| Epilepsy is a group of chronic disease characterized by recurrent episodic central nervous system dysfunction caused by abnormal discharge of neurons. In recent years, along with the development of neurological science, the study of epileptogenic mechanisms have generally focused on how the epileptic brain start recurrent seizures without the stimulation of activating seizures.Nowadays researchers think that in humans with temporal lobe epilepsy (TLE) and in animal models of TLE, neurons form inappropriate synaptic connections and aberrant excitatory circuits in the processof epileptogensis, which increase the brain excitability, cause recurrent seizures, and contribute to the refractory epilepsy. The brain plasticity related with epilepsy play an important role in the pathogenesis of epilepsy, especially in TLE.The relationship between brain plasticity and epilepsy have been received considerable attention. Mossy fiber sprouting in hippocampus , which is the major character of hippocampal plasticity in TLE, is especially paid more attention. The extensive studies about the pathological physiological character of MFS, mechanisms of sprouting and the relationship between MFS and TLE, have been a new trend that explore the pathogensis of refractory TLE.Mossy fibers are the axons of the dentate granule cells, and they normally project to CA3 pyramidal cells and hilar interneurons, forming dense synaptic network in these regions. Seizures can induce MFS, the sprouted fibers follow a new course, projecting aberrantly to the inner molecular layer of dentate gyrus and forming novel synaptic connection in this region. It is not at all clear what is the meaning of MFS and synaptic reorganization induced by seizures.It is also not clear whether the reorganized synapses form recurrent excitatory circuits or recurrent inhibitory circuits in dentate gyrus.Furthermore, the types of new synapses, number and the identity of their postsynaptictargets determine the functional properties of the reorganized circuity. Therefore, it is very significant to study the sprouted mossy fiber' s role in mechanisms ofTLE.What cellular and molecular events trigger MFS and synaptic reorganization is unresolved. Many factors play important role in axon extending, pathfinding, targeting and synapse forming. Neural (N-)cadherin and axon guidance factor —netrin-1 are known to function in neurite outgrowth, synapse formation, and the targeting growth of axon in central nerve system. There is little report whether they involve in seizure-induced MFS and synaptic reorganization.In this study, we firstly establish the chronical seizure model following status epilepticus (SE) induced by Li-pilocarpine, this model is similar to the major character of human TLE. And we dynamicly observe the relationship between seizures and MFS by Timm staining, then we explore the change rule of MFS; Secondly, the sprouted mossy fiber synaptic terminals were labeled by Timm histochemistry and the types, number, location, postsynaptic target of new synapses were observed by electron microscopy , which to study the sprouted mossy fiber' s role in mechanisms of TLE at the synaptic ultrastructure level. Lastly, we investigate the role of N-cadherin and netrin-1 in the process of MFS and synaptic reorganization by immunohistochemical staining and by in situ hybridization.Here are the results:(1) Rats displayed SE in acute period after pilocarpine injection. After SE, the rats went into a relatively unresponsive and akinetic period (latent period) for 5-22 days. At last, the rats went into chronic period, in which category I -III spontaneous recurrent seizures (SRSs) can be observed.The clinical character of this animal model is similar with that of human TLE.(2) Seizures can induce MFS, the sprouted fibers follow a new course, projecting aberrantly to the inner molecular layer of dentate gyrus. MFS have appeared before spontaneous seizures, the staining denseness gradually increasing with time. The intense staining of MFS commonly (not absolutely) correlated with the degree and frequency of seizures.(3) The ultrastructural study of sprouted mossy fiber synapses indicated that The sprouted mossy fiber synapses in epileptic rats most commonly form asymmetric synapses with dendritic spines, less frequently with dendritic shafts ,and we occasionally observed sprouted mossy fiber terminal labeled by silver form synapses with granule cell somata in granule cell layer, not with inhibitory interneuron . In the subtypes of synapse formed by sprouted mossy fiber terminals, axospinous synapses are the most common type, next are axodendritic synapses, less frequently are axosomatic synapses.(4) N-cadherin immunostaining showed an increase ofN-cadherin in the inner molecular of dentate gyrus within 1-4 week after pilocarpine treatment , and the strong staining gradually form a compact layer-shaped strip,a pattern that was watched identically with the Timm' s staining.(5) At 7 days after pilocarpine-induced SE, netrin-1 mRNA content is increased in the dentate granule cell layers of hippocampus, which continue to 28 days after SE. The time course of the increased netrin-1 mRNA content in the dentate granule cell layers is consistent with the time course of MFS and synaptic reorganization in hippocampus.Following points can be concluded from the results of the present studies.(1) Chronic spontaneous recurrent seizures can be found after SE in animal model induced by Li-pilocarpine;The Li-pilocarpine epilepsy model is the ideal TLE model, which meets the needs of exploring mechanisms of TLE and selecting the new antiepileptic drugs.(2) SE can induce MFS, sprouted mossy fibers form novel synaptic connections with target cells that is not dominated by mossy fibers in normal condition, namely synaptic reorganization. MFS and synaptic reorganization plays an important role in the process of TLE.( 3 ) The most sprouted mossy fiber synapses are excitatory synapses, the postsynaptic targets of sprouted...
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