Analgesic responses elicited by endogenous enkephalins (protected by mixed peptidase inhibitors) in a variety of morphine-sensitive noxious tests

Protection of endogenous enkephalin catabolism as natural approach to novel analgesic and antidepressant drugs

The most efficient drugs to alleviate severe pain are opioid compounds. However, their chronic use could be associated with serious drawbacks, such as tolerance, respiratory depression and constipation. Therefore, there is a need for compounds able to efficiently alleviate inflammatory and neurogenic pain following chronic treatment. The discovery that the endogenous opioid peptides, enkephalins, are inactivated by two metallopeptidases, neutral endopeptidase and aminopeptidase N, which can be blocked by synthetic dual inhibitors, represents a promising way to develop 'physiological' analgesics devoid of morphine side effects. These dual inhibitors also have antidepressant-like properties through enkephalin-related activation of delta-opioid receptors. This is expected to reduce the emotional component of pain in humans. This article reviews the promising data obtained for future development of a new class of analgesic that could be of major interest in a number of severe and chronic pain syndromes.

Interaction of endogenous ligands mediating antinociception

It is well known that a multitude of transmitters and receptors are involved in the nociceptive system, some of them increasing and others inhibiting the pain sensation both peripherally and centrally. These substances, which include neurotransmitters, hormones, etc., can modify the activity of nerves involved in the pain pathways. Furthermore, the organism itself can express very effective antinociception under different circumstances (e.g. stress), and, during such situations, the levels of various endogenous ligands change. A very exciting field of pain research relates to the roles of endogenous ligands. Most of them have been suggested to influence pain transmission, but only a few studies have been performed on the interactions of different endogenous ligands. This review focuses on the results of antinociceptive interactions after the co-administration of endogenous ligands. The data based on 55 situations reveal that the interactions between the endogenous ligands are very different, depending on the substances, the pain tests, the species of animals and the route of administrations. It is also revealed that only a few of the possible interactions between endogenous ligands have been investigated to date, in spite of the fact that the type of antinociceptive interaction between different endogenous ligands could hardly be predicted. The results indicate that the combination of endogenous ligands should not be omitted from the pain therapy arsenal. Attention will hopefully be drawn to the complex interdependence of endogenous ligands and their potential use in clinical practice.

Aminopeptidase-N/CD13 (EC 3.4.11.2) inhibitors: chemistry, biological evaluations, and therapeutic prospects

Aminopeptidase N (APN)/CD13 (EC 3.4.11.2) is a transmembrane protease present in a wide variety of human tissues and cell types (endothelial, epithelial, fibroblast, leukocyte). APN/CD13 expression is dysregulated in inflammatory diseases and in cancers (solid and hematologic tumors). APN/CD13 serves as a receptor for coronaviruses. Natural and synthetic inhibitors of APN activity have been characterized. These inhibitors have revealed that APN is able to modulate bioactive peptide responses (pain management, vasopressin release) and to influence immune functions and major biological events (cell proliferation, secretion, invasion, angiogenesis). Therefore, inhibition of APN/CD13 may lead to the development of anti-cancer and anti-inflammatory drugs. This review provides an update on the biological and pharmacological profiles of known natural and synthetic APN inhibitors. Current status on their potential use as therapeutic agents is discussed with regard to toxicity and specificity.

Pain management by a new series of dual inhibitors of enkephalin degrading enzymes: long lasting antinociceptive properties and potentiation by CCK2 antagonist or methadone

The discovery that the endogenous morphine-like peptides named enkephalins are inactivated by two metallopeptidases, neutral endopeptidase and aminopeptidase N, which can be blocked by dual inhibitors, represents a promising way to develop 'physiological' analgesics devoid of the side effects of morphine. A new series of dual aminophosphinic inhibitors of the two enkephalin-catabolizing enzymes has been recently designed. In this study, one of these inhibitors, RB3007, was tested in various assays commonly used to select analgesics (mouse hot-plate test, rat tail-flick test, writhing and formalin tests in mice, and paw pressure test in rats), and the extracellular levels of the endogenous enkephalins in the ventrolateral periaqueductal grey have been measured by microdialysis after systemic administration of RB3007. In the mouse hot-plate test, the dual inhibitor induced long-lasting (2 h) antinociceptive effects with a maximum of 35% analgesia 60 min after i.v. or i.p. administration. These antinociceptive responses were antagonized by prior injection of naloxone (0.1 mg/kg, s.c.). Similar long lasting effects were observed in the other animal models used. Very interestingly, injection of RB3007 (50 mg/kg, i.p.) significantly increased (82%) the extracellular levels of Met-enkephalin with a peak 60 min after i.p. injection. This increase parallels the antinociceptive responses observed. In addition, strong facilitatory effects of subanalgesic doses of the CCK(2) receptor antagonist, PD-134,308 or the synthetic opioid agonist, methadone on RB3007-induced antinociceptive responses were observed. These findings may constitute promising data for future development of a new class of analgesics that could be of major interest in a number of severe and persistent pain syndromes.

Depressant effect on a C-fibre reflex in the rat, of RB101, a dual inhibitor of enkephalin-degrading enzymes

The effect of N-[(R,S)-2-benzyl-3[(S)-(2-amino-4-methylthio)butyldithiol]-1-oxopropyl]-L-phenylalanine benzyl ester (RB101), a dual inhibitor of the enkephalin-degrading enzymes, neutral endopeptidase and aminopeptidase N, was assessed in anaesthetised rats on the C-fibre reflex elicited by electrical stimulation within the sural nerve territory and recorded from the ipsilateral biceps femoris muscle. The temporal evolution of the pharmacological response was monitored by the repeated application of a constant stimulus intensity, namely three times threshold (3 T). In addition, recruitment curves were built by varying the stimulus intensity from 0 to 7 T. RB101 (7.5, 15 and 30 mg kg(-1), i.v.) induced a dose-dependent, naloxone-reversible depression of the reflex, which lasted around 60 min with the highest dose. The ED(50) was calculated as 16.9 mg kg(-1). Analyses of the recruitment curves revealed: (1) a significant increase of threshold; (2) a significant depression of the reflex in the ascending part of the curve; and (3) a lack of major depressive effects on the responses elicited by the strongest stimuli (corresponding to the plateau of the curve). The increase in the nociceptive threshold by enkephalin-degrading enzyme inhibitors, confirms previous data obtained from behavioural tests. In addition, the present study revealed an efficacy of these compounds over a wide range of stimulus intensities, albeit excluding the highest.

Long lasting antinociceptive properties of enkephalin degrading enzyme (NEP and APN) inhibitor prodrugs

Prodrugs of phosphinic dual inhibitors of the enkephalin degrading enzymes, neutral endopeptidase (NEP) and aminopeptidase N (APN), corresponding to the formula H(3)N(+)CH(R(1))P(O)(OR)CH(2)CH(CH(2)Bip)CONHCH(CH(3))COOCH(2)Ph, with R(1) = CH(3) or Ph and R being a benzyl ester, a S-acyl-2-thioethyl derivative, or an acyloxyalkyl group, were synthesized to improve the poor central bioavailability of their precursors. As expected, these compounds (50 mg/kg, iv or ip) induced long lasting ( approximately 2 h) antinociceptive responses in the hot plate test in mice with a ceiling effect varying between 25 and 42% of analgesia. A very rapid hydrolysis of the carboxylate ester contrasting with a slow deprotection of the phosphinate group (t(1/2) approximately 1 h) was observed in serum while 80% of free drug was obtained after 1 h incubation with brain membranes. These results account for the long duration of action observed with these prodrugs.

Phosphinic derivatives as new dual enkephalin-degrading enzyme inhibitors: synthesis, biological properties, and antinociceptive activities

The development of dual inhibitors of the two zinc metallopeptidases, neprilysin (neutral endopeptidase) and aminopeptidase N involved in the inactivation of the opioid peptides, enkephalins, represents an attractive physiological approach in the search for new analgesics devoid of the major drawbacks of morphine. Phosphinic compounds, corresponding to the general formula H(3)N(+)-CH(R(1))-P(O)(OH)-CH(2)-CH(R(2))-CONH-CH(R(3))-COO(-), able to act as transition-state analogues and to fit the S(1), S(1)', and S(2)' subsites of both enzymes were designed. Selection of the R(1), R(2), and R(3) residues for optimal recognition of these enzymes led to the first dual competitive inhibitors with K(i) values in the nanomolar range for neprilysin and aminopeptidase N. These compounds induce potent analgesic responses after intracerebroventricular or intravenous administrations in mice (hot plate test), and several of them were shown to be, at least, 10 times more potent than the previously described dual inhibitors.

Potent and selective inhibition of zinc aminopeptidase A (EC 3.4.11.7, APA) by glutamyl aminophosphinic peptides: importance of glutamyl aminophosphinic residue in the P1 position

Through the development of a new chemical strategy, aminophosphinic peptides containing a pseudoglutamyl residue (Glu Psi(PO2-CH2)Leu-Xaa) in the N-terminal position were synthesized and evaluated as inhibitors of aminopeptidase A (APA). The most potent inhibitor developed in this study, Glu Psi(PO2-CH2)Leu-Ala, displayed a Ki value of 0.8 nM for APA, but was much less effective in blocking aminopeptidase N (APN) (Ki = 31 microM). The critical role of the glutamyl residue in this phosphinic peptide, both in potency and selectivity, is exemplified by the P1 position analogue, Ala Psi(PO2-CH2)Leu-Ala, which exhibited a Ki value of 0.9 microM toward APA but behaved as a rather potent inhibitor of APN (Ki = 25 nM). Glu Psi(PO2-CH2)Leu-Xaa peptides are poor inhibitors of angiotensin converting enzyme (Ki values higher than 1 microM). Depending on the nature of the Xaa residue, the potency of these phosphinic peptides toward neutral endopeptidase 24-11 varied from 50 nM to 3 microM. In view of the in vivo role of APA in the formation of brain angiotensin III, one of the main effector peptides of the renin angiotensin system in the central nervous system, highly potent and selective inhibitors of APA may find important therapeutic applications soon.

Binding properties of a highly potent and selective iodinated aminopeptidase N inhibitor appropriate for radioautography

Aminopeptidase N (APN) is a zinc metallopeptidase involved in the inactivation of biologically active peptides. The knowledge of its precise distribution is crucial to investigate its physiological role. This requires the use of appropriate probes such as the recently developed highly potent and selective radiolabeled APN inhibitor 2(S)-benzyl-3-[hydroxy(1'(R)-aminoethyl)phosphinyl]propanoyl-L-3-[ (12 5)I]iodotyrosine ([(125)I]RB 129). Its binding properties were investigated using rat brain homogenates (K(d)=3.4 nM) or APN expressed in COS-7 cells (K(d)=0.9 nM). The specific binding was 95% at [K(d)], and preliminary autoradiography in intestine is promising. The decreased affinity of [(125)I]RB 129 (=10(-6) M) for the E(350)D APN mutant, supports the critical role of E(350) in the amino-exopeptidase action of APN.

The effects of RB101, a mixed inhibitor of enkephalin-catabolizing enzymes, on carrageenin-induced spinal c-Fos expression are completely blocked by beta-funaltrexamine, a selective mu-opioid receptor antagonist

We have demonstrated that pre-administered RB101 (40 mg/kg, i.v.), a mixed inhibitor of enkephalin-catabolizing enzymes, decreased spinal c-Fos expression induced 1 h and 30 min after intraplantar (i.pl.) carrageenin (41% reduction, p<0.01). These effects were completely blocked by pre-administered beta-funaltrexamine (10 mg/kg, i.v., 24 h prior to stimulation), a selective long-lasting mu-opioid receptor antagonist. In conclusion, these results clearly demonstrate that the effects of endogenous enkephalins on noxiously evoked spinal c-Fos expression are essentially mediated via mu-opioid receptors.

Aminophosphinic inhibitors as transition state analogues of enkephalin-degrading enzymes: a class of central analgesics

Inhibition of aminopeptidase N and neutral endopeptidase-24.11, two zinc metallopeptidases involved in the inactivation of the opioid peptides enkephalins, produces potent physiological analgesic responses, without major side-effects, in all animal models of pain in which morphine is active. Dual inhibitors of both enzymes could fill the gap between opioid analgesics and antalgics. Until now, attempts to find a compound with high affinity both for neutral endopeptidase and aminopeptidase N have failed. We report here the design of dual competitive inhibitors of both enzymes with KI values in the nanomolar range. These have been obtained by selecting R1, R2, and R3 determinants in aminophosphinic-containing inhibitors: NH2---CH(R1)P(O)---(OH)CH2---CH(R2)CONH---CH(R3)COOH, for optimal recognition of the two enkephalin inactivating enzymes, whose active site peculiarities, determined by site-directed mutagenesis, have been taken into account. The best inhibitors were 10x more potent than described dual inhibitors in alleviating acute and inflammatory nociceptive stimuli in mice, thus providing a basis for the development of a family of analgesics devoid of opioid side effects.

Characterization of Glu350 as a critical residue involved in the N-terminal amine binding site of aminopeptidase N (EC 3.4.11.2): insights into its mechanism of action

The molecular components ensuring the strict exopeptidase action of aminopeptidase N (APN) and related zinc aminopeptidases of the M1 family have not yet been clearly established. The specific recognition of the N-terminal amino acid of the substrates by the enzymes has been proposed to involve either the complexation of the free amino group by the catalytic zinc ion or an interaction with an anionic binding site, which could be constituted by an aspartate or glutamate residue. To investigate the existence of such an ionic binding site, site-directed mutagenesis experiments have been performed on acidic residues of pig APN. Given that aminopeptidases of the M1 family are likely to have a common mechanism of action, only strictly conserved residues were mutated. As compared to the wild-type enzyme, the mutation D220E led only to slight modifications in the kinetic parameters of the enzyme and in the Ki values of various inhibitors, indicating that this residue is not critically involved in the hydrolytic mechanism. In contrast, the mutations E350Q and E350D induced a large decrease in enzyme activity, essentially due to modifications in kcat, whereas the E350A mutation led to an almost completely inactive enzyme. Moreover, among the inhibitors tested, only those acting as transition state analogs showed significant increases in their Ki values. These data are in favor of E350 belonging to the "anionic binding site" in APN. A mechanism of action, derived from that of thermolysin, is proposed for these aminopeptidases, which explains the importance of E350 in transition state formation, rather than in the Michaelis complex.

Contribution of endopeptidase 3.4.24.15 to central neurotensin inactivation

The tridecapeptide, neurotensin elicits naloxone-insensitive analgesia after its intracebroventricular administration in mice. We used this central pharmacological effect to assess the putative contribution of the endopeptidase 3.4.24.15 to central inactivation of the peptide. By means of combinatorial chemistry, we previously designed the first potent endopeptidase 3.4.24.15 inhibitor. This agent, Z-(L,D)Phe psi(PO2CH2)(L,D)Ala-Lys-Met (phosphodiepryl 21), is shown here to behave as a fully specific endopeptidase 3.4.24.15 inhibitor, as demonstrated by the absence of effect on a series of other exo- and endopeptidases belonging to various classes of proteolytic activities present in murine brain membranes. Furthermore, central administration of phosphodiepryl 21 drastically prolongs the forepaw licking latency of mice tested on the hot plate and injected with sub-maximally active doses of neurotensin. Altogether, our results demonstrated that, in addition to endopeptidase 3.4.24.16, endopeptidase 3.4.24.15 likely contributes to the physiological termination of the neurotensinergic message in murine brain.

Effect of a novel selective and potent phosphinic peptide inhibitor of endopeptidase 3.4.24.16 on neurotensin-induced analgesia and neuronal inactivation

1. We have examined a series of novel phosphinic peptides as putative potent and selective inhibitors of endopeptidase 3.4.24.16. 2. The most selective inhibitor, Pro-Phe-psi(PO2CH2)-Leu-Pro-NH2 displayed a Ki value of 12 nM towards endopeptidase 3.4.24.16 and was 5540 fold less potent on its related peptidase endopeptidase 3.4.24.15. Furthermore, this inhibitor was 12.5 less potent on angiotensin-converting enzyme and was unable to block endopeptidase 3.4.24.11, aminopeptidases B and M, dipeptidylaminopeptidase IV and proline endopeptidase. 3. The effect of Pro-Phe-psi(PO2CH2)-Leu-Pro-NH2, in vitro and in vivo, on neurotensin metabolism in the central nervous system was examined. 4. Pro-Phe-psi(PO2CHH2)-Leu-Pro-NH2 dose-dependently inhibited the formation of neurotensin 1-10 and concomittantly protected neurotensin from degradation by primary cultured neurones from mouse embryos. 5. Intracerebroventricular administration of Pro-Phe-psi(PO2CH2)-Leu-Pro-NH2 significantly potentiated the neurotensin-induced antinociception of mice in the hot plate test. 6. Altogether, our study has established Pro-Phe-psi(PO2CH2)-Leu-Pro-NH2 as a fully selective and highly potent inhibitor of endopeptidase 3.4.24.16 and demonstrates, for the first time, the contribution of this enzyme in the central metabolism of neurotensin.

RB 101, a purported pro drug inhibitor of enkephalin metabolism, is antinociceptive in pregnant mice

In an earlier study, we demonstrated the enhancement of pregnancy-induced analgesia with an inhibitor of endogenous enkephalin metabolism. The purpose of the present study was to evaluate the antinociceptive effect of another inhibitor of enkephalin metabolism, RB 101, on pregnant mice. Further, since other studies have shown RB 101 to be free of opioid side effects, we examined its effect on respiratory rate. Analgesia was assessed using the hot plate test, and respiratory rate was measured by recording the output from an end-tidal carbon dioxide detector. In pregnant mice, experiments were conducted on Day 17 or Day 18 of pregnancy; mice usually deliver on Day 19. For the hot plate test, animals were tested in the following groups: Group 1, RB 101 150 mg/kg (n = 15); Group 2, RB 101 50 mg/kg (n = 15); Group 3, RB 101 vehicle (n = 15); Group 4, morphine 5 mg/kg (n = 14); and Group 5, RB 101 150 mg/kg + naloxone 5 mg/kg (n = 10). The test was repeated on the second day after delivery in animals in Groups 1 and 3 (given RB 101 150 mg/kg and RB 101 vehicle, respectively). RB 101 150 mg/kg and morphine 5 mg/kg were significantly different (mean percentage of maximum possible effect 30.0 and 37.7, respectively, at 30 min and 41.6 and 32.6, respectively, at 60 min) in their antinociceptive effect in pregnant animals from all other groups. Naloxone, when coadministered with RB 101, prevented the development of antinociception. RB 101 150 mg/kg was not antinociceptive after delivery. Depression of respiratory rate was tested in a separate set of animals in the following groups: Group 1, RB 101 150 mg/kg (n = 16); Group 2, morphine 5 mg/kg (n = 16); Group 3, RB 101 vehicle (n = 15). Morphine 5 mg/kg produced significant depression of respiratory rate at 30 min postinjection when compared with RB 101 150 mg/kg and RB 101 vehicle (mean percent change in respiratory rate was 78.5% compared with 87.7% and 92.4%, respectively, where 100% = no change). These results suggest that drugs such as RB 101 may produce antinociception with minimal effects on respiration.

Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the mu-opioid-receptor gene

Despite tremendous efforts in the search for safe, efficacious and non-addictive opioids for pain treatment, morphine remains the most valuable painkiller in contemporary medicine. Opioids exert their pharmacological actions through three opioid-receptor classes, mu, delta and kappa, whose genes have been cloned. Genetic approaches are now available to delineate the contribution of each receptor in opioid function in vivo. Here we disrupt the mu-opioid-receptor gene in mice by homologous recombination and find that there are no overt behavioural abnormalities or major compensatory changes within the opioid system in these animals. Investigation of the behavioural effects of morphine reveals that a lack of mu receptors abolishes the analgesic effect of morphine, as well as place-preference activity and physical dependence. We observed no behavioural responses related to delta- or kappa-receptor activation with morphine, although these receptors are present and bind opioid ligands. We conclude that the mu-opioid-receptor gene product is the molecular target of morphine in vivo and that it is a mandatory component of the opioid system for morphine action.

Liver transplantation is associated with increased met-enkephalin levels in the pig

BACKGROUND

It has been reported that less postoperative morphine is required following liver transplantation than is required following open cholecystectomy. This may be attributable to endogenous factors rather than to altered morphine pharmacokinetics. We measured the plasma concentrations of two endogenous neuropeptides associated with pain modulation, substance P (SP) and met-enkephalin (ME), in pigs undergoing liver transplantation and in control pigs undergoing laparotomy.

METHODS

With the approval of the institutional Animal Care Committee, pigs were anesthetized with ketamine (30 mg/ kg,i.m.), atropine (0.05 mg/kg, i.m.) and acetylpromazine (0.1 mg/kg, i.m.). Anesthesia was maintained with isoflurane in oxygen. Pigs in the transplantation group (n = 10) underwent liver transplantation and control pigs (n = 10) underwent laparotomy. Blood samples for SP and ME measurement were collected pre-incision (Pre-In), pre-emergence (Pre-Em) from anesthesia, 6-12 hours, 18 hours, and 24 hours after surgery. SP and ME levels were determined by radioimmunoassay. Results are expressed as mean +/- SEM (in pg/ml of plasma for both peptides) and were compared by the non-parametric Mann-Whitney U test. Statistical significance was inferred if P < 0.05.

RESULTS

Plasma ME levels were significantly increased in the transplanted pigs at Pre-Em, 6-12 hours and 18 hours after surgery. No statistically significant difference was observed for plasma SP level between the control and transplant pigs.

CONCLUSIONS

Liver transplantation in the pig model is associated with increased concentrations of endogenous ME (but not SP) in plasma for at least 18 hours after surgery as compared to animals undergoing laparotomy.

Reduction of morphine abstinence in mice with a mutation in the gene encoding CREB

Chronic morphine administration induces an up-regulation of several components of the cyclic adenosine 5'-monophosphate (cAMP) signal transduction cascade. The behavioral and biochemical consequences of opiate withdrawal were investigated in mice with a genetic disruption of the alpha and Delta isoforms of the cAMP-responsive element-binding protein (CREB). In CREBalphadelta mutant mice the main symptoms of morphine withdrawal were strongly attenuated. No change in opioid binding sites or in morphine-induced analgesia was observed in these mutant mice, and the increase of adenylyl cyclase activity and immediate early gene expression after morphine withdrawal was normal. Thus, CREB-dependent gene transcription is a factor in the onset of behavioral manifestations of opiate dependence.

Analgesia produced by immobilization stress and an enkephalinase inhibitor in amphibians

The role of endogenous opioids in modulating pain transmission in amphibians was examined by two methods known to activate endogenous opioids in mammals. Analgesia was assessed using the acetic acid test in the Northern grass frog, Rana pipiens. One or 2 h of immobilization produced a significant analgesia lasting for at least 90 min. Systemic, but not spinal, administration of naloxone before immobilization prevented the analgesic effects seen in saline-pretreated controls. Spinal administration of the enkephalinase inhibitor, thiorphan, but not bestatin (both at 100 nmol/frog), produced significant analgesia. The analgesic effect of thiorphan was blocked by coadministration of intraspinal naloxone. These data are the first to suggest a role for endogenous opioid modulation of noxious stimuli in lower vertebrates by examination of stress-induced analgesia and the action of agents that inhibit enkephalin degradation.

Phosphorus-containing peptides as mixed inhibitors of endopeptidase 3.4.24.15 and 3.4.24.16: effect on neurotensin degradation in vitro and in vivo

1. We have examined several phosphorus-containing peptides as potential mixed inhibitors of two neurotensin-degrading zinc metallopeptidases, endopeptidase 3.4.24.15 and endopeptidase 3.4.24.16. 2. Among a series of 13 phosphonamide peptides, N-(2-(2-naphtyl)ethylphosphonyl-glycyl-prolyl-norleucine (phosphodiepryl 08) was found to inhibit potently the hydrolysis of neurotensin by purified endopeptidase 3.4.24.15 and 3.4.24.16 with an identical Ki value of 0.4 nM. 3. Phosphodiepryl 08 displayed a strong selectivity towards the two peptidases since it failed to inhibit several other zinc-containing peptidases such as endopeptidase 3.4.24.11, angiotensin-converting enzyme, aminopeptidase M, leucine aminopeptidase and carboxypeptidases A and B. 4. The protective effect of phosphodiepryl 08 on neurotensin degradation was examined in vitro and in vivo in central and peripheral bioassays. 5. Phosphodiepryl 08 virtually abolished neurotensin degradation by 4-day-old plated pure cultured neurones from mouse embryos and greatly potentiated neurotensin-induced antinociception in the mouse hot plate test. 6. In the periphery, phosphodiepryl 08 inhibited neurotensin degradation by membranes prepared from isolated longitudinal smooth muscle of guinea-pig ileum and greatly potentiated the neurotensin-induced contraction of the same longitudinal smooth muscle preparation. 7. Our study indicates that phosphodiepryl 08 behaves as a potent and selective mixed inhibitor of endopeptidase 3.4.24.15 and 3.4.24.16 and can be used as a powerful agent to prevent neurotensin degradation, in vitro and in vivo, in central and peripheral assays.

Analysis of nocifensive behavior induced in rats by CO2 laser pulse stimulation

To characterize nocifensive behavior, a laser beam was applied to the hind footpad of nonanesthetized and unrestrained rats and the reaction pattern was analyzed. Fifty-four rats were divided into nine groups of six animals, and each group was given one of nine combinations of laser stimuli: intensity of 4, 8 or 12 W and duration of 10, 30, or 50 ms. A single pulse was applied to a 0.13 cm2 area of right or left footpad and the trial was repeated 20 times with 3 min between trials. The behavior was videotaped and reviewed for a period of 2 min following each stimulation. It seemed to consist of eight discrete responses, and each response was scored for whether it occurred and for its summed duration per trial. The component responses and the behavior as a whole were characterized by their sensitivity in terms of the level of energy required to attain 50% of the maximum response, and their linear or quadratic trends with increasing stimulus energy. The most sensitive index of pain stimulation was the composite score, followed by foot jumping, foot elevation, body movements, licking, and then foot movements. As stimulus energy increased, rats exhibited a greater number of different responses and a greater frequency of each component response. The results suggest that a pool of hierarchically organized responses in the nocifensive motor system are recruited partially or wholly by nociceptive stimuli of varying intensity.

Inhibition of morphine withdrawal by the association of RB 101, an inhibitor of enkephalin catabolism, and the CCKB antagonist PD-134,308

1. The effects induced in rats on naloxone-precipitated morphine withdrawal syndrome by the new mixed inhibitor of enkephalin catabolism able to cross the blood-brain barrier RB 101 (N-((R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyl dithio]-1-ox-opropyl-L-phenylalanine benzyl ester) given alone or associated with the selective CCKB antagonist, PD-134,308, were investigated. 2. The systemic administration of RB 101 (5, 10 and 20 mg kg-1, i.v.) elicited a significant decrease in 8 of the 14 withdrawal signs evaluated. PD-134,308 (3 mg kg-1, i.p.) did not modify the expression of morphine abstinence when given alone, but induced a strong facilitation of RB 101 responses (12 of 14 withdrawal signs were decreased). This potentiation was particularly intense in peripherally mediated withdrawal signs. 3. In order to clarify the biochemical mechanisms implicated in these responses, the effects induced by the association of RB 101 and PD-134,308 on the occupation of brain opioid receptors by endogenous enkephalins were also investigated in mice. PD-134,308, as well as RB 101, inhibited [3H]-diprenorphine binding to opioid receptors. These results suggest that an increase in endogenous enkephalin levels induced by PD-134,308 could participate in the facilitation of RB 101 behavioural responses. 4. RB 101 has a promising potential role in the management of the opiate withdrawal syndrome. CCKB antagonists, such as PD-134,308 may be useful in potentiating this anti-withdrawal effect.

An innovative cold tail-flick test: the cold ethanol tail-flick test

An innovative antinociceptive test, the cold ethanol tail-flick test (CET), was developed for evaluating the actions of opioid analgesics. To select an optimal operation temperature range for the CET, temperatures from -5 degrees C to -30 degrees C were screened. After screening, temperatures ranging between -20 degrees C and -30 degrees C were both strong and effective enough to act as a noxious cold stimulus. In the following study, -20 degrees C was selected as the cold stimulus for the CET. The sensitivity and specificity of this test were challenged by opioid analgesics: an agonist (morphine) and two agonist-antagonists (buprenorphine and nalbuphine), two tranquilizers (droperidol and diazepam), and four nonopioid analgesics (acetaminophen, aspirin, indomethacin, and ketoprofen). The sensitivity of the CET was also compared with the assays using heat (radiant heat and hot water). The AD50 values determined by the CET for morphine, buprenorphine, and nalbuphine were 0.16 mg/kg, 0.22 micrograms/kg, and 0.19 mg/kg, respectively. Naloxone, an opioid antagonist, blocked the antinociceptive effects of these opioids which were determined by the CET. Furthermore, the tranquilizers and nonopioid analgesics did not show any activity in the CET. Our results show that not only can the CET assess the antinociceptive activity of both opioid agonist and mixed agonist-antagonist, it also possess the characteristics of sensitivity, specificity, simplicity, and reproducibility.

Antinociceptive response induced by mixed inhibitors of enkephalin catabolism in peripheral inflammation

RB101 (N-((R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyl dithio]-1-ox-opropyl)-L-phenylalanine benzyl ester) is a recently developed full inhibitor of the enkephalin-catabolizing enzymes able to cross the blood-brain barrier, whereas RB38A ((R)-3-(N-hydroxycarboxamido-2-benzylpropanoyl)-L-phenylalanine) is as potent as RB101 but almost unable to enter the brain. In this study, we have investigated the effects of systemic administration of morphine, RB101 and RB38A on nociception induced by pressure on inflamed peripheral tissues. Antinociceptive test was performed between 4 and 5 days after injection into the rat left hindpaw of Freund's complete adjuvant to produce localized inflammation. Morphine (1, 2 and 4 mg/kg, i.v.) induced antinociception in inflamed paws at all the doses used, and only at the highest dose in non-inflamed paws. RB101 (10 and 20 mg/kg, i.v.) induced an antinociceptive response only in the inflamed paws. RB38A, also induced an antinociceptive effect in the inflamed paws, but only at the highest dose (20 mg/kg, i.v.). The responses induced by morphine and the inhibitors of enkephalin catabolism were antagonized by the systemic administration of naloxone (1 mg/kg) or methylnaloxonium (2 mg/kg) which acts essentially outside the brain. Central injection (i.c.v.) of methylnaloxonium (2 micrograms) blocked the effect of morphine only in non-inflamed paws, and slightly decreased the response induced by RB101 on inflamed paws. These results indicate that the endogenous opioid peptides, probably enkephalins, are important in the peripheral control of nociception from inflamed tissues.

Enkephalin antinociception in mice is mediated by delta 1- and delta 2-opioid receptors in the brain and spinal cord, respectively

Pharmacological evidence for the existence of delta-opioid receptor subtypes has been reported. This study was conducted to determine which type of delta-opioid receptors was involved supraspinally and spinally when antinociception was induced by the natural enkephalins, [Leu5]enkephalin and [Met5]enkephalin. In the mouse tail flick assay, the antinociceptive ED50 values of both intracerebroventricularly (i.c.v.) administered [Leu5]enkephalin and [Met5]enkephalin (together with the peptidase inhibitors, bestatin and thiorphan) were significantly increased by 7-benzylidenenaltrexone (BNTX), a selective delta 1-opioid receptor antagonist but not by naltriben, a selective delta 2-opioid receptor antagonist. On the other hand, when the enkephalins were administered intrathecally (i.t.), the antinociceptive ED50 values of both enkephalins were significantly raised by naltriben but not by BNTX. beta-Endorphin-induced (i.c.v.) antinociception was antagonized by naltriben administered i.t. or s.c. but not by BNTX administered i.t. or s.c. Different delta-opioid receptor subtypes appeared to be involved in supraspinal (delta 1) and spinal (delta 2) antinociception induced by endogenous delta-opioid receptor agonists, [Leu5] and [Met5]enkephalin. The antinociception produced by i.c.v. administered beta-endorphin has been attributed to the release of [Met5]enkephalin in the spinal cord and its antagonism by naltriben support the finding that enkephalins interact with delta 2-opioid receptors in the spinal cord to mediate antinociception. beta-Endorphin may be interacting at receptors other than delta 1- or delta 2-opioid receptors in the brain, perhaps the putative epsilon receptors, to mediate their effects because neither i.c.v. administered BNTX nor naltriben inhibited its activity.

Effect of mixed (RB 38A) and selective (RB 38B) inhibitors of enkephalin degrading enzymes on a model of depression in the rat

This is a study of the effects of the endogenous opioid peptides, enkephalins, on learned helplessness, an experimental model of depression in rats. For this purpose, the responses induced by RB 38A, a mixed inhibitor of enkephalin catabolism, and RB 38B, a selective inhibitor of neutral endopeptidase EC 3.4.24.11, were compared with the antidepressive effect induced by imipramine. RB 38A and RB 38B induced an imipramine-like effect in reducing helpless behavior, as illustrated by the decrease in the number of escape failures. According to the different pharmacological potential of both inhibitors to reduce enkephalin metabolism, complete inhibition of enkephalins (RB 38A) produced a higher response than that obtained with a partial inhibitor (RB 38B). On the other hand, naloxone (NLX) was found to facilitate the induction of learned helplessness, and to antagonize the effect of both enkephalin-degrading enzyme inhibitors. These results suggest that modifications in the activity of the endogenous opioid system could take place in this model of depression. The antidepressant-like effects induced by RB 38B, and especially by RB 38A, in the learned helplessness paradigm suggest that new mixed enkephalinase inhibitors, able to cross the blood-brain barrier, could provide a new strategy in the treatment of affective disorders.

Effect of inhibiting enkephalin catabolism in the VTA on motor activity and extracellular dopamine

The mixed inhibitor of enkephalin catabolism, kelatorphan, was microinjected into the ventral tegmental area (VTA) of rats to determine if endogenous enkephalins can modulate dopamine transmission in the mesoaccumbens projection. The concentration of extracellular dopamine content in the nucleus accumbens was monitored using in vivo microdialysis simultaneously with measuring motor behavior. Kelatorphan microinjection into the VTA produced a dose-related increase in motor activity and extracellular dopamine in the nucleus accumbens. While the change in extracellular dopamine was modest as compared to exogenous stimulation by a mu agonist such as DAMGO, there was a marked increase in the extracellular content of dopamine and serotonin metabolites. This suggests that mesoaccumbens dopamine transmission is under tonic control of endogenous enkephalins at the ventral tegmental area level.

Repeated systemic administration of the mixed inhibitor of enkephalin-degrading enzymes, RB101, does not induce either antinociceptive tolerance or cross-tolerance with morphine

The potent analgesic responses elicited by systemic administration of RB101, N-[(R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyldithio]-1-oxopro pyl]- 1-oxopropyl]-L-phenylalanine benzyl ester, a prodrug able to inhibit enkephalin-degrading enzymes completely after in vivo bioactivation, has made it possible to investigate the development of antinociceptive tolerance after chronic potentiation of endogenous enkephalins. The ED50 values of RB101 obtained 10 min after i.v. injection were not significantly different in mice treated for 4 days with i.p. administered vehicle (ED50 = 9.50 (6.37-14.15) mg/kg), or with 80 mg/kg of RB101 twice daily (ED50 = 9.50 (5.86-15.39) mg/kg). In contrast, a parallel rightwards shift of the dose-response curves, corresponding to a significant 1.92 (1.49-2.52)-fold decrease in analgesic potency, was observed after i.v. administration of morphine in mice chronically treated with morphine (3 mg/kg, twice daily for 4 days) (ED50 = 3.10 (2.52-3.81) mg/kg) vs. saline (ED50 = 1.60 (1.22-2.09) mg/kg). No tolerance to RB101 was observed even after a longer period (8 days) of chronic treatment with the prodrug. Moreover, no cross-tolerance between morphine and RB101 appeared to occur since the ED50 values obtained after i.v. administration of RB101 were not significantly different in mice chronically pretreated with vehicle (ED50 = 9.50 (6.37-14.15) mg/kg) or with morphine (ED50 = 10.00 (6.62-15.10) mg/kg). The analgesic effect of RB101 observed in morphine-tolerant mice was antagonized by prior injection of naloxone, but not naltrindole (delta-selective antagonist), supporting a preferential involvement of mu-opioid receptors in the antinociceptive effect of RB101, at least in mice in the hot-plate test.(ABSTRACT TRUNCATED AT 250 WORDS)

Cultured human synovial fibroblasts rapidly metabolize kinins and neuropeptides

Kinins and substance P have been implicated in the pathogenesis of inflammatory arthritis by virtue of their abilities to induce vasodilation, edema, and pain. The relative biological potencies of these peptides in vivo would depend at least in part upon their rates of catabolism in the joint. We hypothesized that human synovial lining cells may regulate intraarticular levels of kinins and neuropeptides via degradation by cell surface-associated peptidases. We exposed intact human synovial fibroblasts to kinins and substance P, in the presence or absence of specific peptidase inhibitors, and measured the amount of intact substrate remaining and degradation product(s) generated over time. Aminopeptidase M (AmM; EC 3.4.11.2), neutral endopeptidase-24.11 (NEP-24.11; EC 3.4.24.11), and dipeptidyl(amino)peptidase IV (DAP IV; EC 3.4.14.5) were identified on the cell surface of synovial cells. Bradykinin degradation was due entirely to NEP-24.11 (1.39 +/- 0.29 nmol/min per well). Lysylbradykinin was also degraded by NEP-24.11 (0.80 +/- 0.19 nmol/min per well); however, in the presence of phosphoramidon, AmM-mediated conversion to bradykinin (3.74 +/- 0.46 nmol/min per well) could be demonstrated. The combined actions of NEP-24.11 (0.93 +/- 0.15 nmol/min per well) and DAP IV (0.84 +/- 0.18 nmol/min per well) were responsible for the degradation of substance P. AmM (2.44 +/- 0.33 nmol/min per well) and NEP-24.11 (1.30 +/- 0.45 nmol/min per well) were responsible for the degradation of the opioid peptide, [Leu5]enkephalin. The identity of each of the three peptidases was confirmed via synthetic substrate hydrolysis, inhibition profile, and immunological identification. The profiles of peptidase enzymes identified in cells derived from rheumatoid and osteoarthritic joints were identical. These data demonstrate the human synovial fibroblast to be a rich source of three specific peptidases and suggest that it may play a prominent role in regulating peptide levels in the joint.

Antidepressant-type effects of endogenous enkephalins protected by systemic RB 101 are mediated by opioid delta and dopamine D1 receptor stimulation

The role of endogenous enkephalins in behavioural control in mice was investigated by i.v. injection of RB 101 (N-[(R,S)-2-benzyl-3[(S)(2-amino-4- methylthio)butyl dithio]-1-oxopropyl]-L-phenylalanine benzyl ester). RB 101 is a recently reported systemically active mixed inhibitor prodrug of the two enzymes which metabolize the enkephalins neutral endopeptidase 24.11 and aminopeptidase N. RB 101 (2.5-10 mg/kg) induced a dose-dependent long-lasting hyperlocomotion and attenuated the conditioned suppression of motility in mice placed in an environment where they had received footshocks 24 h before. In addition, RB 101 decreased the duration of immobility in the forced swim test. All these actions of RB 101 were antagonized by the selective delta antagonist, naltrindole, supporting the preferential involvement of delta opioid receptors in these enkephalin-controlled behavioural responses. The effects induced by RB 101 were also suppressed by prior administration of the selective dopamine D1 antagonist, SCH 23390, but not by the D2 antagonist, sulpiride. Attenuation of the conditioned suppression of motility was associated with increased striatal dihydroxyphenylacetic acid (DOPAC)/dopamine (DA) and homovanillic acid (HVA)/DA ratios, both effects being antagonized by naltrindole. This latter compound is also efficient to inhibit the effect of imipramine in the mouse forced swim test. Taken together, these results support the occurrence of tonic and phasic controls of mood-related behaviour by endogenous enkephalins through delta and D1 receptor stimulation and suggest a possible future use of these mixed inhibitors as new antidepressants.

Peptidomimetics as receptors agonists or peptidase inhibitors: a structural approach in the field of enkephalins, ANP and CCK

Stabilization of biologically active conformations of native peptides by cyclization or introduction of hindering residues led to peptidominetics endowed with high affinity and selectivity for one class of receptors and able to cross the blood brain barrier. This is the case of BUBU, Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr(OtBu) and BUBUC, Tyr-D-Cys-(OtBu)-Gly-Phe-Leu-Thr(OtBu) for the opioid delta receptors and of BC 254, Boc-gamma-D-Glu-Tyr(SO3H)-Nle-D-Lys-Trp-Nle-Asp-PheNH2 and of BC 264, Boc-Tyr(SO3H)gNle-mGly-Trp-MeNle-Asp-PheNH2 for central CCK-B receptors. Inhibition of metabolizing peptidases such as aminopeptidase N and endopeptidase 24.11 (NEP) for enkephalins and of NEP and ACE for atrial natriuretic peptide and angiotensin I by mixed inhibitors such as kelatorphan and RB 101 or ES14, rationally designed by taking into account the structural differences in the active site of these zinc-metallopeptidases, led to potent analgesics devoid of the major morphine side effects or to new antihypertensives.

In vivo occupation of mouse brain opioid receptors by endogenous enkephalins: blockade of enkephalin degrading enzymes by RB 101 inhibits [3H]diprenorphine binding

With the aim of possibly studying the local activity of brain enkephalinergic pathways by autoradiography and positron emission tomography, preliminary competition experiments of [3H]diprenorphine binding in mouse brain were carried out after i.v. administration of the first systemically-active mixed inhibitor of enkephalin degrading enzymes RB 101 (N(R,S)-2-benzyl-3[(S)-(2-amino-4-methylthiobutyldithio]-1-oxoprop yl]- L-phenylalanine benzyl ester). Although devoid of affinity for the opioid binding sites, RB 101 inhibited the [3H]diprenorphine binding to the opioid receptors in a dose-dependent manner. This effect, very likely due to an RB 101-induced increase in extracellular levels of enkephalins, reached a plateau at a dose of 10 mg/kg, where almost 30% displacement was observed. Intravenous administration of either 5 or 20 mg/kg of RB 101 in mice submitted to warm-swim stress led to an additional [3H]diprenorphine displacement, which reached 45% compared to unstressed controls. This ceiling effect could account for the reported minimal morphine-like side effects induced by mixed inhibitors. A large increase in endogenous enkephalin levels induced by RB 101, associated or not with stress, was also indirectly demonstrated by the analgesic responses elicited by i.v. injection of the mixed inhibitor. This effect was blocked by naloxone but not by the delta antagonist naltrindole (NTI), supporting a preferential implication of mu receptors in supraspinal analgesia. Taken together, these results suggest that RB 101 could be used to determine the precise in vivo localization of enkephalinergic pathways recruited by various stimuli.