Brain c-Fos expression analysis showed that the number of c-Fos-positive cells was higher in the paraventricular thalamus and lateral hypothalamus and lower in the prefrontal cortical regions, nucleus accumbens shell, dorsolateral septal nucleus, hippocampal regions, and ventromedial hypothalamus after exposure to the behavioral paradigm assessing depression-related behavior in 5-HTT−/− mice than in 5-HTT+/+ mice. Here, FOS is linked to depressive disorder.