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The Journal of Neuroscience, October 1, 1999, 19(19):8419-8434
Calcium Entry Related to Active Zones and Differences in Transmitter Release at Phasic and Tonic Synapses
M. Msghina1, A. G. Millar1, M. P. Charlton1, C. K. Govind2, and H. L. Atwood1 1 Department of Physiology, Medical Research Council Neural Group, University of Toronto, Toronto, Ontario, Canada M5S 1A8, and 2 Life Sciences Division, University of Toronto at Scarborough, Toronto, Ontario, Canada M1C 1A4
Synaptic functional differentiation of crayfish phasic and tonic motor neurons is large. For one impulse, quantal release of neurotransmitter is typically 100-1000 times higher for phasic synapses. We tested the hypothesis that differences in synaptic strength are determined by differences in synaptic calcium entry. Calcium signals were measured with the injected calcium indicator dyes Calcium Green-1 and fura-2. Estimated Ca2+ entry increased almost linearly with frequency for both axons and was two to three times larger in phasic terminals. Tonic terminal Ca2+ at 10 Hz exceeded phasic terminal Ca2+ at 1 Hz, yet transmitter release was much higher for phasic terminals at these frequencies. Freeze-fracture images of synapses revealed on average similar numbers of prominent presynaptic active zone particles (putative ion channels) for both neurons and a two- to fourfold phasic/tonic ratio of active zones per terminal volume. This can account for the larger calcium signals seen in phasic terminals. Thus, differences in synaptic strength are less closely linked to differences in synaptic channel properties and calcium entry than to differences in calcium sensitivity of transmitter release.
Key words: crustacea; crayfish; synaptic differentiation; tonic; phasic; ultrastructure; active zone; focal recording; calcium imaging; quantal release
Copyright © 1999 Society for Neuroscience 0270-6474/99/19198419-16$05.00/0
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