Deltakephalin, Tyr-D-Thr-Gly-Phe-Leu-Thr: a new highly potent and fully specific agonist for opiate delta-receptors

N-Guanidyl and C-Tetrazole Leu-Enkephalin Derivatives: Efficient Mu and Delta Opioid Receptor Agonists with Improved Pharmacological Properties

Leu-enkephalin and d-Ala₂-Leu-enkephalin were modified at their N- and C-termini with guanidyl and tetrazole groups. The resulting molecules were prepared in solution or by solid phase peptide synthesis. The affinity of the different analogues at mu (MOP) and delta opioid receptors (DOP) was then assessed by competitive binding in stably transfected DOP and MOP HEK293 cells. Inhibition of cAMP production and recruitment of β-arrestin were also investigated. Finally, lipophilicity (logD_7.4) and plasma stability of each compound were measured. Compared to the native ligands, we found that the replacement of the terminal carboxylate by a tetrazole slightly decreased both the affinity at mu and delta opioid receptors as well as the half-life. By contrast, replacing the ammonium at the N-terminus with a guanidyl significantly improved the affinity, the potency, as well as the lipophilicity and the stability of the resulting peptides. Replacing the glycine residue with a d-alanine in position 2 consistently improved the potency as well as the stability of the analogues. The best peptidomimetic of the whole series, guanidyl-Tyr-d-Ala-Gly-Phe-Leu-tetrazole, displayed sub-nanomolar affinity and an increased lipophilicity. Moreover, it proved to be stable in plasma for up to 24 h, suggesting that the modifications are protecting the compound against protease degradation.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Enterostatin (VPDPR) Has Anti-analgesic and Anti-amnesic Activites

Enterostatin (VPDPR), an anorexigenic peptide derived from the amino terminus of procolipase, significantly inhibited analgesia induced by the mu-opioid agonist morphine (5 mg/kg, s.c.) after i.c.v. administration to mice at a dose of 100 nmol. On the other hand, VPDPR (approximately 200 nmol, i.c.v.) did not attenuate analgesia induced by the kappa-opioid agonist D-Phe-D-Phe-D-Nle-D-Arg-NH2 (100 microg/mouse, i.c.v.) or delta-opioid agonist DTLET (4 nmol/mouse, i.c.v.). VPDPR (100 nmol, i.c.v.) significantly improved amnesia induced by scopolamine (0.2 mg/kg, i.p.) in mice. However, VPDPR did not enhance memory in normal mice at the same dose.

Opioid glycopeptide analgesics derived from endogenous enkephalins and endorphins

Over the past two decades, potent and selective analgesics have been developed from endogenous opioid peptides. Glycosylation provides an important means of modulating interaction with biological membranes, which greatly affects the pharmacodynamics and pharmacokinetics of the resulting glycopeptide analogues. Furthermore, manipulation of the membrane affinity allows penetration of cellular barriers that block efficient drug distribution, including the blood-brain barrier. Extremely potent and selective opiate agonists have been developed from endogenous peptides, some of which show great promise as drug candidates.

Phosphorylation of enkephalins: NMR and CD studies in aqueous and membrane-mimicking environments

Phosphorylation of l-serine-containing enkephalin analogs has been explored as an alternative to glycosylation in an effort to increase blood-brain barrier permeability and CNS bioavailability of peptide pharmacophores. Two enkephalin-based peptides were modified for these studies, a set related to DTLES, a mixed μ/δ-agonist, and one related to DAMGO, a highly selective μ-agonist. Each unglycosylated peptide was compared to its phosphate, its mono-benzylphosphate ester, and its β-d-glucoside. Binding was characterized in membrane preparations from Chinese hamster ovary cells expressing human μ, δ and κ-opiate receptors. Antinociception was measured in mice using the 55 °C tail-flick assay. To estimate bioavailability, the antinociceptive effect of each opioid agonist was evaluated after intracerebroventricular (i.c.v.) or intravenous administration (i.v.) of the peptides. Circular dichroism methods and high-field nuclear magnetic resonance were used in the presence and absence of sodium dodecylsulfate to understand how the presence of a membrane might influence the peptide conformations.

Further studies of tyrosine surrogates in opioid receptor peptide ligands

A series of opioid peptide ligands containing modified N-terminal tyrosine (Tyr) residues was prepared and evaluated against cloned human mu, delta, and kappa opioid receptors. This work extends the recent discovery that (S)-4-carboxamidophenylalanine (Cpa) is an effective tyrosine bioisostere. Amino acids containing negatively charged functional groups in place of tyrosine's phenolic hydroxyl lacked receptor affinity, while exchange of Tyr for (S)-4-aminophenylalanine was modestly successful. Peptides containing the new amino acids, (S)-4-carboxamido-2,6-dimethylphenylalanine (Cdp) and (S)-beta-(2-aminobenzo[d]thiazol-6-yl)alanine (Aba), displayed binding (K(i)) and functional (EC(50)) profiles comparable to the parent ligands at the three receptors. Cdp represents the best performing Tyr surrogate in terms of overall activity, while Cpa and Aba show a subtle proclivity toward the delta receptor.

Glycosylated neuropeptides: a new vista for neuropsychopharmacology?

The application of endogenous neuropeptides (e.g., enkephalins) as analgesics has been retarded by their poor stability in vivo and by their inability to effectively penetrate the blood-brain barrier (BBB). Effective BBB transport of glycosylated enkephalins has been demonstrated in several labs now. Analgesia (antinociception) levels greater than morphine, and with reduced side effects have been observed for several glycopeptides related to enkephalin. Somewhat paradoxically, enhanced BBB transport across this lipophilic barrier is achieved by attaching water-soluble carbohydrate groups to the peptide moieties to produce biousian glycopeptides that can be either water-soluble or membrane bound. Transport is believed to rely on an endocytotic mechanism (transcytosis), and allows for systemic delivery and transport of the water-soluble glycopeptides. Much larger endorphin/dynorphin glycopeptide analogs bearing amphipathic helix address regions also have been shown to penetrate the BBB in mice. This holds forth the possibility of transporting much larger neuropeptides across the BBB, which may encompass a wide variety of receptors beyond the opioid receptors.

Pharmacological and functional biochemical properties of d-Ala2-d-Nle5-enkephalin-Arg-Phe

Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (DADN) a synthetic analogue of the endogenous Met-enkephalin-Arg-Phe (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MERF), was investigated in radioligand binding assays, [(35)S]GTPgammaS stimulation experiments as well as in in vivo algesiometric tests. Binding properties of [(3)H]DADN were measured in crude membrane fractions of rat spinal cord tissues and in homogenates of Chinese hamster ovary (CHO) cells selectively expressing delta-, kappa-or micro-opioid receptors. The highest affinity for [(3)H]DADN binding was observed in membranes from CHO cells transfected with micro-opioid receptors confirming the micro-selectivity of the peptide. Unlabeled DADN was also investigated in functional biochemical experiments by measuring opioid receptor-mediated G-protein activation in rat brain membrane fractions. The peptide stimulated the activity of the regulatory G-proteins in a concentration dependent manner, and the stimulation was efficiently inhibited in the presence of micro-receptor specific antagonist ligands further supporting the selectivity profile of DADN. Intrathecally administered DADN produced a dose-related, naloxone-reversible antinociception in rat hot water tail-flick tests. Among the selective opioid antagonists tested, the delta-selective naltrindole (NTI) and the kappa-specific norbinaltorphimine (norBNI) showed only slight blocking effects compared with naloxone. The results obtained in the in vitro agonist-stimulated [(35)S]GTPgammaS binding assays are in good agreement with the opioid agonist effect seen in the in vivo pain test.

(4-Carboxamido)phenylalanine is a surrogate for tyrosine in opioid receptor peptide ligands

(S)-4-(Carboxamido)phenylalanine (Cpa) is examined as a bioisosteric replacement for the terminal tyrosine (Tyr) residue in a variety of known peptide ligands for the mu, delta and kappa opioid receptors. The Cpa-containing peptides, assayed against cloned human opioid receptors, display comparable binding affinity (Ki), and agonist potency (EC50) to the parent ligands at the three receptors. Cpa analogs of delta selective peptides show an increase in delta selectivity relative to the mu receptor. Cpa is the first example of an amino acid that acts as a surrogate for Tyr in opioid peptide ligands, challenging the long-standing belief that a phenolic residue is required for high affinity binding.

Synthesis and evaluation of derivatives of leucine enkephalin as potential affinity labels for δ opioid receptors

As part of an effort to develop peptide-based affinity labels for opioid receptors, [Leu(5)]enkephalin (LeuEnk) and DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr), potent agonists for delta receptors, were selected as the parent peptides for further modification. The affinity label derivatives were prepared using standard Fmoc solid-phase peptide synthesis in conjunction with Fmoc-Phe(p-NHAlloc) (Fmoc: 9-flourenylmethoxycarbonyl;) and selective modification of the p-amino group on this residue. The electrophilic isothiocyanate and bromoacetamide groups were introduced into the para position of Phe(4); the corresponding free amine-containing peptides were also prepared for comparison. The pure peptides were evaluated in radioligand binding assays using Chinese hamster ovary (CHO) cells expressing delta and micro opioid receptors. Modification of Phe(4) in LeuEnk and DTLET significantly decreased delta-receptor binding affinity (40 to >2,000-fold). Among the synthesized analogues, [Phe(p-NH(2))(4)]DTLET showed the highest delta-receptor binding affinity (IC(50) = 39 nM) and enhanced selectivity for delta receptors compared to DTLET while other derivatives exhibited much lower delta receptor affinity. The differences in affinities between the two series of analogues and between the derivatives of LeuEnk and N,N-dibenzyl[Leu(5)]Enk reported previously suggest subtle differences in interactions of Phe(4) with delta receptors depending on other modifications in the sequences.

Josef Rudinger Memorial Lecture 2002. The chemistry of the opioid receptor binding sites

Since the discovery of the opioid receptors and their endogenous ligands an immense research work has been devoted to the exploration of their specificity, the mechanism of ligand binding and ligand-receptor interactions. One of the main goals has been the location and characterization of the binding sites. The present review compiles the results achieved in this field in the last quarter of a century, and puts some questions concerning the success of these efforts.

Affinity labeling of delta opioid receptors by an enkephalin-derivative alkylating agent, DSLET-Mal

Opioid binding properties of Tyr-D-Ser-Gly-Phe-Leu-Thr-NH-NH-Gly-Mal (DSLET-Mal), a novel enkephalin-framed affinity label, was determined in rat brain membranes. In competition studies the ligand showed high affinity for the delta opioid sites, labelled by [(3)H][Ile(5,6)]deltorphin II (K(i) = 8 nM), whereas its binding to the mu ([(3)H]DAMGO) and kappa ([(3)H]EKC) sites was weaker. Preincubation of the rat brain membranes with DSLET-Mal at micromolar concentrations resulted in a wash-resistant and dose-dependent inhibition of the [(3)H][Ile(5,6)]deltorphin II binding sites (96% blocking at 10 microM concentration). Intracerebroventricular (ICV) administration of DSLET-Mal reduced the density of delta opioid receptors and had no effect on mu and kappa receptors, as determined by saturation binding studies. [Ile(5, 6)]deltorphin II-stimulated [(35)S]GTPgammaS binding was determined in membrane preparations of different brain areas of the ICV-treated animals. In both frontal cortex and hippocampus DSLET-Mal significantly decreased G protein activation by the delta agonist, having no effect on DAMGO stimulated [(35)S]GTPgammaS binding. DSLET-Mal had qualitatively similar effects on both receptor binding and G protein activation. These characteristics of the compound studied suggest that DSLET-Mal can serve as an affinity label for further studies of the delta-opioid receptors.

Structural studies of opioid peptides: a review of recent progress in x-ray diffraction studies

The solid state structures of many opioid peptide agonists have been elucidated by x-ray diffraction analysis. Recently, the first structure of an opioid peptide antagonist has been determined. Theoretically, linear peptides can have many different backbone conformations, yet early x-ray studies (1983-1987) on enkephalin and its analogues showed only two different backbone conformations: extended and single beta-bend. In 1989 enkephalin was observed in a third conformation, a double beta-bend. Since that time diffraction studies have been completed on the rationally designed linear opioid peptide agonists DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr) and DADLE (D-Ala2,D-Leu5-enkephalin) as well as on several cyclic enkephalin analogues including DPDPE (Tyr-[D-Pen-Gly-Phe-D-Pen]) and JOM-13 (Tyr-[D-Cys-Phe-D-Pen]). The most recent review of the x-ray studies on this class of compounds was written in 1988. This paper will update that review to include the results of studies completed since that time.

Dalargin and [Cys-(O2NH2)]2 analogues of enkephalins and their selectivity for mu opioid receptors

1. Effects of the enkephalins Met-enk (M) and Leu-enk (L), of two newly synthesized analogues--[Cys-(O2NH2)]2-Met-enk (CM) and [Cys-(O2NH2)]2-Leu-enk (CL)--and of a hexapeptide--D-Ala2-Leu5-Arg6 (Dalargin; DL) on the spontaneous and electrically stimulated activity were examined with respect to their selectivity for the mu opioid receptors in the longitudinal layer of guinea pig ileum. 2. M and CM exerted relaxing and contractile effects on the spontaneous contractile activity while L, CL and DL produced only relaxation. The order of potency towards the relaxatory phase was DL > M > CM > L > CL and towards the contractile phase CM > M. 3. The effects of enkephalins on the spontaneous activity were naloxone and TTX sensitive except for the contractile phase of M and CM which persisted in the presence of TTX. NO was not involved in the neurotransmission of the relaxatory responses, while the blockade of alpha and beta adrenoceptors showed the participation of adrenergic mechanisms. Relaxation and contraction induced by enkephalins could not be directly attributed to cholinergic neurotransmission. 4. The naloxone-sensitive and concentration-dependent inhibitory effects of enkephalins and their analogues on the electrically stimulated cholinergic contractions were established. The order of the relative potency of opioids was: DL-3.8; M-1.0; L-0.4; CM-0.01; CL-0.005. 5. These data indicated that the D-Ala2 substitution and lengthening of the peptide chain by Arg6 in the molecule of L increased the potency at the mu opiate receptors, while the substitution in position 2 with Cys-(O2NH2) in the molecule of M and L yielded a less potent and selective mu agonists.

Mu and delta opioid receptors mediate opposite modulations by morphine of the spinal release of cholecystokinin-like material

The possible modulations by morphine and various opioids of the spinal release of cholecystokinin-like material (CCKLM) evoked by 30 mM K+ was studied in vitro, using slices of the dorsal part of the rat lumbar enlargement superfused with an artificial cerebrospinal fluid. Addition of the mu agonist, DAGO (0.1-10 microM), to the perfusing fluid produced a concentration-dependent decrease in the peptide release, which could be prevented by the preferential mu antagonist, naloxone. Complex modulations were induced by the delta agonist, DTLET, as this drug inhibited CCKLM release when added at 10 nM-3 microM to the perfusing fluid, but enhanced it at 10 microM. Both effects were preventable by the delta antagonists naltrindole and ICI 154129, suggesting that delta receptors, possibly of different subtypes, mediated the inhibition and stimulation by DTLET. Morphine also exerted a biphasic effect, as the alkaloid decreased CCKLM release at 0.01-0.1 microM and enhanced it at 10 microM. Morphine-induced inhibition was preventable by naloxone, whereas its stimulatory effect could be blocked by naltrindole and ICI 154129. Although inactive on its own on CCKLM release, the selective kappa 1 agonist U 50488H (1 microM) prevented the inhibitory effects of both DAGO (10 microM) and morphine (0.1 microM), suggesting the existence of interactions between kappa 1 and mu receptors within the dorsal zone of the rat spinal cord. These data indicate that low concentrations of morphine exert an inhibitory influence on spinal CCKergic neurons that depends on the stimulation of mu opioid receptors. The excitatory influence of 10 microM morphine likely results from the simultaneous stimulation of mu, delta and kappa receptors, as the inhibitory effect of mu receptor stimulation can be masked by that of kappa 1 receptors, allowing only the expression of a delta-dependent excitatory effect similar to that induced by 10 microM DTLET.

Crystal structure of deltakephalin: a delta-selective opioid peptide with a novel beta-bend-like conformation

The solid-state structure of deltakephalin (Tyr-DThr-Gly-Phe-Leu-Thr) has been determined by single-crystal X-ray diffraction. Deltakephalin (DTLET) is a synthetic opioid peptide which differs from enkephalin in that a D-Thr has been substituted for Gly2 and a sixth residue, L-Thr, has been added. Clear colorless plates obtained using vapor diffusion and macro-seeding crystallization techniques were monoclinic; space group C2 with a = 27.389(5), b = 9.205(2), c = 16.788(2) A, beta = 98.87(2) degrees and V = 4181.4(14) A3. The asymmetric unit contained one molecule of DTLET and six molecules of water, giving a calculated density of 1.28 g cm-3. The crystal structure revealed that DTLET has a pseudo type I' beta-bend which is stabilized by an intramolecular side-chain to backbone hydrogen bond. This is the first reported observation of a pseudo beta-bend conformation in a solid-state structure of an enkephalin analog.

Opioidergic control of the spinal release of neuropeptides. Possible significance for the analgesic effects of opioids

Several neuropeptides play a key role in the transfer (substance P, calcitonin gene-related peptide, etc) and control (enkephalins, cholecystokinin, etc) of nociceptive messages from primary afferent fibres to spino-thalamic neurones in the dorsal horn of the spinal cord. This first relay in nociceptive pathways has been shown to be a major target for opioids such as analgesic drugs, and the effects of exogenous (mainly morphine) and endogenous opioids on the release of neuropeptides within the dorsal horn are reviewed here for a better understanding of the cellular mechanisms responsible for their antinociceptive action. Complex modulations of the in vitro (from tissue slices) and in vivo (in halothane-anaesthetized rats whose intrathecal space was perfused with an artificial cerebrospinal fluid) release of substance P and calcitonin gene-related peptide by opioids have been reported, depending on the opioid receptor (mu, delta, kappa, and their subtypes) stimulated by these compounds. In particular, the inhibition by delta agonists of substance P release from primary afferent fibres, and that by the concomitant stimulation of mu and kappa receptors of the release of calcitonin gene-related peptide are very probably involved in the analgesic action of specific opioids and morphine at the level of the spinal cord. Furthermore, the negative modulation (through presynaptic opioid autoreceptors) by delta and mu agonists of the spinal release of met-enkephalin, and the complex inhibitory/excitatory influence of delta, mu and kappa receptor ligands on the release of cholecystokinin within the dorsal horn very likely also contribute to the antinociceptive action of these drugs and morphine. The reviewed data strongly support the existence of functional interactions between mu and kappa receptors within the spinal cord, and their key role in the analgesic action of non specific opiates (acting on mu, delta and kappa receptors) such as morphine.

Morphine reduces the release of met-enkephalin-like material from the rat spinal cord in vivo by acting at delta opioid receptors

The modulation by morphine of the spinal release of met-enkephalin-like material (MELM) was investigated in anaesthetized rats whose intrathecal space was perfused with an artificial CSF (ACSF). Morphine (10 microM in the ACSF), as well as a mu- (DAGO, 10 microM) or delta opioid receptor agonist (DTLET, 10 microM), significantly decreased the outflow of MELM. The effects of morphine and DTLET were prevented by the delta antagonist, naltrindole (10 microM), but not by naloxone (10 microM). Conversely, naloxone, but not naltrindole, prevented the inhibitory effect of DAGO. Although neither the kappa 1 agonist, U 50488H (10 microM), nor the kappa 1 antagonist, norbinaltorphimine (10 microM), exerted on their own any significant effect, norbinaltorphimine enhanced the inhibitory action of morphine. In contrast to the inhibition induced by morphine (with or without naloxone) which was preventable by 10 microM naltrindole, the inhibition of MELM release by morphine plus norbinaltorphimine was only partly reduced by naltrindole. Thus, concomitant stimulation of mu, delta and kappa 1 receptors might account for the apparent delta opioid receptor-dependent inhibition of MELM release by morphine. Indeed, its potential inhibitory effect through the stimulation of mu receptors (normally prevented by the concomitant stimulation of kappa 1 receptors) becomes efficient only when kappa 1 receptors are blocked.

The in vivo effects of opioid peptides on the murine immune response

We have previously reported that met-enkephalin has dual immunomodulatory properties in vitro. We have continued this investigation using an in vivo system. In this study, Alzet miniosmotic pumps were loaded with either met-enkephalin, DTLET or FK 33-824 and were surgically implanted into BAF1/J mice. Twenty-four hours after pump implantation, mice were challenged with sub-optimal, optimal or supraoptimal immunizing doses of antigen. The immune response was assessed 4 or 5 days after primary immunization. FK 33-824, a met-enkephalin analogue, had no effect on the response of mice challenged with a suboptimal antigen dose. However, FK 33-824, at a pump concentration of 10(-3) M, suppressed the response against optimal challenge doses of antigen. At a pump concentration of 10(-8) M, FK 33-824 suppressed, enhanced or had no effect on the supraoptimal antigen dose-induced immune response. The suppressive effect of FK 33-824 in mice immunized with either optimal or supraoptimal doses of antigen was blocked by naloxone. Met-enkephalin and its delta opioid receptor specific analogue, DTLET, had no effect on the immune response to optimal antigen immunization. These results indicate that FK 33-824 has in vivo immunomodulatory activity and provide evidence that opioid peptides may either upregulate or downregulate the in vivo immune response depending on the strength of the response.

[125I]azido-DTLET as a tool for selective covalent labeling of delta-opioid receptors in rat brain sections

The binding kinetics and pharmacological selectivity of the photoaffinity delta-opioid ligand [125I]azido-DTLET (Tyr-D-Thr-Gly-Phe(pN3)-Leu-Thr) were investigated in serial frozen sections from rat neostriatum prior to ultraviolet irradiation (i.e., in conditions of reversibility). Scatchard analysis of saturation binding experiments indicated that [125I]azido-DTLET binds to both a high (KD = 5.04 nM) and a low (KD = 38 nM) affinity site. Binding to the low-affinity site was no longer detectable in the presence of unlabeled [D-Ala2,N-MePhe4,Gly-Ol5]enkephalin (DAGO), suggesting that this site corresponds to mu-opioid receptors. This interpretation was further supported by the dose-dependent inhibition of the binding of [3H]DAGO by non-radioactive azido-DTLET. Binding to the high-affinity site was totally inhibited, in a dose-dependent fashion, by a variety of opioid drugs among which delta-opioid ligands showed the highest order of potency. It is concluded that, in the nanomolar range, [125I]azido-DTLET constitutes a highly selective tool for covalent labeling of delta-opioid receptors in rat brain sections.

Investigation of the structural parameters involved in the delta-opioid selectivity of several families of opioid peptides

Three series of highly delta-opioid selective peptides are now available, and each family is used as template to investigate the structural parameters involved in delta-receptor recognition and in the modulation of the selectivity of the parent peptide. The first series includes cyclic derivatives such as Tyr-D-Pen-Gly-Phe-D-Pen(DPDPE) and Tyr-D-Pen-Gly-Phe-Pen(DPLPE); the second are the synthetic linear constrained peptides [Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr(DSTBULET), Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr (OtBu)(BUBU) and especially Tyr-D-Cys(StBu)-Gly-Phe-Leu-Thr(OtBu) (BUBUC)] and the last one the natural peptides [Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2 (deltorphin or dermenkephalin) and Tyr-D-Ala-Phe-Asp-Val-Val-GlyNH2 ([D-Ala2] deltorphin I)]. In the present study, the possibly of transposing some of the decisive factors of delta-selectivity evidenced in the two other families, to the linear constrained peptides series was examined. With this aim in view, residues such as Phe3, pClPhe4 or Asp were introduced in the sequence of DSTBULET, BUBU or BUBUC. Direct comparison between the biochemical profiles of the [pClPhe4] analogs of the linear constrained peptides and their parent compounds shows that the addition of an electronegative atom on the Phe4 residue of enkephalin sequences is not an absolute parameter for delta-selectivity improvement. The hydrophobic delta-receptor subsite seems able to receive a range of molecular volumes and electronegativities. By contrast, this subsite cannot interact with a Phe3 aromatic ring introduced in this series of peptides. Moreover, the results obtained with linear peptides including additional negatively charged residues demonstrate that the proposed location of the delta-receptors in a cationic membrane environment is not adequate to explain the selectivity profile of a number of compounds.

Opioid control of the release of calcitonin gene-related peptide-like material from the rat spinal cord in vivo

The possible control by opioids of the spinal release of calcitonin gene-related peptide-like material (CGRPLM) was investigated in halothane-anaesthetized rats whose intrathecal space was perfused with an artificial cerebrospinal fluid. Morphine (20 mg/kg i.v.; or at 10-100 microM added to the perfusing fluid), the mu selective agonist DAGO (10 microM) and the kappa selective agonist U 50488 H (10 microM) did not affect the spontaneous outflow of the CGRPLM. In contrast, the selective delta agonist DTLET (10 microM) significantly increased CGRPLM release. The latter effect could be prevented by the selective delta antagonist naltrindole (10 microM) as expected from the involvement of this class of opioid receptors. However, the addition of naltrindole alone to the perfusing fluid did not modify CGRPLM outflow, indicating that endogenous opioids do not exert a tonic control of CGRP-containing fibers through the stimulation of delta receptors. In contrast, intrathecal perfusion with naloxone (10 microM) or nor-binaltorphimine (10 microM), a selective antagonist of kappa receptors, produced a marked increase in spinal CGRPLM release, suggesting that endogenous opioids acting at mu and kappa receptors, respectively, exert a tonic inhibitory control of CGRP-containing fibers. Indeed, a significant decrease in the spinal release of CGRPLM release could be evoked by the combined addition of U 50488 H (10 microM) plus DAGO (10 microM) to the perfusing medium, indicating that the simultaneous stimulation of both kappa and mu receptors is required for this negative control to occur. This could notably be achieved with morphine (10 microM) in the presence of naltrindole (10 microM) which also produced a significant reduction in the spinal release of CGRPLM. In conclusion, morphine per se did not change CGRPLM release because this drug triggers opposite positive (through the stimulation of delta receptors) and negative (through the concomitant stimulation of both kappa and mu receptors) control mechanisms within the rat spinal cord.

Quantitative autoradiographic mapping of delta-opioid receptors in the rat central nervous system using [125I][D.Ala2]deltorphin-I

The distribution of delta-opioid binding sites was studied by quantitative autoradiography in rat brain and spinal cord using the highly selective ligand [125I][D.Ala2]deltorphin-I. The binding properties of [125I][D.Ala2]deltorphin-I were investigated by microdensitometry of autoradiographic films with the aid of a computer-assisted image-analysis system. [125I][D.Ala2]deltorphin-I appeared to interact with a single class of sites in all brain areas (KD = 0.9 nM). In 23 regions tested, whatever the delta site concentration, DTLET, a delta agonist, appears to be 2 orders of magnitude more effective than DAGO, a mu agonist, in inhibiting specific [125I][D.Ala2]deltorphin-I binding. The distribution of [125I][D.Ala2]deltorphin-I sites is globally consistent with that of other delta ligands and does not support the existence of a delta-receptor subtype recognized by [D.Ala2]deltorphin-I. [125I][D.Ala2]deltorphin-I binding sites were highly confined, exhibiting selective localization in the neocortex and a diffuse pattern in the striatum, accumbens nucleus, claustrum, layer of bulb, amygdaloid nucleus, pontine nuclei, and inferior colliculus. In several areas a rostro-caudal gradient of site concentration was indicated. [D.Ala2]deltorphin-I binding sites were also present in the substantia gelatinosa at all levels of the spinal cord and, unexpectedly, in deeper laminae and the ventral horn. These results demonstrate the ability of [125I][D.Ala2]deltorphin-I to characterize low concentrations of binding sites and to reveal new localizations of delta receptors.

Kappa-/mu-receptor interactions in the opioid control of the in vivo release of substance P-like material from the rat spinal cord

The possible involvement of mu and kappa receptors in the opioid control of the spinal release of substance P-like material was assessed in vivo, in halothane-anaesthetized rats whose intrathecal space was continuously perfused with an artificial cerebrospinal fluid supplemented with various opioid receptor agonists and antagonists. Whereas the intrathecal perfusion with the mu agonist DAGO (10 microM) significantly enhanced (approximately + 50%) the spontaneous release of substance P-like material, that with the kappa agonist U 50488 H (10 microM) produced no change in the peptide outflow. The respective antagonists naloxone (10 microM) for the mu receptors and nor-binaltorphimine (10 microM) for the kappa receptors did not affect the spontaneous release of substance P-like material, indicating that endogenous opioids acting at mu and kappa receptors do not exert a tonic control on substance P-containing neurons in the spinal cord of halothane-anaesthetized rats. However, as expected from the involvement of mu receptors, the stimulatory effect of DAGO on the peptide outflow could be prevented by naloxone but not norbinaltorphimine. Furthermore, instead of an increase with DAGO alone, a significant decrease in the spinal release of substance P-like material was observed upon the intrathecal perfusion with DAGO plus U 50488 H. Additional experiments with the respective mu and kappa antagonists naloxone and nor-binaltorphimine demonstrated that this effect actually resulted from the simultaneous stimulation of mu and kappa receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of opioid binding sites in the superficial dorsal horn of the rat spinal cord following loose ligation of the sciatic nerve: comparison with sciatic nerve section and lumbar dorsal rhizotomy

The aim of the present study was to quantify time-related modifications in mu and delta opioid binding sites in the superficial layers (laminae I and II) of the L4 lumbar segment in a rat model of mononeuropathy induced by loose ligation of the sciatic nerve. We have shown a 28% (P < 0.01) and 24% (P < 0.01) decrease in ipsi/contralateral side binding ratios for tritiated (Tyr*-D-Ala-Gly-NMe-Phe-Gly-ol) ([3H]DAMGO) and tritiated (Tyr*-D-Thr-Gly-Phe-Leu-Thr) ([3H]DTLET) respectively, at two weeks postlesion which correspond to the delay of maximal hyperalgesia and of maximal alteration of fine diameter primary afferent fibers. In contrast, no change in [3H]U.69593 specific binding could be detected at this postlesion delay. For longer survival delays (four, eight and 15 weeks postlesion), mu and delta binding ratios return towards control values (approximately equal to 1), probably reflecting the occurrence of a long-term neuroplasticity (i.e. a new equilibrium in the metabolism of primary neurons, or collateral sprouting from intact primary afferents) following loose nerve ligation. In addition, a comparison of the results obtained in this model with those measured after sciatic nerve section and lumbar dorsal rhizotomy was performed in order to compare the degree of loss in opioid binding sites in these three types of lesion. The section of the sciatic nerve induced at eight days postlesion an 18% (P < 0.01) and 28% (P < 0.01) decrease in binding ratio for [3H]DAMGO and [3H]DTLET, respectively. At two weeks postlesion the loss was 24% (P < 0.01) for the two ligands, and at longer delays (four and 12 weeks), a progressive recovery in binding ratio was observed. Thus, it appears that both sciatic nerve lesions we have studied result in mu and delta binding modifications which have similar intensity and similar time course from two to 12-15 weeks postlesion. In contrast, the unilateral rhizotomy of nine consecutive dorsal roots (T13-S2), which is known to induce a massive degeneration of fine diameter primary afferent fibers, is followed by a dramatic decrease in binding ratios for [3H]DAMGO (53%, P < 0.001) and [3H]DTLET (45%, P < 0.001) at two weeks postlesion. These data suggest that the more deprived the dorsal horn is of fine diameter primary afferent fibers, the more dramatic is the opioid binding loss in the ipsilateral side as compared to the contralateral side.(ABSTRACT TRUNCATED AT 400 WORDS)

Guinea-pig ileum (GPI) and mouse vas deferens (MVD) preparations in the discovery, discrimination and parallel bioassay of opioid peptides

As many as 47 amphibian and mammalian, natural and non-natural opioid peptides have been examined in guinea-pig ileum (GPI) and mouse vas deferens (MVD) preparations. The great value of these extremely simple and accessible tissue models in the identification, isolation and purification of endogenous opioid peptides, in studying structure/activity relationships, and in determining selectivity of the peptide molecules for the various opioid receptors, especially delta- and mu-receptors, is emphasized.

Feedback inhibition of met-enkephalin release from the rat spinal cord in vivo

The possible existence of a feedback control by endogenous opioids of the spinal release of met-enkephalin-like material was assessed in vivo, in halothane-anesthetized rats whose intrathecal space was continuously perfused with an artificial cerebrospinal fluid supplemented with various opioid-related drugs. Both the intrathecal perfusion of the mu agonist D-Ala2-D-MePhe4-Gly-ol5-enkephalin (DAGO) (10 microM) and the delta agonist Tyr-D-Thr-Gly-Phe-Leu-Thr (DTLET) (10 microM) produced a significant inhibition of the spinal outflow of met-enkephalin-like material. The effect of DAGO, but not that of DTLET, could be prevented by naloxone (10 microM), and, conversely, the effect of DLTET, but not that of DAGO, was no longer observed in the presence of naltrindole (10 microM). Therefore naloxone and naltrindole acted as potent and selective mu and delta antagonists, respectively, when perfused at 10 microM in the intrathecal space of halothane-anesthetized rats. As expected from the lack of a tonic opioid control of spinal enkephalinergic neurones, neither naloxone nor naltrindole alone affected the spontaneous outflow of met-enkephalin-like material. However, naltrindole, but not naloxone, markedly increased the spinal overflow of met-enkephalin-like material due to intrathecal administration of either porcine calcitonin (10 microM) or the peptidase inhibitors thiorphan (10 microM) plus bestatin (20 microM). These data suggest that delta, but not mu, receptors are involved in a phasic opioid inhibitory control of the release of met-enkephalin-like material in the rat spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

Up-regulation of [3H]DAMGO and [3H]DTLET opioid binding sites in laminae I-II of the spinal cord in intact and deafferented morphine-tolerant rats

Using quantitative autoradiography and selective opioid ligands, we have measured the effects of morphine-induced tolerance on [3H]DAMGO and [3H]DTLET binding sites in the superficial spinal dorsal horn (laminae I-II) of intact and deafferented rats (unilateral C4-T2 dorsal rhizotomy). In intact rats, the treatment induced an up-regulation of 26% and 39% for [3H]DAMGO and [3H]DTLET binding sites, respectively, without modification of receptor affinity. In deafferented rats, the treatment similarly induced an up-regulation of 31% and 29% for [3H]DAMGO and [3H]DTLET binding sites, respectively, on the contralateral side, and of 21% and 25%, respectively, on the ipsilateral side. These data demonstrate that the up-regulation induced by morphine tolerance is of similar magnitude for both presynaptic (on primary afferent fibers) and postsynaptic (on spinal neurons) opioid binding sites in the rat dorsal horn.

Opioid control of the in vitro release of cholecystokinin-like material from the rat substantia nigra

Possible interactions between Met-enkephalin and cholecystokinin (CCK)-containing neurons in the rat substantia nigra were investigated by looking for the effects of various opioid receptor ligands and inhibitors of enkephalin-degrading enzymes on the K(+)-evoked overflow of CCK-like material (CCKLM) from substantia nigra slices. The delta-opioid agonists D-Pen2, D-Pen5-enkephalin (50 microM) and Tyr-D-Thr-Gly-Phe-Leu-Thr (DTLET; 3 microM) enhanced, whereas the mu-opioid agonists Tyr-D-Ala-Gly-MePhe-Gly-ol (DAGO; 10 microM) and MePhe3, D-Pro4-morphiceptin (PL 017; 10 microM) decreased, the K(+)-evoked release of CCKLM. By contrast, the kappa-opioid agonist U-50488 H (5 microM) was inactive. The stimulatory effect of DTLET could be prevented by the delta antagonist ICI-154129 (50 microM), but not by the mu antagonist naloxone (1 microM). Conversely, the latter drug, but not ICI-154129, prevented the inhibitory effect of DAGO and PL 017. A significant increase in CCKLM overflow was observed upon tissue superfusion with the peptidase inhibitors kelatorphan or bestatin plus thiorphan. This effect probably resulted from the stimulation of delta-opioid receptors by endogenous enkephalins protected from degradation, because it could be prevented by ICI-154129 (50 microM). Furthermore the peptidase inhibitors did not enhance CCKLM release further when delta-opioid receptors were stimulated directly by DTLET (3 microM). These data indicate that opioids acting on delta and mu receptors may exert an opposite influence, i.e., excitatory and inhibitory, respectively, on CCK-containing neurons in the rat substantia nigra.(ABSTRACT TRUNCATED AT 250 WORDS)

Kappa-opioid receptor stimulation abolishes mu- but not delta-mediated inhibitory control of spinal Met-enkephalin release

The possible opioid control through delta, mu and kappa receptors of the spinal release of Met-enkephalin-like material (MELM) was investigated in halothane-anaesthetized rats. The intrathecal perfusion of the delta agonist DTLET (10 microM) or the mu agonist DAGO (10 microM) resulted in a marked inhibition of MELM release, which could be prevented by the selective antagonists naltrindole and naloxone, respectively. Although the kappa agonist U 50488 H (10 microM) was inactive per se, it completely suppressed the inhibitory effect of DAGO, without affecting that of DTLET. As the selective kappa antagonist norbinaltorphimine blocked the action of U 50488 H, it can be concluded that kappa receptors modulate the mu- (but not the delta-) mediated feed back control of spinal enkephalinergic neurones.

A novel opioid structure which accepts protonated as well as non-protonated nitrogen: a family of pure, delta receptor selective antagonists

Conventional opioids including opioid peptides require an "opioid" nitrogen which exists in protonated state while interacting with the receptor. In the present paper we demonstrate that the Tyr-Pro-Gly-Phe-Leu-Thr hexapeptide sequence accepts N-terminal substituents such as N-t-Boc, N-phenylacetyl and N-diphenylacetyl where the N cannot become protonated, as well as "traditional" substitutions such as N,N-diallyl, where protonation is likely under physiological conditions. The opioid peptides bearing these substituents are pure antagonists of medium affinity (Ke values in the mouse vas deferens bioassay against [Met5]-enkephalin are in the 3 x 10(-7)-4 x 10(-6) M range) with a high delta receptor preference (50-350-fold delta over mu selectivity ratios).

Normotensive Wistar rats differ from spontaneously hypertensive and renal hypertensive rats in their cardiovascular responses to opioid agonists

1. The effects of three opioid receptor agonists on the blood pressure and heart rate of anaesthetized normotensive, spontaneously hypertensive and renal hypertensive rats were measured. 2. Mu agonist morphiceptin i.c.v. induced a pressor response and increase in heart rate in hypertensive rats, but hypotension in normotensive rats. After intravenous (i.v.) injection, morphiceptin produced a hypotensive response in all three groups of rats. 3. In contrast, the delta agonist DTLET i.c.v. decreased blood pressure and heart rate in hypertensive rats, but increased both pressure and beat rate in normotensive rats. After i.v. injections DTLET produced a hypertensive response and increase in heart rate in all groups of rats. 4. Kappa agonist U-50, 488H given i.c.v. induced effects similar to morphiceptin: an increase in blood pressure and heart rate in hypertensive and a decrease in normotensive rats. After i.v. injections U-50, 488H produced decreases in blood pressure and heart rate in all treated groups of rats. 5. Pretreatment with naloxone antagonized the activity of morphiceptin but prevented only the stimulating effect of DTLET in normotensive rats. Cardiovascular actions of U-50, 488H were not blocked by naloxone. 6. The results suggest that opioid agonists exert similar changes in cardiovascular function at central and peripheral sites in both models of experimental hypertension and these effects are different in normotensive rats.

Dermenkephalin and deltorphin I reveal similarities within ligand-binding domains of mu- and delta-opioid receptors and an additional address subsite on the delta-receptor

Dermorphin (Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2), dermenkephalin (Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2) and deltorphin I (Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2) are the first naturally occurring peptides highly potent for and almost specific to the mu- and delta-opioid receptors, respectively. The amino-terminal domains Tyr-D-X-Phe (where X is either Ala or Met) of these peptides behave as selective and potent mu-receptor ligands. Routing of Tyr-D-X-Phe to the delta- or the mu- receptor is associated with the presence or the absence at the C-terminus of an additional hydrophobic and negatively charged tetrapeptide by-passing the mu-addressing ability of the amino-terminal moiety. A study of 20 Tyr-D-X-Phe-Y-NH2 analogs with substitution of X and Y by neutral, hydrophobic, aromatic amino acids as well as by charged amino acid residues shows that tetrapeptides maintain high binding affinity and selectivity for the mu-opioid receptor. Although residue in position 4 serves a delta-address function, the tripeptide motif at the C-terminus of dermenkephalin and deltorphin I are critical components for high selectivity at delta-opioid receptor. Results demonstrate that mu- and delta-opioid receptors share topologically equivalent ligand-binding domains, or ligand-binding sequences similarities, that recognized Tyr-D-X-Phe as a consensus message-binding sequence. The delta-receptor additionally contains a unique address subsite at or near the conserved binding domain that accommodates the C-terminal tetrapeptide motif of dermenkephalin and deltorphin I.

[125I][D-Ala2]deltorphin-I: a high affinity, delta-selective opioid receptor ligand

The selective delta opioid agonist [D-Ala2]deltorphin-I was radioiodinated and the product purified using reverse phase HPLC. The binding characteristics and distribution profile of [125I][D-Ala2]deltorphin-I were assessed in mouse brain using homogenate binding techniques and quantitative autoradiography. [125I][D-Ala2]deltorphin-I bound with high affinity to a single class of sites (KD = 0.5 nM) in brain membrane preparations and striatal sections. Competition studies indicated that [125I][D-Ala2]deltorphin-I was selectively labeling delta opioid receptors as shown by the ratio of apparent affinities for mu and delta receptors (KI mu/KI delta = 1388). The autoradiographical distribution profile of [125I][D-Ala2]deltorphin-I binding sites was also consistent with that of other delta-selective radioligands. The data indicate that [125I][D-Ala2]deltorphin-I binds to delta opioid receptors with high affinity and selectivity. Because of its very high specific activity, it can be detected rapidly with high sensitivity by autoradiographic emulsion.

Opioid control of the release of Met-enkephalin-like material from the rat spinal cord

The possible control by opioids of the release of Met-enkephalin-like material (MELM) from the rat spinal cord was investigated in vitro and in vivo. Superfusion of slices of the dorsal zone of the lumbar enlargement with the mu selective agonists DAGO or PL 017 or the delta selective agonist DTLET produced a significant reduction in the K(+)-evoked MELM release from these tissues. These effects persisted in the presence ot tetrodotoxin, as expected from their mediation through presynaptically located opioid autoreceptors. Furthermore, the inhibitory effect of DAGO and PL 017, but not that of DTLET, was prevented by the preferential mu antagonist naloxone. Conversely, the effect of DTLET was prevented by the delta antagonist naltrindole but not by naloxone. In vivo experiments performed in halothane-anaesthetized rats have shown that the intrathecal perfusion of DAGO and DTLET significantly depressed the spontaneous MELM outflow from the whole spinal cord. In contrast to these mu and delta agonists, the kappa selective agonist U 50488 H did not affect the in vivo- and only slightly reduced (at a very high concentration: 50 microM) the in vitro-release of MELM from the rat spinal cord. These data indicate that both mu and delta opioid autoreceptors are involved in a local presynaptic autoinhibitory control of MELM release in the rat dorsal horn.

A supermolecule study of the effect of hydration on the conformational behaviour of leucine-enkephalin

A theoretical conformational study was performed on leu-enkephalin in its zwitterionic form, both in vacuo and in the presence of a number, n, of up to 13 water molecules saturating its first hydration shell. The intramolecular energy of enkephalin as well as the intermolecular enkephalin-water and water-water interaction energies were computed with the SIBFA procedure (Sum of Interactions Between Fragments Ab initio computed), which uses additive ab initio multipole systematics and analytical formulas grounded on ab initio SCF computations. Energy minimizations were performed with a polyvalent minimizer, Merlin, with which three distinct derivative and three distinct nonderivative minimizers can be activated in a sequential fashion. Eight different candidate conformations of enkephalin were used as starting points. These conformations are either those found in distinct X-ray structures, or those proposed on the basis of theoretical computations by other authors. In the absence of hydration, they converged towards distinct folded energy-minima, the best four ones being separated by an energy gap of 8.7 kcal/mol. In marked contrast, with up to n = 13, the energetical separation between the six best conformers narrowed down to congruent to 4 kcal/mol. They can be characterized by: (a) either a direct or a water-mediated ammonium-carboxylate interaction; b) either a close proximity (as in morphine) or a large separation between the aromatic rings of Tyr and Phe (intercenter separations of congruent to 4.5 A and congruent to 10.5 A, respectively), with each of the four mutual combinations of (a) and (b) being represented.

[3H]ohmefentanyl preferentially binds to mu-opioid receptors but also labels sigma-sites in rat brain sections

Ohmefentanyl has been shown to be 6300 times more potent than morphine for analgesia. The receptor binding characteristics and distribution of [3H]ohmefentanyl in rat brain sections are presented. [3H]Ohmefentanyl bound with high affinity to opioid receptors in a saturable manner (Kd = 0.95 +/- 0.08 nM, Bmax = 337 +/- 14 fmol/mg protein). We used various currently available specific mu, delta and kappa ligands to show that [3H]ohmefentanyl has a high selectivity for the mu opioid receptor. However, [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) was unable to completely inhibit [3H]ohmefentanyl specific binding, while complete inhibition was observed with fentanyl derivatives and the benzomorphan derivative, ethylketocyclazocine. This remaining 20% DAGO-inaccessible [3H]ohmefentanyl specific binding did not correspond to either mu1, delta or kappa sites. Haloperidol and 1,3-di-o-tolylguanidine were able to inhibit DAGO-inaccessible [3H]ohmefentanyl specific binding, suggesting that [3H]ohmefentanyl might also bind to haloperidol-sensitive sigma sites. The topographical distribution of [3H]ohmefentanyl found by autoradiography was generally similar to that of [3H]DAGO. However, in agreement with the biochemical results, quantitative analysis revealed additional sites in several rat brain regions, the greatest discrepancies with [3H]DAGO distribution being observed in cerebellum, central grey, hippocampal formation and locus coeruleus. Finally, our results suggest that this capacity of binding to both mu and sigma sites is shared by various fentanyl derivatives.

Development of delta opioid peptides as nonaddicting analgesics

Although much effort has been devoted to opioid research since the identification of enkephalins, understanding of the physiological importance and mechanisms of action of endogenous opioids lags behind understanding of opiate alkaloids such as morphine. In recent years, several novel approaches have been refined with promise for the successful development of the long-awaited nonaddicting analgesics that act at the opioid delta receptor. The present communication reviews these efforts.

Opioid delta agonists and endogenous enkephalins induce different emotional reactivity than mu agonists after injection in the rat ventral tegmental area

The possible role of opioid receptor heterogeneity in the biphasic changes in locomotion (activation and inhibition) induced by non-selective opiates such as morphine, has been investigated by measuring the behaviour of rats exposed to different environments after injection into the ventral tegmental area, of selective mu (DAGO) or delta (DTLET, DSTBULET, BUBU) opioid agonists and of kelatorphan, a complete inhibitor of enkephalin metabolism. delta agonists or kelatorphan-induced hyperactivity in a familiar (actimeter), unfamiliar (four-hole box) and a fear inducing (open-field) environment. These effects were suppressed by naloxone and delta selective antagonists (ICI 174, 864 2 mg/kg SC, naltrindole 7 nmol in the ventral tegmental area). Moreover, the delta agonists and endogenous enkephalins protected by kelatorphan did not affect the emotional state of rats measured in an elevated plus maze. Infused into the ventral tegmental area, DAGO also enhanced locomotion in the actimeter but in contrast to delta agonists and kelatorphan, the mu agonist decreased activity in the open-field and the four-hole box. The hypoactivity observed in these tests could be related to an enhanced emotionality produced by mu receptor stimulation, as shown by the significant decrease in the number of visits and time spent in open arms of the elevated plus maze. Naloxone (0.3 mg/kg SC) but not delta selective antagonists, blocked the various responses induced by DAGO.

In vivo tonic inhibition of spinal substance P (-like material) release by endogenous opioid(s) acting at delta receptors

Although numerous data support the existence of a presynaptic inhibitory control by opioids of substance P-containing primary afferent fibres entering the dorsal horn of the spinal cord, the exact nature of the opioid receptor involved in this control is still a matter of debate. In the present study, the potential role of delta opioid receptors was investigated by looking for the possible effects of selective delta ligands on the in vivo release of substance P-like material from the whole spinal cord in halothane-anaesthetized rats. Perfusion of the intrathecal space allowed the collection of substance P-like material that was released at a constant rate of approximately 0.65 pg substance P equivalents/min for at least 135 min. The addition of Tyr-D-Thr-Gly-Phe-Leu-Thr (10 microM) or dermenkephalin (10 microM), two selective delta agonists, to the perfusing fluid produced a marked reduction (-50-65%) in substance P-like material outflow which could be prevented by the selective delta antagonist naltrindole (10 microM) but not by naloxone (10 microM), which acts preferentially on mu opioid receptors. Furthermore, naltrindole alone (or the association of this antagonist plus dermenkephalin) enhanced the outflow of substance P-like material (+ 170%) as expected from the blockade of a tonic inhibitory control due to the stimulation of delta receptors by endogenous opioids.(ABSTRACT TRUNCATED AT 250 WORDS)