Distinct Neuronal Excitability in the Paraventricular Nucleus of Hypothalamus after Baroreflex Challenge in Salt‐Loaded Rats

Abstract

High salt intake is a determinant factor for hypertension in humans and animals’ models. Many studies indicate that the paraventricular hypothalamus (PVH) contributes to the regulation of body fluids homeostasis and neural control of circulation in salt-induced hypertension, with the involvement of the baroreflex function, as part of a hypothalamic-brainstem circuitry. Here, our aim was to evaluate the autonomic balance and the neuronal activity of PVH in response to baroreflex activation of animals subjected to high salt intake. Ethical protocols were approved by the Institutional Animal Care and Use Committee (n.127/2017) of the ICB/USP. Male Wistar rats (300-400 g) received a solution of NaCl 2% in replacement of the tap water (SALT) for 7 days, while the control (CT) received only tap water. On the sixth day, rats had femoral artery catheterized for monitoring of the MAP and HR, and the femoral vein to infusion of drugs. The next day, baroreflex function was evaluated by bolus injections of phenylephrine (PHE; 0.1-12,8 µg/Kg) and sodium nitroprusside (NPS: 0.2-25.6 µg/Kg)]. Sigmoidal logistic equation was used to analysis baroreflex function. The cardiac autonomic balance was evaluated with injection of muscarinic receptor antagonist (ATROP; methylatropine bromide; 2 mg/Kg) and 15 min later with beta-1-adrenergic receptor antagonist atenolol (ATN; 4 mg/Kg). The neuronal activity in the PVH was evaluated by the FOS expression during baroreflex function challenge [a ramp infusion PHE (18-22 µg/Kg/min)]. After 90 min the animals were euthanized, and brain tissue fixed for immunoperoxidase procedures, and subsequent cells counting analysis. Data were expressed as mean±SEM, analyzed by Student's t-test or ANOVA two-way (*P<0.05). Salt-loading elicited an increase in the MAP (SALT: 115±3 vs. CT: 109±3* mmHg, t-test, n=11) and HR (SALT: 406±10 vs. CT: 318±9* b/min, t-test, n=11) followed by an attenuation on the range of the baroreflex function (SALT: 169±14 vs. CT: 233±22* b/min, t-test, n=6). As for the autonomic balance salt-loaded rats have shown an increase in the sympathetic tonus (SALT: 104±11 vs. CT: 59±7* Δb/min, t-test, n=6), but no change in the parasympathetic tonus (SALT: -68±13 vs. CT: -48±11 Δb/min, t-test, n=6). The activation of the arterial baroreflex resulted in an increase in the number FOS neurons in the PVH in SALT group (SALT-PHE: 1,012±96*) compared to CT-PHE: 345±57, (ANOVA: *P<0,05, n=4), but no change among CT-PHE vs. CT-NOT-PHE neither SALT-NOT-PHE. The large number of FOS-positive neurons occurred in medial level of the PVH in the SALT group, specifically in the neuron clusters of the lateral magnocellular (90%), dorsal cap (83%), medial parvicellular (71%) and ventral part (66%) when compared to the other groups. Taken together, these results show that 7 days of high salt intake elicits hypertension, impairment of baroreflex function and increase in the sympathetic tone to the heart with activation of distinct neuronal populations in the PVN. The precise neuronal mechanisms involved in these responses are still under investigation.