A new study finds that chronic stress leads to recruitment of myeloid macrophages to the hippocampus, weakening the brain’s capacity for spatial memory. This finding may have broad implications for sufferers of stress-induced disorders.
Stress is part of our lives. Whether it occurs over finishing a term paper five milliseconds before the deadline, navigating the complicated social undercurrents in the office, juggling your teenaged offspring’s wishes against the budgetary restrictions of your existence, or getting delayed on the freeway on your way to an interview / concert / meeting / [insert absolutely crucial event here] – we all experience stress to varying degrees. Our bodies are equipped to handle stress, by pumping more oxygen to the brain, making us more alert and responsive.
Unless that stress becomes a regular occurrence. Chronic stress, whether real or perceived, is bad for you. It can cause hypertension, heart problems, muscle pain, sleep disorders - and has a profound effect on your mood and brain functions. Social anxiety and memory loss are often observed in war veterans, victims of repeated bullying, and people with extremely stressful jobs. A new study by researchers from The Ohio State University now sheds light on one surprising cause of the memory loss experienced after chronic stress. 
The researchers investigated the effects of chronic stress on memory and social interactions using a simple mouse model. First, they taught mice how to navigate a Barnes maze and how to locate a platform submerged at a specific constant location in milky liquid (a so-called Morris water maze). To induce chronic stress, the scientists then introduced an aggressive male alpha mouse into a nest of test animals every day for two hours for six days in a row. Then the mice’s performance in the mazes was measured again.
The stressed mice promptly performed badly in the mazes. However, the memory impairment was temporary and disappeared when re-measured 28 days after the stress had been induced. The scientists also found that the chronic stress caused inflammation in the dentate gyrus (DG) of the hippocampus of the mice’s brain, with plenty of CD45+ myeloid macrophages, that is, bone marrow-derived immune cells. Lo and behold, the hippocampus governs mood, social interactions, and, you guessed it, spatial memory.
The researchers then wondered whether preventing this inflammation would thwart the memory loss. They treated mice with minocycline, an anti-inflammatory antibiotic, and repeated the experiments. Jackpot: The stressed mice now performed just like unstressed control animals in the mazes.
The stressed animals also tended to avoid social contact, an effect that persisted for longer than 28 days. Again, the scientists tried minocycline. This time, this intervention did not work – the antibiotic-treated mice remained as aloof as stressed mice that did not receive the minocycline. Therefore, unlike the memory loss, the social avoidance was not caused by the influx of myeloid macrophages.
Bone marrow-derived cells such as the myeloid macrophages that are recruited to the brain by chronic stress can be differentiated in vitro from unprocessed bone marrow or bone marrow-derived mononuclear cells, readily available at HemaCare. Give us a call at (877) 397-3087 if this (or any other product from our extensive catalog) is of interest to you – we are happy to help.
What does this study mean for victims of chronic stress disorders? Anti-inflammatory interventions may be able to counteract some of the memory problems caused by chronic stress, and could therefore be added to therapeutic regimens in the future. Sounds unlikely? Consider this: the most common anti-inflammatory agent, aspirin, has been found to help in seemingly unrelated health problems (such as cardiovascular disease and melanoma) and improves the success rates of cancer immunotherapies. So, relax - keeping inflammation in check may also help you remembering where you left that darn key.
 McKim DB, Niraula A, Tarr AJ, Wohleb ES, Sheridan JF, Godbout JP. Neuroinflammatory Dynamics Underlie Memory Impairments after Repeated Social Defeat. J Neurosci. 2016 Mar 2;36(9):2590-604.