Concussive
brain injury results in neuronal degeneration, microglial activation and enhanced excitability in the hippocampal dentate gyrus, increasing the risk for
epilepsy and memory dysfunction. Endogenous molecules released during injury can activate innate immune responses including
toll-like receptor 4 (TLR4). Recent studies indicate that immune mediators can modulate neuronal excitability. Since non-specific agents that reduce TLR4 signaling can limit post-traumatic neuropathology, we examined whether TLR4 signaling contributes to early changes in dentate excitability after
brain injury. Concussive
brain injury caused a transient increase in hippocampal TLR4 expression within 4h, which peaked at 24h. Post-injury increase in TLR4 expression in the dentate gyrus was primarily neuronal and persisted for one week. Acute, in vitro treatment with TLR4
ligands caused bidirectional modulation of dentate excitability in control and brain-injured rats, with a reversal in the direction of modulation after
brain injury. TLR4 antagonists decreased, and agonist increased, afferent-evoked dentate excitability one week after
brain injury.
NMDA receptor antagonist did not occlude the ability of LPS-RS, a TLR4 antagonist, to decrease post-traumatic dentate excitability. LPS-RS failed to modulate granule cell
NMDA EPSCs but decreased perforant path-evoked non-
NMDA EPSC peak amplitude and charge transfer in both granule cells and mossy cells. Our findings indicate an active role for TLR4 signaling in early post-traumatic dentate hyperexcitability. The novel TLR4 modulation of non-
NMDA glutamatergic currents, identified herein, could represent a general mechanism by which immune activation influences neuronal excitability in
neurological disorders that recruit sterile inflammatory responses.