Biological studies on psychiatric disorders started in the pharmacological field of therapeutic drugs, and once the drug targets were visualized by PET, quantitative data of dopamine D2 receptors in schizophrenia, and serotonin and noradrenaline transporters in depression were reported. However, the measured values of these neurotransmission components largely overlapped with those in normal volunteers and the diagnostic thresholds were unable to be defined. Relationships with personality traits were found in the diverse distribution of these neurotransmission components in normal volunteers, suggesting that neural transmission can be regulated depending on individual variation, such as personality and behavioral traits, and the values in those with psychiatric disorders are on a continuous spectrum. On the other hand, there have been several reports on specific sets of resting state functional connections in certain psychiatric disorders. However, for the biological confirmation of these connections, manipulation of the circuits in animal models is needed. DREADD is a new technique to manipulate neural circuits in large animals such as monkeys. We developed a PET imaging method to visualize the induced receptor and a new agonist, deschloroclozapine (DCZ). These new tools provide high-signal PET images and rapid response of the induced receptor. To evaluate the functions of neural circuits in the human brain, neurodegenerative disorders can be good models because the affected region can be visualized by PET using tau imaging. Biological studies on the mechanisms of psychiatric symptoms are important for both mental disorders and neurodegenerative disorders. Multimodal imaging studies together with translational animal research are important for confirmation of neural circuit functions.
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