VGLUT1 is localized in astrocytic processes in several brain regions

During the last years, the concept of gliotransmission has been established. Glutamate has been shown to be released from astrocytes by different mechanisms, e.g., in an exocytotic manner. The authors have previously shown that astrocytes in the dentate-molecular layers express vesicular glutamate transporters on synaptic-like microvesicles (SLMVs). By confocal microscopy, the authors, in this study, show that vesicles by a family of glutamate transporters 1 (VGLUT1) labeling was clearly present within astrocytic processes (diameter > 1 μm) in several brain regions; the dentate-molecular layers, the stratum radiatum of CA1 hippocampus, the frontal cortex, and the striatum. At the electron microscopic level, immunogold cytochemistry showed the presence of VGLUT1 gold particles over SLMVs in delicate astrocytic processes (cross-sectional diameter < 500 nm) in all the above-mentioned brain regions. When measuring the distance from the membrane of SLMVs in astrocytes to the closest VGLUT1 gold particle, it turned out that most gold particles (above 95 %) were located within 25 nm from the membrane, strongly suggesting that VGLUT1 is present in SLMVs in the astrocytes. Finally, electron microscopic immunocytochemistry shows that VGLUT1 labeling was concentrated in astrocytic processes from wild type, and not in VGLUT1 knock out hippocampus. The authors have concluded that astrocytes not only in the dentate-molecular layers but also in stratum radiatum of CA1 hippocampus, frontal cortex, and the striatum possess SLMVs carrying VGLUT1, suggesting that astrocytes in all these brain regions are capable of vesicular release of glutamate.