Abstracts

Rationale: Traumatic brain injury, a leading cause of acquired temporal lobe epilepsy, results in cellular and synaptic changes including dentate cell loss and hyperexcitability. Release of endogenous molecules from disrupted cells and extracellular matrix can activate toll-like receptors (TLRs) involved in innate immune responses. Brain injury results in an increase in TLR4 expression within 4 hours followed by a return to control levels 7 days post-injury. Since TLR4 signaling has been implicated in hippocampal hyperexcitability in epilepsy, we examined whether increases in TLR4 expression contribute to early increases in dentate excitability after brain injury.Methods: Young adult male Wistar rats (25-27 day old) were subject to sham- or moderate (2 atm) lateral fluid percussion injury (FPI) (Gupta et al., 2012). Immunostaining was used to determine the cell-type specific expression of TLR4. Afferent-evoked granule cell field responses were recorded in acute hippocampal slices prepared 3-7 days after FPI. Whole-cell current and voltage clamp recordings were obtained from granule cells and dentate hilar neurons in slices incubated in control ACSF or in LPS-RS for 2 hours.Results: Expression of TLR4 in the dentate molecular layer and hilus was increased 24 hours after FPI. Hilar expression of TLR4 co-localized primarily with NeuN-positive neurons in both control and injured rats. Hilar neuronal profiles expressing somatostatin but not those expressing parvalbumin showed co-labeling for TLR4 indicating cell-type specific expression of TLR4 in GABAergic interneurons. LPS-RS decreased the amplitude of the perforant path-evoked granule cell population spike amplitude (amplitude in mV evoked by a 4mA stimulation, ACSF: 1.29 0.27, LPS-RS: 0.38 0.11 mA, p<0.05, n=15) in slices from FPI rats but not in sham-controls. The ability of LPS-RS to reduce the granule cell population spike amplitude in slices from injured rats was not altered in the presence of the selective NMDA receptor antagonist AP-5. In whole cell recordings, incubation in LPS-RS reduced the frequency and amplitude of spontaneous EPSCs and reduced firing in response to current injections in granule cells from injured rats. The ability of LPS-RS to reduce the post-FPI increase in granule cell population spike amplitude was decreased in the presence of the GABAA receptor antagonist SR95531 (10 M) indicating that TLR4 signaling modifies dentate inhibition. The frequency and amplitude of sEPSCs in dentate interneurons was increased after FPI. Similar to granule cells, LPS-RS significantly reduced sEPSC frequency in dentate interneurons after FPI.Conclusions: These findings demonstrate that TLR4 signaling contributes to early enhancement of dentate excitability after brain injury through NMDAR-independent mechanisms that likely involve complex cell-type specific modulation of excitatory and inhibitory circuits. Support: CURE Foundation and NJCBIR 09.003-BIR1 to V.S and NJCBIR 11-3223-BIR-E-O to Y.L