Studies of sodium-sensing sodium channel in brain

建立了稳定表达小鼠NAX通道的细胞系。在这些细胞中观察到了细胞外钠水平的敏感反应，但在对照的Nax阴性细胞中没有观察到。这些细胞系可用于NAX通道分子和细胞机制的功能研究。免疫电子显微镜研究表明，Nax通道蛋白定位于室周器官内星形胶质细胞和室管膜细胞的层状突起。这一结果表明，神经元-神经胶质细胞的通讯在大脑的钠感觉中起着至关重要的作用。并利用Nax基因敲除小鼠研究了脑组织的钠敏感机制。在脱水状态下，野生型动物避免摄入高渗钠溶液，而NAX基因敲除小鼠在这种情况下继续饮用高渗钠溶液。向小鼠脑室注入高渗钠溶液可引起野生型小鼠对盐摄取的厌恶行为。另一方面，在NAX基因敲除的小鼠中没有观察到这种厌恶行为。接下来，用腺病毒表达载体将外源Nax通道导入Nax基因敲除小鼠的脑室周围器官。将Nax通道引入穹隆下器，恢复了基因敲除小鼠在脱水条件下的避盐行为。这些结果表明，穹隆下器是钠敏感的主要部位，Nax在脑内钠敏感机制中是不可或缺的。

Stable cell lines which express mouse Nax channel were established here. The extracellular sodium-level sensitive response was observed in these cells, but not in control Nax-negative cells. These cell lines are useful for functional studies of molecular and cellular mechanisms of Nax channel. By immuno-electron microscopic studies, I showed that Nax channel protein was localized to laminate processes of astrocytes and ependymal cells in the circumventricular organs. This result suggests that neuron-glia communication plays crucial role in the sodium sensing of the brain. Sodium sensing mechanism of the brain was also studied using by the Nax-knockout mice. Under dehydration, wild-type animals avoid intake of hypertonic sodium solution, whereas the Nax-knockout mice keep drinking it under such conditions. Infusion of hypertonic sodium solution to the cerebral ventricle of mice caused aversive behavior for salt-intake in wild-type mice. On the other hand, such aversive behavior was not observed in Nax-knockout mice. Next, exogenous Nax channel was introduced to the circumventricular organs of the Nax-knockout mice by using Adenoviral expression vector. The introduction of Nax channel to the subfornical organ recovered the salt-avoiding behavior of the knockout mice under dehydrated conditions. These results indicate that the subfornical organ is the dominant locus of the sodium sensing and that Nax is indispensable for the sodium-sensing mechanism in the brain.