By regulation of synaptic glutamate concentration glutamate

By regulation of synaptic glutamate concentration, glutamate transporters play an important role in limiting glutamate signaling and controlling the activation of glutamate receptors (Rimmele and Rosenberg, 2016). Reduced glutamate transport is thought to underlie the pathogenesis of numerous neurological diseases, such as epilepsy (Coulter and Eid, 2012), amyotrophic lateral sclerosis (ALS) (Maragakis and Rothstein, 2001), and Alzheimer's disease (Scimemi et al., 2013). For example, a significant loss of EAAT2 protein has been found in ALS, in which down-regulation of the transporter is associated with neurodegeneration (Maragakis and Rothstein, 2001). Factors that affect glutamate transporter expression and activity can lead to alterations in excitatory neurotransmission (Rothstein et al., 1995, Shashidharan et al., 1994). The results obtained from our previous studies support the hypothesis that exposure to OP compounds can lead to alterations in the Carmofur GABAergic and glutamatergic systems (Ghasemi et al., 2007, Mohammadi et al., 2008, Mohammadi et al., 2016). Whereas evidence for recruitment of glutamatergic mechanisms after OP intoxication is well known, the effect of OP compounds on glutamate transporters has not been investigated in detail. Recently, we studied alterations in the expression of the glutamate transporters in the rat hippocampus following administration of paraoxon (Mohammadi et al., 2016). Significant increases in the mRNA and protein levels of both glial glutamate transporters were found in rats treated with convulsive doses of paraoxon. Since cerebral cortex is rich in cholinergic and glutamatergic innervations and long-lasting excitability spreads to cortical areas, we thus hypothesized that exposure to paraoxon could cause alterations in cerebral glutamatergic system. In the present study, the expression of glutamate transporters was measured in the cerebral cortex of paraoxon-treated rats at mRNA and protein levels.
Materials and methods
Results
Discussion In the current study, we examined the changes in the expression of neuronal and glial glutamate transporters at mRNA and protein levels as well as mRNA expression of Bax and Bcl2 in rat cerebral cortices at 4 or 18h after intoxication with one of three doses of paraoxon. Compared to unexposed rats, mRNA and protein levels of both glial glutamate transporters (GLAST and GLT-1) were increased in animals receiving 0.7mg/kg of paraoxon. There was a significant decrease in the expression of neuronal (EAAC1) and both glial glutamate transporters in 1mg/kg paraoxon treated group at mRNA and protein levels. The mRNA and protein expression of all glutamate transporters remained unchanged in rats exposed to non-convulsive dose of paraoxon (0.3mg/kg). We have previously measured mRNA and protein expression levels of glutamate transporters in hippocampus, using the same animal model of paraoxon toxicity. The results revealed that 4 and 18h after an exposure to 0.7 and 1mg/kg of paraoxon, the expression GLAST and GLT-1, but not that of EAAC1 was increased at both mRNA and protein levels (Mohammadi et al., 2016). Concurrently, in the present study, we found increased expression of GLAST and GLT-1 mRNA and protein levels in the cerebral cortices of animals intoxicated with 0.7mg/kg of paraoxon. Another study also showed an increased expression of both neuronal and glial glutamate transporters in PC12 cells after chlorpyrifos exposure (Slotkin et al., 2010). A higher expression of glutamate transporters and increased glutamate uptake may reflect a preventive or compensatory response to keep low extracellular glutamate concentration and counteract hyperexcitability. However, glutamate transporters may have adverse effects through reverse transport and release of glutamate following seizures (Crino et al., 2002, Rossi et al., 2000). On the other hand, the expression of all three neuronal and glial and transporters was decreased after exposure to 1mg/kg of paraoxon. Therefore, we decided to examine mRNA expression levels of Bax and Bcl2 genes in the cerebral cortex. Bax, a pro-apoptotic member of the Bcl2 gene family, promote apoptosis by releasing mitochondrial cytochrome c and subsequent activation of caspase pathway. The anti-apoptotic protein, Bcl2, as a member of the Bcl2 family, prevents apoptosis by binding to and inactivating pro-apoptotic proteins (Lowthert et al., 2012). Intracellular balance between Bax and Bcl2 gene expression could control the sensitivity of cells to apoptosis. Our qRT-PCR assays demonstrated that paraoxon increased Bax mRNA and decreased Bcl2 mRNA expression only in animals treated with 1mg/kg of paraoxon. Concurrent with our results, an increase in apoptotic cells and Bax mRNA and protein levels was found in rat frontal cortex after exposure to 1×LD50 of sarin (Lazar et al., 2016). Therefore, paraoxon-induced apoptotic cell death could be a possible mechanism for down-regulation of glutamate transporters in the cerebral cortex.