Nociceptin/orphanin FQ Receptor Research Articles

Synthesis, Biological Activity and Molecular Docking of Chimeric Peptides Targeting Opioid and NOP Receptors.

Recently, mixed opioid/NOP agonists came to the spotlight for their favorable functional profiles and promising outcomes in clinical trials as novel analgesics. This study reports on two novel chimeric peptides incorporating the fragment Tyr-c[D-Lys-Phe-Phe]Asp-NH2 (RP-170), a cyclic peptide with high affinity for µ and κ opioid receptors (or MOP and KOP, respectively), conjugated with the peptide Ac-RYYRIK-NH2, a known ligand of the nociceptin/orphanin FQ receptor (NOP), yielding RP-170-RYYRIK-NH2 (KW-495) and RP-170-Gly3-RYYRIK-NH2 (KW-496). In vitro, the chimeric KW-496 gained affinity for KOP, hence becoming a dual KOP/MOP agonist, while KW-495 behaved as a mixed MOP/NOP agonist with low nM affinity. Hence, KW-495 was selected for further in vivo experiments. Intrathecal administration of this peptide in mice elicited antinociceptive effects in the hot-plate test; this action was sensitive to both the universal opioid receptor antagonist naloxone and the selective NOP antagonist SB-612111. The rotarod test revealed that KW-495 administration did not alter the mice motor coordination performance. Computational studies have been conducted on the two chimeras to investigate the structural determinants at the basis of the experimental activities, including any role of the Gly3 spacer.

In vitro sepsis induces Nociceptin/Orphanin FQ receptor (NOP) expression in primary human vascular endothelial but not smooth muscle cells.

Sepsis is a dysregulated host response to infection that can cause widespread effects on other organs including cardiovascular depression, hypotension and organ failure. The receptor for Nociceptin/Orphanin FQ (N/OFQ), NOP is expressed on immune cells and these cells can release the peptide. Exogenous N/OFQ can dilate blood vessels and this peptide is increased in animal and human sepsis. We hypothesise that NOP receptors are present on vascular endothelial cells and therefore provide the target for released N/OFQ to cause vasodilation and hence hypotension. Using human umbilical vein endothelial cells (HUVEC) and human vascular smooth muscle cells (HVSMC) freshly prepared from umbilical cords and up to passage 4, we assessed NOP mRNA expression by Polymerase Chain Reaction (PCR), NOP surface receptor expression using a fluorescent NOP selective probe (N/OFQATTO594) and NOP receptor function with N/OFQ stimulated ERK1/2 phosphorylation. As an in vitro sepsis mimic we variably incubated cells with 100ng/ml Lipopolysaccharide and Peptidoglycan G (LPS/PepG). HUVECs express NOP mRNA and this was reduced by ~80% (n = 49) after 24–48 hours treatment with LPS/PepG. Untreated cells do not express surface NOP receptors but when treated with LPS/PepG the reduced mRNA was translated into protein visualised by N/OFQATTO594 binding (n = 49). These NOP receptors in treated cells produced an N/OFQ (1μM) driven increase in ERK1/2 phosphorylation (n = 20). One (of 50) HUVEC lines expressed NOP mRNA and receptor protein in the absence of LPS/PepG treatment. In contrast, HVSMC expressed NOP mRNA and surface receptor protein (n = 10) independently of LPS/PepG treatment. These receptors were also coupled to ERK1/2 where N/OFQ (1μM) increased phosphorylation. Collectively these data show that an in vitro sepsis mimic (LPS/PepG) upregulates functional NOP expression in the vascular endothelium. Activation of these endothelial receptors as suggested from in vivo whole animal work may contribute to the hypotensive response seen in sepsis. Moreover, blockade of these receptors might be a useful adjunct in the treatment of sepsis.

Endogenous Opioids and Their Role in Stem Cell Biology and Tissue Rescue.

Opioids are considered the oldest drugs known by humans and have been used for sedation and pain relief for several centuries. Nowadays, endogenous opioid peptides are divided into four families: enkephalins, dynorphins, endorphins, and nociceptin/orphanin FQ. They exert their action through the opioid receptors (ORs), transmembrane proteins belonging to the super-family of G-protein-coupled receptors, and are expressed throughout the body; the receptors are the δ opioid receptor (DOR), μ opioid receptor (MOR), κ opioid receptor (KOR), and nociceptin/orphanin FQ receptor (NOP). Endogenous opioids are mainly studied in the central nervous system (CNS), but their role has been investigated in other organs, both in physiological and in pathological conditions. Here, we revise their role in stem cell (SC) biology, since these cells are a subject of great scientific interest due to their peculiar features and their involvement in cell-based therapies in regenerative medicine. In particular, we focus on endogenous opioids’ ability to modulate SC proliferation, stress response (to oxidative stress, starvation, or damage following ischemia–reperfusion), and differentiation towards different lineages, such as neurogenesis, vasculogenesis, and cardiogenesis.

Buprenorphine Analogs to Treat Relapse

Substance use disorders (SUDs) – the overuse and dependence on at least one substance – have a lifetime prevalence of ~10% in the U.S. While buprenorphine (BUP) treatment reduces relapse rates, it is only marginally effective, as 70% of SUD patients relapse within a year. Due to BUP’s marginal effectiveness and abuse potential, there is a need to improve SUD therapies that prevent relapse to multiple drugs of abuse after exposure to stress, drug‐associated context and drug priming. Here we evaluate BUP and two analogs for their ability to inhibit cocaine‐ and stress‐induced relapse using a conditioned place preference (CPP) paradigm. Pharmacologically, BUP is a potent (sub nM) partial agonist at the m‐opioid receptor (MOR) with potent antagonist activity at the k‐(KOR), and d‐opioid receptors (DOR). In vitro BUP can act as a weak (mM) partial agonist at the nociceptin/OrphaninFQ (NOP) receptor with uncertain in vivo consequences. We measured in vitro potency and efficacy using [35S]GTPgS functional assays in CHO cells separately expressing DOR, KOR, MOR or NOP. Since NOP receptor agonists reduce relapse in animal models, we compared BU10119 with NOP agonist activity (EC50 = 120 nM ± 20; EMAX = 44% ± 5%) or BU12004 with NOP antagonist activity (KB = 190 nM ±30) to BUP (EC50 = 1300 nM ± 320; EMAX = 39% ± 7%). Furthermore, BU12004 and BU10119 were MOR antagonists (KB = 0.42 nM ± 0.08 and KB = 1.1 nM ± 0.2, respectively) with less abuse liability than the MOR partial agonist BUP (EC50 = 0.79 nM ± 0.31; EMAX = 34% ± 6%). BUP, BU10119, and BU12004 were all potent antagonists at DOR and KOR. BU10119 (1.0 mg/kg) attenuated cocaine‐primed reinstatement in CPP, but 10 mg/kg BU12004 did not. BUP trended toward reduced drug‐primed reinstatement. The in vivo rank order of BU10119 >BUP >BU12004 matches the rank order for NOP agonism in vitro. Additionally, 1.0 mg/kg of either BU10119 or BU12004 blocked swim stress‐primed reinstatement, ostensibly due to their KOR antagonist activity. Taken together, BU10119 provides a promising lead to treat SUDs with better effectiveness and reduced abuse potential compared to BUP.Support or Funding InformationSupported by NIDA R01 DA07315 and 035316.

Blockade of NOP receptor modulates anxiety-related behaviors in mice exposed to inescapable stress.

Depression and anxiety frequently co-occur, and this has important clinical implications. Previous studies showed that activation of the nociceptin/orphanin FQ receptor (NOP) elicits anxiolytic effects, while its blockade promotes consistent antidepressant actions. NOP antagonists are effective in reversing footshock-induced depressive-like behaviors, but their effects on stress-induced anxiety are still unclear. This study aimed to investigate the effects of the NOP antagonist SB-612111 on footshock stress-induced anxiety behaviors. Male Swiss mice were exposed to inescapable electric footshock stress, and behavioral phenotype was screened based on the ability to escape from footshock (i.e., helpless or non-helpless). Animals were then treated with diazepam (1mg/kg) and SB-612111 (0.1-10mg/kg), and their behavior was assessed in the elevated plus-maze (EPM) and open field test. When compared with non-stressed mice, helpless, but not non-helpless, animals displayed significant reductions in the time spent in and entries into open arms in the EPM. Diazepam significantly increased open arms exploration in helpless, non-helpless, and non-stressed mice. However, treatment with the NOP antagonist SB-612111 was inactive in naive mice, while it reversed anxiogenic-related behaviors in helpless mice and increased anxiety states in non-helpless mice. No effects on locomotion were observed. Helpless mice displayed increased anxiety compared to non-stressed and non-helpless animals, thus supporting use of this approach as an animal model to investigate anxiety/depression comorbidity. Additionally, SB-612111 modulated anxiety-like behaviors in male mice depending on individual stress susceptibility. Ultimately, NOP antagonists could be useful for treating anxiety in depressed patients.

Endogenous opioid peptides and brain development: Endomorphin-1 and Nociceptin play a sex-specific role in the control of oligodendrocyte maturation and brain myelination.

The generation of fully functional oligodendrocytes, the myelinating cells of the central nervous system, is preceded by a complex maturational process. We previously showed that the timing of oligodendrocyte differentiation and rat brain myelination were altered by perinatal exposure to buprenorphine and methadone, opioid analogs used for the management of pregnant addicts. Those observations suggested the involvement of the μ-opioid receptor (MOR) and the nociceptin/orphanin FQ receptor (NOR). However, it remained to be determined if these receptors and their endogenous ligands could indeed control the timing of myelination under normal physiological conditions of brain development. We now found that the endogenous MOR ligand endomorphin-1 (EM-1) exerts a striking stimulatory action on cellular and morphological maturation of rat pre-oligodendrocytes, but unexpectedly, these effects appear to be restricted to the cells from the female pups. Critically, this stimulation is abolished by coincubation with the endogenous NOR ligand nociceptin. Furthermore, NOR antagonist treatment of 9-day-old female pups results in accelerated brain myelination. Interestingly, the lack of sex-dependent differences in developmental brain levels of EM-1 and nociceptin, or oligodendroglial expression of MOR and NOR, suggests that the observed sex-specific responses may be highly dependent on important intrinsic differences between the male and female oligodendrocytes. The discovery of a significant effect of EM-1 and nociceptin in the developing female oligodendrocytes and brain myelination, underscores the need for further studies investigating brain sex-related differences and their implications in opioid use and abuse, pain control, and susceptibility and remyelinating capacity in demyelinating disease as multiple sclerosis.

Novel Targets to Treat Depression: Opioid-Based Therapeutics.

After participating in this activity, learners should be better able to:• Identify the effects of dysregulated opioid signalling in depression• Evaluate the use of opioid compounds and ketamine in patients with depression ABSTRACT: Major depressive disorder (MDD) remains one of the leading causes of disability and functional impairment worldwide. Current antidepressant therapeutics require weeks to months of treatment prior to the onset of clinical efficacy on depressed mood but remain ineffective in treating suicidal ideation and cognitive impairment. Moreover, 30%-40% of individuals fail to respond to currently available antidepressant medications. MDD is a heterogeneous disorder with an unknown etiology; novel strategies must be developed to treat MDD more effectively. Emerging evidence suggests that targeting one or more of the four opioid receptors-mu (MOR), kappa (KOR), delta (DOR), and the nociceptin/orphanin FQ receptor (NOP)-may yield effective therapeutics for stress-related psychiatric disorders. Furthermore, the effects of the rapidly acting antidepressant ketamine may involve opioid receptors. This review highlights dysregulated opioid signaling in depression, evaluates clinical trials with opioid compounds, and considers the role of opioid mechanisms in rapidly acting antidepressants.

Probing ligand recognition of the opioid pan antagonist AT-076 at nociceptin, kappa, mu, and delta opioid receptors through structure-activity relationships

Few opioid ligands binding to the three classic opioid receptor subtypes, mu, kappa and delta, have high affinity at the fourth opioid receptor, the nociceptin/orphanin FQ receptor (NOP). We recently reported the discovery of AT-076 (1), (R)-7-hydroxy-N-((S)-1-(4-(3-hydroxyphenyl)piperidin-1-yl)-3-methylbutan-2-yl)-1,2,3,4-tetrahydroisoquinoline-3-carboxamide, a pan antagonist with nanomolar affinity for all four subtypes. Since AT-076 binds with high affinity at all four subtypes, we conducted a structure-activity relationship (SAR) study to probe ligand recognition features important for pan opioid receptor activity, using chemical modifications of key pharmacophoric groups. SAR analysis of the resulting analogs suggests that for the NOP receptor, the entire AT-076 scaffold is crucial for high binding affinity, but the binding mode is likely different from that of NOP antagonists C-24 and SB-612111 bound in the NOP crystal structure. On the other hand, modifications of the 3-hydroxyphenyl pharmacophore, but not the 7-hydroxy Tic pharmacophore, are better tolerated at kappa and mu receptors and yield very high affinity multifunctional (e.g. 12) or highly selective (e.g. 16) kappa ligands. With the availability of the opioid receptor crystal structures, our SAR analysis of the common chemotype of AT-076 suggests rational approaches to modulate binding selectivity, enabling the design of multifunctional or selective opioid ligands from such scaffolds.

Novel role of the nociceptin system as a regulator of glutamate transporter expression in developing astrocytes.

Our previous results showed that oligodendrocyte development is regulated by both nociceptin and its G-protein coupled receptor, the nociceptin/orphanin FQ receptor (NOR). The present in vitro and in vivo findings show that nociceptin plays a crucial conserved role regulating the levels of the glutamate/aspartate transporter GLAST/EAAT1 in both human and rodent brain astrocytes. This nociceptin-mediated response takes place during a critical developmental window that coincides with the early stages of astrocyte maturation. GLAST/EAAT1 upregulation by nociceptin is mediated by NOR and the downstream participation of a complex signaling cascade that involves the interaction of several kinase systems, including PI-3K/AKT, mTOR, and JAK. Because GLAST is the main glutamate transporter during brain maturation, these novel findings suggest that nociceptin plays a crucial role in regulating the function of early astrocytes and their capacity to support glutamate homeostasis in the developing brain.

Characterization of the Discriminative Stimulus Effects of a NOP Receptor Agonist Ro 64-6198 in Rhesus Monkeys.

Nociceptin/orphanin FQ receptor (NOP) agonists have been reported to produce antinociceptive effects in rhesus monkeys with comparable efficacy to μ-opioid receptor (MOP) agonists, but without their limiting side effects. There are also known to be species differences between rodents and nonhuman primates (NHPs) in the behavioral effects of NOP agonists. The aims of this study were the following: 1) to determine if the NOP agonist Ro 64-6198 could be trained as a discriminative stimulus; 2) to evaluate its pharmacological selectivity as a discriminative stimulus; and 3) to establish the order of potency with which Ro 64-6198 produces discriminative stimulus effects compared with analgesic effects in NHPs. Two groups of rhesus monkeys were trained to discriminate either fentanyl or Ro 64-6198 from vehicle. Four monkeys were trained in the warm-water tail-withdrawal procedure to measure antinociception. Ro 64-6198 produced discriminative stimulus effects that were blocked by the NOP antagonist J-113397 and not by naltrexone. The discriminative stimulus effects of Ro 64-6198 partially generalized to diazepam, but not to fentanyl, SNC 80, ketocyclazocine, buprenorphine, phencyclidine, or chlorpromazine. Fentanyl produced stimulus effects that were blocked by naltrexone and not by J-113397, and Ro 64-6198 did not produce fentanyl-appropriate responding in fentanyl-trained animals. In measures of antinociception, fentanyl, but not Ro 64-6198, produced dose-dependent increases in tail-withdrawal latency. Together, these results demonstrate that Ro 64-6198 produced stimulus effects in monkeys that are distinct from other opioid receptor agonists, but may be somewhat similar to diazepam. In contrast to previous findings, Ro 64-6198 did not produce antinociception in the majority of animals tested even at doses considerably greater than those that produced discriminative stimulus effects.

Brain Gαi 2 -subunit proteins and the prevention of salt sensitive hypertension.

To counter the development of salt-sensitive hypertension, multiple brain G-protein-coupled receptor (GPCR) systems are activated to facilitate sympathoinhibition, sodium homeostasis, and normotension. Currently there is a paucity of knowledge regarding the role of down-stream GPCR-activated Gα-subunit proteins in these critically important physiological regulatory responses required for long-term blood pressure regulation. We have determined that brain Gαi2-proteins mediate natriuretic and sympathoinhibitory responses produced by acute pharmacological (exogenous central nociceptin/orphanin FQ receptor (NOP) and α2-adrenoceptor activation) and physiological challenges to sodium homeostasis (intravenous volume expansion and 1 M sodium load) in conscious Sprague–Dawley rats. We have demonstrated that in salt-resistant rat phenotypes, high dietary salt intake evokes site-specific up-regulation of hypothalamic paraventricular nucleus (PVN) Gαi2-proteins. Further, we established that PVN Gαi2 protein up-regulation prevents the development of renal nerve-dependent sympathetically mediated salt-sensitive hypertension in Sprague–Dawley and Dahl salt-resistant rats. Additionally, failure to up-regulate PVN Gαi2 proteins during high salt-intake contributes to the pathophysiology of Dahl salt-sensitive (DSS) hypertension. Collectively, our data demonstrate that brain, and likely PVN specific, Gαi2 protein pathways represent a central molecular pathway mediating sympathoinhibitory renal-nerve dependent responses evoked to maintain sodium homeostasis and a salt-resistant phenotype. Further, impairment of this endogenous “anti-hypertensive” mechanism contributes to the pathophysiology of salt-sensitive hypertension.

A novel, potent, orally‐bioavailable nociceptin‐1 receptor antagonist reduces ethanol self‐administration and ethanol‐seeking in animal models of alcohol addiction (656.2)

Background: The receptor for nociceptin/orphanin‐FQ (NOP) is localized in the mesolimbic reward pathway and has been suggested to play a role in addiction and motivated behavior. We characterize here a novel, potent, orally‐bioavailable NOP antagonist in several ethanol self‐administration models.Methods: Compound 1 was evaluated in homecage ethanol self‐administration models, progressive ratio operant self‐administration, stress‐induced reinstatement to ethanol‐seeking, and in vivo microdialysis.Results: Compound 1 dose‐dependently reduced homecage ethanol self‐administration in both P and msP rats, without affecting food or water intake. Compound 1 also attenuated progressive ratio operant responding and breakpoints in P rats. Stress‐induced reinstatement of ethanol‐seeking in msP rats was completely blocked by compound 1. Furthermore, compound 1 blocked ethanol‐induced dopamine release in the nucleus accumbens.Conclusions: Compound 1 demonstrates efficacy in attenuating ethanol self‐administration and stress‐induced ethanol‐seeking in two different lines of rats that exhibit excessive ethanol consumption. Compound 1 also blocks the motivation to consume ethanol and ethanol‐stimulated increases in extracellular dopamine levels in the nucleus accumbens, suggesting potential therapeutic utility of NOP receptor antagonists in treating alcohol addiction.

Examining the reinforcing properties of NOP receptor agonist SCH221510 and its role in attenuating mu‐receptor mediated reinforcement in a rat self‐administration assay

Nociceptin/orphanin FQ receptor (NOP) plays a critical role in regulation of reward and motivation neuropathways. In rodents, NOP agonists do not produce conditioned place preference and block rewarding effects of opiates and psychostimulants. We used a rat self‐administration assay to determine reinforcing properties of NOP agonist SCH221510 and its role in attenuating reinforcing effects of mu‐opioid receptor (MOP) agonist remifentanil. Rats with intravenous catheters were allowed to self‐administer SCH221510 (3–300 μg/kg/infusion) or remifentanil (0.3–10 μg/kg/infusion) under fixed (FR5) and progressive ratio (PR) schedules of reinforcement. All rats self‐administered remifentanil, generating a biphasic dose effect curve typical of intravenous drug self‐administration. SCH221510 did not maintain responding, resulting in a flat dose effect curve. Under PR, break points for SCH221510 were similar to those of vehicle, whereas remifentanil had significantly higher break points. Intracisternal (0.03–3 μg) but not systemic (1–10 mg/kg, s.c.) administration of SCH221510 decreased responding for remifentanil (3.2 μg/kg/infusion). This effect was comparable to that of MOP antagonist naltrexone (0.03– 0.3 mg/kg s.c.). Hence, our data show that NOP agonists do not function as reinforcers and may block reinforcing effects of MOP agonists (Supported by USPHS grant R01‐DA032568).

The Nociceptin/Orphanin FQ Receptor (NOP) as a Target for Drug Abuse Medications

Several studies show that the nociceptin receptor NOP plays a role in the regulation of reward and motivation pathways related to substance abuse. Administration of the NOP's natural peptide ligand, Nociceptin/Orphanin FQ (N/OFQ) or synthetic agonist Ro 64-6198 has been shown to block rewarding effects of cocaine, morphine, amphetamines and alcohol, in various behavioral models of drug reward and reinforcement, such as conditioned place preference and drug self-administration. Administration of N/OFQ has been shown to reduce drug-stimulated levels of dopamine in mesolimbic pathways. The NOP-N/OFQ system has been particularly well examined in the development of alcohol abuse in animal models. Furthermore, the efficacy of the mixed-action opioid buprenorphine, in attenuating alcohol consumption in human addicts and in alcohol-preferring animal models, at higher doses, has been attributed to its partial agonist activity at the NOP receptor. These studies suggest that NOP receptor agonists may have potential as drug abuse medications. However, the pathophysiology of addiction is complex and drug addiction pharmacotherapy needs to address the various phases of substance addiction (craving, withdrawal, relapse). Further studies are needed to clearly establish how NOP agonists may attenuate the drug addiction process and provide therapeutic benefit. Addiction to multiple abused drugs (polydrug addiction) is now commonplace and presents a treatment challenge, given the limited pharmacotherapies currently approved. Polydrug addiction may not be adequately treated by a single agent with a single mechanism of action. As with the case of buprenorphine, a mixed-action profile of NOP/opioid activity may provide a more effective drug to treat addiction to various abused substances and/or polydrug addiction.

Pharmacological blockade or genetic knockout of the NOP receptor potentiates the rewarding effect of morphine in rats

The Nociceptin/OrphaninFQ (NOP) system is believed to be involved in drug abuse and addiction. We have recently demonstrated that activation of the NOP receptor, by systemic administration of the NOP receptor agonist Ro65-6570, attenuated the rewarding effect of various opioids in conditioned place preference (CPP) in rats and this attenuating effect was reversed by the NOP receptor antagonist J-113397. The present study demonstrates that co-administration of J-113397 (4.64 mg/kg, i.p.) during conditioning, facilitates morphine-induced CPP. Moreover, we found that NOP receptor knockout rats (oprl1 −/−) are more sensitive to the rewarding effect of morphine than wildtype control rats. Thus, pharmacological or genetic inactivation of the NOP system rendered rats more susceptible to the rewarding effect of morphine. These findings support the suggestion that the endogenous NOP system attenuates the rewarding effect of opioids and therefore offers a therapeutic target for the treatment of drug abuse and addiction.

3D-QSAR, homology modeling, and molecular docking studies on spiropiperidines analogues as agonists of nociceptin/orphanin FQ receptor

The nociceptin/orphanin FQ receptor (NOP) has been implicated in a wide range of biological functions, including pain, anxiety, depression and drug abuse. Especially, its agonists have a great potential to be developed into anxiolytics. However, the crystal structure of NOP is still not available. In the present work, both structure-based and ligand-based modeling methods have been used to achieve a comprehensive understanding on 67 N-substituted spiropiperidine analogues as NOP agonists. The comparative molecular-field analysis method was performed to formulate a reasonable 3D-QSAR model (cross-validated coefficient q 2 = 0.819 and conventional r 2 = 0.950), whose robustness and predictability were further verified by leave-eight-out, Y-randomization, and external test-set validations. The excellent performance of CoMFA to the affinity differences among these compounds was attributed to the contributions of electrostatic/hydrogen-bonding and steric/hydrophobic interactions, which was supported by the Surflex-Dock and CDOCKER molecular-docking simulations based on the 3D model of NOP built by the homology modeling method. The CoMFA contour maps and the molecular docking simulations were integrated to propose a binding mode for the spiropiperidine analogues at the binding site of NOP.

The nociceptin/orphanin FQ (NOP) receptor antagonist J‐113397 enhances the effects of levodopa in the MPTP‐lesioned nonhuman primate model of Parkinson's disease

The anti-parkinsonian and levodopa-sparing potential of the nociceptin/orphanin FQ receptor (NOP) antagonist J-113397 has been demonstrated in rodent models of Parkinson's disease. Here, we describe the levodopa-sparing potential of J-113397 in MPTP-lesioned marmosets. Coadministration of J-113397 (30 mg/kg) with a sub-therapeutic dose of levodopa (12.5 mg/kg) produced an anti-parkinsonian action equivalent to that of a therapeutic dose of levodopa. However, these effects were accompanied by an equivalent level of dyskinesia. The actions of NOP antagonists seen in rodents translate to nonhuman primates. However, the present study raises the possibility that these levodopa-sparing benefits may be offset by a propensity to exacerbate dyskinesia.

Effects of nociceptin/orphanin FQ receptor (NOP) agonist, Ro64-6198, on reactivity to acute pain in mice: Comparison to morphine

The aim of the present study was to clarify the role of nociceptin system in pain modulation. The effects of the synthetic nociceptin (NOP) receptor agonist, Ro64-6198 ((1S,3aS)-8-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triaza-spiro[4.5]decan-4-one), on reactivity to acute noxious stimuli were assessed in C57BL/6N (B6) mice in tail-flick, hot-plate and shock threshold tests. The mu receptor agonist, morphine, was used in each study for comparison. In the tail-flick test, morphine (4 and 8 mg/kg, i.p.) produced analgesia, while Ro64-6198 (0.3, 1 and 3 mg/kg, i.p.) increased pain sensitivity. The effects of Ro64-6198 were seen in naïve but not in mice previously habituated to testing conditions, indicating that increased pain sensitivity may be due to inhibition of stress-induced analgesia. In the hot-plate and the shock threshold tests, Ro64-6198 produced analgesia in B6 mice, like morphine. These effects were reproduced in wild-type but not in NOP receptor knockout mice. Finally, when injected conjointly at subthreshold doses, Ro64-6198 (1 mg/kg) and morphine (1 mg/kg) acted in additive manner to reduce pain sensitivity in the hot-plate test. Together these results show that systemic activation of NOP receptors produced bidirectional changes in pain sensitivity depending on the experimental conditions. They also suggest that central NOP and mu receptors may inhibit reactivity to acute noxious stimuli via independent neural mechanisms.

Nociceptin/orphanin FQ inhibits glutamate release from rat cerebellar and brain stem slices

We have previously demonstrated that nociceptin/orphanin FQ (N/OFQ), inhibits K + depolarisation-evoked glutamate release from rat cerebrocortical slices. In this study we have examined the effects of N/OFQ on glutamate release from rat cerebellar and brain stem slices as there are regional differences in nociceptin/orphanin FQ receptor (NOP) expression. Slices were depolarised with two pulses of 46 mM K + (S 1 and S 2) with N/OFQ added after S 1. Glutamate (non-radioactive) was measured using a fluorescence-based assay. N/OFQ effects were assessed by measuring area under S 1 and S 2 release curves and calculation of S 2/S 1 ratios. In cerebellar slices K + evoked S 2/S 1 ratio was 1.17±0.10 ( n=28). This was reduced in a concentration dependent (EC 50 22 nM; E max 46%) and naloxone (10 μM) insensitive manner by N/OFQ. In the brain stem K + evoked glutamate release was considerably reduced compared to cerebellum. In several preparations K + failed to evoke a significant release. In those that did K + evoked S 2/S 1 ratio was 1.03±0.07 ( n=13). A total of 100 nM N/OFQ reduced this by 38±12% and this response was naloxone insensitive. Due to this small response and its variability we could not construct a full concentration response curve. In conclusion we have demonstrated a functional NOP in rat cerebellum and brain stem that inhibits the release of glutamate.