Interaction between the mu-agonist dermorphin and the delta-agonist [D-Ala2, Glu4]deltorphin in supraspinal antinociception and delta-opioid receptor binding

Sensitivity to μ-opioid receptor-mediated anti-nociception is determined by cross-regulation between μ- and δ-opioid receptors at supraspinal level

BACKGROUND AND PURPOSE

The perception of pain and its inhibition varies considerably between individuals, and this variability is still unexplained. The aim of the present study is to determine whether functional interactions between opioid receptors are involved in the inter-individual variability in the sensitivity to μ-opioid receptor agonists.

EXPERIMENTAL APPROACH

Anti-nociceptive tests, radioligand binding, stimulation of [(35) S]GTP-γ-S binding, inhibition of cAMP production and co-immunoprecipitation experiments were performed in two strains of rat (Sprague-Dawley bred at our university - SDU - and Wistar) that differ in their sensitivity to opioids.

KEY RESULTS

The increased anti-nociceptive potency of µ-opioid receptor agonists in SDU rats was reversed by the δ-opioid receptor antagonist, naltrindole. Inhibition of the binding of [(3) H] naltrindole by µ-opioid receptor agonists was different in brain membranes from SDU and Wistar rats. Differences were also evident in the effect of δ-opioid receptor ligands on the binding of [(35) S]GTP-γ-S stimulated by µ-opioid receptors agonists. No strain-related differences were detected in spinal cord membranes. The potency of morphine to inhibit cAMP production in brain membranes varied between the strains, in the presence of deltorphin II and naltrindole. Co-immunoprecipitation experiments demonstrated that δ-opioid receptors were associated with μ-opioid receptors to a higher extent in brain synaptosomal fractions from SDU than in those from Wistar rats.

CONCLUSIONS AND IMPLICATIONS

There was increased supraspinal cross-talk between μ and δ-opioid receptors in SDU, as compared with Wistar rats. This was related to an enhanced sensitivity to anti-nociception induced by µ-opioid receptor agonists.

DPI-3290 [(+)-3-((α-R)-α-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N-(3-fluorophenyl)-N-methylbenzamide]. II. A Mixed Opioid Agonist with Potent Antinociceptive Activity and Limited Effects on Respiratory Function

Allyl-2,5-dimethyl-1-piperazines have been of interest as analgesic agents for the management of moderate-to-severe pain. In this study, we compared the antinociceptive properties and respiratory depressant activity of one such agent, (+)-3-((alpha-R)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N-(3-fluorophenyl)-N-methylbenzamide (DPI-3290), with those of established narcotic analgesics, morphine and fentanyl. Intravenous administration of DPI-3290 in conscious laboratory rats increased antinociception in a dose-dependent manner with a corresponding ED(50) value of 0.05 +/- 0.0072 mg/kg. Simultaneous measurement of arterial blood gas in animals treated with DPI-3290 demonstrated dose-dependent increases in pCO2 with an ED(50) value of 0.91 +/- 0.22 mg/kg. In comparison, morphine and fentanyl increased antinociception in rats with ED(50) values of 2.01 +/- 0.0005 and 0.0034 +/- 0.00024 mg/kg, respectively, and the ED(50) value for morphine-induced changes in pCO2 was 4.23 +/- 0.72 mg/kg, whereas the ED(50) value for fentanyl-induced changes in pCO2 was 0.0127 +/- 0.0035 mg/kg. A separate series of experiments were designed to examine the effects of DPI-3290 on mu-opioid receptor induced antinociception and hypercapnia. Intravenous bolus doses of DPI-3290 that ranged from 0.2 to 1.0 mg/kg had no effect on antinociception mediated by alfentanil (2 microg/kg/min i.v.) but reduced hypercapnia by approximately 50%. Results from these studies demonstrate the equivalent antinociceptive efficacy of DPI-3290, morphine, and fentanyl but dramatic differences in the hypercapnia that antinociceptive doses of these drugs produce. When measured simultaneously, DPI-3290 had an 18.2-fold difference in the ratio comparing the ED(50) value for antinociception with the ED(50) value for changes in pCO2; this ratio was 2.1 for morphine and 3.7 for fentanyl. Furthermore, DPI-3290 reduced the alfentanil-mediated hypercapnia without any effect on antinociception. Together, the balanced opioid agonist activity of DPI-3290 may provide a means of powerful analgesia while mitigating the mu-opioid receptor-mediated hypercapnia.

Local μ and δ opioid receptors regulate amphetamine-induced behavior and neuropeptide mRNA in the striatum

The purpose of this study was to investigate the role that mu and delta opioid receptor blockade has upon stimulant-induced behavior and neuropeptide gene expression in the striatum. Acute administration of amphetamine (2.5 mg/kg i.p.) caused an increase in behavioral activity and preprodynorphin, substance P, and preproenkephalin mRNA expression. Intrastriatal infusion of the mu opioid antagonist, H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP), or the delta opioid antagonist, H-Tyr-Tic[CH(2)NH]-Phe-Phe-OH (TIPPpsi), significantly decreased amphetamine-induced vertical activity. However, only CTAP reduced amphetamine-induced distance traveled. Quantitative in situ hybridization histochemistry revealed that CTAP blocked amphetamine-induced preprodynorphin and substance P mRNA. However, preproenkephalin mRNA levels in the dorsal striatum were increased to the same extent by CTAP, amphetamine, or a combination of the two drugs. In contrast, TIPPpsi significantly decreased amphetamine-induced mRNA expression of all three neuropeptides. These data indicate that both mu and delta receptor subtypes differentially regulate amphetamine-induced behavior and neuropeptide gene expression in the rat striatum.

Opioid peptides: synthesis and biological properties of [(Nγ-glucosyl,Nγ-methoxy)-α,γ-diamino-(S)-butanoyl]4-deltorphin-1-neoglycopeptide and related analogues

The [D-Ala2]deltorphin 1 sequence in which the aspartic acid residue is replaced by the N gamma-OCH3-alpha, gamma-diamino (S) butanoyl residue was synthesized using the Fmoc-chemistry-based solid phase procedure. The resulting deltorphin analogue was chemoselectively glucosylated by reaction with unprotected D-glucose (Glc). The Asn4-, (2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-beta-D-galactopyranosyl)-Asn4- and the (2-acetamido-2-deoxy-D-galactopyranosyl)-Asn4-deltorphin I were also prepared for comparison. The affinity of the new compounds for the delta-opioid receptor was expressed by the inhibition constant (Ki) of the binding of the delta-receptor selective ligand [3H]naltrindole (NTI) to rat brain membrane preparations. The in vitro biological activity of the synthetic peptides was compared with that of the mu-opioid receptor agonist dermorphin in guinea pig ileum (GPI) preparations and with that of the delta-opioid receptor agonist deltorphin I in mouse vas deferens (MVD) preparations. The substitution of Asp4 with Asn failed to affect drastically the Ki and IC50 values for delta-sites, suggesting that an electrostatic interaction does not play an essential role in the binding to delta-opioid sites. The steric hindrance of the side chain of the residue in position 4 affects binding to delta-sites. The increase of the Ki value is smaller when the sugar-peptide linkage involves the gamma-nitrogen of the Dab residue in comparison with the Asn amide side chain.

Synthesis, conformation and biological activity of dermorphin and deltorphin I analogues containingN-alkylglycine in place of residues in position 1, 3, 5 and 6

Syntheses are described of new dermorphin and [D-Ala2]deltorphin I analogues in which the phenylalanine, the tyrosine or the valine residues have been substituted by the corresponding N-alkylglycine residues. Structural investigations by CD measurements in different solvents and preliminary pharmacological experiments were carried out on the resulting peptide-peptoid hybrids. The contribution from aromatic side chain residues is prominent in the CD spectra of dermorphin analogues and the assignment of a prevailing secondary structure could be questionable. In the CD spectra of deltorphin analogues the aromatic contribution is lower and the dichroic curves indicate the predominance of random conformer populations. The disappearance of the aromatic contribution in the [Ntyr1,D-Ala2]-deltorphin spectrum could be explained in terms of high conformational freedom of the N-terminal residue. The kinetics of degradation of the synthetic peptoids digestion by rat and human plasma enzymes were compared with that of [Leu5]-enkephalin. The binding to opioid receptors was tested on crude membrane preparations from CHO cells stably transfected with the mu- and delta-opioid receptors. The biological potency of peptoids was compared with that of dermorphin in GPI preparations and with that of deltorphin I in MVD preparations. All the substitutions produced a dramatic decrease in the affinity of the peptide-peptoid hybrids for both the mu- and delta-opioid receptors. Nval5 and/or Nval6 containing hybrids behaved as mu-opioid receptor agonists and elicit a dose-dependent analgesia (tail-flick test) when injected i.c.v. in rats.

Synergy between mu opioid ligands: evidence for functional interactions among mu opioid receptor subtypes

Pharmacological differences among mu opioid drugs have been observed in in vitro and in vivo preclinical models, as well as clinically, implying that all mu opioids may not be working through the same mechanism of action. Here we demonstrate analgesic synergy between L-methadone and several mu opioid ligands. Of the compounds examined, L-methadone selectively synergizes with morphine, morphine-6beta-glucuronide, codeine, and the active metabolite of heroin, 6-acetylmorphine. Morphine synergizes only with L-methadone. In analgesic assays, D-methadone was inactive alone and did not enhance morphine analgesia when the two were given together, confirming that L-methadone was not acting through N-methyl-D-aspartate mechanisms. Both L-methadone and morphine displayed only additive effects when paired with oxymorphone, oxycodone, fentanyl, alfentanyl, or meperidine. Although it displays synergy in analgesic assays, the L-methadone/morphine combination does not exhibit synergy in the gastrointestinal transit assay. This analgesic synergy of L-methadone with selective mu opioid drugs and the differences in opioid-mediated actions suggest that these drugs may be acting via different mechanisms. These findings provide further evidence for the complexity of the pharmacology of mu opioids.

Altered nucleus accumbens circuitry mediates pain-induced antinociception in morphine-tolerant rats

We investigated the effect of chronic administration of morphine on noxious stimulus-induced antinociception (NSIA) produced by intraplantar capsaicin injection. In the untreated (naive) rat, we previously found that NSIA depends on activation of dopamine, nicotinic acetylcholine, and mu- and delta-opioid receptors in nucleus accumbens. Rats chronically implanted with subcutaneous morphine pellets demonstrated tolerance to the antinociceptive effects of acute systemic morphine administration but did not show cross-tolerance to NSIA. Morphine pretreatment, however, significantly reduced NSIA dependence on intra-accumbens opioid receptors but not on dopamine or nicotinic acetylcholine receptors. As observed in naive rats, intra-accumbens microinjection of either the dopamine receptor antagonist flupentixol or the nicotinic receptor antagonist mecamylamine blocked NSIA in rats tolerant to the antinociceptive effects of morphine, but, in contrast to naive rats, intra-accumbens microinjection of either the mu-receptor antagonist Cys2,Tyr3,Orn5,Pen7 amide or the delta-receptor antagonist naltrindole failed to block NSIA. These findings suggest that although NSIA is dependent on nucleus accumbens opioid receptors in the naive state, this dependence disappears in rats tolerant to the antinociceptive effects of morphine, which may account for the lack of NSIA cross-tolerance. In separate experiments, intra-accumbens extracellular dopamine levels were measured using microdialysis. Dopamine levels increased after either capsaicin or systemic morphine administration in naive rats but only after capsaicin administration in morphine pretreated rats. Thus, intra-accumbens dopamine release paralleled antinociceptive responses in naive and morphine pretreated rats.

mu/delta Cooperativity and opposing kappa-opioid effects in nucleus accumbens-mediated antinociception in the rat

We previously demonstrated that noxious peripheral stimulation (e.g. subdermal capsaicin injection in the hind paw) produces antinociception that is mediated by opioid receptors in nucleus accumbens. The current study used the trigeminal jaw-opening nociceptive reflex responses in the rat to assess the role of intra-accumbens mu-, delta- and kappa-opioid receptors in the antinociceptive effect of noxious stimulation and intra-accumbens opioid agonism. Whilst intra-accumbens injection of either the mu-receptor-selective antagonist Cys2,Tyr3,Orn5,Pen7amide (CTOP) or the delta-receptor-selective antagonist naltrindole blocked capsaicin-induced antinociception, neither the selective mu-agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO; 150 or 300 ng) nor the selective delta-agonist D-Pen2,5-enkephalin (DPDPE; 150 or 300 ng) alone induced antinociception. Simultaneous injection of DAMGO and DPDPE (150 ng each), however, produced significant antinociception. Capsaicin-induced antinociception was not blocked by the selective kappa-receptor antagonist nor-binaltorphimine, but was blocked by the kappa-agonist U69,593. U69,593 also antagonized the antinociceptive effect of the DAMGO/DPDPE combination. Thus, in nucleus accumbens, mu- and delta- but not kappa-opioid receptors contributed to capsaicin-induced antinociception; selective activation of individual receptor subtypes was insufficient, but coactivation of mu- and delta-opioid receptors induced antinociception, and kappa-receptors appeared to play an antianalgesic role in nucleus accumbens.

Preferential cytoplasmic localization of delta-opioid receptors in rat striatal patches: comparison with plasmalemmal mu-opioid receptors

The activation of delta-opioid receptors (DORs) in the caudate-putamen nucleus (CPN) produces regionally distinct changes in motor functions, many of which are also influenced by opioids active at micro-opioid receptors (MORs). These actions most likely occur in MOR-enriched patch compartments in the CPN. To determine the functional sites for DOR activation and potential interactions involving MOR in these regions, immunoperoxidase and immunogold-silver labeling methods were applied reversibly for the ultrastructural localization of DOR and MOR in single rat brain sections containing patches of the CPN. DOR immunoreactivity was commonly seen within the cytoplasm of spiny and aspiny neurons, many of which also expressed MOR. In dendrites and spines, DOR labeling was preferentially localized to membranes of the smooth endoplasmic reticulum and spine apparatus, whereas MOR showed a prominent plasmalemmal distribution. DOR- and/or MOR-labeled spines received asymmetric, excitatory synapses, some of which showed notable perforations, suggesting the involvement of these receptors in activity-dependent synaptic plasticity. DORs were more frequently detected than were MORs within axon terminals that formed either asymmetric synapses with spine heads or symmetric synapses with spine necks. Our results suggest that in striatal patches, DORs, often in cooperation with MORs, play a direct modulatory role in controlling the postsynaptic excitability of spines, whereas presynaptic neurotransmitter release onto spines is mainly influenced by DOR activation. In comparison with MOR, the prevalent association of DOR with cytoplasmic organelles that are involved in intracellular trafficking of cell surface proteins suggests major differences in availability of these receptors to extracellular opioids.

Preferential cytoplasmic localization of delta-opioid receptors in rat striatal patches: comparison with plasmalemmal mu-opioid receptors

The activation of delta-opioid receptors (DORs) in the caudate-putamen nucleus (CPN) produces regionally distinct changes in motor functions, many of which are also influenced by opioids active at micro-opioid receptors (MORs). These actions most likely occur in MOR-enriched patch compartments in the CPN. To determine the functional sites for DOR activation and potential interactions involving MOR in these regions, immunoperoxidase and immunogold-silver labeling methods were applied reversibly for the ultrastructural localization of DOR and MOR in single rat brain sections containing patches of the CPN. DOR immunoreactivity was commonly seen within the cytoplasm of spiny and aspiny neurons, many of which also expressed MOR. In dendrites and spines, DOR labeling was preferentially localized to membranes of the smooth endoplasmic reticulum and spine apparatus, whereas MOR showed a prominent plasmalemmal distribution. DOR- and/or MOR-labeled spines received asymmetric, excitatory synapses, some of which showed notable perforations, suggesting the involvement of these receptors in activity-dependent synaptic plasticity. DORs were more frequently detected than were MORs within axon terminals that formed either asymmetric synapses with spine heads or symmetric synapses with spine necks. Our results suggest that in striatal patches, DORs, often in cooperation with MORs, play a direct modulatory role in controlling the postsynaptic excitability of spines, whereas presynaptic neurotransmitter release onto spines is mainly influenced by DOR activation. In comparison with MOR, the prevalent association of DOR with cytoplasmic organelles that are involved in intracellular trafficking of cell surface proteins suggests major differences in availability of these receptors to extracellular opioids.

Pharmacology of amphibian opiate peptides

In 1980 the skin of certain frogs belonging to the genus Phyllomedusinae was found to contain two new peptides that proved to be selective mu-opioid agonists, and named dermorphins. Since 1987 deltorphins, a family of highly selective delta-opioid peptides were identified either by cloning of the cDNA from frog skins or isolation of the peptides. The distinctive feature of opioid peptides is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe. The discovery of the amphibian opiate peptides, provided new insights into the functional role of the mu- and delta-opiate systems. It also provided models for novel analgesics with enhanced therapeutic benefits and reduced toxicity.

Intracerebroventricular injection of clonidine releases beta-endorphin to induce mucosal protection in the rat

The possibility that the endogenous opioid system could be involved in the central nervous system (CNS)-mediated gastroprotective effect of clonidine was investigated. Intracerebroventricularly (i.c.v.) injected clonidine (470 pmol/rat) inhibited the gastric mucosal lesions induced by (orally administered) acidified ethanol in a significant manner in the rat. The gastroprotective effect of the centrally administered clonidine was antagonised by i.c.v. or intracisternally (i.c.) administered presynaptic alpha-2 adrenoceptor antagonist, yohimbine; the non-selective opioid receptor antagonist, naloxone; and the delta opioid receptor antagonist naltrindole. These results suggest that an interaction between central alpha-2 adrenoceptors and endogenous opioid systems is involved in mediating the mucosal protective effect. beta-endorphin antiserum (i.c.) also antagonised the gastroprotection induced by intracerebroventricularly injected clonidine indicating that beta-endorphin release is likely to be a key factor in the gastroprotective effect of clonidine. Furthermore, the i.c.v. or i.c. injection of beta-endorphin produced a potent gastroprotection in the picomolar range. The mucosal protective effect of clonidine was abolished after vagotomy indicating that the central effect may be conveyed to the periphery by vagal efferents. Since atropine (1 mg/kg i.v.) failed to modify, but hexamethonium (10 mg/kg i.v.) antagonised the gastroprotective effect of clonidine, it would appear that in the periphery nicotinic, but not muscarinic, cholinergic receptors are likely to be involved in the mucosal protective effect of clonidine. In conclusion, clonidine (i.c.v.) induces gastroprotective action by releasing an endogenous opioid substance - most likely beta-endorphin - in the rat. The clonidine-induced central gastroprotection requires the integrity of vagal pathway; cholinergic nicotinic - but not muscarinic - receptors might mediate the effect in the periphery.

Effects of antisense oligonucleotides on brain delta-opioid receptor density and on SNC80-induced locomotor stimulation and colonic transit inhibition in rats

1. To reduce the density of delta-opioid receptor protein, five antisense phosphorothioate oligodeoxynucleotides (aODN), targeting the three exons of rat delta-opioid receptor mRNA (DOR), were injected twice daily for 4 days or continuously infused for 7 days into brain lateral ventricles (i.c.v.) of Sprague-Dawley rats. Rats acting as controls were infused or injected with a mismatch sequence (mODN) of each aODN. The density of opioid receptors in rat brain membranes was measured by saturation binding experiments using selective ligands for delta, mu and kappa opioid receptors. 2. aODNs injected twice a day for 4 days left rat brain delta-opioid receptor density unchanged. The ODN targeting the DOR nucleotide sequence 280 - 299 (aODN280 - 299, exon 2), decreased brain delta-opioid receptor density significantly more than aODNs targeting exon 1 (aODN239 - 258), exon 2 (aODN361 - 380), or exon 3 (aODN741 - 760) (to 52% vs 79, 72, and 68%). None of the aODNs to the DOR changed the brain density of mu- or k-opioid receptors. 3. When in a novel environment (but not when kept in their home cages), the locomotor activity of aODN280 - 299 treated rats was significantly lower than that of saline or mODN treated rats. The delta-opioid agonist SNC80 (5 mg kg-1, s.c.) significantly and potently stimulated locomotion and delayed colonic propulsion in saline- and mODN-infused rats, but left motor behaviour and colonic transit of delta-knockdown rats unchanged. 4. The baseline nociceptive threshold and the antinociceptive response to morphine were unchanged in delta-knockdown rats.

delta opioid receptor modulation of several voltage-dependent Ca(2+) currents in rat sensory neurons

Endogenous enkephalins and delta opiates affect sensory function and pain sensation by inhibiting synaptic transmission in sensory circuits via delta opioid receptors (DORs). DORs have long been suspected of mediating these effects by modulating voltage-dependent Ca(2+) entry in primary sensory neurons. However, not only has this hypothesis never been validated in these cells, but in fact several previous studies have only turned up negative results. By using whole-cell current recordings, we show that the delta enkephalin analog [D-Ala(2), D-Leu(5)]-enkephalin (DADLE) inhibits, via DORs, L-, N-, P-, and Q-high voltage-activated Ca(2+) channel currents in cultured rat dorsal root ganglion (DRG) neurons. The percentage of responding cells was remarkably high (75%) within a novel subpopulation of substance P-containing neurons compared with the other cells (18-35%). DADLE (1 microM) inhibited 32% of the total barium current through calcium channels (I(Ba)). A delta (naltrindole, 1 microM), but not a mu (beta-funaltrexamine, 5 microM), antagonist prevented the DADLE response, whereas a DOR-2 subtype (deltorphin-II, 100 nM), but not a DOR-1 (DPDPE, 1 microM), agonist mimicked the response. L-, N-, P-, and Q-type currents contributed, on average, 18, 48, 14, and 16% to the total I(Ba) and 19, 50, 26, and 20% to the DADLE-sensitive current, respectively. The drug-insensitive R-type current component was not affected by the agonist. This work represents the first demonstration that DORs modulate Ca(2+) entry in sensory neurons and suggests that delta opioids could affect diverse Ca(2+)-dependent processes linked to Ca(2+) influx through different high-voltage-activated channel types.

delta opioid receptor modulation of several voltage-dependent Ca(2+) currents in rat sensory neurons

Endogenous enkephalins and delta opiates affect sensory function and pain sensation by inhibiting synaptic transmission in sensory circuits via delta opioid receptors (DORs). DORs have long been suspected of mediating these effects by modulating voltage-dependent Ca(2+) entry in primary sensory neurons. However, not only has this hypothesis never been validated in these cells, but in fact several previous studies have only turned up negative results. By using whole-cell current recordings, we show that the delta enkephalin analog [D-Ala(2), D-Leu(5)]-enkephalin (DADLE) inhibits, via DORs, L-, N-, P-, and Q-high voltage-activated Ca(2+) channel currents in cultured rat dorsal root ganglion (DRG) neurons. The percentage of responding cells was remarkably high (75%) within a novel subpopulation of substance P-containing neurons compared with the other cells (18-35%). DADLE (1 microM) inhibited 32% of the total barium current through calcium channels (I(Ba)). A delta (naltrindole, 1 microM), but not a mu (beta-funaltrexamine, 5 microM), antagonist prevented the DADLE response, whereas a DOR-2 subtype (deltorphin-II, 100 nM), but not a DOR-1 (DPDPE, 1 microM), agonist mimicked the response. L-, N-, P-, and Q-type currents contributed, on average, 18, 48, 14, and 16% to the total I(Ba) and 19, 50, 26, and 20% to the DADLE-sensitive current, respectively. The drug-insensitive R-type current component was not affected by the agonist. This work represents the first demonstration that DORs modulate Ca(2+) entry in sensory neurons and suggests that delta opioids could affect diverse Ca(2+)-dependent processes linked to Ca(2+) influx through different high-voltage-activated channel types.

Differential knockdown of delta-opioid receptor subtypes in the rat brain by antisense oligodeoxynucleotides targeting mRNA

Two antisense oligodeoxynucleotides (A-ODN), targeting delta-opioid receptor mRNA (DOR) and two mismatch ODN sequences (mODN) were continuously infused for 24 days into the lateral brain ventricles of Wistar rats. The density of delta-opioid receptors in rat brain homogenates was measured by saturation binding experiments using four selective ligands, two agonists ([D-Ala2, Glu4]-deltorphin and DPDPE) and two antagonists (Dmt-Tic-OH and naltrindole), and by immunoblotting SDS solubilized receptor protein. In brain membranes of mODN or saline-infused rats, the rank order of delta-opioid receptor density, calculated by Bmax values of the four delta-opioid receptor ligands, was: [D-Ala2, Glu4]deltorphin approximately Dmt-Tic-OH approximately naltrindole (86-118 fmo/mg protein) > DPDPE (73.6+/-6.3 fmol/mg protein). At the end of the 24 day infusion of A-ODN targeting DOR nucleotide sequence 280299 (A-ODN280-299), the Bmax of DPDPE (62.4+/-3.2 fmol/mg protein) was significantly higher than that of Dmt-Tic-OH (31.5+/-3.9 fmol/mg protein). Moreover, both the Kd value for DPDPE saturation binding and the Ki value for Dmt-Tic-OH displacement by DPDPE were halved. In contrast, an A-ODN treatment targeting exon 3 (A-ODN741-760) decreased the specific binding of [D-Ala2, Glu4]deltorphin and Dmt-Tic-OH significantly less (67%-81%) than the binding of DPDPE (53%), without changes in DPDPE Ki and KD values. No A-ODN treatment modified the specific binding of the micro-opioid agonist DAMGO and of the k-selective opioid receptor ligand U69593. On the Western blot of solubilized striatum proteins, A-ODN(280-299) and A-ODN(741-760) downregulated the levels of the DOR protein, whereas the corresponding mODN were inactive. The 24-day infusion of A-ODN(280-299) inhibited the rat locomotor response to [D-Ala2, Glu4]deltorphin but not to DPDPE. Intracerebroventricular (i.c.v.) infusion of A-ODN(741-760) reduced the locomotor responses to both delta-opioid receptor agonists, whereas mODN infusion never affected agonist potencies. In conclusion, these results demonstrate that 24-day continuous i.c.v. infusion of A-ODN targeting the nucleotide sequence 280-299 of DOR can differentially knockdown delta1 and delta2 binding sites in the rat brain.

What peptides these deltorphins be

The deltorphins are a class of highly selective delta-opioid heptapeptides from the skin of the Amazonian frogs Phyllomedusa sauvagei and P. bicolor. The first of these fascinating peptides came to light in 1987 by cloning of the cDNA of from frog skins, while the other members of this family were identified either by cDNA or isolation of the peptides. The distinctive feature of deltorphins is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe, comparable to dermorphin, which is the prototype of a group of mu-selective opioids from the same source. The D-amino acid and the anionic residues, either Glu or Asp, as well as their unique amino acid compositions are responsible for the remarkable biostability, high delta-receptor affinity, bioactivity and peptide conformation. This review summarizes a decade of research from many laboratories that defined which residues and substituents in the deltorphins interact with the delta-receptor and characterized pharmacological and physiological activities in vitro and in vivo. It begins with a historical description of the topic and presents general schema for the synthesis of peptide analogues of deltorphins A, B and C as a means to document the methods employed in producing a myriad of analogues. Structure activity studies of the peptides and their pharmacological activities in vitro are detailed in abundantly tabulated data. A brief compendium of the current level of knowledge of the delta-receptor assists the reader to appreciate the rationale for the design of these analogues. Discussion of the conformation of these peptides addresses how structure leads to further hypotheses regarding ligand receptor interaction. The review ends with a broad discussion of the potential applications of these peptides in clinical and therapeutic settings.

Chronic naltrindole administration does not modify the inhibitory effect of morphine on vocalization responses in the tail electric stimulation test in rats

To address the existence of possible functional interactions between delta- and mu- receptors in relation to the affective component of pain, we have studied the effects of functional blockade of delta-receptors by a chronic treatment with naltrindole (1 mg/kg, 8 consecutive days) on antinociceptive responses to morphine (2 and 5 mg/kg) in the tail electric stimulation test, in adult male rats. The thresholds for the motor response (tail withdrawal), vocalization during stimulus and vocalization afterdischarge were assessed. These responses are considered to be integrated at spinal, medulla oblongata and diencephalon-rhinencephalon levels, respectively. The results show that the vocalization during stimulus and the vocalization afterdischarge were significantly affected by morphine in a dose dependent manner, the latter response being the most sensitive to the effects of the mu-opioid agonist. However, no significant effect was observed on motor responses at the doses used in this study. Chronic naltrindole treatment did not modify the inhibitory effect of morphine on the vocalization responses. Since the vocalization afterdischarge is related to the affective component of pain, the data suggest that the delta-opioid receptor is not involved in the supraspinal mechanisms at which these responses are organized and that there is not a mu-delta interaction in the modulation of the affective responses to noxious electrical stimulation.

Effects of neonatal naltrindole treatment on antinociceptive and behavioral responses to mu and kappa agonists in rats

The effects of a daily injection of the delta selective opioid antagonist naltrindole (1 mg/kg), from birth to postnatal day 19, on antinociceptive and behavioral responses to the mu selective agonist alfentanil (65 microg/kg) and the kappa selective agonist CI-977 (50 microg/kg) in 20-day-old male rats were investigated. Antinociception was assessed using the tail immersion test and behavioral testing was performed by employing an open field. The functional blockade of the delta receptor by naltrindole blocked the antinociceptive response to alfentanil but did not affect the antinociception induced by CI-977. The effects of alfentanil (increased exploration) and CI-977 (a marked hypoactivity) in the open field were not modified by neonatal naltrindole treatment. The results suggest a functional interaction between delta and mu receptors in the postnatal period but not between delta and kappa receptors. The data also suggest differences in the delta and mu receptors interacting in the modulation of antinociception and those involved in behavioral responses in the open field.

Antinociceptive and behavioral effects of synthetic deltorphin analogs

A possible correlation of behavioral, antinociceptive and cataleptic responses with central delta- and mu-opioid receptor stimulation was tested for in the rat by i.c.v. injections of some synthetic deltorphin analogs. At doses ranging from 0.1 to 3.0 nmol/rat, the selective delta-opioid receptor agonist, [D-Ala2,Glu4]deltorphin (Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2), induced a dose-dependent stereotyped pattern of locomotor activity, reaching the maximum in the first 30 min; doses higher than 30 nmol induced early and fleeting antinociception. The replacement of Glu4 by Gly, Ala, Val, His or Asn yielded peptides with a lower delta-selectivity because of a gain in mu-affinity. [D-Ala2,Ala4]deltorphin (0.14-4.0 nmol) induced negligible behavioral stimulation but a rapidly appearing and long-lasting analgesia and catalepsy. The other four synthetic peptides induced biphasic effects: low dosages stimulated locomotion whereas higher doses initially suppressed, then increased locomotor activity. At doses ranging from 1 to 70 nmol all the peptides induced analgesia and catalepsy. In experiments examining the locomotor and antinociceptive effects induced by 14 nmol of [D-Ala2,Gly4]deltorphin in rats pretreated with mu and delta antagonists, the non-selective mu-opioid receptor antagonist, naloxone (1 mg/kg i.p.), reduced analgesia and abolished the initial hypolocomotion. The delta-selective antagonist, naltrindole (10 mg/kg i.p.), abolished locomotor activity without affecting analgesia. The mu1 -selective antagonist, naloxonazine (10 mg/kg i.v.), seemed to prolong analgesia and immobility. Hence this peptide appears to activate, in addition to delta-receptors, mainly the opioid receptor mu2-subtype, which mediates catalepsy in the rat. We suggest that the mu2- and delta-opioid receptors of the rat brain modulate locomotor behavior by activating functionally opposed responses. [D-Ala2,Ala4]deltorphin had an antinociceptive and cataleptic potency higher than would have been expected from its mu-affinity. A possible explanation might be a mu/delta-opioid receptor interaction.