Pharmacological Manipulation of Microglia Regulates Social Stress Susceptibility Among Females in a Region‐Specific Manner
Authors: Pope Brittany S. , Bouknight Samantha , Harrington Evelynn , Martines‐Muniz Gustavo , Augenblick Lee , Mott Sarah , Wood Susan K. 
Published: May 2022

Social stress is a major risk factor for anxiety disorders and co‐morbid cardiovascular (CV) dysfunction. Women are particularly susceptible to these stress‐related pathologies, yet the etiology of this is unclear. Recently, neuroinflammation has been linked with anxiety disorders, yet the therapeutic value of region‐specific microglial manipulation, and thus precise neuroinflammatory modulation, is not known. Our previous findings indicate that female rats repeatedly exposed to witnessing an aggressive social defeat episode between two male rats (WS) develop an anxiety‐like phenotype and robust, long‐lasting increases in inflammatory cytokines in stress‐sensitive brain regions. One region that facilitates behavioral and CV responses to social stress is the locus coeruleus (LC), but the role for LC microglia in regulating stress susceptibility is unknown. Here, we use targeted intra‐LC infusions of lipopolysaccharide (LPS; study 1), a potent microglial activator, and mannosylated lipid‐based nanoparticles containing clodronate (m‐CLD; study 2), a compound toxic to microglia, to manipulate LC microglial activation or expression, respectively, in the presence and absence of WS. In study 1, female rats were treated with intra‐LC vehicle or LPS (0, 1, or 3 μg/side). One hour later, rats underwent 15 min of WS or non‐stress control handling. Blood and brains were collected immediately following WS/control. In study 2, female rats were treated with intra‐LC or intra‐central amygdala (CeA) vehicle or m‐CLD (0 or 25μg/side) 3 days prior to undergoing 5 daily 15‐min WS exposures. In both studies, WS and control sessions were video‐recorded. Freezing, rearing, and burying were quantified, and anhedonia was assessed using a two‐bottle choice sucrose preference test. CV telemetry was used to assess sympathovagal balance via measures of heart rate variability (HRV). Brain and plasma cytokines were quantified to assess the effects of stress and m‐CLD on central and peripheral inflammation. Intra‐LC LPS dose dependently augmented the anxiety‐like burying response induced by WS. The higher (3μg) dose of intra‐LC LPS also increased plasma corticosterone (CORT) and IL‐1β. Conversely, intra‐LC m‐CLD attenuated the anxiety‐like burying response to WS. Intra‐LC m‐CLD did not impact stress‐evoked vagal withdrawal indicated by HRV. Even though m‐CLD reduced neuroinflammation (IL‐1β) in both the LC and CeA, the effects of m‐CLD on stress‐evoked burying were region specific as intra‐CeA m‐CLD had no effect on stress‐evoked behavior nor CV function. Repeated WS increased plasma CORT, but this response was not impacted by intra‐LC nor intra‐CeA m‐CLD. Importantly, this study identified for the first time that m‐CLD can be used to pharmacologically reduce microglial expression in a discrete, region‐specific functional manner. Taken together, these studies highlight a novel role for microglia within the LC in regulating the hypervigilant responses to WS and indicate that microglia may serve as an effective pharmacological target for regulating social stress susceptibility in females.