Nociceptin/orphanin FQ and drugs of abuse

Emerging pharmacological targets for alcohol use disorder

Alcohol Use Disorder (AUD) remains a challenging condition with limited effective treatment options; however new technology in drug delivery and advancements in pharmacology have paved the way for discovery of novel therapeutic targets. This review explores emerging pharmacological targets that offer new options for the management of AUD, focusing on the potential of somatostatin (SST), vasoactive intestinal peptide (VIP), glucagon-like peptide-1 (GLP-1), nociceptin (NOP), and neuropeptide S (NPS). These targets have been selected based on recent advancements in preclinical and clinical research, which suggest their significant roles in modulating alcohol consumption and related behaviors. SST dampens cortical circuits, and targeting both the SST neurons and the SST peptide itself presents promise for treating AUD and various related comorbidities. VIP neurons are modulated by alcohol and targeting the VIP system presents an unexplored avenue for addressing alcohol exposure at various stages of development. GLP-1 interacts with the dopaminergic reward system and reduces alcohol intake. Nociceptin modulates mesolimbic circuitry and agonism and antagonism of nociceptin receptor offers a complex but promising approach to reducing alcohol consumption. NPS stands out for its anxiolytic-like effects, particularly relevant for the anxiety associated with AUD. This review aims to synthesize the current understanding of these targets, highlighting their potential in developing more effective and personalized AUD therapies, and underscores the importance of continued research in identifying and validating novel targets for treatment of AUD and comorbid conditions.

Development of a genetically encoded sensor for probing endogenous nociceptin opioid peptide release

Nociceptin/orphanin-FQ (N/OFQ) is a recently appreciated critical opioid peptide with key regulatory functions in several central behavioral processes including motivation, stress, feeding, and sleep. The functional relevance of N/OFQ action in the mammalian brain remains unclear due to a lack of high-resolution approaches to detect this neuropeptide with appropriate spatial and temporal resolution. Here we develop and characterize NOPLight, a genetically encoded sensor that sensitively reports changes in endogenous N/OFQ release. We characterized the affinity, pharmacological profile, spectral properties, kinetics, ligand selectivity, and potential interaction with intracellular signal transducers of NOPLight in vitro. Its functionality was established in acute brain slices by exogeneous N/OFQ application and chemogenetic induction of endogenous N/OFQ release from PNOC neurons. In vivo studies with fibre photometry enabled direct recording of NOPLight binding to exogenous N/OFQ receptor ligands, as well as detection of endogenous N/OFQ release within the paranigral ventral tegmental area (pnVTA) during natural behaviors and chemogenetic activation of PNOC neurons. In summary, we show here that NOPLight can be used to detect N/OFQ opioid peptide signal dynamics in tissue and freely behaving animals.

Development of a genetically encoded sensor for probing endogenous nociceptin opioid peptide release

Nociceptin/orphanin-FQ (N/OFQ) is a recently appreciated critical opioid peptide with key regulatory functions in several central behavioral processes including motivation, stress, feeding, and sleep. The functional relevance of N/OFQ action in the mammalian brain remains unclear due to a lack of high-resolution approaches to detect this neuropeptide with appropriate spatial and temporal resolution. Here we develop and characterize NOPLight, a genetically encoded sensor that sensitively reports changes in endogenous N/OFQ release. We characterized the affinity, pharmacological profile, spectral properties, kinetics, ligand selectivity, and potential interaction with intracellular signal transducers of NOPLight in vitro. Its functionality was established in acute brain slices by exogeneous N/OFQ application and chemogenetic induction of endogenous N/OFQ release from PNOC neurons. In vivo studies with fibre photometry enabled direct recording of NOPLight binding to exogenous N/OFQ receptor ligands, as well as detection of endogenous N/OFQ release within the paranigral ventral tegmental area (pnVTA) during natural behaviors and chemogenetic activation of PNOC neurons. In summary, we show here that NOPLight can be used to detect N/OFQ opioid peptide signal dynamics in tissue and freely behaving animals.

Genetically selected alcohol-preferring msP rats to study alcohol use disorder: Anything lost in translation?

For several decades, genetically selected alcohol-preferring rats have been successfully used to mimic and study alcohol use disorders (AUD). These rat lines have been instrumental in advancing our understanding of the neurobiology of alcoholism and enabling pharmacological studies to evaluate drug efficacy on alcohol drinking and relapse. Moreover, the results of these studies have identified genetic variables that are linked to AUD vulnerability. This is an up-to-date review that focuses on genetically selected Marchigian Sardinian alcohol-preferring (msP) rats. To support the translational relevance of the findings that are obtained from msP rats and highlight important similarities to AUD patients, we also discuss the results of recent brain imaging studies. Finally, to demonstrate the importance of studying sex differences in animal models of AUD, we present original data that highlight behavioral differences in the response to alcohol in male and female rats. Female msP rats exhibited higher alcohol consumption compared with males. Furthermore, msP rats of both sexes exhibit higher anxiety- and depressive-like behaviors in the elevated plus maze and forced swim test, respectively, compared with unselected Wistar controls. Notably, voluntary alcohol drinking decreases foot-shock stress and depressive-like behavior in both sexes, whereas anxiety-like behavior in the elevated plus maze is attenuated only in males. These findings suggest that male and female msP rats both drink high amounts of alcohol to self-medicate negative affective symptoms. For females, this behavior may be driven by an attempt to treat stress and depressive-like conditions. For males, generalized anxiety appears to be an important additional factor in the motivation to drink alcohol. This article is part of the special issue on 'Vulnerabilities to Substance Abuse.'

Differential Modulation of Ventral Tegmental Area Circuits by the Nociceptin/Orphanin FQ System

The neuropeptide nociceptin/orphanin FQ (N/OFQ) can be released by stressors and is associated with disorders of emotion regulation and reward processing. N/OFQ and its receptor, NOP, are enriched in dopaminergic pathways, and intra-ventricular agonist delivery decreases dopamine levels in the dorsal striatum, nucleus accumbens (NAc), and ventral tegmental area (VTA). We used whole-cell electrophysiology in acute rat midbrain slices to investigate synaptic actions of N/OFQ. N/OFQ was primarily inhibitory, causing outward currents in both immunocytochemically identified dopaminergic (tyrosine hydroxylase positive (TH(+))) and non-dopaminergic (TH(-)) VTA neurons; effect at 1 μm: 20 ± 4 pA. Surprisingly, this effect was mediated by augmentation of postsynaptic GABAAR currents, unlike the substantia nigra pars compacta (SNc), where the N/OFQ-induced outward currents were K+ channel dependent. A smaller population, 17% of all VTA neurons, responded to low concentrations of N/OFQ with inward currents (10 nm: -11 ± 2 pA). Following 100 nm N/OFQ, the response to a second N/OFQ application was markedly diminished in VTA neurons (14 ± 10% of first response) but not in SNc neurons (90 ± 20% of first response). N/OFQ generated outward currents in medial prefrontal cortex (mPFC)-projecting VTA neurons, but inward currents in a subset of posterior anterior cingulate cortex (pACC)-projecting VTA neurons. While N/OFQ inhibited NAc-projecting VTA cell bodies, it had little effect on electrically or optogenetically evoked terminal dopamine release in the NAc measured ex vivo with fast scan cyclic voltammetry (FSCV). These results extend our understanding of the N/OFQ system in brainstem circuits implicated in many neurobehavioral disorders.

Pain, Motivation, Migraine, and the Microbiome: New Frontiers for Opioid Systems and Disease

For decades the broad role of opioids in addiction, neuropsychiatric disorders, and pain states has been somewhat well established. However, in recent years, with the rise of technological advances, not only is the existing dogma being challenged, but we are identifying new disease areas in which opioids play a critical role. This review highlights four new areas of exploration in the opioid field. The most recent addition to the opioid family, the nociceptin receptor system, shows promise as the missing link in understanding the neurocircuitry of motivation. It is well known that activation of the kappa opioid receptor system modulates negative affect and dysphoria, but recent studies now implicate the kappa opioid system in the modulation of negative affect associated with pain. Opioids are critical in pain management; however, the often-forgotten delta opioid receptor system has been identified as a novel therapeutic target for headache disorders and migraine. Lastly, changes to the gut microbiome have been shown to directly contribute to many of the symptoms of chronic opioid use and opioid related behaviors. This review summarizes the findings from each of these areas with an emphasis on identifying new therapeutic targets. SIGNIFICANCE STATEMENT: The focus of this minireview is to highlight new disease areas or new aspects of disease in which opioids have been implicated; this includes pain, motivation, migraine, and the microbiome. In some cases, this has resulted in the pursuit of a novel therapeutic target and resultant clinical trial. We believe this is very timely and will be a refreshing take on reading about opioids and disease.

Selective alterations in endogenous opioid system genes expression in rats selected for high ethanol intake during adolescence

Historically, the roots of alcoholism have been linked to either environment or heredity. However, the interaction between these factors is still largely unexplored. The evidence supports a link between alcohol consumption and the endogenous opioid system. We here studied the opioid genes expression in male and female Wistar rats derived from a short-term breeding program which selected -- at adolescence -- for high (ADHI line) or low (ADLO line) ethanol drinking. Specifically, in this work we analyzed central opioid gene expression in the rats of the second filial generation (S₂-ADLO and S₂-ADHI). Selective downregulation of pronociceptin (Pnoc) and its receptor (Oprl1) mRNA levels were observed in the prefrontal cortex of male S₂-ADHI rats when compared to S₂-ADLO, and for Oprl1 also in the nucleus accumbens. An increase in gene expression was instead observed for pro-opiomelanocortin (Pomc) in the nucleus accumbens of S₂-ADHI males when compared to S₂-ADLO, as well as for mu opioid receptor (Oprm1) but in females. The differences in mRNA levels may be due to the different alcohol consumption between the two groups of rats or may represent pre-existing differences between them. Moreover, we show a sex-specific modulation of the expression of these genes, thus pointing out the importance of sex on ethanol responses. The results might lead to more specific and effective pharmacological treatments for alcoholism.

The kisspeptin derivative kissorphin reduces the acquisition, expression, and reinstatement of ethanol-induced conditioned place preference in rats

Research has shown that opioids are involved in the rewarding effects of ethanol. Neuropeptide FF (NPFF) has been described as an anti-opioid peptide because, in many cases, it inhibits opioid and ethanol effects in rodents. Kissorphin (KSO) is a new peptide derived from kisspeptin-10 with structural similarities to NPFF. This peptide possesses NPFF-like biological activity in vitro. The aim of the current study was to investigate whether KSO (Tyr-Asn-Trp-Asn-Ser-Phe-NH2) influences the acquisition, expression, and reinstatement of ethanol-induced conditioned place preference (ethanol-CPP) in rats. The ethanol-CPP was established (conditioning for 5 days) by intraperitoneal (i.p.) administration of ethanol (1 g/kg, 20%, w/v) using an unbiased procedure. After that, one group of rats was used in final post-conditioning testing (expression of CPP) and the other group received a priming injection of ethanol after 10 days of extinction (reinstatement of CPP). Our experiments showed that KSO, given intravenously (i.v.) at the doses of 1, 3, and 10 nmol before every ethanol administration, inhibited the acquisition and, given acutely before the post-conditioning test or before the priming dose of ethanol, inhibited the expression and reinstatement of ethanol-CPP, respectively, in a dose-dependent manner. KSO given by itself neither induced place preference nor aversion and did not alter locomotor activity and coordination of rats. These results suggest that KSO can alter rewarding/motivational effects of ethanol. These data suggest this peptide possesses an anti-opioid character.

Regulation of the Genes Encoding the ppN/OFQ and NOP Receptor

Over the years, the ability of N/OFQ-NOP receptor system in modulating several physiological functions, including the release of neurotransmitters, anxiety-like behavior responses, modulation of the reward circuitry, inflammatory signaling, nociception, and motor function, has been examined in several brain regions and at spinal level. This chapter collects information related to the genes encoding the ppN/OFQ and NOP receptor, their regulation, and relative transcriptional control mechanisms. Furthermore, genetic manipulations, polymorphisms, and epigenetic alterations associated with different pathological conditions are discussed. The evidence here collected indicates that the study of ppN/OFQ and NOP receptor gene expression may offer novel opportunities in the field of personalized therapies and highlights this system as a good "druggable target" for different pathological conditions.

Nociceptin/orphanin FQ receptors modulate the discriminative stimulus effects of oxycodone in C57BL/6 mice

BACKGROUND

Nociceptin/orphanin FQ (NOP) receptor ligands have shown efficacy as putative analgesics and can modulate the abuse-related effects of opioids, suggesting therapeutic applications. The discriminative stimulus effects of a drug are related to their subjective effects, a predictor of abuse potential. To determine whether activation of NOP receptors could alter the subjective effects of an abused opioid analgesic, a novel oxycodone discrimination was established in mice, characterized with positive and negative controls, and its expression evaluated with a NOP receptor agonist.

METHODS

Adult male C57BL/6 mice were trained to discriminate 1.3 mg/kg oxycodone from vehicle in a two-lever operant procedure. The discrimination was characterized with naloxone challenge, and generalization tests with the μ-opioid receptor agonists, heroin and morphine, and the κ-opioid receptor selective agonist, U50488. Subsequently, effects of the NOP agonist Ro64-6198 were evaluated with and without oxycodone.

RESULTS

Oxycodone generalization occurred in a dose-dependent manner and was reversed by naloxone pretreatment. Heroin and morphine, but not U50488, substituted for oxycodone. Co-treatment of 1 mg/kg Ro64-6198 with the oxycodone training dose reduced % oxycodone lever responding (%OLR) and restored response rates to vehicle control levels. J-113397, a NOP antagonist, reversed these effects. Co-administration of 1 mg/kg Ro64-6198 with a range of oxycodone doses resulted in rightward dose-effect curve shifts in %OLR and response rates compared to oxycodone alone.

CONCLUSIONS

These results provide additional evidence that NOP receptor activation can modulate the subjective effects of opioid analgesics and represent the first characterization of oxycodone's discriminative stimulus effects in mice.

The Nociceptin Receptor (NOP) Agonist AT-312 Blocks Acquisition of Morphine- and Cocaine-Induced Conditioned Place Preference in Mice

Treatment of drug addiction remains an unmet medical need due to the dearth of approved pharmacotherapies. There are no approved treatments for cocaine addiction, whereas the current opioid crisis has revealed the stark reality of the limited options to treat prescription and illicit opioid abuse. Preclinical studies in rodents and nonhuman primates have shown that orphanin FQ/nociceptin (N/OFQ), the endogenous ligand for the nociceptin opioid receptor (NOP) reduces the rewarding effects of several abused substances, including opioids, psychostimulants and alcohol. A few nonpeptide small-molecule NOP agonists have also shown efficacy in attenuating the rewarding effects of various abused drugs. We previously demonstrated that a high affinity small-molecule NOP agonist AT-312 selectively reduced the rewarding effects of ethanol in the conditioned place preference paradigm in mice. In the present study, we examined if AT-312 (3 mg/kg, i.p. or s.c. respectively), would alter the rewarding action of morphine (7.5 mg/kg, s.c.) or cocaine (15 mg/kg, i.p.). The effect of AT-312 on morphine- and cocaine-induced motor stimulation was also assessed on the conditioning days. The role of the NOP receptor in the effects of AT-312 was further confirmed by conducting the place conditioning experiments in NOP knockout mice and compared to their wild-type controls. Our results showed that AT-312 significantly reduced the acquisition of morphine and cocaine CPP in wild-type mice but not in mice lacking NOP receptors. AT-312 also suppressed morphine-induced and completely abolished cocaine-induced motor stimulation in NOP wild-type mice, but not in NOP knockout mice. These results show that small-molecule NOP receptor agonists have promising efficacy for attenuating the rewarding effects of morphine and cocaine, and may have potential as pharmacotherapy for opioid and psychostimulant addiction or for treating polydrug addiction.

Genetic Deletion of the Nociceptin/Orphanin FQ Receptor in the Rat Confers Resilience to the Development of Drug Addiction

The nociceptin (NOP) receptor is a G-protein-coupled receptor whose natural ligand is the NOP/orphanin FQ (N/OFQ) peptide. Evidence from pharmacological studies suggests that the N/OFQ system is implicated in the regulation of several addiction-related phenomena, such as drug intake, withdrawal, and relapse. Here, to further explore the role of NOP system in addiction, we used NOP (-/-) rats to study the motivation for cocaine, heroin, and alcohol self-administration in the absence of N/OFQ function. Conditioned place preference (CPP) and saccharin (0.2% w/v) self-administration were also investigated. Results showed that NOP (-/-) rats self-administer less cocaine (0.25, 0.125, or 0.5 mg/infusion) both under a fixed ratio 1 and a progressive ratio schedule of reinforcement compared with wild-type (Wt) controls. Consistently, cocaine (10 mg/kg, i.p.) was able to induce CPP in Wt but not in NOP (-/-). When NOP (-/-) rats were tested for heroin (20 μg/infusion) and ethanol (10% v/v) self-administration, they showed significantly lower drug intake compared with Wt. Conversely, saccharin self-administration was not affected by NOP deletion, excluding the possibility of nonspecific learning deficits or generalized disruption of reward mechanisms in NOP (-/-) rats. These findings were confirmed with pharmacological experiments using two selective NOP antagonists, SB-612111 and LY2817412. Both drugs attenuated alcohol self-administration in Wt rats but not in NOP (-/-) rats. In conclusion, our results demonstrate that genetic deletion of NOP receptors confers resilience to drug abuse and support a role for NOP receptor antagonism as a potential treatment option for drug addiction.

Evaluation of the Tolerability of Switching Patients on Chronic Full μ-Opioid Agonist Therapy to Buccal Buprenorphine

Objective Assess whether patients with chronic pain receiving 80 to 220 mg oral morphine sulfate equivalent of a full μ-opioid agonist could be transitioned to buccal buprenorphine at approximately 50% of their full dose without inducing opioid withdrawal or sacrificing analgesic efficacy.

Methods. A randomized, double-blind, double-dummy, active-controlled, two-period crossover study in adult patients receiving around-the-clock full opioid agonist therapy and confirmed to be opioid dependent by naloxone challenge. Study doses were substituted at the time of the regular dose schedule for each patient. The primary endpoint was the proportion of patients with a maximum Clinical Opiate Withdrawal Scale score ≥ 13 (moderate withdrawal) or use of rescue medication.

Results. 35 subjects on ≥ 80 mg morphine sulfate equivalent per day were evaluable for opioid withdrawal. One patient during buccal buprenorphine treatment and two during 50% full μ-opioid agonist treatment experienced opioid withdrawal of at least moderate intensity. The mean maximum Clinical Opiate Withdrawal Scale scores were similar, and numerically lower on buccal buprenorphine. There were no significant differences in pain ratings between treatments. The most frequent adverse events with buccal buprenorphine were headache (19%), vomiting (13%), nausea, diarrhea, and drug withdrawal syndrome (each 9%), and with full μ-opioid agonist were headache (16%), drug withdrawal syndrome (13%), and nausea (6%).

Conclusions. Chronic pain patients treated with around-the-clock full μ-opioid agonist therapy can be switched to buccal buprenorphine (a partial μ-opioid agonist) at approximately 50% of the full μ-opioid agonist dose without an increased risk of opioid withdrawal or loss of pain control.

Efficacy and tolerability of buccal buprenorphine in opioid-naive patients with moderate to severe chronic low back pain

OBJECTIVES

Buprenorphine HCl buccal film has been developed for treating chronic pain utilizing BioErodible MucoAdhesive (BEMA(®)) delivery technology. Buccal buprenorphine (BBUP; Belbuca(TM), Endo Pharmaceuticals) was evaluated for the management of moderate to severe chronic low back pain (CLBP) requiring around-the-clock analgesia in a multicenter, double-blind, placebo-controlled, enriched-enrollment, randomized-withdrawal study in opioid-naive patients.

METHODS

Patients (n = 749) were titrated to a dose of BBUP (range, 150-450 µg every 12 h) that was generally well tolerated and provided adequate analgesia for ≥14 days, and then randomized to BBUP (n = 229) or placebo (n = 232), respectively. The primary efficacy variable was the change from baseline to week 12 of double-blind treatment in the mean of daily average pain intensity scores (numeric rating scale from 0 [no pain] to 10 [worst pain imaginable]).

RESULTS

Patients were experiencing moderate to severe pain at study entry: mean (SD) = 7.15 (1.05). Following titration, pain was reduced to the mild range; 2.81 (1.07). After randomization, mean (SD) pain scores increased from baseline to week 12 more with placebo (1.59 [2.04]) versus BBUP: (0.94 [1.85]) with a significant between-group difference (-0.67 [95% CI: -1.07 to -0.26]; p = 0.0012). A significantly larger percentage of patients receiving BBUP versus placebo had ≥30% pain reduction (63% vs 47%; p = 0.0012). During double-blind treatment, the most frequent adverse events (AEs) with BBUP were nausea (10%), constipation (4%) and vomiting (4%). The most common AEs with placebo were nausea (7%), upper respiratory tract infection (4%), headache (3%) and diarrhea (3%).

CONCLUSIONS

These findings demonstrate the efficacy and tolerability of BBUP among opioid-naive patients requiring around-the-clock opioid treatment for CLBP.

Synthesis and biological activity of small peptides as NOP and opioid receptors' ligands: view on current developments

The heptadecapeptide nociceptin, also called orphanin FQ (N/OFQ), is the endogenous agonist of the N/OFQ peptide receptor (NOP receptor) and is involved in several central nervous system pathways, such as nociception, reward, tolerance, and feeding. The discovery of small molecule ligands for NOP is being actively pursued for several therapeutic applications. This review presents overview of the several recently reported NOP ligands (agonists and antagonists), with an emphasis of the structural features that may be important for modulating the intrinsic activity of these ligands. In addition, a brief account on the characterization of newly synthesized ligands of NOP receptor with aminophosphonate moiety and β-tryptophan analogues will be presented.

Endogenous nociceptin system involvement in stress responses and anxiety behavior

The mechanisms underpinning stress-related behavior and dysfunctional events leading to the expression of neuropsychiatric disorders remain incompletely understood. Novel candidates involved in the neuromodulation of stress, mediated both peripherally and centrally, provide opportunities for improved understanding of the neurobiological basis of stress disorders and may represent targets for novel therapeutic development. This chapter provides an overview of the mechanisms by which the opioid-related peptide, nociceptin, regulates the neuroendocrine stress response and stress-related behavior. In our research, we have employed nociceptin receptor antagonists to investigate endogenous nociceptin function in tonic control over stress-induced activity of the hypothalamo-pituitary-adrenal axis. Nociceptin demonstrates a wide range of functions, including modulation of psychological and inflammatory stress responses, modulation of neurotransmitter release, immune homeostasis, in addition to anxiety and cognitive behaviors. Greater appreciation of the complexity of limbic-hypothalamic neuronal networks, together with attention toward gender differences and the roles of steroid hormones, provides an opportunity for deeper understanding of the importance of the nociceptin system in the context of the neurobiology of stress and behavior.

Chronic treatment with novel brain-penetrating selective NOP receptor agonist MT-7716 reduces alcohol drinking and seeking in the rat

Since its discovery, the nociceptin/orphanin FQ (N/OFQ)-NOP receptor system has been extensively investigated as a promising target to treat alcoholism. Encouraging results obtained with the endogenous ligand N/OFQ stimulated research towards the development of novel brain-penetrating NOP receptor agonists with a pharmacological and toxicological profile compatible with clinical development. Here we describe the biochemical and alcohol-related behavioral effects of the novel NOP receptor agonist MT-7716. MT-7716 has high affinity for human NOP receptors expressed in HEK293 cells with a Ki value of 0.21 nM. MT-7716 concentration-dependently stimulated GTPγ(35)S binding with an EC50 value of 0.30 nM and its efficacy was similar to N/OFQ, suggesting that MT7716 is a full agonist at NOP receptors. In the two bottle choice test MT-7716 (0, 0.3, 1, and 3 mg/kg, bid) given orally for 14 days dose-dependently decreased voluntary alcohol intake in Marchigian Sardinian rats. The effect became gradually stronger following repeated administration, and was still significant 1 week after discontinuation of the drug. Oral naltrexone (30 mg/kg, bid) for 14 days also reduced ethanol intake; however, the effect decreased over the treatment period and rapidly disappeared when drug treatment was discontinued. MT-7716 is also effective for preventing reinstatement caused by both ethanol-associated environmental stimuli and stress. Finally, to investigate the effect of MT-7716 on alcohol withdrawal symptoms, Wistar rats were withdrawn from a 7-day alcohol liquid diet. MT-7716 significantly attenuated somatic alcohol withdrawal symptoms. Together these findings indicate that MT-7716 is a promising candidate for alcoholism treatment remaining effective with chronic administration.

Nociceptin/orphanin FQ decreases glutamate transmission and blocks ethanol-induced effects in the central amygdala of naive and ethanol-dependent rats

The central nucleus of the amygdala (CeA) mediates several addiction-related processes and nociceptin/orphanin FQ (nociceptin) regulates ethanol intake and anxiety-like behaviors. Glutamatergic synapses, in the CeA and throughout the brain, are very sensitive to ethanol and contribute to alcohol reinforcement, tolerance, and dependence. Previously, we reported that in the rat CeA, acute and chronic ethanol exposures significantly decrease glutamate transmission by both pre- and postsynaptic actions. In this study, using electrophysiological techniques in an in vitro CeA slice preparation, we investigated the effects of nociceptin on glutamatergic transmission and its interaction with acute ethanol in naive and ethanol-dependent rats. We found that nociceptin (100-1000 nM) diminished basal-evoked compound glutamatergic receptor-mediated excitatory postsynaptic potentials (EPSPs) and spontaneous and miniature EPSCs (s/mEPSCs) by mainly decreasing glutamate release in the CeA of naive rats. Notably, nociceptin blocked the inhibition induced by acute ethanol (44 mM) and ethanol blocked the nociceptin-induced inhibition of evoked EPSPs in CeA neurons of naive rats. In neurons from chronic ethanol-treated (ethanol-dependent) rats, the nociceptin-induced inhibition of evoked EPSP amplitude was not significantly different from that in naive rats. Application of [Nphe1]Nociceptin(1-13)NH2, a nociceptin receptor (NOP) antagonist, revealed tonic inhibitory activity of NOP on evoked CeA glutamatergic transmission only in ethanol-dependent rats. The antagonist also blocked nociceptin-induced decreases in glutamatergic responses, but did not affect ethanol-induced decreases in evoked EPSP amplitude. Taken together, these studies implicate a potential role for the nociceptin system in regulating glutamatergic transmission and a complex interaction with ethanol at CeA glutamatergic synapses.

MT-7716, a novel selective nonpeptidergic NOP receptor agonist, effectively blocks ethanol-induced increase in GABAergic transmission in the rat central amygdala

The GABAergic system in the central amygdala (CeA) plays a major role in ethanol dependence and the anxiogenic-like response to ethanol withdrawal. A large body of evidence shows that Nociceptin/Orphanin FQ (N/OFQ) regulates ethanol intake and anxiety-like behavior. In the rat, ethanol significantly augments CeA GABA release, whereas N/OFQ diminishes it. Using electrophysiological techniques in an in vitro slice preparation, in this study we investigated the effects of a nonpeptidergic NOP receptor agonist, MT-7716 [(R)-2-3-[1-(Acenaphthen-1-yl)piperidin-4-yl]-2-oxo-2,3-dihydro-1H-benzimidazol-1-yl-N-methylacetamide hydrochloride hydrate], and its interaction with ethanol on GABAergic transmission in CeA slices of naïve rats. We found that MT-7716 dose-dependently (100-1000 nM) diminished evoked GABAA receptor-mediated inhibitory postsynaptic potentials (IPSPs) and increased paired-pulse facilitation (PPF) ratio of these evoked IPSPs, suggesting a presynaptic site of action of the MT-7716 by decreasing GABA release at CeA synapses. The presynaptic action of MT-7716 was also supported by the significant decrease in the frequency of miniature inhibitory postsynaptic currents (mIPSCs) induced by the nociceptin receptor (NOP) agonist. Interestingly, MT-7716 prevented the ethanol-induced augmentation of evoked IPSPs. A putative selective NOP antagonist, [Nphe1]Nociceptin(1-13)NH2, totally prevented the MT-7716-induced inhibition of IPSP amplitudes indicating that MT-7716 exerts its effect through NOPs. These data provide support for an interaction between the nociceptin and GABAergic systems in the CeA and for the anti-alcohol properties of the NOP activation. The development of a synthetic nonpeptidergic NOP receptor agonist such as MT-7716 may represent a useful therapeutic target for alcoholism.

Caloric restriction increases the sensitivity to the hyperphagic effect of nociceptin/orphanin FQ limiting its ability to reduce binge eating in female rats

RATIONALE

Nociceptin/orphanin FQ (N/OFQ) is a functional antagonist of corticotrophin-releasing factor, the main mediator of the stress response. Stress represents a key determinant of binge eating (BE) for highly palatable food (HPF).

OBJECTIVES

In relation to the antistress properties of N/OFQ, we evaluated its effect on BE. After the observation that episodes of food restriction increase the sensitivity to its hyperphagic effects, the function of NOP receptor and N/OFQ was investigated after cycles of food restrictions.

MATERIALS AND METHODS

In BE experiments, four groups were used: rats fed normally and not stressed or stressed, rats exposed to cycles of restriction/refeeding and then stressed, or not stressed. In the other experiments, two groups were used: rats exposed or not to food restriction.

RESULTS

Only restricted and stressed rats exhibited BE for HPF (containing chocolate cream). Intracerebroventricular injections of N/OFQ of 0.5 nmol/rat significantly reduced BE. N/OFQ 1 nmol/rat did not reduce BE but significantly increased HPF intake following food restrictions. Cycles of food restriction increased animals' sensitivity to the hyperphagic effect of N/OFQ for HPF. In situ hybridization studies following food restrictions showed decreased ppN/OFQ mRNA expression in the bed nucleus of the stria terminalis and increased expression of ppN/OFQ and NOP receptor mRNA in the ventral tegmental area and in the ventromedial hypothalamus, respectively.

CONCLUSIONS

These findings indicate that N/OFQ slightly reduces BE at low doses, while higher doses increase HPF intake, due to increased sensitivity to its hyperphagic effect following a history of caloric restrictions.

Nociceptin induces hypophagia in the perifornical and lateral hypothalamic area

Nociceptin/orphanin FQ (N/OFQ) is known to induce food intake when administered into the lateral ventricle or certain brain areas. This is somewhat contradictory to its reward-suppressing role, as food is a strong rewarding stimulus. This discrepancy may be due to the functional diversity of N/OFQ's target brain areas. N/OFQ has been shown to inhibit orexin and melanin-concentrating hormone (MCH) neurons, both of which are appetite-inducing cells. As the expression of these neurons is largely confined to the lateral hypothalamus/perifornical area (LH/PFA), we hypothesized that N/OFQ inhibits food intake by acting in this area. To test this hypothesis, we examined the effect of local N/OFQ infusion within the LH/PFA on food intake in the rat and found that N/OFQ decreased sugar pellet as well as chow intake. This effect was not seen when the injection site was outside of the LH/PFA, suggesting a site-specific effect. Next, to determine a possible cellular mechanism of N/OFQ action on food intake, whole cell patch clamp recordings were performed on rat orexin neurons. As previously reported in mice, N/OFQ induced a strong and long lasting hyperpolarization. Pharmacological study indicated that N/OFQ directly inhibited orexin neurons by activating ATP-sensitive potassium (KATP) channels. This effect was partially but significantly attenuated by the inhibitors of PI3K, PKC and PKA, suggesting that the N/OFQ signaling is mediated by these protein kinases. In summary, our results demonstrate a KATP channel-dependent N/OFQ signaling and that N/OFQ is a site-specific anorexic peptide.

Nociceptin/orphanin FQ blockade of corticotropin-releasing factor-induced gamma-aminobutyric acid release in central amygdala is enhanced after chronic ethanol exposure

BACKGROUND

The central nucleus of the amygdala (CeA) mediates stress- and addiction-related processes. Corticotropin-releasing factor (CRF) and nociceptin/orphanin FQ (nociceptin) regulate ethanol intake and anxiety-like behavior. In the rat, CRF and ethanol significantly augment CeA gamma-aminobutyric acid (GABA) release, whereas nociceptin diminishes it.

METHODS

Using electrophysiologic techniques in an in vitro slice preparation, we investigated the interaction of nociceptin and CRF on evoked and spontaneous GABAergic transmission in CeA slices of naive and ethanol-dependent rats and the mechanistic role of protein kinase A.

RESULTS

In neurons from naive animals, nociceptin dose-dependently diminished basal-evoked GABA(A) receptor-mediated inhibitory postsynaptic potentials (IPSPs) by decreasing GABA release and prevented, as well as reversed, CRF-induced augmentation of IPSPs, actions that required PKA signaling. In neurons from ethanol-dependent animals, nociceptin decreased basal GABAergic transmission and blocked the CRF-induced increase in GABA release to a greater extent than in naive controls.

CONCLUSIONS

These data provide new evidence for an interaction between the nociceptin and CRF systems in the CeA. Nociceptin opposes CRF effects on CeA GABAergic transmission with sensitization of this effect in dependent animals. These properties of nociceptin may underlie its anti-alcohol and anxiolytic properties and identify the nociceptin receptor as a useful therapeutic target for alcoholism.

Effects of the NOP receptor agonist Ro65-6570 on the acquisition of opiate- and psychostimulant-induced conditioned place preference in rats

Activation of the Nociceptin/Orphanin FQ (NOP) receptor may have anti-abuse effects. The present study examined the consequence of NOP receptor activation on the rewarding effect of opiates and psychostimulants in the conditioned place preference task in rats. First, the motivational effect of the NOP receptor agonists Ro64-6198 (0.316-3.16 mg/kg i.p.) and Ro65-6570 (1-10mg/kg i.p.) when administered alone, was assessed. Ro65-6570 was selected for further drug combination studies since, unlike Ro64-6198, it was devoid of an intrinsic motivational effect. Next, the minimal effective dose to induce reward for the opiates heroin (0.1-3.16 mg/kg i.p.), morphine (1-10mg/kg i.p.), hydrocodone (0.316-10mg/kg i.p.), tilidine (1-31.6 mg/kg i.p.), hydromorphone (0.1-10mg/kg i.p.), and oxycodone (0.0316-10mg/kg i.p.), as well as for the psychostimulants cocaine (3.16-31.6 mg/kg i.p.) and dexamphetamine (0.316-3.16 mg/kg i.p.) in combination with Ro 65-6570 (0 or 3.16 mg/kg i.p.) was determined. All drugs produced conditioned place preference, and for opiates and cocaine, but not for dexamphetamine, the minimal effective dose was higher when combined with Ro65-6570 (3.16 mg/kg i.p.). Attenuation of the rewarding effect of tilidine (3.16 mg/kg i.p.) and oxycodone (1mg/kg i.p.) by Ro65-6570 (3.16 mg/kg i.p.) could be reversed by pre-treatment with the NOP receptor antagonist J-113397 (4.64 mg/kg i.p.), suggesting that the attenuating effect of Ro65-6570 on opiates is due to activation of the NOP receptor. Taken together, the present study suggests that activation of NOP receptors effectively attenuates the rewarding effect of opiates, but may be less effective in reducing psychostimulant-induced reward.

UFP-112 a potent and long-lasting agonist selective for the Nociceptin/Orphanin FQ receptor

Nociceptin/orphanin FQ (N/OFQ) controls several biological functions via selective activation of the N/OFQ peptide receptor (NOP). [(pF)Phe(4) Aib(7) Arg(14) Lys(15) ]N/OFQ-NH(2) (UFP-112) is an NOP receptor ligand designed using a combination of several chemical modifications in the same peptide sequence that increase NOP receptor affinity/potency and/or reduce susceptibility to enzymatic degradation. In the present review article, we summarize data from the literature and present original findings on the in vitro and in vivo pharmacological features of UFP-112. Moreover, important biological actions and possible therapeutic indications of NOP receptor agonists are discussed based on the results obtained with UFP-112 and compared with other peptide and nonpeptide NOP receptor ligands.

Recently patented and promising ORL-1 ligands: where have we been and where are we going?

IMPORTANCE OF THE FIELD

The interactions of nociceptin/orphanin FQ (N/OFQ) and the opioid receptor-like receptor 1 (nociceptin opioid peptide--NOP) have been implicated in a variety of systems including cardiovascular, respiratory, immune, and the central and peripheral nervous systems.

AREAS COVERED IN THIS REVIEW

To elucidate the endogenous role of the N/OFQ-NOP system through the use of knockout and knockdown animal preparations, though most advances have been made using a host of synthetic agonists and antagonists. This review gives a brief history of the receptor-ligand discovery, the development of these agonists and antagonists within the last 10 years as published, and the therapeutic indications thereof focusing on pain.

WHAT THE READER WILL GAIN

The use of NOP ligands in pain has been controversial at best; however, there are indications that both agonists and antagonists have a place in the clinical setting for acute and chronic pain. NOP ligands have potential as novel therapeutics, interestingly, when incorporated into a rationally-designed multi-target agent.

TAKE HOME MESSAGE

The discovery of N/OFQ and NOP opened a new option for the treatment of pain with the potential for a decreased side effect profile. Numerous compounds have been designed to target this system, the most promising of which have mixed profiles.

Evidence for association of two variants of the nociceptin/orphanin FQ receptor gene OPRL1 with vulnerability to develop opiate addiction in Caucasians

OBJECTIVES

The OPRL1 gene encodes the nociceptin/orphanin FQ receptor, which plays a role in regulating tolerance and behavioral responses to morphine. However, there is limited information on whether variants of OPRL1 are associated with vulnerability to develop opiate addiction. In this study, we examined five variants of OPRL1 and their role in determining vulnerability to develop opiate addiction.

METHODS

We recruited 447 individuals: 271 former severe heroin addicts and 176 healthy controls. Using a 5'-fluorogenic exonuclease assay, we genotyped individuals at five variants in OPRL1. It was then determined whether there was a significant association of allele, genotype, or haplotype frequency with vulnerability to develop opiate addiction.

RESULTS

When the cohort was stratified by ethnicity, we found that, in Caucasians but not in African-Americans or Hispanics, the allele frequency of rs6090041 and rs6090043 were significantly associated point-wise with opiate addiction (P=0.03 and 0.04, respectively). Of the haplotypes formed by these two variants, one haplotype was found to be associated with protection from developing opiate addiction in both African-Americans (point-wise P=0.04) and Caucasians (point-wise P=0.04), and another haplotype with vulnerability to develop opiate addiction in Caucasians only (P=0.020).

CONCLUSION

This study provides evidence for an association of two variants of the OPRL1 gene, rs6090041 and rs6090043, with vulnerability to develop opiate addiction, suggesting a role for nociceptin/orphanin FQ receptor in the development of opiate addiction.

Role of CRF and other neuropeptides in stress-induced reinstatement of drug seeking

A central problem in the treatment of drug addiction is high rates of relapse to drug use after periods of forced or self-imposed abstinence. This relapse is often provoked by exposure to stress. Stress-induced relapse to drug seeking can be modeled in laboratory animals using a reinstatement procedure. In this procedure, drug-taking behaviors are extinguished and then reinstated by acute exposure to stressors like intermittent unpredictable footshock, restraint, food deprivation, and systemic injections of yohimbine, an alpha-2 adrenoceptor antagonist that induces stress-like responses in humans and nonhumans. For this special issue entitled "The role of neuropeptides in stress and addiction", we review results from studies on the role of corticotropin-releasing factor (CRF) and several other peptides in stress-induced reinstatement of drug seeking in laboratory animals. The results of the studies reviewed indicate that extrahypothalamic CRF plays a critical role in stress-induced reinstatement of drug seeking; this role is largely independent of drug class, experimental procedure, and type of stressor. There is also limited evidence for the role of dynorphins, hypocretins (orexins), nociceptin (orphanin FQ), and leptin in stress-induced reinstatement of drug seeking.

Complete knockout of the nociceptin/orphanin FQ receptor in the rat does not induce compensatory changes in mu, delta and kappa opioid receptors

The nociceptin/orphanin FQ (N/OFQ) opioid peptide receptor (NOPr) is a new member of the opioid receptor family consisting of mu, delta and kappa opioid receptors. The anti-opioid properties of its endogenous ligand, N/OFQ provide the receptor interesting potentials in symptoms and processes related to drug addiction, learning and memory, anxiety and depression, and nociception. Using target-selected N-ethyl-N-nitrosourea (ENU)-driven mutagenesis we recently generated a rat model bearing a premature stop codon in the opioid-like receptor (oprl1) gene, and here we describe the primary characterization of this novel model. Data revealed that [(3)H]N/OFQ binding to brain slices was completely absent in rats homozygous for the premature stop codon (oprl1(-/-)). Heterozygous rats displayed an intermediate level of NOPr binding. Oprl1 receptor transcript levels, as determined by Northern blot analysis, were reduced by approximately 50% in oprl1(-/-) rats compared to wild-type controls (oprl1(+/+)), and no alternative spliced transcripts were observed. Quantitative autoradiographic mapping of mu, delta and kappa opioid receptors using [(3)H]DAMGO, [(3)H]deltorphin and [(3)H]CI-977, respectively, did not show any changes in opioid receptor binding. In conclusion, we present a novel mutant rat lacking NOPr without compensatory changes in mu, delta and kappa opioid receptors. We anticipate that this mutant rat will have heuristic value to further understand the function of NOPr.

Development of pharmacotherapies for drug addiction: a Rosetta stone approach

Current pharmacotherapies for addiction represent opportunities for facilitating treatment and are forming a foundation for evaluating new medications. Furthermore, validated animal models of addiction and a surge in understanding of neurocircuitry and neuropharmacological mechanisms involved in the development and maintenance of addiction - such as the neuroadaptive changes that account for the transition to dependence and the vulnerability to relapse - have provided numerous potential therapeutic targets. Here, we emphasize a 'Rosetta Stone approach', whereby existing pharmacotherapies for addiction are used to validate and improve animal and human laboratory models to identify viable new treatment candidates. This approach will promote translational research and provide a heuristic framework for developing efficient and effective pharmacotherapies for addiction.

Activation of nociceptin/orphanin FQ-NOP receptor system inhibits tyrosine hydroxylase phosphorylation, dopamine synthesis, and dopamine D(1) receptor signaling in rat nucleus accumbens and dorsal striatum

Nociceptin/orphanin FQ (N/OFQ) has been reported to inhibit dopamine (DA) release in basal ganglia mainly by acting on NOP receptors in substantia nigra and ventral tegmental area. We investigated whether N/OFQ could affect DA transmission by acting at either DA nerve endings or DA-targeted post-synaptic neurons. In synaptosomes of rat nucleus accumbens and striatum N/OFQ inhibited DA synthesis and tyrosine hydroxylase (TH) phosphorylation at Ser40 via NOP receptors coupled to inhibition of the cAMP/protein kinase A pathway. Immunofluorescence studies showed that N/OFQ preferentially inhibited phospho-Ser40-TH in nucleus accumbens shell and that in this subregion NOP receptors partly colocalized with either TH or DA D(1) receptor positive structures. In accumbens and striatum N/OFQ inhibited DA D(1) receptor-stimulated cAMP formation, but failed to affect either adenosine A(2A) or DA D(2) receptor regulation of cAMP. In accumbens slices, N/OFQ inhibited DA D(1)-induced phosphorylation of NMDA and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate glutamate receptors, whereas in primary cultures of accumbal cells, which were found to coexpress NOP and DA D(1) receptors, N/OFQ curtailed DA D(1) receptor-induced cAMP-response element-binding protein phosphorylation. Thus, in accumbens and striatum N/OFQ exerts an inhibitory constraint on DA transmission by acting on either pre-synaptic NOP receptors inhibiting TH phosphorylation and DA synthesis or post-synaptic NOP receptors selectively down-regulating DA D(1) receptor signaling.

Dysregulation of nociceptin/orphanin FQ activity in the amygdala is linked to excessive alcohol drinking in the rat

BACKGROUND

Alcoholism is a complex behavioral disorder in which interactions between stressful life events and heritable susceptibility factors contribute to the initiation and progression of disease. Neural substrates of these interactions remain largely unknown. Here, we examined the role of the nociceptin/orphanin FQ (N/OFQ) system, with an animal model in which genetic selection for high alcohol preference has led to co-segregation of elevated behavioral sensitivity to stress (Marchigian Sardinian alcohol-preferring [msP]).

METHODS

The msP and Wistar rats trained to self-administer alcohol received central injections of N/OFQ. In situ hybridization and receptor binding assays were also performed to evaluate N/OFQ receptor (NOP) function in naïve msP and Wistar rats.

RESULTS

Intracerebroventricular (ICV) injection of N/OFQ significantly inhibited alcohol self-administration in msP but not in nonselected Wistar rats. The NOP receptor messenger RNA expression and binding was upregulated across most brain regions in msP compared with Wistar rats. However, in msP rats [(35)S]GTPgammaS binding revealed a selective impairment of NOP receptor signaling in the central amygdala (CeA). Ethanol self-administration in msP rats was suppressed after N/OFQ microinjection into the CeA but not into the bed nucleus of the stria terminalis or the basolateral amygdala.

CONCLUSIONS

These findings indicate that dysregulation of N/OFQ-NOP receptor signaling in the CeA contributes to excessive alcohol intake in msP rats and that this phenotype can be rescued by local administration of pharmacological doses of exogenous N/OFQ. Data are interpreted on the basis of the anti-corticotropin releasing factor (CRF) actions of N/OFQ and the significance of the CRF system in promoting excessive alcohol drinking in msP rats.

Association analysis of genes encoding the nociceptin receptor (OPRL1) and its endogenous ligand (PNOC) with alcohol or illicit drug dependence

Recent studies in animal models have shown that the nociceptin system, comprising nociceptin (or OFQ/N, encoded by PNOC) and the nociceptin receptor (an opioid receptor-like protein encoded by OPRL1), may be involved in alcohol and other drug reward pathways. To determine whether the nociceptin system is associated with alcohol or illicit drug dependence in humans, we analyzed 10 single nucleotide polymorphisms (SNPs) in OPRL1 and 15 SNPs in PNOC in a sample of 1923 European Americans from 219 multiplex alcohol dependent families ascertained by the Collaborative Study on the Genetics of Alcoholism. The SNPs spanned both genes and several kb of their flanking sequences, and were in high linkage disequilibrium. Neither gene was associated with alcohol or illicit drug dependence, although two SNPs in PNOC showed marginal association with alcoholism and one with illicit drug dependence (P = 0.04-0.05). Secondary analyses suggested that two adjacent SNPs in intron 1 of OPRL1 were marginally associated with opioid dependence (P = 0.05); none of the SNPs in PNOC were associated with opioid dependence.

Association between the nociceptin receptor gene (OPRL1) single nucleotide polymorphisms and alcohol dependence

OPRL1 encodes the nociceptin receptor, which has been shown to be involved in alcohol dependence in previous studies. In the present study, we investigated the association between genetic polymorphisms of OPRL1 and alcohol dependence in a Scandinavian population. We genotyped 15 single nucleotide polymorphisms (SNPs) spanning the OPRL1 locus and found that SNP rs6010718 was significantly associated with both Type I and Type II alcoholics (P < 0.05). Linkage disequilibrium and haplotype analysis identified two haplotype blocks in this region. Furthermore, two haplotypes composed of five tag SNPs showed significant association with alcohol dependence. These findings suggest that genetic variants of the OPRL1 gene play a role in alcohol dependence in the Scandinavian population, warranting further investigation at the OPRL1 locus.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

The pharmacology of Ro 64-6198, a systemically active, nonpeptide NOP receptor (opiate receptor-like 1, ORL-1) agonist with diverse preclinical therapeutic activity

The NOP receptor (formerly referred to as opiate receptor-like 1, ORL-1, LC132, OP(4), or NOP(1)) is a G protein-coupled receptor that shares high homology to the classic opioid MOP, DOP, and KOP (mu, delta, and kappa, respectively) receptors and was first cloned in 1994 by several groups. The NOP receptor remained an orphan receptor until 1995, when the endogenous neuropeptide agonist, known as nociceptin or orphanin FQ (N/OFQ) was isolated. Five years later, a group at Hoffmann-La Roche reported on the selective, nonpeptide NOP agonist Ro 64-6198, which became the most extensively published nonpeptide NOP agonist and a valuable pharmacological tool in determining the potential of the NOP receptor as a therapeutic target. Ro 64-6198 is systemically active and achieves high brain penetration. It has subnanomolar affinity for the NOP receptor and is at least 100 times more selective for the NOP receptor over the classic opioid receptors. Ro 64-6198 ranges from partial to full agonist, depending on the assay. Preclinical data indicate that Ro 64-6198 may have broad clinical uses, such as in treating stress and anxiety, addiction, neuropathic pain, cough, and anorexia. This review summarizes the pharmacology and preclinical data of Ro 64-6198.

In vitro and in vivo studies on UFP-112, a novel potent and long lasting agonist selective for the nociceptin/orphanin FQ receptor

[(pF)Phe(4)Aib(7)Arg(14)Lys(15)]N/OFQ-NH(2) (UFP-112) has been designed as a novel ligand for the nociceptin/orphanin FQ (N/OFQ) peptide receptor (NOP) by combining into the same peptide different chemical modifications reported to increase N/OFQ potency. In vitro data obtained in the electrically stimulated mouse vas deferens demonstrated that UFP-112 behaved as a high potency (pEC(50) 9.43) full agonist at the NOP receptor. UFP-112 effects were sensitive to the NOP antagonist UFP-101 but not to naloxone and no longer evident in tissues taken from NOP(-/-) mice. In vitro half life of UFP-112 in mouse plasma and brain homogenate was 2.6- and 3.5-fold higher than that of N/OFQ. In vivo, in the mouse tail withdrawal assay, UFP-112 (1-100pmol, i.c.v.) mimicked the actions of N/OFQ producing pronociceptive effects after i.c.v. administration and antinociceptive effects when given i.t.; in both cases, UFP-112 was approximately 100-fold more potent than the natural peptide and produced longer lasting effects. UFP-112 also mimicked the hyperphagic effect of N/OFQ producing a bell shaped dose response curve with the maximum reached at 10pmol. The hyperphagic effects of N/OFQ and UFP-112 were absent in NOP(-/-) mice. Equi-effective high doses of UFP-112 (0.1nmol) and N/OFQ (10nmol) were injected i.c.v. in mice and spontaneous locomotor activity recorded for 16h. N/OFQ produced a clear inhibitory effect which lasted for 60min while UFP-112 elicited longer lasting effects (>6h). In conscious rats, UFP-112 (0.1 and 10nmol/kg, i.v.) produced a marked and sustained decrease in heart rate, blood pressure, and urinary sodium excretion and a profound increase in urine flow. Collectively, these findings demonstrate that UFP-112 behaves in vitro and in vivo as a highly potent and selective ligand able to produce full and long lasting activation of NOP receptors.

The nociceptin/orphanin FQ receptor agonist Ro 64-6198 reduces alcohol self-administration and prevents relapse-like alcohol drinking

Effects of the opioid receptor like-1 (ORL-1) receptor agonist Ro 64-6198 (0.1, 0.3, and 1.0 mg/kg intraperitoneally (i.p.)) on operant ethanol self-administration and activation of self-administration by ethanol deprivation were studied in male Wistar rats. Acute administration of Ro 64-6198 caused a dose-dependent reduction of ethanol self-administration. In comparison, the opioid antagonist naltrexone (0.1, 0.3, and 1.0 mg/kg i.p.) inhibited ethanol self-administration at all doses tested. Ethanol deprivation for 10 days significantly increased ethanol self-administration during the first 2 days after deprivation. Daily pretreatment with Ro 64-6198 (0.3 mg/kg) or naltrexone (0.3 mg/kg) during the last 3 days of ethanol deprivation abolished the deprivation-induced increase in ethanol intake. Thus, stimulation of the ORL-1 receptors by Ro 64-6198 reduced the acute reinforcing effects of ethanol and prevented relapse-like behavior in the ethanol-deprivation model in a similar manner as a blockade of opioid receptors by naltrexone. Ro 64-6198 at 0.1 and 0.3 mg/kg doses did not alter self-administration of 0.2% saccharin solution, indicating an apparent selectivity of this compound in modification of ethanol reward. These findings add further support to the idea that Ro 64-6198 and potentially other synthetic ORL-1 receptor agonists are as effective as naltrexone in blocking the actions of ethanol important for its addictive potential in animal experiments, and therefore may have therapeutic value in the treatment of alcoholism.

Buprenorphine reduces alcohol drinking through activation of the nociceptin/orphanin FQ-NOP receptor system

BACKGROUND

Activation of the NOP receptor by its endogenous ligand nociceptin/orphanin FQ reduces ethanol intake in genetically selected alcohol preferring Marchigian Sardinian alcohol preferring (msP) rats. Here we evaluated whether buprenorphine, a partial agonist at micro-opioid and NOP receptors, would reduce ethanol consumption in msP rats via activation of NOP receptors.

METHODS

Marchigian Sardinian alcohol preferring rats trained to drink 10% alcohol 2 hours/day were injected with buprenorphine (.03, .3, 3.0, or 6.0 mg/kg intraperitoneally [IP]) 90 min before access to ethanol.

RESULTS

Similar to prototypical micro-agonists, the two lowest doses of buprenorphine significantly increased ethanol consumption (p < .01); in contrast, the two highest doses reduced it (p < .05). Pretreatment with naltrexone (.25 mg/kg IP) prevented the increase of ethanol intake induced by .03 mg/kg of buprenorphine (p < .001) but did not affect the inhibition of ethanol drinking induced by 3.0 mg/kg of buprenorphine. Conversely, pretreatment with the selective NOP receptor antagonist UFP-101 (10.0 or 20.0 microg/rat) abolished the suppression of ethanol drinking by 3.0 mg/kg of buprenorphine.

CONCLUSIONS

Buprenorphine has dualistic effects on ethanol drinking; low doses increase alcohol intake via stimulation of classic opioid receptors, whereas higher doses reduce it via activation of NOP receptors. We suggest that NOP agonistic properties of buprenorphine might be useful in the treatment of alcoholism.

Effect of novel nociceptin/orphanin FQ-NOP receptor ligands on ethanol drinking in alcohol-preferring msP rats

Activation of the NOP receptor by the endogenous ligand nociceptin/orphanin FQ (N/OFQ) reduces alcohol consumption in genetically selected alcohol-preferring Marchigian Sardinian (msP) rats. The present study evaluated the effect of three newly synthesized peptidergic and one brain-penetrating heterocyclic NOP receptor agonists on alcohol drinking in the two bottle choice paradigm. MsP rats were intracerebroventricularly (ICV) injected with the NOP receptor agonists OS-462 (0.5 and 1.0 microg), UFP-102 (0.25 and 1.0 microg) or UFP-112 (0.01 and 0.05 microg), or with Ro 64-6198 (0.3 and 1.0 mg/kg) given intraperitoneally (i.p.) and tested for 10% alcohol consumption. Results showed decreased alcohol consumption after treatment with all three peptidergic NOP receptor agonists (OS-462, UFP-102 and UFP-112). OS-462 (at the 1.0 microg dose) and UFP-102 (at the 0.25 microg dose) induced a significant increase in food intake as well. Surprisingly, Ro 64-6198 was ineffective at the 0.3 mg/kg dose, whereas it increased ethanol and food consumption at the 1.0 mg/kg dose. Pre-treatment with the selective mu-receptor antagonist naloxone (0.5 mg/kg, i.p.) reduced these effects of 1.0 mg/kg of Ro 64-6198. These findings confirm that activation of brain NOP receptors reduces alcohol drinking in msP rats and demonstrate that OS-462, UFP-102 and UFP-112 act as potent NOP receptor agonists. On the other hand, Ro 64-6198 increased alcohol drinking, an effect probably induced by a residual agonist activity of this compound at mu-opioid receptors. Overall, the results indicate that OS-462, UFP-102 and UFP-112 may represent valuable pharmacological tools to investigate the functional role of the brain N/OFQ system.

Chronic intracerebroventricular infusion of nociceptin/orphanin FQ increases food and ethanol intake in alcohol-preferring rats

Central administration of low doses of nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the opioid-like orphan receptor NOP, have been shown to reduce ethanol consumption, ethanol-induced conditioned place preference and stress-induced reinstatement of alcohol-seeking behavior in alcohol preferring rats. The present study evaluated the effect of continuous (7 days) lateral brain ventricle infusions of N/OFQ (0, 0.25, 1, 4, and 8 microg/h), by means of osmotic mini-pumps, on 10% ethanol intake in Marchigian-Sardinian alcohol-preferring (msP) rats provided 2h or 24h access to it. N/OFQ dose-dependently increased food intake in msP rats. On the other hand, in contrast to previous studies with acute injections, continuous lateral brain ventricle infusion of high doses of N/OFQ increased ethanol consumption when the ethanol solution was available for 24h/day or 2h/day. The present study demonstrates that continuous activation of the opioidergic N/OFQ receptor does not blunt the reinforcing effects of ethanol. Moreover, the data suggest that continuous activation of the opioidergic N/OFQ receptor is not a suitable way to reduce alcohol abuse.

Nociceptin levels in the cerebrospinal fluid of chronic pain patients with or without intrathecal administration of morphine

The neuropeptide nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the opioid-like receptor ORL-1 and is thought to be involved in pain transmission and modulation. Human studies have not yet defined its role in pain patients. The aims of this study were 1) to verify the presence of N/OFQ in the cerebrospinal fluid (CSF) of human controls and patients with chronic noncancer pain, including those treated with intrathecally administered morphine, and 2) to determine whether pain or treatment with long-term intrathecal morphine influences its levels. The CSF of 27 patients (nine controls and 18 with chronic noncancer pain, of whom 12 were treated chronically with intrathecally administered morphine and six were opioid naïve) was analyzed, blindly, with radioimmunoassay methods. N/OFQ was detected in all patients. Mean CSF concentrations were lowest in the morphine-treated group and highest in the untreated chronic pain patients (12.06+/-1.19 and 57.41+/-10.06 fmol/ml, respectively), and the difference between the morphine-treated group and controls was statistically significant (44.72+/-13.56 fmol/ml, P<0.05). The presence of N/OFQ peptide in human CSF may correlate with biological activities that are influenced by different pain states and long-term intrathecal-morphine treatment. Further studies should verify whether the determination of this peptide CSF level may provide information on opioid treatment efficacy and on the presence of opioid tolerance.

Genetically selected Marchigian Sardinian alcohol-preferring (msP) rats: an animal model to study the neurobiology of alcoholism

The present article provides an up-to-date review summarizing almost 18 years of research in genetically selected Marchigian Sardinian alcohol-preferring (msP) rats. The results of this work demonstrate that msP rats have natural preference for ethanol characterized by a spontaneous binge-type of drinking that leads to pharmacologically significant blood ethanol levels. This rat line is highly vulnerable to relapse and presentation of stimuli predictive of alcohol availability or foot-shock stress can reinstate extinguished drug-seeking up to 8 months from the last alcohol experience. The msP rat is highly sensitive to stress, shows an anxious phenotype and has depressive-like symptoms that recover following ethanol drinking. Interestingly, these animals have an up-regulated corticotrophin releasing factor (CRF) receptor 1 system. Clinical studies have shown that alcoholic patients often drink ethanol in the attempt to self-medicate from negative affective states and to search for anxiety relief. We propose that msP rats represent an animal model that largely mimics the human alcoholic population that due to poor ability to engage in stress-coping strategies drink ethanol as a tension relief strategy and for self-medication purposes.

Nociceptin/orphanin FQ presynaptically decreases GABAergic transmission and blocks the ethanol-induced increase of GABA release in central amygdala

Behavioral studies show that the GABAergic system in the central amygdala (CeA) nucleus has a complex role in the reinforcing effects effects of ethanol and the anxiogenic response to ethanol withdrawal. Opioid peptides and nociceptin/orphanin FQ (nociceptin) within the CeA are implicated also in regulating voluntary ethanol consumption and ethanol relapse. Recently, we reported that basal GABAergic transmission was increased in ethanol-dependent rats, and that acute ethanol increases GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) in CeA neurons from both naïve and ethanol-dependent rats to the same extent, suggesting lack of tolerance for the acute effect of ethanol. Here, we investigated the effect of nociceptin on IPSCs in CeA neurons and its interaction with ethanol effects on these GABA synapses. We found that nociceptin moderately decreased IPSC amplitudes, acting mostly presynaptically as it increased paired-pulse facilitation ratio of IPSCs and decreased miniature IPSC frequencies (but not amplitudes). Nociceptin also prevented the ethanol-induced augmentation of IPSCs in CeA of naïve rats. Interestingly, in CeA of ethanol-dependent rats, the nociceptin-induced inhibition of IPSCs was increased, indicating an enhanced sensitivity to nociceptin. Nociceptin also blocked the ethanol-induced augmentation of IPSCs in ethanol-dependent rats. Our data suggest that nociceptin has a role in regulating the GABAergic system and opposing the effect elicited by ethanol. Thus, nociceptin may represent a therapeutic target for alleviating alcohol dependence.

A neuropeptide FF agonist blocks the acquisition of conditioned place preference to morphine in C57Bl/6J mice

Neuropeptide FF behaves as an opioid-modulating peptide that seems to be involved in morphine tolerance and physical dependence. Nevertheless, the effects of neuropeptide FF agonists on the rewarding properties of morphine remain unknown. C57BL6 mice were conditioned in an unbiased balanced paradigm of conditioned place preference to study the effect of i.c.v. injections of 1DMe (D-Tyr1(NMe)Phe3]NPFF), a stable agonist of the neuropeptide FF system, on the acquisition of place conditioning by morphine or alcohol (ethanol). Morphine (10 mg/kg, i.p.) or ethanol (2 g/kg, i.p.) induced a significant place preference. Injection of 1DMe (1-20 nmol), given 10 min before the i.p. injection of the reinforcing drug during conditioning, inhibited the rewarding effect of morphine but had no effect on the rewarding effect of ethanol. However, a single injection of 1DMe given just before place preference testing was unable to inhibit the rewarding effects of morphine. By itself, 1DMe was inactive but an aversive effect of this agonist could be evidenced if the experimental procedure was biased. These results suggest that neuropeptide FF, injected during conditioning, should influence the development of rewarding effects of morphine and reinforce the hypothesis of strong inhibitory interactions between neuropeptide FF and opioids.