Molecular Adaptations of Striatal Spiny Projection Neurons During Levodopa-Induced Dyskinesia

L-3,4-dihydroxyphenylalanine (levodopa) treatment is the major pharmacotherapy for Parkinson's disease. However, almost all patients receiving levodopa eventually develop debilitating involuntary movements (dyskinesia). While it is known that striatal spiny projection neurons (SPNs) are involved in the genesis of this movement disorder, the molecular basis of dyskinesia is not understood. In this study, we identify distinct cell-type-specific gene expression changes that occur in sub-classes of SPNs upon induction of a parkinsonian lesion followed by chronic levodopa treatment. We identify several hundred genes whose expression is correlated with levodopa dose, many of which are under the control of AP-1 and ERK signaling. In spite of homeostatic adaptations involving several signaling modulators, AP-1-dependent gene expression remains highly dysregulated in direct pathway SPNs (dSPNs) upon chronic levodopa treatment. We also discuss which molecular pathways are most likely to dampen abnormal dopaminoceptive signaling in spiny projection neurons, hence providing potential targets for antidyskinetic treatments in Parkinson's disease. To profile the cell-type-specific responses of striatal spiny projection neurons (SPNs) to striatal dopamine depletion, we conducted TRAP analysis of the two major classes of these neurons: dSPNs that express the dopamine receptor 1a (Drd1a), and iSPNs that express the dopamine receptor 2 (Drd2). To disrupt dopamine innervation to both of these SPN populations that reside in the striatum, we injected the neurotoxin 6-hydroxydopamine (6-OHDA), unilaterally, in the medial forebrain bundle (MFB) in hemizygous Drd1-TRAP and Drd2-TRAP adult (9-14 weeks) male mice (kept on a C57BL/6J genetic background). This lesion procedure causes nigral dopamine cell death within a few days, along with a widespread and near-complete loss of dopaminergic innervation to the entire dorsal striatum on one side of the brain (a hemiparkinsonian model). We first examined the effects of dopamine depletion alone, compared to a mock lesion (ascorbate / saline injected). We then examined the effects of chronic levodopa treatment upon the molecular profiles of dopamine- depleted dSPNs and iSPNs, with two dose regimens. The ‘high-dose’ L-DOPA regimen (3 mg/kg on days 1-3, followed by 6 mg/kg on days 4-9) was expected to induce severe dyskinesia in all MFB-lesioned mice. The low-dose L-DOPA regimen (1 mg/kg on days 1-3, followed by 2 mg/kg on days 4-9) was expected to reverse limb use asymmetry without causing conspicuous dyskinesias. To equalize the effects of stress and handling across all groups, including control groups, all mice were equally handled and thus received saline injections when not receiving levodopa injections. Each treatment group contained 7-10 replicates. TRAP-purified mRNAs from either Drd1a- or Drd2-expressing SPNs were reverse-transcribed, amplified, and used to interrogate Affymetrix 430_2.0 GeneChip microarrays.