Free cytoplasmic Ca2+ and neurotransmitter release: studies on PC12 cells and synaptosomes exposed to alpha-latrotoxin.
The relationship between the free cytoplasmic Ca2+ concentration, [Ca2+]i, and neurotransmitter release was investigated in guinea pig brain synaptosomes and the neurosecretory cell line PC12. Release was induced by alpha-latrotoxin, which acts in both Ca2+ -containing and Ca2+ -free incubation media, or by the classical depolarizing agents high K+ and veratridine, which require extracellular Ca2+. Two complementary approaches were used to reveal changes of [Ca2+]i: (i) direct measurement by a fluorescent Ca2+ indicator (quin2) and (ii) study of the Ca2+ -dependent phosphorylation of a protein, synapsin I, located at the cytoplasmic surface of synaptic vesicles. Depolarizing agents, when applied in Ca2+ -containing medium, induced the [Ca2+]i to increase promptly 3- to 6-fold, drastically increased synapsin I phosphorylation, and caused stimulation of transmitter release. With alpha-latrotoxin, the [Ca2+]i increase was delayed and occurred at a slower rate, the increase of synapsin I phosphorylation was less drastic, and the release response was much more pronounced. In Ca2+ -free medium, depolarizing agents released no transmitter and had no effect on [Ca2+]i or synapsin I phosphorylation, whereas with alpha-latrotoxin these processes were dissociated: considerable stimulation of the release without apparent change of [Ca2+]i and synapsin I phosphorylation. We conclude that the relationship between average [Ca2+]i and transmitter release is not straightforward and, in particular, that the release evoked by alpha-latrotoxin in Ca2+ -free medium is mediated by a factor(s) other than bulk redistribution of Ca2+ from intracellular stores.
D000634:Aminoquinolines; D001180:Arthropod Venoms / Q000494:pharmacology*; D002118:Calcium / Q000502:physiology*; D002460:Cell Line; D003600:Cytosol / Q000502:physiology; D009419:Nerve Tissue Proteins / Q000378:metabolism; D018377:Neurotransmitter Agents / Q000378:metabolism*; D010766:Phosphorylation; D013111:Spider Venoms / Q000494:pharmacology*; D016704:Synapsins; D013574:Synaptosomes / Q000502:physiology; D014701:Veratridine / Q000494:pharmacology
D000634:Aminoquinolines; D001180:Arthropod Venoms; D009419:Nerve Tissue Proteins; D018377:Neurotransmitter Agents; D013111:Spider Venoms; D016704:Synapsins; C020763:alpha-latrotoxin; D014701:Veratridine; C035738:Quin2-acetoxymethyl ester; D002118:Calcium