Phramacological characteristics of acetylcholine receptors in chick ciliary ganglion neurons

急性分离鸡胚睫状神经节神经元，<in>用Fura-2显微荧光法和全细胞钳夹技术研究尼古丁和毒蕈碱引起的细胞内Ca^2+浓度（[Ca]_2）升高的机制。尼古丁和毒蕈碱诱导的[Ca]_2<in>增加可被L型和N型Ca^&lt;2+&gt;通道阻断剂抑制，并被L型Ca^&lt;2+&gt;通道激动剂Bay-K-8644增强。用SBFI显微荧光测定法测得，尼古丁（而非毒蕈碱）可增加细胞内Na^+浓度（[Na]_<in>），这种[Na]_<in>的增加可被烟碱受体通道拮抗剂抑制。这些结果表明，尼古丁增加了Na^+通过烟碱受体通道的内流，导致细胞膜去极化，进而增加了Ca^2+通过Ca^2+通道的内流。然而，在缺乏外部Na^+的情况下，尼古丁仍然增加了Ca^&lt;2+&gt;和Mn^&lt;2+&gt;的内流，这表明尼古丁受体通道不仅对单价阳离子具有渗透性，而且对Ca^&lt;2+&gt;和Mn^&lt;2+&gt;也具有渗透性。毒蕈碱引起一个缓慢的内向电流，该电流在超极化电位下变小但不逆转，表明该电流是通过抑制M电流而引起的。事实上，毒胡萝卜素能<in>阻断咖啡因和毒蕈碱激动剂诱导的细胞内Ca ^2+释放，而毒胡萝卜素能阻断咖啡因和毒蕈碱诱导的细胞内Ca^2+释放。在脱敏浓度的烟碱存在下，毒蕈碱能有效地增加[Ca]_2<in>，同时加入最大剂量的毒蕈碱和烟碱可使[Ca]_2累加增加<in>。另一方面，尼古丁、ATP和高K^+诱导的[Ca]_2增加在<in>毒蕈碱连续刺激期间显著增强。这些结果表明，毒蕈碱通过抑制M-电流引起膜去极化，M-电流反过来增加通过电压依赖性Ca^2+通道的Ca^2+内流。

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.