Enhanced sensitivity to quipazine in the genetically dystonic rat (dt)

The Neuroanatomical Basis of the 5-HT Syndrome and Harmalineinduced Tremor

The 5-HT syndrome in rats is composed of head weaving, body shaking, forepaw treading, flat body posture, hindlimb abduction, and Straub tail. The importance of the brainstem and spinal cord for the syndrome is underlined by findings of 5,7-dihydroxytryptamine (5,7-DHT)-induced denervation supersensitivity in response to 5-HT-stimulant drugs. For head weaving and Straub tail, supersensitivity occurred when the neurotoxin was injected into the cisterna magna or spinal cord, for forepaw treading in cisterna magna, and for hindlimb abduction in the spinal cord. Although 5,7- DHT-related body shaking increased in the spinal cord, the sign decreased when injected into the striatum, indicating the modulatory influence of the basal ganglia. Further details on body shaking are provided by its reduced response to harmaline after 5-HT depletion caused by intraventricular 5,7-DHT, electrolytic lesions of the medial or dorsal raphe, and lesions of the inferior olive caused by systemic injection of 3-acetylpyridine along with those found in Agtpbp1pcd or nr cerebellar mouse mutants. Yet the influence of the climbing fiber pathway on other signs of the 5-HT syndrome remains to be determined.

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.

Serotonergic perturbations in dystonia disorders-a systematic review

Dystonia is a hyperkinetic movement disorder characterized by sustained or intermittent muscle contractions. Emerging data describe high prevalences of non-motor symptoms, including psychiatric co-morbidity, as part of the phenotype of dystonia. Basal ganglia serotonin and serotonin-dopamine interactions gain attention, as imbalances are known to be involved in extrapyramidal movement and psychiatric disorders. We systematically reviewed the literature for human and animal studies relating to serotonin and its role in dystonia. An association between dystonia and the serotonergic system was reported with decreased levels of 5-hydroxyindolacetic acid, the main metabolite of serotonin. A relation between dystonia and drugs affecting the serotonergic system was described in 89 cases in 49 papers. Psychiatric co-morbidity was frequently described, but likely underestimated as it was not systematically examined. Currently, there are no good (pharmaco)therapeutic options for most forms of dystonia or associated non-motor symptoms. Further research using selective serotonergic drugs in appropriate models of dystonia is required to establish the role of the serotonergic system in dystonia and to guide us to new therapeutic strategies.

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.

Behavioural response to pharmacologic manipulation of serotonin receptors in the genetically dystonic hamster

The genetically dystonic (dtsz) hamster is an autosomal recessive mutant that shares several features with paroxysmal dystonia, i.e., a subcategory of inherited idiopathic dystonia in humans. Because the serotonin (5-HT) system has been suggested to be involved in dystonia, we examined the functional responsiveness of the 5-HT system in dystonic hamsters by administering various 5-HT agonists and antagonists selective for different receptor subtypes and observing the effects on dystonic attacks as well as the behavioural responses associated with drug administration. Paradoxically, marked prodystonic effects (i.e., increased severity and/or decreased latency of dystonic attacks) were seen with both the selective 5-HT1A receptor agonist 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) and the selective and "silent" 5-HT1A receptor antagonist, N-tert-butyl-3[4-(2-methoxyphenyl)piperazin-1-yl]-2- phenylpropionamide [(+)-WAY-100135], whereas other 5-HT1A receptor antagonists, i.e., methyl 4[4-(4-[1,1,3-trioxo-2H-1,2-benzoiosothiazol-2-yl]butyl)-1- piperazinyl]1-H-indole-2-carboxylate (SDZ 216-525) and N1-bromoacetyl-N8-3'-(4-indolyloxy)-2'-hydroxypropyl-(Z)-1,8- diamino-p-methane (pindobind-5-HT1A) did not alter dystonia to any comparable extent. Because among these 5-HT1A receptor antagonists, (+)-WAY-100135 is the only drug known to be not only silent at postsynaptic but also presynaptic (somatodendritic) 5-HT1A receptors, the marked prodystonic effect of this drug could relate to increased 5-HT release as a result of the blockade of somatodendritic 5-HT1A receptors. The only 5-HT1A receptor antagonist that exerted antidystonic effects in hamsters was pindolol, which, however, could be related to its beta-adrenoceptor blocking action. The 5-HT1A receptor partial agonist ipsapirone exerted moderate prodystonic activity. Prodystonic activity was also determined for the mixed 5-HT1A/5-HT2 receptor agonist 5-methoxy-N,N-dimethyltryptamine, although this drug was less potent in this regard than 8-OH-DPAT. The 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) exerted prodystonic effects in mutant hamsters, which, however, were also seen after the administration of the 5-HT2 receptor antagonist ritanserin. Collectively, the results of this study demonstrate that dystonia in genetically dystonic hamsters can be affected by pharmacologic manipulation of 5-HT receptors. The data may also indicate that dystonia is not a potential clinical application for selective 5-HT1A or 5-HT2 receptor antagonists.

Serotonin modulation of inferior olivary oscillations and synchronicity: a multiple-electrode study in the rat cerebellum

Simultaneous recording of complex spikes from multiple Purkinje cells (up to 44) in the rat cerebellum was used to examine the effects of 5-hydroxytryptamine (serotonin, 5-HT) on olivocerebellar function. Microinjection into the inferior olive was found to increase the average firing rate of inferior olivary neurons while slowing their oscillation frequency and increasing the coherence of their oscillations. Indeed, while the normal rostrocaudal band of synchronous activity remained unchanged, the degree of synchrony between Purkinje cell complex spikes within this band was enhanced following the 5-HT injections. Multiple-electrode recordings obtained from crus Ila and vermal lobule Vlb yielded qualitatively similar results; however, the effects on vermal activity were more pronounced. The effects of the 5-HT microinjection decayed with a time course of 75 min. The half-maximum effective concentration of 5-HT was between 10 and 100 microM. Injections of various 5-HT agonists and antagonists demonstrated that a 5-HT type-2A (5-HT2A) receptor is the main mediator for the 5-HT effect, which was very similar to the effect produced by injections of harmaline. However, 5-HT and harmaline appear to have independent mechanisms since the action of harmaline was not blocked by the 5-HT2A antagonist LY53857. A possible role for 5-HT, as a physiological enhancer of the timing of motor function of the olivocerebellar system, is discussed.

The novel selective and silent 5-HT1A receptor antagonist (+)-WAY-100135 aggravates dystonic movements in a mutant hamster model

Several clinical and experimental findings suggest that abnormal serotonin (5-HT) function may be involved in movement disorders such as dystonia, and it was proposed that selective 5-HT1A receptor antagonists may be of benefit in treating such disorders. In the present study, the novel, highly selective and silent 5-HT1A receptor antagonist (+)-WAY-100135 (N-tert-butyl-3(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenylprop ionamide) was tested in an inbred line of Syrian hamsters with generalized dystonia, i.e. a frequent movement disorder in humans. In order to demonstrate that WAY-100135 acts as a 5-HT1A receptor antagonist in the hamster, the drug was shown to antagonize the behavioural syndrome induced by 8-hydroxy-2-(di-n-propylamino)tetralin. When administered at 5-HT1A receptor antagonistic doses in dystonic hamsters, (+)-WAY-100135 dramatically aggravated the dystonic attacks. The data thus suggest that, in contrast to previous theoretical proposals, 5-HT1A receptor antagonists provide no novel therapeutic approach to involuntary movement disorders such as dystonia.

Inferior olive serotonin and norepinephrine levels during development in the genetically dystonic rat

The dystonic (dt) rat is an autosomal recessive mutant with a motor syndrome that shares several features with idiopathic torsion dystonia in humans. In the dt rats, marked biochemical and physiological abnormalities have been localized to the olivo-cerebellar system. At the pharmacological level, the dt rats exhibit enhanced sensitivity to the behavioral effects of serotonergic (5HT) agonists, including quipazine, a drug that activates the neurons of the inferior olive (IO). High performance liquid chromatography with electrochemical detection was used to assay 5-HT, 5-hydroxyindoleacetic acid (5HIAA), and norepinephrine (NE) in micropunches of the IO in normal and dt rats at 14, 18 and 22 days of age. Samples of the rostral frontal lobes were used as internal controls. Significant age-dependent effects were seen on 5-HT and 5-HIAA levels in the IO, but not the frontal cortex, in both groups. Although both groups reached similar 5-HT levels by postnatal day 22, a significant interaction effect between age and phenotype indicated a difference in the pattern of development. Administration of quipazine (10 mg/kg, IP) to 18-day-old normal and dt rats 1 h prior to sacrifice caused significant reductions in NE, 5-HIAA and the ratio of 5-HIAA to 5-HT; however, no phenotypic differences were detected. The findings do not suggest that the differential behavioral responses to 5-HT agonists seen in normal and dt rats are the result of global abnormalities in 5-HT systems, nor do they suggest the presence of presynaptic defects in the IO.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal cerebellar output in rats with an inherited movement disorder

Biochemical and metabolic mapping techniques have consistently identified the deep cerebellar nuclei (DCN) of the genetically dystonic rat as a site of abnormality. Extracellular single-unit recording techniques were used to assess the functional significance of these findings in affected rats and normal littermates between 16 and 25 days of age. Cells in the medial nucleus of the mutant rats had significantly increased spontaneous firing rates in comparison with cells from normal rats. In both the medial and the interpositus nuclei, cells from the mutants fired more rhythmically than those from the normal rats. When harmaline was administered systemically to activate the olivo-cerebellar system, in normal rats, increased firing rate and bursting patterns of activity were seen. There was no reliable change in the average firing rate or rhythmicity of cells in the medial nucleus of the dystonic rats, although previous studies have shown that harmaline activates neurons in the inferior olive in the mutants. It is likely that naturally stimulated olivary activity also fails to modulate cerebellar output in this model of inherited movement disorder. Anatomical studies did not reveal any consistent changes in the number of Purkinje cells, the volume of the DCN, or the soma size of DCN neurons. Since the electrophysiological findings cannot be ascribed to a loss of the Purkinje cells that normally provide an inhibitory input to the cerebellar nuclei, the results of this study indicate the presence of a functional defect in the control of cerebellar output in the dystonic rat that accounts for the failure of these animals to display harmaline tremor and which may be critical to the motor syndrome.