An association between genotypic variations and protein expression of the glial glutamate transporter 2 in the human nucleus accumbens

The glial glutamate transporter EAAT2 is responsible for the majority of synaptic glutamate clearance. Dysfunction of EAAT2 is strongly implicated in neuropsychiatric disorders. Single nucleotide polymorphisms (SNPs) in the EAAT2 gene have been associated with an increased risk of pathological conditions that may result from changes in extracellular glutamate levels. Genetic variation in the metabotropic glutamate receptor 3 (GRM3) gene has been reported to affect EAAT2 mRNA. Therefore, the aim of this study was to investigate the association of EAAT2 (rs4755404 and rs1885343) and GRM3 (rs6465084) SNPs and EAAT2 protein expression in healthy subjects. Postmortem nucleus accumbens tissue from 37 normal subjects had EAAT2 protein determined and was genotyped for three SNPs. Expression of EAAT2 protein was observed in both monomeric (70kDa) and multimeric (150kDa) forms. A significantly lower expression of the monomer (P=0.037) was observed with the GG genotype than in A allele carriers of rs1885343. However, there were no differences in EAAT2 expression associated with genotypes or alleles of rs4755404 and rs6465084. This finding indicates an association between EAAT2 protein expression in the human nucleus accumbens and a genetic polymorphism of EAAT2.

Changes in the neuronal glutamate transporter EAAT3 in rat brain after exposure to methamphetamine

Methamphetamine (METH), an addictive psychostimulant, can induce glutamate release in several brain areas such as cerebral cortex, hippocampus and striatum. Excess glutamate is ordinarily removed from the synaptic cleft by glutamate transporters for maintaining homoeostasis. EAAT3, a subtype of glutamate transporter expressed mainly by neurons, is a major glutamate transporter in the hippocampus and cortex. Therefore, this study examined the effects of acute and sub-acute METH administration on the expression of the EAAT3 in the hippocampal formation, striatum and frontal cortex. Male Sprague-Dawley rats received vehicle injections (i.p.) for 13 days followed by one injection of METH (8 mg/kg, i.p.) on day 14 in acute group. Animals received METH (4 mg/kg, i.p.) or vehicle for 14 days in sub-acute and control groups, respectively. EAAT3 immunoreactivity was determined by western blotting followed by measurement of the integrated optical density. A significant increase in EAAT3 was found in the hippocampal formation after sub-acute, but not acute, METH administration. Conversely, a significant decrease in EAAT3 in striatum was observed in both acute and sub-acute groups. A trend towards a decrease in EAAT3 was also found in frontal cortex in the sub-acute group. Our results of decreased EAAT3 in striatum and frontal cortex suggest deficits of cortico-striatal glutamatergic synapses after METH exposure. Increased EAAT3 expression in the hippocampus may be a compensatory response to possible deficits of glutamatergic neurotransmission induced by METH. Moreover, our findings provide further support for glutamatergic dysfunction with abnormalities involving a transporter important in the regulation of neuronal glutamate.