Rat area postrema microglial cells act as sensors for the toll-like receptor-4 agonist lipopolysaccharide

The area postrema (AP) represents the sensory circumventricular organ lacking endothelial blood-brain barrier function in direct vicinity to the 4th cerebral ventricle. Administration of lipopolysaccharide (LPS), as opposed to muramyldipeptide (MDP) or fibroblast-stimulating lipopeptide-1 (FSL-1), caused fast transient rises in intracellular calcium concentrations in 10–12% of the microglial cells investigated in a primary microculture of the rat AP, with limited responses of neurons, astrocytes and oligodendrocytes. In addition, a marked release of the pro-inflammatory cytokines TNF-α and IL-6 was determined in LPS-treated AP microcultures. Pre-incubation of AP microcultures with LPS for 18 h suppressed LPS-induced calcium signaling and attenuated cytokine secretion. Evidently, the AP can act as a sensor for circulating LPS and has the capacity to develop endotoxin-tolerance.