Participation in the study will last a maximum of 14 weeks consisting of a 2 week (maximum) screening/baseline period; a 2 week (maximum) double-blind dose titration; an 8 week double-blind placebo-controlled treatment period; and a 2 week follow-up period. An extension study is also available to continue treatment if determined appropriate by the study doctor. This Study is NCT01132326 sponsored by Chelsea Therapeutics and is enrolling by invitation only.