Subcellular location of astrocytic calcium stores favors extrasynaptic neuron–astrocyte communication

Neuron–astrocyte interactions are important for brain computations and synaptic plasticity. Perisynaptic astrocytic processes (PAPs) contain a high density of transporters that are responsible for neurotransmitter clearance. Metabotropic glutamate receptors are thought to trigger Ca2+ release from Ca2+ stores in PAPs in response to synaptic activity. Our ultrastructural study revealed that PAPs are actually devoid of Ca2+ stores and have a high surface-to-volume ratio favorable for uptake. Astrocytic processes containing Ca2+ stores were located further away from the synapses and could therefore respond to changes in ambient glutamate. Thus, the anatomic data do not support communication involving Ca2+ stores in tripartite synapses, but rather point to extrasynaptic communication.