Homeostatic synaptic downscaling is a compensatory form of synaptic plasticity that stabilizes neuronal activity in response to sustained synaptic potentiation. Here, we investigated the involvement of Toll-like receptor (TLR) signaling in synaptic downscaling induced by bicuculline, a GABAA receptor antagonist. In cultured hippocampal neurons, bicuculline altered mitochondrial morphology and reduced the colocalization of TLR9 with mitochondrial DNA, an endogenous TLR9 ligand. Pharmacological inhibition of TLR9 mimicked bicuculline-induced reductions in surface expression of the AMPA receptor subunit GluA1 (sGluA1), and co-application of bicuculline and TLR9 inhibitors produced no additive effect, suggesting a shared pathway. Inhibition of SARM1, a downstream effector of TLR9, prevented bicuculline-induced sGluA1 reduction. Bicuculline also upregulated another upstream effector of SARM1, TLR7, which is negatively regulated by TLR9. TLR7 inhibition blocked, whereas TLR7 activation mimicked, bicuculline-induced decreases in sGluA1. These findings indicate that the shift from TLR9 to TLR7 signaling contributes to homeostatic synaptic downscaling.
Activation of the cholinergic anti-inflammatory pathway (CAP), including vagus nerve stimulation (VNS), suppresses inflammatory cytokine production and has shown efficacy in experimental sepsis when applied prior to sepsis induction. However, VNS before sepsis onset is rarely feasible in clinical practice, and whether VNS remains effective after sepsis onset is unclear. Using a lipopolysaccharide-induced mouse sepsis model, we evaluated VNS applied before and after sepsis onset. VNS applied before sepsis onset significantly reduced plasma TNF-α levels, whereas VNS applied after sepsis onset failed to suppress inflammation. Although post-sepsis VNS did not worsen hypotension or organ dysfunction, it was associated with increased lactate accumulation. Notably, fluid resuscitation restored the anti-inflammatory effect of VNS and reduced lactate levels. These findings suggest that CAP responsiveness depends on the host’s physiological state. Impaired circulation and metabolic stress may limit CAP activation during sepsis, while circulatory stabilization appears necessary for effective inflammatory control.
