Nonmotor symptoms in Parkinson's disease: investigating early-phase onset of behavioral dysfunction in the 6-hydroxydopamine-lesioned rat model.

To investigate the psychiatric symptoms accompanying the early phases of Parkinson's disease (PD), we injected adult rats with 10.5 microg 6-hydroxydopamine (6-OHDA) bilaterally into the dorsal striatum. The resulting neurodegeneration led, 12 weeks after injection, to a mild (36%) reduction of striatal dopamine. We tested the behavioral response of sham and 6-OHDA-lesioned animals at different time points after injection to evaluate the onset and progression of behavioral abnormalities. The results showed that such a mild reduction of dopamine levels was associated with a decrease in anxiety-like behavior, an increase in "depression"-like behavior, and a marked change in social behavior. Learning and memory abilities were not affected. Overall, the PD rat model used here displays behavioral alterations having face validity with psychiatric symptoms of the pathology and thus appears to be a valuable tool for investigating the neural bases of the early phases of PD.