Glutamate-induced Exocytosis of Glutamate from Astrocytes

Abstract

Recent studies indicate that astrocytes can play a much more active role in neuronal circuits than previously believed, by releasing neurotransmitters such as glutamate and ATP. Here we report that local application of glutamate or glutamine synthetase inhibitors induces astrocytic release of glutamate, which activates a slowly decaying transient inward current ( In addition to their nutritive and metabolic functions, astrocytes have been recently discovered to actively participate in and modify neuronal activity by releasing neurotransmitters, such as glutamate and ATP (1-6). Astrocytic Ca 2ϩ -dependent glutamate release modulates neuronal activity in hippocampal slices (7-9) and cell cultures (10, 11). However, there is still controversy about the mechanism by which astrocytes release glutamate. Several hypotheses have been proposed, including exocytosis of vesicles using a protein docking system similar to synaptic vesicles (12-17), swelling-induced anion channels (18), gap-junction hemichannels (19), P 2X receptor channels (20, 21), reverse transport (22), cystine/glutamate exchangers (23), and volume-sensitive nonselective channels (24). Recently, several groups have reported that a transient astrocytic glutamate release activates a slowly decaying transient inward current (SIC) 2 in hippocampal and thalamic neurons (7,8,25,26). Besides transient astrocytic glutamate release (ϳ1 s), a long lasting release of glutamate (several 10 s) from astrocytes has also been reported (7,21,24). Transient astrocytic glutamate release could play important roles in modulating neuronal activities under both physiological and pathological conditions (26,28,29). In a previous study, we reported that either bath application of 4-aminopyridine (4-AP) or infusion of inositol 1,4,5-trisphosphate (IP 3 ) together with high [glutamate] into astrocytes induced SICs, which were caused by fusion of a high [Ca 2ϩ ] large vesicle in astrocytes (26). In this study, we further demonstrate that local increases in [glutamate] o also induce spontaneous SICs and that SICs are activated by astrocytic release of glutamate through fusion of a large vesicle. EXPERIMENTAL PROCEDURESSlice Preparation-Brain slices were prepared as described previously (9). Briefly, 14 -20-day-old (P14 -P20) SpragueDawley rats of either sex were anesthetized with sodium pentobarbitone (55 mg/kg) and then decapitated. Transverse brain slices of 300 m thickness were cut with a vibratome (TPI, St. Louis, MO) in a cutting solution containing the following (in mM): 2.5 KCl, 1.25 NaH 2 PO 4 , 10 MgSO 4 , 0.5 CaCl 2 , 10 glucose,

Calcium Triggers Exocytosis from Two Types of Organelles in a Single Astrocyte

“…5-7). Gliotransmitter release from astrocytes can be triggered by GPCR agonists (Bowser and Khakh, 2007;Xu et al, 2007;Marchaland et al, 2008) in addition to direct elevation of intracellular calcium levels by mechanical stimulation and the calcium ionophore. It is interesting that activation of different GPCRs in astrocytes favors different forms of small vesicle exocytosis (Bowser and Khakh, 2007).…”

“…Syb2 in synaptic vesicles is essential for vesicle fusion and neurotransmitter release (Schoch et al, 2001). Glutamate release in astrocytes in situ and in culture is sensitive to TeNT treatment as well (Montana et al, 2004(Montana et al, , 2006Haydon and Carmignoto, 2006;Xu et al, 2007;Zhao et al, 2009). We assessed the effect of TeNT on small vesicle and lysosome fusion, as well as Ca 2ϩ -dependent glutamate release in our experimental conditions.…”

The Glutamatergic Neurotransmission in the Central Nervous System

“…Electron lucent vesicles with a size of approximately 50 nm have been interpreted as recycling glutamatergic vesicles which capture the extracellular substance after its release [93]. Much larger vesicles, over 1 μm in diameter, form within minutes of repeated stimulation with pharmacological dosages (5-50 mM) of glutamate [94,95], but it is still unclear if they are relevant to physiological conditions or if they are a sort of "experimental artifact". Intracellular trafficking of glutamatergic vesicles in astrocytes was also demonstrated [93], as well as their fusion to the plasma membrane [90].…”

FM dyes enter via a store-operated calcium channel and modify calcium signaling of cultured astrocytes

“…Their activity-dependent uptake and destaining has made them common probes for synaptic vesicle cycling at the nerve terminals (9-11). Cultured astrocytes internalize FM dyes (3,(12)(13)(14)(15)(16)(17)(18). However, this uptake is not modified by the activation of metabotropic glutamate receptors (19), and its mechanism remains elusive.…”