Pericontusion axon sprouting is spatially and temporally consistent with a growth-permissive environment after traumatic brain injury

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.

sAPPα rescues deficits in amyloid precursor protein knockout mice following focal traumatic brain injury

The amyloid precursor protein (APP) is thought to be neuroprotective following traumatic brain injury (TBI), although definitive evidence at moderate to severe levels of injury is lacking. In the current study, we investigated histological and functional outcomes in APP-/- mice compared with APP+/+ mice following a moderate focal injury, and whether administration of sAPPα restored the outcomes in knockout animals back to the wildtype state. Following moderate controlled cortical impact injury, APP-/- mice demonstrated greater impairment in motor and cognitive outcome as determined by the ledged beam and Barnes Maze tests respectively (p < 0.05). This corresponded with the degree of neuronal damage, with APP-/- mice having significantly greater lesion volume (25.0 ± 1.6 vs. 20.3 ± 1.6%, p < 0.01) and hippocampal damage, with less remaining CA neurons (839 ± 245 vs. 1353 ± 142 and 1401 ± 263). This was also associated with an impaired neuroreparative response, with decreased GAP-43 immunoreactivity within the cortex around the lesion edge compared with APP+/+ mice. The deficits observed in the APP-/- mice related to a lack of sAPPα, as treatment with exogenously added sAPPα post-injury improved APP-/- mice histological and functional outcome to the point that they were no longer significantly different to APP+/+ mice (p < 0.05). This study shows that endogenous APP is potentially protective at moderate levels of TBI, and that this neuroprotective activity is related to the presence of sAPPα. Importantly, it indicates that the mechanism of action of exogenously added sAPPα is independent of the presence of endogenous APP.

Hyperbaric oxygenation improves locomotor ability by enhancing neuroplastic responses after cortical ablation in rats

OBJECTIVE

To investigate whether hyperbaric oxygenation (HBO) can improve the recovery of motor functions in rats after suction ablation of the right sensorimotor cortex.

METHODS

The experimental paradigm implies the following groups: Control animals (C), Control + HBO (CHBO), Sham controls (S), Sham control + HBO (SHBO), Lesion group (L), right sensorimotor cortex was removed by suction, Lesion + HBO (LHBO). Hyperbaric protocol: pressure applied 2.5 atmospheres absolute, for 60 minutes, once a day for 10 days. A beam walking test and grip strength meter were used to evaluate the recovery of motor functions. Expression profiles of growth-associated protein 43 (GAP43) and synaptophysin (SYP) were detected using immunohistochemistry.

RESULTS

The LHBO group achieved statistically superior scores in the beam walking test compared to the L group. Additionally, the recovery of muscle strength of the affected hindpaw was significantly enhanced after HBO treatment. Hyperbaric oxygenation induced over-expression of GAP43 and SYP in the neurons surrounding the lesion site.

CONCLUSIONS

Data presented suggest that hyperbaric oxygen therapy can intensify neuroplastic responses by promoting axonal sprouting and synapse remodelling, which contributes to the recovery of locomotor performances in rats. This provides the perspective for implementation of HBO in clinical strategies for treating traumatic brain injuries.

Influence of the environment on adult CNS plasticity and repair

During developmental critical periods, external stimuli are crucial for information processing, acquisition of new functions or functional recovery after CNS damage. These phenomena depend on the capability of neurons to modify their functional properties and/or their connections, generally defined as "plasticity". Although plasticity decreases after the closure of critical periods, the adult CNS retains significant capabilities for structural remodelling and functional adaptation. At the molecular level, structural modifications of neural circuits depend on the balance between intrinsic growth properties of the involved neurons and growth-regulatory cues of the extracellular milieu. Interestingly, experience acts on this balance, so as to create permissive conditions for neuritic remodelling. Here, we present an overview of recent findings concerning the effects of experience on cellular and molecular processes responsible for producing structural plasticity of neural networks or functional recovery after an insult to the adult CNS (e.g. traumatic injury, ischemia or neurodegenerative disease). Understanding experience-dependent mechanisms is crucial for the development of tailored rehabilitative strategies, which can be exploited alone or in combination with specific therapeutic interventions to improve neural repair after damage.

The contribution of gliosis to diffusion tensor anisotropy and tractography following traumatic brain injury: validation in the rat using Fourier analysis of stained tissue sections

Diffusion tensor imaging is highly sensitive to the microstructural integrity of the brain and has uncovered significant abnormalities following traumatic brain injury not appreciated through other methods. It is hoped that this increased sensitivity will aid in the detection and prognostication in patients with traumatic injury. However, the pathological substrates of such changes are poorly understood. Specifically, decreases in fractional anisotropy derived from diffusion tensor imaging are consistent with axonal injury, myelin injury or both in white matter fibres. In contrast, in both humans and animal models, increases in fractional anisotropy have been suggested to reflect axonal regeneration and plasticity, but the direct histological evidence for such changes remains tenuous. We developed a method to quantify the anisotropy of stained histological sections using Fourier analysis, and applied the method to a rat controlled cortical impact model to identify the specific pathological features that give rise to the diffusion tensor imaging changes in subacute to chronic traumatic brain injury. A multiple linear regression was performed to relate the histological measurements to the measured diffusion tensor changes. The results show that anisotropy was significantly increased (P < 0.001) in the perilesioned cortex following injury. Cortical anisotropy was independently associated (standardized β = 0.62, P = 0.04) with the coherent organization of reactive astrocytes (i.e. gliosis) and was not attributed to axons. By comparison, a decrease in white matter anisotropy (P < 0.001) was significantly related to demyelination (β = 0.75, P = 0.0015) and to a lesser extent, axonal degeneration (β = -0.48, P = 0.043). Gliosis within the lesioned cortex also influenced diffusion tensor tractography, highlighting the fact that spurious tracts in the injured brain may not necessarily reflect continuous axons and may instead depict glial scarring. The current study demonstrates a novel method to relate pathology to diffusion tensor imaging findings, elucidates the underlying mechanisms of anisotropy changes following traumatic brain injury and significantly impacts the clinical interpretation of diffusion tensor imaging findings in the injured brain.

Disturbance of perineuronal nets in the perilesional area after photothrombosis is not associated with neuronal death

Perineuronal nets (PNNs) are a condensed form of extracellular matrix that covers the surface of a subset of neurons. Their presence limits neuronal plasticity and may protect neurons against harmful agents. Here we analyzed the relationship between spatiotemporal changes in PNN expression and cell death markers after focal cortical photothrombotic stroke in rats. We registered a substantial decrease in PNN density using Wisteria floribunda agglutinin staining and CAT-315 and brevican immunoreactivity; the decrease occurred not only in the lesion core but also in the perilesional and remote cortex as well as in homotopic contralateral cortical regions. Fluoro Jade C and TUNEL staining in perilesional and remote areas, however, showed a low density of dying cells. Our results suggest that the PNN reduction was not a result of cellular death and could be considered an attempt to create conditions favorable for synaptic remodeling.

Diffuse traumatic axonal injury in the mouse induces atrophy, c-Jun activation, and axonal outgrowth in the axotomized neuronal population

Traumatic axonal injury (TAI) is a consistent component of traumatic brain injury (TBI) and is associated with much of its morbidity. Little is known regarding the long-term retrograde neuronal consequences of TAI and/or the potential that TAI could lead to anterograde axonal reorganization and repair. To investigate the repertoire of anterograde and retrograde responses triggered by TIA, Thy1-YFP-H mice were subjected to mild central fluid percussion injury and killed at various times between 15 min and 28 d post-injury. Based upon confocal assessment of the endogenous neuronal fluorescence, such injury was found to result in diffuse TAI throughout layer V of the neocortex within yellow fluorescent protein (YFP)-positive axons. When these fluorescent approaches were coupled with various quantitative and immunohistochemical approaches, we found that this TAI did not result in neuronal death over the 28 d period assessed. Rather, it elicited neuronal atrophy. Within these same axotomized neuronal populations, TAI was also found to induce an early and sustained activation of the transcription factors c-Jun and ATF-3 (activating transcription factor 3), known regulators of axon regeneration. Parallel ultrastructural studies confirmed that these reactive changes are consistent with atrophy in the absence of neuronal death. Concurrent with those events ongoing in the neuronal cell bodies, their downstream axonal segments revealed, as early as 1 d post-injury, morphological changes consistent with reactive sprouting that was accompanied by significant axonal elongation over time. Collectively, these TAI-linked events are consistent with sustained neuronal recovery, an activation of a regenerative genetic program, and subsequent axonal reorganization suggestive of some form of regenerative response.

Reorganization of motor cortex after controlled cortical impact in rats and implications for functional recovery

We report the results of controlled cortical impact (CCI) centered on the caudal forelimb area (CFA) of rat motor cortex to determine the feasibility of examining cortical plasticity in a spared cortical motor area (rostral forelimb area, RFA). We compared the effects of three CCI parameter sets (groups CCI-1, CCI-2, and CCI-3) that differed in impactor surface shape, size, and location, on behavioral recovery and RFA structural and functional integrity. Forelimb deficits in the limb contralateral to the injury were evident in all three CCI groups assessed by skilled reach and footfault tasks that persisted throughout the 35-day post-CCI assessment period. Nissl-stained coronal sections revealed that the RFA was structurally intact. Intracortical microstimulation experiments conducted at 7 weeks post-CCI demonstrated that RFA was functionally viable. However, the size of the forelimb representation decreased significantly in CCI-1 compared to the control group. Subdivided into component movement categories, there was a significant group effect for proximal forelimb movements. The RFA area reduction and reorganization are discussed in relation to possible diaschisis, and to compensatory functional behavior, respectively. Also, an inverse correlation between the anterior extent of the lesion and the size of the RFA was identified and is discussed in relation to corticocortical connectivity. The results suggest that CCI can be applied to rat CFA while sparing RFA. This CCI model can contribute to our understanding of neural plasticity in premotor cortex as a substrate for functional motor recovery.

Chondroitinase ABC enhances pericontusion axonal sprouting but does not confer robust improvements in behavioral recovery

Traumatic brain injury (TBI) results in enduring functional deficits. Strategies aimed at promoting plasticity within the injured brain may aid in enhancing functional outcome. We have previously shown that spontaneous pericontusional axon sprouting occurs within 7-14 days after controlled cortical impact injury in the adult rat, but ultimately fails due to an increasingly growth-inhibitory environment. We therefore sought to determine whether acute infusion of chondroitinase ABC into the site of the cortical contusion, to further reduce pericontusional growth-inhibitory chondroitin sulfate proteoglycans (CSPGs), would enhance and prolong the sprouting response. We also wanted to determine if chondroitinase-enhanced sprouting would ameliorate the behavioral deficits in forelimb function that occur in this model. Acute chondroitinase infusion decreased intact CSPGs and significantly increased pericontusional cortical grey and white matter growth-associated protein 43 (GAP43)-positive axon sprouting at 7 days post-injury. A return of intact CSPGs at later time points likely contributed to the absence of persistently increased levels of axon sprouting by 14-21 days post-injury. There was no overall benefit on forelimb function during the time of maximal sprouting or at any subsequent times in three of four behavioral outcome measures. However, there was a chondroitinase-induced improvement in recovery from unskilled limb use deficits on the staircase forelimb reaching test toward sham-injured values at 28 days, which was not achieved by the vehicle-treated rats, indicating that there is some minor functional benefit of the increased sprouting induced by chondroitinase treatment. The current results, together with data from spinal cord injury models after chondroitinase intervention, suggest that a combinatorial approach with the addition of neurotrophins and rehabilitation would result in more robust axon sprouting and consequently improve behavioral outcome.