Expression patterns and regulation of glutamate transporters in the developing and adult nervous system

Glutamate and aspartate are the primary excitatory neurotransmitters in the mammalian central nervous system and have also been implicated as mediators of excitotoxic neuronal injury and death. The precise control of extracellular glutamate and aspartate is crucial to the maintenance of normal synaptic transmission and the prevention of excitotoxicity following acute insults to the brain, such as stroke or head trauma, or during the progression of neurodegenerative diseases such as amyotrophic lateral sclerosis. The removal of excitatory amino acids (EAAs) from the extracellular space is primarily mediated by a family of sodium-dependent glutamate transporters. These transporters use the sodium electrochemical gradients of the cell to actively concentrate EAAs in both neurons and glia. Five members of this transporter family have been cloned recently and include both 'glial'-specific and 'neuron'-specific subtypes. Although these subtypes share many common functional properties, there are considerable differences in developmental expression, chronic and acute regulation by cellular signaling pathways, and contribution to disease processes among the subtypes. In this review recent studies of glutamate transporter expression, regulation, function, and pathological relevance are summarized, and some of the discrepancies and unexpected results common to any rapidly progressing field are discussed.