VGLUT1 is localized in astrocytic processes in several brain regions

Abstract

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.

Neuroglial Transmission

“…This does not contradict the presence of VGLUTs in astrocytes, as they have all been found to be localized in acutely isolated astrocytes both at the protein level (236,268,458) and the mRNA level (458). In line with this are immunohistochemical results, showing that VGLUT1, VGLUT2, and VGLUT3 are indeed located within delicate perisynaptic astrocytic processes in the intact brain (33,37,123,295,296,333,458). Interestingly, VGLUT3 has been shown to be upregulated in astrocytes after focal cerebral ischemia (360).…”

“…Using knockout controls for VGLUT1-3 antibodies, one confocal study concludes that none of the VGLUTs is present in astrocytes (222). In our previous studies (295,296), the VGLUT labeling in astrocytes was also controlled for by using VGLUT1 and VGLUT3 knockout tissue. The reasons for discrepancies between the results of the former and the latter studies are obscure.…”

Astrocytes contribute to gamma oscillations and recognition memory

“…Although these alternative explanations remain possible, exogenous application of the glutamate receptor agonist AMPA was capable of rescuing the duration of oscillations that were reduced with defective vesicle fusion in our model. Coupled with recent evidence that perisynaptic astrocytes express the vesicular glutamate transporters VGLUT1 and VGLUT3 (49,50) and that glutamate accumulates in synaptic-like microvesicles in the perisynaptic processes of astrocytes at a concentration comparable to that in synaptic vesicles of excitatory nerve terminals (51), glutamate may be said to in some way mediate the impact of astrocytes on network dynamics. The fact that the expression of tetanus toxin did not cause evident changes in basal synaptic transmission, as measured in cultured and in acute slices, strongly suggested that the TeNT gene, delivered by lentiviral vectors in slice cultures or integrated into the genome of transgenic mice, was exclusively expressed in astrocytes and not in neurons.…”

What do we know about gliotransmitter release from astrocytes?

“…Moreover, and crucially, good preservation of astrocytes in preparations of the rodent brain using conventional EM methods has not been sufficiently considered. Indeed, most studies to date have shown that the cytoplasm of astrocytes is electron lucent (‘empty’), and this feature has even been used as a criterion to identify and distinguish astrocytes from neurons in the neuropil [52,53]. It should be noted, however, that astrocytes and their fine processes do not always appear empty (see figure 2).…”