Ion changes and signalling in perisynaptic glia

The maintenance of ion gradients across plasma membranes is a prerequisite for the establishment of cellular membrane potentials, electrical signalling, and metabolite transport. At active synapses, pre- and postsynaptic ion gradients are constantly challenged and used for signalling purposes. Perisynaptic glia, mainly represented by fine processes of astrocytes which get into close vicinity to neuronal synapses, are required to normalize the extracellular ionic milieu and maintain ion gradients. On the other hand, perisynaptic glia itself is activated by synaptically released transmitters binding to plasma membrane receptors and transmitter carriers, and experiences significant ion changes as well. In this review we present an overview of dynamic changes of the major ion species in astrocytes in response to neuronal, especially synaptic, activity. We will focus on calcium, sodium, and proton/hydroxyl ions that play key roles in signalling processes, and will discuss the functional consequences of the glial ion signals and homeostatic processes for synaptic transmission.