Distinct changes in cAMP and extracellular signal-regulated protein kinase signalling in L-DOPA-induced dyskinesia

Abstract

Background: In rodents, the development of dyskinesia produced by L-DOPA in the dopamine-depleted striatum occurs in response to increased dopamine D1 receptor-mediated activation of the cAMP - protein kinase A and of the Ras-extracellular signal-regulated kinase (ERK) signalling pathways. However, very little is known, in non-human primates, about the regulation of these signalling cascades and their association with the induction, manifestation and/or maintenance of dyskinesia.

Methodology/results: We here studied, in the gold-standard non-human primate model of Parkinson's disease, the changes in PKA-dependent phosphorylation of DARPP-32 and GluR1 AMPA receptor, as well as in ERK and ribosomal protein S6 (S6) phosphorylation, associated to acute and chronic administration of L-DOPA. Increased phosphorylation of DARPP-32 and GluR1 was observed in both L-DOPA first-ever exposed and chronically-treated dyskinetic parkinsonian monkeys. In contrast, phosphorylation of ERK and S6 was enhanced preferentially after acute L-DOPA administration and decreased during the course of chronic treatment.

Conclusion: Dysregulation of cAMP signalling is maintained during the course of chronic L-DOPA administration, while abnormal ERK signalling peaks during the initial phase of L-DOPA treatment and decreases following prolonged exposure. While cAMP signalling enhancement is associated with dyskinesia, abnormal ERK signalling is associated with priming.

Figure 1. Experimental flowchart illustrating study design, treatments and group assignments.

Figure 2. Western blot analysis of levels of (A) phosphorylated DARPP-32 at Thr34, (B) phosphorylated GluR1 at Ser845, (C) phosphorylated ERK1/2 at Thr202/Tyr204 and (D) phosphorylated S6 at Ser235/Ser236 in the dorsal putamen (mean ± S.E.M.).

Groups were prepared as depicted in figure 1. Representative blots are shown above each panel. Data were analysed using a one-way ANOVA followed by post hoc Tukey-Kramer multiple comparisons test (Graphpad Instat, Graphpad softwares, San Diego, CA). A probability level of 5% (*: P<0.05) was considered statistically significant. (A) F(_6,37) = 3.6, P = 0.008; (B) F(_6,42) = 3.4, P = 0.008; (C) F(6,₃₈) = 3.9, P = 0.004; (D) F(_6,38) = 3.9, P = 0.004.

Figure 3. Western blot analysis of levels of (A) DARPP-32 (B) GluR1 (C) ERK1/2 and (D) S6 in the dorsal putamen (mean ± S.E.M.).

Groups were prepared as depicted in Figure 1. Representative blots are shown above each panel.