Tel.+49 30 838 53221 Fax.+49 30 838 53112
Dystonia is a movement disorder, which is often intractable. Its pathophysiology is poorly understood. Anticholinergic drugs which are selective for the muscarinic acetylcholine receptor subtype 1 (mAChR1) exert beneficial effects in some patients but also provoke a wide range of severe side effects. In order to develop anticholinergics with an improved efficacy and safety, the mAChR4 seems to be an interesting therapeutical target. The aim of this study was to further clarify the pathophysiological role of the cholinergic system by using different methods in two established animal models of dystonia and thus contribute to the improvement of therapeutic strategies.
The dtsz hamster represents an animal model of primary paroxysmal dystonia. As indicated by previous studies, the acute systemic administration of the mAChR1-antagonist trihexyphenidyl (THP) or of the relatively selective mAChR4-antagonist tropicamide exerted only moderate antidystonic effects in dtsz hamsters (Löscher and Fredow, 1992; Smiljanic, 2010). Therefore, it should be examined whether stronger effects could be achieved by (1) the acute combined administration of THP and tropicamide, (2) long-term treatment, or (3) a local manipulation of striatal mAChR1 and mAChR4. Furthermore, the administration of the peripherally active mAChR1-antagonist pirenzepine should elucidate if the known antidystonic efficacy of mAChR1-antagonists may be mediated by peripheral effects. In addition, autoradiographic analysis was performed to clarify if there are distinct alterations of mAChR-subtypes in brain regions of dystonic hamsters.
In the present investigations, the acute combined treatment with THP and tropicamide resulted in a greater efficacy than the application of the single compounds. However, this efficacy could not be potentiated by chronic treatment. Striatal microinjections of both compounds or the mAChR4-selective allosteric modulator VU0152100 exerted weaker effects, indicating that systemic effects are mediated by extrastriatal brain regions. The lack of efficacy of pirenzepine suggests that beneficial effects of clinically used mAChR1-antagonist are not based on peripheral inhibition of the cholinergic system. In summary, the present results did not show a critical involvement of the (striatal) cholinergic system in the dystonic syndrome of the dtsz hamster, as underlined by a widely unchanged density of mAChR1, 2 and 4 in different brain regions. Nevertheless, it cannot be excluded that agents with higher selectivity for mAChR-subtypes, which are not available so far, might exert more pronounced effects.
Different transgenic mouse lines, which carry the human defect gene (DYT1) for the earlyonset torsion dystonia, do not exhibit dystonic symptoms (Sharma et al., 2005; Shashidharan et al., 2005; Grundmann et al., 2007). Nevertheless, these mice may contribute to understand why only 30-40% of the human DYT1 gene carriers develop dystonia and which factors lead to its manifestation. Previous in-vitro studies in DYT1 mice, which were used in this work, suggest that the DYT1 mutation results in an overactivity of the cholinergic system (Martella et al., 2009; Pisani et al., 2006). Pharmacological in-vivo experiments were carried out to investigate the functional relevance of these in-vitro findings, paying particular attention to the possible induction of a dystonic phenotype. Furthermore, it should be clarified by immunohistochemical studies, whether the previously identified cholinergic overactivity is based on an increased density of striatal cholinergic interneurons. Western blot analysis was carried out to elucidate, if the DYT1 mutation is related to an increased expression of choline acetyltransferase (ChAT), the key enzyme for the ACh synthesis.
The acute and chronic administration of the cholinomimetic drug pilocarpine did not provoke dystonia-like movements in DYT1 mice. However, after chronic administration DYT1 mice showed a trend towards more pronounced side effects, which may indicate a disturbance of the cholinergic system in these animals. The slightly increased incidence of epileptic seizures after repeated local striatal administration of pilocarpine let presume an altered synaptic plasticity in DYT1 mice. Together with the findings of an almost unchanged density of striatal cholinergic interneurons as well as the unchanged expression of ChAT in different brain regions in comparison to wild-type mice, the present results do not suggest a functional relevance of a striatal cholinergic overactivity, as previously found in these mice by in-vitro experiments. However, with regard to the use of the non-specific cholinomimetic pilocarpine and to a restriction on systemic and intrastriatal pharmacological manipulations, it cannot be excluded, that only certain mAChR-subtypes or extrastriatal brain regions are pathophysiologically involved. This question should be further investigated by receptor autoradiographic studies.