Approximately 44 patients are to be randomized in a 3:1 ratio to two possible treatment groups, pardoprunox and pramipexole, respectively. The first part of the study will be blinded and consists of a minimum 1-week screening period, a 4-week switch and stabilization period and an 8-week maintenance period. The second part of the study will be open label pardoprunox treatment with a dose adjustment period of 4 weeks followed by long term maintenance treatment.