Muscarinic Ca2+ responses resistant to muscarinic antagonists at perisynaptic Schwann cells of the frog neuromuscular junction

• Published in: The Journal of physiology Vol. 504; no. Pt 2; pp. 337 - 347
• Main Authors: Robitaille, Richard, Jahromi, Babak S., Charlton, Milton P.
• Format: Journal Article
• Language: English
• Published: Oxford, UK The Physiological Society 15.10.1997; Blackwell Science Ltd
• Subjects: Acetylcholine - pharmacology; Adenosine - pharmacology; Aniline Compounds - metabolism; Animals; Calcium - metabolism; Fluorescent Dyes - metabolism; Gallamine Triethiodide - pharmacology; Microscopy, Fluorescence; Muscarine - pharmacology; Muscarinic Agonists - pharmacology; Muscarinic Antagonists - pharmacology; Neuromuscular Junction - drug effects; Neuromuscular Junction - metabolism; Nicotine - pharmacology; Nicotinic Agonists - pharmacology; Nicotinic Antagonists - pharmacology; Pertussis Toxin; Rana pipiens; Receptors, Muscarinic - metabolism; Schwann Cells - cytology; Schwann Cells - drug effects; Schwann Cells - metabolism; Virulence Factors, Bordetella - pharmacology; Xanthenes - metabolism
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1111/j.1469-7793.1997.337be.x

1. Acetylcholine causes a rise of intracellular Ca2+ in perisynaptic Schwann cells (PSCs) of the frog neuromuscular junction. The signalling pathway was characterized using the fluorescent Ca2+ indicator fluo-3 and fluorescence microscopy. 2. Nicotinic antagonists had no effect on Ca2+ responses evoked by ACh and no Ca2+ responses were evoked with the nicotinic agonist nicotine. The muscarinic agonists muscarine and oxotremorine-M induced Ca2+ signals in PSCs. 3. Ca2+ responses remained unchanged when extracellular Ca2+ was removed, indicating that they are due to the release of Ca2+ from internal stores. Incubation with pertussis toxin did not alter the Ca2+ signals induced by muscarine, but did block depression of transmitter release induced by adenosine and prevented Ca2+ responses in PSCs induced by adenosine. 4. The general muscarinic antagonists atropine, quinuclidinyl benzilate and N-methyl-scopolamine failed to block Ca2+ responses to muscarinic agonists. Atropine (at 20,000-fold excess concentration) also failed to reduce the proportion of cells responding to a threshold muscarine concentration sufficient to cause responses in less than 50% of cells. Only the allosteric, non-specific blocker, gallamine (1-10 microM) was effective in blocking muscarine-induced Ca2+ responses. 5. In preparations denervated 7 days prior to experiments, low concentrations of atropine reversibly and completely blocked Ca2+ responses to muscarine. 6. The lack of blockade by general muscarinic antagonists in innervated, in situ preparations suggests that muscarinic Ca2+ responses at PSCs are not mediated by any of the five known muscarinic receptors or that post-translational modification prevented antagonist binding.