Functional Deficits after Sensorimotor Controlled Cortical Impact
The rotarod and beam walk tests confirmed that CCI to the sensorimotor cortex produced substantial motor deficits (). The largest rotarod deficit was seen on D1 after TBI (−56±19%, P<0.0001). Rotarod performance improved from D1 to D7 (P=0.0009) but not subsequently. On D14, rotarod performance remained significantly impaired compared with preinjury baseline (−36±5%, P<0.0001). The largest beam walk deficit was also seen on D1 (−52±12%, P<0.0001). Beam walk scores improved from D1 through D10 (P<0.0001) but not subsequently. On D14, beam walk performance remained significantly impaired compared with preinjury baseline (−14±7%, P=0.002).
Figure 1 Motor behavioral deficits after controlled cortical impact (CCI) of the sensorimotor cortex. Postinjury scores on the rotarod and the beam walk were expressed as a percent of preinjury baseline scores for each animal. After traumatic brain injury (TBI), (more ...)
Lesion Characteristics on T2-Weighted Magnetic Resonance Imaging
T2-weighted MRI verified the tissue effects of CCI and allowed us to follow the longitudinal development of the brain contusion in vivo (). Tissue disruption was visible on D0 (~1 hour after TBI), including cortical surface deformation, ventral shift of the corpus callosum, and frequent small intraparenchymal hemorrhages. On D1 to D3, edema could be seen as a diffuse hyperintensity in the ipsilateral cortex, and tissue swelling was indicated by displacement of the cortical surface and a midline shift toward the contralateral hemisphere. Tissue swelling had subsided by D7, giving way to cortical thinning and ventricular enlargement. On D14, a cortical cavity with discrete boundaries was visible, filled with hyperintense cerebrospinal fluid and hypointense blood products. The cortical cavity frequently appeared to connect with the enlarged ipsilateral ventricle.
Figure 2 T2-weighted magnetic resonance imaging (MRI) of a rat brain after controlled cortical impact (CCI). Representative coronal images (bregma −0.5mm) show the development of the cortical contusion from Day 0 (D0, 1 hour after injury) to Day (more ...)
Quality and Reproducibility of Proton Magnetic Resonance Spectra
High quality spectra with narrow line widths were generally obtained throughout the study. Before injury, mean line widths for the unsuppressed water resonance were 11.6±0.6
Hz in the hippocampal voxel and 12.1±1.0
Hz in the cortical voxel. After TBI, at the earliest time point (D0) mean line widths increased to 14.9±1.6
Hz in the hippocampus and to 17.1±2.3
Hz in the cortex, presumably due to acute tissue swelling and microhemorrhage. Line widths at all subsequent time points were similar to preinjury levels (D1 to D14 mean 12.6±1.2
Hz in hippocampus; 12.1±2.5
Hz in cortex).
Neurochemical concentrations measured with 1
H-MRS in the absence of TBI were highly reproducible. At two time points 7 days apart, mean NAA concentrations were 9.48±0.50 and 9.25±0.28μ
mol/g, Gln was 2.78±0.24 and 2.69±0.23μ
mol/g, Tau was 6.28±0.15 and 6.25±0.16μ
mol/g, and GSH was 0.91±0.08 and 0.92±0.12μ
mol/g, respectively. Neurochemical concentrations were not significantly different between time points (all P
values >0.5, paired t
-test). Although coefficients of variation varied across neurochemicals (2% to 13%), they were similar for each metabolite over time.
The Neurochemical Profile of Traumatic Brain Injury in Contused and Normal-Appearing Brain Regions
We used longitudinal in vivo
1H-MRS to evaluate the postinjury evolution of 20 individual neurochemicals after TBI in two brain regions ( and ). The cortical voxel for MRS was positioned directly under the impact and contained tissue that would degenerate into a visible contusion cavity. In the cortex, TBI led to significant changes in 19 out of 20 neurochemicals measured. presents a series of cortical spectra after TBI, showing dramatically altered spectral profiles.
Rat cortex: time course of neurochemical changes after TBI
Rat hippocampus: time course of neurochemical changes after TBI
Figure 3 Representative proton magnetic resonance spectroscopy (1H-MRS) spectra acquired before and after traumatic brain injury (TBI) in cortex and hippocampus. (A) A T2-weighted coronal image from D3 shows the location of the cortical voxel at the site of the (more ...)
N-acetylaspartate decreased rapidly after injury (−45% on D0), reached its lowest level on D3 (−83%), and remained significantly reduced through D14 (; ). Ins and Tau were also reduced after TBI (Ins, −73% at D3; Tau, −61% at D3) but recovered toward baseline levels after D3. Ins continued to increase from D3 to D14 and was significantly elevated compared with baseline by D14 (+27% ; ).
Figure 4 Changes in selected neurochemicals measured with proton magnetic resonance spectroscopy (1H-MRS) after traumatic brain injury (TBI) (*indicates P<0.05 versus preTBI). Data are expressed as mean±standard error. ctx, cortical voxel, (more ...)
GPC and PCho showed differential changes after TBI (; ). GPC decreased sharply by 1 hour after injury to levels below the detection limit of our system but had recovered to baseline by D7. By contrast, PCho was increased (~270% to 450%) from D1 to D7 followed by a return to baseline by D14.
Figure 5 Increased spectral resolution at high field strength permits independent measurement of choline-containing compounds, glutamate, and glutamine. (A) Changes in choline-containing compounds in the contused cortex after traumatic brain injury (TBI). Compared (more ...)
Neurotransmitters also showed differential changes. Although Glu was decreased in the cortex at all postinjury time points, Gln was increased at D0 (+205%) and D1 (+182%) before returning to baseline levels (; ). Other neurotransmitters were also decreased in the contused cortical voxel, including GABA (maximum change of −46% on D1), NAAG (maximum change of −68% on D7), and Asp (maximum change of −81% on D0).
We observed sharp decreases in the antioxidants GSH and Asc in the injured cortex (; ). GSH decreased on D0 to D3 to levels below the detection limit of our system and recovered to baseline levels by D7. Asc reached its lowest level at D3 (−69%) and remained significantly depleted through D14 (−44%).
Dramatic increases in Lac at the cortical injury site were seen by D0 (). Lac increased by ~600% on D0 to D3 after injury, before falling back toward baseline values (; ). Ala and Ser were also increased over a similar time course. Ala increases ranged from ~350% to 550% on D0 to D3 before falling back to low baseline levels. Significant increases in Ser (120% to 200%) were seen between D0 and D14. By contrast, cortical Glc levels decreased from D0 to D3, reaching their lowest level on D3 (−50%) before recovering.
In summary, we observed substantial neurometabolic changes at the cortical contusion site, with 14 neurochemicals significantly decreased and 5 (Gln, PCho, Lac, Ala, and Ser) increased after TBI.
The hippocampal voxel was positioned ipsilateral to TBI but farther from the impact site. Despite the lack of visible contusion in this location (), 1H-MRS revealed significant changes in nine neurochemicals after TBI. NAA decreased early after TBI (−12% at D0) and remained significantly decreased compared with preinjury through D14 (; ). Ins was decreased on D0 to D3 (−7% to −10%) but returned to control levels by D7 (; ). Similar to our findings in the cortex, Glu was decreased (maximum change of −17% on D0) and Gln was increased (maximum change of +39% on D3) after TBI. Both Asp (−30%) and GPC (−24%) were transiently decreased at D0, returning to baseline levels by D1. By contrast, a postinjury decrease in Cr was sustained from D0 (−10%) until D14 (−15%). Hippocampal Lac was increased after TBI (maximum change of +23% on D1; and ).
Although the hippocampal patterns of change for an additional eight neurochemicals (Ala, Asc, PCho, PCr, GSH, NAAG, Ser, and Tau) were generally similar to those in the cortex, they were of lower magnitude and did not survive rigorous statistical correction for multiple comparisons (). The plots in illustrate the regional difference in severity of neurochemical changes after TBI.