Two properties were found to distinguish neuronal from muscle nicotinic acetylcholine receptors (nAChRs). First, neuronal nAChRs have a greater Ca 2+ permeability. The high Ca2+ flux through neuronal nAChRs activates a Ca2+-dependent CI- conductance, and the Ca 2+ to Cs+ permeability ratio ( PCa PCs) is 7 times greater for neuronal than for muscle nAChRs. A second difference between the receptor types is that neuronal nAChRs are potently modulated by physiological levels of external Ca 2+. Neuronal nAChR currents are enhanced by external Ca2+ in a dose-dependent manner. The results indicate that changes in extracellular Call modulate neuronal nAChRs and may modulate cholinergic synapses in the CNS. Also, activation of neuronal nAChRs produces a significant influx of Ca2+ that could be an important intracellular signal.
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