Persistent cognitive dysfunction after traumatic brain injury: A dopamine hypothesis

Designer Receptor Exclusively Activated by Designer Drug (DREADD)-Mediated Activation of the Periaqueductal Gray Restores Nociceptive Descending Inhibition After Traumatic Brain Injury in Rats

Traumatic brain injury (TBI) patients frequently experience chronic pain that can enhance their suffering and significantly impair rehabilitative efforts. Clinical studies suggest that damage to the periaqueductal gray matter (PAG) following TBI, a principal center involved in endogenous pain control, may underlie the development of chronic pain. We hypothesized that TBI would diminish the usual pain control functions of the PAG, but that directly stimulating this center using a chemogenetic approach would restore descending pain modulation. We used a well-characterized lateral fluid percussion model (1.3 ± 0.1 atm) of TBI in male rats (n = 271) and measured hindpaw mechanical nociceptive withdrawal thresholds using von Frey filaments. To investigate the role of the PAG in pain both before and after TBI, we activated the neurons of the PAG using a Designer Receptor Exclusively Activated by Designer Drug (DREADD) viral construct. Immunohistochemical analysis of brain tissue was used to assess the location and confirm the appropriate expression of the viral constructs in the PAG. Activation of the PAG DREADD using clozapine N-oxide (CNO) caused hindpaw analgesia that could be blocked using opioid receptor antagonist, naloxone, in uninjured but not TBI rats. Due to the importance of descending serotonergic signaling in modulating nociception, we ablated spinal serotonin signaling using 5,7-DHT. This treatment strongly reduced CNO-mediated anti-nociceptive effects in TBI but not uninjured rats. To define the serotonergic receptor(s) required for the CNO-stimulated effects in TBI rats, we administered 5-HT7 (SB-269970) and 5-HT1A (WAY-100635) receptor antagonists but observed no effects. The selective 5-HT2A receptor antagonist ketanserin, however, blocked CNO's effects in the DREADD expressing TBI but not DREADD expressing sham TBI animals. Blockade of alpha-1 adrenergic receptors with prazosin also had no effect after TBI. Descending pain control originating in the PAG is mediated through opioid receptors in uninjured rats. TBI, however, fundamentally alters the descending nociceptive control circuitry such that serotonergic influences predominate, and those are mediated by the 5-HT2A receptor. These results provide further evidence that the PAG is a key target for anti-nociception after TBI.

Cognitive and Motor Function Effects of Antipsychotics in Traumatic Brain Injury: A Systematic Review of Pre-Clinical Studies

Traumatic brain injury (TBI) survivors often suffer from agitated behaviors and will most likely receive pharmacological treatments. Choosing an optimal and safe treatment that will not interfere with neurological recovery remains controversial. By interfering with dopaminergic circuits, antipsychotics may impede processes important to cognitive recovery. Despite their frequent use, there have been no large randomized controlled studies of antipsychotics for the management of agitated behaviors during the acute TBI recovery period. We conducted a systematic review and meta-analysis of pre-clinical studies evaluating the effects of antipsychotics post-TBI on both cognitive and motor recovery. MEDLINE and Embase databases were searched up to August 2, 2023. Pre-clinical studies evaluating the effects of antipsychotics on cognitive and motor functions post-TBI were considered. Risk of bias was evaluated with the Systematic Review Centre for Laboratory Animal Experimentation (SYRCLE) tool. We identified 15 studies including a total of 1188 rodents, mostly conducted in male Sprague-Dawley rats using cortical impact injury. The analysis revealed no consistent effect of haloperidol on motor functions, but risperidone was associated with a significant impairment in motor function on day 5 post-injury (7.05 sec; 95% confidence interval [CI]: 1.47, 12.62; I2 = 92%). Other atypical antipsychotics did not result in impaired motor function. When evaluating cognitive function, haloperidol- (23.00 sec; 95% CI: 17.42-28.59; I2 = 7%) and risperidone-treated rats (24.27 sec; 95% CI: 16.18-32.36; I2 = 0%) were consistently impaired when compared to controls. In studies evaluating atypical antipsychotics, no impairments were observed. Clinicians should avoid the regular use of haloperidol and risperidone, and future human studies should be conducted with atypical antipsychotics.

A bridge to recovery: Acute amantadine prior to environmental enrichment after brain trauma augments cognitive benefit

Environmental enrichment (EE) facilitates motor and cognitive recovery after traumatic brain injury (TBI). Historically, EE has been provided immediately and continuously after TBI, but this paradigm does not model the clinic where rehabilitation is typically not initiated until after critical care. Yet, treating TBI early may facilitate recovery. Hence, we sought to provide amantadine (AMT) as a bridge therapy before commencing EE. It was hypothesized that bridging EE with AMT would augment motor and cognitive benefits. Anesthetized adult male rats received a cortical impact (2.8 mm deformation at 4 m/s) or sham surgery and then were housed in standard (STD) conditions where they received intraperitoneal AMT (10 mg/kg or 20 mg/kg) or saline vehicle (VEH; 1 mL/kg) beginning 24 h after surgery and once daily during the 6-day bridge phase or once daily for 19 days for the non-bridge groups (i.e., continuously STD-housed) to compare the effects of acute AMT plus EE vs. chronic AMT alone. Abbreviated EE, which was presented to closer emulate clinical rehabilitation (e.g., 6 h/day), began on day 7 for the AMT bridge and chronic EE groups. Motor (beam-walking) and cognition (acquisition of spatial learning and memory) were assessed on days 7-11 and 14-19, respectively. Cortical lesion volume and hippocampal cell survival were quantified on day 21. EE, whether provided in combination with VEH or AMT, and AMT (20 mg/kg) + STD, benefitted motor and cognition vs. the STD-housed VEH and AMT (10 mg/kg) groups (p < 0.05). The AMT (20 mg/kg) + EE group performed better than the VEH + EE, AMT (10 mg/kg) + EE, and AMT (20 mg/kg) + STD groups in the acquisition of spatial learning (p < 0.05) but did not differ in motor function (p > 0.05). All groups receiving EE exhibited decreased cortical lesion volumes and increased CA3 neuron survival relative to the STD-housed groups (p < 0.05) but did not differ from one another (p > 0.05). The added cognitive benefit achieved by bridging EE with AMT (20 mg/kg) supports the hypothesis that the temporal separation of combinational therapies is more effective after TBI.

Mechanisms and treatments of chronic pain after traumatic brain injury

While pain after trauma generally resolves, some trauma patients experience pain for months to years after injury. An example, relevant to both combat and civilian settings, is chronic pain after traumatic brain injury (TBI). Headache as well as pain in the back and extremities are common locations for TBI-related chronic pain to be experienced. TBI-related pain can exist alone or can exacerbate pain from other injuries long after healing has occurred. Consequences of chronic pain in these settings include increased suffering, higher levels of disability, serious emotional problems, and worsened cognitive deficits. The current review will examine recent evidence regarding dysfunction of endogenous pain modulatory mechanisms, neuroplastic changes in the trigeminal circuitry and alterations in spinal nociceptive processing as contributors to TBI-related chronic pain. Key pain modulatory centers including the locus coeruleus, periaqueductal grey matter, and rostroventromedial medulla are vulnerable to TBI. Both the rationales and existing evidence for the use of monoamine reuptake inhibitors, CGRP antagonists, CXCR2 chemokine receptor antagonists, and interventional therapies will be presented. While consensus guidelines for the management of chronic post-traumatic TBI-related pain are lacking, several approaches to this clinically challenging situation deserve focused evaluation and may prove to be viable therapeutic options.

Sex-Specific and Traumatic Brain Injury Effects on Dopamine Receptor Expression in the Hippocampus

Traumatic brain injury (TBI) is a major health concern. Each year, over 50 million individuals worldwide suffer from TBI, and this leads to a number of acute and chronic health issues. These include affective and cognitive impairment, as well as an increased risk of alcohol and drug use. The dopaminergic system, a key component of reward circuitry, has been linked to alcohol and other substance use disorders, and previous research indicates that TBI can induce plasticity within this system. Understanding how TBI modifies the dopaminergic system may offer insights into the heightened substance use and reward-seeking behavior following TBI. The hippocampus, a critical component of the reward circuit, is responsible for encoding and integrating the spatial and salient aspects of rewarding stimuli. This study explored TBI-related changes in neuronal D2 receptor expression within the hippocampus, examining the hypothesis that sex differences exist in both baseline hippocampal D2 receptor expression and its response to TBI. Utilizing D2-expressing tdTomato transgenic male and female mice, we implemented either a sham injury or the lateral fluid percussion injury (FPI) model of TBI and subsequently performed a region-specific quantification of D2 expression in the hippocampus. The results show that male mice exhibit higher baseline hippocampal D2 expression compared to female mice. Additionally, there was a significant interaction effect between sex and injury on the expression of D2 in the hippocampus, particularly in regions of the dentate gyrus. Furthermore, TBI led to significant reductions in hippocampal D2 expression in male mice, while female mice remained mostly unaffected. These results suggest that hippocampal D2 expression varies between male and female mice, with the female dopaminergic system demonstrating less susceptibility to TBI-induced plasticity.

Treatment of cognitive and mood disorders secondary to traumatic brain injury by the association of bilateral occipital nerve stimulation and a combined protocol of multisite repetitive transcranial magnetic stimulation and cognitive training: A case report

Purpose

Cognitive impairment secondary to traumatic brain injury (TBI) is difficult to treat and usually results in severe disability.

Method

A 48-year-old man presented with chronic refractory headaches and persistent disabling cognitive impairment after TBI. He was first treated with occipital nerve stimulation (ONS) implanted bilaterally to relieve headaches (8 years after the head trauma). Two years later, he was treated with a 6-week protocol combining repetitive transcranial magnetic stimulation (rTMS) delivered to multiple cortical sites (prefrontal cortex, language areas, and areas involved in visuo-spatial functions) and computerized cognitive training (CogT) (targeting memory, language, and visuo-spatial functions) to improve cognitive performance.

Results

Executive and cognitive functions (attention, ability to perform calculations, and verbal fluency) improved in association with pain relief after ONS (33-42% improvement) and then improved even more after the rTMS-CogT protocol with an additional improvement of 36-40% on apathy, depression, and anxiety, leading to a significant reduction in caregiver burden. The functional improvement persisted and even increased at 6 months after the end of the rTMS-CogT procedure (10 years after the onset of TBI and 2 years after ONS implantation).

Conclusion

This is the first observation describing sustained improvement in post-TBI refractory headache, depression, and cognitive impairment by the association of bilaterally implanted ONS and a combined procedure of multisite rTMS and CogT to target various brain functions.

Computed tomography findings as early predictors of long-term language impairment in patients with traumatic brain injury

This study aims to assess the relationship between computed tomography (CT) findings, during the acute phase of hospitalization, and long-term language impairment in people with traumatic brain injury (TBI). Another aim was to assess the receptive and expressive abilities of subjects with TBI based on the location of the injury. This is a retrospective observational study including 49 participants with TBI due to war injuries. The Arabic Diagnostic Aphasia Battery (A-DAB-1) was administered to the participants and the Helsinki CT score was computed to quantify brain damage. The results showed that the Helsinki CT score was negatively correlated with the total score of the A-DAB-1 (r = -0.544, p-value < 0.0001). Simple linear regression supported such findings and reflected an inversely proportional relationship between both variables (p-value < 0.0001). When compared with subjects having right hemisphere damage, subjects with left hemisphere and bilateral brain damage performed more poorly on language tasks respectively as follows: A-DAB-1 overall score (92.08-66.08-70.28, p-value = 0.021), Content of descriptive speech (9.57-6.69-7.22, p-value = 0.034), Verbal fluency (6.57-3.54-3.89, p-value = 0.002), Auditory comprehension (9.71-7.54-7.78, p-value = 0.039), Complex auditory commands (9.71-7.65-7.56, p-value = 0.043), Repetition (9.75-7.08-7.61, p-value = 0.036), Naming (9.93-7.15-8.11, p-value = 0.046). Following TBI, CT findings on admission can significantly predict long-term language abilities, with left side lesions inducing poorer outcomes.

A placebo-controlled randomized clinical trial of amantadine hydrochloride for evaluating the functional improvement of patients following severe acute traumatic brain injury

BACKGROUND

Considering the known derangements in the dopaminergic neurotransmitter systems following traumatic brain injury (TBI), dopamine agonists are used as a pharmacologic option. In this study, we evaluate the effects of amantadine hydrochloride on the functional improvement of severe TBI patients.

METHODS

Within a triple-blinded (patients, intervention administrators, and outcome assessors) placebo-controlled randomized clinical trial, we evaluated the effects of amantadine (100 mg BD (twice a day) for 14 days, then 150 mg BD for another 7 days, and 200 mg BD for another 21 days) on outcome measurements of weekly mean Glasgow Outcome Scale (GOS) and Disability Rating Scale (DRS), through six weeks of trial for 57 patients (29 amantadine, 28 placeboes) with severe TBI admitted in our hospital.

RESULTS

Although both groups had improvement in their DRS, the change from baseline was significantly better in the amantadine group (10.88±5.24 for amantadine vs. 8.04±4.07 for placebo, P=0.015). No significant difference was observed between groups for GOS (1.04±0.55 for amantadine vs. 1.12±1.05 for placebo, P=0.966).

CONCLUSIONS

Based on our findings, amantadine hydrochloride might improve the speed of functional ability improvement in severe TBI patients, evaluated by DRS, and is also well tolerated by patients. Although, there were some limitations in this study, including small sample size, short time interval, not providing a wash-off period and invalidity of GOS for measuring recovery rates in short-term periods.

Cognitive Rehabilitation: Mild Traumatic Brain Injury and Relevance of OTPF

There is increased awareness of the long-term cognitive sequelae of mild traumatic brain injury (mTBI). Therefore, researchers and clinicians have developed and tested cognitive training protocols to address these challenges. The current review summarized literature that examined existing cognitive rehabilitation/training programs. Specifically, the review listed the impact of these programs on functional domains informed by the Occupational Therapy Practice Framework (OTPF). Literature between the years 2008 and 2022 was gathered from nine databases. Results indicate that several cognitive rehabilitation programs have proven to positively influence domains of occupation, client factors, performance, and context. Occupational therapy practitioners have an opportunity to engage in mTBI management. Furthermore, adopting domains of OTPF may guide assessments, treatment planning, and long-term follow-up.

Can Virtual Reality Cognitive Rehabilitation Improve Executive Functioning and Coping Strategies in Traumatic Brain Injury? A Pilot Study

Executive dysfunction is among the most common and disabling facets of cognitive impairment following traumatic brain injury (TBI), and may include deficits in reasoning, planning, mental flexibility, some aspects of attention and orientation, awareness and behavior. Rehabilitation programs based on cognitive-behavioral approaches to retrain planning and problem-solving and other executive deficits may improve such cognitive dysfunction. The purpose of this study is to investigate the effects of non-immersive virtual reality-based training to improve executive abilities and to reduce anxiety and depression symptoms in patients with TBI. Twenty patients with moderate to severe TBI were enrolled at our Neurorehabilitation Unit and divided to receive either the standard cognitive training or the virtual reality (VR) based cognitive training using the virtual reality rehabilitation system (VRRS-Evo). Each group received the same amount of rehabilitative training, including ROT (Reality Orientation Therapy) and Executive Training (ET), but using a different approach, i.e., a paper and pencil and an advanced approach. All patients were evaluated with a specific psychometric battery before (T0) and after the end (T1) of each program. Comparing pre- and post- treatment scores, in the VR-CT group, we found statistically significant differences in all administered outcome measures for cognitive and executive functioning, i.e., MoCA (p < 0.005), FAB (p < 0.005), TMT-A (p < 0.005), TMT-B (p < 0.005), TMT-BA (p < 0.001), and mood, i.e., HRS-D (p < 0.008). In the Conventional cognitive training (C-CT) group, we found a significant improvement only in MoCA (p < 0.03), FAB (p < 0.02) and in TMT-BA (p < 0.01). Coping strategies also improved, with better results in the VR-CT group. Our results suggest that VR rehabilitation, using the VRRS system, may be a valuable and motivational approach to improve visuo-executive abilities and coping strategies as well as mood in chronic TBI patients.

Cannabidiol's neuroprotective properties and potential treatment of traumatic brain injuries

Cannabidiol (CBD) has numerous pharmacological targets that initiate anti-inflammatory, antioxidative, and antiepileptic properties. These neuroprotective benefits have generated interest in CBD's therapeutic potential against the secondary injury cascade from traumatic brain injury (TBI). There are currently no effective broad treatment strategies for combating the damaging mechanisms that follow the primary injury and lead to lasting neurological consequences or death. However, CBD's effects on different neurotransmitter systems, the blood brain barrier, oxidative stress mechanisms, and the inflammatory response provides mechanistic support for CBD's clinical utility in TBI. This review describes the cascades of damage caused by TBI and CBD's neuroprotective mechanisms to counter them. We also present challenges in the clinical treatment of TBI and discuss important future clinical research directions for integrating CBD in treatment protocols. The mechanistic evidence provided by pre-clinical research shows great potential for CBD as a much-needed improvement in the clinical treatment of TBI. Upcoming clinical trials sponsored by major professional sport leagues are the first attempts to test the efficacy of CBD in head injury treatment protocols and highlight the need for further clinical research.

Activation of the Locus Coeruleus Mediated by Designer Receptor Exclusively Activated by Designer Drug Restores Descending Nociceptive Inhibition after Traumatic Brain Injury in Rats

Disruption of endogenous pain control mechanisms including descending pain inhibition has been linked to several forms of pain including chronic pain after traumatic brain injury (TBI). The locus coeruleus (LC) is the principal noradrenergic (NA) nucleus participating in descending pain inhibition. We therefore hypothesized that selectively stimulating LC neurons would reduce nociception after TBI. All experiments used a well-characterized rat lateral fluid percussion model of TBI. NA neurons were stimulated by administering clozapine N-oxide (CNO) to rats selectively expressing a designer receptor exclusively activated by designer drug (DREADD) viral construct in their LC's. Mechanical nociceptive thresholds were measured using von Frey fibers. The efficacy of diffuse noxious inhibitory control (DNIC), a critical endogenous pain control mechanism, was assessed using the hindpaw administration of capsaicin. Immunohistochemical analyses demonstrated the selective expression of the DREADD construct in LC neurons after stereotactic injection. During the 1st week after TBI, when rats demonstrated hindlimb (HL) nociceptive sensitization, CNO administration provided transient anti-allodynia in DREADD-expressing rats but not in rats injected with control virus. Seven weeks after TBI we observed a complete loss of DNIC in response to capsaicin. However, CNO administration largely restored DNIC in TBI DREADD-expressing rats but not those injected with control virus. Unexpectedly, the effects of LC activation in the DREADD-expressing rats were blocked by the α-1 adrenergic receptor antagonist prazosin, but not the α-2 adrenergic receptor antagonist atipamezole. These results suggest that directly stimulating the LC after TBI can reduce both early and late manifestations of dysfunctional endogenous pain regulation. Clinical approaches to activating descending pain circuits may reduce suffering in those with pain after TBI.

Large-N Rat Data Enables Phenotyping of Risky Decision-Making: A Retrospective Analysis of Brain Injury on the Rodent Gambling Task

Decision-making is substantially altered after brain injuries. Patients and rats with brain injury are more likely to make suboptimal, and sometimes risky choices. Such changes in decision-making may arise from alterations in how sensitive individuals are to outcomes. To assess this, we compiled and harmonized a large dataset from four studies of TBI, each of which evaluated behavior on the Rodent Gambling Task (RGT). We then determined whether the following were altered: (1) sensitivity to overall contingencies, (2) sensitivity to immediate outcomes, or (3) general choice phenotypes. Overall sensitivity was evaluated using the matching law, immediate sensitivity by looking at the probability of switching choices given a win or loss, and choice phenotypes by k-means clustering. We found significant reductions in sensitivity to the overall outcomes and a bias toward riskier alternatives in TBI rats. However, the substantial individual variability led to poor overall fits in matching analyses. We also found that TBI caused a significant reduction in the tendency to repeatedly choose a given option, but no difference in win- or loss-specific sensitivity. Finally, clustering revealed 5 distinct decision-making phenotypes and TBI reduced membership in the “optimal” type. The current findings support a hypothesis that TBI reduces sensitivity to contingencies. However, in the case of tasks such as the RGT, this is not a simple shift to indiscriminate or less discriminate responding. Rather, TBI rats are more likely to develop suboptimal preferences and frequently switch choices. Treatments will have to consider how this behavior might be corrected.

Endogenous Opioid Dynorphin Is a Potential Link between Traumatic Brain Injury, Chronic Pain, and Substance Use Disorder

Traumatic brain injury (TBI) is a serious public health problem associated with numerous physical and neuropsychiatric comorbidities. Chronic pain is prevalent and interferes with post-injury functioning and quality of life, whereas substance use disorder (SUD) is the third most common neuropsychiatric diagnosis after TBI. Neither of these conditions has a clear mechanistic explanation based on the known pathophysiology of TBI. Dynorphin is an endogenous opioid neuropeptide that is significantly dysregulated after TBI. Both dynorphin and its primary receptor, the ĸ-opioid receptor (KOR), are implicated in the neuropathology of chronic pain and SUD. Here, we review the known roles of dynorphin and KORs in chronic pain and SUDs. We synthesize this information with our current understanding of TBI and highlight potential mechanistic parallels between and across conditions that suggest a role for dynorphin in long-term sequelae after TBI. In pain studies, dynorphin/KOR activation has either antinociceptive or pro-nociceptive effects, and there are similarities between the signaling pathways influenced by dynorphin and those underlying development of chronic pain. Moreover, the dynorphin/KOR system is considered a key regulator of the negative affective state that characterizes drug withdrawal and protracted abstinence in SUD, and molecular and neurochemical changes observed during the development of SUD are mirrored by the pathophysiology of TBI. We conclude by proposing hypotheses and directions for future research aimed at elucidating the potential role of dynorphin/KOR in chronic pain and/or SUD after TBI.

Mild repetitive head impacts alter perivascular flow in the midbrain dopaminergic system in awake rats

Head injury is a known risk factor for Parkinson's disease. Disruption in the perivascular clearance of metabolic waste and unwanted proteins is thought to be a contributing factor to disease progression. We hypothesized that repetitive mild head impacts, without evidence of structural brain damage, would increase microgliosis and AQP4 expression and depolarization and alter perivascular flow in the midbrain dopaminergic system. Adult male rats were subjected to sham, or two mild head impacts separated by 48 h. Three weeks later, fully awake rats were imaged using dynamic, contrast-enhanced MRI to follow the distribution of intraventricular gadobenate dimeglumine contrast agent. Images were registered to and analysed using a 3D MRI rat atlas providing site-specific data on 171 different brain areas. Following imaging, rats were tested for cognitive function using the Barnes maze assay. Histological analyses of tyrosine hydroxylase, microglia activation and AQP4 expression and polarization were performed on a parallel cohort of head impacted rats at 20 days post insult to coordinate with the time of imaging. There was no change in the global flux of contrast agent between sham and head impacted rats. The midbrain dopaminergic system showed a significant decrease in the influx of contrast agent as compared to sham controls together with a significant increase in microgliosis, AQP4 expression and depolarization. There were no deficits in cognitive function. The histology showed a significant level of neuroinflammation in the midbrain dopaminergic system 3 weeks post mild repetitive head impact but no loss in tyrosine hydroxylase. MRI revealed no structural brain damage emphasizing the potential serious consequences of mild head impacts on sustained brain neuroinflammation in this area critical to the pathophysiology of Parkinson's.

Early amantadine treatment reduces the risk of death in patients with large hemisphere infarctions:a Chinese hospital-based study

Background

Amantadine hydrochloride is one of the most frequently prescribed drugs for patients with severe traumatic brain injury in restoring consciousness and accelerating the pace of functional recovery. However, there is a paucity of studies on the effectiveness of amantadine in patients with severe stroke especially large hemisphere infarction (LHI). The present study aimed to investigate whether amantadine treatment is associated with better clinical outcomes in conservatively treated LHI patients.

Methods

We retrospectively collected conservatively treated LHI patients according to inclusion/exclusion criteria. The patients were divided into two groups based on the treatment regimen, whether they did receive amantadine hydrochloride in addition to standard therapy (ST) or not. The primary outcomes were in-hospital death, 3-month mortality, and unfavorable outcome (defined as modified Rankin Scale score of 4 to 6). All outcomes were compared between the two groups before and after propensity score matching (PSM). Multivariate logistic regression was performed to identify the association between early amantadine hydrochloride treatment and clinical outcomes in LHI patients.

Results

Thirty-one LHI patients treated with amantadine combined with ST and 127 patients treated with ST were enrolled. Amantadine group had a shorter prehospital delay (median: 2 vs. 10 h), a higher baseline NIHSS score (21.71 ± 4.76 vs. 17.49 ± 5.84), and a higher rate of dominant hemisphere involvement (67.74% vs. 45.67%). After PSM, amantadine treatment significantly reduced the risk of in-hospital death (7.41% vs. 31.11%, p=0.019) and 3-month mortality (25.93% vs. 55.56%, p=0.008). Amantadine treatment yielded a significant decrease in death in-hospital (before PSM: OR 0.143, 95% CI 0.034 to 0.605; after PSM: OR 0.113, 95% CI 0.020 to 0.635) and 3-month mortality (before PSM: OR 0.214, 95% CI 0.077 to 0.598; after PSM: OR 0.176, 95% CI 0.053 to 0.586) in unmatched and matched multivariate analyses.

Conclusion

The results of our study provide initial evidence that early amantadine treatment was associated with a decrease in death in conservatively treated LHI patients. Considering the limitations of observational study, randomized controlled trials with a large sample size may help provide a clearer picture of the utility of amantadine in LHI patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12883-021-02444-w.

A Pilot Trial Examining the Merits of Combining Amantadine and Repetitive Transcranial Magnetic Stimulation as an Intervention for Persons With Disordered Consciousness After TBI

OBJECTIVE

Report pilot findings of neurobehavioral gains and network changes observed in persons with disordered consciousness (DoC) who received repetitive transcranial magnetic stimulation (rTMS) or amantadine (AMA), and then rTMS+AMA.

PARTICIPANTS

Four persons with DoC 1 to 15 years after traumatic brain injury (TBI).

DESIGN

Alternate treatment-order, within-subject, baseline-controlled trial.

MAIN MEASURES

For group and individual neurobehavioral analyses, predetermined thresholds, based on mixed linear-effects models and conditional minimally detectable change, were used to define meaningful neurobehavioral change for the Disorders of Consciousness Scale-25 (DOCS) total and Auditory-Language measures. Resting-state functional connectivity (rsFC) of the default mode and 6 other networks was examined.

RESULTS

Meaningful gains in DOCS total measures were observed for 75% of treatment segments and auditory-language gains were observed after rTMS, which doubled when rTMS preceded rTMS+AMA. Neurobehavioral changes were reflected in rsFC for language, salience, and sensorimotor networks. Between networks interactions were modulated, globally, after all treatments.

CONCLUSIONS

For persons with DoC 1 to 15 years after TBI, meaningful neurobehavioral gains were observed after provision of rTMS, AMA, and rTMS+AMA. Sequencing and combining of treatments to modulate broad-scale neural activity, via differing mechanisms, merits investigation in a future study powered to determine efficacy of this approach to enabling neurobehavioral recovery.

Dopamine-Related Genes Moderate the Association Between Family Environment and Executive Function Following Pediatric Traumatic Brain Injury: An Exploratory Study

OBJECTIVE

This study examined whether carrying dopamine-related "risk" genes-either the dopamine transporter (DAT1) 10-repeat allele or dopamine receptor-4 (DRD4) 7-repeat allele-moderated the association of family environment and executive function (EF) following traumatic brain injury (TBI) in early childhood.

METHODS

Caregivers of children with TBI or orthopedic injury (OI) completed the Behavior Rating Inventory of Executive Function (BRIEF) at postinjury visits. General linear models examined gene by environment interactions as moderators of the effects of TBI on EF at 12 months and 7 years postinjury.

RESULTS

At 12 months, we did not find any significant gene by environment interactions. At 7 years, we found a significant 3-way interaction among combined carrier status, level of permissive parenting, and injury type. For children exposed to more optimal parenting, carriers of DAT1 and/or DRD4 risk alleles with TBI showed significantly worse parent-reported EF than carriers with OI. In those with less optimal parenting, carriers and noncarriers with TBI, as well as carriers with OI, showed significantly worse parent-reported EF than noncarriers with OI, with medium to large effect sizes.

CONCLUSIONS

The findings highlight the importance of considering polygenetic and environmental factors in future studies of recovery following TBI and other injuries in childhood.

Neural correlates of extrinsic and intrinsic outcome processing during learning in individuals with TBI: a pilot investigation

Outcome processing, the ability to learn from feedback, is an important component of adaptive behavior and rehabilitation. Evidence from healthy adults implicates the striatum and dopamine in outcome processing. Animal research shows that damage to dopaminergic pathways in the brain can lead to a disruption of dopamine tone and transmission. Such evidence thus suggests that persons with TBI experience deficits in outcome processing. However, no research has directly investigated outcome processing and associated neural mechanisms in TBI. Here, we examine outcome processing in individuals with TBI during learning. Given that TBI negatively impacts striatal and dopaminergic systems, we hypothesize that individuals with TBI exhibit deficits in learning from outcomes. To test this hypothesis, individuals with moderate-to-severe TBI and healthy adults were presented with a declarative paired-associate word learning task. Outcomes indicating performance accuracy were presented immediately during task performance and in the form of either monetary or performance-based feedback. Two types of feedback provided the opportunity to test whether extrinsic and intrinsic motivational aspects of outcome presentation play a role during learning and outcome processing. Our results show that individuals with TBI exhibited impaired learning from feedback compared to healthy participants. Additionally, individuals with TBI exhibited increased activation in the striatum during outcome processing. The results of this study suggest that outcome processing and learning from immediate outcomes is impaired in individuals with TBI and might be related to inefficient use of neural resources during task performance as reflected by increased activation of the striatum.

Association between Lifetime History of Traumatic Brain Injury, Prescription Opioid Use, and Persistent Pain: A Nationally Representative Study

Pain is common among adults with traumatic brain injury (TBI), yet little data exist regarding prevalence of opioid use in this population. The objective of this retrospective cohort study was to evaluate the association between lifetime TBI exposure, opioid use, and pain in a nationally representative sample of 1022 adults aged 50+ who participated in the Health and Retirement Study (HRS). Our primary exposure was lifetime TBI history measured via the Ohio State University TBI Identification Method. We evaluated three alternate TBI exposures (years since most recent TBI, age at first TBI, and number of lifetime TBIs) in sensitivity analyses. We evaluated two outcomes: recent opioid medication use, and moderate-to-severe pain measured over two HRS waves. We classified three pain groups (persistent, intermittent, and no pain). Prevalences of opioid use among individuals with and without TBI were 19.7% and 13.6%, respectively. After adjustment for age, sex, and race, individuals with TBI had a 52% increased risk for opioid use compared with individuals without TBI (relative risk = 1.52, 95% confidence interval: 1.11, 2.04). Individuals with recent TBI (1-10 years ago), first TBI after age 40+, and 2+ lifetime TBIs had greatest risk for opioid use. Compared with individuals without TBI, individuals with TBI had 4.9-times increased odds for persistent versus no pain, and 1.9-times increased odds of intermittent versus no pain. Persistent pain among adults with lifetime TBI is elevated compared with the general population, which may contribute to increased opioid use among persons with TBI, particularly those with recent injuries or multiple lifetime TBIs.

Pharmacological Treatment of Agitation and/or Aggression in Patients With Traumatic Brain Injury: A Systematic Review of Reviews

OBJECTIVE

To systematically review the available literature on the pharmacological management of agitation and/or aggression in patients with traumatic brain injury (TBI), synthesize the available data, and provide guidelines.

DESIGN

Systematic review of systematic reviews.

MAIN MEASURES

A literature review of the following websites was performed looking for systematic reviews on the treatment of agitation and/or aggression among patients with TBI: PubMed, CINAHL, DynaMed, Health Business Elite, and EBSCO (Psychology and behavioral sciences collection). Two researchers independently assessed articles for meeting inclusion/exclusion criteria. Data were extracted on year of publication, reviewed databases, dates of coverage, search limitations, pharmacological agents of interest, and a list of all controlled studies included. The included controlled studies were then examined to determine potential reasons for any difference in recommendations.

RESULTS

The literature review led to 187 citations and 67 unique publications after removing the duplicates. Following review of the title/abstracts and full texts, a total of 11 systematic reviews were included. The systematic reviews evaluated the evidence for safety and efficacy of the following medications: amantadine, amphetamines, methylphenidate, antiepileptics, atypical and typical antipsychotics, benzodiazepines, β-blockers, and sertraline.

CONCLUSIONS

On the basis of the results of this literature review, the authors recommend avoiding benzodiazepines and haloperidol for treating agitation and/or aggression in the context of TBI. Atypical antipsychotics (olanzapine in particular) can be considered as practical alternatives for the as-needed management of agitation and/or aggression in lieu of benzodiazepines and haloperidol. Amantadine, β-blockers (propranolol and pindolol), antiepileptics, and methylphenidate can be considered for scheduled treatment of agitation and/or aggression in patients with TBI.

Role of the Dopaminergic System in the Striatum and Its Association With Functional Recovery or Rehabilitation After Brain Injury

Disabilities are estimated to occur in approximately 2% of survivors of traumatic brain injury (TBI) worldwide, and disability may persist even decades after brain injury. Facilitation or modulation of functional recovery is an important goal of rehabilitation in all patients who survive severe TBI. However, this recovery tends to vary among patients because it is affected by the biological and physical characteristics of the patients; the types, doses, and application regimens of the drugs used; and clinical indications. In clinical practice, diverse dopaminergic drugs with various dosing and application procedures are used for TBI. Previous studies have shown that dopamine (DA) neurotransmission is disrupted following moderate to severe TBI and have reported beneficial effects of drugs that affect the dopaminergic system. However, the mechanisms of action of dopaminergic drugs have not been completely clarified, partly because dopaminergic receptor activation can lead to restoration of the pathway of the corticobasal ganglia after injury in brain structures with high densities of these receptors. This review aims to provide an overview of the functionality of the dopaminergic system in the striatum and its roles in functional recovery or rehabilitation after TBI.

Progressive Neurodegeneration Across Chronic Stages of Severe Traumatic Brain Injury

OBJECTIVE

To examine the trajectory of structural gray matter changes across 2 chronic periods of recovery in individuals who have sustained severe traumatic brain injury (TBI), adding to the growing literature indicating that neurodegenerative processes occur in the months to years postinjury.

PARTICIPANTS

Patients who experienced posttraumatic amnesia of 1 hour or more, and/or scored 12 or less on the Glasgow Coma Scale at the emergency department or the scene of the accident, and/or had positive brain imaging findings were recruited while receiving inpatient care, resulting in 51 patients with severe TBI.

METHODS

Secondary analyses of gray matter changes across approximately 5 months, 1 year, and 2.5 years postinjury were undertaken, using an automated segmentation protocol with improved accuracy in populations with morphological anomalies. We compared patients and matched controls on regions implicated in poorer long-term clinical outcome (accumbens, amygdala, brainstem, hippocampus, thalamus). To model brain-wide patterns of change, we then conducted an exploratory principal component analysis (PCA) on the linear slopes of all regional volumes across the 3 time points. Finally, we assessed nonlinear trends across earlier (5 months-1 year) versus later (1-2.5 years) time-windows with PCA to compare degeneration rates across time. Chronic degeneration was predicted cortically and subcortically brain-wide, and within specific regions of interest.

RESULTS

(1) From 5 months to 1 year, patients showed significant degeneration in the accumbens, and marginal degeneration in the amygdala, brainstem, thalamus, and the left hippocampus when examined unilaterally, compared with controls. (2) PCA components representing subcortical and temporal regions, and regions from the basal ganglia, significantly differed from controls in the first time-window. (3) Progression occurred at the same rate across both time-windows, suggesting neither escalation nor attenuation of degeneration across time.

CONCLUSION

Localized yet progressive decline emphasizes the necessity of developing interventions to offset degeneration and improve long-term functioning.

Amantadine: reappraisal of the timeless diamond-target updates and novel therapeutic potentials

The aim of the current review was to provide a new, in-depth insight into possible pharmacological targets of amantadine to pave the way to extending its therapeutic use to further indications beyond Parkinson's disease symptoms and viral infections. Considering amantadine's affinities in vitro and the expected concentration at targets at therapeutic doses in humans, the following primary targets seem to be most plausible: aromatic amino acids decarboxylase, glial-cell derived neurotrophic factor, sigma-1 receptors, phosphodiesterases, and nicotinic receptors. Further three targets could play a role to a lesser extent: NMDA receptors, 5-HT3 receptors, and potassium channels. Based on published clinical studies, traumatic brain injury, fatigue [e.g., in multiple sclerosis (MS)], and chorea in Huntington's disease should be regarded potential, encouraging indications. Preclinical investigations suggest amantadine's therapeutic potential in several further indications such as: depression, recovery after spinal cord injury, neuroprotection in MS, and cutaneous pain. Query in the database http://www.clinicaltrials.gov reveals research interest in several further indications: cancer, autism, cocaine abuse, MS, diabetes, attention deficit-hyperactivity disorder, obesity, and schizophrenia.

Differential Change in Oculomotor Performance among Female Collegiate Soccer Players versus Non-Contact Athletes from Pre- to Post-Season

Sensitive and reliable tools are needed to evaluate potential behavioral and cognitive changes following head impact exposure in contact and collision sport participation. We evaluated change in oculomotor testing performance among female, varsity, collegiate athletes following variable exposure to head impacts across a season. Female, collegiate, contact sport (soccer, CONT) and non-contact sport (NON-CONT) athletes were assessed pre-season and post-season. Soccer athletes were grouped according to total season game headers into low dose (≤40 headers; CONT-Low Dose) or high dose (>40 headers; CONT-High Dose) groups. Performance on pro-saccade (reflexive visual response), anti-saccade (executive inhibition), and memory-guided saccade (MGS, spatial working memory) computer-based laboratory tasks were assessed. Primary saccade measures included latency/reaction time, inhibition error rate (anti-saccade only), and spatial accuracy (MGS only). NON-CONT (n = 20), CONT-Low Dose (n = 17), and CONT-High Dose (n = 7) groups significantly differed on pre-season versus post-season latency on tasks with executive functioning demands (anti-saccade and MGS, p ≤ 0.001). Specifically, NON-CONT and CONT-Low Dose demonstrated shorter (i.e., faster) anti-saccade (1.84% and 2.68%, respectively) and MGS (5.74% and 2.76%, respectively) latencies from pre-season to post-season, whereas CONT-High Dose showed 1.40% average longer anti-saccade, and 0.74% shorter MGS, latencies. NON-CONT and CONT-Low Dose demonstrated reduced (i.e., improved) inhibition error rate on the anti-saccade task at post-season versus pre-season, whereas CONT-High Dose demonstrated relative stability (p = 0.021). The results of this study suggest differential exposure to subconcussive head impacts in collegiate female athletes is associated with differential change in reaction time and inhibitory control performances on executive saccadic oculomotor testing.

123I-Iofluopane Single-Photon Emission Computed Tomography as an Imaging Biomarker of Pre-Synaptic Dopaminergic System after Moderate-to-Severe Traumatic Brain Injury

Dopaminergic (DA) system function is frequently disrupted after traumatic brain injury (TBI). However, published interventions that target the DA system with the hope of enhancing functional outcomes are inconclusive, partially because of the lack of DA signaling biomarkers that can be used to select patients likely to benefit from DA-directed therapies or to monitor treatment efficacy. The aim of this study was to evaluate the feasibility of using ¹²³I-iofluopane single-photon emission computerized tomography (SPECT) to assess pre-synaptic DA system dysfunction after severe TBI. Eighteen patients with severe TBI were enrolled in this study. ¹²³I-iofluopane SPECT imaging was performed at baseline and again 2.5 h after a single dose of methylphenidate (MP) administered enterally. DA transporter (DAT) specific binding ratio (SBR) before and after MP was measured. Functional outcomes included the Disability Rating Scale, JFK Coma Recovery Scale-Revised, Functional Independence Measure, and Functional Assessment Measure. Thirteen of 18 patients completed the study. Average time from injury to SPECT scan was 48 days (standard deviation [SD], 24 days; median, 31). Baseline ioflupane striatal SBR was 1.51 ± 0.46 (median, 1.67). A 43.1% (SD, 16; median, 46.5) displacement of ioflupane from pre-synaptic DAT was observed after MP administration. Baseline SBR positively correlated with functional status at baseline and 4 weeks after completion of the study. Serum MP levels correlated with relative change in SBR (r_s = 0.60; p = 0.04). Our findings suggest that ¹²³I-iofluopane SPECT is a promising tool to determine the severity of pre-synaptic DA terminal disruption and for monitoring pharmacokinetics and pharmacodynamics of therapeutic interventions targeting the DA system.

Traumatic brain injury substantially reduces the conditioned reinforcing effects of environmental cues in rats

Traumatic brain injury affects millions of people each year and is an established risk factor for addiction. Recent animal studies have causally demonstrated that injuries can increase drug self-administration across a variety of substances. One potential behavioral mediator for this finding is an increased responsivity to drug-associated cues. This endophenotype can be identified by profiling non-drug-related behaviors. The current study evaluated several paradigms (conditioned approach, conditioned reinforcement, extinction from variable interval responding, conditioned facilitation) to determine how rats with a frontal TBI differed in their response to Pavlovian conditioning in response to food-paired cues. Surprisingly, rats with a TBI demonstrated increased goal-tracking in a conditioned approach paradigm and exerted less effort for a conditioned reinforcer. Moreover, they had slightly facilitated extinction (as demonstrated by significantly larger interresponse times) in the face of reinforcer-associated cues. Despite these effects, TBI rats still demonstrated conditioned facilitation to an auditory stimulus. Together, these effects suggest a phenotype in the opposite direction of what might be anticipated. Cues still served a strong discriminative function and altered behavior; however, they did not function as strong conditioned reinforcers for TBI animals. One potential reason for this is that substantial changes to the dopamine system after TBI may reduce the conditioned reinforcing effects of cues, but sensitize the brain to potent drugs of abuse. More research will be needed to determine whether this is the case.

Loss of diffuse noxious inhibitory control after traumatic brain injury in rats: A chronic issue

Chronic pain is one of the most challenging and debilitating symptoms to manage after traumatic brain injury (TBI), yet the underlying mechanisms remain elusive. The disruption of normal endogenous pain control mechanisms has been linked to several forms of chronic pain and may play a role in pain after TBI. We hypothesized therefore that dysfunctional descending noradrenergic and serotonergic pain control circuits may contribute to the loss of diffuse noxious inhibitory control (DNIC), a critical endogenous pain control mechanism, weeks to months after TBI. For these studies, the rat lateral fluid percussion model of mild TBI was used along with a DNIC paradigm involving a capsaicin-conditioning stimulus. We observed sustained failure of the DNIC response up to 180-days post injury. We confirmed, that descending α₂ adrenoceptor-mediated noradrenergic signaling was critical for endogenous pain inhibition in uninjured rats. However, augmenting descending noradrenergic signaling using reboxetine, a selective noradrenaline reuptake inhibitor, failed to restore DNIC after TBI. Furthermore, blocking serotonin-mediated descending signaling using selective spinal serotonergic fiber depletion with 5, 7-dihydroxytryptamine was also unsuccessful at restoring endogenous pain modulation after TBI. Unexpectedly, increasing descending serotonergic signaling using the selective serotonin reuptake inhibitor escitalopram and the serotonin-norepinephrine reuptake inhibitor duloxetine restored the DNIC response in TBI rats at both 49- and 180- days post injury. Consistent with these observations, spinal serotonergic fiber depletion with 5, 7-dihydroxytryptamine eliminated the effects of escitalopram. Intact α₂ adrenoceptor signaling, however, was not required for the serotonin-mediated restoration of DNIC after TBI. These results suggest that TBI causes maladaptation of descending nociceptive signaling mechanisms and changes in the function of both adrenergic and serotonergic circuits. Such changes could predispose those with TBI to chronic pain.

The closed-head impact model of engineered rotational acceleration (CHIMERA) as an application for traumatic brain injury pre-clinical research: A status report

Closed-head traumatic brain injury (TBI) is a worldwide concern with increasing prevalence and cost to society. Rotational acceleration is a primary mechanism in TBI that results from tissue strains that give rise to diffuse axonal injury. The Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA) was recently introduced as a method for the study of impact acceleration effects in pre-clinical TBI research. This review provides a survey of the published literature implementing the CHIMERA device and describes pathological, imaging, neurophysiological, and behavioral findings. Findings show CHIMERA inflicts damage in white matter tracts as a key area of injury. Behaviorally, repeated studies have shown motor deficits and more chronic cognitive effects after CHIMERA injury. Good progress with model application has been accomplished by investigators attending to what is required for model validation. However, the majority of CHIMERA studies only utilize adult male mice. To further establish this model, more work with female animals and various age groups need to be performed, as well as studies to further establish and standardize methodologies for validation of the models for clinical relevance. Common data elements to standardize the reporting methodology for the CHIMERA literature are suggested.

The use of amantadine in patients with unresponsive wakefulness syndrome after severe cerebral hemorrhage

BACKGROUND

Amantadine is currently recommended for use in patients of post-traumatic brain injury with unresponsive wakefulness syndrome (UWS). However, the application of amantadine in UWS after cerebral hemorrhage has only been rarely reported. This allows for a further exploration of the role of amantadine in the treatment of UWS resulting from a severe cerebral hemorrhage.

METHODS

We observed the changes of seven patients with UWS of intracerebral hemorrhage after taking amantadine. We also carried out a detailed neurological examination of the patient with disorders of consciousness to include or exclude subjects for the study. CRS-R score was used to evaluate the neurological recovery.

RESULTS

An improvement in consciousness was observed within 3-6 days after the start of amantadine administration in all seven cases (n = 7/7; 100%). Five patients recovered conscious and left aphasia, hemiplegia and other sequelae, and two patients recovered from UWS to minimally conscious state (MCS).

CONCLUSIONS

In this study, amantadine administration showed substantial positive effects on recovery following severe cerebral hemorrhage. We recommend further randomized controlled studies to determine the efficacy of amantadine.

Unilateral parietal brain injury increases risk-taking on a rat gambling task

Traumatic brain injury (TBI) affects millions of individuals every year. Many of these injuries lead to lasting effects, particularly impairments in domains broadly classified as executive functions, such as impulse control and decision-making. While these impairments have been historically associated with frontal brain damage, other injuries such as concussion or parietal injury also contribute to similar dysfunction. However, it is unknown whether animal models of TBI would replicate these broad effects that are observed in human patients. In the current study, we delivered a unilateral parietal controlled cortical impact injury and assessed the performance of rats on a motoric task (rotarod) and a test of decision-making and impulsivity (rodent gambling task). TBI rats demonstrated significant motor impairments on the rotarod task; however, this did not extend to difficulties inhibiting motor actions (impulsivity). In addition, TBI caused chronic alterations to risk-based decision-making, extending out to 12 weeks post-injury. Specifically, rats with TBI preferred the riskiest, and most suboptimal option over all others. The current data suggest that models of unilateral TBI are sufficient for replicating some aspects of executive dysfunction (risky decision-making), while others are limited to frontal damage (impulsivity). These models may be used to develop therapeutics targeted at the chronic post-injury period when these symptoms often manifest in patients, a critically understudied area in preclinical TBI research.

Reward presentation reduces on-task fatigue in traumatic brain injury

While cognitive fatigue is experienced by up to 80% of individuals with traumatic brain injury (TBI), little is known about its neural underpinnings. We previously hypothesized that presentation of rewarding outcomes leads to cognitive fatigue reduction and activation of the striatum, a brain region shown to be associated with cognitive fatigue in clinical populations and processing of rewarding outcomes. We have demonstrated this in individuals with multiple sclerosis. Here, we tested this hypothesis in individuals with TBI. Twenty-one individuals with TBI and 24 healthy participants underwent functional magnetic resonance imaging. Participants performed a task during which they were presented with 1) the Outcome condition where they were exposed to monetary rewards, and 2) the No Outcome condition that served as the control condition and was not associated with monetary rewards. In accordance with our hypothesis, results showed that attainment of rewarding outcomes leads to cognitive fatigue reduction in individuals with TBI, as well as activation of the striatum. Specifically, we observed a significant group by condition interaction on fatigue scores driven by the TBI group reporting lower levels of fatigue after the Outcome condition. fMRI data revealed a significant main-effect of condition in regions previously implicated in outcome processing, while a significant group by condition interaction was observed in the left ventral striatum as revealed by a priori region of interest analysis. Results suggest that a salient motivator can significantly reduce fatigue and that outcome presentation leads to increased activation of the ventral striatum in TBI. These findings can inform the development of future non-pharmacological cognitive fatigue treatment methods and contribute to the growing body of evidence showing the association between cognitive fatigue and the striatum.

Approaches to Monitor Circuit Disruption after Traumatic Brain Injury: Frontiers in Preclinical Research

Mild traumatic brain injury (TBI) often results in pathophysiological damage that can manifest as both acute and chronic neurological deficits. In an attempt to repair and reconnect disrupted circuits to compensate for loss of afferent and efferent connections, maladaptive circuitry is created and contributes to neurological deficits, including post-concussive symptoms. The TBI-induced pathology physically and metabolically changes the structure and function of neurons associated with behaviorally relevant circuit function. Complex neurological processing is governed, in part, by circuitry mediated by primary and modulatory neurotransmitter systems, where signaling is disrupted acutely and chronically after injury, and therefore serves as a primary target for treatment. Monitoring of neurotransmitter signaling in experimental models with technology empowered with improved temporal and spatial resolution is capable of recording in vivo extracellular neurotransmitter signaling in behaviorally relevant circuits. Here, we review preclinical evidence in TBI literature that implicates the role of neurotransmitter changes mediating circuit function that contributes to neurological deficits in the post-acute and chronic phases and methods developed for in vivo neurochemical monitoring. Coupling TBI models demonstrating chronic behavioral deficits with in vivo technologies capable of real-time monitoring of neurotransmitters provides an innovative approach to directly quantify and characterize neurotransmitter signaling as a universal consequence of TBI and the direct influence of pharmacological approaches on both behavior and signaling.

Considerations of power and sample size in rehabilitation research

With the current emphasis on power and reproducibility, pressures are rising to increase sample sizes in rehabilitation research in order to reflect more accurate effect estimation and generalizable results. The conventional way of increasing power by enrolling more participants is less feasible in some fields of research. In particular, rehabilitation research faces considerable challenges in achieving this goal. We describe the specific challenges to increasing power by recruiting large sample sizes and obtaining large effects in rehabilitation research. Specifically, we discuss how variability within clinical populations, lack of common standards for selecting appropriate control groups; potentially reduced reliability of measurements of brain function in individuals recovering from a brain injury; biases involved in a priori effect size estimation, and higher budgetary and staffing requirements can influence considerations of sample and effect size in rehabilitation. We also describe solutions to these challenges, such as increased sampling per participant, improving experimental control, appropriate analyses, transparent result reporting and using innovative ways of harnessing the inherent variability of clinical populations. These solutions can improve statistical power and produce reliable and valid results even in the face of limited availability of large samples.

Mild Traumatic Brain Injury Decreases Spatial Information Content and Reduces Place Field Stability of Hippocampal CA1 Neurons

Both clinical and experimental studies have reported that mild traumatic brain injury (mTBI) can result in cognitive impairments in the absence of overt brain damage. Whether these impairments result from neuronal dysfunction/altered plasticity is an area that has received limited attention. In this study, we recorded activity of neurons in the cornu Ammonis (CA)1 subfield of the hippocampus in sham and mild lateral fluid percussion injured (mFPI) rats while these animals were performing an object location task. Electrophysiology results showed that the number of excitatory neurons encoding spatial information (i.e., place cells) was reduced in mFPI rats, and that these cells had broader and less stable place fields. Additionally, the in-field firing rate of place cells in sham operated, but not in mFPI, animals increased when objects within the testing arena were moved. Immunostaining indicated no visible damage or overall neuronal loss in mFPI brain sections. However, a reduction in the number of parvalbumin-positive inhibitory neurons in the CA1 subfield of mFPI animals was observed, suggesting that this reduction could have influenced place cell physiology. Alterations in spatial information content, place cell stability, and activity in mFPI rats coincided with poor performance in the object location task. These results indicate that altered place cell physiology may underlie the hippocampus-dependent cognitive impairments that result from mTBI.

Enhanced descending pain facilitation in acute traumatic brain injury

Acute and persistent pain are recognized consequences of TBI that can enhance suffering and significantly impair rehabilitative efforts. Both experimental models and clinical studies suggest that TBI may result in an imbalance between descending pain facilitatory and inhibitory pathways. The aim of this study was to assess the role of enhanced descending serotonin-mediated pain facilitation in a rat TBI model using selective spinal serotonergic fiber depletion with 5, 7-dihydroxytryptamine (DHT). We observed significant hindpaw allodynia in TBI rats that was reduced after DHT but not vehicle treatment. Immunohistochemical studies demonstrated profound spinal serotonin depletion in DHT-treated rats. Furthermore, lumbar intrathecal administration of the 5-HT₃ receptor antagonist ondansetron at 7 days post-injury (DPI), when hindpaw allodynia was maximal, also attenuated nociceptive sensitization. Additional immunohistochemical analyses of the lumbar spinal cord at 7 DPI revealed a robust bilateral microglial response in the superficial dorsal horns that was significantly reduced with DHT treatment. Furthermore, serotonin depletion also prevented the TBI-induced bilateral increase in c-Fos positive cells within the Rexed laminae I and II of the dorsal horns. These results indicate that in the weeks following TBI, pain may be responsive to 5-HT₃ receptor antagonists or other measures which rebalance descending pain modulation.

Alcohol misuse and traumatic brain injury: a review of the potential roles of dopaminergic dysfunction and physiological underarousal post-injury

Although many researchers have demonstrated an increase in alcohol use following traumatic brain injury (TBI), there is also a body of research indicating that alcohol misuse predisposes one to injury and precedes TBI. Accordingly, various mechanisms have been proposed (e.g., self-medication, dampened levels of arousal, dopaminergic dysfunction, etc.) and variable results have emerged. This paper reviews the empirical evidence, for and against, TBI as a risk factor for alcohol misuse. In particular, this paper focuses on the brain-behavior relationships involved and examines the roles of physiological underarousal and dopaminergic dysfunction in the development of alcohol misuse after injury. Alcohol misuse impedes community reintegration among TBI survivors and creates additional rehabilitative challenges. Thus, in order to inform and improve treatment outcomes among this vulnerable population, a deeper understanding of the neural mechanisms implicated is needed.

Exposure to uncertainty mediates the effects of traumatic brain injury on probabilistic decision-making in rats

Primary Objective: Traumatic brain injury (TBI) is associated with numerous psychiatric comorbidities, and subclinical psychiatric symptoms. While many symptoms have been replicated in animal models of brain injury, a vast majority of studies utilize naïve rats as subjects, which fail to mimic the complex learning history of human patients.Methods and Procedures: In the current study, we evaluated the effects of a brain injury in animals with early exposure to uncertainty on post-injury decision-making in a probabilistic task, the rodent gambling task (RGT).Main Outcomes and Results: Exposure to uncertainty resulted in a heterogeneous sample relative to prior publications, and brain-injured rats showed no deficits in choice behavior compared to shams which contrasts with large, pervasive deficits in previously published work. However, TBI increased impulsivity and caused transient changes in behavioral variables indicative of initial motivational deficits (pellets earned, omitted responses). Notably, effects of amphetamine were similar on this heterogeneous sample of rats relative to a number of other published reports, suggesting consistent effects of gross monoaminergic manipulations on choice behavior, independent of experience.Conclusions: Going forward, translational studies need to consider the heterogeneity that exists at the clinical level and account for these problems when modeling diseases in animals.

Treadmill exercise improves memory by up-regulating dopamine and down-regulating D2 dopamine receptor in traumatic brain injury rats

Traumatic brain injury (TBI) causes a variety of neuropathological manifestations including cognitive, emotional, physiological and psychological deficits. Physical exercise is known to ameliorate neurological impairments induced by various brain injuries. We investigated the effects of treadmill exercise on memory impairments due to TBI in relation to dopamine and D₂ dopamine receptor. TBI was induced with an electromagnetic-controlled cortical impact device. The rats in the exercise groups were scheduled to run on a treadmill for 30 min once a day for 28 days after TBI induction. Then, step-down avoidance task, radial 8-arm maze test, immunohistochemistry for tyrosine hydroxylase (TH), and western blot for D₂ dopamine receptor were performed. TBI impaired short-term and spatial learning memories. TBI decreased TH expressions in the prefrontal cortex (PFC), striatum, hippocampus dentate gyrus, and substantia nigra (SN). By contrast, the expressions of D₂ dopamine receptor in the PFC, striatum, hippocampus, and SN were increased by TBI. Treadmill exercise alleviated the impairments of short-term and spatial learning memories observed in TBI rats. TH expression was decreased and D₂ dopamine receptor expression was increased in TBI rats. Treadmill exercise enhanced TH expression and suppressed D₂ dopamine receptor expression in TBI rats. TBI deteriorated short-term and spatial learning memories, in contrast, treadmill exercise alleviated the TBI-induced memory impairments by up-regulating dopamine level and down-regulating D₂ dopamine receptor expression.

Neuroradiological Changes Following Single or Repetitive Mild TBI

Objectives

To test the hypothesis that there are differences in neuroradiological measures between single and repeated mild traumatic brain injury using multimodal MRI.

Methods

A closed-head momentum exchange model was used to produce one or three mild head injuries in young adult male rats compared to non-injured, age and weight-matched controls. Six-seven weeks post-injury, rats were studied for deficits in cognitive and motor function. Seven-eight weeks post-injury changes in brain anatomy and function were evaluated through analysis of high resolution T2 weighted images, resting-state BOLD functional connectivity, and diffusion weighted imaging with quantitative anisotropy.

Results

Head injuries occurred without skull fracture or signs of intracranial bleeding or contusion. There were no significant differences in cognitive or motors behaviors between experimental groups. With a single mild hit, the affected areas were limited to the caudate/putamen and central amygdala. Rats hit three times showed altered diffusivity in white matter tracts, basal ganglia, central amygdala, brainstem, and cerebellum. Comparing three hits to one hit showed a similar pattern of change underscoring a dose effect of repeated head injury on the brainstem and cerebellum. Disruption of functional connectivity was pronounced with three mild hits. The midbrain dopamine system, hippocampus, and brainstem/cerebellum showed hypoconnectivity. Interestingly, rats exposed to one hit showed enhanced functional connectivity (or hyperconnectivity) across brain sites, particularly between the olfactory system and the cerebellum.

Interpretation

Neuroradiological evidence of altered brain structure and function, particularly in striatal and midbrain dopaminergic areas, persists long after mild repetitive head injury. These changes may serve as biomarkers of neurodegeneration and risk for dementia later in life.

Executive (dys)function after traumatic brain injury: special considerations for behavioral pharmacology

Executive function is an umbrella term that includes cognitive processes such as decision-making, impulse control, attention, behavioral flexibility, and working memory. Each of these processes depends largely upon monoaminergic (dopaminergic, serotonergic, and noradrenergic) neurotransmission in the frontal cortex, striatum, and hippocampus, among other brain areas. Traumatic brain injury (TBI) induces disruptions in monoaminergic signaling along several steps in the neurotransmission process - synthesis, distribution, and breakdown - and in turn, produces long-lasting deficits in several executive function domains. Understanding how TBI alters monoamingeric neurotransmission and executive function will advance basic knowledge of the underlying principles that govern executive function and potentially further treatment of cognitive deficits following such injury. In this review, we examine the influence of TBI on the following measures of executive function - impulsivity, behavioral flexibility, and working memory. We also describe monoaminergic-systems changes following TBI. Given that TBI patients experience alterations in monoaminergic signaling following injury, they may represent a unique population with regard to pharmacotherapy. We conclude this review by discussing some considerations for pharmacotherapy in the field of TBI.

Dopamine D2/D3 receptor abnormalities after traumatic brain injury and their relationship to post-traumatic depression

Objective

To investigate dopamine D2/D3 receptor availability following traumatic brain injury (TBI) and their relationship to the presence of DSM-IV Major Depressive Disorder (MDD) and patterns of axonal injury.

Methods

Twelve moderate-severe TBI patients and 26 controls were imaged using [¹¹C]PHNO positron emission tomography (PET) and structural magnetic resonance imaging (MRI). TBI patients and a second group of 32 controls also underwent diffusion tensor imaging (DTI) and neuropsychological assessment. Patients included six with post-injury MDD (TBI-MDD) and six without (TBI-NON). Non-displaceable binding potential (BP_ND) [¹¹C]PHNO values were used to index D2/D3 receptor availability, and were calculated using a reference region procedure. Differences in BP_ND were examined using voxelwise and region-of-interest analyses. White matter microstructure integrity, quantified by fractional anisotropy (FA), was assessed and correlated with BP_ND.

Results

Lower [¹¹C]PHNO BP_ND was found in the caudate across all TBI patients when compared to controls. Lower [¹¹C]PHNO BP_ND was observed in the caudate of TBI-MDD patients and increased [¹¹C]PHNO BP_ND in the Amygdala of TBI-NON patients compared to controls. There were no significant differences in [¹¹C]PHNO BP_ND between TBI-MDD and TBI-NON patients. Furthermore, DTI provided evidence of axonal injury following TBI. The uncinate fasciculus and cingulum had abnormally low FA, with the uncinate particularly affected in TBI-MDD patients. Caudate [¹¹C]PHNO BP_ND correlated with FA within the nigro-caudate tract.

Conclusions

[¹¹C]PHNO BP_ND is abnormal following TBI, which indicates post-traumatic changes in D2/D3 receptors. Patterns of [¹¹C]PHNO BP_ND seen in patients with and without MDD suggest that further research would be beneficial to determine whether the use of dopaminergic treatment might be effective in the treatment of post-traumatic depression.

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[¹¹C]PHNO PET is used for the first time in traumatic brain injury (TBI) patients.

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Post-traumatic changes in dopamine D2/D3 receptors were observed.

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Patients with major depression showed more prominent reductions in [¹¹C]PHNO BP_ND.

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Non-depressed TBI patients had greater [¹¹C]PHNO BP_ND in the Amygdala.

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These findings suggest a potential role of D2/D3 changes in post-TBI depression.

GABAergic imbalance is normalized by dopamine D1 receptor activation in the striatum contralateral to the cortical injury in motor deficit-recovered rats

RATIONALE

The sensorimotor cortex and the striatum are interconnected by the corticostriatal pathway, suggesting that cortical injury alters the striatal function, which may be modulated by dopamine.

OBJECTIVES

We studied whether the activation of dopamine D₁ receptors (D₁Rs) modulates the γ-aminobutyric acid (GABA) and glutamate levels in the striatum of recovered rats at 192 h after cortical injury.

METHODS

The D₁R agonist SKF-38393 (0, 2, 3, or 4 mg/kg) was administered at 24, 48, 96, and 192 h post-injury, and then rats were decapitated to determine GABA and glutamate levels and the levels of D₁R mRNA on both sides of the striatum.

RESULTS

GABAergic imbalance in the striatum contralateral to the injury site was normalized by the administration of the D₁R agonist, but this treatment did not produce a significant effect on glutamate levels, suggesting that glutamate was metabolized into GABA. The administration of SKF-38393 (2 mg/kg) decreased the levels of D₁R mRNA in the striatum contralateral to the injury, and this effect was blocked by the coadministration of the D₁R antagonist SCH-23390 (2 mg/kg). In the striatum ipsilateral to the injury, the D₁R agonist increased the D₁R mRNA levels, an effect that was blocked by SCH-23390.

CONCLUSION

The reversal of the GABAergic imbalance in the striatum contralateral to the cortical injury can be modulated by extrastriatal D₁R activation, and the D₁R agonist-induced increases in the D₁R mRNA levels in the striatum ipsilateral to the injury suggest that the striatum may be necessary to achieve functional recovery.

Neurotransmitter changes after traumatic brain injury: an update for new treatment strategies

Traumatic brain injury (TBI) is a pervasive problem in the United States and worldwide, as the number of diagnosed individuals is increasing yearly and there are no efficacious therapeutic interventions. A large number of patients suffer with cognitive disabilities and psychiatric conditions after TBI, especially anxiety and depression. The constellation of post-injury cognitive and behavioral symptoms suggest permanent effects of injury on neurotransmission. Guided in part by preclinical studies, clinical trials have focused on high-yield pathophysiologic mechanisms, including protein aggregation, inflammation, metabolic disruption, cell generation, physiology, and alterations in neurotransmitter signaling. Despite successful treatment of experimental TBI in animal models, clinical studies based on these findings have failed to translate to humans. The current international effort to reshape TBI research is focusing on redefining the taxonomy and characterization of TBI. In addition, as the next round of clinical trials is pending, there is a pressing need to consider what the field has learned over the past two decades of research, and how we can best capitalize on this knowledge to inform the hypotheses for future innovations. Thus, it is critically important to extend our understanding of the pathophysiology of TBI, particularly to mechanisms that are associated with recovery versus development of chronic symptoms. In this review, we focus on the pathology of neurotransmission after TBI, reflecting on what has been learned from both the preclinical and clinical studies, and we discuss new directions and opportunities for future work.

Dopaminergic abnormalities following traumatic brain injury

Traumatic brain injury can reduce striatal dopamine levels. The cause of this is uncertain, but is likely to be related to damage to the nigrostriatal system. We investigated the pattern of striatal dopamine abnormalities using 123I-Ioflupane single-photon emission computed tomography (SPECT) scans and their relationship to nigrostriatal damage and clinical features. We studied 42 moderate-severe traumatic brain injury patients with cognitive impairments but no motor parkinsonism signs and 20 healthy controls. 123I-Ioflupane scanning was used to assess dopamine transporter levels. Clinical scan reports were compared to quantitative dopamine transporter results. Advanced MRI methods were used to assess the nigrostriatal system, including the area through which the nigrostriatal projections pass as defined from high-resolution Human Connectome data. Detailed clinical and neuropsychological assessments were performed. Around 20% of our moderate-severe patients had clear evidence of reduced specific binding ratios for the dopamine transporter in the striatum measured using 123I-Ioflupane SPECT. The caudate was affected more consistently than other striatal regions. Dopamine transporter abnormalities were associated with reduced substantia nigra volume. In addition, diffusion MRI provided evidence of damage to the regions through which the nigrostriatal tract passes, particularly the area traversed by dopaminergic projections to the caudate. Only a small percentage of patients had evidence of macroscopic lesions in the striatum and there was no relationship between presence of lesions and dopamine transporter specific binding ratio abnormalities. There was also no relationship between reduced volume in the striatal subregions and reduced dopamine transporter specific binding ratios. Patients with low caudate dopamine transporter specific binding ratios show impaired processing speed and executive dysfunction compared to patients with normal levels. Taken together, our results suggest that the dopaminergic system is affected by a moderate-severe traumatic brain injury in a significant proportion of patients, even in the absence of clinical motor parkinsonism. Reduced dopamine transporter levels are most commonly seen in the caudate and this is likely to reflect the pattern of nigrostriatal tract damage produced by axonal injury and associated midbrain damage.