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Terminal sprouting in mouse neuromuscular junctions poisoned with botulinum type A toxin: morphological and
D Angaut-Petit1, J Molgó, J X Comella
1Laboratorie de Neurobiologie Cellulaire et Moléculaire, C.N.R.S., Gif-sur-Yvette, France.
Abstract:
Functional properties of terminal sprouts elicited by an in vivo injection of Clostridium botulinum type A toxin were studied in endplates of the Levator auris longus muscle of the mouse poisoned from a few days to 28 days beforehand. For this purpose, morphological observations of the extent of terminal sprouts and localization of acetylcholine receptors was performed in whole mount preparations. Sprouts appeared as thin unmyelinated filaments that run usually parallel to the longitudinal axis of the muscle fibres; labelling acetylcholine receptors revealed their line-shaped accumulation co-localized with the sprouts. In addition, presynaptic membrane currents elicited by nerve stimulation were recorded by external electrodes applied under visual control onto the membrane of pre-existing motor endings and newly formed sprouts. These recordings showed the presence of widespread triphasic waveforms which indicated active impulse propagation of the action potential over most of the length of the poisoned endings. Ca2+ influx and Ca2(+)-dependent K+ currents in the sprout membrane were found to be similar to those described in unpoisoned endings. The presence of normal Ca2+ influx, upon active depolarization, in the terminal sprout membranes together with the localization of acetylcholine receptors in front of these membranes, indicates that the terminal sprouts may play a role in the recovery of neuromuscular transmission after Clostridium botulinum poisoning.
Insights
Terminal sprouts in mouse muscle, induced by Clostridium botulinum type A toxin, show normal function. These sprouts, along with acetylcholine receptors, may aid recovery of neuromuscular transmission.
Area of Science:
- Neuroscience
- Muscle Physiology
Background:
- Clostridium botulinum type A toxin disrupts neuromuscular transmission.
- Terminal sprouting is a response to nerve injury or toxin exposure.
Purpose of the Study:
- To investigate the functional properties of terminal sprouts in response to Clostridium botulinum type A toxin.
- To understand the role of these sprouts in neuromuscular transmission recovery.
Main Methods:
- Morphological observation of terminal sprouts and acetylcholine receptor localization in mouse Levator auris longus muscle.
- Recording of presynaptic membrane currents from motor endings and sprouts using external electrodes.
- Analysis of action potential propagation and ion channel activity (Ca2+ influx, Ca2+-dependent K+ currents) in sprouts.
Main Results:
- Terminal sprouts appeared as thin, unmyelinated filaments co-localized with acetylcholine receptors.
- Active impulse propagation and normal Ca2+ influx were observed in sprout membranes.
- Ca2+ influx and Ca2+-dependent K+ currents in sprouts were similar to unpoisoned endings.
Conclusions:
- Terminal sprouts exhibit functional properties similar to normal motor endings.
- The presence of functional sprouts and localized acetylcholine receptors suggests a role in restoring neuromuscular transmission after botulinum toxin poisoning.