Phramacological characteristics of acetylcholine receptors in chick ciliary ganglion neurons

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Chick embryo ciliary ganglion neurons were acutely isolated, and the mechanisms underlying the increases in the cytosolic Ca^<2+> concentration ([Ca]_<in>) induced by nicotine and muscarine were examined using fura-2 microfluorometry and whole cell clamp technique. Both the nicotine- and muscarine-induced [Ca]_<in> increases were inhibited by both L- and N-type Ca^<2+> channel blockers and potentiated by an L-type Ca^<2+> channel agonist, Bay-K-8644. Nicotine, but not muscarine, increased the cytosolic Na^+ concentration ([Na]_<in>) as measured by SBFI microfluorometry, and this [Na]_<in> increase was inhibited by nicotinic receptor channel antagonists. These results suggest that nicotine increased Na^+ influx through nicotinic receptor channels resulting in membrane depolarization, which in turn increased Ca^<2+> influx through Ca^<2+> channels. However, nicotine still increased inflexes of Ca^<2+> and Mn^<2+> in the absence of external Na^+, suggesting that nicotinic receptor channels are permeable not only to monovalent cations but also to Ca^<2+> and Mn^<2+>. Muscarine caused a slow inward current, which became smaller at a hyperpolarized potential but not reversed, suggesting that the current was elicited by the inhibition of M-current. In fact, the muscarine-induced increase in [Ca]_<in> persisted after treatment with thapsigargin which blocked caffeine- and muscarinic agonist-induced intracellular Ca^<2+> release. Muscarine was effective in increasing [Ca]_<in> in the presence of a desensitizing concentration of nicotine, and simultaneous addition of maximal doses of muscarine and nicotine caused an additive increase in [Ca]_<in>. On the other hand, nicotine-, ATP-, and high K^+-induced increase in [Ca]_<in> was markedly potentiated during continuous stimulation with muscarine. These results suggest that muscarine causes membrane depolarization by inhibiting M-current, which in turn increases Ca^<2+> influx passing through voltage-dependent Ca^<2+> channels.