Effects of Alpha-Synuclein Over-Expression and DSP-4 Toxicity on Cellular and Behavioral Correlates of Parkinson's Disease

Parkinson's disease is primarily characterized by dopamine cell loss and protein accumulation in the brain. However, in PD there is also substantial loss of noradrenergic neurons and protein accumulation in the locus coeruleus. Alpha-synuclein is presynaptic protein that abnormally accumulates in both familial and sporadic forms of PD. This study looks at the connection between LC degeneration and abnormal alpha-synuclein aggregation in terms of the neuropsychiatric impairments that develop with PD pathogenesis. We hypothesize that LC degeneration coupled with alpha-synuclein pathology will exacerbate both motor and non-motor symptoms in PD. LC degeneration was induced using three different doses (0, 25, 50 mg/kg) of a selective neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP-4) in two month old male transgenic mice overexpressing human wildtype alpha-synuclein (Thy1-aSyn). The behavioral analysis was comprised of three components that directly address the major aspects of PD symptomatology. For motor performance the tests used were challenging beam traversal, spontaneous activity (cylinder) and locomotion. For emotional reactivity the tests used were sucrose preference and elevated plus maze, and for cognitive function a novel object recognition task was used. We anticipated that Thy1-aSyn mice receiving DSP-4 (25 & 50 mg/kg) would show greater behavioral deficits compared to Thy1-aSyn mice receiving vehicle and wildtype controls.;The present data provide support for the relationship that may exist between alpha-synuclein aggregates and the vulnerability of LC neurons based on the immunohistochemical results. For example, our data indicate that animals with the greatest degree of cell loss in the LC also performed poorer on certain behavioral assays (i.e., cognitive test), compared to those animals with attenuated or no cell loss in the LC. Overall, we found differential effects of DSP-4 on behavioral and histological analyses, which varied between genotypes (i.e., Thy1-aSyn and WT) and drug concentrations. The 3-4-month-old transgenic controls tested in this study displayed the sensorimotor abnormalities associated with PD without any of the neuronal cell loss that is characteristic of this line. As expected for motor coordination, specifically with challenging beam traversal and spontaneous activity; Thy1-aSyn mice showed the same robust deficits in errors per step and hindlimb stepping in comparison with wildtype controls as previously reported (Fleming et al., 2004). In terms of the effect of DSP-4 on motor coordination, animals treated with the highest dose of DSP-4 made more errors than mice treated with the intermediate and vehicle dose. However, DSP-4 seemed to have no effect on the open field or cylinder test (spontaneous activity). Histological analysis on coronal sections of the LC using immunostained Tyrosine Hydroxylase (TH) neurons indicate that DSP-4 administration decreased TH+ cells specifically in two groups. Unbiased stereological cell counts showed that the 25 mg⁄kg DSP-4-treated Thy1-aSyn mice and the 50 mg/kg DSP-4-treated WT mice were the only groups to have lower TH+ cell counts. At present, we are unable to explain this result. Overall, the present findings suggest an increased vulnerability to DSP-4 caused by alpha-synuclein over-expression that leads to deficits in certain aspects of cognition, such as selective attention.