Hypertension and Fear‐Related Neuropsychiatric Disorder Co‐Morbidity: Is Body Fluid Hyperosmolality a Required Link?

Abstract

High salt (NaCl) intake (HSI) is a physiological stressor that can increase the prevalence and severity of arterial hypertension (HTN), which is often comorbid with fear-related psychiatric disorders. Here, we sought to determine the contribution of HSI in HTN and fear disorder co-morbidity. Preliminary studies in male C57Bl6 mice established that salt loading (SL) introduced by replacing drinking water with a 4% NaCl solution induced significant HTN (+ 21 ± 5 mmHg, n=3) (P=0.0437) while consumption of high salt diet (HSD) by provision of chow containing 4% NaCl did not (+2 ± 5 mmHg, n=3) (P=0.06042). We formulated the hypothesis that HSI will enhance fear memory retrieval when accompanied by HTN but will not do so when HSI fails to raise mean arterial pressure (MAP). To test this, we subjected male C57Bl6 mice to 5 weeks of HSD or a normal salt diet (NSD, 0.4% NaCl). Treated mice then underwent contextual fear conditioning and retrieval testing 24 h later. Fear behavior was quantified as the percentage of time spent freezing (i.e., posturally immobile). A third cohort of HSD mice underwent surgical implantation of an osmotic mini-pump to deliver the pro-hypertensive hormone angiotensin II (Ang II, 600 ng/kg/min) or vehicle (Veh) for 14 d prior to fear conditioning. A subset of mice were instrumented with a radio telemetry transmitter to monitor MAP during AngII/Veh treatment. Blood was collected upon completion of experiments to measure serum osmolality. Analysis by unpaired t-test reveal that MAP was significantly increased by treatment with AngII (+28 ± 3 mmHg, n=5) compared to Veh (-1 ± 1 mmHg, n=2) (P=0.0014). During contextual fear memory retrieval, freezing behavior was not different between normotensive HSD mice and normotensive NSD mice (HSD: 45± 6%, n=10, NSD: 44± 7%. n=9) (P=0.9194). Furthermore, treatment of HSD mice with AngII did not significantly increase freezing behavior compared to Veh (AngII: 58 ± 3%, n=9, Veh: 62 ± 16%, n=3) (P=0.7273). These results suggest that unlike SL, which induces HTN and promotes contextual fear memory, HSD does not, even when HTN is induced by co-treatment with AngII. Overall, findings suggest that SL promotes HTN and fear memory through parallel, independent mechanisms, and that consumption of high salt-containing food does not increase fear memory regardless of whether accompanied by HTN or not. Published evidence indicates that unlike HSD alone, or in combination with AngII, SL significantly increases body fluid osmolality. This suggests that body fluid hyperosmolality, rather than salt intake per se, contributes to HTN and fear-related disorder co-morbidity. Consistent with this possibility, serum osmolality in the present study was unaltered relative to NSD (316 ± 1 mOsm, n=27) both by HSD alone (313 ± 2 mOsm, n=19) (P=0.1401) and by HSD with AngII (320 ± 2 mOsm, n=10) (P=0.0801).