Fig. 1.
a Schematic illustration of the actions of endogenous (agonist) ligands, agonist and antagonist drugs at the receptor. Endogenous ligands and agonist drugs bind to and stimulate receptors. Antagonist drugs bind to and block receptors without stimulating them, thereby preventing other ligands (e.g. endorphins, enkephalins) from activating the receptor. b Mu-opioid receptor distribution in the healthy human brain as measured by positron emission tomography using the [11C] carfentanil radioligand BPND (binding potential relative to nondisplaceable radioligand in tissue, see Innis et al. [58]). Images are based on 204 subjects from Aivo database (http://aivo.utu.fi). Key structures implicated in reward and threat processing are densely innervated with MOR receptors, such as the nucleus accumbens (NAc), the nuclei of the amygdala, the thalamus, anterior cingulate cortex (ACC) and periaqueductal gray (PAG). This MNI-space atlas is available on @VaultNeuro (https://neurovault.org/collections/GCELSAIA/)