Abstract 083: Neurons in the Organum Vasculosum of the Lamina Terminalis Contribute to Salt-sensitive Hypertension

Abstract

Excess dietary salt intake raises plasma and cerebrospinal fluid NaCl concentrations to elevate sympathetic nerve activity (SNA) and arterial blood pressure (ABP). Changes in extracellular NaCl concentrations are sensed by neurons in the organum vasculosum of the lamina terminalis (OVLT) - a circumventricular organ that lacks a complete blood-brain barrier. The purpose of the present study was to investigate the hypothesis that salt-sensitive hypertension was mediated, in part, by an elevated activity of OVLT neurons. Dahl-Salt-Sensitive or Sprague-Dawley rats (8-10 weeks) were fed 0.5% or 4.0% NaCl diets for 3-4 weeks. First, in vivo single-unit recordings demonstrate the discharge of OVLT neurons in Dahl-Salt-Sensitive rats was higher after a 4.0% versus 0.5% NaCl diet (4.1±0.4 Hz vs 1.9±0.3 Hz, n=6 per group, P<0.05). OVLT neuronal discharge of Sprague-Dawley rats was not different after a 4.0% or 0.5% NaCl diet (2.1±0.4 Hz vs 1.7±0.3 Hz, n=6-9 per group, P>0.5). In a second set of experiments, injection of hypertonic NaCl (1.0M NaCl, 20nL) into the OVLT produced significantly greater increases in lumbar SNA (131±6% vs 116±3%, n=4 per group, P<0.05) and mean ABP (14±2 vs 8±2 mmHg, n=4 per group, P<0.05) of Dahl-Salt-Sensitive rats fed 4.0% versus 0.5% NaCl respectively. Sprague-Dawley rats fed 4.0% versus 0.5% NaCl exhibited responses of smaller magnitude for both lumbar SNA (115±4 vs 108±3%, n=4 per group, P<0.05) and mean ABP (9±2 vs 6±2 mmHg, n=4 per group, P<0.05). Interestingly, the duration of the response was much longer in Dahl-Salt-Sensitive versus Sprague-Dawley rats (data not shown). Finally, inhibition of neuronal activity by injection of the GABA agonist muscimol (5mM, 20nL) into the OVLT produced a significantly greater fall in lumbar SNA (-25±4% vs -11±3%, n=4 per group, P<0.05) and mean ABP (-19±4 vs -6±2 mmHg, n=4 per group, P<0.05) of Dahl-Salt-Sensitive rats fed 4.0% versus 0.5% NaCl, respectively. Injection of muscimol into the OVLT of Sprague-Dawley rats did not significantly affect SNA or mean ABP. Collectively, these findings suggest a high salt diet increases the activity of OVLT neurons to elevate SNA and ABP in salt-sensitive hypertension.