The Journal of Neuroscience, December 15, 2002, 22(24):10671-10679
Modulation of the Readily Releasable Pool of Transmitter and of
Excitation-Secretion Coupling by Activity and by Serotonin at
Aplysia Sensorimotor Synapses in Culture
Yali
Zhao and
Marc
Klein
Clinical Research Institute of Montreal and University of Montreal,
Montreal, Quebec, Canada H2W IR7, and Department of Physiological
Science, University of California Los Angeles, Los Angeles, California
90095-1606
Short-term homosynaptic depression and heterosynaptic
facilitation of transmitter release from mechanoreceptor sensory
neurons of Aplysia are involved in habituation and
sensitization, respectively, of defensive withdrawal reflexes. We
investigated whether synaptic transmission is regulated in these forms
of plasticity by means of changes in the size of the pool of
transmitter available for immediate release [the readily releasable
pool (RRP)] or in the efficacy of release from an unchanging pool.
Using sensorimotor synapses formed in cell culture, we estimated the
number of transmitter quanta in the RRP from the asynchronous release
of neurotransmitter caused by application of a hypertonic bathing
solution. Our experiments indicate that the transmitter released by
action potentials and by hypertonic solution comes from the same pool.
The RRP was reduced after homosynaptic depression of the EPSP by
low-frequency stimulation and increased after facilitation of the EPSP
by application of the endogenous facilitatory transmitter serotonin
(5-HT) after homosynaptic depression. However, although the fractional
changes in the RRP and in the EPSP were similar for both synaptic
depression and facilitation when depression was induced by repeated
hypertonic stimulation, the changes in the EPSP were significantly
greater than the changes in the RRP when depression was induced by
repeated electrical stimulation. These observations indicate that
homosynaptic depression and restoration of depressed transmission by
5-HT are caused by changes in both the amount of transmitter available for immediate release and in processes involved in the coupling of the
action potential to transmitter release.
Key words:
Aplysia; serotonin; habituation; dishabituation; sensitization; transmitter release; synapses; multivesicular release; synaptic ultrastructure; synaptic plasticity; synaptic vesicles; releasable pool; excitation-secretion coupling; homosynaptic depression; synaptic facilitation
Copyright © 2002 Society for Neuroscience 0270-6474/02/222410671-09$05.00/0
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