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Transmitter Glutamate Release from Isolated Nerve Terminals: Evidence for Biphasic Release and Triggering by Localized Ca 2+
Author(s) -
McMahon Harvey T.,
Nicholls David G.
Publication year - 1991
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1991.tb02566.x
Subject(s) - ionomycin , glutamate receptor , 4 aminopyridine , biophysics , depolarization , exocytosis , chemistry , liberation , vesicle , biochemistry , biology , membrane , potassium channel , intracellular , in vitro , receptor
The kinetics of Ca 2+ ‐dependent release of glutamate from guinea‐pig cerebrocortical synaptosomes evoked by KCl or 4‐aminopyridine are investigated using a continuous flu‐orimetric assay. Release by both agents is biphasic, with a rapid phase complete within 2 s followed by a more extensive slow phase with a half‐maximal release in 52 s for KCl‐evoked release and > 120 s for 4‐aminopyridine‐evoked release. The two phases ofglutamate release may reflect a dual localization of releasable vesicles at the active zone and in the bulk cytoplasm. Decreasing depolarization depresses the extent rather than increasing the time for half‐maximal Ca 2+ ‐dependent release. Both the fast and the slow phases of glutamate release require external Ca 2+ and cytoplasmic ATP. KCl depolarization produces a transient “spike”of cytoplasmic free Ca 2+ ([Ca 2+ ] c ), which recovers to a plateau; the major component of glutamate release occurs during this plateau. Predepolarization in the absence of added external Ca 2+ , to inhibit transient Ca 2+ channels, does not affect the subsequent glutamate release evoked by Ca 2+ readdition. Thus, release involves primarily noninactivating Ca 2+ channels. For a given increase in [Ca 2+ ] c , KCl and 4‐aminopyridine cause equal release of glutamate, while ionomycin releases much less glutamate. This lowered efficiency is not due to ATP depletion. It is concluded that glutamate exocytosis is evoked by localized Ca 2+ entering through noninactivating voltage‐dependent Ca 2+ channels and that nonlocalized Ca 2+ entry with ionomycin is inefficient.