Effects of naloxone benzoylhydrazone on native and recombinant nociceptin/orphanin FQ receptors

Nociceptin and the NOP receptor in aversive learning in mice

The endogenous neuropeptide nociceptin (N/OFQ), which mediates its actions via the nociceptin receptor (NOP), is implicated in multiple behavioural and physiological functions. This study examined the effects of the NOP agonists N/OFQ and the synthetic agonist Ro 64-6198, the antagonists NNN and NalBzoH, as well as deletion of the Pronociceptin gene on emotional memory in mice. The animals were tested in the passive avoidance (PA) task, dependent on hippocampal and amygdala functions. N/OFQ injected intraventricularly (i.c.v.) prior to training produced a biphasic effect on PA retention; facilitation at a low dose and impairment at higher doses. Ro 64-6198 also displayed a biphasic effect with memory facilitation at lower doses and impairment at a high dose. None of the agonists influenced PA training latencies. NNN did not significantly modulate retention in the PA task but antagonized the inhibitory effects of N/OFQ. NalBzoH facilitated memory retention in a dose-dependent manner and blocked the impairing effects of N/OFQ. However, neither NNN nor NalBzoH blocked the memory-impairing effects of Ro 64-6198. Finally, the Pnoc knockout mice exhibited enhanced PA retention latencies compared to the wild type mice. The biphasic effect of the natural ligand and Ro 64-6198 and the failure of the antagonists to block the action of Ro 64-6198 indicate complexity in ligand-receptor interaction. These results indicate that brain nociceptin and its NOP has a subtle role in regulation of mechanisms of relevance for treatment of disorders with processing disturbances of aversive events e.g. Alzheimer's disease, anxiety, depression and PTSD.

Panicolytic-like effects caused by substantia nigra pars reticulata pretreatment with low doses of endomorphin-1 and high doses of CTOP or the NOP receptors antagonist JTC-801 in male Rattus norvegicus

RATIONALE

Gamma-aminobutyric acid (GABA)ergic neurons of the substantia nigra pars reticulata (SNpr) are connected to the deep layers of the superior colliculus (dlSC). The dlSC, in turn, connect with the SNpr through opioid projections. Nociceptin/orphanin FQ peptide (N/OFQ) is a natural ligand of a Gi protein-coupled nociceptin receptor (ORL1; NOP) that is also found in the SNpr. Our hypothesis is that tectonigral opioid pathways and intranigral orphanin-mediated mechanisms modulate GABAergic nigrotectal connections.

OBJECTIVES

Therefore, the aim of this work was to study the role of opioid and NOP receptors in the SNpr during the modulation of defence reactions organised by the dlSC.

METHODS

The SNpr was pretreated with either opioid or NOP receptor agonists and antagonists, followed by dlSC treatment with bicuculline.

RESULTS

Blockade of GABAA receptors in the dlSC elicited fear-related defensive behaviour. Pretreatment of the SNpr with naloxone benzoylhydrazone (NalBzoH), a μ-, δ-, and κ1-opioid receptor antagonist as well as a NOP receptor antagonist, decreased the aversive effect of bicuculline treatment on the dlSC. Either μ-opioid receptor activation or blockade by SNpr microinjection of endomorphin-1 (EM-1) and CTOP promoted pro-aversive and anti-aversive actions, respectively, that modulated the defensive responses elicited by bicuculline injection into the dlSC. Pretreatment of the SNpr with the selective NOP receptor antagonist JTC801 decreased the aversive effect of bicuculline, and microinjections of the selective NOP receptor agonist NNC 63-0532 promoted the opposite effect.

CONCLUSIONS

These results demonstrate that opioid pathways and orphanin-mediated mechanisms have a critical role in modulating the activity of nigrotectal GABAergic pathways during the organisation of defensive behaviours.

Binding of GTPgamma[35S] is regulated by GDP and receptor activation. Studies with the nociceptin/orphanin FQ receptor

BACKGROUND AND PURPOSE

We have examined the effects of ligand efficacy and receptor density on the binding of guanosine 5'-[gamma-thio]triphosphate (GTPgammaS) and GDP to the nociceptin/orphanin FQ (N/OFQ) peptide receptor (NOP)-coupled G-proteins.

EXPERIMENTAL APPROACH

In GTPgamma[(35)S] binding experiments, using stable (CHO(hNOP)) and inducible (CHO(INDhNOP)) recombinant human and rat NOP we have measured: (i) ligand-specific GDP requirements; (ii) the effects of receptor density on guanine nucleotide affinity/capacity; and (iii) the effect of ligand efficacy on GTPgammaS association kinetics.

KEY RESULTS

GTPgammaS competition curves were shallow and modelled by high- and low-affinity components that were relatively consistent between cell types and tissue preparations. In the presence of 1 microM N/OFQ a high-affinity GDP binding site was also present, but the fraction of total binding was reduced. In an efficacy-dependent manner, the partial agonists [F/G]N/OFQ(1-13)NH(2) ([Phe(1)psi(CH(2)-NH)Gly(2)]-nociceptin(1-13)NH(2)) and naloxone benzoylhydrazone both reduced the fraction of high-affinity sites for GDP (relative to basal). While the pIC(50) for high-affinity GDP binding site did not decrease in the presence of 1 microM N/OFQ, N/OFQ produced a significant reduction in pIC(50) for the low-affinity site. Agonist-mediated decrease in affinity for GDP binding was efficacy-dependent. GDP displayed three affinities: high, conserved in the presence and absence of ligand; intermediate, present as a low fraction under basal conditions; low (efficacy-dependent), present during receptor activation representing the majority of binding.

CONCLUSIONS AND IMPLICATIONS

The affinity of GTPgamma[(35)S] was regulated by GDP and receptor activation caused increased binding of GTPgamma[(35)S] through a reduction in GDP affinity.

The nociceptin system and hippocampal cognition in mice: a pharmacological and genetic analysis

This study examines the effects of NOP agonists nociceptin/orphanin FQ (N/OFQ) and Ro 64-6198, NOP antagonists [Nphe(1)]N/OFQ(1-13)-NH(2) Nphe(1) and naloxone benzoylhydrazone (NalBzoH) on spatial memory in NMRI mice and pronociceptin (proNC) knockout (KO) mice using the water maze task. N/OFQ, administered i.c.v. (1, 5 and 10 nmol/mouse) and into hippocampal CA3 (1 nmol/mouse, bilaterally), impaired acquisition and retention in the maze. Impairments were blocked by pre-treatment with Nphe(1) (10 nmol, i.c.v.). Ro 64-6198 (0.1-0.3-1 mg/kg i.p.) also dose-dependently impaired learning. However, pre-treatment with NalBzoH (1 mg/kg, s.c.) failed to modify the effects of Ro 64-6198. Nphe(1) (10 nmol/mouse i.c.v.) and NalBzoH (1 mg/kg, s.c.) by themselves failed to affect maze performance, despite a tendency for enhanced performance. Prepro N/OFQ knockout (ppN/OFQ -/-) showed evidence of improved learning, evident at retention trials and in reversal training. ppN/OFQ -/- mice were not impaired by N/OFQ (10 nmol i.c.v.) in the task, suggesting that changes in postsynaptic NOP receptors may occur in such KO mice. It is concluded that N/OFQ and NOP receptors have an important role in hippocampus-dependent spatial learning and memory, probably by modulation of glutamatergic functions.

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.

Agonist activity of naloxone benzoylhydrazone at recombinant and native opioid receptors

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

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

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

Ro 64-6198 [(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] acts differently from nociceptin/orphanin FQ in rat periaqueductal gray slices

Ro 64-6198 [(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] was developed as a nonpeptide agonist of nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptors, using bioassays at cloned receptors expressed in cell cultures. We have investigated the actions of Ro 64-6198 at native NOP receptors of the ventrolateral periaqueductal gray (PAG), a crucial site for N/OFQ-induced reversal of opioid analgesia, using the patch-clamp recording technique in rat brain slices. Ro 64-6198, like N/OFQ, activated G protein-coupled inwardly rectifying K(+) channels (GIRK) in ventrolateral PAG neurons but displayed only 60% efficacy and 22% potency of N/OFQ. Unlike N/OFQ that activated GIRK through NOP receptors in almost all tested neurons, Ro 64-6198 affected only 62% (114/185) of the neurons recorded, among which 57% were sensitive to CompB (J-113397), a selective NOP receptor antagonist. The effect of Ro 64-6198 was not affected by naloxone (1 microM), sulpiride (10 microM), and [1-(2-methoxyphenyl)-4-[4-2-phthalimido)butyl]piperazine (NAN-190) (1 microM), respectively, the antagonist of opioid, dopamine D(2), and 5-HT(1A) receptors. In Ro 64-6198-unresponsive neurons, N/OFQ activated GIRK through NOP receptors. It is concluded that Ro 64-6198 is a weak agonist of NOP receptors both in terms of potency and efficacy in ventrolateral PAG neurons. Heterogeneity of NOP receptors has been proposed from binding studies and in vivo functional studies. The possibility was discussed that two subsets of NOP receptors exist in ventrolateral PAG neurons, and Ro 64-6198 activates only one subset but N/OFQ activates both of them.

G protein activation and cyclic AMP modulation by naloxone benzoylhydrazone in distinct layers of rat olfactory bulb

1 Naloxone benzoylhydrazone (NalBzoH) has initially been developed as an agonist of the pharmacologically defined kappa3-opioid receptor and has recently been employed as an antagonist at the opioid receptor-like (ORL1) receptor. In the present study, we investigated the ability of NalBzoH to elicit agonist-like effects on receptor signalling in distinct layers of rat olfactory bulb, a brain region where we have demonstrated the presence of opioid and ORL1 receptors coupled to both stimulation and inhibition of cyclic AMP formation. 2 In membranes of the olfactory nerve-glomerular layer (ON-GL), external plexiform layer (EPL) and granule cell layer (GRL), NalBzoH elicited a concentration-dependent stimulation of guanosine-5'-O-(3-[35S]-thio)triphosphate ([35S]GTPgammaS) binding with pEC50 values ranging from 7.36 to 7.86, whereas the kappa1-opioid receptor agonists (-)-U-50,488 and U-69,593 were inactive. 3 In membranes of GRL, but not ON-GL and EPL, NalBzoH stimulated basal adenylyl cyclase activity by 40% with a pEC50 of 8.14, and significantly potentiated the net enzyme stimulation elicited by corticotropin-releasing hormone and pituitary adenylate cyclase-activating peptide 38. Pertussis toxin prevented the NalBzoH stimulations of [35S]GTPgammaS binding and adenylyl cyclase activity. 4 In membranes of EPL and GRL, but not ON-GL, NalBzoH elicited a concentration-dependent inhibition of forskolin-stimulated adenylyl cyclase activity with pEC50 values of 8.07 and 8.08, respectively. 5 At concentrations that completely blocked the actions of nociceptin/orphanin FQ (N/OFQ), the ORL1 receptor antagonists CompB and [Nphe1]N/OFQ(1-13)NH2 failed to antagonize either the stimulatory or the inhibitory effect of NalBzoH on cyclic AMP formation. Similarly, the kappa1-opioid receptor antagonist nor-binaltorphimine counteracted the NalBzoH effects with relatively low potencies (pKi values=7.67-8.09). 6 Conversely, the selective delta-opioid receptor antagonist TIPP (pKi=9.10) and the selective mu-opioid receptor antagonist CTAP (pKi=8.27) reduced the inhibitory effect of NalBzoH by 70 and 30%, respectively. Moreover, TIPP and CTAP potently inhibited the NalBzoH stimulation of cyclic AMP, each antagonist maximally causing 50% blockade of the agonist response. 7These data demonstrate that in the olfactory bulb NalBzoH activates receptor signalling by acting through delta- and mu-opioid receptors and independently of ORL1 and kappa1-opioid receptors.

Nonpeptide/peptide chimeric ligands for the nociceptin/orphanin FQ receptor: design, synthesis and in vitro pharmacological activity

Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the G-protein coupled receptor referred to as N/OFQ peptide (NOP) receptor. NOP receptor activation by N/OFQ modulates several biological functions both at central and peripheral level. Structure activity relationship (SAR) studies demonstrated that the N/OFQ sequence can be divided into a N-terminal tetrapeptide 'message' crucial for receptor activation and a C-terminal 'address' important for receptor binding. On the basis of this message/address concept we synthesized some chimeric compounds in which we substituted the natural message domain with the nonselective nonpeptide NOP ligand (8-Naphthalen-1-yl-methyl-4-oxo-1-phenyl-1,3,8-triaza-spiro[4,5]dec-3-yl)-aceticacid methyl ester (NNC 63-0532) and used as address domain the peptide sequences Thr-NH2, N/OFQ(5-9)-NH2, N/OFQ(5-13)-NH2 and N/OFQ(5-17)-NH2. All the compounds were pharmacologically evaluated in the electrically stimulated guinea-pig ileum. NNC 63-0532 produced a concentration-dependent inhibition of the electrically induced twitches showing, in comparison with N/OFQ, lower potency and higher maximal effects. In addition, contrary to N/OFQ, the effects of NNC 63-0532 were insensitive to the NOP selective antagonist [Nphe1, Arg14, Lys15]N/OFQ-NH2 (UFP-101) while prevented by naloxone. Similar results were obtained with NNC 63-0532/Thr-NH2 and NNC 63-0532/N/OFQ(1-9)-NH2. On the contrary, the inhibitory effects of NNC 63-0532/N/OFQ(5-13)-NH2 and NNC 63-0532/N/OFQ(5-17)-NH2 were slightly antagonized by UFP-101 while naloxone prevented the effects of the high but not of the low concentrations of the two ligands. These data indicate that it is possible to functionalize with the N/OFQ address sequence a nonpeptide NOP ligand for increasing its binding to the NOP receptor. Moreover, these results corroborate the idea that the 5-13 sequence represents the crucial core of the N/OFQ address domain.

β-Phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids derived from them, including further products of oxidation. condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2002 to June 2003 is reviewed, with 568 references cited.

Evidence in locomotion test for the functional heterogeneity of ORL-1 receptors

1. The ORL1 agonists nociceptin and Ro 64-6198 were compared in their ability to modify spontaneous locomotor activity in male NMRI mice not habituated to the test environment. 2. Higher doses of nociceptin (>5 nmol i.c.v.) reduced whereas lower doses (<1 nmol i.c.v.) stimulated locomotor activity. Both effects were blocked by the putative ORL1 antagonists [NPhe1]nociceptin(1-13)NH2 (10 nmol i.c.v.) and UFP101 (10 nmol, i.c.v.). The effects were also blocked by naloxone benzoylhydrazone (1 mg x kg(-1) s.c.), but not by the nonselective opioid antagonist naloxone (1 mg x kg(-1) s.c.). 3 In contrast to nociceptin, the synthetic ORL1 agonist Ro 64-6198 (0.01-1.0 mg x kg(-1) i.p.) produced monophasic inhibition of locomotor activity, which was insensitive to the treatment with [NPhe1]nociceptin(1-13)NH2 or naloxone benzoylhydrazone. Treatment with UFP101 abolished the locomotor inhibition induced by Ro 64-6198 (1.0 mg x kg(-1)), whereas naloxone (1.0 mg x kg(-1), s.c.) further increased the locomotor-inhibitory effects. 4. Naloxone benzoylhydrazone (0.3; 1.0 and 3.0 mg x kg(-1) s.c.) increased locomotor activity, although the effect was statistically significant only with the highest dose used. 5. Pretreatment with the tyrosine hydroxylase inhibitor H44-68 totally eliminated the motor-stimulatory effects of low doses of nociceptin, probably via dopamine depletion. 6. The results suggest that nociceptin stimulates locomotor activity at low doses if dopamine activity is intact. High doses of nociceptin and all the tested doses of Ro 64-6198 seem to interact with a functionally different subset of ORL1 receptors. In addition, the effects of Ro 64-6198 are modulated by tonic opioid receptor activity.

Pharmacological profile of the cyclic nociceptin/orphanin FQ analogues c[Cys10,14]N/OFQ(1-14)NH2 and c[Nphe1,Cys10,14]N/OFQ(1-14)NH2

In this study we describe the activity of two cyclic nociceptin/orphanin FQ (N/OFQ) peptides; c[Cys(10,14)]N/OFQ(1-14)NH(2) (c[Cys(10,14)]) and its [Nphe(1)] derivative c[Nphe(1),Cys(10,14)]N/OFQ(1-14)NH(2) (c[Nphe(1),Cys(10,14)]) in native rat and mouse and recombinant human N/OFQ receptors (NOP). Cyclisation may protect the peptide from metabolic degradation. In competition binding studies of rat, mouse and human NOP the following rank order pK(i) was obtained: N/OFQ(1-13)NH(2)(reference agonist)>N/OFQ=c[Cys(10,14)]>>c[Nphe(1)Cys(10,14)]. In GTPgamma(35)S studies of Chinese hamster ovary cells expressing human NOP (CHO(hNOP)) c[Cys(10,14)] (pEC(50) 8.29) and N/OFQ(1-13)NH(2) (pEC(50) 8.57) were full agonists whilst c[Nphe(1)Cys(10,14)] alone was inactive. Following 30 min pre-incubation c[Nphe(1)Cys(10,14)] competitively antagonised the effects of N/OFQ(1-13)NH(2) with a pA(2) and slope factor of 6.92 and 1.01 respectively. In cAMP assays c[Cys(10,14)] (pEC(50) 9.29, E(max) 102% inhibition of the forskolin stimulated response), N/OFQ(1-13)NH(2) (pEC(50) 10.16, E(max) 103% inhibition) and c[Nphe(1)Cys(10,14)] (~80% inhibition at 10 microM) displayed agonist activity. In the mouse vas deferens c[Cys(10,14)] (pEC(50) 6.82, E(max) 89% inhibition of electrically evoked contractions) and N/OFQ(1-13)NH(2) (pEC(50) 7.47, E(max) 93% inhibition) were full agonists whilst c[Nphe(1)Cys(10,14)] alone was inactive. c[Nphe(1)Cys(10,14)] (10 microM) competitively antagonised the effects of N/OFQ(1-13)NH(2) with a pK(B) of 5.66. In a crude attempt to assess metabolic stability, c[Cys(10,14)] was incubated with rat brain membranes and then the supernatant assayed for remaining peptide. Following 60 min incubation 64% of the 1 nM added peptide was metabolised (compared with 54% for N/OFQ-NH(2)). In summary, we report that c[Cys(10,14)] is a full agonist with a small reduction in potency but no improvement in stability whilst c[Nphe(1)Cys(10,14)] displays tissue (antagonist in the vas deferens) and assay (antagonist in the GTPgamma(35)S assay and agonist in cAMP assay) dependent activity.

Characterization of nociceptin/orphanin FQ binding sites in dog brain membranes

Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the N/OFQ receptor (NOP), whose characteristics in the dog are unknown. We therefore compared [(3)H]N/OFQ binding in dog and rat brain membranes. Radioligand saturation/competition studies with these membranes and leucyl-[(3)H]N/OFQ(1-17)OH or the novel radioligand [(3)H]N/OFQ(1-13)NH(2) were performed to determine receptor density and ligand affinity. The density of classic opioid receptors was determined by using [(3)H]diprenorphine. Leucyl-[(3)H]N/OFQ(1-17)OH binding was concentration dependent and saturable in dog (maximum binding capacity [B(max)], 28.7 +/- 2.8 fmol/mg of protein; equilibrium dissociation constant as negative log [pK(d)], 10.27 +/- 0.11) and rat (B(max), 137.0 +/- 12.9 fmol/mg of protein; pK(d), 10.41 +/- 0.05). In comparison, the B(max) and pK(d) of [(3)H]diprenorphine were, respectively, 77.7 +/- 5.3 fmol/mg of protein and 9.74 +/- 0.09 in dog and 79.1 +/- 18.2 fmol/mg of protein and 9.51 +/- 0.04 in rat. In dog, [(3)H]N/OFQ(1-13)NH(2) binding to NOP receptors was also saturable (B(max), 23.7 +/- 2.0 fmol/mg of protein; pK(d), 10.16 +/- 0.12). In both species, leucyl-[(3)H]N/OFQ(1-17)OH was displaced by various NOP ligands. Dynorphin A, N/OFQ(1-5)NH(2), and nocistatin were essentially inactive. There was a significant positive correlation (r(2) = 0.95; P < 0.0001) between pK(i) values (an estimate of affinity) obtained in displacement studies in rat and dog. We have demonstrated a low density of NOP receptors, measured with two radioligands, in dog, and these receptors display a high degree of pharmacological similarity with those natively expressed in the rat.

Partial agonist behaviour depends upon the level of nociceptin/orphanin FQ receptor expression: studies using the ecdysone-inducible mammalian expression system

(1) Partial agonism is primarily dependent upon receptor density and coupling efficiency. As these parameters are tissue/model dependent, intrinsic activity in different tissues can vary. We have utilised the ecdysone-inducible expression system containing the human nociceptin/orphanin FQ (N/OFQ) peptide receptor (hNOP) expressed in Chinese hamster ovary cells (CHOINDhNOP) to examine the activity of a range of partial agonists in receptor binding, GTPgamma35S binding and inhibition of adenylyl cyclase studies. (2) Incubation of CHOINDhNOP cells with ponasterone A (PON) induced hNOP expression ([leucyl-3H]N/OFQ binding) of 24, 68, 191 and 1101 fmol mg-1 protein at 1, 2, 5 and 10 microm PON, respectively. At 191 fmol mg-1, protein hNOP pharmacology was identical to that reported for other traditional expression systems. (3) pEC50 values for GTPgamma35S binding ranged from 7.23 to 7.72 (2-10 microm PON) for the partial agonist [Phe1psi(CH2-NH)Gly2]N/OFQ(1-13)-NH2 ([F/G]N/OFQ(1-13)-NH2) and 8.12-8.60 (1-10 microm PON) for N/OFQ(1-13)-NH2 and Emax values (stimulation factor relative to basal) ranged from 1.51 to 3.21 (2-10 microm PON) for [F/G]N/OFQ(1-13)-NH2 and 1.28-6.95 (1-10 microm) for N/OFQ(1-13)-NH2. Intrinsic activity of [F/G]N/OFQ(1-13)-NH2 relative to N/OFQ(1-13)-NH2 was 0.3-0.5. [F/G]N/OFQ(1-13)-NH2 did not stimulate GTPgamma35S binding at 1 microm PON, but competitively antagonised the effects of N/OFQ(1-13)-NH2 with a pKB=7.62. (4) pEC50 values for cAMP inhibition ranged from 8.26 to 8.32 (2-10 microm PON) for [F/G]N/OFQ(1-13)-NH2 and 9.42-10.35 for N/OFQ(1-13)-NH2 and Emax values (% inhibition) ranged from 19.6 to 83.2 for [F/G]N/OFQ(1-13)-NH2 and 40.9-86.0 for N/OFQ(1-13)-NH2. The intrinsic activity of [F/G]N/OFQ(1-13)-NH2 relative to N/OFQ(1-13)-NH2 was 0.48-0.97. (5) In the same cellular environment with receptor density as the only variable, we show that the profile of [F/G]N/OFQ(1-13)-NH2 can be manipulated to encompass full and partial agonism along with antagonism.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

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

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

UFP-101, a high affinity antagonist for the nociceptin/orphanin FQ receptor: radioligand and GTPgamma(35)S binding studies

Studies of the pharmacology of nociceptin/orphanin FQ (N/OFQ) and its receptor (NOP) have been hampered by the lack of a range of high potency antagonists. In this study we have examined the effects of a novel N/OFQ analogue [Nphe(1),Arg(14),Lys(15)]N/OFQ NH(2) hereafter referred to as UFP-101. [(3)H]N/OFQ competition binding and GTPgamma(35)S binding assays were performed using CHO cells expressing the human NOP receptor (CHO(hNOP)). UFP-101 (pK(i) of 10.14+/-0.09) and a range of NOP selective agonists displaced [(3)H]N/OFQ binding with the following rank order of affinity: [Arg(14),Lys(15)]N/OFQ>[( pF)Phe(4)]N/OFQ(1-13)NH(2)>N/OFQ(1-13)NH(2)>UFP-101>N/OFQ>Ro64-6198>[Nphe(1)]N/OFQ(1-13)NH(2). N/OFQ, N/OFQ(1-13)NH(2), [( pF)Phe(4)]N/OFQ(1-13)NH(2), [Arg(14),Lys(15)]N/OFQ and Ro64-6198 also produced a concentration dependent (pEC(50) values of 8.75+/-0.11, 9.28+/-0.15, 9.69+/-0.04, 9.12+/-0.11 and 8.09+/-0.07 respectively) and saturable stimulation of GTPgamma(35)S binding and all were full agonists. UFP-101 did not stimulate GTPgamma(35)S binding per se, but produced a concentration dependent and parallel rightward shift in the concentration response curves to all agonists. UFP-101 yielded pA(2) values in the range 8.4-9.0. For comparison a pA(2) for [Nphe(1)]N/OFQ(1-13)NH(2) (the template for UFP-101) against N/OFQ of 7.33+/-0.08 was obtained. Slope factors for the Schild regression lines were approximately 1 indicating competitivity. When UFP-101 is compared with its template molecule [Nphe(1)]N/OFQ(1-13)NH(2), Arg(14),Lys(15) substitution produced approximately 1 log greater potency. We suggest that due to its high potency UFP-101 should prove a further useful tool in the evaluation of the N/OFQ-NOP receptor system.

Inhibition of striatal and retinal dopamine release via nociceptin/orphanin FQ receptors

1. We determined the effects of nociceptin/orphanin FQ and the NOP receptor ligands acetyl-Arg-Tyr-Tyr-Arg-Ile-Lys-NH(2) (Ac-RYYRIK-NH(2)) and naloxone benzoylhydrazone on transmitter release in vitro. 2. The electrically evoked tritium overflow from guinea-pig and mouse striatal slices and guinea-pig retinal discs preincubated with [(3)H]-dopamine was inhibited by nociceptin/orphanin FQ (pEC(50) 7.9, 7.6 and 8.6; E(max) 30, 50 and 55%). Ac-RYYRIK-NH(2) 0.032 microM and naloxone benzoylhydrazone 5 microM antagonized the effect of nociceptin/orphanin FQ in striatal slices of the guinea-pig (apparent pA(2) 9.1 and 6.8) and the mouse (apparent pA(2) 9.2 and 7.5) and strongly attenuated the effect of nociceptin/orphanin FQ 0.1 microM in guinea-pig retinal discs. Ac-RYYRIK-NH(2) 0.032 microM did not affect the evoked overflow by itself whereas naloxone benzoylhydrazone 5 microM inhibited it in each tissue. 3. The electrically evoked tritium overflow from mouse brain cortex slices preincubated with [(3)H]-noradrenaline was inhibited by nociceptin/orphanin FQ (pEC(50) 7.9, E(max) 85%), Ac-RYYRIK-NH(2) (pEC(50) 8.3, E(max) 47%) but not affected by naloxone benzoylhydrazone 5 microM. Ac-RYYRIK-NH(2) and naloxone benzoylhydrazone showed apparent pA(2) values of 8.6 and 6.9. 4. In conclusion, the inhibitory effect of nociceptin/orphanin FQ on dopamine release in the striatum and retina and on noradrenaline release in the cerebral cortex is mediated via NOP receptors. Ac-RYYRIK-NH(2) behaves as an extremely potent NOP receptor antagonist in the striatum and retina and as a partial agonist in the cortex.