Traumatic brain injury and its effects on synaptic plasticity

Electrical stimulation methods and protocols for the treatment of traumatic brain injury: a critical review of preclinical research

BACKGROUND

Traumatic brain injury (TBI) is a leading cause of disabilities resulting from cognitive and neurological deficits, as well as psychological disorders. Only recently, preclinical research on electrical stimulation methods as a potential treatment of TBI sequelae has gained more traction. However, the underlying mechanisms of the anticipated improvements induced by these methods are still not fully understood. It remains unclear in which stage after TBI they are best applied to optimize the therapeutic outcome, preferably with persisting effects. Studies with animal models address these questions and investigate beneficial long- and short-term changes mediated by these novel modalities.

METHODS

In this review, we present the state-of-the-art in preclinical research on electrical stimulation methods used to treat TBI sequelae. We analyze publications on the most commonly used electrical stimulation methods, namely transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), deep brain stimulation (DBS) and vagus nerve stimulation (VNS), that aim to treat disabilities caused by TBI. We discuss applied stimulation parameters, such as the amplitude, frequency, and length of stimulation, as well as stimulation time frames, specifically the onset of stimulation, how often stimulation sessions were repeated and the total length of the treatment. These parameters are then analyzed in the context of injury severity, the disability under investigation and the stimulated location, and the resulting therapeutic effects are compared. We provide a comprehensive and critical review and discuss directions for future research. RESULTS AND CONCLUSION: We find that the parameters used in studies on each of these stimulation methods vary widely, making it difficult to draw direct comparisons between stimulation protocols and therapeutic outcome. Persisting beneficial effects and adverse consequences of electrical simulation are rarely investigated, leaving many questions about their suitability for clinical applications. Nevertheless, we conclude that the stimulation methods discussed here show promising results that could be further supported by additional research in this field.

All-trans Retinoic Acid has Limited Therapeutic Effects on Cognition and Hippocampal Protein Expression After Controlled Cortical Impact

Traumatic brain injury (TBI) is known to impair synaptic function, and subsequently contribute to observed cognitive deficits. Retinoic Acid (RA) signaling modulates expression of synaptic plasticity proteins and is involved in hippocampal learning and memory. All trans-retinoic acid (ATRA), a metabolite of Vitamin A, has been identified as a potential pharmacotherapeutic for other neurological disorders due to this role. This study conducted an ATRA dose response to determine its therapeutic effects on cognitive behaviors and expression of hippocampal markers of synaptic plasticity and RA signaling proteins after experimental TBI. Under isoflurane anesthesia, adult male Sprague Dawley rats received either controlled cortical impact (CCI, 2.5 mm deformation, 4 m/s) or control surgery. Animals received daily intraperitoneal injection of 0.5, 1, 5, or 10 mg/kg of ATRA or vehicle for 2 weeks. Animals underwent motor and spatial learning and memory testing. Hippocampal expression of synaptic plasticity proteins neurogranin (Ng), and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluA1 sub-unit, as well as RA signaling proteins STRA6, ADLH1a1, CYP26A1 and CYP26B1 were evaluated by western blot at 2-weeks post-injury. ATRA treatment significantly recovered Ng synaptic protein expression, while having no effect on motor performance, spatial learning, and memory, and GluA1 expression after TBI. RA signaling protein expression is unchanged 2 weeks after TBI. Overall, ATRA administration after TBI showed limited therapeutic benefits compared to the vehicle.

The impact of high fat consumption on neurological functions following a traumatic brain injury in rats

Traumatic brain injury (TBI) and obesity are two common conditions in modern society; both can impair neuronal integrity and neurological function. However, it is unclear whether the co-existence of both conditions will worsen outcomes. Thus, in a rat model, we aimed to investigate whether the co-existence of TBI and a high-fat diet (HFD) has an additive effect, leading to more severe neurological impairments, and whether they are related to changes in brain protein markers of oxidative stress, inflammation and synaptic plasticity. Sprague-Dawley rats (female, ~250g) were divided into HFD (43% fat) and chow diet (CD, 17% fat) groups for 6 weeks. Within each dietary group, half underwent a TBI by a weight-drop device, and the other half underwent sham surgery. Short-term memory and sensory function were measured at 24 hours, 1 week, 3 weeks and 6 weeks post-TBI. Brain tissues were harvested at 24 hours and 6 weeks post-TBI and markers of oxidative stress, apoptosis, inflammation, and synaptic plasticity were measured via immunostaining and western blotting. In rats without TBI, HFD increased the presynaptic protein synaptophysin. In rats with TBI, HFD resulted in worsened sensory and memory function, an increase in activated macrophages, and a decrease in the endogenous antioxidant manganese superoxide dismutase. Our findings suggest that the additive effect of HFD and TBI worsens short term memory and sensation deficits, and may be driven by enhanced oxidative stress and inflammation.

Plasticity impairment exposes CA3 vulnerability in a hippocampal network model of mild traumatic brain injury

Proper function of the hippocampus is critical for executing cognitive tasks such as learning and memory. Traumatic brain injury (TBI) and other neurological disorders are commonly associated with cognitive deficits and hippocampal dysfunction. Although there are many existing models of individual subregions of the hippocampus, few models attempt to integrate the primary areas into one system. In this work, we developed a computational model of the hippocampus, including the dentate gyrus, CA3, and CA1. The subregions are represented as an interconnected neuronal network, incorporating well-characterized ex vivo slice electrophysiology into the functional neuron models and well-documented anatomical connections into the network structure. In addition, since plasticity is foundational to the role of the hippocampus in learning and memory as well as necessary for studying adaptation to injury, we implemented spike-timing-dependent plasticity among the synaptic connections. Our model mimics key features of hippocampal activity, including signal frequencies in the theta and gamma bands and phase-amplitude coupling in area CA1. We also studied the effects of spike-timing-dependent plasticity impairment, a potential consequence of TBI, in our model and found that impairment decreases broadband power in CA3 and CA1 and reduces phase coherence between these two subregions, yet phase-amplitude coupling in CA1 remains intact. Altogether, our work demonstrates characteristic hippocampal activity with a scaled network model of spiking neurons and reveals the sensitive balance of plasticity mechanisms in the circuit through one manifestation of mild traumatic injury.

Improved spatial memory, neurobehavioral outcomes, and neuroprotective effect after progesterone administration in ovariectomized rats with traumatic brain injury: Role of RU486 progesterone receptor antagonist

OBJECTIVES

The contribution of classic progesterone receptors (PR) in interceding the neuroprotective efficacy of progesterone (P4) on the prevention of brain edema and long-time behavioral disturbances was assessed in traumatic brain injury (TBI).

MATERIALS AND METHODS

Female Wistar rats were ovariectomized and apportioned into 6 groups: sham, TBI, oil, P4, vehicle, and RU486. P4 or oil was injected following TBI. The antagonist of PR (RU486) or DMSO was administered before TBI. The brain edema and destruction of the blood-brain barrier (BBB) were determined. Intracranial pressure (ICP), cerebral perfusion pressure (CPP), and beam walk (BW) task were evaluated previously and at various times post-trauma. Long-time locomotor and cognitive consequences were measured one day before and on days 3, 7, 14, and 21 after the trauma.

RESULTS

RU486 eliminated the inhibitory effects of P4 on brain edema and BBB leakage (P<0.05, P<0.001, respectively). RU486 inhibited the decremental effect of P4 on ICP as well as the increasing effect of P4 on CPP (P<0.001) after TBI. Also, RU486 inhibited the effect of P4 on the increase in traversal time and reduction in vestibulomotor score in the BW task (P<0.001). TBI induced motor, cognitive, and anxiety-like disorders, which lasted for 3 weeks after TBI; but, P4 prevented these cognitive and behavioral abnormalities (P<0.05), and RU486 opposed this P4 effect (P<0.001).

CONCLUSION

The classic progesterone receptors have neuroprotective effects and prevent long-time behavioral and memory deficiency after brain trauma.

Acute Post-Traumatic Sleep May Define Vulnerability to a Second Traumatic Brain Injury in Mice

Chronic neurological impairments can manifest from repetitive traumatic brain injury (rTBI), particularly when subsequent injuries occur before the initial injury completely heals. Herein, we apply post-traumatic sleep as a physiological biomarker of vulnerability, hypothesizing that a second TBI during post-traumatic sleep worsens neurological and histological outcomes compared to one TBI or a second TBI after post-traumatic sleep subsides. Mice received sham or diffuse TBI by midline fluid percussion injury; brain-injured mice received one TBI or rTBIs at 3- or 9-h intervals. Over 40 h post-injury, injured mice slept more than shams. Functional assessments indicated lower latencies on rotarod and increased Neurological Severity Scores for mice with rTBIs within 3 h. Anxiety-like behaviors in the open field task were increased for mice with rTBIs at 3 h. Based on pixel density of silver accumulation, neuropathology was greater at 28 days post-injury (DPI) in rTBI groups than sham and single TBI. Cortical microglia morphology was quantified and mice receiving rTBI were de-ramified at 14 DPI compared to shams and mice receiving a single TBI, suggesting robust microglial response in rTBI groups. Orexin-A-positive cells were sustained in the lateral hypothalamus with no loss detected, indicating that loss of wake-promoting neurons did not contribute to post-traumatic sleep. Thus, duration of post-traumatic sleep is a period of vulnerability that results in exacerbated injury from rTBI. Monitoring individual post-traumatic sleep is a potential clinical tool for personalized TBI management, where regular sleep patterns may inform rehabilitative strategies and return-to-activity guidelines.

Atypical Within-Session Motor Procedural Learning after Traumatic Brain Injury but Well-Preserved Between-Session Procedural Memory Consolidation

Using the finger-to-thumb opposition sequence (FOS) learning task, we characterized motor skill learning in sub-acute patients hospitalized for rehabilitation following traumatic brain injury (TBI). Ten patients (Trained TBI) and 11 healthy participants (Trained Healthy) were trained using a multi-session protocol: a single session was afforded in the first week of the study, and four daily sessions were afforded during the second week. Intensity of practice was adapted to patients. Performance speed and accuracy were tested before and after each session. Retention was tested 1 month later. Ten patients (Control TBI) had no FOS training and were tested only at the beginning and the end of the 6 week period. Although baseline performance on the FOS was very slow, all three phases of skill learning found in healthy adults (acquisition, between-session consolidation gains, and long-term retention) could be identified in patients with TBI. However, their time-course of learning was atypical. The Trained TBI group improved in speed about double the spontaneous improvements observed in the Control TBI group, with no speed-accuracy tradeoff. Normalized to their initial performance on the FOS, the gains accrued by the Trained TBI group after a first training were comparable to those accrued by healthy adults. Only during the second week with daily training, the rate of improvement of the Trained TBI group lagged behind that of the Trained Healthy group, due to increasing within-sessions losses in performance speed; no such losses were found in healthy participants. The Functional Independence Measure scores at the start of the study correlated with the total gains attained at the end of the study; no correlations were found with severity of injury or explicit memory impairments. Despite within-sessions losses in performance, which we propose reflect cognitive fatigue, training resulted in robust overall learning and long-term retention in patients with moderate-severe TBI. Given that the gains in performance evolved mainly between sessions, as delayed, offline, gains, our results suggest that memory consolidation processes can be effectively engaged in patients with TBI. However, practice protocols and schedules may need to be optimized to better engage the potential for long-term plasticity in these patients.

Chronic Subdural Hematoma: A Perspective on Subdural Membranes and Dementia

OBJECTIVE

To review the complex pathogenesis of the subdural membrane and the link between head trauma, dementia, and dural lymphatics.

METHODS

A thorough literature search of published English-language articles was performed using PubMed, Ovid, and Cochrane databases.

RESULTS

Chronic subdural hematoma (cSDH) is a common intracranial pathology and a leading cause of reversible dementia. cSDH is projected to affect at least 60,000 new individuals in the United States annually by 2030. This condition can result from mild to moderate head trauma that leads to hemorrhaging in the dura-arachnoid interface. The short-term and long-term effects of cSDH and the subdural membrane on the pathogenesis of dementia and the newly discovered dural lymphatics is a topic of increasing importance.

CONCLUSIONS

Further research into the possible link between traumatic brain injury and cSDH in particular and dural lymphatics and intracranial fluid dynamics is warranted.

Experimental diffuse brain injury results in regional alteration of gross vascular morphology independent of neuropathology

PRIMARY OBJECTIVE

A dynamic relationship exists between diffuse traumatic brain injury and changes to the neurovascular unit. The purpose of this study was to evaluate vascular changes during the first week following diffuse TBI. It was hypothesized that pathology is associated with modification of the vasculature.

METHODS

Male Sprague-Dawley rats underwent either midline fluid percussion injury or sham-injury. Brain tissue was collected 1, 2 or 7 days post-injury or sham-injury (n = 3/time point). Tissue was collected and stained by de Olmos amino-cupric silver technique to visualize neuropathology or animals were perfused with AltaBlue casting resin before high-resolution vascular imaging. The average volume, surface area, radius, branching and tortuosity of the vessels were evaluated across three regions of interest.

RESULTS

In M2, average vessel volume (p < 0.01) and surface area (p < 0.05) were significantly larger at 1 day relative to 2 days, 7 days and sham. In S1BF and VPM, no significant differences in the average vessel volume or surface area at any of the post-injury time points were observed. No significant changes in average radius, branching or tortuosity were observed.

CONCLUSIONS

Preliminary findings suggest gross morphological changes within the vascular network likely represent an acute response to mechanical forces of injury, rather than delayed or chronic pathological processes.

Resolvins AT-D1 and E1 differentially impact functional outcome, post-traumatic sleep, and microglial activation following diffuse brain injury in the mouse

Traumatic brain injury (TBI) is induced by mechanical forces which initiate a cascade of secondary injury processes, including inflammation. Therapies which resolve the inflammatory response may promote neural repair without exacerbating the primary injury. Specific derivatives of omega-3 fatty acids loosely grouped as specialized pro-resolving lipid mediators (SPMs) and termed resolvins promote the active resolution of inflammation. In the current study, we investigate the effect of two resolvin molecules, RvE1 and AT-RvD1, on post-traumatic sleep and functional outcome following diffuse TBI through modulation of the inflammatory response. Adult, male C57BL/6 mice were injured using a midline fluid percussion injury (mFPI) model (6-10min righting reflex time for brain-injured mice). Experimental groups included mFPI administered RvE1 (100ng daily), AT-RvD1 (100ng daily), or vehicle (sterile saline) and counterbalanced with uninjured sham mice. Resolvins or saline were administered daily for seven consecutive days beginning 3days prior to TBI to evaluate proof-of-principle to improve outcome. Immediately following diffuse TBI, post-traumatic sleep was recorded for 24h post-injury. For days 1-7 post-injury, motor outcome was assessed by rotarod. Cognitive function was measured at 6days post-injury using novel object recognition (NOR). At 7days post-injury, microglial activation was quantified using immunohistochemistry for Iba-1. In the diffuse brain-injured mouse, AT-RvD1 treatment, but not RvE1, mitigated motor and cognitive deficits. RvE1 treatment significantly increased post-traumatic sleep in brain-injured mice compared to all other groups. RvE1 treated mice displayed a higher proportion of ramified microglia and lower proportion of activated rod microglia in the cortex compared to saline or AT-RvD1 treated brain-injured mice. Thus, RvE1 treatment modulated post-traumatic sleep and the inflammatory response to TBI, albeit independently of improvement in motor and cognitive outcome as seen in AT-RvD1-treated mice. This suggests AT-RvD1 may impart functional benefit through mechanisms other than resolution of inflammation alone.

Ellagic acid prevents cognitive and hippocampal long-term potentiation deficits and brain inflammation in rat with traumatic brain injury

AIMS

Traumatic brain injury (TBI) remains one of the main clinical problems globally and is a common cause of death among youth. Cognitive defects such as thinking, memory and behavior or mental health disorders are considered as the most frequent effects of severe and moderate TBI. It has been reported that ellagic acid (EA), a natural polyphenol, exhibits protective effects against oxidative damage. This study was performed to examine the EA preventive effects on cognitive impairments, long-term potentiation (LTP) deficits in hippocampus and brain inflammation induced by diffuse TBI in rat.

MAIN METHODS

Subchronic oral administration of 100 mg/kg EA, 7 consecutive days before induction of trauma (once daily) was used to elucidate the EA effects on passive avoidance memory and hippocampal LTP following TBI. To illustrate the possible mechanisms related to the preventive effects of EA on brain function following TBI, brain content of IL-1β, IL-6 and blood-brain barrier (BBB) permeability were determined.

KEY FINDINGS

EA pretreatment significantly (P<0.001) prevented TBI-induced memory and hippocampal LTP impairments in rat. Furthermore TBI induced elevation in brain content of IL-1β, IL-6 and BBB permeability were decreased significantly (P<0.001) due to EA pre-treatment.

SIGNIFICANCE

Our findings suggest that EA can prevent cognitive and LTP deficits and also prevent brain inflammation following TBI.

Diffuse brain injury induces acute post-traumatic sleep

OBJECTIVE

Clinical observations report excessive sleepiness immediately following traumatic brain injury (TBI); however, there is a lack of experimental evidence to support or refute the benefit of sleep following a brain injury. The aim of this study is to investigate acute post-traumatic sleep.

METHODS

Sham, mild or moderate diffuse TBI was induced by midline fluid percussion injury (mFPI) in male C57BL/6J mice at 9:00 or 21:00 to evaluate injury-induced sleep behavior at sleep and wake onset, respectively. Sleep profiles were measured post-injury using a non-invasive, piezoelectric cage system. In separate cohorts of mice, inflammatory cytokines in the neocortex were quantified by immunoassay, and microglial activation was visualized by immunohistochemistry.

RESULTS

Immediately after diffuse TBI, quantitative measures of sleep were characterized by a significant increase in sleep (>50%) for the first 6 hours post-injury, resulting from increases in sleep bout length, compared to sham. Acute post-traumatic sleep increased significantly independent of injury severity and time of injury (9:00 vs 21:00). The pro-inflammatory cytokine IL-1β increased in brain-injured mice compared to sham over the first 9 hours post-injury. Iba-1 positive microglia were evident in brain-injured cortex at 6 hours post-injury.

CONCLUSION

Post-traumatic sleep occurs for up to 6 hours after diffuse brain injury in the mouse regardless of injury severity or time of day. The temporal profile of secondary injury cascades may be driving the significant increase in post-traumatic sleep and contribute to the natural course of recovery through cellular repair.

Preclinical non-human models to combat dementia

Dementia is characterized by a certain degree of memory loss with disabled intellectual functioning, which mostly presents as Alzheimer's disease. The underlying causes range from gene mutations, lifestyle factors, and other environmental influences to brain injuries and normal aging. Although there have been many rodent and non-human primate models created by various drugs, neurotoxins and genetic ablation but the current scenario does not exhibit a well characterized animal model to evaluate novel compounds and various treatment strategies for dementia. Therefore, a comprehensive model exhibiting the pathologies and neuro-behavioral parameters close to this syndrome is very much needed. This report discusses the various experimental strategies to create animal models of dementia.

Noninvasive brain stimulation to modulate neuroplasticity in traumatic brain injury

OBJECTIVE

To review the use of noninvasive brain stimulation (NBS) as a therapeutic tool to enhance neuroplasticity following traumatic brain injury (TBI).

MATERIALS AND METHODS

Based on a literature search, we describe the pathophysiological events following TBI and the rationale for the use of transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) in this setting.

RESULTS

The pathophysiological mechanisms occurring after TBI vary across time and therefore require differential interventions. Theoretically, given the neurophysiological effects of both TMS and tDCS, these tools may: 1) decrease cortical hyperexcitability acutely after TBI; 2) modulate long-term synaptic plasticity as to avoid maladaptive consequences; and 3) combined with physical and behavioral therapy, facilitate cortical reorganization and consolidation of learning in specific neural networks. All of these interventions may help decrease the burden of disabling sequelae after brain injury.

CONCLUSIONS

Evidence from animal and human studies reveals the potential benefit of NBS in decreasing the extent of injury and enhancing plastic changes to facilitate learning and recovery of function in lesioned neural tissue. However, this evidence is mainly theoretical at this point. Given safety constraints, studies in TBI patients are necessary to address the role of NBS in this condition as well as to further elucidate its therapeutic effects and define optimal stimulation parameters.

Comparison of rat sensory behavioral tasks to detect somatosensory morbidity after diffuse brain-injury

Brain injury disrupts neuronal circuits, impacting neurological function. Selective and sensitive behavioral tests are required to explore neurological dysfunction, recovery and potential therapy. Previously we reported that the Whisker Nuisance Task (WNT), where whiskers are manually stimulated in an open field, shows sensory sensitivity in diffuse brain-injured rats. To further explore this somatosensory morbidity, we evaluated three additional whisker-dependent tasks: Gap Cross Test, a novel Angle Entrance Task and Whisker Guided Exploration Task. Brain-injured (n=11) and sham (n=8) rats were tested before midline fluid percussion brain injury (moderate: 2.0atm) and 1 and 4 weeks after injury. For the WNT, we confirmed that brain-injured rats develop significant sensory sensitivity to whisker stimulation over 28 days. In the Gap Cross Test, where rats cross progressively larger elevated gaps, we found that animals were inconsistent in crossable distance regardless of injury. In the Angle Entrance Task, where rats enter 30°, 40°, 50° or 80° corners, rats performed consistently regardless of injury. In the Whisker Guided Exploration Task, where rats voluntarily explore an oval circuit, we identified significant decreases in the number of rears and reversals and changes in the predominant location (injured rats spend more time in the inside of the turn compared to the outside) after injury and increased thigmotaxis after sham and brain-injury. Both the WNT and Whisker Guided Exploration Task show injury-induced somatosensory behavioral morbidity; however, the WNT remains more sensitive in detecting brain injury, possibly due to imposed whisker stimulation that elicits agitation similar to the human condition.

Chronic dysfunction of astrocytic inwardly rectifying K+ channels specific to the neocortical epileptic focus after fluid percussion injury in the rat

Astrocytic inwardly rectifying K(+) currents (I(KIR)) have an important role in extracellular K(+) homeostasis, which influences neuronal excitability, and serum extravasation has been linked to impaired K(IR)-mediated K(+) buffering and chronic hyperexcitability. Head injury induces acute impairment in astroglial membrane I(KIR) and impaired K(+) buffering in the rat hippocampus, but chronic spontaneous seizures appear in the perilesional neocortex--not the hippocampus--in the early weeks to months after injury. Thus we examined astrocytic K(IR) channel pathophysiology in both neocortex and hippocampus after rostral parasaggital fluid percussion injury (rpFPI). rpFPI induced greater acute serum extravasation and metabolic impairment in the perilesional neocortex than in the underlying hippocampus, and in situ whole cell recordings showed a greater acute loss of astrocytic I(KIR) in neocortex than hippocampus. I(KIR) loss persisted through 1 mo after injury only in the neocortical epileptic focus, but fully recovered in the hippocampus that did not generate chronic seizures. Neocortical cell-attached recordings showed no loss or an increase of I(KIR) in astrocytic somata. Confocal imaging showed depletion of KIR4.1 immunoreactivity especially in processes--not somata--of neocortical astrocytes, whereas hippocampal astrocytes appeared normal. In naïve animals, intracortical infusion of serum, devoid of coagulation-mediating thrombin activity, reproduces the effects of rpFPI both in vivo and at the cellular level. In vivo serum infusion induces partial seizures similar to those induced by rpFPI, whereas bath-applied serum, but not dialyzed albumin, rapidly silenced astrocytic K(IR) membrane currents in whole cell and cell-attached patch-clamp recordings in situ. Thus both acute impairment in astrocytic I(KIR) and chronic spontaneous seizures typical of rpFPI are reproduced by serum extravasation, whereas the chronic impairment in astroglial I(KIR) is specific to the neocortex that develops the epileptic focus.

Pharmacological treatment of neurobehavioural sequelae of traumatic brain injury

Neurobehavioural sequelae of traumatic brain injuries require an appropriate/effective pharmacological response in that they represent an important cause of disability. In this field, there is no evidence that reaches the level of a standard: there are guidelines on the use of methylphenidate, donepezil and bromocriptine for the treatment of cognitive disturbances, for the non-use of phenytoin and for the use of beta-blockers for controlling aggressiveness. Resolving a single symptom is not relevant in a rehabilitation project if it is not in the context of a more complex picture of neurobehavioural recovery, in which the positive and negative effects of every therapeutic choice are considered. For example, phenytoin could be used for the positive control of epileptic crises but is not advised since it impedes the recovery of cognitive functions in general. Analogous effects not yet identified may concern benzodiazepine, neuroleptics and other sedatives usually prescribed in cases of cranial trauma. Psychotropic drugs are considered to be able to influence the neuronal plasticity processes. Studies on animals have shown that the administration of D-amphetamine combined with sensorial-motor exercise produces the steady acceleration of motor recovery, which acts as a catalyst to the neurological recovery process. On the other hand, alpha1-NA receptor antagonist drugs produce negative effects; these include clonidine (antihypertension) and haloperidol (neuroleptic). Studies need to be carried out to evaluate the effectiveness of particular drugs. These studies need to focus not only on the disappearance of symptoms but also on the positive and negative effects on overall rehabilitation and on the neurobiological recovery of the patient.

Metaplasticity: tuning synapses and networks for plasticity

Synaptic plasticity is a key component of the learning machinery in the brain. It is vital that such plasticity be tightly regulated so that it occurs to the proper extent at the proper time. Activity-dependent mechanisms that have been collectively termed metaplasticity have evolved to help implement these essential computational constraints. Various intercellular signalling molecules can trigger lasting changes in the ability of synapses to express plasticity; their mechanisms of action are reviewed here, along with a consideration of how metaplasticity might affect learning and clinical conditions.

The effects of a multimodal intervention on outcomes of persons with early-stage dementia

Theories supporting the existence of a use-dependent neuroplasticity in the older brain were used to guide this pilot study. A repeated-measures randomized design was used to test the effectiveness of a multimodal (Taiji exercises, cognitive-behavioral therapies, support group) intervention on cognitive functioning, physical functioning, and behavioral outcomes in persons with dementia. The treatment group (n = 24 persons with dementia) participated in a 40-week intervention, with outcomes assessed at 20 and 40 weeks to assess optimal treatment length. Control group subjects (n = 19 persons with dementia) received attention-control educational programs. At 20 weeks, differences between groups were found for mental ability and self-esteem, with gains in balance being evident. Also, stability in depression and physical health were evident at 20 and 40 weeks for treatment group subjects. Continued improvement in outcomes was not observed at 40 weeks. However, findings support further testing of the intervention along with potential for achieving positive outcomes in early-stage dementia.

Cognitive neural plasticity during learning and recovery from brain damage

The process of neuroplasticity is the ability of the brain to change, either in response to experience or injury. It is a vital process both during normal development and for the recovery after brain injury. Recent research has emphasized that this takes place via both local restitution as well as reorganization and compensatory reassignment. The fact that the brain can undergo such plastic changes has provided evidence for what underlies developmental brain disorders, as well as the variable response to injury at different points in the lifespan. The factors affecting plasticity and its long-term consequences may have increasing importance in exposing the pattern of changes that occur in the normal brain with aging.

Electrical stimulation protocols for hippocampal synaptic plasticity and neuronal hyper-excitability: are they effective or relevant?

Long-term potentiation (LTP) of synaptic transmission is a widely accepted model that attempts to link synaptic plasticity with memory. LTP models are also now used in order to test how a variety of neurological disorders might affect synaptic plasticity. Interestingly, electrical stimulation protocols that induce LTP appear to display different efficiencies and importantly, some may not be as physiologically relevant as others. In spite of advancements in our understanding of these differences, many types of LTP inducing protocols are still widely used. In addition, in some cases electrical stimulation leads to normal biological phenomena, such as putative memory encoding and in other cases electrical stimulation triggers pathological phenomena, such as epileptic seizures. Kindling, a model of epileptogenesis involving repeated electrical stimulation, leads to seizure activity and has also been thought of, and studied as, a form of long-term neural plasticity and memory. Furthermore, some investigators now use electrical stimulation in order to reduce aspects of seizure activity. In this review, we compare in vitro and in vivo electrical stimulation protocols employed in the hippocampal formation that are utilized in models of synaptic plasticity or neuronal hyperexcitability. Here the effectiveness and physiological relevance of these electrical stimulation protocols are examined in situations involving memory encoding (e.g., LTP/LTD) and epileptiform activity.

Hippocampal vulnerability following traumatic brain injury: a potential role for neurotrophin-4/5 in pyramidal cell neuroprotection

Traumatic brain injury (TBI) causes selective hippocampal cell death, which is believed to be associated with cognitive impairment observed both in clinical and experimental settings. Although neurotrophin administration has been tested as a strategy to prevent cell death following TBI, the potential neuroprotective role of neurotrophin-4/5 (NT-4/5) in TBI remains unknown. We hypothesized that NT-4/5 would offer neuroprotection for selectively vulnerable hippocampal neurons following TBI. Measurements of NT-4/5 in rats subjected to lateral fluid percussion (LFP) TBI revealed two-threefold increases in the injured cortex and hippocampus in the acute period (1-3 days) following brain injury. Subsequently, the response of NT-4/5 knockout (NT-4/5(-/-)) mice to controlled-cortical impact TBI was investigated. NT-4/5(-/-) mice were more susceptible to selective pyramidal cell loss in Ahmon's corn (CA) subfields of the hippocampus following TBI, and showed impaired motor recovery when compared with their brain-injured wild-type controls (NT-4/5(wt)). Additionally, we show that acute, prolonged administration of recombinant NT-4/5 (5 microg/kg/day) prevented up to 50% of the hippocampal CA pyramidal cell death following LFP TBI in rats. These results suggest that post-traumatic increases in endogenous NT-4/5 may be part of an adaptive neuroprotective response in the injured brain, and that administration of this neurotrophic factor may be useful as a therapeutic strategy following TBI.

MONaKA, a novel modulator of the plasma membrane Na,K-ATPase

We have cloned and characterized mouse and human variants of MONaKA, a novel protein that interacts with and modulates the plasma membrane Na,K-ATPase. MONaKA was cloned based on its sequence homology to the Drosophila Slowpoke channel-binding protein dSlob, but mouse and human MONaKA do not bind to mammalian Slowpoke channels. At least two splice variants of MONaKA exist; the splicing is conserved perfectly between mouse and human, suggesting that it serves some important function. Both splice variants of MONaKA are expressed widely throughout the CNS and peripheral nervous system, with different splice variant expression ratios in neurons and glia. A yeast two-hybrid screen with MONaKA as bait revealed that it binds tightly to the beta1 and beta3 subunits of the Na,K-ATPase. The association between MONaKA and Na,K-ATPase beta subunits was confirmed further by coimmunoprecipitation from transfected cells, mouse brain, and cultured mouse astrocytes. A glutathione S-transferase-MONaKA fusion protein inhibits Na,K-ATPase activity from whole brain or cultured astrocytes. Furthermore, transfection of MONaKA inhibits 86Rb+ uptake via the Na,K-ATPase in intact cells. These results are consistent with the hypothesis that MONaKA modulates brain Na,K-ATPase and may thereby participate in the regulation of electrical excitability and synaptic transmission.

Matrix metalloproteinase-7 disrupts dendritic spines in hippocampal neurons through NMDA receptor activation

Dendritic spines are protrusions from the dendritic shaft that host most excitatory synapses in the brain. Although they first emerge during neuronal maturation, dendritic spines remain plastic through adulthood, and recent advances in the molecular mechanisms governing spine morphology have shown them to be exquisitely sensitive to changes in the micro-environment. Among the many factors affecting spine morphology are components and regulators of the extracellular matrix (ECM). Modification of the ECM is critical to the repair of injuries throughout the body, including the CNS. Matrix metalloproteinase (MMP)-7/matrilysin is a key regulator of the ECM during pathogen infection, after nerve crush and in encephalitogenic disorders. We have investigated the effects of MMP-7 on dendritic spines in hippocampal neuron cultures and found that it induces the transformation of mature, short mushroom-shaped spines into long, thin filopodia reminiscent of immature spines. These changes were accompanied by a dramatic redistribution of F-actin from spine heads into thick, rope-like structures in the dendritic shaft. Strikingly, MMP-7 effects on dendritic spines were similar to those of NMDA treatment, and both could be blocked by channel-specific antagonists. These findings are the first direct evidence that MMPs can influence the morphology of mature dendritic spines, and hence synaptic stability.

Newly born granule cells in the dentate gyrus rapidly extend axons into the hippocampal CA3 region following experimental brain injury

We investigated whether new neurons generated in the adult rat brain following lateral fluid percussion traumatic brain injury (TBI) are capable of projecting axons along the mossy fiber pathway to the CA3 region of the hippocampus. Dividing cells were labeled by intraperitoneal injection of bromodeoxyuridine (BrdU) on the day of surgery/injury, and neurons that extended axons to the CA3 region were retrogradely labeled by fluorescent tracers (FluoSpheres), stereotactically injected into the CA3 region of both the ipsi- and contralateral hippocampus at 1 or 12 days following TBI (n = 12) or sham injury (n = 12) in anaesthetized rats. Animals (n = 6 injured and n = 6 sham-injured controls per time point) were sacrificed at either 3 or 14 days post-injury. Another group of animals (n = 3) was subjected to experimental TBI and BrdU administration and sacrificed 3 days after TBI to be processed for BrdU and immunohistochemistry for polysialylated neural cell adhesion molecule (PSA-NCAM), a growth-related protein normally observed during CNS development. A fivefold bilateral increase in the number of mitotically active (BrdU+) cells was noted within the dentate gyrus when compared to uninjured controls as early as 3 days following TBI. A subgroup of dividing cells was also immunoreactive for PSA-NCAM at 3 days following TBI. By 2 weeks post-injury the number of BrdU+ cells within the dentate gyrus was increased twofold compared to the uninjured counterparts and a proportion of these newly generated cells showed cytoplasmic staining for the fluorescent tracer. These findings document rapid neurogenesis following TBI and show, for the first time, that newly generated granule neurons are capable of extending projections along the hippocampal mossy fiber pathway in the acute post-traumatic period.

Head injury and dementia

PURPOSE OF REVIEW

The link between head injury and dementia/Alzheimer's disease is controversial. This review discusses some recent epidemiological, human autopsy and experimental studies on the relationship between traumatic head injury and dementia.

RECENT FINDINGS

Recent epidemiological studies have shown that head injury is a risk factor for the development of dementia/Alzheimer's disease, whereas others have not. After experimental brain trauma the long-term accumulation of amyloid beta peptide suggests that neurodegeneration is influenced by apolipoprotein E epsilon 4, and after human brain injury both amyloid beta peptide deposition and tau pathology are seen, even in younger patients. Amyloid beta peptide levels in the cerebrospinal fluid and the overproduction of beta amyloid precursor protein in humans and animals after traumatic brain injury are increased. Repeated mild head trauma in both animals and humans accelerates amyloid beta peptide accumulation and cognitive impairment. Retrospective autopsy data support clinical studies suggesting that severe traumatic brain injury with long-lasting morphological residuals are a risk factor for the development of dementia/Alzheimer's disease. The influence of the apolipoprotein E genotype on the prognosis of traumatic brain injury is under discussion.

SUMMARY

Although epidemiological studies and retrospective autopsy data provide evidence that a later cognitive decline may occur after severe traumatic brain injury, the relationship between dementia after head/brain trauma and apolipoprotein E status is still ambiguous. Both human postmortem and experimental studies showing apolipoprotein beta deposition and tau pathology after head injury support the link between traumatic brain injury and dementia, and further studies are warranted to clarify this relationship.