Changes In Expression Of Nogo-A And Its Receptor NgR In Hippocampus Of Rats Kindled By Pentylenetetrazol

Background and ObjectiveEpilepsy(EP)is a group of chronic disease characterized by recurrent episodic central nervous system(CNS) dysfunction caused by abnormal discharge of neurons. Although most patients with epilepsy can be effectively controlled by appropriate antiepileptic drugs (AEDs),about 20-25% cases of epilepsy show resistance to medicine drugs,which is considered to be intractable epileps.Temporal lobe epilepsy (TLE) is one of the most common intractable epilepsy in clinical,the pathogenesis and etiology of which have not been well understood up to now.Axonal sprouting and subsequent synaptic reorganization in hippocampus,which is the main pathologic characteristics of TLE,has been received considerable attention in recent years.Mossy fiber sprouting(MFS) in hippocampus, which is the major character of hippocampal plasticity in TLE,is especially paid more attention. Mossy fibers,which are the axons of the dentate granule cells in hippocampus,normally project to CA3 pyramidal cells and hilar interneurons,forming dense synaptic network in these regions.The chronic seizures can induce the neural plasticity changes such as MFS and synaptic reorganization in the hippocampus.Nowadays, it is thought that MFS and synaptic reorganization of hippocampus in the process of epileptogensis of TLE,can form inappropriate synaptic connections and abnormal hyperexcitability circuits in hippocampal formation,which increase the brain excitability,cause recurrent seizures and contribute to the intractable epilepsy.The cellular and molecular mechanism of triggering axonal sprouting and synaptic reorganization in the hippocampus after seizures is still not understood.Axon regeneration and structural plasticity after injury in the adult CNS of higher vertebrate animals is limited by the presence of inhibitory proteins.Nogo-A is a myelin-derived axon growth inhibitory proteins,and a majority of Nogo-A is expressed by oligodendrocytes in the adult CNS of higher vertebrate animals.NgR,the specific receptor of Nogo-66(a transmembrane segment of Nogo-A), has been identified as an axonal glycosyl phosphatidyl inositol (GPI),which is found throughout the neuronal cell bodies and axons in the adult and maturing CNS. Nogo-A on the oligodendrocyte surface interacts with NgR on neurons, resulting in the collapse and retraction of central nervous system (CNS) axonal growth cones,to play an important role both in the restriction of axonal regeneration and in targeting growth of axon, which involves in the reconstruction of the functional neural circuit after injury in the adult CNS.Nogo-A and NgR are widely expressed in the hippocampus of adult animals.but we did not know whether they contribute to axonal sprouting and synaptic reorganization in the hippocampus in TLE.At present,there were few reports about the changes of Nogo-A or NgR in patients with TLE and in animal models of TLE,and the conclusions were different.In thepresent study,we first investigated the dynamic changes in expression of Nogo-A and its receptor NgR in the hippocampus of rats kindled by pentylenetetrazol (PTZ),to explore their effects on axonal sprouting and synaptic reorganization in the hippocampus in TLE.Materials and MethodsThirty six healthy male Sprague-Dawley (SD) rats were randomly divided into experimental group (intraperitoneal injection of 35mg/kg PTZ,daily,n=20) and control (intraperitoneal injection of 3.5mL/kg normal saline,daily,n=16) group.According to the Racine scale of seizure activity,the behavior of rats were observed and recorded for 1 hour after each PTZ-injection or normal saline-injection.When the seizure activity of experimental rats reached about class II and kindling criterion,the rats were anaesthetized and perfused,and the experimental group and control group were equally divided into 2 subgroups according to the different time points.Electroencephalograms (EEGs) of antemortem rats were recorded.The expression of Nogo-A and its receptor NgR in the hippocampus of the rats of each group were determined by immunohistochemistry at eachtime point. The data were indicated by mean±standard deviation (x±s) and analyzed with SPSS 10.0 satatistical software by analysis of variance (ANOVA),and comparison between samples was analyzed with q-test. the significant level isα=0.05.Result(1) Observation of epilepsia praxiology and electroencephalogram (EEG)All rats in the experimental groups reached kindling criterion at week 4,and the seizure activity of experimental rats reached aboutclass II at week 2.Therecorded eletrien-cephalograms(EEGs) of rats in the experimental groups showed more high-amplitude spike (sharp) waves and spike(sharp) and wave complex discharges at week 2 and week 4. The observed behavior and recorded electroencephalograms( EEGs) were normal in all control rats.(2) Nogo-A and its receptor NgR expressionNogo-A was widely expressed in the white matter in hippocampus of control rats, and the considerable Nogo-A-positive neurons was detected in the pyramidal cell layers of CA1 and CA3 region and the multiform layer of hilar region in hippocampus of control rats.NgR was widely expressed by neurons in the pyramidal cell layers of CA1 and CA3 region,multiform layer of hilar region and granular cell layer of dentate gyrus in hippocampus of control rats.The patterns of expression of Nogo-A and its receptor NgR in hippocampus of experimental rats had no changes compared with control rats.Compared with control rats,Nogo-A immunoreactivity was significantly decreased in the inner molecular layer of dentate gyrus,multiform layer of hilar region and CA3 region in hippocampus of experimental rats at week 2 and week 4 (P <0.01),and compared with the control groups, the neurons staining for NgR were markedly decreased in the pyramidal cell layers of CA3 region,multiform layer of hilar region and granular cell layer of dentate gyrus in hippocampus of experimental groups (P<0.01).The expression levels of Nogo-A and its receptor NgR in the CA1 region in hippocampus of the experimental rats had no significant difference compared with the controls at every time point (P>0.05).Conclusions(1) The down-regulation of Nogo-A in the inner molecular layer of dentate gyrus and the down-regulation of NgR in the granular cell layer of dentate gyrus may be one of the endogenous molecular mechanism of MFS into the inner molecular laye of dentate gyrus in TLE.(2) The down-regulation of Nogo-A and NgR in the CA3 region and hilar region maybe participate in the axon sprouting and synaptic reorganization of CA3 region and hilar region in TLE.