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The Journal of Neuroscience, December 20, 2006, 26(51):13240-13249; doi:10.1523/JNEUROSCI.1418-06.2006

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Cellular/Molecular
Ca²⁺ from One or Two Channels Controls Fusion of a Single Vesicle at the Frog Neuromuscular Junction

Vahid Shahrezaei,^1 * Alex Cao,^2 * and Kerry R. Delaney²

¹Department of Physics, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6, and ²Department of Biology, University of Victoria, Victoria, British Columbia, Canada V8W 3N5

Correspondence should be addressed to Dr. Kerry R. Delaney, Department of Biology, University of Victoria, Box 3020 STN CSC, Victoria, British Columbia, Canada V8W 3N5. Email: kdelaney{at}uvic.ca

Neurotransmitter release is triggered by the cooperative action of approximately five Ca²⁺ ions entering the presynaptic terminal through Ca²⁺ channels. Depending on the organization of the active zone (AZ), influx through one or many channels may be needed to cause fusion of a vesicle. Using a combination of experiments and modeling, we examined the number of channels that contribute Ca²⁺ for fusion of a single vesicle in a frog neuromuscular AZ. We compared Ca²⁺ influx to neurotransmitter release by measuring presynaptic action potential-evoked (AP-evoked) Ca²⁺ transients simultaneously with postsynaptic potentials. Ca²⁺ influx was manipulated by changing extracellular [Ca²⁺] (Ca_ext) to alter the flux per channel or by reducing the number of open Ca²⁺ channels with -conotoxin GVIA (-CTX). When Ca_ext was reduced, the exponent of the power relationship relating release to Ca²⁺ influx was 4.16 ± 0.62 (SD; n = 4), consistent with a biochemical cooperativity of 5. In contrast, reducing influx with -CTX yielded a power relationship of 1.7 ± 0.44 (n = 5) for Ca_ext of 1.8 mM and 2.12 ± 0.44 for Ca_ext of 0.45 mM (n = 5). Using geometrically realistic Monte Carlo simulations, we tracked Ca²⁺ ions as they entered through each channel and diffused in the terminal. Experimental and modeling data were consistent with two to six channel openings per AZ per AP; the Ca²⁺ that causes fusion of a single vesicle originates from one or two channels. Channel cooperativity depends mainly on the physical relationship between channels and vesicles and is insensitive to changes in the non-geometrical parameters of our model.

Key words: calcium channels; calcium imaging; colocalization; Monte Carlo; transmitter release; vesicle

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Received April 3, 2006; revised Nov. 3, 2006; accepted Nov. 4, 2006.

Correspondence should be addressed to Dr. Kerry R. Delaney, Department of Biology, University of Victoria, Box 3020 STN CSC, Victoria, British Columbia, Canada V8W 3N5. Email: kdelaney{at}uvic.ca

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