Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. After the release of glutamate from synaptic vesicles into the synaptic cleft during neurotransmission, excitatory amino acid transporters (EAATs) remove extracellular glutamate to avoid excitotoxic levels (1).
Five EAATs with differential expression patterns have been described so far: EAAT1, also referred to as GLAST and SLC1A3, has neuroprotective potential following ischemia and occurs in reactive astrocytes and activated microglia. EAAT2 (GLT-1, SLC1A2) is the most abundant isoform and is primarily expressed in astrocytes. Both variants show high levels in brain and retina. EAAT3 / SLC1A1, EAAT4 / SLC1A6 and EAAT5 / SLC1A7 are expressed in neurons (2). EAAT4 shows weak expression in the forebrain and high levels in the cerebellum, where it mainly locates to Purkinje cells (3). EAAT5 primarily occurs in the retina, where it locates very close to glutamate release sites. In K.O. mice flicker resolution is considerably compromised (4). Recent findings suggest that EAAT5 is an abundant isoform, expressed also in non-neuronal peripheral tissues (5).
