Newman L C, Wallace D R, Stevens C W
Department of Pharmacology and Physiology, Oklahoma State University, College of Osteopathic Medicine, 1111 W. 17th Street, Tulsa, OK, 74107, USA.
Brain Res. 2000 Nov 24;884(1--2):184-91. doi: 10.1016/s0006-8993(00)02967-x.
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.
阿片类药物通过三种不同类型的受体（即μ、κ和δ受体）在哺乳动物中引发镇痛作用。然而，尚不清楚哪种类型的阿片受体介导非哺乳动物脊椎动物的镇痛作用。采用放射性配体结合技术来表征阿片类药物在两栖动物牛蛙（Rana pipiens）中的作用位点。纳洛酮是一种尚未在牛蛙中进行表征的通用阿片拮抗剂。使用非选择性阿片拮抗剂[³H] - 纳洛酮，在两栖动物脊髓中表征阿片结合位点。使用选择性阿片激动剂和高选择性阿片拮抗剂进行竞争性结合测定。纳洛酮与单一位点结合，动力学研究和饱和研究的亲和力分别为11.3 nM和18.7 nM。在脊髓中观察到B(max)值为2725 fmol/mg蛋白质。未标记的μ、κ和δ的竞争常数（K(i)）范围为2.58 nM至84 μM。高选择性阿片拮抗剂产生的K(i)值相似，范围为5.37至31.1 nM。这些研究首次检测了两栖动物脊髓中的阿片结合。结合先前的行为数据，这些结果表明非哺乳动物脊椎动物表达一种独特的阿片受体，该受体介导选择性μ、κ和δ阿片激动剂的作用。
