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

Abstract

The activity pattern of analogues of the enkephalins was determined in four parallel assays, the inhibition of the electrically evoked contraction of the guinea‐pig ileum and mouse vas deferens at 36°C and the inhibition of [3H]‐naltrexone and [3H]‐leucine‐enkephalin binding at 0 to 4°C in homogenates of guinea‐pig brain. The activity pattern was best characterized by the ratio of the potency in the guinea‐pig ileum to that in the mouse vas deferens (G.p.i./M.v.d.) and the ratio of the potency in inhibiting [3H]‐naltrexone binding to that in inhibiting [3H]‐leucine‐enkephalin binding (Nal/Leu). The enkephalins had low G.p.i./M.v.d. (0.02 to 0.09) and low Nal/Leu (0.05 to 0.18) ratios whereas the corresponding values for morphine were 7.0 and 7.5. Analogues obtained by substituting d‐Ala for Gly2and d‐Met or d‐Leu for l‐Met5or l‐Leu5showed only minor changes in G.p.i./M.v.d. (0.01 to 0.11) and in Nal/Leu (0.06 to 0.13) ratios. Analogues in which resistance to enzymatic degradation was brought about by amidation of the C‐terminal carboxylic group or methylation of the amino group of tyrosine or both modifications, had G.p.i./M.v.d. ratios of 1.2 to 5.5 and Nal/Leu ratios of 0.5 to 21. High values (2.1 and 3.4) were found for the potent antinociceptive analogue of Sandoz, Tyr‐d‐Ala‐Gly‐NCH3Phe‐Met(O)‐ol. In the mouse vas deferens, some of the analogues with high G.p.i./M.v.d. and Nal/Leu ratios were tested for antagonism by naloxone and found to require less than the high concentration needed for the natural enkephalins. C57/BL mice, which have a lowered sensitivity to morphine but a normal response to peptides with low G.p.i./M.v.d. and Nal/Leu ratios, had a lowered sensitivity to analogues with high ratios. In the alkaloid‐like series of narcotic analgesic drugs, ketobemidone, levorphanol, methadone, etorphine and the antagonist Mr 2266 had lower Nal/Leu ratios (1.0 to 2.8) than morphine, normorphine, naloxone and naltrexone (8 to 12). It would appear that compounds with low G.p.i./M.v.d. and Nal/Leu ratios interact mainly with δ‐receptors in the brain and peripheral nervous system while compounds with high ratios interact mainly with μ‐receptors. For antinociceptive action μ‐receptors may be more important than δ‐receptors.

Enkephalins modulate inhibitory neuromuscular transmission in circular muscle of human colon via delta‐opioid receptors.

“…The purpose of this study was to confirm that [Met5]enkephalin (MENK) and [Leu5]enkephalin (LENK) act on 8-opioid receptors. To this end, the actions of the 6-opioid receptor-selective agonists enkephalin (DPDPE) and [DAla2, D-Leu5]enkephalin (DADLE), and the 8-opioid receptor-selective antagonist ICI 174,864 were studied (Wiister, Schulz & Herz, 1979;Kosterlitz, Lord, Paterson & Waterfield, 1980;Mosberg, Hurst, Hruby, Gee, Yamamura, Galligan & Burks, 1983;Cotton, Giles, Miller, Shaw & Timms, 1984 (Hoyle, 1987) was used to measure changes in membrane potential in isolated strips of smooth muscle.…”

Comparison of the Binding Characteristics of Tritiated Opiates and Opioid Peptides

“…For the assay of the 6-binding sites, [3H]-D-Ala2-D-Leu5-enkephalin was chosen because it had been shown to have retained an enkephalin-like pattern of activity (Kosterlitz et al, 1980). Further support for the choice of this analogue was obtained by the finding that unlabelled D-Ala2-D-Leu5-enkephalin protected the binding of [3H]-D-Ala2-D-Leu5-enkephalin against the alkylating action of phenoxybenzamine much better than unlabelled dihydromorphine, while the reverse was observed when the binding of [3H]-dihydromorphine had to be protected (Robson & Kosterlitz, 1979).…”

Actions of opioids on primate spinothalamic tract neurons

“…For the present investigation, we wished to examine the actions of other opioid substances. Since there are a number of different opiate receptors, as shown by physiological studies (Martin et al, 1976), binding experiments (Chang et al, 1980;Fields et al, 1980;Kosterlitz et al, 1980;Lord et al, 1977) and smooth muscle bioassays (Hutchinson et al, 1975;Lord et al, 1977;Wtister et al, 1980), we decided to try agents that appear to be agonists for each of several opiate receptor types. Morphine was used as an agonist for the p receptor, dynorphin for the K receptor, methionine enkephalinamide for the 6 receptor, and N-allylnormetazocine (SKF 10047) and phencyclidine for the u receptor (Chavkin et al, 1982;Huidoboro-Toro et al, 198 1;Quirion et al, 198 1;Z&in and Zukin, 1979, 198 1; see also reviews by Martin, 1984;Zieglginsberger, 1984).…”