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Neuron. 1996 Apr;16(4):783-95.

Ca2+ imaging of CNS axons in culture indicates reliable coupling between single action potentials and distal functional release sites.

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Kinsmen Laboratory, Department of Psychiatry, University of British Columbia, Vancouver, Canada.


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.

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