Glial glutamate receptors: likely actors in brain signaling.

It has become clear that the neurotransmitter glutamate does not confine its excitatory effects to central nervous system neurons but interacts also with glial cells. Neurons and glia share the same types of ionotropic and metabotropic glutamate receptors except for the N-methyl-D-aspartate receptor, which is not found on glia. Applied on cultured glial cells, glutamate regulates the opening of receptor channels, activates second messengers, and causes the release of neuroactive compounds. Although glutamate and glutamate receptors confer on cultured glia the ability to receive and emit signals, it remains to be established whether glial signaling takes place in vivo. The chick Bergmann glial cells provide a unique experimental system with which to test the contribution of glial glutamate receptors to neuronal electrical activity. These cells are the exclusive carriers in the cerebellum of functional kainate receptors. The synaptic location of these receptors, their ion channel properties, and their regulation by phosphorylation reactions suggest that glial kainate receptors play a role in regulating synaptic efficacy and plasticity. If proved, this concept may require a modification of the anatomical and functional definition of a synapse to include a glial component as well.