Presynaptic Excitability as a Potential Target for the Treatment of the Traumatic Cerebellum

Pattern of Neuronal and Axonal Damage, Glial Response, and Synaptic Changes in Rat Cerebellum within the First Week following Traumatic Brain Injury

We examined damage and repair processes in the rat cerebellum within the first week following moderate traumatic brain injury (TBI) induced by lateral fluid percussion injury (LFPI) over the left parietal cortex. Rats were killed 1, 3, or 7 days after the injury or sham procedure. Fluoro-Jade B staining revealed 2 phases of neurodegenerative changes in the cell bodies and fibers: first, more focal, 1 day after the LFPI, and second, widespread, starting on post-injury day 3. Purkinje cell loss was detected in posterior lobule IX 1 day following LFPI. Apoptosis was observed in the cerebellar cortex, on days 1 and 7 following LFPI, and was not caspase- or apoptosis-inducing factor (AIF)-mediated. AIF immunostaining indicated axonal damage in the cerebellar white matter tracts 3- and 7-days post-injury. Significant astrocytosis and microgliosis were noticed on day 7 following LFPI at the sites of neuronal damage and loss. Immunohistochemical labeling with the presynaptic markers synaptophysin and growth-associated protein-43 revealed synaptic perturbations already on day 1 that were more pronounced at later time points following LFPI. These results provide new insights into pathophysiological alterations in the cerebellum and their mechanisms following cerebral TBI.

Electrophysiological Correlates of Blast-Wave Induced Cerebellar Injury

Understanding the mechanisms underlying traumatic neural injury and the sequelae of events in the acute phase is important for deciding on the best window of therapeutic intervention. We hypothesized that evoked potentials (EP) recorded from the cerebellar cortex can detect mild levels of neural trauma and provide a qualitative assessment tool for progression of cerebellar injury in time. The cerebellar local field potentials evoked by a mechanical tap on the hand and collected with chronically implanted micro-ECoG arrays on the rat cerebellar cortex demonstrated substantial changes both in amplitude and timing as a result of blast-wave induced injury. The results revealed that the largest EP changes occurred within the first day of injury, and partial recoveries were observed from day-1 to day-3, followed by a period of gradual improvements (day-7 to day-14). The mossy fiber (MF) and climbing fiber (CF) mediated components of the EPs were affected differentially. The behavioral tests (ladder rung walking) and immunohistological analysis (calbindin and caspase-3) did not reveal any detectable changes at these blast pressures that are typically considered as mild (100-130 kPa). The results demonstrate the sensitivity of the electrophysiological method and its use as a tool to monitor the progression of cerebellar injuries in longitudinal animal studies.

Electrophysiological monitoring of injury progression in the rat cerebellar cortex

The changes of excitability in affected neural networks can be used as a marker to study the temporal course of traumatic brain injury (TBI). The cerebellum is an ideal platform to study brain injury mechanisms at the network level using the electrophysiological methods. Within its crystalline morphology, the cerebellar cortex contains highly organized topographical subunits that are defined by two main inputs, the climbing (CFs) and mossy fibers (MFs). Here we demonstrate the use of cerebellar evoked potentials (EPs) mediated through these afferent systems for monitoring the injury progression in a rat model of fluid percussion injury (FPI). A mechanical tap on the dorsal hand was used as a stimulus, and EPs were recorded from the paramedian lobule (PML) of the posterior cerebellum via multi-electrode arrays (MEAs). Post-injury evoked response amplitudes (EPAs) were analyzed on a daily basis for 1 week and compared with pre-injury values. We found a trend of consistently decreasing EPAs in all nine animals, losing as much as 72 ± 4% of baseline amplitudes measured before the injury. Notably, our results highlighted two particular time windows; the first 24 h of injury in the acute period and day-3 to day-7 in the delayed period where the largest drops (~50% and 24%) were observed in the EPAs. In addition, cross-correlations of spontaneous signals between electrode pairs declined (from 0.47 ± 0.1 to 0.35 ± 0.04, p < 0.001) along with the EPAs throughout the week of injury. In support of the electrophysiological findings, immunohistochemical analysis at day-7 post-injury showed detectable Purkinje cell loss at low FPI pressures and more with the largest pressures used. Our results suggest that sensory evoked potentials (SEPs) recorded from the cerebellar surface can be a useful technique to monitor the course of cerebellar injury and identify the phases of injury progression even at mild levels.

Animal models of human cerebellar ataxias: a cornerstone for the therapies of the twenty-first century

Cerebellar ataxias represent a group of disabling neurological disorders. Our understanding of the pathogenesis of cerebellar ataxias is continuously expanding. A considerable number of laboratory animals with neurological mutations have been reported and numerous relevant animal models mimicking the phenotype of cerebellar ataxias are becoming available. These models greatly help dissecting the numerous mechanisms of cerebellar dysfunction, a major step for the assessment of therapeutics targeting a given deleterious pathway and for the screening of old or newly synthesized chemical compounds. Nevertheless, differences between animal models and human disorders should not be overlooked and difficulties in terms of characterization should not be occulted. The identification of the mutations of many hereditary ataxias, the development of valuable animal models, and the recent identifications of the molecular mechanisms underlying cerebellar disorders represent a combination of key factors for the development of anti-ataxic innovative therapies. It is anticipated that the twenty-first century will be the century of effective therapies in the field of cerebellar ataxias. The animal models are a cornerstone to reach this goal.

Cholesterol depletion inhibits electrophysiological changes induced by anoxia in CA1 region of rat hippocampal slices

The hyper-activation of glutamate receptors is a key event in the degenerative processes triggered by ischemia in the brain. Several types of these receptors reside in cholesterol-sphingomyelin rich domains of post-synaptic plasma membranes and have been described to be sensitive to cholesterol depletion. Hence we investigated, by extracellular recordings, the effect of cholesterol depletion on population spikes (PS) during ischemia-like conditions in the CA1 region of rat hippocampal slices using the cholesterol-depleting agent methyl-beta-cyclodextrin (MbetaCD). Results obtained demonstrate that MbetaCD prevents the changes induced by anoxic insult, i.e., depression of the population spike amplitude and insurgence of ischemic long-term potentiation. Furthermore cholesterol depletion prevents the disappearance of population spike induced by anoxia/aglycemia during kainate perfusion. Our data suggest a possible role of MbetaCD in preventing the pathological changes in synaptic activity induced by ischemia and indicate that manipulation of lipid components of membrane rafts might provide a new approach for the treatment of ischemia.

Models of traumatic cerebellar injury

Traumatic brain injury (TBI) is a major cause of morbidity and mortality worldwide. Studies of human TBI demonstrate that the cerebellum is sometimes affected even when the initial mechanical insult is directed to the cerebral cortex. Some of the components of TBI, including ataxia, postural instability, tremor, impairments in balance and fine motor skills, and even cognitive deficits, may be attributed in part to cerebellar damage. Animal models of TBI have begun to explore the vulnerability of the cerebellum. In this paper, we review the clinical presentation, pathogenesis, and putative mechanisms underlying cerebellar damage with an emphasis on experimental models that have been used to further elucidate this poorly understood but important aspect of TBI. Animal models of indirect (supratentorial) trauma to the cerebellum, including fluid percussion, controlled cortical impact, weight drop impact acceleration, and rotational acceleration injuries, are considered. In addition, we describe models that produce direct trauma to the cerebellum as well as those that reproduce specific components of TBI including axotomy, stab injury, in vitro stretch injury, and excitotoxicity. Overall, these models reveal robust characteristics of cerebellar damage including regionally specific Purkinje cell injury or loss, activation of glia in a distinct spatial pattern, and traumatic axonal injury. Further research is needed to better understand the mechanisms underlying the pathogenesis of cerebellar trauma, and the experimental models discussed here offer an important first step toward achieving that objective.

Aldolase C-positive cerebellar Purkinje cells are resistant to delayed death after cerebral trauma and AMPA-mediated excitotoxicity

The cerebellum has been shown to be vulnerable to global ischemic damage in tightly controlled zones of Purkinje cells (PCs) that lack aldolase C, an enzyme critical for glycolysis. Here, we investigated whether aldolase C-negative PCs were more likely to die after cerebral trauma in vivo, and whether this death was mediated by excitotoxic [alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-mediated] means in vitro. Mice were subjected to controlled cortical impact, or remained uninjured, and were killed at 6 h, 24 h or 7 days after injury. Cerebellar sections (both ipsilateral and contralateral to the site of cerebral injury) were stained against aldolase C and calbindin (a marker of PCs). The number of viable, calbindin-positive PCs decreased significantly at 24 h and 7 days after injury, and the percentage of surviving, aldolase C-positive PCs significantly increased at those time-points. In addition, we subjected murine cerebellar cultures to AMPA (30 microm, 20 min), which killed a significant number of PCs at 24 h post-treatment. A similar number of PCs was lost after transfection with aldolase C siRNA, and this effect was exacerbated in transfected cultures treated with AMPA. The results from the present study indicate that aldolase C provides marked neuroprotection to PCs after trauma and excitotoxicity.

Injury severity determines Purkinje cell loss and microglial activation in the cerebellum after cortical contusion injury

Clinical evidence suggests that the cerebellum is damaged after traumatic brain injury (TBI) and experimental studies have validated these observations. We have previously shown cerebellar vulnerability, as demonstrated by Purkinje cell loss and microglial activation, after fluid percussion brain injury. In this study, we examine the effect of graded controlled cortical impact (CCI) injury on the cerebellum in the context of physiologic and anatomical parameters that have been shown by others to be sensitive to injury severity. Adult male rats received mild, moderate, or severe CCI and were euthanized 7 days later. We first validated the severity of the initial injury using physiologic criteria, including apnea and blood pressure, during the immediate postinjury period. Increasing injury severity was associated with an increased incidence of apnea and higher mortality. Severe injury also induced transient hypertension followed by hypotension, while lower grade injuries produced an immediate and sustained hypotension. We next evaluated the pattern of subcortical neuronal loss in response to graded injuries. There was significant neuronal loss in the ipsilateral cortex, hippocampal CA2/CA3, and laterodorsal thalamus that was injury severity-dependent and that paralleled microglial activation. Similarly, there was a distinctive pattern of Purkinje cell loss and microglial activation in the cerebellar vermis that varied with injury severity. Together, these findings emphasize the vulnerability of the cerebellum to TBI. That a selective pattern of Purkinje cell loss occurs regardless of the type of injury suggests a generalized response that is a likely determinant of recovery and a target for therapeutic intervention.

Structural and functional alterations of cerebellum following fluid percussion injury in rats

Cerebellum was shown to be vulnerable to traumatic brain injury (TBI) in experimental animals. However, the detailed pathological and functional changes within the cerebellum following TBI are not known. Using our established cerebellum fluid percussion injury (FPI) model, we characterized the temporal pattern and the nature of structural damage following FPI, as well as the functional changes of Purkinje cells in response to climbing fiber activation. Our results showed that 60% of Purkinje cells died within the first 24 h following moderate FPI. In contrast, clusters of densely stained shrunken granule cells were stained positive for terminal deoxynucleotidyl transferase-mediated UTP nick end labeling (TUNEL) in 1, 3 or 7 days following FPI animals. We also observed an accompanying structural damage to the cerebellar white matter tract. Disconnected axonal fibers appeared 1 day post-FPI, and loss of white matter fibers were visible 3 and 7 days post-FPI. Massive accumulation of beta-amyloid precursor protein (betaAPP) was found in the white matter tracts and molecular layer in the cerebellum of 1, 3 or 7 days FPI animals. Our functional study showed that the majority of Purkinje cells from 1 day and all cells from 3 to 7 days post-FPI had distorted membrane potential and synaptic responses to climbing fiber activation. These results suggested that there is a co-related structural and functional deterioration with a specific temporal pattern in the cerebellum following FPI. These observations provide a basis for future mechanistic investigations aiming to realize neuroprotection from cerebellar neuronal death and loss of cerebellar functionality.

Long-term potentiation of evoked presynaptic response at CA3-CA1 synapses by transient oxygen-glucose deprivation in rat brain slices

Physiological activity-dependent long-term changes in synaptic transmission, as long-term potentiation (LTP) are thought to be the substrate of learning and memory. However, a form of postsynaptic pathological LTP at the CA3-CA1 synapses has been demonstrated following few minutes of anoxia and aglycemia in vitro. The ischemia LTP shared many molecular mechanisms with the physiological LTP, and was believed to be involved in the delayed neuronal death following ischemia. However, the role of the presynaptic component in this regard is not known. Here we show that a short period of oxygen-glucose deprivation can induce a form of LTP (lasting for hours) of the presynaptic response at the CA3-CA1 synapses. This form of LTP is independent of postsynaptic alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors, but Ca(2+) dependent. This presynaptic LTP may represent a presynaptic hyperexcitability of the afferent fibers following ischemia, and responsible for the excitotoxicity to the CA1 neurons (ischemia-induced increases of glutamate release that kills neurons) and the postsynaptic pathological ischemic LTP.