Calcium triggers exocytosis from two types of organelles in a single astrocyte.

Astrocytes release a variety of signaling molecules including glutamate, D-serine, and ATP in a regulated manner. Although the functions of these molecules, from regulating synaptic transmission to controlling specific behavior, are well documented, the identity of their cellular compartment(s) is still unclear. Here we set out to study vesicular exocytosis and glutamate release in mouse hippocampal astrocytes. We found that small vesicles and lysosomes coexisted in the same freshly isolated or cultured astrocytes. Both small vesicles and lysosome fused with the plasma membrane in the same astrocytes in a Ca(2+)-regulated manner, although small vesicles were exocytosed more efficiently than lysosomes. Blockade of the vesicle glutamate transporter or cleavage of synaptobrevin 2 and cellubrevin (both are vesicle-associated membrane proteins) with a clostridial toxin greatly inhibited glutamate release from astrocytes, while lysosome exocytosis remained intact. Thus, both small vesicles and lysosomes contribute to Ca(2+)-dependent vesicular exocytosis, and small vesicles support glutamate release from astrocytes.