Adolescent Traumatic Brain Injury Induces Chronic Mesolimbic Neuroinflammation with Concurrent Enhancement in the Rewarding Effects of Cocaine in Mice during Adulthood

Activation of CB2R by synthetic CB2R agonist, PM289, improves brain endothelial barrier properties, decreases inflammatory response and enhances endothelial repair

The Cannabinoid 2 Receptor (CB2R) has been found to provide immunological modulation in different cell types. More recently, detection of CB2R in the cerebral endothelium suggests a possible role in the resolution of inflammation at the level of the blood-brain-barrier (BBB). Here, the notion that CB2R upregulation in brain endothelial cells could be exploited to promote vascular protection and BBB integrity was evaluated. Targeting and activation of CB2R was accomplished by a novel and highly specific chromenopyrazole based CB2R agonist, PM289. This study demonstrates that CB2R upregulation is induced as early as 8 h in the cortical vasculature in an experimental mouse model of TBI. Unlike CB2R, CB1R was marginally detected and not significantly induced. In the human brain endothelial cell line, hCMEC/D3 cells, similar induction of CB2R was observed upon stimulation with TNFα. Analysis of transendothelial electrical resistance shows that PM289 markedly prevented the barrier-leakiness induced by TNFα. The BBB is also responsible for maintaining an immunological barrier. The five-fold increase in ICAM1 expression in stimulated endothelial cells was significantly diminished due to CB2R activation. Utilizing wounding assays, results showed that wound repair could be accomplished in nearly half the time when the novel CB2R agonist is present compared to the untreated control. Lastly, mechanistically, the effects of CB2R may be explained by the observed inhibition of the p65 NFκB subunit. Overall, these studies support the notion that targeting and activating CB2R in the brain vasculature could aid in BBB and vascular protection in the context of neuroinflammation.

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.

Obesity-mediated Lipoinflammation Modulates Food Reward Responses

Accumulation of white adipose tissue (WAT) during obesity is associated with the development of chronic low-grade inflammation, a biological process known as lipoinflammation. Systemic and central lipoinflammation accumulates pro-inflammatory cytokines including IL-6, IL-1β and TNF-α in plasma and also in brain, disrupting neurometabolism and cognitive behavior. Obesity-mediated lipoinflammation has been reported in brain regions of the mesocorticolimbic reward circuit leading to alterations in the perception and consumption of ultra-processed foods. While still under investigation, lipoinflammation targets two major outcomes of the mesocorticolimbic circuit during food reward: perception and motivation ("Wanting") and the pleasurable feeling of feeding ("Liking"). This review will provide experimental and clinical evidence supporting the contribution of obesity- or overnutrition-related lipoinflammation affecting the mesocorticolimbic reward circuit and enhancing food reward responses. We will also address neuroanatomical targets of inflammatory profiles that modulate food reward responses during obesity and describe potential cellular and molecular mechanisms of overnutrition linked to addiction-like behavior favored by brain lipoinflammation.

Association between pediatric TBI and mental health and substance use disorders: A scoping review

BACKGROUND

The relationship between pediatric Traumatic Brain Injury (TBI) and long-term mental health and substance use disorders is not well known, resulting in inadequate prevention and management strategies. The aim of this scoping review is to review the evidence on pediatric TBI and the development of mental health disorders and substance use later in life and to identify gaps in the literature to inform future research.

METHODS

We searched multiple databases for original articles published between September 2002 and September 2022 on TBI-related mental health and/or substance use disorders in children and youth. Two independent reviewers performed the screening using Arksey and O'Malley and Levac et al.'s scoping review framework.

RESULTS

A total of six papers are included in this scoping review. Studies included are comprised of cross-sectional and prospective longitudinal cohort studies.

DISCUSSION

A correlation between pediatric TBI and development of certain mental health disorders and substance use is suggested, although much of the current evidence is mixed and does not account for confounding variables. Future studies should aim to closely examine these links and identify modifiers that can influence these relationships.

Neuroinflammatory Response in Reward-Associated Psychostimulants and Opioids: A Review

Substance abuse is one of the significant problems in social and public health worldwide. Vast numbers of evidence illustrate that motivational and reinforcing impacts of addictive drugs are primarily attributed to their ability to change dopamine signaling in the reward circuit. However, the roles of classic neurotransmitters, especially dopamine and neuromodulators, monoamines, and neuropeptides, in reinforcing characteristics of abused drugs have been extensively investigated. It has recently been revealed that central immune signaling includes cascades of chemokines and proinflammatory cytokines released by neurons and glia via downstream intracellular signaling pathways that play a crucial role in mediating rewarding behavioral effects of drugs. More interestingly, inflammatory responses in the central nervous system modulate the mesolimbic dopamine signaling and glutamate-dependent currents induced by addictive drugs. This review summarized researches in the alterations of inflammatory responses accompanied by rewarding and reinforcing properties of addictive drugs, including cocaine, methamphetamine, and opioids that were evaluated by conditioned place preference and self-administration procedures as highly common behavioral tests to investigate the motivational and reinforcing impacts of addictive drugs. The neuroinflammatory responses affect the rewarding properties of psychostimulants and opioids.

Chronicity of repeated blast traumatic brain injury associated increase in oxycodone seeking in rats

Numerous epidemiological studies have found co-morbidity between non-severe traumatic brain injury (TBI) and substance misuse in both civilian and military populations. Preclinical studies have also identified this relationship for some misused substances. We have previously demonstrated that repeated blast traumatic brain injury (rbTBI) increased oxycodone seeking without increasing oxycodone self-administration, suggesting that the neurological sequelae of traumatic brain injury can elevate opioid misuse liability. Here, we determined the chronicity of this effect by testing different durations of time between injury and oxycodone self-administration and durations of abstinence. We found that the subchronic (four weeks), but not the acute (three days) or chronic (four months) duration between injury and oxycodone self-administration was associated with increased drug seeking and re-acquisition of self-administration following a 10-day abstinence. Examination of other abstinence durations (two days, four weeks, or four months) revealed no effect of rbTBI on drug seeking at any of the abstinence durations tested. Together, these data indicate that there is a window of vulnerability after TBI when oxycodone self-administration is associated with elevated drug seeking and relapse-related behaviors.

Inflammation in VTA Caused by HFD Induces Activation of Dopaminergic Neurons Accompanied by Binge-like Eating

Binge eating is a characteristic symptom observed in obese individuals that is related to dysfunction of dopaminergic neurons (DNs). Intermittent administration of a high-fat diet (HFD) is reported to induce binge-like eating, but the underlying mechanisms remain unclear. We generated dopaminergic neuron specific IKKβ deficient mice (KO) to examine the effects of inflammation in DNs on binge-like eating under inflammatory conditions associated with HFD. After administration of HFD for 4 weeks, mice were fasted for 24 h, and then the consumption of HFD was measured for 2 h. We also evaluated that the mRNA expressions of inflammatory cytokines, glial markers, and dopamine signaling-related genes in the ventral tegmental area (VTA) and striatum. Moreover, insulin was administered intraventricularly to assess downstream signaling. The consumption of HFD was significantly reduced, and the phosphorylation of AKT in the VTA was significantly increased in female KO compared to wild-type (WT) mice. Analyses of mRNA expressions revealed that DNs activity and inflammation in the VTA were significantly decreased in female KO mice. Thus, our data suggest that HFD-induced inflammation with glial cell activation in the VTA affects DNs function and causes abnormal eating behaviors accompanied by insulin resistance in the VTA of female mice.

Does Traumatic Brain Injury Cause Risky Substance Use or Substance Use Disorder?

There is a high co-occurrence of risky substance use among adults with traumatic brain injury (TBI), although it is unknown if the neurologic sequelae of TBI can promote this behavior. We propose that to conclude that TBI can cause risky substance use, it must be determined that TBI precedes risky substance use, that confounders with the potential to increase the likelihood of both TBI and risky substance use must be ruled out, and that there must be a plausible mechanism of action. In this review, we address these factors by providing an overview of key clinical and preclinical studies and list plausible mechanisms by which TBI could increase risky substance use. Human and animal studies have identified an association between TBI and risky substance use, although the strength of this association varies. Factors that may limit detection of this relationship include differential variability due to substance, sex, age of injury, and confounders that may influence the likelihood of both TBI and risky substance use. We propose possible mechanisms by which TBI could increase substance use that include damage-associated neuroplasticity, chronic changes in neuroimmune signaling, and TBI-associated alterations in brain networks.

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.

Traumatic Brain Injury: An Age-Dependent View of Post-Traumatic Neuroinflammation and Its Treatment

Traumatic brain injury (TBI) is a leading cause of death and disability all over the world. TBI leads to (1) an inflammatory response, (2) white matter injuries and (3) neurodegenerative pathologies in the long term. In humans, TBI occurs most often in children and adolescents or in the elderly, and it is well known that immune responses and the neuroregenerative capacities of the brain, among other factors, vary over a lifetime. Thus, age-at-injury can influence the consequences of TBI. Furthermore, age-at-injury also influences the pharmacological effects of drugs. However, the post-TBI inflammatory, neuronal and functional consequences have been mostly studied in experimental young adult animal models. The specificity and the mechanisms underlying the consequences of TBI and pharmacological responses are poorly understood in extreme ages. In this review, we detail the variations of these age-dependent inflammatory responses and consequences after TBI, from an experimental point of view. We investigate the evolution of microglial, astrocyte and other immune cells responses, and the consequences in terms of neuronal death and functional deficits in neonates, juvenile, adolescent and aged male animals, following a single TBI. We also describe the pharmacological responses to anti-inflammatory or neuroprotective agents, highlighting the need for an age-specific approach to the development of therapies of TBI.

NLRP3 Inflammasome Is Involved in Cocaine-Mediated Potentiation on Behavioral Changes in CX3CR1-Deficient Mice

Microglia, the primary immunocompetent cells of the brain, are suggested to play a role in the development of drug addiction. Previous studies have identified the microglia-derived pro-inflammatory factor IL1β can promote the progression of cocaine addiction. Additionally, the activation status of microglia and “two-hit hypothesis” have been proposed in the field of drug addiction to explain how early life stress (ELS) could significantly increase the incidence of drug addiction in later life. However, the mechanisms underlying microglia prime and full activation and their roles in drug addiction remain greatly unexplored. Here, we employed CX3CR1-GFP mice (CX3CR1 functional deficiency, CX3CR1−/−) to explore whether primed microglia could potentiate cocaine-mediated behavioral changes and the possible underlying mechanisms. CX3CR1−/− mice revealed higher hyperlocomotion activity and conditional place preference than wild-type (WT) mice did under cocaine administration. In parallel, CX3CR1−/− mice showed higher activity of NLR family pyrin domain-containing 3 (NLRP3) inflammasome than WT mice. Interestingly, CX3CR1 deficiency itself could prime NLRP3 signaling by increasing the expression of NLPR3 and affect lysosome biogenesis under basal conditions. Taken together, our findings demonstrated that the functional status of microglia could have an impact on cocaine-mediated reward effects, and NLRP3 inflammasome activity was associated with this phenomenon. This study was consistent with the two-hit hypothesis and provided solid evidence to support the involvement of microglia in drug addiction. Targeting the NLRP3 inflammasome may represent a novel therapeutic approach for ameliorating or blocking the development of drug addiction.

Pathophysiology of Pediatric Traumatic Brain Injury

The national incidence of traumatic brain injury (TBI) exceeds that of any other disease in the pediatric population. In the United States the Centers for Disease Control and Prevention (CDC) reports 697,347 annual TBIs in children ages 0–19 that result in emergency room visits, hospitalization or deaths. There is a bimodal distribution within the pediatric TBI population, with peaks in both toddlers and adolescents. Preclinical TBI research provides evidence for age differences in acute pathophysiology that likely contribute to long-term outcome differences between age groups. This review will examine the timecourse of acute pathophysiological processes during cerebral maturation, including calcium accumulation, glucose metabolism and cerebral blood flow. Consequences of pediatric TBI are complicated by the ongoing maturational changes allowing for substantial plasticity and windows of vulnerabilities. This review will also examine the timecourse of later outcomes after mild, repeat mild and more severe TBI to establish developmental windows of susceptibility and altered maturational trajectories. Research progress for pediatric TBI is critically important to reveal age-associated mechanisms and to determine knowledge gaps for future studies.

CXCR4 inhibition with AMD3100 attenuates amphetamine induced locomotor activity in adolescent Long Evans male rats

Adolescent psychostimulant abuse has been on the rise over the past decade. This trend has demonstrable ramifications on adolescent behavior and brain morphology, increasing risk for development of addiction during adolescence and in later adulthood. Neuroimmune substrates are implicated in the etiology of substance use disorders. To add to this body of work, the current study was developed to explore the role of a chemokine receptor, CXC Chemokine Receptor 4 (CXCR4), in the development of amphetamine (AMPH) sensitization. We targeted CXCR4 as it is implicated in developmental processes, dopaminergic transmission, neuroimmune responses, and the potentiation of psychostimulant abuse pathology. To evaluate the role of CXCR4 activity on the development of AMPH sensitization, a CXCR4 antagonist (Plerixafor; AMD3100) was administered to rats as a pretreatment variable. Specifically, adolescent Long Evans male rats (N = 37) were divided into four groups: (1) AMD3100 (IP, 4.0 mg/kg) + AMPH (IP, 4.0 mg/kg), (2) saline (SAL; 0.9% NaCl) + AMPH, (3) AMD3100 + SAL, and (4) SAL + SAL. Animals were first habituated to locomotor activity (LMA) chambers, then injected with a pretreatment drug (AMD3100 or SAL) followed by AMPH or SAL every other for four days. After a one-week withdrawal period, all animals were administered a low challenge dose of AMPH (IP, 1.0 mg/kg). AMPH-injected rats displayed significantly more locomotor activity compared to controls across all testing days. CXCR4 antagonism significantly attenuated AMPH-induced locomotor activity. On challenge day, AMD3100 pre-treated animals exhibited diminutive AMPH-induced locomotor activity compared to SAL pre-treated animals. Postmortem analyses of brain tissue revealed elevated CXCR4 protein levels in the striatum of all experimental groups. Our results implicate CXCR4 signaling in the development of AMPH sensitization and may represent an important therapeutic target for future research in psychostimulant abuse.

High-fat Feeding Causes Inflammation and Insulin Resistance in the Ventral Tegmental Area in Mice

The reward system plays an important role in the pathogenesis of not only drug addiction, but also diet-induced obesity. Recent studies have shown that insulin and leptin receptor signaling in the ventral tegmental area (VTA) regulate energy homeostasis and that their dysregulation is responsible for obesity and altered food preferences. Although a high-fat diet (HFD) induces inflammation that leads to insulin and leptin resistance in the brain, it remains unclear whether HFD induces inflammation in the VTA. In the present study, we placed male mice on a chow diet or HFD for 3, 7, and 28 days and evaluated the mRNA expression of inflammatory cytokines and microglial activation markers in the VTA. The HFD group showed significantly elevated mRNA expressions of IL1β at 3 days; tumor necrosis factor-alpha (TNFα), IL1β, IL6, Iba1, and CD11b at 7 days; and TNFα, IL1β, Iba1, and CD11b at 28 days. The changes in TNFα were also confirmed in immunohistochemical analysis. Next, after administration of chow or HFD for 7 days, we selected mice with equal weights in both groups. In experiments using these mice, Akt phosphorylation in the VTA was significantly decreased after intracerebroventricular injection of insulin, whereas no change in STAT3 phosphorylation was found with leptin. Taken together, these results suggest that HFD induces inflammation at least partly associated with microglial activation in the VTA leading to insulin resistance, independently of the energy balance. Our data provide new insight into the pathophysiology of obesity caused by a dysfunctional reward system under HFD conditions.

iTRAQ-based proteomic profiling reveals protein alterations after traumatic brain injury and supports thyroxine as a potential treatment

Traumatic brain injury (TBI) is a primary cause of disability and death across the world. Previously, RNA analysis was widely used to study the pathophysiological mechanisms underlying TBI; however, the relatively low correlation between the transcriptome and proteome revealed that RNA transcription abundance does not reliably predict protein abundance, which led to the emergence of proteomic research. In this study, an iTRAQ proteomics approach was applied to detect protein alterations after TBI on a large scale. A total of 3937 proteins were identified, and 146 proteins were significantly changed after TBI. Moreover, 23 upregulated proteins were verified by parallel reaction monitoring (PRM), and fold changes in 16 proteins were consistent with iTRAQ outcomes. Transthyretin (Ttr) upregulation has been demonstrated at the transcriptional level, and this study further confirmed this at the protein level. After treatment with thyroxine (T4), which is transported by Ttr, the effects of T4 on neuronal histopathology and behavioral performance were determined in vivo (TBI + T4 group). Brain edema was alleviated, and the integrity of the blood brain barrier (BBB) improved. Escape latency in the Morris water maze (MWM) declined significantly compared with the group without T4 treatment. Modified neurological severity scores (mNSS) of the TBI + T4 group decreased from day 1 to day 7 post-TBI compared with the TBI + saline group. These results indicate that T4 treatment has potential to alleviate pathologic and behavioral abnormalities post-TBI. Protein alterations after T4 treatment were also detected by iTRAQ proteomics. Upregulation of proteins like Lgals3, Gfap and Apoe after TBI were reversed by T4 treatment. GO enrichment showed T4 mainly affected intermediate filament organization, cholesterol transportation and axonal regeneration. In summary, iTRAQ proteomics provides information about the impact of TBI on protein alterations and yields insight into underlying mechanisms and pathways involved in TBI and T4 treatment. Finally, Ttr and other proteins identified by iTRAQ may become potential novel treatment targets post-TBI.

Sex, Drugs, and TBI: The Role of Sex in Substance Abuse Related to Traumatic Brain Injuries

Traumatic brain injuries (TBI) are a significant public health problem costing billions of dollars in healthcare costs and lost productivity while simultaneously reducing the quality of life for both patients and caregivers. Substance abuse is closely interconnected with TBI, as intoxicated individuals are at a greater risk of suffering brain injuries, and TBI may serve as a risk factor for the subsequent development of substance use disorders. There are also prominent sex differences in the etiology, epidemiology, and consequences of TBI. For instance, men are more likely to be injured on sporting fields or in auto accidents, while women are disproportionately likely to suffer TBI associated with intimate partner violence. Moreover, while men are much more likely to suffer TBI during late adolescence–young adulthood, sex differences in the incidence of TBI are much less prominent during other developmental epochs. Further, there are prominent sex differences in substance abuse biology; for example, while more men meet diagnostic criteria for substance abuse disorders, women tend to advance from casual use to addiction more quickly. In this paper, we will discuss the emerging clinical and preclinical evidence that these sex differences in TBI and substance abuse interact and may be prominent determinates of long-term outcomes.

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.

Is Pediatric Traumatic Brain Injury Associated with Adult Alcohol Misuse?

Evidence suggests that pediatric traumatic brain injury (TBI) may be causally related to alcohol misuse later in life; however, the nature and extent of the association has not been well described. This study examined the relationship between pediatric TBI and adult alcohol misuse in a population sample ≥20 years of age. We sought to determine (1) whether first self-reported incidence of TBI with loss of consciousness (LOC) before the age of 20 increased the risk for alcohol misuse later in life; and (2) whether sex, injury severity, and age at time of injury modified the association. We found a greater likelihood of binge but not heavy drinking for those whose first self-reported TBI with LOC occurred before the age of 20 when compared with those whose first self-reported TBI with LOC occurred later in life (28.5% vs. 20.4%, p = 0.003). When limited to those with only mild TBI, the relationship to binge drinking remained significant (31.9% vs. 19.3%, p < 0.001) and was evident for both males (38.4% vs. 25.6%, p = 0.016) and females (20.9% vs. 12.4%, p = 0.044). When controlling for sex, age, and race/ethnicity, reporting a first TBI with LOC before age 20 was associated with binge drinking only for those with mild TBI (adjusted odds ratio [AOR] = 1.32; 95% confidence interval [CI] = 1.00-1.74). Results also showed that those with first TBI with LOC occurring between the ages of 10 and 19 years were more likely to binge drink as adults than those first injured earlier in life, regardless of TBI severity. Further research is needed at both the epidemiological and pre-clinical levels to better understand this relationship.

Experimental Traumatic Brain Injury during Adolescence Enhances Cocaine Rewarding Efficacy and Dysregulates Dopamine and Neuroimmune Systems in Brain Reward Substrates

Although clinical studies identify traumatic brain injury (TBI) as a risk factor for the development of substance use disorder, much remains unknown about the possible underlying pathogenesis and age-specific effects. Thus, the aim of this study is to test the hypothesis that at an age of ongoing maturation, adolescent TBI alters elements of the reward pathway, resulting in increased sensitivity to the rewarding effects of a subthreshold dose of cocaine that does not induce significant behavioral changes in naïve, non-injured mice. Specifically, these results were derived from the combination of the controlled cortical impact model of TBI, performed on either adolescent (6 weeks) or young adult (8 weeks) mice, followed by the cocaine-induced conditioned place preference assay 2 weeks later. Using three-dimensional isosurface rendering and volumetric image analysis, TBI was found to induce neuromorphological changes such as decreased dendritic complexity and reduced spine density in brain regions essential for reward perception and processing of drug-induced euphoria. Further, we demonstrated that these neuronal changes may affect the differential expression of dopamine-associated genes. Our analysis also provided evidence for age-related differences in immune response and the distinct involvement of augmented microglial phagocytic activity in the remodeling of neuronal structures in the adolescent TBI brain. Our studies suggest that TBI during adolescence, a period associated with ongoing maturation of dopaminergic systems, may subsequently enhance the abuse liability of cocaine in adulthood.

Reward and immune responses in adolescent females following experimental traumatic brain injury

The pathology of traumatic brain injury (TBI) adversely affects many brain regions, often resulting in the development of comorbid psychiatric disorders including substance use disorders (SUD). Although traditionally thought to be an epidemic that predominantly affects males, recent clinical studies report females have higher rates of concussions and longer recovery times than males. Yet, how neurotrauma, particularly deep within the brain, between the sexes is differentially manifested remains largely unknown. The risk of TBI peaks during adolescence when neuronal networks that regulate reward behaviors are not fully developed. Previously, using the conditioned place preference (CPP) assay, we found that adolescent TBI increased susceptibility to the rewarding effects of cocaine in male mice. Further, we observed augmented inflammatory profiles, increased microglial phagocytosis of neuronal proteins, and decreased neuronal spine density in the NAc. Notably, the extent of sex differences in SUD susceptibility following TBI has not be investigated. Thus, here we ask the central question of whether the adolescent TBI-induced neuroinflammatory profile at reward centers is divergent in a sex-dependent manner. Using the CPP assay, we found that female mice with high levels of female sex hormones at the time of adolescent TBI demonstrated neuroprotection against increased sensitivity to the rewarding effects of cocaine. These studies also provide evidence of significantly reduced microglial activation and phagocytosis of neuronal proteins within the NAc of females. Overall, our results offer crucial insight into how adolescent TBI impacts the reward pathway in a sex depending manner that could explain a vulnerability to addiction-like behavior.

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.

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.

Brain interrupted: Early life traumatic brain injury and addiction vulnerability

Recent reports provide evidence for increased risk of substance use disorders (SUD) among patients with a history of early-life traumatic brain injury (TBI). Preclinical research utilizing animal models of TBI have identified injury-induced inflammation, blood-brain barrier permeability, and changes to synapses and neuronal networks within regions of the brain associated with the perception of reward. Importantly, these reward pathway networks are underdeveloped during childhood and adolescence, and early-life TBI pathology may interrupt ongoing maturation. As such, maladaptive changes induced by juvenile brain injury may underlie increased susceptibility to SUD. In this review, we describe the available clinical and preclinical evidence that identifies SUD as a persistent psychiatric consequence of pediatric neurotrauma by discussing (1) the incidence of early-life TBI, (2) how preclinical studies model TBI and SUD, (3) TBI-induced neuropathology and neuroinflammation in the corticostriatal regions of the brain, and (4) the link between childhood or adolescent TBI and addiction in adulthood. In summary, preclinical research utilizes an innovative combination of models of early-life TBI and SUD to recapitulate clinical features and to determine how TBI promotes a risk for the development of SUD. However, causal processes that link TBI and SUD remain unclear. Additional research to identify and therapeutically target underlying mechanisms of aberrant reward pathway development will provide a launching point for TBI and SUD treatment strategies.

Sex differences in pediatric traumatic brain injury

The response of the developing brain to traumatic injury is different from the response of the mature, adult brain. There are critical developmental trajectories in the young brain, whereby injury can lead to long term functional abnormalities. Emerging preclinical and clinical literature supports the presence of significant sex differences in both the response to and the recovery from pediatric traumatic brain injury (TBI). These sex differences are seen at all pediatric ages, including neonates/infants, pre-pubertal children, and adolescents. As importantly, the response to neuroprotective therapies or treatments can differ between male and females subjects. These sex differences can result from several biologic origins, and may manifest differently during the various phases of brain and body development. Recognizing and understanding these potential sex differences is crucial, and should be considered in both preclinical and clinical studies of pediatric TBI.

Spontaneous resurgence of conditioned fear weeks after successful extinction in brain injured mice

Mild traumatic brain injury (TBI) is a major risk factor for post-traumatic stress disorder (PTSD), and both disorders share common symptoms and neurobiological defects. Relapse after successful treatment, known as long-term fear resurgence, is common in PTSD patients and a major therapeutic hurdle. We induced a mild focal TBI by controlled cortical impact (CCI) in male C57BL/6 J mice and used fear conditioning to assess PTSD-like behaviors and concomitant alterations in the fear circuitry. We found for the first time that mild TBI, and to a lesser extent sham (craniotomy), mice displayed a spontaneous resurgence of conditioned fear when tested for fear extinction memory recall, despite having effectively acquired and extinguished conditioned fear 6 weeks earlier in the same context. Other characteristic symptoms of PTSD are risk-taking behaviors and cognitive deficits. CCI mice displayed risk-taking behaviors, behavioral inflexibility and reductions in processing speed compared to naïve mice. In conjunction with these changes there were alterations in amygdala morphology 3 months post-trauma, and decreased myelin basic protein density at the primary lesion site and in distant secondary sites such as the hippocampus, thalamus, and amygdala, compared to sham mice. Furthermore, activity-dependent brain-derived neurotrophic factor (BDNF) transcripts were decreased in the prefrontal cortex, a key region for fear extinction consolidation, following fear extinction training in both TBI and, to a lesser extent, sham mice. This study shows for the first time that a mild brain injury can generate a spontaneous resurgence of conditioned fear associated with defective BDNF signalling in the prefrontal cortex, PTSD-like behaviors, and have enduring effects on the brain.

Repeated blast model of mild traumatic brain injury alters oxycodone self-administration and drug seeking

Each year, traumatic brain injuries (TBI) affect millions worldwide. Mild TBIs (mTBI) are the most prevalent and can lead to a range of neurobehavioral problems, including substance abuse. A single blast exposure, inducing mTBI alters the medial prefrontal cortex, an area implicated in addiction, for at least 30 days post injury in rats. Repeated blast exposures result in greater physiological and behavioral dysfunction than single exposure; however, the impact of repeated mTBI on addiction is unknown. In this study, the effect of mTBI on various stages of oxycodone use was examined. Male Sprague Dawley rats were exposed to a blast model of mTBI once per day for 3 days. Rats were trained to self-administer oxycodone during short (2 h) and long (6 h) access sessions. Following abstinence, rats underwent extinction and two cued reinstatement sessions. Sham and rbTBI rats had similar oxycodone intake, extinction responding and cued reinstatement of drug seeking. A second group of rats were trained to self-administer oxycodone with varying reinforcement schedules (fixed ratio (FR)-2 and FR-4). Under an FR-2 schedule, rbTBI-exposed rats earned fewer reinforcers than sham-exposed rats. During 10 extinction sessions, the rbTBI-exposed rats exhibited significantly more seeking for oxycodone than the sham-injured rats. There was a positive correlation between total oxycodone intake and day 1 extinction drug seeking in sham, but not in rbTBI-exposed rats. Together, this suggests that rbTBI-exposed rats are more sensitive to oxycodone-associated cues during reinstatement than sham-exposed rats and that rbTBI may disrupt the relationship between oxycodone intake and seeking.

Cocaine self-administration is increased after frontal traumatic brain injury and associated with neuroinflammation

Traumatic brain injury (TBI) has been linked to the development of numerous psychiatric diseases, including substance use disorder. However, it can be difficult to ascertain from clinical data whether the TBI is cause or consequence of increased addiction vulnerability. Surprisingly few studies have taken advantage of animal models to investigate the causal nature of this relationship. In terms of a plausible neurobiological mechanism through which TBI could magnify the risk of substance dependence, numerous studies indicate that TBI can cause widespread disruption to monoaminergic signaling in striatal regions, and also increases neuroinflammation. In the current study, male Long-Evans rats received either a mild or severe TBI centered over the frontal cortex via controlled cortical impact, and were subsequently trained to self-administer cocaine over 10 6-hour sessions. At the end of the study, markers of striatal dopaminergic function, and levels of inflammatory cytokine levels in the frontal lobes, were assessed via western blot and multiplex ELISA, respectively. There was significantly higher cocaine intake in a subset of animals with either mild or severe TBI. However, many animals within both TBI groups failed to acquire self-administration. Principal components analysis suggested that both dopaminergic and neuroinflammatory proteins were associated with overall cocaine intake, yet only an inflammatory component was associated with acquisition of self-administration, suggesting neuroinflammation may make a more substantial contribution to the likelihood of drug-taking. Should neuroinflammation play a causal role in mediating TBI-induced addiction risk, anti-inflammatory therapy may reduce the likelihood of substance abuse in TBI populations.

Effects of Mild Blast Traumatic Brain Injury on Cognitive- and Addiction-Related Behaviors

Traumatic brain injury (TBI) commonly results in cognitive and psychiatric problems. Cognitive impairments occur in approximately 30% of patients suffering from mild TBI (mTBI), and correlational evidence from clinical studies indicates that substance abuse may be increased following mTBI. However, understanding the lasting cognitive and psychiatric problems stemming from mTBI is difficult in clinical settings where pre-injury assessment may not be possible or accurate. Therefore, we used a previously characterized blast model of mTBI (bTBI) to examine cognitive- and addiction-related outcomes. We previously demonstrated that this model leads to bilateral damage of the medial prefrontal cortex (mPFC), a region critical for cognitive function and addiction. Rats were exposed to bTBI and tested in operant learning tasks several weeks after injury. bTBI rats made more errors during acquisition of a cue discrimination task compared to sham treated rats. Surprisingly, we observed no differences between groups in set shifting and delayed matching to sample, tasks known to require the mPFC. Separate rats performed cocaine self-administration. No group differences were found in intake or extinction, and only subtle differences were observed in drug-primed reinstatement 3-4 months after injury. These findings indicate that bTBI impairs acquisition of a visual discrimination task and that bTBI does not significantly increase the ability of cocaine exposure to trigger drug seeking.

Dexamethasone Attenuates the Enhanced Rewarding Effects of Cocaine Following Experimental Traumatic Brain Injury

Clinical studies have identified traumatic brain injury (TBI) as a risk factor for the development of cocaine dependence. This claim is supported by our recent preclinical studies showing enhancement of the rewarding effects of cocaine in mice sustaining moderate controlled cortical impact (CCI) injury during adolescence. Here we test the efficacy of dexamethasone, an anti-inflammatory corticosteroid, to attenuate augmentation of the behavioral response to cocaine observed in CCI-TBI animals using the conditioned place preference (CPP) assay. These studies were performed in order to determine whether proinflammatory activity in the nucleus accumbens (NAc), a key brain nucleus in the reward pathway, mediates enhanced cocaine-induced CPP in adolescent animals sustaining moderate CCI-TBI. Our data reveal robust glial activation in the NAc following CCI-TBI and a significant increase in the cocaine-induced CPP of untreated CCI-TBI mice. Furthermore, our results show that dexamethasone treatment following CCI-TBI can attenuate the cocaine place preference of injured animals without producing aversion in the CPP assay. Our studies also found that dexamethasone treatment significantly reduced the expression of select immune response genes including Monocyte chemoattractant protein-1 (MCP-1/CCL2) and intercellular adhesion molecule-1 ( ICAM-1), returning their expression to control levels, which prompted an investigation of peripheral blood monocytes in dexamethasone-treated animals. Experimental findings showed that no craniectomy/dexamethasone mice had a significant increase, while CCI-TBI/dexamethasone animals had a significant decrease in the percentage of circulating nonclassical patrolling monocytes. These results suggest that a portion of these monocytes may migrate to the brain in response to CCI-TBI, potentially sparing the development of chronic neuroinflammation in regions associated with the reward circuitry such as the NAc. Overall, our findings indicate that anti-inflammatory agents, such as dexamethasone, may be effective in normalizing the rewarding effects of cocaine following CCI-TBI.

Impact of Traumatic Brain Injury on Dopaminergic Transmission

Brain trauma is often associated with severe morbidity and is a major public health concern. Even when injury is mild and no obvious anatomic disruption is seen, many individuals suffer disabling neuropsychological impairments such as memory loss, mood dysfunction, substance abuse, and adjustment disorder. These changes may be related to subtle disruption of neural circuits as well as functional changes at the neurotransmitter level. In particular, there is considerable evidence that dopamine (DA) physiology in the nigrostriatal and mesocorticolimbic pathways might be impaired after traumatic brain injury (TBI). Alterations in DA levels can lead to oxidative stress and cellular dysfunction, and DA plays an important role in central nervous system inflammation. Therapeutic targeting of DA pathways may offer benefits for both neuronal survival and functional outcome after TBI. The purpose of this review is to discuss the role of DA pathology in acute TBI and the potential impact of therapies that target these systems for the treatment of TBI.

Traumatic Brain Injuries during Development: Implications for Alcohol Abuse

Traumatic brain injuries are strongly related to alcohol intoxication as by some estimates half or more of all brain injuries involve at least one intoxicated individual. Additionally, there is mounting evidence that traumatic brain injuries can themselves serve as independent risk factors for the development of alcohol use disorders, particularly when injury occurs during juvenile or adolescent development. Here, we will review the epidemiological and experimental evidence for this phenomenon and discuss potential psychosocial mediators including attenuation of negative affect and impaired decision making as well as neurochemical mediators including disruption in the glutamatergic, GABAergic, and dopaminergic signaling pathways and increases in inflammation.

Acute administration of catalase targeted to ICAM-1 attenuates neuropathology in experimental traumatic brain injury

Traumatic brain injury (TBI) contributes to one third of injury related deaths in the US. Treatment strategies for TBI are supportive, and the pathophysiology is not fully understood. Secondary mechanisms of injury in TBI, such as oxidative stress and inflammation, are points at which intervention may reduce neuropathology. Evidence suggests that reactive oxygen species (ROS) propagate blood-brain barrier (BBB) hyperpermeability and inflammation following TBI. We hypothesized that targeted detoxification of ROS may improve the pathological outcomes of TBI. Following TBI, endothelial activation results in a time dependent increase in vascular expression of ICAM-1. We conjugated catalase to anti-ICAM-1 antibodies and administered the conjugate to 8 wk old C57BL/6J mice 30 min after moderate controlled cortical impact injury. Results indicate that catalase targeted to ICAM-1 reduces markers of oxidative stress, preserves BBB permeability, and attenuates neuropathological indices more effectively than non-targeted catalase and anti-ICAM-1 antibody alone. Furthermore, the study of microglia by two-photon microscopy revealed that anti-ICAM-1/catalase prevents the transition of microglia to an activated phenotype. These findings demonstrate the use of a targeted antioxidant enzyme to interfere with oxidative stress mechanisms in TBI and provide a proof-of-concept approach to improve acute TBI management that may also be applicable to other neuroinflammatory conditions.

Factors affecting increased risk for substance use disorders following traumatic brain injury: What we can learn from animal models

Recent studies have helped identify multiple factors affecting increased risk for substance use disorders (SUDs) following traumatic brain injury (TBI). These factors include age at the time of injury, repetitive injury and TBI severity, neurocircuits, neurotransmitter systems, neuroinflammation, and sex differences. This review will address each of these factors by discussing 1) the clinical and preclinical data identifying patient populations at greatest risk for SUDs post-TBI, 2) TBI-related neuropathology in discrete brain regions heavily implicated in SUDs, and 3) the effects of TBI on molecular mechanisms that may drive substance abuse behavior, like dopaminergic and glutamatergic transmission or neuroimmune signaling in mesolimbic regions of the brain. Although these studies have laid the groundwork for identifying factors that affect risk of SUDs post-TBI, additional studies are required. Notably, preclinical models have been shown to recapitulate many of the behavioral, cellular, and neurochemical features of SUDs and TBI. Therefore, these models are well suited for answering important questions that remain in future investigations.