Functional properties of the Na^+-sensitive Na^+ channel in tanycytes

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Methods : Brain slices, sectioned coronally at the level of the median eminence(ME) with 200μm thickness, were obtained from adult mice(C57BL/6J). Slices were loaded with the Na^+-sensitive fluorescent dye SBFI(sodium-binding benzofuran isophthalate), and intracellular Na^+ images were monitored using an image analyzer ARGUS-50(Hamamatsu Photonics, Japan).Resulfls : 1)In all areas studied, the intracellular Na^+ concentration([Na^+]_i) was greatly increased by 1mM ouabain(a Na^+ pump blocker). 2)[Na^+]_i was decreased either by 1-10μM benzamil(a blocker of Na^+/Ca^<2+> exchanger), 100μM amiloride(an inhibitor of epithelial Na^+ channels(ENaCs)) or 1mM phloridzin(a blocker of Na^+/ghicose co-transporter). 3)When [Na^+]_o was raised from a control value of 140mM to 160-200mM, a sustained elevation in [Na^+]_i was observed in ME but only small responses were observed in the other regions. 4)The responses were abolished when Na^+ was substituted with choline or mannitol. 5)Very small changes were seen in [Na^+]_i when [Na^+]_o was decreased from 140mM to 100mM. 6)The transitional area between the lateral wall of third ventricle (3V) and the ME responded inversely, i.e. this area showed a decrease in [Na^+]_i when [Na^+]_o was elevated.Conclusions : The data indicate that (a)the basal level of [Na^+]_i in the brain regions lining 3V and ME is maintained chiefly by Na^+ pumps, Na^+/Ca^<2+> exchangers, ENaCs and Na^+/glucose co-transporters, and (b)a Na^+-level sensor, present in ME where the blood-brain barrier is absent, works when [Na^+]_o Aviates from the maintained [Na^+]_i level.