Dopamine-Induced Changes in Gα Protein Levels in Striatonigral and Striatopallidal Medium Spiny Neurons Underlie the Genesis of l-DOPA-Induced Dyskinesia in Parkinsonian Mice.

The dopamine precursor, l-3,4-dihydroxyphenylalanine (l-DOPA), exerts powerful therapeutic effects but eventually generates l-DOPA-induced dyskinesia (LID) in patients with Parkinson's disease (PD). LID has a close link with deregulation of striatal dopamine/cAMP signaling, which is integrated by medium spiny neurons (MSNs). Olfactory type G-protein α subunit (Gα), a stimulatory GTP-binding protein encoded by the gene, is highly concentrated in the striatum, where it positively couples with dopamine D (DR) receptor and adenosine A receptor (AR) to increase intracellular cAMP levels in MSNs. In the striatum, DRs are mainly expressed in the MSNs that form the striatonigral pathway, while DRs and ARs are expressed in the MSNs that form the striatopallidal pathway. Here, we examined the association between striatal Gα protein levels and the development of LID. We used a hemi-parkinsonian mouse model with nigrostriatal lesions induced by 6-hydroxydopamine (6-OHDA). Using quantitative immunohistochemistry (IHC) and a dual-antigen recognition proximity ligation assay (PLA), we here found that in the dopamine-depleted striatum, there appeared increased and decreased levels of Gα protein in striatonigral and striatopallidal MSNs, respectively, after a daily pulsatile administration of l-DOPA. This leads to increased responsiveness to dopamine stimulation in both striatonigral and striatopallidal MSNs. Because Gα protein levels serve as a determinant of cAMP signal-dependent activity in striatal MSNs, we suggest that l-DOPA-induced changes in striatal Gα levels in the dopamine-depleted striatum could be a key event in generating LID.