The role of the subiculum in epilepsy and epileptogenesis

The Peptide Network between Tetanus Toxin and Human Proteins Associated with Epilepsy

Sequence matching analyses show that Clostridium tetani neurotoxin shares numerous pentapeptides (68, including multiple occurrences) with 42 human proteins that, when altered, have been associated with epilepsy. Such a peptide sharing is higher than expected, nonstochastic, and involves tetanus toxin-derived epitopes that have been validated as immunopositive in the human host. Of note, an unexpected high level of peptide matching is found in mitogen-activated protein kinase 10 (MK10), a protein selectively expressed in hippocampal areas. On the whole, the data indicate a potential for cross-reactivity between the neurotoxin and specific epilepsy-associated proteins and may help evaluate the potential risk for epilepsy following immune responses induced by tetanus infection. Moreover, this study may contribute to clarifying the etiopathogenesis of the different types of epilepsy.

Anterior-posterior cerebral blood volume gradient in human subiculum

The human hippocampal formation is characterized by anterior-posterior gradients of cell density, neurochemistry, and hemodynamics. In addition, some functions are associated with specific subfields (subiculum, CA1-4, dentate gyrus) and regions (anterior and posterior). We performed contrast-enhanced, high-resolution T1-weighted 3T steady state (SS) imaging to investigate cerebral blood volume (CBV) gradients of the hippocampal formation. We studied 14 healthy subjects and found significant CBV gradients (anterior > posterior) in the subiculum but not in other hippocampal subfields. Since CBV is a marker of basal metabolism, these results indicate a greater baseline activity in the anterior compared with the posterior subiculum. This gradient might be related to the role of the subiculum as the main outflow station of the hippocampal formation and might have implications for the mechanisms of neuropsychiatric disorders.

Postnatal loss of P/Q-type channels confined to rhombic-lip-derived neurons alters synaptic transmission at the parallel fiber to purkinje cell synapse and replicates genomic Cacna1a mutation phenotype of ataxia and seizures in mice

Ataxia, episodic dyskinesia, and thalamocortical seizures are associated with an inherited loss of P/Q-type voltage-gated Ca(2+) channel function. P/Q-type channels are widely expressed throughout the neuraxis, obscuring identification of the critical networks underlying these complex neurological disorders. We showed recently that the conditional postnatal loss of P/Q-type channels in cerebellar Purkinje cells (PCs) in mice (purky) leads to these aberrant phenotypes, suggesting that intrinsic alteration in PC output is a sufficient pathogenic factor for disease initiation. The question arises whether P/Q-type channel deletion confined to a single upstream cerebellar synapse might induce the pathophysiological abnormality of genomically inherited P/Q-type channel disorders. PCs integrate two excitatory inputs, climbing fibers from inferior olive and parallel fibers (PFs) from granule cells (GCs) that receive mossy fiber (MF) input derived from precerebellar nuclei. In this study, we introduce a new mouse model with a selective knock-out of P/Q-type channels in rhombic-lip-derived neurons including the PF and MF pathways (quirky). We found that in quirky mice, PF-PC synaptic transmission is reduced during low-frequency stimulation. Using focal light stimulation of GCs that express optogenetic light-sensitive channels, channelrhodopsin-2, we found that modulation of PC firing via GC input is reduced in quirky mice. Phenotypic analysis revealed that quirky mice display ataxia, dyskinesia, and absence epilepsy. These results suggest that developmental alteration of patterned input confined to only one of the main afferent cerebellar excitatory synaptic pathways has a significant role in generating the neurological phenotype associated with the global genomic loss of P/Q-type channel function.

Electrode location and clinical outcome in hippocampal electrical stimulation for mesial temporal lobe epilepsy

PURPOSE

To study the clinical outcome in hippocampal deep brain stimulation (DBS) for the treatment of patients with refractory mesial temporal lobe epilepsy (MTLE) according to the electrode location.

METHODS

Eight MTLE patients implanted in the hippocampus and stimulated with high-frequency DBS were included in this study. Five underwent invasive recordings with depth electrodes to localize ictal onset zone prior to chronic DBS. Position of the active contacts of the electrode was calculated on postoperative imaging. The distances to the ictal onset zone were measured as well as atlas-based hippocampus structures impacted by stimulation were identified. Both were correlated with seizure frequency reduction.

RESULTS

The distances between active electrode location and estimated ictal onset zone were 11±4.3 or 9.1±2.3mm for patients with a >50% or <50% reduction in seizure frequency. In patients (N=6) showing a >50% seizure frequency reduction, 100% had the active contacts located <3mm from the subiculum (p<0.05). The 2 non-responders patients were stimulated on contacts located >3mm to the subiculum.

CONCLUSION

Decrease of epileptogenic activity induced by hippocampal DBS in refractory MTLE: (1) seems not directly associated with the vicinity of active electrode to the ictal focus determined by invasive recordings; (2) might be obtained through the neuromodulation of the subiculum.

Complexity and multifractality of neuronal noise in mouse and human hippocampal epileptiform dynamics

Fractal methods offer an invaluable means of investigating turbulent nonlinearity in non-stationary biomedical recordings from the brain. Here, we investigate properties of complexity (i.e. the correlation dimension, maximum Lyapunov exponent, 1/f(γ) noise and approximate entropy) and multifractality in background neuronal noise-like activity underlying epileptiform transitions recorded at the intracellular and local network scales from two in vitro models: the whole-intact mouse hippocampus and lesional human hippocampal slices. Our results show evidence for reduced dynamical complexity and multifractal signal features following transition to the ictal epileptiform state. These findings suggest that pathological breakdown in multifractal complexity coincides with loss of signal variability or heterogeneity, consistent with an unhealthy ictal state that is far from the equilibrium of turbulent yet healthy fractal dynamics in the brain. Thus, it appears that background noise-like activity successfully captures complex and multifractal signal features that may, at least in part, be used to classify and identify brain state transitions in the healthy and epileptic brain, offering potential promise for therapeutic neuromodulatory strategies for afflicted patients suffering from epilepsy and other related neurological disorders.

Wide therapeutic time-window of low-frequency stimulation at the subiculum for temporal lobe epilepsy treatment in rats

Low-frequency stimulation (LFS) has been considered as an option for the treatment of intractable epilepsy. However, previous data showed that LFS of certain brain regions only exerts its effect within a very narrow therapeutic time window, which lasts from seconds to tens of seconds, thus restricting its clinical application. The present study was designed to determine whether there exists a target with a wider therapeutic window for LFS treatment. Therefore, evoked seizures in the rat were induced by amygdala kindling and spontaneous seizures were induced by pilocarpine. The effects of different modes of LFS at the subiculum on the progression and severity of evoked seizures and the frequency of spontaneous seizure were evaluated. We found that (i) LFS at 1Hz delivered to the subiculum before and immediately after the kindling stimulations, or after the cessation of afterdischarge (afterdischarge duration, ADD) decreased the seizure stages and shortened the ADD both in seizure acquisition and expression in amygdaloid-kindled seizures. In addition, even LFS delivered after duration of double the ADD prolonged the kindling progression. (ii) LFS delivered at 1Hz, but not 0.5, 3 or 130Hz, immediately after the cessation of kindling stimulations retarded the progression of kindling seizures. (iii) Pilocarpine-induced spontaneous seizures were completely inhibited by 1Hz LFS. Thus, these results demonstrated that LFS of the subiculum has a wide therapeutic time-window for temporal lobe epilepsy treatment in rats, suggesting that the subiculum may be a promising and suitable target for clinical application.

A stereological study of synapse number in the epileptic human hippocampus

Hippocampal sclerosis is the most frequent pathology encountered in resected mesial temporal structures from patients with intractable temporal lobe epilepsy (TLE). Here, we have used stereological methods to compare the overall density of synapses and neurons between non-sclerotic and sclerotic hippocampal tissue obtained by surgical resection from patients with TLE. Specifically, we examined the possible changes in the subiculum and CA1, regions that seem to be critical for the development and/or maintenance of seizures in these patients. We found a remarkable decrease in synaptic and neuronal density in the sclerotic CA1, and while the subiculum from the sclerotic hippocampus did not display changes in synaptic density, the neuronal density was higher. Since the subiculum from the sclerotic hippocampus displays a significant increase in neuronal density, as well as a various other neurochemical changes, we propose that the apparently normal subiculum from the sclerotic hippocampus suffers profound alterations in neuronal circuits at both the molecular and synaptic level that are likely to be critical for the development or maintenance of seizure activity.

A knock-in reporter mouse model for Batten disease reveals predominant expression of Cln3 in visual, limbic and subcortical motor structures

Juvenile neuronal ceroid lipofuscinosis (JNCL) or Batten disease is an autosomal recessive neurodegenerative disorder of children caused by mutation in CLN3. JNCL is characterized by progressive visual impairment, cognitive and motor deficits, seizures and premature death. Information about the localization of CLN3 expressing neurons in the nervous system is limited, especially during development. The present study has systematically mapped the spatial and temporal localization of CLN3 reporter neurons in the entire nervous system including retina, using a knock-in reporter mouse model. CLN3 reporter is expressed predominantly in post-migratory neurons in visual and limbic cortices, anterior and intralaminar thalamic nuclei, amygdala, cerebellum, red nucleus, reticular formation, vestibular nuclei and retina. CLN3 reporter in the nervous system is mainly expressed during the first postnatal month except in the dentate gyrus, parasolitary nucleus and retina, where it is still strongly expressed in adulthood. The predominant distribution of CLN3 reporter neurons in visual, limbic and subcortical motor structures correlates well with the clinical symptoms of JNCL. These findings have also revealed potential target brain regions and time periods for future investigations of the disease mechanisms and therapeutic intervention.

Persistent sodium current and its role in epilepsy

Sodium currents are essential for the initiation and propagation of neuronal firing. Alterations of sodium currents can lead to abnormal neuronal activity, such as occurs in epilepsy. The transient voltage-gated sodium current mediates the upstroke of the action potential. A small fraction of sodium current, termed the persistent sodium current (I(NaP)), fails to inactivate significantly, even with prolonged depolarization. I(NaP) is activated in the subthreshold voltage range and is capable of amplifying a neuron's response to synaptic input and enhancing its repetitive firing capability. A burgeoning literature is documenting mutations in sodium channels that underlie human disease, including epilepsy. Some of these mutations lead to altered neuronal excitability by increasing I(NaP). This review focuses on the pathophysiological effects of I(NaP) in epilepsy.

Morpho-physiologic characteristics of dorsal subicular network in mice after pilocarpine-induced status epilepticus

The goal of this study was to examine the morpho-physiologic changes in the dorsal subiculum network in the mouse model of temporal lobe epilepsy using extracellular recording, juxtacellular and immunofluorescence double labeling, and anterograde tracing methods. A significant loss of total dorsal subicular neurons, particularly calbindin, parvalbumin (PV) and immunopositive interneurons, was found at 2 months after pilocarpine-induced status epilepticus (SE). However, the sprouting of axons from lateral entorhinal cortex (LEnt) was observed to contact with surviving subicular neurons. These neurons had two predominant discharge patterns: bursting and fast irregular discharges. The bursting neurons were mainly pyramidal cells, and their dendritic spine density and bursting discharge rates were increased significantly in SE mice compared with the control group. Fast irregular discharge neurons were PV-immunopositive interneurons and had less dendritic spines in SE mice when compared with the control mice. When LEnt was stimulated, bursting and fast irregular discharge neurons had much shorter latency and stronger excitatory response in SE mice compared with the control group. Our results illustrate that morpho-physiologic changes in the dorsal subiculum could be part of a multilevel pathologic network that occurs simultaneously in many brain areas to contribute to the generation of epileptiform activity.

The NMDAR subunit NR2B expression is modified in hippocampus after repetitive seizures

NMDA receptor is involved in synaptic plasticity, learning, memory and neurological diseases like epilepsia and it is the major mediator of excitotoxicity. NR2B-containing NMDA receptors may be playing a crucial role in epileptic disorders. In the present study the effect of the convulsant drug 3-mercaptopropionic acid (MP) repetitive administration (4-7 days) on the hippocampal NR2B subunit was studied. A significant decrease in NR2B in the whole hippocampus was observed after MP4 with a tendency to recover to normal values in MP7 by western blot assay. Immunohistochemical studies showed a decrease in several CA1 and CA2/3 strata (21-73%). MP7 showed a reversion of the drop observed at 4 days in stratum oriens, pyramidal cell layer in CA1, CA2/3 and CA1 stratum radiatum. A significant fall in the lacunosum molecular layer of both areas and stratum radiatum of CA2/3 was observed. The immunostaining in MP4 showed a decrease in the granulare layer from dentate gyrus (20%), in hillus (71%) and subicullum (63%) as compared with control and these decreases were similar at MP7 values. Results showed decreases in NR2B subunit expression in different areas following repeated MP-induce seizures, suggesting that NR2B expression is altered depending on the diverse hippocampal input and output signals of each region that could be differently involved in modulating MP-induced hyperactivity.

Hippocampal cell loss in posttraumatic human epilepsy

Hippocampal Cell Loss in Posttraumatic Human Epilepsy. Swartz BE, Houser CR, Tomiyasu U, Walsh GO, DeSalles A, Rich JR, Delgado-Escueta A. Epilepsia 2006;47(8): 1373–1382. Purpose: We performed this study to determine whether significant head trauma in human adults can result in hippocampal cell loss, particularly in hilar (polymorph) and CA3 neurons, similar to that observed in animal models of traumatic brain injury. We examined the incidence of hippocampal pathology and its relation to temporal neocortical pathology, neuronal reorganization, and other variables. Methods: Twenty-one of 200 sequential temporal lobectomies had only trauma as a risk factor for epilepsy. Tissue specimens from temporal neocortex and hippocampus were stained with glial fibrillary acidic protein (GFAP) and hematoxylin and eosin (H&E). Eleven hippocampal specimens had additional analysis of neuronal distributions by using cresyl violet and immunolabeling of a neuron-specific nuclear protein. Results: The median age at onset of trauma was 19 years, the median time between trauma and onset of seizures was 2 years, and the median epilepsy duration was 16 years. The length of the latent period was inversely related to the age at the time of trauma ( r = 0.75; Spearman). The neocortex showed gliosis in all specimens, with hemosiderosis ( n = 8) or heterotopias ( n = 6) in some, a distribution differing from chance ( p = 0.02; Fisher). Hippocampal neuronal loss was found in 94% of specimens, and all of these had cell loss in the polymorph (hilar) region of the dentate gyrus. Hilar cell loss ranged from mild, when cell loss was confined to the hilus, to severe, when cell loss extended into CA3 and CA1. Some degree of mossy fiber sprouting was found in the dentate gyrus of all 10 specimens in which it was evaluated. Granule cell dispersion ( n = 4) was seen only in specimens with moderate to severe neuronal loss. Conclusions: Neocortical pathology was universally present after trauma. Neuronal loss in the hilar region was the most consistent finding in the hippocampal formation, similar to that found in the fluid-percussion model of traumatic head injury. These findings support the idea that head trauma can induce hippocampal epilepsy in humans in the absence of other known risk factors.

Quantitative analysis of parvalbumin-immunoreactive cells in the human epileptic hippocampus

Hippocampal sclerosis is the most frequent pathology encountered in mesial temporal structures resected from patients with intractable temporal lobe epilepsy and it mainly involves hippocampal neuronal loss and gliosis. These alterations are accompanied by changes in the expression of a variety of molecules in the surviving neurons, as well as axonal reorganization in both excitatory and inhibitory circuits. The alteration of a subpopulation of GABAergic interneurons that expresses the calcium binding protein parvalbumin (PV) is thought to be a key factor in the epileptogenic process. We investigated the distribution and density of parvalbumin-immunoreactive (PV-ir) neurons in surgically resected hippocampal tissue from epileptic patients with and without sclerosis. Using quantitative stereological methods, we show for the first time that there is no correlation between total neuronal loss and PV-ir neuronal loss in any of the hippocampal fields. We also observed higher values of the total neuronal density in the sclerotic subiculum, which is accompanied by a lower density of PV-ir when compared with non-sclerotic epileptic and autopsy hippocampi. These findings suggest that, the apparently normal subiculum from sclerotic patients also shows unexpected changes in the density and proportion of PV-ir neurons.

Synaptic reorganization in subiculum and CA3 after early-life status epilepticus in the kainic acid rat model

PURPOSE

The immature rat brain is highly susceptible to seizures, but has a resistance to pathological changes induced by seizures as compared to adult rats. However, prolonged seizures during early-life enhance cellular injury and hyperexcitability induced by convulsive insults later in adulthood. The mechanisms underlying these phenomena are not understood. In adult models, the CA1 axons reorganize their projections to subiculum. Seizure induced plasticity in this pathway has not been investigated in immature seizure models, and may contribute to the vulnerability to later seizures.

METHODS

On postnatal day 15, rats experienced convulsive status epilepticus with kainic acid (KA). Seizure induced plasticity was examined with Timm histochemistry and iontophoretic injections of sodium selenite, a retrograde tracer. Cellular injury was evaluated with Fluoro-Jade B histochemistry.

RESULTS

Retrograde tracing experiments determined a 67% larger dorsoventral extent of retrograde labeling in the CA1 pyramidal region after tracer injections in subiculum. The synaptic reorganization of the CA1 projection to subiculum was noted in the absence of overt neuronal injury in subiculum or CA1. In contrast, mossy fiber sprouting was detected into the stratum oriens of CA3 with limited neuronal injury to CA3 pyramidal neurons. No mossy fiber sprouting into the inner molecular layer of the dentate gyrus, or CA1 sprouting into the stratum moleculare of CA1 were noted.

CONCLUSIONS

The results indicate that the developing brain has distinct mechanisms of seizure induced reorganization as compared to the adult brain. Our experiments show that the concept of "resistance of the immature brain to excitotoxicity" is considerably more complicated than generally believed. Morphological plasticity in the immature brain appears more extensive in distal, but not proximal, projections of hippocampal pathways, and across hippocampal lamellae. The abnormal connectivity between hippocampal lamellae might play a role in the increased susceptibility to injury and hyperexcitability associated with later convulsive insults.

Epilepsy: a review of selected clinical syndromes and advances in basic science

Epilepsy is a common neurologic disorder that manifests in diverse ways. There are numerous seizure types and numerous mechanisms by which the brain generates seizures. The two hallmarks of seizure generation are hyperexcitability of neurons and hypersynchrony of neural circuits. A large variety of mechanisms alters the balance between excitation and inhibition to predispose a local or widespread region of the brain to hyperexcitability and hypersynchrony. This review discusses five clinical syndromes that have seizures as a prominent manifestation. These five syndromes differ markedly in their etiologies and clinical features, and were selected for discussion because the seizures are generated at a different 'level' of neural dysfunction in each case: (1) mutation of a specific family of ion (potassium) channels in benign familial neonatal convulsions; (2) deficiency of the protein that transports glucose into the CNS in Glut-1 deficiency; (3) aberrantly formed local neural circuits in focal cortical dysplasia; (4) synaptic reorganization of limbic circuitry in temporal lobe epilepsy; and (5) abnormal thalamocortical circuit function in childhood absence epilepsy. Despite this diversity of clinical phenotype and mechanism, these syndromes are informative as to how pathophysiological processes converge to produce brain hyperexcitability and seizures.