Exploring cognitive and affective outcomes following bilateral 6-hydroxydopamine lesions of the substance nigra or striatum in rats

Abstract Purpose: The majority of animal models of Parkinson’s disease (PD) focus on motor symptoms that are induced by unilateral injections of such neurotoxins as 6-hydroxydopamine (6-OHDA) in nigrostriatal dopaminergic pathways. However, motor changes that are induced by unilateral 6-OHDA injections may interfere with the identification of cognitive and affective dysfunction induced by dopaminergic neurodegeneration. Material and Methods: To select an appropriate method for studying nonmotor symptoms of PD and potential neuroprotective treatments, the present study compared behavioral effects of bilateral 6-OHDA infusions directly in the substantia nigra pars compacta (SNpc) or striatum in rats. A battery of behavioral tests, including affective and cognitive tasks, was performed for 22 days after nigrostriatal lesions. Results:The massive degeneration of tyrosine hydroxylase-immunoreactive neurons was observed in the SNpc, striatum, and ventral tegmental area with 6-OHDA infusions in either the SNpc or striatum. With regard to functional outcomes, 6-OHDA infusions in the striatum decreased general exploratory activity 7 days after the lesion. Rats that received 6-OHDA in the SNpc exhibited cognitive impairments and despair-like behavior. A decrease in the number of newborn neurons was found in the hippocampus in rats that received 6-OHDA in the striatum, indicating a decrease in neurogenesis. 6-OHDA infusions in both the SNpc and striatum impacted the maturation of newborn hippocampal neurons. Conclusions: These results indicate that bilateral injections of 6-OHDA in the SNpc might be appropriate for studying nonmotor symptoms of PD.