Changes in AMPA receptor binding in an animal model of inborn paroxysmal dystonia

Expression of metabotropic glutamate 5 receptors in the striatum and cortex and effects of modulators on the severity of dystonia in the phenotypic dtsz model

The metabotropic glutamate 5 (mGlu₅) receptor has been suggested as therapeutic target for L-Dopa-induced dyskinesia which is often associated with dystonic symptoms. Therefore, we investigated the acute effects of the non-competitive mGlu₅ receptor antagonist fenobam as well as the positive modulator 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) on the severity of inherited dystonia in the mutant dt^sz hamster, a phenotypic model with age-dependent episodes of dystonia. Fenobam did not exert significant antidystonic effects (20-50 mg/kg intraperinoneal, i.p.). CDPPB (10, 20 mg/kg i.p.) which was expected to worsen dystonia also failed to show any effects on the severity of dystonia. Interestingly, CDPPB caused axial dyskinesia in addition to the dystonic symptoms in mutant hamsters. This adverse effect could not be observed in non-dystonic control hamsters, indicating possible changes in the expression of mGlu₅ receptors in dystonic hamsters. The mGlu₅ receptor mRNA did not differ between the dt^sz mutant and control hamsters, while immunohistochemical studies indicated that the mGlu₅ receptor expression was about 35% higher in striatum and cortex of mutant hamsters at the age of high dystonia severity scores, notably not after spontaneous remission of dystonia, compared to age-matched controls. This difference in mGlu₅ receptor protein may be due to altered protein conformation instead of protein level, as western blots revealed similar amounts of monomeric and dimeric protein in mutant hamsters versus control. Thus, the present data do not provide clear evidence for an important role of the mGlu5 receptor in the pathophysiology and as a therapeutic target for types of inherited dystonia.

A rat model of hemidystonia induced by 3-nitropropionic acid

OBJECTIVE

Secondary dystonia commonly presents as hemidystonia and is often refractory to current treatments. We aimed to establish an inducible rat model of hemidystonia utilizing 3-nitropropionic acid (3-NP) and to determine the pathophysiology of this model.

METHODS

Two different doses of 3-NP were stereotactically administered into the ipsilateral caudate putamen (CPu) of Wistar rats. Behavioral changes and alterations in the neurotransmitter levels in the basal ganglia were analyzed. We also performed an electromyogram, 7.0-T magnetic resonance imaging and transmission electron microscopy examination to determine the pathophysiology of the model.

RESULTS

In the CPu region, 3-NP produced mitochondrial cristae rupture, axonal degeneration, increased excitatory synaptic vesicles and necrosis. The extracellular concentrations of excitatory amino acids increased, whereas the inhibitory amino acids decreased in the CPu. Furthermore, an imbalance of neurotransmitters was found in other regions of the basal ganglia with the exception of the external globus pallidus. This study demonstrated that 3-NP administration results in CPu damage, and combined with a neurotransmitter imbalance in the basal ganglia, it produces specific neurobehavioral changes in rats. Right limb (contralateral side of CPu lesion) and trunk dystonic postures, shortened step length and ipsiversive dystonic posturing were observed in these rats. Furthermore, EMG recordings confirmed that co-contraction of the agonist and antagonist muscles could be seen for several seconds in right limbs.

CONCLUSIONS

Stereotactic injection of 3-NP into the ipsilateral CPu of rats established an inducible model for hemidystonia. This effect might result from an imbalance of neurotransmitter levels, which induce dysfunctional activity of the basal ganglia mainly via the cortico-striato-GPi direct pathway. Symptoms in this model were present for 1 week. Activation of the cortico-striato-GPe indirect pathway and rebalance of neurotransmitters may lead to recovery. This rat model may be a suitable tool used to understand and further investigate the pathophysiology of dystonia.

AMPA receptors mediate passive avoidance deficits induced by sleep deprivation

The present study addressed the effects of sleep deprivation (SD) on AMPA receptor (AMPAR) binding in brain regions associated with learning and memory, and investigated whether treatment with drugs acting on AMPAR could prevent passive avoidance deficits in sleep deprived animals. [(3)H]AMPA binding and GluR1 in situ hybridization signals were quantified in different brain regions of male Wistar rats either immediately after 96 h of sleep deprivation or after 24h of sleep recovery following 96 h of sleep deprivation. Another group of animals were sleep deprived and then treated with either the AMPAR potentiator, aniracetam (25, 50 and 100mg/kg, acute administration) or the AMPAR antagonist GYKI-52466 (5 and 10mg/kg, acute and chronic administration) before passive avoidance training. Task performance was evaluated 2h and 24h after training. A significant reduction in [(3)H]AMPA binding was found in the hippocampal formation of SD animals, while no alterations were observed in GluR1 mRNA levels. The highest dose of aniracetam (100mg/kg) reverted SD-induced impairment of passive avoidance performance in both retention tests, whereas GYKI-52466 treatment had no effect. Pharmacological enhancement of AMPAR function may revert hippocampal-dependent learning impairments produced after SD. We argue that such effects might be associated with reduced AMPAR binding in the hippocampus of sleep deprived animals.

Dyschromatosis symmetrica hereditaria associated with neurological disorders

Dyschromatosis symmetrica hereditaria (DSH) is a pigmentary genodermatosis of autosomal dominant inheritance caused by a mutation of adenosine deaminase acting on the RNA 1 gene (ADAR1). It is characterized by a mixture of hyper- and hypopigmented macules on the back of the hands and feet. The pathomechanism by which the ADAR1 gene mutation induces DSH has not been clarified yet. We experienced an 11-year-old male DSH patient associated with dystonia, mental deterioration and brain calcification, who had a mutation of p.G1007R in the ADAR1 gene. This mutation had already been reported in a patient with similar neurological symptoms by Tojo et al. Additionally, a patient with DSH associated with torsion dystonia was reported by Patrizi et al., but gene analysis was not carried out. Only three cases with neurological disorders have been reported, although more than 50 mutations of the ADAR1 gene causing DSH have been reported and none of them had any neurological symptoms. Therefore, we suggest that neurological disorders rarely develop in DSH.

Extracellular amino acid levels in the striatum of the dt(sz) mutant, a model of paroxysmal dystonia

The pathophysiology of idiopathic dystonia is still unknown, but it is regarded as a basal ganglia disorder. Previous studies indicated an involvement of a striatal GABAergic disinhibition and a cortico-striatal glutamatergic overactivity in the manifestation of stress-inducible dystonic episodes in the dt(sz) hamster, a model of idiopathic paroxysmal dystonia. These investigations were carried out postmortem or in anesthetized animals. In the present study, in vivo microdialysis in conscious, freely-moving dt(sz) and non-dystonic control hamsters was used to examine the levels of GABA, aspartate, glutamate, glutamine, glycine and taurine in each animal during following conditions: (1) at baseline in the absence of dystonia, (2) during an episode of paroxysmal dystonia precipitated by stressful stimuli, (3) during a recovery period and (4) at baseline after complete recovery. In comparison to non-dystonic controls, which were treated in the same manner as the dystonic animals, no differences could be detected under basal conditions. The induction of a dystonic episode in mutant hamsters led to higher contents of glycine in these animals in comparison to stressed but non-dystonic controls. Significant changes of glycine levels within the animal groups were not detected. The levels of the excitatory amino acids glutamate, glutamine and aspartate as well as the levels of the inhibitory amino acids GABA and taurine did not differ between the animal groups or between the periods of measurement. The higher levels of glycine might contribute to the manifestation of paroxysmal dystonia in dt(sz) hamsters, although unaltered glutamate, glutamine and aspartate levels do not support the hypothesis of a critical involvement of a cortico-striatal overactivity. It seems that a deficiency of GABAergic interneurons, found by previous immunohistochemical examinations, does not lead to reduced extracellular GABA levels in the striatum.

Effects of intrastriatal injections of glutamate receptor antagonists on the severity of paroxysmal dystonia in the dtsz mutant

Imbalances of the glutamatergic system are implicated in the pathophysiology of various basal ganglia disorders, but few is known about their role in dystonia, a common neurological syndrome in which involuntary muscle co-contractions lead to twisting movements and abnormal postures. Previous systemic administrations of glutamate receptor antagonists in dtsz hamsters, an animal model of primary paroxysmal dystonia, exerted antidystonic effects and electrophysiological experiments pointed to an enhanced corticostriatal glutamatergic activity. In order to examine the pathophysiological relevance of these findings, we performed striatal microinjections of the alpha-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazolepropanoic acid (AMPA) receptor antagonist 2,3-dioxo-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX) and the N-methyl-D-aspartate (NMDA) receptor antagonists D(-)-2-amino-5-phosphopentanoic acid (AP-5), (R)-(+)-3-amino-1-hydroxypyrrolidin-2-one (HA-966) and dizocilpine (MK-801). The striatal application of NBQX reduced the severity and increased the latency to onset of dystonia significantly only at a dosage of 0.08 microg per hemisphere, lower (0.03 microg) and higher dosages (0.16 microg and 0.32 microg) failed to exert comparable effects on the severity. None of the striatal injected NMDA receptor antagonists influenced the severity of the dystonic attacks in the mutant hamster. The combined application of NBQX (0.08 microg) with AP-5 (1.0 microg) failed to exert synergistic antidystonic effects, but the beneficial effect on the severity of dystonia of the single application of NBQX was reproduced. Therefore, corticostriatal glutamatergic overactivity mediated by AMPA receptors, but not by NMDA receptors, is possibly important for the manifestation of dystonic attacks in the dtsz hamster mutant.

Age-related changes in parvalbumin-positive interneurons in the striatum, but not in the sensorimotor cortex in dystonic brains of the dt mutant hamster

In the dt(sz) hamster, a model of paroxysmal dystonia, an age-dependent increase in the activity of striatal projection neurons has been hypothesized to be based on a deficit of striatal parvalbumin-immunoreactive (PV(+)) interneurons at an age of most marked expression of dystonia (30-40 days of life). In the present study, the spontaneous age-dependent remission of paroxysmal dystonia in older dt(sz) hamsters (age>90 days) was found to coincide with a normalization of the density of striatal PV(+) interneurons. Furthermore, the arborization of these interneurons was lower in 31 day old dt(sz) hamsters, but was even higher in dt(sz) mutant at an age of >90 days than in control animals. Double-labeling with bromodeoxyuridine failed to show a retarded proliferation, while the number of interneurons with strong expression of PV mRNA was lower in young mutant hamsters. As shown by unaltered density of PV(+) interneurons in sensorimotor cortex of 31 day old dt(sz) hamsters, PV containing interneurons are not reduced throughout the whole brain at the sensitive age. The present data suggest that a retarded postnatal maturation of striatal PV(+) interneurons plays a critical role in paroxysmal dystonia.

Dystonia, mental deterioration, and dyschromatosis symmetrica hereditaria in a family with ADAR1 mutation

A family with dystonia associated with dyschromatosis symmetrica hereditaria (DSH), mental deterioration, and tissue calcification is described. The proband possessed an adenosine deaminase acting on the RNA 1 gene (ADAR1) mutation Gly1007Arg. This ADAR1 mutation could disturb RNA editing at Q/R sites of glutamate receptor in the brain and increase Ca(2+) influx into neurons, which is thought to induce dystonia and mental deterioration. The observations in our family raise the possibility that the ADAR1 mutation might be a direct cause or a predisposing factor for heredodegenerative dystonia. Further investigation of ADAR1 mutations will shed light on the genotype-phenotype correlation in DSH.

The kynurenine 3-hydroxylase inhibitor Ro 61-8048 improves dystonia in a genetic model of paroxysmal dyskinesia

The effects of the novel kynurenine 3-hydroxylase inhibitor 3,4-dimethoxy-N-[4-(3-nitrophenyl)thiazol-2-yl]benzenesulfonamide (Ro 61-8048) on severity of dystonia were examined in dt(sz) mutant hamsters, an animal model of paroxysmal dystonia, in which stress precipitates dystonic episodes. Ro 61-8048 (50, 100 and 150 mg/kg i.p.) significantly reduced the severity of dystonia in dt(sz) hamsters without leading to marked central side effects. Determinations of kynurenic acid concentrations in brain homogenates demonstrated that Ro 61-8048 (100 mg/kg i.p.) provoked a two- to threefold increase of the endogeneous broad spectrum glutamate receptor antagonist kynurenic acid in the striatum, cerebellum and brainstem of mutant hamsters. The antidystonic efficacy of Ro 61-8048 at well-tolerated doses suggests that kynurenine 3-hydroxylase inhibitors should be considered as new therapeutic candidates for the treatment of dyskinesias.

Increased excitability in cortico-striatal synaptic pathway in a model of paroxysmal dystonia

Dystonias are movement disorders whose pathomechanism is largely unknown. Dystonic dt(sz) hamsters represent a model of primary dystonias, where alterations of striatal interneuron density and sodium channel function in projection neurones were described. Here, using cortico-striatal slices, we explore whether also the communication between neocortex and striatum is altered in dt(sz) hamsters. Field and intracellular recordings were done in dorsomedial striatum. Electrical stimulation was used to mimic neocortical afferents. Neuronal characteristics, synaptic connections, input-output relations and short- and long-term plasticity were analysed. Regarding cellular properties, striatal neurons of affected animals showed no alterations. Concerning network properties, evoked responses at threshold stimulation were mediated by (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptors. In dt(sz) slices, field responses, paired-pulse accentuation and LTP were larger than in control, possibly by an increase in presynaptic release probability at glutamatergic synapses. In summary, the study indicates that a change of cortico-striatal communication is involved in the manifestation of paroxysmal dystonia in the dt(sz) mutant.

Polyamines in a genetic animal model of paroxysmal dyskinesia

Previous studies suggested that glutamatergic overactivity contributes to the manifestation of dystonia in the dt(sz) mutant hamster, a model of idiopathic paroxysmal dyskinesia in which dystonic episodes occur in response to mild stress. Therefore, the role of polyamines, known as positive modulators of NMDA receptors, was examined in the present study. The levels of polyamines (putrescine, spermidine, spermine) were determined in forebrain, cerebellum and brainstem in dt(sz) hamsters at an age of most marked expression of dystonia (32 days) and in age-matched non-dystonic control hamsters. Spermine was found to be significantly increased in the forebrain (35%) of dystonic animals, while spermidine was unaltered in dystonic brains and only a moderate increase in putrescine (12%) was detected in the cerebellum of dt(sz) mutants. In view of enhanced spermine levels, the effect of the putative polyamine receptor antagonist ifenprodil on the severity of dystonia was examined in dystonic hamsters. Ifenprodil (5-40 mg/kg i.p.) failed to exert a beneficial effect, but even aggravated dystonia in the dt(sz) mutant at higher doses. These data together with previous pharmacological findings in mutant hamsters do not completely exclude a pathophysiological role of enhanced polyamine levels but suggest that overstimulation of NMDA receptors which contain NR2B subunits by enhanced spermine levels is not involved in the dystonic syndrome.

The NMDA receptor NR2B subtype selective antagonist Ro 25-6981 aggravates paroxysmal dyskinesia in the dt(sz) mutant

Previously, enhanced levels of spermine which stimulates N-methyl-D-aspartate (NMDA) receptors, particularly those containing the NR2B subunit, were found in brains of dt(sz) mutant hamsters, a model of paroxysmal dyskinesia in which dystonic episodes occur in response to stress. Therefore, the effects of the NR2B selective NMDA receptor antagonist Ro 25-6981 ([R-(R,S)]-alpha-(4-hydroxyphenyl)-beta-methyl-4-phenyl-methyl)-1-piperidine-propanol] on severity of dystonia were investigated in the dt(sz) hamster. Ro 25-6981 failed to exert antidystonic effects, but even caused a moderate aggravation at higher doses (10.0, 12.5 mg/kg). This result indicates that overstimulation of receptors that include the NR2B subunit by polyamines is not involved in the dystonic syndrome. NR2B-selective NMDA receptor antagonists seem not to provide a novel approach in the treatment of hereditary paroxysmal dyskinesias.

Regional decreases in NK-3, but not NK-1 tachykinin receptor binding in dystonic hamster (dt(sz)) brains

Although the pathophysiology of primary dystonias is currently unknown, it is thought to involve changes in the basal ganglia-thalamus-cortex circuit, particularly activity imbalances between direct and indirect striatal pathways. Substance P, a member of the tachykinin family of neuropeptides, is a major component in the direct pathway from striatum to basal ganglia output nuclei. In the present study quantitative autoradiography was used to examine changes in neurokinin-1 (NK-1) and neurokinin-3 (NK-3) receptors in mutant dystonic hamsters (dt(sz)), a well characterized model of paroxysmal dystonia. NK-1 receptors were labeled in 10 dystonic brains and 10 age-matched controls with 3 nM [(3)H]-[Sar(9), Met(O(2))(11)]-SP. NK-3 binding sites were labeled in adjacent sections with 2.5 nM [(3)H]senktide. NK-1 binding was found to be unaltered in 27 brain areas examined. In contrast, NK-3 binding was significantly reduced in layers 4 and 5 of the prefrontal (-46%), anterior cingulate (-42%) and parietal (-45%) cortices, ventromedial thalamus (-42%) and substantia nigra pars compacta (-36%) in dystonic brains compared to controls. The latter effects may be particularly relevant in view of evidence that activation of NK-3 receptors on dopaminergic neurons in the substantia nigra pars compacta can increase nigrostriatal dopaminergic activity. Since previous studies indicated that a reduced basal ganglia output in mutant hamsters is based on an overactivity of the direct pathway which also innervates substantia nigra pars compacta neurons, the decreased NK-3 binding could be related to a receptor down-regulation. The present finding of decreased NK-3 receptor density in the substantia nigra pars compacta, thalamic and cortical areas substantiates the hypothesis that disturbances of the basal ganglia-thalamus-cortex circuit play a critical role in the pathogenesis of paroxysmal dystonia.