Increased sensitivity to d-and l-amphetamine action after midbrain raphe lesions as measured by locomotor activity

This study examined the effects of d- and l-amphetamine on locomotor activity and stereotyped behavior in control rats and rats with lesions in the raphe nuclei (RN) and dorsolateral tegmentum (DLT). Raphe nuclei lesions produced a large (93%) and selective depletion of the telencephalic content of serotonin and an enhancement of both d- and l-amphetamine action on locomotor activity that was characterized by a shift in the dose-effect curve to the left of controls. The ED50 for the locomotor increasing effects of both d- and l-amphetamine was significantly reduced by the RN lesions to approximately one-third of control values. In addition, RN lesions significantly increased the maximum levels of locomotor responding to d- but not l-amphetamine. Finally, rats with RN lesions demonstrated a significantly greater incidence of stereotyped behavior at lower dosages of d- but not l-amphetamine than did controls. Dorsolateral tegmentum lesions that produced a selective but small (54%) decrease in the telencephalic content of norepinephrine had no effect on either d- or l-amphetamine action.