Social stress exposure precipitates the onset of anxiety disorders and women are more likely to suffer from these stress-related pathologies. The stress-sensitive neural mechanisms promoting anxiety in females are not well understood, thus preventing the development of therapies targeting this hyper-aroused state. The locus coeruleus (LC) is a stress-sensitive brain region known to facilitate behavioral and cardiovascular (CV) responses to social stress, two endpoints altered under conditions of hyperarousal. Importantly, social stressors induce microglial activation, and the resulting neuroinflammation plays a causative role in generating anxiety-like behavior. However, it is unknown whether pharmacological manipulation of microglia within the LC serves as a method to regulate stress susceptibility in a female population and is tested in two ways in this study. In study 1 we use lipopolysaccharide (LPS), a potent microglial activator, and in study 2, we administer mannosylated lipid-based nanoparticles containing clodronate (CLD), a compound toxic to microglia, to manipulate microglial expression in the presence and absence of witness stress (WS). In study 1, following recovery from implantation of bilateral LC cannulae, female rats were treated with intra-LC vehicle or LPS (0, 1, 3, or 10 μg/side). After a 45-min rest period, rats were briefly handled, placed back into their home cage for a 15-min non-stress control period. Immediately following, rats were subjected to witnessing an aggressive social defeat encounter between two male rats (WS). Behavior was video-recorded during control and WS, and blood and brain were collected following WS. This study demonstrated that WS increased stress-evoked burying (an anxiety-like response) and intra-LC LPS dose dependently augments this anxiety-like response during WS. Intra-LC LPS (1 and 3μg) reduced WS-evoked rearing behavior but had no effect on freezing. Additionally, intra-LC LPS (3 μg) increases plasma corticosterone and interleukin-1β. Alternatively, to investigate whether decreased microglial expression in the LC inhibits these stress-evoked behaviors, study 2 examined the effect of partial microglial depletion (intra-LC CLD) on behavioral and CV responses to repeated WS. Following recovery from implantation of CV transmitters (HD-S11, DSI) and bilateral LC cannulae, female rats were treated with intra-LC vehicle or CLD (0 or 25 μg/side). 3 days post injection, rats were subjected to WS for 15 min/day on 5 consecutive days. While WS exposure increased anxiety-like burying, partial microglial depletion in the LC attenuated this response. Interestingly, intra-LC CLD's effect on hyperaroused behavioral responses was independent of sympathetically driven CV responses as there was no effect on heart rate or blood pressure on day 1 of WS, but there was an exaggerated pressor response following repeated stress exposure (day 5). Taken together, these studies highlight a novel role for microglia in regulating behavioral responses to WS and indicate that microglial targeting may be effective in regulating social stress susceptibility.
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