6-OHDA induced effects upon the acquisition and performance of specific locomotor tasks in rats

The effect of norepinephrine (NE) depletion on acquisition and performance of locomotor tasks requiring precise paw placement was tested. Running times (RT, 25 trials/day, 4 consecutive days) of water-deprived rats trained to transverse horizontal rods in an equally spaced regular rod arrangement (REG) were obtained before and after (REG/REG) intracisternal 6-hydroxydopamine (6-OHDA, 3 X 25 micrograms free base) infusion. No significant differences from ascorbate (0.1%) vehicle controls were seen. Additional rats were tested using the same protocol except a more difficult, irregularly spaced rod arrangement (IRR) was used. These IRR/IRR rats also revealed no significant differences. However, testing on the REG task before, and the new IRR task after infusion produced impaired performance on days 3 and 4 when 6-OHDA and vehicle treated rats were compared. These REG/IRR rats also showed a significant difference in the slope of the line reflecting the decrease in RT over the 4 day post-infusion period. Since no differences in intertrial intervals or extinction behavior were seen, the effect was not attributed to differences in arousal or motivational state. This effect could not be attributed to a simple reduction in non-specific activity, since significant differences in spontaneous locomotor activity or open field behavior were not seen. Assays verified the severe reduction of cerebellar NE to 14.5% of vehicle controls, and the smaller reduction in limbic forebrain NE and dopamine (53.8% and 75.2% of controls respectively). These findings suggest that NE deafferentation of the cerebellum causes impaired acquisition of locomotor behavior rather than an impairment of post-acquisitional performance.