Activation of the dentate gyrus by pentylenetetrazol evoked seizures induces mossy fiber synaptic reorganization

Kindled seizures evoked by electrical stimulation of limbic pathways in the rat induce sprouting and synaptic reorganization of the mossy fiber pathway in the dentate gyrus (DG). To investigate whether seizures evoked by different methods also induce reorganization of this pathway, the distribution of mossy fiber terminals in the DG was examined with Timm histochemistry after systemic administration of pentylenetetrazol, a chemoconvulsant that reduces Cl- mediated GABAergic inhibition. Myoclonic seizures evoked by subconvulsant doses of pentylenetetrazol (24 mg/kg i.p.) were not accompanied by electrographic seizures in the DG, and did not induce mossy fiber sprouting. Generalized tonic-clonic seizures evoked by repeated administration of PTZ (24 mg/kg i.p.) were consistently accompanied by electrographic seizure activity in the DG, and induced sprouting and synaptic reorganization of the mossy fiber pathway. The results demonstrated that repeated generalized tonic-clonic seizures evoked by pentylenetetrazol induced mossy fiber synaptic reorganization when ictal electrographic discharges activated the circuitry of the DG.     

Mossy fiber synaptic reorganization induced by kindling: time course of development, progression, and permanence

Recent studies have revealed that mossy fiber axons of granule cells in the dentate gyrus undergo reorganization of their terminal projections in both animal models of epilepsy and human epilepsy. This synaptic reorganization has been demonstrated by the Timm method, a histochemical technique that selectively labels synaptic terminals of mossy fibers because of their high zinc content. It has been generally presumed that the reorganization of the terminal projections of the mossy fiber pathway is a consequence of axonal sprouting and synaptogenesis by mossy fibers. To evaluate this possibility further, the time course for development of Timm granules, which correspond ultrastructurally to mossy fiber synaptic terminals, was examined in the supragranular layer of the dentate gyrus at the initiation of kindling stimulation with an improved scoring method for assessment of alterations in Timm histochemistry. The progression and permanence of this histological alteration were similarly evaluated during the behavioral and electrographic evolution of kindling evoked by perforant path, amygdala, or olfactory bulb stimulation. Mossy fiber synaptic terminals developed in the supragranular region of the dentate gyrus by 4 d after initiation of kindling stimulation in a time course compatible with axon sprouting. The induced alterations in the terminal projections of the mossy fiber pathway progressed with the evolution of behavioral kindled seizures, became permanent in parallel with the development of longlasting susceptibility to evoked seizures, and were observed as long as 8 months after the last evoked kindled seizure. The results demonstrated a strong correlation between mossy fiber synaptic reorganization and the development, progression, and permanence of the kindling phenomenon.     

Mossy fiber sprouting interacts with sodium channel mutations to increase dentate gyrus excitability

Purpose:  Idiopathic epilepsy is caused by the complex interaction of genetic and environmental factors. The purpose of this study was to use computational approaches to explore the interaction between changes in sodium channel availability caused by mutations and mossy fiber sprouting.
Methods:  We used a previously published biophysically realistic computer model of dentate gyrus neurons and networks. A sensitivity analysis probed the effects of typical mutation-like changes in either single- or multiple-gating parameters. Isolated neuron models were stimulated with current injections, and networks were stimulated with perforant path synaptic input. The gene–environment interaction was studied by including mossy fiber sprouting into these networks.
Results:  Single neuron responses to current injections were complex, with increased sodium channel availability paradoxically reducing firing rates. In the absence of mossy fiber sprouting, control networks showed strong accommodation supporting the role of the dentate gyrus as a gate. Availability changes alone were not able to drive the networks into ictal states, even though they reduced the effectiveness of the dentate gyrus gate. Interestingly, the presence of electrophysiologic changes substantially increased the ability of mossy fiber sprouting to induce ictal activity.
Conclusion:  (1) Increased sodium channel availability does not necessarily lead to increased firing rates, (2) network excitability is most sensitive to changes in steady state activation of sodium channels, (3) mutation-induced changes in availability reduce the effectiveness of the dentate gyrus gate, and (4) mutations interact strongly with structural network changes to allow ictal-like activity in the dentate gyrus.     

Amygdala kindling develops without mossy fiber sprouting and hippocampal neuronal degeneration in rats

Repeated electrical stimulation of limbic structures has been reported to produce the kindling effect together with morphological changes in the hippocampus such as mossy fiber sprouting and/or neuronal loss. However, to argue against a causal role of these neuropathological changes in the development of kindling-associated seizures, we examined mossy fiber sprouting in amygdala (AM)-kindled rats using Timm histochemical staining, and evaluated the hippocampal neuronal degeneration in AM-kindled rats by terminal deoxynucleotidyl transferase-mediated digoxigenin-11-dUTP nick end labelling (TUNEL). Amygdala kindling was established by 10.3 +/- 0.7 electrical stimulations, and no increase in Timm granules (neuronal sprouting) was observed up to the time of acquisition of a fully kindled state. However, the density and distribution of Timm granules increased significantly in the dentate gyrus compared with unkindled rats after 29 after-discharges or more than 10 kindled convulsions. In addition, no significant increase in TUNEL-positive cells was found in the hilar polymorphic neurons or in CA3 pyramidal neurons of the kindled rats that had fewer than 29 after-discharges. However, a significant increase of TUNEL-positive cells was found in the granule cell layer in the dentate gyrus of the stimulated side after 18 after-discharges or 10 kindled convulsions. Our result show that AM kindling develops without evidence of mossy fiber sprouting, and that mossy fiber sprouting may appear after repeated kindled convulsions, following death of the granule cells in the dentate gyrus.     

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.     

Origin of microglia in the quail retina: Central-to-peripheral and vitreal-to-scleral migration of microglial precursors during development

Collateral sprouting of dentate granule cell axons, the mossy fibers, occurs in response to denervation, kindling, or excitotoxic damage to the hippocampus. Organotypic slice culture of rodent hippocampal tissue is a model system for the controlled study of collateral sprouting in vitro. Organotypic roller-tube cultures were prepared from hippocampal slices derived from postnatal day 7 mice. The Timm heavy metal stain and densitometry were used to assay the degree of mossy fiber collateral sprouting in the molecular layer of the hippocampal dentate gyrus. Factors influencing mossy fiber collateral sprouting were time in culture, positional origin of the slice culture along the septotemporal axis of the hippocampus, and presence of attached subicular-entorhinal cortical tissues. Collateral sprouting in the molecular layer was first detected after 6 days in culture and increased steadily thereafter. By 2 weeks considerable sprouting was apparent, and at 3 weeks intense sprouting was observed within the molecular layer. An intrinsic septal-to-temporal gradient of collateral sprouting was apparent at 14 days in culture. To determine whether differential damage to the mossy fibers was the basis for the differences in collateral sprouting along the septotemporal axis, we made complete transections of the mossy fiber projection as it exited the dentate hilus at various levels along the septotemporal axis; no differences were found on subsequent collateral sprouting in the dentate molecular layer. Timm-stained hippocampal cultures with an attached entorhinal cortex, a major source of afferent innervation to the dentate granule cells, displayed significantly less collateral sprouting at 10 days in culture compared to that in cultures from adjacent sections without attached subicular-entorhinal tissues present. Thus, time in culture, position along the septotemporal axis, and presence of afferent cortical tissues influence aberrant neurite collateral sprouting in organotypic slice cultures of neonatal mouse hippocampus. © Wiley-Liss, Inc.     

Fetal Hippocampal Cells Grafted to Kainate-Lesioned CA3 Region of Adult Hippocampus Suppress Aberrant Supragranular Sprouting of Host Mossy Fibers

Selective lesion of the rat hippocampus using an intracerebroventricular administration of kainic acid (KA) represents an animal model for studying both lesion recovery and temporal lobe epilepsy. This KA lesion leads initially to loss of CA3 hippocampal neurons, the postsynaptic target of mossy fibers, and later results in aberrant mossy fiber sprouting into the dentate supragranular layer (DSGL). Because of the close association of this aberrant mossy fiber sprouting with an increase in the seizure susceptibility of the dentate gyrus, delayed therapeutic strategies capable of suppressing the sprouting of mossy fibers into the DSGL are of significant importance. We hypothesize that neural grafting can restore the disrupted hippocampal mossy fiber circuitry in this model through the establishment of appropriate mossy fiber projections onto grafted pyramidal neurons and that these appropriate projections will lead to reduced inappropriate sprouting into the DSGL. Large grafts of Embryonic Day 19 hippocampal cells were transplanted into adult hippocampus at 4 days post-KA lesion. Aberrant mossy fiber sprouting was quantified after 3–4 months survival using three different measures of Timm's staining density. Grafts located near the degenerated CA3 cell layer showed dense ingrowth of host mossy fibers compared to grafts elsewhere in the hippocampus. Aberrant mossy fiber sprouting throughout the dentate gyrus was dramatically and specifically reduced in animals with grafts near the degenerated CA3 cell layer compared to “lesion only” animals and those with ectopic grafts away from the CA3 region. These results reveal the capability of appropriately placed fetal hippocampal grafts to restore disrupted hippocampal mossy fiber circuitry by attracting sufficient host mossy fibers to suppress the development of aberrant circuitry in hippocampus. Thus, providing an appropriate postsynaptic target at early postlesion periods significantly facilitates lesion recovery. The graft-induced long-term suppression of aberrant sprouting shown here may provide a new avenue for amelioration of hyperexcitability that occurs following hippocampal lesions.     

Repeated long-term potentiation induces mossy fibre sprouting and changes the sensibility of hippocampal granule cells to subconvulsive doses of pentylenetetrazol

Electrical and chemical kindling induces sprouting of the mossy fibre system and potentiation of evoked field potentials in the dentate gyrus. It has been postulated that such changes may also be induced by repeated induction of long-term potentiation (LTP) with tetanic stimulation of the perforant pathway. LTP was induced in rats chronically implanted with stimulation electrodes in the ipsilateral and contralateral angular bundles and with a recording electrode in the ipsilateral dorsal dentate gyrus. The animals were stimulated 10 times on 10 consecutive days but with different tetanization strengths. Sprouting of the mossy fibres terminating in the CA3 region was significantly induced only in the group of 'strongly' tetanized animals, but not in that of 'weakly' tetanized animals, or in low-frequency stimulated animals. Additionally, a novel form of potentiation which was previously found in pentylenetetrazol (PTZ)-kindled animals was also observed in the group of 'strongly' and 'weakly' tetanized rats. Differences in duration of this potentiation were found between the two groups of animals tetanized with different strengths. The results further demonstrate that morphological and functional changes in the hippocampus, similar to those seen after kindling, can also occur in an activation paradigm leading to long-lasting synaptic plasticity but not accompanied by seizure activity.     

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.     

Effects of mineralocorticoid and glucocorticoid receptors on long-term potentiation in the CA3 hippocampal field

We have previously shown that the two types of adrenal steroid receptors, mineralocorticoid MR. and glucocorticoid GR. produce opposite effects on long-term potentiation LTP. in the dentate gyrus in vivo. and CA1 hippocampal field in vitro. More specifically, MR activation enhanced and prolonged LTP, whereas GR activation suppressed LTP in these areas and also produced a long-term depression LTD. of the synaptic response. In the present experiment we investigated acute effects of MR and GR activation on LTP induction in the mossy fiber and commissural associational input to the CA3 hippocampal field, since the mechanisms underlying LTP induction in these two pathways differ, the former being N-methyl-D-aspartate receptor NMDAR. independent while the latter being NMDAR-dependent. Rats were either adrenalectomized ADX or adrenally intact. ADX animals were acutely injected with either the specific MR agonist, aldosterone, the specific GR agonist RU 28362 or vehicle. One hour following the injection, the animals were prepared for electrophysiological recording stimulation. Field potential recordings were performed in the radiatum or laconosum moleculare layers of the CA3 field, with stimulation of either the mossy fibers or the commissural associational input from the contralateral hemisphere. We also replicated our previous findings by recording in the dentate gyrus with stimulation of the medial perforant pathway, in the same animals. As observed in our previous study in the dentate gyrus, we found an enhancement and a suppression of LTP with MR and GR activation, respectively. Similarly, for the commissural associational input to CA3, MR activation enhanced LTP, while GR activation reduced it. In contrast, for the mossy fiber input to CA3, neither MR nor GR activation significantly affected LTP induction. These results indicate that adrenal steroids may modulate LTP induction in the hippocampus via an interaction with glutamatergic NMDAR.     

Long- and short-term plasticity at mossy fiber synapses on mossy cells in the rat dentate gyrus

Mossy cells give rise to the commissural and associational pathway of the dentate gyrus, and receive their major excitatory inputs from the mossy fibers of granule cells. Through these feed-back excitatory connections, mossy cells have been suggested to play important roles in both normal signal processing in learning and memory, as well as in seizure propagation. However, the nature of the activity-dependent modifications of the mossy fiber inputs to mossy hilar cells is not well understood. We studied the long- and short-term plasticity properties of the mossy fiber-mossy cell synapse, using the minimal stimulation technique in slices in whole cell recorded mossy cells retrogradely prelabeled with the fluorescent dye DiO from the contralateral dentate gyrus. Following tetanic stimulation, mossy fiber synapses showed significant NMDA receptor-independent long-term potentiation (LTP), associated with increased excitatory postsynaptic currents (EPSC) amplitude and decreased failure rates. Coefficient of variance and failure rate analyses suggested a presynaptic locus of LTP induction. Mossy fiber synapses on mossy cells also showed activity-dependent short-term modification properties, including both frequency-dependent facilitation (stimuli at higher frequencies evoked larger EPSCs with lower failure rates) and burst facilitation (each EPSC in a burst had a larger amplitude and higher probability of occurrence than the preceding EPSCs within the burst). The data show that mossy fiber-mossy cell synapses exhibit both long- and short-term plasticity phenomena that are generally similar to the mossy fiber synapses on CA3 pyramidal cells.     

Increased granule cell neurogenesis in the adult dentate gyrus following mossy fiber stimulation sufficient to induce long-term potentiation

Neurons are continually added at a low rate to the granule cell layer of the dentate gyrus during adulthood in rats. The functional significance of this unusual feature is not completely understood, although recent studies suggest continued granule cell neurogenesis is essential for normal learning and memory. We report here that, in the adult rat, stimulation of the granule cell mossy fibers sufficient to induce long-term potentiation (LTP) increases the number of newly formed granule cells in the dentate gyrus, indicating that granule cell neurogenesis is regulated by efferent activity and, possibly, the induction of LTP.     

Prolonged infusion of cycloheximide does not block mossy fiber sprouting in a model of temporal lobe epilepsy

PURPOSE: The role of protein synthesis in mossy fiber sprouting is unclear. Conflicting reports exist on whether a single dose of the protein synthesis-blocker cycloheximide administered around the time of an epileptogenic injury can block the eventual development of mossy fiber sprouting. METHODS: In rats, osmotic minipumps and cannulae were implanted to deliver 8 mg/ml cycloheximide to one dentate gyrus and vehicle to the other. This method has been used to block protein synthesis in the infused region for up to 5 days with minimal neurotoxic effects (Taha and Stryker, Neuron 2002;34:425-36). After 2 days of infusion, rats were treated with pilocarpine to induce status epilepticus. Pumps were removed 3 days later. Thirty days after pilocarpine treatment, rats were perfused, and hippocampal sections were processed for Timm staining. RESULTS: Timm staining revealed aberrant mossy fiber sprouting in the inner molecular layer regardless of whether hippocampi were treated with cycloheximide or vehicle. Cycloheximide-treated hippocampi displayed more aberrant Timm staining and more tissue damage around the infusion site than did vehicle-treated hippocampi. CONCLUSIONS: Prolonged infusion of cycloheximide, spanning the period of pilocarpine treatment, did not block mossy fiber sprouting. This finding suggests that protein-dependent mechanisms around the time of an epileptogenic injury are not necessary for the eventual development of synaptic reorganization.     

N-cadherin在颞叶癫痫海马苔藓纤维出芽和突触重组中的作用

目的 探讨神经性钙粘附分子(N-cadherin)在癫痫状态后海马苔藓纤维出芽和突触重组中的作用。方法取锂一匹罗卡品诱导大鼠癫痫持续状态及慢性自发性颞叶癫痫发作期的大鼠脑片，用Timm染色和免疫组化的方法分别检测苔藓纤维出芽和N-cadherin在大鼠海马组织中的表达。结果癫痫状态后第2周和第4周的实验组大鼠可见到苔藓纤维出芽，穿越齿状回颗粒细胞层到达内分子层，并在此形成一条致密的层状带(Timm染色)。免疫组化染色发现实验组大鼠在第2周和第4周，海马齿状回内分子层可以看到强染色，并形成一条致密带，与Timm染色时观察到的条带一致。结论癫痫状态后在海马齿状回内分子层N-cadherin的表达上调．N-cadherin可能参与了癫痫后苔藓纤维出芽和突触重组过程。     

Electrophysiology of dentate granule cells after kainate-induced synaptic reorganization of the mossy fibers.

Morphological data from humans with temporal lobe epilepsy and from animal models of epilepsy suggest that seizure-induced damage to dentate hilar neurons causes granule cells to sprout new axon collaterals that innervate other granule cells. This aberrant projection has been suggested to be an anatomical substrate for epileptogenesis. This hypothesis was tested in the present study with intra- and extracellular recordings from granule cells in hippocampal slices removed from rats 1-4 months after kainate treatment. In this animal model, hippocampal cell loss leads to sprouting of mossy fiber axons from the granule cells into the inner molecular layer of the dentate gyrus. Unexpectedly, when slices with mossy fiber sprouting were examined in normal medium, extracellular stimulation of the hilus or perforant path evoked relatively normal responses. However, in the presence of the GABAA-receptor antagonist, bicuculline, low-intensity hilar stimulation evoked delayed bursts of action potentials in about one-quarter of the slices. In one-third of the bicuculline-treated slices with mossy fiber sprouting, spontaneous bursts of synchronous spikes were superimposed on slow negative field potentials. Slices from normal rats or kainate-treated rats without mossy fiber sprouting never showed delayed bursts to weak hilar stimulation or spontaneous bursts in bicuculline. These data suggest that new local excitatory circuits may be suppressed normally, and then emerge functionally when synaptic inhibition is blocked. Therefore, after repeated seizures and excitotoxic damage in the hippocampus, synaptic reorganization of the mossy fibers is consistently associated with normal responses; however, in some preparations, the mossy fibers may form functional recurrent excitatory connections, but synaptic inhibition appears to mask these potentially epileptogenic alterations.     

The effects of fimbria/fornix transections on perforant path kindling and mossy fiber sprouting

Various clinical and experimental studies of epilepsy have described synaptic reorganization in the dentate gyrus of hippocampus, in the form of collateral sprouting of the mossy fibers. These reports have led to the hypothesis that reorganized mossy fibers form a functional excitatory feedback circuit that contributes to local circuit hyperexcitability and chronic seizures. Much of the evidence supporting the sprouting hypothesis has been derived from kindling. We recently reported that transection of the fimbria/fornix (FF), which produces chronic epileptiform activity in the hippocampus, also induces mossy fiber sprouting in the inner molecular layer of the dentate gyrus. In the present study, we attempted to determine whether mossy fiber sprouting contributes to epileptiform activity, by examining the effects FF transections on perforant path (PP) kindling and associated mossy fiber sprouting. We found that FF transections and PP kindling produced moderate levels of sprouting, whereas the combination of the two treatments produced significantly denser sprouting. FF transections had mixed effects on kindling: afterdischarge thresholds were decreased and clonus and afterdischarge durations were increased, suggesting increased local excitation, whereas the kindling of behavioral seizures was delayed, suggesting decreased epileptogenesis.     

Mossy fiber synaptic reorganization in the epileptic human temporal lobe

The distribution of the mossy fiber synaptic terminals was examined using the Timm histochemical method in surgically excised hippocampus and dentate gyrus from patients who underwent lobectomy of the anterior part of the temporal lobe for refractory partial complex epilepsy. The dentate gyrus of epileptic patients demonstrated intense Timm granules and abundant mossy fiber synaptic terminals in the supragranular region and the inner molecular layer. In contrast, the dentate gyrus of presenescent nonepileptic primates demonstrated no Timm granules in the supragranular region. In nonepileptic senescent primates, occasional very sparse supragranular Timm granules were results are morphological evidence of mossy fiber synaptic reorganization in the temporal lobe of epileptic humans, and suggest the intriguing possibility that mossy fiber sprouting and synaptic reorganization induced by repeated partial complex seizures may play a role in human epilepsy.     

Posttraumatic epilepsy after controlled cortical impact injury in mice

Many patients develop temporal lobe epilepsy after trauma, but basic mechanisms underlying the development of chronic seizures after head injury remain poorly understood. Using the controlled cortical impact injury model we examined whether mice developed spontaneous seizures after mild (0.5 mm injury depth) or severe (1.0 mm injury depth) brain injury and how subsequent posttraumatic mossy fiber sprouting was associated with excitability in the dentate gyrus 42–71 d after injury. After several weeks, spontaneous behavioral seizures were observed in 20% of mice with mild and 36% of mice with severe injury. Mossy fiber sprouting was typically present in septal slices of the dentate gyrus ipsilateral to the injury, but not in control mice. In slices with mossy fiber sprouting, perforant path stimulation revealed a significant reduction (P < 0.01) in paired-pulse ratios in dentate granule cells at 20 ms and 40 ms interpulse intervals, but not at 80 ms or 160 ms intervals. These slices were also characterized by spontaneous and hilar-evoked epileptiform activity in the dentate gyrus in the presence of Mg²⁺-free ACSF containing 100 μM picrotoxin. In contrast, paired-pulse and hilar-evoked responses in slices from injured animals that did not display mossy fiber sprouting were not different from controls. These data suggest the development of spontaneous posttraumatic seizures as well as structural and functional network changes associated with temporal lobe epilepsy in the mouse dentate gyrus by 71 d after CCI injury. Identifying experimental injury models that exhibit similar pathology to injury-induced epilepsy in humans should help to elucidate the mechanisms by which the injured brain becomes epileptic.     

Locus Coeruleus and Neuronal Plasticity in a Model of Focal Limbic Epilepsy

Summary:  Purpose: A lesion of the noradrenergic nucleus Locus Coeruleus (LC) converts sporadic seizures evoked by microinfusion of bicuculline into the anterior piriform cortex (APC) of rats into limbic status epilepticus (SE). The purpose of this study was to evaluate the chronic effects of this new model of SE on the onset of secondary epileptogenesis. We further related the loss of noradrenaline (NE) with hippocampal mossy fiber sprouting.
Methods: Male Sprague Dawley rats were treated with systemic saline or DSP-4 (a neurotoxin selective for noradrenergic terminals originating from the LC), microinfused with bicuculline into the APC three days later, and sacrificed after 45 days. Naïve and DSP-4 pretreated sham-operated rats served as respective controls. The following evaluations were performed: (a) monitoring of acute seizures and delayed occurrence of spontaneous recurrent seizures (SRS); (b) NE levels in the hippocampus, frontal and olfactory cortex; (c) occurrence of mossy fiber sprouting into the inner molecular layer of the dentate gyrus of the dorsal hippocampus.
Results: In 30% of rats lacking noradrenergic terminals, SE evoked from the APC was followed by SRS. Conversely, seizures evoked in intact rats did not result in chronic epileptogenesis. Seizures/SE did not modify NE levels as compared with baseline levels both in naïve and DSP-4-pretreated rats. Rats undergoing SE following DSP-4 + bicuculline developed SRS which were accompanied by hippocampal mossy fiber sprouting.
Conclusions: Noradrenergic loss converts focally induced sporadic seizures into an epileptogenic SE, which is accompanied by mossy fiber sprouting within the dentate gyrus.     

Localized overexpression of FGF-2 and BDNF in hippocampus reduces mossy fiber sprouting and spontaneous seizures up to 4 weeks after pilocarpine-induced status epilepticus

Purpose: We have recently reported that viral vector–mediated supplementation of fibroblast growth factor‐2 (FGF‐2) and brain‐derived neurotrophic factor (BDNF) in a lesioned, epileptogenic rat hippocampus limits neuronal damage, favors neurogenesis, and reduces spontaneous recurrent seizures. To test if this treatment can also prevent hippocampal circuit reorganization, we examined here its effect on mossy fiber sprouting, the best studied form of axonal plasticity in epilepsy. Methods: A herpes‐based vector expressing FGF‐2 and BDNF was injected into the rat hippocampus 3 days after an epileptogenic insult (pilocarpine‐induced status epilepticus). Continuous video–electroencephalography (EEG) monitoring was initiated 7 days after status epilepticus, and animals were sacrificed at 28 days for analysis of cell loss (measured using NeuN immunofluorescence) and mossy fiber sprouting (measured using dynorphin A immunohistochemistry). Key Findings: The vector expressing FGF‐2 and BDNF decreased both mossy fiber sprouting and the frequency and severity of spontaneous seizures. The effect on sprouting correlated strictly with the cell loss in the terminal fields of physiologic mossy fiber innervation (mossy cells in the dentate gyrus hilus and CA3 pyramidal neurons). Significance: These data suggest that the supplementation of FGF‐2 and BDNF in an epileptogenic hippocampus may prevent epileptogenesis by decreasing neuronal loss and mossy fiber sprouting, that is, reducing some forms of circuit reorganization.