DIFFERENCES IN MICROGLIAL PRIMING BY LPS AND IFNΓ IN RAT HIPPOCAMPAL SLICE CULTURES

Activation of microglia through innate Toll-like receptors (TLR) recognizing pathogen- or damage-associated molecular patterns plays an important role in many CNS diseases. Recent evidence suggests that microglia have the ability to form an immune memory. After priming with a primary stimulus microglia are known to react differently to a secondary stimulus than unprimed microglia. We explore how priming of microglia by the cytokine interferon-gamma (IFNγ) and by lipopolysaccharide (LPS), a bacterial cell wall component stimulating TLR4, affects neuronal network function in rat organotypic hippocampal slice cultures. Microglia were primed by LPS at low (1ng/ml) and high (100ng/ml) concentrations or by IFNγ (100ng/ml) for 24h, then maintained in standard culture medium for 72h, and finally exposed to LPS (100ng/ml) as a secondary inflammatory stimulus. The effects of exposures were tested with electrophysiology by characterizing neuronal gamma oscillations (30-70 Hz) that are essential for higher cognitive functions, such as perception and memory. We find that (1) priming by IFNγ induces severe neuronal network dysfunction, with the majority of slice cultures showing no electrical activity. (2) Priming by LPS (1ng/ml) induces advanced network dysfunction reflected by neural bursting, beta oscillations (15-30 Hz) or low activity. (3) Priming by LPS (100ng/ml) had no significant effect. (4) Priming by IFNγ is associated with much higher release of nitric oxide compared with LPS whereas release of IL-6 and TNFα was similar. Our data suggest that the different effects of microglia priming on neuronal network activity depend on both ligand type and ligand concentration.