Is mossy fiber sprouting present at the time of the first spontaneous seizures in rat experimental temporal lobe epilepsy?

Inhibition of the mammalian target of rapamycin signaling pathway suppresses dentate granule cell axon sprouting in a rodent model of temporal lobe epilepsy

Dentate granule cell axon (mossy fiber) sprouting is a common abnormality in patients with temporal lobe epilepsy. Mossy fiber sprouting creates an aberrant positive-feedback network among granule cells that does not normally exist. Its role in epileptogenesis is unclear and controversial. If it were possible to block mossy fiber sprouting from developing after epileptogenic treatments, its potential role in the pathogenesis of epilepsy could be tested. Previous attempts to block mossy fiber sprouting have been unsuccessful. The present study targeted the mammalian target of rapamycin (mTOR) signaling pathway, which regulates cell growth and is blocked by rapamycin. Rapamycin was focally, continuously, and unilaterally infused into the dorsal hippocampus for prolonged periods beginning within hours after rats sustained pilocarpine-induced status epilepticus. Infusion for 1 month reduced aberrant Timm staining (a marker of mossy fibers) in the granule cell layer and molecular layer. Infusion for 2 months inhibited mossy fiber sprouting more. However, after rapamycin infusion ceased, aberrant Timm staining developed and approached untreated levels. When onset of infusion began after mossy fiber sprouting had developed for 2 months, rapamycin did not reverse aberrant Timm staining. These findings suggest that inhibition of the mTOR signaling pathway suppressed development of mossy fiber sprouting. However, suppression required continual treatment, and rapamycin treatment did not reverse already established axon reorganization.

Amyloid beta-induced neuronal hyperexcitability triggers progressive epilepsy

Alzheimer's disease is associated with an increased risk of unprovoked seizures. However, the underlying mechanisms of seizure induction remain elusive. Here, we performed video-EEG recordings in mice carrying mutant human APPswe and PS1dE9 genes (APdE9 mice) and their wild-type littermates to determine the prevalence of unprovoked seizures. In two recording episodes at the onset of amyloid beta (Abeta) pathogenesis (3 and 4.5 months of age), at least one unprovoked seizure was detected in 65% of APdE9 mice, of which 46% had multiple seizures and 38% had a generalized seizure. None of the wild-type mice had seizures. In a subset of APdE9 mice, seizure phenotype was associated with a loss of calbindin-D28k immunoreactivity in dentate granular cells and ectopic expression of neuropeptide Y in mossy fibers. In APdE9 mice, persistently decreased resting membrane potential in neocortical layer 2/3 pyramidal cells and dentate granule cells underpinned increased network excitability as identified by patch-clamp electrophysiology. At stimulus strengths evoking single-component EPSPs in wild-type littermates, APdE9 mice exhibited decreased action potential threshold and burst firing of pyramidal cells. Bath application (1 h) of Abeta1-42 or Abeta25-35 (proto-)fibrils but not oligomers induced significant membrane depolarization of pyramidal cells and increased the activity of excitatory cell populations as measured by extracellular field recordings in the juvenile rodent brain, confirming the pathogenic significance of bath-applied Abeta (proto-)fibrils. Overall, these data identify fibrillar Abeta as a pathogenic entity powerfully altering neuronal membrane properties such that hyperexcitability of pyramidal cells culminates in epileptiform activity.

Hippocampal epileptogenesis in animal models of mesial temporal lobe epilepsy with hippocampal sclerosis: the importance of the "latent period" and other concepts

Prolonged chemoconvulsant-induced status epilepticus in rats has long been promoted as an animal model of mesial temporal lobe epilepsy with hippocampal sclerosis, under the assumption that these animals involve: (1) pathology similar to that of the human neurologic condition; (2) a seizure-free, "preepileptic" latent period of several weeks duration after injury, during which a secondary epileptogenic process gradually develops; and (3) a chronic epileptic state in which the hippocampus, in general, and the dentate gyrus, in particular, becomes a source of the spontaneous behavioral seizures that define these animals as "epileptic." Retrospective analysis suggests that all of these assumptions are in doubt. Neuropathologic studies have shown that prolonged status epilepticus causes greater extrahippocampal than hippocampal damage, and does not produce classic hippocampal sclerosis. In vivo electrophysiologic studies suggest that the hippocampus of these animals may not be "epileptic." Most importantly, studies using continuous video monitoring to detect spontaneous behavioral seizures indicate that these rats become epileptic soon after insult, before any delayed secondary processes have time to develop. High mortality, significant variability, and the lack of an extended "therapeutic window" after brain injury suggest the need to develop animal models that more closely resemble the human neurologic condition.

Molecular and cellular basis of epileptogenesis in symptomatic epilepsy

Epileptogenesis refers to a process in which an initial brain-damaging insult triggers a cascade of molecular and cellular changes that eventually lead to the occurrence of spontaneous seizures. Cellular alterations include neurodegeneration, neurogenesis, axonal sprouting, axonal injury, dendritic remodeling, gliosis, invasion of inflammatory cells, angiogenesis, alterations in extracellular matrix, and acquired channelopathies. Large-scale molecular profiling of epileptogenic tissue has provided information about the molecular pathways that can initiate and maintain cellular alterations. Currently we are learning how these pathways contribute to postinjury epileptogenesis and recovery process and whether they could be used as treatment targets.

Minimal latency to hippocampal epileptogenesis and clinical epilepsy after perforant pathway stimulation-induced status epilepticus in awake rats

Hippocampal epileptogenesis is hypothesized to involve secondary mechanisms triggered by initial brain injury. Chemoconvulsant-induced status epilepticus has been used to identify secondary epileptogenic mechanisms under the assumption that a seizure-free, preepileptic "latent period" exists that is long enough to accommodate delayed mechanisms. The latent period is difficult to assess experimentally because early spontaneous seizures may be caused or influenced by residual chemoconvulsant that masks the true duration of the epileptogenic process. To avoid the use of chemoconvulsants and determine the latency to hippocampal epileptogenesis and clinical epilepsy, we developed an electrical stimulation-based method to evoke hippocampal discharges in awake rats and produce hippocampal injury and hippocampal-onset epilepsy reliably. Continuous video monitoring and granule cell layer recording determined whether hippocampal epileptogenesis develops immediately or long after injury. Bilateral perforant pathway stimulation for 3 hours evoked granule cell epileptiform discharges and convulsive status epilepticus with minimal lethality. Spontaneous stage 3-5 behavioral seizures reliably developed within 3 days poststimulation, and all 72 spontaneous behavioral seizures recorded in 10 animals were preceded by spontaneous granule cell epileptiform discharges. Histological analysis confirmed a reproducible pattern of limited hippocampal and extrahippocampal injury, including an extensive bilateral loss of hilar neurons throughout the hippocampal longitudinal axis. These results indicate that hippocampal epileptogenesis after convulsive status epilepticus is an immediate network defect coincident with neuron loss or other early changes. We hypothesize that the latent period is directly related and inversely proportional to the extent of neuron loss in brain regions involved in seizure initiation, spread, and clinical expression.

Prolonged infusion of inhibitors of calcineurin or L-type calcium channels does not block mossy fiber sprouting in a model of temporal lobe epilepsy

PURPOSE

It would be useful to selectively block granule cell axon (mossy fiber) sprouting to test its functional role in temporal lobe epileptogenesis. Targeting axonal growth cones may be an effective strategy to block mossy fiber sprouting. L-type calcium channels and calcineurin, a calcium-activated phosphatase, are critical for normal growth cone function. Previous studies have provided encouraging evidence that blocking L-type calcium channels or inhibiting calcineurin during epileptogenic treatments suppresses mossy fiber sprouting.

METHODS

Rats were treated systemically with pilocarpine to induce status epilepticus, which lasted at least 2 h. Then, osmotic pumps and cannulae were implanted to infuse calcineurin inhibitors (FK506 or cyclosporin A) or an L-type calcium channel blocker (nicardipine) into the dorsal dentate gyrus. After 28 days of continuous infusion, extent of mossy fiber sprouting was evaluated with Timm staining and stereological methods.

RESULTS

Percentages of volumes of the granule cell layer plus molecular layer that were Timm-positive were similar in infused and noninfused hippocampi.

CONCLUSIONS

These findings suggest inhibiting calcineurin or L-type calcium channels does not block mossy fiber sprouting in the pilocarpine-treated rat model of temporal lobe epilepsy.

Unilateral hippocampal CA3-predominant damage and short latency epileptogenesis after intra-amygdala microinjection of kainic acid in mice

Mesial temporal lobe epilepsy is the most common, intractable seizure disorder in adults. It is associated with an asymmetric pattern of hippocampal neuron loss within the endfolium (hilus and CA3) and CA1, with limited pathology in extra-hippocampal regions. We previously developed a model of focally-evoked seizure-induced neuronal death using intra-amygdala kainic acid (KA) microinjection and characterized the acute hippocampal pathology. Here, we sought to characterize the full extent of hippocampal and potential extra-hippocampal damage in this model, and the temporal onset of epileptic seizures. Seizure damage assessed at four stereotaxic levels by FluoroJade B staining was most prominent in ipsilateral hippocampal CA3 where it extended from septal to temporal pole. Minor but significant neuronal injury was present in ipsilateral CA1. Extra-hippocampal neuronal damage was generally limited in extent and restricted to the lateral septal nucleus, injected amygdala and select regions of neocortex ipsilateral to the seizure elicitation side. Continuous surface EEG recorded with implanted telemetry units in freely-moving mice detected spontaneous, epileptic seizures by five days post-KA in all mice. Epileptic seizure number averaged 1-4 per day. Hippocampi from epileptic mice 15 days post-KA displayed unilateral CA3 lesions, astrogliosis and increased neuropeptide Y immunoreactivity suggestive of mossy fiber rearrangement. These studies characterize a mouse model of unilateral hippocampal-dominant neuronal damage and short latency epileptogenesis that may be suitable for studying the cell and molecular pathogenesis of human mesial temporal lobe epilepsy.

The adenosine kinase hypothesis of epileptogenesis

Current therapies for epilepsy are largely symptomatic and do not affect the underlying mechanisms of disease progression, i.e. epileptogenesis. Given the large percentage of pharmacoresistant chronic epilepsies, novel approaches are needed to understand and modify the underlying pathogenetic mechanisms. Although different types of brain injury (e.g. status epilepticus, traumatic brain injury, stroke) can trigger epileptogenesis, astrogliosis appears to be a homotypic response and hallmark of epilepsy. Indeed, recent findings indicate that epilepsy might be a disease of astrocyte dysfunction. This review focuses on the inhibitory neuromodulator and endogenous anticonvulsant adenosine, which is largely regulated by astrocytes and its key metabolic enzyme adenosine kinase (ADK). Recent findings support the "ADK hypothesis of epileptogenesis": (i) Mouse models of epileptogenesis suggest a sequence of events leading from initial downregulation of ADK and elevation of ambient adenosine as an acute protective response, to changes in astrocytic adenosine receptor expression, to astrocyte proliferation and hypertrophy (i.e. astrogliosis), to consequential overexpression of ADK, reduced adenosine and - finally - to spontaneous focal seizure activity restricted to regions of astrogliotic overexpression of ADK. (ii) Transgenic mice overexpressing ADK display increased sensitivity to brain injury and seizures. (iii) Inhibition of ADK prevents seizures in a mouse model of pharmacoresistant epilepsy. (iv) Intrahippocampal implants of stem cells engineered to lack ADK prevent epileptogenesis. Thus, ADK emerges both as a diagnostic marker to predict, as well as a prime therapeutic target to prevent, epileptogenesis.

Stereological analysis of GluR2-immunoreactive hilar neurons in the pilocarpine model of temporal lobe epilepsy: correlation of cell loss with mossy fiber sprouting

Mossy fiber sprouting and the genesis of ectopic granule cells contribute to reverberating excitation in the dentate gyrus of epileptic brain. This study determined whether the extent of sprouting after status epilepticus in rats correlates with the seizure-induced degeneration of GluR2-immunoreactive (GluR2+) hilar neurons (presumptive mossy cells) and also quantitated granule cell-like GluR2-immunoreactive hilar neurons. Stereological cell counting indicated that GluR2+ neurons account for 57% of the total hilar neuron population. Prolonged pilocarpine-induced status epilepticus killed 95% of these cells. A smaller percentage of GluR2+ neurons (74%) was killed when status epilepticus was interrupted after 1-3.5 h with a single injection of phenobarbital, and the number of residual GluR2+ neurons varied among animals by a factor of 6.2. GluR2+ neurons were not necessarily more vulnerable than other hilar neurons. In rats administered phenobarbital, the extent of recurrent mossy fiber growth varied inversely and linearly with the number of GluR2+ hilar neurons that remained intact (P=0.0001). Thus the loss of each GluR2+ neuron was associated with roughly the same amount of sprouting. These findings support the hypothesis that mossy fiber sprouting is driven largely by the degeneration of and/or loss of innervation from mossy cells. Granule cell-like GluR2-immunoreactive neurons were rarely encountered in the hilus of control rats, but increased 6- to 140-fold after status epilepticus. Their number did not correlate with the extent of hilar cell death or mossy fiber sprouting in the same animal. The morphology, number, and distribution of these neurons suggested that they were hilar ectopic granule cells.

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 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.

Animal models of post-traumatic epilepsy

Epilepsy is a major unfavorable long-term consequence of traumatic brain injury (TBI). Moreover, TBI is one of the most important predisposing factors for the development of epilepsy, particularly in young adults. Understanding the molecular and cellular cascades that lead to the development of post-traumatic epilepsy (PTE) is key for preventing its development or modifying the disease process in such a way that epilepsy, if it develops, is milder and easier-to-treat. Tissue from TBI patients undergoing epileptogenesis is not available for such studies, which underscores the importance of developing clinically relevant animal models of PTE. The goal of this review is to (1) provide a description of PTE in humans, which is critical for the development of clinically relevant models of PTE, (2) review the characteristics of currently available PTE models, and (3) provide suggestions for the development of future models of PTE based on our current understanding of the mechanisms of TBI and epilepsy. The development of clinically relevant models of PTE is critical to advance our understanding of the mechanisms of post-traumatic epileptogenesis and epilepsy, as well as for producing breakthroughs in the development and testing of novel antiepileptogenic treatments.

A model of posttraumatic epilepsy induced by lateral fluid-percussion brain injury in rats

Although traumatic brain injury is a major cause of symptomatic epilepsy, the mechanism by which it leads to recurrent seizures is unknown. An animal model of posttraumatic epilepsy that reliably reproduces the clinical sequelae of human traumatic brain injury is essential to identify the molecular and cellular substrates of posttraumatic epileptogenesis, and perform preclinical screening of new antiepileptogenic compounds. We studied the electrophysiologic, behavioral, and structural features of posttraumatic epilepsy induced by severe, non-penetrating lateral fluid-percussion brain injury in rats. Data from two independent experiments indicated that 43% to 50% of injured animals developed epilepsy, with a latency period between 7 weeks to 1 year. Mean seizure frequency was 0.3+/-0.2 seizures per day and mean seizure duration was 113+/-46 s. Behavioral seizure severity increased over time in the majority of animals. Secondarily-generalized seizures comprised an average of 66+/-37% of all seizures. Mossy fiber sprouting was increased in the ipsilateral hippocampus of animals with posttraumatic epilepsy compared with those subjected to traumatic brain injury without epilepsy. Stereologic cell counts indicated a loss of dentate hilar neurons ipsilaterally following traumatic brain injury. Our data suggest that posttraumatic epilepsy occurs with a frequency of 40% to 50% after severe non-penetrating fluid-percussion brain injury in rats, and that the lateral fluid percussion model can serve as a clinically-relevant tool for pathophysiologic and preclinical studies.

Repair of the injured adult hippocampus through graft-mediated modulation of the plasticity of the dentate gyrus in a rat model of temporal lobe epilepsy

Intracerebroventricular kainate administration in rat, a model of temporal lobe epilepsy (TLE), causes degeneration of the hippocampal CA3 pyramidal and dentate hilar neurons. This leads to a robust but aberrant sprouting of the granule cell axons (mossy fibers) into the dentate supragranular layer and the CA3 stratum oriens. Because this plasticity is linked to an increased seizure susceptibility in TLE, strategies that restrain the aberrant mossy fiber sprouting (MFS) are perceived to be important for preventing the TLE development after the hippocampal injury. We ascertained the efficacy of fetal hippocampal CA3 or CA1 cell grafting into the kainate-lesioned CA3 region of the adult rat hippocampus at early post-kainic acid injury for providing a lasting inhibition of the aberrant MFS. Analyses at 12 months after grafting revealed that host mossy fibers project vigorously into CA3 cell grafts but avoid CA1 cell grafts. Consequently, in animals receiving CA3 cell grafts, the extent of aberrant MFS was minimal, in comparison with the robust MFS observed in both "lesion-only" animals and animals receiving CA1 cell grafts. Analyses of the graft axon growth revealed strong graft efferent projections into the dentate supragranular layer with CA3 cell grafting but not with CA1 cell grafting. Thus, the formation of reciprocal circuitry between the dentate granule cells and the grafted CA3 pyramidal neurons is likely the basis of inhibition of the aberrant MFS by CA3 cell grafts. The results also underscore that grafting of cells capable of differentiating into CA3 pyramidal neurons is highly efficacious for a lasting inhibition of the abnormal mossy fiber circuitry development in the injured hippocampus.

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.

Manganese-enhanced magnetic resonance imaging of mossy fiber plasticity in vivo

Mn(2+)-enhanced magnetic resonance imaging (MEMRI) was used to characterize activity-dependent plasticity in the mossy fiber pathway after intraperitoneal kainic acid (KA) injection. Enhancement of the MEMRI signal in the dentate gyrus and the CA3 subregion of the hippocampus was evident 3 to 5 days after injection of MnCl(2) into the entorhinal cortex both in control and KA-injected rats. In volume-rendered three-dimensional reconstructions, Mn(2+)-induced signal enhancement revealed the extent of the mossy fiber pathway throughout the septotemporal axis of the dentate gyrus. An increase in the number of Mn(2+)-enhanced pixels in the dentate gyrus and CA3 subfield of rats with KA injection correlated (P < 0.05) with histologically verified mossy fiber sprouting. These data demonstrate that MEMRI can be used to detect specific changes at the cellular level during activity-dependent plasticity in vivo. The present findings also suggest that MEMRI signal changes can serve as an imaging marker of epileptogenesis.

Hippocampal granule cell activity and c-Fos expression during spontaneous seizures in awake, chronically epileptic, pilocarpine-treated rats: Implications for hippocampal epileptogenesis

The process of postinjury hippocampal epileptogenesis may involve gradually developing dentate granule cell hyperexcitability caused by neuron loss and synaptic reorganization. We tested this hypothesis by repeatedly assessing granule cell excitability after pilocarpine-induced status epilepticus (SE) and monitoring granule cell behavior during 235 spontaneous seizures in awake, chronically implanted rats. During the first week post-SE, granule cells exhibited diminished paired-pulse suppression and decreased seizure discharge thresholds in response to afferent stimulation. Spontaneous seizures often began during the first week after SE, recruited granule cell discharges that followed behavioral seizure onsets, and evoked c-Fos expression in all hippocampal neurons. Paired-pulse suppression and epileptiform discharge thresholds increased gradually after SE, eventually becoming abnormally elevated. In the chronic epileptic state, interictal granule cell hyperinhibition extended to the ictal state; granule cells did not discharge synchronously before any of 191 chronic seizures. Instead, granule cells generated only low-frequency voltage fluctuations (presumed "field excitatory postsynaptic potentials") during 89% of chronic seizures. Granule cell epileptiform discharges were recruited during 11% of spontaneous seizures, but these occurred only at the end of each behavioral seizure. Hippocampal c-Fos after chronic seizures was expressed primarily by inhibitory interneurons. Thus, granule cells became progressively less excitable, rather than hyperexcitable, as mossy fiber sprouting progressed and did not initiate the spontaneous behavioral seizures. These findings raise doubts about dentate granule cells as a source of spontaneous seizures in rats subjected to prolonged SE and suggest that dentate gyrus neuron loss and mossy fiber sprouting are not primary epileptogenic mechanisms in this animal model.

Genetic background regulates semaphorin gene expression and epileptogenesis in mouse brain after kainic acid status epilepticus

The host response to neural injury, which can include axonal sprouting and synaptic reorganization is likely to be under tight genetic regulatory control at the level of the genome and may be implicated in epileptogenesis. Despite its importance, however, the molecular basis of synaptic reorganization is unclear. We have studied the development of synaptic reorganization, semaphorin gene expression, and epileptogenesis in hippocampus of epileptogenic sensitive (FVB/NJ) and epileptogenic resistant (C57BL/6J) mice (i.e. distinct genetic backgrounds) after kainic acid-induced status epilepticus. Our results support the hypothesis that disruption of transcriptional regulation of axon guidance genes leads to a differential loss of tonic neuropilin-2 dependent activation of semaphorin 3F receptors on hippocampal neurons on distinct genetic backgrounds. This results in rearranged synaptic circuitry and thus promotes epileptogenesis. These findings may define biologic principles underlying the role of semaphorin signaling which may broadly apply to other systems undergoing neural regeneration.

Atipamezole, an alpha(2)-adrenoceptor antagonist, has disease modifying effects on epileptogenesis in rats

Stimulation of alpha(2)-adrenoceptors delays the development of kindling, a model of epileptogenesis in humans. Blocking alpha(2)-adrenoceptors is proconvulsant, but has beneficial effects on somatomotor recovery after experimental stroke. We investigated whether atipamezole, a selective alpha(2)-adrenoceptor antagonist, affects the recovery process from status epilepticus (SE)-induced brain damage, which affects the risk of epileptogenesis. Vehicle or atipamezole (100 microg/kg/h) treatment was started 1 week after the induction of SE and continued for 9 weeks using Alzet minipumps (n = 70). Development and severity of epilepsy, spatial and emotional learning, and histologic analysis were used as outcome measures. There were no differences in the percentage of animals with epilepsy in the different treatment groups. In the atipamezole group, however, daily seizure frequency was lower (P < 0.01), a higher percentage of epileptic animals had mild epilepsy (<1 seizure/day; P < 0.01), and seizure frequency did not increase over time compared with the vehicle group. The atipamezole group had milder hilar cell damage (P < 0.05) and less intense mossy fiber sprouting (P < 0.05). Behavioral impairments were similar between groups. Our data indicate that chronic treatment with atipamezole does not prevent epileptogenesis. There is, however, a disease-modifying effect; that is, the epilepsy that develops is milder and non-progressive. These data warrant further studies.

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.

Kindling and status epilepticus models of epilepsy: rewiring the brain

This review focuses on the remodeling of brain circuitry associated with epilepsy, particularly in excitatory glutamate and inhibitory GABA systems, including alterations in synaptic efficacy, growth of new connections, and loss of existing connections. From recent studies on the kindling and status epilepticus models, which have been used most extensively to investigate temporal lobe epilepsy, it is now clear that the brain reorganizes itself in response to excess neural activation, such as seizure activity. The contributing factors to this reorganization include activation of glutamate receptors, second messengers, immediate early genes, transcription factors, neurotrophic factors, axon guidance molecules, protein synthesis, neurogenesis, and synaptogenesis. Some of the resulting changes may, in turn, contribute to the permanent alterations in seizure susceptibility. There is increasing evidence that neurogenesis and synaptogenesis can appear not only in the mossy fiber pathway in the hippocampus but also in other limbic structures. Neuronal loss, induced by prolonged seizure activity, may also contribute to circuit restructuring, particularly in the status epilepticus model. However, it is unlikely that any one structure, plastic system, neurotrophin, or downstream effector pathway is uniquely critical for epileptogenesis. The sensitivity of neural systems to the modulation of inhibition makes a disinhibition hypothesis compelling for both the triggering stage of the epileptic response and the long-term changes that promote the epileptic state. Loss of selective types of interneurons, alteration of GABA receptor configuration, and/or decrease in dendritic inhibition could contribute to the development of spontaneous seizures.

Mechanisms of epilepsy progression: current theories and perspectives from neuroplasticity in adulthood and development

Clinical and epidemiological studies have repeatedly demonstrated that a subset of patients with epilepsy have progressive syndromes with increasing seizure frequency and cumulative adverse effects despite optimal anticonvulsant therapy. Recent longitudinal imaging studies and long-term neuropsychological studies have confirmed that a substantial subset of people with epilepsy undergo progressive brain atrophy accompanied by functional declines that worsen with duration of epilepsy. As further evidence of the progressive and adverse effects of inadequately controlled epilepsy, chronic experimental models of epilepsy and the phenomenon of kindling have provided abundant evidence that neural circuits undergo long-term progressive structural and functional alterations in response to seizures. This long-term seizure-induced plasticity in neural circuits appears to be "bidirectional", inducing progressive damage while also inducing resistance to additional damage, as a function of timing or inter-seizure interval. Seizure-induced plasticity has pronounced age-dependence, and influences long-term cognitive consequences of seizures during early life and acquired susceptibility to epilepsy in adulthood. While it is clear from clinical and epidemiological studies that human epilepsy is a heterogeneous disorder and that not all epileptic syndromes are progressive, emerging results from studies of activity-dependent and seizure-induced plasticity and perspectives from "complex systems" analysis are providing new insights into systematic neurobiological processes that are likely to influence the progressive features of epileptic syndromes and patterns of progression in individual patients. The emerging perspective is that phenomena of plasticity and genetic background exert powerful effects in development and adulthood through regulation of activity-dependent structural and functional remodeling of neural circuitry, and that these effects not only influence progression and consequences of seizures, but also offer new opportunities for therapeutic intervention.

Prolonged infusion of tetrodotoxin does not block mossy fiber sprouting in pilocarpine-treated rats

PURPOSE

Mossy fiber sprouting is a common abnormality found in patients and models of temporal lobe epilepsy. The role of mossy fiber sprouting in epileptogenesis is unclear, and its blockade would be useful experimentally and perhaps therapeutically. Results from previous attempts to block mossy fiber sprouting have been disappointing or controversial. In some brain regions, prolonged application of the sodium channel blocker tetrodotoxin prevents axon sprouting and posttrauma epileptogenesis. The present study tested the hypothesis that prolonged, focal infusion of tetrodotoxin would block mossy fiber sprouting after an epileptogenic treatment.

METHODS

Adult rats were treated with pilocarpine to induce status epilepticus. Several hours to 3 days after pilocarpine treatment, a pump with a cannula directed toward the dentate gyrus was implanted to deliver 10 microM tetrodotoxin or vehicle alone at 0.25 microl/h. This method blocks local EEG activity in the hippocampus (Galvan et al. J Neurosci 2000; 20:2904-16). After 28 days of continuous infusion, rats were perfused with fixative, and their hippocampi analyzed anatomically with stereologic techniques.

RESULTS

Tetrodotoxin infusion was verified immunocytochemically in tetrodotoxin-treated but not vehicle-treated hippocampi. Tetrodotoxin-infused and vehicle-infused hippocampi displayed similar levels of hilar neuron loss. The Timm stain revealed mossy fiber sprouting regardless of whether hippocampi were treated with tetrodotoxin infusion, vehicle infusion, or neither.

CONCLUSIONS

Prolonged infusion of tetrodotoxin did not block mossy fiber sprouting. This finding suggests that sodium channel-mediated neuronal activity is not necessary for mossy fiber sprouting after an epileptogenic treatment.

Perforant path activation of ectopic granule cells that are born after pilocarpine-induced seizures

Granule cells in the dentate gyrus are born throughout life, and various stimuli can affect their development in the adult brain. Following seizures, for instance, neurogenesis increases greatly, and some new cells migrate to abnormal (ectopic) locations, such as the hilus. Previous electrophysiological studies of this population have shown that they have intrinsic properties that are similar to normal granule cells, but differ in other characteristics, consistent with abnormal integration into host circuitry. To characterize the response of ectopic hilar granule cells to perforant path stimulation, intracellular recordings were made in hippocampal slices from rats that had pilocarpine-induced status epilepticus and subsequent spontaneous recurrent seizures. Comparisons were made with granule cells located in the granule cell layer of both pilocarpine- and saline-treated animals. In addition, a few ectopic hilar granule cells were sampled from saline-treated rats. Remarkably, hilar granule cells displayed robust responses, even when their dendrites were not present within the molecular layer, where perforant path axons normally terminate. The evoked responses of hilar granule cells were similar in several ways to those of normally positioned granule cells, but there were some differences. For example, there was an unusually long latency to onset of responses evoked in many hilar granule cells, especially those without molecular layer dendrites. Presumably this is due to polysynaptic activation by the perforant path. These results indicate that synaptic reorganization after seizures can lead to robust activation of newly born hilar granule cells by the perforant path, even when their dendrites are not in the terminal field of the perforant path. Additionally, the fact that these cells can be found in normal tissue and develop similar synaptic responses, suggests that seizures, while not necessary for their formation, strongly promote their generation and the development of associated circuits, potentially contributing to a lowered seizure threshold.

Administration of caspase 3 inhibitor during and after status epilepticus in rat: effect on neuronal damage and epileptogenesis

Symptomatic temporal lobe epilepsy typically develops in three phases: brain damage --> epileptogenesis --> spontaneous seizures (epilepsy). The challenge is to prevent epileptogenesis after injury. We hypothesized that alleviation of damage by caspase inhibitors will reduce epileptogenesis or at least have disease-modifying effects (less severe epilepsy, milder cognitive decline). Epileptogenesis was triggered by amygdala stimulation-induced status epilepticus (SE) in rats and spontaneous seizures were monitored with video-electroencephalography (EEG). First, we tested the neuroprotective effect of a 1-week treatment with caspase 1, 3 or 9 inhibitors (3 micro g/d/i.c.v., started 3 h after the beginning of SE). The least damage to the hippocampus was observed in animals treated with the caspase 3 inhibitor (z-DEVD-fmk) which reduced the enzyme activity to 6% of that in the vehicle group. Thus, z-DEVD-fmk was chosen for long-term studies, in which the treatment regime remained the same except the dose was doubled (6 micro g/d/i.c.v.). Video-EEG monitoring was performed for 3 to 4 weeks, starting either 8 or 14 weeks after SE. One group of animals was tested in water-maze and fear-conditioning tests, and all animals were perfused for histological analysis. Treatment with the caspase 3 inhibitor neither prevented the development of epilepsy, nor had any disease-modifying effects. Mossy fibre sprouting, however, was reduced. The present data indicate that administration of z-DEVD-fmk monotherapy was not antiepileptogenic despite its short-term neuroprotective effects. These findings challenge the idea that prevention of cell death is the primary target for the development of antiepileptogenic compounds.

GAP-43 mRNA and protein expression in the hippocampal and parahippocampal region during the course of epileptogenesis in rats

In order to reveal axonal rewiring in the hippocampal and parahippocampal regions after status epilepticus, we investigated the temporal evolution of growth-associated protein-43 (GAP-43) mRNA and protein expression in two rat models of mesial temporal lobe epilepsy (MTLE). Status epilepticus (SE) was induced by electrical stimulation of the angular bundle or by intraperitoneal kainic acid (KA) injections. Despite increased GAP-43 mRNA expression in dentate granule cells at 24 h after SE, GAP-43 protein expression in the inner molecular layer (IML) of the dentate gyrus decreased progressively after 24 h after SE in both models. Nevertheless robust mossy fiber sprouting (MFS) was evident in the IML of chronic epileptic rats. Remaining GAP-43 protein expression in the IML in chronic epileptic rats did not correlate with the extent of MFS, but with the number of surviving hilar neurons. In the parahippocampal region, GAP-43 mRNA expression was decreased in layer III of the medial entorhinal area (MEAIII) in parallel with extensive neuronal loss in this layer. There was a tendency of GAP-43 mRNA up-regulation in the presubiculum, a region that projects to MEAIII. With regard to this parahippocampal region, however, changes in GAP-43 mRNA expression were not followed by protein changes. The presence of the presynaptic protein GAP-43 in a neurodegenerated MEAIII indicates that fibers still project to this layer. Whether reorganization of fibers has occurred in this region after SE needs to be investigated with tools other than GAP-43.

Sprouting of mossy fibers and the vacating of postsynaptic targets in the inner molecular layer of the dentate gyrus

Aberrant mossy fiber sprouting, which presumably results from hilar mossy cell death after status epilepticus (SE), is a frequently studied feature of temporal lobe epilepsy. Although mossy fiber sprouting can be suppressed by the protein synthesis inhibitor cycloheximide, spontaneous seizures remain unaltered. We have investigated the mechanisms underlying the ability of cycloheximide to block SE-induced mossy fiber sprouting in the inner molecular layer of dentate gyrus (IML). Pilocarpine-induced SE in the presence of cycloheximide resulted in a reduced number of injured hilar cells compared to rats not pretreated with cycloheximide. Presumed mossy cells, identified by calcitonin gene related peptide (CGRP) immunohistochemistry, were not significantly reduced in either group 60 days after SE. Whereas controls had a strong band of CGRP-positive fibers (putative mossy cell axons) and no neo-Timm stained fibers in the IML, pilocarpine-treated rats had no CGRP fibers and strong neo-Timm staining. Cycloheximide-pilocarpine-treated animals, in contrast, had CGRP and neo-Timm staining similar to controls. Cycloheximide might protect hilar CGRP-positive cells during SE and, by allowing those cells to retain their normal axonal projection, prevent mossy fiber sprouting. The recently suggested "irritable" mossy cell hypothesis relies on the survival of mossy cells for network hyperexcitability. We hypothesized that CGRP may be a marker for a subpopulation of relatively resistant mossy cells in rats, which, if they survive injury, may become irritable and contribute to hyperexcitability. We suggest that cycloheximide prevents SE-induced mossy fiber sprouting by preventing the loss of hilar CGRP-positive cells (putative mossy cells).

"Dormant basket cell" hypothesis revisited: relative vulnerabilities of dentate gyrus mossy cells and inhibitory interneurons after hippocampal status epilepticus in the rat

The "dormant basket cell" hypothesis suggests that postinjury hippocampal network hyperexcitability results from the loss of vulnerable neurons that normally excite insult-resistant inhibitory basket cells. We have reexamined the experimental basis of this hypothesis in light of reports that excitatory hilar mossy cells are not consistently vulnerable and inhibitory basket cells are not consistently seizure resistant. Prolonged afferent stimulation that reliably evoked granule cell discharges always produced extensive hilar neuron degeneration and immediate granule cell disinhibition. Conversely, kainic acid-induced status epilepticus in chronically implanted animals produced similarly extensive hilar cell loss and immediate granule cell disinhibition, but only when granule cells discharged continuously during status epilepticus. In both preparations, electron microscopy revealed degeneration of presynaptic terminals forming asymmetrical synapses in the mossy cell target zone, including some terminating on gamma-aminobutyric acid-immunoreactive elements, but no evidence of axosomatic or axoaxonic degeneration in the adjacent granule cell layer. Although parvalbumin immunocytochemistry and in situ hybridization revealed decreased staining, this apparently was due to altered parvalbumin expression rather than basket cell death, because substance P receptor-positive interneurons, some of which contained residual parvalbumin immunoreactivity, survived. These results confirm the inherent vulnerability of dendritically projecting hilar mossy cells and interneurons and the relative resistance of dentate inhibitory basket and chandelier cells that target granule cell somata. The variability of hippocampal cell loss after status epilepticus suggests that altered hippocampal structure and function cannot be assumed to cause the spontaneous seizures that develop in these animals and highlights the importance of confirming hippocampal pathology and pathophysiology in vivo in each case.

Epilepsy after early-life seizures can be independent of hippocampal injury

Prolonged early-life seizures are considered potential risk factors for later epilepsy development, but mediators of this process remain largely unknown. Seizure-induced structural damage in hippocampus, including cell loss and mossy fiber sprouting, is thought to contribute to the hyperexcitability characterizing epilepsy, but a causative role has not been established. To determine whether early-life insults that lead to epilepsy result in similar structural changes, we subjected rat pups to lithium-pilocarpine-induced status epilepticus during postnatal development (day 20) and examined them as adults for the occurrence of spontaneous seizures and alterations in hippocampal morphology. Sixty-seven percent of rats developed spontaneous seizures after status epilepticus, yet only one third of these epileptic animals exhibited visible hippocampal cell loss or mossy fiber sprouting in dentate gyrus. Most epileptic rats had no apparent structural alterations in the hippocampus detectable using standard light microscopy methods (profile counts and Timm's staining). These results suggest that hippocampal cell loss and mossy fiber sprouting can occur after early-life status epilepticus but may not be necessary prerequisites for epileptogenesis in the developing brain.

Mossy fiber plasticity and enhanced hippocampal excitability, without hippocampal cell loss or altered neurogenesis, in an animal model of prolonged febrile seizures

Seizures induced by fever (febrile seizures) are the most frequent seizures affecting infants and children; however, their impact on the developing hippocampal formation is not completely understood. Such understanding is highly important because of the potential relationship of prolonged febrile seizures to temporal lobe epilepsy. Using an immature rat model, we have previously demonstrated that prolonged experimental febrile seizures render the hippocampus hyperexcitable throughout life. Here we examined whether (1) neuronal loss, (2) altered neurogenesis, or (3) mossy fiber sprouting, all implicated in epileptogenesis in both animal models and humans, were involved in the generation of a pro-epileptic, hyperexcitable hippocampus by these seizures. The results demonstrated that prolonged experimental febrile seizures did not result in appreciable loss of any vulnerable hippocampal cell population, though causing strikingly enhanced sensitivity to hippocampal excitants later in life. In addition, experimental febrile seizures on postnatal day 10 did not enhance proliferation of granule cells, whereas seizures generated by kainic acid during the same developmental age increased neurogenesis in the immature hippocampus. However, prolonged febrile seizures resulted in long-term axonal reorganization in the immature hippocampal formation: Mossy fiber densities in granule cell- and molecular layers were significantly increased by 3 months (but not 10 days) after the seizures. Thus, the data indicate that prolonged febrile seizures influence connectivity of the immature hippocampus long-term, and this process requires neither significant neuronal loss nor altered neurogenesis. In addition, the temporal course of the augmented mossy fiber invasion of the granule cell and molecular layers suggests that it is a consequence, rather than the cause, of the hyperexcitable hippocampal network resulting from these seizures.

Axon sprouting in a model of temporal lobe epilepsy creates a predominantly excitatory feedback circuit

The most common type of epilepsy in adults is temporal lobe epilepsy. After epileptogenic injuries, dentate granule cell axons (mossy fibers) sprout and form new synaptic connections. Whether this synaptic reorganization strengthens recurrent inhibitory circuits or forms a novel recurrent excitatory circuit is unresolved. We labeled individual granule cells in vivo, reconstructed sprouted mossy fibers at the EM level, and identified postsynaptic targets with GABA immunocytochemistry in the pilocarpine model of temporal lobe epilepsy. Granule cells projected an average of 1.0 and 1.1 mm of axon into the granule cell and molecular layers, respectively. Axons formed an average of one synapse every 7 microm in the granule cell layer and every 3 microm in the molecular layer. Most synapses were with spines (76 and 98% in the granule cell and molecular layers, respectively). Almost all of the synapses were with GABA-negative structures (93 and 96% in the granule cell and molecular layers, respectively). By integrating light microscopic and EM data, we estimate that sprouted mossy fibers form an average of over 500 new synapses per granule cell, but <25 of the new synapses are with GABAergic interneurons. These findings suggest that almost all of the synapses formed by mossy fibers in the granule cell and molecular layers are with other granule cells. Therefore, after epileptogenic treatments that kill hilar mossy cells, mossy fiber sprouting does not simply replace one recurrent excitatory circuit with another. Rather, it replaces a distally distributed and disynaptic excitatory feedback circuit with one that is local and monosynaptic.

Seizure-Induced Axonal Sprouting: Assessing Connections Between Injury, Local Circuits, and Epileptogenesis

Neurons and neural circuits undergo extensive structural and functional remodeling in response to seizures. Sprouting of axons in the mossy fiber pathway of the hippocampus is a prominent example of a seizure-induced structural alteration which has received particular attention because it is easily detected, is induced by intense or repeated brief seizures in focal chronic models of epilepsy, and is also observed in the human epileptic hippocampus. During the last decade the association of mossy fiber sprouting with seizures and epilepsy has been firmly established. Many anatomical features of mossy fiber sprouting have been described in considerable detail, and there is evidence that sprouting occurs in a variety of other pathways in association with seizures and injury. There is uncertainty, however, about how or when mossy fiber sprouting may contribute to hippocampal dysfunction and generation of seizures. Study of mossy fiber sprouting has provided a strong theoretical and conceptual framework for efforts to understand how seizures and injury may contribute to epileptogenesis and its consequences. It is likely that investigation of mossy fiber sprouting will continure to offer significant opportunities for insights into seizure-induced plasticity of neural circuits at molecular, cellular, and systems levels.

Seizure-Induced Axonal Sprouting: Assessing Connections between Injury, Local Circuits, and Epileptogenesis

Neurons and neural circuits undergo extensive structural and functional remodeling in response to seizures. Sprouting of axons in the mossy fiber pathway of the hippocampus is a prominent example of a seizure-induced structural alteration which has received particular attention because it is easily detected, is induced by intense or repeated brief seizures in focal chronic models of epilepsy, and is also observed in the human epileptic hippocampus. During the last decade the association of mossy fiber sprouting with seizures and epilepsy has been firmly established. Many anatomical features of mossy fiber sprouting have been described in considerable detail, and there is evidence that sprouting occurs in a variety of other pathways in association with seizures and injury. There is uncertainty, however, about how or when mossy fiber sprouting may contribute to hippocampal dysfunction and generation of seizures. Study of mossy fiber sprouting has provided a strong theoretical and conceptual framework for efforts to understand how seizures and injury may contribute to epileptogenesis and its consequences. It is likely that investigation of mossy fiber sprouting will continure to offer significant opportunities for insights into seizure-induced plasticity of neural circuits at molecular, cellular, and systems levels.