title: Spontaneous glutamate release is independent of calcium influx and tonically activated by the calcium-sensing receptor.
abstract: Spontaneous release of glutamate is important for maintaining synaptic strength and controlling spike timing in the brain. Mechanisms regulating spontaneous exocytosis remain poorly understood. Extracellular calcium concentration ([Ca(2+)](o)) regulates Ca(2+) entry through voltage-activated calcium channels (VACCs) and consequently is a pivotal determinant of action potential-evoked vesicle fusion. Extracellular Ca(2+) also enhances spontaneous release, but via unknown mechanisms. Here we report that external Ca(2+) triggers spontaneous glutamate release more weakly than evoked release in mouse neocortical neurons. Blockade of VACCs has no effect on the spontaneous release rate or its dependence on [Ca(2+)](o). Intracellular [Ca(2+)] slowly increases in a minority of neurons following increases in [Ca(2+)](o). Furthermore, the enhancement of spontaneous release by extracellular calcium is insensitive to chelation of intracellular calcium by BAPTA. Activation of the calcium-sensing receptor (CaSR), a G-protein-coupled receptor present in nerve terminals, by several specific agonists increased spontaneous glutamate release. The frequency of spontaneous synaptic transmission was decreased in CaSR mutant neurons. The concentration-effect relationship for extracellular calcium regulation of spontaneous release was well described by a combination of CaSR-dependent and CaSR-independent mechanisms. Overall these results indicate that extracellular Ca(2+) does not trigger spontaneous glutamate release by simply increasing calcium influx but stimulates CaSR and thereby promotes resting spontaneous glutamate release.
authors: Division of Pulmonary and Critical Care Medicine, Oregon Health & Science University, Portland, Oregon 97239, USA.
authors: Nicholas P
authors: NP
authors: Vyleta
authors: Stephen M
authors: SM
authors: Smith
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000465:Algorithms; D000818:Animals; D000831:Animals, Newborn / Q000502:physiology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology; D020013:Calcium Signaling / Q000235:genetics / Q000502:physiology*; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D055724:Electrophysiological Phenomena; D019706:Excitatory Postsynaptic Potentials / Q000502:physiology; D005260:Female; D005838:Genotype; D018698:Glutamic Acid / Q000378:metabolism*; D007091:Image Processing, Computer-Assisted; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D051379:Mice; D018408:Patch-Clamp Techniques; D044169:Receptors, Calcium-Sensing / Q000235:genetics / Q000502:physiology*; D043562:Receptors, G-Protein-Coupled / Q000502:physiology; D019831:Sodium-Calcium Exchanger / Q000502:physiology; D013572:Synaptic Vesicles / Q000187:drug effects
chemical_list: D015220:Calcium Channels; D002614:Chelating Agents; D044169:Receptors, Calcium-Sensing; D043562:Receptors, G-Protein-Coupled; D019831:Sodium-Calcium Exchanger; D018698:Glutamic Acid; D004533:Egtazic Acid; D008274:Magnesium; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium