Alterations in the noradrenergic projection to the cerebellum of the dystonic (dt) rat

Physiology of Dystonia: Animal Studies

Dystonia is currently ranked as the third most prevalent motor disorder. It is typically characterized by involuntary muscle over- or co-contractions that can cause painful abnormal postures and jerky movements. Dystonia is a heterogenous disorder-across patients, dystonic symptoms vary in their severity, body distribution, temporal pattern, onset, and progression. There are also a growing number of genes that are associated with hereditary dystonia. In addition, multiple brain regions are associated with dystonic symptoms in both genetic and sporadic forms of the disease. The heterogeneity of dystonia has made it difficult to fully understand its underlying pathophysiology. However, the use of animal models has been used to uncover the complex circuit mechanisms that lead to dystonic behaviors. Here, we summarize findings from animal models harboring mutations in dystonia-associated genes and phenotypic animal models with overt dystonic motor signs resulting from spontaneous mutations, neural circuit perturbations, or pharmacological manipulations. Taken together, an emerging picture depicts dystonia as a result of brain-wide network dysfunction driven by basal ganglia and cerebellar dysfunction. In the basal ganglia, changes in dopaminergic, serotonergic, noradrenergic, and cholinergic signaling are found across different animal models. In the cerebellum, abnormal burst firing activity is observed in multiple dystonia models. We are now beginning to unveil the extent to which these structures mechanistically interact with each other. Such mechanisms inspire the use of pre-clinical animal models that will be used to design new therapies including drug treatments and brain stimulation.

Calcium-induced calcium release in noradrenergic neurons of the locus coeruleus

The locus coeruleus (LC) is a nucleus within the brainstem that consists of norepinephrine-releasing neurons. It is involved in broad processes including cognitive and emotional functions. Understanding the mechanisms that control the excitability of LC neurons is important because they innervate widespread brain regions. One of the key regulators is cytosolic calcium concentration ([Ca²⁺]_c), the increases in which can be amplified by calcium-induced calcium release (CICR) from intracellular calcium stores. Although the electrical activities of LC neurons are regulated by changes in [Ca²⁺]_c, the extent of CICR involvement in this regulation has remained unclear. Here we show that CICR hyperpolarizes acutely dissociated LC neurons of the rat and demonstrate the underlying pathway. When CICR was activated by extracellular application of 10 mM caffeine, LC neurons were hyperpolarized in the current-clamp mode of patch-clamp recording, and the majority of neurons showed an outward current in the voltage-clamp mode. This outward current was accompanied by increased membrane conductance, and its reversal potential was close to the K⁺ equilibrium potential, indicating that it is mediated by opening of K⁺ channels. The outward current was generated in the absence of extracellular calcium and was blocked when the calcium stores were inhibited by applying ryanodine. Pharmacological blockers indicated that it was mediated by Ca²⁺-activated K⁺ channels of the non-small conductance type. The application of caffeine increased [Ca²⁺]_c, as visualized by fluorescence microscopy. These findings show CICR suppresses LC neuronal activity, and indicate its dynamic role in modulating the LC-mediated noradrenergic tone in the brain.

The Dystonias

Dystonia is a difficult problem for both the clinician and the scientist. It is sufficiently common to be seen by almost all physicians, yet uncommon enough to prevent any physician from gaining broad experience in its diagnosis and treatment. Each case represents a difficult challenge even to the specialist. The basic scientist is faced with investigating a disorder that is without relevant animal models and which is so rare that obtaining suitable tissue for study is a major obstacle. Dystonia may be idiopathic, or associated with lesions from many sources, including a variety of rare diseases. If idiopathic, it may be genetically transmitted or sporadic. If genetically transmitted, it may be generalized or focal, with symptoms varying in different members of the same family. It may be refractory to treatment, or it may respond to any one of a number of individual drugs that have very different mechanisms of action. For idiopathic dystonias, no clear method of genetic transmission has been established and no consistent pathology identified.

Treatment of myoclonus-dystonia syndrome with tetrabenazine

BACKGROUND

Many cases of myoclonus-dystonia (M-D) are due to mutations in SGCE (DYT11). For the majority of patients, myoclonus is relatively more severe than dystonia and can lead to significant functional disability. Deep brain stimulation has been chosen as a treatment option in some patients given that M-D often responds poorly to oral pharmacotherapy.

METHODS

Two siblings with M-D due to the same SGCE deletion mutation were evaluated with the Global Dystonia Rating Scale (GDRS), Fahn-Marsden Rating Scale (FM) and Unified Myoclonus Rating Scale (UMRS) on and off tetrabenazine.

RESULTS

Both subjects showed marked improvement in myoclonus and mild-to-moderate improvement in dystonia with tetrabenazine. In addition, the response to tetrabenazine has been sustained for years.

CONCLUSIONS

A therapeutic trial of tetrabenazine should be considered in patients with M-D, especially before consideration of deep brain stimulation. An adequately powered multi-center, double-blind study of tetrabenazine will be required to determine the relative contributions of tetrabenazine therapy to myoclonus, dystonia, quality of life, and activities of daily living in patients with M-D.

Dystonia and the cerebellum: a new field of interest in movement disorders?

Although dystonia has traditionally been regarded as a basal ganglia dysfunction, recent provocative evidence has emerged of cerebellar involvement in the pathophysiology of this enigmatic disease. This review synthesizes the data suggesting that the cerebellum plays an important role in dystonia etiology, from neuroanatomical research of complex networks showing that the cerebellum is connected to a wide range of other central nervous system structures involved in movement control to animal models indicating that signs of dystonia are due to cerebellum dysfunction and completely disappear after cerebellectomy, and finally to clinical observations in secondary dystonia patients with various types of cerebellar lesions. We propose that dystonia is a large-scale dysfunction, involving not only cortico-basal ganglia-thalamo-cortical pathways, but the cortico-ponto-cerebello-thalamo-cortical loop as well. Even in the absence of traditional "cerebellar signs" in most dystonia patients, there are more subtle indications of cerebellar dysfunction. It is clear that as long as the cerebellum's role in dystonia genesis remains unexamined, it will be difficult to significantly improve the current standards of dystonia treatment or to provide curative treatment.

Animal models of generalized dystonia

Dystonia is a prevalent neurological disorder characterized by abnormal co-contractions of antagonistic muscle groups that produce twisting movements and abnormal postures. The disorder may be inherited, arise sporadically, or result from brain insult. Dystonia is a heterogeneous disorder because patients may exhibit focal or generalized symptoms associated with abnormalities in many brain regions including basal ganglia and cerebellum. Elucidating the pathogenic mechanisms underlying dystonia has therefore been challenging. Animal models of dystonia exhibit similar heterogeneity and are useful for understanding pathogenesis. The neurochemical and neurophysiological abnormalities in rodents with idiopathic generalized dystonia suggest that dysfunctional output from basal ganglia, cerebellum, or from multiple systems is the cause of motor dysfunction. Findings from drug- or toxin-induced dystonia in rodents and nonhuman primates mirror the genetic models. The parallels between dystonia in humans and animals suggest that the models will continue to prove useful in determining pathogenesis. Furthermore, detailed characterization of the existing models of dystonia and the development of new models hold promise for the identification of novel therapeutics.

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.

Motor abnormalities and changes in the noradrenaline content and the cytoarchitecture of developing cerebellum following X-irradiation at birth

We have studied the developmental time-course of changes in the noradrenaline (NA) content of cerebellum (CE), cytoarchitecture of the cerebellar cortex, and motor abnormalities induced by the exposure of the cephalic end of rats to a single dose (5 Gy) of X-irradiation immediately after birth. At all ages examined, i.e., from postnatal (PN) d 5 to 90, CE from exposed animals show a marked atrophy, with an agranular cortex that has lost its layered structure. Purkinje cells are scattered at all depths in the cortex, and their primary dendrite is randomly oriented. The motor syndrome includes dystonia-like movements, a fine tremor, and an ataxic gait. Being progressive, the abnormal movements are evident from PN d 10, and fully developed by d 30. On the other hand, no differences in cerebellar NA content between X-irradiated rats and age-matched nonirradiated controls were detected from PN d 5 to 60. However, at PN d 90 a significant increase in NA content of CE from exposed animals is found when compared to either age-matched controls (+36%, p < 0.01), or data from irradiated rats obtained at PN d 5 to 60 (p < 0.01). These results indicate a temporal dissociation between the motor and cytoarchitectural abnormalities and the increase in cerebellar NA content produced by a single dose of X-rays at birth. The late increase in cerebellar NA content might represent a compensatory response of noradrenergic terminals to an altered information flow out of the cerebellar cortex induced by the ionizing noxa.

Neurochemical characterization of the alterations in the noradrenergic afferents to the cerebellum of adult rats exposed to X-irradiation at birth

A single dose of x-irradiation was applied on the cephalic end of newborn rats, and the alterations in the noradrenergic afferents to the cerebellum were studied 180 days later. A net increase in the noradrenaline content of cerebellum was found (122% of nonirradiated controls). The response of noradrenaline content to reserpine injection (0.9 mg/kg, i.p.) was similar in exposed and control rats. Likewise, the 3H release induced by Ro 4-1284 from cerebellar cortex slices labeled with [3H]noradrenaline was unmodified by x-rays, although a mild increase in the spontaneous efflux of 3H was found. The retention of 3H by the slices was reduced in exposed animals (58% of controls). Both the in vitro activity of tyrosine hydroxylase and the accumulation of L-3,4-dihydroxyphenylalanine (L-DOPA) were not significantly different between x-treated rats and controls. In contrast, monoamine oxidase activity was markedly reduced in x-irradiated cerebellum (38% of controls). The x-ray-induced decrease in cerebellar weight (-60%) resulted in marked increases in noradrenaline concentration (223%), tyrosine hydroxylase activity per milligram of protein (206%), and 3H retention (50%). The accumulation of L-DOPA per gram of tissue was also increased at every time considered. These data indicate that x-irradiation at birth produces a cerebellar loss not completely shared by the noradrenergic afferents, and a permanent imbalance between the noradrenergic afferent input and its target cells might eventually result.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased activity of tyrosine hydroxylase in the cerebellum of the X-irradiated dystonic rat

The exposure of the cephalic end of rats to repeated doses of X-irradiation (150 rad) immediately after birth induces a long-term increase in the noradrenaline (NA) content of cerebellum (CE) (+ 37.8%), and a decrease in cerebellar weight (65.2% of controls), which results in an increased NA concentration (+ 109%). This increase in the neurotransmitter level is accompanied by a dystonic syndrome and histological abnormalities: Purkinje cells (the target cells for NA afferents to CE) fail to arrange in a characteristic monolayer, and their primary dendritic tree appears randomly oriented. The injection of reserpine 0.9 and 1.2 mg/kg ip to adult rats for 18 h depletes cerebellar NA content in both controls (15.7 +/- 4 ng/CE and 2.8 +/- 1.5 ng/CE, respectively) and X-irradiated rats (17.1 +/- 1 ng/CE and 8.3 +/- 2 ng/CE, respectively). The activity of tyrosine hydroxylase (TH) in CE of adult rats, measured by an in vitro assay, is significantly increased in neonatally X-irradiated animals when compared to age-matched controls (16.4 +/- 1.4 vs 6.32 +/- 0.6 nmol CO2/h/mg prot., p less than 0.01). As observed for NA levels, a net increase in TH activity induced by the ionizing radiation is also measured: 308.9 +/- 23.8 vs 408.2 +/- 21.5 nmol CO2/h/CE, p less than 0.01 (controls and X-treated, respectively). These results suggest that X-irradiation at birth may induce an abnormal sprouting of noradrenergic afferents to CE. The possibility that these changes represent a response of the NA system to the dystonic syndrome is discussed.

Effect of reserpine on retention of the conditioned NK cell response

The effect of reserpine and 6-hydroxydopamine on the learned conditioned natural killer (NK) cell response was investigated in mice. Reserpine given at 2.5 mg/kg, 24 hr prior to reexposure to camphor-conditioned stimulus on days 6 and 8 blocked the recall of conditioned NK cell response to a significant extent. In other words, the NK cell activity of conditioned mice, treated with reserpine and reexposed to the conditioned stimulus, was similar to the nonconditioned (NC) group. A conditioned increase in NK cell response was still evident in mice treated with 6-OHDA.

Effect of paraventricular nucleus lesions on body weight, food intake and insulin levels

Lesions of the paraventricular nucleus of the hypothalamus (PVN) produce obesity and hyperphagia. However, the underlying mechanism is unknown. The connections of the PVN with brainstem centers for autonomic control suggest that a change in autonomic function could mediate the PVN obesity syndrome. We examined this hypothesis in a series of 3 experiments, searching specifically for changes in insulin secretion. Rats with PVN lesions were hyperphagic and hyperinsulinemic, when obese. However, hyperinsulinemia could not be detected prior to the onset of obesity or following weight reduction. Subdiaphragmatic vagotomy reversed the PVN obesity and lowered insulin levels below those of sham-vagotomized rats. Since noradrenergic innervation of the hypothalamus is implicated in feeding, hypothalamic norepinephrine (NE) was depleted by injection of 6-hydroxydopamine into the central tegmental tract, posterior to the hypothalamus. The effects of NE depletion was compared with those of PVN lesions. Loss of hypothalamic NE resulted in hyperphagia with no increase in body weight and no change in insulin. Histological analyses indicated that the posterior PVN was the most effective lesion focus for producing disturbances in body weight and food intake. Although the results of these experiments implicate the autonomic nervous system in PVN obesity, basal hyperinsulinemia does not appear to be a primary feature of the syndrome.

Glutamic acid decarboxylase activity in micropunches of the deep cerebellar nuclei of the genetically dystonic (dt) rat

Glutamic acid decarboxylase (GAD) activity was measured in specific divisions of the deep cerebellar nuclei of rats with an inherited dystonia. In 16-day-old dystonic rats there was a significant increase in GAD activity only in the nucleus interpositus (+26%). In 20-day-old dystonic rats GAD activity in all 3 cerebellar nuclei (fastigial, interpositus, dentate) was significantly increased compared to normal controls. The results indicate a spread of the anatomical locus of the neurochemical abnormality with time. During this period (postnatal days 16-20) there is a progressive worsening of the motor disorder in the affected animals.