Prodystonic effects of riluzole in an animal model of idiopathic dystonia related to decreased total power in the red nucleus?

Effects of rubral microinjections of muscimol and bicuculline in a genetic animal model of paroxysmal dystonia

Previous studies suggested that GABAergic dysfunctions within the red nucleus are involved in stress-inducible paroxysmal dystonia of the dt(sz) mutant hamster. In the present study, rubral microinjections of the GABAA receptor agonist muscimol exerted only moderate antidystonic effects and the antagonist bicuculline failed to show significant effects on the severity of dystonia. These data indicate that disturbed rubral GABAergic inhibition is not important for the manifestation of dystonia in the dt(sz) mutant.

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.

Altered discharge pattern of basal ganglia output neurons in an animal model of idiopathic dystonia

A decreased activity of basal ganglia output neurons is thought to underlie idiopathic dystonias and other hyperkinetic movement disorders. We found recently a reduced spontaneous discharge rate of entopeduncular neurons (internal globus pallidus in primates) in dt(sz) hamsters, an unique model for idiopathic paroxysmal dystonia in which stress-inducible attacks show an age-dependent severity. Otherwise, it has been suggested that an altered discharge pattern may be more important for the occurrence of dystonia than a reduced discharge rate. Based on qualitative and computerized quantitative evaluations of interspike interval histograms and spike trains of extracellularly recorded single neurons, we investigated the spontaneous discharge pattern of GABAergic entopeduncular and nigral neurons in dt(sz) hamsters at different ages. The discharge pattern of entopeduncular neurons was highly irregular and showed an altered burst-like firing in dt(sz) hamsters at the age of the most marked expression of dystonia when compared with age-matched nondystonic controls. In line with a recently reported normalization of discharge rates after age-dependent disappearance of dystonia, we found an almost complete normalization of the discharge pattern of entopeduncular neurons after remission of dystonia in dt(sz) hamsters. Investigations of GABAergic nigral neurons, reported recently to have the same spontaneous discharge rates in dystonic and nondystonic hamsters, did not show an altered firing pattern in dt(sz) hamsters. The present data clearly indicate the fundamental importance of an altered discharge pattern of entopeduncular neurons for the expression of paroxysmal dystonia, and probably also for other dyskinesias, and may explain the improvements obtained by pallidotomy in dystonic patients despite an obviously reduced pallidal output.

Paradoxical aggravation of paroxysmal dystonia during chronic treatment with phenobarbital in a genetic rodent model

Recent studies in mutant hamsters (dt(sz)), an animal model of primary paroxysmal dystonia, indicated that altered function of the gamma-aminobutyric acid (GABA)ergic system plays a critical role in the pathogenesis of dystonia. In the present study, dt(sz) hamsters were chronically treated with phenobarbital, which has been found to exert antidystonic effects in mutant hamsters after acute administration. In untreated dt(sz) hamsters, the severity of dystonia follows an age-dependent time course with a maximum between the 30th and 40th day of life, followed by a continuous decline of severity until complete remission occurs at the age of about 70 days. In contrast to acute effects, chronic treatment with phenobarbital via drinking water starting at an age of 21 days (i.e., after weaning) worsened dystonia and retarded the spontaneous remission. The unexpected prodystonic effect was more marked after administration of higher doses and when chronic treatment with phenobarbital started at an age of 1 day (neonatal administration via breast milk). After withdrawal of phenobarbital at the age of 70 days, the severity rapidly declined in all treated groups. When phenobarbital was readministered 1 week later, the hamsters again exhibited severe dystonia. The mechanism of these unexpected findings is unknown. Tentatively, activity-dependent GABA-mediated excitation caused by chronic treatment with phenobarbital may be important for the prodystonic effects under pathological conditions in dt(sz) hamsters.

Gabapentin decreases the severity of dystonia at low doses in a genetic animal model of paroxysmal dystonic choreoathetosis

The effects of the gamma-aminobutyric acid (GABA)-potentiating drug gabapentin (1-(aminomethyl) cyclohexaneacetic acid) on severity of dystonia were examined in a hamster model of idiopathic paroxysmal dystonic choreoathetosis. In the genetically dystonic hamster (dt(sz)) recent pharmacological and neurochemical studies suggested that disturbed GABAergic inhibition is involved in the pathogenesis. In line with a case report of beneficial effects in human paroxysmal dystonic choreoathetosis, gabapentin reduced the severity of dystonia in mutant hamsters at doses of 5 and 10 mg kg(-1) i.p. At higher doses (20 and 100 mg kg(-1)), gabapentin, however, failed to exert antidystonic effects. The GABApotentiating activity of gabapentin could explain the antidystonic effects of low doses, while the loss of efficacy at higher doses may be due to other mechanisms of gabapentin.

Pathology of idiopathic dystonia: findings from genetic animal models

Dystonia is a common movement disorder which is thought to represent a disease of the basal ganglia. However, the pathogenesis of the idiopathic dystonias, i.e. the neuroanatomic and neurochemical basis, is still a mystery. Research in dystonia is complicated by the existence of various phenotypic and genotypic subtypes of idiopathic dystonia, probably related to heterogeneous dysfunctions. In neurological diseases in which no obvious neuronal degeneration can be found, such as in idiopathic dystonia, the identification of a primary defect is difficult, because of the large number of chemically distinct, but functionally interrelated, neurotransmitter systems in the brain. The variable response to pharmacological agents in patients with idiopathic dystonia supports the notion that the underlying biochemical dysfunctions vary in the subtypes of idiopathic dystonia. Hence, in basic research it is important to clearly define the involved type of dystonia. Animal models of dystonias were described as limited. However, over the last years, there has been considerable progress in the evaluation of animal models for different types of dystonia. Apart from animal models of symptomatic dystonia, genetic animal models with inherited dystonia which occurs in the absence of pathomorphological alterations in brain and spinal cord are describe. This review will focus mainly on genetic animal models of different idiopathic dystonias and pathophysiological findings. In particular, in the case of the mutant dystonic (dt) rat, a model of generalized dystonia, and in the case of the genetically dystonic hamster (dt(sz)), a model of paroxysmal dystonic choreoathetosis has been used, as these show great promise in contributing to the identification of underlying mechanisms in idiopathic dystonias, although even a proper animal model will probably never be equivalent to a human disease. Several pathophysiological findings from animal models are in line with clinical observations in dystonic patients, indicating abnormalities not only in the basal ganglia and thalamic nuclei, but also in the cerebellum and brainstem. Through clinical studies and neurochemical data several similarities were found in the genetic animal models, although the current data indicates different defects in dystonic animals which is consistent with the notion that dystonia is a heterogenous disorder. Different supraspinal dysfunctions appear to lead to manifestation of dystonic movements and postures. In addition to increasing our understanding of the pathophysiology of idiopathic dystonia, animal models may help to improve therapeutic strategies for this movement disorder.