These revealed that, surprisingly, compounds of the formula III with n=O, R1 =isobutyl, R2 =H, R4 =H, CH3, R5 =Cl, R6 =H lead to a large increase in the extracellular levels of the endogenous neurotransmitters serotonin and norepinephrine in the CNS. Compounds where R4 is a hydrogen atom in particular show a good effect in these experiments.