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-housed control (SHC) and SDL mice. In contrast, PER2::LUC rhythms in the SCN of SDL mice were not affected. However, SDL mice exhibited a 2-hour phase advance of the PER2::LUC rhythm in the adrenal gland compared to SHC mice. Furthermore, plasma levels of brain-derived neurotrophic factor (BDNF) and BDNF mRNA in the SCN were elevated in SDL mice. Taken together, these results show that the SCN molecular rhythmicity is affected by repeated SDD, but not SDL, while the adrenal peripheral clock is influenced mainly by SDL. The observed increase in BDNF in the SDL group may act to protect against the negative consequences of repeated psychosocial stress.