Astroglia and glutamate in physiology and pathology: aspects on glutamate transport, glutamate-induced cell swelling and gap-junction communication

Astroglia have the capacity to monitor extracellular glutamate (Glu) and ma intain it at low levels, metabolize Glu, or release it back into the extrac ellular space. Glu can induce an increase in astroglial cell volume with a resulting decrease of the extracellular space, and thereby alter the concen tration of extracellular substances. Many lines of evidence show that K+ ca n be buffered within the astroglial gap-junction-coupled network. and recen t results show that gay junctions are permeable for Glu. All these events o ccur dynamically: the astroglial network has the capacity to interfere acti vely with neurotransmission, thereby contributing to a high signal-to-noise ratio for the Gln transmission. High-quality neuronal messages during norm al physiology can then be maintained. With the same mechanisms, astroglia m ight exert a neuroprotective function in situations of moderately increased extracellular Glu concentrations, i.e,, corresponding to conditions of pat hological hyper-excitability, or corresponding to early stages of an acute brain injury. If the astroglial functions are failing, neuronal dysfunction can be reinforced. (C) 2000 Elsevier Science Ltd. All rights reserved.