Salt Loading Activates Microglia and Increases Grooming Time and Time Spent Mobile in the Forced Swim Test: Is High Salt Intake a Causal Factor Linking Hypertension and Anxiety?
Authors: Mitchell Nathan C. , Gilman Lee T. , Daws Lynette C. , Toney Glenn M. 
Published: November 2023

Hypertension and psychiatric disorders are major contributors to worldwide disability and are often co‐morbid; however, a shared pathogenic mechanism has not been identified. In models of neurogenic hypertension, high dietary salt (NaCl) drives microglial activation and stimulates production of proinflammatory cytokines. Cytokines promote action potential firing of pre‐sympathetic neurons in brain regions that regulate blood pressure, such as the hypothalamic paraventricular nucleus (PVN), and thereby likely contribute to development of neurogenic hypertension. Of special note is that neuroinflammation has likewise been implicated in the pathogenesis of depression and anxiety. Therefore, we hypothesized that high dietary salt concurrently drives microglial activation in blood pressure and mood regulating and synaptically connected brain regions, such as the PVN and amygdala, thereby promoting hypertension and yielding depression‐ and/or anxiety‐like behaviors. To test this, C57BL/6 mice were salt loaded (drinking 2 or 4% NaCl in water) for 1 week. Mice were then subjected to the forced swim test (FST), a screen for behavioral despair, and time spent grooming was assessed as a measure of anxiety‐like behavior. We found salt loading increased time spent mobile in the FST (P<0.01) and time spent grooming (P<0.01), both of which also result from acute administration of anxiogenic drugs, such as pentylenetrazole. Although findings suggest that salt loading induces anxiety‐like effects, additional studies utilizing alternative and complementary measures of anxiety‐like behavior are needed to affirm this interpretation. Hematology data support an anxiogenic action of salt loading since time spent mobile in the FST was positively correlated with the increase of plasma osmolality (P<0.01). Immunohistochemistry staining for ionized calcium‐binding adaptor molecule 1 (Iba1), a marker of microglial activation, was used to index tissue inflammation, and c‐Fos staining was used to identify transcriptionally activated neurons. Preliminary data showed salt loading increased Iba1 and c‐Fos positive cell staining in the PVN and the basolateral nucleus of the amygdala. To determine if behavioral effects of salt loading were dependent on microglial activation, minocycline, an inhibitor of microglial activation, was administered (100 mg/kg/day) in drinking water for 36 h before and during salt loading for 1 week. Preliminary data showed minocycline attenuated salt loading induced increases in time spent mobile in the FST and time spent grooming. These data suggest that even short‐term salt loading prompts microglial activation and transcriptional activation in limbic brain regions capable of producing anxiety‐like behavior. Future studies are needed to determine transcriptional consequences of microglial activation and to identify synaptic/intrinsic neuronal adaptations that generate this apparent salt loading induced anxiety phenotype in mice.

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Support: 5T32HL007446 (NCM), HL088052 (GMT)