Pharmacology and electrophysiology of a synchronous GABA-mediated potential in the human neocortex

Bursting of excitatory cells is linked to interictal epileptic discharge generation in humans

Knowledge about the activity of single neurons is essential in understanding the mechanisms of synchrony generation, and particularly interesting if related to pathological conditions. The generation of interictal spikes—the hypersynchronous events between seizures—is linked to hyperexcitability and to bursting behaviour of neurons in animal models. To explore its cellular mechanisms in humans we investigated the activity of clustered single neurons in a human in vitro model generating both physiological and epileptiform synchronous events. We show that non-epileptic synchronous events resulted from the finely balanced firing of excitatory and inhibitory cells, which was shifted towards an enhanced excitability in epileptic tissue. In contrast, interictal-like spikes were characterised by an asymmetric overall neuronal discharge initiated by excitatory neurons with the presumptive leading role of bursting pyramidal cells, and possibly terminated by inhibitory interneurons. We found that the overall burstiness of human neocortical neurons is not necessarily related to epilepsy, but the bursting behaviour of excitatory cells comprising both intrinsic and synaptically driven bursting is clearly linked to the generation of epileptiform synchrony.

Synaptic plasticity in human cortical circuits: cellular mechanisms of learning and memory in the human brain?

Synaptic plasticity is the cellular basis of learning and memory, but to what extent this holds for the adult human brain is not known. To study synaptic plasticity in human neuronal circuits poses a huge challenge, since live human neurons and synapses are not readily accessible. Despite this, various lines of research have provided insights in properties of adult human synapses and their plasticity both in vitro and in vivo, with some unexpected surprises. We first discuss the experimental approaches to study activity-dependent plasticity of adult human synapses, and then highlight rules and mechanisms of Hebbian spike timing-dependent plasticity (STDP) found in these synapses. Finally, we conclude with thoughts on how these synaptic principles can underlie human learning and memory.

Optogenetic dissection of roles of specific cortical interneuron subtypes in GABAergic network synchronization

KEY POINTS

An increase in the excitability of GABAergic cells has typically been assumed to decrease network activity, potentially producing overall anti-epileptic effects. Recent data suggest that inhibitory networks may actually play a role in initiating epileptiform activity. We show that activation of GABAergic interneurons can elicit synchronous long-lasting network activity. Specific interneuron subpopulations differentially contributed to GABA network synchrony, indicating cell type-specific contributions of interneurons to cortical network activity. Interneurons may critically contribute to the generation of aberrant network activity characteristic of epilepsy, warranting further investigation into the contribution of distinct cortical interneuron subpopulations to the propagation and rhythmicity of epileptiform activity.

ABSTRACT

In the presence of the A-type K+ channel blocker 4-aminopyrdine, spontaneous synchronous network activity develops in the neocortex of mice of either sex. This aberrant synchrony persists in the presence of excitatory amino acid receptor antagonists (EAA blockers) and is considered to arise from synchronous firing of cortical interneurons (INs). Although much attention has been given to the mechanisms underlying this GABAergic synchrony, the contribution of specific IN subtypes to the generation of these long-lasting discharges (LLDs) is incompletely understood. We employed genetically-encoded channelrhodopsin and archaerhodopsin opsins to investigate the sufficiency and necessity, respectively, of activation of parvalbumin (PV), somatostatin (SST) and vasointestinal peptide (VIP)-expressing INs for the generation of synchronous neocortical GABAergic discharges. We found light-induced activation of PV or SST INs to be equally sufficient for the generation of LLDs, whereas activation of VIP INs was not. By contrast, light-induced inhibition of PV INs strongly reduced LLD initiation, whereas suppression of SST or VIP IN activity only partially attenuated LLD magnitude. These results suggest neocortical INs perform cell type-specific roles in the generation of aberrant GABAergic cortical network activity.

Hyperexcitability of the network contributes to synchronization processes in the human epileptic neocortex

KEY POINTS

Hyperexcitability and hypersynchrony of neuronal networks are thought to be linked to the generation of epileptic activity in both humans and animal models. Here we show that human epileptic postoperative neocortical tissue is able to generate two different types of synchronies in vitro. Epileptiform bursts occurred only in slices derived from epileptic patients and were hypersynchronous events characterized by high levels of excitability. Spontaneous population activity emerged in both epileptic and non-epileptic tissue, with a significantly lower degree of excitability and synchrony, and could not be linked to epilepsy. These results help us to understand better the role of excitatory and inhibitory neuronal circuits in the generation of population events, and to define the subtle border between physiological and pathological synchronies.

ABSTRACT

Interictal activity is a hallmark of epilepsy diagnostics and is linked to neuronal hypersynchrony. Little is known about perturbations in human epileptic neocortical microcircuits, and their role in generating pathological synchronies. To explore hyperexcitability of the human epileptic network, and its contribution to convulsive activity, we investigated an in vitro model of synchronous burst activity spontaneously occurring in postoperative tissue slices derived from patients with or without preoperative clinical and electrographic manifestations of epileptic activity. Human neocortical slices generated two types of synchronies. Interictal-like discharges (classified as epileptiform events) emerged only in epileptic samples, and were hypersynchronous bursts characterized by considerably elevated levels of excitation. Synchronous population activity was initiated in both epileptic and non-epileptic tissue, with a significantly lower degree of excitability and synchrony, and could not be linked to epilepsy. However, in pharmacoresistant epileptic tissue, a higher percentage of slices exhibited population activity, with higher local field potential gradient amplitudes. More intracellularly recorded neurons received depolarizing synaptic potentials, discharging more reliably during the events. Light and electron microscopic examinations showed slightly lower neuron densities and higher densities of excitatory synapses in the human epileptic neocortex. Our data suggest that human neocortical microcircuits retain their functionality and plasticity in vitro, and can generate two significantly different synchronies. We propose that population bursts might not be pathological events while interictal-like discharges may reflect the epileptogenicity of the human cortex. Our results show that hyperexcitability characterizes the human epileptic neocortical network, and that it is closely related to the emergence of synchronies.

The piriform, perirhinal, and entorhinal cortex in seizure generation

Understanding neural network behavior is essential to shed light on epileptogenesis and seizure propagation. The interconnectivity and plasticity of mammalian limbic and neocortical brain regions provide the substrate for the hypersynchrony and hyperexcitability associated with seizure activity. Recurrent unprovoked seizures are the hallmark of epilepsy, and limbic epilepsy is the most common type of medically-intractable focal epilepsy in adolescents and adults that necessitates surgical evaluation. In this review, we describe the role and relationships among the piriform (PIRC), perirhinal (PRC), and entorhinal cortex (ERC) in seizure-generation and epilepsy. The inherent function, anatomy, and histological composition of these cortical regions are discussed. In addition, the neurotransmitters, intrinsic and extrinsic connections, and the interaction of these regions are described. Furthermore, we provide evidence based on clinical research and animal models that suggest that these cortical regions may act as key seizure-trigger zones and, even, epileptogenesis.

Differential modulation of repetitive firing and synchronous network activity in neocortical interneurons by inhibition of A-type K(+) channels and Ih

GABAergic interneurons provide the main source of inhibition in the neocortex and are important in regulating neocortical network activity. In the presence 4-aminopyridine (4-AP), CNQX, and D-APV, large amplitude GABAA-receptor mediated depolarizing responses were observed in the neocortex. GABAergic networks are comprised of several types of interneurons, each with its own protein expression pattern, firing properties, and inhibitory role in network activity. Voltage-gated ion channels, especially A-type K(+) channels, differentially regulate passive membrane properties, action potential (AP) waveform, and repetitive firing properties in interneurons depending on their composition and localization. HCN channels are known modulators of pyramidal cell intrinsic excitability and excitatory network activity. Little information is available regarding how HCN channels functionally modulate excitability of individual interneurons and inhibitory networks. In this study, we examined the effect of 4-AP on intrinsic excitability of fast-spiking basket cells (FS-BCs) and Martinotti cells (MCs). 4-AP increased the duration of APs in both FS-BCs and MCs. The repetitive firing properties of MCs were differentially affected compared to FS-BCs. We also examined the effect of Ih inhibition on synchronous GABAergic depolarizations and synaptic integration of depolarizing IPSPs. ZD 7288 enhanced the amplitude and area of evoked GABAergic responses in both cell types. Similarly, the frequency and area of spontaneous GABAergic depolarizations in both FS-BCs and MCs were increased in presence of ZD 7288. Synaptic integration of IPSPs in MCs was significantly enhanced, but remained unaltered in FS-BCs. These results indicate that 4-AP differentially alters the firing properties of interneurons, suggesting MCs and FS-BCs may have unique roles in GABAergic network synchronization. Enhancement of GABAergic network synchronization by ZD 7288 suggests that HCN channels attenuate inhibitory network activity.

Hippocampus and epilepsy: Findings from human tissues

Surgical removal of the epileptogenic zone provides an effective therapy for several focal epileptic syndromes. This surgery offers the opportunity to study pathological activity in living human tissue for pharmacoresistant partial epilepsy syndromes including temporal lobe epilepsies with hippocampal sclerosis, cortical dysplasias, epilepsies associated with tumors and developmental malformations. Slices of tissue from patients with these syndromes retain functional neuronal networks and may generate epileptic activities. The properties of cells in this tissue may not be greatly changed, but excitatory synaptic transmission is often enhanced and GABAergic inhibition is preserved. Typically epileptic activity is not generated spontaneously by the neocortex, whether dysplastic or not, but can be induced by convulsants. The initiation of ictal discharges in the neocortex depends on both GABAergic signaling and increased extracellular potassium. In contrast, a spontaneous interictal-like activity is generated by tissues from patients with temporal lobe epilepsies associated with hippocampal sclerosis. This activity is initiated, not in the hippocampus but in the subiculum, an output region, which projects to the entorhinal cortex. Interictal events seem to be triggered by GABAergic cells, which paradoxically excite about 20% of subicular pyramidal cells while simultaneously inhibiting the majority. Interictal discharges thus depend on both GABAergic and glutamatergic signaling. The depolarizing effects of GABA depend on a pathological elevation in levels of chloride in some subicular cells, similar to those of developmentally immature cells. Such defect is caused by a perturbed expression of the cotransporters regulating intracellular chloride concentration, the importer NKCC1 and the extruder KCC2. Blockade of NKCC1 actions by the diuretic bumetanide restores intracellular chloride and thus hyperpolarizing GABAergic actions and consequently suppressing interictal activity.

Transgenic mouse lines for non-invasive ratiometric monitoring of intracellular chloride

Chloride is the most abundant physiological anion and participates in a variety of cellular processes including trans-epithelial transport, cell volume regulation, and regulation of electrical excitability. The development of tools to monitor intracellular chloride concentration ([Cli]) is therefore important for the evaluation of cellular function in normal and pathological conditions. Recently, several Cl-sensitive genetically encoded probes have been described which allow for non-invasive monitoring of [Cli]. Here we describe two mouse lines expressing a CFP-YFP-based Cl probe called Cl-Sensor. First, we generated transgenic mice expressing Cl-Sensor under the control of the mouse Thy1 mini promoter. Cl-Sensor exhibited good expression from postnatal day two (P2) in neurons of the hippocampus and cortex, and its level increased strongly during development. Using simultaneous whole-cell monitoring of ionic currents and Cl-dependent fluorescence, we determined that the apparent EC 50 for Cli was 46 mM, indicating that this line is appropriate for measuring neuronal [Cli] in postnatal mice. We also describe a transgenic mouse reporter line for Cre-dependent conditional expression of Cl-Sensor, which was targeted to the Rosa26 locus and by incorporating a strong exogenous promoter induced robust expression upon Cre-mediated recombination. We demonstrate high levels of tissue-specific expression in two different Cre-driver lines targeting cells of the myeloid lineage and peripheral sensory neurons. Using these mice the apparent EC 50 for Cli was estimated to be 61 and 54 mM in macrophages and DRG, respectively. Our data suggest that these mouse lines will be useful models for ratiometric monitoring of Cli in specific cell types in vivo.

GABAergic synchronization in the limbic system and its role in the generation of epileptiform activity

GABA is the main inhibitory neurotransmitter in the adult forebrain, where it activates ionotropic type A and metabotropic type B receptors. Early studies have shown that GABA(A) receptor-mediated inhibition controls neuronal excitability and thus the occurrence of seizures. However, more complex, and at times unexpected, mechanisms of GABAergic signaling have been identified during epileptiform discharges over the last few years. Here, we will review experimental data that point at the paradoxical role played by GABA(A) receptor-mediated mechanisms in synchronizing neuronal networks, and in particular those of limbic structures such as the hippocampus, the entorhinal and perirhinal cortices, or the amygdala. After having summarized the fundamental characteristics of GABA(A) receptor-mediated mechanisms, we will analyze their role in the generation of network oscillations and their contribution to epileptiform synchronization. Whether and how GABA(A) receptors influence the interaction between limbic networks leading to ictogenesis will be also reviewed. Finally, we will consider the role of altered inhibition in the human epileptic brain along with the ability of GABA(A) receptor-mediated conductances to generate synchronous depolarizing events that may lead to ictogenesis in human epileptic disorders as well.

Functional phase response curves: a method for understanding synchronization of adapting neurons

Phase response curves (PRCs) for a single neuron are often used to predict the synchrony of mutually coupled neurons. Previous theoretical work on pulse-coupled oscillators used single-pulse perturbations. We propose an alternate method in which functional PRCs (fPRCs) are generated using a train of pulses applied at a fixed delay after each spike, with the PRC measured when the phasic relationship between the stimulus and the subsequent spike in the neuron has converged. The essential information is the dependence of the recovery time from pulse onset until the next spike as a function of the delay between the previous spike and the onset of the applied pulse. Experimental fPRCs in Aplysia pacemaker neurons were different from single-pulse PRCs, principally due to adaptation. In the biological neuron, convergence to the fully adapted recovery interval was slower at some phases than that at others because the change in the effective intrinsic period due to adaptation changes the effective phase resetting in a way that opposes and slows the effects of adaptation. The fPRCs for two isolated adapting model neurons were used to predict the existence and stability of 1:1 phase-locked network activity when the two neurons were coupled. A stability criterion was derived by linearizing a coupled map based on the fPRC and the existence and stability criteria were successfully tested in two-simulated-neuron networks with reciprocal inhibition or excitation. The fPRC is the first PRC-based tool that can account for adaptation in analyzing networks of neural oscillators.

Synchronous GABA-receptor-dependent potentials in limbic areas of the in-vitro isolated adult guinea pig brain

Epileptiform discharges are known to reflect the hypersynchronous glutamatergic activation of cortical neurons. However, experimental evidence has revealed that epileptiform synchronization is also contributed to by population events mediated by GABA(A) receptors. Here, we analysed the spatial distribution of GABA(A)-receptor-dependent interictal events in the hippocampal/parahippocampal region of the adult guinea pig brain isolated in vitro. We found that arterial perfusion of this preparation with 4-aminopyridine caused the appearance of glutamatergic-independent interictal potentials that were reversibly abolished by GABA(A) receptor antagonism. Laminar profiles and current source density analysis performed in different limbic areas demonstrated that these GABA(A)-receptor-mediated events were independently generated in different areas of the hippocampal/parahippocampal formation (most often in the medial entorhinal cortex) and propagated between interconnected limbic structures of both hemispheres. Finally, intracellular recordings from principal neurons of the medial entorhinal cortex demonstrated that the GABAergic field potential correlated to inhibitory postsynaptic potentials (membrane potential reversal, -68.12 +/- 8.01 mV, n = 5) that were interrupted by ectopic spiking. Our findings demonstrate that, in an acute seizure model developed in the adult guinea pig brain, hypersynchronous GABA(A)-receptor-mediated interictal events are generated from independent sources and propagate within limbic cortices in the absence of excitatory synaptic transmission. As spared or enhanced inhibition was reported in models of epilepsy, our data may support a role of GABA-mediated signaling in ictogenesis and epileptogenesis.

Diminished presynaptic GABA(B) receptor function in the neocortex of a genetic model of absence epilepsy

Changes in GABA(B) receptor subunit expression have been recently reported in the neocortex of epileptic WAG/Rij rats that are genetically prone to experience absence seizures. These alterations may lead to hyperexcitability by downregulating the function of presynaptic GABA(B) receptors in neocortical networks as suggested by a reduction in paired-pulse depression. Here, we tested further this hypothesis by analyzing the effects induced by the GABA(B) receptor agonist baclofen (0.1-10 microM) on the inhibitory events recorded in vitro from neocortical slices obtained from epileptic (>180 day-old) WAG/Rij and age-matched, non-epileptic control (NEC) rats. We found that higher doses of baclofen were required to depress pharmacologically isolated, stimulus-induced IPSPs generated by WAG/Rij neurons as compared to NEC. We also obtained similar evidence by comparing the effects of baclofen on the rate of occurrence of synchronous GABAergic events recorded by WAG/Rij and NEC neocortical slices treated with 4-aminopyridine + glutamatergic receptor antagonists. In conclusion, these data highlight a decreased function of presynaptic GABA(B) receptors in the WAG/Rij rat neocortex. We propose that this alteration may contribute to neocortical hyperexcitability and thus to absence seizures.

Epileptiform synchronization in the cingulate cortex

PURPOSE

The anterior cingulate cortex (ACC)--which plays a role in pain, emotions and behavior--can generate epileptic seizures. To date, little is known on the neuronal mechanisms leading to epileptiform synchronization in this structure. Therefore, we investigated the role of excitatory and inhibitory synaptic transmission in epileptiform activity in this cortical area. In addition, since the ACC presents with a high density of opioid receptors, we studied the effect of opioid agonism on epileptiform synchronization in this brain region.

METHODS

We used field and intracellular recordings in conjunction with pharmacological manipulations to characterize the epileptiform activity generated by the rat ACC in a brain slice preparation.

RESULTS

Bath-application of the convulsant 4-aminopyridine (4AP, 50 microM) induced both brief and prolonged periods of epileptiform synchronization resembling interictal- and ictal-like discharges, respectively. Interictal events could occur more frequently before the onset of ictal activity that was contributed by N-methyl-D-aspartate (NMDA) receptors. Mu-opioid receptor activation abolished 4AP-induced ictal events and markedly reduced the occurrence of the pharmacologically isolated GABAergic synchronous potentials. Ictal discharges were replaced by interictal events during GABAergic antagonism; this GABA-independent activity was influenced by subsequent mu-opioid agonist application.

CONCLUSIONS

Our results indicate that both glutamatergic and GABAergic signaling contribute to epileptiform synchronization leading to the generation of electrographic ictal events in the ACC. In addition, mu-opioid receptors appear to modulate both excitatory and inhibitory mechanisms, thus influencing epileptiform synchronization in the ACC.

NKCC1 and KCC2 prevent hyperexcitability in the mouse hippocampus

During postnatal development of the central nervous system (CNS), the response of GABA(A) receptors to its agonist undergoes maturation from depolarizing to hyperpolarizing. This switch in polarity is due to the developmental decrease of the intracellular Cl concentration in neurons. Here we show that absence of NKCC1 in P9-P13 CA3 pyramidal neurons, through genetic manipulation or through bumetanide inhibition, results in a significant increase in cell excitability. Furthermore, the pro-convulsant agent 4-aminopyridine induces seizure-like events in NKCC1-null mice but not in wild-type mice. Measurements of muscimol responses in the presence and absence of NKCC1 shows that the Na-K-2Cl cotransporter only marginally affects intracellular Cl(-) in P9-P13 CA3 principal neurons. However, large increases in intracellular Cl(-) are observed in CA3 pyramidal neurons following increased hyperexcitability, indicating that P9-P13 CA3 pyramidal neurons lack robust mechanisms to regulate intracellular Cl(-) during high synaptic activity. This increase in the Cl(-) concentration is network-driven and activity-dependent, as it is blocked by the non-NMDA glutamate receptor antagonist DNQX. We also show that expression of the outward K-Cl cotransporter, KCC2, prevents the development of hyperexcitability, as a reduction of KCC2 expression by half results in increased susceptibility to seizure under control and 4-AP conditions.

[Epileptiform activities generated in vitro by human temporal lobe tissue]

Drug-resistant partial epilepsies, including temporal lobe epilepsies with hippocampal sclerosis and cortical dysplasias, offer the opportunity to study human epileptic activity in vitro since the preferred therapy often consists of the surgical removal of the epileptogenic zone. Slices of this tissue retain functional neuronal networks and may generate epileptic activity. The properties of cells in this tissue do not seem to be significantly changed, but excitatory synaptic characteristics are enhanced and GABAergic inhibition is preserved. Typically, epileptic activity is not generated spontaneously by the neocortex, whether dysplastic or not, but can be induced by convulsants. The initiation of ictal discharges in neocortex depends on both GABAergic signaling and increased extracellular potassium. In contrast, a spontaneous interictal-like activity is generated by tissues from patients with temporal lobe epilepsies associated with hippocampal sclerosis. This activity is initiated not in the hippocampus but in the subiculum, an output region that projects to the entorhinal cortex. Interictal events seem to be triggered by GABAergic cells, which paradoxically excite approximately 20% of subicular pyramidal cells, while simultaneously inhibiting the majority. Interictal discharges are therefore sustained by both GABAergic and glutamatergic signaling. The atypical depolarizing effects of GABA depend on a pathological elevation in the basal levels of chloride in some subicular cells, similar to those of developmentally immature cells. This defect is caused by the perturbation of the expression of the cotransporters regulating the intracellular chloride concentration, the importer NKCC1, and the extruder KCC2. Blockade of excessive NKCC1 by the diuretic bumetanide restores intracellular chloride and thus hyperpolarizing GABAergic actions, suppressing interictal activity.

Epileptiform synchronization in the rat insular and perirhinal cortices in vitro

The hippocampus plays a primary role in temporal lobe epilepsy, a common form of partial epilepsy in adults. Recent studies, however, indicate that extrahippocampal areas such as the perirhinal and insular cortices represent important participants in this epileptic disorder. By employing field potential recordings in the in vitro 4-aminopyridine model of temporal lobe epilepsy, we have investigated here the contribution of glutamatergic and GABAergic signaling to epileptiform activity in these structures. First, we provide evidence of epileptiform synchronicity between the perirhinal and insular cortices, and resolve some pharmacological and network mechanisms involved in sustaining the interictal- and ictal-like discharges recorded there. Second, we report that in the absence of ionotropic glutamatergic transmission, GABAergic networks produce synchronous potentials that spread between the perirhinal and insular cortices. Finally, we have established that such activity is modulated by activating micro-opioid receptors. Our findings support clinical and experimental evidence concerning the involvement of the perirhinal and insular cortex networks in temporal lobe epilepsy, and provide observations that may impact research focussing on the role of the insular cortex in nociception.

Methodological approaches to exploring epileptic disorders in the human brain in vitro

Brain surgery, and in particular epilepsy surgery, offers the unique opportunity to study viable human central nervous tissue in vitro. This does not only open a window to address the basic mechanisms underlying human disease, such as epilepsy, but it allows to venture into investigating neurophysiological functions per se. In the present paper, we describe the most commonly used methods in the electrophysiological (and, at least to some extent, also histochemical and molecular) analysis of human tissue in vitro. In addition, we consider the pitfalls and limitations of such studies, in particular regarding the issue of tissue sampling procedures and control experiments.

Developmental changes in somatostatin-positive interneurons in a freeze-lesion model of epilepsy

Somatostatin-expressing (SS) cells are inhibitory interneurons critical to the regulation of excitability in the cerebral cortex. It has been suggested in several animal models of epilepsy that the activity of these neurons reduces the occurrence and strength of epileptiform activity. The physiological properties of SS cells further support these hypotheses. Freeze lesions of neonatal rats serve as a model of human polymicrogyria, which is often characterized by severe seizures. Here we investigate the effects of neonatal freeze lesions on SS-expressing neurons by measuring their densities in control and lesioned hemispheres at two ages. We found that in late juveniles (P30-P32), SS-expressing neurons were depleted by 20% in areas adjacent to the freeze lesion, but at an earlier developmental age (P14-15), there was no significant loss. Since the deficit in SS-expressing neurons occurs well after the onset of epileptiform activity (P12-P18), we conclude that the death of these interneurons does not initiate hyperexcitability in this model.

Effects of gap junction blockers on human neocortical synchronization

Field potentials and intracellular recordings were obtained from human neocortical slices to study the role of gap junctions (GJ) in neuronal network synchronization. First, we examined the effects of GJ blockers (i.e., carbenoxolone, octanol, quinine, and quinidine) on the spontaneous synchronous events (duration = 0.2-1.1 s; intervals of occurrence = 3-27 s) generated by neocortical slices obtained from temporal lobe epileptic patients during application of 4-aminopyridine (4AP, 50 muM) and glutamatergic receptor antagonists. The synchronicity of these potentials (recorded at distances up to 5 mm) was decreased by GJ blockers within 20 min of application, while prolonged GJ blockers treatment at higher doses made them disappear with different time courses. Second, we found that slices from patients with focal cortical dysplasia (FCD) could generate in normal medium spontaneous synchronous discharges (duration = 0.4-8 s; intervals of occurrence = 0.5-90 s) that were (i) abolished by NMDA receptor antagonists and (ii) slowed down by carbenoxolone. Finally, octanol or carbenoxolone blocked 4AP-induced ictal-like discharges (duration = up to 35 s) in FCD slices. These data indicate that GJ play a role in synchronizing human neocortical networks and may implement epileptiform activity in FCD.

Functional Pharmacology in Human Brain

Most neurological and psychiatric disorders involve selective or preferential impairments of neurotransmitter systems. Therefore, studies of functional transmitter pathophysiology in human brain are of unique importance in view of the development of effective, mechanism-based, therapeutic modalities. It is well known that central nervous system functional proteins, including receptors, transporters, ion channels, and enzymes, can exhibit high heterogeneity in terms of structure, function, and pharmacological profile. If the existence of types and subtypes of functional proteins amplifies the possibility of developing selective drugs, such heterogeneity certainly increases the likelihood of interspecies differences. It is therefore essential, before choosing animal models to be used in preclinical pharmacology experimentation, to establish whether functionally corresponding proteins in men and animals also display identical pharmacological profiles. Because of evidence that scaffolding proteins, trafficking between plasma membrane and intracellular pools, phosphorylation and allosteric modulators can affect the function of receptors and transporters, experiments with human clones expressed in host cells where the environment of native receptors is rarely reproduced should be interpreted with caution. Thus, the use of neurosurgically removed fresh human brain tissue samples in which receptors, transporters, ion channels, and enzymes essentially retain their natural environment represents a unique experimental approach to enlarge our understanding of human brain processes and to help in the choice of appropriate animal models. Using this experimental approach, many human brain functional proteins, in particular transmitter receptors, have been characterized in terms of localization, function, and pharmacological properties.

Cellular and molecular mechanisms of epilepsy in the human brain

Animal models have provided invaluable data for identifying the pathogenesis of epileptic disorders. Clearly, the relevance of these experimental findings would be strengthened by the demonstration that similar fundamental mechanisms are at work in the human epileptic brain. Epilepsy surgery has indeed opened the possibility to directly study the functional properties of human brain tissue in vitro, and to analyze the mechanisms underlying seizures and epileptogenesis. Here, we summarize the findings obtained over the last 40 years from electrophysiological, histochemical and molecular experiments made with the human brain tissue. In particular, this review will focus on (i) the synaptic and non-synaptic properties of neocortical neurons along with their ability to produce synchronous activity; (ii) the anatomical and functional alterations that characterize limbic structures in patients presenting with mesial temporal lobe epilepsy; (iii) the issue of antiepileptic drug action and resistance; and (iv) the pathophysiology of seizure genesis in Taylor's type focal cortical dysplasia. Finally, we will address some of the problems that are inherent to this type of experimental approach, in particular the lack of proper controls and possible strategies to obviate this limitation.

Endogenous acetylcholine enhances synchronized interneuron activity in rat neocortex

Application of 4-aminopyridine (4-AP) along with EAA) receptor antagonists produces gamma-aminobutyric acid (GABAA) receptor-dependent synchronized activity in interneurons. This results in waves of activity propagating through upper cortical layers. Because interneurons in the neocortex are excited by nicotinic acetylcholine receptor (nAChR) agonists, ACh may influence synchronization of these local neocortical interneuronal networks. To study this possibility, we have used voltage-sensitive dye imaging using the fluorescent dye RH 414 (30 microM) in rat neocortical slices. Recordings were obtained in the presence of 4-AP (100 microM) and the EAA receptor antagonists D-2-amino-5-phosphonvaleric acid (20 microM) and 6-cyano-7-nitro-quinoxaline-2,3-dione (10 microM). In response to intracortical stimulation, localized or propagated activity restricted to upper cortical layers was seen. Bath application of the ACh esterase inhibitor neostigmine (10 microM) and the nAChR agonist 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP; 10 microM) increased the response amplitude, the extent of spread, and the duration of this activity. These changes were seen in 13 of 16 slices tested with neostigmine (10 microM) and 4 of 7 slices tested with DMPP (10 microM). Application of the muscarinic AChR antagonist atropine (1 microM) did not block the enhancement of activity by neostigmine (n = 7). Application of dihydro-beta-erythroidine (10 microM), known, at this concentration, to selectively antagonize alpha4beta2-like nAChRs, blocked the effect of neostigmine (n = 5). The selective alpha7-like nAChR antagonist methyllycaconitine (50 nM) was ineffective (n = 5). These results suggest that activation of alpha4beta2-like nAChRs by endogenously released ACh can enhance synchronized activity in local neocortical inhibitory networks.

Modification of epileptiform discharges in neocortical neurons following glutamate uptake inhibition

Sodium-dependent high-affinity glutamate transporters regulate synaptic glutamate levels to maintain low ambient levels of glutamate and prevent excitotoxicity. Most studies using pharmacological inhibition of glutamate transport to examine the involvement of glutamate transporters in regulating synaptic activity have examined small synaptic currents. Using in vitro brain slices, we investigated the effects of uptake inhibition on two types of epileptiform activity, bicuculline-induced paroxysmal activity, and epileptiform responses in the freeze-lesion epilepsy model. In layer II/III pyramidal cells of the prefrontal cortex, inhibiting uptake with low concentrations of DL-threo-ss-benzyloxyaspartic acid (TBOA) (20 or 30 microM) prolonged bicuculline-induced epileptiform activity. At higher concentrations, TBOA (150 or 300 microM) caused a transient enhancement of epileptiform discharges that was followed by a decrease. In the freeze-lesion model, inhibiting uptake also increased the amplitude and response area of evoked activity. The prolongation of epileptiform activity exhibited by the inhibition of glutamate uptake (TBOA 20 or 30 microM) is attributed to an increase in the level of glutamate extracellularly during uptake blockade, resulting in sustained activation of glutamate receptors. The decrease in epileptiform activity at higher TBOA concentration could be due to glutamate receptor desensitization or loss of excitability due to a depolarization block. The present results suggest that decreases in glutamate uptake can be proconvulsant in the two models of epilepsy examined.

Horizontal spread of activity in neocortical inhibitory networks

In the presence of 4-aminopyridine (4-AP) and excitatory amino acid receptor blockers, GABAergic networks in the neocortex give rise to large spontaneous GABA-mediated depolarizations. We used voltage-sensitive dye techniques to explore the network properties of depolarizing GABA responses. Voltage-sensitive dye signals demonstrated that the superficial layers support the propagation of depolarizing GABA responses, with only minimal signals detected in deeper cortical layers. GABA responses propagated at a speed of 2.7 +/- 0.2 mm/s, a rate intermediate to fast synaptic transmission and spreading depression. Changes in the extracellular potassium concentration altered the propagation speed of the depolarizing GABA response. Taken together, these data support a role for both direct synaptic action of GABA at GABA(A) receptors and nonsynaptic mechanisms in the generation and propagation of depolarizing GABA responses.

Rat subicular networks gate hippocampal output activity in an in vitro model of limbic seizures

Evidence obtained from human epileptic tissue maintained in vitro indicates that the subiculum may play a crucial role in initiating epileptiform discharges in patients with mesial temporal lobe epilepsy. Hence, we used rat hippocampus-entorhinal cortex (EC) slices to identify the role of subiculum in epileptiform synchronization during bath application of 4-aminopyridine (4AP, 50 microM). In these slices, fast CA3-driven interictal-like events were restricted to the hippocampal CA3/CA1 areas and failed to propagate to the EC where slow interictal-like and ictal-like epileptiform discharges were recorded. However, antagonizing GABA(A) receptors with picrotoxin (50 microM) made CA3-driven interictal activity spread to EC. Sequential field potential analysis along the CA3-CA1-subiculum axis revealed that the amplitude of CA3-driven interictal discharges recorded in the presence of 4AP only diminished within the subiculum. Furthermore, CA1 electrical stimulation under control conditions elicited little or no subicular activation and never any response in EC; in contrast, robust subicular discharges that spread to EC could be evoked after picrotoxin. Intracellular recordings indicated that potentiation by picrotoxin was associated with blockade of hyperpolarizing IPSPs in subicular cells. Finally, when surgically isolated from adjacent structures, the subiculum generated low-amplitude synchronous discharges that corresponded to an intracellular hyperpolarization-depolarization sequence, were resistant to glutamatergic antagonists, and represented the activity of synchronized interneuronal networks. Bath application of picrotoxin abolished these 4AP-induced events and in their place robust network bursting occurred. In conclusion, our study demonstrates that the subiculum plays a powerful gating role on hippocampal output activity. This function depends on GABA(A) receptor-mediated inhibition and controls hippocampal-parahippocampal interactions that are known to modulate limbic seizures.

Ectopic action potential generation in cortical interneurons during synchronized GABA responses

In the presence of 4-aminopyridine and excitatory amino acid receptor antagonists, individual neurons in brain slice preparations exhibit large gamma aminobutyric acid (GABA)-mediated responses as a consequence of synchronous GABA release from a network of interneurons. These synchronized GABA responses are frequently associated with ectopic action potentials (EAPs), which are thought to be action potentials initiated in distal axon terminals which subsequently travel antidromically toward the soma. Ectopic action potentials feature prominently in some models of epilepsy. Neocortical synchronized GABA responses propagate across the cortex, predominantly in superficial layers. The role that EAPs may play in contributing to laminar differences in the synchronized GABA response has not been addressed. Here we examined the occurrence of EAPs during synchronized GABA responses in neurons within layers I and II/III. EAPs occurred in 78% of layer I interneurons and in 25% of layer II/III interneurons (including chandelier cells). EAPs were not observed in layer II/III pyramidal neurons. The prevalence of EAPs in layer I interneurons provides a mechanism by which layer I can support both the initiation and propagation of synchronized GABA responses. Thus, layer I interneurons are a critical component of a network capable of synchronizing a propagating wave of GABA release across the neocortex.

GABAA receptor-dependent synchronization leads to ictogenesis in the human dysplastic cortex

Patients with Taylor's type focal cortical dysplasia (FCD) present with seizures that are often medically intractable. Here, we attempted to identify the cellular and pharmacological mechanisms responsible for this epileptogenic state by using field potential and K+-selective recordings in neocortical slices obtained from epileptic patients with FCD and, for purposes of comparison, with mesial temporal lobe epilepsy (MTLE), an epileptic disorder that, at least in the neocortex, is not characterized by any obvious structural aberration of neuronal networks. Spontaneous epileptiform activity was induced in vitro by applying 4-aminopyridine (4AP)-containing medium. Under these conditions, we could identify in FCD slices a close temporal relationship between ictal activity onset and the occurrence of slow interictal-like events that were mainly contributed by GABAA receptor activation. We also found that in FCD slices, pharmacological procedures capable of decreasing or increasing GABAA receptor function abolished or potentiated ictal discharges, respectively. In addition, the initiation of ictal events in FCD tissue coincided with the occurrence of GABAA receptor-dependent interictal events leading to [K+]o elevations that were larger than those seen during the interictal period. Finally, by testing the effects induced by baclofen on epileptiform events generated by FCD and MTLE slices, we discovered that the function of GABAB receptors (presumably located at presynaptic inhibitory terminals) was markedly decreased in FCD tissue. Thus, epileptiform synchronization leading to in vitro ictal activity in the human FCD tissue is initiated by a synchronizing mechanism that paradoxically relies on GABAA receptor activation causing sizeable increases in [K+]o. This mechanism may be facilitated by the decreased ability of GABAB receptors to control GABA release from interneuron terminals.

Glutamate transporters regulate excitability in local networks in rat neocortex

Excitatory postsynaptic currents (EPSCs) in the neocortex are principally mediated by glutamate receptors. Termination of excitation requires rapid removal of glutamate from the synaptic cleft following release. Glutamate transporters are involved in EPSC termination but the effect of uptake inhibition on excitatory neurotransmission varies by brain region. Epileptiform activity is largely mediated by a synchronous synaptic activation of cells in local cortical circuits, presumably associated with a large release of glutamate. The role of glutamate transporters in regulating epileptiform activity has not been addressed. Here we examine the effect of glutamate transport inhibition on EPSCs and epileptiform events in layer II/III pyramidal cells in rat neocortex. Inhibiting glutamate transporters with DL-threo-beta-benzyloxyaspartic acid (TBOA; 30 microM) had no effect on the amplitude or decay time of evoked, presumably alpha-amino-3-hydroxyl-5-methyl-isoxazolepropionic acid-mediated, EPSCs. In contrast, the amplitude and duration of epileptiform discharges were significantly enhanced. TBOA resulted also in a decreased threshold for evoking epileptiform activity and an increased probability of occurrence of spontaneous epileptiform discharges. TBOA's effects were not inhibited by the group I and II metabotropic glutamate receptors antagonist (S)-alpha-methyl-4-carboxyphenylglycine or the kainate receptor antagonist [(3S,4aR, 6S, 8aR)-6-((4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid]. D-(-)-2-amino-5-phosphonovaleric acid could both prevent excitability changes by TBOA and block already induced changes. Dihydrokainate (300 microM) had effects similar to TBOA suggesting involvement of the glial transporter GLT-1. Inhibiting glutamate transport increases local network excitability under conditions where there is an enhanced release of glutamate. Our results indicate that uptake inhibition produces an elevation of extracellular glutamate levels and activation of N-methyl-D-aspartate receptors.

Synaptic and non-synaptic mechanisms of amygdala recruitment into temporolimbic epileptiform activities

Lateral amygdala (LA) activity during synchronized-epileptiform discharges in temporolimbic circuits was investigated in rat horizontal slices containing the amygdala, hippocampus (Hip), perirhinal (Prh) and lateral entorhinal (LEnt) cortex, through multiple-site extra- and intracellular recording techniques and measurement of the extracellular K+ concentration. Application of 4-aminopyridine (50 microm) induced epileptiform discharges in all regions under study. Slow interictal-like burst discharges persisted in the Prh/LEnt/LA after disconnection of the Hip, seemed to originate in the Prh as shown from time delay analyses, and often preceded the onset of ictal-like activity. Disconnection of the amygdala resulted in de-synchronization of epileptiform discharges in the LA from those in the Prh/LEnt. Interictal-like activity was intracellularly reflected in LA projection neurons as gamma-aminobutyric acid (GABA)A/B receptor-mediated synaptic responses, and depolarizing electrogenic events (spikelets) residing on the initial phase of the GABA response. Spikelets were considered antidromically conducted ectopic action potentials generated at axon terminals, as they were graded in amplitude, were not abolished through hyperpolarizing membrane responses (which effectively blocked evoked orthodromic action potentials), lacked a clear prepotential or synaptic potential, were not affected through blockers of gap junctions, and were blocked through remote application of tetrodotoxin at putative target areas of LA projection neurons. Remote application of a GABAB receptor antagonist facilitated spikelet generation. A transient elevation in the extracellular K+ level averaging 3 mm above baseline occurred in conjunction with interictal-like activity in all areas under study. We conclude that interictal-like discharges in the LA/LEnt/Prh spread in a predictable manner through the synaptic network with the Prh playing a leading role. The rise in extracellular K+ may provide a depolarizing mechanism for recruitment of interneurons and generation of ectopic action potentials at axon terminals of LA projection neurons. Antidromically conducted ectopic action potentials may provide a spreading mechanism of seizure activity mediated by diffuse axonal projections of LA neurons.

GABAC receptors in the rat superior colliculus and pretectum participate in synaptic neurotransmission

In mammals, GABA(C) receptors seem to be specifically expressed in the retina and the subcortical visual system, with highest extraretinal expression levels in the superior colliculus (SC). Although its presence in the superficial SC has been demonstrated physiologically, a direct involvement of this receptor type in fast synaptic neurotransmission still awaits verification. We addressed the question of a possible synaptic localization of GABA(C) receptors by performing in vitro whole-cell patch-clamp recordings of inhibitory postsynaptic currents (IPSCs) in single neurons of the rat SC and the neighboring pretectal nuclear complex, where GABA(C) receptors are also expressed at significant levels. To increase the likelihood to record IPSCs we induced spontaneous activity by application of the potassium channel blocker 4-aminopyridine (4-AP) and blocked glutamate-mediated excitatory neurotransmission with kynurenic acid. All 4-AP-induced postsynaptic currents were of synaptic origin because they were completely suppressed by lidocaine or by substitution of extracellular calcium with cobalt. In 40% of the SC cells and in 60% of the pretectal neurons, IPSCs in the presence of 4-AP and kynurenic acid were only partly blocked by the selective GABA(A) receptor antagonist bicuculline. Inhibitory currents that were insensitive to bicuculline, however, could be blocked by coapplication of either the specific GABA(C) receptor antagonist 1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid or picrotoxin, an unselective GABA(A) and GABA(C) receptor antagonist. We conclude that GABA(C) receptors are, at least partially, located synaptically in SC and pretectal neurons in the rat, which indicates a direct function of this receptor type for synaptic processing in both structures.

Laminar properties of 4-aminopyridine-induced synchronous network activities in rat neocortex

We examined the effects of 4-aminopyridine (4-AP) on isolated horizontal (superficial, middle and deep) rat neocortical slices in order to study laminar synchronous network behavior directly. Application of 4-AP induced spontaneous synchronized activity in all of these types of slices. In middle and deep layer slices the activities were similar to those of coronal slices, consisting of periodic short- and long-duration discharges. In superficial slices distinct spontaneous rhythmic multiphasic burst discharges were induced. Ionotropic glutamate receptor antagonists blocked the 4-AP-induced synchronous activities in middle and deep layer slices, but those in superficial slices persisted. The GABA(A) receptor antagonist picrotoxin suppressed this spontaneous synchronous activity resistant to 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (a NMDA receptor antagonist) and 6-cyano-7-nitroquinoxaline-2,3-dione (a non-NMDA receptor antagonist), in superficial slices, leaving small, slow spontaneous events. In superficial slices with intact excitatory amino acid transmission, picrotoxin attenuated the 4-AP-induced spontaneous synchronous discharges, even in this highly convulsant environment. By contrast, conventional coronal slices showed robust spontaneous epileptiform discharges under these circumstances. In intact coronal slices focal 4-AP application in superficial layers induced spontaneous inhibitory GABAergic events, while delivery into deep layers led to epileptiform discharges. From these results we conclude that: (1) 4-AP-induced population discharges are driven by glutamatergic transmission in middle and deep layer horizontal slices, and by GABAergic transmission in superficial layers; (2) only superficial layers are capable of supporting synchronized GABAergic activity independent of excitatory amino acid transmission; (3) superficial layers do not sustain epileptiform activity in the absence of deep layer neurons; and (4) synchronized superficial networks can inhibit deep layer neuronal activity.

GABA-mediated synchronization in the human neocortex: elevations in extracellular potassium and presynaptic mechanisms

Field potential and extracellular [K(+)] ([K(+)](o)) recordings were made in the human neocortex in an in vitro slice preparation to study the synchronous activity that occurs in the presence of 4-aminopyridine (50 microM) and ionotropic excitatory amino acid receptor antagonists. Under these experimental conditions, negative or negative-positive field potentials accompanied by rises in [K(+)](o) (up to 4.1 mM from a baseline of 3.25 mM) occurred spontaneously at intervals of 3-27 s. Both field potentials and [K(+)](o) elevations were largest at approximately 1000 microm from the pia. Similar events were induced by neocortical electrical stimuli. Application of medium containing low [Ca(2+)]/high [Mg(2+)] (n=3 slices), antagonism of the GABA(A) receptor (n=7) or mu-opioid receptor activation (n=4) abolished these events. Hence, they represented network, GABA-mediated potentials mainly reflecting the activation of type A receptors following GABA release from interneurons. The GABA(B) receptor agonist baclofen (10-100 microM, n=11) reduced and abolished the GABA-mediated potentials (ID(50)=18 microM). Baclofen effects were antagonized by the GABA(B) receptor antagonist CGP 35348 (0.1-1 mM, n=6; ID(50)=0.19 mM). CGP 38345 application to control medium increased the amplitude of the GABA-mediated potentials and the concomitant [K(+)](o) rises without modifying their rate of occurrence. The GABA-mediated potentials were not influenced by the broad-spectrum metabotropic glutamate agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (100 microM, n=10), but decreased in rate with the group I receptor agonist (S)-3,5-dihydroxyphenylglycine (10-100 microM, n=9). Our data indicate that human neocortical networks challenged with 4-aminopyridine generate glutamatergic-independent, GABA-mediated potentials that are modulated by mu-opioid and GABA(B) receptors presumably located on interneuron terminals. These events are associated with [K(+)](o) elevations that may contribute to interneuron synchronization in the absence of ionotropic excitatory synaptic transmission.

Immature neocortical neurons exist as extensive syncitial networks linked by dendrodendritic electrical connections

The properties of immature cortex that may enable it to exhibit large-scale wavelike activity during a brief critical developmental period were investigated by imaging neuronal calcium signals in neonatal cortical slices under conditions of artificially enhanced excitability, conditions that produce a more frequent and robust version of the naturally occurring waves. Using pharmacological manipulation to probe the underlying mechanisms, I show that waves can propagate effectively when excitatory synaptic transmission is blocked. In contrast, propagation is very sensitive to reductions in gap junctional communication. In the barrel field cortex wave propagation is affected by the underlying cytoarchitecture in a way that is consistent with a role for dendrodendritic gap junctions. The ability of cortex to sustain wave activity ends around postnatal day 12, precisely when a major reduction in neuronal gap junctions takes place in cortex. These results suggest that in immature cortex gap junctions link neurons into extensive networks that may allow electrical activity to spread over long distances.

Mechanisms underlying partial (focal, or lesional) epilepsy

Neuronal mechanisms underlying focal convulsions and secondary generalised convulsions have been extensively investigated at many different levels, from the biochemical, through the cellular to the intact brain. Numerous pathogenic processes relevant to epilepsy are now known. Experimental models suggest that alterations to the shape, connectivity and receptor-chemistry of individual neurons increase their excitability and the resultant increases in activity lead to changes of the local ionic environment that further causes enhanced excitability. As a consequence of the latter, neuronal firing rates increase and there is a shift from single-firing- to burst-firing-behaviours in populations of neurons and, possibly, also several changes in the way populations of neurons communicate, namely, from 'synaptic' to 'electric' and from orthodromic to antidromic. As a consequence, massive neuronal synchronisation occurs, the correlate of the focal or secondarily generalised attack. Because of experimental difficulties in studying the widespread and evolving neuronal activities in freely behaving animals, a precise correlation between states of neuronal activation and convulsive behaviour is still unclear.

Epileptiform discharges in the human dysplastic neocortex: in vitro physiology and pharmacology

Field potential and intracellular recordings were made in slices of human neocortical tissue obtained during surgery for the treatment of seizures associated with focal cortical dysplasia. Ictal-like epileptiform discharges, along with isolated field potentials, were induced by bath application of 4-aminopyridine (50-100 microM). Some of the isolated field potentials were associated with fast transients representing population spikes. Field potential profile analysis indicated that both types of synchronous activity had maximal negative values at 1,400 to 1,600 microm from the pia. The intracellular counterpart of the ictal-like discharge was a prolonged membrane depolarization capped by repetitive action potential burst firing. By contrast, the isolated field potentials were mirrored by long-lasting depolarizations with minimal action potential firing; only when population spikes occurred, the isolated field potentials were associated with epileptiform action potential bursting. Ictal-like discharges were abolished by either N-methyl-D-aspartate or non-N-methyl-D-aspartate receptor antagonists. In contrast, the isolated field potentials continued to occur synchronously during excitatory transmission blockade (although they lacked fast transients) but were abolished by the gamma-aminobutyric acid(A) receptor antagonist bicuculline methiodide (n = 2 slices). Our study demonstrates that focal cortical dysplasia tissue maintained in vitro has an intrinsic ability to generate ictal-like epileptiform events when challenged with 4-aminopyridine. These discharges depend on excitatory amino acid receptor-mediated mechanisms. Our results also show the presence in focal cortical dysplasia tissue of glutamatergic-independent synchronous potentials that are mainly contributed by gamma-aminobutyric acid(A) receptor-mediated conductances.

Excitability changes in freeze-induced neocortical microgyria

A freezing probe was placed on the skull of postnatal day (PN) 1 rats to induce formation of a cerebrocortical microsulcus. Experimental studies were performed on PN days 21-24. At that time point, Nissl-stained sections revealed the presence of a microsulcus similar to that described in human dysplastic cortex. Immunocytochemical staining for parvalbumin, calretinin and calbindin indicated a significant decrease in the number of immunoreactive neurons within the microsulcus and non-significant decreases in regions adjacent to the microsulcus. Staining for the glial markers GFAP and vimentin was increased near the microsulcus. Using in vitro brain slices, recordings were made in cortex adjacent to the microsulcus. Epileptiform activity was observed in response to electrical stimulation near the microsulcus. Analysis of the voltage dependence of evoked epileptiform discharges suggested the presence of an inhibitory component. As previously observed in non-lesioned animals, bath application of 4-aminopyridine induced bicuculline-sensitive spontaneous burst discharges in the presence of excitatory amino acid antagonists. These results suggest that cortical freeze lesions associated with abnormal neuronal migration produce a chronic hyperexcitable state. The findings are consistent with a mechanism involving an alteration, not loss, of inhibition in this model.

Function and organization in dysgenic cortex. Case report

Cerebral dysgenesis is a subject of interest because of its relationship to cerebral development and dysfunction and to epilepsy. The authors present a detailed study of a 16-year-old boy who underwent surgery for a severe seizure disorder. This patient had dysgenesis of the right hemisphere, which was composed of a giant central frontoparietal nodular gray matter heterotopia with overlying large islands of cortical dysplasia around a displaced central fissure. Exceptional insight into the function, biochemistry, electrophysiology, and histological structure of this lesion was obtained from neurological studies that revealed complementary information: magnetic resonance (MR) imaging, [18]fluoro-2-deoxy-D-glucose positron emission tomography (PET), functional PET scanning, proton MR spectroscopic (1H-MRS) imaging, intraoperative cortical mapping and electrocorticography, in vitro electrophysiology, and immunocytochemistry. These studies demonstrated compensatory cortical reorganization and showed that large areas of heterotopia and cortical dysplasia in the central area may retain normal motor and sensory function despite strikingly altered cytoarchitectonic organization and neuronal metabolism. Such lesions necessitate appropriate functional imaging studies prior to surgery and cortical mapping to avoid creating neurological deficits. Integrated studies, such as PET, 1H-MRS imaging, cortical mapping, immunocytochemistry, and electrophysiology may provide information on the function of developmental disorders of cerebral organization.

Recruitment of GABAergic inhibition and synchronization of inhibitory interneurons in rat neocortex

Intracellular recordings were obtained from pyramidal and interneuronal cells in rat neocortical slices to examine the recruitment of GABAergic inhibition and inhibitory interneurons. In the presence of the convulsant agent 4-aminopyridine (4-AP), after excitatory amino acid (EAA) ionotropic transmission was blocked, large-amplitude triphasic inhibitory postsynaptic potentials (IPSPs) occurred rhythmically (every 10-40 s) and synchronously in pyramidal neurons. After exposure to the gamma-aminobutyric acid-A (GABA(A)) receptor antagonist picrotoxin, large-amplitude monophasic slow IPSPs persisted in these cells. In the presence of 4-AP and EAA blockers, interneurons showed periodic spike firing. Although some spikes rode on an underlying synaptic depolarization, much of the rhythmic firing consisted of spikes having highly variable amplitudes, arising abruptly from baseline, even during hyperpolarization. The spike firing and depolarizing synaptic potentials were completely suppressed by picrotoxin exposure, although monophasic slow IPSPs persisted in interneurons. This suggests that this subset of interneurons may participate in generating fast GABA(A) IPSPs, but not slow GABA(B) IPSPs. Cell morphology was confirmed by intracellular injection of neurobiotin or the fluorescent dye Lucifer yellow CH. Dye injection into interneurons often (>70%) resulted in the labeling of two to six cells (dye coupling). These findings suggest that GABA(A)ergic neurons may be synchronized via recurrent collaterals through the depolarizing action of synaptically activated GABA(A) receptors and a mechanism involving electrotonic coupling. Although inhibitory neurons mediating GABA(B) IPSPs may be entrained by the excitatory GABA(A) mechanism, they appear to be a separate subset of GABAergic neurons capable of functioning independently with autonomous pacing.

Tigabine hydrochloride, an inhibitor of gamma-aminobutyric acid (GABA) uptake, induces cortical depolarizations in vitro

The effect of the gamma-aminobutyric acid uptake inhibitor tiagabine hydrochloride was studied on electrical responses in cortical wedges prepared from 20-30 day-old, audiogenic seizure-prone DBA/2 mice. Perfusion of tiagabine (50 microM) for 15 min, evoked large, slow depolarizations with a frequency of 6-8/h which persisted for 4-5 h. The GABA(A) receptor antagonists, bicuculline (10 microM) and picrotoxin (100 microM), inhibited established depolarizations. These depolarizations were also calcium-dependent and blocked by tetrodotoxin. The non-opioid antitussive, dextromethorphan, which has been shown to inhibit glutamate release, irreversibly blocked the depolarizations. Conversely, 4-aminopyridine (50 microM), a potassium channel antagonist, markedly potentiated the responses. The NMDA receptor antagonist, 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid, had no effect on the depolarizations at concentrations up to 100 microM but the AMPA/kainate receptor antagonist, 6,7-dinitroquinoxaline-2.3-dione at high concentrations (100 and 200 microM), reversibly decreased the frequency without affecting the amplitude. It is concluded that the tiagabine-induced depolarizations in this in vitro preparation were initiated through GABA(A) receptors leading, possibly, to a release of excitatory amino acids.

GABA-mediated synchronous potentials and seizure generation

This article summarizes findings related to a synchronous, GABA-mediated potential that may contribute to the initiation and spread of epileptiform discharges within the brain. This phenomenon is observed in cortical structures such as the hippocampus, the entorhinal cortex, and the neocortex during application of low concentrations of 4-aminopyridine and is characterized at the intracellular level by a long-lasting membrane depolarization. The synchronous, GABA-mediated potential continues to occur after blockade of excitatory synaptic transmission and relays on the synchronous firing of inhibitory interneurons and consequent activation of postsynaptic (mainly type A) GABA receptors leading to a transient elevation of [K+]O. Studies performed in young rat hippocampus indicate that the synchronous, GABA-mediated potential may play a role in initiating ictal discharges under normal conditions (i.e., when excitatory amino acid receptors are operant). Moreover, a similar phenomenon may also occur in adult rat entorhinal cortex. These findings therefore indicate a novel role that is played by GABAA receptors in limbic structures. The ability of this synchronous GABA-mediated potential to propagate in the absence of excitatory synaptic transmission may also be relevant for the propagation of synchronous activity outside conventional neuronal-synapse dependent pathways. This condition may occur in brain structures with neuronal loss and consequent disruption of normal excitatory synaptic connections such as mesial limbic structures of temporal lobe epilepsy patients with Ammon's horn sclerosis.

Synchronization of rat hippocampal neurons in the absence of excitatory amino acid-mediated transmission

Extracellular and intracellular recordings and measurements of extracellular K+ concentration ([K+]o) were performed in the adult rat hippocampus in an in vitro slice preparation. Excitatory amino acid receptor antagonists, as well as the K(+)-channel blockers 4-aminopyridine (4AP, 50 microM) and/or tetraethylammonium (TEA, 5 mM), were added to the bath. Synchronous, negative-going field potentials were recorded in the CA3 stratum radiatum during application of 4AP and excitatory amino acid receptor antagonists. Each of these events was associated with an intracellular long-lasting depolarization and a concomitant rise in [K+]o that attained peak values of 4.3 +/- 0.1 mM (mean +/- S.E.M., n = 6 slices) and lasted 29 +/- 3 s. These field potentials were still recorded in CA3 stratum radiatum after addition of TEA. Under these conditions, prolonged field potentials (40.2 +/- 4.5 s, n = 18) characterized by a prominent positive component; discharge of population spikes also occurred. [K+]o increases associated with these prolonged field-potential discharges had a considerable variability in magnitude (peak value = 3.8-14.1 mM, 6.1 +/- 0.7 mM, n = 5) and duration (14-210 s; 48 +/- 13 s, n = 5). In 8% of the cases spreading depression-like episodes were observed. [K+]o increases during spreading depression-like episodes attained peak values of 11-27 mM (22.8 +/- 0.2 mM, n = 2) and had a duration of 160-396 s (244 +/- 29 s, n = 2). All types of synchronous activity were abolished by the GABAA-receptor antagonist bicuculline methiodide (10 microM) (n = 11). A similar effect was obtained by applying Ca(2+)-free/high-Mg2+ medium (n = 5). Simultaneous field-potential recordings in CA3, CA1, dentate area and subiculum demonstrated that negative-going potentials and prolonged field-potential discharges occurred in all areas in a synchronous fashion. Spreading depression-like episodes were more frequently recorded in the CA1 than in the CA3 area and were not seen in the subiculum or dentate area. These experiments indicate that a glutamatergic-independent, synchronous GABA-mediated potential which is elicited by 4AP in the adult rat hippocampus continues to occur in the presence of TEA. In addition, concomitant application of these K(+)-channel blockers induces a novel type of prolonged field-potential discharge as well as spreading depression-like episodes. Since all synchronous potentials (including spreading depression-like episodes) were abolished by bicuculline methiodide, we conclude that their occurrence is presumably dependent upon the post-synaptic activation of GABAA receptors located on neuronal and glial elements. As excitatory synaptic transmission was nominally blocked under our experimental conditions, we also propose that rises in [K+]o and consequent redistribution processes are per se sufficient to make all types of synchronous activity propagate.

Synchronous GABA-mediated potentials and epileptiform discharges in the rat limbic system in vitro

Application of 4-aminopyridine (4AP, 50 microM) to combined slices of adult rat hippocampus-entorhinal cortex-induced ictal and interictal epileptiform discharges, as well as slow field potentials that were abolished by the mu-opioid agonist [D-Ala2,N-Me-Phe4,Gly-ol5] enkephalin (DAGO, 10 microM) or the GABAA receptor antagonist bicuculline methiodide (BMI, 10 microM); hence, they represented synchronous GABA-mediated potentials. Ictal discharges originated in the entorhinal cortex and propagated to the hippocampus, whereas interictal activity of CA3 origin was usually recorded in the hippocampus. The GABA-mediated potentials had no fixed site of origin or modality of propagation; they closely preceded (0.2-5 sec) and thus appeared to initiate ictal discharges. Only ictal discharges were blocked by the antagonist of the NMDA receptor 3,3-(2-carboxypiperazine-4-yl)propyl-1-phosphonate (CPP, 10 microM), whereas the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) abolished all epileptiform activities. The GABA-mediated potentials continued to occur synchronously in all regions even after concomitant application of CNQX and CPP. [K+]o elevations were recorded in the entorhinal cortex during the ictal discharge (peak values = 13.9 +/- 0.9 mM) and the synchronous GABA-mediated potentials (peak values = 4.2 +/- 0.1 mM); the latter increases were presumably attributable to postsynaptic GABAa-receptor activation because they were abolished by DAGO or BMI. Their role in initiating ictal activity was demonstrated by using DAGO, which abolished both GABA-mediated synchronous potentials and ictal discharges. These data indicate that NMDA-mediated ictal discharges induced by 4AP originate in the entorhinal cortex; such a conclusion is in line with clinical evidence obtained in temporal lobe epilepsy patients. 4AP also induces GABA-mediated potentials that spread within the limbic system when excitatory transmission is blocked and may play a role in initiating ictal discharge by increasing [K+]o.

Reverberation of chloride-dependent synaptic potentials in the rat entorhinal cortex in vitro

The spontaneous activity generated by rat entorhinal neurons during application of 4-aminopyridine (4AP; 50 microM) was studied with intracellular and extracellular field-potential recordings in an vitro slice preparation. Long-lasting depolarizations (LLDs) with amplitudes of 15 +/- 7.6 mV (mean +/- SD; n = 14) and durations of 1.65 +/- 0.77 s (n = 14) occurred at 0.036 +/- 0.01/s (n = 14). Each LLD was followed by a rhythmic sequence of depolarizing potentials (up to 22 events) with amplitudes of 4-30 mV, durations of 40-500 ms and frequency of 0.9 +/- 0.2/s (n = 14). These intracellular potentials were mirrored by negative-going field potentials, suggesting that they represented synchronous events. Membrane input resistance decreased by 79-86% during both LLDs and subsequent rhythmic depolarizations. Intracellular injection of steady depolarizing or hyperpolarizing current modified the amplitude of these potentials in a similar manner: the reversal potential of the LLDs and of the rhythmic depolarizations was -66.4 +/- 4 mV and -67.9 +/- 3.2 mV, respectively (n = 7). Intracellular injection of Cl- increased the amplitude of both types of potentials. Spontaneous LLDs continued to occur during application of the non-N-methyl-D-aspartate (NMDA) receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (10 microM), a procedure that abolished the subsequent rhythmic depolarizations (n = 3). LLDs were blocked by further addition of the gamma-aminobutyric acid (GABA)A receptor antagonist bicuculline methiodide (10 microM, n = 3). Our findings demonstrate that during 4AP application entorhinal neurons generate glutamatergic-independent LLDs as well as synchronous, Cl(-)-dependent depolarizations that reverberate through non-NMDA-mediated excitatory circuits.

Ischemia and lesion induced imbalances in cortical function

Cortical structures are often critically affected by ischemic and traumatic lesions which may cause transient or permanent functional disturbances. These disorders consist of changes in the membrane properties of single cells and alterations in synaptic network interactions within and between cortical areas including large-scale reorganizations in the representation of the peripheral input. Prominent functional modifications consisting of massive membrane depolarizations, suppression of intracortical inhibitory synaptic mechanisms and enhancement of excitatory synaptic transmission can be observed within a few minutes following the onset of cortical hypoxia or ischemia and probably represent the trigger signals for the induction of neuronal hyperexcitability, irreversible cellular dysfunction and cell death. Pharmacological manipulation of these early events may therefore be the most effective approach to control ischemia and lesion induced disturbances and to attenuate long-term neurological deficits. The complexity of secondary structural and functional alterations in cortical and subcortical structures demands an early and powerful intervention before neuronal damage expands to intact regions. The unsatisfactory clinical experience with calcium and N-methyl-D-aspartate antagonists suggests that this result might be achieved with compounds that show a broad spectrum of actions at different ligand-activated receptors, voltage-dependent channels and that also act at the vascular system. Whether the same therapy strategies developed for the treatment of ischemic injury in the adult brain may be applied for the immature cortex is questionable, since young cortical networks with a high degree of synaptic plasticity reveal a different response pattern to hypoxic and ischemic insults. Age-dependent molecular biological, morphological and physiological parameters contribute to an enhanced susceptibility of the immature brain to these noxae during early ontogenesis and have to be investigated in more detail for the development of adequate clinical therapy.