In one study [21] investigating the role of M1 and M4 muscarinic receptors in L-DOPA-induced dyskinesia, it was shown that both M1 and M4 muscarinic receptor antagonists reduced D1 dopamine receptor agonist-induced dyskinesia but not D2 dopamine receptor agonist-induced dyskinesia, suggesting that muscarinic blockade differentially affects medium spiny neuron firing in the absence of postsynaptic dopamine. This evidence concerns the gene DRD2 and drug-induced dyskinesia.