Glutamate on demand: astrocytes as a ready source.

The past decade of studies has changed our view of the integrative capacities and roles of glia. A picture is emerging in which neurons and astrocytes, a subtype of glial cell, are in a continuous regulatory dialogue. Initial studies demonstrated that chemical transmitters, which are released from neurons, induce elevations of astrocytic calcium. Furthermore, stimulation of neuronal afferents at modest frequencies induces a calcium response in astrocytes that is graded with stimulation frequency. The consequence of this astrocytic calcium response is now beginning to be appreciated in that changes in calcium level can induce the release of the chemical transmitter glutamate from this nonneuronal cell. During the past few years, it has been shown that by releasing glutamate, astrocytes can regulate synaptic transmission and contribute to certain forms of synaptic plasticity. The roles played in information processing by this glial feedback loop remain to be determined. However, it is likely that the results of these recent studies will signal a new way of thinking about the nervous system, in which the glial cell comes to the forefront of our attention.