Astrocytic control of synaptic function

Abstract

Astrocytes intimately interact with synapses, both morphologically and, as evidenced in the past 20 years, at the functional level. Ultrathin astrocytic processes contact and sometimes enwrap the synaptic elements, sense synaptic transmission and shape or alter the synaptic signal by releasing signalling molecules. Yet, the consequences of such interactions in terms of information processing in the brain remain very elusive. This is largely due to two major constraints: (i) the exquisitely complex, dynamic and ultrathin nature of distal astrocytic processes that renders their investigation highly challenging and (ii) our lack of understanding of how information is encoded by local and global fluctuations of intracellular calcium concentrations in astrocytes. Here, we will review the existing anatomical and functional evidence of local interactions between astrocytes and synapses, and how it underlies a role for astrocytes in the computation of synaptic information.This article is part of the themed issue 'Integrating Hebbian and homeostatic plasticity'.

Keywords: gliotransmission; neuron–glia interactions; sleep; synaptic homeostasis; synaptic islands; tripartite synapse.

Figure 1.

(a) Layout of neuron cell bodies, dendritic network and axonal fibres (grey) transmitting action potentials (red arrows). One dendrite of interest is designated by arrowheads (see below). (b) The mosaic of astrocytic domains is superimposed to the background neuronal network. (c) Different contiguous segments of the same dendrite (arrowheads in A) intersect distinct astrocytic domains. Synapses are controlled independently within each of these domains (synaptic islands). (d) One astrocyte covers portions of different dendrites, belonging to four distinct neurons, surveying the activity of a total 20 000–160 000 synapses.