Furthermore, higher mu-opioid receptor availability was negatively correlated with the affective/sensory pain ratio in the medial frontal gyrus, posterior cingulate cortex, ACC, and precentral gyrus.56 Based on such findings, the authors hypothesized that in patients with fibromyalgia, tonic higher levels of endogenous opioids would promote a lower affinity or a downregulation of mu-opioid receptors on GABAergic interneurons of brain regions involved in nociception. Here, OPRM1 is linked to fibromyalgia.