Sodium appetite and thirst do not require angiotensinogen production in astrocytes or hepatocytes.

In addition to its renal and cardiovascular functions, angiotensin signaling is thought to be responsible for the increases in salt and water intake caused by hypovolemia. However, it remains unclear whether these behaviors require angiotensin production in the brain or liver. Here, we use in situ hybridization to identify tissue-specific expression of the genes required for producing angiotensin peptides, then use conditional genetic deletion of angiotensinogen (Agt) to test whether production in the brain or liver is necessary for sodium appetite and thirst. In the mouse brain, we identified expression of Agt (the precursor of all angiotensin peptides) in a large subset of astrocytes. We also identified Ren1 and Ace (enzymes required to produce angiotensin II) in the choroid plexus, and Ren1 in neurons within the nucleus ambiguus compact formation. In the liver, we confirmed that Agt is expressed in widespread hepatocytes. We next tested whether thirst and sodium appetite require angiotensinogen production in astrocytes or hepatocytes. Despite virtually eliminating expression in the brain, deleting astrocytic Agt did not reduce thirst or sodium appetite. Despite markedly reducing angiotensinogen in the blood, eliminating Agt from hepatocytes did not reduce thirst or sodium appetite, and in fact, these mice consumed the largest amounts of salt and water after sodium deprivation. Deleting Agt from both astrocytes and hepatocytes also did not prevent thirst or sodium appetite. Our findings suggest that angiotensin signaling is not required for sodium appetite or thirst and highlight the need to identify alternative signaling mechanisms. KEY POINTS: Angiotensin signaling is thought to be responsible for the increased thirst and sodium appetite caused by hypovolemia, producing elevated water and sodium intake. Specific cells in separate brain regions express the three genes needed to produce angiotensin peptides, but brain-specific deletion of the angiotensinogen gene (Agt), which encodes the lone precursor for all angiotensin peptides, did not reduce thirst or sodium appetite. Double-deletion of Agt from brain and liver also did not reduce thirst or sodium appetite. Liver-specific deletion of Agt reduced circulating angiotensinogen levels without reducing thirst or sodium appetite. Instead, these angiotensin-deficient mice exhibited an enhanced sodium appetite. Because the physiologic mechanisms controlling thirst and sodium appetite continued functioning without angiotensin production in the brain and liver, understanding these mechanisms requires a renewed search for the hypovolemic signals necessary for activating each behavior. Abstract figure legend Angiotensinogen is the precursor for angiotensin II, which is considered important for thirst and sodium appetite. In the brain, astrocytes express the angiotensinogen gene (Agt), but using GFAP-Cre to delete this gene from astrocytes did not reduce thirst or sodium appetite. In the liver, hepatocytes express this gene and produce circulating angiotensinogen. Using Alb-Cre to delete Agt from hepatocytes also did not reduce (and in fact enhanced) salt and water consumption. This article is protected by copyright. All rights reserved.