The invention is directed to a compound of formula I, as defined herein, or a pharmaceutically acceptable salt thereof a pharmaceutical composition containing a compound of formula I, a method of treatment of a disorder or condition that may be treated by administration of the compound, the method comprising administering to a mammal in need of such treatment a compound of formula I as described above, and a method of treatment of a disorder or condition selected from the group consisting of depression, mood disorders, schizophrenia, Alzheimers disease, anxiety and addiction, the method comprising administering to a mammal, including a human, in need of such treatment a compound of formula I as described above.
