This study aims to investigate the contribution of nigral dopaminergic (DA) cell loss, repeated exposure to DA medication and the combination of both to the development of neuropsychiatric symptoms observed in Parkinson's disease (PD). A bilateral 6-OHDA lesion of the substantia nigra pars compacta (SNc) was performed in rats. A set of animals was repeatedly administered with L-dopa (20 mg/kg/day) and benserazide (5 mg/kg/day) over 10 days starting from day 11 post-lesion. Behavioural testing was performed in week 3 postlesion: novel object recognition (NOR), elevated plus maze (EPM) social interaction (SI) tests, and amphetamine-induced hyperlocomotion (AIH). Immunohistochemical analysis revealed a significant partial lesion (48%) in 6-OHDA versus sham rats. This lesion was not associated with motor impairment. However, lesioned rats displayed a significant deficit in the NOR, which was reversed by acute treatment with L-dopa/benserazide (12.5 mg/kg and 15 mg/kg respectively). Lesioned rats also displayed a deficit in the EPM which was not reversed by acute treatment with L-dopa. No difference was observed in the SI test or in the AIH assay. In all assays, no effect of chronic L-dopa exposure was observed. This study provides new insights into the neuropathophysiology associated with neuropsychiatric symptoms of PD. Our data strongly emphasises a not previously clearly identified critical role in cognition for the SNc. The results suggest that DA pathways were less directly involved in lesioninduced anxiety-like behaviour. We did not report any effect of chronic L-dopa exposure in the context of partial nigral cell loss.

The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated non-human primate (NHP) has been described as the most translatable model for experimental reproduction of L-dopa-induced dyskinesia (LID). However, from a drug discovery perspective, the risk associated with investment in this type of model is high due to the time and cost. The 6-hydroxydopamine (6-OHDA) rat dyskinesia model is recommended for testing compounds but relies on onerous, and non-standard behavioral rating scales. We sought to develop a simplified and sensitive method aiming at assessing LID in the rat. The purpose was to validate a reliable tool providing earlier insight into the antidyskinetic potential of compounds in a time/cost-effective manner before further investigation in NHP models.Unilaterally 6-OHDA-lesioned rats were administered L-dopa (20 mg/kg) and benserazide (5 mg/kg) daily for 3 weeks starting 4 weeks post-lesion, then co-administered with amantadine (20-30-40 mg/kg). An adapted rating scale was used to score LID frequency and a severity coefficient was applied depending on the features of the observed behavior.A gradual increase (about 3-fold) in LID score was observed over the 3 weeks of L-dopa treatment.The rating scale was sensitive enough to highlight a dose-dependent amantadine-mediated decrease (about 2.2-fold) in LID score.We validated a simplified method, able to reflect different levels of severity in the assessment of LID and, thus, provide a reliable tool for drug discovery.

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