Microglial regulation of social stress susceptibility in Female Rats

There is a significant link between stress and the development of psychiatric diseases such as depression and anxiety. In addition, females exhibit increased susceptibility for the development of depression and anxiety; however, the reasons for this are unclear. The locus coeruleus (LC) is known to regulate the stress response, in part, through its projections to the central amygdala (CeA). All LC neurons contain norepinephrine (NE) and thus project NE to the CeA, which in turn activates the stress‐sensitive corticotropin‐releasing factor (CRF), which can mediate the anxiety‐like response to stress. Importantly, social stress elicits robust activation of the brain's resident immune cells (microglia) and overactivation of these microglia are implicated in stress‐related disorders. These studies seek to understand the molecular cascade between the stress sensitive LC‐NE system and the CeA. Therefore, here we test the hypothesis that microglia within the LC may be regulating, in part, the stress‐evoked LC to CeA projections. Our lab has identified that a witness‐stress (WS) paradigm is capable of producing stress related behavioral dysfunction in female rats including anxiety‐like behaviors. In this WS paradigm, a female rat is placed behind a clear partition in a dominant and territorial male's cage (termed the “resident”) and observes social defeat between the resident and a smaller male “intruder”. The female “witness” responds to this exposure with increased stress‐evoked burying behavior, stress hormone release and increased heart rate and blood pressure. Our previous work identified a method that was capable of reducing the inflammatory cells in the LC by administering mannosylated liposomal clodronate (Encapsula NanoSciences). We identified that 25 μg of clodronate (CLD) administered directly into the LC via the mannosylated lipid‐based nanoparticle was selectively toxic to microglia of the LC. When administered intra‐LC, this dose of CLD decreased microglial expression by approximately 50%. (LC microglial number ±SEM; vehicle: 55.3 ± 5.3, 25 μg CLD: 27.4 ±2.6). In the current study, rats received an empty liposome (vehicle) or 25 μg CLD three days before repeated WS or control (Con) exposure (5 daily exposures, 15 mins/day). 5 days after WS/Con, rats in all stress/treatment conditions were either euthanized under resting conditions or 1 hour following a WS exposure. Blood and brain tissue were collected. Preliminary results demonstrate elevated IL‐1B assessed in the LC 5 days after stress in vehicle‐treated witness stressed rats, an effect that was absent in CLD‐treated rats. We also identified that the WS‐induced anxiety‐like response (burying) was reduced in the rats that were treated with intra‐LC CLD. Ongoing studies are assessing NE and CRF expression in the CeA in these studies. These data lay the foundation to begin to explain the underlying causes of stress susceptibility in females. By understanding neuroinflammation's role in susceptibility in regions such as the LC and it's stress sensitive projections, this project could have implications to improve treatments of psychiatric disorders in females.