molecules Review A Survey of Molecular Imaging of Opioid Receptors Paul Cumming 1,2,* ,János Marton 3, Tuomas O. Lilius 4 , Dag Erlend Olberg 5 and Axel Rominger 1,* 1 Department of Nuclear Medicine, University of Bern, Inselspital, Freiburgstraße 18, 3010 Bern, Switzerland 2 School of Psychology and Counselling and IHBI, Queensland University of Technology, QLD 4059, Brisbane, Australia 3 ABX Advanced Biochemical Compounds, Biomedizinische Forschungsreagenzien GmbH, Heinrich-Glaeser-Strasse 10-14, D-1454 Radeberg, Germany;
[email protected] 4 Center for Translational Neuromedicine, Faculty of Health and Medical Sciences, University of Copenhagen, 2200 Copenhagen N, Denmark;
[email protected] 5 School of Pharmacy, University of Oslo, Norwegian Medical Cyclotron Centre, N-0372 Oslo, Norway and Norwegian Medical Cyclotron Centre Ltd., Sognsvannsveien 20, N-0372 Oslo, Norway;
[email protected] * Correspondence:
[email protected] (P.C.);
[email protected] (A.R.); Tel.: +41-31-664-04-98 (P.C.); +41-31-632-26-10 (A.R.) Academic Editor: Peter Brust Received: 21 October 2019; Accepted: 13 November 2019; Published: 19 November 2019 Abstract: The discovery of endogenous peptide ligands for morphine binding sites occurred in parallel with the identification of three subclasses of opioid receptor (OR), traditionally designated as µ, δ, and κ, along with the more recently defined opioid-receptor-like (ORL1) receptor. Early efforts in opioid receptor radiochemistry focused on the structure of the prototype agonist ligand, morphine, although N-[methyl-11C]morphine, -codeine and -heroin did not show significant binding in vivo. [11C]Diprenorphine ([11C]DPN), an orvinol type, non-selective OR antagonist ligand, was among the first successful PET tracers for molecular brain imaging, but has been largely supplanted in research studies by the µ-preferring agonist [11C]carfentanil ([11C]Caf).