Enduring changes in tonic GABAA receptor signaling in dentate granule cells after controlled cortical impact brain injury in mice

Abstract

Changes in functional GABAAR signaling in hippocampus have previously been evaluated using pre-clinical animal models of either diffuse brain injury or extreme focal brain injury that precludes measurement of cells located ipsilateral to injury. As a result, there is little information about the status of functional GABAAR signaling in dentate granule cells (DGCs) located ipsilateral to focal brain injury, where significant cellular changes have been documented. We used whole-cell patch-clamp recordings from hippocampal slices to measure changes in GABAARs in dentate granule cells (DGCs) at 1-2, 3-5, and 8-13 weeks after controlled cortical impact (CCI) brain injury. Synaptic and tonic GABAAR currents (ITonicGABA) were measured in DGCs at baseline conditions and during application of the GABAAR agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol hydrochloride (THIP) to assess in the function of δ subunit-containing GABAARs. DGCs ipsilateral to CCI exhibited no changes in the amplitude of resting ITonicGABA relative to DGCs after sham-injury or contralateral to CCI. In contrast, there was a significant reduction in the THIP-evoked ITonicGABA in DGCs ipsilateral to CCI at both time-points. Tonic GABAergic inhibition of DGCs ipsilateral to injury also exhibited reduced responsiveness to the neurosteroid THDOC. ITonicGABA in DGCs ipsilateral to CCI did not exhibit a change in sensitivity to L655,708, an inverse agonist with selectivity for α5 subunit-containing GABAARs, suggesting a lack of functional change in GABAARs containing this subunit. At the 8-13 week time-point, gene expression of GABAAR subunits expected to contribute to ITonicGABA (i.e., α4, α5 and δ) was not significantly altered by CCI injury in isolated dentate gyrus. Collectively, these results demonstrate enduring functional changes in ITonicGABA in DGCs ipsilateral to focal brain injury that occur independent of altered gene expression.

Differential effects of rapamycin treatment on tonic and phasic GABAergic inhibition in dentate granule cells after focal brain injury in mice

“…THIP-induced tonic GABA A R-mediated currents in DGCs are altered in rodent models of TBI and TLE (Boychuk et al, 2016; Gupta et al, 2012; Mtchedlishvili et al, 2010; Pavlov et al, 2011). In mice, resting tonic GABA A R current is not altered after CCI, but the THIP- or neurosteroid-induced tonic GABA A R current amplitude is reduced in the ipsilateral hemisphere after CCI, and this reduction is sustained for at least 3 months (Boychuk et al, 2016).…”

“…mossy fiber sprouting) leads to recurrent excitation of DGCs and disrupts the normal information processing of the hippocampus. Synaptic inhibition of DGCs is also altered after TBI, and GABA A receptors (GABA A R's) undergo functional changes in models of TBI and epilepsy (Boychuk et al, 2016; Hunt et al, 2011; Mtchedlishvili et al, 2010; Raible et al, 2012; Raible et al, 2015). …”

“…The effect of mTOR inhibition in TBI and TLE models on neuron loss is controversial (Buckmaster and Wen, 2011; Butler et al, 2015; Guo et al, 2013), but rapamycin treatment suppresses TLE-related morphological changes in hilar inhibitory interneurons (Buckmaster and Wen, 2011). Hilar interneuron loss likely contributes to altered GABAergic circuitry function and receptor responsiveness after TBI (Boychuk et al, 2016; Hunt et al, 2011; Mtchedlishvili et al, 2010), but effects of mTOR modulation on inhibitory signaling in the dentate gryus after focal brain injury have not been adequately described. We hypothesized that chronic rapamycin treatment following CCI injury leads to reduced TBI-induced changes in synaptic and non-synaptic inhibition of DGCs in the ipsilateral hemisphere.…”

How and Why Study Posttraumatic Epileptogenesis in Animal Models?

“…Mossy fiber sprouting is localized to areas near the injury epicenter after focal injury (29) and is less severe and sparser after TBI than, for example, in chemoconvulsant models of TLE, but its presence, however limited, is a consistent feature of both models. Other cellular outcomes identified in both TLE and PTE models include an increase in postinjury granule cell progenitor proliferation and a reorganization of GABA A receptors in granule cells (33,(37)(38)(39)(40). Increased adult neurogenesis has been hypothesized to contribute to aberrant mossy fiber connectivity and an eventual increase in seizure susceptibility but may also contribute to cognitive recovery shortly after TBI (40,41).…”

“…Increased adult neurogenesis has been hypothesized to contribute to aberrant mossy fiber connectivity and an eventual increase in seizure susceptibility but may also contribute to cognitive recovery shortly after TBI (40,41). Reorganization of GABA A receptors might also serve a compensatory function, particularly since model-and location-dependent increases or decreases in receptor function have been observed (37)(38)(39). As for most other outcomes, cellular changes are correlated with epileptogenesis, but definitive studies to determine whether there is a causal role for these events in PTE development are lacking.…”