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Distribution of Ca2+ channels on frog motor nerve terminals revealed by fluorescent omega-conotoxin
M W Cohen1, O T Jones, K J Angelides
1Department of Physiology, McGill University, Montreal, Quebec, Canada.
Abstract:
Tetramethylrhodamine-conjugated omega-conotoxin was used as a fluorescent stain (Jones et al., 1989) to determine the spatial distribution of voltage-gated Ca2+ channels along frog motor nerve terminals. Like native omega-conotoxin, the fluorescent toxin blocked neuromuscular transmission irreversibly. The fluorescent staining was confined to the neuromuscular junction and consisted of a series of narrow bands (in face views) or dots (in side views) approximately 1 micron apart. This characteristic staining pattern was prevented by pretreatment with omega-conotoxin and by prior denervation for 5-7 d. Combined fluorescence and phase-contrast optics indicated that the stain was on the synaptic rather than the nonsynaptic side of the nerve terminal. The bands and dots of stain proved to be in spatial register with the postsynaptic junctional folds, as revealed by combined staining of ACh receptors. It is concluded that the voltage-gated Ca2+ channels on frog motor nerve terminals are concentrated at active zones. The findings are consistent with the suggestion (Heuser et al., 1974; Pumplin et al., 1981) that the large intramembraneous particles seen at freeze-fractured active zones are voltage-gated Ca2+ channels.
Insights
Voltage-gated calcium channels in frog motor nerve terminals are concentrated at active zones. Tetramethylrhodamine-conjugated omega-conotoxin revealed their precise location at the neuromuscular junction, aiding in understanding nerve signal transmission.
Area of Science:
- Neuroscience
- Cell Biology
- Biochemistry
Background:
- Voltage-gated calcium channels are crucial for neurotransmitter release.
- Their precise localization at nerve terminals is essential for synaptic function.
- Previous studies suggested a link between active zones and calcium channel distribution.
Purpose of the Study:
- To determine the spatial distribution of voltage-gated calcium channels in frog motor nerve terminals.
- To investigate the relationship between calcium channels and active zones.
- To utilize a novel fluorescently labeled toxin for channel visualization.
Main Methods:
- Employing tetramethylrhodamine-conjugated omega-conotoxin as a fluorescent stain.
- Utilizing combined fluorescence and phase-contrast microscopy.
- Comparing staining patterns with acetylcholine receptor distribution and effects of toxin pretreatment and denervation.
Main Results:
- Fluorescent staining for voltage-gated calcium channels was localized to the neuromuscular junction in distinct bands or dots.
- The staining pattern was abolished by omega-conotoxin pretreatment and denervation.
- The distribution of calcium channels colocalized with postsynaptic junctional folds and acetylcholine receptors.
Conclusions:
- Voltage-gated calcium channels are concentrated at active zones of frog motor nerve terminals.
- This localization supports the hypothesis that intramembranous particles at active zones represent voltage-gated calcium channels.
- The findings provide direct evidence for the role of active zones in synaptic transmission regulation.