We can postulate that vulnerability to cell death might be a common mechanism of these disorders. In the case of Parkinson’s disease, dopaminergic neurons are susceptible to cell death because of the metabolism of dopamine. On the other hand, insulin-producing cells of the pancreatic islet are susceptible to cell death due to overload by sustained elevation of glucose levels.Despite extensive studies, a brain structure responsible for bipolar disorder has not been identified yet. If we can identify the neuronal system impaired by mitochondrial dysfunction in bipolar disorder, it would help to develop a new treatment strategy and understand the pathophysiological mechanism. Our model mice may provide a clue to identifying such a brain structure. Because our transgenic mice showed characteristic circadian rhythm abnormality, the retino-hypothalamic pathway might be a candidate system for further investigation.