Role of calcium in neurotransmitter release evoked by alpha-latrotoxin or hypertonic sucrose.
At the synapse, neurotransmitter release is triggered physiologically by Ca(2+) influx through voltage-gated Ca(2+) channels. Non-physiologically, release can be evoked by a potent neurotoxin, alpha-latrotoxin, and by hypertonic sucrose. Controversy has arisen on whether release evoked by alpha-latrotoxin and hypertonic sucrose requires extracellular Ca(2+) or Ca(2+) from intracellular stores. Using synaptosomes, we have studied the Ca(2+) dependence of alpha-latrotoxin and sucrose action in different neurotransmitter systems. In agreement with previous data, no requirement for extracellular Ca(2+) in sucrose-induced secretion of norepinephrine, dopamine, glutamate or GABA was detected. Unexpectedly, we observed large differences between these neurotransmitters in the Ca(2+) dependence of alpha-latrotoxin-stimulated release: norepinephrine release required Ca(2+), dopamine release was only partially Ca(2+) dependent, and glutamate and GABA release did not require Ca(2+). To test if Ca(2+) derived from intracellular Ca(2+) stores participates in neurotransmitter release triggered by alpha-latrotoxin or hypertonic sucrose, we employed thapsigargin, a Ca(2+)-ATPase inhibitor that empties Ca(2+) stores. Thapsigargin did not induce neurotransmitter release, nor did it inhibit subsequent release stimulated by KCl depolarization, hypertonic sucrose or alpha-latrotoxin. However, intracellular Ca(2+) performs an important regulatory function, since thapsigargin increased the size of the readily releasable pool as measured by stimulation with hypertonic sucrose. This effect required extracellular Ca(2+) and protein kinase C, suggesting that depletion of internal Ca(2+) stores leads to store-operated Ca(2+) entry. The resulting Ca(2+) influx does not trigger release by itself, but activates protein kinase C that increases the readily releasable pool of neurotransmitters. Our data show that internal and external Ca(2+) is not acutely involved in hypertonic sucrose-evoked neurotransmitter release, while alpha-latrotoxin-triggered release requires external Ca(2+) for a subset of neurotransmitters. Although internal Ca(2+) is not essential for release, it modulates its extent, implying that the emptying of intracellular stores by activation of presynaptic receptors plays an important regulatory role in neurotransmitter release.
Khvotchev
M
M
Center for Basic Neuroscience, Department of Molecular Genetics, and Howard Hughes Medical Institute, The University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75390, USA.
Lonart
G
G
Südhof
T C
TC
D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D020013:Calcium Signaling / Q000502:physiology; D018698:Glutamic Acid / Q000378:metabolism; D006982:Hypertonic Solutions / Q000494:pharmacology; D007424:Intracellular Fluid / Q000187:drug effects / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018377:Neurotransmitter Agents / Q000378:metabolism*; D009638:Norepinephrine / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013111:Spider Venoms / Q000494:pharmacology*; D013395:Sucrose / Q000494:pharmacology*; D009435:Synaptic Transmission / Q000187:drug effects* / Q000502:physiology; D013572:Synaptic Vesicles / Q000378:metabolism*; D013574:Synaptosomes / Q000378:metabolism; D019284:Thapsigargin / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D015220:Calcium Channels; D006982:Hypertonic Solutions; D018377:Neurotransmitter Agents; D013111:Spider Venoms; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; D013395:Sucrose; C020763:alpha-latrotoxin; D019284:Thapsigargin; D002118:Calcium; D009638:Norepinephrine