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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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Effect of BFA on  ACh does not involve the cell  body. The axon was transected  with a sharp microelectrode in  the vicinity of the cell body as  in Fig. 4. Myocytes were manipulated into contact with the  middle of axonal fragment 15  min after transection, the time  when the SSC frequency returns to control (before transection) values (time 0 on the plot).  Spontaneous neurotransmitter secretion was measured for ∼25  min by patch clamp recordings from the myocytes (control). The  average SSC frequency was calculated for 3-min intervals and  normalized to the SSC frequency at time point 0 (solid circles).  Bath application of BFA (10 μg/ml) 5 min after the start of recording resulted in a decrease in the SSC frequency (open circles). Each bar represents the average of 10 experiments ± SEM.  *, P < 0.05, ANOVA.
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Figure 10: Effect of BFA on ACh does not involve the cell body. The axon was transected with a sharp microelectrode in the vicinity of the cell body as in Fig. 4. Myocytes were manipulated into contact with the middle of axonal fragment 15 min after transection, the time when the SSC frequency returns to control (before transection) values (time 0 on the plot). Spontaneous neurotransmitter secretion was measured for ∼25 min by patch clamp recordings from the myocytes (control). The average SSC frequency was calculated for 3-min intervals and normalized to the SSC frequency at time point 0 (solid circles). Bath application of BFA (10 μg/ml) 5 min after the start of recording resulted in a decrease in the SSC frequency (open circles). Each bar represents the average of 10 experiments ± SEM. *, P < 0.05, ANOVA.

Mentions: Brefeldin A treatment induces a collapse of the Golgi complex into ER (Doms et al., 1989; Lippincott-Schwartz et al., 1989; Dascher and Balch, 1994), and rapidly arrests axonal growth (Jareb and Banker, 1997; Chang et al., 1998), presumably by blocking the supply of newly synthesized membranes from the trans-Golgi network (Craig et al., 1995). Although we cannot completely exclude the direct contribution of the Golgi-derived vesicles to the ACh secretion along the axon, it appears that SSCs, both at the preformed synapses and along the axon, reflect local recycling of synaptic vesicles. This exoendocytic cycle does not directly depend on the supply of Golgi-derived material (see Figs. 2–4). Additional support for this model is provided by a series of experiments in which we measured the SSC frequency along the distal axonal segments that were transected from the soma. Recordings were started 20 min after the transection and the SSC frequency was constant throughout the period of recording (Fig. 10 and also see Fig. 4). Within 5–10 min after BFA application (10 μg/ml), the spontaneous neurotransmitter secretion along the axonal fragments was significantly inhibited (Fig. 10). 25 min after the onset of BFA treatment, the frequency of SSCs along the distal axonal fragment dropped to 27% of that at the start of recording (Fig. 10). Since the transected axonal fragments lack the Golgi apparatus, the results strongly suggest an inhibitory action of BFA on local synaptic vesicle recycling along the axon.


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

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

Effect of BFA on  ACh does not involve the cell  body. The axon was transected  with a sharp microelectrode in  the vicinity of the cell body as  in Fig. 4. Myocytes were manipulated into contact with the  middle of axonal fragment 15  min after transection, the time  when the SSC frequency returns to control (before transection) values (time 0 on the plot).  Spontaneous neurotransmitter secretion was measured for ∼25  min by patch clamp recordings from the myocytes (control). The  average SSC frequency was calculated for 3-min intervals and  normalized to the SSC frequency at time point 0 (solid circles).  Bath application of BFA (10 μg/ml) 5 min after the start of recording resulted in a decrease in the SSC frequency (open circles). Each bar represents the average of 10 experiments ± SEM.  *, P < 0.05, ANOVA.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2132923&req=5

Figure 10: Effect of BFA on ACh does not involve the cell body. The axon was transected with a sharp microelectrode in the vicinity of the cell body as in Fig. 4. Myocytes were manipulated into contact with the middle of axonal fragment 15 min after transection, the time when the SSC frequency returns to control (before transection) values (time 0 on the plot). Spontaneous neurotransmitter secretion was measured for ∼25 min by patch clamp recordings from the myocytes (control). The average SSC frequency was calculated for 3-min intervals and normalized to the SSC frequency at time point 0 (solid circles). Bath application of BFA (10 μg/ml) 5 min after the start of recording resulted in a decrease in the SSC frequency (open circles). Each bar represents the average of 10 experiments ± SEM. *, P < 0.05, ANOVA.
Mentions: Brefeldin A treatment induces a collapse of the Golgi complex into ER (Doms et al., 1989; Lippincott-Schwartz et al., 1989; Dascher and Balch, 1994), and rapidly arrests axonal growth (Jareb and Banker, 1997; Chang et al., 1998), presumably by blocking the supply of newly synthesized membranes from the trans-Golgi network (Craig et al., 1995). Although we cannot completely exclude the direct contribution of the Golgi-derived vesicles to the ACh secretion along the axon, it appears that SSCs, both at the preformed synapses and along the axon, reflect local recycling of synaptic vesicles. This exoendocytic cycle does not directly depend on the supply of Golgi-derived material (see Figs. 2–4). Additional support for this model is provided by a series of experiments in which we measured the SSC frequency along the distal axonal segments that were transected from the soma. Recordings were started 20 min after the transection and the SSC frequency was constant throughout the period of recording (Fig. 10 and also see Fig. 4). Within 5–10 min after BFA application (10 μg/ml), the spontaneous neurotransmitter secretion along the axonal fragments was significantly inhibited (Fig. 10). 25 min after the onset of BFA treatment, the frequency of SSCs along the distal axonal fragment dropped to 27% of that at the start of recording (Fig. 10). Since the transected axonal fragments lack the Golgi apparatus, the results strongly suggest an inhibitory action of BFA on local synaptic vesicle recycling along the axon.

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