The Subfornical Organ is the Primary Locus of Sodium-Level Sensing by Na_xSodium Channels for the Control of Salt-Intake Behavior

Abstract

Dehydration causes an increase in the sodium (Na) concentration and osmolarity of body fluid. For Na homeostasis of the body, controls of Na and water intake and excretion are of prime importance. However, the system for sensing the Na level within the brain that is responsible for the control of Na-and water-intake behavior remains to be elucidated. We reported previously that the Na x channel is preferentially expressed in the circumventricular organs (CVOs) in the brain and that Na x knock-out mice ingest saline in excess under dehydrated conditions. Subsequently, we demonstrated that Na x is a Na-level-sensitive Na channel. Here we show that the subfornical organ (SFO) is the principal site for the control of salt-intake behavior, where the Na x channel is the Na-level sensor. Infusion of a hypertonic Na solution into the cerebral ventricle induced extensive water intake and aversion to saline in wild-type animals but not in the knock-out mice. Importantly, the aversion to salt was not induced by the infusion of a hyperosmotic mannitol solution with physiological Na concentration in either genotype of mice. When Na x cDNA was introduced into the brain of the knock-out mice with an adenoviral expression vector, only animals that received a transduction of the Na x gene into the SFO among the CVOs recovered salt-avoiding behavior under dehydrated conditions. These results clearly show that the SFO is the center of the control of salt-intake behavior in the brain, where the Na-level-sensitive Na x channel is involved in sensing the physiological increase in the Na level of body fluids.

Distinct neural mechanisms for the control of thirst and salt appetite in the subfornical organ

“…The sensitivity of Na x to [Na + ] was revealed to be upregulated by endothelin-3 through activation of endothelin receptor B (Hiyama et al, 2013). Noda and his colleagues subsequently revealed that the SFO is the principal site for sensing [Na + ] in body fluids to generate salt-avoidance behavior in dehydrated animals (Watanabe et al, 2000;Hiyama et al, 2004;Hiyama et al, 2010).…”

“…Viral injections were performed as described previously, with minor modifications (Hiyama et al, 2004). Briefly, a pulled-glass pipette with a tip diameter of 20-40 µm was inserted into the brain, and virus was injected with a microsyringe pump (Ultra Micro Pump III, World Precision Instruments; AAV, 0.1 µL/min for 5 min).…”

“…The amounts of fluid intake were automatically monitored by measurements were performed using a previously described system (Hiyama et al, 2004). Mice were acclimated to the spouts providing distilled water and salt solutions for more than 3 d before the tests.…”

“…Noda's laboratory previously revealed that Na x -KO (Na x lacZ/lacZ ) mice did not show salt aversion under the water-depleted condition (Watanabe et al, 2000;Hiyama et al, 2004). To examine the relationship between Na x signals and the AT1a-dependent control of salt appetite, I examined 0.3 M NaCl intake by WT, Na x -KO, AT1a-KO, and Na x -AT1a-double KO (DKO) mice in the two-bottle test before and after 48 h of water depletion ( Fig.…”

“…Previous studies on Na x -KO mice by my laboratory revealed that Na x channels expressed in specific glial cells sense an increase in [Na + ] in body fluids to generate salt-avoidance behavior in dehydrated animals (Hiyama et al, 2004). In the SFO, activation of Na x leads to release of lactate from Na x -positive glial cells, which functions as a gliotransmitter for the activation of the GABAergic inhibitory neurons (Shimizu, et al, 2007).…”