Abstract
Although L-3,4-dihydroxyphenylalanine (L-DOPA) remains the reference treatment of Parkinson's disease, its long-term beneficial effects are hindered by L-DOPA-induced dyskinesia (LID). In the dopamine (DA)-denervated striatum, L-DOPA activates DA D&sub1; receptor(D&sub1;R) signaling, including cAMP-dependent protein kinase A (PKA) and extracellular signal-regulated kinase (ERK), two responses associated with LID. However, the cause of PKA and ERK activation, their respective contribution to LID, and their relationship are not known. In striatal neurons, D&sub1;R activates adenylyl-cyclase through Gα(olf), a protein upregulated after lesion of DA neurons in rats and inpatients. We report here that increased Gα(olf) levels in hemiparkinsonian mice are correlated with LID after chronic L-DOPA treatment. To determine the role of this upregulation, we performed unilateral lesion in mice lacking one allele of the Gnal gene coding for Gα(olf) (Gnal⁺/⁻). Despite an increase in the lesioned striatum,Gα(olf) levels remained below those of unlesioned wild-type mice. In Gnal⁺/⁻ mice, the lesion-induced L-DOPA stimulation of cAMP/PKA-mediated phosphorylation of GluA1 Ser845 and DARPP-32 (32 kDa DA- and cAMP-regulated phosphoprotein) Thr34 was dramatically reduced, whereas ERK activation was preserved. LID occurrence was similar in Gnal⁺/⁺ and Gnal⁺/⁻ mice after a 10-d L-DOPA (20 mg/kg) treatment. Thus, in lesioned animals, Gα(olf) upregulation is critical for the activation by L-DOPA of D&sub1;R-stimulated cAMP/PKA but not ERK signaling. Although the cAMP/PKA pathway appears to be required for LID development, our results indicate that its activation is unlikely to be the main source of LID. In contrast, the persistence of L-DOPA-induced ERK activation in Gnal⁺/⁻ mice supports its causal role in LID development.