Astrocytic Synapses

Abstract

The idea that astrocytes, as well as neurons, are actively signaling is becoming more accepted. Indeed, there is evidence that astrocytes release glutamate through a calcium-dependent mechanism. Bezzi et al . performed ultrastructural analysis of hippocampal astrocytes in situ to show that a subset of these glial cells possess a vesicular compartment that is positive for the vesicular glutamate transporter and various components of the SNARE fusion complex. This compartment is juxtaposed to the plasma membrane adjacent to neuronal structures. Indeed, some of the vesicles were located across from neuronal membranes positive for the glutamate N -methyl-D-aspartate (NMDA) receptor. Glutamate release and vesicle fusion were stimulated by application of dihydroxyphenylglycine (DHPG) to cultured astrocytes. Differences between astrocytes and neurons included the density of vesicles (astrocytes had a lower density of vesicles than that in neuronal synapses) and the rate of vesicle fusion (cultured astrocytes were slower than activated neurons). Nevertheless, these results suggest that glial-neuronal communication may occur by a synapse-like mechanism, whereby the glia stimulate the adjacent neuron by releasing neurotransmitter through calcium-stimulated vesicular fusion at sites of membrane apposition. P. Bezzi, V. Gundersen, J. L. Galbete, G. Seifert, C. Steinhäuser, E. Pilati, A. Volterra, Astrocytes contain a vesicular compartment that is competent for regulated exocytosis of glutamate. Nat. Neurosci. 7 , 613-620 (2004). [Online Journal]