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Neurotransmitter secretion along growing nerve processes: comparison with synaptic vesicle exocytosis.

Zakharenko S, Chang S, O'Donoghue M, Popov SV - J. Cell Biol. (1999)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics M/C 901, University of Illinois, Chicago, Illinois 60612, USA.

ABSTRACT
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 (Matteoli et al., 1992; Kraszewski et al., 1995). Moreover, in growing Xenopus spinal cord neurons in culture, acetylcholine (ACh) is spontaneously secreted in the quantal fashion along the axonal shaft (Evers et al., 1989; Antonov et al., 1998). 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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Induction of SSCs by application of hypertonic solution. Current traces are representative examples of whole-cell recordings from myocytes at the preformed synapses (A) and the  middle axon (B). Hypertonic solution containing 300 mM sucrose  in culture medium was applied to the neurons using a local perfusion system. Superfusion of the neuron with hyperosmotic solution (horizontal black bars) resulted in a rapid increase in the  SSC frequency. (C) Normalized frequency of SSCs after application of hypertonic solution to the preformed synapses (solid bar)  and the middle axon (open bar). For each recording the average  frequency of SSCs during a period of 20–60 s after the onset of  sucrose application was normalized to that before the application  of hyperosmotic solution. Data are presented as a mean ± SEM  of 10 (preformed synapse) and 14 (middle axon) experiments.
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Figure 8: Induction of SSCs by application of hypertonic solution. Current traces are representative examples of whole-cell recordings from myocytes at the preformed synapses (A) and the middle axon (B). Hypertonic solution containing 300 mM sucrose in culture medium was applied to the neurons using a local perfusion system. Superfusion of the neuron with hyperosmotic solution (horizontal black bars) resulted in a rapid increase in the SSC frequency. (C) Normalized frequency of SSCs after application of hypertonic solution to the preformed synapses (solid bar) and the middle axon (open bar). For each recording the average frequency of SSCs during a period of 20–60 s after the onset of sucrose application was normalized to that before the application of hyperosmotic solution. Data are presented as a mean ± SEM of 10 (preformed synapse) and 14 (middle axon) experiments.

Mentions: The induction of ESCs by electrical stimulation of the neuron suggests that a population of fusion-competent synaptic vesicles is docked at the plasma membrane throughout the axon. To further test this prediction, we applied a pulse of hypertonic solution to neuronal cultures, while continuously recording SSCs at the preformed synapse or at the middle axon (Fig. 8). Application of a hypertonic solution is known to induce an immediate exocytosis of the fusion-competent vesicles at the nerve terminal (Stevens and Tsujimoto, 1995; Rosenmund and Stevens, 1996). The readily releasable pool of quanta defined in this assay appears to be identical to the one drawn upon by action potential-evoked release (Stevens and Tsujimoto, 1995; Rosenmund and Stevens, 1996). We found that a hypertonic solution containing 300 mM sucrose in the culture medium induced a rapid and highly reproducible increase in the frequency of SSCs both at the nerve terminal (Fig. 8 A) and at the middle axon (Fig. 8 B). For a period of 20–60 s after the onset of sucrose application, the average frequency of SSCs was 526 ± 87 events/min (mean ± SEM, n = 10) and 373 ± 112 events/min (n = 14) in recordings from the preformed synapses, and from the middle axon, respectively. These values were ∼50-fold higher than that before the application of the hypertonic solution (Fig. 8 C). The increase in the SSC frequency at the preformed synapse induced by the hypertonic solution showed no statistically significant difference compared with the middle axonal segment.


Neurotransmitter secretion along growing nerve processes: comparison with synaptic vesicle exocytosis.

Zakharenko S, Chang S, O'Donoghue M, Popov SV - J. Cell Biol. (1999)

Induction of SSCs by application of hypertonic solution. Current traces are representative examples of whole-cell recordings from myocytes at the preformed synapses (A) and the  middle axon (B). Hypertonic solution containing 300 mM sucrose  in culture medium was applied to the neurons using a local perfusion system. Superfusion of the neuron with hyperosmotic solution (horizontal black bars) resulted in a rapid increase in the  SSC frequency. (C) Normalized frequency of SSCs after application of hypertonic solution to the preformed synapses (solid bar)  and the middle axon (open bar). For each recording the average  frequency of SSCs during a period of 20–60 s after the onset of  sucrose application was normalized to that before the application  of hyperosmotic solution. Data are presented as a mean ± SEM  of 10 (preformed synapse) and 14 (middle axon) experiments.
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Related In: Results  -  Collection

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Figure 8: Induction of SSCs by application of hypertonic solution. Current traces are representative examples of whole-cell recordings from myocytes at the preformed synapses (A) and the middle axon (B). Hypertonic solution containing 300 mM sucrose in culture medium was applied to the neurons using a local perfusion system. Superfusion of the neuron with hyperosmotic solution (horizontal black bars) resulted in a rapid increase in the SSC frequency. (C) Normalized frequency of SSCs after application of hypertonic solution to the preformed synapses (solid bar) and the middle axon (open bar). For each recording the average frequency of SSCs during a period of 20–60 s after the onset of sucrose application was normalized to that before the application of hyperosmotic solution. Data are presented as a mean ± SEM of 10 (preformed synapse) and 14 (middle axon) experiments.
Mentions: The induction of ESCs by electrical stimulation of the neuron suggests that a population of fusion-competent synaptic vesicles is docked at the plasma membrane throughout the axon. To further test this prediction, we applied a pulse of hypertonic solution to neuronal cultures, while continuously recording SSCs at the preformed synapse or at the middle axon (Fig. 8). Application of a hypertonic solution is known to induce an immediate exocytosis of the fusion-competent vesicles at the nerve terminal (Stevens and Tsujimoto, 1995; Rosenmund and Stevens, 1996). The readily releasable pool of quanta defined in this assay appears to be identical to the one drawn upon by action potential-evoked release (Stevens and Tsujimoto, 1995; Rosenmund and Stevens, 1996). We found that a hypertonic solution containing 300 mM sucrose in the culture medium induced a rapid and highly reproducible increase in the frequency of SSCs both at the nerve terminal (Fig. 8 A) and at the middle axon (Fig. 8 B). For a period of 20–60 s after the onset of sucrose application, the average frequency of SSCs was 526 ± 87 events/min (mean ± SEM, n = 10) and 373 ± 112 events/min (n = 14) in recordings from the preformed synapses, and from the middle axon, respectively. These values were ∼50-fold higher than that before the application of the hypertonic solution (Fig. 8 C). The increase in the SSC frequency at the preformed synapse induced by the hypertonic solution showed no statistically significant difference compared with the middle axonal segment.

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics M/C 901, University of Illinois, Chicago, Illinois 60612, USA.

ABSTRACT
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 (Matteoli et al., 1992; Kraszewski et al., 1995). Moreover, in growing Xenopus spinal cord neurons in culture, acetylcholine (ACh) is spontaneously secreted in the quantal fashion along the axonal shaft (Evers et al., 1989; Antonov et al., 1998). 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.

Show MeSH
Related in: MedlinePlus