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

NOE data at 500 MHz reveal the proximity of phenyl and tyrosine rings in enkephalin

Met5-enkephalin-a pentapeptide (Tyr-Gly-Gly-Phe-Met)-can exist in two possible folded arrangements with a rigid two-hydrogen-bonded network. In one arrangement, a Gly 2-Gly 3 beta-bend is formed and in the other a Gly 3-Phe 4 beta-bend. The two conformations are distinguished by the spatial relation of Tyr 1 and Phe 4: in the Gly 2-Gly 3 beta-bend, Tyr 1 and Phe 4 can be brought close to each other while in the Gly 3-Phe 4 beta-bend they are far apart (greater than 5 A). We have utilized one-dimensional (1D) nuclear Overhauser effect (NOE) measurements between the ring protons of Tyr 1 and Phe 4 to determine their proximity. The NOE data clearly show that a pair protons, one each from Tyr 1 and Phe 4, are as close as 3.3 A while other inter-proton distances are beyond 4.5 A. Therefore, we propose the presence of a Gly 2-Gly 3 beta-bend (in which Tyr 1 and Phe 4 are spatially close) for Met5-enkephalin in solution. The structure of Met5-enkephalin in solution is very similar to the single crystal structure of Leu5-enkephalin and tends to explain the biological activity data of several modified enkephalins.

125I-FK 33-824: a selective probe for radioautographic labeling of mu opioid receptors in the brain

The selectivity of the Met-enkephalin analog FK 33-824 (FK) for mu opioid receptors has been, over the years, a matter of controversy. We report here pharmacological and radioautographic data demonstrating that at nanomolar concentrations. 125I-FK interacts exclusively with mu sites. (1) Specific binding of 125I-FK to rat striatal membranes is totally inhibited by mu- and/or delta-preferring ligands according to monovalent, Michaelian kinetics, with a potency proportional to the affinity of competing drugs for mu receptors. (2) Unlabeled FK competes only at high concentration with the delta-selective ligand 3H-DPLPE and according to the same kinetics as the mu-selective agonist DAGO. (3) 125I-FK generates the same regional radioautographic labeling pattern as 3H-DAGO. We conclude that when used at nanomolar concentrations 125I-FK constitutes a selective probe for the radioautographic detection of mu opioid receptors at both light and electron microscopic levels.

The contribution of intrinsic activity to the action of opioids in vitro

The effects of opioids were compared in five field-stimulated isolated tissue models, the guinea-pig ileum and vasa deferentia from rat, rabbit and mice of the Alderley Park and C57BL/6 strains. Although the mu-receptor agonist [D-Ala2, MePhe4, Gly-ol5] enkephalin appeared to act at similar receptors in the guinea-pig ileum, rat vas deferens, mouse vas deferens and C57BL/6 mouse vas deferens preparations, its potency varied considerably between these preparations. Similar potency differences were also observed with the kappa-agonist, ethylketocyclazocine. It is proposed that these variations in potency reflect differences in the number of spare receptors present in each model. The finding that some drugs which have agonist activity in the more sensitive preparations behave as antagonists in the less sensitive tissues supports this proposal and highlights the importance of intrinsic activity in determining the action of opioids. Many of the prototypic opioid agonists were found to be either partial agonists (eg. morphine and bremazocine) or to possess affinity for more than one receptor type (eg. ethylketocyclazocine, Mr 2034).

125I-DPDYN, monoiodo[D-Pro10]dynorphin(1-11): a highly radioactive and selective probe for the study of kappa opioid receptors

The mono- and diiodinated derivatives of the kappa-selective ligand [D-Pro10]dynorphin(1-11), DPDYN, were prepared. Their binding properties at the three opioid receptor types (mu, delta and kappa) were examined and compared to those of the parent peptide. The monoiodo derivative shows a general although moderate decrease in affinity and retains high kappa selectivity (KI mu/KI kappa = 48 and KI delta/KI kappa = 140). The binding properties of the diiodo derivative are found to be dramatically decreased. Radioiodination of DPDYN leads to the monoiodinated peptide with high specific activity (700-800 Ci/mmol). In guinea-pig cerebellum membranes, a kappa-specific tissue, [125I]-labelled monoiodo[D-Pro10]dynorphin(1-11), 125I-DPDYN, interacts specifically and reversibly with a single class of binding sites (Bmax = 118 fmol/mg protein) with a high affinity (KD = 0.12 nM from equilibrium experiments, 0.18 nM from kinetics studies). Therefore, because of its high specific radioactivity, high affinity and reasonably good selectivity, 125I-DPDYN designates itself as the probe of the k-opioid receptor type.

Synthesis and binding properties of specific photoaffinity ligands for mu and delta opioid receptor subtypes

We report the synthesis and binding properties of specific photoaffinity ligands for mu and delta opioid receptor subtypes. These ligands are derived from DAGO: Tyr-D-Ala-Gly-NMePhe-Gly-ol, a mu selective probe and DTLET: Tyr-D-Thr-Gly-Phe-Leu-Thr, a delta selective probe by modifying the Phe 4 residue. These modifications are: i) a nitro group on the para position of Phe ring as Phe(4 NO2) or Nip, ii) an azido group as Phe(4 N3) or AZ. Pharmacological responses on mouse vas deferens (delta sites) and guinea pig ileum (mu sites), as well as competition experiments with [3H] DAGO and [3H] DTLET on crude rat brain membranes have been performed. The nitro group on the phenyl ring of the Phe residue preserves the affinity and selectivity of each probe: NipDAGO for the mu sites, NipDTLET for the delta ones. However the nitro probes do not appear to be photoactivable by u.v. irradiation. Likewise, azidation of the phenyl ring of the Phe residue does not change the receptor selectivity of each probe, but AZDAGO has less affinity than its parent molecule DAGO, while AZDTLET has more affinity than DTLET. These compounds are photoactivable and provide an efficient tool to characterize and isolate the different receptor subtypes, especially the delta site.

Characterisation of the delta-opioid receptor on the hamster vas deferens

It has recently been reported that the hamster vas deferens contains only delta-opioid receptors. We have demonstrated that three delta-receptor agonists [D-Ala2, D-Leu5]enkephalin (DADLE), [D-Pen2, D-Pen5]enkephalin (DPDPE) and [D-Thr2, Leu5, Thr6] enkephalin (DTLET) appear to mediate their effects via the delta-receptor since they are readily reversed by the selective delta-receptor antagonist ICI 174864. In addition, a number of classical mu and k receptor compounds were devoid of activity in this preparation. However it was observed that some compounds such as etorphine and MR 2034 reported to possess delta-receptor affinity in other assay systems were weak or inactive on the hamster vas deferens.

NMB: a human neuroblastoma cell line with specific opiate binding sites

The human neuroblastoma cell line designated NMB (Brodeur et al., 1977, Cancer 40: 2256) has been shown to have specific opiate binding sites. These sites are highly stereospecific. Two characteristic delta specific peptides, D-Ala2-D-Leu5 enkephalin and D-Thr2-D-Thr6 enkephalin, have high affinity for the binding sites. Morphine binds specifically but with a much lower affinity. Dextrorphan and the mu specific peptide morphiceptin (Tyr-Pro-Phe-Pro-CO-NH2) do not bind to the site. The binding sites are heat and trypsin sensitive. Sodium ions specifically lower agonist binding to the sites. Approximately 14,000 binding sites per cell are found. The binding characteristics of these sites are very similar to those of the delta sites characterized on mouse neuroblastoma cell lines.

Localization of mu- and delta-opioid receptors in cat respiratory areas: an autoradiographic study

Autoradiography after in vitro binding with selective ligands for either mu ([3H](Tyr-D-Ala-Gly-(NMePhe)-Gly-ol] or delta ([3H](Tyr-D-Thr-Gly-Phe-Leu-Thr)) opioid receptor types revealed the presence of variable amounts of radioactive labeling in the cat brainstem. Areas involved in the respiratory rhythmogenesis were among the most prominently labeled structures. The pneumotaxic center, including the nucleus parabrachialis medialis and the Kolliker-Füse nucleus, contains a very high density of delta binding sites while the dorsal respiratory nucleus which corresponds to the nucleus tractus solitarius, is more heavily labeled by the mu ligand. The neuroanatomical differences in the distribution of opioid receptors correlates well with the pharmacological responses induced by administration of specific mu- or delta ligands.

Opiate binding sites in the chick, rabbit and goldfish retina

The characteristics of opiate binding sites in the retina of the chick, rabbit and goldfish have been investigated. In the newly hatched chick retina, 131 fmol/mg of binding sites for [D-Ala2-D-Leu5]-[3H]enkephalin are present; competition studies with the delta selective peptide [D-Thr-Leu5]-enkephalin (DTLET) and the mu selective peptide morphiceptin show that all of the [D-Ala2-D-Leu5]-[3H]-enkephalin binding sites are of the delta subtype. Dihydro[3H]morphine binds poorly to the chick retina; 13.2 fmol/mg of this binding is displaceable by morphiceptin and corresponds to mu binding sites. Benzomorphan sites are defined as sites occupied by [3H]diprenorphine which is displaceable by low concentrations of ethylketocyclozacine but not by high concentrations of D-Ala2-D-Leu5-enkephalin and morphiceptin. At least 88 fmol/mg of benzomorphan sites are present in the chick retina. [3H]diprenorphine binding to the rabbit and fish retina was measured. The rabbit retina bound 60 fmol/mg, and the fish retina 42 fmol/mg of [3H]diprenorphine. These findings are discussed in the light of the studies on the localization and physiological effects of enkephalin in the retina.

Differences in binding properties of mu and delta opioid receptor subtypes from rat brain: kinetic analysis and effects of ions and nucleotides

Differences in binding properties of mu and delta opioid receptors were investigated using DAGO (Tyr-D-Ala-Gly-MePhe-Gly-ol) and DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr), which occur, respectively, as the most selective mu and delta radioligands available. At high concentration, each agonist is able to interact with its nonspecific sites. Competition experiments indicated that a two-site competitive model was adequate to explain the interactions of DAGO and DTLET with [3H]DTLET and [3H]DAGO binding sites, respectively. The weak cross-reactivity (congruent to 10%) of DTLET for mu sites was taken into account in these experiments. On the other hand, DAGO and DTLET exhibit differential binding kinetics. Thus, at 35 degrees C, the lifetime of DTLET within its receptor site is about 14 times longer than that of the mu agonist. Sodium and manganese ions decrease the maximal number of high affinity mu and delta sites, but the sensitivity of mu receptors is three times higher towards Na+ and 20-fold higher towards Mn2+ than that of delta receptors. GTP reduces similarly the mu and delta binding whereas only the DAGO binding was modified by the nonhydrolyzable analogue guanylylimidodiphosphate [GMP-P(NH)P]. However, in the presence of Na+ ions, GMP-P(NH)P inhibits the DTLET binding in a concentration-dependent manner. The effects of Na+ and GMP-P(NH)P could be explained by a sequential transformation of delta receptors to low-affinity states. This model predicts that Na+, by lowering the affinity of a fraction of sites, produces a decrease in the maximal number of high-affinity delta receptors and that GMP-P(NH)P enhances the Na+ effect. Moreover, the binding kinetic to this high-affinity state was also modified by Na+ and nucleotides. All of these data support the existence of two independent mu and delta binding sites, the properties of which are differentially regulated by these endogenous effectors.

Comparative binding properties of linear and cyclic delta-selective enkephalin analogues: [3H]-[D-Thr2, Leu5] enkephalyl-Thr6 and [3H]-[D-Pen2, D-Pen5] enkephalin

The range of delta-selectivity of linear and cyclic analogues of enkephalin in rat brain was found to be: [D-Pen2, L-Pen5] enkephalin (DPLPE) greater than [D-Pen2, D-Pen5] enkephalin (DPDPE) greater than [D-Thr2, Leu5] enkephalyl-Thr6 (DTLET) greater than [D-Ser2, Leu5] enkephalyl-Thr6 (DSLET). Saturation experiments performed with [3H]DPDPE and [3H]DTLET in NG108-15 cells and rat brain showed similar binding capacities for both the ligands, but the delta-affinity of [3H]DTLET (KD approximately 1.2 nM) was much better than that of [3H]DPDPE (KD approximately 7.2 nM). The rather low delta-affinity of DPDPE induced high experimental errors cancelling the benefit of its better delta-selectivity. Binding experiments in rat or guinea-pig brains showed, in both cases, the better delta-selectivity of [3H]DTLET compared to [3H]DSLET. The former peptide remains at this time the most appropriate radioactive probe for binding studies of delta-receptor.

Possible interactions between dynorphin and dopaminergic systems in rat basal ganglia and substantia nigra

Chronic haloperidol treatment markedly increases dynorphin-related peptide contents in caudate-putamen, globus pallidus and substantia nigra. Leu-enkephalin levels follow dynorphin-related peptide concentrations in these areas while Met-enkephalin-related peptide contents are unchanged in the substantia nigra following a similar treatment. An acute haloperidol injection had no effect on any opioid peptide levels in the basal ganglia. This suggests that Leu-enkephalin is likely to be derived from prodynorphin in the rat striatonigral pathway. Moreover, the Leu-enkephalin/dynorphin projection appears to be under striatal dopaminergic control.

Characterization of [3H][2-D-penicillamine, 5-D-penicillamine]-enkephalin binding to delta opiate receptors in the rat brain and neuroblastoma--glioma hybrid cell line (NG 108-15)

Specific binding properties of the tritium-labeled delta opiate receptor agonist [3H][2-D-penicillamine, 5-D-penicillamine]enkephalin [( 3H][D-Pen2, D-Pen5]enkephalin) were characterized in the rat brain and in a mouse neuroblastoma-rat glioma hybrid cell line (NG 108-15). Saturation isotherms of [3H][D-Pen2, D-Pen5]enkephalin binding to rat brain and NG 108-15 membranes gave apparent Kd values of 1-6 nM. These values are in good agreement with the Kd value obtained from the kinetic studies. The Bmax value in NG 108-15 membranes was 235.3 fmol/mg of protein. An apparent regional distribution of [3H][D-Pen2, D-Pen5]enkephalin binding was observed in the rat brain. A number of enkephalin analogues inhibited [3H][D-Pen2, D-Pen5]enkephalin binding with high affinity (IC50 values of 0.5-5.0 nM) in both NG 108-15 and rat brain membranes. However, putative mu receptor-selective ligands such as morphine, [D-Ala2, MePhe4, Gly5-ol]enkephalin, [MePhe3, D-Pro4]morphiceptin, and naloxone were less effective inhibitors of [3H][D-Pen2, D-Pen5]enkephalin binding in both systems tested. These data suggest that [3H][D-Pen2, D-Pen5]enkephalin is a potent and selective ligand for the delta opioid receptor.

Two new opioid delta-receptor ligands: a highly selective agonist and a potent selective antagonist in in vitro isolated preparations

N,N-Diallyl derivatives of enkephalin analogues were chemically synthesized, and their biological activities were estimated in vitro isolated preparations. N,N-Diallyl-[D-Ala2, D-Leu5]-enkephalin [test compound I] at doses up to 10 microM did not inhibit the electrically-evoked contractions of guinea-pig ileum, which had been suggested to contain opioid mu- and kappa-receptors, but it significantly depressed the contractions of mouse vas deferens, which had been indicated to contain mu-, kappa- and delta-receptors, suggesting that test compound I did not act on both mu- and kappa-receptors, but acted on delta-receptors. Additionally, the Ke (equilibrium dissociation constant) values against test compound I of naloxone were approximately 30 nM and similar to those of Mr 2266, also indicating that test compound I acted as a delta agonist. Moreover, the Ke values of ICI 154129 against compound I were approximately 340 nM, strongly suggesting that test compound I acted as a delta agonist. The Ke values of bis-[N,N-diallyl-[D-Ala2, Leu5]-enkephalyl]-cystine [test compound II] against [D-Ala2, D-Leu5]-enkephalin in mouse vas deferens and morphine or ethylketocyclazocine in guinea-pig ileum were 44.9 nM and 5.00 or 11.3 microM, respectively, showing that test compound II was a potent selective opioid delta antagonist. In conclusion, among compounds synthesized, two new opioid delta-receptor ligands, one being a highly selective agonist and the other being a potent selective antagonist in in vitro isolated preparations, were found in the present study.

Highly selective photoaffinity labeling of mu and delta opioid receptors

We report the synthesis and photolabeling properties of two highly selective ligands for mu and delta opioid-binding sites: Tyr-D-Ala-Gly-MePhe (pN3)-Gly-ol (AZ-DAMGE) and Tyr-D-Thr-Gly-Phe (pN3)-Leu-Thr (AZ-DTLET). An irreversible inhibition of the electrically induced contractions of mouse vas deferens is caused by irradiation (at 254 nm) of the muscle strip in the presence of AZ-DTLET (1 nM). This phenomenon is antagonized only at large concentrations (10 microM) of naloxone, in accordance with the well-known lower selectivity of naloxone for delta sites. Competition experiments with [3H]DAMGE and [3H]DTLET on crude rat brain membranes showed that the azido photoprobes display a similar (AZ-DAMGE) and even a better (AZ-DTLET) selectivity than their respective parent compounds DAMGE and DTLET. Up to 25 nM, AZ-DTLET irreversibly and selectively photolabels the delta sites of crude rat brain homogenates. Due to its lower affinity AZ-DAMGE provides similar selective photolabeling of the mu sites but at higher concentrations (approximately equal to 0.3 microM). When [3H]DAMGE and [3H]DTLET were used as ligands for mu and delta binding subtypes, respectively, no important change in binding capacity and affinity of one receptor type was observed after photolabeling of the other.

Anatomical relationship between opioid peptides and receptors in rhesus monkey brain

To determine whether opioid peptide-receptor pharmacological association found in vitro (e.g., enkephalin-delta, dynorphin-kappa) predict anatomical relationships in situ, immunocytochemical and receptor autoradiographic studies were carried out on adjacent sections from the same brains of formaldehyde-perfused rhesus monkeys. Apparent mu and kappa opioid receptors (labeled, respectively, by [3H] naloxone and [3H]bremazocine under different incubation conditions), but not delta opioid receptors (labeled by [3H]D-Ala2, D-Leu5-enkephalin), survived the fixation procedure, and were found to be colocalized throughout the brain. We have observed complex associations between these binding sites and one, two, or all three opioid peptide systems (i.e., proopiomelanocortin, proenkephalin, and prodynorphin) in different brain regions. These multiple opioid peptide-receptor subtype associations are apparent, for example, in neural systems involved in the processing of pain stimuli, and may be important for mediating different types of analgesia. Since differential processing of proenkephalin and prodynorphin can give rise to opioids of varying receptor selectivities, the colocalization of opioid receptor subtypes may signify that such processing is a key regulatory event in determining which receptor subtype is activated and, thus, the physiological consequences of opioid neurotransmission.

Endogenous opiates: 1983

This paper is the sixth in an annual series of reviews of research involving the endogenous opiates, each installment being restricted to work published during the previous year. Although the early articles in the series attempted to be comprehensive and cover the complete range of research with the opiate peptides, in the last two years we have limited our coverage to non-analgesic and behavioral work due to the enormous number of articles published in the field. The specific areas discussed here include stress, tolerance and dependence, consummatory responses, other gastrointestinal functions, interactions with alcohol, mental illness, learning and memory, cardiovascular responses, respiratory effects, thermoregulation, neurological disorders, activity, and miscellaneous other topics. As in previous years, we have attempted to present a relatively complete review of the subjects covered only for the previous year and generally have not tried to evaluate their contributions relative to those of past years.

Selectivities of opioid peptide analogues as agonists and antagonists at the delta-receptor

The endogenous opioid ligands interact with more than one of the mu-, delta- and kappa-binding sites. By the use of binding assays and bioassays, enkephalin analogues have been assessed for their selectivity for binding at the delta-binding site and for their agonist and antagonist activities at the delta-receptor. The electrically-induced contractions of myenteric plexus-longitudinal muscle preparations of the guinea-pig ileum were inhibited by mu- and kappa-receptor ligands. Inhibitions were seen with mu-, delta- and kappa-receptor ligands in the mouse vas deferens, mainly with mu-receptor ligands in the rat vas deferens and only with kappa-receptor ligands in the rabbit vas deferens. From observations on a considerable number of [Leu5] enkephalin analogues, it has been concluded that [D-Pen2, D-Pen5] enkephalin and [D-Pen2, L-Pen5] enkephalin are the most selective delta-agonists and that N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH is the most selective antagonist (Aib = alpha-aminoisobutyric acid). The binding of these peptides at the delta-site is 99% of the total binding. As to potency, the agonists are superior to the antagonists.

Solubilization and characterization of mu, delta, and kappa opioid binding sites from guinea pig brain: physical separation of kappa receptors

Sucrose density gradient centrifugation of digitonin-solubilized opioid binding sites from guinea pig brain and cerebellum was carried out. Centrifugation of extracts of whole brain into a gradient devoid of sodium and low in digitonin revealed the presence of two well-separated peaks of opioid binding activity. Peak A was shown to have the binding characteristics of kappa sites, whereas peak B seems to be a mixture of mu and delta sites. When extracts of guinea pig cerebellum were treated in the same manner, a single peak of binding activity was obtained that coincided with peak A from guinea pig brain and exhibited the characteristics of kappa binding sites. All three sites closely resemble their membrane-bound counterparts, retaining good affinity and selectivity for their appropriate ligands. The apparent sedimentation coefficients (S20,w) of the digitonin-solubilized binding sites present in the two peaks are 19 s for peak A and 34-39 s for peak B, and the estimated apparent molecular weights are 400,000 for kappa sites and 750,000-875,000 for the mixture of mu and delta sites. Our results suggest that kappa sites constitute separate molecular entities from mu and delta sites.

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

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

Pain, nociception and spinal opioid receptors

Opioid peptides derived from proenkephalin and prodynorphin are differentially distributed in the spinal cord. Proenkephalin peptides are preferentially located in the sacral portion of the cord while prodynorphin peptides are concentrated in the cervical spinal cord. Mu opioid receptor are highly concentrated in superficial layers of the dorsal horn in all the spinal cord. Delta opioid receptor are more diffusely distributed in the gray matter of the spinal cord. These sites are principally located in cervical and thoracic portions of the spinal cord. Kappa opioid receptors are highly concentrated in the superficial layers of the lumbo-sacral spinal cord. Its density decreased in the upper levels of the spinal cord. It appears that mu opioid receptors are indifferentially activated by thermal, pressure and visceral nociceptive inputs. Delta receptors are more likely to be involved in thermal nociception while kappa opioid binding sites are associated to visceral pain nociceptive inputs.

Bis-penicillamine enkephalins possess highly improved specificity toward delta opioid receptors

The conformationally restricted, cyclic, disulfide-containing, enkephalin analogs [2-D-penicillamine, 5-L-penicillamine]enkephalin [(D-Pen2,L-Pen5]enkephalin) and [2-D-penicillamine, 5-D-penicillamine]enkephalin [(D-Pen2,D-Pen5]enkephalin) were synthesized by solid-phase methods. Selectivities of these analogs for a single class of opioid receptor were investigated by examining relative potencies in the mouse vas deferens assay, in which the functional receptor is the delta receptor, versus the guinea pig ileum assay, in which the mu receptor is the functional receptor, and by determining their relative abilities to displace the prototypical delta receptor ligand [D-Ala2, D-Leu5]enkephalin and the prototypical mu receptor ligand naloxone from rat brain membrane preparations. Based on these comparisons [D-Pen2,L-Pen5]- and [D-Pen2,D-Pen5]enkephalin exhibited delta receptor selectivities of 1,088 and 3,164, respectively, in the bioassays, and 371 and 175, respectively, in the binding assays. Compared with the previously reported delta receptor selective analogs, [D-Ala2,D-Leu5]enkephalin, [D-Ser2,Leu5,Thr6]enkephalin, and [D-Thr2,Leu5,Thr6]enkephalin, the bis-Pen-containing analogs provide an order of magnitude increase in delta receptor selectivity.

Pain control by endogenous enkephalins is mediated by mu opioid receptors

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

Differential properties of mu and delta opiate binding sites studied with highly selective ligands

Differences in binding kinetics, ions and nucleotides interactions with rat brain opiate mu and delta receptor subtypes were investigated using respectively DAGO (Tyr-D-Ala-Gly-MePhe-Gly-ol) and DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr) as highly selective mu and delta ligands. On the basis of kinetic experiments the delta-agonist binding is an extremely slow process as compared to that of the mu-agonist. Displacement experiments by DTLET of [3H] DAGO binding and Scatchard analysis of the binding of [3H] DTLET in the absence and presence of DAGO demonstrated that an independent model of interaction to two independent sites best describes the observed data. Steady state binding of [3H] DAGO is decreased by GMP-P(NH)P while this nucleotide is ineffective in reducing the binding of DTLET. However in presence of NaCl, GMP-P(NH)P reduces the specific binding at both sites in a concentration dependent manner. The analysis of the nucleotide inhibition of delta-binding at various concentrations of NaCl suggests that the non-hydrolyzable nucleotide can exerts its effects only on a prebound Na+ receptor. These data suggest that mu and delta-receptors may assume different affinity states depending upon the presence of Na+ and nucleotide.

Cyclic penicillamine containing enkephalin analogs display profound delta receptor selectivities

The cyclic, penicillamine(beta, beta dimethylcysteine)-containing enkephalin analogs, [D-Cys2, L-Pen5]-and [D-Cys2, D-Pen5]enkephalin and the corresponding bis-penicillamine analogs, [D-Pen2, L-Pen5]-and [D-Pen2, D-Pen5]enkephalin were synthesized and evaluated for opioid activity in the guinea pig ileum (GPI) and mouse vas deferens (MVD) bioassays and in rat brain and neuroblastoma-glioma cell membrane binding assays. These analogs all displayed delta receptor selectivity as assessed by IC50(GPI)/IC50(MVD) ratios and by their relative potencies for displacing [3H]naloxone (NAL) vs. [3H] [D-Ala2, D-Leu5]enkephalin (DADLE) from rat brain membrane preparations. For [D-Pen2, L-Pen5]- and [D-Pen2, D-Pen5]enkephalin the observed IC50(GPI)/IC50 (MVD) ratios (1088 and 3164) and IC50NAL/IC50DADLE ratios (371 and 175) represent a vast improvement over previously reported delta receptor selective ligands.

Autoradiographic distribution of mu and delta opiate receptors in rat brain using highly selective ligands

We have characterized the autoradiographic distribution of mu and delta opiate receptors in rat brain using [3H]-Tyr-D-Ala-Gly-NMe-Phe-Gly-ol (DAGO) and [3H]D-Thr2, Thr6leu-enkephalin (DTLET), two highly selective ligands for mu and delta opiate receptors, respectively. Both ligands label an apparent single class of sites. Ligand selectivity shows that [3H]DAGO and [3H]DTLET are more selective ligands for their respective receptors than [3H]dihydromorphine (DHM) and [3H][D-Ala2,D-Leu5]enkephalin (DADLE). Autoradiographically, mu opiate receptors are highly discretely distributed in certain areas. High densities are found in "patches" in caudate-putamen, nucleus accumbens, cingulate cortex, habenula, various thalamic nuclei, amygdala, superior colliculus, and interpeduncular nucleus. Delta opiate receptors are mostly concentrated in the external plexiform layers of the olfactory bulb, caudate-putamen, olfactory tubercule, claustrum, and deep layers (layers V and VI) of the cortex. Since [3H]DAGO and [3H]DTLET are more selective ligands for their respective opiate receptor subtypes, they should be better ligands to characterize the fine distribution of mu and delta opiate receptors.