Modulation of 5-hydroxytryptamine efflux from rat cortical synaptosomes by opioids and nociceptin

Role of Nociceptin/Orphanin FQ-NOP Receptor System in the Regulation of Stress-Related Disorders

Nociceptin/orphanin FQ (N/OFQ) is a 17-residue neuropeptide that binds the nociceptin opioid-like receptor (NOP). N/OFQ exhibits nucleotidic and aminoacidics sequence homology with the precursors of other opioid neuropeptides but it does not activate either MOP, KOP or DOP receptors. Furthermore, opioid neuropeptides do not activate the NOP receptor. Generally, activation of N/OFQ system exerts anti-opioids effects, for instance toward opioid-induced reward and analgesia. The NOP receptor is widely expressed throughout the brain, whereas N/OFQ localization is confined to brain nuclei that are involved in stress response such as amygdala, BNST and hypothalamus. Decades of studies have delineated the biological role of this system demonstrating its involvement in significant physiological processes such as pain, learning and memory, anxiety, depression, feeding, drug and alcohol dependence. This review discusses the role of this peptidergic system in the modulation of stress and stress-associated psychiatric disorders in particular drug addiction, mood, anxiety and food-related associated-disorders. Emerging preclinical evidence suggests that both NOP agonists and antagonists may represent a effective therapeutic approaches for substances use disorder. Moreover, the current literature suggests that NOP antagonists can be useful to treat depression and feeding-related diseases, such as obesity and binge eating behavior, whereas the activation of NOP receptor by agonists could be a promising tool for anxiety.

Role of nociceptin/orphanin FQ and nociceptin opioid peptide receptor in depression and antidepressant effects of nociceptin opioid peptide receptor antagonists

Nociceptin/orphanin FQ (N/OFQ) and its receptor, nociceptin opioid peptide (NOP) receptor, are localized in brain areas implicated in depression including the amygdala, bed nucleus of the stria terminalis, habenula, and monoaminergic nuclei in the brain stem. N/OFQ inhibits neuronal excitability of monoaminergic neurons and monoamine release from their terminals by activation of G protein-coupled inwardly rectifying K⁺ channels and inhibition of voltage sensitive calcium channels, respectively. Therefore, NOP receptor antagonists have been proposed as a potential antidepressant. Indeed, mounting evidence shows that NOP receptor antagonists have antidepressant-like effects in various preclinical animal models of depression, and recent clinical studies again confirmed the idea that blockade of NOP receptor signaling could provide a novel strategy for the treatment of depression. In this review, we describe the pharmacological effects of N/OFQ in relation to depression and explore the possible mechanism of NOP receptor antagonists as potential antidepressants.

Multiple pharmacological and toxicological investigations on Tanacetum parthenium and Salix alba extracts: Focus on potential application as anti-migraine agents

Migraine is one of the most common neurological disorder, which has long been related to brain serotonin (5-HT) depletion and neuro-inflammation. Despite many treatment options are available, the frequent occurrence of unacceptable adverse effects further supports the research toward nutraceuticals and herbal preparations, among which Tanacetum parthenium and Salix alba showed promising anti-inflammatory and neuro-modulatory activities. The impact of extract treatment on astrocyte viability, spontaneous migration and apoptosis was evaluated. Anti-inflammatory/anti-oxidant effects were investigated on isolated rat cortexes exposed to a neurotoxic stimulus. The lactate dehydrogenase (LDH) release, nitrite levels and 5-HT turnover were evaluated, as well. A proteomic analysis was focused on specific neuronal proteins and a fingerprint analysis was carried out on selected phenolic compounds. Both extracts appeared able to exert in vitro anti-oxidant and anti-apoptotic effects. S. alba and T. parthenium extracts reduced LDH release, nitrite levels and 5-HT turnover induced by neurotoxic stimulus. The downregulation of selected proteins suggest a neurotoxicity, which could be ascribed to an elevated content of gallic acid in both S. alba and T. parthenium extracts. Concluding, both extracts exert neuroprotective effects, although the downregulation of key proteins involved in neuron physiology suggest caution in their use as food supplements.

Comprehensive Chemical Profiling and Multidirectional Biological Investigation of Two Wild Anthemis Species (Anthemis tinctoria var. Pallida and A. cretica subsp. tenuiloba): Focus on Neuroprotective Effects

Ethyl acetate (EA), methanol (MeOH), and aqueous extracts of aerial parts of Anthemis tinctoria var. pallida (ATP) and A. cretica subsp. tenuiloba (ACT) were investigated for their phenol and flavonoid content, antioxidant, and key enzyme inhibitory potentials. All extracts displayed antiradical effects, with MeOH and aqueous extracts being a superior source of antioxidants. On the other hand, EA and MeOH extracts were potent against AChE and BChE. Enzyme inhibitory effects against tyrosinase and α-glucosidase were observed, as well. We also studied Anthemis extracts in an ex vivo experimental neurotoxicity paradigm. We assayed extract influence on oxidative stress and neurotransmission biomarkers, including lactate dehydrogenase (LDH) and serotonin (5-HT), in isolated rat cortex challenged with K⁺ 60 mM Krebs-Ringer buffer (excitotoxicity stimulus). An untargeted proteomic analysis was finally performed in order to explore the putative mechanism in the brain. The pharmacological study highlighted the capability of ACT water extract to blunt K⁺ 60 mM increase in LDH level and 5-HT turnover, and restore physiological activity of specific proteins involved in neuron morphology and neurotransmission, including NEFMs, VAMP-2, and PKCγ, thus further supporting the neuroprotective role of ACT water extract.

Nociceptin/orphanin FQ receptor agonists increase aggressiveness in the mouse resident-intruder test

Aggressive behaviors can be considered symptoms of bipolar disorder, schizophrenia, post-traumatic stress, intermittent explosive, and personality disorders. Nociceptin/orphanin FQ (N/OFQ) is a peptide acting as endogenous ligand of the NOP receptor. Preclinical and clinical findings suggest the NOP receptor as an innovative target for the treatment of psychopathologies, such as anxiety, depression, and drug abuse. This study investigated the effects of NOP ligands and the behavioral phenotype of mice lacking the NOP receptor in an animal model of aggressiveness, the resident-intruder test. Mood stabilizers, such as valproate, lithium, and carbamazepine reduced aggressive behaviors of resident mice, while diazepam was inactive. In contrast, para-chlorophenylalanine (PCPA), an inhibitor of 5-HT synthesis, increased aggressiveness in mice. Similar to PCPA, the treatment with the NOP agonists Ro 65-6570 and AT-090 also increased aggressive behaviors. The systemic administration of the NOP antagonist SB-612111 did not modify the behavior of resident mice, but it prevented the aggressive behavior of Ro 65-6570. NOP receptor knockout mice did not display any behavioral difference compared to wild-type animals in the resident-intruder test. None of the treatments affected non-agonistic behaviors and spontaneous locomotion. In conclusion, NOP receptor agonists increased aggressiveness, while the pharmacological and genetic blockade of NOP receptor signaling did not modify agonistic behaviors. Ultimately, the aggressive profile of action of NOP agonists should be taken into account in the development of innovative psychiatric drugs targeting the NOP receptor.

NOP receptors in the prelimbic cortex have an inhibitory influence on cardiovascular responses induced by restraint stress

Nociceptin/orphanin FQ (N/OFQ) and its receptor (NOP) have structural homology with classic opioids, but constitute a distinct neurotransmitter system because they lack affinity for the opioid peptides and receptors. This neurotransmission is implicated in several physiologic processes, but the role played by NOP receptors during stress situations remains unclear. The acute restraint stress (RS) is a model of unavoidable stress, characterized by sustained increases in mean arterial pressure (MAP), heart rate (HR) and a drop in tail temperature. On another side, the prelimbic (PL) and infralimbic (IL) cortices, subdivisions of the medial prefrontal cortex (MPFC), are implicated in the modulation of functional responses caused by RS. Considering that, the objective of the present study was to investigate the involvement of PL and IL NOP receptors in the control of autonomic responses induced by RS. Bilateral microinjection of nociceptin (NOP agonist) into the PL reduced the cardiovascular responses evoked by RS. Bilateral microinjection of UPF-101 (NOP antagonist) into the PL potentiated the pressor and tachycardiac responses evoked by RS, in a dose-dependent manner. Local pretreatment with UPF-101 blocked the RS-evoked changes following nociceptin administration into the PL. None of these treatments affected the drop in tail temperature induced by RS. Otherwise, the administration of nociceptin or UPF-101 into the IL had no effect on RS-evoked autonomic changes. To investigate the peripheral mechanism involved in the increase in the RS-evoked cardiovascular responses induced by the blockade of PL NOP receptors, rats were intravenous pretreated with either homatropine or atenolol. The intravenous treatment with homatropine blunted the increase in the RS-evoked pressor and tachycardiac response induced by the PL treatment with UPF-101, while the intravenous treatment with atenolol did not affect the RS-evoked pressor and tachycardiac response induced by the PL treatment with UPF-101. In conclusion, our study shows an influence of the PL N/OFQ neurotransmission, but not the IL NOP receptors, in the control of cardiovascular responses observed during acute stress, by increasing cardiac parasympathetic activity.

Safety, tolerability, and clinical effect of low-dose buprenorphine for treatment-resistant depression in midlife and older adults

OBJECTIVE

To describe the clinical effect and safety of low-dose buprenorphine, a κ-opioid receptor antagonist, for treatment-resistant depression (TRD) in midlife and older adults.

METHOD

In an 8-week open-label study, buprenorphine was prescribed for 15 adults aged 50 years or older with TRD, diagnosed with the Structured Clinical Interview for DSM-IV, between June 2010 and June 2011. The titrated dose of buprenorphine ranged from 0.2-1.6 mg/d. We assessed clinical change in depression, anxiety, sleep, positive and negative affect, and quality of life. The Montgomery-Asberg Depression Rating scale (MADRS) served as the main outcome measure. Tolerability was assessed by documenting side effects and change in vital signs, weight, and cognitive function. Clinical response durability was assessed 8 weeks after discontinuation of buprenorphine.

RESULTS

The mean dose of buprenorphine was 0.4 mg/d (mean maximum dose = 0.7 mg/d). The mean depression score (MADRS) at baseline was 27.0 (SD = 7.3) and at week 8 was 9.5 (SD = 9.5). A sharp decline in depression severity occurred during the first 3 weeks of exposure (mean change = -15.0 [SD = 7.9]). Depression-specific items measuring pessimism and sadness indicated improvement during exposure, supporting a true antidepressant effect. Treatment-emergent side effects (in particular, nausea and constipation) were not sustained, vital signs and weight remained stable, and executive function and learning improved from pretreatment to posttreatment.

CONCLUSION

Low-dose buprenorphine may be a novel-mechanism medication that provides a rapid and sustained improvement for older adults with TRD. Placebo-controlled trials of longer duration are required to assess efficacy, safety, and physiologic and psychological effects of extended exposure to this medication.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT01071538.

Nociceptin/orphanin FQ receptor antagonists as innovative antidepressant drugs

Nociceptin/orphanin FQ (N/OFQ) and its receptor (NOP) were identified in the mid 90s as a novel peptidergic system structurally related to opioids. A growing body of preclinical evidence suggests that blockade of NOP receptors evokes antidepressant-like actions. These have been explored using a range of compounds (peptide and non peptide antagonists), across different species (rat and mouse) and assays (behavioral despair and chronic mild stress) suggesting a robust and consistent antidepressant-like effect. Moreover, rats and mice knockout for the NOP receptor gene display an antidepressant-like phenotype in behavioral despair assays. Electrophysiological, immunohistochemical and neurochemical studies point to an important role played by monoaminergic systems, particularly 5-HTergic, in mediating the antidepressant-like properties of NOP antagonists. However other putative mechanisms of action, including modulation of the CRF system, circadian rhythm and a possible neuroendocrine-immune control might be involved. A close relationship between the N/OFQ-NOP receptor system and stress responses is well described in the literature. Stressful situations also alter endocrine, behavioral and neurochemical parameters in rats and chronic administration of a NOP antagonist restored these alterations. Interestingly, clinical findings showed that plasma N/OFQ levels were significantly altered in major and post-partum depression, and bipolar disease patients. Collectively, data in the literature support the notion that blockade of NOP receptor signaling could be a novel and interesting strategy for the development of innovative antidepressants.

Mapping neurotransmitter networks with PET: an example on serotonin and opioid systems

All functions of the human brain are consequences of altered activity of specific neural pathways and neurotransmitter systems. Although the knowledge of "system level" connectivity in the brain is increasing rapidly, we lack "molecular level" information on brain networks and connectivity patterns. We introduce novel voxel-based positron emission tomography (PET) methods for studying internal neurotransmitter network structure and intercorrelations of different neurotransmitter systems in the human brain. We chose serotonin transporter and μ-opioid receptor for this analysis because of their functional interaction at the cellular level and similar regional distribution in the brain. Twenty-one healthy subjects underwent two consecutive PET scans using [(11)C]MADAM, a serotonin transporter tracer, and [(11)C]carfentanil, a μ-opioid receptor tracer. First, voxel-by-voxel "intracorrelations" (hub and seed analyses) were used to study the internal structure of opioid and serotonin systems. Second, voxel-level opioid-serotonin intercorrelations (between neurotransmitters) were computed. Regional μ-opioid receptor binding potentials were uniformly correlated throughout the brain. However, our analyses revealed nonuniformity in the serotonin transporter intracorrelations and identified a highly connected local network (midbrain-striatum-thalamus-amygdala). Regionally specific intercorrelations between the opioid and serotonin tracers were found in anteromedial thalamus, amygdala, anterior cingulate cortex, dorsolateral prefrontal cortex, and left parietal cortex, i.e., in areas relevant for several neuropsychiatric disorders, especially affective disorders. This methodology enables in vivo mapping of connectivity patterns within and between neurotransmitter systems. Quantification of functional neurotransmitter balances may be a useful approach in etiological studies of neuropsychiatric disorders and also in drug development as a biomarker-based rationale for targeted modulation of neurotransmitter networks.

Nociceptin/orphanin FQ receptor agonists attenuate L-DOPA-induced dyskinesias

In the present study we investigated whether the neuropeptide nociceptin/orphanin FQ (N/OFQ), previously implicated in the pathogenesis of Parkinson's disease, also affects L-DOPA-induced dyskinesia. In striatal slices of naive rodents, N/OFQ (0.1-1 μm) prevented the increase of ERK phosphorylation and the loss of depotentiation of synaptic plasticity induced by the D1 receptor agonist SKF38393 in spiny neurons. In vivo, exogenous N/OFQ (0.03-1 nmol, i.c.v.) or a synthetic N/OFQ receptor agonist given systemically (0.01-1 mg/Kg) attenuated dyskinesias expression in 6-hydroxydopamine hemilesioned rats primed with L-DOPA, without causing primary hypolocomotive effects. Conversely, N/OFQ receptor antagonists worsened dyskinesia expression. In vivo microdialysis revealed that N/OFQ prevented dyskinesias simultaneously with its neurochemical correlates such as the surge of nigral GABA and glutamate, and the reduction of thalamic GABA. Regional microinjections revealed that N/OFQ attenuated dyskinesias more potently and effectively when microinjected in striatum than substantia nigra (SN) reticulata, whereas N/OFQ receptor antagonists were ineffective in striatum but worsened dyskinesias when given in SN. Quantitative autoradiography showed an increase in N/OFQ receptor binding in striatum and a reduction in SN of both unprimed and dyskinetic 6-hydroxydopamine rats, consistent with opposite adaptive changes of N/OFQ transmission. Finally, the N/OFQ receptor synthetic agonist also reduced dyskinesia expression in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated dyskinetic macaques without affecting the global parkinsonian score. We conclude that N/OFQ receptor agonists may represent a novel strategy to counteract L-DOPA-induced dyskinesias. Their action is possibly mediated by upregulated striatal N/OFQ receptors opposing the D1 receptor-mediated overactivation of the striatonigral direct pathway.

Nociceptin/Orphanin FQ in PAG modulates the release of amino acids, serotonin and norepinephrine in the rostral ventromedial medulla and spinal cord in rats

High density Nociceptin/Orphanin FQ (N/OFQ) and its receptor (NOPr) have been found in the ventrolateral periaqueductal gray (vlPAG), a main output pathway involved in the descending pain-control system. Our previous study demonstrated that the microinjection of N/OFQ into the vlPAG markedly facilitated nociceptive responses of spinal dorsal horn neurons. The aim of the present work was to further provide evidence for the supraspinal mechanisms of action for N/OFQ-mediated nociceptive facilitation by examining the effect of N/OFQ in the vlPAG on neurotransmitter release in the descending pain-control system, including the nucleus raphe magnus (NRM), nucleus reticularis gigantocellularis (NGC) and dorsal horn of the spinal cord. The results showed that the microinjection of N/OFQ into the vlPAG produced robust decreases in 5-hydroxytryptamine (5-HT, serotonin), norepinephrine (NE), and gamma-aminobutyric acid (GABA), and increase in glutamate (Glu) release in the spinal dorsal horn. Spinal application of 5-HT, 2-Me-5-HT (5-HT(3) receptor agonist), muscimol (GABA(A) receptor agonist), and baclofen (GABA(B) receptor agonist) significantly blocked intra-vlPAG-induced facilitation on nociceptive responses. However, the extracellular concentrations of these neurotransmitters in the NRM and NGC exhibited diversity following intra-vlPAG of N/OFQ. In the NRM, intra-vlPAG injection of N/OFQ significantly decreased 5-HT, NE, and Glu, but increased GABA release. Differently, in the NGC, both NE and GABA releases were attenuated by intra-vlPAG of N/OFQ, whereas the concentration of 5-HT and Glu exhibited a trend to increase. These findings provide direct support for the hypothesis that intra-PAG of N/OFQ-induced facilitation of nociceptive responses is associated with the release of 5-HT, NE, and amino acids.

Inhibition of serotonin outflow by nociceptin/orphaninFQ in dorsal raphe nucleus slices from normal and stressed rats: Role of corticotropin releasing factor

In the dorsal raphe nucleus (DRN) many inputs converge and interact to modulate serotonergic neuronal activity and the behavioral responses to stress. The effects exerted by two stress-related neuropeptides, corticotropin releasing factor (CRF) and nociceptin/orphaninFQ (N/OFQ), on the outflow of [(3)H]5- hydroxytryptamine were investigated in superfused rat dorsal raphe nucleus slices. Electrical stimulation (100 mA, 1 ms for 2 min) evoked a frequency-dependent peak of [(3)H]5- hydroxytryptamine outflow, which was sodium and calcium-dependent. Corticotropin releasing factor (1-100 nM), concentration-dependently inhibited the stimulation (3 Hz)-evoked [(3)H]5-hydroxytryptamine outflow; the inhibition by 30 nM corticotropin releasing factor (to 68 +/- 5.7%) was prevented both by the non selective CRF receptor antagonist alpha-helicalCRF(9-41) (alpha-HEL) (300 nM) and by the CRF(1) receptor antagonist antalarmin (ANT) (100 nM). The CRF(2) agonist urocortin II (10 nM) did not modify [(3)H]5- hydroxytryptamine outflow, ruling out the involvement of CRF(2) receptors. Bicuculline (BIC), a GABAA antagonist (10 microM), prevented the inhibitory effect of corticotropin releasing factor (30 nM), supporting the hypothesis that the inhibition was mediated by increased gamma-aminobutyric acid (GABA) release. Nociceptin/ orphaninFQ (1 nM-1 microM) exerted an antalarmin- and bicuculline-insensitive inhibition on [(3)H]5- hydroxytryptamine outflow, with the maximum at 100 nM (to 63+/- 4.2%), antagonized by the NOP receptor antagonist UFP-101 (1 microM). Dorsal raphe nucleus slices prepared from rats exposed to 15 min of forced swim stress displayed a reduced [(3)H]5-hydroxytryptamine outflow, in part reversed by antalarmin and further inhibited by nociceptin/orphaninFQ. These findings indicate that (i) both corticotropin releasing factor and nociceptin/orphaninFQ exert an inhibitory control on dorsal raphe nucleus serotonergic neurons; (ii) the inhibition by corticotropin releasing factor involves gamma-aminobutyric acid neurons; (iii) nociceptin/ orphaninFQ inhibits dorsal raphe nucleus serotonin system in a corticotropin releasing factor- and gamma-aminobutyric acid-independent manner; (iv) nociceptin/orphaninFQ modulation is still operant in slices prepared from stressed rats. The nociceptin/orphaninFQ-NOP receptor system could represent a new target for drugs effective in stress-related disorders.

Rewarding morphine-induced synaptic function of delta-opioid receptors on central glutamate synapses

The rewarding effect of opioids, the driving force for compulsive behaviors of opioid abuse and addiction, is primarily mediated by the mu-opioid receptor. However, the role of the delta-opioid receptor (DOR) in opioid reward and addiction is still poorly understood. The recently discovered adaptive DOR property of exocytotic translocation in sensory neurons after chronic opioid exposure provides a new avenue of conceptual thoughts to exploring the DOR function in this psychoneurological disease. In this study, we investigated potential adaptive function of DOR in neurons of the central nucleus of the amygdala (CeA), a forebrain structure increasingly recognized for mediating stimulus reward learning in drug addiction. Using whole-cell recordings in CeA slices, we found that in rats displaying morphine-induced behavior of conditioned place preference, a behavioral measure of drug reward, the overall synaptic strength of glutamate synapses in CeA neurons was significantly enhanced. The selective DOR agonist [D-Pen(2),D-Pen(5)]-enkephalin, having no apparent effect on glutamatergic excitatory postsynaptic current (EPSC) in neurons from control rats, produced a significant, dose-dependent inhibition of the synaptic current in neurons from those morphine-treated rats. Detailed analyses of EPSC properties revealed that DOR activation inhibited the EPSC by reducing presynaptic release of glutamate, indicating functional DOR emerging on presynaptic glutamate terminals. The morphine treatment also significantly increased DOR proteins in CeA preparations of synaptosomes. These findings provide functional evidence for an adaptive modulation by presynaptic DOR of a key synaptic activity altered by morphine, thus implying likely important involvement of DOR in opioid reward and addiction.

Nociceptin/orphanin FQ modulates motor behavior and primary motor cortex output through receptors located in substantia nigra reticulata

This study was set to investigate whether motor effects of nociceptin/orphanin FQ (N/OFQ) can be related to changes in primary motor cortex output. N/OFQ injected i.c.v. biphasically modulated motor performance, low doses being facilitating and higher ones inhibitory. These effects were counteracted by the N/OFQ receptor antagonist [Nphe(1) Arg(14),Lys(15)]N/OFQ-NH(2) (UFP-101) confirming the specificity of N/OFQ action. However, UFP-101 alone facilitated motor performance, suggesting that endogenous N/OFQ inhibits motor function. N/OFQ and UFP-101 injected into the substantia nigra reticulata but not motor cortex replicated these effects, suggesting motor responses were mediated by subcortical circuits involving the basal ganglia. Intracortical microstimulation technique showed that i.c.v. N/OFQ also biphasically modulated motor cortex excitability and movement representation. Low N/OFQ doses caused a leftward shift of threshold distribution curve in the forelimb area without affecting the number of effective sites. Conversely, high N/OFQ doses increased unresponsive and reduced excitable (movement) sites in vibrissa but not forelimb area. However, increased threshold currents and rightward shift of threshold distribution curve were observed in both areas, suggesting an overall inhibitory effect on cortical motor output. UFP-101 alone evoked effects similar to low N/OFQ doses, suggesting tonic inhibitory control over forelimb movement by endogenous N/OFQ. As shown in behavioral experiments, these effects were replicated by intranigral, but not intracortical, N/OFQ or UFP-101 injections. We conclude that N/OFQ receptors located in the substantia nigra reticulata mediate N/OFQ biphasic control over motor behavior, possibly through changes of primary motor cortex output.

The orphanin FQ/nociceptin (OFQ/N) system

Orphanin FQ/nociceptin (OFQ/N) was the first novel neuropeptide discovered as the natural ligand of an orphan G protein-coupled receptor (GPCR). Orphan GPCRs are proteins classified as receptors on the basis of their sequence similarities to known GPCRs but that lack the ligands that activate them in vivo. One such orphan GPCR exhibited sequence similarities with the opioid receptors. OFQ/N was isolated as its natural ligand and shown to also share sequence similarities to the opioid peptides. This led to numerous studies attempting to find functional similarities and differences between the OFQ/N and opioid systems. This chapter will summarize our knowledge of the OFQ/N system and of its roles in the organism.

Nocistatin inhibits 5-hydroxytryptamine release in the mouse neocortex via presynaptic Gi/o protein linked pathways

BACKGROUND AND PURPOSE

Nocistatin (NST) is a neuropeptide generated from cleavage of the nociceptin/orphanin FQ (N/OFQ) precursor. Evidence has been presented that NST acts as a functional antagonist of N/OFQ, although NST receptor and transduction pathways have not yet been identified. We previously showed that N/OFQ inhibited [(3)H]5-hydroxytryptamine ([(3)H]5-HT) release from mouse cortical synaptosomes via activation of NOP receptors. We now investigate whether NST regulates [(3)H]5-HT release in the same preparation.

EXPERIMENTAL APPROACH

Mouse and rat cerebrocortical synaptosomes in superfusion, preloaded with [(3)H]5-HT and stimulated with 1 min pulses of 10 mM KCl, were used.

KEY RESULTS

Bovine NST (b-NST) inhibited the K(+)-induced [(3)H]5-HT release, displaying similar efficacy but lower potency than N/OFQ. b-NST action underwent concentration-dependent and time-dependent desensitization, and was not prevented either by the NOP receptor antagonist [Nphe(1) Arg(14),Lys(15)]N/OFQ(1-13)-NH(2) (UFP-101) or by the non-selective opioid receptor antagonist, naloxone. Contrary to N/OFQ, b-NST reduced [(3)H]5-HT release from synaptosomes obtained from NOP receptor knockout mice. However, both N/OFQ and NST were ineffective in synaptosomes pre-treated with the G(i/o) protein inhibitor, Pertussis toxin. NST-N/OFQ interactions were also investigated. Co-application of maximal concentrations of both peptides did not result in additive effects, whereas pre-application of maximal b-NST concentrations partially attenuated N/OFQ inhibition.

CONCLUSIONS AND IMPLICATIONS

We conclude that b-NST inhibits [(3)H]5-HT release via activation of G(i/o) protein linked pathways, not involving classical opioid receptors and the NOP receptor. The present data strengthen the view that b-NST is, per se, a biologically active peptide endowed with agonist activity.

Nociceptin/orphanin FQ decreases serotonin efflux in the rat brain but in contrast to a kappa-opioid has no antagonistic effect on mu-opioid-induced increases in serotonin efflux

Similar to kappa-opioids, nociceptin/orphanin FQ (OFQ) exerts anti-mu-opioid actions. This may involve interactions within the circuitry controlling 5-HT neurons in the dorsal raphe nucleus (DRN) that project to the nucleus accumbens (NAcc). To test this hypothesis, we compared the effects of OFQ and kappa-opioids on 5-HT efflux in the CNS of freely behaving rats. First, OFQ (30-300 microM) infused into the DRN for 120 min dose-dependently decreased 5-HT efflux in the DRN. The opioid receptor-like 1 (ORL-1) antagonist [Nphe(1)]nociceptin(1-13)NH(2) blocked this effect. Using dual-probe microdialysis we observed that OFQ (300 microM) infused into the DRN for 120 min produced parallel decreases in 5-HT efflux in the DRN and NAcc, suggesting that ORL-1 receptors in the DRN inhibit serotonergic neurons projecting to the NAcc. Also, 5-HT efflux in the NAcc was dose-dependently decreased during OFQ (30-300 microM) infusion into the NAcc. This suggests that OFQ can reduce 5-HT efflux in the NAcc both by inhibiting serotonergic neurons in the DRN and by stimulating ORL-1 receptors in the NAcc. Similar to OFQ, the kappa-opioids U-50,488 (300 microM) and dynorphin A(1-13) (300 microM) infused into the DRN for 120 min decreased 5-HT efflux in the DRN. This effect was blocked only by the kappa-opioid receptor antagonist nor-BNI. Lastly, we compared the ability of OFQ and U-50,488 to block mu-opioid-induced increases in 5-HT. The kappa-opioid U-50,488 (1000 microM) attenuated the increase in 5-HT induced by the mu-opioid agonist endomorphin-1 (300 microM) in the DRN. In contrast, OFQ (300-1000 microM) did not alter mu-opioid-induced increases in 5-HT efflux. In summary, kappa-opioids and OFQ both decreased 5-HT efflux in the CNS. However, in contrast to kappa-opioids, which reversed mu-opioid-induced increases in 5-HT efflux, the anti-mu-opioid effects of OFQ apparently do not involve changes in 5-HT transmission under our experimental conditions.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

Presynaptic modulation of 5-HT release in the rat septal region

5-HT released from serotonergic axon terminals in the septal nuclei modulates the activity of septal output neurons (e.g. septohippocampal cholinergic neurons) bearing somatodendritic 5-HT receptors. Therefore, we studied the mechanisms involved in the presynaptic modulation of 5-HT release in the lateral (LS) and medial septum (MS), and the diagonal band of Broca (DB). HPLC analysis showed that tissue concentrations of noradrenaline, dopamine and 5-HT were highest in DB (DB>MS>LS). Slices prepared from LS, MS and DB regions were preincubated with [(3)H]5-HT, superfused in the presence of 6-nitro-2-(1-piperazinyl)-quinoline (6-nitroquipazine) and electrically stimulated up to three times (first electrical stimulation period (S(1)), S(2), S(3); 360 pulses, 3 Hz, 2 ms, 26-28 mA). In all septal regions the Ca(2+)-dependent and tetrodotoxin-sensitive electrically-evoked overflow of [(3)H] was inhibited by the 5-HT(1B) agonist CP-93,129 and the alpha(2)-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline tartrate (UK-14,304). Also the mu- and kappa-opioid receptor agonists (d-Ala(2), N-Me-Phe(4), glycinol(5))-enkephalin (DAMGO) and [trans-(1S,2S(-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzenacetamide hydro-chloride] (U-50,488H), respectively, acted inhibitory (although less potently), whereas the delta-opioid receptor agonist (d-Pen(2), d-Pen(5))-enkephalin (DPDPE), the dopamine D(2) receptor agonist quinpirole and the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine were all ineffective; the GABA(B) receptor agonist baclofen had weak effects. All inhibitory effects of the agonists were antagonized by the corresponding antagonists (3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR-55,562), idazoxan, naloxone, nor-binaltorphimine), which also significantly enhanced the evoked release of 5-HT at S(1). It is concluded that 5-HT release in septal nuclei of the rat is modulated by presynaptic 5-HT(1B) autoreceptors, as well as by alpha(2)-, mu- and kappa-opioid heteroreceptors. All of these receptors seem to be under a tonic inhibitory influence of the corresponding endogenous agonists and show qualitatively comparable modulatory properties along the dorso-ventral distribution of the 5-HT terminals.

Pharmacological characterization of the nociceptin/orphanin FQ receptor antagonist SB-612111 [(-)-cis-1-methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol]: in vitro studies

The compound SB-612111 [(-)-cis-1-methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol] was recently identified as a selective antagonist for the nociceptin/orphanin FQ (N/OFQ) peptide receptor (NOP). In the present study, the in vitro pharmacological profile of SB-612111 at human recombinant NOP receptors expressed in Chinese hamster ovary (CHO) cells [receptor binding, guanosine 5'-O-(3-[(35)S]thio)triphosphate (GTPgamma[(35)S]) binding, and cAMP level experiments] as well as at native NOP receptors expressed in peripheral (mouse and rat vas deferens, guinea pig ileum) and central (mouse cerebral cortex synaptosomes releasing [(3)H]5-HT) preparations was evaluated and compared with that of the standard nonpeptide antagonist (+/-)J-113397 [(+/-)-trans-1-[1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one]. SB-612111 produced a concentration-dependent displacement of [(3)H]N/OFQ binding to CHO(hNOP) cell membranes, showing higher affinity and NOP selectivity over classical opioid receptors than (+/-)J-113397. SB-612111 and (+/-)J-113397 competitively antagonized the effects of N/OFQ on GTPgamma[(35)S] binding in CHO(hNOP) cell membranes (pK(B), 9.70 and 8.71, respectively) and on cAMP accumulation in CHO(hNOP) cells (pK(B), 8.63 and 7.95, respectively), being per se inactive. In isolated peripheral tissues of mice, rats, and guinea pigs and in mouse cerebral cortex synaptosomes preloaded with [(3)H]5-HT, SB-612111 competitively antagonized the inhibitory effects of N/OFQ, with pA(2) values in the range of 8.20 to 8.50. In parallel experiments, (+/-)J-113397 was found to be 2- to 9-fold less potent than SB-612111. In the electrically stimulated tissues, 1 microM SB-612111 did not modify the effects of classical opioid receptor agonists. In conclusion, the results of the present study demonstrated that SB-612111 is among the most potent and NOP-selective nonpeptide antagonists identified to date.

Involvement of local orphanin FQ in the development of analgesic tolerance induced by morphine microinjections into the dorsal raphe nucleus of rats

It is well known that dorsal raphe nucleus (DRN) is one of the key structures for the development of opioid analgesia and tolerance. An increased activity of 'antiopioids' like orphanin-FQ (OFQ) has been proposed as a possible mechanism for opioid tolerance. The present study evaluates the role of DRN-located OFQ in the opioid analgesic tolerance induced by repeated microinjections of morphine (MOR) into DRN. Male rats were implanted with chronic guide cannulae aimed at the DRN. Microinjection of MOR (0.5 microg in 0.5 microl) into DRN caused antinociception as quantified with the tail flick and the hot plate tests. When MOR microinjection was repeated twice daily, the antinociceptive effect disappeared within 2 days (tolerance). However, if each MOR microinjection was preceded (within 15 min) by a microinjection of the OFQ receptor antagonist nocistatin (NST) (1 ng in 0.5 microl) into the same DRN site, the microinjections of MOR always produced antinociception and did not induce tolerance. If NST microinjections were suspended, subsequent MOR microinjections induced tolerance. In MOR-tolerant rats, a single NST microinjection into the same DRN site was enough to restore the antinociceptive effect of MOR. On the other hand, if OFQ (1 ng in 0.5 microl) was microinjected into DRN, then MOR microinjection administered 15 min later into the same DRN site did not elicit antinociception. Finally, opioid tolerance induced by repeated systemic MOR injections (5 mg/kg, i.p.) was reversed by a single microinjection of NST into DRN. This emphasizes the central importance of DRN-located OFQ in the MOR analgesic tolerance.

Effects of pre-germinated brown rice on depression-like behavior in mice

We investigated the antidepressant-like effects of pre-germinated brown rice (PGBR) and polished rice (PR) pellets, respectively, in comparison with control (AIN-93G) pellets in the forced swimming test and the learned helplessness paradigm in mice. Mice were fed respective pellets for 30 days. The immobility time on the 2nd day of the forced swimming test was shorter in mice fed with PR or PGBR pellets than in mice fed with control pellets. In the learned helplessness paradigm, the number of escape failures in mice fed with PGBR pellets was significantly smaller than that in mice fed with control pellets. Compared to the control group, an increase in serotonin (5-HT) levels, but not in 5-hydroxyindoleacetic acid (5-HIAA) levels, and a decrease in the 5-HIAA/5-HT ratio were observed in the frontal cortex of the PGBR group. There were no differences among the three groups in terms of 5-HT and 5-HIAA levels and their ratios in the hippocampus and striatum. The levels of noradrenaline and 3-methoxy-4-hydroxyphenylglycol were not affected by the food pellets in all the brain regions tested. Additionally, we could not detect any differences in the expression of the 5-HT1A receptor and the 5-HT transporter in the frontal cortex of the three groups. These results suggest that the increase of 5-HT levels in the mouse frontal cortex contributes to the antidepressant-like effects of PGBR pellets.

Presynaptic opioid receptors on noradrenergic and serotonergic neurons in the human as compared to the rat neocortex

1. Electrically evoked release of [3H]-noradrenaline ([3H]-NA) or [3H]-5-hydroxytryptamine ([3H]-5-HT) in slices of human and the rat neocortex was used to characterize presynaptic opioid receptors. 2. Release of [3H]-NA in rat neocortical slices was reduced only by the mu-receptor agonist DAMGO (pIC50: 7.27, CI95: [7.22, 7.32]; Imax: 77.6+/-1.6%; antagonized by naloxone: pA2: 8.88, CI95: [8.78, 8.98]). 3. Release of [3H]-NA in human neocortical slices was unaffected by DAMGO, but inhibited by the delta-receptor agonist DPDPE (Imax: 25.7+/-2.2%) and the kappa-receptor agonist U-50,488H (19.7+/-2.7% inhibition at 1 microM). Both effects were antagonized by naltrindole (1 microM). 4. Release of [3H]-5-HT in rat neocortical slices, was inhibited by DAMGO (10 microM) and U-50,488H (1 and 10 microM) only in the presence of the 5-HT receptor antagonist methiotepin (1 microM). 5. Release of [3H]-5-HT in human neocortical slices was unaffected by DPDPE, but U-50,488H (Imax: 40.8+/-8.3%; antagonized by 0.1 microM norbinaltorphimine) and DAMGO (16.4+/-3.9% inhibition at 1 microM; antagonized by 0.1 microM naloxone) acted inhibitory. 6. Release of [3H]-5-HT in human neocortical slices was reduced by nociceptin/orphanin (0.1 and 1 microM). These effects were antagonized by the ORL1 antagonist J-113397 (1-[(3R,4R)-1-cyclo-octylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one; 0.1 microM). 7. This study provides evidence for significant species differences in opioid receptor-mediated modulation of NA and 5-HT-release in human vs rat neocortex. In rats, mu-opioid receptors modulate NA release, but 5-HT release is only weakly affected by mu- and kappa-opioids. In contrast, NA release in human neocortex is modulated via delta-opioid receptors, but 5-HT release mainly via kappa-opioid receptors. In addition also the ORL1 receptor seems to be involved in 5-HT release modulation.

Antidepressant- and anxiolytic-like effects of nociceptin/orphanin FQ receptor ligands

Many studies point toward the nociceptin/orphanin FQ (N/OFQ) and the N/OFQ peptide receptor (NOP) as targets for the development of innovative drugs for treating affective disorders. It has been reported that the activation of NOP receptors produces anxiolytic-like effects in rodents in a large series of behavioral assays, i.e., elevated plus maze, light-dark aversion, operant conflict, fear-potentiated startle, pup ultrasonic vocalizations, and hole board tests. In contrast, the blockade of N/OFQ signaling obtained with NOP-selective antagonists promotes antidepressant-like effects in the forced swimming and tail suspension tests. In these assays, N/OFQ is inactive per se, but reverses the antidepressant-like effects of NOP antagonists. NOP receptor knockout mice show an antidepressant-like phenotype, and NOP antagonists are inactive in these animals. Thus, the activation of the NOP receptor seems to evoke anxiolytic-like effects while its blockade antidepressant-like effects. This appears to be a rather unique behavioral profile since the activation or the blockade of a given neuropeptide receptor produces, in most of the cases, both antidepressant- and anxiolytic-like effects. This particular behavioral profile, the possible mechanisms of action, and the therapeutic potential of NOP receptor ligands for the treatment of depression and anxiety disorders are discussed in this review article.

Acute noise stress analgesia in relation to 5-HT2 and μ-opioid receptor changes in the frontal cortex of young mice

A number of studies have reported that exposure to stress provoked behavioural changes, including analgesia, in rodents. Differences have been observed in these responses to different types of stress and a link between hormones and neurotransmitters proposed. We studied the effect of acute noise stress on nociception and the possible changes in the serotonergic and opioidergic systems in young mice of both sexes. Naloxone pre-treatment was also investigated. Noise stress was produced by a sound source, nociception was measured by the hot-plate test and binding characteristics were evaluated by a radioligand binding technique using membrane preparation from the total frontal cortex. Acute noise stress provoked an antinociceptive effect, associated with an increase in plasma corticosterone levels, a decrease in the number of 5-HT2 receptors in stressed male and female mice and a decrease in the number of mu receptors in both sexes. The behavioural and biochemical effects were antagonized by 1 mg/kg of naloxone. Acute noise stress behaves like other types of stress on nociception. The opioidergic system seems to be involved in this behaviour but also the serotonergic system may play a role. Sex differences were detected in the number of 5-HT2 and mu receptors between male and female mice not subjected to stress, while the percentage decrease in 5-HT2 and mu receptors did not differ significantly between the two sexes.

Opioid receptor-like 1 (NOP) receptors in the rat dorsal raphe nucleus: evidence for localization on serotoninergic neurons and functional adaptation after 5,7-dihydroxytryptamine lesion

A high density of opioid receptor-like 1 (ORL1) receptor (also referred to as NOP receptor) is found in limbic areas and in regions containing monoamines, which are implicated in emotional activity and physiopathology of depression and anxiety. We aimed at defining precisely the localization of ORL1 receptors in dorsal raphe nucleus, by means of a lesion strategy and autoradiographic studies. In control rats, [3H]nociceptin and nociceptin-stimulated [35S]GTPgammaS bindings were found to be correlated in several brain regions. We performed in rats a selective destruction of serotoninergic neurons by surgical stereotaxic injection of 5,7-dihydroxytryptamine (5,7-DHT) in dorsal raphe nucleus. This led to a marked decrease in serotonin contents in striata and frontal cortices (about -60%) and in autoradiographic [3H]citalopram binding in posterior regions. In dorsal raphe nucleus, [3H]nociceptin binding was decreased to the same extent as [3H]citalopram binding, whereas it was unchanged in the other regions studied. Nevertheless, in the dorsal raphe, nociceptin-stimulated [35S]GTPgammaS binding was decreased to a lesser extent than [3H]nociceptin binding in 5,7-DHT-lesioned rats. The ratio between nociceptin-stimulated [35S]GTPgammaS binding and [3H]nociceptin binding was significantly increased in 5,7-DHT-lesioned rats compared with controls in this region. These data demonstrate 1) that ORL1 receptors are located on serotoninergic neurons in the dorsal raphe nucleus and 2) that, after a lesion, the functionality of remaining ORL1 receptors appears to be up-regulated, which could correspond to a compensatory mechanism.

Buprenorphine in cancer pain

Buprenorphine is a broad spectrum, highly lipophilic, and long-acting partial mu opioid receptor agonist that is noncross tolerant to other opioids. Buprenorphine can be given by several routes. Metabolism is through CYP3A4 and CYP2C8 and by conjugases. Constipation and sexual dysfunction appear to be less with buprenorphine than with other opioids. The recent development of a polymer matrix patch delivery system for buprenorphine prevents "dose dumping" and facilitates pain management in those unable to take oral analgesics. Sublingual buprenorphine has been combined with naloxone to prevent illicit conversion to parenteral administration. Buprenorphine has been used extensively to control cancer pain. In certain clinical situations, buprenorphine may have particular advantages over other opioids.

Depressive symptoms during buprenorphine vs. methadone maintenance: findings from a randomised, controlled trial in opioid dependence

Research suggests that buprenorphine may possess antidepressant activity. The Beck Depression Inventory was completed at baseline and 3 months by heroin dependent subjects receiving either buprenorphine or methadone maintenance as part of a larger, pre-existing, double blind trial conducted by NDARC (Australia). Depressive symptoms improved in all subjects, with no difference between methadone and buprenorphine groups, suggesting no differential benefit on depressive symptoms for buprenorphine compared to methadone.

The interaction of highly helical structural mutants with the NOP receptor discloses the role of the address domain of nociceptin/orphanin FQ

Nociceptin is a heptadecapeptide whose sequence is similar to that of Dynorphin A, sharing a message domain characterized by two glycines and two aromatic residues, and a highly basic C-terminal address domain but, in spite of these similarities, displays no opioid activity. Establishing the relative importance of the message and address domains of nociceptin has so far been hampered by its extreme conformational flexibility. Here we show that mutants of this peptide, designed to increase the helical content in the address domain, can be employed to explain the mode of interaction with the NOP receptor. Nociceptin analogues in which Ala residues are substituted with aminoisobutyric acid (Aib) show a substantial increment of activity in their interaction with the NOP receptor. The increment of biological activity was attributed to the well-documented ability of Aib to induce helicity. Here we have verified this working hypothesis by a conformational investigation extended to new analogues in which the role of Aib is taken up by Leu. The NMR conformational analysis confirms that all Ala/Aib peptides as well as [Leu(7,11)]-N/OFQ-amide and [Leu(11,15)]-N/OFQ-amide mutants (N/OFQ=nociceptin/orphanin FQ) have comparable helix content in helix-promoting media. We show that the helical address domain of nociceptin can place key basic residues at an optimal distance from complementary acidic groups of the EL(2) loop of the receptor. Our structural data are used to rationalize pharmacological data which show that although [Leu(11,15)]-N/OFQ-amide has an activity comparable to those of Ala/Aib peptides, [Leu(7,11)]-N/OFQ-amide is less active than N/OFQ-amide. We hypothesize that bulky residues cannot be hosted in or near the hinge region (Thr(5)-Gly(6)-Ala(7)) without severe steric clash with the receptor. This hypothesis is also consistent with previous data on this hinge region obtained by systematic substitution of Thr, Gly, and Ala with Pro.

Nociceptin/orphanin FQ prevents the antinociceptive action of paracetamol on the rat hot plate test

Nociceptin/orphanin FQ (N/OFQ) is involved in many behavioural patterns; in particular, it exerts a modulating effect on nociception. Like other proposed antiopiates, nociceptin/orphanin FQ has been shown to have analgesic, hyperalgesic as well as antianalgesic properties. Among the various effects proposed on nociceptive sensitivity at supraspinal level, the antagonistic activity toward morphine analgesia seems to be of interest. Therefore, we decided to investigate whether nociceptin/orphanin FQ and [Arg14, Lys15] nociceptin/orphanin FQ (R-K, a nociceptin analogue) can have the same effect on the analgesia produced by nonopioid analgesics. In this study, we examined the antianalgesic effect of nociceptin/orphanin FQ and its analogue R-K on paracetamol-induced analgesia and evaluated by means of the hot plate test in rats. Nociceptin/orphanin FQ was intracerebroventricularly administered, and, after 5 min, a dose of 400 mg/kg paracetamol was injected intraperitoneally, 30 min before the hot plate test. Nociceptin/orphanin FQ and R-K showed a dose-dependent antagonism on the antinociceptive effect of paracetamol, and the activity of both drugs was significantly reduced by the antagonist [Nphe1] Arg14, Lys15-N/OFQ-NH2 (UFP-101). These data indicate that nociceptin/orphanin FQ and R-K have an antianalgesic effect on the analgesia produced by a nonopioid analgesic drug, like paracetamol, that seems to develop within the brain.

Effects of nociceptin/orphanin FQ and endomorphin-1 on glutamate and GABA release, intracellular [Ca2+] and cell excitability in primary cultures of rat cortical neurons

The effects of nociceptin/orphanin FQ (N/OFQ) and endomorphin-1 (EM-1) on glutamate and GABA release, intracellular calcium, neuronal excitability and glutamate current were investigated in rat primary cortical neuronal cultures. Through their specific receptors N/OFQ and EM-1 (0.02-1 microM) inhibited the electrically evoked outflow of [3H]D-aspartate at most to -50% and that of [3H]GABA to -30%. In addition, at 1 microM, both peptides induced a decrease of the firing rate caused by electrical depolarization. N/OFQ 1-10 microM did not influence either the electrically evoked calcium influx or the glutamate-evoked currents, whereas EM-1 1 microM significantly inhibited them. Thus, in cortical neurons in culture, both N/OFQ and EM-1 inhibited the secretory process and neuronal excitability but EM-1 also affected calcium influx and cell body responsiveness to glutamate. Consequently, EM-1 appeared to dampen this excitatory signal more then N/OFQ did.

Influence of nociceptin(1-17) fragments and its tyrosine-substituted derivative on morphine-withdrawal signs in rats

Our previous studies demonstrated that endogenous ligand of nociceptin (NOP) receptor, nociceptin(1-17) (also known as orphanin FQ), inhibits morphine-withdrawal syndrome measured as wet dog shakes in rats [Life Sci. 66 (2000) PL119]. This peptide is metabolized in the spinal cord, both in vitro and in vivo, to shorter fragments, including nociceptin(1-11) and nociceptin(1-6). These fragments, formed after cleavage by endogenous peptidase, are behaviorally active and modulate nociception in a bi-phasic process [Peptides 20 (1999) 239]. As these peptides induced transient naloxone-reversible analgesia in behavioral tests [Peptides 20 (1999) 239], in the present study we tested the influence of nociceptin(1-11) (10 and 20 microg) and nociceptin(1-6) (10, 20 and 40 microg) on the morphine-withdrawal syndrome in rats. Furthermore, the modified fragment of nociceptin(1-6) with an opioid-message domain achieved by replacement of Phe1 with Tyr was tested. Morphine-withdrawal syndrome was precipitated by the i.p. injection of naloxone hydrochloride (2 mg/kg), 72 h after implantation of morphine pellets. The wet-dog shakes were chosen for statistical analyses of the abstinence signs. The results show that nociceptin(1-11) and (1-6) attenuate this morphine-withdrawal symptom. The replacement of Phe1 with Tyr in nociceptin(1-6) fragment did not potentiate the influence of nociceptin(1-6) on wet dog shakes precipitated by naloxone in morphine-dependent rats.

Effects of nociceptin on the exploratory behavior of mice in the hole-board test

The effects of nociceptin on the exploratory behavior of mice were examined using an automatic hole-board apparatus. A low dose of nociceptin (0.01 nmol, i.c.v.) had an anxiolytic effect, as reflected by an increase in head-dipping behavior. However, high doses of nociceptin (1-5 nmol, i.c.v.) produced a dose-dependent anxiogenic effect, as reflected by a decrease in head-dipping behavior. Both the anxiolytic and anxiogenic effects of nociceptin were antagonized by nocistatin, an opioid receptor-like 1 (ORL1) receptor antagonist. Although a low dose (0.01 nmol, i.c.v.) of nociceptin significantly increased the rate of serotonin (5-hyroxytryptamine, 5-HT) turnover in the hippocampus, a high dose (5 nmol, i.c.v.) of nociceptin significantly decreased this turnover in the amygdala. Furthermore, the anxiolytic effect of nociceptin at a low dose was antagonized by N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide 3HCl (WAY100635), a 5-HT1A receptor antagonist. On the other hand, the anxiogenic effect of nociceptin at a high dose was antagonized by R(+)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT), a 5-HT1A receptor agonist. In conclusion, the results of this study suggest that nociceptin has dose-related anxiolytic and anxiogenic effects as a result of the activation of ORL1 receptors. The present results also suggest that a low dose of nociceptin has an anxiolytic effect via the activation of 5-HT ergic function in the hippocampus, while a high dose of nociceptin has an anxiogenic effect via the inhibition of 5-HT ergic function in the amygdala.

Pharmacological profile of nociceptin/orphanin FQ receptors regulating 5-hydroxytryptamine release in the mouse neocortex

A synaptosomal preparation was employed to pharmacologically characterize the role of presynaptic nociceptin/orphanin FQ (N/OFQ) receptors (NOP receptors) in the regulation of 5-hydroxytryptamine release in the Swiss mouse neocortex. In the present study, the NOP receptor ligands N/OFQ, Ac-RYYRWK-NH(2) and [Phe(1)psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)-NH(2) inhibited the K(+)-induced [(3)H]-5-HT overflow with similar maximal effects ( approximately -35%) but different potencies (pEC(50) of 8.56, 8.35 and 7.23, respectively). The novel agonist [Arg(14),Lys(15)]N/OFQ also inhibited [(3)H]-5-HT overflow, but the concentration-response curve was biphasic and the efficacy higher ( approximately -45%). Receptor selectivity of NOP receptor agonists was demonstrated by showing that synaptosomes from NOP receptor knockout mice were unresponsive to N/OFQ, [Arg(14),Lys(15)]N/OFQ and [Phe(1)psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)-NH(2) but maintained full responsiveness to endomorphin-1. Moreover, the inhibitory effect of N/OFQ was prevented by peptide ([Nphe(1)]N/OFQ(1-13)-NH(2) and UFP-101) and nonpeptide (J-113397 and JTC-801) NOP receptor selective antagonists. Desensitization occurred under perfusion with high (3 and 10 microm) N/OFQ concentrations. This phenomenon was prevented by the protein kinase C inhibitor, bisindolylmaleimide. Moreover, N/OFQ-induced desensitization did not affect mu opioid receptor responsiveness. Finally, it was observed in a similar preparation of rat cerebrocortical synaptosomes, although it was induced by higher N/OFQ concentrations than that used in the mouse. Together, these findings indicate that presynaptic NOP receptors inhibit 5-hydroxytryptamine release in the mouse neocortex. Based on present and previous studies, we conclude that NOP receptors in the mouse are subtly different from the homologous receptor population in the rat, strengthening the view that there exist species differences in the pharmacology of central NOP receptors.

Systemic morphine-induced release of serotonin in the rostroventral medulla is not mimicked by morphine microinjection into the periaqueductal gray

We used in vivo microdialysis in awake rats to test the hypothesis that intravenous morphine increases serotonin (5-HT) release within the rostral ventromedial medulla (RVM). We also injected morphine into various sites along the rostrocaudal extent of the periaqueductal gray (PAG), and examined the extent of its diffusion to the RVM. Intravenous morphine (3.0 mg/kg) produced thermal antinociception and increased RVM dialysate 5-HT, 5-hydroxyindole acetic acid (5-HIAA), and homovanillic acid (HVA) in a naloxone-reversible manner. As neither PAG microinjection of morphine (5 micro g/0.5 micro L) nor RVM administration of fentanyl or d-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) increased RVM 5-HT, we were unable to determine the precise site of action of morphine. Surprisingly, peak morphine levels in the RVM were higher after microinjection into the caudal PAG as compared to either intravenous injection or microinjection into more rostral sites within the PAG. Naloxone-precipitated withdrawal in morphine-tolerant rats not only increased extracellular 5-HT in the RVM, but also dopamine (DA) and HVA. We conclude that substantial amounts of morphine diffuse from the PAG to the RVM, and speculate that opioid receptor interactions at multiple brain sites mediate the analgesic effects of PAG morphine. Further studies will be required to elucidate the contribution of 5-HT and DA release in the RVM to opioid analgesia and opioid withdrawal.

Differential responsiveness of rat striatal nerve endings to the mitochondrial toxin 3-nitropropionic acid: implications for Huntington's disease

Rat striatal synaptosomes and slices were used to investigate the responsiveness of different populations of nerve terminals to 3-nitropropionic acid (3-NP), a suicide inhibitor of the mitochondrial enzyme succinate dehydrogenase, and to elucidate the ionic mechanisms involved. 3-NP (0.3-3 mm) stimulated spontaneous gamma-aminobutyric acid (GABA), glutamate and [3H]-dopamine efflux but left unchanged acetylcholine efflux from synaptosomes. This effect was associated with a >70% inhibition of succinate dehydrogenase, as measured in the whole synaptosomal population. The facilitation was not dependent on extracellular Ca2+ but relied on voltage-dependent Na+ channel opening, because it was prevented by tetrodotoxin and riluzole. 3-NP also elevated spontaneous glutamate efflux from slices but in a tetrodotoxin-insensitive way. To investigate whether energy depletion could change the responsiveness of nerve endings to a depolarizing stimulus, synaptosomes were pretreated with 3-NP and challenged with pulses of KCl evoking 'quasi-physiological' neurotransmitter release. 3-NP potentiated the K+-evoked GABA, glutamate and [3H]-dopamine release but inhibited the K+-evoked acetylcholine release. The 3-NP induced potentiation of GABA release was Ca2+-dependent and prevented by tetrodotoxin and riluzole whereas the 3-NP-induced inhibition of acetylcholine release was tetrodotoxin- and riluzole-insensitive but reversed by glipizide, an ATP-dependent K+ channel inhibitor. We conclude that the responsiveness of striatal nerve endings to 3-NP relies on activation of different ionic conductances, and suggest that the selective survival of striatal cholinergic interneurons following chronic 3-NP treatment (as in models of Huntington's disease) may rely on the opening of ATP-dependent K+ channels, which counteracts the fall in membrane potential as a result of mitochondrial impairment.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Blockade of nociceptin/orphanin FQ-NOP receptor signalling produces antidepressant-like effects: pharmacological and genetic evidences from the mouse forced swimming test

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the NOP receptor, regulates several central functions such as pain transmission, learning and memory, fear and anxiety and feeding and locomotor activity. It has been recently reported that NOP receptor antagonists induce antidepressant-like effects in the mouse forced swimming test (FST), i.e. reduce immobility time. This assay was used in the present study for further investigating the involvement of the NOP receptor in depression states. In male Swiss mice, intracerebroventricular injection (i.c.v) of the novel NOP receptor antagonist, UFP-101 (1-10 nmol) dose-dependently reduced the immobility time (control 192 +/- 14 s, UFP-101 91 +/- 15 s). The effect of 3 or 10 nmol UFP-101 was fully or partially reversed, respectively, by the coadministration of 1 nmol N/OFQ, which was inactive per se. NOP receptor knockout mice showed a reduced immobility time compared with their wild-type littermates (wild-type 215 +/- 10 s, knockout 143 +/- 12 s). Moreover, i.c.v. injected UFP-101 (10 nmol) significantly reduced immobility time in wild-type mice but not in NOP receptor knockout animals. In conclusion, these results, obtained using a combined pharmacological and genetic approach, indicate that blockade of the N/OFQ-NOP receptor signalling in the brain produces antidepressant-like effects in the mouse FST. These findings support the NOP receptor as a candidate target for the development of innovative antidepressant drugs.

Pharmacological profiles of presynaptic nociceptin/orphanin FQ receptors modulating 5-hydroxytryptamine and noradrenaline release in the rat neocortex

1 The pharmacological profiles of presynaptic nociceptin/orphanin FQ (N/OFQ) peptide receptors (NOP) modulating 5-hydroxytryptamine (5-HT) and noradrenaline (NE) release in the rat neocortex were characterized in a preparation of superfused synaptosomes challenged with 10 mM KCl. 2 N/OFQ concentration-dependently inhibited K(+)-evoked [(3)H]-5-HT and [(3)H]-NE overflow with similar potency (pEC(50) approximately 7.9 and approximately 7.7, respectively) and efficacy (maximal inhibition approximately 40%). 3 N/OFQ (0.1 micro M) inhibition of [(3)H]-5-HT and [(3)H]-NE overflow was antagonized by selective NOP receptor antagonists of peptide ([Nphe(1)]N/OFQ(1-13)NH(2) and UFP-101; 10 and 1 microM, respectively) and non-peptide (J-113397 and JTC-801; both 0.1 microM) nature. Antagonists were routinely applied 3 min before N/OFQ. However, a 21 min pre-application time was necessary for J-113397 and JTC-801 to prevent N/OFQ inhibition of [(3)H]-NE overflow. 4 The NOP receptor ligand [Phe(1)psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)NH(2) ([F/G]N/OFQ(1-13)NH(2); 3 microM) did not affect K(+)-evoked [(3)H]-NE but inhibited K(+)-evoked [(3)H]-5-HT overflow in a UFP-101 sensitive manner. [F/G]N/OFQ(1-13)NH(2) antagonized N/OFQ actions on both neurotransmitters. 5 The time-dependency of JTC-801 action was studied in CHO cells expressing human NOP receptors. N/OFQ inhibited forskolin-stimulated cAMP accumulation and JTC-801, tested at different concentrations (0.1-10 microM) and pre-incubation times (0, 40 and 90 min), antagonized this effect in a time-dependent manner. The Schild-type analysis excluded a competitive type of antagonism. 6 We conclude that presynaptic NO receptors inhibiting 5-HT and NE release in the rat neocortex have similar pharmacological profiles. Nevertheless, they can be differentiated pharmacologically on the basis of responsiveness to [F/G]N/OFQ(1-13)NH(2) and time-dependent sensitivity towards non-peptide antagonists.

Direct and indirect inhibition by nociceptin/orphanin FQ on noradrenaline release from rodent cerebral cortex in vitro

1 The modulation exerted by nociceptin/orphanin FQ (NC) on noradrenaline (NE) release in rodent cerebral cortex slices and synaptosomes was studied. 2 Rat, mouse and guinea-pig cortical slices and synaptosomes were preincubated with 0.1 micro M [(3)H]-NE and superfused. NE release was evoked by 2 min of electrical (3 Hz) stimulation in slices and by 1 min pulse of 10 mM KCl in synaptosomes. 3 In rat cortical slices, 0.01-3 micro M NC reduced the evoked [(3)H]-NE efflux (E(max)-54%), with a bell-shaped concentration-response curve, which regained its monotonic nature in the presence of either 0.1 micro M naloxone (NX) or 30 micro M bicuculline. In synaptosomes, the NC effect curve was sygmoidal in shape and reached a plateau at 1 micro M concentration. 4 In the rat, both 1 micro M [Phe(1)psi(CH(2)-NH)Gly(2)]NC(1-13)NH(2) and 10 micro M [Nphe(1)]NC(1-13)NH(2) (NPhe) antagonised NC-induced inhibition, without per se modifying [(3)H]-NE efflux. The effects of 0.3-1 micro M NC concentrations were partially prevented by 1 micro M NX; 1 micro M D-Phe-Cys-Thr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP) was also an effective antagonist, but 0.1 micro M norbinaltorphimine was not. 5 In the mouse cerebral cortex, NC-induced inhibition of NE release (pEC(50) 6.87, E(max)-61%, in the slices) was prevented by Nphe but was NX-insensitive. In guinea-pig cortical slices, NC effect (pEC(50) 6.22, E(max)-38%) was prevented by Nphe, but was NX-insensitive. 6 These findings demonstrate that NC inhibits NE release from rodent cerebral cortex via presynaptically located ORL(1) receptors. In the rat, micro opioid and GABA(A) receptors are involved as well.

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

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

Endomorphins 1 and 2 reduce relaxant non-adrenergic, non-cholinergic neurotransmission in rat gastric fundus

It is now well established that opioids modulate cholinergic excitatory neurotransmission in the gastrointestinal tract. The aim of the present study was to characterize a possible effect of endomorphins on nonadrenergic, noncholinergic (NANC) relaxant neurotransmission in the rat gastric fundus in vitro. The drugs used in the experiments were the endogenous mu-opioid receptors (MORs) endomorphin 1 and 2 and the mu-opioid receptor antagonist CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2). CTAP left the basal tonus and the spontaneous activity of the preparation unchanged. Electrical field stimulation (EFS) under NANC conditions at frequencies ranging from 0.5 to 16 Hz caused a frequency-dependent relaxant response on the 5-hydoxytryptamine (5-HT) (10(-7) M) precontracted smooth-muscle strip. Both endomorphin 1 and endomorphin 2 significantly reduced this relaxation in a concentration-dependent manner. Endomorphin 1 proved to be more potent in reducing the relaxant responses. The endomorphin effects were significantly reversed by the MOR antagonist CTAP. CTAP itself did not influence the EFS-induced relaxation. In summary, these data provide evidence that the endogenous MOR agonists endomorphin 1 and 2 can reduce nonadrenergic, noncholinergic neurotransmission in the rat gastric fundus smooth muscle via a pathway involving MORs. The physiological relevance of these findings remains to be established, since the data presented suggest that the endomorphins act as neuromodulators within NANC relaxant neurotransmission.

[Nphe1,Arg14,Lys15]nociceptin-NH2, a novel potent and selective antagonist of the nociceptin/orphanin FQ receptor

1. Nociceptin/orphanin FQ (N/OFQ) modulates several biological functions by activating a specific G-protein coupled receptor (NOP). Few molecules are available that selectively activate or block the NOP receptor. Here we describe the in vitro and in vivo pharmacological profile of a novel NOP receptor ligand, [Nphe(1),Arg(14),Lys(15)]N/OFQ-NH(2) (UFP-101). 2. UFP-101 binds to the human recombinant NOP receptor expressed in Chinese hamster ovary (CHO) cells with high affinity (pK(i) 10.2) and shows more than 3000 fold selectivity over classical opioid receptors. UFP-101 competitively antagonizes the effects of N/OFQ on GTPgamma(35)S binding in CHO(hNOP) cell membranes (pA(2) 9.1) and on cyclic AMP accumulation in CHO(hNOP) cells (pA(2) 7.1), being per se inactive at concentrations up to 10 microM. 3. In isolated peripheral tissues of mice, rats and guinea-pigs, and in rat cerebral cortex synaptosomes preloaded with [(3)H]-5-HT, UFP-101 competitively antagonized the effects of N/OFQ with pA(2) values in the range of 7.3 - 7.7. In the same preparations, the peptide was inactive alone and did not modify the effects of classical opioid receptor agonists. 4. UFP-101 is also active in vivo where it prevented the depressant action on locomotor activity and the pronociceptive effect induced by 1 nmol N/OFQ i.c.v. in the mouse. In the tail withdrawal assay, UFP-101 at 10 nmol produces per se a robust and long lasting antinociceptive effect. 5. UFP-101 is a novel, potent and selective NOP receptor antagonist which appears to be a useful tool for future investigations of the N/OFQ-NOP receptor system.

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

We have studied the effects of naloxone benzoylhydrazone (NalBzoH) at recombinant human OP4 receptors expressed in Chinese hamster ovary (CHO) cells (CHOhOP4) and native OP4 sites in isolated tissues from various species. In CHOhOP4 membranes, nociceptin (NC) and NalBzoH displaced [125I]Tyr14-NC with pKi values of 10.1 and 7.3. In the presence of 100 microM GDP, NC stimulated GTPgamma35S binding (pEC50 = 8.5). NalBzoH was ineffective but antagonized the effects of NC (pA2 = 6.9). At 5 microM GDP, there was an increase in potency (pEC50 = 9.3) and efficacy (4.3-fold) of NC. NalBzOH was a partial agonist (pEC50 = 7.0, Emax = 13% relative to NC). In CHOhOP4 cells, NC and NalBzoH inhibited cAMP formation with pEC50 and Emax values of 9.8 and 100% and 6.0 and 44%, respectively. In the rat vas deferens, NalBzoH (10 microM) did not modify electrically induced twitches but competitively antagonized the inhibitory action of NC (pA2 = 6.2). In the mouse vas deferens (mVD) and guinea pig ileum (gpI), NalBzoH inhibited twitches with pEC50 and Emax values of 7.6 and 78% and 8.5 and 77%, respectively. The effect of 3 microM NalBzoH was fully inhibited by 3 microM naloxone in mVD and 30 microM in gpI. Under these conditions, NalBzoH antagonized the actions of NC in both preparations with pA2 values of 6.3 and 6.8, respectively. Collectively, these data demonstrate that NalBzoH is a nonselective OP4 ligand with system-dependent behaviour.

Pharmacological profile of nociceptin/orphanin FQ receptors

1. Nociceptin/orphanin FQ (NC) and its receptor (OP4) represent a novel peptide/receptor system pharmacologically distinct from classical opioid systems. 2. Via OP4 receptor activation, NC regulates several biological functions, both at peripheral and central levels; therefore, the OP4 receptor may be viewed as a novel target for drug development. However, the pharmacology of this receptor is still in its infancy, with few molecules interacting selectively with this receptor. 3. In the present article, we review the findings of studies that have investigated the pharmacological profile of ligands selective for the OP4 receptor, these being two antagonists, the peptide [Nphe1]NC(1-13)NH2 and the non-peptide J-113397, and two agonists, the peptide [Arg14,Lys15]NC, and the non- peptide Ro 64-6198. 4. The results of these studies indicate that agents that selectively activate or block the OP4 receptor may represent new potential drugs for the treatment of human diseases.

[Nphe(1)]N/OFQ-(1-13)-NH(2) is a competitive and selective antagonist at nociceptin/orphanin FQ receptors mediating K(+) channel activation in rat periaqueductal gray slices

A novel member of the opioid related receptor family, the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor was identified and demonstrated to be involved in many physiological functions including pain regulation. [Nphe(1)]N/OFQ-(1-13)-NH(2) (Nphe) is a novel peptide antagonist of NOP receptors, developed using peripheral preparations. We have quantitatively investigated the interaction of Nphe with N/OFQ, the endogenous ligand of NOP receptors, in the midbrain ventrolateral periaqueductal gray (PAG), a crucial brain region for N/OFQ-induced reversal of opioid analgesia, using the patch-clamp recording technique in brain slices. N/OFQ concentration-dependently activated an inwardly rectifying K(+) current in response to hyperpolarization ramps from -60 to -140 mV. Nphe concentration-dependently attenuated the K(+) current activated by N/OFQ without changing its reversal potential. The presence of Nphe right-shifted the concentration-response curve to N/OFQ in a parallel manner. The Schild plot analysis yielded a slope of 1.16 and a pA(2) value of 6.64 that is similar to those obtained in peripheral preparations. At concentrations up to 3 microM, Nphe affected neither the membrane current per se, nor the inwardly rectifying K(+) current activated by [D-Ala(2), N-Me-Phe(4),Gly-ol(5)]-enkephalin or baclofen, a mu-opioid and GABA(B) receptor agonist, respectively. It is concluded that Nphe acts as a pure, selective and competitive antagonist at native NOP receptors of ventrolateral PAG neurons.

Nociceptin/orphanin FQ receptors modulate glutamate extracellular levels in the substantia nigra pars reticulata. A microdialysis study in the awake freely moving rat

Intracerebral microdialysis was employed in awake freely moving rats to investigate the effects of nociceptin/orphanin FQ receptor ligands on glutamate extracellular levels in the substantia nigra pars reticulata. Nociceptin/orphanin FQ, ineffective at 0.1 microM, induced a prolonged stimulation of nigral glutamate levels at 1 and 10 microM (mean effect of 137+/-9 and 167+/-13%, respectively, of basal values). These effects were prevented by the novel nociceptin/orphanin FQ receptor antagonist [Nphe(1)]nociceptin/orphanin FQ(1-13)NH(2) (100 and 300 microM, respectively) but not by the non-selective opioid receptor antagonist naloxone (10 microM). [Nphe(1)]nociceptin/orphanin FQ(1-13)NH(2) (100 microM) inhibited by itself glutamate outflow (maximal reduction to 71+/-4%) while naloxone was ineffective. The nociceptin/orphanin FQ receptor ligand [Phe(1)psi(CH(2)-NH)Gly(2)]nociceptin/orphanin FQ(1-13)NH(2) also facilitated glutamate outflow at 10 microM (mean effect of 145+/-10%). Intranigral perfusion with tetrodotoxin (1 microM) or with the dopamine D(2) receptor antagonist raclopride (1 microM), failed to affect basal glutamate output and prevented the facilitatory effect of nociceptin/orphanin FQ (10 microM). However, perfusion with the GABA(A) receptor antagonist bicuculline (10 microM) increased local glutamate extracellular levels by itself and attenuated the effect of the peptide. Our data suggest that nociceptin/orphanin FQ increases glutamate extracellular levels in the substantia nigra pars reticulata via activation of nociceptin/orphanin FQ receptors located on non-glutamatergic, possibly dopaminergic and GABAergic, neuronal elements.