“Kiss-and-Run” Glutamate Secretion in Cultured and Freshly Isolated Rat Hippocampal Astrocytes

Abstract

Under physiological conditions, astrocytes not only passively support and nourish adjacent neurons, but also actively modulate neuronal transmission by releasing "glial transmitters," such as glutamate, ATP, and D-serine. Unlike the case for neurons, the mechanisms by which glia release transmitters are essentially unknown. Here, by using electrochemical amperometry and frequency-modulated singlevesicle imaging, we discovered that hippocampal astrocytes exhibit two modes of exocytosis of glutamate in response to various stimuli. After physiological stimulation, a glial vesicle releases a quantal content that is only 10% of that induced by nonphysiological, mechanical stimulation. The small release event arises from a brief (ϳ2 ms) opening of the fusion pore. We conclude that, after physiological stimulation, astrocytes release glutamate via a vesicular "kiss-and-run" mechanism.

Two forms of single-vesicle astrocyte exocytosis imaged with total internal reflection fluorescence microscopy

“…6). Our data do not rule out the possibility that astrocytes also express vesicles that are Ϸ300 nm in diameter (21). However, the data are most consistent with the view that the SpH laden vesicles we have imaged represent Ϸ30-50 nm glutamatergic vesicles.…”

“…The fastconfined SpH events are similar to kiss and run exocytosis, whereas the slow-spreading events likely represent full vesicle fusion. Recent amperometry experiments have provided evidence for kiss and run exocytosis from astrocytes (21). Do the distinct events described in the present study correspond to two different pools of vesicles or different modes of release of a common vesicle?…”

“…Astrocytes contain vesicles that are between 30 and 300 nm in size (13,21), and possess the vesicular machinery needed to package and release glutamate (13,(22)(23)(24)(25)(26)(27)(28)(29)(30). We transfected astrocytes with plasmids encoding SpH (18), in the expectation that SpH would become incorporated within astrocyte vesicles.…”

“…It is likely that these differences arise because acridine orange labels multiple acid compartments in the astrocytes, only some of which are transmittercontaining vesicles (37). Previously membrane capacitance measurements (30), acridine orange imaging (13) and amperometry (21) have been used to study astrocyte exocytosis. Our experiments extend those studies and indicate that SpH imaging with TIRF microscopy is useful to study astrocyte exocytosis, with the unexpected finding that two forms of exocytosis can be imaged simultaneously from the same astrocytes.…”

Dynamics of a minimal neural model consisting of an astrocyte, a neuron, and an interneuron

“…The main neurotransmitters that are released by astrocytes are glutamate and ATP, and in what follows their release and modulation properties will be briefly described. Thanks to important experimental studies, it was shown that glutamate is released by these cells with mechanisms similar to that regulating its release from the presynaptic terminal [24][25][26][27][28][29]. The glutamate released from astrocytes can act on the presynaptic neuron by modulating its synaptic activity, or on the postsynaptic neuron by determining its depolarization.…”