Differential sensitivity to acute administration of cocaine, GBR 12909, and fluoxetine in mice selectively bred for hyperactive wheel-running behavior

Rationale: To study the neural basis of genetic hyperactivity, we measured acute drug responses of mice (Mus domesticus) from four replicate lines tha t had been selectively bred (23-24 generations) for increased running-wheel activity. Objectives: We tested the hypothesis that the high-running lines would respond differently to cocaine, GBR 12909, and fluoxetine (Prozac) c ompared with four replicate, random-bred. control lines. We also tested the hypothesis that the high-running lines would display hyperactivity in cage s without wheels. Methods: Drug trials were conducted at night, during peak activity, after animals were habituated (3 weeks) to their cages with atta ched wheels. Revolutions on wheels 10-40 min post-injection were used to qu antify drug responses. In a separate study, total photobeam breaks (produce d on the first and second 24-h period of exposure) were used to quantify ba sal activity in animals deprived of wheels. Results: Cocaine and GBR 12909 decreased wheel running in selected lines by reducing the average speed but not the duration of running, but these drugs had little effect in control lines. Fluoxetine reduced running speed and duration in both selected and c ontrol animals, and the magnitude of the reduction was proportional to base line activity. Basal activity in animals deprived of wheels (quantified usi ng photobeam breaks) was significantly higher in selected than control line s on the second day of testing. Conclusions: These results suggest an assoc iation between genetically determined hyperactive wheel-running behavior an d dysfunction in the dopaminergic neuromodulatory system. Our selected line s may prove to be a useful genetic model for attention deficit hyperactivit y disorder.