An MRI and Immunohistochemical Study of Striatal Changes Accompanying Levodopa-Induced Dyskinesia in a Rat Model of Parkinson’s Disease

Chronic levodopa (L-DOPA) treatment leads to abnormal involuntary movements or L-DOPA-induced dyskinesia (LID), in a significant proportion of patients with Parkinson’s disease. Whilst the mechanisms underlying LID remain enigmatic, accumulating experimental evidence suggests permanent structural remodelling of the striatum may be involved (1). The aim of this study was to examine MRI and cellular changes in the striatum of rats exhibiting LID to provide supporting evidence of such structural plasticity.

Adult male Sprague Dawley rats (270-300g, Harlan, UK) were subject to a full 6-hydroxydopamine-induced unilateral lesion of the nigrostriatal tract (2). Three-weeks post lesion, rats were treated daily for 21 days with either L-DOPA/benserazide (6.25 mg/kg and 15 mg/kg respectively, s.c.) (n=9) to induce dyskinesia or saline (n=10). Animals were scored every two days for abnormal involuntary movements (AIMS; rodent equivalent of LIDs) (3). One week later, animals were terminally anaesthetised, transcardially perfused with 4% PFA and heads removed and prepared for ex vivo structural MRI at 7T (4). Neuroanatomical differences between levodopa and saline-treated groups were assessed using tensor-based morphometry using a pre-defined striatal mask and corrected for multiple comparisons (q=0.05; False discovery rate) (5). The brains were then removed and 40 μm sections taken through the striatum for immunohistochemistry, to investigate cellular changes related to striatal volume, including: vascular endothelial cells (RECA1); astrocytes (GFAP) and microglia (Iba1). Densitometry was used to quantify changes in expression. Levels of staining between lesioned and intact hemispheres and between treatment groups were assessed using 2-way ANOVA and Bonferroni post-hoc tests (P<0.05). All procedures were performed in accordance with the U.K. Animals (Scientific Procedures) Act, 1986.

Chronic L-DOPA treatment led to LID, reaching a maximum AIMS score of 173±13 on day 21 compared to a complete absence of dyskinesia in the saline-treated group. MRI revealed a significant increase in volume in the dorsal striatum of the lesioned hemisphere of L-DOPA treated rats compared to the intact hemisphere of L-DOPA treated rats and the lesioned hemisphere of saline injected rats. No significant changes in RECA1 levels were found either between hemispheres or between treatment groups. GFAP expression was significantly elevated in the lesioned versus intact hemisphere of both treatment groups (Saline, 19% increase; L-DOPA, 22% increase), but did not significantly differ between treatment groups in either hemisphere. In contrast, the optical density of Iba1+ microglia, although elevated in the lesioned versus intact hemispheres of both treatment groups (Saline, 106% increase; L-DOPA 113% increase), was significantly higher (by 50%) in the lesioned hemisphere of the L-DOPA group versus the lesioned hemisphere of the saline group), an increase not related to changes in Iba1+ cell numbers.

In summary, structural MRI has revealed an increase in striatal volume in the lesioned hemisphere of L-DOPA-treated dyskinetic rats, supporting the notion that the combination of lesion and L-DOPA treatment, which is essential for LID development, induces plasticity in this key motor region of the brain. The present data rule against a contribution from increased density of either vasculature (RECA1) or astrocytes (GFAP) in this volume increase, but suggest a contribution from striatal microglia.

1) Finlay CJ et al. (2014). Frontiers in Neurology 5(95):1-11.

2) Messenger MJ et al. (2002). Neuropharmacology 43:261-271.

3) Winkler C et al. (2002). Neurobiology of Disease 10:165–186.

4) Vernon AC et al. (2011). Biol Psychiatry 69:936-944

5) Vernon AC et al. (2014). Biol Psychiatry 75:982-990