Abstracts

Rationale:
Reorganization of neuronal circuits in the dentate gyrus (DG) develops after traumatic brain injury (TBI). Dentate granule cells (DGCs) are generated throughout life and normally integrate into neuronal circuits in the DG, but aberrant circuit integration of granule cells born during the peri-injury period contributes to hyperexcitability in posttraumatic epilepsy (PTE). Conversely, excitation of a subset of hilar GABAergic interneurons by DGCs is upregulated after TBI, which may compensate for GABAergic interneuron loss. However, it remains unclear how DGCs born at different ages contribute to increased excitation of GABAergic interneurons and recurrent excitation in TBI. We hypothesized that activation of adult born DGCs induces both larger inhibition and recurrent excitation in DGCs through their increased innervation of GABAergic interneurons and aberrant innervation of other granule cells, respectively.
Method:
Gli1CreERT2::channelrhodopsin 2 (ChR2) mice were used to selectively express ChR2 in newborn DGCs by Tamoxifen treatment during one of three periods: Postnatal days 8-12 (early born) or for two weeks immediately before or after CCI (adult born). Mice were subjected to severe controlled cortical impact (CCI) at 7 weeks of age to induce focal unilateral brain injuries or craniotomy alone (sham control). At 15-17 weeks of age, we performed whole-cell patch-clamp recordings from ChR2-negative mature DGCs in hippocampal slices to determine whether optogenetic activation of ChR2-expressing DGCs from CCI-injured mice induced more evoked IPSCs or EPSCs, compared to those from sham controls.
Results:
Ipsilateral to CCI, optogenetic activation of ChR2-expressing DGCs evoked IPSCs in a significantly higher proportion of mature DGCs, compared to those from sham controls and contralateral hippocampi of CCI mice. These results were similar across all groups, with response magnitudes varying with DGC age or expression timing. In addition, activation of ChR2-expressing DGCs evoked more IPSCs with larger amplitude in mature DGCs ipsilateral to CCI compared to DGCs from sham controls. Evoked IPSCs were blocked by a GABAA receptor antagonist or an AMPA receptor antagonist, suggesting that light-evoked IPSCs arise through excitation of GABAergic interneurons by ChR2-expressing DGCs. In sharp contrast, similar optogenetic activation of ChR2-expressing DGCs rarely evoked EPSCs in mature DGCs from ipsilateral hippocampi of CCI mice of both early born and adult born groups, which was similar to results from sham controls.
Conclusion:
Activation of DGCs born early in life or near injury produced significantly increased inhibition of mature DGCs, but not recurrent excitation after CCI. Upregulated inhibition of mature DGCs may help compensate for inhibitory neuron loss after TBI, but could also lower seizure threshold and contribute to synchronization of epileptiform discharges in PTE, hypotheses which will be tested in future studies.
Funding:
:NIH NINDS R01 NS092552