Comparison of the Binding Characteristics of Tritiated Opiates and Opioid Peptides

Abstract

The binding capacities of the natural methionine-and leucine-enkephalins measured at 0°C were 5 to 6 pmol/g brain. At this temperature, the binding capacity of dihydromorphine, D-Ala2-D-Leu5-enkephalin and of the two enkephalin amides was only slightly lower than at 25°C. 4 In assays in which unlabelled ligand competed with the same labelled ligand, the inhibition constants (K,) were equal to or not more than twice as large as the equilibrium dissociation constant (KD) determined in saturation assays. In contrast, the K, of unlabelled dihydromorphine againstmorphine were about 20 times higher than the respective KD values.5 When for a given compound the ratio of the K, value against [3H]-D-Ala2-D-Leu5-enkephalin to the K, value against [3H]-dihydromorphine (discrimination ratio) is calculated, a high value indicates selectivity in favour of the t-receptor and a low value selectivity in favour of the 6-receptor. The most selective p-agonist known so far is normorphine with a discrimination ratio of 70 and the most selective 6-agonist is D-Ala2-D-Leu5-enkephalin with a ratio of 0.11. The selectivity of the known antagonists is in favour of the j-receptor, since their discrimination ratios are larger than 1, varying between 10 for naloxone and 4 for Mr 2266.

Determination of the receptor selectivity of opioid agonists in the guinea‐pig ileum and mouse vas deferens by use of β‐funaltrexamine

“…Concentration-response curves to DSLET, DAGO, normorphine, fentanyl, FK33824, dihydrocodeine and pentazocine were shifted to the right indicating that they are I-selective opioid agonists in this tissue. This is supported by evidence from tissue preparations in vitro and from binding studies (Lord et al, 1977;Gacel et al, 1980;Gillan et al, 1980;Leslie et al, 1980). However, the small shift for pentazocine may indicate that although it is "-receptor selective, it also has appreciable activity at K-opioid receptors.…”

Effects of Changes in the Structure of Enkephalins and of Narcotic Analgesic Drugs on Their Interactions With Μ‐ and Δ‐receptors

“…Thus, in the biophase of the guinea-pig ileum and the mouse vas deferens, there will be considerable degradation of the natural enkephalins but not, or at least to a much lesser extent, of the analogues with D-alanine in position 2 which are protected aginst the action of aminopeptidases. From the data presented it is likely that the tritiated D-Ala2-D-Met'-enkephalin or D-Ala2-D-Leu5-enkephalin may be the ligands most suitable for the investigation of the enkephalin binding sites since they are sufficiently stable for binding assays to be performed at 25°C (Gillan, Kosterlitz & Paterson, 1979).…”

Alkylation with β‐funaltrexamine suggests differences between μ‐opioid receptor systems in guinea‐pig brain and myenteric‐plexus

“…Homogenates of tissues, either strips of myenteric plexus longitudinal muscle from the ileum or whole brain (without cerebellum), were prepared in Tris buffer (pH 7.4, 50mM), or Krebs-HEPES, essentially as described by Kosterlitz and colleagues (Gillan et al, 1980;Corbett et al, 1985 (Handa et al, 1981 (Corbett et al, 1985). Assuming affinities of [3H]-DAMGO C-1 Macmillan Press Ltd, 1991 p-OPIOID RECEPTOR SYSTEMS AND /i-FUNALTREXAMINE 719 for the p-site in myenteric plexus of 2.27 nM (Corbett et al, 1985) and for the b-and K-sites of 407 nM (Cotton et al, 1985) and 4960 (Kosterlitz et al, 1981) respectively and a p: 6 : K binding site ratio of 25%: 26%: 49% (Corbett et al, 1985) (McPherson, 1985).…”