Excitatory synaptic transmission mediated by NMDA and non-NMDA receptors in the superficial/middle layers of the epileptogenic human neocortex maintained in vitro

Synaptic alterations and neuronal firing in human epileptic neocortical excitatory networks

Epilepsy is a prevalent neurological condition, with underlying neuronal mechanisms involving hyperexcitability and hypersynchrony. Imbalance between excitatory and inhibitory circuits, as well as histological reorganization are relatively well-documented in animal models or even in the human hippocampus, but less is known about human neocortical epileptic activity. Our knowledge about changes in the excitatory signaling is especially scarce, compared to that about the inhibitory cell population. This study investigated the firing properties of single neurons in the human neocortex in vitro, during pharmacological blockade of glutamate receptors, and additionally evaluated anatomical changes in the excitatory circuit in tissue samples from epileptic and non-epileptic patients. Both epileptic and non-epileptic tissues exhibited spontaneous population activity (SPA), NMDA receptor antagonization reduced SPA recurrence only in epileptic tissue, whereas further blockade of AMPA/kainate receptors reversibly abolished SPA emergence regardless of epilepsy. Firing rates did not significantly change in excitatory principal cells and inhibitory interneurons during pharmacological experiments. Granular layer (L4) neurons showed an increased firing rate in epileptic compared to non-epileptic tissue. The burstiness of neurons remained unchanged, except for that of inhibitory cells in epileptic recordings, which decreased during blockade of glutamate receptors. Crosscorrelograms computed from single neuron discharge revealed both mono- and polysynaptic connections, particularly involving intrinsically bursting principal cells. Histological investigations found similar densities of SMI-32-immunopositive long-range projecting pyramidal cells in both groups, and shorter excitatory synaptic active zones with a higher proportion of perforated synapses in the epileptic group. These findings provide insights into epileptic modifications from the perspective of the excitatory system and highlight discrete alterations in firing patterns and synaptic structure. Our data suggest that NMDA-dependent glutamatergic signaling, as well as the excitatory synaptic machinery are perturbed in epilepsy, which might contribute to epileptic activity in the human neocortex.

Valeriana officinalis Root Extract Modulates Cortical Excitatory Circuits in Humans

BACKGROUND

Valeriana officinalis extract (VE) is a popular herbal medicine used for the treatment of anxiety and sleep disorders. Although the anxiolytic and sedative effects are mainly attributed to the modulation of GABA-ergic transmission, the mechanism of action has not been fully investigated in humans. Noninvasive brain stimulation protocols can be used to elucidate the mechanisms of action of psychoactive substances at the cortical level in humans. In this study, we investigated the effects of a single dose of VE on cortical excitability as assessed with transcranial magnetic stimulation (TMS).

METHODS

Fifteen healthy volunteers participated in a double-blind, randomized, cross-over, placebo-controlled study. Subjects were required to take either 900 mg of VE (valerenic acid 0.8%) or placebo (an equal dose of vitamin E). Motor cortex excitability was studied by single and paired TMS before and at 1 h and 6 h after the oral administration. Cortical excitability was assessed using different TMS parameters: resting motor threshold, motor-evoked potential amplitude, cortical silent period, short-interval intracortical inhibition, and intracortical facilitation. Furthermore, we assessed sensorimotor integration by short-latency and long-latency afferent inhibition.

RESULTS

We found a significant reduction in ICF, without any significant changes in other TMS measures of motor cortex excitability. The amount of ICF returned to baseline value 6 h after the intake of the VE.

CONCLUSION

A single oral dose of VE modulates intracortical facilitatory circuits. Our results in healthy subjects could be predictive markers of treatment response in patients and further support the use of pharmaco-TMS to investigate the neuropsychiatric effects of herbal therapies in humans.

Profile of perampanel and its potential in the treatment of partial onset seizures

Perampanel (PER) is a novel antiepileptic compound that decreases neuronal excitability by modulating glutamatergic transmission through selective noncompetitive blockade of AMPA receptors. PER has been evaluated in three pivotal placebo-controlled randomized trials as adjunctive therapy in adult drug-resistant partial epilepsy. In comparison to placebo, adjunctive PER effectively reduces seizure frequency. The relative risk of the responder rate (95% confidence interval [CI]) was thus 1.60 (1.08-2.36), 1.79 (1.42-2.25) and 1.66 (1.24-2.23) for once-daily PER 4 mg/day, 8 mg/day and 12 mg/day, respectively. The most common adverse events associated with PER were nonspecific central nervous system side effects. Some concerns have been raised about risk of clinically significant weight gain and of psychiatric adverse events. Long-term open-label extensions of the three pivotal trials are underway. PER has recently been approved both in Europe and in the USA for the adjunctive treatment of partial onset seizures in patients aged 12 years and above. However, in the absence of a direct comparison between PER and other licensed antiepileptic drugs' efficacy and tolerability, the clinical advantages of PER over the other drugs in intractable partial epilepsy remains to be determined.

Revisiting AMPA receptors as an antiepileptic drug target

In the 1990s there was intense interest in ionotropic glutamate receptors as therapeutic targets for diverse neurological disorders, including epilepsy. NMDA receptors were thought to play a key role in the generation of seizures, leading to clinical studies of NMDA receptor blocking drugs in epilepsy. Disappointing results dampened enthusiasm for ionotropic glutamate receptors as a therapeutic target. Eventually it became appreciated that another type of ionotropic glutamate receptor, the AMPA receptor, is actually the predominant mediator of excitatory neurotransmission in the central nervous system and moreover that AMPA receptors are critical to the generation and spread of epileptic activity. As drugs became available that selectively target AMPA receptors, it was possible to demonstrate that AMPA receptor antagonists have powerful antiseizure activity in in vitro and in vivo models. A decade later, promising clinical studies with AMPA receptor antagonists, including the potent noncompetitive antagonist perampanel, are once again focusing attention on AMPA receptors as a drug target for epilepsy therapy.

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.

Intrinsic excitability, synaptic potentials, and short-term plasticity in human epileptic neocortex

Although studies of epileptic human hippocampus suggest changes of synaptic and intrinsic excitability, few changes, save the appearance of spontaneous field/synaptic potentials, are known in epileptic neocortical tissue. However, invasive EEG and histological studies suggest that neocortical tissue, even in mesial temporal lobe epilepsy, can play an important role as an irritative zone or extrahippocampal focus. We hypothesized that intrinsic neuronal and synaptic excitability, as well as short-term plasticity, are altered in neocortical areas, particularly with elevated K+ levels as occur during seizures. We analyzed neuronal firing properties, synaptic responses, and paired-pulse plasticity in human neocortical slices from tissue resected during epilepsy surgery, both under normal and under pathological conditions, i.e., after elevating K+ (4/8 mM), with rat neocortical slices as controls. Neuronal firing properties were not different. We did find, however, alterations of synaptic responsiveness in epileptic tissue, i.e., an elevated network excitability with K+ elevations, and reduction of paired-pulse depression.

The NMDA receptor NR2B subunit contributes to epileptogenesis in human cortical dysplasia

Cortical dysplasia (CD) is often associated with pharmacoresistant epilepsy. Previous studies showed increased expression of the NMDA receptor subunit NR2B in dysplastic and epileptic human neocortex. We tested the hypothesis that differential increase of NR2B constitutes an epileptogenic mechanism in humans. Dysplastic neocortex and lateral temporal lobe regions resected for treatment of pharmacoresistant seizures were processed for electrophysiological, histological, and immunocytochemical studies. Assignment to the "dysplastic" (n = 8) and "non-dysplastic" (n = 8) groups was based on histology. Neurons in "dysplastic" samples differentially stained for NR2B. Western blot (n = 6) showed an immunoreactive band for NR2B in three out of four "dysplastic" samples. Epileptiform field potentials (EFP) were elicited in vitro by omission of magnesium from the bath. EFP in "dysplastic" slices were characterized by multiple afterdischarges, occurring at a significantly higher repetition rate than EFP in non-dysplastic slices. The NR2B-specific NMDA receptor inhibitor ifenprodil (10muM) suppressed EFP in dysplastic slices. In non-dysplastic slices, burst repetition rate did not change with ifenprodil application. In both dysplastic and non-dysplastic slices, EFP were suppressed by a non-specific NMDAR antagonist (APV) or AMPA receptor antagonist (CNQX). These results provide additional evidence that the differential expression of NR2B in dysplastic human neocortex may play a role in the expression of in-situ epileptogenesis in human CD. NR2B may constitute a target for new diagnostic and pharmacotherapeutic approaches.

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.

Properties of single NMDA receptor channels in human dentate gyrus granule cells

1. Cell-attached single-channel recordings of NMDA channels were carried out in human dentate gyrus granule cells acutely dissociated from slices prepared from hippocampi surgically removed for the treatment of temporal lobe epilepsy (TLE). The channels were activated by L-aspartate (250-500 nM) in the presence of saturating glycine (8 microM). 2. The main conductance was 51 +/- 3 pS. In ten of thirty granule cells, clear subconductance states were observed with a mean conductance of 42 +/- 3 pS, representing 8 +/- 2 % of the total openings. 3. The mean open times varied from cell to cell, possibly owing to differences in the epileptogenicity of the tissue of origin. The mean open time was 2.70 +/- 0.95 ms (range, 1.24-4.78 ms). In 87 % of the cells, three exponential components were required to fit the apparent open time distributions. In the remaining neurons, as in control rat granule cells, two exponentials were sufficient. Shut time distributions were fitted by five exponential components. 4. The average numbers of openings in bursts (1.74 +/- 0.09) and clusters (3.06 +/- 0.26) were similar to values obtained in rodents. The mean burst (6.66 +/- 0.9 ms), cluster (20.1 +/- 3.3 ms) and supercluster lengths (116.7 +/- 17.5 ms) were longer than those in control rat granule cells, but approached the values previously reported for TLE (kindled) rats. 5. As in rat NMDA channels, adjacent open and shut intervals appeared to be inversely related to each other, but it was only the relative areas of the three open time constants that changed with adjacent shut time intervals. 6. The long openings of human TLE NMDA channels resembled those produced by calcineurin inhibitors in control rat granule cells. Yet the calcineurin inhibitor FK-506 (500 nM) did not prolong the openings of human channels, consistent with a decreased calcineurin activity in human TLE. 7. Many properties of the human NMDA channels resemble those recorded in rat hippocampal neurons. Both have similar slope conductances, five exponential shut time distributions, complex groupings of openings, and a comparable number of openings per grouping. Other properties of human TLE NMDA channels correspond to those observed in kindling; the openings are considerably long, requiring an additional exponential component to fit their distributions, and inhibition of calcineurin is without effect in prolonging the openings.

Actions of 5-HT on human neocortical neurones in vitro

Using intracellular recordings, we have studied the action of 5-hydroxytryptamine (5-HT) on slices of human temporal, occipital and frontal cortex maintained in vitro. The recordings were usually made 1.2 to 1.5 mm down from the pial surface, in or around layer III. The action of 5-HT (30-50 microM) was studied on 21 cells (from 12 individuals) which had electrophysiological characteristics of glutamatergic pyramidal neurones. 5-HT depolarised the majority (11) of these cells with a median response of 5 mV. It produced a hyperpolarising response in five neurones (median=-4 mV) and a combined hyperpolarising/depolarising response in two others. No response was detected in three cells. The depolarising response was probably mediated by reducing a resting potassium conductance. Ketanserin (0.1 and 1.0 microM) and spiperone (1 microM) reduced the response indicating that it was likely mediated by 5-HT2A receptors. The hyperpolarising response was associated with the opening of ion channels and was blocked by the selective 5-HT1A receptor antagonist WAY-100635 (100 nM). 5-HT inhibited spontaneous synaptic potentials. This effect was reduced by ketanserin (1 microM) but not by WAY-100635 (100 nM). It is concluded that human neocortical neurones in vitro can be depolarised via 5-HT2A receptors and hyperpolarised via 5-HT1A receptors.

Lack of correlation between neuronal hyperexcitability and electrocorticographic responsiveness in epileptogenic human neocortex

OBJECT

The purpose of this study was to determine whether intrinsic neuronal properties and synaptic responses differed between interictally active and inactive tissue removed in neocortical resections from patients undergoing surgical treatment for epilepsy.

METHODS

Whole-cell patch recordings were performed in layer 2 or 3 and layer 5 pyramidal cells in neocortical slices obtained from tissue surgically removed from patients for the treatment of medically intractable seizures. Synaptic responses to stimulation at the layer 6-white matter border were used to classify cells as nonbursting if they responded with only a single action potential for all above-threshold stimuli (80%). These responses were usually followed by biphasic inhibitory postsynaptic potentials (IPSPs). Cells were classified as bursting if they fired at least three action potentials in response to synaptic stimulation (20%). These cells typically showed no IPSPs and responded in either an all-or-nothing or graded fashion. Approximately twice as many cells at layer 2 or 3 (29%) than cells at layer 5 (14%) fired synaptic bursts. Synaptic bursting was not associated with an alteration in a cell's response properties to gamma-aminobutyric acid. It was notable that, in tissue samples determined by electrocorticography (ECoG) to be either interictally active or not active, the proportion of cells that burst was exactly the same in both groups (24%). We found no cells with intrinsic burst firing.

CONCLUSIONS

We conclude that synaptic bursting was characteristic of a small proportion of cells from epileptic tissue; however, this did not correlate with interictal spikes on ECoG.

Epileptiform discharge induced by 4-aminopyridine in magnesium-free medium in neocortical neurons: physiological and pharmacological characterization

An in vitro model of epileptiform activity was developed to study the role of excitatory and inhibitory neurotransmitters in the epileptogenesis. Intracellular recordings were obtained from rat neocortical slices exposed to 4-aminopyridine in a magnesium-free solution. Spontaneous epileptiform activity consisting of paroxysmal depolarization shifts with associated spontaneous depolarizing postsynaptic potentials were observed. The paroxysmal depolarization shifts were blocked either by D,L-2-amino-5-phosphonovalerate (50 microM), an N-methyl-D-aspartate receptor antagonist, or by 6-cyano-7-nitroquinoxaline-2.3-dione (10 microM), a non-N-methyl-D-aspartate receptor antagonist. These glutamate receptor antagonists also reduced the occurrence of spontaneous depolarizing postsynaptic potentials. Bicuculline methiodide, an antagonist of GABAA receptors, suppressed spontaneous depolarizing postsynaptic potentials, while it reduced the frequency of paroxysmal depolarization shifts and increased their duration. Hyperpolarization of the membrane potential by continuous current injection increased the frequency of paroxysmal depolarization shifts and reduced their duration, but it reduced the occurrence of spontaneous postsynaptic potentials. Paroxysmal depolarization shifts were blocked by tetrodotoxin (1 microM). The duration and the frequency of paroxysmal depolarization shift were reduced by dopamine (30-300 microM) in a dose-dependent manner. Our model suggests a different involvement of excitatory and inhibitory processes in the generation of epileptiform activity.

Quantitative localization of NMDAR1 receptor subunit immunoreactivity in inferotemporal and prefrontal association cortices of monkey and human

The cellular and synaptic localization of immunoreactivity for the N-methyl-D-aspartate (NMDA) receptor subunit, NMDAR1, was investigated in inferotemporal and prefrontal association neocortices of monkeys and humans. In all monkey association areas examined, the laminar distribution patterns of NMDAR1 immunoreactivity were similar, and characterized by predominant pyramidal-like neuronal labeling in layers II, III, V and VI and a dense neuropil labeling consisting of intensely stained puncta and fine-caliber processes present throughout layers I-III, and V-VI. Layer IV, in contrast, contained only very lightly immunostained neurons which mostly lacked extensive dendritic staining. The laminar distribution of NMDAR1 immunolabeling in human association cortex was similar to that observed in monkeys. Electron microscopy of monkey areas 46 and TE1 confirmed that intensely immunoreactive asymmetrical postsynaptic densities were present throughout all cell-dense layers of prefrontal and inferotemporal association cortex. Quantitative analyses of the laminar proportions of immunoreactive synapses demonstrated that in both areas examined, the percentages of immunolabeled synapses were mostly similar across superficial layers, layer IV and infragranular layers. Finally, quantitative double-labeling immunofluorescence for non-NMDA receptor subunits or calcium-binding proteins demonstrated that virtually all GluR2/3 or GluR5/6/7-immunoreactive neurons were also labeled for NMDAR1, while regionally-specific subsets of parvalbumin-, calbindin- and calretinin-immunoreactive neurons were co-labeled. These data indicate that in primate association cortex, NMDA receptors are heterogeneously distributed to subsets of functionally distinct types of neurons and subsets of excitatory synapses, suggesting a critical and highly specific role in mediating the activity of excitatory connectivity which converges on cortical association areas.

Microzonal decreases in the immunostaining for non-NMDA ionotropic excitatory amino acid receptor subunits GluR 2/3 and GluR 5/6/7 in the human epileptogenic neocortex

Potential alterations in glutamate-utilizing excitatory circuits in resected human epileptogenic frontal and temporal neocortex were investigated by using immunocytochemical methods to visualize receptor subunits which comprise the AMPA/kainate (GluR2/3) and kainate (GluR5/6/7) receptor subtypes. Examination of the patterns of immunostaining in regions of neocortex that were identified as spiking and non-spiking based on intraoperative electrocorticography revealed dramatic, microzonal decreases in immunoreactivity for the receptor subunits examined. The patches of decreased immunostaining for GluR2/3 and for GluR5/6/7 were often coincident with respect to each other. However, such abnormal regions were not necessarily correlated with any particular electrocorticographically defined regions nor any overtly abnormal cytoarchitectural features in adjacent Nissl-stained sections. Moreover in many but not all cases, the focal regions of decreased receptor subunit immunoreactivity coincided with small patches of decreased parvalbumin immunoreactivity a calcium-binding protein which labels a subpopulation of powerful inhibitory GABAergic interneurons. These results indicate that in the human epileptogenic neocortex there may be alterations in particular excitatory and/or inhibitory synaptic systems at small, multiple neocortical foci, and that these alterations are found mostly in the same regions. We suggest that these alterations may contribute to the initiation and/or propagation of seizure activity.

Functional studies of neurotransmitter receptors in human brain

Understanding synaptic transmission in the human brain is of the uppermost importance due to the involvement of neurotransmitters in several neurological and psychiatric disorders. Studies of animal pharmacology and of molecular biology are revealing that transmitter receptors are highly heterogeneous. It is therefore essential, also in view of using animal models in the development of therapeutically useful drugs, to establish if functionally corresponding receptors in men and animals also display identical pharmacological profiles. Using human brain tissue samples removed during neurosurgery and monitoring transmitter release as a functional response, a number of neurotransmitter receptors have been identified, localized and pharmacologically characterized as types and subtypes.