Role of the activated microglia in hippocampal synaptic plasticity alteration in in vitro inflammatory models

Abstract Background Microglia cells are primary players in neuroinflammation, which plays a key role in the pathogenesis of Alzheimer's disease (AD). Under pathological inflammatory conditions, microglia activation leads to synaptic alterations thereby influencing the progression of AD. Increased activation of microglial colony stimulating factor 1 receptor (CSF1R) can lead to pathological microglia activation in experimental models of AD (Olmos‐Alonso et al., 2016). Using electrophysiological and immunohistochemical analysis, we investigated hippocampal synaptic transmission and plasticity in experimental models of neuroinflammation. We then tested whether pharmacological manipulation of microglia activation could restore the observed neuroplasticity alterations. Method C57BL6/J mices of both sexes aged between 30‐ and 40‐days were used. Employing electrophysiological approaches ( field and whole cell patch clamp ), we recorded in area CA1 in response to stimulation of Schaffer collaterals. We evaluated Long‐Term Potentiation (LTP) induced by theta‐burst stimulation measuring field Excitatory Post‐Synaptic Potentials (fEPSPs) 1 hour after tetanization. We also investigated paired‐pulse ratio (PPR) and spontaneous EPSCs (sEPSCs). Two in vitro inflammatory models were obtained applying Lipopolysaccharide (LPS) (10 μg/ml) or Aβ 1‐42 oligomers (200 nM). As a control, we tested the effect of scrambled Aβ 1‐42. To inhibit the activation of microglia we used Pexidartinib (PLX‐3397) (10μM) and Minocycline (500 nM). For immunohistochemical experiments free‐floating sections were incubated with primary antibodies overnight at 4°C followed by Alexa‐conjugated secondary antibody and DAPI for 2 hours at room temperature in the dark. VGLUT1, Iba1 and CD68 were used as synaptic marker in a triple staining to study pathological pruning and its reversal by PLX‐3397. Result Both inflammatory models induced LTP impairment, that was restored by application of a no‐specific blocker, Minocycline, or a specific inhibitor of activated microglia, PLX‐3397.Using whole cell patch clamp recordings, we observed that Aβ produces a reduction in glutamatergic transmission, evaluated as amplitude or frequency of sEPSCs and PPR, that is prevented by PLX‐3397. The data were confirmed using imaging studies, by which count of CA1 microglia and spine remodeling were evaluated. Conclusion Our results demonstrate a functional role of activated microglia in synaptic plasticity alteration that occurs in neuroinflammation suggesting CSF1R as new pharmacological target for AD treatment.