The Journal of Neuroscience, September 1, 2000, 20(17):6326-6332
Analysis of Presynaptic Ca2+ Influx and Transmitter
Release Kinetics during Facilitation at the Inhibitor of the Crayfish
Neuromuscular Junction
Andrey
Vyshedskiy,
Tariq
Allana, and
Jen-Wei
Lin
Department of Biology, Boston University, Boston, Massachusetts
02215
The inhibitory synapse of the crayfish neuromuscular junction was
used to examine mechanisms underlying the F2 component of synaptic
facilitation. Because previous studies have shown accelerated transmitter release during facilitation, we examined whether an activity-dependent plasticity in ICa could
underlie this acceleration. We established that fluorescent transients
generated by Magnesium Green can resolve small differences in
presynaptic Ca2+ influx that correlate with changes
in IPSC waveform. However, there was no change in
Ca2+ transients associated with the accelerated
release. Analyzing the initial rise of IPSC and the duration of the
presynaptic spike yielded a depolarization-release coupling plot that
captures the impact of spike waveform on the initial rate of release.
We conclude that accelerated release during F2 facilitation cannot be
attributed to plasticity of ICa or
modulation of spike waveform. Kinetic analysis showed a reduction in
synaptic delay during facilitation only when broad action potentials
were used. In unfacilitated release, synaptic delay increased as spike
duration lengthened. We propose that small single
Ca2+ channel currents during the plateau phase of
broad action potentials raise local Ca2+
concentration only enough to fill a high-affinity site. Occupation of
this site in itself, or events downstream, would convert a vesicle from
control to facilitated state. If the conversion were a slow process, it
could explain the changes in synaptic delay reported here. This
hypothesis can also account for a number of observations related to
Ca2+ cooperativity and synaptic facilitation.
Key words:
synaptic delay; synapse; crayfish inhibitor; neuromuscular junction; facilitation; calcium indicator
Copyright © 2000 Society for Neuroscience 0270-6474/00/20176326-07$05.00/0
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