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. While transporter-dependent efflux of some lipophilic substrates across the BBB shows circadian variation due to rhythmic transporter expression, basal transporter-independent permeability and leakage is nonrhythmic. Whether daily timing influences BBB permeability in response to inflammation is unknown. Here, we induced systemic inflammation through repeated LPS injections either in the morning (ZT1) or evening (ZT13) under standard lighting conditions; we then examined BBB permeability to a polar molecule that is not a transporter substrate, 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 as well as 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 reveal time of day as a key regulator of inflammatory BBB disruption.