**4. Conclusion**

Given that microglia play major roles in brain inflammation and express many receptors for neurotransmitters in addition to stress hormones, we speculate that microglia are deeply involved in the pathophysiology of stress-related psychiatric disorders. In this chapter, we described findings regarding microglia ever obtained from animal and human studies of stress-related psychiatric disorders. The animal study is an indispensable research method for stress-related psychiatric disorders because it can control the type and intensity of stress to be applied and the molecular mechanism related to the pathological condition and therapeutic mechanism can be investigated in detail. There is a limit to extrapolating human higher brain dysfunction from animal behaviors. On the other hand, the greatest advantage of a humansamples study is the ability to capture detailed psychopathological symptoms. There are also drawbacks in human imaging studies, postmortem brain studies, and studies using peripheral blood. Current PET imaging with TSPO ligands can measure only a small portion of microglial changes and has an inadequate resolution. Postmortem brain study is not easy to carry out due to various technical and ethical constraints. It is not clear how far we can infer events in the central nervous system from the results of studies using peripheral blood. It has been shown that human iMG cells made from monocytes in our laboratory share many features with microglia [56, 57]. We expect that iMG cells play a role as a biomarker for psychiatric disorders, and have

*Stress, Microglial Activation, and Mental Disorders DOI: http://dx.doi.org/10.5772/intechopen.103784*

the potential to reproduce in vitro the dynamics of microglia actually occurring in the brain. We are planning to generate iMG cells in patients with stress-related psychiatric disorders and perform morphological and molecular analysis, and further analyze the correlation with clinical findings such as diagnosis, various test scores, severity. We hope that progress in bidirectional research between animals and humans by making the best use of the strengths of each research method and improving the weaknesses elucidates the role of microglia in stress-related psychiatric disorders and develops treatment targeting them.
