Allosteric modulation of dopamine D2 receptors by homocysteine

It has been suggested that L-DOPA-induced hyperhomocysteinemia can increase the risk of stroke, heart disease, and dementia and is an additional pathogenetic factor involved in the progression of Parkinson's disease. In Chinese hamster ovary (CHO) cells stably cotransfected with adenosine A(2A) and dopamine D-2 receptors, homocysteine selectively decreased the ability of D-2 receptor stimulation to internalize adenosine A(2A)-dopamine D-2 receptor complexes. Radioligand-binding experiments in the same cell line demonstrated that homocysteine acts as an allosteric D-2 receptor antagonist, by selectively reducing the affinity of D-2 receptors for agonists but not for antagonists. Mass spectrometric analysis showed that, by means of an arginine (Arg)-thiol electrostatic interaction, homocysteine forms noncovalent complexes with the two Arg-rich epitopes of the third intracellular loop of the D-2 receptor, one of them involved in A(2A)-D-2 receptor heteromerization. However, homocysteine was unable to prevent or disrupt A(2A)-D-2 receptor heteromerization, as demonstrated with Fluorescence Resonance Energy Transfer (FRET) experiments in stably cotransfected HEK cells. The present results could have implications for Parkinson's disease.