Microglia drive diurnal variation in susceptibility to inflammatory blood-brain barrier breakdown

The blood-brain barrier (BBB) is critical for maintaining brain homeostasis but is susceptible to inflammatory dysfunction. Permeability of the BBB to lipophilic molecules shows circadian variation due to rhythmic transporter expression, while basal permeability to polar molecules is non-rhythmic. Whether daily timing influences BBB permeability in response to inflammation is unknown. Here, we induced systemic inflammation through repeated lipopolysaccharide (LPS) injections either in the morning (ZT1) or evening (ZT13) under standard lighting conditions, then examined BBB permeability to a polar molecule, sodium fluorescein. We observed clear diurnal variation in inflammatory BBB permeability, with a striking increase in paracellular leak across the BBB specifically following evening LPS injection. Evening LPS led to persisting glia activation and inflammation in the brain that was not observed in the periphery. The exaggerated evening neuroinflammation and BBB disruption were suppressed by microglial depletion or through keeping mice in constant darkness. Our data show that diurnal rhythms in microglial inflammatory responses to LPS drive daily variability in BBB breakdown and reveals time-of-day as a key regulator of inflammatory BBB disruption. ### Competing Interest Statement The authors have declared no competing interest.