Emotional, cognitive and neurochemical alterations in a premotor stage model of Parkinson's disease

In addition to classic motor symptoms, Parkinson's disease (PD) is characterized by cognitive and emotional deficits, which have been demonstrated to precede motor impairments. The present study addresses the question of whether a partial degeneration of dopaminergic neurons using 6-hydroxydopamine (6-OHDA) in rats is able to induce premotor behavioral signs. The time-course of nigrostriatal damage was evaluated by tyrosine hydroxylase immunohistochemistry and the levels of dopamine, noradrenaline, and 5-HT in various brain regions were analyzed by high performance liquid chromatography (HPLC). Behavioral tests that assessed a variety of psychological functions, including locomotor activity, emotional reactivity and depression, anxiety and memory were conducted on 6-OHDA lesioned rats. Bilateral infusion of 6-OHDA in the striatum of rats caused early (1 week) damage of dopaminergic terminals in striatum and in cell bodies in substantia nigra pars compacta. The nigrostriatal lesion was accompanied by early loss of dopamine in the striatum, which remained stable through a 3-week period of observation. In addition, a late (3 weeks) loss of dopamine in the prefrontal cortex, but not in the hippocampus, was seen. Additional noradrenergic and serotonergic alterations were observed after 6-OHDA administration. The results indicated that 6-OHDA lesioned rats show decreased sucrose consumption and an increased immobility time in the forced swimming test, an anhedonic-depressive-like effect. In addition, an anxiogenic-like activity in the elevated plus maze test and cognitive impairments were observed on the cued version of the Morris water maze and social recognition tests. These findings suggest that partial striatal dopaminergic degeneration and parallel dopaminergic, noradrenergic and serotonergic alterations in striatum and prefrontal cortex may have caused the emotional and cognitive deficits observed in this rat model of early phase PD.