Traumatic brain injury modifies synaptic plasticity in newly-generated granule cells of the adult hippocampus

Abstract

The hippocampus is vulnerable to traumatic brain injury (TBI), and hippocampal damage is associated with cognitive deficits that are often the hallmark of TBI. Recent studies have found that TBI induces enhanced neurogenesis in the dentate gyrus (DG) of the hippocampus, and this cellular response is related to innate cognitive recovery. However, cellular mechanisms of the role of DG neurogenesis in post-TBI recovery remain unclear. This study investigated changes in long-term potentiation (LTP) within the DG in relation to TBI-induced neurogenesis. Adult male rats received a moderate TBI or sham injury and were sacrificed for brain slice recordings at 30 or 60 days post-injury. Recordings were taken from the medial perforant path input to DG granule cells in the presence or absence of the GABAergic antagonist picrotoxin, reflecting activity of either all DG granule cells or predominately newborn granule cells, respectively. Measurements of LTP observed in the total granule cell population (with picrotoxin) showed a prolonged impairment which worsened between 30 and 60 days post-TBI. Under conditions which predominantly reflected the LTP elicited in newly born granule cells (no picrotoxin), a strikingly different pattern of post-TBI changes was observed, with a time-dependent cycle of functional impairment and recovery. At 30 days after injury this cell population showed little or no LTP, but by 60 days the capacity for LTP of the newly born granule cells was no different from that of sham controls. The time-frame of LTP improvements in the newborn cell population, comparable to that of behavioral recovery reported previously, suggests the unique functional properties of newborn granule cells enable them to contribute to restorative change following brain injury.

Keywords: Dentate gyrus; Long-term potentiation; Neurogenesis; Recovery of function; Traumatic brain injury.

Figure 1.. LTP measured in the presence of picrotoxin (+)Pic.

Mean fEPSP slopes (+SEM), expressed as percent of baseline level, are plotted for sham-injury, TBI 30 dpi, and TBI 60 dpi, showing the final 10 minutes of baseline recording and 60 minutes of post-HFS recording. Responses were evoked and collected at a rate of 1/30 sec, but for statistical and graphical analyses were aggregated into 2-minute epochs. The magnitude of LTP in the sham injury group, averaged over the 60 minutes of post-HFS recording, was significantly larger than in the TBI 60 dpi group (*p<0.05

Figure 2.. Analysis of Input-Output relationships in (+)Pic study groups.

A. Evoked waveforms show representative input-output series evoked in sham and injury groups during baseline recording and at 60 min post-LTP. B. Mean fEPSP slopes (+SEM) are plotted for graded current intensities, spanning threshold to the maximum response obtained during baseline recording. Significant tetanus-induced upward shifts were observed for sham and TBI-30d (+)Pic slices (Mann-Whitney U statistic shown as Z values).

Figure 3.. LTP measured in the absence of picrotoxin (−)Pic.

Mean fEPSP slopes (+SEM), expressed as percent of baseline level, are plotted for sham-injury, TBI 30 dpi, and TBI 60 dpi, showing the final 10 minutes of baseline recording and 60 minutes of post-HFS recording. Responses were evoked and collected at a rate of 1/30 sec, but for statistical and graphical analyses were aggregated into 2-minute epochs. The magnitude of LTP in the TBI 60 dpi group, averaged over the 60 minutes of post-HFS recording, was significantly larger than in the TBI 30 dpi group (*p<0.05).

Figure 4.. Analysis of Input-Output relationships in (−)Pic study groups.

A. Evoked waveforms show representative input-output series evoked in sham and injury groups during baseline recording and at 60 min post-LTP. B. Mean fEPSP slopes (+SEM) are plotted for graded current intensities, spanning threshold to the maximum response obtained during baseline recording. For the lower signal amplitudes of the (−)Pic group, the tetanus-induced curve shifts were not significant (Mann-Whitney U statistic shown as Z values).

Figure 5.. Analysis of responses evoked using 100μA pulses.

A. Representative responses, from each of the (+)Pic and (−)Pic groups evoked with uniform 100μA pulses, acquired during input-output recording. B. Mean group fEPSP slopes were not significantly different for either (+)Pic or (−)Pic conditions, suggesting basal excitability did not differ among study groups.

Figure 6.. Analysis of responses to paired-stimulus presentations.

A. Paired responses in a representative (+)Pic sham rat, evoked at interpulse intervals of 50, 100, and 150 ms. B. Group mean paired-pulse responses, plotting the ratio [fEPSP (2)/fEPSP(1) × 100] at the three interpulse intervals. There were no differences among groups in this index of presynaptic excitability for either (+)Pic groups or (−)Pic groups.

Figure 7.. Changes in tissue volume after LFPI.

A-C. Representative H&E stained sections showing sham injury (A), 30 dpi TBI (B), and 60 dpi TBI (C). A delayed tissue shrinkage was observed ipsilateral to the injury (arrowhead). Group means for the volume ratio (%IPSI / CONTRA) are plotted below for cortical tissue (D), hippocampal tissue (E), and dentate gyrus tissue (F). D: For cortical tissue, the volume ratios observed at 60 dpi were significantly below sham values (*p<0.05). E: For total hippocampus, the volume ratios observed at 60 dpi were significantly lower than the sham and 30 dpi (*p<0.05), whereas no difference was found between 30 dpi and sham. F: For DG, the volume ratios in injured groups at 30- and 60 dpi were significantly lower than the sham group (**p<0.01), no difference was found between 30- and 60 dpi. Bar = 2mm.