OPRM1 and fibromyalgia: In one of the pioneer studies using selective (MOR) radioligands, a decreased nondisplacable binding potential (BPND) of the MOR selective radiotracer [11C] carfentanil was found in several brain areas directly related to different aspects of pain, including the nucleus accumbens (NAc), amygdala, and dorsal ACC (dACC), of patients diagnosed with fibromyalgia syndrome, an idiopathic chronic pain disorder of obscure etiology and difficult control.