Multiple Lines of Evidence Indicate That Gliotransmission Does Not Occur under Physiological Conditions

Abstract

A major controversy persists within the field of glial biology concerning whether or not, under physiological conditions, neuronal activity leads to Ca²⁺-dependent release of neurotransmitters from astrocytes, a phenomenon known as gliotransmission. Our perspective is that, while we and others can apply techniques to cause gliotransmission, there is considerable evidence gathered using astrocyte-specific and more physiological approaches which suggests that gliotransmission is a pharmacological phenomenon rather than a physiological process. Approaches providing evidence against gliotransmission include stimulation of Gq-GPCRs expressed only in astrocytes, as well as removal of the primary proposed source of astrocyte Ca²⁺ responsible for gliotransmission. These approaches contrast with those supportive of gliotransmission, which include mechanical stimulation, strong astrocytic depolarization using whole-cell patch-clamp or optogenetics, uncaging Ca²⁺ or IP3, chelating Ca²⁺ using BAPTA, and nonspecific bath application of agonists to receptors expressed by a multitude of cell types. These techniques are not subtle and therefore are not supportive of recent suggestions that gliotransmission requires very specific and delicate temporal and spatial requirements. Other evidence, including lack of propagating Ca²⁺ waves between astrocytes in healthy tissue, lack of expression of vesicular release machinery, and the demise of the d-serine gliotransmission hypothesis, provides additional evidence against gliotransmission. Overall, the data suggest that Ca²⁺-dependent release of neurotransmitters is the province of neurons, not astrocytes, in the intact brain under physiological conditions.Dual Perspectives Companion Paper: Gliotransmission: Beyond Black-and-White, by Iaroslav Savtchouk and Andrea Volterra.

Keywords: GPCR; IP3R; astrocyte; calcium; d-serine; glutamate.

Figure 1.

Stimulation of astrocytic MrgA1 receptors produces a widespread Ca²⁺ elevation that propagates throughout the cell, including the fine processes and with a pattern similar to a native Gq-GPCR. A, Numbered regions of interest over astrocytic compartments correspond to the fluorescence over time measurements recorded in the same regions (B, C). Increases in fluorescence indicate astrocyte Ca²⁺ elevations. B, Application of the MrgA1 receptor agonist FLRFa produced a Ca²⁺ response very similar to the one produced by application of the endogenous Group I mGluR agonist DHPG. Hatched boxes represent regions of expanded timescale shown in C. Stimulation of astrocytic MrgA1Rs evoked a Ca²⁺ elevation that initiated in a process and then propagated throughout the visible extent of the astrocyte, including the fine processes (see Movie 1 for this cell, where it is clearly evident that the Ca²⁺ elevation enters the fine astrocyte compartments, even though fluorescence intensity was recorded only in a small number of regions of interest). C, The Ca²⁺ elevation evoked by MrgA1R stimulation (top) produced a pattern nearly identical to that produced by DHPG application (bottom). Scale bars, 10 μm. Reprinted with permission from Elsevier Limited, copyright 2007.