Abstract 107: Neurons in the Organum Vasculosum of the Lamina Terminalis Sense NaCl and Angiotensin II to Regulate Sympathetic Nerve Activity and Arterial Blood Pressure

Abstract

The organum vasculosum of the lamina terminalis (OVLT) contains specialized neurons that sense changes in extracellular NaCl concentrations to regulate sympathetic nerve activity (SNA) and arterial blood pressure (ABP). However, OVLT neurons also abundantly express angiotensin II (AngII)-type I receptors. Therefore, we performed a series of in vitro and in vivo studies in adult male Sprague-Dawley rats to determine whether OVLT neurons sense both NaCl and AngII to impact SNA and ABP. First, in vitro whole-cell recordings revealed the majority of OVLT neurons (66%, 23/35) showed an increased discharge frequency to both +7.5mM NaCl (1.1±0.2 to 2.2±0.3Hz, P<0.001) and 100pM AngII (1.3±0.3 to 2.2±0.3Hz, P<0.001). Second, in vivo single-unit recordings demonstrate the majority of OVLT neurons (61%, 11/18) displayed an increase in cell discharge to intracarotid injection of 0.5M NaCl (3.6±0.6 to 9.0±1.6Hz, P<0.01) and 200ng AngII (3.2±0.7 to 9.2±1.9Hz, P<0.01). Third, optogenetic inhibition of OVLT neurons (rAAV2-CaMKII-eNpHR3.0-mCherry, 5x10 12 particles/mL, 561nm, 10mW) attenuated thirst responses to IV infusion of 2M NaCl (1.25mL/30min; Laser OFF: 6.1±0.5mL vs ON: 2.0±0.7mL, P<0.05) and AngII (Laser OFF 7.2±0.7mL vs ON: 2.2±1.2mL; n=5 per group, P<0.05). Fourth, OVLT injection (20nL) of 0.5M NaCl or 10pmol AngII increased SNA (121±6% or 117±5%, n=4/group; P<0.05 respectively) and mean ABP (7±1 or 9±2mmHg, n=4/group; P<0.05 respectively). To determine whether chronic activation of OVLT neurons produces hypertension, rats received an OVLT injection of rAAV2-hSyn-HM3D(Gq)-mCherry (3x10 12 particles/mL, 50nL) and given access to 0.1% NaCl chow and 0.9% NaCl drinking solution. After 4 weeks, daily administration of clozapine-N-oxide to the drinking solution (3mg/kg/day) significantly increased 24-h fluid intake (baseline: 60±10mL vs Day 7: 178±29mL; n=6, P<0.01) and 24-h mean ABP (baseline: 97±2mmHg vs Day 7: 108±1 mmHg; n=6, P<0.05). Ganglionic blockade with hexamethonium (30mg/kg, ip) produced a greater fall in mean ABP at Day 7 (baseline: -38±6 mmHg vs Day 7: -53±5 mmHg, P<0.05). These findings suggest OVLT neurons sense both NaCl and AngII to regulate body fluid homeostasis and SNA to produce a sympathetically-mediated hypertension.