Behavioral and cellular dopamine D and D receptor-mediated synergy: Implications for L-DOPA-induced dyskinesia.

Individually, D1 and D3 dopamine receptors (DR and DR, respectively) have been implicated in L-DOPA-induced dyskinesia (LID). Of late, direct DR-DR interactions have been linked to LID yet remain enigmatic. Therefore, the current research sought to characterize consequences of putative DR-DR interactions in dyskinesia expression and in LID-associated downstream cellular signaling. To do so, adult male Sprague-Dawley hemi-parkinsonian rats were given daily L-DOPA (6 mg/kg; s.c.) for 2 weeks to establish stable LID, as measured via the abnormal voluntary movements (AIMs) scale. Thereafter, rats underwent dose-response AIMs testing for the DR agonist SKF38393 (0, 0.3, 1.0, 3.0 mg/kg) and the DR agonist, PD128907 (0, 0.1, 0.3, 1.0 mg/kg). Each agonist dose-dependently induced dyskinesia, implicating individual receptor involvement. More importantly, when threshold doses were co-administered, rats displayed synergistic exacerbation of dyskinesia. Interestingly, this observation was not mirrored in general locomotor behaviors, highlighting a potentially dyskinesia-specific effect. To illuminate the mechanisms by which DR-DR co-stimulation led to in vivo synergy, levels of striatal phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2) were quantified after administration of SKF38393 and/or PD128907. Combined agonist treatment synergistically drove striatal pERK1/2 expression. Together, these results support the presence of a functional, synergistic interaction between DR and DR that manifests both behaviorally and biochemically to drive dyskinesia in hemi-parkinsonian rats.