OR06-4 Brain Derived Neurotrophic Factor in the Regulation of Supraoptic Vasopressin Neurons in Development of Hyponatremia

Abstract

Dilutional hyponatremia due to elevated arginine vasopressin (AVP) secretion increases mortality in patients with liver failure. The current conventional therapies are palliative and frequently ineffective. The neural mechanisms causing dysregulation of AVP secretion are not completely understood. Based on previous studies, we hypothesis that inappropriate AVP release associated with liver failure is due to increased Brain Derived Neurotrophic Factor (BDNF) in the supraoptic nucleus (SON). BDNF diminishes GABAA inhibition in SON AVP neurons by increasing intracellular chloride ([Cl]i) through tyrosine receptor kinase B (TrkB) activation and downregulation of K+/Cl- co-transporter 2 (KCC2). This loss of inhibition could increase AVP secretion. To test this hypothesis, chronic bile duct ligated (BDL) rats, a model of dilutional hyponatremia associated with liver failure, were injected with shRNA to prevent increased BDNF in the SON. Adult male Sprague Dawley rats were bilaterally injected in the SON with AAV2- shRNA-BDNF-mCherry or AAV2- shRNA-SCR-mCherry. After two weeks, the rats had BDL or sham ligation surgery. Four weeks later, SON regions were collected by punches for western blot analysis. Fluid accumulation, liver and body weight were recorded to verify if the rats develop liver failure. Blood samples were collected to measure plasma osmolality, hematocrit, and circulating AVP concentration. Data were analyzed by one-way ANOVA with Bonferroni comparisons. The BDL rats had significant increases in liver to body weight ratio (p< 0.05; 6-9) compared to sham rats indicating liver failure and ascites with bile duct ligation. BDL rats with scrambled injections (BDL SCR) developed hyponatremia with significant decrease in plasma osmolality and hematocrit along with significant increase in plasma AVP concentration (all p< 0.05; 6-9) compared to sham groups. This is the first study to show that phosphorylation of TrkB (pTrkBY515) is significantly increased along with significant decrease in phosphorylation of KCC2 (pKCC2S940) in BDL SCR rats compared to the sham rats (p< 0.05; 6-8). Knockdown of BDNF in the SON of BDL rats (BDL shBDNF) prevented the rats from developing hyponatremia and caused significant increase in plasma osmolality and hematocrit compared to BDL SCR rats (p< 0.05; 6-9). The BDL shBDNF rats had significant (p< 0.05; 6-9) decreases in plasma AVP concentration compared to BDL SCR rats. shBDNF injections in the SON of BDL rats significantly blocked the increase in TrkB phosphorylation and decrease in KCC2 phosphorylation (p< 0.05;6-8) associated with bile duct ligation. mCherry expression was used to verify the specificity of stereotaxic injections. Rats that did not had successful virus injections in the SON were analyzed separately. The results indicate that BDNF produced in the SON contributes to increased AVP secretion and hyponatremia during liver cirrhosis.