Repeated psychosocial stress at night, but not day, affects the central molecular clock

We have recently demonstrated that the outcome of repeated social defeat (SD) on behavior, physiology and immunology is more negative when applied during the dark/active phase as compared with the light/inactive phase of male C57BL/6 mice. Here, we investigated the effects of the same stress paradigm, which combines a psychosocial and novelty stressor, on the circadian clock in transgenic PERIOD2::LUCIFERASE (PER2::LUC) and wildtype (WT) mice by subjecting them to repeated SD, either in the early light phase (social defeat light = SDL) or in the early dark phase (social defeat dark = SDD) across 19 days. The PER2::LUC rhythms and clock gene mRNA expression were analyzed in the suprachiasmatic nucleus (SCN) and the adrenal gland, and PER2 protein expression in the SCN was assessed. SDD mice showed increased PER2::LUC rhythm amplitude in the SCN, reduced Per2 and Cryptochrome1 mRNA expression in the adrenal gland, and increased PER2 protein expression in the posterior part of the SCN compared with single-ho...