Tyrosine hydroxylase immunoreactivity and [3H]WIN 35,428 binding to the dopamine transporter in a hamster model of idiopathic paroxysmal dystonia

Role of striatal NMDA receptor subunits in a model of paroxysmal dystonia

Dystonia is a movement disorder in which abnormal plasticity in the basal ganglia has been hypothesized to play a critical role. In a model of paroxysmal dystonia, the dt(sz) mutant hamster, previous studies indicated striatal dysfunctions, including an increased long-term potentiation (LTP). Beneficial effects were exerted by subunit-unspecific antagonists at NMDA receptors, which blocked LTP. NR2B subtype selective antagonists aggravated dystonia after systemic treatment in dt(sz) hamsters, suggesting that beneficial effects involved the NR2A receptor subtype. In the present study, NVP-AAM077, an antagonist with preferential activity on NR2A-containing NMDA receptors, exerted significant antidystonic effects in mutant hamsters after systemic administration (20 and 30mg/kg i.p.) and delayed the onset of a dystonic episode after intrastriatal injections (0.12 and 0.24μg). As shown by present electrophysiological examinations in corticostriatal slices of dt(sz) hamsters and non-dystonic control hamsters, NVP-AAM077 (50nM) completely blocked LTP in dt(sz) slices, but did not exert significant effects on LTP in non-dystonic controls. In contrast, the NR2B antagonist Ro 25-6981 (1-10μmol) reduced LTP to a lower extent in dt(sz) mutant hamsters than in control animals. By using quantitative RT-PCR, the NR2A/NR2B ratio was found to be increased in the striatum, but not in the cortex of mutant hamsters in comparison to non-dystonic controls. These data indicate that NR2A-mediated activation may be involved in the pathophysiology of paroxysmal dystonia. Since significant antidystonic effects were observed after systemic administration of NVP-AAM077 already at well tolerated doses, antagonists with preferential activity on NR2A-containing NMDA receptors could be interesting candidates for the treatment of dystonia.

Genetic animal models of dystonia: common features and diversities

Animal models are pivotal for studies of pathogenesis and treatment of disorders of the central nervous system which in its complexity cannot yet be modeled in vitro or using computer simulations. The choice of a specific model to test novel therapeutic strategies for a human disease should be based on validity of the model for the approach: does the model reflect symptoms, pathogenesis and treatment response present in human patients? In the movement disorder dystonia, prior to the availability of genetically engineered mice, spontaneous mutants were chosen based on expression of dystonic features, including abnormal muscle contraction, movements and postures. Recent discovery of a number of genes and gene products involved in dystonia initiated research on pathogenesis of the disorder, and the creation of novel models based on gene mutations. Here we present a review of current models of dystonia, with a focus on genetic rodent models, which will likely be first choice in the future either for pathophysiological or for preclinical drug testing or both. In order to help selection of a model depending on expression of a specific feature of dystonia, this review is organized by symptoms and current knowledge of pathogenesis of dystonia. We conclude that albeit there is increasing need for research on pathogenesis of the disease and development of improved models, current models do replicate features of dystonia and are useful tools to develop urgently demanded treatment for this debilitating disorder.

Striatal dopaminergic dysfunction at rest and during task performance in writer's cramp

Writer's cramp is a task-specific focal hand dystonia characterized by involuntary excessive muscle contractions during writing. Although abnormal striatal dopamine receptor binding has been implicated in the pathophysiology of writer's cramp and other primary dystonias, endogenous dopamine release during task performance has not been previously investigated in writer's cramp. Using positron emission tomography imaging with the D2/D3 antagonist 11C-raclopride, we analysed striatal D2/D3 availability at rest and endogenous dopamine release during sequential finger tapping and speech production tasks in 15 patients with writer's cramp and 15 matched healthy control subjects. Compared with control subjects, patients had reduced 11C-raclopride binding to D2/D3 receptors at rest in the bilateral striatum, consistent with findings in previous studies. During the tapping task, patients had decreased dopamine release in the left striatum as assessed by reduced change in 11C-raclopride binding compared with control subjects. One cluster of reduced dopamine release in the left putamen during tapping overlapped with a region of reduced 11C-raclopride binding to D2/D3 receptors at rest. During the sentence production task, patients showed increased dopamine release in the left striatum. No overlap between altered dopamine release during speech production and reduced 11C-raclopride binding to D2/D3 receptors at rest was seen. Striatal regions where D2/D3 availability at rest positively correlated with disease duration were lateral and non-overlapping with striatal regions showing reduced D2/D3 receptor availability, except for a cluster in the left nucleus accumbens, which showed a negative correlation with disease duration and overlapped with striatal regions showing reduced D2/D3 availability. Our findings suggest that patients with writer's cramp may have divergent responses in striatal dopamine release during an asymptomatic motor task involving the dystonic hand and an unrelated asymptomatic task, sentence production. Our voxel-based results also suggest that writer's cramp may be associated with reduced striatal dopamine release occuring in the setting of reduced D2/D3 receptor availability and raise the possibility that basal ganglia circuits associated with premotor cortices and those associated with primary motor cortex are differentially affected in primary focal dystonias.

Neuroanatomical substrates for paroxysmal dyskinesia in lethargic mice

The paroxysmal dyskinesias are a group of neurological disorders described by intermittent attacks of involuntary abnormal movements superimposed on a relatively normal baseline. The neuroanatomical substrates for these attacks are not fully understood, though available evidence from studies of affected people and animal models points to dysfunction in the basal ganglia or cerebellum. In the current studies, the anatomical basis for paroxysmal dyskinesias in lethargic mice was determined via histochemical methods sensitive to changes in regional brain activity followed by surgical elimination of the suspected source. Cytochrome oxidase histochemistry revealed increased activity in the red nucleus. Surgical removal of the cerebellum worsened ataxia but eliminated paroxysmal dyskinesias. These studies support the hypothesis that abnormal cerebellar output contributes to paroxysmal dyskinesias.

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.

Age-related changes in striatal nitric oxide synthase-immunoreactive interneurones in the dystonic dt sz mutant hamster

The dt(sz) mutant hamster represents a model of paroxysmal dyskinesia in which dystonic episodes can be age-dependently induced by stress. GABAergic interneurones which co-express calcium binding proteins were found to be reduced in the striatum of the dt(sz) mutant. Other types of striatal interneurones have so far not been examined. In the present study, we therefore determined the density of nitric oxide synthase (NOS)-immunoreactive interneurones in the striatum of the dt(sz) mutant in comparison with nondystonic control hamsters. At the age of most marked expression of dystonia (30-40 days of life), the density of NOS-positive interneurones was decreased in the striatum of dt(sz) hamsters (-21%) in comparison with age-matched nondystonic control hamsters. Spontaneous remission of dystonia (age >90 days) coincided with a normalization of the density of NOS-reactive interneurones within the whole striatum of dt(sz) hamsters, but there remained a reduced density in distinct subregions. Together with previous findings the present data indicate that the development of striatal interneurones is retarded in mutant hamsters. The age-related deficit of NOS-reactive interneurones may at least in part contribute to an abnormal activity of striatal GABAergic projection neurones and thereby to the age-dependent dystonic syndrome in the dt(sz) mutant.

Age-Dependent Alterations of Striatal Calretinin Interneuron Density in a Genetic Animal Model of Primary Paroxysmal Dystonia

Various types of hereditary dystonia are regarded as a basal ganglia disorder, but the underlying mechanisms are still unknown. In the dt hamster, a genetic animal model of age-dependent paroxysmal dystonia, recent studies demonstrated a reduced density of striatal parvalbumin-immunoreactive (PV) GABAergic interneurons at an age of maximum severity of dystonia in comparison with age-matched nondystonic controls. So far, alterations of other types of striatal interneurons in dt hamsters cannot be excluded. Therefore, we determined the density of calretinin-immunoreactive (CR) interneurons in the dt mutant at an age of maximum severity and after spontaneous remission of dystonia in comparison with age-matched nondystonic controls using an image analysis system and a stereologic counting method in a blinded fashion. At an age of maximum severity of dystonia, CR interneuron density was significantly lower in dt hamsters in comparison with controls (-20%), whereas no significant differences between the animal groups could be detected after spontaneous remission of dystonia. The comparison of CR interneuron density between young hamsters with those at an age of > 90 days revealed a significant ontogenetic decrease of CR interneurons in both animal groups (dt hamsters: -38%, controls: -54%). These results demonstrate that alterations of striatal interneuron density in dt mutants are not restricted to PV ones. A deficit of CR interneurons that coexpress GABA may contribute to previous findings of disinhibition of striatal projection neurons in the dt mutant at an age of maximum expression of dystonia.

Altered expression of preproenkephalin and prodynorphin mRNA in a genetic model of paroxysmal dystonia

The dtsz mutant hamster represents a model of primary paroxysmal dystonia, in which dystonic episodes occur in response to stress. Previous examinations demonstrated striatal dysfunctions in dtsz hamsters. In the present study, in situ hybridization was used to examine preproenkephalin and prodynorphin expression as potential indices of imbalances between the striatopallidal and striatonigral pathways. Brain analyses were performed in dtsz hamsters under basal conditions, i.e., in the absence of dystonia, as well as mutant hamsters that exhibited severe stress-induced dystonic attacks immediately prior to sacrifice. In the striatum the basal expression of prodynorphin tended to be higher, while that of preproenkephalin tended to be lower in mutant hamsters in comparison to non-dystonic control hamsters. Significant basal changes were restricted to higher levels of prodynorphin in the ventrolateral striatum and lower prodynorphin and preproenkephalin mRNA expression in the hippocampus and/or in subregions of the hypothalamus. After stressful stimulation, the neuropeptides increased in several regions in both animals groups. In comparison to stimulated control hamsters, a significantly lower prodynorphin expression was found in several limbic areas of stimulated mutant hamsters during the manifestation of dystonia, while preproenkephalin mRNA was significantly lower in the anterior and dorsal striatal subregions and in nucleus accumbens. Since changes in the expression of these opioid peptides have been suggested to be related to abnormal dopaminergic activity, the present findings may reflect disturbances in striatal dopaminergic systems, and also in limbic structures in the dtsz mutant, particularly during the expression of dystonia.

Striatal increase of extracellular dopamine levels during dystonic episodes in a genetic model of paroxysmal dyskinesia

In vivo microdialysis was used to examine the levels of dopamine, serotonin, and their metabolites dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in the striatum of dt(sz) mutant hamsters, an animal model of paroxysmal dyskinesia, in which stress can precipitate dystonic episodes. Measurements were made under three different conditions in each animal: (1) at baseline in the absence of abnormal involuntary movements, (2) during an episode of paroxysmal dystonia precipitated by handling, and (3) during the recovery (postdystonic) period. In comparison to nondystonic control hamsters, which were treated in the same manner as dystonic animals, no changes could be detected under basal conditions, although the levels of DOPAC and HVA tended to be higher in mutant hamsters. Significantly elevated striatal levels of dopamine and DOPAC became evident during the period of stress-induced dystonic attacks in mutant hamsters. During dystonic episodes, dopamine levels were approximately 6.5-fold higher (followed by a 2.5-fold increase of DOPAC) in dt(sz) hamsters than in normal controls. Before the disappearance of dystonia, the levels of dopamine returned to basal concentrations in mutant hamsters. Consistent with previous pharmacologic findings, paroxysmal dystonia in mutant hamsters is associated with temporary increases of extracellular dopamine levels in the striatum.

Motor disturbances in mice with deficiency of the sodium channel gene Scn8a show features of human dystonia

The med(J) mouse with twisting movements related to deficiency of the sodium channel Scn8a has been proposed as a model of kinesiogenic dystonia. This prompted us to examine the phenotype of these mice in more detail. By cortical electroencephalographic (EEG) recordings, we could not detect any changes, demonstrating that the motor disturbances are not epileptic in nature, an important similarity to human dystonia. The significantly decreased body weight of med(J) mice was related to reduced food intake. Observations in the open field and by video recordings revealed that the mice exhibit sustained abnormal postures and movements of limbs, trunk and tail not only during locomotor activity but also at rest. With the exception of the head tremor, the other motor impairments were persistent rather than paroxysmal. When several neurological reflexes were tested, alterations were restricted to the posture and righting reflexes. Results of the wire hang test confirmed the greatly reduced muscle strength in the med(J) mouse. In agreement with different types of human dystonia, biperiden, haloperidol and diazepam moderately reduced the severity of motor disturbances in med(J) mice. In view of the sodium channel deficiency in med(J) mice, the beneficial effects of the sodium channel blocker phenytoin was an unexpected finding. By immunohistochemical examinations, the density of nigral dopaminergic neurons was found to be unaltered, substantiating the absence of pathomorphological abnormalities within the brain of med(J) mice shown by previous studies. With the exception of muscle weakness, many of the features of the med(J) mouse are similar to human idiopathic dystonia.

A critical role for nucleus accumbens dopamine in partner-preference formation in male prairie voles

Although the role of nucleus accumbens (NAcc) dopamine (DA) in reward learning has been extensively studied, few investigations have addressed its involvement in learning socially relevant information. Here, we have examined the involvement of NAcc DA in social attachment of the "monogamous" prairie vole (Microtus orchrogaster). We first demonstrated that DA is necessary for the formation of social attachment in male prairie voles, because administration of haloperidol blocked, whereas apomorphine induced, partner-preference formation. We then provided the first descriptions of DA neuroanatomy and tissue content in vole NAcc, and mating appeared to induce a 33% increase in DA turnover. We also showed that administration of haloperidol directly into the NAcc blocked partner preferences induced by mating and apomorphine. In addition, administration of apomorphine into the NAcc but not the caudate putamen induced partner preferences in the absence of mating. Together, our data support the hypothesis that NAcc DA is critical for pair-bond formation in male prairie voles.

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.

Ataxia and paroxysmal dyskinesia in mice lacking axonally transported FGF14

Fibroblast growth factor 14 (FGF14) belongs to a distinct subclass of FGFs that is expressed in the developing and adult CNS. We disrupted the Fgf14 gene and introduced an Fgf14(N-beta-Gal) allele that abolished Fgf14 expression and generated a fusion protein (FGF14N-beta-gal) containing the first exon of FGF14 and beta-galactosidase. Fgf14-deficient mice were viable, fertile, and anatomically normal, but developed ataxia and a paroxysmal hyperkinetic movement disorder. Neuropharmacological studies showed that Fgf14-deficient mice have reduced responses to dopamine agonists. The paroxysmal hyperkinetic movement disorder phenocopies a form of dystonia, a disease often associated with dysfunction of the putamen. Strikingly, the FGF14N-beta-gal chimeric protein was efficiently transported into neuronal processes in the basal ganglia and cerebellum. Together, these studies identify a novel function for FGF14 in neuronal signaling and implicate FGF14 in axonal trafficking and synaptosomal function.

Effects of striatal injections of GABA(A) receptor agonists and antagonists in a genetic animal model of paroxysmal dystonia

The underlying mechanisms of idiopathic dystonias are poorly understood. The dystonic phenotype in the dt(sz) mutant hamster, a model of paroxysmal dystonia, has been suggested to be based on a deficit of gamma-aminobutyric acid (GABA)ergic interneurons and changes of the GABA(A)-benzodiazepine receptor complex in the striatum. In order to confirm and extend previous observations, the effects of compounds which bind to different sites of the GABA(A) receptor on the severity of dystonia were determined after striatal microinjections in comparison to systemic treatments in dt(sz) mutants. The GABA(A) receptor agonist (muscimol) and the benzodiazepine (flurazepam) reduced the severity of dystonia after striatal and systemic injections. The antidystonic effects of the barbiturate phenobarbital were less marked both after striatal and intraperitoneal administration of drugs. Intrastriatal injections of GABA delayed the onset of dystonic attacks. Striatal and systemic treatments with the GABA(A) receptor antagonist, bicuculline, and with pentylenetetrazole, which reduces GABAergic function, accelerated the onset of dystonia at subconvulsant doses. The benzodiazepine receptor antagonists flumazenil aggravated dystonia after systemic and intrastriatal injections. In all, the present data substantiate the relevance of striatal GABAergic disinhibition in the pathogenesis of paroxysmal dystonia in dt(sz) mutants.

Deficit of striatal parvalbumin-reactive GABAergic interneurons and decreased basal ganglia output in a genetic rodent model of idiopathic paroxysmal dystonia

The underlying mechanisms of various types of hereditary dystonia, a common movement disorder, are still unknown. Recent findings in a genetic model of a type of paroxysmal dystonia, the dt(sz) mutant hamster, pointed to striatal dysfunctions. In the present study, immunhistochemical experiments demonstrated a marked decrease in the number and density of parvalbumin-immunoreactive GABAergic interneurons in all striatal subregions of mutant hamsters. To examine the functional relevance of the reduction of these inhibitory interneurons, the effects of the GABA(A) receptor agonist muscimol on severity of dystonia were examined after microinjections into the striatum and after systemic administrations. Muscimol improved the dystonic syndrome after striatal injections to a similar extent as after systemic treatment, supporting the importance of the deficiency of striatal GABAergic interneurons for the occurrence of the motor disturbances. The disinhibition of striatal GABAergic projection neurons, as suggested by recent extracellular single-unit recordings in dt(sz) hamsters, should lead to an abnormal neuronal activity in the basal ganglia output nuclei. Indeed, a significantly decreased basal discharge rate of entopeduncular neurons was found in dt(sz) hamsters. We conclude that a deficit of striatal GABAergic interneurons leads by disinhibition of striatal GABAergic projection neurons to a reduced activity in the entopeduncular nucleus, i.e., to a decreased basal ganglia output. This finding is in line with the current hypothesis about the pathophysiology of hyperkinesias. The results indicate that striatal interneurons deserve attention in basic and clinical research of those movement disorders.

Evidence for striatal dopaminergic overactivity in paroxysmal dystonia indicated by microinjections in a genetic rodent model

Mutant dystonic hamsters (dt(sz)), a model of primary paroxysmal dystonia, display attacks of generalized dystonia in response to mild stress in an age-dependent manner. Recent studies in dystonic hamsters have revealed decreased densities of dopamine D(1) and D(2) in the dorsal striatum. This finding has been interpreted as a down-regulation in response to enhanced dopamine release because systemic treatments with neuroleptics reduced the severity of dystonia while levodopa exerted prodystonic effects. Therefore, in the present study we investigated the effects of amphetamine as well as of selective D(1) or D(2) receptor agonists and antagonists on the severity of dystonia after systemic administrations and after microinjections into the dorsal striatum. Amphetamine and the dopamine D(2) agonist quinpirole increased the severity of dystonia after systemic and striatal injections, while the dopamine D(1) agonist SKF 38393 exerted only moderate prodystonic effects after systemic administration of a high dose but not after striatal injections. These results suggest that a predominant overstimulation of D(2) receptors is pathogenetically involved in the dystonic syndrome. Combined systemic or striatal administrations of the D(1) and D(2) receptor agonists did not reveal synergistic prodystonic effects at the examined doses. The selective D(1) antagonist SCH 23390 as well as the D(2) antagonist raclopride tended to decrease the severity of dystonia after systemic administration but failed to exert significant effects after striatal injection. The coadministration of ineffective doses of the antagonists SCH 23390 and raclopride, however, exerted an enormous antidystonic efficacy after both systemic and striatal injections. Since striatal injections of compounds which enhance dopaminergic activity aggravated dystonia, while coinjections of dopamine D1 and D2 receptor antagonists reduced the severity of dystonia, the present findings clearly support the hypothesis that striatal dopaminergic overactivity plays a crucial role for the manifestation of dystonic attacks in the hamster model of paroxysmal dystonia.

Paroxysmal dyskinesias as a paradigm of paroxysmal movement disorders

Paroxysmal dyskinesias are genetically and clinically heterogeneous. Paroxysmal kinesigenic choreoathetosis is frequently familial, with autosomal-dominant transmission. Benign infantile convulsions can be observed in these families and both diseases as linked to the pericentromeric region of chromosome 16. Two different forms of paroxysmal dystonic choreoathetosis are distinguished on clinical grounds, by the presence or absence of spasticity, and genetically, as they are linked with loci on different chromosomes. Among the paroxysmal disorders, these diseases may belong to the group of channelopathies.