Dopaminergic Neurotransmission in a Model of DOPA-responsive Dystonia

Dystonia is characterized by involuntary muscle contractions that cause debilitating twisting movements and postures. Abnormal dopamine (DA) neurotransmission is consistently observed across many different forms of dystonia. However, the DA defects that give rise to dystonia are poorly understood. L-DOPA-responsive dystonia (DRD) is considered a prototype for understanding how abnormal DA neurotransmission evokes dystonia. DRD is characterized by childhood onset dystonia with diurnal fluctuation whereby symptoms worsen throughout the course of the day. The distinguishing feature of DRD is the dramatic improvement in symptoms after restoration of DA signaling with L-DOPA or DA agonists, demonstrating a clear link between abnormal DA and dystonia. Indeed, DRD is caused by mutations in genes critical for DA synthesis, including tyrosine hydroxylase (TH). DRD-causing TH mutations are associated with some residual TH activity whereas mutations that abolish TH activity cause childhood parkinsonism suggesting that TH activity and [DA] are critical determinants in the development of dystonia. However, there are few tools available for understanding the DA signaling dysfunction that gives rise to dystonia. To fill this gap, we generated a knockin mouse bearing the DRD-causing Q381K TH mutation (DRD mice). Like the human disorder, DRD mice exhibit a reduction in TH activity leading to a reduction in [DA]. Similar to the diurnal fluctuations in DRD patients, DRD mice exhibit dystonic-like movements that are relatively mild at the beginning of the active (nocturnal) period but worsen by the end of the night. The abnormal movements are alleviated by L-DOPA. Additionally, our preliminary data demonstrate that the dystonic movements are also alleviated by D1 DA eceptor (DAR) agonists and potentiated by D1 DAR antagonism. DRD mice are the first and, so far, the only mouse model of dystonia that is a genocopy of a human dystonic disorder and exhibits dystonia. The lack of an animal model with both face and etiologic validity had been an extraordinary barrier to progress in dystonia research. However, we now have an unprecedented opportunity to dissect the mechanisms underlying dystonia using this powerful novel model. Therefore, using a multidisciplinary approach, we will address the hypothesis supported by our preliminary data suggesting that dystonia is, at least in part, mediated by abnormal D1DAR signaling combined with a deficit in DA. The Specific Aims are: 1. To assess the diurnal fluctuations in DA and DA metabolism in DRD mice. 2. To determine the DAR subtype(s) and signaling defects that contribute to the abnormal movements. 3. To delineate alterations in the intrinsic and synaptic properties of D1 vs. D2 DAR-expressing medium spiny neurons in DRD mice. 4. To examine the morphological and ultrastructural features of MSNs in DRD mice.