Regulatory Role of Synaptic Proteins in Chronic Medial Temporal Lobe Epilepsy
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Knowledge of the regulatory function of synaptic proteins in neurodegenerative diseases can be useful in developing neuroprotective measures against these diseases. Therefore, the regulatory role of some synaptic proteins, namely GluA1, GluA2, PICK1 and synaptotagmin I in chronic medial temporal lobe epilepsy (MTLE) was investigated in the present study using a kainic acid (KA)-induced animal model of chronic MTLE (eight weeks after first chronic MTLE). In addition, the possible regulatory role of some important synaptic proteins, namely β-tubulin, synaptophysin, PSD-95, NR2B, and GluK2 was examined. Western blotting technique was used to examine these proteins in synaptic fractions (synaptosomes) of the hippocampus and immunogold electron microscopy (EM) technique was used for synaptotagmin I as well. Seven and eight animals were used in the control and the chronic MTLE groups respectively, in the western blotting experiments. For the immunogold EM experiments, the synapses in the stratum radiatum of the CA1 and the CA3 areas of the hippocampus were examined in three animals in each of the control and the chronic MTLE groups in both areas. The independent t-test used to analyse the mean band intensities of the western blots revealed a significant reduction in the AMPA receptor subunits GluA1 and GluA2 as well as PICK1 and synaptotagmin I in the chronic MTLE groups. Also, no significant difference was obtained for β-tubulin, synaptophysin, PSD-95, NR2B and GluK2. Due to the highly significant reduction observed for synaptotagmin I as well as its role as a Ca2+ -sensor, quantitative immunogold analysis was carried out to determine the subsynaptic sites for this reduction. The EM results showed significant reductions of synaptotagmin I concentration at the active zone and the postsynaptic density of the CA1 area as well as in the presynaptic cytoplasm and the postsynaptic cytoplasm of the CA3 area of the hippocampus. Interestingly, PICK1, which plays a role in the internalisation of GluA2 subunit from the postsynaptic plasma membrane, was also reduced, possibly as an adaptive response to curb further retraction of GluA2 from the synapse. Synaptic synaptotagmin I reductions, also, point to the possible adaptive adjustments in order to reduce both presynaptic glutamate release and postsynaptic trafficking. These findings will be useful in understanding the molecular mechanisms underlying chronic MTLE.