M. Julia García‐Fuster

Fas‐Associated protein with Death Domain (FADD), a key molecule controlling cell fate by balancing apoptotic versus non‐apoptotic functions, is dysregulated in post‐mortem brains of subjects with psychopathologies, in animal models capturing certain aspects of these disorders, and by several pharmacological agents. Since persistent disruptions in normal functioning of daily rhythms are linked with these conditions, oscillations over time of key biomarkers, such as FADD, could play a crucial role in balancing the clinical outcome. Therefore, we characterized the 24‐h regulation of FADD (and linked molecular partners: p‐ERK/t‐ERK ratio, Cdk‐5, p35/p25, cell proliferation) in key brain regions for FADD regulation (prefrontal cortex, striatum, hippocampus). Samples were collected during Zeitgeber time (ZT) 2, ZT5, ZT8, ZT11, ZT14, ZT17, ZT20, and ZT23 (ZT0, lights‐on or inactive period; ZT12, lights‐off or active period). FADD showed similar daily fluctuations in all regions analyzed, with higher values during lights off, and opposite to p‐ERK/t‐ERK ratios regulation. Both Cdk‐5 and p35 remained stable and did not change across ZT. However, p25 increased during lights off, but exclusively in striatum. Finally, no 24‐h modulation was observed for hippocampal cell proliferation, although higher values were present during lights off. These results demonstrated a clear daily modulation of FADD in several key brain regions, with a more prominent regulation during the active time of rats, and suggested a key role for FADD, and molecular partners, in the normal physiological functioning of the brain's daily rhythmicity, which if disrupted might participate in the development of certain pathologies.