Rathouz M M, Berg D K
Department of Biology, University of California, San Diego, La Jolla 92093-0357.
J Neurosci. 1994 Nov;14(11 Pt 2):6935-45. doi: 10.1523/JNEUROSCI.14-11-06935.1994.
Nicotinic acetylcholine receptors (AChRs) located in the postsynaptic membrane on neurons are responsible for mediating fast, excitatory synaptic transmission. If synaptic AChRs are also highly permeable to calcium as reported recently for several kinds of neuronal AChRs, the synaptic receptors could regulate calcium-dependent events in the neurons in concert with normal transmission. Chick ciliary ganglion neurons have two classes of AChRs, one located predominantly in the synaptic membrane and responsible for synaptic signaling through the ganglion and the other located almost exclusively in nonsynaptic membrane and having no known function. The nonsynaptic receptors can readily elevate intracellular calcium concentrations. The experiments reported here indicate that synaptic-type receptors can raise intracellular calcium levels to the same extent as the nonsynaptic receptors and that they do so not only by being permeable to calcium themselves but also by activating voltage-dependent calcium channels (VDCCs). Currents of equivalent amplitude are obtained through the synaptic-type receptors when neurons are bathed in solutions containing either sodium or calcium as the sole extracellular cation. Measuring the effect of ion substitutions on the reversal potential of the receptors and applying the Goldman-Hodgkin-Katz constant field equation indicates the receptors are at least as permeable to calcium as to sodium. When neurons are loaded with the calcium-sensitive dye fluo-3 and challenged with nicotine, both the synaptic-type and nonsynaptic AChRs substantially elevate intracellular calcium levels under physiological conditions, and do so largely by activating VDCCs. Confirmation that synaptic-type AChRs can elevate intracellular calcium levels in the absence of contributions from VDCCs was obtained from voltage-clamp experiments on neurons loaded with fluo-3. The fluorescence signals indicate that the nicotine-induced calcium increases in neurons voltage clamped at rest are nearly as great as those induced in the same neurons when VDCCs are maximally activated by a voltage step. Calcium flux through AChRs may be particularly important for mediating local changes in calcium concentrations near the plasma membrane, which, in turn, could regulate specific membrane-associated calcium-dependent events.
位于神经元突触后膜上的烟碱型乙酰胆碱受体（AChRs）负责介导快速的兴奋性突触传递。如果如最近针对几种神经元型AChRs所报道的那样，突触型AChRs对钙也具有高度通透性，那么突触受体就可以在正常传递的同时调节神经元中依赖钙的事件。鸡睫状神经节神经元有两类AChRs，一类主要位于突触膜，负责通过神经节进行突触信号传递，另一类几乎只位于非突触膜，且功能未知。非突触受体能轻易提高细胞内钙浓度。本文报道的实验表明，突触型受体能够将细胞内钙水平提高到与非突触受体相同的程度，而且它们不仅自身对钙具有通透性，还能激活电压依赖性钙通道（VDCCs）。当神经元浸浴在以钠或钙作为唯一细胞外阳离子的溶液中时，通过突触型受体可获得幅度相当的电流。测量离子置换对受体反转电位的影响并应用戈德曼-霍奇金- Katz恒定场方程表明，受体对钙的通透性至少与对钠的通透性一样高。当神经元加载钙敏感染料fluo-3并用尼古丁刺激时，在生理条件下，突触型和非突触型AChRs都能大幅提高细胞内钙水平，且主要是通过激活VDCCs来实现的。通过对加载了fluo-3的神经元进行电压钳实验，证实了突触型AChRs在没有VDCCs参与的情况下也能提高细胞内钙水平。荧光信号表明，在静息状态下进行电压钳制的神经元中，尼古丁诱导的钙增加量与通过电压阶跃使VDCCs最大程度激活时在同一神经元中诱导的钙增加量几乎一样大。通过AChRs的钙通量对于介导质膜附近钙浓度的局部变化可能尤为重要，而这反过来又可能调节特定的膜相关钙依赖性事件。
