The active role of astrocytes in synaptic transmission

In the central nervous system, astrocytes form an intimately connected netw ork with neurons, and their processes closely enwrap synapses. The critical role of these cells in metabolic and trophic support to neurons, ion buffe ring and clearance of neurotransmitters is well established. However, recen t accumulating evidence suggests that astrocytes are active partners of neu rons in additional and more complex functions. In particular, astrocytes ex press a repertoire of neurotransmitter receptors mirroring that of neighbou ring synapses. Such receptors are stimulated during synaptic activity and s tart calcium signalling into the astrocyte network. Intracellular oscillati ons and intercellular calcium waves represent the astrocyte's own form of e xcitability, as they trigger release of transmitter (i.e. glutamate) via a novel process sensitive to blockers of exocytosis and involving cyclooxygen ase eicosanoids. Astrocyte-released glutamate activates receptors on the su rrounding neurons and modifies their electrical and intracellular calcium ( [Ca2+](i)) state. These exciting new findings reveal an active participatio n of astrocytes in synaptic transmission and the involvement of neuron-astr ocyte circuits in the processing of information in the brain.