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

Receptor expression and signaling properties in the brain, and structural ligand motifs that contribute to delta opioid receptor agonist-induced seizures

The δ opioid receptor (δOR) is a therapeutic target for the treatment of various neurological disorders, such as migraines, chronic pain, alcohol use, and mood disorders. Relative to μ opioid receptor agonists, δOR agonists show lower abuse liability and may be potentially safer analgesic alternatives. However, currently no δOR agonists are approved for clinical use. A small number of δOR agonists reached Phase II trials, but ultimately failed to progress due to lack of efficacy. One side effect of δOR agonism that remains poorly understood is the ability of δOR agonists to produce seizures. The lack of a clear mechanism of action is partly driven by the fact that δOR agonists range in their propensity to induce seizure behavior, with multiple δOR agonists reportedly not causing seizures. There is a significant gap in our current understanding of why certain δOR agonists are more likely to induce seizures, and what signal-transduction pathway and/or brain area is engaged to produce these seizures. In this review we provide a comprehensive overview of the current state of knowledge of δOR agonist-mediated seizures. The review was structured to highlight which agonists produce seizures, which brain regions have been implicated and which signaling mediators have been examined in this behavior. Our hope is that this review will spur future studies that are carefully designed and aimed to solve the question why certain δOR agonists are seizurogenic. Obtaining such insight may expedite the development of novel δOR clinical candidates without the risk of inducing seizures. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

The inhibition of enkephalin catabolism by dual enkephalinase inhibitor: A novel possible therapeutic approach for opioid use disorders

Despite the increasing impact of opioid use disorders on society, there is a disturbing lack of effective medications for their clinical management. An interesting innovative strategy to treat these disorders consists in the protection of endogenous opioid peptides to activate opioid receptors, avoiding the classical opioid-like side effects. Dual enkephalinase inhibitors (DENKIs) physiologically activate the endogenous opioid system by inhibiting the enzymes responsible for the breakdown of enkephalins, protecting endogenous enkephalins and increasing their half-lives and physiological actions. The activation of opioid receptors by the increased enkephalin levels, and their well-demonstrated safety, suggests that DENKIs could represent a novel analgesic therapy and a possible effective treatment for acute opioid withdrawal, as well as a promising alternative to opioid substitution therapy minimizing side effects. This new pharmacological class of compounds could bring effective and safe medications avoiding the major limitations of exogenous opioids, representing a novel approach to overcome the problem of opioid use disorders. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Deep brain stimulation of the periaqueductal gray releases endogenous opioids in humans

Deep brain stimulation (DBS) of the periaqueductal gray (PAG) is used in the treatment of severe refractory neuropathic pain. We tested the hypothesis that DBS releases endogenous opioids to exert its analgesic effect using [¹¹C]diprenorphine (DPN) positron emission tomography (PET). Patients with de-afferentation pain (phantom limb pain or Anaesthesia Dolorosa (n=5)) who obtained long-lasting analgesic benefit from DBS were recruited. [¹¹C]DPN and [¹⁵O]water PET scanning was performed in consecutive sessions; first without, and then with PAG stimulation. The regional cerebral tracer distribution and kinetics were quantified for the whole brain and brainstem. Analysis was performed on a voxel-wise basis using statistical parametric mapping (SPM) and also within brainstem regions of interest and correlated to the DBS-induced improvement in pain score and mood. Brain-wide analysis identified a single cluster of reduced [¹¹C]DPN binding (15.5% reduction) in the caudal, dorsal PAG following DBS from effective electrodes located in rostral dorsal/lateral PAG. There was no evidence for an accompanying focal change in blood flow within the PAG. No correlation was found between the change in PAG [¹¹C]DPN binding and the analgesic effect or the effect on mood (POMS_SV) of DBS. The analgesic effect of DBS in these subjects was not altered by systemic administration of the opioid antagonist naloxone (400 ug). These findings indicate that DBS of the PAG does indeed release endogenous opioid peptides focally within the midbrain of these neuropathic pain patients but we are unable to further resolve the question of whether this release is responsible for the observed analgesic benefit.

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Sequential opioid-PET imaging study of deafferentation pain patients.

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All obtained analgesic benefit from deep brain stimulators (DBS) in periaqueductal grey (PAG).

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PET imaging with diprenorphine showed DBS reduced binding of the radioligand in the PAG.

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Change in binding consistent with DBS-evoked release of endogenous opioids.

Imaging endogenous opioid peptide release with [11C]carfentanil and [3H]diprenorphine: influence of agonist-induced internalization

Understanding the cellular processes underpinning the changes in binding observed during positron emission tomography neurotransmitter release studies may aid translation of these methodologies to other neurotransmitter systems. We compared the sensitivities of opioid receptor radioligands, carfentanil, and diprenorphine, to amphetamine-induced endogenous opioid peptide (EOP) release and methadone administration in the rat. We also investigated whether agonist-induced internalization was involved in reductions in observed binding using subcellular fractionation and confocal microscopy. After radioligand administration, significant reductions in [(11)C]carfentanil, but not [(3)H]diprenorphine, uptake were observed after methadone and amphetamine pretreatment. Subcellular fractionation and in vitro radioligand binding studies showed that amphetamine pretreatment only decreased total [(11)C]carfentanil binding. In vitro saturation binding studies conducted in buffers representative of the internalization pathway suggested that μ-receptors are significantly less able to bind the radioligands in endosomal compared with extracellular compartments. Finally, a significant increase in μ-receptor-early endosome co-localization in the hypothalamus was observed after amphetamine and methadone treatment using double-labeling confocal microscopy, with no changes in δ- or κ-receptor co-localization. These data indicate carfentanil may be superior to diprenorphine when imaging EOP release in vivo, and that alterations in the ability to bind internalized receptors may be a predictor of ligand sensitivity to endogenous neurotransmitter release.

New orally active dual enkephalinase inhibitors (DENKIs) for central and peripheral pain treatment

Protecting enkephalins, endogenous opioid peptides released in response to nociceptive stimuli, is an innovative approach for acute and neuropathic pain alleviation. This is achieved by inhibition of their enzymatic degradation by two membrane-bound Zn-metallopeptidases, neprilysin (NEP, EC 3.4.24.11) and aminopeptidase N (APN, EC 3.4.11.2). Selective and efficient inhibitors of both enzymes, designated enkephalinases, have been designed that markedly increase extracellular concentrations and half-lives of enkephalins, inducing potent antinociceptive effects. Several chemical families of Dual ENKephalinase Inhibitors (DENKIs) have previously been developed but devoid of oral activity. We report here the design and synthesis of new pro-drugs, derived from co-drugs combining a NEP and an APN inhibitor through a disulfide bond with side chains improving oral bioavailability. Their pharmacological properties were assessed in various animal models of pain targeting central and/or peripheral opioid systems. Considering its efficacy in acute and neuropathic pain, one of these new DENKIs, 19-IIIa, was selected for clinical development.

Effects and underlying mechanisms of human opiorphin on colonic motility and nociception in mice

In the present study, we investigated the effects of human opiorphin on colonic motility and nociception in mice. In in vitro bioassay, opiorphin (10(-6) to 10(-4)M) caused colonic contraction in a concentration-dependent manner, which was completely blocked by naloxone and partially attenuated by beta-funaltrexamine and naltrindole. Moreover, opiorphin (10(-4)M) significantly enhanced the contractile response induced by Met-enkephalin. The data suggested that the effect of opiorphin on colonic contraction may be due to the protection of enkephalins. In in vivo bioassay, intracerebroventricular (i.c.v.) administration of opiorphin (1.25-10 microg/kg) dose- and time-dependently induced potent analgesic effect (ED(50)=3.22 microg/kg). This effect was fully blocked by naloxone and significantly inhibited by co-injection (i.c.v.) with beta-funaltrexamine or naltrindole, but not by nor-binaltorphimine, indicating the involvement of both mu- and delta-opioid receptors in the analgesic response evoked by opiorphin. In addition, i.c.v. administration of 5 microg/kg opiorphin produced the comparative effect as 10 microg/kg morphine on the analgesia, suggesting that opiorphin displayed more potent analgesic effect than that induced by morphine.

RB101-mediated protection of endogenous opioids: potential therapeutic utility?

The endogenous opioids met- and leu-enkephalin are inactivated by peptidases preventing the activation of opioid receptors. Inhibition of enkephalin-degrading enzymes increases endogenous enkephalin levels and stimulates robust behavioral effects. RB101, an inhibitor of enkephalin-degrading enzymes, produces antinociceptive, antidepressant, and anxiolytic effects in rodents, without typical opioid-related negative side effects. Although enkephalins are not selective endogenous ligands, RB101 induces these behaviors through receptor-selective activity. The antinociceptive effects of RB101 are produced through either the mu-opioid receptor alone or through activation of both mu- and delta-opioid receptors; the antidepressant-like and anxiolytic effects of RB101 are mediated only through the delta-opioid receptor. Although little is known about the effects of RB101 on other physiologically and behaviorally relevant peptides, these findings suggest that RB101 and other inhibitors of enkephalin-degrading enzymes may have potential as novel therapeutic compounds for the treatment of pain, depression, and anxiety.

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.

Stimulation of δ-Opioid Receptors Reduces the In Vivo Binding of the Cholecystokinin (CCK)-B-Selective Agonist [3H]pBC 264: Evidence for a Physiological Regulation of CCKergic Systems by Endogenous Enkephalins

Cholecystokinin (CCK) and enkephalins appear to be colocalized in several brain structures, and a physiological interaction between these peptides has been suggested by a large number of pharmacological studies. In this work we have shown, by in vivo binding experiments, that the endogenous enkephalins, protected from degrading enzymes by mixed inhibitors such as kelatorphan and N-[(R,S)-2-benzyl-3-[(S)-2-amino-4-methylthiobutyldithio]-1-oxo pro pyl]- L-phenylalanine benzyl ester (RB 101), a systemically active prodrug, modulate CCK release in mouse brain, leading to an overall increase in the extracellular levels of CCK. This was quantified by measuring the effects of both inhibitors on the in vivo binding of [3H]propionyl-Tyr(SO3H)-gNle-mGly-Trp-(N-Me)Nle-Asp-Phe-NH2 ([3H]pBC 264), a selective and highly potent CCK-B agonist. Thus, intracerebroventricular injection of kelatorphan produced a dose-dependent inhibition of the in vivo binding of [3H]pBC 264 with a maximal effect (40%) at 50 nmol. A similar response was observed after intravenous injection of RB 101 (40 mg/kg). The specific binding of [3H]pBC 264 was also inhibited (25%) by intravenous injection of the selective delta-opioid agonist H-Tyr-D-Cys(StBu)-Gly-Phe-Leu-Thr(OtBu)-OH (BUBUC; 2 mg/kg) but not by the mu-agonist H-Tyr-D-Ala-Gly-(N-Me)Phe-Gly-ol (5 mg/kg), suggesting a preferential involvement of delta-opioid receptors in the modulation of CCK release. This was confirmed by using the selective delta-opioid antagonist naltrindole, which prevented the inhibitory effects of BUBUC and of enkephalin-degrading enzyme inhibitors on [3H]pBC 264 binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Central poststroke pain and reduced opioid receptor binding within pain processing circuitries: a [11C]diprenorphine PET study

Based on concepts that endogenous opioids participate in neural transmission of pain, the present study in central poststroke pain (CPSP) patients investigated changes in opioid receptor (OR) binding in neural structures centrally involved in the processing of pain. Five patients with central pain after lesions in the brain stem, thalamus or parietal cortex and twelve healthy volunteers underwent a [11C]diprenorphine positron emission tomography study. Binding potentials were calculated using a reference region model in all subjects. Statistical parametric mapping was applied for t-statistical analysis on voxel-basis. Binding potential values for each individual were extracted from a volume of interest at each identified significant peak. Spectral analysis was applied for quantification of global values. Significant regional reduced 11C-diprenorphine binding (corrected for multiple tests) was detected in contralateral thalamus, parietal, secondary somatosensory, insular and lateral prefrontal cortices, and along the midline in anterior cingulate, posterior cingulate and midbrain gray matter. Individual extracted binding values disclosed a reduced binding in these regions in all patients independent from the particular lesion site. The poststroke pain syndrome is associated with a characteristic pattern of reduced OR binding within the neural circuitry processing pain. It is suggested that an imbalance of excitatory-inhibitory mechanisms in certain brain structures, as evidenced in decreased [11C]diprenorphine binding, is one of the causes or the consequences of poststroke pain.

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.

RB101(S), a dual inhibitor of enkephalinases does not induce antinociceptive tolerance, or cross-tolerance with morphine: a c-Fos study at the spinal level

In behavioural tests, RB101 (N-[(S)-2-benzyl-3[(S)(2-amino-4-methyl-thio)butyldithio]-1-oxopropyl]-L-phenylalanine benzyl ester), a mixed inhibitor of enkephalin-degrading enzymes, induces antinociceptive effects without producing tolerance, or cross-tolerance with morphine. In the present experiments, the acute or chronic effects of enantiomer RB101(S) were examined on the response of spinal cord neurons to nociceptive inflammatory stimulation (intraplantar injection of carrageenin) using c-Fos studies in awake rats. The number of c-Fos immunoreactive nuclei was evaluated in the lumbar spinal cord 90 min after carrageenin. c-Fos-immunoreactive nuclei were preferentially located in the superficial (I-II) and deep (V-VI) laminae of segments L4-L5 (areas containing numerous neurones responding exclusively, or not, to nociceptive stimuli). In the first experimental series, acute RB101(S) (30 mg/kg, i.v.), morphine (3 mg/kg, i.v.), or respective vehicles were injected in rats chronically treated with RB101(S) (160 mg/kg/day for 4 days, s.c.). In chronically treated RB101(S) rats, both acute RB101(S) and morphine reduced the total number of carrageenin-evoked c-Fos-immunoreactive nuclei. In the second experimental series, acute RB101(S) (30 mg/kg, i.v.) reduced the total number of carrageenin-evoked c-Fos-immunoreactive nuclei with similar magnitude in naive and in morphine-tolerant (100 mg/kg/day for 3 days, s.c.) rats. These data provide further evidence that different cellular mechanisms occurred after chronic stimulation of opioid receptors by morphine or endogenous enkephalins.

Deletion of CCK2 receptor in mice results in an upregulation of the endogenous opioid system

Stimulation of the brain CCK2 receptor by the C-terminal octapeptide CCK8 of cholecystokinin (CCK) negatively modulates opioid responses. This suggests the existence of physiologically relevant interactions between endogenous CCK and opioid peptides, opening new perspectives particularly in the treatment of pain or drug addiction. CCK2 receptor-deficient mice were used to analyze the incidence of this gene invalidation on opioid system. Compared with wild-type mice, mutants exhibited the following: (1) a hypersensitivity to the locomotor activity induced by inhibitors of enkephalin catabolism or by morphine; (2) a spontaneous hyperalgesia to thermal nociceptive stimulus, which was reversed by previous administration of the NMDA antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate], and a large reduction in analgesic effects of endogenous or exogenous opioids; and (3) a more severe withdrawal syndrome after chronic morphine treatment. As expected, stimulation of mu, delta, and D2 receptors on brain tissue of wild-type animals induced a dose-dependent decrease in adenylate cyclase activity, whereas a striking mirror effect was observed in mutants. All of these results suggest that the absence, in knock-out mice, of the negative feedback control on the opioid system, normally performed out by CCK2 receptor stimulation, results in an upregulation of this system. These biochemical and pharmacological results demonstrate the critical role played by CCK2 receptors in opioid-dependent responses.

Delta9-tetrahydrocannabinol releases and facilitates the effects of endogenous enkephalins: reduction in morphine withdrawal syndrome without change in rewarding effect

Recent studies have suggested that cannabinoids might initiate the consumption of other highly addictive substances, such as opiates. In this work, we show that acute administration of Delta9-tetrahydrocannabinol in mice facilitates the antinociceptive and antidepressant-like responses elicited by the endogenous enkephalins protected from their degradation by RB 101, a complete inhibitor of enkephalin catabolism. This emphasizes the existence of a physiological interaction between endogenous opioid and cannabinoid systems. Accordingly, Delta9-tetrahydrocannabinol increased the release of Met-enkephalin-like material in the nucleus accumbens of awake and freely moving rats measured by microdialysis. In addition, this cannabinoid agonist displaced the in vivo [3H]diprenorphine binding to opioid receptors in total mouse brain. The repetitive pretreatment during 3 weeks of Delta9-tetrahydrocannabinol in mice treated chronically with morphine significantly reduces the naloxone-induced withdrawal syndrome. However, this repetitive administration of Delta9-tetrahydrocannabinol did not modify or even decrease the rewarding responses produced by morphine in the place preference paradigm. Taken together, these behavioural and biochemical results demonstrate the existence of a direct link between endogenous opioid and cannabinoid systems. However, chronic use of high doses of cannabinoids does not seem to potentiate the psychic dependence to opioids.

Enhancement of the effects of a complete inhibitor of enkephalin-catabolizing enzymes, RB 101, by a cholecystokinin-B receptor antagonist in diabetic rats

1. RB 101, a complete inhibitor of enkephalin-catabolizing enzymes, has been previously shown to produce antinociception in normal rats after systemic administration. Moreover, its coadministration with a cholecystokinin-B (CCK-B) receptor antagonist has been shown to strongly enhance its antinociceptive effect in normal rats. In this work, we determined whether RB 101 was able to reduce hyperalgesia and allodynia in diabetic rats, a model of neuropathic pain. The type of opioid receptors (mu or delta) involved was determined using naloxone and naltrindole, respectively, and the interactions between endogenous enkephalins and CCK on nociception control was investigated using coadministration of RB 101 and the CCK-B receptor antagonist CI-988. 2. RB 101 suppressed mechanical hyperalgesia (paw pressure-induced vocalization test), partially alleviated mechanical allodynia (von Frey hair test), and was ineffective in thermal allodynia (tail immersion test). The analgesic effect was completely cancelled by naloxone or naltrindole, suggesting that is requires the availability of mu- and/or delta-opioid receptors. 3. The combination of an inactive dose of CI-988 with the lowest effective dose of RB 101 resulted in a stronger increase in the vocalization threshold comparatively to RB 101 alone. 4. The present study demonstrates that the antinociception generated by RB 101 induced by elevation of extracellular levels of endogenous enkephalins, can be extended to neuropathic pain in diabetic rats and that blockade of CCK-B receptors potentiated antinociceptive effects elicited by RB 101.

Novel approaches to targeting neuropeptide systems

Generation and/or interruption of cell signalling by neuropeptides has been shown to be essentially, although not exclusively, mediated by one or several membrane-bound enzymes, giving rise to the concept of selective versus dual enzyme inhibitors. Because most of these enzymes are zinc metallopeptidases, novel inhibitors are now being designed based on the structure of these proteins. The physiological role of neuropeptides and their relationships with other peptide systems can be investigated by comparing results obtained using peptidase inhibitors and selective receptor antagonists with those obtained using mice in which genes encoding the various components of a peptide system have been deleted. The potential use of peptidase inhibitors, compared with exogenous agonists, as therapeutic agents (particularly as analgesics or antidepressants) and their use in the investigation of the neurobiology of drug abuse will be discussed with particular focus on enkephalins and cholecystokinin 8 (CCK-8).

Analgesic doses of the enkephalin degrading enzyme inhibitor RB 120 do not have discriminative stimulus properties

The systemically active mixed inhibitor of enkephalin metabolism, N-((S)-2-benzyl-3[(S) 2-amino-4-methylthio)butyldithio-]-1-oxopropyl)-L-alanine benzylester (RB 120), alone or in combination with 4-¿[2-[[3-(1H-indol-3-yl))-2-methyl-1-oxo-2-[[(tricyclo[3.3.1.1. ]dec-2-yloxy) carbonyl]amino¿propyl]amino]-1-phenylethyl]amino¿-4-oxo-[R-(R*, R*)]-butanoate N-methyl-D-glucamine (CI 988; CCK(1) receptor antagonist) was investigated for discriminative and morphine generalisation effects using an operant drug discrimination paradigm in rats. Animals dosed with RB 120 (10 mg/kg) failed to develop a discriminative response. Combined CI 988 (0.3 mg/kg) and RB 120 (10 mg/kg) also failed to elicit a discriminative response. Morphine-trained animals (3.0 mg/kg) did not generalise to RB 120 (10 and 20 mg/kg). Similarly, subsequent retraining of the same animals with 1.5 mg/kg of morphine did not elicit generalisation to RB 120 (10 or 20 mg/kg). Combined RB 120 (10 or 20 mg/kg) and CI 988 (0.3 or 3.0 mg/kg) treatment produced no notable drug lever selection in rats able to discriminate morphine (1.5 mg/kg) from saline. These results suggest that RB 120 may have low abuse potential at analgesic doses.

Reduction of stress-induced analgesia but not of exogenous opioid effects in mice lacking CB1 receptors

CB1 cannabinoid receptors are widely distributed in the central nervous system where they mediate most of the cannabinoid-induced responses. Here we have evaluated the interactions between the CB1 cannabinoid receptors and the endogenous opioid system by assaying a number of well-characterized opioid responses, e.g. antinociception and stress-mediated effects, on mutant mice in which the CB1 receptor gene was invalidated. The spontaneous responses to various nociceptive stimuli (thermal, mechanical and visceral pain) were not changed in mutant CB1 mice. Furthermore, the absence of the CB1 cannabinoid receptor did not modify the antinociceptive effects induced by different opioid agonists: morphine (preferential mu opioid agonist), D-Pen2-D-Pen5-enkephalin (DPDPE) and deltorphin II (selective delta opioid agonists), and U-50,488H (selective kappa opioid agonist) in the hot-plate and tail-immersion tests. In contrast, the stress-induced opioid mediated responses were modified in CB1 mutants. Indeed, these mutants did not exhibit antinociception following a forced swim in water at 34 degrees C and presented a decrease in the immobility induced by the previous exposure to electric footshock. However, the antinociception induced by a forced swim in water at 10 degrees C was preserved in CB1 mutants. These results indicate that CB1 receptors are not involved in the antinociceptive responses to exogenous opioids, but that a physiological interaction between the opioid and cannabinoid systems is necessary to allow the development of opioid-mediated responses to stress.

Evidence for tonic activation of NK-1 receptors during the second phase of the formalin test in the Rat

Behavioral, electrophysiological, and autoradiographic experiments were done to study the second nociceptive phase in the formalin test. In initial experiments, this second phase was attenuated by 1-10 mg of the NK-1 receptor antagonist CP-99,994, given subcutaneously 10, 30, or 60 min before formalin (n = 8-10) and by 20 microgram given intrathecally 20 min after formalin (n = 13); the inactive isomer CP-100,263 was ineffective. In electrophysiological experiments on single dorsal horn neurons in vivo, the excitatory responses to subcutaneous formalin injection (50 microliter, 2.5%) were attenuated by subsequent intravenously administration of the NK-1 receptor antagonist CP-96,345 (0.5 mg/kg; n = 8), given 35-40 min after formalin, but not by the inactive enantiomer CP-96,344 (0.5 mg/kg; n = 9). Finally, autoradiographic binding of exogenous [(125)I]BH-substance P in the lumbar cord was reduced at 5 and 25 min after formalin (50 microliter, 1 or 5%), with an intermediate level of reduction at 12 min. These data are interpreted as evidence that the second phase of nociceptive scores in the formalin test is attributable at least partially to tonic activation of NK-1 receptors at the spinal level, whether because of a temporally limited release of substance P, for example only during the first phase, but a slow removal or breakdown of substance P, or, more likely, because of tonic release from primary afferents throughout the second phase. Irrespective of the mechanism, it can be concluded that at least some of the persistent nociceptive effects associated with peripheral inflammation, or at least those provoked by subcutaneous injection of formalin, are mediated via continuous activation of NK-1 receptors at the level of the spinal dorsal horn; this may relate directly to mechanisms underlying prolonged nociceptive pains in humans.

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.

Measurement of changes in opioid receptor binding in vivo during trigeminal neuralgic pain using [11C] diprenorphine and positron emission tomography

The binding of [11C]diprenorphine to mu, kappa, and delta subsites in cortical and subcortical structures was measured by positron emission tomography in vivo in six patients before and after surgical relief of trigeminal neuralgia pain. The volume of distribution of [11C]diprenorphine binding was significantly increased after thermocoagulation of the relevant trigeminal division in the following areas: prefrontal, insular, perigenual, mid-cingulate and inferior parietal cortices, basal ganglia, and thalamus bilaterally. In addition to the pain relief associated with the surgical procedure, there also was an improvement in anxiety and depression scores. In the context of other studies, these changes in binding most likely resulted from the change in the pain state. The results suggest an increased occupancy by endogenous opioid peptides during trigeminal pain but cannot exclude coexistent down-regulation of binding sites.

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.

Antinociception produced by the peptidase inhibitor, RB 101, in rats with adrenal medullary transplant into the spinal cord

This study was undertaken to investigate the effects induced by the systemic administration of RB 101 [N-[(R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyl dithio]-1-oxoprpyl]-L-phenylalanine benzyl ester], a mixed inhibitor of the enkephalin catabolism able to cross the blood-brain barrier, in antinociception produced by adrenal medullary tissue transplanted in the rat spinal subarachnoid space. For this purpose, the antinociceptive responses induced by intravenous (i.v.) administration of RB 101 were evaluated in the tail-flick in rats transplanted 28 and 56 days before the test. Systemic administration of RB 101 induced antinociceptive effects in sham-operated rats, as previously reported. RB 101 also enhanced significantly the antinociception produced by the autotransplant 28 and 56 days after surgery. The antinociceptive responses of RB 101 in both sham-operated and autotransplanted rats were blocked by naloxone, but were not modified by the noradrenergic antagonist, phentolamine, suggesting a selective involvement of opioid mechanisms. The present results indicate that the inhibitors of enkephalin catabolism enhance the antinociception induced by adrenal medullary transplants.

Involvement of delta-opioid receptors in the effects induced by endogenous enkephalins on learned helplessness model

Pharmacological, neurochemical and behavioural findings support a possible role of endogenous opioids in clinical depression. There is evidence from animal studies that delta-opioid receptors are involved in several behavioural responses to opioids, including motivational activities. In the present study, the mixed enkephalin catabolism inhibitor, RB 101 (N(R,S)-2-benzyl-3[(S)-(2-amino-4-methylthiobutyldithio]-1-oxoprop yl)-L-phenylalanine benzyl ester) (1.25, 2.5 and 5 mg/kg), induced a dose-dependent antidepressant-like effect in a learned helplessness model. Thus, RB 101 reversed escape deficits in rats previously subjected to inescapable shocks, suggesting the involvement of endogenous enkephalins in depression. Similar effects were observed after administration of the selective delta-opioid receptor agonist, BUBU (Tyr-D.Ser-(O-tert-butyl)-Gly-Phe-Leu-Thr(O-Tet-butyl-OH) (1 and 2 mg/kg). Moreover, RB 101 effects were antagonized by administration of naltrindole (NTI) (0.1 mg/kg), which points to a preferential involvement of delta-opioid receptors in this enkephalin-controlled behaviour. As RB 101 has been reported to be almost devoid of opiate-related side-effects, it could represent a promising alternative in the treatment of depressive patients who are unresponsive to, or intolerant of, classical antidepressants.

Inhibition of opioid-degrading enzymes potentiates delta9-tetrahydrocannabinol-induced antinociception in mice

Delta9-tetrahydrocannabinol (delta9-THC) elicits antinociception in rodents through the central CB1 cannabinoid receptor subtype. In addition. Delta9-THC stimulates the release of dynorphin-related peptides leading to kappa-opioid spinal antinociception. In this work we describe the effect of a mixture of thiorphan (a neutral endopeptidase EC3.4.24.11 inhibitor) and bestatin (an aminopeptidase inhibitor), administered i.c.v., on the antinociceptive effect of peripherally administered delta9-THC in mice. As in the case of morphine or DAMGO ([D-Ala2.N-Me-Phe4,Gly-ol]enkephalin), a mu-selective opioid receptor agonist, the mixture of enkephalin-degrading enzyme inhibitors also enhanced the antinociceptive effect of delta9-THC. This effect was blocked by the CB1 cannabinoid receptor antagonist, SR-141,716-A, as well as by naloxone. The kappa-opioid receptor antagonist nor-binaltorphimine, administered i.t., also antagonized the effect of this combination. Similar results were obtained with the mu-opioid receptor antagonist beta-funaltrexamine after i.c.v. administration. These results demonstrate the involvement of both mu-opioid supraspinal and kappa-opioid spinal receptors in the interaction of both opioid and cannabinoid systems regulating nociception in mice.

Pain-suppressive effects on various nociceptive stimuli (thermal, chemical, electrical and inflammatory) of the first orally active enkephalin-metabolizing enzyme inhibitor RB 120

RB 101 (N-((R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyldithio]-1-oxopr opyl)-L-phenylalanine benzyl ester) is a full inhibitor of the enkephalin-catabolizing enzymes, which induces strong naloxone-reversible antinociceptive responses after i.v. or i.p. administration, but is only slightly active after oral administration. Chemical modifications were introduced on this compound, resulting in molecules such as RB 120 (N-((S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyldithio]-1-oxoprop yl)-L-alanine benzyl ester), which was selected for a complete study, after oral administration, in various assays commonly used to select analgesics: mouse hot plate test, rat tail-flick test, electrical stimulation of the tail in rats, paw pressure test on inflamed paws in rats, acetic acid-induced writhing test and the formalin test in mice. RB 120 induced potent dose-dependent antinociceptive responses in all these tests after oral administration. The differences in antinociceptive effects induced by RB 120 in the various assays is probably 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. The goal of discovering orally active analgesics endowed with a potency similar to that of morphine but devoid of its major side-effects, seems now to have been reached with mixed neutral endopeptidase/aminopeptidase N (NEP/APN) inhibitors, although these compounds have yet to be evaluated in clinical trials.

Similar involvement of several brain areas in the antinociception of endogenous and exogenous opioids

The complete inhibitor of the enkephalin degrading enzymes, RB 101, N-{(R,S)-2-benzyl-3[(S)-(2-amino-4-methylthio)butyldithio]-1- oxopropyl}-L-phenylalanine benzyl ester, which crosses the blood-brain barrier, induced antinociceptive effects similar to those of exogenous opiates. The almost complete absence of tolerance and dependence after chronic administration of RB 101 is therefore due to limited stimulation of opioid receptors by 'protected' endogenous enkephalins. In order to clarify the mechanisms involved in these response, we have investigated the participation of several brain structures in the antinociceptive effects induced by systemic administration of morphine or RB 101. Rats were implanted with bilateral cannulae into the ventro-basal thalamus, central amygdala and periaqueductal gray matter, or with a cannula into the raphe magnus nucleus. The antinociceptive responses induced by systemic morphine or RB 101 were measured by using the tail-electrical stimulation test, where three different thresholds were determined: motor response, vocalization and vocalization post-discharge. The ability of the opioid receptor antagonist methylnaloxonium to block these antinociceptive responses was evaluated after local injection into the different brain structures. The blockade of morphine- and RB 101-induced antinociception was similar, and was stronger when methylnaloxonium was injected into the periaqueductal gray matter and raphe magnus nucleus than when it was injected into the ventro-basal thalamus and amygdala. These results suggest that brain structures related to the control of pain seem to be the same for the antinociception induced by exogenous opiates and endogenous opioids.

Similar decrease in spontaneous morphine abstinence by methadone and RB 101, an inhibitor of enkephalin catabolism

1. The dual inhibitor of enkephalin degrading enzymes, RB 101, is able to block endogenous enkephalin metabolism completely, leading to potent antinociceptive responses potentiated by blockade of CCKB receptors. In this study we have investigated the effects induced by RB 101 given alone, or with the CCKB antagonist, PD-134,308, on a model of spontaneous morphine withdrawal and substitutive maintenance in rats. 2. Animals were chronically treated with morphine for 7 days followed, 36 h after the interruption of drug administration, by a maintenance treatment for 5 days with methadone (2 mg kg-1, i.p.), clonidine (0.025 mg kg-1, i.p.), RB 101 (40 mg kg-1, i.p.), PD-134,308 (3 mg kg-1, i.p.) or a combination of RB 101 plus PD-134,308. Several behavioural observations were made during this period in order to evaluate the acute effects as well as the consequence of chronic maintenance induced on spontaneous withdrawal by the different treatments. 3. Methadone was the most effective compound in decreasing the spontaneous withdrawal syndrome after acute administration. Both, methadone and RB 101 had similar effectiveness in reducing opiate abstinence during the period of substitutive treatment. PD-134,308 did not show any effect when administered alone and did not modify the effect of RB 101. 4. Naloxone (1 mg kg-1, s.c.) failed to precipitate any sign of withdrawal when injected at the end of the chronic maintenance treatment suggesting that, under the present conditions, methadone and RB 101 did not induce significant physical opiate-dependence. 5. The mildness of the side effects induced by chronic RB 101, suggests that systemically active inhibitors of enkephalin catabolism could represent a promising treatment in the maintenance of opiate addicts.

An endogenous opiate mechanism seems to be involved in stress-induced anhedonia

This study assessed the effect of an uncontrollable stressor on the preference for a palatable solution (sucrose 1%), and on the preference for a context associated with a single administration of D-amphetamine (3 mg/kg i.p.) by means of the conditioning place preference test. We also evaluated the effect of prior naloxone (2 mg/kg, i.p.) administration on the influence of this stressful stimulus in both tests. Animals previously submitted to a 120-min--but not 60-min--restraint period showed a selective reduction in the preference for sucrose intake as compared to unstressed animals. Similarly, an identical restraint exposure elicited a diminished preference for the place previously paired with amphetamine. Both stress-induced effects were blocked by prior naloxone administration. These data demonstrate that a highly aversive experience decreased the reinforcing efficacy of sucrose and amphetamine, suggesting that uncontrollable stress may lead to an impaired capacity to experience pleasure, which could resemble the anhedonia observed in clinical depression. Furthermore, an endogenous opiate mechanism activated by stress seems to be involved in stress-induced anhedonia since naloxone normalized the reduction of the rewarding induced by both reinforcers.

The CCKB antagonist PD-134,308 facilitates rewarding effects of endogenous enkephalins but does not induce place preference in rats

The interaction between cholecystokinin and endogenous opioid systems on rewarding responses was examined. Motivational effects induced by peripheral administration of a complete inhibitor of enkephalin catabolism, RB 101 or the CCKB antagonist PD-134,308, and by both compounds in combination were evaluated in the conditioned place preference test in rats. RB 101 (5, 10, 20, 40 and 80 mg/kg, IP, and 20 mg/kg, IV) given alone produced a bell-shaped dose-effect function. A significant increase of the preference for the drug-associated compartment was only observed at doses of 10 and 20 mg/kg (IP). The effect observed with morphine was stronger, and all the doses used of this compound (1.25, 2.5 and 5 mg/kg, SC) were found to be active. These results suggest that the inhibitor of enkephalin catabolism has weak rewarding properties. Pretreatment with the CCKB antagonist PD-134,308 (0.1, 0.3, 1 and 3 mg/kg, IP) alone failed to produce a reliable aversion or preference on the paradigm studied. When PD-134,308 (0.3 mg/kg, IP) was coadministered with a subthreshold dose of morphine (0.6 mg/kg, SC) or RB 101 (5 mg/kg, IP), a conditioned place preference was observed, indicating that the CCKB antagonist facilitated the motivational responses induced by endogenous enkephalins as compared to morphine. This suggests that endogenous cholecystokinin, acting through CCKB receptors, modulates the rewarding effects of endogenous enkephalins.

Weak tolerance to the antinociceptive effect induced by the association of a peptidase inhibitor and a CCKB receptor antagonist

We have recently shown that CCKB receptor antagonists such as PD-134,308, 4-([2-[[3-(1H-indol-3-yl)-2-methyl-1-oxo-2-[[tricyclo[3.3.1.1]dec - 2-yloxy)carbonyl]amino]propyl]amino]-1-phenylethyl]amino)-4-oxo[R- (R*,R*)]-butanoate-N-methyl-D-glucamine, are able to strongly potentiate antinociception induced by endogenous enkephalins, protected from degrading enzymes by the mixed inhibitor RB 101, N-[(R,S)-2-benzyl-3[(S)-(2-amino-4- methylthio)butyldithio]-1-oxopropyl)-L-phenylalanine benzyl ester, at both spinal and supraspinal levels. In this study, the duration of this facilitatory response and the possible development of tolerance to this synergistic effect were investigated in the rat tail-flick test after acute and chronic treatment with PD-134,308 and RB 101. PD-134,308 facilitated and prolonged the antinociceptive responses induced by RB 101 (20 mg/kg, i.v.). The duration of the effect induced by PD-134,308 was also investigated by injecting this compound at different times before RB 101 administration. In the case of the tail-flick test, the improvement of RB 101 antinociceptive response was still significant 6 h after PD-134,308 (3 mg/kg, i.p.), whereas in the hot-plate test, this enhancement was only effective for 3 h after CCKB receptor antagonist administration. In the case of a repeated administration of RB 101, the potentiation induced by PD-134,308 on the antinociceptive effect produced by the first injection of RB 101 (20 mg/kg, i.v.), was found almost identical after a second administration of RB 101 performed 190 min later. Chronic administration of RB 101 (20 mg/kg, i.v.) plus PD-134,308 (3 mg/kg, i.p.) administered for 5 days both once or twice per day, did not induce the development of tolerance to antinociception at the peak effect time. However, a decrease in the duration of the antinociceptive response was observed. These results indicate that the potent and long-lasting antinociceptive response induced by the coadministration of the peptidase inhibitor and the CCKB receptor antagonist could have interesting perspectives in the clinical treatment of pain.

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.

A selective CCKB receptor antagonist potentiates, mu-, but not delta-opioid receptor-mediated antinociception in the formalin test

The endogenous peptides enkephalins and cholecystokinin appear to play an opposite role in the control of pain. In this work, the effect of the selective CCKB receptor antagonist PD-134,308 on antinociceptive effects induced by morphine or by a complete inhibitor of enkephalin-metabolizing enzymes, RB 101, was studied using the formalin test. In mice, s.c. injection of formalin into the dorsal surface of the hindpaw had a biphasic effect: an early nociceptive response followed by a late response. Morphine (2 mg/kg i.p.) caused naloxone (0.5 mg/kg s.c.) but not naltrindole (0.5 mg/kg s.c.) reversible antinociceptive responses in the early and late phases of the assay, suggesting a preferential involvement of mu-opioid receptors in these responses. In contrast, RB 101 (50 mg/kg i.p.) produced antinociceptive effects in the early and late phases which were both antagonized by the delta-selective opioid receptor antagonist naltrindole (0.5 mg/kg s.c.). The antinociceptive response elicited by morphine on the late but not the early phase of the formalin test was potentiated by the CCKB antagonist PD-134,308 (1 mg/kg i.p.). This compound was unable to facilitate the analgesic effects produced by RB 101 on both phases, in contrast to what was observed in the hot plate test with mice and the tail flick test with rats. Therefore, in the formalin test with mice, the facilitating effects of opiate-induced analgesia by CCKB receptor antagonists seem to be restricted to mu-opioid receptor-mediated responses.

Antinociceptive effect of systemic kelatorphan, in mononeuropathic rats, involves different opioid receptor types

The antinociceptive effect of i.v. kelatorphan (2.5, 5, 10 and 15 mg/kg), a mixed inhibitor of enkephalin degrading enzymes, was studied in a rat model of peripheral unilateral mononeuropathy (chronic constriction of the common sciatic nerve). Kelatorphan at 2.5 and 5 mg/kg had no significant effect on the vocalization threshold to paw pressure test, but higher doses (10 mg/kg) produced a significant antinociceptive effect which plateaued at 15 mg/kg, on both hindpaws. Kelatorphan (10 mg/kg) was co-injected with the specific mu-, delta- and kappa-opioid receptor antagonists naloxone (0.1 mg/kg), naltrindole (1 mg/kg) or nor-binaltorphimine (1 mg/kg). The effect of kelatorphan 10 mg/kg was completely prevented by naloxone, reduced by 75% by naltrindole (both hindpaws), and reduced by 50% by nor-binaltorphimine (contralateral paw only).

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.

Chronic variable stress or chronic morphine facilitates immobility in a forced swim test: reversal by naloxone

The behaviors displayed in a forced swim test were investigated in rats previously exposed to a chronic variable stress treatment or chronic administration of morphine. In addition, to further explore the participation of an endogenous opiate mechanism in these behavioral effects, naloxone was either administered during the chronic treatment (prior to each stress or morphine exposure) or immediately prior to the forced swim test. Animals were submitted daily to a different stressor for 1 week or injected with morphine (10 mg/kg, IP) for 6 days, whereas controls were unmanipulated except for the injection process. On the day following the last stressor, control and stressed animals were administered saline or naloxone (2 mg/kg, IP) 15 min prior to the forced swim test. Morphine treated animals were similarly tested on the third day following the last morphine injection. In a separate group of rats, naloxone (2 mg/kg, IP) was administered daily 10 min prior to each stressor of the chronic stress regime or each daily morphine injection. A significant increase in the time spent in immobility was observed in stressed animals as well as in rats chronically treated with morphine. In both groups, this potentiated immobility was attenuated by naloxone pretreatment prior to the forced swim test or when given before each daily stressor or morphine injection. In addition, the concurrent exposure to stress or morphine along with naloxone administration enhanced struggling in the first 5 min of the forced swim test.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of feeding by multiple opioid receptors in cingulate cortex; follow-up to an in vivo autoradiographic study

A previous in vivo autoradiographic study demonstrated reduced 3H-diprenorphine binding in anterior cingulate cortex of rats that were injected (i.v.) with the radiolabeled opiate during lateral hypothalamic stimulation-induced feeding (SIF). This suggests that an opioid peptide is released in cingulate cortex during feeding and excludes binding of the tracer. The aim of the present study was to determine whether opioid activity in cingulate cortex contributes to the expression of SIF. Agonists and antagonists for multiple opioid receptors were microinjected into cingulate cortex and effects on stimulation frequency threshold for SIF were determined. Although the universal opioid antagonist naloxone (20.0 micrograms) increased threshold, high doses of selective antagonists for mu, delta, and kappa receptors--D-Tic-CTAP, natrindole and norbinaltorphimine, respectively--had no effect. The unique efficacy of naloxone may be due to this lipophilic compound's rapid diffusion throughout an extensive volume of anterior cingulate tissue. While high doses of the kappa agonist U50,488 and the delta agonist DPDPE had no effect, the mu agonist, DAGO (1.0 microgram), decreased the SIF threshold. Moreover, the threshold-lowering effect of DAGO was blocked by pretreatment with the irreversible mu antagonist beta-FNA. These results suggest that mu opioid activity in cingulate cortex can facilitate SIF but that under basal conditions endogenous opioid activity in this brain region makes only a small positive contribution, if any, to the expression of SIF.

Paradoxical analgesia induced by low doses of naloxone is not potentiated by complete inhibition of enkephalin degradation

The involvement of a presynaptic autoinhibition of endogenous enkephalin release has been suggested as a possible explanation for the paradoxical analgesia induced by low doses of naloxone. This hypothesis was investigated by using the systemically active mixed inhibitor of enkephalin degrading enzymes, RB 101. As already described, in both hot plate (55 +/- 0.5 degrees C) and acetic acid (0.6%) abdominal constriction tests in mice, subcutaneous administration of naloxone produced biphasic effects, with antinociceptive responses at very low doses (microgram range) and hyperalgesia at higher dose (mg range). However at concentrations producing an extracellular increase in enkephalin levels and subsequent analgesia, the mixed inhibitor prodrug of the enkephalin-metabolizing enzymes RB 101 (20 or 100 mg/kg i.v. and 5 or 10 mg/kg i.v., in the hot plate test and in the abdominal constriction test, respectively) did not potentiate the paradoxical analgesia induced by naloxone. These results are inconsistent with a negative autoregulation of endogenous enkephalin release and could suggest the involvement of the diffuse noxious inhibitory controls (DNIC). Indeed, the finding that low doses of RB 101 (1 mg/kg i.v. in the hot plate test, and 250 micrograms/kg i.v. in the abdominal constriction test) were able to induce hyperalgesic responses could indicate that the DNIC are tonically activated by endogenous enkephalins accounting for the antinociceptive responses elicited by low doses of naloxone.

Antinociceptive effect of systemic PC 12, a prodrug mixed inhibitor of enkephalin-degrading enzymes, in normal and arthritic rats

The antinociceptive effects of various i.v. doses (2.5, 3.75, 5, 10, 15 and 20 mg/kg) of a prodrug mixed inhibitor of enkephalin degrading enzymes, PC 12, were tested in normal and Freund's adjuvant-induced arthritic rats, using vocalization thresholds to paw pressure as a nociceptive test. In normal animals, PC 12 produced a dose-dependent antinociceptive effect over the 2.5-15 mg/kg range, but the time-course of the response was shorter with the dose of 20 mg/kg. In arthritic rats, PC 12 had no significant effect at 2.5 mg/kg, but induced a highly significant dose-dependent antinociceptive action with 3.5 and 5 mg/kg, which decreased with the highest doses. The antinociceptive effect of PC 12 (10 mg/kg i.v.) was prevented by naloxone (0.5 mg/kg i.v.) in the two categories of rats, providing evidence for the involvement of opioid receptors. These results are discussed in relation with the increased level of endogenous opioid substances in the spinal dorsal horn of arthritic rats and the nociceptive test used.

Hypoalgesic action of bestatin analogues that inhibit central aminopeptidases, but not neutral endopeptidase

Two analogues of the aminopeptidase inhibitor bestatin, Z 4212 (N-[(2S, 3R)-3-Amino-2-hydroxy-4-(4-methylsulphonyl-phenyl)-1-oxobutyl]-1- aminocyclopentanecarboxylic) and Z 1796 ((2S)-N-[(2S,3R)-3-Amino-2-hydroxy-4-(4-methylsulphonyl-phenyl)-1- oxobutyl]-L-leucine) were found to behave as hypoalgesics when intracerebroventricularly (i.c.v.) administered to mice in the hot-plate test. At high doses, Z 4212 was also found to reduce the pain threshold after intraventricular (i.v.) administration. Hypoalgesia induced by bestatin analogues was prevented by prior treatment with the opiate receptor blocker naloxone. Thiorphan, a potent inhibitor of NEP, was found to enhance the hypoalgesic effect of low doses of either Z 4212 or Z 1796. These results indicate that both the major opioid-degrading peptidases, i.e. aminopeptidases and neutral endopeptidase (NEP), are individually implicated in the hypoalgesia induced by peptidase inhibitors. In vitro studies showed that these new bestatin analogues readily inhibit aminopeptidases in membranes from mouse c. striatum whereas more than 1000 times the concentration was required for NEP to be blocked. Ex vivo experiments showed that, at variance with bestatin, the hypoalgesic action of Z 4212 or Z 1796 appeared to implicate central aminopeptidases but not NEP, so partially sparing the metabolism of other NEP substrates that might produce additional alterations (substance P and ANP). On the basis of the antitumour and immunomodulatory actions of bestatin, these new analogues might be potentially useful as mixed antitumour and hypoalgesic agents in malignancy.

Modulation of opioid antinociception by CCK at the supraspinal level: evidence of regulatory mechanisms between CCK and enkephalin systems in the control of pain

1. Much evidence in the literature supports the idea that cholecystokinin (CCK) interacts with opioids in pain mechanisms. In this work, we have investigated the supraspinal interactions between enkephalins and CCK, using the hot plate test in mice. 2. Intracerebroventricular (i.c.v.) administration of BDNL (a mixed CCKA/CCKB agonist) induced dose-dependent antinociceptive responses on both paw lick and jump responses. In contrast, using the same test, the i.c.v. injection of BC 264 (a selective CCKB agonist) induced a hyperalgesic effect, which was restricted to paw licking and occurred only at a high dose of 2.5 nmol. 3. In addition, i.c.v. administration of BDNL potentiated the antinociceptive effects of the mixed inhibitor of enkephalin degrading enzymes, RB 101 and of the mu-agonist, DAMGO, while BC 264 reduced these effects. 4. Furthermore, at a dose where it interacts selectively with delta-opioid receptors, the opioid agonist BUBU reversed the hyperalgesic responses of BC 264 (2.5 nmol) but was unable to modify the effects induced by BDNL. 5. Taken together, these results suggest the existence of regulatory mechanisms between CCK and enkephalin systems in the control of pain. These regulatory loops could enhance the antinociceptive effects of morphine allowing the opiate doses used to be reduced and thus, possibly, the side-effects to be minimized.

Unlike morphine the endogenous enkephalins protected by RB101 are unable to establish a conditioned place preference in mice

The mixed inhibitor prodrug, RB101, was used to study the motivational properties of the endogenous opioid peptides, the enkephalins. In the conditioned place preference test, which is commonly used to investigate the reinforcing properties of drugs, mice alternately treated with morphine (3 mg/kg i.p.) on the initially non-preferred compartment and with saline on the preferred one, for four place pairings, spent more time in the drug-associated compartment. This shift in place preference after the conditioning procedure was not found after treatment with RB101 (80 mg/kg i.p.). Administration of naloxone (1 mg/kg s.c.) after the conditioning phase increased the preference for the drug-associated compartment of mice treated with 6 mg/kg (i.p.) of morphine. This illustrates the negative motivational properties of morphine withdrawal or the establishment of psychic dependence on the drug. In contrast, no modification of preference was observed after injection of naloxone in animals treated with a high dose of RB101 (160 mg/kg i.p.). The failure to establish conditioned place preference by inhibiting endogenous enkephalin metabolism, and the lack of development of psychic dependence after RB101 administration demonstrate for the first time the interest of mixed inhibitors of enkephalin-degrading enzymes as potent new non-addictive analgesics.

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)

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.