Endogenous nociceptin/orphanin FQ (N/OFQ) contributes to haloperidol-induced changes of nigral amino acid transmission and parkinsonism: a combined microdialysis and behavioral study in naïve and nociceptin/orphanin FQ receptor knockout mice

Endometriosis and Opioid Receptors: Are Opioids a Possible/Promising Treatment for Endometriosis?

Endometriosis (EM), defined as the presence of endometrial-like tissue with surrounding smooth muscle cells outside the uterus, is a disregarded gynecological disease reported to affect 6-10% of women of reproductive age, with 30-50% of them suffering from chronic pelvic pain and infertility. Since the exact pathogenic mechanisms of EM are still unclear, no curative therapy is available. As pain is an important factor in EM, optimal analgesia should be sought, which to date has been treated primarily with non-steroidal anti-inflammatory drugs (NSAIDs), metamizole or, in extreme cases, opioids. Here, we review the pain therapy options, the mechanisms of pain development in EM, the endogenous opioid system and pain, as well as the opioid receptors and EM-associated pain. We also explore the drug abuse and addiction to opioids and the possible use of NOP receptors in terms of analgesia and improved tolerability as a target for EM-associated pain treatment. Emerging evidence has shown a promising functional profile of bifunctional NOP/MOP partial agonists as safe and nonaddictive analgesics. However, until now, the role of NOP receptors in EM has not been investigated. This review offers a thought which still needs further investigation but may provide potential options for relieving EM-associated pain.

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.

Haloperidol-induced catalepsy as an animal model for parkinsonism: A systematic review of experimental studies

Several useful animal models for parkinsonism have been developed so far. Haloperidol-induced catalepsy is often used as a rodent model for the study of motor impairments observed in Parkinson's disease and related disorders and for the screening of potential antiparkinsonian compounds. The objective of this systematic review is to identify publications that used the haloperidol-induced catalepsy model for parkinsonism and to explore the methodological characteristics and the main questions addressed in these studies. A careful systematic search of the literature was carried out by accessing articles in three different databases: Web of Science, PubMed and SCOPUS. The selection and inclusion of studies were performed based on the abstract and, subsequently, on full-text analysis. Data extraction included the objective of the study, study design and outcome of interest. Two hundred and fifty-five articles were included in the review. Publication years ranged from 1981 to 2020. Most studies used the model to explore the effects of potential treatments for parkinsonism. Although the methodological characteristics used are quite varied, most studies used Wistar rats as experimental subjects. The most frequent dose of haloperidol used was 1.0 mg/kg, and the horizontal bar test was the most used to assess catalepsy. The data presented here provide a framework for an evidence-based approach to the design of preclinical research on parkinsonism using the haloperidol-induced catalepsy model. This model has been used routinely and successfully and is likely to continue to play a critical role in the ongoing search for the next generation of therapeutic interventions for parkinsonism.

Managing Parkinson's disease: moving ON with NOP

The opioid-like neuropeptide nociceptin/orphanin FQ (N/OFQ) and its receptor (NOP receptor) contribute to Parkinson's disease (PD) and motor complications associated with levodopa therapy. The N/OFQ-NOP receptor system is expressed in cortical and subcortical motor areas and, notably, in dopaminergic neurons of the substantia nigra compacta. Dopamine depletion, as in rodent models of PD results in up-regulation of N/OFQ transmission in the substantia nigra and down-regulation of N/OFQ transmission in the striatum. Consistent with this, NOP receptor antagonists relieve motor deficits in PD models by reinstating the physiological balance between excitatory and inhibitory inputs impinging on nigro-thalamic GABAergic neurons. NOP receptor antagonists also counteract the degeneration of nigrostriatal dopaminergic neurons, possibly by attenuating the excitotoxicity or modulating the immune response. Conversely, NOP receptor agonists attenuate levodopa-induced dyskinesia by attenuating the hyperactivation of striatal D1 receptor signalling in neurons of the direct striatonigral pathway. The N/OFQ-NOP receptor system might represent a novel target in the therapy of PD.

NOP Receptor Ligands and Parkinson's Disease

Nociceptin/Orphanin FQ (N/OFQ) and its NOP receptor are highly expressed in motor areas of the rodent, nonhuman, and human primate brain, such as primary motor cortex, thalamus, globus pallidus, striatum, and substantia nigra. Endogenous N/OFQ negatively regulates motor behavior and dopamine transmission through NOP receptors expressed by dopaminergic neurons of the substantia nigra compacta. Consistent with the existence of an N/OFQ tone over dopaminergic transmission, blockade of NOP receptor antagonists increases striatal dopamine release. In this chapter, we will review the evidence linking the N/OFQ-NOP receptor system to Parkinson's disease (PD). We will first discuss data showing that the central N/OFQ-NOP receptor system undergoes plastic changes in different basal ganglia nuclei following dopamine depletion. Then we will show that NOP receptor antagonists relieve motor deficits in different rodent and nonhuman primate models of PD. Mechanistically, NOP receptor blockade in substantia nigra reticulata results in rebalancing of the inhibitory GABAergic and excitatory glutamatergic inputs impinging on nigro-thalamic GABAergic neurons, leading to thalamic disinhibition. We will also present data showing that, in addition to motor symptoms, N/OFQ also plays a role in the parkinsonian neurodegeneration. In fact, NOP receptor antagonists possess neuroprotective/neurorescue properties in in vitro and in vivo models of PD.

A systematic review of the role of the nociceptin receptor system in stress, cognition, and reward: relevance to schizophrenia

Schizophrenia is a debilitating neuropsychiatric illness that is characterized by positive, negative, and cognitive symptoms. Research over the past two decades suggests that the nociceptin receptor system may be involved in domains affected in schizophrenia, based on evidence aligning it with hallmark features of the disorder. First, aberrant glutamatergic and striatal dopaminergic function are associated with psychotic symptoms, and the nociceptin receptor system has been shown to regulate dopamine and glutamate transmission. Second, stress is a critical risk factor for first break and relapse in schizophrenia, and evidence suggests that the nociceptin receptor system is also directly involved in stress modulation. Third, cognitive deficits are prevalent in schizophrenia, and the nociceptin receptor system has significant impact on learning and working memory. Last, reward processing is disrupted in schizophrenia, and nociceptin signaling has been shown to regulate reward cue salience. These findings provide the foundation for the involvement of the nociceptin receptor system in the pathophysiology of schizophrenia and outline the need for future research into this system.

Olanzapine, but not clozapine, increases glutamate release in the prefrontal cortex of freely moving mice by inhibiting D-aspartate oxidase activity

D-aspartate levels in the brain are regulated by the catabolic enzyme D-aspartate oxidase (DDO). D-aspartate activates NMDA receptors, and influences brain connectivity and behaviors relevant to schizophrenia in animal models. In addition, recent evidence reported a significant reduction of D-aspartate levels in the post-mortem brain of schizophrenia-affected patients, associated to higher DDO activity. In the present work, microdialysis experiments in freely moving mice revealed that exogenously administered D-aspartate efficiently cross the blood brain barrier and stimulates L-glutamate efflux in the prefrontal cortex (PFC). Consistently, D-aspartate was able to evoke L-glutamate release in a preparation of cortical synaptosomes through presynaptic stimulation of NMDA, mGlu5 and AMPA/kainate receptors. In support of a potential therapeutic relevance of D-aspartate metabolism in schizophrenia, in vitro enzymatic assays revealed that the second-generation antipsychotic olanzapine, differently to clozapine, chlorpromazine, haloperidol, bupropion, fluoxetine and amitriptyline, inhibits the human DDO activity. In line with in vitro evidence, chronic systemic administration of olanzapine induces a significant extracellular release of D-aspartate and L-glutamate in the PFC of freely moving mice, which is suppressed in Ddo knockout animals. These results suggest that the second-generation antipsychotic olanzapine, through the inhibition of DDO activity, increases L-glutamate release in the PFC of treated mice.

Age-dependent dopamine transporter dysfunction and Serine129 phospho-α-synuclein overload in G2019S LRRK2 mice

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common genetic cause of Parkinson’s disease. Here, we investigated whether the G2019S LRRK2 mutation causes morphological and/or functional changes at nigro-striatal dopamine neurons. Density of striatal dopaminergic terminals, nigral cell counts, tyrosine hydroxylase protein levels as well as exocytotic dopamine release measured in striatal synaptosomes, or striatal extracellular dopamine levels monitored by in vivo microdialysis were similar between ≥12-month-old G2019S knock-in mice and wild-type controls. In vivo striatal dopamine release was insensitive to the LRRK2 inhibitor Nov-LRRK2-11, and was elevated by the membrane dopamine transporter blocker GBR-12783. However, G2019S knock-in mice showed a blunted neurochemical and motor activation response to GBR-12783 compared to wild-type controls. Western blot and dopamine uptake analysis revealed an increase in dopamine transporter levels and activity in the striatum of 12-month-old G2019S KI mice. This phenotype correlated with a reduction in vesicular monoamine transporter 2 levels and an enhancement of vesicular dopamine uptake, which was consistent with greater resistance to reserpine-induced hypolocomotion. These changes were not observed in 3-month-old mice. Finally, Western blot analysis revealed no genotype difference in striatal levels of endogenous α-synuclein or α-synuclein bound to DOPAL (a toxic metabolite of dopamine). However, Serine129-phosphorylated α-synuclein levels were higher in 12-month-old G2019S knock-in mice. Immunohistochemistry confirmed this finding, also showing no genotype difference in 3-month-old mice. We conclude that the G2019S mutation causes progressive dysfunctions of dopamine transporters, along with Serine129-phosphorylated α-synuclein overload, at striatal dopaminergic terminals, which are not associated with dopamine homeostasis dysregulation or neuron loss but might contribute to intrinsic dopaminergic terminal vulnerability. We propose G2019S knock-in mice as a presymptomatic Parkinson’s disease model, useful to investigate the pathogenic interaction among genetics, aging, and internal or environmental factors leading to the disease.

Expanding the repertoire of L-DOPA's actions: A comprehensive review of its functional neurochemistry

Though a multi-facetted disorder, Parkinson's disease is prototypically characterized by neurodegeneration of nigrostriatal dopaminergic neurons of the substantia nigra pars compacta, leading to a severe disruption of motor function. Accordingly, L-DOPA, the metabolic precursor of dopamine (DA), is well-established as a treatment for the motor deficits of Parkinson's disease despite long-term complications such as dyskinesia and psychiatric side-effects. Paradoxically, however, despite the traditional assumption that L-DOPA is transformed in residual striatal dopaminergic neurons into DA, the mechanism of action of L-DOPA is neither simple nor entirely clear. Herein, focussing on its influence upon extracellular DA and other neuromodulators in intact animals and experimental models of Parkinson's disease, we highlight effects other than striatal generation of DA in the functional profile of L-DOPA. While not excluding a minor role for glial cells, L-DOPA is principally transformed into DA in neurons yet, interestingly, with a more important role for serotonergic than dopaminergic projections. Moreover, in addition to the striatum, L-DOPA evokes marked increases in extracellular DA in frontal cortex, nucleus accumbens, the subthalamic nucleus and additional extra-striatal regions. In considering its functional profile, it is also important to bear in mind the marked (probably indirect) influence of L-DOPA upon cholinergic, GABAergic and glutamatergic neurons in the basal ganglia and/or cortex, while anomalous serotonergic transmission is incriminated in the emergence of L-DOPA elicited dyskinesia and psychosis. Finally, L-DOPA may exert intrinsic receptor-mediated actions independently of DA neurotransmission and can be processed into bioactive metabolites. In conclusion, L-DOPA exerts a surprisingly complex pattern of neurochemical effects of much greater scope that mere striatal transformation into DA in spared dopaminergic neurons. Their further experimental and clinical clarification should help improve both L-DOPA-based and novel strategies for controlling the motor and other symptoms of Parkinson's disease.

Genetic and pharmacological evidence that endogenous nociceptin/orphanin FQ contributes to dopamine cell loss in Parkinson's disease

To investigate whether the endogenous neuropeptide nociceptin/orphanin FQ (N/OFQ) contributes to the death of dopamine neurons in Parkinson's disease, we undertook a genetic and a pharmacological approach using NOP receptor knockout (NOP(-/-)) mice, and the selective and potent small molecule NOP receptor antagonist (-)-cis-1-methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol (SB-612111). Stereological unbiased methods were used to estimate the total number of dopamine neurons in the substantia nigra of i) NOP(-/-) mice acutely treated with the parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP), ii) naïve mice subacutely treated with MPTP, alone or in combination with SB-612111, iii) rats injected with a recombinant adeno-associated viral (AAV) vector overexpressing human mutant p.A53T α-synuclein, treated with vehicle or SB-612111. NOP(-/-) mice showed a 50% greater amount of nigral dopamine neurons spared in response to acute MPTP compared to controls, which was associated with a milder motor impairment. SB-612111, given 4 days after MPTP treatment to mimic the clinical condition, prevented the loss of nigral dopamine neurons and striatal dopaminergic terminals caused by subacute MPTP. SB-612111, administered a week after the AAV injections in a clinically-driven protocol, also increased by 50% both the number of spared nigral dopamine neurons and striatal dopamine terminals, and prevented accompanying motor deficits induced by α-synuclein. We conclude that endogenous N/OFQ contributes to dopamine neuron loss in pathogenic and etiologic models of Parkinson's disease through NOP receptor-mediated mechanisms. NOP receptor antagonists might prove effective as disease-modifying agents in Parkinson's disease, through the rescue of degenerating nigral dopamine neurons and/or the protection of the healthy ones.

Differential involvement of Ras-GRF1 and Ras-GRF2 in L-DOPA-induced dyskinesia

Objective

Recent findings have shown that pharmacogenetic manipulations of the Ras-ERK pathway provide a therapeutic means to tackle l-3,4-dihydroxyphenylalanine (l-DOPA)-induced dyskinesia (LID). First, we investigated whether a prolonged l-DOPA treatment differentially affected ERK signaling in medium spiny neurons of the direct pathway (dMSNs) and in cholinergic aspiny interneurons (ChIs) and assessed the role of Ras-GRF1 in both subpopulations. Second, using viral-assisted technology, we probed Ras-GRF1 and Ras-GRF2 as potential targets in this pathway. We investigated how selective blockade of striatal Ras-GRF1 or Ras-GRF2 expression impacted on LID (induction, maintenance, and reversion) and its neurochemical correlates.

Methods

We used both Ras-GRF1 knockout mice and lentiviral vectors (LVs) delivering short-hairpin RNA sequences (shRNAs) to obtain striatum-specific gene knockdown of Ras-GRF1 and Ras-GRF2. The consequences of these genetic manipulations were evaluated in the 6-hydroxydopamine mouse model of Parkinson’s disease. Escalating doses of l-DOPA were administered and then behavioral analysis with immunohistochemical assays and in vivo microdialysis were performed.

Results

Ras-GRF1 was found essential in controlling ERK signaling in dMSNs, but its ablation did not prevent ERK activation in ChIs. Moreover, striatal injection of LV-shRNA/Ras-GRF1 attenuated dyskinesia development and ERK-dependent signaling, whereas LV-shRNA/Ras-GRF2 was without effect, ruling out the involvement of Ras-GRF2 in LID expression. Accordingly, Ras-GRF1 but not Ras-GRF2 striatal gene-knockdown reduced l-DOPA-induced GABA and glutamate release in the substantia nigra pars reticulata, a neurochemical correlate of dyskinesia. Finally, inactivation of Ras-GRF1 provided a prolonged anti-dyskinetic effect for up to 7 weeks and significantly attenuated symptoms in animals with established LID.

Interpretation

Our results suggest that Ras-GRF1 is a promising target for LID therapy based on Ras-ERK signaling inhibition in the striatum.

Nociceptin/orphanin FQ-NOP receptor system in inflammatory and immune-mediated diseases

The neuropeptide nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand of the G-protein-coupled receptor NOP. Cells from the immune system express the precursor preproN/OFQ and the NOP receptor, as well as secrete N/OFQ. The activation of the N/OFQ-NOP pathway can regulate inflammatory and immune responses. Several immune activities, including leukocyte migration, cytokine and chemokine production, and lymphocytes proliferation are influenced by NOP activation. It was demonstrated that cytokines and other stimuli such as Toll-like receptor agonist (e.g., lipopolysaccharide) induce N/OFQ production by cells from innate and adaptive immune response. In this context, N/OFQ could modulate the outcome of inflammatory diseases, such as sepsis and immune-mediated pathologies by mechanisms not clearly elucidated. In fact, clinical studies revealed increased levels of N/OFQ under sepsis, arthritis, and Parkinson's disease. Preclinical and clinical studies pointed to the blockade of NOP receptor signaling as successful strategy for the treatment of inflammatory diseases. This review is focused on experimental and clinical data that suggest the participation of N/OFQ-NOP receptor activation in the modulation of the immune response, highlighting the immunomodulatory potential of NOP antagonists in the inflammatory and immunological disturbances.

Acute and chronic antiparkinsonian effects of the novel nociceptin/orphanin FQ receptor antagonist NiK-21273 in comparison with SB-612111

BACKGROUND AND PURPOSE

Nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor antagonists have been proposed as a novel therapeutic approach to Parkinson's disease. Main limitations of previous studies were the use of structurally similar compounds and the evaluation of their acute effects only. We report here on the acute and long-term antiparkinsonian effects of the novel compound 2-[3-[4-(2-chloro-6-fluoro-phenyl)-piperidin-1-ylmethyl]-2-(morpholine-4-carbonyl)-indol-1-yl]-acetamide (NiK-21273) in comparison with the potent and selective NOP receptor antagonist SB-612111.

EXPERIMENTAL APPROACH

Basic pharmacological properties of NiK-21273 were studied in cell lines and isolated tissues (mouse and rat vas deferens). Antiparkinsonian effects were studied in reserpinized mice and 6-hydroxydopamine hemilesioned rats under both acute and chronic administration protocols.

KEY RESULTS

In vitro, NiK-21273 behaved as a potent (pA(2) 7.7) and selective NOP receptor antagonist. In vivo, it reduced hypokinesia in reserpinized mice at 0.1 and 1 but not 10 mg·kg(-1), whereas SB-612111 (0.01-1 mg·kg(-1)) provided a dose-dependent antiparkinsonian effect. NiK-21273 ameliorated motor performance in 6-hydroxydopamine hemilesioned rats at 0.5 and 5 but not 15 mg·kg(-1). SB-612111 replicated these effects in the 0.01-1 mg·kg(-1) range without loss of efficacy. Both antagonists synergized with L-DOPA at subthreshold doses. Chronic administration of NiK-21273 provided delayed improvement in baseline activity at 0.5 and 1.5 mg·kg(-1), although tolerance to the higher dose was observed. Conversely, SB-612111 (1 mg·kg(-1)) maintained its effects over time without modifying baseline activity.

CONCLUSIONS AND IMPLICATIONS

NOP receptor antagonists provide motor benefit in parkinsonism models although the 'therapeutic' window and long-term effects may vary between compounds.

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.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

NOP Receptor Ligands as Potential Agents for Inflammatory and Autoimmune Diseases

Nociceptin/orphanin FQ (N/OFQ) is a seventeen-amino acid peptide that is the endogenous ligand of a G-protein-coupled receptor (NOP). Various immune cells express the precursor protein and secrete N/OFQ as well as display binding sites for this peptide. The functional capacity of NOP receptor was demonstrated in vitro and in vivo studies by the ability of N/OFQ to induce chemotaxis of immune cells, to regulate the expression of cytokines and other inflammatory mediators, and to control cellular and humoral immunity. In this context, N/OFQ could modulate the outcome of some inflammatory diseases, such as sepsis and autoimmune pathologies by mechanisms not clearly elucidated yet. In fact, human body fluid revealed increased levels of N/OFQ under sepsis, arthritis, and Parkinson's diagnose. Preclinical studies pointed to the blockade of NOP receptor signaling as successful in treating these experimental conditions. Further preclinical and clinical studies are required to investigate the potential of NOP ligands in treating inflammatory diseases.

Amantadine attenuates levodopa-induced dyskinesia in mice and rats preventing the accompanying rise in nigral GABA levels

Amantadine is the only drug marketed for treating levodopa-induced dyskinesia. However, its impact on basal ganglia circuitry in the dyskinetic brain, particularly on the activity of striatofugal pathways, has not been evaluated. We therefore used dual probe microdialysis to investigate the effect of amantadine on behavioral and neurochemical changes in the globus pallidus and substantia nigra reticulata of 6-hydroxydopamine hemi-lesioned dyskinetic mice and rats. Levodopa evoked abnormal involuntary movements (AIMs) in dyskinetic mice, and simultaneously elevated GABA release in substantia nigra reticulata (∼3-fold) but not globus pallidus. Glutamate levels were unaffected in both areas. Amantadine (40 mg/kg, i.p.), ineffective alone, attenuated (∼50%) AIMs expression and prevented the GABA rise. Moreover, it unraveled a facilitatory effect of levodopa on pallidal glutamate levels. Levodopa also evoked AIMs expression and a GABA surge (∼2-fold) selectively in the substantia nigra of dyskinetic rats. However, different from mice, glutamate levels rose simultaneously. Amantadine, ineffective alone, attenuated (∼50%) AIMs expression preventing amino acid increase and leaving unaffected pallidal glutamate. Overall, the data provide neurochemical evidence that levodopa-induced dyskinesia is accompanied by activation of the striato-nigral pathway in both mice and rats, and that the anti-dyskinetic effect of amantadine partly relies on the modulation of this pathway.

Nociceptin/orphanin FQ receptor knockout rats: in vitro and in vivo studies

Nociceptin/orphanin FQ (N/OFQ) regulates several biological functions via selective activation of the N/OFQ peptide (NOP) receptor. Recently knockout rats for the NOP receptor gene (NOP(-/-)) have been generated; these animals were used in the present study to investigate their emotional (open field, elevated plus maze, and forced swimming test), locomotor (drag and rotarod test), and nociceptive (plantar and formalin test) phenotypes in comparison with their NOP(+/+) littermates. In addition, N/OFQ sensitivity has been assessed in electrically stimulated vas deferens tissues taken from NOP(+/+) and NOP(-/-) rats. In the elevated plus maze and forced swimming tests NOP(-/-) rats showed anxiety- and anti-depressant-like phenotype, respectively. No differences were found in the open field test. NOP(-/-) rats outperformed their NOP(+/+) littermates in two motor behaviour assays. Genetic ablation of the NOP receptor gene produced a statistically significant increase in nociceptive behaviour of the mutant rats in the formalin test. Finally, in the electrically stimulated rat vas deferens taken from NOP(+/+) tissues, N/OFQ inhibited in a concentration-dependent manner the electrically induced twitches while the peptide was inactive in tissues taken from NOP(-/-) animals. These results, in line with previous findings obtained with selective NOP receptor antagonists in mice and rats and with mouse knockout studies, clearly indicate that endogenous N/OFQ-NOP receptor signalling plays an important role in controlling anxiety- and mood-related behaviours, exercise-driven locomotor activity and nociception. These observations are relevant for defining the therapeutic indications (and contraindications) of NOP receptor antagonists.

Dopamine-nociceptin/orphanin FQ interactions in the substantia nigra reticulata of hemiparkinsonian rats: involvement of D2/D3 receptors and impact on nigro-thalamic neurons and motor activity

Nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor antagonists proved to be effective in alleviating experimental parkinsonism. Nonetheless, loss of effectiveness or even worsening of parkinsonian symptoms have been observed at high doses. With the aim of clarifying the circuitry underlying the dual action of NOP receptor antagonists and the role of endogenous dopamine, the NOP receptor antagonist 1-benzyl-N-[3-[spiroisobenzofuran-1(3H),4'-piperidin-1-yl]propyl]pyrrolidine-2-carboxamide (Compound 24) and the D(2)/D(3) receptor antagonist raclopride were used in 6-hydroxydopamine hemilesioned rats. Systemically administered Compound 24 improved motor activity in the 0.1-10mg/kg dose range being ineffective at 30 mg/kg. To confirm NOP selectivity, Compound 24 improved motor performance in wild-type mice at 1 and 10mg/kg and inhibited it at 60 mg/kg, being ineffective in NOP receptor knockout mice. To prove that the bell-shaped profile was mediated by nigral NOP receptors, reverse dialysis of Compound 24 (0.03 μM) in substantia nigra reticulata ameliorated akinesia whereas Compound 24 (3 μM) was ineffective. To demonstrate that motor responses were mediated by tuning inhibitory and excitatory inputs to nigro-thalamic neurons, the low concentration elevated GABA and reduced glutamate in substantia nigra, simultaneously reducing GABA levels in ventro-medial thalamus. Conversely, the higher concentration reduced nigral and elevated thalamic GABA, without affecting nigral glutamate levels. Co-perfusion with raclopride (1 μM) abolished the antiakinetic action of Compound 24 (0.03 μM) and turned the ineffectiveness of Compound 24 (3 μM) into an antiakinetic effect. The low concentration reduced nigral but did not affect thalamic GABA whereas the higher concentration elevated nigral and reduced thalamic GABA. Neither concentration affected nigral glutamate. We conclude that dual motor effects of Compound 24 in hemiparkinsonian rats are accomplished through blockade of nigral NOP receptors resulting in opposite modulation of nigro-thalamic neurons. Endogenous dopamine contributes to these responses affecting the level of GABAergic inhibition of the nigral output via D(2)/D(3) receptors.

Novel helix-constrained nociceptin derivatives are potent agonists and antagonists of ERK phosphorylation and thermal analgesia in mice

The nociceptin opioid peptide receptor (NOP, NOR, ORL-1) is a GPCR that recognizes nociceptin, a 17-residue peptide hormone. Nociceptin regulates pain transmission, learning, memory, anxiety, locomotion, cardiovascular and respiratory stress, food intake, and immunity. Nociceptin was constrained using an optimized helix-inducing cyclization strategy to produce the most potent NOP agonist (EC50 = 40 pM) and antagonist (IC50 = 7.5 nM) known. Alpha helical structures were measured in water by CD and 2D (1)H NMR spectroscopy. Agonist and antagonist potencies, evaluated by ERK phosphorylation in mouse neuroblastoma cells natively expressing NOR, increased 20-fold and 5-fold, respectively, over nociceptin. Helix-constrained peptides with key amino acid substitutions had much higher in vitro activity, serum stability, and thermal analgesic activity in mice, without cytotoxicity. The most potent agonist increased hot plate contact time from seconds up to 60 min; the antagonist prevented this effect. Such helix-constrained peptides may be valuable physiological probes and therapeutics for treating some forms of pain.

Further evidence for an involvement of nociceptin/orphanin FQ in the pathophysiology of Parkinson's disease: a behavioral and neurochemical study in reserpinized mice

The contribution of nociceptin/orphanin FQ (N/OFQ) to reserpine-induced Parkinsonism was evaluated in mice. A battery of motor tests revealed that reserpine caused dose-dependent and long-lasting motor impairment. Endogenous N/OFQ sustained this response because N/OFQ peptide (NOP) receptor knockout (NOP(-/-) ) mice were less susceptible to the hypokinetic action of reserpine than wild-type (NOP(+/+) ) animals. Microdialysis revealed that reserpine elevated glutamate and reduced GABA levels in substantia nigra reticulata, and that resistance to reserpine in NOP(-/-) mice was accompanied by a milder increase in glutamate and lack of inhibition of GABA levels. To substantiate this genetic evidence, the NOP receptor antagonist 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one (J-113397) simultaneously reduced akinesia and nigral glutamate levels in reserpinized NOP(+/+) mice, being ineffective in NOP(-/-) mice. Moreover, repeated J-113397 administration in reserpinized mice resulted in faster recovery of baseline motor performance which was, however, accompanied by a loss of acute antiakinetic response. The short-term beneficial effect of J-113397 was paralleled by normalization of nigral glutamate levels, whereas loss of acute response was paralleled by loss of the ability of J-113397 to inhibit glutamate levels. We conclude that endogenous N/OFQ contributes to reserpine-induced Parkinsonism, and that sustained NOP receptor blockade produces short-term motor improvement accompanied by normalization of nigral glutamate release.

Brain interstitial nociceptin/orphanin FQ levels are elevated in Parkinson's disease

Expression and release of nociceptin/orphanin FQ (N/OFQ) are elevated in the substantia nigra reticulata of 6-hydroxydopamine-hemilesioned rats, suggesting a pathogenic role for N/OFQ in Parkinson's disease. In this study, we investigated whether elevation of N/OFQ expression in 6-hydroxydopamine-hemilesioned rats selectively occurs in substantia nigra and whether hypomotility following acute haloperidol administration is accompanied by a rise in nigral N/OFQ levels. Moreover, to prove a link between N/OFQ and idiopathic Parkinson's disease in humans, we measured N/OFQ levels in the cerebrospinal fluid of parkinsonian patients undergoing surgery for deep brain stimulation. In situ hybridization demonstrated that dopamine depletion was associated with increase of N/OFQ expression in substantia nigra (compacta +160%, reticulata +105%) and subthalamic nucleus (+45%), as well as reduction in caudate putamen (-20%). No change was observed in globus pallidus, nucleus accumbens, thalamus, and motor cortex. Microdialysis coupled to the bar test allowed to demonstrate that acute administration of haloperidol (0.8 and 3 mg/kg) increased nigral N/OFQ levels (maximally of +47% and +53%, respectively) in parallel with akinesia. A correlation with preclinical studies was found by analyzing N/OFQ levels in humans. Indeed, N/OFQ levels were found to be approximately 3.5-fold elevated in the cerebrospinal fluid of parkinsonian patients (148 fmol/ml) compared with nonparkinsonian neurologic controls (41 fmol/ml). These data represent the first clinical evidence linking N/OFQ to idiopathic Parkinson's disease in humans. They strengthen the pathogenic role of N/OFQ in the modulation of parkinsonism across species and provide a rationale for developing N/OFQ receptor antagonists as antiparkinsonian drugs.