Granule cell dispersion in the dentate gyrus of humans with temporal lobe epilepsy.

The distribution of granule cells in the dentate gyrus of the hippocampal formation was studied in control autopsy and temporal lobe epilepsy (TLE) specimens. In control tissue, the granule cell somata were closely approximated and formed a narrow lamina with a distinct, regular border with the molecular layer. In 11 of 15 TLE specimens, the granule cell somata were dispersed and formed a wider than normal granule cell layer. The granule cell somata extended into the molecular layer to varying extents, creating an irregular boundary between the lamina. The dispersed granule cells were frequently aligned in columns, and many of these neurons displayed elongated bipolar forms. The extent of granule cell dispersion appeared to be related to the amount of cell loss in the polymorph layer of the dentate gyrus. Granule cell dispersion was not consistently associated with granule cell loss although 5 of the 11 specimens with granule cell dispersion also showed moderate to marked granule cell loss. The most common features in the histories of the TLE cases with granule cell dispersion were severe febrile seizures or seizures associated with meningitis or encephalitis during the first 4 years of life. The dispersion of the granule cells suggests that there has been some alteration in the patterns of cell migration in a subpopulation of cases with severe TLE. The resultant ectopic positions of the granule cells could lead to changes in both the afferent and efferent connections of these neurons and, thus, contribute to the altered circuitry of the hippocampal formation in TLE.     

Increased expression of Nogo-A in hippocampal neurons of patients with temporal lobe epilepsy

Mesial temporal lobe epilepsy (TLE) is associated with pronounced anatomical and biochemical changes in the hippocampal formation including extensive neurodegeneration, reorganization of mossy fibres and sprouting of interneurons. Although the anatomical features and some of the physiological consequences of hippocampal remodeling have been well documented, the molecular mechanisms underlying the profound and orientated outgrowth of hippocampal neurons in TLE are not yet understood. The reticulon protein Nogo-A has been associated with an inhibitory action on axon growth and plasticity. Using immunohistochemistry and in situ hybridization, we investigated the expression of Nogo-A in specimens obtained at surgery from patients with TLE compared with those obtained from autopsy controls. In control specimens, Nogo-A immunoreactivity and mRNA were mainly confined to oligodendrocytes. Only approximately 40% of the specimens revealed low expression of Nogo-A mRNA in neurons. In contrast, in TLE patients with and without Ammon's horn sclerosis, Nogo-A mRNA and immunoreactivity were markedly up-regulated in most neurons (3.6- and 4.4-fold increases in Nogo-A mRNA in granule cells of sclerotic and nonsclerotic specimens) and their processes throughout the hippocampal formation. Similar elevations in Nogo-A mRNA and protein levels were determined by quantitative RT-PCR and Western blotting. Since Nogo-A expression was also up-regulated in specimens without hippocampal sclerosis, it may be induced by seizures prior to progressing neurodegeneration.     

Altered distribution of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor subunit GluR2(4) and the N-methyl-d-aspartate receptor subunit NMDAR1 in the hippocampus of patients with temporal lobe epilepsy

In patients with therapy-refractory temporal lobe epilepsy (TLE), alterations of glutamate receptors have been proposed as a mechanism for enhanced excitability. Using commercially available monoclonal antibodies specific for the N-methyl-d-aspartate (NMDA) receptor subunit NMDAR1 and for the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor subunit GluR2(4), we have examined the distribution of these polypeptides in human hippocampal tissue that was surgically removed from patients with intractable TLE. Surgical specimens were classified according to the presence of Ammon’s horn sclerosis (AHS) or a focal lesion in the temporal lobe. Cell counts and a densitometric analysis of the immunoreactivity patterns were carried out for all hippocampal subfields. NMDAR1 and GluR2(4) levels were markedly reduced in patients with AHS, primarily in those subfields with substantial neuronal cell loss (in particular CA1, CA4 and CA3), compared to those seen in patients with focal lesions and in control specimens obtained at autopsy. In contrast, the molecular layer of the dentate gyrus (DG-ML) showed significantly higher levels of GluR2(4) immunoreactivity in AHS compared to control tissue, while NMDAR1 showed no significant up-regulation in this sublayer. When the receptor staining intensity was normalized for alterations in neuronal density, no significant alterations could be detected except for an increase in GluR2(4) in the DG-ML of patients with AHS. These changes may reflect synaptic reorganization observed in the DG-ML of specimens from patients with chronic intractable TLE. Received: 25 March 1996 / Revised, accepted: 10 June 1996     

Altered patterns of dynorphin immunoreactivity suggest mossy fiber reorganization in human hippocampal epilepsy.

Dynorphin A(1-17), an opioid peptide that is normally present in the hippocampal mossy fiber system, was localized immunocytochemically in the hippocampal formation of control autopsy and temporal lobe epilepsy (TLE) specimens. In control tissue, dynorphin-like immunoreactive (Dyn-IR) structures were confined to the mossy fiber path and were most highly concentrated in the polymorph (hilar) region of the dentate gyrus. Very few Dyn-IR structures were present in the molecular and granule cell layers of the dentate gyrus. In contrast, in all TLE specimens, Dyn-IR elements were present in these layers. The extent of aberrant staining varied among the TLE specimens, and 2 major patterns were observed. The first was a relatively wide band of reaction product in the inner one-third to one-fourth of the molecular layer (8 cases), and the second was a more limited distribution of immunoreactive fibers and presumptive terminals in the granule cell and immediately adjacent supragranular regions (2 cases). The extent of aberrant Dyn-IR structures appeared to be related to the amount of cell loss in the polymorph and CA3 fields and to dispersion of the granule cell somata. Specimens processed with the Timm's sulfide silver method for heavy metals provided independent evidence for the distribution of mossy fibers. In both control and TLE specimens, the patterns of labeling were virtually identical to those of dynorphin localization. These findings suggest that sprouting of mossy fibers or their axon collaterals has occurred in hippocampal epilepsy and that the reorganized fibers contain at least one of the neuropeptides that are normally present in this system. Such fibers could form recurrent excitatory circuits and contribute to synchronous firing and epileptiform activity, as suggested in studies of experimental models of epilepsy.     

An increase of hippocampal calretinin-immunoreactive neurons correlates with early febrile seizures in temporal lobe epilepsy

Numerous studies indicate that initial precipitating injuries (IPI) such as febrile seizures during early childhood may play a pivotal role in the pathogenesis of temporal lobe epilepsy (TLE) and Ammon’s horn sclerosis (AHS). Previous data demonstrate an increase of horizontally oriented neurons in molecular layers of hippocampal subfields, which are immunoreactive for calretinin (CR-ir) and resemble Cajal-Retzius-like cells. Cajal-Retzius cells are transiently expressed in the murine developing hippocampus and are critically involved in neuronal pattern formation. Here we investigated a potential relationship between the distribution of horizontally oriented calretinin-immunoreactive neurons and the clinical history of TLE patients with AHS. Horizontally oriented neurons in the molecular layer of the hippocampal formation have been visualized by antibodies against the calcium-binding proteins calretinin and calbindin D-28k. Cell counts derived from 27 epilepsy patients with AHS were compared with autopsy specimens from developing and adult normal human hippocampus (n = 26). During ontogeny, CR-ir cells showed a marked perinatal peak in the CA1 and dentate gyrus molecular layer (CA1-ML, DG-ML) followed by a gradual postnatal decline. In hippocampal specimens from TLE patients with AHS and seizure onset before the age of 4 years, significantly higher levels of CR-ir neurons in CA1-ML (P = 0.05) and DG-ML (P < 0.05) were encountered than in AHS patients without precipitating seizures or with an uneventful early medical history. However, all three groups had higher levels of CR-ir neurons compared to adult controls obtained at autopsy (P < 0.01). In addition, AHS specimens showed increased CR-ir neuropil staining throughout the DG-ML compared with the restricted distribution of CR-ir fibers within the superficial granule cell layer visible in controls. These findings suggest that a condsiderable number of TLE patients with AHS display signs of impaired hippocampal maturation and circuitry formation as indicated by increased numbers of Cajal-Retzius like cells. It remains to be elucidated, how these changes contribute to the pathogenesis of TLE. Received: 23 April 1998 / Revised, accepted: 29 June 1998     

Remodeling of neuronal circuitries in human temporal lobe epilepsy: Increased expression of highly polysialylated neural cell adhesion molecule in the hippocampus and the entorhinal cortex

Neuronal loss and axonal sprouting are the most typical histopathological findings in the hippocampus of patients with drug-refractory temporal lobe epilepsy (TLE). It is under dispute, however, whether remodeling of neuronal circuits is a continuous process or whether it occurs only during epileptogenesis. Also, little is known about the plasticity outside of the hippocampus. We investigated the immunoreactivity of the highly polysialylated neural cell adhesion molecule (PSA-NCAM) in the surgically removed hippocampus and the entorhinal cortex of patients with drug-refractory TLE (n = 25) and autopsy controls (n = 7). Previous studies have shown that the expression of PSA-NCAM is associated with the induction of synaptic plasticity, neurite outgrowth, neuronal migration, and events requiring remodeling or repair of tissue. In patients with TLE, the optical density (OD) of punctate PSA-NCAM immunoreactivity was increased both in the inner and outer molecular layers of the dentate gyrus, compared with controls. The intensity of PSA-NCAM immunoreactivity in the inner molecular layer correlated with the duration of epilepsy, severity of hippocampal neuronal loss, density of mossy fiber sprouting, and astrogliosis. In TLE patients with only mild neuronal loss in the hippocampus, the density of infragranular immunopositive neurons was increased twofold compared with controls, whereas in TLE patients with severe neuronal loss, the infragranular PSA-NCAM–positive cells were not present. In the hilus, the somata and tortuous dendrites of some surviving neurons were intensely stained in TLE. PSA-NCAM immunoreactivity was also increased in CA1 and in layer II of the rostral entorhinal cortex, where immunopositive neurons were surrounded by PSA-NCAM-positive fibers and puncta. Our data provide evidence that synaptic reorganization is an active process in human drug-refractory TLE. Moreover, remodeling is not limited to the dentate gyrus, but also occurs in the CA1 subfield and the entorhinal cortex.     

Decreased expression of synaptic vesicle protein 2A, the binding site for levetiracetam, during epileptogenesis and chronic epilepsy

Purpose:   We previously showed that gene expression of synaptic vesicle protein 2A (SV2A), the binding site for the antiepileptic drug levetiracetam, is reduced during epileptogenesis in the rat. Since absence of SV2A has been associated with increased epileptogenicity, changes in expression of SV2A could have consequences for the progression of epilepsy. Therefore we investigated hippocampal SV2A protein expression of temporal lobe epilepsy (TLE) patients and in rats during epileptogenesis and in the chronic epileptic phase.
Methods:   SV2A immunocytochemistry and Western blot analysis were performed on the hippocampus of autopsy controls, patients that died from status epilepticus (SE), and pharmacoresistant TLE patients. In addition, in epileptic rats, SV2A expression was determined after SE during the acute, latent, and chronic epileptic phase.
Results:   In control tissue, presynaptic SV2A was expressed in all hippocampal subfields, with strongest expression in mossy fiber terminals. SV2A positive puncta were distributed in a patchy pattern over the somata and dendrites of neurons. SV2A decreased throughout the hippocampus of TLE patients with hippocampal sclerosis (HS), compared to autopsy control, SE, and non-HS tissue. In most rats, SV2A was already decreased in the latent period especially in the inner molecular layer and stratum lucidum. Similarly as in humans, SV2A was also decreased throughout the hippocampus of chronic epileptic rats, specifically in rats with a progressive form of epilepsy.
Discussion:   These data support previous findings that reduced expression of SV2A could contribute to the increased epileptogenicity. Whether this affects the effectiveness of levetiracetam needs to be further investigated.     

Increased expression of the neuronal glutamate transporter (EAAT3/EAAC1) in hippocampal and neocortical epilepsy

PURPOSE: To define the changes in gene and protein expression of the neuronal glutamate transporter (EAAT3/EAAC1) in a rat model of temporal lobe epilepsy as well as in human hippocampal and neocortical epilepsy. METHODS: The expression of EAAT3/EAAC1 mRNA was measured by reverse Northern blotting in single dissociated hippocampal dentate granule cells from rats with pilocarpine-induced temporal lobe epilepsy (TLE) and age-matched controls, in dentate granule cells from hippocampal surgical specimens from patients with TLE, and in dysplastic neurons microdissected from human focal cortical dysplasia specimens. Immunolabeling of rat and human hippocampi and cortical dysplasia tissue with EAAT3/EAAC1 antibodies served to corroborate the mRNA expression analysis. RESULTS: The expression of EAAT3/EAAC1 mRNA was increased by nearly threefold in dentate granule cells from rats with spontaneous seizures compared with dentate granule cells from control rats. EAAT3/EAAC1 mRNA levels also were high in human dentate granule cells from patients with TLE and were significantly elevated in dysplastic neurons in cortical dysplasia compared with non-dysplastic neurons from postmortem control tissue. No difference in expression of another glutamate transporter, EAAT2/GLT-1, was observed. Immunolabeling demonstrated that EAAT3/EAAC1 protein expression was enhanced in dentate granule cells from both rats and humans with TLE as well as in dysplastic neurons from human cortical dysplasia tissue. CONCLUSIONS: Elevations of EAAT3/EAAC1 mRNA and protein levels are present in neurons from hippocampus and neocortex in both rats and humans with epilepsy. Upregulation of EAAT3/EAAC1 in hippocampal and neocortical epilepsy may be an important modulator of extracellular glutamate concentrations and may occur as a response to recurrent seizures in these cell types.     

Possible role of the innate immunity in temporal lobe epilepsy

Purpose: Temporal lobe epilepsy (TLE) is a multifactorial disease often involving the hippocampus. So far the etiology of the disease has remained elusive. In some pharmacoresistant TLE patients the hippocampus is surgically resected as treatment. To investigate the involvement of the immune system in human TLE, we performed large-scale gene expression profiling on this human hippocampal tissue.
Methods: Microarray analysis was performed on hippocampal specimen from TLE patients with and without hippocampal sclerosis and from autopsy controls (n = 4 per group). We used a common reference pool design to perform an unbiased three-way comparison between the two patient groups and the autopsy controls. Differentially expressed genes were statistically analyzed for significant overrepresentation of gene ontology (GO) classes.
Results: Three-way analysis identified 618 differentially expressed genes. GO analysis identified immunity and defense genes as most affected in TLE. Particularly, the chemokines CCL3 and CCL4 were highly (>10-fold) upregulated. Other highly affected gene classes include neuropeptides, chaperonins (protein protection), and the ubiquitin/proteasome system (protein degradation).
Discussion: The strong upregulation of CCL3 and CCL4 implicates these chemokines in the etiology and pathogenesis of TLE. These chemokines, which are mainly expressed by glia, may directly or indirectly affect neuronal excitability. Genes and gene clusters identified here may provide targets for developing new TLE therapies and candidates for genetic research.     

Hippocampal loss of tenascin boundaries in Ammon's horn sclerosis

Ammon's horn sclerosis (AHS) is a common finding in patients with temporal lobe epilepsy (TLE). In addition to selective neuronal cell loss and axonal reorganization, AHS is also characterized by a striking astroglial reaction. However, the functional significance of reactive astrogliosis in the pathogenesis of TLE remains to be determined. Reactive astrocytes produce a variety of cell adhesion molecules and other extracellular matrix (ECM) components with potential effects on axonal growth, axonal branching, and neosynaptogenesis in the central nervous system (CNS). In the present study we describe the distribution of the ECM glycoprotein tenascin/cytotactin (TN-C) in 44 human hippocampal specimens from patients with TLE. The distribution of TN-C immunoreactivity was evaluated with the anti-human TN-C monoclonal antibody K8 by densitometrical analysis, and TN-C protein levels were detected by immunoblotting. In the normal human hippocampus, there were distinctive boundaries between areas of high and low TN-C expression. These border zones demarcated areas with major synaptic input, i.e., the dentate gyrus molecular layer (DG-ML) and the gray matter of the Ammon's horn. TN-C and the neurite growth-associated protein GAP-43 exhibited a complementary pattern of distribution. Densitometric and protein biochemical analysis showed a significant, 4.3-fold increase of TN-C in the hippocampus of TLE patients with AHS compared with normal hippocampus obtained at autopsy. This increase in TN-C immunoreactivity was accompanied by a loss of TN-C boundaries and closely correlated with the extent of reactive gliosis, as indicated by immunoreactivity for glial fibrillary acidic protein. Furthermore, a striking colocalization between TN-C and GAP-43 was observed in the DG-ML of patients with AHS. These observations raise the intriguing possibility of pathogenetically relevant glio-neuronal interactions in human TLE. GLIA 19:35–46, 1997. © 1997 Wiley-Liss, Inc.     

Muscarinic Receptor Loss and Preservation of Presynaptic Cholinergic Terminals in Hippocampal Sclerosis

Summary: Purpose: Prior single-photon emission tomography studies showed losses of muscarinic acetylcholine receptor (MAChR) binding in patients with refractory mesial temporal lobe epilepsy. Experimental animal studies demonstrated transient losses of MAChR due to electrically induced seizures originating in the amygdala. However, the relations between cholinergic synaptic markers, seizures, and underlying neuro-pathology in human temporal lobe epilepsy are unknow. We tested the hypotheses that human brain MAChR changes are attributable to hippocampal sclerosis (HS), and that HS resembles axon-sparing lesions in experimental animal models.
Methods: We measured MAChR binding-site density, an intrinsic neuronal marker, within the hippocampal formation (HF) in anterior temporal lobectomy specimens from 10 patients with HS and in 10 autopsy controls. Binding-site density of the presynaptic vesicular acetylcholine transporter (VAChT) was measured as a marker of extrinsic cholinergic afferent integrity. MAChR and VAChT results were compared with neuronal cell counts to assess their relations to local neuronal losses.
Results: Reduced MAChR binding-site density was demonstrated throughout the HF in the epilepsy specimens compared with autopsy controls and correlated in severity with reductions in cell counts in several HF regions. In contrast to MAChR, VAChT binding-site density was unchanged in the epilepsy specimens compared with autopsy controls.
Conclusions: Reduction in MAChR binding in HS is attributable to intrinsic neuronal losses. Sparing of afferent septal cholinergic terminals is consistent with the hypothesis that an excitotoxic mechanism may contribute to the development of HS and refractory partial epilepsy in humans.     

Neuropathologische Befunde bei Fieberkrämpfen

Patients suffering from drug-resistant temporal lobe epilepsy (TLE) frequently reported about prolonged febrile seizures during their early childhood. Neuropathological evaluation of neurosurgical specimens obtained from TLE patients with prolonged febrile seizures before the age of 4 years compared with specimens from TLE patients without febrile seizures has revealed a few interesting differences: Segmental neuronal cell loss is usually more severe, the architecture of the granule cell layer is usually more disturbed, and postsurgical seizure control is usually more successful. On the other hand, hippocampal cell loss was not observed in a rat model of febrile seizures. In this model, young rats were submitted to hyperthermia and experienced prolonged febrile convulsions. One-third of the animals developed chronic limbic epilepsy. This animal model, by unraveling the effects of prolonged febrile seizures on neuronal function, thereby identified several molecular pathomechanisms and the potential contribution of such seizures to epilepsy.     

Modulation of androgen and estrogen receptor expression by antiepileptic drugs and steroids in hippocampus of patients with temporal lobe epilepsy

Purpose:   Many of the antiepileptic drugs (AED) used in therapy of temporal lobe epilepsy (TLE) are known as cytochrome P450 (CYP, P450) inducers. These AEDs are thought to modulate androgen and estrogen pathways in hippocampus, and therefore cause mental and reproductive disorders found in TLE patients. In the present study, we analyzed expression of androgen receptor (AR), estrogen receptor α (ERα), and CYP3A in the hippocampus of TLE patients and in murine hippocampal cell line HN25.1.
Methods:   Patients and cell lines had been treated with P450-inducing or noninducing AEDs, or with prednisolone, applied to prevent oedema formation prior to neurosurgical resection of the epileptic hippocampus. Human patient samples were analyzed by immunohistochemical approach, the HN25.1 cell line by quantitative RT-PCR, CAT reporter gene assay, and immunoblot.
Results:   In both, humans and cell lines, the expression of testosterone metabolising CYP3A4 (human) or CYP3A11 (mouse) and AR was up-regulated when P450-inducing AEDs and/or prednisolone had been applied. AR responsive CAT reporter gene assay indicated an increase of AR-signalling after treatment of the HN25.1 cells with the P450-inducers phenytoin and carbamazepine. ERα expression was increased only by the P450-inducing AEDs, but not by prednisolone, which indicates that pathways different from CYP3A4/11 led to ERα enhancement.
Discussion:   We conclude that P450-inducing AEDs influence AR expression and signalling in hippocampus most likely via CYP3A4/11-induction. The HN25.1 cell line holds promise to investigate the correlation between drug application and AR regulation, and to specifically address issues that are relevant to human TLE patients.     

Hippocampal loss of N-methyl-D-aspartate receptor subunit 1 mRNA in chronic temporal lobe epilepsy

The hippocampal distribution of mRNA for the N-methyl-D-aspartate (NMDA) receptor subunit 1 (NR 1) was examined by non-radioactive in situ hybridization in 21 archival formalin-fixed and paraffin-embedded surgical specimens from patients with pharmacoresistant chronic epilepsy and in normal control specimens obtained at autopsy. Using the digoxigenin-labeling procedure, ribonucleotide probes were found to be significantly more sensitive than synthetic oligonucleotide probes. In normal autopsy specimens and in surgical specimens without Ammon's horn sclerosis there was intense NR 1 expression in a great majority of the dentate gyrus granular cells. Many neurons in the hippocampal pyramidal cell layer also revealed a strong signal intensity. The strata oriens and moleculare of Ammon's horn and the molecular layer of the dentate gyrus contained only few labeled neurons. In the subiculum and entorhinal cortex most neurons throughout various layers were positive. In hippocampal specimens of patients with chronic epilepsy there was a loss of NR 1-positive cells that was closely related to the overall neuronal loss in the respective specimen and to Ammon's horn sclerosis. These data suggest that the loss of NR 1 expression is a secondary phenomenon rather than an event that is relevant for the pathogenesis of epileptic seizures.     

Alterations of Neuronal Connectivity in Area CA1 of Hippocampal Slices from Temporal Lobe Epilepsy Patients and from Pilocarpine-Treated Epileptic Rats

Summary: Purpose : Neuronal network reorganization might be involved in epileptogenesis in human and rat limbic epilepsy. Apart from aberrant mossy fiber sprouting, a more wide-spread fiber rearrangement in the hippocampal formation might occur. Therefore, we studied sprouting in area CA1 because this region is most affected in human temporal lobe epilepsy.
Methods : In slices from hippocampi of patients operated on for temporal lobe epilepsy (n = 134), from pilocarpine-treated rats (n = 74), and from control rats (n = 15), viable neurons were labeled with fluorescent dextran amines.
Results : In human hippocampi as well as in pilocarpine-treated rats, the degree of nerve cell loss varied. In 67 of 134 slices from human specimens with distinct Ammon's horn sclerosis and in 23 of 74 slices from pilocarpine-treated rats, a severe shrunken area CA1 presented with a similar picture: few damaged neurons were labeled, and aberrant fiber connections were not visible. This was in contrast to human resected hippocampi and hippocampi from pilocarpine-treated rats with no or moderate loss of neurons. In these cases, pyramidal cells remote from the injection site were labeled (human tissue, n = 59 of 134; pilocarpine-treated rats, n = 39 of 74). In human resected hippocampi without obvious pathology and in control animals, no pyramidal neurons were labeled apart from the injection site.
Conclusions : Axon collaterals of CA1 pyramidal cells are increased in human temporal lobe epilepsy and in pilocarpine-treated rats. Adjacent CA1 pyramidal cells project via aberrant collaterals to the stratum pyramidale and the stratum radiatum of area CA1. This network reorganization can contribute to hyperexcitability via increased backward excitation.     

Increased expression of GABA(A) receptor beta-subunits in the hippocampus of patients with temporal lobe epilepsy

It has been postulated that dysfunction of the GABA-ergic transmission is causatively related to the development of epilepsy. Animal models of temporal lobe epilepsy (TLE) revealed considerable changes in the expression of GABA(A) receptor subunits in the hippocampus. Using immunocytochemistry, we investigated the expression of GABA(A) receptor subunits alpha1, alpha3, beta1-3, and gamma2 in hippocampal specimens obtained at surgery from TLE patients with and without hippocampal sclerosis and in autopsy controls. Consistent with the severe neurodegeneration in the CA1 sector, significant decreases in alpha1-, alpha3-, beta3-, and gamma2-subunit immunoreactivity (IR) were detected in sclerotic but not in nonsclerosic specimens. In contrast, pronounced increases in IR of all 3 beta-subunits were observed in most sectors of the hippocampal formation both in sclerotic and nonsclerotic specimens, being especially pronounced in the dentate molecular layer and in the subiculum where subunit alpha3- and gamma2-IR were also elevated. Using in situ hybridization for subunits beta2 and beta3, increased expression of the respective mRNAs was detected in dentate granule cells of patients with and without hippocampal sclerosis. Beta-subunits are important constituents of the GABA(A) receptor and contribute to the binding site of GABA. Our data indicate pronounced adaptive changes in the expression of these GABA(A) receptor subunits related to seizure activity and indicate altered assembly of GABA(A) receptors in TLE.     

Distinct increased metabotropic glutamate receptor type 5 (mGluR5) in temporal lobe epilepsy with and without hippocampal sclerosis

Metabotropic glutamate receptor type 5 (mGluR5) upregulation in temporal lobe epilepsy (TLE) and the correlation of its expression with features of hippocampal sclerosis (HS) remains unclear. Here we characterized mGluR5 immunoreactivity in hippocampus, entorhinal cortex (EC), and subiculum of TLE specimens with confirmed HS, with neocortical TLE (non‐HS) and necropsy controls. We correlated mGluR5 immunoreactivity with neuronal density, mossy fiber sprouting, astrogliosis (GFAP), and dendritic alterations (MAP2). TLE specimens showed increased mGluR5 expression, which was most pronounced in the EC, subiculum, CA2, and dentate gyrus outer molecular layer. Increased mGluR5 expression was seen in hippocampal head and body segments and was independent of neuronal density, astrogliosis, or dendritic alterations. Positive correlation between mGluR5 expression with mossy fiber sprouting and with MAP2 in CA3 and CA1 was found only in HS specimens. Negative correlation between mGluR5 expression with seizure frequency and epilepsy duration was found only in non‐HS cases. Specimens from HS patients without previous history of febrile seizure (FS) showed higher mGluR5 and MAP2 expression in CA2. Our study suggests that mGluR5 upregulation is part of a repertoire of post‐synaptic adaptations that might control overexcitation and excessive glutamate release rather than a dysfunction that leads to seizure facilitation. That would explain why non‐HS cases, on which seizures are likely to originate outside the hippocampal formation, also exhibit upregulated mGluR5. On the other hand, lower mGluR5 expression was related to increased seizure frequency. In addition to its role in hyperexcitability, mGluR5 upregulation could play a role in counterbalance mechanisms along the hyperexcitable circuitry uniquely altered in sclerotic hippocampal formation. Inefficient post‐synaptic compensatory morphological (dendritic branching) and glutamatergic (mGluR5 expression) mechanisms in CA2 subfield could potentially underlie the association of FS with HS and TLE. © 2013 The Authors. Hippocampus Published by Wiley Periodicals, Inc.