Journal of Neuroscience, Vol 8, 4667-4674, Copyright © 1988 by Society for Neuroscience
Presynaptic long-term facilitation at the crayfish neuromuscular junction: voltage-dependent and ion-dependent phases
JM Wojtowicz and HL Atwood
Department of Physiology, University of Toronto, Ontario, Canada.
Long-term facilitation (LTF) of synaptic transmission was investigated in
the crayfish opener muscle to determine the factors necessary for its
induction and expression. LTF was induced without action potentials by
intracellular depolarization of presynaptic nerve terminals. Following
induction, the synaptic transmission was enhanced by about 80% for a period
of several hours. Intracellular recordings from pre- and postsynaptic
cells, combined with ionic and pharmacological tests, permitted dissection
of LTF into 2 phases: an initial tetanic phase that depended on the
presence of both sodium and calcium ions and a subsequent long-lasting
phase. This latter long-lasting enhancement of synaptic transmission was
induced by repeated depolarizations of synaptic terminals but did not
depend on the influx of sodium or calcium ions or on intracellular release
of calcium ions. Both tetanic and long-lasting phases of LTF are
attributable to activity of a single neuron, i.e., they are homosynaptic
phenomena. Furthermore, LTF is associated with an increase of quantal
release, whereas the size of quanta remains unchanged. During the
long-lasting phase of LTF, the nerve terminal releases more transmitter for
a given depolarization than before induction of LTF. Thus, the locus of LTF
is presynaptic. Our findings suggest the presence of a voltage-dependent
mechanism in the presynaptic membrane different from voltage-gating of Na
or Ca channels. Such a mechanism may be important in the establishment of
long-lasting synaptic changes at the crayfish neuromuscular junction and
perhaps in other neural systems.
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