Abstracts

Rationale: A single episode of moderate traumatic brain injury (TBI) causes loss of GABAergic neurons and upsets GABAergic inhibition in hippocampal dentate gyrus. Alterations of the homeostatic balance between inhibition and excitation in hippocampal dentate granule cells (DGCs) after TBI have been linked to posttraumatic epilepsy, but the long-term fate of postsynaptic GABAARs after TBI remains unclear. DGCs are principal neurons in the dentate gyrus and gate propagation of abnormal excitation in limbic system. DGCs receive two forms of GABAAR-mediated inhibition - phasic (mediated by synaptic receptors) and tonic (mediated by extrasynaptic receptors). These two forms of inhibition are maintained by GABAARs with distinct subunit compositions. We characterized long term alterations in both forms of inhibition in DGCs in the Controlled Cortical Impact (CCI) model of TBI in a rat. Methods: Male Sprague-Dawley rats were sacrificed 90 days after CCI. DGCs were voltage clamped to -70 mV at room temperature in 350 μ coronal slices, under isotonic chloride conditions, ipsilateral to the injury site. Results: Miniature inhibitory postsynaptic currents (mIPSCs) were recorded in the presence of blockers or glutamate receptors and 1 μM tetrodotoxin. The mIPSCs were randomly selected from pooled recordings from naive and CCI groups (~5000 events in each group). No differences were found between means of medians of peak amplitudes (27.43 ± 5.9 pA in control, n = 9; and 35.6 ± 4.1 pA in CCI, n = 12, p = 0.14, t-test), frequencies of mIPSCs (0.95 ± 0.15 Hz in control, n = 9, and 1.09 ± 0.11 Hz, CCI, n = 12, p = 0.51), and the decay time constants (5.42 ± 0.18 msec in control, n = 9, and 5.15 ± 0.0.5 msec in CCI, n=12, p=0.28). 300 nM diazepam prolonged decay time in naive DGCs (5.61 ± 0.08 msec and 6.59 ± 0.08 msec, p < 0.001), but not in CCI DGCs (7.47 ± 0.09 msec and 7.68 ± 0.11, p = 0.15). Potentiation of mean tonic current and baseline noise by the low affinity GABAAR agonist THIP (1 μM) was increased in CCI DGCs by 30.6% and 35%, respectively, compared to naive DGCs. THIP did not alter frequency, peak amplitude, or decay time in mIPSCs in control and CCI DGCs. 20 μM bicuculline inhibited mean current in naïve DGCs by 9.5%, and by 61.4% in CCI DGCs. There was an increase of GABAAR δ subunit expression in the granule cell layer and in the molecular layers of dentate gyrus in rats 90 days after CCI. Conclusions: In summary, our study demonstrates for the first time alterations of tonic GABAergic inhibition in DGCs after a single episode of TBI. Supported by Health Research Formula Fund RFA 01-07-26 from the Pennsylvania Department of Health, Tobacco Settlement Fund (ZM).