Sodium dynamics: another key to astroglial excitability?

Astroglial excitability is largely mediated by fluctuations in intracellular ion concentrations. In addition to generally acknowledged Ca2+ excitability of astroglia, recent studies have demonstrated that neuronal activity triggers transient increases in the cytosolic Na+ concentration ([Na+](i)) in perisynaptic astrocytes. These [Na+](i) transients are controlled by multiple Na+-permeable channels and Na+-dependent transporters; spatiotemporally organized [Na+](i) dynamics in turn regulate diverse astroglial homeostatic responses such as metabolic/signaling utilization of lactate and glutamate, transmembrane transport of neurotransmitters and K+ buffering. In particular, near-membrane [Na+](i) transients determine the rate and the direction of the transmembrane transport of GABA and Ca2+. We discuss here the role of Na+ in the regulation of various systems that mediate fast bidirectional communication between neurones and glia at the single synapse level.