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细胞外钠水平敏感性反应，但在对照Nax阴性细胞中未观察到。这些细胞系可用于Nax通道的分子和细胞机制的功能研究。通过免疫电镜研究，我发现Nax通道蛋白定位于室周器的星形胶质细胞和室管膜细胞的层压过程中。这一结果表明，神经元-胶质细胞通讯在脑钠敏感中起着至关重要的作用。利用Nax基因敲除小鼠研究脑钠敏感机制。在脱水的情况下，野生型动物避免摄入高渗钠溶液，而敲除Nax的小鼠在这种条件下继续饮用。向小鼠脑室输注高渗钠溶液可引起野生型小鼠对盐摄入的厌恶行为。另一方面，在Nax基因敲除小鼠中没有观察到这种厌恶行为。接着，通过使用腺病毒表达载体将外源性Nax通道导入到Nax敲除小鼠的脑室周围器官。将Nax通道引入穹窿下器官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，穹窿下器官是钠敏感的优势位点，Nax是不可缺少的钠敏感机制在大脑中。

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.