br Acknowledgements This work was
Acknowledgements This work was supported by the Science and Technology Department of Hubei Province (No.2016CFB368) and the National Natural Science Foundation of China (No. 81201053).
Introduction L-Glutamate (glutamate) is the most important excitatory neurotransmitter of the central nervous system (CNS); (Fonnum, 1984; Bennett and Balcar, 1999; Danbolt, 2014). Excessive extracellular concentrations of glutamate are, however, potentially neurotoxic and could be involved in etiology of many diseases from neuroinflammatory to neurodegenerative (reviews: Sheldon and Robinson, 2007; Getts et al., 2008; Lewerenz and Maher, 2015). Glutamate is removed by rapid transport mediated by specialized protein molecules (glutamate transporters) residing in plasma membranes, mostly in cells surrounding glutamatergic synapses. Five subtypes of glutamate transporters have been identified and named EAAT1 – EAAT5 (products of genes SLC1A3, SLC1A2, SLC1A1, SLC1A6, SLC1A7). The subtypes EAAT1 and EAAT2 are located in glial cells (astrocytes, microglia and oligodendrocytes), however, a variant of EAAT2 has also been found in axon-terminals (Danbolt et al., 2016). EAAT2 is responsible for over 90% of glutamate reuptake within the central nervous system (CNS) (reviews: Zhou and Danbolt, 2013; Danbolt et al., 2016). EAAT3 and EAAT4 are present in neurons and have variously been reported as expressed in axon terminals, cell bodies, and dendrites (Danbolt, 2001; Danbolt et al., 2016). EAAT5 is located mainly in the retina but could have much wider distribution both elsewhere in the CNS and in the periphery (Danbolt, 2001; Lee et al., 2012, 2013; 2016). The nomenclature based on EAAT (“excitatory amino SJB3-019A transporter”) is used mainly for the glutamate transporters isolated from human or guinea pig brain tissue; in rodents EAAT1 is referred to as GLAST; EAAT2 is called GLT1 and EAAT3 is known as EAAC3 (reviews: Danbolt, 2001; Balcar, 2002; Šerý et al., 2015). Repeated exposure to ethanol has been shown to elevate extracellular glutamate levels while reducing glutamate uptake without changing the expression of GLAST (EAAT1) in the nucleus accumbens (NAc); (Melendez et al., 2005). In contrast, using human brain post-mortem tissue, Flatscher-Bader and Wilce (2008) found dramatic increases in EAAT1 expression in deep layers of the prefrontal cortex whilst Rimondini et al. (2002) reported that chronic intermittent ethanol self-administration by rats induced GLAST (EAAT1) gene expression in the frontal cortex 5.7-fold. Experiments with GLAST knockout mice, however, indicated that GLAST −/− animals had lower alcohol consumption with no impact on ethanol preference in the conditioned place preference (CPP) paradigm (Karlsson et al., 2012). There is no easy and straightforward interpretation of the above findings. The data may have been influenced by species differences, by brain regional variations in the sensitivity to ethanol, by the differences in the modes of ethanol administration as well as by actual doses (particularly difficult to determine accurately when using human post-mortem tissue) and the length of time for which it was given. The above data, nevertheless, point to GLAST as potentially one of the key proteins involved in the response of brain tissue to ethanol, particularly when ethanol is administered repeatedly for a long time. As such, GLAST (EAAT1) should be considered a molecule of interest when investigating mechanisms of alcoholism. There is evidence that glutamate transport acts in a close association with other proteins, particularly in conjunction with Na+,K+-dependent ATPase that produces Na+ and K+ gradients which provide the main driving force for the glutamate transport (Pellerin and Magistretti, 1986; Nanitsos et al., 2004). GLAST (EAAT1) in particular has been shown to bind and, possibly, cluster with a number of proteins, probably forming a complex (“transportosome”) thus perhaps facilitating its normal functioning (Bauer et al., 2012).