Selective antagonists at the opiate delta-receptor

The mu rather than the delta subtype of opioid receptors appears to be involved in enkephalin-induced analgesia

The analgesic activity of some opioid peptides which display a relative selectivity for either the mu-receptor subtype, [D-Ala2, MePhe4, Gly-ol5]enkephalin (DAGO) or the delta-receptor subtype. [D-Ala2, D-Leu5]enkephalin (DADLE), [D-Ser2, Leu5]enkephalyl-Thr (DSLET) and [D-Thr2, Leu5]enkephalyl-Thr (DTLET) is highly correlated with their affinity for central or peripheral mu- but not delta-receptors. Moreover their analgesic effects as well as those elicited by degrading enzyme inhibitors (bestatin + thiorphan) of endogenous enkephalins were easily antagonized by naloxone with similar pA2 values but not by the delta-antagonist ICI 154,129. Therefore the analgesia produced by opioid peptides including endogenous enkephalins is likely connected to mu-receptor stimulation. Finally, there was no obvious potentiation by delta-agonists of the analgesia resulting from either administration of the mu-agonist morphine or endogenous enkephalins. This suggested that in the hot plate test, there is no modulation of the effect resulting from mu-receptor stimulation by a delta-receptor interaction. Likewise, enkephalinergic activity such as that due to thiorphan + bestatin does not appear to be regulated through mu- or delta-receptor stimulation.

Synthesis and biological activity of a cyclic pseudohexapeptide analog of somatostatin

A cyclic pseudohexapeptide analog of somatostatin, cyclo(Pro psi[ CH2S ]Phe-D-Trp-Lys-Thr-Phe) was synthesized by solid phase methods and diphenylphosphoryl azide ring closure. The resulting crystalline compound possessed 23% of the growth hormone inhibitory activity of the parent tetradecapeptide and approximately 6% of the activity of the all-amide cyclic hexapeptide analog in spite of the absence of one of two postulated intramolecular hydrogen bonds.

Differential role of the opioid mu and delta receptors in the activation of prolactin (PRL) and growth hormone (GH) secretion by morphine in the male rat

Administration of naloxazone (50 mg/kg i.v.), an irreversible, selective and long acting antagonist of the mu 1 subclass of the opioid receptors, strongly reduced stimulation of PRL secretion by morphine (5.0 mg/kg i.v.) injected 24 hours later into conscious, unrestrained rats. In contrast, the effect of morphine on PRL release was unimpaired in rats treated 24 hours beforehand with either the reversible opioid antagonist naloxone (50 mg/kg i.v.), or the vehicle for naloxazone. A complete suppression of the PRL response to morphine (3.0 mg/kg i.v.) was observed in animals given intraventricular (IVT) injection of beta- funaltrexamine (beta-FNA, 2.5 micrograms), another selective, irreversible and long acting antagonist of the mu receptors, 24 hours beforehand. Neither naloxazone nor beta-FNA had any effect on the activation of GH secretion by morphine, which, however, was conspicuously reduced by ICI 154, 129, a preferential delta receptor antagonist, injected IVT (50 micrograms) 5 minutes before morphine. ICI 154, 129 had no effect on the PRL response to morphine. It is concluded that the PRL stimulating effect of morphine is mediated by the mu receptors, whereas activation of GH probably involves the delta sites.

Multiple opioid receptors in endotoxic shock: evidence for delta involvement and mu-delta interactions in vivo

The use of selective delta and mu opioid antagonists has provided evidence that delta opioid receptors within the brain mediate the endogenous opioid component of endotoxic shock hypotension. The selectivity of these delta and mu antagonists was demonstrated by their differing effects upon morphine analgesia and endotoxic hypotension. The mu antagonist beta-funaltrexamine, at doses that antagonized morphine analgesia, failed to alter shock, whereas the delta antagonist M 154,129: [N,N-bisallyl-Tyr-Gly-Gly-psi-(CH2S)-Phe-Leu-OH] (ICI) reversed shock at doses that failed to block morphine analgesia. Therefore, selective delta antagonists may have therapeutic value in reversing circulatory shock without altering the analgesic actions of endogenous or exogenous opioids. Additional data revealed that prior occupancy of mu binding sites by irreversible opioid antagonists may allosterically attenuate the actions of antagonists with selectivity for delta binding sites. For endogenous opioid systems, this observation provides an opportunity to link in vivo physiological responses with receptor-level biochemical interactions.

ICI 154,129, a delta-opioid receptor antagonist raises seizure threshold in rats

Acute i.c.v. administration of ICI 154,129 (100-600 micrograms), a delta-opioid receptor antagonist, raised the seizure threshold in a dose-related manner in rats exposed to flurothyl, a volatile convulsant. Pretreatment with naloxone or beta-funaltrexamine (beta-FNA) antagonized this effect. Lower doses of ICI 154,129 (12.5-50 micrograms), which did not influence seizure threshold, selectively antagonized the anticonvulsant action of [D-Ala2,D-Leu5]enkephalin (DADLE) in the same procedure. Consequently, it may be inferred that ICI 154,129 at high doses has mu-agonist and at low doses delta-antagonist properties in the rat flurothyl test.

The involvement of mu- and kappa- but not delta-opioid receptors in the body weight gain of suckling rats

The effects of the benzomorphan antagonist Mr 2266 and the selective delta-antagonist ICI 154,129 on the body weight gain of 6-day-old suckling rat pups was observed. Mr 2266 significantly reduced body weight gain in these animals, though ICI 154,129 had no affect on this variable. These findings suggest that mu- and kappa- but probably not delta-opioid receptors are involved in the regulation of ingestive behaviours in infant rats. The results are discussed in relation to the development of opioid-receptor subtypes in the neonatal rat brain.

Opioid peptides and central control of blood pressure

The role of endogenous opioids in central modulation of baroreceptor reflex function has been assessed in rabbits and in man using stable enkephalin analogues and synthetic opiates with a range of mu, delta and kappa opiate receptor agonist specificity. In addition the effects of naloxone, a mu opiate receptor antagonist, have been studied. In rabbits descarboxy-leu-enkephalin (RX783016) given by intracisternal injection reduced baroreflex sensitivity as assessed by heart rate responses to phenylephrine, sodium nitroprusside and controlled haemorrhage. These effects were prevented by intravenous naloxone. Naloxone alone increased the slope of the heart period: mean arterial pressure relationship and thus increased baroreflex sensitivity. In conscious man essentially similar results were found following intravenous dosing with a stable met-enkephalin analogue (DAMME, FK33824) or naloxone with decreases and increases respectively in the sensitivity of baroreflex responses to sodium nitroprusside. In rabbits and man arterial baroreceptor reflexes mediating heart rate responses can be pharmacologically modified by exogenous opiates and may be under some tonic endogenous opiate peptide influences.

Comparison of the influence of the opiate delta receptor antagonist, ICI 154,129, and naloxone on social interaction and behaviour in an open field

The influence of a novel opiate delta receptor antagonist, ICI 154,129, and naloxone on social interaction and on behaviour in an open field were compared. At a dose thought to optimally block the delta receptor, ICI 154,129 produced open field behaviour typical of a non-emotional animal: this drug decreased social investigation and the latency to attack. In contrast, naloxone was largely without effect in these tests.

The stimulation of glycogenolysis and gluconeogenesis in isolated hepatocytes by opioid peptides

The opioid agonists [leucine]enkephalin, [D-Ala2,D-Leu5]enkephalin and dynorphin-(1-13)-peptide, but not morphine, stimulated the conversion of [2-14C]pyruvate into glucose and glycogenolysis when added directly to isolated hepatocytes. Naloxone produced a small but significant inhibition of both the basal and stimulated rate of incorporation of label into glucose but had no effect on the total glucose output by the cells. The effects of the opioid peptides were mediated by a cyclic AMP-independent mechanism.

Inhibition of the antisecretory effects of [D-Ala2,D-Leu5]enkephalin in the guinea-pig ileum by a selective delta opioid antagonist

The novel opioid antagonist [diallyl-Tyr1,(CH2)S-Phe4,Leu5]enkephalin (M154,129) was examined for its ability to block the antisecretory effects of [D-Ala2,D-Leu5]enkephalin (DADLE) in isolated mucosal segments of the guinea-pig ileum, actions mediated predominantly through the delta opiate receptor. DADLE reduced transepithelial potential difference and short-circuit current (ED50 18 nM); these effects were competitively antagonized by M154,129 with a Ke value of 746 nM. These results are consistent with others demonstrating M154,129 is a relatively selective although not highly potent blocker of delta opiate receptors.

Inhibition of an opioid-evoked vagal reflex in rats by naloxone, SMS 201-995 and ICI 154, 129

Intravenous injection of opioid agonists in rats evokes a vagal reflex resulting in a fall in heart rate, and blood pressure. Three opioid antagonists, naloxone, SMS 201-995, and ICI 154,129 were used to assess the nature of the opioid receptors that mediate the vagal reflex. The agonists used were morphine, Tyr-Pro-NMePhe-D-Pro-NH2 (PLO17), and D-Ala2-Leu5-enkephalin (DADL). At challenge doses of morphine, PLO17, and DADL at five times the ED50 for bradycardia, the naloxone ED50 for DADL was nine times greater than that for morphine and PLO17. The pA2 value of naloxone against DADL was significantly less than that for morphine and PLO17. The antagonist properties of SMS 201-995 were similar to those of naloxone. ICI 154,129, a putative delta receptor antagonist, was not, however, selective in its antagonism of opioid bradycardia. Both SMS 201-995 and ICI 154,129, when injected alone, produced changes in heart rate and blood pressure. The cardiovascular actions of the peptide antagonists were not affected by naloxone hydrochloride at doses up to 4 mg/kg i.v.

ACTH-(1-24) and RX 336-M induce excessive grooming in rats through different mechanisms

ACTH-(1-24) (0.03-6 micrograms i.c.v.) and RX 336-M (7,8-dihydro-5',6'-dimethylcyclohex-5'-eno-1',2',8',14 codeinone) (1.5-6 mg/kg i.p.) induce dose-related excessive grooming and 'wet-dog' shaking in rats. In the present study, the grooming associated with these compounds was compared and analyzed pharmacologically. Grooming caused by RX 336-M and by ACTH-(1-24) was antagonized when rats were pretreated with comparable doses of morphine (0.5-4 mg/kg s.c.), however, only ACTH-(1-24)-induced grooming was attenuated by naloxone (1 and 10 mg/kg s.c.). ICI 154,129 (N,N-bisallyl-Tyr-Gly-Gly-psi-(CH2S)-Phe-Leu-OH) (30 mg/kg s.c.), a selective delta-opiate receptor antagonist, was ineffective against both ACTH-(1-24) and RX 336-M. Although haloperidol is known to antagonize grooming elicited by ACTH-(1-24) (e.g., Wiegant et al., 1977, European J. Pharmacol. 41, 343), even a high dose of this neuroleptic agent (5 mg/kg s.c.) only partially attenuated grooming caused by RX 336-M. Tolerance developed to the grooming elicited by RX 336-M, and by ACTH-(1-24), but there was no cross-tolerance. Both agents were active in genetically hypotrichotic rats; and, again in such animals, even after numbing the area caudal to the shoulders with lidocaine. Given the divergent results with naloxone, and, possibly, with haloperidol, and the lack of cross-tolerance, we conclude that the excessive grooming induced in rats by ACTH-(1-24) and by RX 336-M is mediated by different mechanisms.

Pain control by endogenous enkephalins is mediated by mu opioid receptors

The analgesic effects of bestatin and thiorphan, two enzymatic inhibitors protecting endogenous enkephalins from their degradation, and those of DAGO and deltakephalin, respectively mu and delta opioid peptides, are assessed on the electrical stimulation test of the mouse tail. The relative analgesic potency of DAGO and deltakephalin is in good agreement with their relative potency on mu pharmacological assays: inhibition of electrically-induced contractions of guinea-pig ileum, displacement of 3H DAGO on rat brain. Finally, the analgesic effects of DAGO, deltakephalin and bestatin + thiorphan, are antagonized by the mu antagonist naloxone with similar pA2, and they are not modified by the delta antagonist ICI 154, 129. We conclude that only mu and not delta receptors are involved in the analgesic effects of enkephalins.

Multiple opioid receptors: evidence for mu-delta binding site interactions in endotoxic shock

Using antagonists with selectivity for the delta (ICI 154,129) and mu (beta-funaltrexamine) binding sites, evidence was obtained to indicate that delta receptors within the brain mediate the endogenous opioid component of endotoxic hypotension. The therapeutic actions of intravenous ICI 154,129 were dose related, with effective doses between 15-60 mg/kg. Evidence for a functional interaction between mu and delta binding sites was obtained: prior occupancy of the mu binding site by beta-funaltrexamine prevented the usual therapeutic response to the delta antagonist ICI 154,129 in endotoxemic rats. These data indicate that mu and delta binding sites may be a part of the same macromolecular complex which interact through allosteric coupling.

Evidence for delta-receptor involvement in the post-ictal antinociceptive responses to electroconvulsive shock in rats

Studies were conducted with the putative delta-opioid receptor antagonist lCl 154,129 (lCl) and the putative mu-receptor antagonist beta-funaltrexamine (beta-FNA) to investigate their ability to block the acute opioid-like effects of a single electroconvulsive shock (ECS). lCl, but not beta-FNA, attenuated the increase in hot-plate escape latencies seen following ECS. From these data, we conclude that ECS activates endogenous opioids which act upon delta rather than mu receptors to result in an increase in hot-plate escape latencies.

Divergent structure activity relationships in series of enkephalin agonists and cognate antagonists

Diallylation of the amino group of [Leu]enkephalin methyl ester yields a moderately potent, delta-selective opiate receptor antagonist. The diallyl congeners of a larger range of potent mu-and delta-selective enkephalin agonists have been prepared and were found to be weak, non-selective antagonists as assessed by their ability to antagonise the effects of normorphine and [Leu]enkephalin on the field-stimulated mouse vas deferens. Conversely, whereas [Gly3 psi (CH2S)Phe4,Leu5]enkephalin and [Gly3 psi(CH2S)-D-Phe4,Leu5]enkephalin are virtually inactive as opiate agonists the corresponding diallyl analogues are moderately potent, highly selective delta-antagonists. Analogues of diallyl[Leu]enkephalin in which the Gly2 and Gly3 residues have been replaced by D- and L- Ala have also been prepared and tested as delta-receptor antagonists. In addition, the empiric energy program ECEPP has been used to derive eighteen low energy conformations of diallyl[Leu]enkephalin and to estimate the effect of the D- and L-Ala substitutions on the conformer energies. Two conformers were identified for which there was a partial correlation between the variations in conformational energy and delta-antagonist potency.

A selective role for delta-receptors in the regulation of opioid-induced changes in seizure threshold

In an effort to identify delta-receptor-specific properties for opioid modulation of seizure activity, studies were conducted with ICI 154,129, a putative delta-receptor antagonist, in the rat flurothyl test. Rats were pretreated i.c.v. with ICI 154,129 (50 micrograms) which, at this dose, does not alter normal seizure thresholds. Mean seizure thresholds for control groups (i.c.v. saline) ranged between 323-349 sec. In this test, D-Ala2-D-Leu5 enkephalin (20 micrograms, i.c.v.), metkephamid (40 mg/kg, s.c.), and etorphine (20 micrograms/kg, s.c.) raised seizure thresholds by 117, 128, and 140% of control, respectively. Meperidine (25 mg/kg, s.c.) lowered seizure thresholds by 14% less than control. Pretreatment with ICI 154,129 failed to antagonize the proconvulsant action of meperidine or the anticonvulsant and behavioral depressant actions of etorphine. The increases in seizure threshold produced by DADL and metkephamid (two delta-directed ligands) were significantly attenuated by ICI 154,129. However, the DADL-induced wet-shakes, rigid immobility, and behavioral depression were insensitive to ICI 154,129. These data indicate that ICI 154,129 possesses delta-receptor antagonistic properties in this in vivo model of seizure activity. Furthermore, since only the changes in seizure threshold were antagonized, it may be inferred that opioid-induced behavioral depression and DADLE wet-shakes are not a function of delta-receptor activity.

In vivo studies with ICI 154,129, a putative delta receptor antagonist

We studied the in vivo pharmacology of ICI 154,129, a new antagonist that is claimed to show selectivity for delta opiate receptors. At s.c. doses of 30 and 100 mg/kg, ICI 154,129 had no marked effect on the gastrointestinal transit of a charcoal meal in mice. In this test, ICI 154,129 reversed the inhibitory action of metkephamid (a proposed delta receptor agonist) but not that of levorphanol. ICI 154,129 was proconvulsant in the mouse picrotoxin potentiation test; the dose-response curve had a low ceiling and was biphasic. Naloxone (1 mg/kg, s.c.) enhanced the proconvulsant action of ICI 154,129 (40 mg/kg, s.c.) by an unknown mechanism.