Dissociated effects of inhibitors of enkephalin-metabolising peptidases or naloxone on various nociceptive responses

Positive allosteric modulation of the mu-opioid receptor produces analgesia with reduced side effects

Positive allosteric modulators (PAMs) of the mu-opioid receptor (MOR) have been hypothesized as potentially safer analgesics than traditional opioid drugs. This is based on the idea that PAMs will promote the action of endogenous opioid peptides while preserving their temporal and spatial release patterns and so have an improved therapeutic index. However, this hypothesis has never been tested. Here, we show that a mu-PAM, BMS-986122, enhances the ability of the endogenous opioid Methionine-enkephalin (Met-Enk) to stimulate G protein activity in mouse brain homogenates without activity on its own and to enhance G protein activation to a greater extent than β-arrestin recruitment in Chinese hamster ovary (CHO) cells expressing human mu-opioid receptors. Moreover, BMS-986122 increases the potency of Met-Enk to inhibit GABA release in the periaqueductal gray, an important site for antinociception. We describe in vivo experiments demonstrating that the mu-PAM produces antinociception in mouse models of acute noxious heat pain as well as inflammatory pain. These effects are blocked by MOR antagonists and are consistent with the hypothesis that in vivo mu-PAMs enhance the activity of endogenous opioid peptides. Because BMS-986122 does not bind to the orthosteric site and has no inherent agonist action at endogenously expressed levels of MOR, it produces a reduced level of morphine-like side effects of constipation, reward as measured by conditioned place preference, and respiratory depression. These data provide a rationale for the further exploration of the action and safety of mu-PAMs as an innovative approach to pain management.

Identification and functional characterization of a stable, centrally active derivative of the neurotensin (8-13) fragment as a potential first-in-class analgesic

The neurotensin hexapapetide fragment NT(8-13) is a potent analgesic when administered directly to the central nervous system but does not cross the blood-brain barrier. A total of 43 novel derivatives of NT(8-13) were evaluated, with one, ABS212 (1), being most active in four rat models of pain when administered peripherally. Compound 1 binds to human neurotensin receptors 1 and 2 with IC(50) of 10.6 and 54.2 nM, respectively, and tolerance to the compound in a rat pain model did not develop after 12 days of daily administration. When it was administered peripherally, serum levels and neurotensin receptor binding potency of 1 peaked within 5 min and returned to baseline within 90-120 min; however, analgesic activity remained near maximum for >240 min. This could be due to its metabolism into an active fragment; however, all 4- and 5-mer hydrolysis products were inactive. This pharmacokinetic/pharmacodynamic dichotomy is discussed. Compound 1 is a candidate for development as a first-in-class analgesic.

Interactions between NTS2 neurotensin and opioid receptors on two nociceptive responses assessed on the hot plate test in mice

The intracerebroventricular administration of the tridecapeptide neurotensin (NT) produces strong analgesic effects in tests evaluating acute pain. We investigated whether these effects are mediated by the opioid receptors. In the hot plate test, the NT receptors agonist NT1 (N(alpha)Me-Arg-Lys-Pro-Trp-Tle-Leu), s.c. injected (0.3-3 mg/kg), increased paw licking and jump latencies. These effects were inhibited by the NTS2 antagonist levocabastine (2.5 mg/kg, i.p.) but not by the selective NTS1 antagonist SR48692 (3 mg/kg, i.p.). The opioid receptor antagonist naloxone did not modify (up to the dose of 4.5 mg/kg, s.c.) the NT1 effect on licking, but abolished the increase in the jump latency (from the dose of 1.5 mg/kg). In mice made tolerant to the analgesic effect of morphine (2 mg/kg, s.c.) by previous morphine injections (32 mg/kg, s.c., twice a day, 4 days), NT1 maintained its effect on licking, but its effect on jump latency was suppressed. Levocabastine (up to the dose of 4.5 mg/kg) failed to antagonize the effects of morphine (2 mg/kg, s.c.) on both licking and jump latencies. In mice made tolerant to the analgesic effect of NT1 (0.3 mg/kg, s.c.) by previous NT1 injections (3 mg/kg, s.c., twice a day, 4 days) morphine maintained its analgesic effects both on licking and jumping latencies. We can conclude that neurotensinergic and opioidergic transmissions are functionally independent as regards the licking response. However, in the jump response, neurotensinergic transmission seems to regulate opioidergic transmission, inducing its stimulation.

Acetorphan, an Enkephalinase Inhibitor, Modulates Dopaminergic Transmission in Rat Olfactory Tubercle, but not in the Nucleus Accumbens and Striatum

The present study focused on the effects of acetorphan, a parenterally active enkephalinase inhibitor, on dopaminergic transmission in rat olfactory tubercle, nucleus accumbens and striatum. Acetorphan was administered i.v. (10 mg/kg) 15 min before measurement of the in vivo specific binding of [3H]N-propylnorapomorphine ([3H]NPA) or measurement of the levels of dopamine (DA) and its metabolites 3-methoxytyramine-homovanillic acid (3MT-HVA) in the three areas. Acetorphan decreased the in vivo specific binding of [3H]NPA in the olfactory tubercle, this effect being antagonized by naloxone 1.5 mg/kg s.c. DA release in this brain structure was also significantly increased by acetorphan 10 mg/kg, as indicated by the 3MT:DA and HVA:DA ratios. Neither the specific binding of [3H]NPA nor DA metabolism and release were modified by the inhibitor in the striatum and the nucleus accumbens. The stimulant effect of acetorphan was significantly decreased in rats in which a bilateral lesion of dopaminergic endings in the olfactory tubercle had been produced by 6-hydroxydopamine (6-OHDA). These results suggest that dopaminergic transmissions in the olfactory tubercle are particularly sensitive to the modulation exerted by endogenous enkephalins, this modulation being at least partly involved in the increased locomotion induced by the enkephalinase inhibitor.

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.

Neutral endopeptidase knockout induces hyperalgesia in a model of visceral pain, an effect related to bradykinin and nitric oxide

Neutral endopeptidase (EC3.4.24.11, NEP, enkephalinase) is a zinc-metalloendopeptidase, cleaving a variety of substrates like enkephalins, substance P, and bradykinin. In the brain, NEP is a key enzyme in the degradation of enkephalins. Pharmacological inhibition of NEP-activity causes analgesia resulting from enhanced extracellular enkephalin concentrations. Recently, transgenic mice lacking the enzyme NEP have been developed (Lu, 1995). The present study was designed to investigate the nociceptive behavior of these NEP-knockout mice. Interestingly, NEP-deficient mice did not respond with decreased pain perception, but exhibited hyperalgesia in the hot-plate jump, warm-water tail-withdrawal, and mostnotablyin theacetic-acid writhing test. Inhibition of aminopeptidase N by bestatin reduced writhing in both strains, whereas NEP-inhibition by thiorphan reduced writhing selectively in wild-type mice. Naloxone increased writhing in wild-type but not in knockouts, whereas the bradykinin B2-receptor antagonist HOE140 reduced writhing selectively in NEP-knockouts. Similarly, the nitric oxide synthase inhibitor L-NAME reduced writhing in NEP-knockouts. These results indicate that genetic elimination of NEP, in contrast to pharmacological inhibition, leads to bradykinin-induced hyperalgesia instead of enkephalin-mediated analgesia. Nitric oxide (NO) is suggested to be involved in this process.

Diprotin A, an inhibitor of dipeptidyl aminopeptidase IV(EC 3.4.14.5) produces naloxone-reversible analgesia in rats

The dipeptidyl aminopeptidase IV (DP IV) inhibitor Diprotin A produces a full, dose-dependent, short-lasting and naloxone-reversible analgesia in the rat tail-flick test when given intracerebroventricularly, with an ED50 of 295 nmol/rat but it has no direct opioid agonist activity in the longitudinal muscle strip of guinea-pig ileum bioassay. Two of the potential DP IV substrates, morphiceptin and endomorphin 1, identified recently in bovine brain were also analgesic given by similar route. The action of endomorphin 1 was more potent (ED50 = 7.9 nmol/rat) and slightly but significantly more sustained than that of Diprotin A. Diprotin A neither potentiated nor prolonged the effect of a marginally analgesic dose of endomorphin 1. The distinct time course and the lack of potentiation indicate that in the analgesic effect of Diprotin A in rats the protection of a brain Tyr-Pro-peptide other than endomorphin 1 is involved.

Comparison of behavioural effects of NocII or NocIII, two related pronociceptin-derived peptides

Prepronociceptin contains, in addition to nociceptin, other potentially excisable peptides which may have physiological significance. We have here considered NocII, a heptadecapeptide whose sequence lies immediately downstream of that of nociceptin in the precursor polypeptide, as well as NocIII which corresponds to NocII extended by a stretch of three arginine residues. When i.c.v.-administered in mice, NocII (10-10,000 ng) stimulated horizontal locomotor activity and decreased the latency to paw licking but neither to rearing nor escape jumping in the hot plate test (55 degrees C). When nociceptin (100 ng) and NocII (100 ng) were simultaneously intracerebroventricularly injected, each peptide produced its own effect without modifying the effect of the other. NocII was ineffective in the tail flick and writhing tests. NocIII (NocII-Arg-Arg-Arg) was inactive in all tests, even when assayed as long as 40 min following i.c.v. administration. The fact that NocII, but not its very close structural analogue NocII, is biologically active indicates that their may exist a specific receptor to NocII.

The proenkephalin A-processing product peptide E, which encompasses two enkephalin sequences, has a much lower opioid activity than beta-endorphin

Peptide E is a 25-amino acid peptide derived from proenkephalin A that was originally isolated from the bovine adrenal medulla. Bovine peptide E (BPE), which possesses a Met- and a Leu-enkephalin sequence at its N- and C-terminus, respectively, has been described as a highly potent and selective mu-opioid receptor agonist. Paradoxically, the frog counterpart of peptide E (FPE), which exhibits only two amino acid substitutions (Met15-->Gln and Leu25-->Met) compared with BPE, was found to be totally devoid of antinociceptive activity. To decipher this apparent discrepancy, we have decided to compare the structural and pharmacological characteristics of FPE, BPE, and the chimeric peptide [Gln15]BPE (Q15BPE). In methanol, all three peptides exhibited virtually the same conformation, the central region of each peptide (residues 10-20) being involved in a regular helix. Intracerebroventricular administration of FPE, BPE, or Q15BPE, at doses up to 1000 ng per mouse, did not induce any analgesic effects, as evaluated by the hot plate and writhing tests, whereas, in the same tests, beta-endorphin at a dose of 100 ng provoked profound analgesia. Concomitant administration of FPE, BPE, or Q15BPE (100 ng) with the aminopeptidase-N inhibitor bestatin (50 microg) or the endopeptidase 24-11 inhibitor thiorphan (10 microg) did not produce analgesic responses. Antinociceptive effects were only observed when very high doses of FPE, BPE, and Q15BPE (10000 ng per mouse) were administered. These data clearly demonstrate that, contrary to what has been previously reported, peptide E is virtually devoid of opioid activity.

Effects of peptidase inhibitors on the enkephalin-induced anti-nociception in rats

The intra-third-ventricular (i.t.v.) administration of [Met5]-enkephalin (enk) to rats pretreated i.t.v. with three peptidase inhibitors (PIs), amastatin, captopril and phosphoramidon, inhibited the tail-flick response. The enk-induced inhibition was augmented by increasing the doses of the three PIs, with the maximum inhibition being attained at the doses of 10 nmol each. The enk-induced inhibition in rats pretreated with any combination of two PIs, however, were markedly smaller than that in rats pretreated with all three PIs, indicating that three kinds of enzymes all played important roles in the inactivation of enk. The inhibitory effect of enk on the tail-flick response in rats pretreated with the three PIs at doses of 10 nmol each was approximately tenfold higher than that of morphine. The relative anti-nociceptive potencies of enk and morphine were similar to the relative inhibitory potencies obtained previously with the isolated guinea pig ileum pretreated with the three PIs, indicating that the hydrolysis of the i.t.v. administered enk was largely prevented by the three PIs. However, the magnitude of the enk-induced inhibition in rats pretreated s.c. with the three PIs indicated that the hydrolysis of enk injected i.t.v. was not largely prevented by the s.c. administration of three PIs at doses up to 10 micromol each/kg.

Antinociceptive and antidepressant-like profiles of BL-2401, a novel enkephalinase inhibitor, in mice and rats

To clarify the properties of BL-2401 ((+/-)-3-[2-benzyl-3-(propionylthio) propionyl]amino-5-methylbenzoic acid), a novel enkephalinase inhibitor, we examined its antinociceptive and antidepressant-like activities after oral administration, along with their association with endogenous opioid systems. BL-2401 produced an antinociceptive effect after oral administration in the mouse phenylbenzoquinone writhing test (ED50: 12.4 mg/kg) and the rat acetic acid writhing test (ED50: 55.8 mg/kg), the antinociceptive effect being antagonized by naloxone hydrochloride. BL-2401 also relieved arthritis-induced hyperalgesia in rats. In the mouse hot-plate and tail pressure tests, BL-2401 showed significant but modest antinociception at higher doses (200 and 400 mg/kg). In addition, BL-2401 (100 mg/kg) produced a naloxone-reversible antidepressant-like effect in the mouse forced swimming test. As for the mechanism of the action, the active metabolite of BL-2401, BL-2240 ((+/-)-3-(2-benzyl-3-mercaptopropionyl) amino-5-methylbenzoic acid), selectively inhibited enkephalinase in vitro (IC50: 5.2 nM). Oral administration of BL-2401 to mice significantly inhibited the enkephalinase activity in the striatum and also potentiated the antinociceptive effect of (D-Ala2,Met5)-enkephalin given intracisternally. These findings indicate that BL-2401 is an orally active enkephalinase inhibitor and may produce antinociceptive and antidepressant-like effects in association with endogenous opioid systems.

Opioid-related changes in nociceptive threshold and in tissue levels of enkephalins after target disruption of the gene for neutral endopeptidase (EC 3.4.24.11) in mice

Neutral endopeptidase EC 3.4.24.11 (NEP) is localized in peptidergic neurons and various colocalized peptides or other humoral mediators may serve as substrates. Target disruption of the NEP gene was reported to enhance the lethal response to endotoxin shock in mice. We examined thermonociceptive thresholds and enkephalin (ENK) tissue levels in transgenic NEP (-/-) and control wild type NEP (+/+) mice. Hot plate (52 degrees C) latency was 13.1 +/- 1.4 s in NEP (+/+) mice (n = 16) while latency increased significantly (P = 0.031) to 17.7 +/- 1.6 s in NEP (-/-) mice. Naloxone (10 mg/kg) had no effect on hot plate latency in NEP (+/+) mice (12.5 s, n = 8), but significantly decreased the latency in NEP (-/-) mice compared to untreated NEP (-/-) deficient mice (10.5 s, n = 8). Morphine (3 or 10 mg/kg) analgesic response was similar in knockout mice and wild type mice. Methionine-ENK (MET-ENK) and leucine-ENK (LEU-ENK) levels were determined in extracts from cortex, brain stem, hypothalamus, striatum, spinal cord, trigeminal ganglion and heart in treated and untreated mice. ENK-levels varied in a regionally-dependent manner and were significantly decreased in hypothalamus and spinal cord. We conclude that deletion of the NEP gene results in an opioid-related increase in thermonociceptive threshold. Regional differences in opioid metabolism indicate that NEP evokes tissue-specific patterns of ENK-regulation. NEP selectively controls opioid biosynthesis in hypothalamus and spinal cord presumably by feedback regulation.

Aggressiveness, hypoalgesia and high blood pressure in mice lacking the adenosine A2a receptor

Adenosine is released from metabolically active cells by facilitated diffusion, and is generated extracellularly by degradation of released ATP. It is a potent biological mediator that modulates the activity of numerous cell types, including various neuronal populations, platelets, neutrophils and mast cells, and smooth muscle cells in bronchi and vasculature. Most of these effects help to protect cells and tissues during stress conditions such as ischaemia. Adenosine mediates its effects through four receptor subtypes: the A1, A2a, A2b and A3 receptors. The A2a receptor (A2aR) is abundant in basal ganglia, vasculature and platelets, and stimulates adenylyl cyclase. It is a major target of caffeine, the most widely used psychoactive drug. Here we investigate the role of the A2a receptor by disrupting the gene in mice. We found that A2aR-knockout (A2aR-/-) mice were viable and bred normally. Their exploratory activity was reduced, whereas caffeine, which normally stimulates exploratory behaviour, became a depressant of exploratory activity. Knockout animals scored higher in anxiety tests, and male mice were much more aggressive towards intruders. The response of A2aR-/- mice to acute pain stimuli was slower. Blood pressure and heart rate were increased, as well as platelet aggregation. The specific A2a agonist CGS 21680 lost its biological activity in all systems tested.

Antinociceptive effects of ONO-9902, an enkephalinase inhibitor, after visceral stress condition in rats

PURPOSE

The present study was designed to examine the antinociceptive effects of orally administered ONO-9902, an enkephalinase inhibitor, on both somatic and visceral pain after visceral stress conditions.

METHODS

Twenty six male rats were examined. Tail-flick (TF) and colorectal distension (CD) tests were used to determine somatic and visceral antinociceptive effects, respectively. Measurements were performed in rats under immediate post-stress conditions (group ST; n = 14) and in rats nor under stress conditions (group NST; n = 12). In the stressed group, the same device, CD, for visceral antinociceptive effects was used for visceral stress and was applied with an intracolonic pressure of 60 mmHg for 20 min after drug administration. The TF latency and CD threshold were measured before and at 30, 40, 50, 60 and 90 min after administration of ONO-9902 300 mg.kg-1 or distilled water.

RESULTS

Orally administered ONO-9902 did not produce any changes in the % maximum possible effect (%MPE) in either TF or CD tests in the unstressed group. In the stressed group, %MPE in the CD test increased 18% and 31% at 30 and 40 min, respectively, after oral administration of ONO-9902 compared with the control group (P < 0.05). However, %MPE to TF test did not alter even after the CD-induced stress condition.

CONCLUSION

These results suggest that ONO-9902 may have analgesic effects on visceral pain but not on somatic pain under immediate post-stress conditions.

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.

SCH 32615, an enkephalinase inhibitor, enhances pregnancy-induced analgesia in mice

Increased tolerance to noxious stimuli during pregnancy has been demonstrated. The purpose of this study was to examine the effect of SCH 32615, an inhibitor of one of the enzymes (enkephalinase) responsible for the degradation of endogenous enkephalins, on pregnancy-induced analgesia in mice. Analgesia was tested using the hot-plate and tail-flick tests. For the hot-plate test, animals were tested in late pregnancy (Day 17 or Day 18 of pregnancy; mice deliver on Day 19) and in the postpartum period (Days 2 and 8 after delivery) in the following groups: i) no treatment (n = 15); ii) vehicle only (n = 15); iii) SCH 32615 250 mg/kg (n = 20), 150 mg/kg (n = 15), 50 mg/kg (n = 14); iv) naloxone 5 mg/kg (n = 15); v) naloxone 5 mg/kg+SCH 32615 150 mg/kg (n = 10); vi) nonpregnant control given SCH 32615 150 mg/kg (n = 14). All drugs were given subcutaneously. Hot-plate latency (HPL) was significantly higher in pregnant mice (mean hot-plate latency 17.5 s) than postpartum mice (mean hot-plate latency 11 s on Day 2 and 8.5 s on Day 8). SCH 32615 250 mg/kg and 150 mg/kg significantly enhanced this analgesia in pregnant mice (mean percent of maximum possible effect 24.2 and 29.9, respectively) but not SCH 32615 50 mg/kg or the vehicle alone (mean percent of maximum possible effect 12.4 and 0.5, respectively). Naloxone significantly lowered HPL in pregnant mice (19.8 s-16.2 s) and antagonized the effect of SCH 32615.(ABSTRACT TRUNCATED AT 250 WORDS)

Analgesic effect of the direct D2 dopamine receptor agonist RU 24926 and cross tolerance with morphine

The direct D2 dopamine receptor agonist RU 24926, administered subcutaneously to mice, elicited, starting at the dose of 0.125 mg/kg, a dose dependent analgesic effect, assessed as the jump latency from a hot plate (55 degrees C). The analgesic effect induced by 0.25 mg/kg RU 24926 was dose dependently antagonized by the preferential D2 dopamine receptor antagonist haloperidol (ID50 = 15.1 +/- 3.3 micrograms/kg sc) as well as by the opioid receptor antagonist naloxone (ID50 = 0.59 +/- 0.17 mg/kg sc). The reversion of RU 24926-induced analgesia by naloxone was not accompanied by a reversion of hypothermia. Semi-chronic administration of RU 24926 (2.5 mg/kg, sc, 3 times a day for 3 days) completely desensitized to the analgesic effect induced by a 0.25 mg/kg test dose of RU 24926 and partially reduced the analgesic effect of low doses of morphine (0.5, 1, 1.5 mg/kg). Conversely, semi-chronic administration of morphine (32 mg/kg sc, twice daily for 4 days) completely desensitized the analgesic effect induced by a 2 mg/kg test dose of morphine and partially reduced the analgesic effect of RU 24926 (0.25, 0.5 and 1 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Locomotor and analgesic effects of morphine and acetorphan in rats chronically treated with morphine or thiorphan

A continuous 8-day s.c. administration of morphine (450 microgram/kg/h) sensitized rats to the morphine-induced stimulation of locomotion (morphine test dose = 3 mg/kg, s.c.) but not to the acetorphan (5 mg/kg, i.v.)-induced stimulation of locomotion. On the other hand, a continuous 10-day intracerebroventricular infusion of the enkephalinase inhibitor, thiorphan (25 micrograms/rat/h), known to desensitize the acetorphan-induced stimulation of locomotion, also desensitized the morphine (3 mg/kg, s.c.)-induced stimulation of locomotion. The continuous 10-day, s.c. administration of morphine desensitized to the morphine (3 mg/kg, s.c.)-but not acetorphan (5 mg/kg, i.v.)-induced analgesia, as measured by the latency to jump from a hot plate (55 degrees C). On the other hand, the continuous 10-day intracerebroventricular infusion of thiorphan did not desensitize to morphine (3 mg/kg, s.c.)-induced analgesia. Thus, the chronic actions of morphine and thiorphan, according to the tested function, did not result in cross-sensitization (locomotion) or cross-tolerance (nociception). These differences could depend on the involvement of different opioid receptors (mu vs. delta) and/or on different functional organizations.

Tolerance to the hypothermic but not to the analgesic effect of [D-Trp11]neurotensin during the semichronic intracerebroventricular infusion of the peptide in rats

The peptidase-resistant derivative of neurotensin, [D-Trp11]neurotensin, has been continuously infused intracerebroventricularly (75 ng/h) with an osmotic minipump for 10 days. On several days during this infusion the locomotor activity, the body temperature, the food intake, the body weight, and the nociceptive response in the plantar test were measured. A nonsignificant decrease of body temperature and a sustained analgesic effect were observed at each time considered. The response to a test dose of [D-Trp11]neurotensin (75 ng per rat) injected intracerebroventricularly at the 10th day of the chronic infusion revealed a complete tolerance to its hypothermic effect. Thus, it appears that the analgesic effect of [D-Trp11]neurotensin is independent of a hypothermic or an incapacitating effect of the peptide and does not give rise to tolerance after a 10-day continuous administration, in contrast to the hypothermic effect.

Inhibitors of enkephalin-degrading enzymes as potential therapeutic agents

A limited number of enzymes such as membrane metalloendopeptidase (enkephalinase) and angiotensin converting enzyme appear to be involved in deactivation and modulation of circulatory regulatory peptides. Peptides such as the enkephalins are also involved in a large number of physiological processes. This multiplicity of physiological roles has made it difficult to establish the therapeutic role of enkephalin-degrading enzyme inhibitors. Other factors such as difficulty in quantification and thus measurement of processes involved in pain and mental illness have also hindered the process of establishing any therapeutic role of enkephalin-degrading enzyme inhibitors in these conditions. However, they have proved to be useful pharmacological 'tools'. The most likely therapeutic role at present appears to be in the treatment of cardiovascular disorders. As a 'profile' of pharmacological actions of enkephalin-degrading enzymes emerges, it is becoming apparent that bioavailability rather than a high degree of specificity or inhibitory potency may be the most important factor. This may be used to an advantage in future developments by the use of less specific or combined inhibitors in the form of prodrugs, designed to be active at specific sites such as the central nervous system.

Desensitization of mu-opioid receptors does not modify the analgesia induced by an enkephalinase inhibitor

Acetorphan, an enkephalinase inhibitor, or morphine was injected in mice which had received saline or morphine (32 mg/kg s.c. twice a day on 8 consecutive days) chronically. In the hot-plate test, the analgesia (increase in jump latency) induced by morphine (2 mg/kg i.p.) or by the mu selective opioid agonist, [D-Ala2,N-Me-Phe4, Gly5-ol]enkephalin (DAGO) (1.5, 3 or 6 ng/mouse i.c.v.), was significant in the saline group but was strongly decreased in morphine-pretreated mice. In contrast the analgesic effect of acetorphan (5 mg/kg i.v.) or of the delta selective opioid agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) (0.75, 1.5 or 3 micrograms/mouse i.c.v.) was similar in both groups. These results suggest that the enkephalins protected by acetorphan act on the delta receptor site to produce antinociception.

Chronic inhibition of enkephalinase induces changes in the antinociceptive and locomotor effects of the enkephalinase inhibitor acetorphan in rats

The enkephalinase inhibitor thiorphan was infused intracerebroventricularly in rats during 14 days (25 micrograms/5 microliters/hr), inducing an average inhibition of cerebral enkephalinase of about 65%. Animals were tested during the infusion for their response to acetorphan, a parenterally active derivative of thiorphan. When administered intravenously on day 8 of the infusion, acetorphan (5 mg/kg) significantly increased locomotion in chronic saline-infused rats but not in animals receiving thiorphan. Furthermore, when injected at the same dose on day 10, acetorphan did not modify the latency to jump, in the hot plate test, in thiorphan-treated rats, whereas it elicited a significant analgesia in chronic saline-treated controls. These data show that the effects induced by the administration of an enkephalinase inhibitor were diminished after a period of chronic inhibition of the enzyme, suggesting the development of tolerance.

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

It has been suggested that the endogenous opioid peptides, methionine and leucine enkephalin, participate only in naloxone-facilitated antinociceptive responses. To reassess this proposal, analgesic effects resulting from complete inhibition of enkephalin metabolism by intracerebroventricular (i.c.v.) administration of the mixed inhibitor RB 38A (R,S)HONHCOCH2CH(CH2 phi)CONHCH(CH2 phi)COOH) were compared to the effects of morphine (i.c.v.) in various assays commonly used to select analgesics: mouse hot plate-test, tail flick test with mice and rats, electrical stimulation of the tail (TES), paw pressure test with rats, and phenylbenzoquinone-induced writhing test with mice. The ED50s of morphine vs. ED50s of RB 38A in the writhing, hot plate (jumping) and tail flick tests with mice were 0.24 nmol vs. 38 nmol, 1 nmol vs. 36 nmol and 3.2 nmol vs. 285 nmol, respectively. RB 38A (ED30 153 nmol) was only 15 times less active in the tail flick test with rats than morphine and only halve as active in the paw pressure test. Noxious TES in rat was very sensitive to the inhibitory action of endogenous opioids protected by RB 38A, particularly the post-vocalization response which was also shown to be alleviated by antidepressants. All the analgesic effects observed were reversed by naloxone. This first direct evidence of analgesia resulting from peptidase inhibition, in the tail flick test with mice and rats, hot plate (paw lick) and TES shows that the pain suppressive effects of endogenous opioid peptides are not restricted to naloxone-facilitated noxious stimuli but occur more generally, in all morphine-sensitive tests. The differential effects of RB 38A in the various assays is likely to be related to the amount of enkephalins released and to the efficiency of peptidase inactivation in particular brain regions implicated in the control of a given nociceptive input. This mechanism could account for the reduction in side-effects compared to those of morphine following chronic administration of RB 38A.

Stimulation of dopamine D2 receptors induces an analgesia involving an opioidergic but non enkephalinergic link

In the hot plate test, the dopamine D2 receptor agonist RU 24926 as well as the mixed dopamine D1/D2 receptor agonist apomorphine dose dependently increased the nociceptive threshold of mice, as expressed by the jump latency. The dopamine D1 receptor agonist SKF 38393 was ineffective on this parameter. The effect of RU 24926 was antagonized by the dopamine D2 specific receptor antagonist sulpiride but not by the dopamine D1 specific receptor antagonist SCH 23390. It was not increased by SKF 38393. However, the effect of apomorphine was partially but significantly reduced by SCH 23390. Inhibitors of enkephalin-degrading peptidases (thiorphan and bestatin injected i.c.v. or acetorphan injected i.v.) did not potentiate the effect of apomorphine whereas the delta opioid antagonist IC 154, 129 did not reverse the apomorphine-induced analgesia. Finally, the effect of apomorphine was significantly decreased in mice rendered tolerant to morphine. It is concluded that, in mice, the antinociceptive effect induced by apomorphine results mainly from stimulation of D2 receptors. This stimulation probably involves an endogenous opioid, different from enkephalins, which acts at mu opioid receptors.

The antinociceptive effects of SCH-32615, a neutral endopeptidase (enkephalinase) inhibitor, microinjected into the periaqueductal, ventral medulla and amygdala

The local effects of SCH-32615, an inhibitor of enkephalinase (EC 3.4.24.11) on the hot-plate (HP) and tail-flick (TF) responses were examined following unilateral intracerebral microinjection into the periaqueductal brain (PAG), the medial ventral medulla (VM) and bilateral microinjection into the amygdala (AM) of the rat. In the PAG and VM, SCH-32615 resulted in a dose-dependent increase in HP and TF response latencies over a dose range of 1-30 micrograms with the ED50 values (micrograms) being PAG-TF = 17; PAG-HP = 11; VM-TF = 7; VM-HP = 6. In the AM, dose-dependent increases were only observed on the HP. (ED50 (micrograms) HP = 17). Peak effects were observed within 10 min and response latencies remained elevated for 45-60 min. Injections of SCH-32615 at sites outside of the PAG or VM were considerably less effective. All antinociceptive effects were antagonized by naloxone (1 mg/kg, i.p.). Twenty-four hours following the microinjection of beta-funaltrexamine (an irreversible opioid antagonist) into the PAG or the VM, the effects of SCH-32615 in the PAG were virtually abolished while in the VM, its effects were only moderately reduced. These data suggest that in the presence of a strong thermal stimulus, the behavioral response is subject to a tonically active or stimulus-evoked modulation by the local release in the PAG, VM and AM of an agent, presumably an enkephalin peptide, the degradation of which is altered by enkephalinase inhibition.

Differences in physical dependence induced by selective mu or delta opioid agonists and by endogenous enkephalins protected by peptidase inhibitors

The aim of the present study was to investigate if a physical dependence could be induced by chronic activation of the endogenous enkephalinergic system. We have therefore evaluated naloxone-induced withdrawal syndrome in rats after central infusion during 7 days of comparable antinociceptive doses of RB 38 A ((R,S)HONH-CO-CH2-CH(CH2C6H5)-CONH-CH(CH2C6H5)-COOH), a mixed enkephalin catabolism blocker and of the selective mu, DAGO (Tyr-D-Ala-Gly-(Me)Phe-Gly-ol) and delta, DSTBULET (Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr), opioid agonists. The responses were compared to those induced by RB 38 B ((S,S)HONH-CO-CH2-CH(CH2C6H5)-CONH-CH(CH2C6H5)-COOH), a selective inhibitor of the 24.11 neutral endopeptidase (NEP) 'enkephalinase'. DAGO induced a severe withdrawal syndrome evidenced by a large weight loss, hypothermia, jumping, mastication, teeth chattering, diarrhoea, lacrimation and salivation. In contrast, DSTBULET and RB 38 A produced only a moderate physical dependence. Only two signs were statistically different in these two groups: wet dog shakes and temperature. Chronic i.c.v. administration of DAGO, DSTBULET and RB 38 A produced a time-dependent reduction in analgesia, but 120 h after continuous infusion only RB 38 A was able to still induce a significative antinociceptive effect. The present data suggest that even in the drastic conditions used here long-term complete inhibition of enkephalin catabolism induces a weak tolerance and a moderate physical dependence, similar to that produced by delta opioid agonists. This effect was not observed after chronic selective inhibition of NEP by RB 38 B.

Neurobiological background of pain and analgesia: the attempt at revaluation according to position of the organism's adaptive activity

The most adequate and successful way to understand the essence of any complex psychophysiological phenomenon, including pain, is obviously the study of its origin, its genesis, i.e., its biological background. Based on critical analysis of recent literature and our own electrophysiological, biochemical and pharmacological data we tried to overcome the difficulties and contradictions derived from the traditional reflex approach and analytical orientation in understanding the experimental investigation of pain-related problems and to determine the neurobiological background of pain and analgesia through the notion of the organism's adaptive activity. Interrelations between the notion of pain and other biological and psychological ideas, the place and functional significance of pain and endogenous analgesic mechanisms in the organization, maintenance and regulation of the organism's adaptive activity, characterization of the involvement of endogenous opioid peptides and monoamines in central processes associated with pain and analgesia, the essence and mechanisms of pain-depressing activity of the opiates are the main stages in our neurobiological consideration of the phenomenon of pain and its natural and pharmacological regulation.

The antinociceptive effects of intrathecally administered SCH32615, an enkephalinase inhibitor in the rat

The presence of opioid receptors and enkephalin-releasing neurons in the spinal dorsal horn prompted us to examine whether an enkephalinase inhibitor, SCH32615, given intrathecally would alter the response of the rat on several nociceptive endpoints: 49 degrees C hot plate (HP), 52 degrees C HP; tail flick (TF) and the paw pressure (PP) test. SCH (16-320 nmol, i.t.) resulted in a submaximal dose-dependent increase in the 49 degrees C HP, 52 degrees C HP, TF, and PP measures with the respective ED50 values being 40, 74, 68 and 83 nmol, respectively. AT 320 nmol, no additional increment in effect was observed. Concurrent examination of the effects of 0.1-10 nmol morphine on the 49 degrees C HP, 52 degrees C HP, TF and PP measures revealed i.t. ED50 values of 0.2, 0.8, 0.9 and 0.6 nmol, respectively, with the maximum dose leading to a complete block on all measures. The effects of SCH were totally reversed by naloxone (1 mg/kg, i.p.), a dose which had no detectable effect upon baseline nociceptive response measures. The effects of SCH, even at the highest dose were not accompanied by motor dysfunction or catalepsy. These results are consistent with the hypothesis that high-threshold thermal and mechanical stimuli will evoke the release of an opioid in the spinal space, the metabolism of which is significantly influenced by enkephalinase inhibition.

Role of delta opioid receptors in the effects of inhibitors of enkephalin-degrading peptidases on the horizontal and vertical components of locomotion in mice

In the present study we report the effects of inhibitors of enkephalin-degrading peptidases on spontaneous locomotion in mice and the involvement of delta opioid receptors in these effects. Animals received intracerebroventricularly (i.c.v.) or intravenously (i.v.) enkephalinase inhibitors (thiorphan and acetorphan), aminopeptidase inhibitors (bestatin and carbaphethiol) or mixed peptidase inhibitors (kelatorphan). The i.c.v. co-administration of bestatin and thiorphan (50 micrograms + 50 micrograms) induced an increase in both the horizontal and vertical components of locomotion. A similar pattern was observed after the i.c.v. administration of kelatorphan (8.5-50 micrograms) or the i.v. co-administration of acetorphan and carbaphethiol (5 mg/kg + 10 mg/kg). The opiate antagonist naltrexone (1 mg/kg, s.c.) failed to reverse the excitolocomotor effects of kelatorphan or of bestatin and thiorphan and antagonized only partially the effects of acetorphan and carbaphethiol. Naloxone (2 mg/kg-10 mg/kg, s.c.) partially reversed the increase in locomotion elicited by bestatin and thiorphan. The pretreatment with the delta opioid antagonists ICI 154,129 (20 micrograms, i.c.v.) or ICI 174,864 (2-4 micrograms, i.c.v.) strongly decreased the effects of all the peptidase inhibitors we tested. These results suggest that endogenous enkephalins may control via delta opioid receptors the horizontal and vertical components of locomotor activity in mice.

Binding in vivo of selective mu and delta opioid receptor agonists: opioid receptor occupancy by endogenous enkephalins

The in vivo binding properties of cerebral mu and delta opioid receptors were investigated in mice after the intrastriatal injection of [3H][D-Ala2, MePhe4, Gly-ol5]enkephalin (DAGO) or [3H][D-Thr2,Leu5]enkephalyl-Thr (DTLET). Both peptides exhibited similar diffusion kinetics in the brain and 30-40% of [3H]DAGO or [3H]DTLET was shown to be present in the tissue 15 min after injection when maximal binding was observed. The specific binding of both agonists, defined as the fraction of the radioactivity bound to brain membranes which was displaced by 10 nmol of cold ligand, was reversible, saturable and displayed a pharmacological profile similar to that found in in vitro experiments. At doses producing a similar analgesic effect in the hot-plate test in mice, DTLET occupied 64% of delta sites and DAGO 15% of mu sites. However, because of the residual cross-reactivity of DTLET for mu sites, it appeared that both ligands occupied a similar number of mu receptors at their ED50 values, thus supporting a preferential involvement of mu opioid binding sites in the supraspinal pain control. [Met5]enkephalin inhibited the in vivo binding of both agonists only when the peptide was protected from degradation by the co-administration of a mixed inhibitor of enkephalin degrading enzymes RB38A (N[3(R)(hydroxyaminocarbonyl)-2-benzyl-1-oxopropyl]- L-phenylalanine). Unlike thiorphan, 5 nmol RB38A alone was able to inhibit [3H]DAGO binding by 60%. This result is the first direct demonstration of the existence of an in vivo tonic control of mu opioid receptor occupation by endogenous opioid peptides.

Studies on the effect of SCH-34826 and thiorphan on [Met5]enkephalin levels and release in rat spinal cord

SCH-34826 and thiorphan are inhibitors of the neutral endopeptidase (NEP; E.C. 3.4.24.11;) that cleaves the opiate peptides [Met5]- and [Leu5]enkephalin at the glycinylphenylalanine bond. These compounds were evaluated for their ability to affect the levels of [Met5]enkephalin-like immunoreactivity (MELI) in the brain and the spinal cord and the release into the extracellular space under resting and K+-evoked conditions. The results showed that oral administration of SCH-34826 (30-100 mg/kg p.o.) or thiorphan (10-30 mg/kg p.o.) had no effect on tissue levels of MELI. In contrast, both agents caused a dose-dependent increase in both the resting and the K+-evoked levels in spinal perfusates, which reached up to 10 times the control values. These data indicate that tissue (presumably intracellular) stores of [Met5]enkephalin are not affected by NEP inhibition and that it is the extracellular effects of the peptide that are potentiated by enzyme blockade. This agrees with the prior results demonstrating that NEP inhibitors require a nociceptive stimulus sufficient to release endogenous stores of [Met5]enkephalins for their actions to be observed.

Changes in levels of the tripeptide Tyr-Gly-Gly as an index of enkephalin release in the spinal cord: effects of noxious stimuli and parenterally-active peptidase inhibitors

The tripeptide Tyr-Gly-Gly (YGG), representing the product of enkephalin hydrolysis by enkephalinase (EC 3.4.24.11), was characterized and its levels measured in spinal cord perfusates of halothane-anaesthetized rats. During noxious pinching of the muzzle, which is known to trigger enkephalin release, YGG levels were enhanced more markedly and for longer than were those of [Met5]enkephalin (YGGFM), in the same samples. By contrast, neither YGG nor YGGFM levels were affected by pinching the tail. Treatment with carbaphethiol, a parenterally-active aminopeptidase inhibitor, markedly increased YGG levels and lengthened the duration of the increase produced by pinching the muzzle. Treatment with acetorphan, a parenterally-active enkephalinase inhibitor, given alone or in combination with carbaphethiol, completely prevented the rise in YGG triggered by noxious stimulation. By contrast, [Met5]enkephalin levels in the perfusates were increased by the combined administration of the two peptidase inhibitors but these levels were not further enhanced by noxious stimulation. Thus, spinal cord YGG appears to be formed under the influence of enkephalinase and to constitute a sensitive index of enkephalin release.

Tail immersion test for the evaluation of a nociceptive reaction in mice. Methodological considerations

The effect of two commonly used methods to immobilize the animals, viz. tube restrainer and wrapping in a diaper (chux) on the tail flick latency in immersion test, was evaluated in mice using a stimulus temperature of 50 degrees C. The animals were immobilized either in the tube or chux briefly (25-30 sec) during the tail flick measurements. The basal tail flick latency was 2.8 +/- 0.2 in the tube restrained and 5.5 +/- 0.3 sec in chux restrained groups (p less than 0.001). The analgesic effect of morphine (1, 3, 4, 7, and 10 mg/kg) was significantly higher in the chux-restrained animals as indicated by the dose ratio of 2.16 for the 50% analgesic response in the chux versus tube restrained mice. The tail flick latency, 15 min after naloxone injection (1 and 3 mg/kg), expressed as % of predrug latency was significantly reduced in the chux- but not the tube-restrained group. The hyperalgesic effect of naloxone could not be detected in chux-restrained animals, when the water temperature was increased to 55 degrees C. The results demonstrate that the restraining procedure will influence the analgesic effects of test drugs in tail immersion test. Furthermore, the stimulus temperature appears to be an important variant that could influence the results in this test. The present results demonstrate the hyperalgesic effect of naloxone after systemic administration in the tail immersion test and supports the concept that tail flick response is tonically inhibited by endogenous opioid systems.

Role of endogenous enkephalins in locomotion and nociception studied with peptidase inhibitors in two inbred strains of mice (C57BL/6J and DBA/2J)

Acetorphan, a parenterally active enkephalinase inhibitor, induced dose-dependently a naloxone-reversible analgesia on the hot-plate jump test in DBA/2J (DBA2) mice but was devoid of effects in C57BL/6J (C57) mice. By contrast, acetorphan increased locomotion in both strains; however, the DBA2 strain was much more sensitive than C57 mice to the locomotor stimulant effect. The increased locomotion was antagonized by naloxone in both strains. These data suggest that endogenous enkephalins modulate nociception and locomotion in the two inbred strains differently.

Detailed immunoautoradiographic mapping of enkephalinase (EC 3.4.24.11) in rat central nervous system: comparison with enkephalins and substance P

The metallopeptidase enkephalinase known to participate in the inactivation of endogenous enkephalins and, possibly, other neuropeptides such as tachykinins, was visualized by autoradiography using a [125I]iodinated monoclonal antibody. A detailed mapping of the enzyme in rat brain and spinal cord was established on 10-micron serial sections prepared in a frontal plane as well as a few sections in a sagittal plane. On adjacent sections, and for the purpose of comparison, substance P-like and enkephalin-like immunoreactivities were also visualized by autoradiography using a 125I-monoclonal antibody and a polyclonal antibody detected by a secondary 125I-anti-rabbit antibody respectively. Histological structures were identified on adjacent Nissl-stained sections. Using the highly sensitive 125I-probe, enkephalinase immunoreactivity was found to be distributed in a markedly heterogeneous manner in all areas of the central nervous system. Immunoreactivity was undetectable in white matter areas, for example the corpus callosum or fornix, and had a laminar pattern in, for example, the cerebral cortex or hippocampal formation. Hence, although immunodetection was not performed at the cellular level, a major neuronal localization of the peptidase is suggested. The latter is consistent with the detection of a strong immunoreactivity in a pathway linking the striatum to the globus pallidum, the entopeduncular nucleus and the substantia nigra, as well as with a series of biochemical and lesion data. The strong immunoreactivity also present in choroid plexuses and ependymal cells as well as in the intermediate lobe and in scattered cells of the anterior lobe of the pituitary suggests that populations of glial and endocrine cells also express the peptidase. The highest density of enkephalinase immunoreactivity was observed in basal ganglia and limbic areas (caudate putamen, globus pallidus, nucleus accumbens, olfactory tubercles) as well as in areas involved in pain control mechanisms (superficial layers of the spinal nucleus of the trigeminal nerve or of the dorsal horn of the spinal cord) which also display the highest immunoreactivities for both enkephalins and substance P (except in globus pallidus for the latter). These localizations account for the opioid-like analgesic and motor effects of enkephalinase inhibitors inasmuch as a selective or predominant participation of the peptidase in enkephalin inactivation is assumed. A number of other areas appear richly endowed in both enkephalinase and enkephalins whereas substance P is hardly detectable. This is particularly the case for the olfactory bulb, bed nucleus of the accessory olfactory tract, the cerebellum (where enkephalinase mainly occurs in the molecular layer) and the hippocampal formation (namely in the molecular layer of the dentate gyrus).(ABSTRACT TRUNCATED AT 400 WORDS)

Dopaminergic involvement in nociceptive sensitivity/behavioral reactivity regulation during aversive states of different nature in the rat

To investigate the involvement of dopamine (DA) in nociceptive sensitivity-behavioral reactivity regulation in animals during aversive states of different nature, the influence of pharmacologically-induced decrease and increase of DA neurotransmission on vocalization and movement reactivity were studied in rats in free behavior, during restraint stress, after acute trauma of an extremity and under intraperitoneal acetic acid administration. The influence of longterm increase (apomorphine in a high dose) and decrease (haloperidol, apomorphine in a low dose) on suprarenals weight and gastric ulceration in animals exposed by polymodal aversive stimulation was also studied. The data obtained are discussed in relation with; 1. DA involvement in regulation of nociceptive sensitivity and behavioral reactivity in aversive environment; 2. the role of DA and endogenous opioid peptides in endogenous analgesic mechanisms; 3. the functional significance of cerebral DA in organization and realization of various types of an organism's adaptive activity produced by different environmental and homeostatic variables; and 4. the interaction of DA and endogenous opioid peptides in mediation of this activity.

Novel inhibitors of enkephalin-degrading enzymes. II: N5'-substituted-4-thioxohydantoic acids as aminopeptidase inhibitors

Some 2-substituted-(2'-aminophenyl)-4-thioxohydantoic acids (o-amino PTC-amino acids) have antinociceptive activity when administered (icv) alone (IC50 = 0.04-0.87 microM/animal) and show a striking prolongation of the antinociceptive action of (D-Ala-2 D-Leu5)-enkephalin (DADL) in combination. The effects are thought to be mediated via opioid receptors since they are naloxone-reversible. Although inhibitors of the enkephalin degrading puromycin-insensitive, bestatin-sensitive aminopeptidase (possibly aminopeptidase M) their action is weak (IC50 = 32 microM leucine, 536 microM, glycine) and they might be considered to have a direct antinociceptive effect on opioid receptors. The titled compounds constitute novel 'lead' compounds for the development of potent aminopeptidase M inhibitors.

Potent inhibition of cerebral aminopeptidases by carbaphethiol, a parenterally active compound

We designed phethiol (1-amino-1-benzyl-2-mercaptoethane) as a potent and selective inhibitor of Zn-containing aminopeptidases. This compound inhibited purified aminopeptidase M (EC.3.4.11.2) with a Ki of 5 nM but was at least 1000 times less potent against other metallopeptidases comprising angiotensin-converting enzyme EC 3.4.15.1), enkephalinase (EC 3.4.24.11), thermolysin (EC 3.4.24.4), or dipeptidylaminopeptidases. Phethiol alone significantly but partially protected endogenous (Met5) enkephalin released from depolarized brain slices, total protection being achieved when it was associated with an enkephalinase inhibitor. In order to obtain a parenterally-active inhibitor of cerebral aminopeptidases, the prodrug carbaphetiol, a readily hydrolyzable S-phenylcarbamoyl derivative of phethiol, was designed. Carbaphethiol (i.v.) elicited a rapid rise in mouse striatal level of Tyr-Gly-Gly, a characteristic extracellular metabolite of enkephalins. Carbapethiol alone and, even more, when associated with an enkephalinase inhibitor, exerted a potent naloxone-reversible antinociceptive activity. Carbaphethiol appears as the first parenterally-active inhibitor of cerebral aminopeptidases, potentially useful in neuropeptides degradation studies and as a pain-suppressing agent.

Opposite effects of restraint on morphine analgesia and naloxone-induced jumping

It has been demonstrated that the effects of exogenous opiates like morphine could be modified by exposure of an organism to stress, but it is uncertain whether this modification is due to the action of endogenous opioid peptides released by stressful stimuli. The stress of restraint produced an antinociceptive response in mice measured by a latency to escape from a hot plate and, in addition, markedly potentiated analgesia induced by low doses of morphine. Both effects were antagonized by naloxone in a dose-dependent manner. On the other hand, restraint reduced the naloxone-precipitated jumping after single morphine injection. Morphine analgesia and a jumping response were not correlated when tested in two different strains of mice. It is suggested that the enhancement of morphine analgesia by restraint and the reduction in naloxone-induced jumping are mediated via independent mechanisms.

An automated method to analyze vocalization of unrestrained rats submitted to noxious electrical stimuli

Unrestrained rats were subjected to electrical stimuli applied to their paws via an electrified cage floor. Intensity, duration, and order of stimulation were chosen after preliminary tests. Vocalization threshold and vocalization as a behavioral response were studied. The vocalization was recorded and the signal analyzed by a simple computerized method that calculated five parameters: delay, maximal amplitude, duration, area, and maximal derivative with respect to time. The last four parameters increased with increasing intensity of stimulation and remained stable when the same stimulation was given repeatedly. Sensitivity to morphine (2.5 and 5 mg/kg s.c.) was tested. Morphine raised the threshold and lowered vocalization parameters, and it was antagonized by naloxone, thus validating the method. The sensitivity of the test and its capacity to separate sensory and affective components of pain are discussed.

The role of neuropeptides in regulating airway function. Postsecretory metabolism of peptides

Peptide hormones and neurotransmitters play an essential role in regulation of cellular metabolism. Once released from an endocrine cell or nerve ending, peptides encounter membrane-bound and soluble peptidases. The peptidases inactivate peptides or form fragments with novel biologic activity. Therefore, peptidases must play a major role in homeostatic control, but this aspect of regulation has been a neglected area. This review examines the postsecretory metabolism of biologically active peptides in the brain and alimentary tract, 2 organs in which peptide regulation is of crucial importance.

Naloxone-reversible antidiarrheal effects of enkephalinase inhibitors

Thiorphan and acetorphan, two potent inhibitors of enkephalinase (EC 3.4.24.11 membrane-metalloendopeptidase) significantly reduced the castor oil-induced diarrhea in rats when administered intravenously (or orally, for acetorphan) but not when administered intracerebroventricularly. These effects were more marked during the 90 min period following the castor oil challenge but were still significant up to 4-8 h after the latter. Acetorphan was about 6 times more potent than thiorphan. The antidiarrheal activity of both compounds was completely prevented in rats receiving naloxone subcutaneously but not intracerebroventricularly (in the case of thiorphan). In contrast to loperamide, a peripherally acting opiate receptor agonist, the enkephalinase inhibitors did not significantly reduce gastrointestinal transit as measured in the charcoal meal test. The antidiarrheal activity of enkephalinase inhibitors therefore seems attributable to protection of endogenous opioids, presumably outside the brain, and to reduction of intestinal secretion rather than transit.

Analgesic effects of metapramine and evidence against the involvement of endogenous enkephalins in the analgesia induced by tricyclic antidepressants

In several pain models, tricyclic antidepressants (TCAs) have been shown to reduce nociception. In the present study, we evaluated the antinociceptive effect of metapramine (META) in 4 nociception tests: (1) the hot plate test; (2) the phenylbenzoquinone-induced writhing; (3) the tail flick test; and (4) the test of electrical stimulation of the tail. We further analysed, using META and clomipramine (CLOM), the eventual role of endogenous opioids in analgesia induced by TCAs. The analgesic effects of META and CLOM in the hot plate test and in the test of electrical stimulation of the tail were reversed by naloxone. On the other hand, we failed to demonstrate a potentiation of META- or CLOM-induced analgesia by acetorphan, an inhibitor of 'enkephalinase.' We also failed to show a potentiation of Met5-enkephalin intracerebroventricularly injected by the two TCAs. Moreover, the administration of the enzymatic inhibitor or of Met5-enkephalin led to a slight decrease of the analgesic effect of the TCAs. These results (1) indicate that in our 4 pain tests, META clearly reduces nociception and (2) provide evidence that the involvement of endogenous enkephalins in the analgesia induced by TCAs is improbable.

Characterisation of two probes for the localisation of enkephalinase in rat brain: [3H]thiorphan and a 125I-labeled monoclonal antibody

We studied the binding of two radioactive probes, i.e. [3H]thiorphan and a 125I-labeled monoclonal antibody raised against the rabbit kidney enzyme, to enkephalinase (EC 3.4.24.11, membrane metalloendopeptidase) from rat cerebral membranes. [3H]Thiorphan binding at equilibrium to striatal membranes was monophasic with a KD (0.7 nM) and a pharmacology consistent with a selective labeling of the enzyme. The ratio of Vmax/Bmax was in the same range as the Kcat of the enzyme purified from peripheral tissues. The monoclonal antibody immunoprecipitated to a similar extent the solubilised enkephalinase activity and [3H]thiorphan binding sites from striatum. The regional distributions of binding sites for the two probes established either on isolated membranes or autoradiographic sections were highly heterogeneous and similar to that of enkephalinase activity. Hence the two probes appear to label membrane-bound enkephalinase in rat brain but, from a technical point of a view, the 125I-monoclonal antibody is a more sensitive and flexible tool.