Opioid peptides were assayed by inhibition of 3H-naloxone and 3H-leu-enkephalin binding in brain homogenates and by depression of contractions of the guinea pig ileum and mouse vas deferens. We conclude that the opioid peptidergic system has agonists of different characteristics which interact with more than one type of receptor.

Evidence has been adduced for the view that the morphiness receptor or receptors in the myenteric plexus of guinea pig ileum are very similar to the brain receptors that mediate the analgesic action of the narcotic alagesics. Such an in vitro model is very suitable for the study of structure-activity relationships because the angonist and antagonist activities of compounds can readily be assessed. One example is the investigation of the effects of changes at the N atom. Another point of interest is the role of substitutions at the C14 atom in morphine and morphinans. Such alterations in structure may have a profound effect on the relative agonist and antagonist activities and may convert a drug with dual agonist and antagonist actions into an antagonist devoid of agonist action.

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.

No abstract

Four benzomorphans which have potent antinociceptive activity in the hot‐plate and writhing tests in the mouse but do not suppress or precipitate withdrawal symptoms in the morphine‐dependent monkey, have been examined for their pharmacological actions in the guinea‐pig ileum and mouse vas deferens. In the guinea‐pig ileum their agonist potencies are 1.5 to 400 times greater than that of normorphine or morphine whereas in the mouse vas deferens their potencies relative to morphine are 0.3 to 100. They exhibit no antagonist activity in either preparation. Benzomorphans which substitute for morphine in the morphine‐dependent monkey do not show such differences between their relative potencies in the guinea‐pig ileum and mouse vas deferens. The relative potencies of the four benzomorphans to inhibit stereospecific [3H]‐dihydromorphine binding by membrane fragments from rat brain, are more closely related to their relative agonist potencies in the mouse vas deferens than to those found in the guinea‐pig ileum. In order to antagonize the agonist actions of these benzomorphans, naloxone is required in concentrations which are 3 to 7 times higher than those needed for the antagonism of normorphine or morphine or of benzomorphans which suppress abstinence in morphine‐dependent monkeys. It may be possible to use the three assays, namely, ratio of relative agonist potency in mouse vas deferens to that in guinea‐pig ileum, ratio of relative agonist potency to relative affinity to opiate receptors and the concentration of naloxone required for antagonism, for the prediction of the potential of new compounds to produce physical dependence.