Astrocyte plasticity: implications for synaptic and neuronal activity.

Astrocytes are increasingly implicated in a range of functions in the brain, many of which were previously ascribed to neurons. Much of the prevailing interest centers on the role of astrocytes in the modulation of synaptic transmission and their involvement in the induction of forms of plasticity such as long-term potentiation and long-term depression. However, there is also an increasing realization that astrocytes themselves can undergo plasticity. This plasticity may be manifest as changes in protein expression which may modify calcium activity within the cells, changes in morphology that affect the environment of the synapse and the extracellular space, or changes in gap junction astrocyte coupling that modify the transfer of ions and metabolites through astrocyte networks. Plasticity in the way that astrocytes release gliotransmitters can also have direct effects on synaptic activity and neuronal excitability. Astrocyte plasticity can potentially have profound effects on neuronal network activity and be recruited in pathological conditions. An emerging principle of astrocyte plasticity is that it is often induced by neuronal activity, reinforcing our emerging understanding of the working brain as a constant interaction between neurons and glial cells.