CHRM2 and postural orthostatic tachycardia syndrome: Serum activity against several GPCRs—including α1-adrenergic receptors (ADRA1), β2-adrenergic receptors (ADRB2), muscarinic M2 receptors (CHRM2), and opioid receptors OPRL1—was significantly higher in patients with POTS compared with controls, with p-values ranging from <0.05 to <0.001.