OPRM1 and major depressive disorder: For example, numerous reports had linked anti-opioid receptor (OPRM1) IgG autoantibodies [48], antibodies to 5HT-1A or 5-hydroxytryptamine receptor 1A (HTR-1A), DRD2, and antibodies directed against the GPCR muscarinic cholinergic receptor 1 (CHRM1), to MDD [49].