The present invention relates to compounds of general formula wherein R1 hydrogen, lower alkyl, halogen or lower alkyl substituted by halogen； R2 is hydrogen or halogen； X1 is N or CH； X2 is N or CH； with the proviso that only one of X1 or X2 is N； X3 is C(R) or N； and R is hydrogen, lower alkyl, halogen, lower alkyl substituted by halogen, lower alkoxy or SO2-lower alkyl； or to a pharmaceutically acceptable acid addition salt, to a racemic mixture or to its corresponding enantiomer and/or optical isomers thereof. The compounds may be used for the treatment of schizophrenia, obsessive-compulsive personality disorder, major depression, bipolar disorders, anxiety disorders, normal aging, epilepsy, retinal degeneration, traumatic brain injury, spinal cord injury, post-traumatic stress disorder, panic disorder, Parkinson's disease, dementia, Alzheimer's disease, mild cognitive impairment, chemotherapy-induced cognitive dysfunction, Down syndrome, autism spectrum disorders, hearing loss, tinnitus, spinocerebellar ataxia, amyotrophic lateral sclerosis, multiple sclerosis, Huntington's disease, stroke, radiation therapy, chronic stress, abuse of neuro-active drugs, such as alcohol, opiates, methamphetamine, phencyclidine and cocaine.
