Abstract 072: Brain Epithelial Sodium Channels on Organum Vasculosum of the Lamina Terminalis Neurons Mediate Sympathoexcitatory Pressor Responses and Neuronal Excitation by Hypertonic NaCl

Abstract

High dietary salt intake raises cerebrospinal fluid (CSF) [Na + ] in salt sensitive subjects to elevate sympathetic nerve activity (SNA), and arterial blood pressure (ABP). This occurs through excitation of NaCl-sensitive sites in the brain, including the organum vasculosum of the lamina terminalis (OVLT). Intriguingly, intracerebroventricular (ICV) pretreatment with benzamil (a non-voltage gated Na + channel blocker) also attenuates those systemic responses to central NaCl. Thus, I hypothesized that benzamil acts on NaCl-sensitive OVLT neurons to attenuate neuronal excitation and elevated SNA and ABP in response to hypertonic NaCl. To evaluate this hypothesis, lumbar SNA and ABP were measured in anesthetized adult rats in response to ICV infusion of 0.15, 0.5, and 1.0 M NaCl with and without prior OVLT microinjection of benzamil (5nmol per 20nL). ICV infusion of NaCl produced concentration-dependent increases in lumbar SNA (0.15M: 101±3%, 0.5M: 118±3%, 1.0M: 130±9% n=4 per group, P<0.05) and mean ABP (0.15M: 1±1mmHg; 0.5M: 5±1mmHg, 1.0M: 12±2mmHg). OVLT microinjection of benzamil significantly attenuated the increase in lumbar SNA (0.15M: 100±2%, 0.5M: 108±2%, 1.0M: 115±3% n=4 per group, P<0.05) and mean ABP (0.15M: 1±0mmHg; 0.5M: 2±1mmHg, 1.0M: 6±2mmHg). In a parallel set of experiments, in vitro whole-cell recordings of OVLT neurons in slices were performed to assess whether blockade of epithelial sodium channels (ENaC) attenuated NaCl-induced excitation. Bath application of +7.5mM NaCl increase action potential (AP) discharge of OVLT neurons (n=11) from 0.46±0.16 Hz to 1.09±0.26 Hz (P<0.05). Subsequent addition of an ENaC selective concentration of benzamil (0.5μM) reversed AP discharge (0.75±0.24 Hz, P<0.05). Isotonic 0.5μM benzamil did not significantly change NaCl-sensitive OVLT neuron (n=13) AP discharge from baseline (0.54±0.14 Hz to 0.60±0.16 Hz, P>0.05). Collectively, this data indicates that elevations in CSF NaCl concentrations excite OVLT neurons via ENaC to elevate SNA and ABP.