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J. Cell Biol.,
Volume 144, Number 3, February 8, 1999 507-518
Department of Physiology and Biophysics M/C 901, University of Illinois, Chicago, Illinois 60612
In mature neurons, synaptic vesicles continuously recycle within the presynaptic nerve terminal. In
developing axons which are free of contact with a
postsynaptic target, constitutive membrane recycling is
not localized to the nerve terminal; instead, plasma membrane components undergo cycles of exoendocytosis throughout the whole axonal surface (; ). Moreover, in growing
Xenopus spinal cord neurons in culture, acetylcholine
(ACh) is spontaneously secreted in the quantal fashion
along the axonal shaft (; ). Here we demonstrate that in Xenopus neurons ACh secretion is mediated by vesicles which recycle locally within the axon. Similar to neurotransmitter
release at the presynaptic nerve terminal, ACh secretion along the axon could be elicited by the action potential or by hypertonic solutions. We found that the
parameters of neurotransmitter secretion at the nerve
terminal and at the middle axon were strikingly similar.
These results lead us to conclude that, as in the case of
the presynaptic nerve terminal, synaptic vesicles involved in neurotransmitter release along the axon contain a complement of proteins for vesicle docking and
Ca2+-dependent fusion. Taken together, our results
support the idea that, in developing axons, the rudimentary machinery for quantal neurotransmitter secretion is distributed throughout the whole axonal surface.
Maturation of this machinery in the process of synaptic
development would improve the fidelity of synaptic
transmission during high-frequency stimulation of the
presynaptic cell.
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