Up to 50% of traumatic brain injury (TBI) survivors demonstrate persisting and late-onset anxiety disorders indicative of limbic system dysregulation, yet the pathophysiology underlying the symptoms is unclear. We hypothesize that the development of TBIinduced anxiety-like behavior in an experimental model of TBI is mediated by changes in glutamate neurotransmission within the amygdala. Adult, male Sprague-Dawley rats underwent midline fluid percussion injury or sham surgery. Anxiety-like behavior was assessed at 7 and 28 days post-injury (DPI) followed by assessment of real-time glutamate neurotransmission in the basolateral amygdala (BLA) and central nucleus of the amygdala (CeA) using glutamate-selective microelectrode arrays. The expression of anxiety-like behavior at 28 DPI coincided with decreased evoked glutamate release and slower glutamate clearance in the CeA, not BLA. Numerous factors contribute to the changes in glutamate neurotransmission over time. In two additional animal cohorts, protein levels of glutamatergic transporters (Glt-1 and GLAST) and presynaptic modulators of glutamate release (mGluR2, TrkB, BDNF, and glucocorticoid receptors) were quantified using automated capillary western techniques at 28 DPI. Astrocytosis and microglial activation have been shown to drive maladaptive glutamate signaling and were histologically assessed over 28 DPI. Alterations in glutamate neurotransmission could not be explained by changes in protein levels for glutamate transporters, mGluR2 receptors, astrocytosis, and microglial activation. Presynaptic modulators, BDNF and TrkB, were significantly decreased at 28 DPI in the amygdala. Dysfunction in presynaptic regulation of glutamate neurotransmission may contribute to anxiety-related behavior and serve as a therapeutic target to improve circuit function.

Doxorubicin (DOX) is a potent chemotherapeutic agent known to cause acute and long-term cognitive impairments in cancer patients. Cognitive function is presumed to be primarily mediated by neuronal circuitry in the frontal cortex (FC) and hippocampus, where glutamate is the primary excitatory neurotransmitter. Mice treated with DOX (25 mg/kg i.p.) were subjected to in vivo recordings under urethane anesthesia at 24h post-DOX injection or 5 consecutive days of cognitive testing (Morris Water Maze; MWM). Using novel glutamate-selective microelectrode arrays, amperometric recordings measured parameters of extracellular glutamate clearance and potassium-evoked release of glutamate within the medial FC and dentate gyrus (DG) of the hippocampus. By 24h post-DOX injection, glutamate uptake was 45% slower in the FC in comparison to saline-treated mice. In the DG, glutamate took 48% longer to clear than saline-treated mice. Glutamate overflow in the FC was similar between treatment groups, however, it was significantly increased in the DG of DOX treated mice. MWM data indicated that a single dose of DOX impaired swim speed without impacting total length traveled. These data indicate that systemic DOX treatment changes glutamate neurotransmission in key nuclei associated with cognitive function within 24h, without a lasting impact on spatial learning and memory. Understanding the functional effects of DOX on glutamate neurotransmission may help us understand and prevent some of the debilitating side effects of chemotherapeutic treatment in cancer survivors.

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.

The aim of this review is to discuss the research presented in a symposium entitled "Current progress in characterizing therapeutic strategies and challenges in experimental CNS injury" which was presented at the 2016 International Behavioral Neuroscience Society annual meeting. Herein we discuss diffuse and focal traumatic brain injury (TBI) and ensuing chronic behavioral deficits as well as potential rehabilitative approaches. We also discuss the effects of stress on executive function after TBI as well as the response of the endocrine system and regulatory feedback mechanisms. The role of the endocannabinoids after CNS injury is also discussed. Finally, we conclude with a discussion of antipsychotic and antiepileptic drugs, which are provided to control TBI-induced agitation and seizures, respectively. The review consists predominantly of published data.

Diffuse brain injury is better described as multi-focal, where pathology can be found adjacent to seemingly uninjured neural tissue. In experimental diffuse brain injury, pathology and pathophysiology have been reported far more lateral than predicted by the impact site. We hypothesized that local thickening of the rodent skull at the temporal ridges serves to focus the intracranial mechanical forces experienced during brain injury and generate predictable pathology. We demonstrated local thickening of the skull at the temporal ridges using contour analysis on magnetic resonance imaging. After diffuse brain injury induced by midline fluid percussion injury (mFPI), pathological foci along the anterior-posterior length of cortex under the temporal ridges were evident acutely (1, 2, and 7 days) and chronically (28 days) post-injury by deposition of argyophilic reaction product. Area CA3 of the hippocampus and lateral nuclei of the thalamus showed pathological change, suggesting that mechanical forces to or from the temporal ridges shear subcortical regions. A proposed model of mFPI biomechanics suggests that injury force vectors reflect off the skull base and radiate toward the temporal ridge, thereby injuring ventral thalamus, dorsolateral hippocampus, and sensorimotor cortex. Surgically thinning the temporal ridge before injury reduced injury-induced inflammation in the sensorimotor cortex. These data build evidence for temporal ridges of the rodent skull to contribute to the observed pathology, whether by focusing extracranial forces to enter the cranium or intracranial forces to escape the cranium. Pre-clinical investigations can take advantage of the predicted pathology to explore injury mechanisms and treatment efficacy.

Background Multifarious clinical presentations of traumatic brain injury (TBI) makes detection difficult. Acceptance of the Fencing Response as an indicator of moderate TBI with localization to the brainstem expanded interest towards other possible indicators. Methods We hypothesized that an individual experiencing traumatic forces to the head resulting in concussion could display additional brainstem-mediated responses. Using YouTube™, videos were systematically evaluated for mechanical forces imposed on the head with a subsequent, observable behavior. Searches identified 9.9 million non-unique videos in which 0.01% were viewed and 79 met inclusion criteria. Videos of head injuries occurred during athletic activity (57%), assaults (38%), automobile accidents (4%) and impact by an inanimate object (1%). Results Individuals with acute head injury were identified as adults (70%; n = 55), teens (29%; n = 23), and children (1.2%; n = 1). Those identified as males made up majority of injured persons (n = 77♂, 2♀). Individuals in the videos were observed to demonstrate the Fencing Response (47%; n = 37), seizing (44%; n = 35), snoring (24%; n = 19), crying (7.6%; n = 6), and vomiting (3.8%; n = 3). Conclusion Each response, which together comprise the “Pentagram of Concussion”, indicates the presence of traumatic forces to the head that present with one or more pentagram signs that would localize dysfunction to the brainstem. Clinical consideration of these responses helps to immediately identify patients at high risk for a brain injury with brainstem involvement that may have otherwise been mistaken for a different diagnosis.

Introduction:Affective disorders, including generalized anxiety disorder and post-traumatic stress disorder (PTSD), are reported in 18.1% of United States adults. Additionally, affective disorders develop and persist in up to 50% of traumatic brain injury (TBI) survivors, however, few common biological mechanisms between TBI and non-TBI patients have been elucidated 1, 2 .The incidence of TBI continues to rise, with at least 2.5 million Americans reporting a TBI each year, costing the American health care system 76.5 billion dollars 3 . Three-quarters of all TBIs are diffuse TBIs (dTBI) in which the signature pathology is multi-focal diffuse axonal injury with no overt pathology detected by CT or MRI 4-6 . dTBI subsequently induces secondary sequelae that occur seconds to months following the initial injury, leading to the development of affective, cognitive, and somatic symptoms 7, 8 . Despite clinical prevalence, the pathophysiology contributing to affective symptoms following dTBI is poorly understood, resulting in misdiagnosis and ineffective treatments 9-11 . Ultimately, affective symptoms impact a patient's ability to return to work, daily function, and social interactions, and thus severely impair the quality of life for both the patient and caregivers 12, 13 .Clinical and preclinical data demonstrate how the amygdala changes following dTBI. In veterans, early amygdala fMRI reactivity post-injury is predictive of the development of PTSD and could contribute to the bilateral reduction of amygdala size observed in patients diagnosed with both TBI and PTSD 14,15 . A study in collegiate football players report a positive correlation between amygdala shape and mood states 16 . TBI survivors with major depressive disorder had smaller lateral and dorsal prefrontal cortex, which mediates ventral-limbic and paralimbic pathways and influences amygdala circuitry 17 . Diffuse and focal experimental models of TBI also identify alterations in amygdala circuitry, including increased neuronal hyperexcitability and GABA production proteins in the absence of overt neuropathology 18,19 . In addition, pyramidal and stellate neurons in the basolateral amygdala (BLA) demonstrate increased complexity distal and proximal to the soma as early as 1 day post-injury (DPI) and lasted until at least 28DPI indicating BLA-central nucleus of the amygdala (CeA) circuit reorganization after dTBI 20 .Affective symptoms are controlled by limbic system circuitry with evidence that the direct stimulation of glutamatergic neurons originating in the BLA and projecting into the CeA 4 produces reversible anxiolytic symptoms 21 22 . BLA neurons are 90% glutamatergic, highlighting the role of glutamate neurotransmission in displays of affective behavior 23,24 . Glutamate neurotransmission is regulated by astrocytes and microglia processes surrounding the synapse 25 .Glutamate transporters, Glt-1 and GLAST (located on astrocytes), rapidly remove glutamate from the extracellular space, restricting glutamate to the synaptic cleft. 26,27 . When spillover...

Aspects of glutamate neurotransmission implicated in normal and pathological conditions are often evaluated using in vivo recording paradigms in rats anesthetized with isoflurane or urethane. Urethane and isoflurane anesthesia influence glutamate neurotransmission through different mechanisms; however real-time outcome measures of potassium chloride (KCl)-evoked glutamate overflow and glutamate clearance kinetics have not been compared within and between regions of the brain. In the following experiments, in vivo amperometric recordings of KCl-evoked glutamate overflow and glutamate clearance kinetics (uptake rate and T80) in the cortex, hippocampus and thalamus were performed using glutamate-selective microelectrode arrays (MEAs) in young adult male, Sprague-Dawley rats anesthetized with isoflurane or urethane. Potassium chloride (KCl)-evoked glutamate overflow was similar under urethane and isoflurane anesthesia in all brain regions studied. Analysis of glutamate clearance determined that the uptake rate was significantly faster (53.2%, p<0.05) within the thalamus under urethane compared to isoflurane, but no differences were measured in the cortex or hippocampus. Under urethane, glutamate clearance parameters were region dependent, with significantly faster glutamate clearance in the thalamus compared to the cortex but not the hippocampus (p<0.05). No region dependent differences were measured for glutamate overflow using isoflurane. These data support that amperometric recordings of glutamate under isoflurane and urethane anesthesia result in mostly similar and comparable data. However, certain parameters of glutamate uptake vary based on choice of anesthesia and brain region. Special considerations must be given to these areas when considering comparison to previous literature and when planning future experiments.