Norepinephrine innervation of the supraoptic nucleus contributes to dilutional hyponatremia in male BDL rats

Purpose: Dilutional hyponatremia is a common complication associated with liver cirrhosis that is linked to inappropriate release of arginine vasopressin (AVP). Elevated plasma AVP causes water retention and hypoosmolality. In the cirrhotic liver, there is increase in resistance to blood flow resulting in portal hypertension, ascites formation, mesenteric vasodilation due to release of vasodilators and increased pooling of blood in the splanchnic circulation. The fluid redistribution decreases central vascular blood volume which is sensed by peripheral baroreceptors and volume receptors located in the aortic arch and heart. This could be relayed to A1 neurons in the caudal ventrolateral medulla (CVLM) and the A2 neurons in the nucleus tractus solitarius (NTS). The A1/A2 neurons stimulate the release of AVP from the supraoptic nucleus (SON). We propose that the A1 and A2 norepinephrine neurons in the hindbrain contribute to the activation of AVP-secreting neurons in the supraoptic nucleus (SON) leading to inappropriate AVP release and dilutional hyponatremia. Method: Anti-DBH saporin [IT-03] (Advanced Targeting Systems), a cytotoxin conjugated to an antibody against DBH was injected to the SON to lesion the norepinephrine innervation of SON including A1/A2 neurons. After two weeks, adult male rats received bile duct ligation surgery (BDL) which was used to model liver cirrhosis. In this model, the common duct that drains bile from the liver to the intestine is cauterized between two ligatures leading to obstructive cholestasis and liver cirrhosis. Four weeks after BDL surgery, rats were anesthetized with inactin (thiobutabarbital sodium salt hydrate; 100 mg/kg, i.p.). Blood samples were taken for plasma copeptin, osmolality, and hematocrit measurements. The rats were then perfused transcardially with 1M phosphate-buffered saline (PBS) followed by 4% paraformaldehyde (4% PFA) in 1M PBS. Plasma copeptin concentration was measured as a surrogate marker for AVP using commercially available copeptin ELISA kits. The brains were removed and processed for delta FosB (a marker of chronic activation), dopamine β-hydroxylase (DBH) and AVP immunohistochemistry. Results: Anti-DBH saporin (ADS) injection in the SON of BDL rats significantly decreased the number of cells positive for both delta FosB and DBH in the A1 and A2 cell groups as compared to vehicle injection (A1 neurons, ADS/BDL vs Vehicle/BDL P< 0.001; A2 neurons, ADS/BDL vs Vehicle/BDL P< 0.05). ADS treated BDL rats had fewer SON cells positive for both AVP and delta FosB as compared to vehicle injection (ADS/BDL vs Vehicle/BDL P< 0.05). Reduced colocalization of delta FosB and DBH in A1/A2 neurons was associated with a significantly lower plasma copeptin concentration in BDL rats. (ADS/BDL vs Vehicle/BDL P< 0.05). Similar effects were seen for plasma osmolality and hematocrit (ADS/BDL vs Vehicle/BDL P< 0.05). Conclusion: The result suggests that an increase in cells positive for both delta FosB and DBH in A1/A2 neurons is associated with an increase in plasma AVP and hypoosmolality in male BDL rats. Anti-DBH saporin lesions of SON prevented increases in plasma copeptin and neural activation of A1/A2 and AVP SON neurons associated with BDL.