Decreased time constant in hippocampal dentate granule cells in pilocarpine-treated rats with progressive seizure frequencies

Potassium Channels in Epilepsy

This review attempts to give a concise and up-to-date overview on the role of potassium channels in epilepsies. Their role can be defined from a genetic perspective, focusing on variants and de novo mutations identified in genetic studies or animal models with targeted, specific mutations in genes coding for a member of the large potassium channel family. In these genetic studies, a demonstrated functional link to hyperexcitability often remains elusive. However, their role can also be defined from a functional perspective, based on dynamic, aggravating, or adaptive transcriptional and posttranslational alterations. In these cases, it often remains elusive whether the alteration is causal or merely incidental. With ∼80 potassium channel types, of which ∼10% are known to be associated with epilepsies (in humans) or a seizure phenotype (in animals), if genetically mutated, a comprehensive review is a challenging endeavor. This goal may seem all the more ambitious once the data on posttranslational alterations, found both in human tissue from epilepsy patients and in chronic or acute animal models, are included. We therefore summarize the literature, and expand only on key findings, particularly regarding functional alterations found in patient brain tissue and chronic animal models.

Interplay between interictal spikes and behavioral seizures in young, but not aged pilocarpine-treated epileptic rats

Interictal spike activity is commonly observed in the EEG of patients with epilepsy, but the causal interrelationship between interictal spikes and behavioral seizures is poorly understood. We performed long-term video-EEG monitoring of 16 epileptic rats after pilocarpine-induced status epilepticus and five control animals. To quantify the interplay between periods of spikes and seizures, we calculated the time spent with spikes as well as the time spent with seizures for each animal. Within a given subject, we found a significant correlation between these two measures in 7/11 young epileptic rats (<400 days); this correlation was positive in six cases and negative in one. By contrast, none of five aged pilocarpine-treated animals exhibited significant correlation coefficients between spike periods and seizures (>600 days, P<0.05). Instead, aged epileptic rats showed a prominent predominance for either spike periods or seizures, which might explain the absence of significant correlation in this population. We found that there is a significant interplay between interictal periods of spikes and behavioral seizures in young epileptic animals, but this association is absent during aging.

Functional, metabolic, and synaptic changes after seizures as potential targets for antiepileptic therapy

Little is known about how the brain limits seizure duration and terminates seizures. Depending on severity and duration, a single seizure is followed by various functional, metabolic, and synaptic changes that may form targets for novel therapeutic strategies. It is long known that most seizures are followed by a period of postictal refractoriness during which the threshold for induction of additional seizures is increased. The endogenous anticonvulsant mechanisms involved in this phenomenon may be relevant for both spontaneous seizure arrest and increase of seizure threshold after seizure arrest. Postictal refractoriness has been extensively studied in various seizure and epilepsy models, including electrically and chemically induced seizures, kindling, and genetic animal models of epilepsy. During kindling development, two antagonistic processes occur simultaneously, one responsible for kindling-like events and the other for terminating ictus and postictal refractoriness. Frequently occurring seizures may lead to an accumulation of postictal refractoriness that may last weeks. The mechanisms involved in seizure termination and postictal refractoriness include changes in ionic microenvironment, in pH, and in various endogenous neuromodulators such as adenosine and neuropeptides. In animal models, the anticonvulsant efficacy of several antiepileptic drugs (AEDs) is increased during postictal refractoriness, which is a logical consequence of the interaction between endogenous anticonvulsant processes and the mechanism of AEDs. As discussed in this review, enhanced understanding of these endogenous processes may lead to novel targets for AED development.

A review of current applications of mass spectrometry for neuroproteomics in epilepsy

The brain is unquestionably the most fascinating organ, and the hippocampus is crucial in memory storage and retrieval and plays an important role in stress response. In temporal lobe epilepsy (TLE), the seizure origin typically involves the hippocampal formation. Despite tremendous progress, current knowledge falls short of being able to explain its function. An emerging approach toward an improved understanding of the complex molecular mechanisms that underlie functions of the brain and hippocampus is neuroproteomics. Mass spectrometry has been widely used to analyze biological samples, and has evolved into an indispensable tool for proteomics research. In this review, we present a general overview of the application of mass spectrometry in proteomics, summarize neuroproteomics and systems biology-based discovery of protein biomarkers for epilepsy, discuss the methodology needed to explore the epileptic hippocampus proteome, and also focus on applications of ingenuity pathway analysis (IPA) in disease research. This neuroproteomics survey presents a framework for large-scale protein research in epilepsy that can be applied for immediate epileptic biomarker discovery and the far-reaching systems biology understanding of the protein regulatory networks. Ultimately, knowledge attained through neuroproteomics could lead to clinical diagnostics and therapeutics to lessen the burden of epilepsy on society.

Upregulation of inward rectifier K+ (Kir2) channels in dentate gyrus granule cells in temporal lobe epilepsy

In humans, temporal lobe epilepsy (TLE) is often associated with Ammon's horn sclerosis (AHS) characterized by hippocampal cell death, gliosis and granule cell dispersion (GCD) in the dentate gyrus. Granule cells surviving TLE have been proposed to be hyperexcitable and to play an important role in seizure generation. However, it is unclear whether this applies to conditions of AHS. We studied granule cells using the intrahippocampal kainate injection mouse model of TLE, brain slice patch-clamp recordings, morphological reconstructions and immunocytochemistry. With progressing AHS and GCD, 'epileptic' granule cells of the injected hippocampus displayed a decreased input resistance, a decreased membrane time constant and an increased rheobase. The resting leak conductance was doubled in epileptic granule cells and roughly 70-80% of this difference were sensitive to K(+) replacement. Of the increased K(+) leak, about 50% were sensitive to 1 mm Ba(2+). Approximately 20-30% of the pathological leak was mediated by a bicuculline-sensitive GABA(A) conductance. Epileptic granule cells had strongly enlarged inwardly rectifying currents with a low micromolar Ba(2+) IC(50), reminiscent of classic inward rectifier K(+) channels (Irk/Kir2). Indeed, protein expression of Kir2 subunits (Kir2.1, Kir2.2, Kir2.3, Kir2.4) was upregulated in epileptic granule cells. Immunolabelling for two-pore weak inward rectifier K(+) channels (Twik1/K2P1.1, Twik2/K2P6.1) was also increased. We conclude that the excitability of granule cells in the sclerotic focus of TLE is reduced due to an increased resting conductance mainly due to upregulated K(+) channel expression. These results point to a local adaptive mechanism that could counterbalance hyperexcitability in epilepsy.

Molecular mechanisms of dendritic spine development and remodeling

Dendritic spines are small protrusions that cover the surface of dendrites and bear the postsynaptic component of excitatory synapses. Having an enlarged head connected to the dendrite by a narrow neck, dendritic spines provide a postsynaptic biochemical compartment that separates the synaptic space from the dendritic shaft and allows each spine to function as a partially independent unit. Spines develop around the time of synaptogenesis and are dynamic structures that continue to undergo remodeling over time. Changes in spine morphology and density influence the properties of neural circuits. Our knowledge of the structure and function of dendritic spines has progressed significantly since their discovery over a century ago, but many uncertainties still remain. For example, several different models have been put forth outlining the sequence of events that lead to the genesis of a spine. Although spines are small and apparently simple organelles with a cytoskeleton mainly composed of actin filaments, regulation of their morphology and physiology appears to be quite sophisticated. A multitude of molecules have been implicated in dendritic spine development and remodeling, suggesting that intricate networks of interconnected signaling pathways converge to regulate actin dynamics in spines. This complexity is not surprising, given the likely importance of dendritic spines in higher brain functions. In this review, we discuss the molecules that are currently known to mediate the exquisite sensitivity of spines to perturbations in their environment and we outline how these molecules interface with each other to mediate cascades of signals flowing from the spine surface to the actin cytoskeleton.

Sex differences in models of temporal lobe epilepsy: role of testosterone

Kainic acid and pilocarpine were used to assess sex differences in temporal lobe seizures. Adult Sprague-Dawley rats were injected with kainic acid (10-12 mg/kg) or with pilocarpine (380 mg/kg) and behavior was recorded for the next 3 h. Trunk blood was collected for hormonal measurements. Our data indicate that the male is more susceptible to the convulsant effects of agents that produce temporal lobe-like seizures. Males presented a higher amount of full limbic convulsions than females. To assess the role of plasma testosterone levels in kainate-induced seizures, a group of males was gonadectomized and half received testosterone replacement. The presence of testosterone, in intact and in gonadectomized males with testosterone replacement, increased the susceptibility to seizure. Seizures were either stronger (full limbic) or more frequent in animals with testosterone compared to animals devoid of testosterone. These results suggest that differences in plasma levels of testosterone may be partially responsible for the observed gender differences in seizure susceptibility. Our data reveal a reciprocal relationship between kainic acid-induced temporal lobe seizures and plasma testosterone. Testosterone enhances the occurrence and the severity of seizures. Conversely, kainic-acid-induced seizures decrease plasma testosterone. The higher plasma corticosterone levels found in these males suggest that kainic acid-induced seizures activate the hypothalamic-pituitary-adrenal axis which may induce alterations in plasma levels of male reproductive hormones.

Altered pattern of light transmittance and resistance to AMPA-induced swelling in the dentate gyrus of the epileptic hippocampus

Glutamate receptor-mediated changes in light transmittance were imaged in the dentate gyri of the epileptic hippocampi, taken from patients with temporal lobe epilepsy and the rat pilocarpine model, to investigate epilepsy-associated alterations in activity-induced cell swelling. A static pattern of light transmittance corresponded to the layered structure of dentate gyrus and reflected epilepsy-associated alterations. Hypoosmotic stress produced more than 35% of dynamic changes in the increase of light transmittance as a reflection of osmotic swelling in the epileptic dentate gyri. This degree of increase was not different from the increase observed in control dentate gyri, suggesting that the capability of osmotically regulating cell volume was preserved in the epileptic dentate gyri. In contrast, alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) induced activity-dependent swelling and an increase in light transmittance by 60.5% in the control dentate gyri, whereas the degree of increase in the epileptic dentate gyri remained 17.9% in response to AMPA. Selective attenuation of light transmittance in response to AMPA in the epileptic but not control dentate gyri suggested a possible alteration in the swelling properties of the epileptic dentate gyri that are linked to the AMPA receptor activation. Surviving cells in the epileptic hippocampus may have a mechanism of downregulating neuronal activity-dependent swelling to maintain optimal cell volume during repeated network hyperexcitation in epilepsy.

Relationship between neuronal loss and interictal glucose metabolism during the chronic phase of the lithium-pilocarpine model of epilepsy in the immature and adult rat

The lithium-pilocarpine (Li-Pilo) model of epilepsy reproduces most of the features of human temporal lobe epilepsy. After having studied the metabolic changes occurring during the silent phase, in the present study, we explored the relationship between interictal metabolic changes and neuronal loss during the chronic phase following status epilepticus (SE) induced by Li-Pilo in 10-day-old (P10), 21-day-old (P21), and adult rats. Rats were observed and their EEG was recorded to detect the occurrence of spontaneous recurrent seizures (SRS). Local cerebral glucose utilization was measured during the interictal period of the chronic phase, between 2 and 7 months after SE, by the [(14)C]2-deoxyglucose method in rats subjected to SE at P10, P21, or as adults. Neuronal damage was assessed by cell counting on adjacent cresyl violet stained sections. When SE was induced at P10, rats did not become epileptic, did not develop lesions and cerebral glucose utilization was in the normal range 7 months later. When SE was induced in adult rats, they all became epileptic after a mean duration of 25 days and developed lesions in the forebrain limbic areas, which were hypometabolic during the interictal period of the chronic phase, 2 months after SE. When SE was induced in P21 rats, 24% developed SRS, and in 43% seizures could be triggered (TS) by handling, after a mean delay of 74 days in both cases. The remaining 33% did not become epileptic (NS). The three groups of P21 rats developed quite comparable lesions mainly in the hilus of the dentate gyrus, lateral thalamus, and entorhinal cortex; at 6 months after SE, the forebrain was hypometabolic in NS and TS rats while it was normo- to slightly hypermetabolic in SRS rats. These data show that interictal metabolic changes are age-dependent. Moreover, there is no obvious correlation, in this model, between interictal hypometabolism and neuronal loss, as reported previously in human temporal lobe epilepsy.

Remodeling dendritic spines of dentate granule cells in temporal lobe epilepsy patients and the rat pilocarpine model

PURPOSE

To study when dendritic alteration can occur in the epileptic hippocampus and how it is influenced by epileptic axonal reorganization.

METHODS

Human specimens and the rat pilocarpine model were used. Dentate granule cells (DGCs) were visualized by intracellular biocytin injection for spine count.

RESULTS

In the rat pilocarpine model, dendrites of DGCs revealed a generalized spine loss immediately after the acute status epilepticus induced by pilocarpine. However, this generalized damage was transient and was followed by recovery and plastic changes in spine shape and density, which occurred 15 to 35 days after the initial acute status, i.e., during the period of establishing a chronic phase of this model with the induction of spontaneous seizures. In human epileptic hippocampi, spine density was significantly higher when DGCs generated aberrant mossy fiber collaterals. This was particularly so in the proximal dendrite of DGCs, where the aberrant collaterals were densely localized. These findings suggest that initial acute seizures do not cause permanent damage in dendrites and spines of DGCs and that dendritic spines of epileptic neurons can respond to changes in the local cellular environment, including newly formed afferents, in a plastic manner.

CONCLUSION

Dendritic spines are dynamically maintained in chronic epilepsy during the course of establishment and maintenance of spontaneous seizures. Local dendritic spine alteration, detected later in the chronic phase of epilepsy, must have a separate cause from initial acute insults.

Granule-like neurons at the hilar/CA3 border after status epilepticus and their synchrony with area CA3 pyramidal cells: functional implications of seizure-induced neurogenesis

A group of neurons with the characteristics of dentate gyrus granule cells was found at the hilar/CA3 border several weeks after pilocarpine- or kainic acid-induced status epilepticus. Intracellular recordings from pilocarpine-treated rats showed that these "granule-like" neurons were similar to normal granule cells (i. e., those in the granule cell layer) in membrane properties, firing behavior, morphology, and their mossy fiber axon. However, in contrast to normal granule cells, they were synchronized with spontaneous, rhythmic bursts of area CA3 pyramidal cells that survived status epilepticus. Saline-treated controls lacked the population of granule-like cells at the hilar/CA3 border and CA3 bursts. In rats that were injected after status epilepticus with bromodeoxyuridine (BrdU) to label newly born cells, and also labeled for calbindin D(28K) (because it normally stains granule cells), many double-labeled neurons were located at the hilar/CA3 border. Many BrdU-labeled cells at the hilar/CA3 border also were double-labeled with a neuronal marker (NeuN). Taken together with the recent evidence that granule cells that are born after seizures can migrate into the hilus, the results suggest that some newly born granule cells migrate as far as the CA3 cell layer, where they become integrated abnormally into the CA3 network, yet they retain granule cell intrinsic properties. The results provide insight into the physiological properties of newly born granule cells in the adult brain and suggest that relatively rigid developmental programs set the membrane properties of newly born cells, but substantial plasticity is present to influence their place in pre-existing circuitry.

Remodeling dendritic spines in the rat pilocarpine model of temporal lobe epilepsy

Dendritic degeneration is a common pathology in temporal lobe epilepsy and its animal models. However, little is known when and how the degeneration occurs. In the present study of the rat pilocarpine model, visualization of dendrites of the hippocampal dentate granule cells (DGCs) by biocytin revealed a generalized spine loss immediately after the acute seizure induced by pilocarpine. However, this generalized damage was followed by recovery and plastic changes in spine shape and density, which occurred 15-35 days after the initial acute seizure, i.e., during the period of establishing a chronic phase of this model with the induction of spontaneous seizures. The present finding suggests that initial acute seizures do not cause permanent damages in dendrites and spines of DGCs; instead, dendritic spines are dynamically maintained in the course of the establishment and maintenance of spontaneous seizures. Local dendritic spine degeneration, detected later in the chronic phase of epilepsy, is likely to have a separate cause from initial acute insults.

Supragranular mossy fiber sprouting is not necessary for spontaneous seizures in the intrahippocampal kainate model of epilepsy in the rat

In a previous study, we suggested a dissociation between spontaneous recurrent epileptic seizures (SRS) and hippocampal supragranular mossy fiber sprouting (MFS) in the pilocarpine model of epilepsy (PILO). One possible explanation, would be that SRS in the PILO model do not originate in the hippocampus and thus would not depend on MFS. In the present study, we investigated whether MFS is necessary for the SRS that develop after a small intrahippocampal dose of kainic acid (KA), a model where seizures are more likely to start in the hippocampus. Intrahippocampal injections of KA were performed in rats, with and without the concomitant administration of cycloheximide (CHX) (0.5 microg of KA and 6 microg of CHX). After injection, recording electrodes were positioned in the same stereotaxic location. Here again, CHX was able to completely block (5/8 animals) MFS, visualized by neo-Timm staining, without altering the frequency and intensity of spontaneous ictal and interictal EEG events. From these data, we can conclude that, in the intra-hippocampal KA model, MFS is not necessary for the occurrence of ictal events. We suggest that CHX can be used together with classic epileptogenic agents, as a means to study temporal lobe epilepsy (TLE) without the contributing effect of MFS--as seen in TLE patients with mass lesions in the lateral temporal lobe.

Glutamate currents in morphologically identified human dentate granule cells in temporal lobe epilepsy

Glutamate-receptor-mediated synaptic transmission was studied in morphologically identified hippocampal dentate granule cells (DGCs; n = 31) with the use of whole cell patch-clamp recording and intracellular injection of biocytin or Lucifer yellow in slices prepared from surgically removed medial temporal lobe specimens of epileptic patients (14 specimens from 14 patients). In the current-clamp recording, low-frequency stimulation of the perforant path generated depolarizing postsynaptic potentials that consisted of excitatory postsynaptic potentials and phase-inverted inhibitory postsynaptic potentials mediated by the gamma-aminobutyric acid-A (GABA(A)) receptor at a resting membrane potential of -62.7 +/- 2.0 (SE) mV. In the voltage-clamp recording, two glutamate conductances, a fast alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-receptor-mediated excitatory postsynaptic current (EPSC; AMPA EPSC) and a slowly developing N-methyl-D-aspartate (NMDA)-receptor-mediated EPSC (NMDA EPSC), were isolated in the presence of a GABA(A) receptor antagonist. NMDA EPSCs showed a voltage-dependent increase in conductance with depolarization by exhibiting an N-shaped current-voltage relationship. The slope conductance of the NMDA EPSC ranged from 1.1 to 9.4 nS in 31 DGCs, reaching up to twice the size of the AMPA conductance. This widely varying size of the NMDA conductance resulted in the generation of double-peaked EPSCs and a nonlinear increase of the slope conductance of up to 37.5 nS with positive membrane potentials, which resembled "paroxysmal currents," in a subpopulation of the neurons. In contrast, AMPA EPSCs, which were isolated in the presence of an NMDA receptor antagonist (2-amino-5-phosphonovaleric acid), showed voltage-independent linear changes in the current-voltage relationship and were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. The AMPA conductance showed little variance, regardless of the size of the NMDA conductance of a given neuron. The average AMPA slope conductance was 5.28 +/- 0.65 (SE) nS in 31 human DGCs. This value was similar to AMPA EPSC conductances in normal rat DGCs (5.35 +/- 0.52 nS, mean +/- SE; n = 55). Dendritic morphology and spine density were quantified in the individual DGCs to assess epileptic pathology. Dendritic spine density showed an inverse correlation (r2 = 0.705) with a slower rise time and a longer half-width of the excitatory postsynaptic potentials mediated by the NMDA receptor. It is concluded that both AMPA and NMDA EPSCs contribute to human DGC synaptic transmission in epileptic hippocampus. However, a wide range of changes in the slope conductance of the NMDA EPSCs suggests that the NMDA-receptor-mediated conductance could be altered in human epileptic DGCs. These changes may influence the generation of chronic subthreshold epileptogenic synaptic activity and give rise to pathological excitation leading to epileptic seizures and dendritic pathology.

Membrane time constant as a tool to assess cell degeneration

Changes in neuronal surface area may be monitored by measuring the plasma membrane capacitance [8]. Membrane time constant (tao m) is given by the product of the membrane resistance (rm) and membrane capacitance (Cm), tao m = rm Cm. Thus, when membrane resistance is kept constant at a steady state (resting), membrane time constant can reflect the size of neuronal surface area. Membrane time constant is the time for the potential to fall from the resting to a fraction (1-l/e), or 63%, of its final value in the charging curve during the application of a small negative current pulse. Negative voltage shift from the resting potential hardly activates any voltage-dependent ion channel, resulting in nominal changes in cell membrane resistance. Although elaborated methods for mathematical models and simulations are available for the electrophysiological assessment of neuron geometry in order to estimate subthreshold potential attenuation during the propagation of synaptically mediated electrical signals, they involve a number of critical assumptions for the convenience to each model, and some of these assumptions are unlikely to be valid. With these restrictive assumptions, very little can be determined about the electronic structure of a neuron beyond the measurement of neuronal membrane resistance and membrane time constant. Alternatively, numerous tracers are available to visualize morphologies of neurons intracellularly and extracellularly. These anatomical methods provide direct and quantitative evidence for neuron geometry; however, they involve tissue processing and a series of chemical reactions, some of which are time- and effort-demanding. The purpose of the present paper is to show that membrane time constant can be effectively used as a tool to assess diminution in cell surface area without involving extensive mathematical theories and/or neuroanatomical techniques. This approach is particularly effective in electrotonically compact cells such as hippocampal neurons. Recent development in the technique of the whole-cell patch clamp recording in the slice preparation yielded longer time constant with better resolution due to the absence of the leak conductance associated with microelectrode impalement. Indeed, when membrane time constant was measured with the whole-cell patch clamp recording technique, it successfully detected the reduction in dendritic arbors (dendritic degeneration) in dentate granule cells in the pilocarpine model of chronic epilepsy, and this finding is supported by the neuroanatomical evidence that was obtained from the same specimen samples. Membrane time constant is an easy-to-measure "passive membrane property" and can be used as a reliable probe by itself for detecting dendritic degeneration or as a tool for decision-making in introducing neuroanatomical technique in combination with slice neurophysiology.

Preservation of dendrites with the presence of reorganized mossy fiber collaterals in hippocampal dentate granule cells in patients with temporal lobe epilepsy

Dendritic morphology was studied in human hippocampal dentate granule cells (DGCs) by intracellularly-injecting biocytin in slice preparations that were obtained from temporal lobe epilepsy patients who underwent a surgical treatment for medically-intractable seizures. These DGCs had a fan-shaped dendritic domain of 54.1 degrees +/- 4.1 S.E.M. with 13.8 +/- 1.1 branch points and an estimated total dendritic length of 11535.6 microns +/- 3045.4. Dendritic spines were counted, and spine density was calculated to be 0.25 spines/microns +/- 0.16 S.E.M.. However, when the cells were categorized into two groups based on the presence or absence of the aberrant mossy fiber collaterals, the number of dendritic branches was significantly lower and spine density was significantly higher in DGCs that had aberrant collaterals. In particular, in the proximal dendrite, the spine density was 5 times higher in DGCs whose own mossy fibers were reorganized sending aberrant collaterals to this dendritic region (0.750 spines/microns +/- 0.203 S.E.M.: P < 0.01) than the DGCs without such collaterals (0.082 spines/microns +/- p.021 S.E.M.). These results suggest that the axonal reorganization may have an effect on the morphology of DGC dendrites directly or indirectly in such a way that dendritic structure and spines could be protected from seizure-induced excitotoxic cell damage.