Further demonstration of kappa opioid binding sites in the brain: evidence for heterogeneity.

By selectively blocking cross-interferences from other types of binding sites, a binding site which likely represents kappa opioid binding sites was obtained in the guinea-pig brain suspension of the particulate fraction. Selective ligands for mu, sigma, delta and epsilon opioid binding sites were poor inhibitors for inhibiting [3H]ethylketocyclazocine binding to this site, whereas kappa opioids like oxilorphan, dynorphin(1-13), ethylketocyclazocine, butorphanol, cyclazocine, ketocyclazocine, tifluadom, nalorphine, pentazocine, U-50-488, nalbuphine and naloxone were potent ligands. Buprenorphine, generally believed to be a mu opiate, was the most potent inhibitor at the kappa site. Scatchard analysis of the saturation curve of [3H]ethylketocyclazocine binding revealed two subtypes of kappa binding sites: a high-affinity site and a low-affinity site with Kd = 0.7 and 78 nM and maximum binding = 22 and 101 fmol/mg of protein, respectively. Analysis of the inhibition curves suggested that tifluadom may be a selective ligand for the high-affinity site and that dynorphin(1-13) and U-50-488 may bind preferentially the high-affinity site but still possess appreciable affinity for the low-affinity site. This study demonstrates a selective assay for kappa opioid binding sites and indicates a possibility of the heterogeneity of kappa opioid binding sites in the brain.