Altered excitatory amino acid function and morphology of the cerebellum of the spastic Han-Wistar rat

Transplanted Human Neural Progenitor Cells Attenuate Motor Dysfunction and Lengthen Longevity in a Rat Model of Ataxia

The spastic Han Wistar (sHW) rat serves as a model for human ataxia presenting symptoms of motor deterioration, weight loss, shortened lifespan, and Purkinje neuron loss. Past studies revealed that human neural progenitor cells (NPCs) improved ataxic symptoms at 20 d posttransplantation in sHW rats. In this study, we investigated the fate and longer-term effectiveness of these transplanted NPCs. Rats were placed into four treatment groups: an untreated normal control group (n = 10), an untreated mutant rat control (n = 10), a mutant group that received an injection of dead NPCs (n = 9), and a mutant group that received live NPCs (n = 10). Bilateral cerebellar injections containing 500,000 of either live or dead NPCs were performed on mutant sHW rats at 40 d of age. Motor activity for all mutant rats started to decline in open field testing around day 35. However, at day 45, the live NPC-treated mutants exhibited significant improvements in open field activity. Similar improvements were observed during rotarod testing and weight gain through the completion of the experiments (100 d). Immunohistochemistry revealed few surviving human NPCs in the cerebella of 80- and 100-d-old NPC-treated mutants; while cresyl violet staining revealed that live NPC-treated mutants had significantly more surviving Purkinje neurons compared to mutants that were untreated or received dead NPCs. Direct stereotactic implantation of NPCs alleviated the symptoms of ataxia, acting as a neuroprotectant, supporting future clinical applications of these NPCs in the areas of ataxia as well as other neurodegenerative diseases.

Exercise-induced neuroprotection in the spastic Han Wistar rat: the possible role of brain-derived neurotrophic factor

Moderate aerobic exercise has been shown to enhance motor skills and protect the nervous system from neurodegenerative diseases, like ataxia. Our lab uses the spastic Han Wistar rat as a model of ataxia. Mutant rats develop forelimb tremor and hind limb rigidity and have a decreased lifespan. Our lab has shown that exercise reduced Purkinje cell degeneration and delayed motor dysfunction, significantly increasing lifespan. Our study investigated how moderate exercise may mediate neuroprotection by analyzing brain-derived neurotrophic factor (BDNF) and its receptor TrkB. To link BDNF to exercise-induced neuroprotection, mutant and normal rats were infused with the TrkB antagonist K252a or vehicle into the third ventricle. During infusion, rats were subjected to moderate exercise regimens on a treadmill. Exercised mutants receiving K252a exhibited a 21.4% loss in Purkinje cells compared to their controls. Cerebellar TrkB expression was evaluated using non-drug-treated mutants subjected to various treadmill running regimens. Running animals expressed three times more TrkB than sedentary animals. BDNF was quantified via Sandwich ELISA, and cerebellar expression was found to be 26.6% greater in mutant rats on 7-day treadmill exercise regimen compared to 30 days of treadmill exercise. These results suggest that BDNF is involved in mediating exercise-induced neuroprotection.

Muscle-specific changes in length-force characteristics of the calf muscles in the spastic Han-Wistar rat

The purpose of the present study was to investigate muscle mechanical properties and mechanical interaction between muscles in the lower hindlimb of the spastic mutant rat. Length-force characteristics of gastrocnemius (GA), soleus (SO), and plantaris (PL) were assessed in anesthetized spastic and normally developed Han-Wistar rats. In addition, the extent of epimuscular myofascial force transmission between synergistic GA, SO, and PL, as well as between the calf muscles and antagonistic tibialis anterior (TA), was investigated. Active length-force curves of spastic GA and PL were narrower with a reduced maximal active force. In contrast, active length-force characteristics of spastic SO were similar to those of controls. In reference position (90° ankle and knee angle), higher resistance to ankle dorsiflexion and increased passive stiffness was found for the spastic calf muscle group. At optimum length, passive stiffness and passive force of spastic GA were decreased, whereas those of spastic SO were increased. No mechanical interaction between the calf muscles and TA was found. As GA was lengthened, force from SO and PL declined despite a constant muscle-tendon unit length of SO and PL. However, the extent of this interaction was not different in spastic rats. In conclusion, the effects of spasticity on length-force characteristics were muscle specific. The changes observed for GA and PL muscles are consistent with the changes in limb mechanics reported for human patients. Our results indicate that altered mechanics in spastic rats cannot be attributed to differences in mechanical interaction, but originate from individual muscular structures.

Neuronal nicotinic receptor agonists improve gait and balance in olivocerebellar ataxia

Clinical studies have reported that the nicotinic receptor agonist varenicline improves balance and coordination in patients with several types of ataxia, but confirmation in an animal model has not been demonstrated. This study investigated whether varenicline and nicotine could attenuate the ataxia induced in rats following destruction of the olivocerebellar pathway by the neurotoxin 3-acetylpyridine (3-AP). The administration of 3-AP (70 mg/kg followed by 300 mg niacinamide/kg; i.p.) led to an 85% loss of inferior olivary neurons within one week without evidence of recovery, and was accompanied by a 72% decrease in rotorod activity, a 3-fold increase in the time to traverse a stationary beam, a 19% decrease in velocity and 31% decrease in distance moved in the open field, and alterations in gait parameters, with a 19% increase in hindpaw stride width. The daily administration of nicotine (0.33 mg free base/kg) for one week improved rotorod performance by 50% and normalized the increased hindpaw stride width, effects that were prevented by the daily preadministration of the nicotinic antagonist mecamylamine (0.8 mg free base/kg). Varenicline (1 and 3 mg free base/kg daily) also improved rotorod performance by approximately 50% following one week of administration, and although it did not alter the time to traverse the beam, it did improve the ability to maintain balance on the beam. Neither varenicline nor nicotine, at doses that improved balance, affected impaired locomotor activity in the open field. Results provide evidence that nicotinic agonists are of benefit for alleviating some of the behavioral deficits in olivocerebellar ataxia and warrant further studies to elucidate the specific mechanism(s) involved.

Neuroprotective effects of moderate aerobic exercise on the spastic Han-Wistar rat, a model of ataxia

Research has shown that physical exercise may reduce degeneration in certain brain regions experiencing ataxia. Our laboratory utilized mutant spastic Han-Wistar rats (sHW) that display developmental abnormalities, including spastic paresis, fore limb tremors, hind limb rigidity, and a reduced life span (60-65 days of age). Concomitant neurodegeneration has been observed in the cerebellum (Purkinje cells). The purpose of this study was to investigate if moderate, aerobic exercise could reduce Purkinje cell neurodegeneration and improve the motor ability and survival of the mutant sHW rat. Mutant male littermates at the ages of 20 (n=11 pairs) and 30 (n=13 pairs) days old were divided into running groups and non-running groups. Mutant rats were run on a motorized treadmill at the rate of 15 m/min with a 10% slope. The "running" group ran for 30 min per day, 5 days a week; the "non-runners" remained nearby in the training facility. These conditions were held constant until the mutant runners could no longer run due to disease progression. Moderate exercise increased the lifespan of running mutant rats in both the 20-day start group (14% increase) and 30-day start group (13% increase). The rats exhibited improved motor function as open-field tests showed higher activity scores for runners after 50 days. Histological examination of the cerebellum revealed a 62% increase in Purkinje cell survival of the runners. These results suggest that aerobic exercise ameliorates, at least partially, cerebellar dysfunction in the sHW rat, an excellent model of ataxia.

Don't get too excited: mechanisms of glutamate-mediated Purkinje cell death

Purkinje cells (PCs) present a unique cellular profile in both the cerebellum and the brain. Because they represent the only output cell of the cerebellar cortex, they play a vital role in the normal function of the cerebellum. Interestingly, PCs are highly susceptible to a variety of pathological conditions that may involve glutamate-mediated 'excitotoxicity', a term coined to describe an excessive release of glutamate, and a subsequent over-activation of excitatory amino acid (NMDA, AMPA, and kainite) receptors. Mature PCs, however, lack functional NMDA receptors, the means by which Ca(2+) enters the cell in classic hippocampal and cortical models of excitotoxicity. In PCs, glutamate predominantly mediates its effects, first via a rapid influx of Ca(2+)through voltage-gated calcium channels, caused by the depolarization of the membrane after AMPA receptor activation (and through Ca(2+)-permeable AMPA receptors themselves), and second, via a delayed release of Ca(2+) from intracellular stores. Although physiological levels of intracellular free Ca(2+) initiate vital second messenger signaling pathways in PCs, excessive Ca(2+) influx can detrimentally alter dendritic spine morphology via interactions with the neuronal cytoskeleton, and thus can perturb normal synaptic function. PCs possess various calcium-binding proteins, such as calbindin-D28K and parvalbumin, and glutamate transporters, in order to prevent glutamate from exerting deleterious effects. Bergmann glia are gaining recognition as key players in the clearance of extracellular glutamate; these cells are also high in S-100beta, a protein with both neurodegenerative and neuroprotective abilities. In this review, we discuss PC-specific mechanisms of glutamate-mediated excitotoxic cell death, the relationship between Ca(2+) and cytoskeleton, and the implications of glutamate, and S-100beta for pathological conditions, such as traumatic brain injury.

Enhanced epileptogenic susceptibility in a genetic model of reactive synaptogenesis: the spastic Han-Wistar rat

Our laboratory has been studying the spastic Han-Wistar (sHW) rat as a model of neuronal degeneration. Mutant sHW rats display a number of developmental abnormalities that eventually lead to hippocampal pyramidal cell death and synaptic reorganization starting around 30 days of age. The present study examined the contribution of hippocampal reorganization to the expression of seizures induced by systemic injections of kainic acid. Behavioral observations, EEG recordings and hippocampal Fos protein expression in these animals indicated that mutants develop paroxysmal discharges and seizures earlier than controls and the intensity of epileptic manifestations is greater. Kainate injections were lethal in 50% of mutants compared to only 5% of controls. Fos expression was increased approximately twofold in the mutant hippocampus, implicating abnormal excitation in this region. Additional studies in untreated animals indicated that GluR2 mRNA expression was significantly increased throughout the hippocampus in mutant animals, possibly contributing to the enhanced susceptibility to kainate treatment. These results confirm the role of synaptic reorganization in the increased propensity to develop epileptic discharges. Our data also underscore the usefulness of this natural model of cell degeneration and reactive synaptogenesis for understanding the mechanisms of neuronal hyperexcitability.

Co-localization of tyrosine hydroxylase and zebrin II immunoreactivities in Purkinje cells of the mutant mice, tottering and tottering/leaner

The mutant mice tottering, leaner and the compound heterozygous tottering/leaner exhibit varying degrees of several abnormal neurological phenotypes including petit mal-like epilepsy, ataxia and an intermittent myoclonus-like movement disorder. Aberrant expression of tyrosine hydroxylase in cerebellar Purkinje cells of tottering, leaner and tottering/leaner mice has been observed previously [Austin M. C. et al. (1992) Molec. Brain Res. 15, 227-240; Hess E. J. and Wilson M. C. (1991) Neuron 6, 123-132]. In the present study, the distribution of tyrosine hydroxylase expression was compared with that of Zebrin II in Purkinje cells of adult homozygous tottering and compound heterozygous tottering/leaner mutant mice using single and double immunocytochemistry and double immunofluorescence. The pattern of Zebrin II expression in the cerebella of the mutant mice was identical to that described for normal mice [Hawkes R. et al. (1985) Brain Res. 333, 359-365; Hawkes R. and Leclerc N. (1987) J. comp. Neurol. 256, 29-41]. In addition, sections through tottering and tottering/leaner cerebella demonstrated an exact correspondence between the bands of tyrosine hydroxylase immunoreactivity and bands of Zebrin II immunoreactivity. Similarly, the compartments of the Purkinje cell layer which were negative for Zebrin II staining were also negative for tyrosine hydroxylase immunoreactivity. This study provides evidence that the cerebellar Purkinje cells of tottering and tottering/leaner mice were able to express a normal gene product, Zebrin II, in a normal spatial pattern and the same Purkinje cells can also express an aberrant gene product, tyrosine hydroxylase. This abnormal gene expression may indicate that at least some Purkinje cells in these mutant mice are not functioning normally. This possibility, taken together with the morphological changes observed in many mutant Purkinje cell axons, suggests that Purkinje cell function could be altered in tottering and tottering/leaner mice, thereby contributing to the neurological abnormalities exhibited by these mice. It is also possible that alteration in function of mutant Purkinje cells could correlate with the rostrocaudal zonation pattern described in this study.

Altered pattern of immunohistochemical staining for glial fibrillary acidic protein (GFAP) in the forebrain and cerebellum of the mutant spastic rat

The spastic rat is a neurological mutant of the Han-Wistar strain with prominent spasticity, tremor, and ataxia. Neurodegeneration is found in the CA3 sector of the hippocampus and in Purkinje cells of the cerebellum. We examined the forebrain and cerebellum of spastic rats for glial reactions by using immunolabelling for the astrocytic marker, glial fibrillary acidic protein (GFAP). First, a map of the GFAP-distribution was made representing a systematic series of frontal sections in controls. Reactive astrocytes with increased GFAP should occur in the areas with established neuronal degeneration, but they could also demarcate further regions with pathology in this rat strain. Since the baseline levels of GFAP-immunoreactivity differ between brain regions, control rats and clinically normal littermates served as controls to judge relative increases in major structures. In the CA3 sector and hilus of the dorsal hippocampus, a massive gliosis was detected. In the cerebellum, a patchy increase of GFAP labelling in Bergmann glia was found. Further increases of GFAP-labelling in reactive astrocytes occurred in fiber tracts, the ventral thalamic nuclei, medial geniculate nuclei, pontine region and optic layer of the superior colliculus. Inconsistent changes were noted in cortex and pallidum. No defects of glial labelling or malformations in glial architectonics were found. The reactive changes of astroglial cells in hippocampus and cerebellum are in proportion to the neuronal degeneration. The glial reactions in the other brain regions possibly reflect a reaction to fiber degeneration and incipient neuronal degeneration or functional alterations of glial cells in response to neuronal dysfunction.

Non-NMDA receptors in motor neuron disease (MND): a quantitative autoradiographic study in spinal cord and motor cortex using [3H]CNQX and [3H]kainate

The distribution and density of non-NMDA receptors in spinal cord and motor cortex was compared in 10 cases of motor neuron disease (MND) and 8 neurologically normal controls by quantitative autoradiography using [3H]CNQX and [3H]kainate. In the motor cortex of MND cases, an increased density of [3H]kainate binding sites was observed which was most marked in the deep layers. No significant differences were observed in [3H]CNQX binding in the motor cortex between MND and control cases. In the spinal cord significantly increased densities of both [3H]CNQX and [3]kainate binding sites were found in the substantia gelatinosa and the intermediate grey matter in the MND group. The changes in [3H]kainate binding were observed only in the amyotrophic lateral sclerosis (ALS) subgroup of MND, while the changes in [3H]CNQX binding in the spinal cord were more marked in ALS compared to progressive muscular atrophy (PMA) cases. These findings provide evidence in support of a disturbance of glutamatergic neurotransmission in MND and suggest that there may be an increased excitatory drive to motor neurons via non-NMDA receptors. It is unclear at present whether the changes observed represent a compensatory response to loss of motor neurons in MND or a pathophysiological phenomenon contributing to motor neuron degeneration. Modulation of non-NMDA receptor activity may represent a possible target for therapeutic intervention in this disease.

Excitotoxicity and motor neurone disease: a review of the evidence

Excitotoxic mechanisms have a well established role in the pathogenesis of neuronal injury following acute CNS insults such as ischaemia and trauma. Their role in the selective cell death which occurs in chronic neurodegenerative disorders such as motor neurone disease (MND) is more speculative. The traditional classification of glutamate receptor subtypes which mediate excitotoxicity requires modification in the light of new molecular data. There is much greater structural and functional diversity in this receptor family than previously envisaged and it is quite possible that specific populations of neurones will be characterised by a unique profile of glutamate receptor subtypes which may be a factor determining their selective vulnerability. The molecular mechanisms underlying excitotoxic neuronal injury are still being elucidated but it is clear that the cascade of events resulting from elevation of intracellular free calcium is likely to play a major role. As well as being a primary mechanism of neuronal injury, excitotoxicity can secondarily damage neurones whose energy metabolism is impaired from some primary pathological process. The 8 lines of evidence that primary or secondary excitotoxic mechanisms may be involved in the selective neuronal injury of MND are discussed. The evidence, while still circumstantial, is sufficient to warrant further research effort in this field, not least because the emergence of pharmacological agents which modify specific aspects of excitatory amino acid neurotransmission offer the possibility of therapeutic intervention in MND.

Functional consequences of expression of the neuron-specific, protein kinase C substrate RC3 (neurogranin) in Xenopus oocytes

RC3 (neurogranin) is a neuron-specific substrate of protein kinase C (PKC) that accumulates predominantly in dendritic spines of forebrain neurons and undergoes long-term potentiation (LTP)-associated increases in PKC-phosphorylation in hippocampal slices. Here the hypothesis that RC3 functions by modulating the IP3/DAG second messenger pathway after its phosphorylation by DAG-activated PKC was tested by heterologous expression in Xenopus oocytes. Acetylcholine-evoked inward chloride (Cl-) currents, dependent on both IP3 release and intracellular calcium (Ca2+), were 2- to 3-fold higher in RC3-injected oocytes than in uninjected control oocytes. RC3-oocytes did not exhibit enhanced currents when preincubated with the protein kinase inhibitor H-7 or when a glycine residue was substituted for serine, the PKC phosphorylation site of RC3. Activation of endogenous oocyte PKC by phorbol esters generated inward Cl- currents in RC3 oocytes but not in control oocytes. RC3-dependent Cl- currents were also elicited by phorbol ester in Ca(2+)-free media. We propose that PKC-phosphorylated RC3 is capable of enhancing the mobilization of intracellular Ca2+ in Xenopus oocytes and, by inference, may play a role in Ca2+ homeostasis in dendrites of forebrain neurons.