title: Selective opioid agonist and antagonist competition for [3H]-naloxone binding in amphibian spinal cord.
abstract: Opioids elicit antinociception in mammals through three distinct types of receptors designated as mu, kappa and delta. However, it is not clear what type of opioid receptor mediates antinociception in non-mammalian vertebrates. Radioligand binding techniques were employed to characterize the site(s) of opioid action in the amphibian, Rana pipiens. Naloxone is a general opioid antagonist that has not been characterized in Rana pipiens. Using the non-selective opioid antagonist, [3H]-naloxone, opioid binding sites were characterized in amphibian spinal cord. Competitive binding assays were done using selective opioid agonists and highly-selective opioid antagonists. Naloxone bound to a single-site with an affinity of 11.3 nM and 18.7 nM for kinetic and saturation studies, respectively. A B(max) value of 2725 fmol/mg protein in spinal cord was observed. The competition constants (K(i)) of unlabeled mu, kappa and delta ranged from 2.58 nM to 84 microM. The highly-selective opioid antagonists yielded similar K(i) values ranging from 5.37 to 31.1 nM. These studies are the first to examine opioid binding in amphibian spinal cord. In conjunction with previous behavioral data, these results suggest that non-mammalian vertebrates express a unique opioid receptor which mediates the action of selective mu, kappa and delta opioid agonists.
authors.affiliation: Department of Pharmacology and Physiology, Oklahoma State University, College of Osteopathic Medicine, 1111 W. 17th Street, Tulsa, OK, 74107, USA.
authors.forename: L C
authors.initials: LC
authors.lastname: Newman
authors.forename: D R
authors.initials: DR
authors.lastname: Wallace
authors.forename: C W
authors.initials: CW
authors.lastname: Stevens
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects* / Q000502:physiology; D007700:Kinetics; D023421:Models, Animal; D009270:Naloxone / Q000493:pharmacokinetics*; D009292:Narcotic Antagonists / Q000494:pharmacology*; D009474:Neurons / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D009619:Nociceptors / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D010146:Pain / Q000188:drug therapy / Q000378:metabolism / Q000503:physiopathology; D011894:Rana pipiens / Q000033:anatomy & histology / Q000378:metabolism*; D011957:Receptors, Opioid / Q000187:drug effects* / Q000378:metabolism; D013116:Spinal Cord / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D014316:Tritium
chemical_list: D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D011957:Receptors, Opioid; D014316:Tritium; D009270:Naloxone