DRD2 and depressive symptom measurement: Examples of this include the 5-HT2A-mGlu2 receptor complex in schizophrenia [34], the μ-opioid-α2a-adrenergic receptor complex in depression and opioid addiction [45], the adenosine A2A-dopamine D2 receptor complex in Parkinson's disease [59], and the κ-δ-opioid receptor complex in analgesia and drug abuse [23].