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

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.

Crosstalk between Opioid and Anti-Opioid Systems: An Overview and Its Possible Therapeutic Significance

Opioid peptides and receptors are broadly expressed throughout peripheral and central nervous systems and have been the subject of intense long-term investigations. Such studies indicate that some endogenous neuropeptides, called anti-opioids, participate in a homeostatic system that tends to reduce the effects of endogenous and exogenous opioids. Anti-opioid properties have been attributed to various peptides, including melanocyte inhibiting factor (MIF)-related peptides, cholecystokinin (CCK), nociceptin/orphanin FQ (N/OFQ), and neuropeptide FF (NPFF). These peptides counteract some of the acute effects of opioids, and therefore, they are involved in the development of opioid tolerance and addiction. In this work, the anti-opioid profile of endogenous peptides was described, mainly taking into account their inhibitory influence on opioid-induced effects. However, the anti-opioid peptides demonstrated complex properties and could show opioid-like as well as anti-opioid effects. The aim of this review is to detail the phenomenon of crosstalk taking place between opioid and anti-opioid systems at the in vivo pharmacological level and to propose a cellular and molecular basis for these interactions. A better knowledge of these mechanisms has potential therapeutic interest for the control of opioid functions, notably for alleviating pain and/or for the treatment of opioid abuse.

Bi- or multifunctional opioid peptide drugs

Strategies for the design of bi- or multifunctional drugs are reviewed. A distinction is made between bifunctional drugs interacting in a monovalent fashion with two targets and ligands containing two distinct pharmacophores binding in a bivalent mode to the two binding sites in a receptor heterodimer. Arguments are presented to indicate that some of the so-called "bivalent" ligands reported in the literature are unlikely to simultaneously interact with two binding sites. Aspects related to the development of bi- or multifunctional drugs are illustrated with examples from the field of opioid analgesics. The drug-like properties of the tetrapeptide Dmt(1)[DALDA] with triple action as a micro opioid agonist, norepinephrine uptake inhibitor and releaser of endogenous opioid peptides to produce potent spinal analgesia are reviewed. Rationales for the development of opioid peptides with mixed agonist/antagonist profiles as analgesics with reduced side effects are presented. Progress in the development of mixed micro opioid agonist/delta opioid antagonists with low propensity to produce tolerance and physical dependence is reviewed. Efforts to develop bifunctional peptides containing a micro opioid agonist and a cholecystokinin antagonist or an NK1 receptor antagonist as analgesics expected to produce less tolerance and dependence are also reviewed. A strategy to improve the drug-like properties of bifunctional opioid peptide analgesics is presented.

Transgene-mediated enkephalin expression attenuates signs of naloxone-precipitated morphine withdrawal in rats with neuropathic pain

Chronic morphine exposure induces physical dependence and tolerance. Previous studies have shown that there is a decrease in met-enkephalin levels in states of morphine physical dependence, and that increasing enkephalin during opiate physical withdrawal ameliorates the severity of the morphine withdrawal syndrome. In order to investigate the role of spinal opioid peptide in the phenomenon of naloxone-precipitated withdrawal we examined the effect of herpes simplex virus vector-mediated overexpression of proenkephalin in lumbar dorsal root ganglia in rats with neuropathic pain treated with morphine. The morphine physical dependence was induced by chronic administration of intraperitoneal (IP) morphine for 2 weeks. Rats with neuropathic pain inoculated subcutaneously with the vector-mediated overexpression of proenkephalin showed a significant reduction in jumps, 'wet-dog' shakes, diarrhea and ptosis precipitated by naloxone after 2 weeks of morphine treatment. The global withdrawal score was also reduced significantly by vector-mediated overexpression of proenkephalin. These studies demonstrate a role for opioid peptide in the spinal cord in mediating some of the withdrawal response.

Identification of bikunin as an endogenous inhibitor of dynorphin convertase in human cerebrospinal fluid

Dynorphin-converting enzymes constitute a group of peptidases capable of converting dynorphins to enkephalins. Through the action of these enzymes, the dynorphin-related peptides bind to delta-opioid instead of kappa-opioid receptors, leading to a change in the biological function of the neuropeptides. In this article, we describe the identification of the protein bikunin as an endogenous, competitive inhibitor of a dynorphin-converting enzyme in human cerebrospinal fluid. This protein is present together with its target enzyme in the same body fluids. The K(M) value of the convertase was found to be 9 microm, and the K(i) value of the inhibitor was 1.7 nm. The finding indicates that bikunin may play a significant role as a regulatory mechanism of neuropeptides, where one bioactive peptide is converted to a shorter sequence, which in turn, can affect the action of its longer form.

The role of withdrawal in heroin addiction: enhances reward or promotes avoidance?

The compulsive nature of heroin abuse has been attributed to the fact that drug self-administration enables an addict to escape from and avoid the severe withdrawal symptoms resulting from opiate dependence. However, studies of incentive learning under natural motivational states suggest an alternative hypothesis, that withdrawal from heroin functions as a motivational state that enhances the incentive value of the drug, thereby enabling it to function as a much more effective reward for self-administration. In support of this hypothesis, we show here that previous experience with heroin in withdrawal is necessary for subsequent heroin-seeking behavior to be enhanced when dependent rats once again experience withdrawal.

Different role of cholecystokinin (CCK)-A and CCK-B receptors in relapse to morphine dependence in rats

The possible effect of different cholecystokinin (CCK) receptor antagonists (MK-329 and L-365260) on the maintenance and reactivation of morphine conditioned place preference (CPP) were investigated in rats, respectively. The results show that the maintenance of morphine CPP could be induced by injection of morphine (10 mg/kg, s.c.) once for 3 days and this effects were significantly attenuated by pretreatment with 1 but not by 0.1 mg/kg L-365260. Furthermore, following a 28-day extinction, the morphine CPP disappeared and then reactivated again by a single injection of morphine (10 mg/kg). Pretreatment with L-365260 (1 and 0.1 mg/kg) significantly blocked this reactivation of morphine CPP. In contrast, pretreatment of MK-329 (1 and 0.1 mg/kg) failed to do so. The present study demonstrated that CCK-B receptor but not CCK-A receptor is involved in the maintenance and reactivation of morphine CPP. These findings suggest that CCK-B receptor antagonists might be of some value in the treatment and prevention of relapse to drug dependence long after detoxification.

Circuitry underlying antiopioid actions of cholecystokinin within the rostral ventromedial medulla

It is now well established that the analgesic actions of opioids can be modified by "anti-analgesic" or "antiopioid" peptides, among them cholecystokinin (CCK). Although the focus of much recent work concerned with CCK-opioid interactions has been at the level of the spinal cord, CCK also acts within the brain to modify opioid analgesia. The aim of the present study was to characterize the actions of CCK in a brain region in which the circuitry mediating the analgesic actions of opioids is relatively well understood, the rostral ventromedial medulla (RVM). Single-cell recording was combined with local infusion of CCK in the RVM and systemic administration of morphine in lightly anesthetized rats. The tail-flick reflex was used as a behavioral index of nociceptive responsiveness. Two classes of RVM neurons with distinct responses to opioids have been identified. OFF cells are activated, indirectly, by morphine and mu-opioid agonists, and there is strong evidence that this activation is crucial to opioid antinociception. ON cells, thought to facilitate nociception, are directly inhibited by opioids. Cells of a third class, NEUTRAL cells, do not respond to opioids, and whether they have any role in nociceptive modulation is unknown. CCK microinjected into the RVM by itself had no effect on tail flick latency or the firing of any cell class but significantly attenuated opioid activation of OFF cells and inhibition of the tail flick. Opioid suppression of ON-cell firing was not significantly altered by CCK. Thus CCK acting within the RVM attenuates the analgesic effect of systemically administered morphine by preventing activation of the putative pain inhibiting output neurons of the RVM, the OFF cells. CCK thus differs from another antiopioid peptide, orphanin FQ/nociceptin, which interferes with opioid analgesia by potently suppressing all OFF-cell firing.

Cocaine, but not morphine, induces conditioned place preference and sensitization to locomotor responses in CB1 knockout mice

The involvement of cannabinoid CB1 receptors in morphine and cocaine motivational effects was investigated using CB1 knockout mice. For this purpose, we evaluated the rewarding effects in the place conditioning paradigm and the sensitization to the locomotor responses induced by these drugs. The hyperlocomotion induced by acute morphine administration (15 mg/kg, s.c.) was preserved, but the sensitization to this locomotor response induced by chronic morphine treatment was abolished in CB1 mutant mice. Morphine (5 mg/kg, s.c.) induced conditioned place preference in wild-type mice but failed to produce any response in knockout mice, indicating the inability of morphine to induce rewarding effects in the absence of CB1 cannabinoid receptors. When the aversive effects of morphine withdrawal were investigated using the place aversion paradigm, no differences between genotypes were observed. Acute cocaine (10 mg/kg, i.p.) induced hyperlocomotor responses in wild-type and knockout mice and a chronic cocaine treatment produced a similar sensitization to this response in both genotypes. In the conditioning place preference paradigm, cocaine (20 mg/kg, i.p.) produced rewarding responses in both wild-type and knockout mice. These results demonstrate that CB1 receptors are essential for adaptive responses produced by chronic morphine but not by chronic cocaine treatment.

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.

Potentiation of opioid-induced conditioned place preference by the selective serotonin reuptake inhibitor fluoxetine

The ability of the selective serotonin reuptake inhibitor, fluoxetine, to modify the effects of morphine, N-((S)-2-benzyl-3[(S) 2-amino-4-methylthio)butyldithio-]-1-oxopropyl)-L-alanine benzylester (RB 120; mixed inhibitor of enkephalin metabolism), and 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; cholecystokinin receptor subtype [CCK(2)] antagonist), was assessed using conditioned place preference. RB 120 and morphine both induced significant, dose-dependent conditioned place preference, whilst CI 988 failed to elicit conditioned place preference. A subthreshold dose of fluoxetine (2.5 mg/kg) potentiated the morphine submaximal response. Notably, the combination of a subthreshold dose of fluoxetine (2.5 mg/kg) with RB 120 (5 mg/kg) or CI 988 (3 mg/kg) was devoid of any significant conditioned place preference properties. Fluoxetine may act via enhanced serotonergic activity to modulate enkephalinergic tone. Agents that increase enkephalinergic tone more directly such as RB 120 and CI 988, at submaximal doses, did not induce conditioned place preference when co-administered with fluoxetine. These data suggest that fluoxetine, in combination with CI 988 or RB 120, might prove to be a beneficial treatment strategy for opioid drug addiction, though further studies are necessary.

CCK-B receptor: chemistry, molecular biology, biochemistry and pharmacology

Cholecystokinin (CCK) is a peptide originally discovered in the gastrointestinal tract but also found in high density in the mammalian brain. The C-terminal sulphated octapeptide fragment of cholecystokinin (CCK8) constitutes one of the major neuropeptides in the brain; CCK8 has been shown to be involved in numerous physiological functions such as feeding behavior, central respiratory control and cardiovascular tonus, vigilance states, memory processes, nociception, emotional and motivational responses. CCK8 interacts with nanomolar affinities with two different receptors designated CCK-A and CCK-B. The functional role of CCK and its binding sites in the brain and periphery has been investigated thanks to the development of potent and selective CCK receptor antagonists and agonists. In this review, the strategies followed to design these probes, and their use to study the anatomy of CCK pathways, the neurochemical and pharmacological properties of this peptide and the clinical perspectives offered by manipulation of the CCK system will be reported. The physiological and pathological implication of CCK-B receptor will be confirmed in CCK-B receptor deficient mice obtained by gene targeting (Nagata el al., 1996. Proc. Natl. Acad. Sci. USA 93, 11825-11830). Moreover, CCK receptor gene structure, deletion and mutagenesis experiments, and signal transduction mechanisms will be discussed.

Interactions between cholecystokinin and opioids in the isolated guinea-pig ileum

1. Although cholecystokinin octapeptide sulphate (CCK-8) activates the opioid system of isolated guinea-pig ileum (GPI) whether it activates the mu- or kappa-system, or both, remains unclear. Neither is it known whether CCK-8 influences the withdrawal responses in GPI preparations briefly exposed to opioid agonists. This study was designed to clarify whether CCK-8 activates mu- or kappa-opioid systems or both; and to investigate its effect on the withdrawal contractures in GPI exposed to mu- or kappa-agonists and on the development of tolerance to the withdrawal response. 2. In GPI exposed to CCK-8, the selective kappa-antagonist nor-binaltorphimine elicited contractile responses that were concentration-related to CCK-8 whereas the selective mu-antagonist cyprodime did not. 3. In GPI preparations briefly exposed to the selective mu-agonist, dermorphin, or the selective kappa-agonist, U-50, 488H, and then challenged with naloxone, CCK-8 strongly enhanced the withdrawal contractures. 4. During repeated opioid agonist/CCK-8/opioid antagonist tests tolerance to opioid-induced withdrawal responses did not develop. 5. These results show that CCK-8 preferentially activates the GPI kappa-opioid system and antagonizes the mechanism(s) that control the expression of acute dependence in the GPI.

Cholecystokinin modulates the aversive component of morphine withdrawal syndrome in rats

The conditioned place aversion paradigm was used to investigate the role of cholecystokinin in the aversive/dysphoric component of morphine abstinence. Several cholecystokinin ligands were chronically administered during the development of morphine dependence: the CCKA antagonist devazepide, the CCKB antagonists PD-134,308 and L-365,260, and the CCKB agonist BC 264. The CCK-B antagonists L-365,260 and PD-134,308 decreased and completely blocked (respectively) the place aversion induced by naloxone in morphine dependent animals whereas BC 264 and devazepide were inactive in this model. No effect was observed in non-dependent animals after chronic administration of these CCK-ligands. These results show a distinct role for CCK receptors in the regulation of the motivational component of morphine abstinence, probably related to their differential effects in the regulation of limbic dopaminergic neurons.

Association of enkephalin catabolism inhibitors and CCK-B antagonists: a potential use in the management of pain and opioid addiction

The overlapping distribution of opioid and cholecystokinin (CCK) peptides and their receptors (mu and delta opioid receptors; CCK-A and CCK-B receptors) in the central nervous system have led to a large number of studies aimed at clarifying the functional relationships between these two neuropeptides. Most of the pharmacological studies devoted to the role of CCK and enkephalins have been focused on the control of pain. Recently the existence of regulatory mechanisms between both systems have been proposed, and the physiological antagonism between CCK and endogenous opioid systems has been definitely demonstrated by coadministration of CCK-B selective antagonists with RB 101, a systemically active inhibitor, which fully protects enkephalins from their degradation. Several studies have also been done to investigate the functional relationships between both systems in development of opioid side-effects and in behavioral responses. This article will review the experimental pharmacology of association of enkephalin-degrading enzyme inhibitors and CCK-B antagonists to demonstrate the interest of these molecules in the management of both pain and opioid addiction.

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.