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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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Evoked synaptic  currents (ESCs) at different  axonal segments. Continuous  traces depict the membrane  current recorded at the preformed synapse (A) and at  the middle axon (B). The  neurons were extracellularly  stimulated (0.5 ms duration,  0.2 Hz) to generate action potentials. ESCs (arrows) are shown  as downward deflections among randomly occurring SSCs.
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Figure 6: Evoked synaptic currents (ESCs) at different axonal segments. Continuous traces depict the membrane current recorded at the preformed synapse (A) and at the middle axon (B). The neurons were extracellularly stimulated (0.5 ms duration, 0.2 Hz) to generate action potentials. ESCs (arrows) are shown as downward deflections among randomly occurring SSCs.

Mentions: As a more direct test for the Ca2+ dependence of synaptic vesicle exocytosis, we investigated whether ACh release can be induced by the action potential. Electrical stimulation of the neuronal cell body results in evoked synaptic currents (ESCs) in recordings from the postsynaptic myocyte in Xenopus neuromuscular synapses (Fig. 6 A). Action potential-evoked currents reflect simultaneous release of a number of ACh quanta from the nerve terminal. ESCs follow the excitation pulse with a characteristic delay of a few milliseconds (Sun and Poo, 1987). To investigate whether ACh secretion along the axon can be induced by an action potential, we stimulated the neuron at the soma with an extracellular patch electrode. Simultaneously we recorded spontaneous and evoked currents from the myocyte manipulated into contact with the middle axonal segment. Low frequency electrical stimulation of the neuron consistently elicited ESCs in the manipulated myocyte (Fig. 6 B). The average amplitude of ESCs was 2.3 ± 0.2 nA (mean ± SEM , n = 16) and 1.1 ± 0.1 nA (n = 14) at the preformed synapses and the middle axon, respectively (Table I). The somewhat higher ESC amplitude at the preformed synapses may reflect a tighter excitation-secretion coupling, or higher density of docked vesicles at the nerve terminal, as compared with that at the middle axon. However, we noticed that the ESC amplitude at the middle axonal segment seemed to depend on the contact area between axonal plasmalemma and the myocyte (see Evers et al., 1989). To take into account the differences in the contact area between the myocyte and the axon, we calculated the ratio of the average ESCs to the average SSC frequency for each recording. Since both of these parameters are expected to be proportional to the area of contact between the myocyte and the neuron, this ratio may serve as an indicator of the efficacy of the excitation-secretion coupling at different axonal segments. The average ratio ESC amplitude/SSC frequency at the preformed synapses and the middle axon showed no significant difference (Table I), suggesting a similar efficiency of evoked neurotransmitter secretion at the middle axon and at the preformed synapses. This conclusion was further supported by the analysis of the delay of ESC onset (as defined by the time between the end of 0.5-ms stimulus and the onset of ESCs), and the fluctuation of the ESC amplitude (as assessed by calculating the coefficient of variation, or SD/mean, of the ESC amplitude observed in each recording). The delay of ESC onset and the fluctuations of the ESC amplitude are believed to reflect the speed and reliability of evoked neurotransmitter secretion (Sun and Poo, 1987; Wang et al., 1995). No statistically significant differences in these parameters were found between preformed synapses and the middle axon (Table I).


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

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

Evoked synaptic  currents (ESCs) at different  axonal segments. Continuous  traces depict the membrane  current recorded at the preformed synapse (A) and at  the middle axon (B). The  neurons were extracellularly  stimulated (0.5 ms duration,  0.2 Hz) to generate action potentials. ESCs (arrows) are shown  as downward deflections among randomly occurring SSCs.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2132923&req=5

Figure 6: Evoked synaptic currents (ESCs) at different axonal segments. Continuous traces depict the membrane current recorded at the preformed synapse (A) and at the middle axon (B). The neurons were extracellularly stimulated (0.5 ms duration, 0.2 Hz) to generate action potentials. ESCs (arrows) are shown as downward deflections among randomly occurring SSCs.
Mentions: As a more direct test for the Ca2+ dependence of synaptic vesicle exocytosis, we investigated whether ACh release can be induced by the action potential. Electrical stimulation of the neuronal cell body results in evoked synaptic currents (ESCs) in recordings from the postsynaptic myocyte in Xenopus neuromuscular synapses (Fig. 6 A). Action potential-evoked currents reflect simultaneous release of a number of ACh quanta from the nerve terminal. ESCs follow the excitation pulse with a characteristic delay of a few milliseconds (Sun and Poo, 1987). To investigate whether ACh secretion along the axon can be induced by an action potential, we stimulated the neuron at the soma with an extracellular patch electrode. Simultaneously we recorded spontaneous and evoked currents from the myocyte manipulated into contact with the middle axonal segment. Low frequency electrical stimulation of the neuron consistently elicited ESCs in the manipulated myocyte (Fig. 6 B). The average amplitude of ESCs was 2.3 ± 0.2 nA (mean ± SEM , n = 16) and 1.1 ± 0.1 nA (n = 14) at the preformed synapses and the middle axon, respectively (Table I). The somewhat higher ESC amplitude at the preformed synapses may reflect a tighter excitation-secretion coupling, or higher density of docked vesicles at the nerve terminal, as compared with that at the middle axon. However, we noticed that the ESC amplitude at the middle axonal segment seemed to depend on the contact area between axonal plasmalemma and the myocyte (see Evers et al., 1989). To take into account the differences in the contact area between the myocyte and the axon, we calculated the ratio of the average ESCs to the average SSC frequency for each recording. Since both of these parameters are expected to be proportional to the area of contact between the myocyte and the neuron, this ratio may serve as an indicator of the efficacy of the excitation-secretion coupling at different axonal segments. The average ratio ESC amplitude/SSC frequency at the preformed synapses and the middle axon showed no significant difference (Table I), suggesting a similar efficiency of evoked neurotransmitter secretion at the middle axon and at the preformed synapses. This conclusion was further supported by the analysis of the delay of ESC onset (as defined by the time between the end of 0.5-ms stimulus and the onset of ESCs), and the fluctuation of the ESC amplitude (as assessed by calculating the coefficient of variation, or SD/mean, of the ESC amplitude observed in each recording). The delay of ESC onset and the fluctuations of the ESC amplitude are believed to reflect the speed and reliability of evoked neurotransmitter secretion (Sun and Poo, 1987; Wang et al., 1995). No statistically significant differences in these parameters were found between preformed synapses and the middle axon (Table I).

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