颞叶癫痈大鼠海马内Rnd1 mRNA及蛋白表达变化

目的：动态观察小分子GTPase Rho家族的Rnd1 mRNA及其蛋白在氯化锂-毛果芸香碱（匹罗卡品）致痫大鼠模型海马中的表达变化，探讨其在颞叶癫痫发生发展中的作用。方法：在氯化锂-毛果芸香碱颞叶癫痫模型中应用逆转录聚合酶链反应（RT—PCR）检测癫痫持续发作（SE）后各时间点海马内Rnd1 mRNA的表达变化，并运用免疫组织化学染色法及Neo—Timm染色法分别检测齿状回门区、CA1区及CA3区中该蛋白在不同时间点的表达变化及苔藓纤维出芽（MFS）情况。结果：实验发现模型组于SE后8h内即出现Rnd1表达上调，SE后约1d达高峰，7d左右回复至对照组水平，此后其mRNA表达水平与对照组相似；而免疫组化染色发现Rnd1蛋白表达从SE后8h内即开始上调，约3d达高峰，至7d虽略有回落，但仍高于对照组水平，且这种情况可一直持续至慢性期。结论：急性期海马齿状回门区Rnd1表达上调可能通过促进MFS的发生参与了颞叶癫痫的发生。     

Neuropathological profile of the pentylenetetrazol (PTZ) kindling model

ABSTRACT

Introduction: There are three phases of seizure developing in pentylenetetrazol (PTZ)-induced kindling animal model: (i) pre-kindling phase; (ii) kindling phase or after animals are fully kindled; (iii) post-kindling phase with non-provoked spontaneous recurrent seizures. The aims of this review were to summarize the progress over time of the electroencephalographic features and neuropathological alterations in kindled PTZ treated animals.

Materials and methods: Keywords relevant to PTZ kindling were used to a guide a literature search on Pubmed, Medline and Cochrane Library.

Results: Clonic seizures induced PTZ at kindling phase led to a strong c-Fos expression in the hippocampus. Although, decline hippocampal neuron and metabolism disturbances were detected at pre-kindlig phase. Repeated PTZ induced seizures alter the GABA-mediated inhibition and glutamate-mediated excitation, which may contribute to increased seizure susceptibility. Similar to chemical animal models such as the pilocarpine and the kainic acid models, mossy fiber sprouting, hippocampal damage, and glucose hypometabolism had been seen after PTZ induced seizures.

Conclusion: PTZ kindling model may improve understanding of the seizures development provided that the differences existing between the phases of kindling model are taken into account.     

Differential regulation of cadherins and catenins during axonal reorganization in the adult rat CNS

The cadherin family consists of several homophilic adhesion molecules that, together with their intracellular binding partners the catenins, are known to mediate axonal navigation, target recognition, and synapse formation during development. Here, we have examined the potential role of these molecules in axonal sprouting induced in the adult brain. Over a period of 3 to 60 days, an episode of pilocarpine-induced status epilepticus (SE) led to sprouting of hippocampal mossy fibers both into the CA3 pyramidal cell layer and the inner molecular layer of the dentate gyrus (DG). We found focal up-regulation of N-cadherin, beta-catenin, and alpha-catenin immunoreactivity within segments of the CA3 pyramidal cell layer with pronounced neuron loss that was associated with the development of mossy fiber sprouting. In contrast, expression of these 3 molecules was unaltered in the DG molecular layer despite mossy fiber sprouting in this area. The levels of E-cadherin immunoreactivity were altered prior to the detection of mossy fiber sprouting, with a general reduction in the neuropil and increased expression in CA1/CA3 pyramidal cell somata. Our results imply that members of the cadherin/catenin families undergo specific spatiotemporal patterns of regulation, which may be important in axon target recognition and synapse formation during lesion-induced sprouting.     

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.     

Effects of vigabatrin treatment on status epilepticus-induced neuronal damage and mossy fiber sprouting in the rat hippocampus

Selective neuronal damage and mossy fiber sprouting may underlie epileptogenesis and spontaneous seizure generation in the epileptic hippocampus. It may be beneficial to prevent their development after cerebral insults that are known to be associated with a high risk of epilepsy later in life in humans. In the present study, we investigated whether chronic treatment with an anticonvulsant, vigabatrin (gamma-vinyl GABA), would prevent the damage to hilar neurons and the development of mossy fiber sprouting. Vigabatrin treatment was started either 1 h, or 2 or 7 days after the beginning of kainic acid-induced (9 mg/kg, i.p.) status epilepticus and continued via subcutaneous osmotic minipumps for 2 months (75 mg/kg per day). Thereafter, rats were perfused for histological analyses. One series of horizontal sections was stained with thionine to estimate the total number of hilar neurons by unbiased stereology. One series was prepared for somatostatin immunohistochemistry and another for Timm histochemistry to detect mossy fiber sprouting. Our data show that vigabatrin treatment did not prevent the decrease in the total number of hilar cells, nor the decrease in hilar somatostatin-immunoreactive (SOM-ir) neurons when SOM-ir neuronal numbers were averaged from all septotemporal levels. However, when vigabatrin was administered 2 days after the onset of status epilepticus, we found a mild neuroprotective effect on SOM-ir neurons in the septal end of the hippocampus (92% SOM-ir neurons remaining; P < 0.05 compared to the vehicle group). Vigabatrin did not prevent mossy fiber sprouting regardless of when treatment was started. Rather, sprouting actually increased in the septal end of the hippocampus when vigabatrin treatment began 1 h after the onset of status epilepticus (P < 0.05 compared to the vehicle group). Our data show that chronic elevation of brain GABA levels after status epilepticus does not have any substantial effects on neuronal loss or mossy fiber sprouting in the rat hippocampus.     

Stereologic estimation of hippocampal GluR2/3- and calretinin-immunoreactive hilar neurons (presumptive mossy cells) in two mouse models of temporal lobe epilepsy

Purpose: Hippocampal mossy cells receive dense innervation from dentate granule cells and, in turn, mossy cells innervate both granule cells and interneurons. Mossy cell loss is thought to trigger granule cell mossy fiber sprouting, which may affect granule cell excitability. The aim of this study was to quantify mossy cell loss in two animal models of temporal lobe epilepsy, and determine whether there exists a relationship between mossy cell loss, mossy fiber sprouting, and granule cell dispersion. Methods: Representative hippocampal sections from p35 knockout mice and mice with unilateral intrahippocampal kainate injection were immunolabeled for GluR2/3, two subunits of the amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionate (AMPA) receptor and calretinin to identify mossy cells. Mossy fibers were immunostained against synaptoporin. Key Findings: p35 Knockout mice showed no hilar cell death, but moderate mossy fiber sprouting and granule cell dispersion. In the kainate‐injected hippocampus, there was an 80% and 85% reduction of GluR2/3‐ and GluR2/3/calretinin‐positive hilar neurons, respectively, and dense mossy fiber sprouting and significant granule cell dispersion. In the contralateral hippocampus there was a 52% loss of GluR2/3‐, but only a 20% loss of GluR2/3‐calretinin‐immunoreactive presumptive mossy cells, and granule cell dispersion; no mossy fiber sprouting was observed. Significance: These results indicate a probable lack of causality between mossy cell death and mossy fiber sprouting.     

法舒地尔对戊四氮点燃大鼠海马丝切蛋白及苔藓纤维出芽的影响

目的 探讨法舒地尔对戊四氮(PTZ)点燃大鼠海马组织中丝切蛋白(cofilin,非磷酸化形式)表达与苔藓纤维出芽程度关系的影响.方法 210只SD雄性大鼠分成戊四氮组、法舒地尔干预组和生理盐水对照组,采用PTZ慢性点燃癫癎模型,应用SABC法检测cofilin表达,用Timm染色检测苔鲜纤维出芽情况.结果 PTZ组大鼠点燃率、病死率与法舒地尔组比较差异无统计学意义.PTZ组和法舒地尔组CA3区苔藓纤维出芽评分差异无统计学意义,与对照组相比差异均有统计学意义(P＜0.05).PTZ组和法舒地尔组海马非磷酸化cofilin表达差异无统计学意义.结论 丝切蛋白可能通过苔藓纤维出芽与癫癎的发生相关.     

Granule cell mRNA levels for BDNF, NGF, and NT-3 correlate with neuron losses or supragranular mossy fiber sprouting in the chronically damaged and epileptic human hippocampus

This study determined in temporal lobe epilepsy patients if there were correlations among hippocampal granule cell expression of neurotrophin mRNAs, aberrant supragranular mossy fiber sprouting, and neuron losses. Consecutive surgically resected hippocampi (n=9) and comparison tissue from autopsies (n=3) were studied for:

1. Granule cell mRNA levels usingin situ hybridization for brainderived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin-3 (NT-3);
2. neo-Timm supragranular mossy fiber sprouting; and
3. Ammon’s horn neuron densities.

Clinically, patients were classified into those with hippocampal sclerosis (HS;n=7) and non-HS cases (i.e., mass lesions and autopsies;n=5). Results showed that compared to non-HS cases, HS patients showed increased granule cell mRNA levels for BDNF, NGF, and NT-3 (p=0.035,p=0.04,p=0.045 respectively; one-tail directional test). Moreover, granule cell BDNF mRNA levels correlated inversely with Ammon’s horn neuron densities (p=0.02) and correlated positively with greater supragranular mossy fiber sprouting (p=0.02). NGF mRNA levels correlated inversely with Ammon’s horn neuron densities (p=0.02), and TN-3 mRNA levels correlated inversely with age at surgery (p=0.04) and correlated positively with greater mossy fiber sprouting (p=0.026). These results indicate in the chronically damaged human hippocampus that granule cells express neurotrophin mRNAs, and mRNA levels correlate with either hippocampal neuron losses or aberrant supragranular mossy fiber sprouting. These data support the hypothesis that in the epileptic human hippocampus, there may be pathophysiologic associations among mossy fiber synaptic plasticity, hippocampal neuron damage, and granule cell mRNA neurotrophin levels.     

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.     

创伤性癫痫大鼠前脑海马CA3区突触蛋白表达与突触重建的相关性

摘要 目的：动态观察突触后密度蛋白-95(PSD-95)和突触囊泡蛋白(SYN)在外伤性癫痫(PTE)大鼠前脑的表达,探讨有关发病机制。方法：立体定向注射1000 nmol FeCl2：于大鼠右侧运动皮层致痫,在不同时间点行为学视频、脑电图监测,用免疫组织化学法检测24h、7d、14d、21d、30d大鼠前脑内PSD-95及SYN的表达。结果;所有大鼠在注射FeC12后不久记录到癫痫样放电,7d达高峰;致痫后24h即可见前脑神经元PSD-95表达下降,7d达最低值,14d后回升;SYN表达在24h明显下降,7d后逐渐回升。结论：FeC12皮层注射可以制成PTE动物模型。与PSD-95 及SYN表达相关的神经元及胶质细胞的动态变化在PTE的发病机制中起重要作用。PSD-95和SYN表达减少可能为FeCl2致痫的共同途径。     

Manganese-enhanced magnetic resonance imaging detects mossy fiber sprouting in the pilocarpine model of epilepsy

Purpose: Mossy fiber sprouting (MFS) is a frequent finding following status epilepticus (SE). The present study aimed to test the feasibility of using manganese‐enhanced magnetic resonance imaging (MEMRI) to detect MFS in the chronic phase of the well‐established pilocarpine (Pilo) rat model of temporal lobe epilepsy (TLE). Methods: To modulate MFS, cycloheximide (CHX), a protein synthesis inhibitor, was coadministered with Pilo in a subgroup of animals. In vivo MEMRI was performed 3 months after induction of SE and compared to the neo‐Timm histologic labeling of zinc mossy fiber terminals in the dentate gyrus (DG). Key Findings: Chronically epileptic rats displaying MFS as detected by neo‐Timm histology had a hyperintense MEMRI signal in the DG, whereas chronically epileptic animals that did not display MFS had minimal MEMRI signal enhancement compared to nonepileptic control animals. A strong correlation (r = 0.81, p < 0.001) was found between MEMRI signal enhancement and MFS. Significance: This study shows that MEMRI is an attractive noninvasive method for detection of mossy fiber sprouting in vivo and can be used as an evaluation tool in testing therapeutic approaches to manage chronic epilepsy.     

The cholinergic system modulates kindling and kindling-induced mossy fiber sprouting

In a previous study, our laboratory demonstrated that the intraventricular infusion of nerve growth factor (NGF) accelerated kindling rates and enhanced mossy fiber sprouting in the absence of noticeable kindling-associated neuronal loss. The purpose of the present study was to investigate whether these NGF effects were mediated via the cholinergic system. This study evaluated the effects of the cholinergic agonist pilocarpine and the cholinergic antagonist scopolamine on kindling rates and kindling-induced mossy fiber sprouting in adult rats. The results showed that pilocarpine accelerated kindling rates and enhanced kindling-induced mossy fiber sprouting in the CA3 region of the hippocampus, whereas scopolamine retarded kindling rates and blocked kindling-induced mossy fiber sprouting in the CA3 and IML regions. These findings suggest that the cholinergic system may contribute to the long-term structural and functional alterations that are characteristic of the kindled state. Moreover, these data provide support for the hypothesis that NGF infusions may mediate kindling and kindling-induced mossy fiber sprouting via regulation of the cholinergic system.     

Aberrant plasticity in the hippocampus after neonatal seizures

Purpose/aim of the study: Neonatal seizures are the most frequent type of neurological disorder, and those newborn babies that experience seizures carry an increased risk of epileptogenesis and other long-term morbidities. The aims of this review were to summarize the different effects on hippocampal neurogenesis between neonatal seizures and adult seizures.

Materials and methods: Pubmed, Medline and Cochrane Library were searched for articles that examined the aberrant plasticity in the hippocampus after neonatal seizures.

Results: Different from the adult animals, the proliferation of newly formed dentate granule cells is reduced in the acute phase and enhanced in the chronic phase after neonatal seizures. Neonatal seizures cause less ectopic migration of newborn neurons. Recurrent neonatal seizures also switch the effect of gamma-aminobutyric acid receptor activation from inhibition to excitation, which may contribute to increased seizure susceptibility. Similar to adult seizures, mossy fiber sprouting and sprouting of basal dendrites had been seen after neonatal seizures.

Conclusions: The effects of seizures on neurogenesis in the hippocampus are age-dependent.     

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.     

匹罗卡品癫痫模型中突触素Ⅰ的表达及苔藓纤维出芽的动态变化

目的探讨癫痫发生过程中突触素Ⅰ(SYNⅠ)在海马和齿状回的表达及齿状回苔藓纤维出芽的动态变化。方法建立匹罗卡品癫痫持续状态模型,用图像分析系统测定海马和齿状回不同时点SYNⅠ免疫反应吸光度值．Neo-timms’染色观察齿状回苔藓出芽的演变。结果SYNⅠ在海马和齿状回的表达于癫痫状态后2d、7d出现降低,14d开始升高,30d、60d表达明显增高；齿状回内分子层于14d开始出现苔藓纤维出芽,大鼠在同期开始出现自发发作。结论在癫痫状态后2d即出现了突触可塑性的变化,14d后由于神经轴突的再生,齿状回内分子层出现苔藓纤维出芽,形成了兴奋性的环路,可能是癫痫反复自发发作的病理基础,SYNⅠ及苔藓纤维出芽较好的反应了神经可塑性的变化。     

Timing of cognitive deficits following neonatal seizures: relationship to histological changes in the hippocampus.

Neonatal seizures are frequently associated with cognitive impairment and reduced seizure threshold. Previous studies in our laboratory have demonstrated that rats with recurrent neonatal seizures have impaired learning, lower seizure thresholds, and sprouting of mossy fibers in CA3 and the supragranular region of the dentate gyrus in the hippocampus when studied as adults. The goal of this study was to determine the age of onset of cognitive dysfunction and alterations in seizure susceptibility in rats subjected to recurrent neonatal seizures and the relation of this cognitive impairment to mossy fiber sprouting and expression of glutamate receptors. Starting at postnatal day (P) 0, rats were exposed to 45 flurothyl-induced seizures over a 9-day period of time. Visual-spatial learning in the water maze and seizure susceptibility were assessed in subsets of the rats at P20 or P35. Brains were evaluated for cell loss, mossy fiber distribution, and AMPA (GluR1) and NMDA (NMDAR1) subreceptor expression at these same time points. Rats with neonatal seizures showed significant impairment in the performance of the water maze and increased seizure susceptibility at both P20 and P35. Sprouting of mossy fibers into the CA3 and supragranular region of the dentate gyrus was seen at both P20 and P35. GluR1 expression was increased in CA3 at P20 and NMDAR1 was increased in expression in CA3 and the supragranular region of the dentate gyrus at P35. Our findings indicate that altered seizure susceptibility and cognitive impairment occurs prior to weaning following a series of neonatal seizures. Furthermore, these alterations in cognition and seizure susceptibility are paralleled by sprouting of mossy fibers and increased expression of glutamate receptors. To be effective, our results suggest that strategies to alter the adverse outcome following neonatal seizures will have to be initiated during, or shortly following, the seizures.     

Immunocytochemical analysis of glutamate and GABA in hippocampus of genetic absence epilepsy rats (GAERS)

In the present study, we used an immunocytochemical technique at the electron microscopic level to determine if there are changes in the glutamate and GABA neurotransmitter content of the hippocampus of genetic absence epilepsy rats from Strasbourg (GAERS). We also investigated if there was mossy fiber reorganization. After perfusion fixation, brains were removed and cryostat sections were stained according to the neo-Timm's procedure. High-resolution electron microscopy was used for ultrastructural examination of the hippocampus of GAERS and non-epileptic control Wistar animals. For ultrastructural and immunocytochemical studies, ultrathin-cut sections were obtained and immunolabeled with anti-glutamate and anti-GABA antibodies. The number of gold particles per nerve terminal was counted and the area of the nerve terminal was determined using the program NIH Image Analysis. No mossy fiber sprouting was detected in the hippocampus of GAERS. GABA and glutamate immunoreactivity were observed in the mossy fiber terminals of both the control and GAERS groups. Glutamate density in the CA3 region of GAERS hippocampus was found to be significantly increased compared to the control group. However, there was no difference in the GABA density of nerve terminals and in areas of GABAergic and mossy terminals between GAERS and the control group. The difference in glutamate level may merely be due to strain differences between the GAERS strain and the original Wistar strain or it is also possible that it appears after seizures have started.     

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.     

Circuit Mechanisms of Seizures in the Pilocarpine Model of Chronic Epilepsy: Cell Loss and Mossy Fiber Sprouting

We used the pilocarpine model of chronic spontaneous recurrent seizures to evaluate the time course of supragranular dentate sprouting and to assess the relation between several changes that occur in epilep tic tissue with different behavioral manifestations of this experimental model of temporal lobe epilepsy. Pilo carpine-induced status epilepticus (SE) invariably led to cell loss in the hilus of the dentate gyrus (DG) and to spontaneous recurrent seizures. Cell loss was often also noted in the DG and in hippocampal subfields CA1 and CA3. The seizures began to appear at a mean of 15 days after SE induction (silent period), recurred at variable frequencies for each animal, and lasted for as long as the animals were allowed to survive (325 days). The granule cell layer of the DG was dispersed in epileptic animals, and neo-Timm stains showed supra-and intragranular mossy fiber sprouting. Supragranular mossy fiber sprout ing and dentate granule cell dispersion began to appear early after SE (as early as 4 and 9 days, respectively) and reached a plateau by 100 days. Animals with a greater degree of cell loss in hippocampal field CAS showed later onset of chronic epilepsy (r= 0.83, p < 0.0005), suggest ing that CA3 represents one of the routes for seizure spread. These results demonstrate that the pilocarpine model of chronic seizures replicates several of the fea tures of human temporal lobe epilepsy (hippocampal cell loss, suprar and intragranular mossy fiber sprouting, den tate granule cell dispersion, spontaneous recurrent sei zures) and that it may be a useful model for studying this human condition. The results also suggest that even though a certain amount of cell loss in specific areas may be essential for chronic seizures to occur, excessive cell loss may hinder epileptogenesis.     

Mossy fiber sprouting and pyramidal cell dispersion in the hippocampal CA2 region in a mouse model of temporal lobe epilepsy

Dentate granule cells and the hippocampal CA2 region are resistant to cell loss associated with mesial temporal lobe epilepsy (MTLE). It is known that granule cells undergo mossy fiber sprouting in the dentate gyrus which contributes to a recurrent, proepileptogenic circuitry in the hippocampus. Here it is shown that mossy fiber sprouting also targets CA2 pyramidal cell somata and that the CA2 region undergoes prominent structural reorganization under epileptic conditions. Using the intrahippocampal kainate mouse model for MTLE and the CA2‐specific markers Purkinje cell protein 4 (PCP4) and regulator of G‐Protein signaling 14 (RGS14), it was found that during epileptogenesis CA2 neurons survive and disperse in direction of CA3 and CA1 resulting in a significantly elongated CA2 region. Using transgenic mice that express enhanced green fluorescent protein (eGFP) in granule cells and mossy fibers, we show that the recently described mossy fiber projection to CA2 undergoes sprouting resulting in aberrant large, synaptoporin‐expressing mossy fiber boutons which surround the CA2 pyramidal cell somata. This opens up the potential for altered synaptic transmission that might contribute to epileptic activity in CA2. Indeed, intrahippocampal recordings in freely moving mice revealed that epileptic activity occurs concomitantly in the dentate gyrus and in CA2. Altogether, the results call attention to CA2 as a region affected by MTLE‐associated pathological restructuring. © 2015 Wiley Periodicals, Inc.