Anti-opioid System Research Articles

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.

Mechanism of action of anti-opioid peptides at pain syndrome

An important goal of modern medicinal chemistry is to study the mechanisms of pain and analgesia in order to design effective analgesic drugs. Opioid analgesics are the gold standard for the treatment of severe pain; however, the use of opiates is associated with the development of side effects which are, in particular, related to the activation of the anti-opioid system. Mammalians synthesize a number of endogenous peptides, such as orphanin FGGFTGARKSARKLANQ, neuropeptide FF (FLFQPQRF-NH2), tripeptide melanostatin (MIF) PLG-NH2, as well as related compounds. These anti-opioid peptides are to one extent or another involved in homeostatic control of transmission of pain impulses. The present review includes the data published to date in domestic and foreign literature on the involvement of these peptides in such undesirable phenomena as inhibition of opioid analgesia, development of opioid tolerance and dependence, and hyperalgesia. Cell-cell and molecular ligand-receptor and receptor-receptor interactions of the opioid and anti-opioid systems are considered. These data can be useful for the design of new pharmaceuticals for pain relief. The generalization and study of these mechanisms are reflected in various approaches to treatment of pain syndromes and require analysis and further investigation.

The NOP receptor involvement in both withdrawal- and CCk-8-induced contracture responses of guinea pig isolated ileum after acute activation of κ-opioid receptor

In isolated guinea-pig ileum (GPI), the κ-opioid acute withdrawal response is under the control of several neuronal signaling systems, including the μ-opioid, the A1-adenosine and the CB1 receptors, which are involved in the inhibitory control of the κ-withdrawal response. After κ-opioid system stimulation, indirect activation of μ-opioid, A1-adenosine and CB1 systems is prevented by the peptide cholecystokinin-8 (CCk-8). In the present study, we have investigated whether the NOP system is also involved in the regulation of the acute κ-withdrawal response. Interestingly, we found that in GPI preparation, the NOP system is not indirectly activated by the κ-opioid receptor stimulation, but instead this system is able by itself to directly regulate the acute κ-withdrawal response. Specifically, our results clearly highlight first the existence of an endogenous tone of the NOP system in GPI, and second that it behaves as a functional anti-opioid system. We also found that, the NOP receptor system is involved in the regulation of the CCk-8-induced contracture intensity, only when in the presence of the κ-opioid receptor stimulation. This effect seems to be regulated by an activation threshold mechanism. In conclusion, the NOP system could act as neuromodulatory system, whose action is strictly related to the modulation of both excitatory and inhibitory neurotransmitters released in GPI enteric nervous system.

Nonpeptide ligands of neuropeptide FF: current status and structural insights.

Neuropeptide FF (NPFF) was first isolated from the bovine brain in 1985 and is linked with a variety of biological activities. NPFF, which belongs to the RF-amide family of peptides, interacts with two distinct G-protein-coupled receptors, NPFF(1) and NPFF(2). These receptors are distributed throughout the body. The NPFF system was initially described as an anti-opioid system and, while the NPFF system does affect the opioid system, it also has been implicated in pain modulation, changes in arterial blood pressure, feeding behavior and regulation of core body temperature and of monoamine systems. Most of this pharmacology has been realized from the peptide NPFF itself or through peptide analogs. The quest for nonpeptide tools for this receptor system has been limited by low selectivity and poor pharmacokinetic properties. Herein, we summarize the current knowledge from the scientific and patent literature that demonstrates a clear need for future medicinal chemistry efforts.

Mechanisms underlying morphine analgesic tolerance and dependence

The mechanisms underlying opioid tolerance are not fully understood, but appear to be comprised of two types of plasticity or counter-adaptation, at the cellular level and through neuronal circuits. Current studies mostly emphasize the cellular adaptation mechanisms, which include altered gene expression and receptor desensitization due to phosphorylation and endocytosis. However, the mechanisms underlying opioid tolerance and dependence are not always explained by cellular adaptation mechanisms alone. This review focuses on the plasticity in neuronal circuits achieved through an enhancement of synaptic activities between glutamate and NMDA receptor due to up-regulation of receptor and racemase to produce D-serine, an allosteric NMDA receptor agonist, and down-regulation of glutamate transporter, all which contribute to the counterbalance of opioid actions or anti-opioid mechanisms underlying opioid tolerance. This anti-opioid system is supposed to be also augmented by altered expression of key molecules regulating through neuron-glial networks. This review also introduces a new approach using in vivo electroporation to identify the brain loci responsible for morphine tolerance and dependence.

Preemptive Antihyperalgesia to Improve Preemptive Analgesia

Preemptive Antihyperalgesia to Improve Preemptive Analgesia

RF9, a potent and selective neuropeptide FF receptor antagonist, prevents opioid-induced tolerance associated with hyperalgesia

Neuropeptide FF (NPFF) has been proposed to play a role in pain modulation, opioid tolerance, and several other physiological processes. However, pharmacological agents that would help define physiological roles for this peptide are still missing. Here we report the discovery of a potent and selective NPFF receptor antagonist, RF9, that can be administered systemically. This compound does not show any effects by itself but can block efficiently the increase in blood pressure and heart rate evoked by NPFF. When chronically coinjected with heroin, RF9 completely blocks the delayed and long-lasting paradoxical opioid-induced hyperalgesia and prevents the development of associated tolerance. Our data indicate that NPFF receptors are part of a bona fide antiopioid system and that selective antagonists of these receptors could represent useful therapeutic agents for improving the efficacy of opioids in chronic pain treatment.

Locus‐Specific Involvement of Anti‐Opioid Systems in Morphine Tolerance and Dependence

Opioid tolerance and addiction could be discussed as two types of plasticity or counteradaptation, at the cellular level and through neuronal circuits. Cellular counteradaptation mechanisms include receptor desensitization through phosphorylation and endocytosis and through altered gene expression. The former mechanisms are related to the acute tolerance mechanisms, while the latter to chronic one. From current studies, it is known that various phosphorylation steps, such as protein kinase C (PKC) and G protein-coupled receptor (GPCR) kinase (GRK) regulate endocytosis. Of interest is that there are some differences in the physiological roles between opioid receptor endocytosis and other GPCR ones. Endocytosis of the opioid receptor is conceived as a recycling and resensitization step rather than the desensitization step. PKC phosphorylation inhibits endocytosis (PKC hypothesis). Therefore the PKC inhibitor attenuates acute analgesic tolerance. The agonist, which shows high-endocytosis stimulation, therefore makes less significant tolerance liability (RAVE hypothesis). Chronic tolerance is more likely related to the mechanisms through plastic modulation of neuronal circuits, where anti-opioidergic neurons are involved. The knockout mice lacking the receptors for anti-opioidergic nociceptin/orphanin FQ (N/OFQ) or glutamatergic neurons show weak or no morphine tolerance and dependence. As their gene expression or protein expression increases during chronic morphine treatments, we propose the hypothesis that the enhanced anti-opioid system may cause a counteradaptation to show tolerance and dependence. By a novel electroporation technique to deliver the receptor into the brain of knockout mice, we succeeded in determining the specific locus for the site of anti-opioid (through GluRepsilon1 or NR2A) action. All these results suggest that enhanced anti-opioid systems may contribute to the development of morphine tolerance and dependence, and their contributions could be brain locus specific.

Locus-Specific Rescue of GluRϵ1 NMDA Receptors in Mutant Mice Identifies the Brain Regions Important for Morphine Tolerance and Dependence

Tolerance and physical dependence caused by chronic treatment of narcotics are good models to study basic neuronal plasticity. Activation of the NMDA subtype of the glutamate receptor has been implicated as an anti-opioid system in the development of morphine analgesic tolerance and dependence. The present study examines the specific role of the epsilon1 subunit of the NMDA receptor using mice lacking the gene encoding epsilon1 subunit of the NMDA receptor (GluRepsilon1-/- mice). GluRepsilon1-/- mice showed significant enhancement and prolongation of morphine anti-nociception, compared with wild-type GluRepsilon1+/+ mice. GluRepsilon1-/- mice also showed a marked loss of the analgesic tolerance after repeated morphine treatments. In C57BL/6J mice treated with chronic morphine after tolerance paradigm, the GluRepsilon1 protein expression significantly increased in periaqueductal gray matter (PAG), ventral tegmental area (VTA) and nucleus accumbens (NAc), but not amygdala or hippocampus. The rescue of GluRepsilon1 protein by electroporation into the PAG and VTA, but not NAc of GluRepsilon1-/- mice significantly reversed morphine analgesic tolerance liability. Similar attempts were also performed in the naloxone-precipitated physical dependence paradigm. GluRepsilon1-/- mice showed marked loss of typical withdrawal abstinence behaviors, and significant enhancement of GluRepsilon1 protein expression was only observed in NAc by chronic morphine treatments after dependence paradigm. The rescue of GluRepsilon1 protein by electroporation into the NAc of GluRepsilon1-/- mice significantly reversed the loss of abstinence behaviors. These findings suggest that GluRepsilon1 has locus-specific roles in the development of morphine analgesic tolerance and physical dependence.

Dynorphin A Inhibits Nociceptin-Converting Enzyme from the Rat Spinal Cord

Cysteine proteinase found in the spinal cord of rat, called nociceptin-converting enzyme (NCE), is competitively inhibited by dynorphin A and its fragment des-[Tyr1]-DYN A. This proteinase converts orphanin FQ/nociceptin (OFQ/N) to two major fragments: OFQ/N(1-11) and further OFQ/N(1-6) with analgesic properties. Dynorphin A at the concentration of 10 μM increases KM from 15.0 to 55.9 μM. The calculated Ki for this interaction was estimated at 3.7 μM. This observation may suggest an interaction between opioid and nociceptive systems which may be affected by the balance between opioid and antiopioid systems. This balance between particular OFQ/N sequences that are derived from the same precursor and regulated by proteinases may play an important role in pain. Interestingly, dynorphin B does not reveal a similar action on the NCE.

Morphine tolerance and dependence in nociceptin/orphanin FQ transgenic knock-out mice.

It has been hypothesized that morphine tolerance and dependence in mice following chronic exposure may reflect increased compensatory activity of antiopioid systems. The endogenous peptide nociceptin/orphanin FQ has been shown to have anti-opioid effects, for example antagonizing morphine analgesia. Moreover, chronic morphine administration increases synthesis of the peptide, and morphine tolerance and dependence can be attenuated or reversed by antagonists and agonists of the nociceptin/orphanin FQ receptor, respectively. The present study seeks to confirm a role for nociceptin/orphanin FQ in opioid tolerance and dependence by comparing morphine ED(50) values and naloxone-precipitated withdrawal jumping in mice homozygous (knock-out) and heterozygous for a null mutation of the Npnc1 gene encoding the nociceptin/orphanin FQ propeptide, and their wild type littermates, following chronic morphine exposure. Relative to morphine-naive control mice, significant rightward shifts in the morphine dose-response curve, resulting in increased morphine ED(50) values (approximately two to three-fold), was observed for all genotypes following three days of repeated systemic morphine injections. However, no differences between genotypes in the magnitude of tolerance were observed. In contrast, knock-out mice displayed significantly increased naloxone-precipitated withdrawal jumping relative to heterozygous and wild-type mice following implantation with a morphine pellet (25mg) for 72h. Use of nociception/orphaninFQ transgenic knock-out mice thus demonstrate the differential involvement of nociceptin/orphanin FQ in morphine tolerance and dependence.

Effects of naloxone on c-jun/AP-1 in met-enkephalin- and FMRFamide-immunoreactive neurons of a gastropod snail

1. Opioid- and FMRFamide (FMRFa)-ergic systems are believed to play antagonistic behavioral roles in both higher and lower animals. In our previous experiments on a snail, behavioral choice has been demonstrated to be dependent on a balance between FMRFa and enkephalins [7]. Here, we examined if the disturbance of the balance causes changes in the activity of both systems. Opiate receptor blocker naloxone was applied and its effect on c-jun expression of met-enkephalin (MEnk)- and FMRFa-ergic neurons was examined immunocytochemically in terrestrial gastropod snail Cepaea nemoralis. 2. In control, untreated snails, central neurons with c-jun/AP-1-like-immunoreactivity were found to occur. These included MEnk-, FMRFa- and 5HT-immunoreactive (-ir) neurons, as was revealed by double-labelling. 3. After treatment with naloxone for 4 h, the following changes were observed: (i) increase in the number of MEnk-ir neurons; increase in the number of neurons showing c-jun/AP-1 and MEnk double-labeling; (ii) disappearance of c-jun/AP-1-immunoreactivity from some FMRFa-ir neurons. 4. It is suggested that immediate early genes are involved in the mechanisms responsible for the reciprocal regulation of the opioid and antiopioid neuropeptide systems.

Simultaneous activation of spinal antiopioid system (neuropeptide FF) and pain facilitatory circuitry by stimulation of opioid receptors in rats

Neuropeptide FF (NPFF) is a mammalian FMRFamide-like octapeptide with antiopioid properties that inhibits morphine-induced analgesia but also produces hyperalgesia. In the present study, a series of three experiments was carried out to investigate the interactions between opioid receptor stimulation and antiopioid systems. First, by using an in vitro superfusion system with rat spinal cord slices, we showed that morphine stimulated NPFF release in a dose-dependent manner. The stimulating effect which was observed with morphine concentrations as low as 100 fM reached a maximum at 0.1 nM, then decreased and was ineffective at 10 μM. The morphine-induced release of NPFF was abolished by naloxone (1 μM) but unaltered by tetrodotoxin. Second, by an in vivo approach, we showed that a single heroin administration (2.5 mg/kg, s.c.) elicited in 30 min a drastic drop (38%) in spinal NPFF content. In a third experiment, we evaluated the capacity of naloxone in revealing an antiopioid component associated with opioid receptor stimulation. The administration of naloxone (1 mg/kg, s.c.) 25 min following that of heroin (2.5 mg/kg, s.c.), not only abolished the heroin-induced increase of tail-flick latency, but also lowered it under the basal value by 30%. These results indicate that opioid receptor stimulation activates both pain inhibitory and pain facilitatory systems in which NPFF may play a significant role and that opiate-induced analgesia is always partly masked.

Changes in CNS neuropeptide FF-like material, pain sensitivity, and opiate dependence following chronic morphine treatment.

Tolerance and dependence to opiates may be an adaptive process that limits excessive effects of morphine on the CNS. Because no consistent opiate receptor reduction in chronically treated rats seems to underlie the hyposensitivity to morphine, an alternative hypothesis has postulated a role of "antiopioid" peptides. It is possible to speculate that the administration of morphine stimulates antiopioid systems such as neuropeptide FF (NPFF), as part of an homeostatic mechanism contributing to the development of tolerance. To test this hypothesis, pain sensitivity, opiate dependence, and CNS NPFF-IR levels were estimated at different times after implantation of morphine pellets (2 x 75 mg; NIDA). Three hours after morphine pellet treatment the analgesic effect was maximum and it decreased rapidly during the following 12 h. Naloxone-precipitated withdrawal syndrome was detected as soon as 3 h after morphine pellet implantation and was maximal after 24 h. NPFF-IR levels were measured in the spinal cord, brain stem, and hypothalamus. A significant decrease of NPFF-IR was observed 1 h after morphine pellet implantation (-25% to -45% depending on the structures) followed by a drastic increase of NPFF-IR levels (+60 to +140%) between 3 and 6 h. NPFF-IR levels rapidly returned to baseline after 24-36 h. It is suggested that the activity of these NPFF-IR neurones may increase gradually as a consequence of the continuous stimulation of opiate receptors and be part of an adaptive process that is able to counteract morphine effects and to induce dependence and tolerance to the analgesic effects of opiates.

Anti-opioid activity of sigma 1 systems

(+)Pentazocine is a potent sigma 1 ligand which antagonizes mu, kappa 1, kappa 3 and delta analgesia. This antagonism is reversed by haloperidol, which blocks both D 2 and sigma receptors, but not by sulpiride, a selective D 2 drug without sigma activity. Haloperidol (< 1 mg/kg) shifts the dose-response curves for morphine in CD-1 mice approximately 2-fold and is more effective against kappa analgesics. These shifts argue for a tonic activity of this anti-opioid system. This anti-opioid system also contributes to the differences in opioid sensitivity among strains of mice.

Antinociception induced by endogenous enkephalins, protected from their catabolism by the peptidase inhibitor RB 101, is potentiated by CCK-B antagonists

The peptide cholecystokinin (CCK) has been shown to counteract the analgesic effects of morphine suggesting a physiological antagonism between opioid and CCK neural systems. The implications that could result from the management of this antiopioid system are very important since it might be possible to modify opioid analgesia and/or to prevent the development of opioid tolerance. In this work we have studied the effects of selective antagonists of CCK-A and CCK-B receptors on the antinociception induced by RB 101, a new systemically active full inhibitor of the enkephalin-catabolizing enzimes (1). The naloxone reversible antinociceptive effects induced by RB101 in rat tail-flick test were strongly increased by low doses of the CCK-B antagonists L-365,260 and PD-134,308

A review of the role of anti-opioid peptides in morphine tolerance and dependence.

Studies on the mechanisms of tolerance and dependence have mostly focused on changes at the receptor level. These experiments, conducted with model systems ranging from clonal cell lines to whole animals, have identified a number of important adaptive mechanisms which occur at the receptor level. However, none of these adaptive mechanisms can completely account for the phenomena which serve to define the state of morphine tolerance and dependence, especially the observation that as an animal becomes more tolerant to morphine, less naloxone is required to trigger withdrawal. The data reviewed in this paper provide strong support for the hypothesis that the brain synthesizes and secretes neuropeptides which act as part of a homeostatic system to attenuate the effects of morphine and endogenous opioid peptides. According to this model, administration of morphine releases anti-opioid peptides (AOP), which then attenuate the effects of morphine. As more morphine is given, more AOP are released, thereby producing tolerance to the effects of morphine. Cessation of morphine administration, or administration of naloxone, produces a relative excess of anti-opioid, which is in part responsible for the withdrawal syndrome. Since endogenous and exogenous antagonists might together produce synergistic effects, less naloxone might be required to trigger withdrawal in the presence of higher levels of AOPs. Although the study of AOP is in its infancy, a deeper understanding of the central nervous system (CNS) anti-opioid systems may lead to new treatments for chronic pain, substance abuse, and psychiatric disorders.

Possibility of Opioid and Anti-Opioid System

Possibility of Opioid and Anti-Opioid System

Hibernation: An opioid-dependent state?

Hibernation reduces substantially the heart rate of hamsters as well as the respiratory rate, the body temperature and the arousal level. The heart rate is reversed dramatically by the injection of low doses of Naloxone and in some cases the hamster arouses prematurely from hibernation. The effect is not due to the pain of the injection because saline injections do not produce such changes. The effect requires a pre-existing state of hibernation because Naloxone has no cardioacceleratory or arousal effect in non-hibernating hamsters that have had their heart rate and body temperature decreased substantially during hypothermia. These results suggest that endogenous opioids may comtribute specifically to the state of hibernation. Moreover, a physiological role may exist for an anti-opioid system in the promotion of arousal from hibernation.