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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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Spontaneous neurotransmitter secretion along  the axon can be detected at the  distal axonal fragments separated from the soma. (A) ACh  release along the axon was  measured using patch clamp  recordings from the myocyte  (M) manipulated into contact  with the middle axon. The  axon was transected with a  sharp microelectrode in the vicinity of the cell body. (B) The  time dependence of the SSC  frequency in recordings from  myocytes normalized to the control frequency before transection.  Typically, the SSC frequency dramatically increased immediately  after transection, and returned to the baseline level within 15–20  min after transection. Each data point is a mean ± SEM of nine  different experiments. *, significantly different from control values (P < 0.05, ANOVA).
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Figure 4: Spontaneous neurotransmitter secretion along the axon can be detected at the distal axonal fragments separated from the soma. (A) ACh release along the axon was measured using patch clamp recordings from the myocyte (M) manipulated into contact with the middle axon. The axon was transected with a sharp microelectrode in the vicinity of the cell body. (B) The time dependence of the SSC frequency in recordings from myocytes normalized to the control frequency before transection. Typically, the SSC frequency dramatically increased immediately after transection, and returned to the baseline level within 15–20 min after transection. Each data point is a mean ± SEM of nine different experiments. *, significantly different from control values (P < 0.05, ANOVA).

Mentions: To directly demonstrate that ACh secretion along the axon was due to the local recycling of ACh-containing vesicles, we transected the axon from the soma with a microelectrode. Previously it has been shown that this procedure results in the transient increase in SSC frequency at the preformed synapses (Stoop and Poo, 1995) due to the influx of Ca2+ (Stoop and Poo, 1995; Ziv and Spira, 1997). Within 15–20 min after transection, both the concentration of cytoplasmic Ca2+ and SSC frequency return to control (before transection) values (Stoop and Poo, 1995). To investigate whether spontaneous ACh secretion persisted along the distal axonal fragments after axotomy, we manipulated the myocyte into contact with the middle axon and recorded SSCs before and for a period of 30 min after transection (Fig. 4). In agreement with previously reported data (Stoop and Poo, 1995), we observed a dramatic increase in the SSC frequency immediately after transection (Fig. 4 B). With time, the frequency of SSC decreased. For a period of 20–30 min after transection, the average frequency of SSCs, determined for 3-min bins, was not significantly different from that recorded at the middle axonal segment before transection (Fig. 4 B).


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

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

Spontaneous neurotransmitter secretion along  the axon can be detected at the  distal axonal fragments separated from the soma. (A) ACh  release along the axon was  measured using patch clamp  recordings from the myocyte  (M) manipulated into contact  with the middle axon. The  axon was transected with a  sharp microelectrode in the vicinity of the cell body. (B) The  time dependence of the SSC  frequency in recordings from  myocytes normalized to the control frequency before transection.  Typically, the SSC frequency dramatically increased immediately  after transection, and returned to the baseline level within 15–20  min after transection. Each data point is a mean ± SEM of nine  different experiments. *, significantly different from control values (P < 0.05, ANOVA).
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Related In: Results  -  Collection

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Figure 4: Spontaneous neurotransmitter secretion along the axon can be detected at the distal axonal fragments separated from the soma. (A) ACh release along the axon was measured using patch clamp recordings from the myocyte (M) manipulated into contact with the middle axon. The axon was transected with a sharp microelectrode in the vicinity of the cell body. (B) The time dependence of the SSC frequency in recordings from myocytes normalized to the control frequency before transection. Typically, the SSC frequency dramatically increased immediately after transection, and returned to the baseline level within 15–20 min after transection. Each data point is a mean ± SEM of nine different experiments. *, significantly different from control values (P < 0.05, ANOVA).
Mentions: To directly demonstrate that ACh secretion along the axon was due to the local recycling of ACh-containing vesicles, we transected the axon from the soma with a microelectrode. Previously it has been shown that this procedure results in the transient increase in SSC frequency at the preformed synapses (Stoop and Poo, 1995) due to the influx of Ca2+ (Stoop and Poo, 1995; Ziv and Spira, 1997). Within 15–20 min after transection, both the concentration of cytoplasmic Ca2+ and SSC frequency return to control (before transection) values (Stoop and Poo, 1995). To investigate whether spontaneous ACh secretion persisted along the distal axonal fragments after axotomy, we manipulated the myocyte into contact with the middle axon and recorded SSCs before and for a period of 30 min after transection (Fig. 4). In agreement with previously reported data (Stoop and Poo, 1995), we observed a dramatic increase in the SSC frequency immediately after transection (Fig. 4 B). With time, the frequency of SSC decreased. For a period of 20–30 min after transection, the average frequency of SSCs, determined for 3-min bins, was not significantly different from that recorded at the middle axonal segment before transection (Fig. 4 B).

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