discharge of the CA1 network. Although many CA1 pyramidal neurons discharge during the time (50-100 msec) of each sharp wave, each wave of a ripple (approximately 5 msec) reflects the synchronization of more discrete subsets of CA1 neurons. When we used multi-site recordings in freely behaving rats, we observed ripples throughout the longitudinal extent (approximately 4-5 mm) of the dorsal CA1 region that were coherent for multiple cycles of each ripple. High-frequency ripples were also observed throughout the hippocampal-entorhinal output pathway that were concurrent but less coherent on a cycle-by-cycle basis. Single and multiunit neuronal activity was phase-related to local ripples throughout the hippocampal-entorhinal output pathway. Entorhinal ripples occurred 5-30 msec after the CA1 ripples and were related to the occurrence of an entorhinal sharp wave. Thus, during each hippocampal sharp wave, there is powerful synchronization among the neuronal networks that connect the hippocampus to the neocortex. We suggest that this population interaction (1) biologically constrains theoretical models of hippocampal function and dysfunction and (2) has the capacity to support an "off-line" memory consolidation process.

GABAergic inhibition of granule cells and hilar neuronal synchrony following ischemia-induced hilar neuronal loss

In the dentate gyrus, granule cells are ischemia-resistant, but at least five types of predominantly spiny hilar neurons are extremely vulnerable to ischemia. Many of the ischemia-sensitive subtypes of hilar neurons appear to be involved in: (i) the regulation of GABAergic inhibition in the dentate gyrus, and (ii) the generation of hilar neuronal synchrony. The present study examined functional consequences of ischemia-induced hilar neuronal loss on GABAergic inhibition of granule cells and hilar neuronal synchrony. Transient (15 min) forebrain ischemia was induced by a modification of the four-vessel-occlusion method producing a substantial hilar neuronal loss as demonstrated by the Gallyas silver stain method. Three months later, we have examined spontaneous and stimulus-evoked inhibitory postsynaptic currents mediated by both GABA(A) and GABA(B) receptors, and inhibitory bursts induced by 4-aminopyridine (50 microM) using whole-cell recordings in coronal brain slices maintained at 34-36 degree C in the presence of excitatory amino acid receptor blockers. Spontaneous dentate spikes reflecting hilar neuronal synchrony and synaptic responses evoked by perforant path stimulation were also recorded in vivo to assess synchrony and inhibition in the dentate gyrus. In spite of significant damage to several types of hilar neurons, there were no marked differences in the conductance, kinetics, and 4-aminopyridine-induced burst frequencies of synaptic GABA(A) and GABA(B) responses in granule cells. Furthermore, both paired-pulse inhibition and dentate spikes appeared to be normal in vivo. We conclude that there appears to be little impairment of GABAergic inhibition of granule cells or of hilar neuronal synchrony three months following a massive ischemic damage to spiny hilar neurons.

Intracellular correlates of hippocampal theta rhythm in identified pyramidal cells, granule cells, and basket cells

The cellular-synaptic generation of rhythmic slow activity (RSA or theta) in the hippocampus has been investigated by intracellular recording from principal cells and basket cells in anesthetized rats. In addition, the voltage-, coherence-, and phase versus depth profiles were examined by simultaneously recording field activity at 16 sites in the intact rat, during urethane anesthesia, and after bilateral entorhinal cortex lesion. In the extracellular experiments the large peak of theta at the hippocampal fissure was attenuated by urethane anesthesia and abolished by entorhinal cortex lesion. The phase versus depth profiles were similar during urethane anesthesia and following entorhinal cortex lesion but distinctly different in the intact, awake rat. These observations suggest that dendritic currents underlying theta in the awake rat may not be revealed under urethane anesthesia. The frequency of theta-related membrane potential oscillation was voltage-independent in pyramidal neurons, granule cells, and basket cells. On the other hand, the phase and amplitude of intracellular theta were voltage-dependent in all three cell types with an almost complete phase reversal at chloride equilibrium potential in pyramidal cells and basket cells. At strong depolarization levels (less than 30 mV) pyramidal cells emitted calcium spike oscillations, phase-locked to theta. Basket cells possessed the most regular membrane oscillations of the three cell types. All neurons of this study were verified by intracellular injection of biocytin. The observations provide direct evidence that theta-related rhythmic hyper-polarization of principal cells is brought about by the rhythmically discharging basket neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of VA-045, a novel apovincaminic acid derivative, on age-related impairment evidence in electroencephalograph, caudate spindle, a passive avoidance task and cerebral blood flow in rats

1. The ability of VA-045 to improve aged-related impairment on electroencephalograph (EEG), caudate spindle, performance on a passive avoidance task and cerebral blood flow (CBF) were evaluated in rats. 2. The cortical EEG of the aged rats showed a higher incidence of spontaneous spindle burst (SSB) than seen in young rats. VA-045 decreased the incidence of SSB in aged rats. In contrast, vinpocetine increased the incidence of SSB in aged rats. 3. Electrical stimulation of the striatum in aged rats lead to a higher incidence of neocortical high voltage spindle (CS) than seen in young rats. In young rats, VA-045 had no effect on the CS, whereas an age-related increase in CS was blocked by VA-045, but was enhanced by vinpocetine. 4. There were no differences in the cortical EEG arousal response elicited by stimulation of the reticular formation of the brain stem in rats of all ages. VA-045 and vinpocetine had no effect on the cortical EEG arousal response in both young and aged rats. 5. VA-045, but not vinpocetine, attenuated the age-related decreased step through latency (STL) on a passive avoidance task. VA-045 and vinpocetine did not enhance the acquisition of learning behavior in a passive avoidance task in young rats. 6. VA-045 increased the cerebral blood flow (CBF) in both young and aged rats and the potency in aged rats was greater than that in young rats. Vinpocetine had no effect on CBF in either young or aged rats. 7. The pharmacological effects of VA-045 on age-related neuronal dysfunction are discussed.

A low-noise preamplifier for multisite recording of brain multi-unit activity in freely moving animals

A novel FET instrumentation amplifier is described which, as compared to most traditional operational FET preamplifiers, is characterized by an about 7-10 times lower intrinsic electronic noise and a higher common mode rejection. This allows discrimination of single units from multi-unit recording, even if the action potential amplitudes are as small as 20-30 microV and the units are located more than 100 microns away from the electrode tips. Such a distant and chronic recording may be expected to reduce the possibility of mechanical interference with functions of neuronal membrane and its immediate environment, and may be suitable for studying changes in functional connectivities among neurons during the animal's behavior and learning.

Hippocampal grafts into the intact brain induce epileptic patterns

Spontaneous hippocampal EEG activity and evoked field potentials were investigated in intact rats and in animals with fetal hippocampal grafts. Pieces of hippocampal grafts, derived from 15- to 16-day-old fetuses, were used to prepare cell suspensions and grafted directly into the intact hippocampus. Control animals received suspension grafts of the cerebellum derived from fetuses of identical age. Host hippocampal electrical patterns were monitored with chronic single electrodes or with a 16-microelectrode probe from 7 to 10 months after grafting. In contrast to previously reported high survival rates of fetal grafts in studies with damage to the host brain prior to grafting, survival of both hippocampal (60%) and cerebellar grafts (20%) was very poor in the intact hippocampus. In animals with cerebellar transplants or without surviving grafted neurons the electrical activity of the host hippocampus was indistinguishable from normal controls. In rats with hippocampal grafts short duration, large amplitude EEG spikes (up to 10 mV) were recorded, predominantly during immobility. When the EEG spikes (putative interictal spikes) were of large amplitude and contained population spikes, test evoked responses delivered to the perforant path were suppressed after the spontaneous events. In contrast, evoked responses were facilitated by interictal spikes without population spikes. The threshold of electrically induced afterdischarges did not differ significantly between groups of intact rats and animals with or without hippocampal grafts. However, in three rats with hippocampal grafts the evoked afterdischarges were associated with behavioral seizures. In two of these rats spontaneously occurring seizures were also observed. Synaptophysin-immunoreactivity demonstrated growth of the host mossy fibers into the graft.(ABSTRACT TRUNCATED AT 250 WORDS)

Emergence and propagation of interictal spikes in the subcortically denervated hippocampus

Spontaneous and evoked field potentials and cellular discharges of the subcortically denervated dorsal hippocampus were studied by multisite recordings in the freely behaving rat. Characteristic short-duration (< 100 ms), large-amplitude (up to 10 mV) transients, termed interictal spikes (IIS), were seen after fimbria-fornix (FF) lesion. Both pyramidal cells and putative interneurons fired maximally during IIS, with some interneurons sustaining long bursts (up to 400 ms) of high-frequency discharges (400-600 Hz) after the IIS. The speed of propagation of IIS along the longitudinal axis of the hippocampus varied from 0.2 m/s to > 3 m/s. The majority of IIS (type 1) could be accounted for by an enhanced activity of the intrahippocampal associational systems; a second class of IIS (type 2) had positive polarities in the stratum radiatum of CA1 and CA3 and propagated very rapidly (> 1.5 m/s). The authors propose that type 2 IIS reflect somatic depolarization and discharge of pyramidal neurons due to nonsynaptic (probably ephaptic) effects. Ephaptic interactions may also explain the longitudinal propagation of IIS at speeds higher than the conduction velocities (0.5 m/s) of hippocampal fiber systems. IIS emerged during the first 3 weeks after fimbria-fornix lesion, their incidence reaching a plateau of 2/min thereafter. During the same time period, paired-pulse suppression increased in the dentate gyrus. The amplitude of test responses to angular bundle stimulation was potentiated by small-amplitude IIS but suppressed by large-amplitude IIS. The incidence of IIS was significantly suppressed during walking relative to standing still. Tetanic stimulation of the angular bundle or handling-induced stress resulted in a 10- to 20-fold increase in the incidence of IIS that lasted for about 30 minutes. There was a negative correlation between evoked field PSP slope and population spike amplitude in the dentate gyrus of FF-lesioned rats; this correlation was positive in intact rats. The authors attribute the above pathophysiological changes to sprouting of both excitatory and inhibitory GABAergic pathways as a result of denervation of the intrahippocampal circuitry. They hypothesize that the majority of the observed physiological alterations can be traced to a weakening of feedforward inhibition coupled with an enhancement of feedback inhibition and excitation.

A method for rapid implantation of multielectrode systems

The implantation of several chronic electrodes at the same time was needed for the studies in which both evoked slow potential and multiple unit neural activity of different brain sites were recorded during classical conditioning in freely moving cats. The recording electrodes were made of 25-75 microns insulated stainless steel or nichrome wires which were inserted into a thin hypodermic needle of a fixed, calibrated length. The fast implantation system described here is based on the use of a stereotaxic frame as a stand for a drill and as a support for an electrode holder with which the electrodes were implanted through precision holes drilled directly according to the coordinates of the brain atlas. The connecting pins of each electrode were inserted to a connector during implantation. The described implantation system allows short distances (0.5 mm) between electrode shafts and free determination of the depth of each electrode. Because fixing of an individual electrode with dental cement to the skull is not needed during the initial phase and the precalibrated electrodes need no adjustments during implantation, considerable time savings can be achieved.

The thalamic clock: emergent network properties

Rhythmical oscillation of thalamic neuronal populations occurs under physiological conditions and in several disease states. In the present experiments we examined the network properties of population rhythmicity and the possible involvement of N-methyl-D-aspartate receptors in the frequency regulation and maintenance of rhythmic thalamic bursts. Multisite recording of neuronal activity and local microinjections of drugs were performed on the freely moving rat. Rhythmic thalamic population bursts at 6 to 9 Hz and concurrent neocortical high voltage spike-and-wave spindles were observed during awake immobility, with the thalamic rhythm leading the neocortical high voltage spindle. Even though all individual thalamocortical neurons fired in a phase-locked manner to the high-voltage spindle, the majority discharged at a significantly lower frequency than that of the population (multiunit) activity. In contrast, neurons in the nucleus reticularis thalami discharged at the frequency of the population bursts. Neurons in the extrapyramidal system and neocortex but not the hippocampal formation also fired in a phase-locked manner to the high-voltage spindle. Systemic administration or local microinjection of either non-competitive or competitive N-methyl-D-aspartate blockers (ketamine or ap-5) slowed the frequency of thalamic multiunit bursts and associated high-voltage spindles from 8 to 2 Hz, or completely blocked rhythmicity. Unilateral thalamic injection of ketamine or ap-5 resulted in a suppression of the amplitude of high-voltage spindles in the injected hemisphere. It is concluded that thalamic rhythmicity is not due to the "pacemaker" properties of thalamic cells but is rather an emergent property of the relay thalamus-nucleus reticularis network. Furthermore, we hypothesize that the frequency of network oscillation is regulated by the interplay between two major classes of voltage-dependent conductances in the thalamocortical cells: low-threshold calcium channels and high-threshold N-methyl-D-aspartate channels.

Petit mal epilepsy and parkinsonian tremor: hypothesis of a common pacemaker

Rhythmic oscillation in neuronal systems may serve physiological purposes or may interfere with normal functions of the brain. In disorders of petit mal epilepsy and parkinsonian tremor, centrally and peripherally observable rhythmic patterns are due to network oscillations of thalamocortical cells. This article reviews the afferent mechanisms that might be critically involved in controlling the ionic conductances of thalamic neurons in the behaving organism. We propose that during active behavior the subcortical aminergic and cholinergic inputs to the thalamus act as anti-burst and anti-oscillation mechanisms. We suggest further that the thalamopetal GABAergic inputs (pars reticulata of substantia nigra, entopeduncular nucleus, pallidum) are burst- and oscillation-promoting systems, whose output is controlled by the striatum. Experimental or disease-related decrease of the striatal dopamine levels is hypothesized to increase the efficacy of the GABAergic burst-promoting systems resulting in rhythmic network oscillation of thalamocortical neurons during rest. The recognition of the overlapping neuronal mechanisms in petit mal epilepsy and parkinsonian tremor, and the multistage control of thalamic oscillation suggests that drugs effectively used in petit mal attacks may be effective in levodopa-refractory parkinsonian tremor, and conversely, epileptic patients may benefit from drugs acting on the extrapyramidal system.

Coherence of compound field potentials reveals discontinuities in the CA1-subiculum of the hippocampus in freely-moving rats

The ongoing micro-electroencephalogram was recorded with a chronically implanted comb-like array of 16 tungsten semi-microelectrodes 0.2 or 0.25 mm apart, spanning CA1 strata oriens, pyramidale and radiatum and into subiculum, in four behavioral states: walking, standing still, paradoxical and slow wave sleep and under scopolamine. Power, phase and coherence spectra were computed, the latter two for each of the 120 pairs, in frequency bands from 1 to 64 Hz. (1) Coherence is high for all frequencies within the same subfield, e.g. stratum radiatum, but falls with distance. Theta frequency (8 Hz), when prominent and widespread (during "theta states" walking and paradoxical sleep), shows the most widespread synchrony: coherence falls slowly, from 1.0 at 0.2 mm to 0.7 at c. 2 mm longitudinally within stratum radiatum; all other frequencies fall two or three times faster. (2) An abrupt drop in coherence occurs across field borders (CA1-subiculum) and between stratum oriens and radiatum, across a line just under stratum pyramidale, between high coherence regions on each side of the coherence discontinuity. A less extreme drop occurs in stratum radiatum 0.4 mm from the subiculum border, without obvious histological correlate. The discontinuities in coherence are stable through all four behavioral states as well as under scopolamine. (3) Phase profiles diagonally across CA1 and into subiculum show abrupt, local shifts of phase (up to 125) at these same levels. No gradual shift reaching 180 (phase reversal) occurs in the span of loci examined. (4) The theta power peak in theta states is not necessarily due to additional energy in that band; in some conditions it is mainly due to reduced power in other frequencies. Root mean square voltage is generally less in the high theta ("synchronized") than in the non-theta states. Only the theta peak correlates with a peak in coherence. (5) Significant microstructure in the dynamics of neuronal cooperativity distinguishes behavioral states and regions of the hippocampal cortex.

Spatial organization of physiological activity in the hippocampal region: relevance to memory formation

Based on a review of anatomical and physiological findings, we suggest that the hippocampus may be viewed as a positive feedback device (autoassociator), which is capable of modifying the activity of the neocortical neurons. We examine the three-dimensional organization of evoked and spontaneous physiological patterns of the hippocampus and suggest rules how these patterns emerge during different behaviors from a hard-wired structural network. The high spatial coherence of theta activity is due to an external pacemaker, while the high synchrony of population bursts underlying hippocampal sharp waves is explained by the similar probability of recruitment of neurons by the burst-initiator cells along the whole extent of the hippocampus. We suggest that the burst-initiator cells are a group of CA3 neurons whose excitability is increased by a transient potentiation action of the neocortical activity during theta-concurrent exploratory behaviors. We hypothesize that sharp wave-concurrent population bursts result in a highly synchronous hippocampal output, converging preferentially on those entorhinal neurons which were instrumental in the creation of the burst-initiator neurons. The feedback action of population activity thus provides a selective mechanism for potentiation of connections between information-carrying neurons in the hippocampus and entorhinal cortex. The state-dependent operations of the anatomical hardware also point to the importance and advantage of studying the physiological activity of the intact brain.

Spike-and-wave neocortical patterns in rats: genetic and aminergic control

Spontaneously occurring and drug-induced high voltage spike-and-wave electroencephalogram patterns were examined in inbred rats of the Fischer 344 and Buffalo strains and of the random-bred Sprague-Dawley strain at different ages. In addition, tyrosine hydroxylase activity and dopamine D2 receptor density were determined in the substantia nigra, corpus striatum, olfactory tubercle and ponsmedulla areas of Fisher 344 and Buffalo animals. High voltage spike-and-wave episodes were present in 87.5% of the 3-month-old and in 100% of the older Fischer 344 rats. High voltage spike-and-wave episodes were completely absent in 3-month-old Buffalo and Sprague-Dawley animals but could be induced by systemic injection of pentylenetetrazol and at an older age they appeared in 58.3% (12-month) and 71.4% (greater than 26-month) of the subjects of these strains. The incidence and duration of high voltage spike-and-wave episodes were significantly higher/longer in Fischer 344 rats than in the age-matched Buffalo and Sprague-Dawley animals. The dopamine blocker acepromazine induced a several-fold increase of the incidence and duration of high voltage spike-and-wave episodes in 3-month-old Fischer 344 rats, but failed to induce high voltage spike-and-wave episodes in Buffalo animals at this age. However, acepromazine also triggered high voltage spike-and-wave episodes in Buffalo rats when they were pretreated with subthreshold doses of pentylenetetrazol. Tyrosine hydroxylase activity was significantly higher in the substantia nigra, corpus striatum and olfactory tubercle of the Fischer 344 strain than in Buffalo rats. The higher tyrosine hydroxylase activity was paralleled with significantly higher D2 binding values in the corpus striatum and olfactory tubercle of Fischer 344 rats. These findings suggest that the neocortical high voltage spike-and-wave phenotype is genetically mediated and that the inbred Fischer 344 and Buffalo rats with defined bilineal origin will facilitate future works aimed at the identification of genetic elements involved in the generation of neocortical high voltage spike-and-wave episodes. The significant genotype x age interaction supports the suggestion, however, that high voltage spike-and-wave episodes are likely to be influenced by more than one gene; some of them are probably related to the regulation of brain aminergic systems.

The grafted hippocampus: an epileptic focus

Field potentials and unitary activity were investigated in the grafted and the host hippocampi in freely moving rats and in vitro. The subcortical afferents and efferents of the hippocampus (fimbria-fornix, FF) were removed by aspiration. Solid pieces of hippocampal grafts derived from 15- to 16-day-old fetuses were placed in the lesion cavity in rats with unilateral FF lesions, and cell suspensions prepared from fetal hippocampi were grafted directly into the host hippocampi in animals with bilateral FF lesions. Reciprocal communication between the grafted and the host hippocampi was monitored with a 16-microelectrode probe from 7 to 10 months after grafting. The fluorescent retrograde tracer, Fluorogold, was used to examine graft-host projections and acetylcholinesterase staining to reveal host-derived fibers in the graft. The most typical neuronal pattern of the hippocampal graft was a highly synchronous population burst with concurrent EEG spike. The speed of propagation of the EEG spike within the graft and across the graft-host interface was either fast (greater than 3 m/s) or slow (less than 0.5 m/s). Large amplitude, short duration EEG spikes usually propagated with a high speed, while smaller amplitude, wider spikes with broad population bursts spread at a lower velocity. The direction of propagation was usually uniform indicating that the population burst was triggered by a localized subgroup of highly excitable neurons ("focus"). Spontaneous seizures were also present in the solid graft which frequently invaded the host hippocampus. The incidence of EEG spikes was three times higher in rats with bilateral suspension grafts than in animals with FF lesion only. In about half of the grafted rats spontaneous behavioral seizures were also observed. Intracellular recordings from putative pyramidal cells in the graft and in the host revealed large amplitude (10-12 mV), spontaneously occurring EPSPs. IPSPs were difficult to detect even during depolarizations of up to 20 mV from rest. We suggest that the increased excitability of the hippocampal graft is due to the high incidence of recurrent excitatory collaterals terminating on or close to the somata of pyramidal neurons. Population bursts may spread fast via extensively arborizing axon collaterals or slowly by successively activating new sets of neighboring neurons. Spontaneous behavioral convulsions are explained by assuming that the grafted hippocampus serves as an epileptic focus which is capable of kindling the host brain by repeated seizure induction.(ABSTRACT TRUNCATED AT 400 WORDS)

Two-stage model of memory trace formation: a role for "noisy" brain states

Review of the normally occurring neuronal patterns of the hippocampus suggests that the two principal cell types of the hippocampus, the pyramidal neurons and granule cells, are maximally active during different behaviors. Granule cells reach their highest discharge rates during theta-concurrent exploratory activities, while population synchrony of pyramidal cells is maximum during immobility, consummatory behaviors, and slow wave sleep associated with field sharp waves. Sharp waves reflect the summed postsynaptic depolarization of large numbers of pyramidal cells in the CA1 and subiculum as a consequence of synchronous discharge of bursting CA3 pyramidal neurons. The trigger for the population burst in the CA3 region is the temporary release from subcortical tonic inhibition. An overview of the experimentally explored criteria of synaptic enhancement (intensity, frequency, and pattern of postsynaptic depolarization, calcium influx, cooperativity, threshold) suggests that these requirements may be present during sharp wave-concurrent population bursts of pyramidal cells. Experimental evidence is cited showing that (a) population bursts in CA3 may lead to long-term potentiation in their postsynaptic CA1 targets, (b) tetanizing stimuli are capable of increasing the synchrony of the sharp wave-burst, and (c) activity patterns of the neocortical input to the hippocampus determine which subgroup of CA3 neurons will trigger subsequently occurring population bursts (initiator cells). Based on the experimental evidence reviewed a formal model of memory trace formation is outlined. During exploratory (theta) behaviors the neocortical information is transmitted to the hippocampus via the fast-firing granule cells which may induce a weak and transient heterosynaptic potentiation in a subgroup of CA3 pyramidal cells. The weakly potentiated CA3 neurons will then initiate population bursts upon the termination of exploratory activity (sharp wave state). It is assumed that recurrent excitation during the population burst is strongest on those cells which initiated the population event. It is suggested that the strong excitatory drive brought about by the sharp wave-concurrent population bursts during consummatory behaviors, immobility, and slow wave sleep may be sufficient for the induction of long-term synaptic modification in the initiator neurons of the CA3 region and in their targets in CA1. In this two-stage model both exploratory (theta) and sharp wave states of the hippocampus are essential and any interference that might modify the structure of the population bursts (e.g. epileptic spikes) is detrimental to memory trace formation.