Studies of sodium-sensing sodium channel in brain

建立了表达小鼠Nax通道的稳定细胞系。在这些细胞中观察到细胞外钠水平的敏感反应，但在对照nax阴性细胞中没有观察到。这些细胞系对研究Nax通道的分子和细胞机制具有重要意义。通过免疫电镜研究，我发现Nax通道蛋白定位于脑室周围器官星形胶质细胞和室管膜细胞的层压突。这一结果表明，神经元-神经胶质的交流在大脑的钠感知中起着至关重要的作用。通过敲除nax基因的小鼠，研究了脑内钠的感知机制。在脱水情况下，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。小鼠脑室输注高渗钠溶液引起野生型小鼠对盐摄入的厌恶行为。另一方面，在敲除nax的小鼠中没有观察到这种厌恶行为。然后，利用腺病毒表达载体将外源性Nax通道引入Nax基因敲除小鼠的心室周围器官。将Nax通道引入皮质下器官，恢复了敲除小鼠在脱水条件下的避盐行为。这些结果表明，皮质下器官是钠敏感的主要位点，并且在脑内钠敏感机制中，钠氨酸是不可或缺的。

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.