DADLE promotes motor function recovery by inhibiting cytosolic phospholipase A2 mediated lysosomal membrane permeabilization after spinal cord injury

BACKGROUND AND PURPOSE

Autophagy is a protective factor for controlling neuronal damage, while necroptosis promotes neuroinflammation after spinal cord injury (SCI). DADLE (D-Ala2 , D-Leu5 ]-enkephalin) is a selective agonist for delta (δ) opioid receptor and has been identified as a promising drug for neuroprotection. The aim of this study was to investigate the mechanism/s by which DADLE causes locomotor recovery following SCI.

EXPERIMENTAL APPROACH

Spinal cord contusion model was used and DADLE was given by i.p. (16 mg·kg-1 ) in mice for following experiments. Motor function was assessed by footprint and Basso mouse scale (BMS) score analysis. Western blotting used to evaluate related protein expression. Immunofluorescence showed the protein expression in each cell and its distribution. Network pharmacology analysis was used to find the related signalling pathways.

KEY RESULTS

DADLE promoted functional recovery after SCI. In SCI model of mice, DADLE significantly increased autophagic flux and inhibited necroptosis. Concurrently, DADLE restored autophagic flux by decreasing lysosomal membrane permeabilization (LMP). Additionally, chloroquine administration reversed the protective effect of DADLE to inhibit necroptosis. Further analysis showed that DADLE decreased phosphorylated cPLA2 , overexpression of cPLA2 partially reversed DADLE inhibitory effect on LMP and necroptosis, as well as the promotion autophagy. Finally, AMPK/SIRT1/p38 pathway regulating cPLA2 is involved in the action DADLE on SCI and naltrindole inhibited DADLE action on δ receptor and on AMPK signalling pathway.

CONCLUSION AND IMPLICATION

DADLE causes its neuroprotective effects on SCI by promoting autophagic flux and inhibiting necroptosis by decreasing LMP via activating δ receptor/AMPK/SIRT1/p38/cPLA2 pathway.

Multifaceted Effects of Delta Opioid Receptors and DADLE in Diseases of the Nervous System

BACKGROUND

The opioid system is considered a potential therapeutic target in a variety of neurological disorders. Delta opioid receptors (DORs) are broadly expressed in the brain, and their activation protects cells from hypoxic/ischemic insults by counteracting disruptions of ionic homeostasis and initiating neuroprotective pathways. The DOR agonist D-Ala2-D-Leu2-Enkephalin (DADLE) promotes neuronal survival, mitigates apoptotic pathways, and protects neurons and glial cells from ischemia-induced cell death, thus making DADLE a promising therapeutic option for stroke. The significant amount of research regarding DORs and DADLE in the last decades also suggests their potential in treating other neurological disorders.

METHODS

This review compiled relevant literature detailing the role of DORs and agonists in central nervous system function and neuropathologies.

RESULTS

Several studies demonstrate potential mechanisms implicating a key interaction between DORs and DADLE in conferring neuroprotective benefits. A better understanding of DOR function in disease-specific contexts is critical to transitioning DOR agonists into the clinic as a therapy for stroke and other neurological diseases.

CONCLUSION

Evidence-based studies support the potential of the delta-opioid family of receptors and its ligands in developing novel therapeutic strategies for stroke and other brain disorders.

Crosstalk between delta opioid receptor and nerve growth factor signaling modulates neuroprotection and differentiation in rodent cell models

Both opioid signaling and neurotrophic factor signaling have played an important role in neuroprotection and differentiation in the nervous system. Little is known about whether the crosstalk between these two signaling pathways will affect neuroprotection and differentiation. Previously, we found that nerve growth factor (NGF) could induce expression of the delta opioid receptor gene (Oprd1, dor), mainly through PI3K/Akt/NF-κB signaling in PC12h cells. In this study, using two NGF-responsive rodent cell model systems, PC12h cells and F11 cells, we found the delta opioid neuropeptide [d-Ala², d-Leu⁵] enkephalin (DADLE)-mediated neuroprotective effect could be blocked by pharmacological reagents: the delta opioid antagonist naltrindole, PI3K inhibitor LY294002, MAPK inhibitor PD98059, and Trk inhibitor K252a, respectively. Western blot analysis revealed that DADLE activated both the PI3K/Akt and MAPK pathways in the two cell lines. siRNA Oprd1 gene knockdown experiment showed that the upregulation of NGF mRNA level was inhibited with concomitant inhibition of the survival effects of DADLE in the both cell models. siRNA Oprd1 gene knockdown also attenuated the DADLE-mediated neurite outgrowth in PC12h cells as well as phosphorylation of MAPK and Akt in PC12h and F11 cells, respectively. These data together strongly suggest that delta opioid peptide DADLE acts through the NGF-induced functional G protein-coupled Oprd1 to provide its neuroprotective and differentiating effects at least in part by regulating survival and differentiating MAPK and PI3K/Akt signaling pathways in NGF-responsive rodent neuronal cells.

Nanofiber-based delivery of therapeutic peptides to the brain

The delivery of therapeutic peptides and proteins to the central nervous system is the biggest challenge when developing effective neuropharmaceuticals. The central issue is that the blood-brain barrier is impermeable to most molecules. Here we demonstrate the concept of employing an amphiphilic derivative of a peptide to deliver the peptide into the brain. The key to success is that the amphiphilic peptide should by design self-assemble into nanofibers wherein the active peptide epitope is tightly wrapped around the nanofiber core. The nanofiber form appears to protect the amphiphilic peptide from degradation while in the plasma, and the amphiphilic nature of the peptide promotes its transport across the blood-brain barrier. Therapeutic brain levels of the amphiphilic peptide are achieved with this strategy, compared with the absence of detectable peptide in the brain and the consequent lack of a therapeutic response when the underivatized peptide is administered.

Naloxone-blocked depriming effect of anxiolytic selank on apomorphine-induced behavioral manifestations of hyperfunction of dopamine system

Peptide anxiolytic selank (Thr-Lys-Pro-Arg-Pro-Gly-Pro) applied intraperitoneally in doses of 0.01, 0.1, 1.0, and 10.0 mg/kg to mice reduces behavioral manifestations of dopaminergic system induced by apomorphine in the verticalization test. This effect was comparable to that of atypical antipsychotic olanzapine in near-therapeutic doses (0.1 and 1.0 mg/kg, intraperitoneally) and was blocked with nonselective opioid receptor antagonist naloxone (10 mg/kg, intraperitoneally). Radioreceptor assay showed that selank did not displace nonselective D2-dopamine receptor antagonist (3)H-spiperone (EC50>100 microM) and delta- and micro-opioid receptor ligand 3H-DADLE (EC50>40 microM) from specific binding sites on rat brain membranes. It is hypothesized that the revealed behavioral effect of selank is mediated by its modulating effect on the endogenous opioid system and specifically, by its effect on activity of enkephalin-degrading enzymes.

Rapid hypoxia preconditioning protects cortical neurons from glutamate toxicity through delta-opioid receptor

BACKGROUND AND PURPOSE

Hypoxia preconditioning (HPC), rapid or delayed, has been reported to induce neuroprotection against subsequent severe stress. Because delta-opioid receptor (DOR) plays an important role in delayed HPC-induced neuroprotection against severe hypoxic injury, we asked whether DOR is also involved in the rapid HPC-induced neuroprotection.

METHODS

Cultured rat cortical neurons at culture days 8 to 9 were exposed to a short-term hypoxia (1% O2 for 30 minutes) to induce HPC followed by 30-minute normoxia before exposing to glutamate toxicity (100 micromol/L; 4 hours). Neuronal viability was assessed by lactate dehydrogenase leakage and morphological assessment. Protein and mRNA levels of DOR were detected by receptor binding and RT-PCR, respectively. Naltrindole was used to block DOR. Developmental changes in NMDA receptor expression was measured by Western blots.

RESULTS

HPC significantly reduced the glutamate-induced neuronal injury. Receptor binding showed that HPC increased DADLE (a DOR ligand) binding density in the cultured cortical neurons by >90% over control level (P<0.05), although RT-PCR did not detect any appreciable change in DOR mRNA. DOR inhibition with naltrindole had no effect on neuronal injury and completely abolished the HPC-induced neuroprotection. In contrast to HPC-induced increase in DADLE binding density, prolonged hypoxia caused severe neuronal injury with a significant decrease in DADLE binding density and DOR mRNA level.

CONCLUSIONS

DOR is involved in neuroprotection induced by rapid HPC in cortical neurons.

Stability of oxymethyl-modified coumarinic acid cyclic prodrugs of diastereomeric opioid peptides in biological media from various animal species including human

In vitro stability studies of oxymethyl-modified coumarinic acid (OMCA) cyclic prodrugs of the diastereomeric opioid peptides DADLE ([D-Ala2,D-Leu5]-Enk, H-Tyr-D-Ala-Gly-Phe-D-Leu-OH), [Ala2,D-Leu5]-Enk (H-Tyr-Ala-Gly-Phe-D-Leu-OH), [D-Ala2,Leu5]-Enk (H-Tyr-D-Ala-Gly-Phe-Leu-OH), and [Ala2,Leu5]-Enk (H-Tyr-Ala-Gly-Phe-Leu-OH) were conducted to evaluate how the chirality of specific amino acid residues (Ala2 and Leu5) in the peptide portion affects their bioconversion by esterases. The stability studies were conducted at 37 degrees C in plasma and tissue homogenates (liver and brain) from five animal species (rat, mouse, canine, guinea pig, and hamster) and human in an attempt to identify an animal species that had a "prodrug bioconversion profile" comparable to that of humans. Initially, the total esterase activity in these biological media was measured using p-nitrophenyl butyrate (PNPB) as a substrate. By repeating this activity assay in the presence of paraoxon, a potent esterase B inhibitor, it was possible to estimate the relative amounts of esterases B and esterases A/C in a biological sample. Stability studies of the cyclic prodrugs were carried out under identical conditions, that is, in the presence and absence of paraoxon. Significant differences in the rates of hydrolysis of the cyclic prodrugs were observed, particularly between cyclic prodrugs with differences in the chirality of the amino acid on the C-terminus of the peptide portion, for example, L-amino acids at the C-terminus hydrolyzed more rapidly than D-amino acids. This stereoselective hydrolysis was independent of the animal species but tended to be more pronounced in brain and liver homogenates compared to plasma. Increased esterase specific activity, as measured by PNPB, in the biological media did not necessarily correlate with increased bioconversion rates of the cyclic prodrugs. The enzymatic stability profiles of the cyclic prodrugs in biological media from canine and guinea pig most closely resembled the profiles from human biological media. Therefore, canine and guinea pig appear to be the most relevant animal models for conducting pharmacokinetic studies on these cyclic prodrugs of opioid peptides.

Opioid modulation of GABA release in the rat inferior colliculus

BACKGROUND

The inferior colliculus, which receives almost all ascending and descending auditory signals, plays a crucial role in the processing of auditory information. While the majority of the recorded activities in the inferior colliculus are attributed to GABAergic and glutamatergic signalling, other neurotransmitter systems are expressed in this brain area including opiate peptides and their receptors which may play a modulatory role in neuronal communication.

RESULTS

Using a perfusion protocol we demonstrate that morphine can inhibit KCl-induced release of [3H]GABA from rat inferior colliculus slices. DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin) but not DADLE ([D-Ala2, D-Leu5]-enkephalin or U69593 has the same effect as morphine indicating that micro rather than delta or kappa opioid receptors mediate this action. [3H]GABA release was diminished by 16%, and this was not altered by the protein kinase C inhibitor bisindolylmaleimide I. Immunostaining of inferior colliculus cryosections shows extensive staining for glutamic acid decarboxylase, more limited staining for micro opiate receptors and relatively few neurons co-stained for both proteins.

CONCLUSION

The results suggest that micro-opioid receptor ligands can modify neurotransmitter release in a sub population of GABAergic neurons of the inferior colliculus. This could have important physiological implications in the processing of hearing information and/or other functions attributed to the inferior colliculus such as audiogenic seizures and aversive behaviour.

The effect of substitution patterns on the release rates of opioid peptides DADLE and [Leu5]-enkephalin from coumarin prodrug moieties

A coumarin-based prodrug system has been developed in our laboratory for the preparation of esterase-sensitive prodrugs of amines, peptides, and peptidomimetics. The drug release rates from this prodrug system were found to be dependent on the structural features of the drug moiety. The effect of the phenyl ring substitutions on the release kinetics of such prodrugs of model amines was examined recently and it was found that appropriately positioned alkyl substituents on the phenyl ring could help to facilitate the release. Aimed at further understanding the structure-release rate relationship of the coumarin-based cyclic prodrugs, we synthesized and examined a series of substituted coumarinic acid derivatives of opioid peptides, DADLE, and [Leu(5)]-enkephalin.

New development in the tritium labelling of peptides and proteins using solid catalytic isotopic exchange with spillover-tritium

The mechanism of the reaction of high temperature solid state catalytic isotope exchange (HSCIE) of hydrogen in peptides with spillover-tritium at 140-180 degrees C was analyzed. This reaction was used for preparing [(3)H]enkephalins such as [(3)H]DALG with specific activity of 138 Ci/mmol and [(3)H]LENK with specific activity of 120 Ci/mmol at 180 degrees C. The analogues of [(3)H]ACTG(4-10) with specific activity of 80 Ci/mmol, [(3)H]zervamicin IIB with specific activity of 70 Ci/mmol and [(3)H]conotoxin G1 with specific activity 35 Ci/mmol were produced. The obtained preparations completely retained their biological activity. [(3)H]Peptide analysis using (3)H NMR spectroscopy on a Varian UNITY-600 spectrometer at 640 MHz was carried out. The reaction ability of amino fragments in HSCIE was shown to depend both of their structures and on the availability and the mobility of the peptide chain. The reaction of HSCIE with the beta-galactosidase from Termoanaerobacter ethanolicus was studied. The selected HSCIE conditions allow to prepare [(3)H] beta-galactosidase with specific activity of 1440 Ci/mmol and completely retained its the enzymatic activity.

Angiotensin 1-9 and 1-7 release in human heart: role of cathepsin A

Human heart tissue enzymes cleave angiotensin (Ang) I to release Ang 1-9, Ang II, or Ang 1-7. In atrial homogenate preparations, cathepsin A (deamidase) is responsible for 65% of the liberated Ang 1-9. Ang 1-7 was released (88% to 100%) by a metallopeptidase, as established with peptidase inhibitors. Ang II was liberated to about equal degrees by ACE and chymase-type enzymes. Cathepsin A's presence in heart tissue was also proven because it deamidated enkephalinamide substrate by immunoprecipitation of cathepsin A with antiserum to human recombinant enzyme and by immunohistochemistry. In immunohistochemistry, cathepsin A was detected in myocytes of atrial tissue. The products of Ang I cleavage, Ang 1-9 and Ang 1-7, potentiated the effect of an ACE-resistant bradykinin analog and enhanced kinin effect on the B(2) receptor in Chinese hamster ovary cells transfected to express human ACE and B(2) (CHO/AB), and in human pulmonary arterial endothelial cells. Ang 1-9 and 1-7 augmented arachidonic acid and nitric oxide (NO) release by kinin. Direct assay of NO liberation by bradykinin from endothelial cells was potentiated at 10 nmol/L concentration, 2.4-fold (Ang 1-9) and 2.1-fold (Ang 1-7); in higher concentrations, Ang 1-9 was significantly more active than Ang 1-7. Both peptides had traces of activity in the absence of bradykinin. Ang 1-9 and Ang 1-7 potentiated bradykinin action on the B(2) receptor by raising arachidonic acid and NO release at much lower concentrations than their 50% inhibition concentrations (IC(50)s) with ACE. They probably induce conformational changes in the ACE/B(2) receptor complex via interaction with ACE.

Receptor specificity of the antiarrhythmic effect produced by opioid peptides Dalargin and DADLE during myocardial reperfusion

Nonselective agonists of mu- and delta-opioid receptors dalargin (D-Ala2,Leu5,Arg6-enkephalin) and DADLE (D-Ala2,D-Leu5-enkephalin) administered immediately before coronary reperfusion in a dose of 0.1 mg/kg prevented the development of ventricular arrhythmias. Blockade of mu-opioid receptors abolished the antiarrhythmic effect of these peptides. Hence, antiarrhythmic activity of dalargin and DADLE is primarily associated with activation of mu-opioid receptors.

[State of the opioid system in rats with different sensitivity to ethanol]

In this work we investigated the content of opioids in plasma blood and the changes caused by ethanol (1.5 g/kg, i.p.) in midbrain opioid receptors of rats with different sensitivity to ethanol, as determined according to the duration of ethanol-induced sleep (DEIS). A receptor binding technique with selective delta-DADLE and mu-DAGO radioligands were used. Ethanol even at low dose produced changes in the midbrain opioid receptors which appeared after 6 hours. The response of the receptors and the content of opioids in plasma blood of rats with different DEIS were not identical.

Expression of functional delta opioid receptors in vascular smooth muscle

We evaluated smooth muscle from human internal mammary artery and rat aorta for the presence of delta opioid receptors. Radioligand receptor competition studies using the delta-receptor selective agonist, [3H]-[D-Ala2, Met5] enkephalinamide (DAMA) suggested the expression of a high affinity binding site in rat and human blood vessels that was consistent with the delta-2 opioid receptor subtype. Using RT-PCR with primers to the cloned delta opioid receptor (DOR), a cDNA fragment identical to the known DOR sequence was obtained from the smooth muscle cell line, A-10. Stimulation of A-10 cells with DAMA resulted in a significant mobilization of intracellular calcium and membrane depolarization. Exposure of aortic rings denuded of endothelium to DAMA induced a naltrindole-senstive increase in contractile tone. These data demonstrate the presence of a functional DOR in vascular smooth muscle and a direct impact of opioids on vascular contractile tone.

[Solid phase catalytic hydrogen isotope exchange in dalargin]

A [3H]Dalargin preparation with a molar radioactivity of 52 Ci/mmol was obtained by the high temperature solid-state catalytic isotope exchange (HSCIE) of tritium for hydrogen at 150 degrees C. This tritium-labeled peptide was shown to completely retain its biological activity in the test of binding to opioid receptors from rat brain. The dissociation constant of the Dalargin-opioid receptor complex was found to be 4.3 nM. The dependencies of the chemical yield and the molar radioactivity on the reaction time and temperature of HSCIE were determined. The activation energy of the HSCIE reaction for the peptide was calculated to be 32 kcal/mol. The amino acid analysis showed that tritium is distributed between all the amino acid residues of [3H]Dalargin at the HSCIE reaction, with the temperature growth significantly increasing the total tritium incorporation and, especially, enhancing the radioactivity incorporation into aromatic residues.

Molecular cloning and biochemical characterization of a new mouse testis soluble-zinc-metallopeptidase of the neprilysin family

Because of their roles in controlling the activity of several bio-active peptides, members of the neprilysin family of zinc metallopeptidases have been identified as putative targets for the design of therapeutic agents. Presently, six members have been reported, these are: neprilysin, endothelin-converting enzyme (ECE)-1 and ECE-2, the Kell blood group protein, PHEX (product of the phosphate-regulating gene with homologies to endopeptidase on the X chromosome) and X-converting enzyme (XCE). In order to identify new members of this important family of peptidases, we designed a reverse transcriptase-PCR strategy based on conserved amino acid sequences of neprilysin, ECE-1 and PHEX. We now report the cloning from mouse testis of a novel neprilysin-like peptidase that we called NL1. NL1 is a glycoprotein that, among the members of the family, shows the strongest sequence identity with neprilysin. However, in contrast with neprilysin and other members of the family which are type II integral membrane proteins, NL1 was secreted when expressed in cultured mammalian cells, likely due to cleavage by a subtilisin-like convertase at a furin-like site located 22 amino acid residues in the C-terminus of the transmembrane domain. The recombinant enzyme exhibited neprilysin-like peptidase activity and was efficiently inhibited by phosphoramidon and thiorphan, two inhibitors of neprilysin. Northern blot analysis and in situ hybridization showed that NL1 mRNA was found predominantly in testis, specifically in round and elongated spermatids. This distribution of NL1 mRNA suggests that it could be involved in sperm formation or other processes related to fertility.

Characterization of delta-opioid receptors and effect of enkephalins on IRD 98 rat epithelial intestinal cell line

Using 3H-Tyr-D-Ala-Gly-Phe-D-Leu-OH (3H-DADLE) as a radioligand, delta-opioid binding sites on the IRD 98 rat epithelial cell line were identified. These sites were found to be reversible, saturable, specific and displayed high affinity for DADLE. Scatchard analysis revealed a dissociation constant (Kd) of 4.9+/-0.5 nmol/l, a maximum binding capacity (Bmax) of 1.7 pmol/mg protein, and 5x10(5) binding sites per cell. The presence of opioid receptors suggests the possibility that enkephalins directly control ion transport in enterocytes. In order to verify this hypothesis, investigations were designed to determine whether these receptors are functional and whether enkephalins can inhibit the stimulation of adenosine 3',5' cyclic monophosphate (cAMP) synthesis induced by cholera toxin. The increase in cAMP synthesis induced by cholera toxin was inhibited in a dose-dependent manner by H-Tyr-D-Ser-Gly-Phe-Leu-Thr-OH (DSLET), a delta-agonist. The enkephalinase inhibitor thiorphan potentiated this effect on IRD 98 cells, which contain enkephalinase. The action of DSLET was increased by 40% in the presence of this inhibitor. This effect was reversed by naltrindole, a potent delta-antagonist. Enkephalins can regulate intestinal secretion by acting directly on enterocytes: they thus have an antidiarrheal role, especially in the presence of an enkephalinase inhibitor.

Synthesis, opioid receptor binding, and biological activities of naltrexone-derived pyrido- and pyrimidomorphinans

A series of pyrido- and pyrimidomorphinans (6a-h and 7a-g) were synthesized from naltrexone and evaluated for binding and biological activity at the opioid receptors. The unsubstituted pyridine 6a displayed high affinities at opioid delta, mu, and kappa receptors with K(i) values of 0.78, 1.5, and 8.8 nM, respectively. Compound 6a was devoid of agonist activity in the mouse vas deferens (MVD) and guinea pig ileum (GPI) preparations but was found to display moderate to weak antagonist activity in the MVD and GPI with K(e) values of 37 and 164 nM, respectively. The pyrimidomorphinans in general displayed lower binding potencies and delta receptor binding selectivities than their pyridine counterparts. Incorporation of aryl groups as putative delta address mimics on the pyrido- and pyrimidomorphinan framework gave ligands with significant differences in binding affinity and intrinsic activity. Attachment of a phenyl group at the 4'-position of 6a or the equivalent 6'-position of 7a led to dramatic reduction in binding potencies at all the three opioid receptors, indicating the existence of a somewhat similar steric constraint at the ligand binding sites of delta, mu, and kappa receptors. In contrast, the introduction of a phenyl group at the 5'-position of 6a did not cause any reduction in the binding affinity at the delta receptor. In comparison to the unsubstituted pyridine 6a, the 5'-phenylpyridine 6c showed improvements in mu/delta and kappa/delta binding selectivity ratios as well as in the delta antagonist potency in the MVD. Interestingly, introduction of a chlorine atom at the para position of the pendant 5'-phenyl group of 6c not only provided further improvements in delta antagonist potency in the MVD but also shifted the intrinsic activity profile of 6c from an antagonist to that of a mu agonist in the GPI. Compound 6d thus possesses the characteristics of a nonpeptide mu agonist/delta antagonist ligand with high affinity at the delta receptor (K(i) = 2.2 nM), high antagonist potency in the MVD (K(e) = 0.66 nM), and moderate agonist potency in the GPI (IC(50) = 163 nM). Antinociceptive evaluations in mice showed that intracerebroventricular (icv) injections of 6d produced a partial agonist effect in the 55 degrees C tail-flick assay and a full agonist effect in the acetic acid writhing assay (A(50) = 7.5 nmol). No signs of overt toxicity were observed with this compound in the dose ranges tested. Moreover, repeated icv injections of an A(90) dose did not induce any significant development of antinociceptive tolerance in the acetic acid writhing assay. The potent delta antagonist component of this mixed mu agonist/delta antagonist may be responsible for the diminished propensity to produce tolerance that this compound displays.

Constitutive activation of the delta opioid receptor by mutations in transmembrane domains III and VII

We have investigated whether transmembrane amino acid residues Asp128 (domain III), Tyr129 (domain III) [corrected], and Tyr308 (domain VII) in the mouse delta opioid receptor play a role in receptor activation. To do so, we have used a [35S]GTPgammaS (where GTPgammaS is guanosine 5'-3-O-(thio)triphosphate) binding assay to quantify the activation of recombinant receptors transiently expressed in COS cells and compared functional responses of D128N, D128A, Y129F, Y129A, and Y308F point-mutated receptors to that of the wild-type receptor. In the absence of ligand, [35S]GTPgammaS binding was increased for every mutant receptor under study (1.6-2.6-fold), suggesting that all mutations are able to enhance constitutive activity at the receptor. In support of this finding, the inverse agonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu (where Aib represents alpha-aminobutyric acid) efficiently reduced basal [35S]GTPgammaS binding in the mutated receptor preparations. The potent agonist BW373U86 stimulated [35S]GTPgammaS binding above basal levels with similar (D128N, Y129F, and Y129A) or markedly increased (Y308F) efficacy compared with wild-type receptor. BW373U86 potency was maintained or increased. In conclusion, our results demonstrate that the mutations under study increase functional activity of the receptor. Three-dimensional modeling suggests that Asp128 (III) and Tyr308 (VII) interact with each other and that Tyr129 (III) undergoes H bonding with His278 (VI). Thus, Asp128, Tyr129, and Tyr308 may be involved in a network of interhelical bonds, which contributes to maintain the delta receptor under an inactive conformation. We suggest that the mutations weaken helix-helix interactions and generate a receptor state that favors the active conformation and/or interacts with heterotrimeric G proteins more effectively.

Kappa1-opioid binding sites are the dominant opioid binding sites in surgical specimens of human pheochromocytomas and in a human pheochromocytoma (KAT45) cell line

The adrenal medulla produces opioids which exert paracrine effects on adrenal cortical and chromaffin cells and on adrenal splanchnic nerves, via specific binding sites. The opioid binding sites in the adrenals are detectable mainly in the medullary part of it and differ in type between species. Thus, the bovine adrenal medulla contains mostly kappa-opioid binding sites and fewer delta- and mu-opioid binding sites while primate adrenals contain mainly delta sites and few kappa-opioid binding sites. Most chromaffin cell tumors, the pheochromocytomas, produce opioids which suppress catecholamine production by the tumor. The aim of the present work was to identify the types of opioid binding sites in human pheochromocytomas. For this purpose, we characterized the opioid binding sites on crude membrane fractions prepared from 14 surgically excised pheohromocytomas and on whole KAT45 cells, a recently characterized human pheochromocytoma cell line. Our data showed that human pheohromocytomas are heterogeneous, as expected, with regard to the production of catecholamines and the distribution and profile of their opioid binding sites. Indeed, only one out of the 14 pheochromocytomas expressed exclusively delta and mu opioid sites, while in the remaining 13 tumors kappa-type binding sites were dominant. The KAT45 cell line possessed a significant number of kappa1 binding sites, fewer kappa2-opioid binding sites and kappa3-opioid binding sites, and minimal binding capacity for delta- and mu-opioid receptor agonists sites. More specifically, the kappa1 sites/cell were approximately 18,000, the kappa2 4500/cell and the kappa3 sites 2000/cell. Our findings for the surgical specimens and the cell line combined with previously published pharmacological data obtained from KAT45 cells suggest that kappa sites appear to be the most prevalent opioid binding sites in pheochromocytomas. Finally, in normal bovine adrenals the profile of opioid binding sites differs in adrenaline and noradrenaline producing chromaffin cells. To test the hypothesis that the type of catecholamine produced by a pheochromocytoma depends on its cell of origin, we compared our binding data with the catecholamine content of each pheochromocytoma examined. We found no correlation between the type of the predominant catecholamine produced and the opioid binding profile of each tumor suggesting that this hypothesis may not be valid.

Phenylpropionic acid-based cyclic prodrugs of opioid peptides that exhibit metabolic stability to peptidases and excellent cellular permeation

PURPOSE

To evaluate the cellular permeation characteristics and the chemical and enzymatic stability of phenylpropionic acid-based cyclic prodrugs 1 and 2 of opioid peptides [Leu5]-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH) and DADLE (H-Tyr-D-Ala-Gly-Phe-D-Leu-OH), respectively.

METHODS

The rates of conversion of cyclic prodrugs 1 and 2 to [Leu5]-enkephalin and DADLE, respectively, in HBSS, pH 7.4 (Caco-2 cell transport buffer) and in various biological media having measurable esterase activity were determined by HPLC. The cell permeation characteristics of [Leu5]-enkephalin, DADLE, and cyclic prodrugs 1 and 2 were measured using Caco-2 cell monolayers grown onto microporus membranes and monitored by HPLC.

RESULTS

In HBSS, pH 7.4, cyclic prodrugs 1 and 2 degraded to [Leu5]-enkephalin and DADLE, respectively, in stoichiometric amounts. In 90% human plasma, the rates of disappearance of cyclic prodrugs 1 and 2 were slightly faster than in HBSS, pH 7.4. These accelerated rates of disappearance in 90% human plasma could be reduced to the rates observed in HBSS, pH 7.4, by pretreatment of the plasma with paraoxon, a known inhibitor of serine-dependent esterases. In homogenates of Caco-2 cells and rat liver, accelerated rates of disappearance of cyclic prodrugs 1 and 2 were not observed. When applied to the AP side of a Caco-2 cell monolayer, cyclic prodrug 1 exhibited significantly greater stability against peptidase metabolism than did [Leu5]-enkephalin. Cyclic prodrug 2 and DADLE exhibited stability similar to prodrug 1 when applied to the AP side of the Caco-2 cell monolayers. Prodrug 1 was 1680 fold more able to permeate the Caco-2 cell monolayers than was [Leu5]-enkephalin, in part because of its increased enzymatic stability. Prodrug 2 was shown to be approximately 77 fold more able to permeate a Caco-2 cell monolayer than was DADLE.

CONCLUSIONS

Cyclic prodrugs 1 and 2, prepared with the phenylpropionic acid promoiety, were substantially more able to permeate Caco-2 cell monolayers than were the corresponding opioid peptides. Prodrug 1 exhibited increased stability to peptidase metabolism compared to [Leu5]-enkephalin. In 90% human plasma but not in Caco-2 cell and rat liver homogenates, the opioid peptides were released from the cyclic prodrugs by an esterase-catalyzed reaction that is sensitive to paraoxon inhibition. However, the rate of this bioconversion appears to be extremely slow.

On the kinetic theory of the extracellular signal transduction in native cells: I. Hormone-receptor interaction

The hormone-receptor interaction is reconsidered and a minimal kinetic theory for the signal transduction in native cells is proposed. The basic equation for the time evolution of the concentration of the hormone-receptor complex is obtained. It is shown, that the hormone-receptor interaction is regulated by a cycle of G protein, i.e. kinetic parameters associated with the G protein cycle enter this equation. The existence of exponential and oscillatory regimes in kinetics of the G protein cycle, and hormone-receptor interactions are predicted. Experimental kinetic curves for agonist binding are computer-simulated, and numerical values for various kinetic and equilibrium parameters are obtained.

Synthesis and biological activity of 3-substituted 3-desoxynaltrindole derivatives

The 3-unsubstituted and substituted analogs of naltrindole (NTI) were synthesized using palladium-catalyzed transformations, and their binding affinity to opioid receptors was determined. Although the 3-desoxy analog showed comparable delta selectivity with that of NTI, all of the novel compounds possessed low affinity for delta receptors indicating the important role of the 3-oxygen atom of NTI for interaction with delta-opioid receptors.

Opioid peptide receptor studies. 8. One of the mouse brain deltaNCX binding sites is similar to the cloned mouse opioid delta receptor: further evidence for heterogeneity of delta opioid receptors

Quantitative ligand binding studies resolved two subtypes of the delta opioid receptor, termed delta(ncx1) and delta(ncx2), in mouse brain membranes depleted of mu receptors by pretreatment with the irreversible ligand, BIT. The purpose of the present study was to compare the binding parameters, ligand-selectivity profile and pharmacological properties of the cloned mouse delta receptor (MDOR) stably expressed in a cell line to the delta(ncx) binding sites of mouse brain. [3H][D-Ala2,D-Leu5]enkephalin labeled a single binding site in membranes prepared from MDOR cells under several different assay conditions including BIT-pretreatment. The MDOR had high affinity for delta agonists and antagonists. [3H][D-Ala2,D-Leu5]enkephalin labeled two binding sites in mouse brain membranes depleted of mu receptors by pretreatment with BIT: the delta(ncx1) site (high affinity for DPDPE and deltorphin) and the delta(ncx2) site (low affinity for DPDPE and deltorphin). Some agents were moderately selective for the delta(ncx2) site: [pCl]DPDPE (10.9-fold), JP41 (5.9-fold) and JP45 (3.8-fold). The Ki values of 12 opioids at the mouse MDOR were determined. These values were highly correlated with their values at the delta(ncx1) site but not the delta(ncx2) site. These data suggest that the delta(ncx2) site may be distinct from the cloned delta opioid receptor.

Opioid peptide receptor studies. 9. Identification of a novel non-mu- non-delta-like opioid peptide binding site in rat brain

Quantitative binding studies resolved two high-affinity [3H][D-Ala2,D-Leu5]enkephalin binding sites in rat brain membranes depleted of mu binding sites by pretreatment with the irreversible agent BIT. The two binding sites had lower (delta ncx-2, Ki = 96.6 nM) and higher (delta ncx-1, Ki = 1.55 nM) affinity for DPDPE. The ligand-selectivity profile of the delta ncx-1 site was that of a classic delta binding site. The ligand-selectivity profile of the delta ncx-2 site was neither mu- or delta-like. The Ki values of selected agents for the delta ncx-2 site were: [pCl]DPDPE (3.9 nM), DPLPE (140 nM), and DAMGO (2.6 nM). Under these assay conditions, [3H][D-Ala2,D-Leu5]enkephalin binding to the cells expressing the cloned mu receptor is very low and pretreatment of cell membranes with BIT almost completely inhibits [3H]DAMGO and [3H][D-Ala2,D-Leu5]enkephalin binding. Intracerebroventricular administration of antisense DNA to the cloned delta receptor selectively decreased [3H][D-Ala2,D-Leu5]enkephalin binding to the delta ncx-1 site. Administration of buprenorphine to rats 24 h prior to preparation of membranes differentially affected mu, delta ncx-1, and delta ncx-2 binding sites. Viewed collectively, these studies have identified a novel non-mu- non-delta-like binding site in rat brain.

The effects of postmortem delay on mu, delta and kappa opioid receptor subtypes in rat brain and guinea pig cerebellum evaluated by radioligand receptor binding

Studies of human and animal central nervous system receptor degradation and measurement postmortem are important as human tissue can rarely be collected under ideal experimental conditions. Correlating the change in binding of opioid receptor subtypes over time will help define the conditions under which human studies may be valid. The present study was designed to investigate the rate at which opioid receptor subtypes degrade postmortem. Brains from rats or cerebelli from guinea pigs were kept at 22 degrees C or 4 degrees C at times from zero to 24 hours to simulate two common human collection techniques; room temperature and morgue refrigeration. Tissue homogenates from these brains were analysed for mu1, mu2, delta, kappa1 and kappa3 opioid receptor binding using standard radioligand binding techniques. At room temperature mu1, mu2 and delta opioid receptor binding was reduced between 6 and 12 hours, whereas kappa1 and kappa3 binding was reduced after 24 hours. At 4 degrees C mu1, mu2, delta, kappa1 and kappa3 binding remained constant over the 24 hour period.

Mutagenesis of Glu403 to Cys in rabbit neutral endopeptidase-24.11 (neprilysin) creates a disulphide-linked homodimer: analogy with endothelin-converting enzyme

Neutral endopeptidase-24.11 (NEP; neprilysin; EC 3.4.24.11) and endothelin-converting enzyme (ECE) are related zinc metallopeptidases involved in the processing of biologically active peptides. Only ECE, however, exists as a disulphide-linked homodimer. The covalent linkage in rat ECE is between Cys412 in each subunit, which is equivalent to Glu403 in rabbit NEP. Here we report that directed mutagenesis of Glu403 to cysteine in rabbit NEP creates a disulphide-linked homodimer, as revealed by transient transfection in COS-1 cells and SDS/PAGE of a membrane fraction. Under reducing conditions, both the mutant (E403C) and the wild-type NEP migrate as a polypeptide of 92 kDa. However, under non-reducing conditions, the Mr of the wild type remains unchanged, whereas that of the mutant is doubled. Co-transfection of wild-type ECE and E403C NEP cDNA did not result in the production of a NEP-ECE heterodimer. Comparison of the kinetic constants for wild-type and E403C mutant NEP with either [D-Ala2,Leu5]enkephalin or 3-carb oxypropanoyl-alanyl-alanyl- leucine-4-nitroanilide(Suc-Ala-Ala-Leu-NH-Np) as substrate show a decrease of approx. 50% in Vmax/Km for the mutant form. The IC50 value for inhibition of the mutant by phosphoramidon or thiorphan is increased 3-fold and 5-fold respectively. Although NEP and ECE exhibit only about 40% identity and differ substantially in substrate specificity and some other characteristics, these data indicate that they have considerable similarity in three-dimensional structure, allowing dimer formation in the mutant NEP with the disulphide link probably occurring in a hydrophilic surface loop.

Discovery of a novel class of substituted pyrrolooctahydroisoquinolines as potent and selective delta opioid agonists, based on an extension of the message-address concept

This paper describes the design and synthesis of compounds belonging to a novel class of substituted pyrrolooctahydroisoquinolines which are potent and selective delta opioid agonists. Molecular modeling studies performed on known, selective delta ligands such as (+)-3 and the potent delta agonists SNC 80 led to the identification of the carboxamido moiety of the latter as a putative nonaromatic delta address. Insertion of this moiety onto the octahydroisoquinoline opioid message resulted in (+/-)-5b, a potent and selective delta ligand. The active enantiomer, (-)-5b, displayed nanomolar affinity for the delta receptor (Ki = 0.9 nM) with good mu/delta and kappa/delta binding selectivity ratios (140 and 1480, respectively). In addition, (-)-5b behaved as a full delta agonist in the mouse vas deferens bioassay having an IC50 = 25 nM and being antagonised in the presence of 30 nM naltrindole (NTI). These studies, based on the message-address concept, indicated that the nonaromatic (N,N-diethylamino)carbonyl moiety is a viable alternative to the classical benzene ring as a delta opioid address. Preliminary in vivo studies showed that (+/-)-5b produced a dose-related antinociception in the mouse abdominal constriction test after intracerebroventricular administration (ED50 = 1.6 micrograms/mouse).

Binding properties of C-truncated delta opioid receptors

AIM

To study the role of C-terminal delta opioid receptor involved in ligand binding affinity and selectivity.

METHODS

The 31 amino acid residues of C-terminal truncated delta opioid receptors and the wild-type were expressed stably in Chinese hamster ovary (CHO) cells, respectively. Then the ligand binding properties of the products were studied by receptor binding assay.

RESULTS

A typical mutated receptor clone CHO-T and a wild-type receptor clone CHO-W were obtained. The Kd values of [3H] diprenorphine (Dip) and [3H]leucine-2-alanine enkephalin (DADLE) bound to CHO-T were similar to CHO-W. Both the specific [3H]Dip bindings of CHO-T and CHO-W were strongly inhibited by delta selective agonists with similar Ki, but neither by mu nor kappa selective agonists.

CONCLUSION

The C-terminal of the delta opioid receptor is not involved in the ligands binding affinity and selectivity.

Differences in the hydrolysis of enkephalin congeners by the two domains of angiotensin converting enzyme

The hydrolysis of enkephalin (Enk) congeners by the isolated N- (N-ACE) and C-domain of angiotensin I converting enzyme (ACE) and by the two-domain somatic ACE was investigated. Both Leu5- and Met5-Enk were cleaved faster by the C-domain than by N-ACE; rates with somatic ACE were 1600 and 2500 nmol/min/nmol enzyme with both active sites being involved. Substitution of Gly2 by D-Ala2 reduced the rate to 1/3rd to 1/7th of that of the Enks. N-ACE cleaved Met5-Enk-Arg6-Phe7 faster than the C-domain, probably with the highest turnover number of any naturally occurring ACE substrate (7600 min(-1)). This heptapeptide is also hydrolyzed in the absence of Cl-, but the activation by Cl- is unique; Cl- enhances the hydrolysis of the heptapeptide by N-ACE but inhibits it by the C-domain, yielding about a 5-fold difference in the turnover number at physiological pH. This difference may result in the predominant role of the N-domain in converting Met5-Enk-Arg6-Phe7 to Enk in vivo.

[The effect of thymogen on the heart in ischemia and reperfusion]

The anti-ischemic properties of thymogen were studied on an isolated heart model. Its cardioprotective effect was higher than that of dalargin and verapamil. The mechanism of the thymogen anti-ischemic action is realized without the participation of the opiate receptors and blockade of calcium entrance into the cardiomyocytes.

Probes for narcotic receptor mediated phenomena. 23. Synthesis, opioid receptor binding, and bioassay of the highly selective delta agonist (+)-4-[(alpha R)-alpha-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]- N,N-diethylbenzamide (SNC 80) and related novel nonpeptide delta opioid receptor ligands

The highly selective delta (delta) opioid receptor agonist SNC 80 [(+)-4- [(alpha R)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N ,N- diethylbenzamide, (+)-21] and novel optically pure derivatives were synthesized from the enantiomers of 1-allyl-trans-2,5-dimethylpiperazine (2). The piperazine (+/-)-2 was synthesized, and its enantiomers were obtained on a multigram scale in > 99% optical purity by optical resolution of the racemate with the camphoric acids. The absolute configuration of (+)-2 was determined to be 2S,5R by X-ray analysis of the salt with (+)-camphoric acid. Since the chirality of the starting material was known, and the relative configuration of compounds (-)-21, (-)-22, and (+)-23 were obtained by single-crystal X-ray analysis, the assignment of the absolute stereochemistry of the entire series could be made. Radioreceptor binding studies in rat brain preparations showed that methyl ethers (+)-21 (SNC 80) and (-)-25 exhibited strong selectivity for rat delta receptors with low nanomolar affinity to delta receptors and only micromolar affinity for rat mu (mu) opioid receptors. Compounds (-)-21, (-)-22, and (-)-23 showed micromolar affinities for delta opioid receptors. The unsubstituted derivative (+)-22 and the fluorinated derivative (-)-27 showed > 2659- and > 2105-fold delta/mu binding selectivity, respectively. The latter derivatives are the most selective ligands described in the new series. Studies with some of the compounds described in the isolated mouse vas deferens and guinea pig ileum bioassays revealed that all were agonists with different degrees of selectivity for the delta opioid receptor. These data show that (+)-21 and (+)-22 are potent delta receptor agonists and suggest that these compounds will be valuable tools for further study of the delta opioid receptor at the molecular level, including its function and role in analgesia and drug abuse.

Metabolism of enkephalins in head membranes of the leech Theromyzon tessulatum by peptidases: isolation of an enkephalin-degrading aminopeptidase

Metabolism of leucine and methionine enkephalins by enzyme preparations from head parts of the leech Theromyzon tessulatum was investigated. Leech homogenate degraded enkephalins by cleavage of the Tyr1-Gly2 and Gly3-Phe4 bonds. The Tyr1-Gly2-Gly3 was detected as a major metabolite when amastatin (aminopeptidase inhibitor) was present to prevent Tyr1-Gly2 breakdown. Around 50% of enkephalin-degrading activity was isolated in a 20000 x g membrane fraction and was shown to be almost entirely due to an aminopeptidase activity. This enzyme, a homodimer of approx. 70 kDa, has been purified to homogeneity by a combined approach including gel permeation and anion exchange chromatographies followed by reversed-phase HPLC. This enkephalin-degrading aminopeptidase is a typical integral membrane 'zincin' metalloprotein with an apparent k(m) of 30 microM, a specific activity of 12 nmol GGFM min-1 mg protein-1 and a catalytic efficiency (kcat/k(m)) of 46 x 10(6) mol-1 min-1. This enzyme is specifically inhibited by amastatin (IC50 = 0.5 microM), but not by bestatin and actinonin. In leech membranes, the other degrading activities performed at the same time were due to a neuropeptide-endopeptidase (NEP)-like enzyme attack, inhibited by phosphoramidon (IC50 = 0.1 microM) and in the case of the Met-enkephalin by a combined action of an angiotensin-converting-like enzyme, inhibited by captopril (IC50 = 0.2 microM) and the NEP-like enzyme. These two enzymes were previously isolated from head membranes of T. tessulatum and possess towards Met-enkephalin a catalytic efficiency (kcat/k(m)) of, respectively, 12 x 10(6) mol-1 min-1 and 78 x 10(6) mol-1 min-1. These findings constitute the first report in leeches on the nature and the sites of attack of the membrane peptidases involved in the metabolism of enkephalins and also the first biochemical evidence for a novel member of the aminopeptidase family.

Synthesis, biological evaluation, and quantitative receptor docking simulations of 2-[(acylamino)ethyl]-1,4-benzodiazepines as novel tifluadom-like ligands with high affinity and selectivity for kappa-opioid receptors

The synthesis and biological evaluation of a series of 2-substituted 5-phenyl-1,4-benzodiazepines, structurally related to tifluadom (5), the only benzodiazepine that acts simultaneously as a kappa-opioid agonist and a cholecystokinin-A (CCK-A) antagonist, are reported. The radioligand binding models used in these studies were [(125)I](BH)-CCK-8 in rat pancreas (CCK-A), [(3)H]-(MENLE(28,31))-cck-8 in guinea pig cerebral cortex (CCK-B), and [(3)H]U-69593 (kappa(1)), [(3)H]DAMGO (mu), and [(3)H]DADLE (delta) in guinea pig brain. All the title compounds were devoid of significant affinity for both CCK-A and CCK-B receptors, while some of them bound with nanomolar affinity and high selectivity for kappa-opioid receptors. In particular, the 2-thienyl derivative 7A(X = H) with a K(i) = 0.50 nM represents a clear improvement with respect to tifluadom, showing a comparable potency but higher selectivity. The application of computational simulations and linear regression analysis techniques to the complexes between guinea pig kappa (kappa(1))-receptor and the title compounds allowed the identification of the structural determinants for recognition and quantitative elucidation of the structure-affinity relationships in this class of receptors.

Biochemical properties of the angiotensin-converting-like enzyme from the leech Theromyzon tessulatum

This article reports the evidence and the biochemical properties of an angiotensin-converting (ACE)-like enzyme from head parts of the leech Theromyzon tessulatum. After solubilization from membranes with Triton X-114, the ACE-like enzyme was purified from the detergent-poor fraction. Four steps of purification including gel permeation and anion exchange chromatographies followed by a reversed-phase HPLC were needed. This poor glycosylated peptidyl dipeptidase (of ca. 120 kDa) hydrolyzes, at pH 8.4 and at 37 degrees C, the Phe8-His9 bond of angiotensin I with a high catalytic activity (i.e., K(m): 830 microM and Kcat/K(m): 153 s-1 mM-1). The hydrolysis of angiotensin I is inhibitable at 80% by captopril (IC50 = 175 nM) and lisinopril (IC50 = 35 nM). This activity is strictly dependent on the presence of NaCl and is increased by Zn2+. This zinc metallopeptidase also attacks peptides that have in their sequence either Gly-His, Gly-Phe, or Phe-His bond [e.g., enkephalins (Kcat/K(m): 12 s-1 mM-1) or bradykinin (Kcat/K(m): 2200 s-1 mM-1]. Taken together, these arguments are consistent with an ACE-like activity implicated in metabolism of angiotensins and bradykinin in leeches.

Identification and properties of a neuropeptide-degrading endopeptidase (neprilysin) of Ascaris suum muscle

We have previously identified in membranes of the locomotory muscle of Ascaris suum a phosphoramidon-sensitive endopeptidase which hydrolyses the neuropeptide AF1 (Lys-Asn-Glu-Phe-Ile-Arg-Phe-NH2) by cleavage of the Glu3-Phe4 bond (Sajid & Isaac, 1994). We have determined the properties of this neuropeptide-degrading enzyme of A. suum muscle using AKH-1 (pGlu-Leu-Asn-Phe-Thr-Pro-Asn-Trp-Gly-Thr-NH2) and [D-Ala2, Leu5]enkephalin as convenient endopeptidase substrates. Phosphoramidon, thiorphan and SQ 28603, potent inhibitors of mammalian neprilysin (neutral endopeptidase, endopeptidase 24.11), inhibited the endopeptidase activity towards AKH-I with IC50 values of 0.13 microM, 22 microM and 6.3 microM, respectively. Two other neprilysin inhibitors (SCH 32615 and SCH 39370) and the bivalent metal ion chelators, EDTA (1 mM) and 1, 10 bis-phenanthroline (1 mM) failed to inhibit the nematode enzyme. The endopeptidase had a neutral pH optimum and a significant proportion (45%) of the enzyme activity partitioned into the detergent-rich phase of Triton X-114, indicating that the enzyme is an integral membrane protein. The muscle enzyme also attacked [D-Ala2, Leu5]enkephalin cleaving the Gly3-Phe4 bond and this hydrolytic activity was inhibited by phosphoramidon and thiorphan (IC50, 0.28 microM and 15.8 microM, respectively) but not by EDTA and 1, 10 bis-phenanthroline. The phosphoramidon-sensitive endopeptidase activity was detected on intact muscle cells prepared by collagenase treatment of the body wall musculature, indicating that endopeptidase is accessible to peptide molecules that interact with the cell surface.

Opioid antagonist properties of the highly delta-receptor-selective BOC-Tyr-Pro-Gly-Phe-Leu-Thr (OtBu) peptide and of its Phe1 and Mel1 analogues

BOC-Tyr-Pro-Gly-Phe-Leu-Thr(OtBu) is a potent, highly delta-opioid receptor-selective competitive antagonist, the Ke values in the mouse vas deferens in vitro assay against [D-Ala2, D-Leu5]-enkephalin [D-Pen2, D-Pen5]enkephalin and deltorphin-II being 39.5, 38.7 and 27.3 nM, respectively, whereas those against [D-Ala2, MePhe4-Gly5-ol]enkephalin (DAMGO) and ethylketocyclazocine in the guinea-pig ileum are 368,000 and > 200,000 nM, giving a higher than 9000-fold delta- vs mu- and a higher than 5000-fold delta- vs kappa-selectivity ratio. The Ki values against various labeled delta-ligands in the rat brain receptor binding assay were in the 300-1000 nM range, whereas the Ki against [3H]-DAMGO was higher than 30,000 nM. The striking discrepancies between bioassay and receptor binding data show another aspect of already recognized differences of mouse vas deferens and rat brain delta-receptors. With the aim of producing a delta-selective affinity ligand, we synthesized the BOC-Mel1 derivative; however, there was a 175-fold loss of delta-receptor affinity in the bioassay and no indication of an irreversible interaction, but a delta-agonist effect appeared in spite of nonprotonated nitrogen. The corresponding BOC-Phe1 derivative had a 10 times higher affinity and, apparently, no agonist activity.

Anti-idiotypic antibodies: a useful alternative for studying the biochemical expression of mu/delta opioid binding sites in mammalian brain

A polyclonal antiserum was produced against opioid binding sites using an anti-idiotypic approach whereby antibodies directed against the opioid agonist DSLET were used as immunogen. The anti-idiotypic antiserum recognized specific brain proteins with molecular masses of 76 +/- 4, 73 +/- 4 and 59 +/- 3 kDa, respectively. The immunolabeling of these proteins was mainly inhibited by mu, delta opioid agonists and a general antagonist, naloxone. The inhibition of immunoprecipitation by opioid agonists and antagonist and the developmental expression of these immunoreactive proteins found to occur during brain ontogeny strongly suggest that these three proteins were mu, delta but not kappa opioid binding sites. The anti-idiotypic antiserum both inhibits 3H-DADLE binding and mimics the inhibitory agonist effects on the stimulated cAMP level of NG 108-15 cells which expressed delta opiate receptors. Numerous mammalian brain opioid binding sites were labeled, due to the fact that the binding site was the epitope recognized by the anti-idiotypic antibodies. From the numerous studies performed with a view to characterizing the specificity of the anti-idiotypic antibodies, it was strongly suggested that the anti-idiotypic antibodies specifically recognize mu/delta opioid binding sites and they can therefore be powerful tools for studying the biochemical expression of these opioid binding sites in mammalian brains.

Evidence that Asn542 of neprilysin (EC 3.4.24.11) is involved in binding of the P2' residue of substrates and inhibitors

Neprilysin (EC 3.4.24.11) is a Zn2+ metallopeptidase involved in the degradation of biologically active peptides, e.g. enkephalins and atrial natriuretic peptide. The substrate specificity and catalytic activity of neprilysin resemble those of thermolysin, a crystallized bacterial Zn2+ metalloprotease. Despite little overall homology between the primary structures of thermolysin and neprilysin, many of the amino acid residues involved in catalysis, as well as Zn2+ and substrate binding, are highly conserved. Most of the active-site residues of neprilysin have their homologues in thermolysin and have been characterized by site-directed mutagenesis. Furthermore, hydrophobic cluster analysis has revealed some other analogies between the neprilysin and thermolysin sequences [Benchetrit, Bissery, Mornon, Devault, Crine and Roques (1988) Biochemistry 27, 592-596]. According to this analysis the role of Asn542 in the neprilysin active site is analogous to that of Asn112 of thermolysin, which is to bind the substrate. Site-directed mutagenesis was used to change Asn542 to Gly or Gln residues. The effect of these mutations on substrate catalysis and inhibitor binding was examined with a series of thiorphan-like compounds containing various degrees of methylation at the P2' residue. For both mutated enzymes, determination of kinetic parameters with [D-Ala2,Leu5]enkephalin as substrate showed that the large decrease in activity was attributable to an increase in Km (14-16-fold) whereas kcat values were only slightly affected (2-3-fold decrease). This is in agreement with Asn542 being involved in substrate binding rather than directly in catalysis. Finally, the IC50 values for thiorphan and substituted thiorphans strongly suggest that Asn542 of neprilysin binds the substrate on the amino side of the P2' residue by formation of a unique hydrogen bond.

Isolation of a neuropeptide-degrading endopeptidase from the leech Theromyzon tessulatum

Extracts of head parts prepared from the leech Theromyzon tessulatum hydrolyse the Gly3-Phe4 bond of synthetic [D-Ala2, Leu5]enkephalin and the Gly-His bond of benzoyl-Gly-His-Leu. The metabolism of benzoyl-Gly-His-Leu was completely inhibited by captopril, consistent with an angiotensin-converting enzyme activity. Such an enzyme has recently been isolated from T. tessulatum. However, the enkephalin hydrolysis by captopril (100 microM) was inhibited to a maximum of 70%. The residual activity hydrolyzing enkephalin was inhibited by phosphoramidon, consistent with the presence of endopeptidase-24.11, a mammalian enzyme implicated in the metabolism of neuropeptides. This enzyme was isolated using four steps of purification including gel-permeation and anion-exchange chromatographies followed by reverse-phase HPLC. This neuropeptide endopeptidase (of approximate molecular mass 45 kDa) hydrolyses, at pH 7 and 37 degrees C, both the Gly3-Phe4 bond of synthetic [D-Ala2, Leu5]enkephalin and the Phe8-His9 bond of angiotensin I. Cleavage of [D-Ala2, Leu5]enkephalin yields, respectively, the Tyr-D-Ala-Gly and Phe-Leu peptides with a specific activity of 29 nmol Tyr-D-Ala-Gly.min-1.mg protein-1 (Km 95 microM). The hydrolysis of angiotensin I yields angiotensin II and the dipeptide His-Leu with a specific activity of 1.2 nmol angiotensin min-1.mg protein-1 (Km 330 microM). The metabolism of these peptides was totally inhibited by phosphoramidon. This study therefore provides biochemical evidence for neuropeptide-degrading endopeptidases in leeches.

Immunoprecipitation of opioid receptor-Go-protein complexes using specific GTP-binding-protein antisera

Solubilization of opioid receptors from rat cortical membranes that retained high-affinity guanine nucleotide-sensitive agonist binding was achieved using 10 mM CHAPS. We report the nature of the interactions of mu and delta opioid receptors with the guanine nucleotide-binding protein G(o) by immunoprecipitation of CHAPS extracts with selective G(o)alpha-subunit protein antisera. Antiserum IM1 raised against amino acids 22-35 of G(o)alpha selectively co-immunoprecipitated G(o)alpha-mu and G(o)alpha-delta opioid receptor complexes detected in the immunoprecipitates by specific [3H][D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin and [3H][D-Ser2,Leu5,Thr6]enkephalin binding respectively. By contrast, antisera directed against the C-terminal decapeptide (OC2) and the N-terminal hexadecapeptide (ON1) of isoforms of G(o)alpha were unable to immunoprecipitate solubilized opioid receptor-G(o) complexes, although both were able to immunoprecipitate solubilized G(o)alpha and have been shown to reduce the affinity of [D-Ala2,D-Leu5]enkephalin for opioid receptors in rat cortical membranes [Georgoussi, Carr and Milligan (1993) Mol. Pharmacol. 44, 62-69]. These findings demonstrate that CHAPS-solubilized mu and delta opioid receptors from rat cortical membranes form stable complexes with one or more variants of G(o).

Identification and characterization of opioid-binding sites present in the Ishikawa human endometrial adenocarcinoma cell line

Normal epithelial cells of human endometrium, and Ishikawa human endometrial adenocarcinoma cells (an in vitro model for the study of steroid hormone effects on human endometrium) have been found to express and secrete opioid peptides deriving from proenkephalin, prodynorphin, and proopiomelanocortin. These opioids may act locally, affecting the uterine tissues. In the present study, we identified and characterized opioid-binding sites on the Ishikawa cell line, producing evidence for the mechanism of local opioid action. We used an acid shock before the receptor assay to dissociate any endogenously bound peptide. The acidification improved specific binding by 2- to 4.5-fold. Characterization of opioid binding using different radiolabeled opioids and effectors has shown the existence of a low concentration of delta-sites (Kd, 6.20 nmol/L; 4,890 sites/cell), no mu-sites, low affinity kappa 1-sites (Kd, 10.8 nmol/L; 276,000 sites/cell), kappa 2-sites with high affinity for ethylketocyclazocine (Kd, approximately 1 nmol/L) and low affinity for diprenorphine (Kd, approximately 8 nmol/L) at a concentration of 93,000 sites/cell, and high affinity kappa 3-sites (Kd, 3.6 nmol/L; 77,000 sites/cell). In conclusion, our report characterizes opioid sites in a particular and homogeneous cell type of human endometrium, i.e. in epithelial cells. The coexistence of opioid sites and their endogenous ligands in the Ishikawa cell line makes these cells a good model for the study of autocrine/paracrine interactions of opioids in nonneural tissues.

Dopamine D1 and opioid receptor binding changes in the limbic system of sleep deprived rats

Sleep deprivation induced by the platform technique is considered to be a heavy stressful situation in rats. At the end of the sleep deprivation period (72 h) the rat displayed particular behavior characterized by wakefulness, a high degree of motor and exploratory activity, increased alertness and reactivity to environmental stimuli. Our previous results indicated that this behavior was potently antagonized by the administration of the D1 selective antagonist SCH 23390 and by the opioid antagonist naloxone. In this paper we show that concomitantly to this behavior, an increased number of D1 receptors associated with an increased dopamine-stimulated adenylate cyclase activity is present in the limbic system but not in the striatum of these animals. On the contrary, a decreased Bmax of mu and delta opioid receptors was found in the same brain area. These data suggest an active role of limbic dopamine and opioid systems in the generation of arousal and insomnia related to sleep deprivation-induced stress.

Opioid peptide receptor studies. 1. Identification of a novel delta-opioid receptor binding site in rat brain membranes

Our laboratory was among the first to propose the existence of delta receptor subtypes: a delta site thought to be associated with a mu-delta-opioid receptor complex termed the delta cx binding site and delta site not associated with the mu-delta-opioid receptor complex, termed the delta ncx site. In previous studies, we assayed the delta cx site with [3H][D-Ala2,D-Leu5]enkephalin using rat brain membranes depleted of delta ncx sites by pretreatment with the site-directed acylating agent, (+)-trans-SUPERFIT. In the present study, we investigated, using (+)-trans-SUPERFIT-pretreated membranes, the possibility of heterogeneity of the delta cx binding site. Two sites were resolved: the delta cx-1 site at which mu ligands are potent noncompetitive inhibitors and delta ligands are weak competitive inhibitors, and the delta cx-2 site where delta ligands are potent and mu ligands are weak, mixed competitive-noncompetitive inhibitors. Although the delta cx-2 site has a delta-like ligand-selectivity profile, several experiments distinguished it from the delta ncx site. Two lines of evidence suggest that the delta ncx site corresponds to the cloned delta receptor. One, the delta receptor was cloned from the NG108-15 cell line, and this receptor, like the delta ncx binding site, irreversibly binds SUPERFIT and (+)-trans-SUPERFIT. Secondly, administration of delta-antisense DNA selectively decreases delta ncx binding. Viewed collectively, the major finding of this study is the discovery of a novel SUPERFIT-insensitive and delta-antisense-insensitive delta cx-2 binding site.

Presence in neuroblastoma cells of a mu 3 receptor with selectivity for opiate alkaloids but without affinity for opioid peptides

Evidence is presented for the occurrence of a unique opiate alkaloid-selective, opioid peptide-insensitive binding site in N18TG2 mouse neuroblastoma cells and in late passage hybrid F-11 cells, derived from N18TG2 neuroblastoma cells and rat dorsal root ganglion cells. Those cells lacked classical opioid peptide-sensitive receptor subtypes, but contained [3H]morphine and [3H]diprenorphine binding sites with affinity for certain opiate alkaloids but not for any endogenously occurring opioid peptide or peptide analog tested, including D-ala2-D-leu5-enkephalin (DADLE), D-Ala2,N-Me-Phe4,Gly5-ol (DAGO) and dynorphin A(1-17). The binding site differed from hitherto described mu, delta and kappa neuronal opioid receptors not only on the basis of peptide insensitivity, but also on the basis of selectivity and affinities of alkaloids. Saturation experiments with [3H]morphine indicated the presence of a single site with Kd = 49 nM and Bmax = 1510 fmol/mg protein. This novel binding site was not present in F-11 hybrid cells at early passage. Instead the hybrid cells contained conventional opioid receptors (predominantly delta and also mu) capable of binding DADLE and other peptides as well as opiate alkaloids. With additional passage (cell divisions) of the hybrid cells, during which a limited change occurred in mouse chromosome number, the peptide-insensitive binding appeared and the opioid peptide-binding (delta and mu) receptors were lost reciprocally. Thus, expression of the peptide-insensitive binding normally may be repressed when conventional opioid receptors are expressed. The peptide-insensitive opiate binding site described here appears to correspond to the mu 3 receptor subtype, recently identified pharmacologically and functionally in several cell types of the immune system.(ABSTRACT TRUNCATED AT 250 WORDS)

IL-1 beta-like Freund's adjuvant enhances axonal transport of opiate receptors in sensory neurons

Chronic pain and inflammation increase substance P in sensory fibres of peripheral nerves in which opiate receptors are known to undergo axonal transport. The aim of the present study was to evaluate a possible modulation of axonal transport of opiate receptors in peripheral nerves during inflammation. After intraplantar injection of Freund's adjuvant to rats, the accumulation of mu and kappa opiate receptors increased on both sides of ligature in sciatic nerves of the injected paw. The contralateral side was unaffected and may serve as control. When IL-1 beta was injected into rat paws, the axonal transport of opiate receptors was increased in a similar way. This suggests that IL-1 beta represents a major mediator to sensitize nociceptors during inflammation through a process requiring retrograde signals.

In vitro and in vivo expression of opioid and sigma receptors in rat C6 glioma and mouse N18TG2 neuroblastoma cells

Mouse N18TG2 neuroblastoma and rat C6 glioma cell lines were injected into male nude mice, and the tumors were passaged serially. At each generation, tumors were analyzed for delta opioid binding using [3H][D-Ala2,D-Leu5]enkephalin and for sigma 1 and sigma 2 binding with 1,3-[3H]di-o-tolylguanidine in the presence and absence of 1 microM pentazocine. Receptor density (Bmax) and affinity (KD) were estimated by homologous competition binding assays. Opioid and sigma Bmax values in the solid tumors were significantly lower than their original levels in vitro. KD values for opioid/sigma ligands were similar in vitro and in vivo. With successive passages in the murine host, delta opioid and sigma 1 binding of the neuroblastoma-derived solid tumors became undetectable. In contrast, sigma 2 receptor Bmax values were unchanged with successive passages of the neuroblastoma-derived tumors and doubled in the nude mouse-borne gliomas. When neuroblastoma-derived solid tumors that were devoid of delta opioid binding were returned to culture, opioid receptors appeared to be up-regulated as compared with their original in vitro levels. Serial passaging of these recultured cells in vivo again resulted in a rapid decline in opioid receptor content. The opioid data are consistent with our prior findings on opioid binding diminution in human brain tumors. The pattern of change for sigma binding was more complex, with the sigma 2 response in late passages of the glioma being reminiscent of the formerly observed increase in number of sigma sites in transformed human meninges, kidney, and colon tissue.

Identification and properties of a peptidyl dipeptidase in the housefly, Musca domestica, that resembles mammalian angiotensin-converting enzyme

[D-Ala2,Leu5]Enkephalin was readily metabolized by membranes (40,000 g pellet) prepared from heads of the housefly, Musca domestica, with Gly3-Phe4 being the major site of cleavage. This hydrolysis was only partially inhibited (40%) by 10 microM phosphoramidon, an inhibitor of endopeptidase-24.11, but was almost totally abolished in the presence of a mixture of 10 microM phosphoramidon and 10 microM captopril, a potent inhibitor of mammalian angiotensin-converting enzyme (ACE). An assay for ACE employing Bz-Gly-His-Leu as the substrate was used to confirm the presence of an ACE-like peptidyl dipeptidase activity in fly head membranes. The peptidase had a Km of 1.91 mM for Bz-Gly-His-Leu and a pH optimum of 8.2. The activity was inhibited by 100 microM EDTA and was greatly activated by ZnCl2 but not other bivalent metal ions. Captopril, lisinopril, fosinoprilat and enalaprilat, all selective inhibitors of mammalian ACE, were also good inhibitors of the insect enzyme with IC50 values of 400 nM, 130 nM, 16 nM and 290 nM respectively. An M(r) value of around 87,000 was obtained for this enzyme from gel-filtration chromatography, indicating that the insect enzyme is similar in size to mammalian testicular ACE (M(r) = 90,000-110,000) and not the larger form of the enzyme (M(r) = 150,000-180,000) found in mammalian somatic tissues. The fly peptidyl dipeptidase was released from membranes into a soluble fraction by incubating the head membranes at 37 degrees C but not at 0 degree C, suggesting that the insect ACE-like enzyme can be solubilized from cell surfaces through the activity of a membrane-bound enzyme activity. In conclusion, we have shown the existence of a peptidyl dipeptidase in membranes from the heads of M. domestica, which has similar properties to those of mammalian ACE.