Synaptic and axonal remodeling of mossy fibers in the hilus and supragranular region of the dentate gyrus in kainate-treated rats

Diffusion Tensor Orientation as a Microstructural MRI Marker of Mossy Fiber Sprouting After TBI in Rats

Diffusion tensor imaging (DTI) metrics are highly sensitive to microstructural brain alterations and are potentially useful imaging biomarkers for underlying neuropathologic changes after experimental and human traumatic brain injury (TBI). As potential imaging biomarkers require direct correlation with neuropathologic alterations for validation and interpretation, this study systematically examined neuropathologic abnormalities underlying alterations in DTI metrics in the hippocampus and cortex following controlled cortical impact (CCI) in rats. Ex vivo DTI metrics were directly compared with a comprehensive histologic battery for neurodegeneration, microgliosis, astrocytosis, and mossy fiber sprouting by Timm histochemistry at carefully matched locations immediately, 48 hours, and 4 weeks after injury. DTI abnormalities corresponded to spatially overlapping but temporally distinct neuropathologic alterations representing an aggregate measure of dynamic tissue damage and reorganization. Prominent DTI alterations of were observed for both the immediate and acute intervals after injury and associated with neurodegeneration and inflammation. In the chronic period, diffusion tensor orientation in the hilus of the dentate gyrus became prominently abnormal and was identified as a reliable structural biomarker for mossy fiber sprouting after CCI in rats, suggesting potential application as a biomarker to follow secondary progression in experimental and human TBI.

mTOR-Related Cell-Clearing Systems in Epileptic Seizures, an Update

Recent evidence suggests that autophagy impairment is implicated in the epileptogenic mechanisms downstream of mTOR hyperactivation. This holds true for a variety of genetic and acquired epileptic syndromes besides malformations of cortical development which are classically known as mTORopathies. Autophagy suppression is sufficient to induce epilepsy in experimental models, while rescuing autophagy prevents epileptogenesis, improves behavioral alterations, and provides neuroprotection in seizure-induced neuronal damage. The implication of autophagy in epileptogenesis and maturation phenomena related to seizure activity is supported by evidence indicating that autophagy is involved in the molecular mechanisms which are implicated in epilepsy. In general, mTOR-dependent autophagy regulates the proliferation and migration of inter-/neuronal cortical progenitors, synapse development, vesicular release, synaptic plasticity, and importantly, synaptic clustering of GABA_A receptors and subsequent excitatory/inhibitory balance in the brain. Similar to autophagy, the ubiquitin–proteasome system is regulated downstream of mTOR, and it is implicated in epileptogenesis. Thus, mTOR-dependent cell-clearing systems are now taking center stage in the field of epilepsy. In the present review, we discuss such evidence in a variety of seizure-related disorders and models. This is expected to provide a deeper insight into the molecular mechanisms underlying seizure activity.

Early detonation by sprouted mossy fibers enables aberrant dentate network activity

In temporal lobe epilepsy, sprouting of hippocampal mossy fiber axons onto dentate granule cell dendrites creates a recurrent excitatory network. However, unlike mossy fibers projecting to CA3, sprouted mossy fiber synapses depress upon repetitive activation. Thus, despite their proximal location, relatively large presynaptic terminals, and ability to excite target neurons, the impact of sprouted mossy fiber synapses on hippocampal hyperexcitability is unclear. We find that despite their short-term depression, single episodes of sprouted mossy fiber activation in hippocampal slices initiated bursts of recurrent polysynaptic excitation. Consistent with a contribution to network hyperexcitability, optogenetic activation of sprouted mossy fibers reliably triggered action potential firing in postsynaptic dentate granule cells after single light pulses. This pattern resulted in a shift in network recruitment dynamics to an "early detonation" mode and an increased probability of release compared with mossy fiber synapses in CA3. A lack of tonic adenosine-mediated inhibition contributed to the higher probability of glutamate release, thus facilitating reverberant circuit activity.

Signaling Pathways and Cellular Mechanisms Regulating Mossy Fiber Sprouting in the Development of Epilepsy

The sprouting of hippocampal dentate granule cell axons, termed mossy fibers, into the dentate inner molecular layer is one of the most consistent findings in tissue from patients with mesial temporal lobe epilepsy. Decades of research in animal models have revealed that mossy fiber sprouting creates de novo recurrent excitatory connections in the hippocampus, fueling speculation that the pathology may drive temporal lobe epileptogenesis. Conducting definitive experiments to test this hypothesis, however, has been challenging due to the difficulty of dissociating this sprouting from the many other changes occurring during epileptogenesis. The field has been largely driven, therefore, by correlative data. Recently, the development of powerful transgenic mouse technologies and the discovery of novel drug targets has provided new tools to assess the role of mossy fiber sprouting in epilepsy. We can now selectively manipulate hippocampal granule cells in rodent epilepsy models, providing new insights into the granule cell subpopulations that participate in mossy fiber sprouting. The cellular pathways regulating this sprouting are also coming to light, providing new targets for pharmacological intervention. Surprisingly, many investigators have found that blocking mossy fiber sprouting has no effect on seizure occurrence, while seizure frequency can be reduced by treatments that have no effect on this sprouting. These results raise new questions about the role of mossy fiber sprouting in epilepsy. Here, we will review these findings with particular regard to the contributions of new granule cells to mossy fiber sprouting and the regulation of this sprouting by the mTOR signaling pathway.

Hippocampal Pathophysiology: Commonality Shared by Temporal Lobe Epilepsy and Psychiatric Disorders

Accumulating evidence points to the association of epilepsy, particularly, temporal lobe epilepsy (TLE), with psychiatric disorders, such as schizophrenia. Among these illnesses, the hippocampus is considered the regional focal point of the brain, playing an important role in cognition, psychosis, and seizure activity and potentially suggesting common etiologies and pathophysiology of TLE and schizophrenia. In the present review, we overview abnormal network connectivity between the dentate gyrus (DG) and the Cornus Ammonis area 3 (CA3) subregions of the hippocampus relative to the induction of epilepsy and schizophrenia. In light of our recent finding on the misguidance of hippocampal mossy fiber projection in the rodent model of schizophrenia, we discuss whether ectopic mossy fiber projection is a commonality in order to evoke TLE as well as symptoms related to schizophrenia.

Diffusion MRI and the detection of alterations following traumatic brain injury

This article provides a review of brain tissue alterations that may be detectable using diffusion magnetic resonance imaging MRI (dMRI) approaches and an overview and perspective on the modern dMRI toolkits for characterizing alterations that follow traumatic brain injury (TBI). Noninvasive imaging is a cornerstone of clinical treatment of TBI and has become increasingly used for preclinical and basic research studies. In particular, quantitative MRI methods have the potential to distinguish and evaluate the complex collection of neurobiological responses to TBI arising from pathology, neuroprotection, and recovery. dMRI provides unique information about the physical environment in tissue and can be used to probe physiological, architectural, and microstructural features. Although well‐established approaches such as diffusion tensor imaging are known to be highly sensitive to changes in the tissue environment, more advanced dMRI techniques have been developed that may offer increased specificity or new information for describing abnormalities. These tools are promising, but incompletely understood in the context of TBI. Furthermore, model dependencies and relative limitations may impact the implementation of these approaches and the interpretation of abnormalities in their metrics. The objective of this paper is to present a basic review and comparison across dMRI methods as they pertain to the detection of the most commonly observed tissue and cellular alterations following TBI.

Diffusion tensor MRI shows progressive changes in the hippocampus and dentate gyrus after status epilepticus in rat - histological validation with Fourier-based analysis

Imaging markers for monitoring disease progression, recovery, and treatment efficacy are a major unmet need for many neurological diseases, including epilepsy. Recent evidence suggests that diffusion tensor imaging (DTI) provides high microstructural contrast even outside major white matter tracts. We hypothesized that in vivo DTI could detect progressive microstructural changes in the dentate gyrus and the hippocampal CA3bc in the rat brain after status epilepticus (SE). To test this hypothesis, we induced SE with systemic kainic acid or pilocarpine in adult male Wistar rats and subsequently scanned them using in vivo DTI at five time-points: prior to SE, and 10, 20, 34, and 79 days post SE. In order to tie the DTI findings to changes in the tissue microstructure, myelin- and glial fibrillary acidic protein (GFAP)-stained sections from the same animals underwent Fourier analysis. We compared the Fourier analysis parameters, anisotropy index and angle of myelinated axons or astrocyte processes, to corresponding DTI parameters, fractional anisotropy (FA) and the orientation angle of the principal eigenvector. We found progressive detectable changes in DTI parameters in both the dentate gyrus (FA, axial diffusivity [D_||], linear anisotropy [CL] and spherical anisotropy [CS], p<0.001, linear mixed-effects model [LMEM]) and the CA3bc (FA, D_||, CS, and angle, p<0.001, LMEM; CL and planar anisotropy [CP], p<0.01, LMEM) post SE. The Fourier analysis revealed that both myelinated axons and astrocyte processes played a role in the water diffusion anisotropy changes detected by DTI in individual portions of the dentate gyrus (suprapyramidal blade, mid-portion, and infrapyramidal blade). In the whole dentate gyrus, myelinated axons markedly contributed to the water diffusion changes. In CA3bc as well as in CA3b and CA3c, both myelinated axons and astrocyte processes contributed to water diffusion anisotropy and orientation. Our study revealed that DTI is a promising method for noninvasive detection of microstructural alterations in the hippocampus proper. These alterations may be potential imaging markers for epileptogenesis.

Mesial Temporal Lobe Epilepsy: Pathogenesis, Induced Rodent Models and Lesions

Mesial temporal lobe epilepsy (MTLE), the most common epilepsy in adults, is generally intractable and is suspected to be the result of recurrent excitation or inhibition circuitry. Recurrent excitation and the development of seizures have been associated with aberrant mossy fiber sprouting in the hippocampus. Of the animal models developed to investigate the pathogenesis of MTLE, post-status epilepticus models have received the greatest acceptance because they are characterized by a latency period, the development of spontaneous motor seizures, and a spectrum of lesions like those of MTLE. Among post-status epilepticus models, induction of systemic kainic acid or pilocarpine-induced epilepsy is less labor-intensive than electrical-stimulation models and these models mirror the clinicopathologic features of MTLE more closely than do kindling, tetanus toxin, hyperthermia, post-traumatic, and perinatal hypoxia/ischemia models. Unfortunately, spontaneous motor seizures do not develop in kindling or adult hyperthermia models and are not a consistent finding in tetanus toxin-induced or perinatal hypoxia/ischemia models. This review presents the mechanistic hypotheses for seizure induction, means of model induction, and associated pathology, especially as compared to MTLE patients. Animal models are valuable tools not only to study the pathogenesis of MTLE, but also to evaluate potential antiepileptogenic drugs.

Axonal plasticity of age-defined dentate granule cells in a rat model of mesial temporal lobe epilepsy

Dentate granule cell (DGC) mossy fiber sprouting (MFS) in mesial temporal lobe epilepsy (mTLE) is thought to underlie the creation of aberrant circuitry which promotes the generation or spread of spontaneous seizure activity. Understanding the extent to which populations of DGCs participate in this circuitry could help determine how it develops and potentially identify therapeutic targets for regulating aberrant network activity. In this study, we investigated how DGC birthdate influences participation in MFS and other aspects of axonal plasticity using the rat pilocarpine-induced status epilepticus (SE) model of mTLE. We injected a retrovirus (RV) carrying a synaptophysin-yellow fluorescent protein (syp-YFP) fusion construct to birthdate DGCs and brightly label their axon terminals, and compared DGCs born during the neonatal period with those generated in adulthood. We found that both neonatal and adult-born DGC populations participate, to a similar extent, in SE-induced MFS within the dentate gyrus inner molecular layer (IML). SE did not alter hilar MF bouton density compared to sham-treated controls, but adult-born DGC bouton density was greater in the IML than in the hilus after SE. Interestingly, we also observed MF axonal reorganization in area CA2 in epileptic rats, and these changes arose from DGCs generated both neonatally and in adulthood. These data indicate that both neonatal and adult-generated DGCs contribute to axonal reorganization in the rat pilocarpine mTLE model, and indicate a more complex relationship between DGC age and participation in seizure-related plasticity than was previously thought.

PTEN deletion from adult-generated dentate granule cells disrupts granule cell mossy fiber axon structure

Dysregulation of the mTOR-signaling pathway is implicated in the development of temporal lobe epilepsy. In mice, deletion of PTEN from hippocampal dentate granule cells leads to mTOR hyperactivation and promotes the rapid onset of spontaneous seizures. The mechanism by which these abnormal cells initiate epileptogenesis, however, is unclear. PTEN-knockout granule cells develop abnormally, exhibiting morphological features indicative of increased excitatory input. If these cells are directly responsible for seizure genesis, it follows that they should also possess increased output. To test this prediction, dentate granule cell axon morphology was quantified in control and PTEN-knockout mice. Unexpectedly, PTEN deletion increased giant mossy fiber bouton spacing along the axon length, suggesting reduced innervation of CA3. Increased width of the mossy fiber axon pathway in stratum lucidum, however, which likely reflects an unusual increase in mossy fiber axon collateralization in this region, offsets the reduction in boutons per axon length. These morphological changes predict a net increase in granule cell innervation of CA3. Increased diameter of axons from PTEN-knockout cells would further enhance granule cell communication with CA3. Altogether, these findings suggest that amplified information flow through the hippocampal circuit contributes to seizure occurrence in the PTEN-knockout mouse model of temporal lobe epilepsy.

Age-dependent long-term structural and functional effects of early-life seizures: evidence for a hippocampal critical period influencing plasticity in adulthood

Neural activity promotes circuit formation in developing systems and during critical periods permanently modifies circuit organization and functional properties. These observations suggest that excessive neural activity, as occurs during seizures, might influence developing neural circuitry with long-term outcomes that depend on age at the time of seizures. We systematically examined long-term structural and functional consequences of seizures induced in rats by kainic acid, pentylenetetrazol, and hyperthermia across postnatal ages from birth through postnatal day 90 in adulthood (P90). Magnetic resonance imaging (MRI), diffusion tensor imaging (DTI), and electrophysiological methods at ⩾P95 following seizures induced from P1 to P90 demonstrated consistent patterns of gross atrophy, microstructural abnormalities in the corpus callosum (CC) and hippocampus, and functional alterations in hippocampal circuitry at ⩾P95 that were independent of the method of seizure induction and varied systematically as a function of age at the time of seizures. Three distinct epochs were observed in which seizures resulted in distinct long-term structural and functional outcomes at ⩾P95. Seizures prior to P20 resulted in DTI abnormalities in CC and hippocampus in the absence of gross cerebral atrophy, and increased paired-pulse inhibition (PPI) in the dentate gyrus (DG) at ⩾P95. Seizures after P30 induced a different pattern of DTI abnormalities in the fimbria and hippocampus accompanied by gross cerebral atrophy with increases in lateral ventricular volume, as well as increased PPI in the DG at ⩾P95. In contrast, seizures between P20 and P30 did not result in cerebral atrophy or significant imaging abnormalities in the hippocampus or white matter, but irreversibly decreased PPI in the DG compared to normal adult controls. These age-specific long-term structural and functional outcomes identify P20-30 as a potential critical period in hippocampal development defined by distinctive long-term structural and functional properties in adult hippocampal circuitry, including loss of capacity for seizure-induced plasticity in adulthood that could influence epileptogenesis and other hippocampal-dependent behaviors and functional properties.

Blockade of excitatory synaptogenesis with proximal dendrites of dentate granule cells following rapamycin treatment in a mouse model of temporal lobe epilepsy

Inhibiting the mammalian target of rapamycin (mTOR) signaling pathway with rapamycin blocks granule cell axon (mossy fiber) sprouting after epileptogenic injuries, including pilocarpine-induced status epilepticus. However, it remains unclear whether axons from other types of neurons sprout into the inner molecular layer and synapse with granule cell dendrites despite rapamycin treatment. If so, other aberrant positive-feedback networks might develop. To test this possibility stereological electron microscopy was used to estimate the numbers of excitatory synapses in the inner molecular layer per hippocampus in pilocarpine-treated control mice, in mice 5 days after pilocarpine-induced status epilepticus, and after status epilepticus and daily treatment beginning 24 hours later with rapamycin or vehicle for 2 months. The optical fractionator method was used to estimate numbers of granule cells in Nissl-stained sections so that numbers of excitatory synapses in the inner molecular layer per granule cell could be calculated. Control mice had an average of 2,280 asymmetric synapses in the inner molecular layer per granule cell, which was reduced to 63% of controls 5 days after status epilepticus, recovered to 93% of controls in vehicle-treated mice 2 months after status epilepticus, but remained at only 63% of controls in rapamycin-treated mice. These findings reveal that rapamycin prevented excitatory axons from synapsing with proximal dendrites of granule cells and raise questions about the recurrent excitation hypothesis of temporal lobe epilepsy.

Morphologic integration of hilar ectopic granule cells into dentate gyrus circuitry in the pilocarpine model of temporal lobe epilepsy

After pilocarpine-induced status epilepticus, many granule cells born into the postseizure environment migrate aberrantly into the dentate hilus. Hilar ectopic granule cells (HEGCs) are hyperexcitable and may therefore increase circuit excitability. This study determined the distribution of their axons and dendrites. HEGCs and normotopic granule cells were filled with biocytin during whole-cell patch clamp recording in hippocampal slices from pilocarpine-treated rats. The apical dendrite of 86% of the biocytin-labeled HEGCs extended to the outer edge of the dentate molecular layer. The total length and branching of HEGC apical dendrites that penetrated the molecular layer were significantly reduced compared with apical dendrites of normotopic granule cells. HEGCs were much more likely to have a hilar basal dendrite than normotopic granule cells. They were about as likely as normotopic granule cells to project to CA3 pyramidal cells within the slice, but were much more likely to send at least one recurrent mossy fiber into the molecular layer. HEGCs with burst capability had less well-branched apical dendrites than nonbursting HEGCs, their dendrites were more likely to be confined to the hilus, and some exhibited dendritic features similar to those of immature granule cells. HEGCs thus have many paths along which to receive synchronized activity from normotopic granule cells and to transmit their own hyperactivity to both normotopic granule cells and CA3 pyramidal cells. They may therefore contribute to the highly interconnected granule cell hubs that have been proposed as crucial to development of a hyperexcitable, potentially seizure-prone circuit.

Regional changes in gene expression after limbic kindling

Repeated electrical stimulation results in development of seizures and a permanent increase in seizure susceptibility (kindling). The permanence of kindling suggests that chronic changes in gene expression are involved. Kindling at different sites produces specific effects on interictal behaviors such as spatial cognition and anxiety, suggesting that causal changes in gene expression might be restricted to the stimulated site. We employed focused microarray analysis to characterize changes in gene expression associated with amygdaloid and hippocampal kindling. Male Long-Evans rats received 1 s trains of electrical stimulation to either the amygdala or hippocampus once daily until five generalized seizures had been kindled. Yoked control rats carried electrodes but were not stimulated. Rats were euthanized 14 days after the last seizures, both amygdala and hippocampus dissected, and transcriptome profiles compared. Of the 1,200 rat brain-associated genes evaluated, 39 genes exhibited statistically significant expression differences between the kindled and non-kindled amygdala and 106 genes exhibited statistically significant differences between the kindled and non-kindled hippocampus. In the amygdala, subsequent ontological analyses using the GOMiner algorithm demonstrated significant enrichment in categories related to cytoskeletal reorganization and cation transport, as well as in gene families related to synaptic transmission and neurogenesis. In the hippocampus, significant enrichment in gene expression within categories related to cytoskeletal reorganization and cation transport was similarly observed. Furthermore, unique to the hippocampus, enrichment in transcription factor activity and GTPase-mediated signal transduction was identified. Overall, these data identify specific and unique neurochemical pathways chronically altered following kindling in the two sites, and provide a platform for defining the molecular basis for the differential behaviors observed in the interictal period.

The ratio of 'deleted in colorectal cancer' to 'uncoordinated-5A' netrin-1 receptors on the growth cone regulates mossy fibre directionality

Proper axonal targeting is fundamental to the establishment of functional neural circuits. The hippocampal mossy fibres normally project towards the CA3 region. In the hippocampi of patients with temporal lobe epilepsy and related animal models, however, mossy fibres project towards the molecular layer and produce the hyperexcitable recurrent networks. The cellular and molecular mechanisms underlying this aberrant axonal targeting, known as mossy fibre sprouting, remain unclear. Netrin-1 attracts or repels axons depending on the composition of its attraction-mediating receptor, deleted in colorectal cancer, and its repulsion-mediating receptor, uncoordinated-5, on the growth cone; but the roles of netrin-1-dependent guidance in pathological conditions are largely unknown. In this study, we examined the role of netrin-1 and its receptors in mossy fibre guidance and report that enhanced neuronal activity changes netrin-1-mediated cell targeting by the axons under hyperexcitable conditions. Netrin-1 antibody or Dcc ribonucleic acid interference attenuated mossy fibre growth towards CA3 in slice overlay assays. The axons were repelled from CA3 and ultimately innervated the molecular layer when hyperactivity was pharmacologically introduced. We first hypothesized that a reduction in netrin-1 expression in CA3 underlies the phenomenon, but found that its expression was increased. We then examined two possible activity-dependent changes in netrin-1 receptor expression: a reduction in the deleted in colorectal cancer receptor and induction of uncoordinated-5 receptor. Hyperactivity did not affect the surface expression of the deleted in colorectal cancer receptor on the growth cone, but it increased that of uncoordinated-5A, which was suppressed by blocking cyclic adenosine monophosphate signalling. In addition, Dcc knockdown did not affect hyperactivity-induced mossy fibre sprouting in the slice cultures, whereas Unc5a knockdown rescued the mistargeting. Thus, netrin-1 appears to attract mossy fibres via the deleted in colorectal cancer receptor, while it repels them via cyclic adenosine monophosphate-induced uncoordinated-5A under hyperexcitable conditions, resulting in mossy fibre sprouting.

Dentate granule cells form hilar basal dendrites in a rat model of hypoxia-ischemia

Hilar basal dendrites form on dentate granule cells following seizures. To determine whether other brain insults cause the formation of hilar basal dendrites, a model of global cerebral hypoxia/ischemia was used. Rats underwent a transient induction of ischemia by occlusion of both common carotid arteries followed by reperfusion. Hippocampal slices were prepared from these animals 1 month after the ischemic insult, and granule cells were labeled with a retrograde tracing technique after biocytin injections into stratum lucidum of CA3b. Ischemic rats had numerous biocytin-labeled granule cells with hilar basal dendrites located at the hilar border of the granule cell layer. Quantitative analysis of ischemic rats compared to controls showed a significant increase in the percentage of biocytin-labeled granule cells with hilar basal dendrites. These data demonstrate that other brain insults in addition to epilepsy may result in the formation of hilar basal dendrites on granule cells.

Temporal Profile of Clinical Signs and Histopathologic Changes in an F-344 Rat Model of Kainic Acid–induced Mesial Temporal Lobe Epilepsy

Since there is limited information in the literature, the purpose of this study was to investigate clinical signs, morphology, and temporal progression of lesions from Days 3 to 168 in a kainic acid (KA)-induced Fischer-344 (F-344) rat model of mesial temporal lobe epilepsy (MTLE). Following a single KA subcutaneous dose of 9 mg/kg to young adult male rats, 95% survived, 93% exhibited status epilepticus, and 80% eventually developed spontaneous motor seizures. Histopathology included hematoxylin and eosin (H&E), autofluorescence, Fluoro-Jade B, Timm’s, ED-1/CD68, GFAP, doublecortin, and Ki-67. Neuronal degeneration occurred on Day 3 in the hippocampal CA1, CA3, and dentate hilar regions; amyg-daloid and thalamic nuclei; and frontoparietotemporal, entorhinal and piriform cortices. Degeneration severity peaked on Day 6 and decreased progressively until Day 168. Aberrant mossy fiber (MF) sprouting was present in the inner molecular layer of dentate gyrus on Days 6–168. Microliosis and astrogliosis peaked on Day 28 and generally colocalized with the distribution of neuronal degeneration. Important correlates to human MTLE included induction of spontaneous seizures, more severe neuronal damage of CA1 than CA3 (in contrast to other animal models but similar to humans), hilar neuronal loss, activated microgliosis and astrogliosis, aberrant MF sprouting, and dentate granule cell neurogenesis. Aberrant MF sprouting prior to spontaneous motor seizures and reduced seizure frequency with a decrease in aberrant MF sprouting support the hypothesis that MF sprouts are necessary for spontaneous seizure generation and maintenance.

Important role of matrix metalloproteinase 9 in epileptogenesis

Temporal lobe epilepsy (TLE) is a devastating disease in which aberrant synaptic plasticity plays a major role. We identify matrix metalloproteinase (MMP) 9 as a novel synaptic enzyme and a key pathogenic factor in two animal models of TLE: kainate-evoked epilepsy and pentylenetetrazole (PTZ) kindling–induced epilepsy. Notably, we show that the sensitivity to PTZ epileptogenesis is decreased in MMP-9 knockout mice but is increased in a novel line of transgenic rats overexpressing MMP-9. Immunoelectron microscopy reveals that MMP-9 associates with hippocampal dendritic spines bearing asymmetrical (excitatory) synapses, where both the MMP-9 protein levels and enzymatic activity become strongly increased upon seizures. Further, we find that MMP-9 deficiency diminishes seizure-evoked pruning of dendritic spines and decreases aberrant synaptogenesis after mossy fiber sprouting. The latter observation provides a possible mechanistic basis for the effect of MMP-9 on epileptogenesis. Our work suggests that a synaptic pool of MMP-9 is critical for the sequence of events that underlie the development of seizures in animal models of TLE.

Synaptic localization of seizure-induced matrix metalloproteinase-9 mRNA

The phenomenon of dendritic transport and local translation of mRNA is considered to be one of the most fundamental mechanisms underlying long-term synaptic plasticity. Matrix metalloproteinase 9 (gelatinase B) (MMP-9) is a matrix metalloproteinase implicated in synaptic long-term potentiation and hippocampus-dependent memory. It was recently shown to be prominently up-regulated in the hippocampal dentate gyrus (DG) upon kainate-mediated seizures. Here, using a high resolution nonradioactive in situ hybridization at the light- and electron-microscopic levels, as well as subcellular fractionation, we provide evidence that in the rat hippocampus, MMP-9 mRNA is associated with dendrites and dendritic spines bearing asymmetric (excitatory) synapses. Moreover we observe that after kainate treatment the number of dendrites and synapses containing MMP-9 mRNA increases markedly. Our results indicate that we are observing the phenomenon of dendritic transport of seizure-induced MMP-9 mRNA.

Doublecortin-positive newly born granule cells of hippocampus have abnormal apical dendritic morphology in the pilocarpine model of temporal lobe epilepsy

Here, we describe dentate gyrus newly born granule cells morphology in rats' temporal lobe epilepsy pilocarpine model. Digital reconstruction of doublecortin-positive neurons revealed that apical dendrites had the same total length and number of nodes in epileptic and control animals. Nonetheless, concentric spheres analyses revealed that apical dendrites spatial distribution was radically altered in epileptic animals. The apical dendrites had more bifurcations inside the granular cell layer and more terminations in the inner molecular layer of epileptic dentate gyrus. Branch order analyses showed that second- and third-order dendrites were shorter in epileptic animals. Apical dendrites were concentrated in regions like the inner molecular layer where granular neuron axons, named mossy fibers, sprout in epileptic animals. The combination of altered dendritic morphology and number enhancement of the new granular neurons suggests a deleterious role of hippocampal neurogenesis in epileptogenesis. Being more numerous and with dendrites concentrated in regions where aberrant axon terminals sprout, the new granular neurons could contribute to the slow epileptogenesis at hippocampal circuits commonly observed in temporal lobe epilepsy.

Changes in vesicular transporters for gamma-aminobutyric acid and glutamate reveal vulnerability and reorganization of hippocampal neurons following pilocarpine-induced seizures

The reorganizations of the overall intrinsic glutamatergic and gamma-aminobutyric acid (GABA)-ergic hippocampal networks as well as the time course of these reorganizations during development of pilocarpine-induced temporal lobe epilepsy were studied with in situ hybridization and immunohistochemistry experiments for the vesicular glutamate transporter 1 (VGLUT1) and the vesicular GABA transporter (VGAT). These transporters are particularly interesting as specific markers for glutamatergic and GABAergic neurons, respectively, whose expression levels could reflect the demand for synaptic transmission and their average activity. We report that 1) concomitantly with the loss of some subpopulations of VGAT-containing neurons, there was an up-regulation of VGAT synthesis in all remaining GABA neurons as early as 1 week after pilocarpine injection. This enhanced synthesis is characterized by marked increases in the relative amount of VGAT mRNAs in interneurons associated with increased intensity of axon terminal labeling for VGAT in all hippocampal layers. 2) There was a striking loss of mossy cells during the latent period, demonstrated by a long-term decrease of VGLUT1 mRNA-containing hilar neurons and associated loss of VGLUT1-containing terminals in the dentate gyrus inner molecular layer. 3) There were aberrant VGLUT1-containing terminals at the chronic stage resulting from axonal sprouting of granule and pyramidal cells. This is illustrated by a recovery of VGLUT1 immunoreactivity in the inner molecular layer and an increased VGLUT1 immunolabeling in the CA1-CA3 dendritic layers. These data indicate that an increased activity of remaining GABAergic interneurons occurs during the latent period, in parallel with the loss of vulnerable glutamatergic and GABAergic neurons preceding the reorganization of glutamatergic networks.

Unmasking recurrent excitation generated by mossy fiber sprouting in the epileptic dentate gyrus: an emergent property of a complex system

Seizure-induced sprouting of the mossy fiber pathway in the dentate gyrus has been observed nearly universally in experimental models of limbic epilepsy and in the epileptic human hippocampus. The observation of progressive mossy fiber sprouting induced by kindling demonstrated that even a few repeated seizures are sufficient to alter synaptic connectivity and circuit organization. As it is now recognized that seizures induce synaptic reorganization in hippocampal and cortical pathways, the implications of seizure-induced synaptic reorganization for circuit properties and function have been subjects of intense interest. Detailed anatomical characterization of the sprouted mossy fiber pathway has revealed that the overwhelming majority of sprouted synapses in the inner molecular layer of the dentate gyrus form recurrent excitatory connections, and are thus likely to contribute to recurrent excitation and potentially to enhanced susceptibility to seizures. Nevertheless, difficulties in detecting functional abnormalities in circuits reorganized by mossy fiber sprouting and the fact that some sprouted axons appear to form synapses with inhibitory interneurons have been cited as evidence that sprouting may not contribute to seizure susceptibility, but could form recurrent inhibitory circuits and be a compensatory response to prevent seizures. Quantitative analysis of the synaptic connections of the sprouted mossy fiber pathway, assessment of the functional features of sprouted circuitry using reliable physiological measures, and the perspective of complex systems analysis of neural circuits strongly support the view that the functional effects of the recurrent excitatory circuits formed by mossy fiber sprouting after seizures or injury emerge only conditionally and intermittently, as observed with spontaneous seizures in human epilepsy. The recognition that mossy fiber sprouting is induced after hippocampal injury and seizures and contributes conditionally to emergence of recurrent excitation has provided a conceptual framework for understanding how injury and seizure-induced circuit reorganization may contribute to paroxysmal network synchronization, epileptogenesis, and the consequences of repeated seizures, and thus has had a major influence on understanding of fundamental aspects of the epilepsies.

Hippocampal mossy fiber sprouting and elevated trkB receptor expression following systemic administration of low dose domoic acid during neonatal development

We have previously reported that serial systemic injections of low-dose (subconvulsive) domoic acid (DOM) during early postnatal development produces changes in both behavior and hippocampal cytoarchitecture in aged rats (17 months) that are similar to those seen in existing animal models of temporal lobe epilepsy. Herein we report further hippocampal changes, consisting of mossy fiber sprouting and associated changes in the trkB receptor population in young adult (3 months) rats, and further, report that these changes show regional variation throughout the septo-temporal axis of the hippocampus. Groups of Sprague Dawley rat pups were injected daily from postnatal day 8-14 with either saline (n = 23) or 20 microg/kg DOM (n = 25), tested for key indicators of neonatal neurobehavioral development, and then left undisturbed until approximately 90 days of age, at which time brain tissue was removed, hippocampi were dissected, fixed and processed using either Timm's stain to visualize hippocampal mossy fiber sprouting (MFS) or trkB immunohistochemistry to visualize full length trkB receptors. Multiple sections from dorsal, mid, and ventral hippocampus were analyzed separately and all measures were conducted using image analysis software. The results indicate significant increases in MFS in the inner molecular layer in treated animals with corresponding changes in trkB receptor density. Further we identified significant increases in trkB receptor density in the hilus of the dentate gyrus and area CA3 and report increased mossy fiber terminal density in the stratum lucidum in treated rats. The magnitude of these changes differed between sections from dorsal, mid, and ventral hippocampus. We conclude that low dose neonatal DOM produces cytoarchitectural changes indicative of abnormal development and/or synaptic plasticity that are progressive with age and show regional variation within the hippocampal formation.

Injury-induced axonal sprouting in the hippocampus is initiated by activation of trkB receptors

Penetrating head injuries are often accompanied by the delayed development of post-traumatic epilepsy. Schaffer collateral transection leads to axonal sprouting and hyperexcitability in area CA3 of hippocampal slice cultures. We used this model to test the hypothesis that the injury-induced axonal sprouting results from increased neurotrophin signaling via trkB receptors near the lesion. Using rats and mice, we established that sprouting CA3 pyramidal cell axons are labeled with an antibody to the growth-associated protein GAP-43. We observed two- to threefold increases in the level of brain-derived neurotrophic factor and trkB protein in area CA3 by 24-48 h after Schaffer collateral transection, preceding the onset of axonal sprouting. Finally, we demonstrated that injury-induced axonal sprouting of GAP-43-immunoreactive axons is impaired in hippocampal slice cultures from mice expressing low levels of trkB receptors. We conclude that injury-induced axonal sprouting is initiated by brain-derived neurotrophic factor-trkB signaling and suggest that this process may be critical for the genesis of post-traumatic epilepsy.

Hippocampal neurodegeneration, spontaneous seizures, and mossy fiber sprouting in the F344 rat model of temporal lobe epilepsy

The links among the extent of hippocampal neurodegeneration, the frequency of spontaneous recurrent motor seizures (SRMS), and the degree of aberrant mossy fiber sprouting (MFS) in temporal lobe epilepsy (TLE) are a subject of contention because of variable findings in different animal models and human studies. To understand these issues further, we quantified these parameters at 3-5 months after graded injections of low doses of kainic acid (KA) in adult F344 rats. KA was administered every 1 hr for 4 hr, for a cumulative dose of 10.5 mg/kg bw, to induce continuous stages III-V motor seizures for >3 hr. At 4 days post-KA, the majority of rats (77%) exhibited moderate bilateral neurodegeneration in different regions of the hippocampus; however, 23% of rats exhibited massive neurodegeneration in all hippocampal regions. All KA-treated rats displayed robust SRMS at 3 months post-KA, and the severity of SRMS increased over time. Analyses of surviving neurons at 5 months post-KA revealed two subgroups of rats, one with moderate hippocampal injury (HI; 55% of rats) and another with widespread HI (45%). Rats with widespread HI exhibited greater loss of CA3 pyramidal neurons and robust aberrant MFS than rats with moderate HI. However, the frequency of SRMS (approximately 3/hr) was comparable between rats with moderate and widespread HI. Thus, in comparison with TLE model using Sprague-Dawley rats (Hellier et al. [1998] Epilepsy Res. 31:73-84), a much lower cumulative dose of KA leads to robust chronic epilepsy in F344 rats. Furthermore, the occurrence of SRMS in this model is always associated with considerable bilateral hippocampal neurodegeneration and aberrant MFS. However, more extensive hippocampal CA3 cell loss and aberrant MFS do not appear to increase the frequency of SRMS. Because most of the features are consistent with mesial TLE in humans, the F344 model appears ideal for testing the efficacy of potential treatment strategies for mesial TLE.

Enhanced excitatory synaptic connectivity in layer v pyramidal neurons of chronically injured epileptogenic neocortex in rats

Formation of new recurrent excitatory circuits after brain injuries has been hypothesized as a major factor contributing to epileptogenesis. Increases in total axonal length and the density of synaptic boutons are present in layer V pyramidal neurons of chronic partial isolations of rat neocortex, a model of posttraumatic epileptogenesis. To explore the functional consequences of these changes, we used laser-scanning photostimulation combined with whole-cell patch-clamp recording from neurons in layer V of somatosensory cortex to map changes in excitatory synaptic connectivity after injury. Coronal slices were submerged in artificial CSF (23 degrees C) containing 100 microM caged glutamate, APV (2-amino-5-phosphonovaleric acid), and high divalent cation concentration to block polysynaptic responses. Focal uncaging of glutamate, accomplished by switching a pulsed UV laser to give a 200-400 micros light stimulus, evoked single- or multiple-component composite EPSCs. In neurons of the partially isolated cortex, there were significant increases in the fraction of uncaging sites from which EPSCs could be evoked ("hot spots") and a decrease in the mean amplitude of individual elements in the composite EPSC. When plotted along the cortical depth, the changes in EPSCs took place mainly between 150 and 200 microm above and below the somata, suggesting a specific enhancement of recurrent excitatory connectivity among layer V pyramidal neurons of the undercut neocortex. These changes may shift the balance within cortical circuits toward increased synaptic excitation and contribute to epileptogenesis.

The role of synaptic reorganization in mesial temporal lobe epilepsy

The mechanisms underlying mesial temporal lobe epilepsy (MTLE) remain uncertain. Putative mechanisms should account for several features characteristic of the clinical presentation and the neurophysiological and neuropathological abnormalities observed in patients with intractable MTLE. Synaptic reorganization of the mossy fiber pathway has received considerable attention over the past two decades as a potential mechanism that increases the excitability of the hippocampal network through the formation of new recurrent excitatory collaterals. Morphological plasticity beyond the mossy fiber pathway has not been as thoroughly investigated. Recently, plasticity of the CA1 pyramidal axons has been demonstrated in acute and chronic experimental models of MTLE. As the hippocampal formation is topographically organized in stacks of slices (lamellae), synaptic reorganization of CA1 axons projecting to subiculum appears to increase the connectivity between lamellae, providing a mechanism for translamellar synchronization of cellular hyperexcitability, leading to pharmacologically intractable seizures.

Chronic ethanol exposure and protracted abstinence alter NMDA receptors in central amygdala

We recently reported that chronic ethanol treatment (CET) and early withdrawal (2-8 h) altered glutamatergic transmission at both pre- and postsynaptic sites in central nucleus of the amygdala (CeA). Acute ethanol (44 mM) inhibited the NMDA receptor (NMDAR)-mediated EPSCs (NMDA-EPSCs) more in CeA neurons from CET rats than from naïve rats and also decreased paired-pulse facilitation (PPF) of NMDA-EPSCs only in CET rats. To determine whether these CET effects persisted after prolonged withdrawal, we recorded intracellularly in rat CeA slices and measured mRNA and protein expression of CeA NMDAR subunits from CET rats and those withdrawn from ethanol for 1 or 2 weeks. At 1 week withdrawal, acute ethanol decreased evoked NMDA-EPSC amplitudes and NMDA currents induced by exogenous NMDA ( approximately 20%) equally to that in naïve rats, indicating that CET effects on postsynaptic mechanisms reversed 1 week after CET cessation. However, acute ethanol still decreased PPF of NMDA-EPSCs, indicating that the acute ethanol-induced increase in glutamate release in CeA seen in CET rats was still present at this time. CET also significantly increased mRNA levels of NR1 and NR2B NMDAR subunits compared to control rats. At 1 week withdrawal, mRNA levels for NR1 and NR2B subunits were significantly decreased. These changes reversed at 2 weeks withdrawal. In Western blots, a significant increase in protein for all three subunits occurred in CeA from CET rats, but not after 1 and 2 weeks of withdrawal. These data indicate that CET induces reversible neuroadaptations in synaptic function, gene expression, and protein composition of NMDAR at CeA synapses.

Mossy fibers are the primary source of afferent input to ectopic granule cells that are born after pilocarpine-induced seizures

Granule cell (GC) neurogenesis increases following seizures, and some newborn GCs develop in abnormal locations within the hilus. These ectopic GCs (EGCs) display robust spontaneous and evoked excitatory activity. However, the pattern of afferent input they receive has not been fully defined. This study used electron microscopic immunolabeling to quantitatively evaluate mossy fiber (MF) input to EGCs since MFs densely innervate the hilus normally and undergo sprouting in many animal models of epilepsy. EGC dendrites were examined in tissue from epileptic rats that had initially been treated with pilocarpine to induce status epilepticus and subsequently had spontaneous seizures. MF terminals were labeled with a zinc transporter-3 antibody, and calbindin immunoreactivity was used to label hilar EGCs and GC layer GCs. The pattern of input provided by sprouted MF terminals to EGC dendrites was then compared to the pattern of MF input to GC dendrites in the inner molecular layer (IML), where most sprouted fibers are thought to project. Analysis of EGC dendrites demonstrated that MF terminals represented their predominant source of afferent input: they comprised 63% of all terminals and, on average, occupied 40% and 29% of the dendritic surface in the dorsal and ventral dentate gyrus, respectively, forming frequent synapses. These measures of connectivity were significantly greater than comparable values for MF innervation of GC dendrites located in the IML of the same tissue sections. Thus, EGCs develop a pattern of synaptic connections that could help explain their previously identified predisposition to discharge in epileptiform bursts and suggest that they play an important role in the generation of seizure activity in the dentate gyrus.

Axonal sprouting of GABAergic interneurons in temporal lobe epilepsy

Temporal lobe epilepsy is one of the most common forms of epilepsy. Numerous contributing factors and compensatory mechanisms have been associated with temporal lobe epilepsy. One feature found in both humans and animal models is sprouting of hippocampal principal cell axons, which suggests that axonal sprouting may be a general phenomenon associated with temporal lobe epilepsy. This article highlights the evidence showing that hippocampal GABAergic interneurons also undergo axonal sprouting in temporal lobe epilepsy. The caveats and unanswered questions associated with the current data and the potential physiological consequences of reorganizations in GABAergic circuits are discussed.

Gene profiling the response to kainic acid induced seizures

Kainic acid activates non-N-methyl-d-aspartate (NMDA) glutamate receptors where it increases synaptic activity resulting in seizures, neurodegeneration, and remodeling. We performed microarray analysis on rat hippocampal tissue following kainic acid treatment in order to study the signaling mechanisms underlying these diverse processes in an attempt to increase our current understanding of mechanisms contributing to such fundamental processes as neuronal protection and neuronal plasticity. The kainic acid-treated rats used in our array experiments demonstrated severe seizure behavior that was also accompanied by neuronal degeneration which is suggested by fluoro-jade B staining and anti-caspase-3 immunohistochemistry. The gene profile revealed 36 novel kainic acid regulated genes along with additional genes previously reported. The functional roles of these novel genes are discussed. These genes mainly have roles in transcription and to a lesser extent have roles in cell death, extracellular matrix remodeling, cell cycle progression, neuroprotection, angiogenesis, and synaptic signaling. Gene regulation was confirmed via quantitative real time polymerase chain reaction and in situ hybridization.

Differential upregulation of extracellular matrix molecules associated with the appearance of granule cell dispersion and mossy fiber sprouting during epileptogenesis in a murine model of temporal lobe epilepsy

We have investigated changes in the extracellular matrix of the hippocampus associated with the early progression of epileptogenesis in a murine model of temporal lobe epilepsy using immunohistochemistry. In the first week following intrahippocampal injection of the glutamate agonist, domoate, there is a latent period at the end of which begins a sequential upregulation of extracellular matrix (ECM) molecules in the granule cell layer of the dentate gyrus, beginning with neurocan and tenascin-C. This expression precedes the characteristic dispersion of the granule cell layer which is evident at 14 days post-injection when the first recurrent seizures can be recorded. At this stage, an upregulation of the chondroitin sulfate proteoglycan, phosphacan, the DSD-1 chondroitin sulfate motif, and the HNK-1 oligosaccharide are also observed. The expression of these molecules is localized differentially in the epileptogenic dentate gyrus, especially in the sprouting molecular layer, where a strong upregulation of phosphacan, tenascin-C, and HNK-1 is observed but there is no expression of the proteoglycan, neurocan, nor of the DSD-1 chondroitin sulfate motif. Hence, it appears that granule cell layer dispersion is accompanied by a general increase in the ECM, while mossy fiber sprouting in the molecular layer is associated with a more restricted repertoire. In contrast to these changes, the expression of the ECM glycoproteins, laminin and fibronectin, both of which are frequently implicated in tissue remodelling events, showed no changes associated with either granule cell dispersion or mossy fiber sprouting, indicating that the epileptogenic plasticity of the hippocampus is accompanied by ECM interactions that are characteristic of the CNS.

Contributions of Mossy Fiber and CA1 Pyramidal Cell Sprouting to Dentate Granule Cell Hyperexcitability in Kainic Acid–Treated Hippocampal Slice Cultures

Axonal sprouting like that of the mossy fibers is commonly associated with temporal lobe epilepsy, but its significance remains uncertain. To investigate the functional consequences of sprouting of mossy fibers and alternative pathways, kainic acid (KA) was used to induce robust mossy fiber sprouting in hippocampal slice cultures. Physiological comparisons documented many similarities in granule cell responses between KA- and vehicle-treated cultures, including: seizures, epileptiform bursts, and spontaneous excitatoty postsynaptic currents (sEPSCs) >600pA. GABAergic control and contribution of glutamatergic synaptic transmission were similar. Analyses of neurobiotin-filled CA1 pyramidal cells revealed robust axonal sprouting in both vehicle- and KA-treated cultures, which was significantly greater in KA-treated cultures. Hilar stimulation evoked an antidromic population spike followed by variable numbers of postsynaptic potentials (PSPs) and population spikes in both vehicle- and KA-treated cultures. Despite robust mossy fiber sprouting, knife cuts separating CA1 from dentate gyrus virtually abolished EPSPs evoked by hilar stimulation in KA-treated but not vehicle-treated cultures, suggesting a pivotal role of functional afferents from CA1 to dentate gyrus in KA-treated cultures. Together, these findings demonstrate striking hyperexcitability of dentate granule cells in long-term hippocampal slice cultures after treatment with either vehicle or KA. The contribution to hilar-evoked hyperexcitability of granule cells by the unexpected axonal projection from CA1 to dentate in KA-treated cultures reinforces the idea that axonal sprouting may contribute to pathologic hyperexcitability of granule cells.

Differential Expression of Phospholipase D Isozymes in the Hippocampus Following Kainic Acid-Induced Seizures

To investigate the pathophysiological role of phospholipase D (PLD)-mediated signaling, changes in the expression of the PLD isozymes PLD1 and PLD2 were investigated in the rat kainic acid (KA) model of human temporal lobe epilepsy. Western blot analysis showed a significant increase in the expression of PLD1 and PLD2 in the postictal hippocampus. PLD1 immunoreactivity increased preferentially in the CA3 and CA1 regions, where pyramidal neurons are susceptible to temporal lobe epilepsy. Experiments employing double immunofluorescence revealed that the cells expressing PLD1 were GFAP-expressing reactive astrocytes. By contrast, PLD2 immunoreactivity increased strikingly in infrapyramidal, but not in suprapyramidal granule cells of the postictal dentate gyrus, fitting well with results of the PLD activity assay. Considering that PLD belongs to a key signaling pathway, this result suggests that changes in granule cell activity in the dentate gyrus after seizures occurs specifically between the supra- and infrapyramidal blades. In addition, enhanced immunoreactivity of PLD2 was observed in the reactive astrocytes of the CA1, CA3, and hilar subregions, but its temporal pattern is different from that of PLD1. Taken together, our results suggest that PLD1 and PLD2 exercise their unique pathophysiological functions in the rat hippocampus after KA-induced seizures.

Epilepsy, CNS viral injury and dynorphin

Epilepsy is a significant health problem. Despite the widespread use of both classic and newer pharmacological agents that target ion channels, amino acid transmission or receptors, there are numerous examples of mono- or polytherapy being ineffective. Seizures that are secondary to CNS infections are among the most refractory medically, and thus insult-specific agents are desirable. Recently, the study of the neuropharmacological actions of dynorphin in CNS viral injury has yielded new insights into epileptogenesis and epilepsy treatment. The opioid neuropeptide dynorphin modulates neuronal excitability in vitro in hippocampal slices and potentiates endogenous anti-ictal (i.e. protective) processes in animal models and humans. This work has renewed interest in the role of dysregulation of dynorphin in the pathogenesis of refractory seizures, including encephalitic seizures. The important role of dynorphin in epilepsy is also supported by new models of symptomatic epilepsies based on viral-induced seizures.

Increased excitatory synaptic activity and local connectivity of hippocampal CA1 pyramidal cells in rats with kainate-induced epilepsy

Formation of local excitatory circuits may contribute to epileptogenesis. We tested the hypothesis that epileptogenesis is associated with increased recurrent excitation in the hippocampal CA1 area of rats with kainate-induced epilepsy. Whole cell recordings were obtained during focal flash photolysis of caged glutamate, which served as a focal excitant to activate local pyramidal cells and to study possible connections between neurons. Kainate-treated rats with spontaneous seizures were studied months after status epilepticus and were compared with saline-injected control rats. Experiments were done in isolated CA1 minislices and in bicuculline to block GABA(A) receptors. Spontaneous excitatory postsynaptic currents (sEPSCs) were present in 42% of the CA1 pyramidal cells from controls and 62% from kainate-treated rats. The frequency of sEPSCs in the kainate group was significantly higher than that in the control group, but mean amplitude was not different. Flash photolysis of caged glutamate on the somatodendritic area of CA1 pyramidal neurons caused a burst of action potentials. Local excitatory connections between CA1 pyramidal cells were found in 4 of 48 neurons (8%) in slices from control animals, but in significantly more neurons (12 of 37; 32%) from rats with kainate-induced epilepsy exhibited interconnections (P < 0.001). Photoactivation of glutamate on recorded CA1 pyramidal cells in the kainate group sometimes caused afterdischarges, but not in controls. The kainate-treated rats with pyramidal cells that responded to photostimulaltion with repetitive EPSCs appeared to have experienced more severe seizures. These data provide new electrophysiological evidence for the formation of recurrent excitatory circuits in the CA1 area of rats with kainate-induced epilepsy.

Translamellar disinhibition in the rat hippocampal dentate gyrus after seizure-induced degeneration of vulnerable hilar neurons

Longitudinally restricted axonal projections of hippocampal granule cells suggest that transverse segments of the granule cell layer may operate independently (the "lamellar" hypothesis). Longitudinal projections of excitatory hilar mossy cells could be viewed as antithetical to lamellar function, but only if longitudinal impulse flow effectively excites distant granule cells. We, therefore, determined the effect of focal granule cell discharges on granule cells located >2 mm along the longitudinal axis. During perforant pathway stimulation in urethane-anesthetized rats, passive diffusion of the GABA(A) receptor antagonist bicuculline methiodide from the tip of a glass recording electrode evoked granule cell discharges and c-Fos expression in granule cells, mossy cells, and inhibitory interneurons, within a approximately 400 microm radius. This focally evoked activity powerfully suppressed distant granule cell-evoked responses recorded simultaneously approximately 2.5-4.5 mm longitudinally. Three days after kainic acid-induced status epilepticus or prolonged perforant pathway stimulation, translamellar inhibition was intact in rats with <40% hilar neuron loss but was consistently abolished after extensive (>85%) hilar cell loss. Retrograde transport of Fluoro-Gold (FG) from the rostral dentate gyrus revealed that few inhibitory interneurons were among the many retrogradely labeled hilar neurons 2.5-4.5 mm longitudinally. Although many somatostatin-positive hilar interneurons effectively transported FG from the distant septum, few of these neurons transported detectable FG from much closer hippocampal injection sites. Inhibitory basket and chandelier cells also exhibited minimal longitudinal FG transport. These findings suggest that translamellar disinhibition may result from the loss of vulnerable, longitudinally projecting mossy cells and may represent a network-level mechanism underlying postinjury hippocampal dysfunction and epileptic network hyperexcitability.

Electrophysiological evidence of monosynaptic excitatory transmission between granule cells after seizure-induced mossy fiber sprouting

Mossy fiber sprouting is a form of synaptic reorganization in the dentate gyrus that occurs in human temporal lobe epilepsy and animal models of epilepsy. The axons of dentate gyrus granule cells, called mossy fibers, develop collaterals that grow into an abnormal location, the inner third of the dentate gyrus molecular layer. Electron microscopy has shown that sprouted fibers from synapses on both spines and dendritic shafts in the inner molecular layer, which are likely to represent the dendrites of granule cells and inhibitory neurons. One of the controversies about this phenomenon is whether mossy fiber sprouting contributes to seizures by forming novel recurrent excitatory circuits among granule cells. To date, there is a great deal of indirect evidence that suggests this is the case, but there are also counterarguments. The purpose of this study was to determine whether functional monosynaptic connections exist between granule cells after mossy fiber sprouting. Using simultaneous recordings from granule cells, we obtained direct evidence that granule cells in epileptic rats have monosynaptic excitatory connections with other granule cells. Such connections were not obtained when age-matched, saline control rats were examined. The results suggest that indeed mossy fiber sprouting provides a substrate for monosynaptic recurrent excitation among granule cells in the dentate gyrus. Interestingly, the characteristics of the excitatory connections that were found indicate that the pathway is only weakly excitatory. These characteristics may contribute to the empirical observation that the sprouted dentate gyrus does not normally generate epileptiform discharges.

Tetanus neurotoxin-insensitive vesicle-associated membrane protein localizes to a presynaptic membrane compartment in selected terminal subsets of the rat brain

Tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP) is a vesicular soluble N-ethyl maleimide-sensitive fusion protein attachment protein receptor (SNARE) that has been implicated in neurite outgrowth. It has previously been reported that TI-VAMP is localised in the somatodendritic compartment of neurons indicating a role in membrane fusion events within dendrites. Using a newly produced monoclonal antibody to TI-VAMP that improves signal/noise immunodetection, we report that TI-VAMP is also present in subsets of axon terminals of the adult rat brain. Four distinctive populations of labelled axon terminals were identified: 1) the hippocampal mossy fibres of the dentate gyrus and of CA3, 2) the striatal peridendritic terminal plexuses in the globus pallidus (GP), substantia nigra pars reticulata (SNr), 3) peridendritic plexuses in the central nucleus of the amygdala, and 4) the primary sensory afferents in the dorsal horn of the spinal cord. The presynaptic localisation of TI-VAMP in these locations was demonstrated by co-localisation with synaptophysin. Ultrastructural studies showed TI-VAMP labelling over synaptic vesicles in the mossy fibres, whereas it was localised in tubulo-vesicular structures and multivesicular bodies in the pyramidal cell dendrites. The presynaptic localisation of TI-VAMP occurred by P15, so relatively late during development. In contrast, dendritic labelling was most prominent during the early post-natal period. Co-localisation with markers of neurotransmitters showed that TI-VAMP-positive terminals are GABAergic in the GP and SNr and glutamatergic in the mossy fibre system and in the dorsal root afferents. Most of these terminals are known to co-localise with neuropeptides. We found met-enkephalin-immunoreactivity in a sizeable fraction of the TI-VAMP positive terminals in the GP, amygdala, and dorsal horn, as well as in a few mossy fibre terminals. The function of TI-VAMP in subsets of mature axon terminals remains to be elucidated; it could participate in the exocytotic molecular machinery and/or be implicated in particular growth properties of the mature axon terminals. Thus, the presence of TI-VAMP in the mossy fibres may correspond to the high degree of plasticity that characterises this pathway throughout adult life.

Spontaneous seizures and loss of axo-axonic and axo-somatic inhibition induced by repeated brief seizures in kindled rats

Repeated brief seizures evoked by kindling progressively increase seizure susceptibility and eventually induce spontaneous seizures. Previous studies have demonstrated that the initial seizures evoked by kindling increase paired-pulse inhibition at 15-25 msec interpulse intervals in the dentate gyrus and also induce apoptosis, progressive neuronal loss, mossy fiber sprouting, and neurogenesis, which could potentially alter the balance of excitation and/or inhibition and modify functional properties of hippocampal circuits. In these experiments, paired-pulse inhibition in the dentate gyrus was reduced or lost after approximately 90-100 evoked seizures in association with emergence of spontaneous seizures. Evoked IPSCs examined by single electrode voltage-clamp methods in granule cells from kindled rats experiencing spontaneous seizures demonstrated altered kinetics (reductions of approximately 48% in 10-90% decay time, approximately 40% in tau, and approximately 65% in charge transfer) and confirmed that decreased inhibition contributed to the reduced paired-pulse inhibition. The loss of inhibition was accompanied by loss of subclasses of inhibitory interneurons labeled by cholecystokinin and the neuronal GABA transporter GAT-1, which project axo-somatic and axo-axonic GABAergic inhibitory terminals to granule cells and axon initial segments. Seizure-induced loss of interneurons providing axo-somatic and axo-axonic inhibition may regulate spike output to pyramidal neurons in CA3 and could play an important role in generation of spontaneous seizures. The sequence of progressive cellular alterations induced by repeated seizures, particularly loss of GABAergic interneurons providing axo-somatic and axo-axonic inhibition, may be important in the development of intractable epilepsy.

Temporal profile of hilar basal dendrite formation on dentate granule cells after status epilepticus

Granule cells with hilar basal dendrites (HBDs) are found after status epilepticus (SE) in three rat models of temporal lobe epilepsy. These granule cells are commonly located at the hilar border and could be newly born granule cells based on their location. The aim of this study was to determine how long it takes for HBDs to form on granule cells after SE. Pilocarpine was injected to induce SE and rats were killed at different times: 3 days, 1, 2, and 3 weeks after SE. Biocytin was injected into CA3 stratum lucidum of hippocampal slices to label granule cells with HBDs. The number, morphology, and length of HBDs were analyzed at the different time points. Basal processes of granule cells from rats killed 3 days after pilocarpine injection were judged not to be HBDs because they were short in length and did not ramify in the hilus. "True" HBDs were detected as early as 7 and 8 days after pilocarpine-induced SE. Similar frequencies of granule cells with HBDs were observed at the later time points. This study shows that HBDs can form on granule cells as early as 1 week following SE. These results are consistent with the hypothesis that HBDs on granule cells may be generated from seizure-induced, de novo granule cells, however, alternative explanations that some or all HBDs arise from pre-SE generated granule cells cannot be ruled out at this time and will require further examination.

Reduced inhibition of dentate granule cells in a model of temporal lobe epilepsy

Patients and models of temporal lobe epilepsy have fewer inhibitory interneurons in the dentate gyrus than controls, but it is unclear whether granule cell inhibition is reduced. We report the loss of GABAergic inhibition of granule cells in the temporal dentate gyrus of pilocarpine-induced epileptic rats. In situ hybridization for GAD65 mRNA and immunocytochemistry for parvalbumin and somatostatin confirmed the loss of inhibitory interneurons. In epileptic rats, granule cells had prolonged EPSPs, and they discharged more action potentials than controls. Although the conductances of evoked IPSPs recorded in normal ACSF were not significantly reduced and paired-pulse responses showed enhanced inhibition of granule cells from epileptic rats, more direct measures of granule cell inhibition revealed significant deficiencies. In granule cells from epileptic rats, evoked monosynaptic IPSP conductances were <40% of controls, and the frequency of GABA(A) receptor-mediated spontaneous and miniature IPSCs (mIPSCs) was <50% of controls. Within 3-7 d after pilocarpine-induced status epilepticus, miniature IPSC frequency had decreased, and it remained low, without functional evidence of compensatory synaptogenesis by GABAergic axons in chronically epileptic rats. Both parvalbumin- and somatostatin-immunoreactive interneuron numbers and the frequency of both fast- and slow-rising GABA(A) receptor-mediated mIPSCs were reduced, suggesting that loss of inhibitory synaptic input to granule cells occurred at both proximal/somatic and distal/dendritic sites. Reduced granule cell inhibition in the temporal dentate gyrus preceded the onset of spontaneous recurrent seizures by days to weeks, so it may contribute, but is insufficient, to cause epilepsy.

Ultrastructural features of sprouted mossy fiber synapses in kindled and kainic acid-treated rats

The mossy fiber pathway in the dentate gyrus undergoes sprouting and synaptic reorganization in response to seizures. The types of new synapses, their location and number, and the identity of their postsynaptic targets determine the functional properties of the reorganized circuitry. The goal of this study was to characterize the types and proportions of sprouted mossy fiber synapses in kindled and kainic acid-treated rats. In normal rats, synapses labeled by Timm histochemistry or dynorphin immunohistochemistry were rarely observed in the supragranular region of the inner molecular layer when examined by electron microscopy. In epileptic rats, sprouted mossy fiber synaptic terminals were frequently observed. The ultrastructural analysis of the types of sprouted synapses revealed that 1) in the supragranular region, labeled synaptic profiles were more frequently axospinous than axodendritic, and many axospinous synapses were perforated; 2) sprouted mossy fiber synaptic terminals formed exclusively asymmetric, putatively excitatory synapses with dendritic spines and shafts in the supragranular region and with the soma of granule cells in the granule cell layer; 3) in contrast to the large sprouted mossy fiber synapses in resected human epileptic hippocampus, the synapses formed by sprouted mossy fibers in rats were smaller; and 4) in several cases, the postsynaptic targets of sprouted synapses were identified as granule cells, but, in one case, a sprouted synaptic terminal formed a synapse with an inhibitory interneuron. The results demonstrate that axospinous asymmetric synapses are the most common type of synapse formed by sprouted mossy fiber terminals, supporting the viewpoint that most sprouted mossy fibers contribute to recurrent excitation in epilepsy.