Research report
Sub-concussive brain injury in the Long-Evans rat induces acute neuroinflammation in the absence of behavioral impairments

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Abstract

Sub-concussive brain injuries may result in neurophysiological changes, cumulative effects, and neurodegeneration. The current study investigated the effects of a mild lateral fluid percussion injury (0.50–0.99 atm) on rat behavior and neuropathology to address the need to better understand sub-concussive brain injury. Male Long-Evans rats received either a single mild lateral fluid percussion injury or a sham-injury, followed by either a short (24 h) or long (4 weeks) recovery period. After recovery, rats underwent extensive behavioral testing consisting of tasks for rodent cognition, anxiety- and depression-like behaviors, social behavior, and sensorimotor function. At the completion of behavioral testing rats were sacrificed and brains were examined immunohistochemically with markers for neuroinflammation and axonal injury. No significant group differences were found on behavioral and axonal injury measures. However, rats given one mild fluid percussion injury displayed an acute neuroinflammatory response, consisting of increased microglia/macrophages and reactive astrogliosis, at 4 days post-injury. Neuroinflammation is a mechanism with the potential to contribute to the cumulative and neurodegenerative effects of repeated sub-concussive injuries. The current findings are consistent with findings in humans experiencing a sub-concussive blow, and provide support for the use of mild lateral fluid percussion injury in the rat as a model of sub-concussive brain injury.

Highlights

► Detailed analysis of sub-concussive mild lateral fluid percussion injury in the rat. ► Mild lateral fluid percussion injury induced acute neuroinflammatory response. ► No evidence of behavioral impairments or axonal injury. ► Neuroinflammation may contribute to neurodegenerative effects of repeated mild TBI. ► Supports use of mild fluid percussion injury as a model of sub-concussive injury.

Introduction

Brain concussion, or mild TBI, is a serious public health concern with an annual injury rate of approximately 1 in 150 individuals [1]. Concussion can be defined as a transient neurological dysfunction resulting from a biomechanical force and is often associated with behavioral symptoms, such as short-term cognitive and emotional disturbances [2], [3]. While evidence suggests that structural brain damage is rare following concussion [3], other pathologies such as axonal injury and neuroinflammatory processes may occur [4], [5], [6], [7]. Individuals who engage in daily combative and bodily contact activities, such as athletes and military personnel, are not only at an increased risk of suffering concussion but can also regularly experience sub-concussive blows to the head that are below the threshold of force required to cause concussion [8], [9], [10], [11], [12]. In a recent study by Talavage et al. [12], significant neurophysiological changes were detected in high school football players who had experienced an impact to the head but displayed no clinical symptoms of concussion. In addition, post-mortem studies on brain tissue obtained from individuals who had experienced repeated sub-concussive head traumas have found evidence that such injuries may have cumulative effects and could ultimately manifest as CTE, a neurodegenerative disorder associated with severe neurological impairments [8], [10], [13], [14]. In light of these findings, there is a need to better understand the effects of sub-concussive brain injuries [3], [8], [10], [11], [15]. Given the current lack of understanding and difficulties associated with assessing sub-concussive impacts in clinical populations, the use of an animal model of sub-concussive brain trauma may be beneficial.

LFP is one of the most commonly used and well-characterized rodent models of TBI [16]. Previous studies administering a single mLFP with a force greater than 1 atm have found that a single mLFP can induce behavioral and pathological changes in the rat consistent with those that occur in humans suffering a single concussion [2], [3], [16], [17], [18]. However, to date, no study has performed a detailed behavioral and pathological examination of the effects of a single mLFP below the force of 1 atm to determine whether it might be used to model sub-concussive brain injury. In an attempt to assess the use of a mLFP as an animal model of sub-concussive brain injury, the current study gave young-adult male Long-Evans rats a mLFP with a force of 0.50–0.99 atm. Following the injury, rats underwent testing on behavioral tasks of anxiety-like behavior, spatial cognition, depression-like behavior, social behavior, locomotion, and sensorimotor ability, as well as immunohistochemical evaluation of neuroinflammation and axonal injury. To assess recovery after mLFP, these analyses were conducted at both short- and long-term recovery intervals.

Section snippets

Subjects

Subjects were 64 adult male Long-Evans hooded rats obtained from Charles River Labs (Quebec). Prior to surgery rats weighed between 250–300 g, were housed in pairs in standard acrylic cages (26 cm × 48 cm × 21 cm) at 21 ± 1.0 °C, and were experimentally naïve. Rats were housed individually post-surgery to maintain the integrity of the injury cap. The light/dark cycle was 12:12 (lights on 07:00–19:00 h) and animals had access to food and water ad libitum. Procedures were in accordance with Canadian Council

Injury force and acute post-injury measures

As shown in Table 1, ANOVA revealed a significant main effect of injury/recovery group (F(3, 63) = 33,276.580, p < .001) on the measure of injury force. Post hoc tests revealed that the SUB-SR and SUB-LR groups experienced a significantly greater injury force than the SHAM groups (all ps < .001). There were no group differences in the duration of apnea, the time to regain hind-limb withdrawal, and the return of the self-righting reflex (all ps > .05). There were no cases of mortality resulting from

Discussion

A single mLFP with a force of 0.5–0.99 atm in the male Long-Evans rat produced a short-term increase in microglia/macrophages and reactive astrogliosis, indicating an acute neuroinflammatory response. This neuroinflammatory response occurred in the absence of any significant axonal injury, or cognitive, emotional, or sensorimotor disturbances. There were no long-term group differences found on any of the behavioral or neuropathological measures at 4 weeks post-injury.

Conclusions

A single mLFP at a force of 0.5–0.99 atm induced a transient neuroinflammatory response in the absence of behavioral impairments. These findings appear consistent with what is observed in humans experiencing a single sub-concussive blow and support the use of mLFP to model sub-concussive brain injury. The finding that a single sub-concussive brain injury can result in neuroinflammation may have important implications for understanding the cumulative and neurodegenerative effects of repeated mild

Acknowledgements

This research was supported by a grant from the Natural Science and Engineering Research Council (NSERC) to Dr. Donald P. Cain, a scholarship from NSERC to Dr. Sandy R. Shultz, and contributions from GoodLife Childrens's Foundation and Round for a Reason Charities to Dr. Derrick F. MacFabe. We thank Drs. Richard Sutton and David Hovda from the UCLA Brain Injury Research Center for their help in LFP training. Additional thanks go to Lisa Tichenoff, Francis Boon, and Acia Blank for their

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