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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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Treatment with nocodazole inhibits transport of mitochondria along Xenopus neurites. Representative fluorescent images of mitochondria in a control neuron and in a neuron treated  for 30 min with nocodazole (5 μg/ml). Mitochondria were stained  with Rhodamine 123 and visualized with digital fluorescence microscopy. Transport of individual mitochondria (arrows) could  be detected in control cells. In neurons treated with nocodazole,  none of the ∼300 observed mitochondria displayed episodes of  long-range transport. Numbers indicate the time in seconds after  the start of experiment.
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Figure 3: Treatment with nocodazole inhibits transport of mitochondria along Xenopus neurites. Representative fluorescent images of mitochondria in a control neuron and in a neuron treated for 30 min with nocodazole (5 μg/ml). Mitochondria were stained with Rhodamine 123 and visualized with digital fluorescence microscopy. Transport of individual mitochondria (arrows) could be detected in control cells. In neurons treated with nocodazole, none of the ∼300 observed mitochondria displayed episodes of long-range transport. Numbers indicate the time in seconds after the start of experiment.

Mentions: The vesicles involved in spontaneous ACh secretion along the axon may recycle locally within the axon, similar to synaptic vesicles in the nerve terminal. Alternatively, these vesicles may be directly transported from the soma via a constitutive biosynthetic pathway (Nakata et al., 1998). In the latter scenario, the cell body-derived vesicles would be expected to carry the molecules of the ACh transporter to produce a detectable SSC upon their exocytosis (Song et al., 1997). To distinguish between the two possibilities, we treated neuronal cultures with 5 μg/ml nocodazole. This treatment resulted in the loss of axonal microtubules (Fig. 2 C), rapidly arrested axonal growth (Fig. 2 D), and inhibited transport of mitochondria along the axon (Fig. 3). Therefore, the treatment with nocodazole is expected to disrupt the delivery of cell body-derived vesicles to the growing axon. However, spontaneous neurotransmitter secretion persisted after nocodazole application both along the axon (Fig. 2 B) and at the preformed synapses (data not shown). Moreover, the disruption of axonal microtubules resulted in a significant increase in the SSC frequency at the middle axonal segment (detailed quantitative analysis of the effects of axonal microtubules on neurotransmitter secretion will be presented elsewhere). These results strongly suggest that constitutive ACh secretion along the axon is not directly related to the exocytosis of cell body-derived vesicles. Instead, ACh secretion is likely to be mediated by a local exoendocytic recycling of ACh-containing vesicles.


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

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

Treatment with nocodazole inhibits transport of mitochondria along Xenopus neurites. Representative fluorescent images of mitochondria in a control neuron and in a neuron treated  for 30 min with nocodazole (5 μg/ml). Mitochondria were stained  with Rhodamine 123 and visualized with digital fluorescence microscopy. Transport of individual mitochondria (arrows) could  be detected in control cells. In neurons treated with nocodazole,  none of the ∼300 observed mitochondria displayed episodes of  long-range transport. Numbers indicate the time in seconds after  the start of experiment.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2132923&req=5

Figure 3: Treatment with nocodazole inhibits transport of mitochondria along Xenopus neurites. Representative fluorescent images of mitochondria in a control neuron and in a neuron treated for 30 min with nocodazole (5 μg/ml). Mitochondria were stained with Rhodamine 123 and visualized with digital fluorescence microscopy. Transport of individual mitochondria (arrows) could be detected in control cells. In neurons treated with nocodazole, none of the ∼300 observed mitochondria displayed episodes of long-range transport. Numbers indicate the time in seconds after the start of experiment.
Mentions: The vesicles involved in spontaneous ACh secretion along the axon may recycle locally within the axon, similar to synaptic vesicles in the nerve terminal. Alternatively, these vesicles may be directly transported from the soma via a constitutive biosynthetic pathway (Nakata et al., 1998). In the latter scenario, the cell body-derived vesicles would be expected to carry the molecules of the ACh transporter to produce a detectable SSC upon their exocytosis (Song et al., 1997). To distinguish between the two possibilities, we treated neuronal cultures with 5 μg/ml nocodazole. This treatment resulted in the loss of axonal microtubules (Fig. 2 C), rapidly arrested axonal growth (Fig. 2 D), and inhibited transport of mitochondria along the axon (Fig. 3). Therefore, the treatment with nocodazole is expected to disrupt the delivery of cell body-derived vesicles to the growing axon. However, spontaneous neurotransmitter secretion persisted after nocodazole application both along the axon (Fig. 2 B) and at the preformed synapses (data not shown). Moreover, the disruption of axonal microtubules resulted in a significant increase in the SSC frequency at the middle axonal segment (detailed quantitative analysis of the effects of axonal microtubules on neurotransmitter secretion will be presented elsewhere). These results strongly suggest that constitutive ACh secretion along the axon is not directly related to the exocytosis of cell body-derived vesicles. Instead, ACh secretion is likely to be mediated by a local exoendocytic recycling of ACh-containing vesicles.

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