Frontal Traumatic Brain Injury in Rats Causes Long-Lasting Impairments in Impulse Control That Are Differentially Sensitive to Pharmacotherapeutics and Associated with Chronic Neuroinflammation

Abstract

Traumatic brain injury (TBI) affects millions yearly, and is increasingly associated with chronic neuropsychiatric symptoms. We assessed the long-term effects of different bilateral frontal controlled cortical impact injury severities (mild, moderate, and severe) on the five-choice serial reaction time task, a paradigm with relatively independent measurements of attention, motor impulsivity, and motivation. Moderately- and severely injured animals exhibited impairments across all cognitive domains that were still evident 14 weeks postinjury, while mild-injured animals only demonstrated persistent deficits in impulse control. However, recovery of function varied considerably between subjects such that some showed no impairment ("TBI-resilient"), some demonstrated initial deficits that recovered ("TBI-vulnerable"), and some never recovered ("chronically-impaired"). Three clinically relevant treatments for impulse-control or TBI, amphetamine, atomoxetine, and amantadine, were assessed for efficacy in treating injury-induced deficits. Susceptibility to TBI affected the response to pharmacological challenge with amphetamine. Whereas sham and TBI-resilient animals showed characteristic impairments in impulse control at higher doses, amphetamine had the opposite effect in chronically impaired rats, improving task performance. In contrast, atomoxetine and amantadine reduced premature responding but increased omissions, suggesting psychomotor slowing. Analysis of brain tissue revealed that generalized neuroinflammation was associated with impulsivity even when accounting for the degree of brain damage. This is one of the first studies to characterize psychiatric-like symptoms in experimental TBI. Our data highlight the importance of testing pharmacotherapies in TBI models in order to predict efficacy, and suggest that neuroinflammation may represent a treatment target for impulse control problems following injury.

Keywords: Controlled cortical impact; amphetamine; cytokine; impulsivity; prelimbic.

Figure 1.

Study overview, including injury location, experimental timeline and five-choice serial reaction time (5CSRT) task description. A) Injury coordinates in stereotaxic space. Injuries were centered on the midline at +3.0 mm from bregma. Severe injuries impacted to a depth of −2.5 mm at 3 m/s, moderate injuries were −1.7 mm at 2 m/s (44% of severe force) and mild injuries were −0.8 mm at 1 m/s (11% of severe force). Adapted from Paxinos and Watson’s The Rat Brain in Stereotaxic Coordinates, 4^th ed. B) Experimental timeline showing when training, assessment, pharmacological challenges and end points occurred. C) Task diagram for the 5CSRT task. After initiating a trial by making a nose poke response at the illuminated food tray, rats must wait 5 s for the brief stimulus light to appear at one of the five holes. Once that occurs, a nose poke at the correct hole is reinforced with a 0.45 mg sugar pellet. Incorrect or omitted responses are punished with a 5 s time-out in which the houselight comes on and no pellets may be earned. Responses made prematurely at the 5-hole array, before the stimulus light appears, are also punished with a 5 s time-out. Correct responses provide a measure of attention, and premature responses provide a measure of motor impulsivity. Latencies to make a choice and to collect the reinforcer were also recorded.

Figure 2.

Effects of injury on 5CSRT performance at acute (week 2–5) and chronic (week 5–14) time points. Deficits in all domains were tiered by injury severity. A) Mild-injured rats demonstrated significant acute deficits in attention (p = 0.004) which recovered over time (p = 0.189), while moderate- and severe-injured rats had significant acute (p < 0.001; p < 0.001) and continuing chronic deficits (p < 0.001; p < 0.001). B) Mild-, moderate- and severe-injured rats showed increased impulsive responding in the acute period (p = 0.001; p < 0.001; p < 0.001), which remained elevated throughout chronic testing (p = 0.052; p < 0.001; p < 0.001). C) Mild-injured rats had no significant change in omitted trials (p = 0.192; p = 0.899), yet moderate- and severe-injured rats showed increased omissions at both the acute (p < 0.001; p < 0.001) and chronic (p < 0.040; p < 0.001) time points. D) Mild-injured animals were initially impaired in overall task efficacy (p = 0.001), but recovered during chronic testing (p = 0.115), while moderate- and severe-injured animals demonstrated initial deficits (p < 0.001; p < 0.001) lasting into the chronic period (p < 0.001; p < 0.001). Data shown are mean + SEM.

Figure 3.

Individual differences in 5CSRT performance and response to brain injury at acute (week 2–5) and chronic (week 5–14) time points. Although sham data is shown for reference in panel C, only injured rats were included in analyses. A) Independent of injury conditions, rats were categorized as “resilient” if they recovered within 5 weeks, “vulnerable” if they recovered by the end of testing (14 weeks), or “chronically impaired” if they never recovered. B) Resilient rats showed acute reductions in task efficacy (p = 0.032) which resolved over time (p = 0.940), while vulnerable rats had continuing deficits despite their recovery (p < 0.001), and chronically impaired rats had unrecovered deficits (p < 0.001). C) The left side of the panel shows raw data for each subject in terms of standard deviations from baseline performance (overall task efficacy), while the right side shows regression fits. Recovery was defined as within 3 standard deviations of (individual) baseline performance (dashed lines). Rats in each injury group show a highly variable response to brain injury. D) Resilient rats demonstrated acute deficits in attention (p = 0.045), which quickly recovered to baseline levels (p = 0.770), while neither vulnerable nor chronically impaired rats fully recovered (p < 0.001; p < 0.001). E) Resilient rats had no change in impulsivity across testing (p = 0.145; p = 0.529), but both vulnerable and chronically impaired rats showed increased impulsivity in the acute period (p < 0.001; p < 0.001), which extended to chronic testing (p = 0.012; p < 0.001). F) Resilient rats showed no change in omitted trials during acute or chronic phases (p = 0.565; p = 0.186), while vulnerable rats showed deficits in the acute (p < 0.001), but not chronic time points (p = 0.445), and chronically impaired rats demonstrated increases throughout testing (p < 0.001). Data shown are individual subjects’ data points and group means.

Figure 4.

Effects of amphetamine on 5CSRT performance tiered by injury severity, as determined by impact force, vs. injury susceptibility, as determined by trajectory of recovery. A) Severe-injured rats had improved attention at 1.0 mg/kg (p = 0.002), moderate-injured rats showed no change at any dose (p’s > 0.151), mild-injured rats approached impairment at 1.0 mg/kg (p = 0.052) and sham rats were impaired at the 0.6 or 1.0 mg/kg (p = 0.009; p = 0.012). B) Severe-injured rats exhibited reduced impulsivity at 1.0 mg/kg (p = 0.015), moderate-injured rats showed no change across doses (p’s > 0.128), while impulsivity increased in both mild-injured and sham rats at all doses compared to saline (p’s < 0.040). C) Overall, omissions increased at the 1.0 mg/kg dose (p = 0.004). D) In general, rats showed reduced task efficacy at all doses (p’s < 0.045). E) Susceptibility subgroups demonstrated differential effects, with resilient rats showing reduced accuracy at 1.0 mg/kg (p = 0.002), vulnerable rats showing no change at any dose (p’s > 0.137), and chronically impaired rats showing improved function at 1.0 mg/kg (p = 0.002). F) Subgroups also demonstrated similar effects with regards to impulsivity with resilient and vulnerable rats showing increased impulsivity as a function of increasing dose (p’s < 0.021) and chronically impaired rats demonstrating reduced impulsive responding at the 1.0 mg/kg dose (p = 0.012). Data shown are mean + SEM and individual data points in panels E and F, * = p < 0.05, ** = p < 0.01, ***= p < 0.001.

Figure 5.

Histological and immune markers and their relationship to functional outcome. A) Lesion cavitation and ventricle size were significantly increased in a severity-dependent manner (p’s < 0.001). B) MRI histoplate demonstrating representative brains from each group. Minor cavitation was evident in mild-injured rats, with increasing damage and ventricular enlargement visible in moderate- and severe-injured rats. C) There were no group differences in IL-1α levels (p = 0.307), however, IL-1α was significantly correlated with attention, impulsivity, and lesion size (p = 0.003; p = 0.001; p = 0.016). D) IL-6 levels were not significantly different across the groups (p = 0.190), however, they were significantly correlated with attention, impulsivity, and lesion size (p < 0.001; p < 0.001; p = 0.008). E) There were no group differences in IL-10 levels (p = 0.172), however, IL-10 was significantly correlated with attention, impulsivity, and lesion size (p’s < 0.001). F) IL-12 levels were significantly increased in mild and moderate TBI groups (p = 0.004; p = 0.040), and approached significance for severe (p = 0.053); IL-12 levels were also significantly correlated with attention, impulsivity, and lesion size (p = 0.027; p = 0.038; p = 0.011). Data shown are mean + SEM in panel A and C-F and raw data points in panels C-F, * = p < 0.05, ** = p < 0.01, ***= p < 0.001.

Figure 6.

Comparison of neuroinflammation principal components across injury groups and regression analyses of lesion and principal component data and their relationship to behavioral outcomes. A) PC1 did not differ across injury groups (p = 0.719). B) PC2 was significantly lower in the severe TBI group (p = 0.014). C) Both the mild and moderate TBI group had significantly lower levels of PC3 (p < 0.001; p = 0.019). E) Multiple regression revealed that lesion size was most strongly associated with accuracy (p < 0.001). F) A regression with both lesion size (p = 0.030) and principal component 2 (p = 0.036) best accounted for premature responses. G) Multiple regression showed that lesion size (p = 0.001) was significantly associated with task efficacy, although with a much poorer model fit compared to other measures. Data shown are mean + SEM in panels A-C, and raw versus predicted data points in D-F; the dashed line demonstrates perfect prediction, while the solid line represents the actual model, * = p < 0.05, ** = p < 0.01, ***= p < 0.001.