Module 7: Figure astrocyte tripartite synapse
Functional interactions between neurons and astrocytes at the tripartite synapse.
Action potentials travelling down the presynaptic ending release glutamate that activates postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPARs) during the typical process of synaptic transmission. Some of the glutamate spills out from the synapse and diffuses across to the astrocyte surface, where it activates either metabotropic glutamate receptors (mGluRs) to stimulate the formation of inositol 1,4,5-trisphosphate (InsP3) to release Ca2+ from the endoplasmic reticulum (ER) or AMPARs to increase Ca2+ entry. This Ca2+ can stimulate the release of glutamate from the astrocyte and this feeds back to the neuronal synapse, where it has two actions: it can either inhibit or activate transmission. In the example provided by this figure, it is shown to be stimulating N-methyl-D-aspartate (NMDA) receptors (NMDARs) to enhance transmission. Ca2+ can also trigger a regenerative Ca2+ wave that spreads throughout the neuron. To see what happens when this wave reaches the endfoot, see Module 7: Figure astrocyte endfoot signalling.