Ca2+ imaging of CNS axons in culture indicates reliable coupling between single action potentials and distal functional release sites.
A combination of Ca2+ imaging and current clamp recording in cultured cortical neurons was used to evaluate the reliability of coupling between the action potential and rises in Ca2+ at distal release sites as a possible source of variability in CNS synaptic transmission. Local domains of enhanced Ca2+ influx were observed at varicosities on axon collaterals. Functional assay of vesicle turnover using FM1-43 and parallel electron microscopy confirmed that these varicosities were release sites. Single action potentials reliably ( > 95% of the time) resulted in a presynaptic Ca2+ transient at all presumed release sites including those on distal collaterals. Variability in the amplitude of presynaptic Ca2+ transients at individual boutons was estimated to be on average less than 20%. We conclude that the coupling of somatic action potentials to distal release sites is generally a reliable process, although nonlinearity in the relationship between Ca2+ influx and neurotransmitter release may amplify the effects of relatively small fluctuations in Ca2+ influx.
Mackenzie
P J
PJ
Kinsmen Laboratory, Department of Psychiatry, University of British Columbia, Vancouver, Canada.
Umemiya
M
M
Murphy
T H
TH
D000200:Action Potentials; D000818:Animals; D001369:Axons / Q000502:physiology*; D002118:Calcium / Q000032:analysis / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000196:embryology / Q000648:ultrastructure*; D005456:Fluorescent Dyes; D007091:Image Processing, Computer-Assisted; D008854:Microscopy, Electron; D009474:Neurons / Q000502:physiology / Q000648:ultrastructure; D051381:Rats; D013569:Synapses / Q000502:physiology
D015220:Calcium Channels; D005456:Fluorescent Dyes; D002118:Calcium