Blockade of adenosine A2A receptors downregulates DARPP-32 but increases ERK1/2 activity in striatum of dopamine deficient “weaver” mouse

In the present study we investigated the signal transduction cascade modulated by adenosine A2A receptors under chronic dopamine deficiency in the “weaver” mouse. We determined the phosphorylation state of cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) at Thr34 and of Extracellular Signal-regulated Protein Kinases 1/2 (ERK1/2), under basal conditions and after in vivo stimulation of A2A receptors by administration of the agonist CGS21680. Our results revealed that the endogenous levels of phospho-DARPPP-32 and phospho-ERK1/2 are elevated in “weaver” striatum probably as an adaptation phenomenon to gradual dopaminergic neurodegeneration appearing in this animal model, characterized as phenocopy of Parkinson's disease. Stimulation of A2A receptors by CGS21680 further increases phospho-DARPP-32 but downregulates significantly the elevated phospho-ERK1/2 levels bringing them close to those observed in wild type animals. Consistently, blockade of A2A receptors by MSX-3 (A2A receptor antagonist) downregulates phospho-DARPP-32 but significantly increases even more the phosphorylation/activation of ERK1/2. These results indicate that under chronic dopamine deficiency (a) the A2A/cAMP/PKA/DARPP-32 cascade is overactive due to the elevated endogenous phospho-DARPP-32 levels and (b) the A2A receptor modulatory effect on ERK1/2 signaling is dysregulated exerting opposing action compared to that observed in normal animals (Quiroz et al., 2006), i.e. in “weaver” animals A2A receptor blockade increases the activity of ERK1/2 cascade. This could be of clinical relevance since A2A antagonists are already used in clinical trials for ameliorating Parkinson's disease (PD) symptoms.