Opioid activity profiles indicate similarities between the nociceptin/orphanin FQ and opioid receptors

Effectiveness of two extended-release buprenorphine formulations during postoperative period in neonatal rats

Information on the effectiveness of a new long-lasting buprenorphine formulation, extended-release buprenorphine, in the neonatal rat is very limited. This study compares whether a high dose of extended-release buprenorphine (XR-Hi) attenuates thermal hypersensitivity for a longer period than a low dose of extended-release buprenorphine (XR-Lo) in a neonatal rat incisional pain model. Two experiments were performed. Experiment one: Male and female postnatal day-5 rat pups (n = 38) were randomly assigned to 1 of 4 treatment groups and received a subcutaneous administration of one of the following: 1) 0.9%NaCl (Saline), 0.1 mL; 2) sustained release buprenorphine (Bup-SR), 1 mg/kg; 3) XR-Lo, 0.65 mg/kg; and 4) XR-Hi, 1.3 mg/kg. Pups were anesthetized with sevoflurane in 100% O₂ and a 5 mm long skin incision was made over the left lateral thigh and underlying muscle dissected. The skin was closed with surgical tissue glue. Thermal hypersensitivity testing (using a laser diode) and clinical observations were conducted 1 hour (h) prior to surgery and subsequently after 1, 4, 8, 24, 48, 72 h of treatment. Experiment two: The plasma buprenorphine concentration level was evaluated at 1, 4, 8, 24, 48, 72 h on five-day-old rat pups. Plasma buprenorphine concentration for all treatment groups remained above the clinically effective concentration of 1 ng/mL for at least 4 h in the Bup-SR group, 8 h in XR-Lo and 24 h in XR-Hi group with no abnormal clinical observations. This study demonstrates that XR-Hi did not attenuate postoperative thermal hypersensitivity for a longer period than XR-Lo in 5-day-old rats; XR-Hi attenuated postoperative thermal hypersensitivity for up to 4 h while Bup-SR and XR-Lo for at least 8 h in this model.

Is a Highly Selective and Potent κ Opioid Receptor (KOR) Agonist with an Unexpected Nonreduction in Locomotor Activity

Undue central nervous system (CNS) side effects including dysphoria and sedation remain to be a challenge for the development of κ opioid receptor (KOR) agonists as effective and safe analgesics. On the basis of our previous work on morphinan-based KOR agonists, a series of 7α-methyl-7β-substituted northebaine derivatives were designed, synthesized, and biologically assayed. Among others, compound 4a (SLL-627) has been identified as a highly selective and potent KOR agonist both in vitro and in vivo, and its molecular basis was also examined and discussed. Besides low liability to conditioned place aversion (CPA) test, treatment of SLL-627 was associated with a nonreduction in locomotor activity, compared to most of the other arylacetamide- or morphinan-based KOR agonists which generally exhibited apparently sedative effects. This unexpected finding provides new insights to dissociate analgesia from sedation for future discovery of innovative KOR agonists.

NOP-Targeted Nonpeptide Ligands

The development of nonpeptide systemically active small-molecule NOP-targeted ligands has contributed tremendously to validating the NOP receptor as a promising target for therapeutics. Although a NOP-targeted compound is not yet approved for clinical use, a few NOP ligands are in clinical trials for various indications. Both successful and failed human clinical trials with NOP ligands provide opportunities for rational development of new and improved NOP-targeted compounds. A few years after the discovery of the NOP receptor in 1994, and its de-orphanization upon discovery of the endogenous peptide nociceptin/orphanin FQ (N/OFQ) in 1995, there was a significant effort in the pharmaceutical industry to discover nonpeptide NOP ligands from hits obtained from high-throughput screening campaigns of compound libraries. Depending on the therapeutic indication to be pursued, NOP agonists and antagonists were discovered, and some were optimized as clinical candidates. Advances such as G protein-coupled receptor (GPCR) structure elucidation, functional selectivity in ligand-driven GPCR activation, and multi-targeted ligands provide new scope for the rational design of novel NOP ligands fine-tuned for successful clinical translation. This article reviews the field of nonpeptide NOP ligand drug design in the context of these exciting developments and highlights new optimized nonpeptide NOP ligands possessing interesting functional profiles, which are particularly attractive for several unmet clinical applications involving NOP receptor pharmacomodulation.

The biology of Nociceptin/Orphanin FQ (N/OFQ) related to obesity, stress, anxiety, mood, and drug dependence

Nociceptin/Orphanin FQ (N/OFQ) is a 17 amino acid peptide that was deorphanized in 1995. The generation of specific agonists, antagonists and receptor deficient mice and rats has enabled progress in elucidating the biological functions of N/OFQ. Additionally, radio-imaging technologies have been advanced for investigation of this system in animals and humans. Together with traditional neurobehavioral techniques, these tools have been utilized to identify the biological significance of the N/OFQ system and its interacting partners. The present review focuses on the role of N/OFQ in the regulation of feeding, body weight homeostasis, stress, the stress-related psychiatric disorders of depression and anxiety, and in drug and alcohol dependence. Critical evaluation of the current scientific preclinical literature suggests that small molecule modulators of nociceptin opioid peptide receptors (NOP) might be useful in the treatment of diseases related to these biological functions. In particular, the literature data suggest that antagonism of NOP receptors will produce anti-obesity and antidepressant activities in humans. However, there are also contradictory data discussed. The current literature on the role of N/OFQ in anxiety and addiction, on the other hand points primarily to a role of agonist modulation being potentially therapeutic. Some drug-like molecules that function either as agonists or antagonists of NOP receptors have been optimized for human clinical study to test some of these hypotheses. The discovery of PET ligands for NOP receptors, combined with the pharmacological tools and burgeoning preclinical data set discussed here bodes well for a rapid advancement of clinical understanding and potential therapeutic benefit.

Use of butorphanol during immobilization of free-ranging white rhinoceros (Ceratotherium simum)

Forty free-ranging white rhinoceros (Ceratotherium simum) were anesthetized with etorphine, azaperone, and hyaluronidase in Kruger National Park, South Africa, between February and August 2009. Eighteen rhinoceros received butorphanol in the dart combination, and 22 rhinoceros had butorphanol administered intravenously within 15 min of darting. Body position, blood gas values, heart rate, respiratory rate, and temperature were measured at two time points after darting, approximately 10 min apart (sample 1 mean collection time after darting, 9.4 +/- 2.7 min; sample 2 mean collection time, 18.6 +/- 2.8 min). A significant number of field-captured rhinoceros remained standing at the first sample period when butorphanol was administered in the dart. Higher median values for arterial partial pressure of oxygen (PaO2) in combination with lower arterial partial pressure of carbon dioxide (PaCO2) in standing versus recumbent rhinoceros suggested improved ventilation in this posture (P < 0.05). When the effect of time, body position, and age was controlled, median values for respiratory rate, lactate, and pH were better in rhinoceros that received butorphanol in the dart (P < 0.05). There was also a trend toward higher median values for SO2 and bicarbonate in rhinoceros receiving butorphanol in the dart. Intravenous administration of butorphanol resulted in significantly decreased median PaCO2 and heart rate in recumbent rhinoceros (P < 0.05) without changes in PaO2 between sample periods 1 and 2. However, rhinoceros remained hypoxemic during the short anesthetic procedure despite butorphanol administration. Preliminary results suggest that administration of butorphanol (either in the dart or intravenously) improves some metabolic parameters in free-ranging recumbent white rhinoceros without significantly affecting ventilation. It is hypothesized that this may be due to a lighter state of immobilization. Addition of butorphanol to the dart provides handling and physiologic advantages because the majority of rhinoceros remain standing.

Parallel buprenorphine phMRI responses in conscious rodents and healthy human subjects

Pharmacological magnetic resonance imaging (phMRI) is one method by which a drug's pharmacodynamic effects in the brain can be assessed. Although phMRI has been frequently used in preclinical and clinical settings, the extent to which a phMRI signature for a compound translates between rodents and humans has not been systematically examined. In the current investigation, we aimed to build on recent clinical work in which the functional response to 0.1 and 0.2 mg/70 kg i.v. buprenorphine (partial µ-opioid receptor agonist) was measured in healthy humans. Here, we measured the phMRI response to 0.04 and 0.1 mg/kg i.v. buprenorphine in conscious, naive rats to establish the parallelism of the phMRI signature of buprenorphine across species. PhMRI of 0.04 and 0.1 mg/kg i.v. buprenorphine yielded dose-dependent activation in a brain network composed of the somatosensory cortex, cingulate, insula, striatum, thalamus, periaqueductal gray, and cerebellum. Similar dose-dependent phMRI activation was observed in the human phMRI studies. These observations indicate an overall preservation of pharmacodynamic responses to buprenorphine between conscious, naive rodents and healthy human subjects, particularly in brain regions implicated in pain and analgesia. This investigation further demonstrates the usefulness of phMRI as a translational tool in neuroscience research that can provide mechanistic insight and guide dose selection in drug development.

Modulation of CNS pain circuitry by intravenous and sublingual doses of buprenorphine

Buprenorphine (BUP) is a partial agonist at μ-, δ- and ORL1 (opioid receptor-like)/nociceptin receptors and antagonist at the κ-opioid receptor site. BUP is known to have both analgesic as well as antihyperalgesic effects via its central activity, and is used in the treatment of moderate to severe chronic pain conditions. Recently, it was shown that intravenous (IV) administration of 0.2mg/70 kg BUP modulates the blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) response to acute noxious stimuli in healthy human subjects. The present study extends these observations by investigating the effects of BUP dose and route of administration on central nervous system (CNS) pain circuitry. Specifically, the modulation of evoked pain BOLD responses and resting state functional connectivity was measured following IV (0.1 and 0.2mg/70 kg) and sublingual (SL) (2mg) BUP administration in healthy human subjects. While 0.1mg/70 kg IV BUP is sub-analgesic, both 0.2mg/70 kg IV BUP and 2.0mg SL BUP are analgesic doses of the drug. Evoked BOLD responses were clearly modulated in a dose-dependent manner. The analgesic doses of BUP by both routes of administration yielded a potentiation in limbic/mesolimbic circuitry and attenuation in sensorimotor/sensory-discriminative circuitry. In addition, robust decreases in functional connectivity between the putamen and the sensorimotor/sensory-discriminative structures were observed at the two analgesic doses subsequent to measuring the maximum plasma BUP concentrations (C(max)). The decreases in functional connectivity within the sensorimotor/sensory-discriminative circuitry were also observed to be dose-dependent in the IV administration cohorts. These reproducible and consistent functional CNS measures at clinically effective doses of BUP demonstrate the potential of evoked pain fMRI and resting-state functional connectivity as objective tools that can inform the process of dose selection. Such methods may be useful during early clinical phase evaluation of potential analgesics in drug development.

Buprenorphine: a unique drug with complex pharmacology

Buprenorphine, an opioid with mixed agonist-antagonist activity at classical opioid receptors, has been approved recently for the treatment of opioid dependency. Buprenorphine is also used as an analgesic. The buprenorphine dose-response curve is sometimes submaximal, or even bell-shaped, in nociceptive assays, depending upon the nature and intensity of the noxious stimulus. Moreover, buprenorphine, when administered with full agonists, such as morphine, antagonizes the action of these drugs. Partial agonism at the mu opioid receptor and, in some cases, antagonism at the kappa or delta opioid receptor have been considered as possible underlying mechanisms for the ceiling effect and bell-shaped dose-response curve of buprenorphine. While ceiling effects can be explained by partial agonist activity of buprenorphine, the bell-shaped dose-response curve cannot be a consequence of this property of the drug. Recently, buprenorphine has been shown to activate the opioid receptor-like (ORL-1; also known as NOP) receptor. Supraspinal activation of the ORL-1 receptor counteracts the antinociceptive and rewarding actions of morphine, raising the possibility that these actions of buprenorphine can also be altered by its ability to concomitantly activate the ORL-1 receptor. The use of molecular biological techniques has advanced our knowledge regarding the role of opioid receptors in modulation of pain and reward. In particular, generation of opioid receptor knockout mice has proven useful in this regard. Indeed, using knockout mice, we have recently shown that the antinociceptive effect of buprenorphine mediated primarily by the mu opioid receptor is attenuated by the ability of the drug to activate the ORL-1 receptor. Thus, the goal of this review is to provide evidence demonstrating that the ORL-1 receptor plays a functional role not only in the antinociceptive effect of buprenorphine but also in other actions of the drug as well.

Transdermal buprenorphine - a critical appraisal of its role in pain management

This paper reviews the current clinical data for the role of transdermal buprenorphine (BUP TDS) in the treatment of diverse acute and chronic pain syndromes. Literature searches were carried out using PubMed (1988 to June 2009). The published findings seem to support hypotheses regarding the rather unique analgesic mechanisms of buprenorphine as compared with pure μ-opioids like morphine and fentanyl. However, the exact mechanism of this analgesic efficacy still remains largely unknown despite recent advances in preclinical pharmacological studies. Such assessments have demonstrated the sustained antihyperalgesic effect of buprenorphine in diverse animal pain models. These findings are supported in a growing number of clinical studies of oral, intrathecal, intravenous, and Bup TDS. This review paper focuses almost entirely on the clinical experience concerning the transdermal administration of buprenorphine, although preclinical aspects are also addressed in order to provide a complete picture of the unique pharmacological properties of this analgesic drug. Mounting evidence indicates the appropriateness of Bup TDS in the treatment of diverse acute and chronic pain syndromes which have been less or not responsive to other opioids. Additionally, BUP TDS seems to hold great promise for other difficult-to-treat (pain) conditions, such as patients in the intensive care setting. However, its use is somewhat tempered by the occurrence of local skin reactions which have been shown to be often therapy resistant. Further studies are certainly warranted to identify even more precisely the clinical syndromes that are most sensitive to buprenorphine treatment, and to compare buprenorphine to other opioids in head-to-head trials of acute and chronic pain conditions.

Buprenorphine inhibits bradykinin-induced release of calcitonin gene-related peptide from rat trigeminal neurons via both mu-opioid and nociceptin/orphanin peptide receptors

In this study we used the dual opioid and nociceptin/orphanin peptide (NOP) agonist buprenorphine to investigate the relative contributions of opioid and NOP systems in regulating bradykinin-stimulated calcitonin-gene related peptide (CGRP) release from primary cultures of neonatal rat trigeminal neurons. We found that: bradykinin stimulates CGRP secretion either by a direct effect or after applying so-called "bradykinin-priming" protocol. In both cases, buprenorphine was able to inhibit bradykinin-stimulated CGRP secretion; however, inhibition was mediated by NOP receptors when buprenorphine was added to the incubation medium along with bradykinin, whereas it appeared to be mediated by mu-opioid receptors in bradykinin priming experiments. Bradykinin treatments also caused an increase in neuronal prostaglandin production; prostanoids appeared to be involved in the stimulatory effects of bradykinin as well as in buprenorphine inhibition, through apparently unrelated mechanisms.

The role of the opioid receptor-like (ORL1) receptor in motor stimulatory and rewarding actions of buprenorphine and morphine

We have previously shown that the ability of buprenorphine to activate the opioid receptor-like (ORL1) receptor compromises its antinociceptive effect. Furthermore, morphine has been shown to alter the level of orphanin FQ/nociceptin (OFQ/N), the endogenous ligand of the ORL1 receptor, raising the possibility that the endogenous OFQ/N/ORL1 receptor system may be involved in the actions of these opioids. Thus, using mice lacking the ORL1 receptor and their wild-type littermates, the present study assessed the role of the ORL1 receptor in psychomotor stimulant and rewarding actions of buprenorphine and morphine. Morphine (5, 10 mg/kg) dose-dependently increased motor activity and induced conditioned place preference. However, the magnitude of each response was comparable for the mutant mice and their wild-type littermates. In contrast, buprenorphine (1 mg/kg) induced greater motor stimulation in ORL1 receptor knockout mice as compared with their wild-type littermates. Further, single conditioning with buprenorphine (3 mg/kg) induced place preference in mutant mice but not in their wild-type littermates. The results of binding assay showed that buprenorphine concentration-dependently (0-1000 nM) displaced specific binding of [(3)H]-OFQ/N in brain membrane of wild-type mice. Together, the present results suggest that the ability of buprenorphine to interact with the ORL1 receptor modulates its acute motor stimulatory and rewarding effects.

Anxiolytic-like effects of morphine and buprenorphine in the rat model of fear-potentiated startle: tolerance, cross-tolerance, and blockade by naloxone

RATIONALE

Morphine and buprenorphine have analgesic and anxiolytic-like properties. While their analgesic effects have been well characterized, their anxiolytic-like properties have not.

OBJECTIVES

Effects of acute morphine and buprenorphine on the expression of acoustic fear-potentiated startle (FPS) and naloxone pretreatment were assessed. Effects of chronic morphine and buprenorphine on tolerance, cross-tolerance, and withdrawal were also examined.

MATERIALS AND METHODS

Fear-conditioned rats were given subcutaneous drug treatment immediately before testing for FPS. Experiment 1, rats were administered morphine (0.03, 0.25, 0.63, 2.5, or 10 mg/kg) or buprenorphine (0.004, 0.0075, 0.015, 0.03, or 0.25 mg/kg). Experiment 2, rats were given saline or naloxone (0.5 mg/kg) and 5 min later given saline, morphine (2.5 mg/kg), or buprenorphine (0.03 mg/kg). Experiment 3, rats received once-daily injections of saline, morphine (10 mg/kg), or buprenorphine (0.25 mg/kg) for 7 days. Immediately before testing, saline-treated rats were given saline, morphine (2.5 mg/kg), or buprenorphine (0.03 mg/kg), morphine-treated rats were given morphine (2.5 mg/kg) or buprenorphine (0.03 mg/kg), and buprenorphine-treated rats were given buprenorphine (0.03 mg/kg) or morphine (2.5 mg/kg). Tolerance and cross-tolerance in analgesia were assessed via the tail-flick test, as were naloxone-precipitated withdrawal.

RESULTS

Morphine and buprenorphine had parallel dose-response curves in blocking FPS, with buprenorphine 40 times more potent than morphine. Naloxone reversed these effects. Morphine and buprenorphine showed tolerance and cross-tolerance in their anxiolytic-like and analgesic effects. Chronic buprenorphine produced less withdrawal than chronic morphine.

CONCLUSIONS

Cross-tolerance between morphine and buprenorphine suggests a common receptor mediating their anxiolytic-like and analgesic effects.

The mu opioid receptor is involved in buprenorphine-induced locomotor stimulation and conditioned place preference

The analgesic effect of buprenorphine is mediated via the mu opioid receptor (MOP). In the present study, using mice lacking the MOP and their wild-type littermates, we determined the role of the MOP in buprenorphine-induced locomotor stimulation and conditioned place preference (CPP). Buprenorphine (3 mg/kg) increased motor activity in wild-type but not in MOP knockout mice, showing the motor stimulatory action of buprenorphine is mediated via the MOP. When the mice were given the same treatment once daily for 5 consecutive days and challenged with buprenorphine on day 11, the motor stimulatory action of buprenorphine was enhanced in wild-type but not in MOP knockout mice, showing sensitization developed to the motor stimulatory action of buprenorphine and this phenomenon was mediated via the MOP. Likewise, buprenorphine induced CPP in wild-type mice after four alternate-day saline/buprenorphine (3 mg/kg) injections paired with olfactory and visual cues. However, buprenorphine failed to induce CPP in MOP knockout mice. In contrast, amphetamine (1 mg/kg) induced a comparable CPP in wild-type and MOP knockout mice. Together, the present results suggest that the ability of buprenorphine to increase motor activity and induce locomotor sensitization and CPP is mediated via the MOP.

Buprenorphine duration of action: mu-opioid receptor availability and pharmacokinetic and behavioral indices

BACKGROUND

Buprenorphine (BUP) is effective in the treatment of opioid dependence when given on alternating days, probably as a result of long-lasting occupation of micro opioid receptors (microORs). This study examined the duration of action of BUP at microORs and correlations with pharmacokinetic and pharmacodynamic outcomes in 10 heroin-dependent volunteers.

METHODS

Availability of microOR (measured with positron emission tomography and [(11)C]-carfentanil), plasma BUP concentration, opioid withdrawal symptoms, and blockade of hydromorphone (HYD; heroin-like agonist) effects were measured at 4, 28, 52, and 76 hours after omitting the 16 mg/d dose of BUP in a study reported elsewhere.

RESULTS

Relative to heroin-dependent volunteers maintained on BUP placebo, whole-brain microOR availability was 30%, 54%, 67%, and 82% at 4, 28, 52, and 76 hours after BUP. Regions of interest showed similar effects. Plasma concentrations of BUP were time dependent, as were withdrawal symptoms, carbon dioxide sensitivity and extent of HYD blockade. Availability of microOR was also correlated with BUP plasma concentration, withdrawal symptoms, and HYD blockade.

CONCLUSIONS

Together with our previous findings, it appears that microOR availability predicts changes in pharmacokinetic and pharmacodynamic measures and that about 50%-60% BUP occupancy is required for adequate withdrawal symptom suppression (in the absence of other opioids) and HYD blockade.

Agonist activity of naloxone benzoylhydrazone at recombinant and native opioid receptors

1. In the present study, we examined the pharmacological activity of the putative kappa3-opioid receptor agonist naloxone benzoylhydrazone (NalBzoH) at recombinant human opioid receptors individually expressed in Chinese hamster ovary (CHO) cells and native opioid receptors present in rat striatum. 2. At the mu-opioid receptor (MOR), NalBzoH stimulated guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding (pEC50=8.59) and inhibited cyclic AMP accumulation (pEC50=8.74) with maximal effects (Emax) corresponding to 55 and 65% of those obtained with the MOR agonist DAMGO, respectively. The MOR antagonist CTAP blocked the stimulatory effects of NalBzoH and DAMGO with similar potencies. 3. At the kappa-opioid receptor (KOR), NalBzoH stimulated [35S]GTPgammaS binding (pEC50=9.70) and inhibited cyclic AMP formation (pEC50=9.45) as effectively as the selective KOR agonist (-)-U-50,488. The NalBzoH effect was blocked by the KOR antagonist nor-binaltorphimine (nor-BNI) (pKi=10.30). 4. In CHO cells expressing the delta-opioid receptor (DOR), NalBzoH increased [35S]GTPgammaS binding (pEC50=8.49) and inhibited cyclic AMP formation (pEC50=8.61) almost as effectively as the DOR agonist DPDPE. Naltrindole (NTI), a selective DOR antagonist, completely blocked the response to NalBzoH (pKi of 10.40). 5. In CHO cells expressing the nociceptin/orphanin FQ (N/OFQ) receptor (NOP), NalBzoH failed to exert agonist effects and antagonized the agonist-induced receptor activation. 6. When compared to other opioid receptor ligands, NalBzoH showed an efficacy that was lower than that of morphine at MOR, but higher at KOR and DOR. 7. In rat striatum, NalBzoH enhanced [35S]GTPgammaS binding and inhibited adenylyl cyclase activity. These effects were antagonized by either CTAP, nor-BNI or NTI, each antagonist blocking a fraction of the NalBzoH response. 8. These data demonstrate that NalBzoH displays agonist activity at MOR, DOR and KOR expressed either in a heterologous cell system or in a native environment.

Comparison of the in vitro efficacy of mu, delta, kappa and ORL1 receptor agonists and non-selective opioid agonists in dog brain membranes

Morphine and related opioid agonists are frequently used in dogs for their analgesic properties, their sedative effects and as adjuncts to anesthesia. Such compounds may be effective through a combined action at mu-, delta- and kappa-opioid receptors. In this work, the in vitro relative agonist efficacy of ligands selective for mu (DAMGO)-, delta (SNC80)- and kappa (U69593)-opioid receptors as well as the opioid receptor-like receptor ORL(1) (orphaninFQ/nociceptin) which may mediate nociceptive or antinociceptive actions was determined using the [35S]GTPgammaS binding assay in membrane homogenates from the frontal cortex, thalamus and spinal cord of beagle dogs. In addition, other analgesics commonly used in the dog were investigated. For the receptor-selective compounds, maximum stimulation of [35S]GTPgammaS binding decreased in the order kappa > ORL1 > delta > mu in cortical homogenates, compared with mu > ORL1 > kappa > delta in thalamic and spinal cord homogenates. For other opioids examined, efficacy decreased in the order etorphine >> morphine > fentanyl = oxymorphine > butorphanol = oxycodone = nalbuphine. There was no significant difference in the potency of compounds to stimulate [35S]GTPgammaS binding between cortex and thalamus, with the exception of etorphine. Buprenorphine, the partial mu-opioid receptor agonist and kappa-, delta-opioid receptor antagonist, which does have analgesic efficacy in the dog, showed no agonism in any tissue but was an effective mu-opioid receptor > ORL1 receptor antagonist. The results show that the ability of agonists to stimulate [35S]GTPgammaS binding relates to the receptor distribution of opioid and ORL1 receptors in the dog.

Autoradiography in opioid triple knockout mice reveals opioid and opioid receptor like binding of naloxone benzoylhydrazone

Naloxone benzoylhydrazone (NalBzoH) is a ligand used to study opioid receptors. It has been suggested to act at a novel kappa3 receptor but also appears to bind to classical opioid receptors, and possibly the ORL1 receptor. We have used opioid receptor triple knockout mice, deficient in genes coding for the mu, delta and kappa-receptor, to characterise the relative contributions of opioid and ORL1 activity to the binding of this ligand, by carrying out receptor autoradiography with [3H]NalBzoH. As competing ligands we have used diprenorphine and nociceptin at 1 microM, alone or in combination, to determine the contribution of opioid and ORL1 receptor binding. At 4 nM [3H]NalBzoH showed labelling in wild-type brains indicative of broad spectrum classical opioid receptor binding. In the triple knockout brains all labelling was completely absent, suggesting that at this concentration there is no binding to ORL1 sites. However at 50 nM [3H]NalBzoH showed labelling in triple knockout brains with a distribution pattern indicative of ORL1 labelling. Quantitative analysis showed that nociceptin displaced typically 30% of the residual labelling in knockout brains whilst diprenorphine had relatively little effect. The data show that at 50 nM NalBzoH no binding was detected other than to classical opioid receptors or to ORL1 in an approximate ratio of 2:1.

Differential effects of gestational buprenorphine, naloxone, and methadone on mesolimbic mu opioid and ORL1 receptor G protein coupling

In addition to its use for heroin addiction pharmacotherapy in general, buprenorphine has advantages in treating maternal heroin abuse. To examine the gestational effects of buprenorphine on opioid receptor signaling, the [(35)S]-GTP gamma S in situ binding induced by the mu agonist [D-Ala(2),MePhe(4),Gly(5)-ol] enkephalin (DAMGO) or the nociceptin/orphanin FQ (N/OFQ) agonist was measured in mesolimbic structures of pup brains from pregnant rats administered with buprenorphine +/- naloxone, naloxone, or methadone by osmotic minipump. Drug- and gender-based changes in DAMGO- and N/OFQ-induced GTP gamma S binding were discovered in mesolimbic regions of dam, P2, and P7 brains. Buprenorphine and/or methadone gestational treatment attenuated DAMGO-induced GTP gamma S binding in some dam and male P2 mesolimbic regions. Methadone diminished DAMGO-induced GTP gamma S binding in almost all monitored brain regions of the dam but had few effects on their N/OFQ-induced GTP gamma S binding. Naloxone used in combination with buprenorphine blocked the inhibition by buprenorphine alone on DAMGO-induced GTP gamma S binding. In contrast to its inhibitory effects on DAMGO-induced GTP gamma S binding, buprenorphine stimulated N/OFQ-induced GTP gamma S binding in male P2 nucleus accumbens and lateral septum. Brain region-dependent gender differences in DAMGO-induced GTP gamma S binding were seen in P2 pups, and males showed greater sensitivity to buprenorphine and methadone than females. Our findings on mu opioid receptor (MOR) GTP-binding regulatory protein (G protein) coupling and its gender dependency are consistent with our earlier studies on mu receptor binding adaptation induced by buprenorphine in dams and neonatal rats after in utero treatment regimens, and they extend the gestational effects of this opiate to mu and N/OFQ receptor functionality.

Buprenorphine-induced antinociception is mediated by mu-opioid receptors and compromised by concomitant activation of opioid receptor-like receptors

Buprenorphine is a mixed opioid receptor agonist-antagonist used clinically for maintenance therapy in opiate addicts and pain management. Dose-response curves for buprenorphine-induced antinociception display ceiling effects or are bell shaped, which have been attributed to the partial agonist activity of buprenorphine at opioid receptors. Recently, buprenorphine has been shown to activate opioid receptor-like (ORL-1) receptors, also known as OP4 receptors. Here we demonstrate that buprenorphine, but not morphine, activates mitogen-activated protein kinase and Akt via ORL-1 receptors. Because the ORL-1 receptor agonist orphanin FQ/nociceptin blocks opioid-induced antinociception, we tested the hypothesis that buprenorphine-induced antinociception might be compromised by concomitant activation of ORL-1 receptors. In support of this hypothesis, the antinociceptive effect of buprenorphine, but not morphine, was markedly enhanced in mice lacking ORL-1 receptors using the tail-flick assay. Additional support for a modulatory role for ORL-1 receptors in buprenorphine-induced antinociception was that coadministration of J-113397, an ORL-1 receptor antagonist, enhanced the antinociceptive efficacy of buprenorphine in wild-type mice but not in mice lacking ORL-1 receptors. The ORL-1 antagonist also eliminated the bell-shaped dose-response curve for buprenorphine-induced antinociception in wild-type mice. Although buprenorphine has been shown to interact with multiple opioid receptors, mice lacking micro-opioid receptors failed to exhibit antinociception after buprenorphine administration. Our results indicate that the antinociceptive effect of buprenorphine in mice is micro-opioid receptor-mediated yet severely compromised by concomitant activation of ORL-1 receptors.

Peptide and nonpeptide ligands for the nociceptin/orphanin FQ receptor ORL1: research tools and potential therapeutic agents

The 17-amino acid neuropeptide nociceptin/Orphanin FQ (N/OFQ) was recently identified as the endogenous ligand for the opioid receptor-like (ORL1) receptor, a fourth member of the classical mu, delta, and kappa opioid receptor family. Although ORL1 clearly belongs to the opioid receptor family, it does not bind classical opiates and the ORL1-N/OFQ system has pharmacological actions distinct from the opioid receptor system. This new ligand-receptor system has generated active interest in the opioid community because of its wide distribution and involvement in a myriad of neurological pathways. The past two years have witnessed tremendous advances in the design and discovery of very potent and selective peptide and nonpeptide agonist and antagonist ligands at ORL1. These discoveries have facilitated the understanding of the role of the ORL1-N/OFQ system in a variety of processes such as pain modulation, anxiety, food intake, learning, memory, neurotransmitter release, reward pathways, and tolerance development. The ORL1 receptor therefore represents a new molecular target for the design of novel agents for anxiety, analgesia, and drug addiction. Indeed, there is tremendous interest in the pharmaceutical industry in the development of nonpeptide ligands such as the potent ORL1 agonist, Ro 64-6198, as anxiolytics and the ORL1 antagonist JTC-801 as novel analgesics. This review presents an overview of the various peptide and nonpeptide ORL1 ligands with an emphasis on their potential therapeutic utility in various human disorders.

Nociceptin/orphanin FQ knockout mice display up-regulation of the opioid receptor-like 1 receptor and alterations in opioid receptor expression in the brain

The opioid receptor-like 1 receptor is a novel member of the opioid receptor family and its endogenous peptide ligand has been termed nociceptin and orphanin FQ. Activation of the opioid receptor-like 1 receptor by nociceptin/orphanin FQ in vivo produces hyperalgesia when this peptide is given supraspinally but analgesia at the spinal level. Nociceptin/orphanin FQ also reverses stress-induced analgesia, suggesting that the peptide has anti-opioid properties. Nociceptin/orphanin FQ knockout mice show alterations in pain sensitivity and stress responses and display increased morphine dependence, suggesting an interaction of the nociceptin/orphanin FQ system with classical opioid receptor function. To determine if the behavioural phenotype of nociceptin/orphanin FQ knockout mice reflects changes in either opioid receptor-like 1 or classical opioid receptor expression, we have carried out quantitative autoradiography of the opioid receptor-like 1, mu-, delta- and kappa-opioid receptors in the brains of these animals. Receptor density was measured on coronal sections from wild-type, heterozygous and homozygous mice using [(3)H]nociceptin, [(3)H][D-Ala(2)-N-methyl-Phe(4)-Gly(5) ol] enkephalin, [(3)H]deltorphin-I, or [(3)H](-)-N-methyl-N-[7-(1-pyrrodinyl)-1-oxospiro[4,5]dec-8-yl]-4-benzofuranacetamide to label opioid receptor-like 1, mu-, delta- and kappa-receptors, respectively. A region-specific up-regulation of the opioid receptor-like 1 receptor (up to 135%) was seen in brains from homozygous mice. Mu-Receptors also showed significant differences between genotypes whilst changes in delta- and kappa- receptors were minor. In conclusion the region-specific up-regulation of the opioid receptor-like 1 receptor indicates a tonic role for nociceptin/orphanin FQ in some brain structures and may suggest the peptide regulates the receptor expression in these regions. The changes in the opioid receptor-like 1 receptor may relate to the anxiogenic phenotype of these animals but the observed change in mu-receptors does not correlate with altered morphine responses.

Buprenorphine: an analgesic with an expanding role in the treatment of opioid addiction

Buprenorphine, a long-acting opioid with both agonist and antagonist properties, binds to mu-opioid (OP(3)), kappa-opioid (OP(2)), delta-opioid (OP(1)), and nociceptin (ORL-1) receptors. Its actions at these receptors have not been completely characterized, although buprenorphine is generally regarded as a mu-opioid receptor partial agonist and a kappa-opioid receptor antagonist. Its pharmacology is further complicated by an active metabolite, norbuprenorphine. Although buprenorphine can be used as an analgesic agent, it is of greater importance in the treatment of opioid abuse. Because of its partial agonist activity at mu-opioid receptors and its long half-life, buprenorphine has proven to be an excellent alternative to methadone for either maintenance therapy or detoxification of the opioid addict. Although buprenorphine may ultimately prove to be superior to methadone in the maintenance of the pregnant addict, its effects on the developing fetus must be carefully evaluated.

Inhibitory effect of nociceptin on somatostatin secretion of the isolated perfused rat stomach

The heptadecapeptide nociceptin/orphanin FQ (N/OFQ) has recently been isolated from porcine and rat brain and identified as the endogenous ligand of the N/OFQ receptor (NOP). It shows structural similarity with opioid peptides. N/OFQ has also been demonstrated in the gastrointestinal tract, where it inhibits gastrointestinal motility. The effect of N/OFQ on gastric neuroendocrine function is unknown as yet. In the isolated perfused rat stomach, N/OFQ 10(-6) M shows a small, but not significant decrease of basal somatostatin (SRIF) secretion. At the doses of 10(-12) M, 10(-10) and 10(-8) M N/OFQ has neither an effect on basal SRIF nor on basal vasoactive intestinal polypeptide (VIP), gastrin, substance P or bombesin secretion, respectively. However, gastric inhibitory polypeptide (GIP) 10(-9) M prestimulated SRIF secretion is significantly inhibited by N/OFQ 10(-8) M (-45+/-11%; p<0.05 vs. GIP). During concomitant infusion of the specific competitive NOP receptor antagonist [Nphe(1)]nociceptin(1-13)NH(2) 10(-6) M, the effect of N/OFQ is abolished (6+/-11%; p<0.05 vs. GIP and N/OFQ) while the opiate receptor antagonist naloxone 10(-6) M has no significant effect (-32+/-9%; ns vs. GIP and N/OFQ). At the higher concentration of N/OFQ 10(-6) M, the inhibition of prestimulated SRIF secretion (-58+/-6%; p<0.05 vs. GIP) is not influenced by the NOP receptor antagonist at the concentration of 10(-6) M (-49+/-9%; ns vs. GIP and N/OFQ) and 10(-5) M (-69+/-10%; ns vs. GIP and N/OFQ), respectively. On the other hand, infusion of naloxone 10(-6) M attenuates the inhibitory effect of N/OFQ 10(-6) M significantly (-21+/-6%; p<0.05 vs. GIP and N/OFQ).Thus, N/OFQ is an inhibitor of gastric somatostatin secretion. At the lower dose, this effect is transmitted via NOP receptors, while at the higher dose of 10(-6) M, the effect is at least in part mediated via opiate receptors.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Characterization of opiates, neuroleptics, and synthetic analogs at ORL1 and opioid receptors

Nociceptin/orphanin FQ (N/OFQ) was recently identified as the endogenous ligand for the opioid-receptor like (ORL1) receptor. Although the ORL1 receptor shows sequence homology with the opioid receptors, the nociceptin/ORL1 ligand-receptor system has very distinct pharmacological actions compared to the opioid receptor system. Recently, several small-molecule ORLI receptor ligands were reported by pharmaceutical companies. Most of these ligands had close structural similarities with known neuroleptics and opiates. In this study, we screened several available neuroleptics and opiates for their binding affinity and functional activity at ORL1 and the opioid receptors. We also synthesized several analogs of known opiates with modified piperidine N-substituents in order to characterize the ORL1 receptor ligand binding pocket. Substitution with the large, lipophilic cyclooctylmethyl moiety increased ORL1 receptor affinity and decreased mu receptor affinity and efficacy in the fentanyl series of ligands but had a different effect in the oripavine class of opiate ligands. Our results indicate that opiates and neuroleptics may be good starting points for ORL1 receptor ligand design, and the selectivity may be modulated by appropriate structural modifications.

Autoradiographic mapping of the opioid receptor-like 1 (ORL1) receptor in the brains of mu-, delta- or kappa-opioid receptor knockout mice

The opioid receptor-like 1 (ORL1) receptor shares a high degree of sequence homology with the classical mu-, delta- and kappa-opioid receptors and a functional mutual opposition between these receptors has been suggested. To further address this possible interaction we have used mu-, delta- and kappa-opioid receptor knockout mice to determine autoradiographically if there are any changes in the number or distribution of the ORL1 receptor, labelled with [(3)H]nociceptin, in the brains of mice deficient in each of the opioid receptors. An up-regulation of ORL1 expression was observed across all brain regions in delta-knockouts with cortical regions typically showing a 15-30% increase in binding that was most marked in heterozygous mice. In contrast, ORL1 receptor expression was down-regulated in virtually all brain structures in heterozygous kappa-knockouts although the magnitude of this change was not as great as for the delta-knockouts. No significant alterations in ORL1 receptor expression were observed across brain regions in mu-receptor knockout mice and there were no qualitative differences in ORL1 receptor expression in any groups. These data suggest there are interactions between the ORL1 system and the classical opioid receptors and that the interactions are receptor-specific. The greater differences observed in heterozygous mice suggest that these interactions might be most relevant when there is only partial loss of receptor function.