A Study On The Effects And Mechanisms Of The Metabotropic Glutamate Receptors Participating In The Induction Of Brain Ischemic Tolerance

Glutamate receptors (GluRs), widespread in the brain, can be classified into ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs). The iGluRs can be further classified into N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazol e (AMPA), and kainate receptors. mGluRs family comprises 8 subtypes, mGluR1, mGluR2, mGluR3, mGluR4, mGluR5, mGluR6, mGluR7 and mGluR8. According to the degree of sequence homology, pharmacological properties and signal transduction mechanisms, they can be classified into three groups: group Ⅰ, including mGluR1/5; group Ⅱ, including mGluR2/3; and group Ⅲ, including mGluR4/6/7/8. These receptors participate in many physiological or pathological processes via different intracelluar signal transduction systems after activated by glutamic acid and aspartic acid, the specific ligands of the receptors. Recently, the roles of these receptors, especially the iGluRs in the induction of brain ischemic tolerance (BIT) have aroused the attention of the scholars all over the world. Many experimental results indicated that NMDA receptors participated in the induction of BIT to some extent. However, few researches addressed the problem whether mGluRs participate in the induction of BIT. The mere relevant report showed that there were minor differences in the expressions of group Ⅰ mGluRs between the global cerebral ischemic and BIT models in gerbils by using immunohistochemistry method, which suggested that mGluR1/5 might participate in the induction of BIT. No further relevant experimental data has been reported until now. There is even no report about whether group Ⅱ mGluRs really participate in the induction of BIT. Since mGluRs widely distribute in the brain, andthey have comprehensive influence on the neuronal plasticity, excitability and ionic channels on the cell membrane, it is reasonable to speculate that the mGluRs may play roles in the induction of BIT. Therefore, the aim of our study is to make sure the role of mGluRs in the induction of BIT, thereby providing experimental evidence for clarifying the mechanism of BIT.1　The effect of mGluRs on the induction of BIT and the expression of glial fibrillary acidic proteinTo determine the role of mGluRs in the induction of BIT, we observed the influence of (s)-4-carboxy-3-hydroxy-phenylglycine ((s)-4C3HPG), antagonist of mGluR1/5, and α-methyl-(4-tetrazolyl-phenyl)glycine (MTPG), antagonist of mGluR2/3, on the induction of BIT and the expression of glial fibrillary acidic protein (GFAP) by using thionine staining and immunohistochemistry in a rat model of four-vessel occlusion global cerebral ischemia. 1.1　Effects of (s)-4C3HPG (antagonist of mGluR1/5) on the induction of BIT and the expression of GFAPThirty-six Spague Dawley (SD) rats (weighting 280g～320g), whose vertebral arteries were permanently occluded, were randomly divided into 4 groups: ①sham group (n=6):exposing the bilateral common carotid arteries (BCCA) but without blocking the blood flow; ②ischemic insult group(n=6):blocking the blood flow of the BCCA for 8 min；③CIP+ischemic insult group ( n=6):blocking the blood flow of the BCCA for 3 min as a cerebral ischemic preconditioning (CIP), then an ischemic insult was given by blocking the blood flow for 8 min 24 h after CIP; ④ (s)-4C3HPG group(n=18):20 min after the administration of (s)-4C3HPG via the right lateral cerebral ventricle (RLCV) route, procedures for CIP and ischemic insult were performed as in CIP+ischemic insult group;According to the dosage of (s)-4C3HPG used, the group was further divided into 0.2 mg(n=6), 0.04 mg(n=6)and 0.008 mg(n=6)groups. During the occlusion of BCCA, the pupil enlarged and EEG showed decreases in frequency and amplitude, and even an isoelectric level. These phenomena indicated the occurrence of global brain ischemia. Examination for morphological changes in CA1 subfield was performed 7 days after the sham operation or last time of ischemia using thionine staining.It was found by thionine...