The purpose of this research study is to determine whether AZD6765 has an effect on the patient's depression when taken together with current depression medication. In addition, information will be gathered on how well AZD6765 is tolerated, investigate the levels of AZD6765 and the levels of the current depression medication in the blood. In addition, the research staff will determine if AZD6765 has any mood or calming effects (how you feel).