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.

The nociceptin orphanin FQ peptide receptor agonist, Ro64-6198, impairs recognition memory formation through interaction with glutamatergic but not cholinergic receptor antagonists.

We previously reported that the selective nociceptin orphanin peptide (NOP) receptor agonist, Ro64-6198, impairs mnemonic function through glutamatergic-dependent mechanisms. The aim of the current study was to determine whether the amnesic effects of Ro64-6198 involve a cholinergic component. The effects of systemic administration of Ro64-6198 (0.3 and 1 mg/kg, i.p.), the cholinergic nicotinic receptor antagonist, mecamylamine (0.1 and 1 mg/kg, s.c.), the cholinergic muscarinic receptor antagonist, scopolamine (0.1 and 0.3 mg/kg, s.c.), and the glutamatergic NMDA receptor antagonist, MK-801 (0.03 and 0.1 mg/kg, s.c.), were studied in the mouse object recognition task. All compounds tested were effective in disrupting formation of long-term (24-h delay) recognition memory. Drug interaction studies were then conducted to reveal the existence of functional interactions between NOP receptors and cholinergic and/or NMDA receptors. Co-administration of silent doses of Ro64-6198 (0.3 mg/kg) and MK-801 (0.01 mg/kg) produced clear-cut memory impairment. Similar synergistic effects were observed with the combination of mecamylamine (0.03 mg/kg) and scopolamine (0.1 mg/kg). In contrast, co-administration of Ro64-6198 (0.3 mg/kg) with either mecamylamine (0.03 and 0.1 mg/kg) or scopolamine (0.1 mg/kg) was without any effect on recognition memory. These findings suggest that NOP receptor may modulate memory formation through a functional interaction with glutamatergic but not cholinergic receptors.

Activity and Resistance of a Brain-Permeable Paradox Breaker BRAF Inhibitor in Melanoma Brain Metastasis.

The therapeutic benefit of approved BRAF and MEK inhibitors (BRAFi/MEKi) in patients with brain metastatic BRAF V600E/K-mutated melanoma is limited and transient. Resistance largely occurs through the restoration of MAPK signaling via paradoxical BRAF activation, highlighting the need for more effective therapeutic options. Aiming to address this clinical challenge, we characterized the activity of a potent, brain-penetrant paradox breaker BRAFi (compound 1a, C1a) as first-line therapy and following progression upon treatment with approved BRAFi and BRAFi/MEKi therapies. C1a activity was evaluated in vitro and in vivo in melanoma cell lines and patient-derived models of BRAF V600E-mutant melanoma brain metastases following relapse after treatment with BRAFi/MEKi. C1a showed superior efficacy compared with approved BRAFi in both subcutaneous and brain metastatic models. Importantly, C1a manifested potent and prolonged antitumor activity even in models that progressed on BRAFi/MEKi treatment. Analysis of mechanisms of resistance to C1a revealed MAPK reactivation under drug treatment as the predominant resistance-driving event in both subcutaneous and intracranial tumors. Specifically, BRAF kinase domain duplication was identified as a frequently occurring driver of resistance to C1a. Combination therapies of C1a and anti-PD-1 antibody proved to significantly reduce disease recurrence. Collectively, these preclinical studies validate the outstanding antitumor activity of C1a in brain metastasis, support clinical investigation of this agent in patients pretreated with BRAFi/MEKi, unveil genetic drivers of tumor escape from C1a, and identify a combinatorial treatment that achieves long-lasting responses. SIGNIFICANCE: A brain-penetrant BRAF inhibitor demonstrates potent activity in brain metastatic melanoma, even upon relapse following standard BRAF inhibitor therapy, supporting further investigation into its clinical utility.

Preclinical Characterization of a Next-Generation Brain Permeable, Paradox Breaker BRAF Inhibitor.

PURPOSE: Disease progression in BRAF V600E/K positive melanomas to approved BRAF/MEK inhibitor therapies is associated with the development of resistance mediated by RAF dimer inducing mechanisms. Moreover, progressing disease after BRAFi/MEKi frequently involves brain metastasis. Here we present the development of a novel BRAF inhibitor (Compound Ia) designed to address the limitations of available BRAFi/MEKi. EXPERIMENTAL DESIGN: The novel, brain penetrant, paradox breaker BRAFi is comprehensively characterized in vitro, ex vivo, and in several preclinical in vivo models of melanoma mimicking peripheral disease, brain metastatic disease, and acquired resistance to first-generation BRAFi. RESULTS: Compound Ia manifested elevated potency and selectivity, which triggered cytotoxic activity restricted to BRAF-mutated models and did not induce RAF paradoxical activation. In comparison to approved BRAFi at clinical relevant doses, this novel agent showed a substantially improved activity in a number of diverse BRAF V600E models. In addition, as a single agent, it outperformed a currently approved BRAFi/MEKi combination in a model of acquired resistance to clinically available BRAFi. Compound Ia presents high central nervous system (CNS) penetration and triggered evident superiority over approved BRAFi in a macro-metastatic and in a disseminated micro-metastatic brain model. Potent inhibition of MAPK by Compound Ia was also demonstrated in patient-derived tumor samples. CONCLUSIONS: The novel BRAFi demonstrates preclinically the potential to outperform available targeted therapies for the treatment of BRAF-mutant tumors, thus supporting its clinical investigation.

Negative allosteric modulators of metabotropic glutamate receptor 3 target the stem-like phenotype of glioblastoma.

Glioblastoma is an invariably deadly disease. A subpopulation of glioma stem-like cells (GSCs) drives tumor progression and treatment resistance. Two recent studies demonstrated that neurons form oncogenic glutamatergic electrochemical synapses with post-synaptic GSCs. This led us to explore whether glutamate signaling through G protein-coupled metabotropic receptors would also contribute to the malignancy of glioblastoma. We found that glutamate metabotropic receptor (Grm)3 is the predominantly expressed Grm in glioblastoma. Associations of GRM3 gene expression levels with survival are confined to the proneural gene expression subtype, which is associated with enrichment of GSCs. Using multiplexed single-cell qRT-PCR, GSC marker-based cell sorting, database interrogations, and functional assays in GSCs derived from patients' tumors, we establish Grm3 as a novel marker and potential therapeutic target in GSCs. We confirm that Grm3 inhibits adenylyl cyclase and regulates extracellular signal-regulated kinase. Targeting Grm3 disrupts self-renewal and promotes differentiation of GSCs. Thus, we hypothesize that Grm3 signaling may complement oncogenic functions of glutamatergic ionotropic receptor activity in neuroglial synapses, supporting a link between neuronal activity and the GSC phenotype. The novel class of highly specific Grm3 inhibitors that we characterize herein have been clinically tested as cognitive enhancers in humans with a favorable safety profile.

Synthesis and characterization of 1,3-dihydro-benzo[b][1,4]diazepin-2-one derivatives: Part 4. In vivo active potent and selective non-competitive metabotropic glutamate receptor 2/3 antagonists.

This study completes a series of papers devoted to the characterization of the non-competitive mGluR2/3 antagonist properties of 1,3-dihydro-benzo[b][1,4]diazepin-2-one derivatives with particular emphasis on derivatizations compatible with brain penetration and in vivo activity. Especially the compounds bearing a para-pyridine consistently showed in vivo activity in rat behavioral models after oral administration, for example, blockade of the mGluR2/3 agonist LY354740-induced hypoactivity and improvement of a working memory deficit induced either by LY354740 or scopolamine in the delayed match to position task (DMTP). Moreover, combination studies with a cholinesterase inhibitor show apparent synergistic effects on working memory impairment induced by scopolamine.

Nociceptin receptor impairs recognition memory via interaction with NMDA receptor-dependent mitogen-activated protein kinase/extracellular signal-regulated kinase signaling in the hippocampus.

Strong evidence suggests a role for nociceptin/orphanin FQ (N/OFQ) neuropeptide and its receptor (NOP) in cognition. However, the signaling mechanisms underlying N/OFQ modulation of memory are less understood. Here, we show that intracerebroventricular or intrahippocampal infusions of N/OFQ impair long-term memory formation in the mouse object recognition task. The synthetic NOP receptor agonist, (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 (Ro64-6198), administered systemically, also produced amnesic effects that were blocked by coinfusion of the NOP receptor antagonist, [Nphe1,Arg14,Lys15]nociceptin-NH2 (UFP-101), into the dorsal hippocampus. In contrast, Ro64-6198 had no effect on short-term memory or recall performances. Immunoblotting analysis revealed a strong suppressive action of Ro64-6198 on learning-induced upregulation of hippocampal extracellular signal-regulated kinase (ERK) phosphorylation, which is crucial for long-term information storage. Accordingly, pharmacological inhibition of ERK activation after systemic injection of SL327 [alpha-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl)benzene acetonitrile], a selective inhibitor of the upstream kinase MEK (mitogen-activated protein kinase kinase), abolished long-term recognition memory formation. The noncompetitive NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine maleate (MK-801), given systemically, also suppressed ERK activation and disrupted recognition memory. In contrast, no effect of MK-801 was observed on recall, as for Ro64-6198. When administered concurrently at subthreshold doses, Ro64-6198 and MK-801 synergistically suppressed hippocampal ERK activation and impaired long-term memory formation. Under resting conditions, neither Ro64-6198 nor MK-801 affected spontaneous ERK activity in the hippocampus at the amnesic doses whereas at higher doses, only MK-801 had a suppressive effect. We conclude that N/OFQ-NOP receptor system negatively regulates long-term recognition memory formation through hippocampal ERK signaling mechanisms. This modulation may in part take place by inhibiting glutamatergic function at the NMDA receptor.

Activation of nociceptin opioid peptide (NOP) receptor impairs contextual fear learning in mice through glutamatergic mechanisms.

The present study investigated whether the selective nociceptin opioid peptide (NOP) receptor agonist, Ro64-6198, impairs acquisition of fear conditioning through glutamatergic mechanisms. Systemic administration of Ro64-6198 (0.3 and 1mg/kg) or the non-competitive NMDA receptor antagonist, MK-801 (0.03 and 0.1mg/kg) prior to conditioning severely impaired contextual but not cued fear learning in C57BL/6N mice. When administered together at sub-effective doses, Ro64-6198 (0.5mg/kg) and MK-801 (0.05mg/kg), synergistically impaired contextual fear learning, but left cued fear learning intact. We next used the immediate shock deficit paradigm (ISD) to examine the effects of Ro64-6198 and MK-801 on contextual memory formation in the absence of the foot-shock. As expected, naive mice that were shocked briefly after being placed in the training chamber displayed no contextual fear conditioning. This learning deficit was elevated by prior exposure of mice to the training context. Furthermore, administration of Ro64-6198 and MK-801, either separately at amnesic doses (1mg/kg and 0.1mg/kg, respectively) or concomitantly at sub-effective doses (0.5mg/kg and 0.05mg/kg, respectively) significantly reduced the facilitating effects of context preexposure. These findings demonstrate the existence of functional antagonism between NOP and NMDA receptors that predominantly contributes to modulation of conditioned fear learning which involves spatial-processing demands.

Fluorinated 9H-xanthene-9-carboxylic acid oxazol-2-yl-amides as potent, orally available mGlu1 receptor enhancers.

Small molecule mGluR1 enhancers, which are 9H-xanthene-9-carboxylic acid [1,2,4]oxadiazol-3-yl- and (2H-tetrazol-5-yl)-amides, have been previously reported. Fluorinated 9H-xanthene-9-carboxylic acid oxazol-2-yl-amides with improved pharmacokinetic properties have been designed and synthesized as useful pharmacological tools for the study of the physiological roles mediated by mGlu1 receptors. The synthesis and the structure-activity relationship of this class of positive allosteric modulators of mGlu1 receptors will be discussed in detail.

Discovery of the imidazo[1,5-a][1,2,4]-triazolo[1,5-d][1,4]benzodiazepine scaffold as a novel, potent and selective GABA(A) alpha5 inverse agonist series.

Through iterative design cycles we have discovered a number of novel new classes where the imidazo[1,5-a][1,2,4]-triazolo[1,5-d][1,4]benzodiazepine was deemed the most promising GABA(A) alpha5 inverse agonist class with potential for cognitive enhancement. This class combines a modest subtype binding selectivity with inverse agonism and has the most favourable molecular properties for further lead optimisation towards a central nervous system (CNS) acting medicine.

Synthesis and characterization of 8-ethynyl-1,3-dihydro-benzo[b][1,4]diazepin-2-one derivatives: part 2. New potent non-competitive metabotropic glutamate receptor 2/3 antagonists.

A series of 1,3-dihydro-benzo[b][1,4]diazepin-2-one derivatives was evaluated as non-competitive mGluR2/3 antagonists. Replacement of a cyano group by a five-membered heterocycle produced compounds inhibiting the binding of [(3)H]-LY354740 to rat mGluR2 with low nanomolar affinity and consistent functional effect at both mGluR2 and mGluR3. Further modification to improve the physicochemical properties led eventually to compounds with the ability to reverse LY354740-mediated inhibition of field excitatory postsynaptic potentials in the rat dentate gyrus.

Synthesis and characterization of 1,3-dihydro-benzo[b][1,4]diazepin-2-one derivatives: Part 3. New potent non-competitive metabotropic glutamate receptor 2/3 antagonists.

A series of 1,3-dihydro-benzo[b][1,4]diazepin-2-one derivatives was evaluated as non-competitive mGluR2/3 antagonists. Replacement of the (2-aryl)-ethynyl-moiety in 8-position with smaller less lipophilic substituents produced compounds inhibiting the binding of [3H]-LY354740 to rat mGluR2 with low nanomolar affinity and consistent functional effect at both mGluR2 and mGluR3. These compounds were able to reverse LY354740-mediated inhibition of field excitatory postsynaptic potentials in the rat dentate gyrus and in vivo activity could be demonstrated by reversal of the LY354740-induced hypoactivity in mice after oral administration.

Synthesis and characterization of 8-ethynyl-1,3-dihydro-benzo[b][1,4]diazepin-2-one derivatives: new potent non-competitive metabotropic glutamate receptor 2/3 antagonists. Part 1.

A series of 1,3-dihydrobenzo[b][1,4]diazepin-2-one derivatives was evaluated as non-competitive mGluR2/3 antagonists. Attachment of an 8-(2-aryl)-ethynyl-moiety produced compounds inhibiting the binding of [(3)H]-LY354740 to rat mGluR2 with low nanomolar affinity and consistent functional effect at both mGluR2 and mGluR3.

Distribution and abundance of metabotropic glutamate receptor subtype 2 in rat brain revealed by [3H]LY354740 binding in vitro and quantitative radioautography: correlation with the sites of synthesis, expression, and agonist stimulation of [35S]GTPgammas binding.

Until recently, there was a lack of selective radioligands for the subtypes of metabotropic glutamate (mGlu) receptors. [(3)H]LY354740 ((+)-2-aminobicyclo[3,1,0]hexane-2,6-dicarboxylic acid), a selective agonist for group II receptors (mGlu2 and -3, which are negatively coupled to cAMP production), has now been used to map their brain distribution and abundance by in vitro binding and quantitative radioautography. The selective cation dependence of its binding allowed the discrimination between mGlu2 and mGlu3 receptor labeling. Thus, in the presence of Ca(2+) and Mg(2+) ions, the agonist bound selectively to mGlu2 receptors as evidenced by: 1) the correlative distribution and abundance of binding sites (highest in the lacunosum moleculare of the hippocampus and lowest in white matter) with mGlu2 receptor mRNA and protein revealed by in situ hybridization histochemistry and immunohistochemistry, respectively; 2) its selective pharmacology; and 3) the distribution of LY354740-stimulated [(35)S]GTPgammaS binding (25-97% above basal, according to the brain region), revealing G protein-coupled receptor coupling to G(i) proteins. Nonspecific binding (in the presence of 10 muM DCG-IV, a group II-selective, mGlu2-preferring, receptor agonist) was <10% of total. In adjacent sections, the distribution of binding sites for [(3)H]DCG-IV was very similar. This extensive study paves the way for investigations of the regional expression and regulation of mGlu2 receptors in human CNS diseases, such as Alzheimer's disease, which may reveal their functional roles and identify potential therapeutic drug targets. Indeed, it has recently been demonstrated (Higgins et al. [2004] Neuropharmacology 46:907-917) that pharmacological manipulation of mGlu2 receptors influences cognitive performance in the rodent.

The effect of a systemically active ORL-1 agonist, Ro 64-6198, on the acquisition, expression, extinction, and reinstatement of morphine conditioned place preference.

ORL-1 agonists have been proposed as potential therapeutics for substance abuse based on their propensity to counter the effects of mu opioid agonists in several systems, and to inhibit mesolimbic dopamine release, while mostly being devoid of aversive properties. In support of this, ORL-1 agonists have been shown to block the acquisition of morphine conditioned place preference (CPP). We investigated the effect of Ro 64-6198, a systemically active ORL-1 agonist, on the acquisition, expression, extinction, and reinstatement of morphine (20 mg/kg, s.c.) CPP in C57BL6/J mice. Similar to effects obtained with nociceptin/orphanin FQ, Ro 64-6198 (1 mg/kg, i.p.) blocked the acquisition of morphine CPP when given 15 min prior to each drug and vehicle conditioning session. This effect was not due to state dependent learning, since when tested again in the presence of Ro 64-6198 or vehicle no CPP was observed. Administration of Ro 64-6198 (0.3 or 1 mg/kg, i.p.) on the test day alone, in a separate group of animals, failed to block the expression of morphine CPP. Another group of mice was conditioned to morphine to develop CPP, and then exposed to the CPP chambers in the absence of drug once a day for 30 min to extinguish the CPP. Ro 64-6198 (1 mg/kg, i.p.) given 15 min prior to each session during extinction did not affect the rate of extinction. Finally, another group was conditioned to morphine, their CPP extinguished and subsequently reinstated by a priming injection of morphine (20 mg/kg, s.c.). Ro 64-6198 (1 mg/kg, i.p.), given 15 min prior to the priming injection, blocked reinstatement of morphine CPP. These results suggest that Ro 64-6198's effects may be limited to attenuation of the acute rewarding effects of morphine.

Effects of the mGluR2/3 agonist LY354740 on computerized tasks of attention and working memory in marmoset monkeys.

RATIONALE: LY354740 is a recently developed metabotropic glutamatergic receptor 2 and 3 (mGluR2/3) agonist. A high density of mGluR2 has been reported in terminal fields of the perforant path in rodents and humans, suggesting its involvement in cognitive functions mediated by the temporal lobe, including memory. A small number of in vivo studies in rodents have assessed the effects of LY354740 on memory tasks, reporting the induction of impaired memory for spatial orientation in a water maze task and for delayed match and non-match to position in an operant version of these tasks. OBJECTIVE: In the present primate study, we used radioautography to describe the distribution and intensity of (3)H-LY354740 binding in the hippocampal formation of the common marmoset monkey (Callithrix jacchus) relative to the rat. In the major, in vivo part of the study, the effects of systemic LY354740 on computerized tasks of attention and memory were investigated. METHODS: Adult common marmosets were trained to perform a five-choice serial reaction time (5-CSRT) task and a concurrent delayed match-to-position (CDMP) task from the Cambridge Neuropsychological Automated test Battery (CANTAB). Filter tests of LY354740 effects on motor dexterity and motivation for reward revealed high inter-individual variation in sensitivity; therefore, on the 5-CSRT, subjects were tested at a dose range of 3--10 mg/kg, and on the CDMP, subjects were tested at 1--3 or 3--10 mg/kg. RESULTS: Radioautography revealed a relatively low level of (3)H-LY354740 binding in the marmoset hippocampal formation compared to the rat. Despite low binding, LY354740 reduced sustained-attention accuracy in the 5-CSRT, and reduced accuracy in two stages of the CDMP. CONCLUSIONS: The current study provides novel evidence for the importance of mGluR2/3 in the regulation of primate cognitive functioning.

Metabotropic glutamate receptor 5 negative allosteric modulators: discovery of 2-chloro-4-[1-(4-fluorophenyl)-2,5-dimethyl-1H-imidazol-4-ylethynyl]pyridine (basimglurant, RO4917523), a promising novel medicine for psychiatric diseases.

Negative allosteric modulators (NAMs) of metabotropic glutamate receptor 5 (mGlu5) have potential for the treatment of psychiatric diseases including depression, fragile X syndrome (FXS), anxiety, obsessive-compulsive disorders, and levodopa induced dyskinesia in Parkinson's disease. Herein we report the optimization of a weakly active screening hit 1 to the potent and selective compounds chloro-4-[1-(4-fluorophenyl)-2,5-dimethyl-1H-imidazol-4-ylethynyl]pyridine (basimglurant, 2) and 2-chloro-4-((2,5-dimethyl-1-(4-(trifluoromethoxy)phenyl)-1H-imidazol-4-yl)ethynyl)pyridine (CTEP, 3). Compound 2 is active in a broad range of anxiety tests reaching the same efficacy but at a 10- to 100-fold lower dose compared to diazepam and is characterized by favorable DMPK properties in rat and monkey as well as an excellent preclinical safety profile and is currently in phase II clinical studies for the treatment of depression and fragile X syndrome. Analogue 3 is the first reported mGlu5 NAM with a long half-life in rodents and is therefore an ideal tool compound for chronic studies in mice and rats.

Novel hexahydrospiro[piperidine-4,1'-pyrrolo[3,4-c]pyrroles]: highly selective small-molecule nociceptin/orphanin FQ receptor agonists.

Novel hexahydrospiro[piperidine-4,1'-pyrrolo[3,4-c]pyrroles that act as potent and selective orphanin FQ/nociceptin (N/OFQ) receptor (NOP) agonists were identified. The best compound, (+)-5a, potently inhibited 3H-N/OFQ binding to the NOP receptor (K(i) = 0.49 nM) but was >1000-fold less potent in binding to MOP, KOP, and DOP opiate receptors. Further, (+)-5a potently stimulated GTP gamma S binding to NOP membranes (EC50 = 65 nM) and inhibited forskolin-mediated cAMP accumulation in NOP-expressing cells (EC50 = 9.1 nM) with a potency comparable to that of the natural peptide agonist N/OFQ. These results indicate that (+)-5a is a highly selective and potent small-molecule full agonist of the NOP receptor.

Alkyl diphenylacetyl, 9H-xanthene- and 9H-thioxanthene-carbonyl carbamates as positive allosteric modulators of mGlu1 receptors.

Starting from the random-screening hit 1a, a series of alkyl diphenylacetyl, 9H-xanthene- and 9H-thioxanthene-carbonyl carbamates 1 has been prepared. These derivatives turned out to be selective positive allosteric modulators of mGlu1 receptors. These compounds do not directly activate mGlu1 receptors but markedly potentiate agonist stimulated responses, increasing potency and maximum efficacy.

Further characterization of the prototypical nociceptin/orphanin FQ peptide receptor agonist Ro 64-6198 in rodent models of conflict anxiety and despair.

RATIONALE: Ro 64-6198, the prototypical non-peptide nociceptin/orphanin FQ peptide (NOP) receptor agonist, has potent anxiolytic-like effects in several preclinical models and species. However the effects of Ro 64-6198 on distinctive anxiety-provoking conditions related to unconditioned conflict behavior as well as its role in despair-like behavior remain to be addressed. OBJECTIVE: Here we examined the effects of Ro 64-6198 on unconditioned conflict anxiety using stimuli with different salience and on regulation of autonomic reactivity and compared these to the effects of benzodiazepine receptor agonists. We also addressed the potential effects of Ro 64-6198 on despair-like behavior. MATERIALS AND METHODS: Ro 64-6198 (0.1 to 10 mg/kg i.p.) and either diazepam or chlordiazepoxide were tested in the Vogel conflict punished drinking test (VCT) in Sprague Dawley rats, in the social approach-avoidance (SAA) test in Lewis rats, in the novelty-induced hypophagia (NIH) in C57BL/6J mice, and in stress-induced hyperthermia in NMRI mice, as well as in the forced swim test (FST) in Sprague Dawley rats and the tail suspension test (TST) in C57BL/6J mice. RESULTS: Ro 64-6198 (0.3 to 3 mg/kg) dose-dependently produced anxiolytic-like effects in the VCT, SAA, NIH, and SIH, similar to benzodiazepine receptor agonists. Ro 64-6198 did not alter immobility time in the FST and TST. CONCLUSIONS: Ro 64-6198 produced marked anxiolytic-like effects in response to a variety of mild to strong anxiogenic stimuli, whereas it did not facilitate depression-related behaviors. This data extend previous literature suggesting that NOP receptors are a viable target for the treatment of anxiety disorders.