SYNTHESIS AND CHARACTERIZATION OF 4,4-DIFLUORO-4-BORA-3A,4A-DIAZA-S-INDACENE (BODIPY)-LABELED FLUORESCENT LIGANDS FOR THE MU-OPIOID RECEPTOR

A series of opioid ligands utilizing the di,4-difluoro-4-bora-3a,4a-di aza-s-indacene (BODIPY) fluorophores dimethyl-4-bora-3a,4a-diaza-s-ind acene-3-propionic acid or adienyl)-4-bora-3a,4a-diaza-s-indacene-3-pro pionic acid were synthesized and characterized for their ability to ac t as a suitable fluorescent label for the mu opioid receptor. All comp ounds displaced the mu opioid receptor binding of [H-3]Tyr-D-Ala-Gly-( Me)Phe Gly-ol in monkey brain membranes with high affinity. The bindin g of fluorescent ligands to delta and kappa receptors was highly varia ble. 5,7-Dimethyl-BODIPY naltrexamine, ''6-BNX,'' displayed subnanomol ar affinities for the mu and kappa opioid receptors (K-i 0.07 and 0.43 nM, respectively) and nanomolar affinity at the delta (K-i 1.4 nM) re ceptor. Using fluorescence spectroscopy, the binding of 6-BNX in membr anes from C-6 glioma cells transfected with the cloned mu opioid recep tor was investigated. In these membranes containing a high receptor de nsity (10-80 pmol/mg protein), 6-BNX labeling was saturable, mu opioid specific, stereoselective (as determined with the isomers dextrorphan and levorphanol), and more than 90% specific. The results describe a series of newly developed fluorescent ligands for the mu opioid recept or and the use of one of these ligands as a label for the cloned mu re ceptor. These ligands provide a new approach for studying the structur al and biophysical nature of opioid receptors. (C) 1997 Elsevier Scien ce Inc.