Channel Properties of Na(x) Expressed in Neurons

Na(x) is a sodium-concentration ([Na(+)])-sensitive Na channel with a gating threshold of ~150 mM for extracellular [Na(+)] ([Na(+)](o)) in vitro. We previously reported that Na(x) was preferentially expressed in the glial cells of sensory circumventricular organs including the subfornical organ, and was involved in [Na(+)] sensing for the control of salt-intake behavior. Although Na(x) was also suggested to be expressed in the neurons of some brain regions including the amygdala and cerebral cortex, the channel properties of Na(x) have not yet been adequately characterized in neurons. We herein verified that Na(x) was expressed in neurons in the lateral amygdala of mice using an antibody that was newly generated against mouse Na(x). To investigate the channel properties of Na(x) expressed in neurons, we established an inducible cell line of Na(x) using the mouse neuroblastoma cell line, Neuro-2a, which is endogenously devoid of the expression of Na(x). Functional analyses of this cell line revealed that the [Na(+)]-sensitivity of Na(x) in neuronal cells was similar to that expressed in glial cells. The cation selectivity sequence of the Na(x) channel in cations was revealed to be Na(+) ≈ Li(+) > Rb(+) > Cs(+) for the first time. Furthermore, we demonstrated that Na(x) bound to postsynaptic density protein 95 (PSD95) through its PSD95/Disc-large/ZO-1 (PDZ)-binding motif at the C-terminus in neurons. The interaction between Na(x) and PSD95 may be involved in promoting the surface expression of Na(x) channels because the depletion of endogenous PSD95 resulted in a decrease in Na(x) at the plasma membrane. These results indicated, for the first time, that Na(x) functions as a [Na(+)]-sensitive Na channel in neurons as well as in glial cells.