Functional features of the rat subicular microcircuits studied in vitro.

The subiculum has a strategic position in controlling hippocampal activity and is now receiving much experimental attention. However, information regarding this structure remains fragmented and there are important gaps in our knowledge between what we know about the subicular architecture and its biological function. In recent years a substantial amount of in vitro experimentation has explored many aspects of the functional organization of the subicular microcircuits. Here we review these recent findings. We aim to identify the rules that govern the operation of subicular microcircuits in vitro and to relate these to the role of the subiculum in the intact brain.

[The pathophysiological foundations of temporal-lobe epilepsy: studies in humans and animals]. / Bases fisiopatológicas de la epilepsia del lóbulo temporal: estudios en humanos y animales.

INTRODUCTION: Temporal lobe epilepsy (TLE) is the most frequent form of pharmaco-resistant epilepsy in human. Research using material from TLE patients undergoing surgery and animal models has significantly increased in the last decade. DEVELOPMENT: We review recent findings obtained over the last years from electrophysiological and anatomical studies in human and animal models of TLE. Data suggest a large heterogeneity and inter-individual variability depending on the model and the system under study. However, a common principle that appears to underlie the epileptic condition is the reorganization of excitation and inhibition resulting in hyperexcitability. Recent research combining in vitro electrophysiology together with depth recordings in vivo and new analytical methodologies is also discussed. CONCLUSIONS: A multidisciplinary approach using both human and animal models can help to fill gaps in our knowledge and to provide unique insights into the pathophysiology of TLE.

Bases fisiopatológicas de la epilepsia del lóbulo temporal: estudios en humanos y animales / The pathophysiological foundations of temporal-lobe epilepsy: studies in humans and animals

Introducción. La epilepsia del lóbulo temporal (ELT) esel tipo más frecuente de epilepsia farmacorresistente en humanos.El tratamiento quirúrgico de estos pacientes permite estudios de granimportancia para conocer los mecanismos fisiopatológicos subyacentes.Desarrollo. Se revisan algunos de los datos y teorías másrecientes sobre la fisiopatología de la ELT, tanto en modelos animalescomo en humanos. Aunque existen similitudes electrofisiológicas,no hay una relación perfecta entre los datos hallados en modelos animales y en humanos, lo que dificulta la extrapolación de losresultados. No obstante, ambos tipos de estudios sugieren una considerableheterogeneidad en las alteraciones responsables de la epilepsia,si bien se acepta comúnmente que existe una remodelaciónde la excitación glutamatérgica y la inhibición gabérgica que derivaen hiperexcitabilidad. El papel de la esclerosis mesial como procesooriginario de la ELT está cada vez más discutido. El desarrollo deestudios electrofisiológicos in vitro y la aplicación de técnicas conmayor poder de resolución, como los registros con microelectrodoso nuevas herramientas matemáticas, pueden aportar importantesdatos al conocimiento fisiopatológico de este síndrome. Conclusiones.El estudio multidisciplinar de la fisiopatología de la ELT en laúltima década ha permitido aumentar el conocimiento sobre los procesosque subyacen a la génesis de las crisis, su clínica y evolución.Este conocimiento es de gran importancia porque abre nuevas opcionesterapéuticas de la ELT

Introduction. Temporal lobe epilepsy (TLE) is the most frequent form of pharmaco-resistant epilepsy in human.Research using material from TLE patients undergoing surgery and animal models has significantly increased in the lastdecade. Development.We review recent findings obtained over the last years from electrophysiological and anatomical studiesin human and animal models of TLE. Data suggest a large heterogeneity and inter-individual variability depending on themodel and the system under study. However, a common principle that appears to underlie the epileptic condition is thereorganization of excitation and inhibition resulting in hyperexcitability. Recent research combining in vitro electrophysiologytogether with depth recordings in vivo and new analytical methodologies is also discussed. Conclusions. A multidisciplinaryapproach using both human and animal models can help to fill gaps in our knowledge and to provide unique insights into thepathophysiology of TLE

Synaptic contributions to focal and widespread spatiotemporal dynamics in the isolated rat subiculum in vitro.

The subiculum, which has a strategic position in controlling hippocampal activity, is receiving significant attention in epilepsy research. However, the functional organization of subicular circuits remains unknown. Here, we combined different recording and analytical methods to study focal and widespread population activity in the isolated subiculum in zero Mg2+ media. Patch and field recordings were combined to examine the contribution of different cell types to population activity. The properties of cells leading field activity were examined. Predictive factors for a cell to behave as leader included exhibiting the bursting phenotype, displaying a low firing threshold, and having more distal apical dendrites. A subset of bursting cells constituted the first glutamatergic type that led a recruitment process that subsequently activated additional excitatory as well as inhibitory cells. This defined a sequence of synaptic excitation and inhibition that was studied by measuring the associated conductance changes and the evolution of the composite reversal potential. It is shown that inhibition was time-locked to excitation, which shunted excitatory inputs and suppressed firing during focal activity. This was recorded extracellularly as a multi-unit ensemble of active cells, the spatial boundaries of which were controlled by inhibition in contrast to widespread epileptiform activity. Focal activity was not dependent on the preparation or the developmental state because it was also recorded under 5 mm [K+]o and in adult tissue. Our data indicate that the subicular networks can be spontaneously organized as leader-follower local circuits in which excitation is mainly driven by a subset of bursting cells and inhibition controls spatiotemporal firing.

Electrophysiological and morphological diversity of neurons from the rat subicular complex in vitro.

We combined whole-cell recordings with Neurobiotin labeling to examine the electrophysiological and morphological properties of neurons from the ventral subicular complex in vitro (including the subicular, presubicular, and parasubicular areas). No a priori morphological sampling criteria were used to select cells. Cells were classified as bursting (IB), regular-spiking (RS), and fast-spiking (FS) according to their firing patterns in response to depolarizing current pulses. A number of cells remained unclassified. We found 54% RS, 26% IB, 11% FS, and 9% unclassified cells out of a total of 131 neurons examined. We also found cells showing intrinsic membrane potential oscillations (MPO) (6%), which represented a subgroup of the unclassified cells. We analyzed several electrophysiological parameters and found that RS and IB cells can be subclassified into two separate subgroups. RS cells were subclassified as tonic and adapting, according to the degree of firing adaptation. Both responded with single spikes to orthodromic stimulation. IB cells were subclassified in two subgroups according to their capacity to fire more than one burst, and showed different responses to orthodromic stimulation. We observed that bursting in these two subgroups appeared to involve both Ca2+ and persistent Na+ components. Both IB and RS cells, as well as MPO neurons, were projecting cells. FS cells were morphologically identified as local circuit interneurons. We also analyzed the spatial distribution of these cell types from the vicinity of CA1 to the parasubicular areas. We conclude that, in contrast to the commonly accepted idea of the subicular complex as a bursting structure, there is a wide electrophysiological variability even within a given cellular group.

Control of bursting by local inhibition in the rat subiculum in vitro.

The subiculum, which provides the major hippocampal output, contains different cell types including weak/strong bursting and regular-spiking cells, and fast-spiking interneurons. These cellular populations play different roles in the generation of physiological rhythms and epileptiform activity. However, their intrinsic connectivity and the synaptic regulation of their discharge patterns remain unknown. In the present study, the local synaptic responses of subicular cell types were examined in vitro. To this purpose, slices were prepared at a specific orientation that permitted the antidromic activation of projection cells as a tool to examine local circuits. Patch recordings in cell-attached and whole-cell configurations were combined with neurobiotin labelling to classify cell types. Strong (approximately 75 %), but not weak (approximately 22 %), bursting cells typically fired bursts in response to local synaptic excitation, whereas the majority of regular-spiking cells (approximately 87 %) remained silent. Local excitation evoked single spikes in more than 70 % of fast-spiking interneurons. This different responsiveness was determined by intrinsic membrane properties and not by the amplitude and pharmacology of synaptic currents. Inhibitory GABAergic responses were also detected in some cells, typically as a component of an excitatory/inhibitory sequence. A positive correlation between the latency of the excitatory and inhibitory responses, together with the glutamatergic control (via non-NMDA receptors) of inhibition, suggested a local mechanism. The effect of local inhibition on synaptically activated firing of different cell types was evaluated. It is shown that projection bursting cells of the subiculum are strongly controlled by local inhibitory circuits.

[In vivo and in vitro neurophysiological features of epilepsy]. / Aspectos neurofisiológicos in vivo e in vitro de la epilepsia.

INTRODUCTION: Some electrophysiological aspects of the interictal activity in temporal lobe epilepsy (TLE) are under debate. However, surgery is often guided by both, pre and intra operative recordings of interictal activity. Some of these studies include intraoperative electrocorticography (ECoG) that guide tailored resection. OBJECTIVE: To analyze several aspects of interictal activity recorded by ECoG and to explore its electrophysiological basis in vitro. PATIENTS AND METHODS: ECoGs from thirteen patients suffering drug resistant TLE were analyzed. Both linear and non linear methods were introduced to explore the ECoG signals. In two cases, neocortical tissue was also electrophysiologically studied in vitro. RESULTS: Interictal ECoG activity showed a mesial origin, a cortical origin or a mixed mesial/temporal origin. Both in cortical and mesial/cortical origin, cortical interictal ECoG signals are characterized by spatiotemporal clusters of activity. In vitro study of cortical tissue from these clusters showed alterations in the synaptic control of excitability. CONCLUSION: Our results suggest an electrophysiological basis of interictal activity. Combination of linear and non linear methods is a very useful tool to explore on line the interictal ECoG activity during surgery.

Excitatory and inhibitory control of epileptiform discharges in combined hippocampal/entorhinal cortical slices.

We examined whether epileptiform activity can be induced and prevented by mild reduction of GABA(A) receptor-mediated inhibition and non-NMDA receptor-mediated excitation, respectively, in different regions of combined hippocampal/entorhinal cortical slices from juvenile rats (P15-21). We used the receptor antagonists bicuculline (GABA(A)) and CNQX (non-NMDA) as tools to investigate the sensitivities of the CA1, the subiculum (SUB) and the medial entorhinal cortex (MEC) for generating epileptiform discharges upon extracellular stimulation. We found that low concentrations of bicuculline (<3.5 microM) were enough to induce epileptiform discharges in the three regions. These discharges were similar to those observed under high concentrations of bicuculline (>10 microM) and consisted of stereotyped population bursts, recorded both extra- and intracellularly. Interestingly, the CA1 and SUB were more susceptible to generate discharges compared to the MEC in the same slices. We also found that non-NMDA excitation was critical in controlling discharges, as they were blocked by CNQX in a concentration-dependent manner. The sensitivity of the CA1 region to CNQX was lower than that of the SUB and MEC. Based on these regional differences, we show that epileptiform activity can be pharmacologically isolated within the CA1 region in the hippocampal-entorhinal circuitry in vitro.

The effect of different morphological sampling criteria on the fraction of bursting cells recorded in the rat subiculum in vitro.

In spite of the large variability of the fraction of bursting cells reported in the subiculum (from 54 to 100%), this structure has been considered as intrinsically bursting. Here, using visually assisted whole-cell recordings and Neurobiotin labeling in vitro, we correlated the electrophysiological firing modes of subicular cells (bursting, regular-spiking and fast-spiking) with their morphological characteristics (somatic size and shape). We then examined how different morphological sampling criteria for patching affect cell classification. We found a dramatic variability in the fraction of bursting cells, which ranged from 30 to 76% depending on the sampling criteria. We discuss the implications of these findings for the notion of the subiculum as an intrinsically bursting structure.

Aspectos neurofisiológicos in vivo e in vitro de la epilepsia / In vivo and in vitro neurophysiological features of epilepsy

Introducción. Algunos aspectos electrofisiológicos de la actividad intercrítica en la epilepsia del lóbulo temporal (ELT) están sujetos a debate. Sin embargo, la cirugía de la ELT está basada en diferentes estudios tanto preoperatorios como intraoperatorios de esta actividad. Entre estos estudios destaca la electrocorticografía (ECoG) intraoperatoria que es utilizada para la delimitación final del área que se va a resecar. Objetivo. En este trabajo se discuten algunos aspectos de la actividad intercrítica registrados in vivo a través de la ECoG, así como sus bases electrofisiológicas estudiadas in vitro. Pacientes y métodos. Se estudiaron los registros ECoG intraoperatorios de 13 pacientes que padecen ELT farmacorresistente. Se desarrollaron técnicas de análisis lineal y no lineal para estudiar las propiedades de la actividad intercrítica. En dos de los pacientes se llevaron a cabo estudios electrofisiológicos in vitro del tejido cortical. Resultados. La actividad intercrítica registrada por ECoG muestra complejos patrones electrofisiológicos, tanto espacial como temporalmente. Esta actividad puede originarse mesialmente, corticalmente o de forma mixta mesial/cortical. Tanto en los casos de origen mixto, como cortical, la actividad intercrítica en corteza temporal se caracteriza por la existencia de agregados espaciales de actividad. El estudio electrofisiológico in vitro de estos agregados demuestra la presencia de anomalías en el control sináptico de la excitabilidad. Conclusión. Estos resultados sugieren la existencia de una base electrofisiológica de la actividad intercrítica. El análisis combinado lineal y no lineal proporciona un instrumento de exploración que puede aplicarse en quirófano para guiar la cirugía (AU)

Heterogeneous populations of cells mediate spontaneous synchronous bursting in the developing hippocampus through a frequency-dependent mechanism.

Under normal conditions, hippocampal slices from newborn rats and rabbits (postnatal days 0-8) show spontaneous synchronous bursts known as giant depolarizing potentials. These bursts are recorded from CA3, CA1 and the fascia dentata in both intact slices and isolated hipocampal regions. Giant depolarizing potentials are network-driven events resulting from the synergistic activation of N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxadepropionate and GABA(A) receptors, the latter playing an excitatory role. Recently, we showed that they spontaneously emerge in an all-or-none manner after the increase of synaptic and cellular activity beyond a threshold frequency [Menendez de la Prida L. and Sanchez-Andres J. V. (1999) J. Neurophysiol. 82, 202-208]. Under this framework, background levels of spontaneous activity at individual neurons build up network synchronization 100-300ms prior to the onset of giant depolarizing potentials. However, the role of distinct cellular populations and connectivity in determining the threshold frequency has not been examined. By performing simultaneous intracellular recordings from pyramidal cells, non-pyramidal cells and interneurons, we investigated their participation in the generation of giant depolarizing potentials. Electrodes containing Neurobiotin were used to examine the cellular morphology. We found that giant depolarizing potentials were not initiated from a single pacemaker cellular group; instead, they involved recurrent cooperation among these groups, which contributed differently according to their intrinsic firing capability. In all the neurons examined, the onset of these bursts took place in an all-or-none frequency-dependent manner, both spontaneously (depending on the frequency of the excitatory postsynaptic potentials) or when triggered by extracellular stimulation. The CA3 threshold of frequency was at 12Hz in both pyramidal cells and interneurons, while in the fascia dentata it was 17Hz. The application of 6-cyano-7-nitroquinoxaline-2,3-dione increased CA3 threshold of frequency up to 50Hz, suggesting that it is determined by combined synaptic components. We examined the role of postsynaptic summation on the threshold of frequency. Heterogeneity is present among the cellular groups, pyramidal neurons from CA1 and CA3 showing less evidence of postsynaptic summation prior to giant depolarizing potentials. Cells showing stronger evidence of postsynaptic summation were more typically recorded at the hilus, the granule layer of the fascia dentata and the CA3/CA4 area. Nevertheless, for a given cell, not all the giant depolarizing potentials were preceded by summation of postsynaptic potentials. These outcomes, together with the long and variable time delays recorded between different areas, strongly suggest that giant depolarizing potentials are locally generated from different initiation sites and not from a single region. We discuss these results in view of the principles underlying hyperexcitability in hippocampal slices, i.e. the intrinsic firing properties of individual cells and the connectivity patterns.

Nonlinear transfer function encodes synchronization in a neural network from the mammalian brain.

Synchronization is one of the mechanisms by which the brain encodes information. The observed synchronization of neuronal activity has, however, several levels of fluctuations, which presumably regulate local features of specific areas. This means that biological neural networks should have an intrinsic mechanism able to synchronize the neuronal activity but also to preserve the firing capability of individual cells. Here, we investigate the input-output relationship of a biological neural network from developing mammalian brain, i.e., the hippocampus. We show that the probability of occurrence of synchronous output activity (which consists in stereotyped population bursts recorded throughout the hippocampus) is encoded by a sigmoidal transfer function of the input frequency. Under this scope, low-frequency inputs will not produce any coherent output while high-frequency inputs will determine a synchronous pattern of output activity (population bursts). We analyze the effect of the network size (N) on the parameters of the transfer function (threshold and slope). We found that sigmoidal functions realistically simulate the synchronous output activity of hippocampal neural networks. This outcome is particularly important in the application of results from neural network models to neurobiology.

Origin of the synchronized network activity in the rabbit developing hippocampus.

Rhythmic spontaneous bursting is a fundamental hallmark of the immature hippocampal activity recorded in vitro. These bursts or giant depolarizing potentials (GDPs) are GABA- and glutamatergic-driven events. The mechanisms of GDPs generation are still controversial, since although a hilar origin has been suggested, GDPs were also recorded from isolated CA3 area. Here, we have investigated the origin of GDPs in hippocampal slices from newborn rabbits. Simultaneous intracellular recordings were performed in CA3, CA1 and the fascia dentata. We found a high degree of correlation between the spontaneous GDPs present in CA3 and CA1 regions. Cross-correlation analysis demonstrated that CA3 firing precedes CA1 by about 192 ms, although a significant population of discharges was recorded first in CA1 (20%). Granule cells (GCs) in the fascia dentata also showed GDPs. The frequency of these events (1.46 +/- 1.25 GDPs/min, n = 7) is significantly lower when compared with that from CA3 (3.13 +/- 1.43 GDPs/min, n = 10) or CA1 (2.94 +/- 1.36 GDPs/min, n = 17). Dual recordings from CA3 and fascia dentata cells showed synchronous bursts in both regions with no prevalent preceding area. By recording from isolated areas we found that CA1, CA3 and the fascia dentata can produce GDPs, suggesting that they emerge as a property of local circuits present throughout the hippocampus.

Analytical characterization of spontaneous activity evolution during hippocampal development in the rabbit.

We have analyzed the postnatal evolution of the spontaneous electrical activity in pyramidal neurons from rabbit hippocampal slices. The firing mode of CA1 neurons changes from bursting to regular spiking along the first postnatal month. Interspike intervals (ISIs) were used to account for the dynamical structure of the firing behavior. Histograms and joint interval scattergrams show that the firing mode from (P0-P7) cells has a different distribution from that obtained in (P15-P25) neurons. We have used a mathematical measure called the product of inertia to quantify this difference. Our findings demonstrate that the spontaneous activity changes along the maturational process.