Nociceptin/orphanin FQ inhibits stress- and CRF-induced anorexia in rats

Do categorically distinct stressors alter the attention to visual food cues?

Stressor exposure affects food intake as well as the preference for high or low palatability foods, but little is known about how stressor types impact the visual attention to food images. We used eye tracking methodology in humans to determine if activation of the hypothalamus-pituitary-adrenal (HPA) axis and sympathetic nervous system is associated with changes in attention to food images as determined by measuring changes in oculomotor activity. Specifically, we tested two questions: 1) Do categorically distinct stressors alter aspects of visual attention to food images as determined by oculomotor activity (i.e., saccade latency, gaze duration, and saccade bouts)? 2) Do categorically distinct stressors differentially affect visual attention to food images of high or low palatability? A total of sixty participants were randomly divided into one of three test groups: controls, an anticipatory stressor group, or a reactive stressor group. We measured salivary cortisol and salivary alpha-amylase (sAA) before and after stressor exposure to confirm activation of the HPA axis and sympathetic nervous system, respectively. Following stressor exposure participants performed an eye-tracking test using a standardized food picture database (Food-pics). We analyzed saccade latency, gaze duration, and saccade bouts in balanced pairs of food and non-food images. Salivary cortisol was elevated by both stressors, although the elevation in salivary cortisol to the reactive stressor was driven by women only. sAA was elevated only by the anticipatory stressor. There were main effects of image type for all three eye-tracking variables, with initial saccades of shorter latency to food images and longer gaze duration and more saccade bouts with food images. Participants exposed to the reactive stressor reduced gaze duration on food images relative to controls, and this affect was not linked to palatability or salivary cortisol levels. We conclude that the reactive stressor decreased time spent looking at food, but not non-food, images. These data are partly consistent with the idea that reactive stressors reduce attention to non-critical visual signals.

A systematic review and meta-analysis of animal models of binge eating - Part 1: Definitions and food/drink intake outcomes

Binge eating involves consuming excessive amounts of food within a discrete period of time and is associated with significant impairments in binge-eating disorder and bulimia nervosa. While research on clinical binge eating has provided valuable aetiological insights, animal models allow for closer examination of environmental, biological, and developmental risk factors. Numerous animal models of binge eating exist and differ widely in operational definitions of bingeing, animal characteristics and methodological parameters. The current review aimed to synthesise the available published evidence on these models. A systematic review of binge definitions in 170 articles found most studies displayed good face validity. Meta-analyses on 150 articles confirmed that the amount of food or drink consumed by animals under binge conditions was larger than that of non-binge conditions across many protocols. The meta-regression revealed species, strain, and sex moderated binge effect size, with the largest effect observed in studies with female animals and mice. Risk of bias assessment identified that improved reporting of allocation, baseline characteristics and outcome assessment is required in future studies.

NOP receptor antagonism attenuates reinstatement of alcohol-seeking through modulation of the mesolimbic circuitry in male and female alcohol-preferring rats

In patients suffering from alcohol use disorder (AUD), stress and environmental stimuli associated with alcohol availability are important triggers of relapse. Activation of the nociceptin opioid peptide (NOP) receptor by its endogenous ligand Nociceptin/Orphanin FQ (N/OFQ) attenuates alcohol drinking and relapse in rodents, suggesting that NOP agonists may be efficacious in treating AUD. Intriguingly, recent data demonstrated that also blockade of NOP receptor reduced alcohol drinking in rodents. To explore further the potential of NOP antagonism, we investigated its effects on the reinstatement of alcohol-seeking elicited by administration of the α2 antagonist yohimbine (1.25 mg/kg, i.p.) or by environmental conditioning factors in male and female genetically selected alcohol-preferring Marchigian Sardinian (msP) rats. The selective NOP receptor antagonist LY2817412 (0.0, 3.0, 10.0, and 30.0 mg/kg) was first tested following oral (p.o.) administration. We then investigated the effects of LY2817412 (1.0, 3.0, 6.0 μg/μl/rat) microinjected into three candidate mesolimbic brain regions: the ventral tegmental area (VTA), the central nucleus of the amygdala (CeA), and the nucleus accumbens (NAc). We found that relapse to alcohol seeking was generally stronger in female than in male rats and oral administration of LY2817412 reduced yohimbine- and cue-induced reinstatement in both sexes. Following site-specific microinjections, LY2817412 reduced yohimbine-induced reinstatement of alcohol-seeking when administered into the VTA and the CeA, but not in the NAc. Cue-induced reinstatement was suppressed only when LY2817412 was microinjected into the VTA. Infusions of LY2817412 into the VTA and the CeA did not alter saccharin self-administration. These results demonstrate that NOP receptor blockade prevents the reinstatement of alcohol-seeking through modulation of mesolimbic system circuitry, providing further evidence of the therapeutic potential of NOP receptor antagonism in AUD.

Nociceptin/orphanin FQ receptor system blockade as an innovative strategy for increasing resilience to stress

The ability to successfully cope with stress is known as 'resilience', and resilient individuals are less prone to develop psychopathologies. Understanding the neurobiological mechanisms of resilience may be instrumental to improve current therapies and benefit high-risk subjects. This review summarizes the complex interplay that exists between physiological and pathological responses to stressful events and the nociceptin/orphanin FQ (N/OFQ) - N/OFQ receptor (NOP) system, including: the effects of stress in regulating N/OFQ release and NOP expression; the ability of the N/OFQ-NOP system to modulate the hypothalamic-pituitary-adrenal axis; behavioral studies; and evidence in humans correlating this peptidergic system with psychopathologies. Available findings support the view that N/OFQ signaling stimulates the hypothalamic-pituitary-adrenal axis, thus increasing stress circulating hormones and corticotropin-releasing factor signaling. Additionally, activation of the NOP receptor inhibits monoamine transmission, including 5-HT, and this may contribute to maladaptive outcomes of stress. Ultimately, the N/OFQ system seems to have an important role in stress vulnerability, and blockade of NOP signaling may provide an innovative strategy for the treatment of stress related psychopathologies.

Blockade of nociceptin/orphanin FQ signaling facilitates an active copying strategy due to acute and repeated stressful stimuli in mice

The role of stress in the etiology of depression has been largely reported. In this line, exogenous glucocorticoids are employed to mimic the influence of stress on the development of depression. The N/OFQ-NOP receptor system has been implicated in the modulation of stress and emotional behaviors. In fact, the blockade of NOP receptors induces antidepressant effects and increases resilience to acute stress. This study investigated the effects of the NOP receptor blockade on dexamethasone-treated mice exposed to acute and prolonged swimming stress. Swiss and NOP(+/+) and NOP(−/−) mice were treated with dexamethasone, and the protective effects of the NOP antagonist SB-612111 (10 mg/kg, ip) or imipramine (20 mg/kg, ip) were investigated in three swimming sessions. The re-exposure to swim stress increased immobility time in Swiss and NOP(+/+), but not in NOP(−/−) mice. Acute and repeated dexamethasone administration induced a further increase in the immobility time, and facilitated body weight loss in Swiss mice. Single administration of SB-612111, but not imipramine, prevented swimming stress- and dexamethasone-induced increase in the immobility time. Repeated administrations of SB-612111 prevented the deleterious effects of 5 days of dexamethasone treatment. Imipramine also partially prevented the effects of repeated glucocorticoid administration on the immobility time, but did not affect the body weight loss. NOP(−/−) mice were more resistant than NOP(+/+) mice to inescapable swimming stress, but not dexamethasone-induced increase in the immobility time and body weight loss. In conclusion, the blockade of the NOP receptor facilitates an active stress copying response and attenuates body weight loss due to repeated stress.

Acute Elevations in Cortisol Increase the In Vivo Binding of [11C]NOP-1A to Nociceptin Receptors: A Novel Imaging Paradigm to Study the Interaction Between Stress- and Antistress-Regulating Neuropeptides

BACKGROUND

An imbalance between neuropeptides that promote stress and resilience, such as corticotropin-releasing factor and nociceptin, has been postulated to underlie relapse in addiction. The objective of this study was to develop a paradigm to image the in vivo interaction between stress-promoting neuropeptides and nociceptin (NOP) receptors in humans.

METHODS

[11C]NOP-1A positron emission tomography was used to measure the binding to NOP receptors at baseline (BASE) and following an intravenous hydrocortisone challenge (CORT) in 19 healthy control subjects. Hydrocortisone was used as a challenge because in microdialysis studies it has been shown to increase corticotropin-releasing factor release in extrahypothalamic brain regions such as the amygdala. [11C]NOP-1A total distribution volume (VT) in 11 regions of interest were measured using a 2-tissue compartment kinetic analysis. The primary outcome measure was hydrocortisone-induced ΔVT calculated as (VT CORT - VT BASE)/VT BASE.

RESULTS

Hydrocortisone led to an acute increase in plasma cortisol levels. Regional [11C]NOP-1A VT was on average 11% to 16% higher in the post-hydrocortisone condition compared with the baseline condition (linear mixed model, condition, p = .005; region, p < .001; condition × region, p < .001). Independent Student's t tests in all regions of interest were statistically significant and survived multiple comparison correction. Hydrocortisone-induced ΔVT was significantly negatively correlated with baseline VT in several regions of interest.

CONCLUSIONS

Hydrocortisone administration increases NOP receptor availability. Increased NOP in response to elevated cortisol might suggest a compensatory mechanism in the brain to counteract corticotropin-releasing factor and/or stress. The [11C]NOP-1A and hydrocortisone imaging paradigm should allow for the examination of interactions between stress-promoting neuropeptides and NOP in addictive disorders.

Endogenous opioid modulation of food intake and body weight: Implications for opioid influences upon motivation and addiction

This review is part of a special issue dedicated to Opioid addiction, and examines the influential role of opioid peptides, opioid receptors and opiate drugs in mediating food intake and body weight control in rodents. This review postulates that opioid mediation of food intake was an example of "positive addictive" properties that provide motivational drives to maintain opioid-seeking behavior and that are not subject to the "negative addictive" properties associated with tolerance, dependence and withdrawal. Data demonstrate that opiate and opioid peptide agonists stimulate food intake through homeostatic activation of sensory, metabolic and energy-related In contrast, general, and particularly mu-selective, opioid receptor antagonists typically block these homeostatically-driven ingestive behaviors. Intake of palatable and hedonic food stimuli is inhibited by general, and particularly mu-selective, opioid receptor antagonists. The selectivity of specific opioid agonists to elicit food intake was confirmed through the use of opioid receptor antagonists and molecular knockdown (antisense) techniques incapacitating specific exons of opioid receptor genes. Further extensive evidence demonstrated that homeostatic and hedonic ingestive situations correspondingly altered the levels and expression of opioid peptides and opioid receptors. Opioid mediation of food intake was controlled by a distributed brain network intimately related to both the appetitive-consummatory sites implicated in food intake as well as sites intimately involved in reward and reinforcement. This emergent system appears to sustain the "positive addictive" properties providing motivational drives to maintain opioid-seeking behavior.

NOP-Related Mechanisms in Substance Use Disorders

Nociceptin/orphanin FQ (N/OFQ) is a 17 amino acid peptide that was deorphanized in 1995 and has been widely studied since. The role of the N/OFQ system in drug abuse has attracted researchers' attention since its initial discovery. The first two scientific papers describing the effect of intracranial injection of N/OFQ appeared 20 years ago and reported efficacy of the peptide in attenuating alcohol intake, whereas heroin self-administration was insensitive. Since then more than 100 scientific articles investigating the role of the N/OFQ and N/OFQ receptor (NOP) system in drug abuse have been published. The present article provides an historical overview of the advances in the field with focus on three major elements. First, the most robust data supportive of the efficacy of NOP agonists in treating drug abuse come from studies in the field of alcohol research, followed by psychostimulant and opioid research. In contrast, activation of NOP appears to facilitate nicotine consumption. Second, emerging data challenge the assumption that activation of NOP is the most appropriate strategy to attenuate consumption of substances of abuse. This assumption is based first on the observation that animals carrying an overexpression of NOP system components are more prone to consume substances of abuse, whereas NOP knockout rats are less motivated to self-administer heroin, alcohol, and cocaine. Third, administration of NOP antagonists also reduces alcohol consumption. In addition, NOP blockade reduces nicotine self-administration. Hypothetical mechanisms explaining this apparent paradox are discussed. Finally, we focus on the possibility that co-activation of NOP and mu opioid (MOP) receptors is an alternative strategy, readily testable in the clinic, to reduce the consumption of psychostimulants, opiates, and, possibly, alcohol.

N/OFQ-NOP System in Food Intake

While lifestyle modifications should be the first-line actions in preventing and treating obesity and eating disorders, pharmacotherapy also provides a necessary tool for the management of these diseases.However, given the limitations of current anti-obesity drugs, innovative treatments that improve efficacy and safety are needed.Since the discovery that the activation of the Nociceptin/Orphanin (N/OFQ) FQ peptide (NOP) receptor by N/OFQ induces an increase of food intake in laboratory animals, and the finding that this effect can be blocked by NOP antagonists, many NOP agonists and antagonists have been synthesized and tested in vitro and in vivo for their potential regulation of feeding behavior. Promising results seem to suggest that the N/OFQergic system may be a potential therapeutic target for the neural control of feeding behavior and related pathologies, especially in binge-like eating behavior.

Nociceptin Receptors in Alcohol Use Disorders: A Positron Emission Tomography Study Using [11C]NOP-1A

BACKGROUND

The neuropeptide transmitter nociceptin, which binds to the nociceptin/orphanin FQ peptide (NOP) receptor, is a core component of the brain's antistress system. Nociceptin exerts its antistress effect by counteracting the functions of corticotropin-releasing factor, the primary stress-mediating neuropeptide in the brain. Basic investigations support a role for medications that target nociceptin receptors in the treatment of alcohol use disorders. Thus, it is of high interest to measure the in vivo status of NOP receptors in individuals with alcohol use disorders.

METHODS

Here, we used [11C]NOP-1A and positron emission tomography to measure the in vivo binding to NOP receptors in 15 alcohol-dependent humans as identified by DSM-IV and 15 healthy control subjects matched for age, sex, and smoking status. Alcohol-dependent individuals with no comorbid psychiatric, medical, or drug abuse disorders were scanned following 2 weeks of outpatient monitored abstinence (confirmed with three times per week urine alcohol metabolite testing). [11C]NOP-1A distribution volume in regions of interest (including the amygdala, hippocampus, and midbrain, striatal, and prefrontal cortical subdivisions) was measured with kinetic analysis using the arterial input function.

RESULTS

Regional [11C]NOP-1A distribution volume in alcohol dependence was not significantly different compared with healthy control subjects. No relationship between [11C]NOP-1A distribution volume and other clinical measures (including duration and severity of alcohol abuse, craving, and anxiety or depressive symptoms) were significant after correction for the multiple hypotheses tested.

CONCLUSIONS

The results of this study do not support alterations in the binding to NOP receptors in alcohol dependence. However, this finding does not necessarily rule out alterations in nociceptin transmission in alcohol dependence.

Hypothalamic CRF1 receptor mechanisms are not sufficient to account for binge-like palatable food consumption in female rats

OBJECTIVE

The present study evaluated the effect of systemic injection of the CRF1 receptor antagonist R121919, the corticosterone synthesis inhibitor metyrapone and central amygdala (CeA) injections of the nonselective CRF antagonist D-Phe-CRF_(12-41) in rats in which binge eating was evoked by stress and cycles of food restriction.

METHOD

Female rats were subjected or not to repeated cycles of regular chow food restriction/ad libitum feeding during which they were also given limited access (2 h) to palatable food. On the test day, rats were either exposed or not to the sight of the palatable food for 15 min without allowing access, before assessing food consumption.

RESULTS

Systemic injections of R121919, but not of the metyrapone, blocked binge-like eating behavior. Restricted and stressed rats showed up-regulation of crh1 receptor mRNA signal in the bed nucleus of the stria terminalis and CeA but not in basolateral amygdala (BLA) or in the paraventricular nucleus. Injection D-Phe-CRF_(12-41) in CeA but not in the BLA-blocked binge-like eating behavior.

DISCUSSION

These findings demonstrate that extra-hypothalamic CRF1 receptors, rather than those involved in endocrine functions, are involved in binge eating and the crucial role of CRF receptors in CeA. CRF1 receptor antagonism may represent a novel pharmacological treatment for binge-related eating disorders.

N/OFQ system in brain areas of nerve-injured mice: its role in different aspects of neuropathic pain

Several studies showed that chronic pain causes reorganization and functional alterations of supraspinal brain regions. The nociceptin-NOP receptor system is one of the major systems involved in pain control and much evidence also suggested its implication in stress, anxiety and depression. Therefore, we investigated the nociceptin-NOP system alterations in selected brain regions in a neuropathic pain murine model. Fourteen days after the common sciatic nerve ligature, polymerase chain reaction (PCR) analysis indicated a significant decrease of pronociceptin and NOP receptor mRNA levels in the thalamus; these alterations could contribute to the decrease of the thalamic inhibitory function reported in neuropathic pain condition. Nociceptin peptide and NOP mRNA increased in the anterior cingulate cortex (ACC) and not in the somatosensory cortex, suggesting a peculiar involvement of this system in pain regulating circuitry. Similarly to the ACC, an increase of nociceptin peptide levels was observed in the amygdala. Finally, the pronociceptin and NOP mRNAs decrease observed in the hypothalamus reflects the lack of hypothalamus-pituitary-adrenal axis activation, already reported in neuropathic pain models. Our data indicate that neuropathic pain conditions affect the supraspinal nociceptin-NOP system which is also altered in regions known to play a role in emotional aspects of pain.

Epigenetic regulation of nociceptin/orphanin FQ and corticotropin-releasing factor system genes in frustration stress-induced binge-like palatable food consumption

Evidence suggests that binge eating may be caused by a unique interaction between dieting and stress. We developed a binge-eating model in which female rats with a history of intermittent food restriction show binge-like palatable food consumption after a 15-minute exposure to the sight of the palatable food (frustration stress). The aim of the present study was to investigate the regulation of the stress neurohormone corticotropin-releasing factor (CRF) system and of the nociceptin/orphanin FQ (N/OFQ) system genes in selective rat brain regions, using our animal model. Food restriction by itself seems to be responsible in the hypothalamus for the downregulation on messenger RNA levels of CRF-1 receptor, N/OFQ and its receptor (NOP). For the latter, this alteration might be due to selective histone modification changes. Instead, CRF gene appears to be upregulated in the hypothalamus as well as in the ventral tegmental area only when rats are food restricted and exposed to frustration stress, and, of relevance, these changes appear to be due to a reduction in DNA methylation at gene promoters. Moreover, also CRF-1 receptor gene resulted to be differentially regulated in these two brain regions. Epigenetic changes may be viewed as adaptive mechanisms to environmental perturbations concurring to facilitate food consumption in adverse conditions, that is, in this study, under food restriction and stressful conditions. Our data on N/OFQ and CRF signaling provide insight on the use of this binge-eating model for the study of epigenetic modifications in controlled genetic and environmental backgrounds.

The Nociceptin Receptor as an Emerging Molecular Target for Cocaine Addiction

Cocaine addiction is a global public health and socioeconomic issue that requires pharmacological and cognitive therapies. Currently there are no FDA-approved medications to treat cocaine addiction. However, in preclinical studies, interventions ranging from herbal medicine to deep-brain stimulation have shown promise for the therapy of cocaine addiction. Recent developments in molecular biology, pharmacology, and medicinal chemistry have enabled scientists to identify novel molecular targets along the pathways involved in drug addiction. In 1994, a receptor that showed a great deal of homology to the traditional opioid receptors was characterized. However, endogenous and exogenous opioids failed to bind to this receptor, which led scientists to name it opioid receptor-like receptor, now referred to as the nociceptin receptor. The endogenous ligand of NOPr was identified a year later and named orphanin FQ/nociceptin. Nociceptin and NOPr are widely distributed throughout the CNS and are involved in many physiological responses, such as food intake, nociceptive processing, neurotransmitter release, etc. Furthermore, exogenous nociceptin has been shown to regulate the activity of mesolimbic dopaminergic neurons, glutamate, and opioid systems, and the stress circuit. Importantly, exogenous nociceptin has been shown to reduce the rewarding and addictive actions of a number of drugs of abuse, such as psychostimulants, alcohol, and opioids. This paper reviews the existing literature on the role of endogenous nociceptin in the rewarding and addictive actions of cocaine. The effect of exogenous nociceptin on these processes is also reviewed. Furthermore, the effects of novel small-molecule NOPr ligands on these actions of cocaine are discussed. Overall, a review of the literature suggests that NOPr could be an emerging target for cocaine addiction pharmacotherapy.

Emerging targets for addiction neuropharmacology: From mechanisms to therapeutics

Drug abuse represents a considerable burden of disease and has enormous economic impacts on societies. Over the years, few medications have been developed for clinical use. Their utilization is endowed with several limitations, including partial efficacy or significant side effects. On the other hand, the successful advancement of these compounds provides an important proof of concept for the feasibility of drug development programs in addiction. In recent years, a wealth of information has been generated on the psychological mechanisms, genetic or epigenetic predisposing factors, and neurobiological adaptations induced by drug consumption that interact with each other to contribute to disease progression. It is now clear that addiction develops through phases, from initial recreational use to excessive consumption and compulsive drug seeking, with a shift from positive to negative reinforcement driving motivated behaviors. A greater understanding of these mechanisms has opened new vistas in drug development programs. Researchers' attention has been shifted from investigation of classical targets associated with reward to biological substrates responsible for negative reinforcement, impulse loss of control, and maladaptive mechanisms resulting from protracted drug use. From this research, several new biological targets for the development of innovative therapies have started to emerge. This chapter offers an overview of targets currently under scrutiny for the development of new medications for addiction. This work is not exhaustive but rather it provides a few examples of how this research has advanced in recent years by virtue of studies carried out in our laboratory.

MT-7716, a potent NOP receptor agonist, preferentially reduces ethanol seeking and reinforcement in post-dependent rats

Dysregulation of the nociceptin (N/OFQ) system has been implicated in alcohol abuse and alcoholism, and growing evidence suggests that targeting this system may be beneficial for treating alcoholism. To further explore the treatment target potential of the N/OFQ system, the novel non-peptide, small-molecule N/OFQ (NOP) agonist MT-7716, (R)-2-{3-[1-(Acenaphthen-1-yl)piperidin-4-yl]-2-oxo-2,3-dihydro-1H-benzimidazol-1-yl}-N-methylacetamide hydrochloride hydrate, was examined for its effects on ethanol self-administration and stress-induced reinstatement of alcohol seeking in non-dependent and post-dependent rats. Male Wistar rats were trained to self-administer ethanol and then made ethanol dependent via repeated intragastric ethanol intubation. The effects of MT-7716 (0.3 and 1 mg/kg; PO) on alcohol self-administration were determined 2 weeks following dependence induction, when baseline self-administration was restored. Effects of MT-7716 on stress-induced reinstatement were tested in separate cohorts of rats, 1 and 3 weeks post-withdrawal. MT-7716 reduced alcohol self-administration and stress-induced reinstatement of alcohol seeking in post-dependent rats, but was ineffective in non-dependent animals. Moreover, the prevention of stress-induced reinstatement by MT-7716 was more pronounced at 3 weeks post-dependence. The results further confirm treatment target potential for the NOP receptor and identify non-peptide NOP agonists as promising potential treatment drugs for alcohol abuse and relapse prevention. The findings also support dysregulation of the N/OFQ system as a factor in alcohol seeking and reinforcement.

Effects of electromagnetic radiation exposure on stress-related behaviors and stress hormones in male wistar rats

Studies have demonstrated that electromagnetic waves, as the one of the most important physical factors, may alter cognitive and non-cognitive behaviors, depending on the frequency and energy. Moreover, non-ionizing radiation of low energy waves e.g. very low frequency waves could alter this phenomenon via alterations in neurotransmitters and neurohormones. In this study, short, medium, and long-term exposure to the extremely low frequency electromagnetic field (ELF-EMF) (1 and 5 Hz radiation) on behavioral, hormonal, and metabolic changes in male Wistar rats (250 g) were studied. In addition, changes in plasma concentrations for two main stress hormones, noradrenaline and adrenocorticotropic hormone (ACTH) were evaluated. ELF-EMF exposure did not alter body weight, and food and water intake. Plasma glucose level was increased and decreased in the groups which exposed to the 5 and 1Hz wave, respectively. Plasma ACTH concentration increased in both using frequencies, whereas nor-adrenaline concentration showed overall reduction. At last, numbers of rearing, sniffing, locomotor activity was increased in group receiving 5 Hz wave over the time. In conclusions, these data showed that the effects of 1 and 5 Hz on the hormonal, metabolic and stress-like behaviors may be different. Moreover, the influence of waves on stress system is depending on time of exposure.

Functional antagonism between nociceptin/orphanin FQ and corticotropin-releasing factor in rat anxiety-related behaviors: involvement of the serotonergic system

Nociceptin/orphanin FQ (N/OFQ) acts as an anxiolytic-like agent in the rat and behaves as a functional antagonist of corticotropin-releasing factor (CRF) due to its ability to oppose CRF biological actions. In response to stress, CRF triggers changes in neurotransmitter systems including serotonin (5-HT). The role of 5-HT1A receptor in anxiety has been supported by preclinical and clinical studies. The present study investigated the possible functional antagonism between N/OFQ (1nmol/rat) and CRF (0.2nmol/rat) in anxiety-related conditions in rats, using elevated plus maze and defensive burying tests, in order to confirm previous literature results. Moreover, possible changes in the serotonergic system were studied in areas rich of serotonergic neurons: frontal cortex and pons. In both tests N/OFQ showed anxiolytic-like effects while CRF displayed anxiogenic-like effects. N/OFQ before CRF treatment counteracted the anxiogenic-like effects evoked by CRF. In frontal cortex, N/OFQ significantly decreased 5-HT levels but did not modify the hydroxyindoleacetic acid (5-HIAA) ones; CRF modified neither 5-HT nor 5-HIAA content but counteracted changes induced by N/OFQ alone. In pons, N/OFQ induced no change in serotonergic activity while CRF significantly decreased 5-HT levels and increased 5-HIAA content. The two peptides' combination reinstated serotonergic parameters to controls. In frontal cortex, N/OFQ increased the 5HT1A receptor density but reduced its affinity, while CRF alone did not induce any change. In pons, CRF decreased 5HT1ABmax and KD whereas N/OFQ was ineffective. All biochemical modifications were reverted by N/OFQ plus CRF treatment. The present study confirms that N/OFQ counteracts CRF anxiogenic-like effects in the behavioral tests evaluated. These effects may involve central serotonergic mechanisms since N/OFQ plus CRF induces a reversion of serotonergic changes provoked by single peptide. Our data support the hypothesis that N/OFQ may behave as functional CRF antagonist, this action being of interest for the treatment of anxiety disorders.

The biology of Nociceptin/Orphanin FQ (N/OFQ) related to obesity, stress, anxiety, mood, and drug dependence

Nociceptin/Orphanin FQ (N/OFQ) is a 17 amino acid peptide that was deorphanized in 1995. The generation of specific agonists, antagonists and receptor deficient mice and rats has enabled progress in elucidating the biological functions of N/OFQ. Additionally, radio-imaging technologies have been advanced for investigation of this system in animals and humans. Together with traditional neurobehavioral techniques, these tools have been utilized to identify the biological significance of the N/OFQ system and its interacting partners. The present review focuses on the role of N/OFQ in the regulation of feeding, body weight homeostasis, stress, the stress-related psychiatric disorders of depression and anxiety, and in drug and alcohol dependence. Critical evaluation of the current scientific preclinical literature suggests that small molecule modulators of nociceptin opioid peptide receptors (NOP) might be useful in the treatment of diseases related to these biological functions. In particular, the literature data suggest that antagonism of NOP receptors will produce anti-obesity and antidepressant activities in humans. However, there are also contradictory data discussed. The current literature on the role of N/OFQ in anxiety and addiction, on the other hand points primarily to a role of agonist modulation being potentially therapeutic. Some drug-like molecules that function either as agonists or antagonists of NOP receptors have been optimized for human clinical study to test some of these hypotheses. The discovery of PET ligands for NOP receptors, combined with the pharmacological tools and burgeoning preclinical data set discussed here bodes well for a rapid advancement of clinical understanding and potential therapeutic benefit.

Restraint stress alters nociceptin/orphanin FQ and CRF systems in the rat central amygdala: significance for anxiety-like behaviors

Corticotropin releasing factor (CRF) is the primary mediator of stress responses, and nociceptin/orphanin FQ (N/OFQ) plays an important role in the modulation of these stress responses. Thus, in this multidisciplinary study, we explored the relationship between the N/OFQ and the CRF systems in response to stress. Using in situ hybridization (ISH), we assessed the effect of body restraint stress on the gene expression of CRF and N/OFQ-related genes in various subdivisions of the amygdala, a critical brain structure involved in the modulation of stress response and anxiety-like behaviors. We found a selective upregulation of the NOP and downregulation of the CRF1 receptor transcripts in the CeA and in the BLA after body restraint. Thus, we performed intracellular electrophysiological recordings of GABAA-mediated IPSPs in the central nucleus of the amygdala (CeA) to explore functional interactions between CRF and N/OFQ systems in this brain region. Acute application of CRF significantly increased IPSPs in the CeA, and this enhancement was blocked by N/OFQ. Importantly, in stress-restraint rats, baseline CeA GABAergic responses were elevated and N/OFQ exerted a larger inhibition of IPSPs compared with unrestraint rats. The NOP antagonist [Nphe1]-nociceptin(1-13)NH2 increased the IPSP amplitudes in restraint rats but not in unrestraint rats, suggesting a functional recruitment of the N/OFQ system after acute stress. Finally, we evaluated the anxiety-like response in rats subjected to restraint stress and nonrestraint rats after N/OFQ microinjection into the CeA. Intra-CeA injections of N/OFQ significantly and selectively reduced anxiety-like behavior in restraint rats in the elevated plus maze. These combined results demonstrate that acute stress increases N/OFQ systems in the CeA and that N/OFQ has antistress properties.

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.

Caloric restriction increases the sensitivity to the hyperphagic effect of nociceptin/orphanin FQ limiting its ability to reduce binge eating in female rats

RATIONALE

Nociceptin/orphanin FQ (N/OFQ) is a functional antagonist of corticotrophin-releasing factor, the main mediator of the stress response. Stress represents a key determinant of binge eating (BE) for highly palatable food (HPF).

OBJECTIVES

In relation to the antistress properties of N/OFQ, we evaluated its effect on BE. After the observation that episodes of food restriction increase the sensitivity to its hyperphagic effects, the function of NOP receptor and N/OFQ was investigated after cycles of food restrictions.

MATERIALS AND METHODS

In BE experiments, four groups were used: rats fed normally and not stressed or stressed, rats exposed to cycles of restriction/refeeding and then stressed, or not stressed. In the other experiments, two groups were used: rats exposed or not to food restriction.

RESULTS

Only restricted and stressed rats exhibited BE for HPF (containing chocolate cream). Intracerebroventricular injections of N/OFQ of 0.5 nmol/rat significantly reduced BE. N/OFQ 1 nmol/rat did not reduce BE but significantly increased HPF intake following food restrictions. Cycles of food restriction increased animals' sensitivity to the hyperphagic effect of N/OFQ for HPF. In situ hybridization studies following food restrictions showed decreased ppN/OFQ mRNA expression in the bed nucleus of the stria terminalis and increased expression of ppN/OFQ and NOP receptor mRNA in the ventral tegmental area and in the ventromedial hypothalamus, respectively.

CONCLUSIONS

These findings indicate that N/OFQ slightly reduces BE at low doses, while higher doses increase HPF intake, due to increased sensitivity to its hyperphagic effect following a history of caloric restrictions.

Participation of the nociceptin/orphanin FQ receptor in ethanol-mediated locomotor activation and ethanol intake in preweanling rats

Activation of nociceptin/orphanin FQ (NOP) receptors seems to attenuate ethanol-induced reinforcement in adult rodents. Since early ethanol exposure results in later increased responsiveness to ethanol, it is important to analyze NOP receptor modulation of ethanol-related behaviors during early ontogeny. By measuring NOP involvement in ethanol intake and ethanol-induced locomotor activation, we analyzed the specific participation of NOP receptors on these ethanol-related behaviors in two-week-old rats. In each experiment animals were pre-treated with the endogenous ligand for this receptor (nociceptin/orphanin FQ at 0.0, 0.5, 1.0 or 2.0 μg) or a selective NOP antagonist (J-113397 at 0.0, 0.5, 2.0 or 5.0 mg/kg). Results indicated that activation of the nociceptin receptor system had no effect on ethanol or water intake, while blockade of the NOP receptor has an unspecific effect on consummatory behavior: J-113397 increased ethanol (at a dose of 0.5 mg/kg) and water intake (at 0.5 and 5.0 mg/kg). Ethanol-mediated locomotor stimulation was attenuated by activation of the NOP system (nociceptin at 1.0 and 2.0 μg). Nociceptin had no effect on basal locomotor activity. Blockade of NOP receptors did not modify ethanol-induced locomotor activation. Contrary to what has been reported for adult rodents, nociceptin failed to suppress intake of ethanol in infants. Attenuation of ethanol-induced stimulation by activation of NOP receptor system suggests an early role of this receptor in this ethanol-related behavior.

Nociceptin/orphanin FQ blockade of corticotropin-releasing factor-induced gamma-aminobutyric acid release in central amygdala is enhanced after chronic ethanol exposure

BACKGROUND

The central nucleus of the amygdala (CeA) mediates stress- and addiction-related processes. Corticotropin-releasing factor (CRF) and nociceptin/orphanin FQ (nociceptin) regulate ethanol intake and anxiety-like behavior. In the rat, CRF and ethanol significantly augment CeA gamma-aminobutyric acid (GABA) release, whereas nociceptin diminishes it.

METHODS

Using electrophysiologic techniques in an in vitro slice preparation, we investigated the interaction of nociceptin and CRF on evoked and spontaneous GABAergic transmission in CeA slices of naive and ethanol-dependent rats and the mechanistic role of protein kinase A.

RESULTS

In neurons from naive animals, nociceptin dose-dependently diminished basal-evoked GABA(A) receptor-mediated inhibitory postsynaptic potentials (IPSPs) by decreasing GABA release and prevented, as well as reversed, CRF-induced augmentation of IPSPs, actions that required PKA signaling. In neurons from ethanol-dependent animals, nociceptin decreased basal GABAergic transmission and blocked the CRF-induced increase in GABA release to a greater extent than in naive controls.

CONCLUSIONS

These data provide new evidence for an interaction between the nociceptin and CRF systems in the CeA. Nociceptin opposes CRF effects on CeA GABAergic transmission with sensitization of this effect in dependent animals. These properties of nociceptin may underlie its anti-alcohol and anxiolytic properties and identify the nociceptin receptor as a useful therapeutic target for alcoholism.

Neuropeptide modulation of central amygdala neuroplasticity is a key mediator of alcohol dependence

Alcohol use disorders are characterized by compulsive drug-seeking and drug-taking, loss of control in limiting intake, and withdrawal syndrome in the absence of drug. The central amygdala (CeA) and neighboring regions (extended amygdala) mediate alcohol-related behaviors and chronic alcohol-induced plasticity. Acute alcohol suppresses excitatory (glutamatergic) transmission whereas chronic alcohol enhances glutamatergic transmission in CeA. Acute alcohol facilitates inhibitory (GABAergic) transmission in CeA, and chronic alcohol increases GABAergic transmission. Electrophysiology techniques are used to explore the effects of neuropeptides/neuromodulators (CRF, NPY, nociceptin, dynorphin, endocannabinoids, galanin) on inhibitory transmission in CeA. In general, pro-anxiety peptides increase, and anti-anxiety peptides decrease CeA GABAergic transmission. These neuropeptides facilitate or block the action of acute alcohol in CeA, and chronic alcohol produces plasticity in neuropeptide systems, possibly reflecting recruitment of negative reinforcement mechanisms during the transition to alcohol dependence. A disinhibition model of CeA output is discussed in the context of alcohol dependence- and anxiety-related behaviors.

The nociceptin/orphanin FQ receptor (NOP) as a target for drug abuse medications

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

Effect of salidroside, active principle of Rhodiola rosea extract, on binge eating

Stress is a key determinant of binge eating (BE). Since Rhodiola rosea is known to modulate stress responses, its effect in a model of BE was investigated. BE for highly palatable food (HPF) was evoked in female rats by three 8-day cycles of food restriction/re-feeding (for 4days 66% of the usual chow intake; for 4days food ad libitum) and acute stress on the test day (day 25). R. rosea dry extract (3% rosavin, 3.12% salidroside) or its active principles were given by gavage 1h before access to HPF. Only rats exposed to both food restrictions and stress exhibited BE in the first 15-60min after the stressful procedure. R. rosea extract 10mg/kg significantly reduced and 20mg/kg abolished the BE episode. R. rosea extract 20mg/kg abolished also stress-induced increase in serum corticosterone levels. The R. rosea active principle salidroside, but not rosavin, at doses present in the extract, dose-dependently reduced or abolished BE for the period in which it was elicited. In conclusion results indicate that R. rosea extracts may have therapeutic properties in bingeing-related eating disorders and that salidroside is the active principle responsible for this effect.

Central nociceptin/orphanin FQ system elevates food consumption by both increasing energy intake and reducing aversive responsiveness

Nociceptin/orphanin FQ (N/OFQ), the nociceptin opioid peptide (NOP) receptor ligand, increases feeding when injected centrally. Initial data suggest that N/OFQ blocks the development of a conditioned taste aversion (CTA). The current project further characterized the involvement of N/OFQ in the regulation of hunger vs. aversive responses in rats by employing behavioral, immunohistochemical, and real-time PCR methodology. We determined that the same low dose of the NOP antagonist [Nphe(1)]N/OFQ(1-13)NH(2) delivered via the lateral ventricle diminishes both N/OFQ- and deprivation-induced feeding. This anorexigenic effect did not stem from aversive consequences, as the antagonist did not cause the development of a CTA. When [Nphe(1)]N/OFQ(1-13)NH(2) was administered with LiCl, it moderately delayed extinction of the LiCl-induced CTA. Injection of LiCl + antagonist compared with LiCl alone generated an increase in c-Fos immunoreactivity in the central nucleus of the amygdala. The antagonist alone elevated Fos immunoreactivity in the paraventricular nucleus of the hypothalamus, nucleus of the solitary tract, and central nucleus of the amygdala. Hypothalamic NOP mRNA levels were decreased during energy intake restriction induced by aversion, as well as in non-CTA rats food-restricted to match CTA-reduced consumption. Brain stem NOP was upregulated only in aversion. Prepro-N/OFQ mRNA showed a trend toward upregulation in restricted rats (P = 0.068). We conclude that the N/OFQ system promotes feeding by affecting the need to replenish lacking calories and by reducing aversive responsiveness. It may belong to mechanisms that shift a balance between the drive to ingest energy and avoidance of potentially tainted food.

Effects of the mGlu2/3 agonist LY379268 and the mGlu5 antagonist MTEP on ethanol seeking and reinforcement are differentially altered in rats with a history of ethanol dependence

BACKGROUND

Growing evidence supports a role of metabotropic glutamate receptors (mGluRs) in ethanol reinforcement, ethanol seeking, and ethanol withdrawal. To extend the understanding of the role of mGluRs in the addiction-relevant effects of ethanol as well as of the treatment target potential of these receptors for alcohol abuse, the effects of a selective mGlu2/3 agonist (LY379268) and a selective mGlu5 antagonist (MTEP) were tested on two processes central to alcohol addiction: ethanol reinforcement and stress-induced reinstatement of ethanol seeking in rats with a history of ethanol dependence.

METHODS

Following operant ethanol self-administration training, male Wistar rats were made dependent by intragastric ethanol intubations. Ethanol dependence was confirmed by the presence of somatic withdrawal signs. Following 2 weeks of withdrawal, stable ethanol self-administration was reestablished, and the effects of LY379268 (0-3 mg/kg subcutaneous) and MTEP (0-3 mg/kg, intraperitoneal) on ethanol self-administration were determined in both nondependent and postdependent rats. A second set of rats underwent extinction training and then was tested for the effects of LY379268 or MTEP on reinstatement of ethanol seeking induced by footshock stress.

RESULTS

LY379268 and MTEP dose-dependently reduced both ethanol self-administration and reinstatement of ethanol seeking induced by footshock stress. Additionally, LY379268 was more effective than MTEP in inhibiting both behaviors in postdependent than in nondependent animals.

CONCLUSIONS

These findings suggest that neuroadaptation associated with chronic ethanol exposure or withdrawal alters the sensitivity of mGlu2/3 receptors, with implications for the understanding of the neural basis of alcohol dependence and the treatment target potential of these receptors.

Role of innate and drug-induced dysregulation of brain stress and arousal systems in addiction: Focus on corticotropin-releasing factor, nociceptin/orphanin FQ, and orexin/hypocretin

Stress-like symptoms are an integral part of acute and protracted drug withdrawal, and several lines of evidence have shown that dysregulation of brain stress systems, including the extrahypothalamic corticotropin-releasing factor (CRF) system, following long-term drug use is of major importance in maintaining drug and alcohol addiction. Recently, two other neuropeptide systems have attracted interest, the nociceptin/orphanin FQ (N/OFQ) and orexin/hypocretin (Orx/Hcrt) systems. N/OFQ participates in a wide range of physiological responses, and the hypothalamic Orx/Hcrt system helps regulate several physiological processes, including feeding, energy metabolism, and arousal. Moreover, these two systems have been suggested to participate in psychiatric disorders, including anxiety and drug addiction. Dysregulation of these systems by chronic drug exposure has been hypothesized to play a role in the maintenance of addiction and dependence. Recent evidence demonstrated that interactions between CRF-N/OFQ and CRF-Orx/Hcrt systems may be functionally relevant for the control of stress-related addictive behavior. The present review discusses recent findings that support the hypotheses of the participation and dysregulation of these systems in drug addiction and evaluates the current understanding of interactions among these stress-regulatory peptides.

Swim stress enhances nociceptin/orphanin FQ-induced inhibition of rat dorsal raphe nucleus activity in vivo and in vitro: role of corticotropin releasing factor

The effects of nociceptin/orphanin FQ on putative serotonin (5HT) neurons of the dorsal raphe nucleus (DRN), known to modulate the behavioral responses to stress, were investigated in vivo and in vitro. In DRN slices from unstressed rats, nociceptin/orphanin FQ concentration-dependently inhibited the firing rate of putative 5HT neurons (EC(50) = 21.6 +/- 1.21 nM) and the selective NOP receptor antagonist UFP-101 shifted the concentration-response curve to the right (estimated pA(2) 6.86). Nociceptin/orphanin FQ potency was enhanced in slices prepared from rats previously subjected to a 15 min swim stress (EC(50) = 1.98 +/- 0.11 nM). Swim stress did not change the number or affinity of NOP receptors in DRN. Stress-elicited potentiation involved corticotropin-releasing factor (CRF)(1) receptors, GABA signaling and protein synthesis, being attenuated by pre-treatment with antalarmin (20 mg/kg, i.p.), diazepam (2.4 mg/kg, i.p.) and cycloheximide (2.5 mg/kg, i.p.), respectively. In anesthetized unstressed rats, locally applied nociceptin/orphanin FQ (0.03 and 0.1 ng/30 nl) inhibited the firing rate of DRN neurons (to 80 +/- 7 and 54 +/- 10% of baseline, respectively). Nociceptin/orphanin FQ inhibition was potentiated both 24 h after swim stress and 1 h after CRF (30 ng/30 nl intra-DRN). Stress-induced potentiation was prevented by the selective CRF(1) receptor antagonist, NBI 30755 (20 mg/kg, i.p.). In contrast, the inhibitory response of DRN neurons to the 5HT(1A) agonist, 8OH-DPAT (1 microg/1 microl, intra-DRN) was not potentiated by swim stress, ruling out a non-specific enhanced permeability of GIRK channel. Together, these findings suggest that CRF and the nociceptin/orphanin FQ/NOP system interact in the DRN during stress to control 5HT transmission; this may play a role in stress-related neuropsychopathologies.

Endogenous nociceptin modulates diet preference independent of motivation and reward

Previous studies show that the opioid peptide nociceptin stimulates food intake. Here, we studied nociceptin receptor knockout (NOP KO) mice in various behavioral paradigms designed to differentiate psychological and physiological loci at which endogenous nociceptin might control feeding. When presented a choice under food restriction, NOP KO mice displayed reduced preference for high sucrose diet, but lower intake of high fat diet under no-choice conditions. These responses were absent under ad libitum feeding conditions. Conditioned place preference to high fat diet under food-deprived conditions was unaltered in NOP KO mice, suggesting no difference in reward responses. Furthermore, operant food self-administration under a variety of conditions showed no genotype-dependent differences, suggesting no differences in the motivational properties of food. Taste reactivity to sucrose was unchanged in NOP KO mice, though NOP KO mice had altered aversive reactions to quinine solutions under ad libitum feeding, suggesting minor differences in the affective impact of palatable and unpalatable tastants. Although NOP KO mice re-fed following food-deprivation showed normal increases in plasma glucose and insulin, multidimensional scaling analysis showed that the relationship between these measures, body weight and plasma leptin was substantially disrupted in NOP KO, particularly in fasted mice. Additionally, the typical positive relationship between body weight and plasma leptin was considerably weaker in NOP KO mice. Together, these findings suggest that endogenous nociceptin differentially modulates diet preference depending on macronutrient content and homeostatic state, independently of the motivating, rewarding or orosensory properties of food, but may involve metabolic or postingestive processes.

Dysregulation of nociceptin/orphanin FQ activity in the amygdala is linked to excessive alcohol drinking in the rat

BACKGROUND

Alcoholism is a complex behavioral disorder in which interactions between stressful life events and heritable susceptibility factors contribute to the initiation and progression of disease. Neural substrates of these interactions remain largely unknown. Here, we examined the role of the nociceptin/orphanin FQ (N/OFQ) system, with an animal model in which genetic selection for high alcohol preference has led to co-segregation of elevated behavioral sensitivity to stress (Marchigian Sardinian alcohol-preferring [msP]).

METHODS

The msP and Wistar rats trained to self-administer alcohol received central injections of N/OFQ. In situ hybridization and receptor binding assays were also performed to evaluate N/OFQ receptor (NOP) function in naïve msP and Wistar rats.

RESULTS

Intracerebroventricular (ICV) injection of N/OFQ significantly inhibited alcohol self-administration in msP but not in nonselected Wistar rats. The NOP receptor messenger RNA expression and binding was upregulated across most brain regions in msP compared with Wistar rats. However, in msP rats [(35)S]GTPgammaS binding revealed a selective impairment of NOP receptor signaling in the central amygdala (CeA). Ethanol self-administration in msP rats was suppressed after N/OFQ microinjection into the CeA but not into the bed nucleus of the stria terminalis or the basolateral amygdala.

CONCLUSIONS

These findings indicate that dysregulation of N/OFQ-NOP receptor signaling in the CeA contributes to excessive alcohol intake in msP rats and that this phenotype can be rescued by local administration of pharmacological doses of exogenous N/OFQ. Data are interpreted on the basis of the anti-corticotropin releasing factor (CRF) actions of N/OFQ and the significance of the CRF system in promoting excessive alcohol drinking in msP rats.

Functional antagonism between nociceptin/orphanin FQ (N/OFQ) and corticotropin-releasing factor (CRF) in the rat brain: evidence for involvement of the bed nucleus of the stria terminalis

RATIONALE

Nociceptin/orphanin FQ (N/OFQ) has been proposed to be a functional antagonist of corticotropin-releasing factor (CRF) in relation to its anti-stress action and its ability to antagonize the anorectic effect of CRF in rats without exhibiting affinity for CRF receptors. The bed nucleus of the stria terminalis (BST) is highly sensitive to the inhibitory effect of N/OFQ on CRF-induced anorexia.

OBJECTIVE

The present study was aimed at further evaluating the role of the BST in the functional antagonism between N/OFQ and CRF by examining it at molecular level and in the context of CRF-induced anxiety in the rat.

MATERIALS AND METHODS

First, in situ hybridization experiments investigated the expression of the pro-N/OFQ precursor and of NOP receptors in several brain areas 6 h after injection of CRF (0.2 and 1 microg/rat) into the lateral cerebroventricle (LV). Second, the elevated plus maze test was used to evaluate whether N/OFQ, injected into the BST (0.05 and 0.5 microg/rat) or into the LV (0.5, 1.8, and 2.4 microg/rat), inhibits the anxiogenic-like effect evoked by LV injection of CRF (1 microg/rat) in rats.

RESULTS

The in situ hybridization study showed that LV injection of CRF 1 microg/rat increases NOP receptor expression in the BST, while no change of the N/OFQ precursor was observed. On the other hand, N/OFQ injection into the BST blocks the anxiogenic effect of CRF at doses lower than those required by LV injection (0.5 vs 1.8 microg/rat, respectively).

CONCLUSION

These data provide further support for the hypothesis that N/OFQ may behave as functional antagonist of CRF and suggest that this antagonism may occur within the BST.

Rhodiola rosea L. extract reduces stress- and CRF-induced anorexia in rats

Rhodiola rosea L. is one of the most popular adaptogen and anti-stress plants in European and Asiatic traditional medicine. Its pharmacological properties appear to depend on its ability to modulate the activation of several components of the complex stress-response system. Exposure to both physical and psychological stress reduces feeding in rodents. The aim of this work was thus to determine whether in rats an hydroalcoholic R. rosea extract standardized in 3% rosavin and 1% salidroside (RHO) reverses hypophagia induced by (1) physical stress due to 60 min immobilization; (2) intracerebroventricular injection of corticotrophin-releasing factor (CRF, 0.2 microg/rat), the major mediator of stress responses in mammals; (3) intraperitoneal injection of Escherichia coli Lipopolysaccharide (LPS, 100 microg/kg); (4) intraperitoneal administration of fluoxetine (FLU, 8 mg/kg). The effect of the same doses of the plant extract was also tested in freely-feeding and in 20 h food-deprived rats. RHO was administered acutely by gavage to male Wistar rats 1 h before the experiments. The results show that at 15 and 20 mg/kg, RHO reversed the anorectic effects induced both by immobilization and by intracerebroventricular CRF injection. Moreover, at the same doses, RHO failed to reduce the anorectic effect induced both by LPS and FLU, and did not modify food intake in both freely-feeding and food-deprived rats. These findings strongly demonstrated that RHO is able selectively to attenuate stress-induced anorexia, providing functional evidence of claimed adaptogen and anti-stress properties of Rhodiola rosea L.

The nociceptin/orphanin FQ antagonist UFP-101 differentially modulates the glucocorticoid response to restraint stress in rats during the peak and nadir phases of the hypothalamo–pituitary–adrenal axis circadian rhythm

The involvement of nociceptin (N/OFQ) and the nociceptin/orphanin FQ peptide (NOP) receptor in behavior associated with stress and anxiety has been established but their role in the regulation of the hypothalamo-pituitary-adrenal (HPA) axis under conditions of stress has not been fully investigated. We used the selective NOP receptor antagonist UFP-101 to examine the contribution of endogenous N/OFQ to HPA axis control under conditions of restraint stress in the morning and the evening. We found that in the morning during the HPA axis circadian nadir rats exposed to restraint stress in both the presence and absence of UFP-101 exhibited significantly elevated plasma corticosterone at 30 min post-i.c.v. injection compared to the home cage control group. Additionally, rats treated with UFP-101 and exposed to restraint had significantly elevated corticosterone levels at 60 min post-i.c.v. injection compared to all other treatment groups. Interestingly, while there was a significant increase in the expression of CRF mRNA in the paraventricular nucleus (PVN) of rats exposed to restraint stress only, there was no comparable increase in those co-treated with UFP-101. There was no change in the expression of AVP or POMC mRNA in any of the treatment groups. In contrast, when carried out in the evening we observed significantly elevated plasma corticosterone in the vehicle-treated restraint group only at 30 min post-i.c.v. injection. There was no significant difference between the UFP-101-treated restraint group and either of the home cage control groups or the vehicle-treated restraint group. Additionally, in contrast to the morning study, UFP-101 did not prolong glucocorticoid release at the 60 min time-point. These results demonstrate for the first time a differential effect of UFP-101 on restraint stress-induced HPA axis activity characterized by significant prolongation of stress-induced activity in the morning but no significant effect on the response to restraint in the evening.

Activation of the nociceptin/orphanin FQ system is unable to reverse CRF2 receptor mediated anorexia in the rat

Central injection of Nociceptin/Orphanin FQ (N/OFQ), inhibits the anorectic effect of corticotropin-relasing factor (CRF) and stress in rats. Recently, Urocortin II (Ucn II) and Urocortin III (Ucn III), two selective CRF(2) receptor agonists, have been identified. Here, we investigated the effect of intracerebroventricular (ICV) injection of 0.25, 0.75, 1.50 or 3 nmol/rat of Ucn II or Ucn III on food and water intake in food deprived rats. The effect of N/OFQ on Ucn II and UCNIII-induced anorexia was also studied. Results showed a greater inhibition of food consumption by Ucn II than Ucn III. Pretreatment with N/OFQ (0.25-2.0 nmol/rat) did not block the effects of Ucn II and UCNIII. Conversely, injection of N/OFQ (0.25-2.0 nmol/rat) blocked the anorectic effect of CRF (0.1 nmol/rat). These findings suggest that N/OFQ selectively prevent the anorectic effect mediated by activation of the CRF(1) receptor system.

Nociceptin/orphanin FQ presynaptically decreases GABAergic transmission and blocks the ethanol-induced increase of GABA release in central amygdala

Behavioral studies show that the GABAergic system in the central amygdala (CeA) nucleus has a complex role in the reinforcing effects effects of ethanol and the anxiogenic response to ethanol withdrawal. Opioid peptides and nociceptin/orphanin FQ (nociceptin) within the CeA are implicated also in regulating voluntary ethanol consumption and ethanol relapse. Recently, we reported that basal GABAergic transmission was increased in ethanol-dependent rats, and that acute ethanol increases GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) in CeA neurons from both naïve and ethanol-dependent rats to the same extent, suggesting lack of tolerance for the acute effect of ethanol. Here, we investigated the effect of nociceptin on IPSCs in CeA neurons and its interaction with ethanol effects on these GABA synapses. We found that nociceptin moderately decreased IPSC amplitudes, acting mostly presynaptically as it increased paired-pulse facilitation ratio of IPSCs and decreased miniature IPSC frequencies (but not amplitudes). Nociceptin also prevented the ethanol-induced augmentation of IPSCs in CeA of naïve rats. Interestingly, in CeA of ethanol-dependent rats, the nociceptin-induced inhibition of IPSCs was increased, indicating an enhanced sensitivity to nociceptin. Nociceptin also blocked the ethanol-induced augmentation of IPSCs in ethanol-dependent rats. Our data suggest that nociceptin has a role in regulating the GABAergic system and opposing the effect elicited by ethanol. Thus, nociceptin may represent a therapeutic target for alleviating alcohol dependence.

Effect of novel NOP receptor ligands on food intake in rats

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand for the NOP opioid receptor, stimulates feeding in rats. The present study evaluated the effect of three newly synthesized NOP receptor agonists and two NOP receptor antagonist on food intake. Freely feeding rats were tested with intracerebroventricular (ICV) injections of the NOP receptor agonists OS-500, OS-462 and OS-461. OS-500 and OS-462 evoked a hyperphagic effect more potent and far more pronounced than that of N/OFQ, while OS-461 was ineffective. OS-500 and OS-462 were also tested by intraperitoneal injection, but were unable to evoke hyperphagia following this route of administration. The NOP receptor antagonist NC-797 and UFP-101 did not modify feeding in freely feeding rats while fully antagonized the hyperphagic effect of N/OFQ. Pre-treatment with UFP-101 but not with NC-797 antagonized the hyperphagic effect of OS-462 and OS-500. The present findings indicate that OS-500, OS-462 may act as potent and long-lasting NOP receptor agonists, whereas UFP-101 and NC-797 show antagonistic properties. The higher efficacy of UFP-101 in blocking the hyperphagic effect of OS-462 and OS-500 may be linked to the better pharmacokinetic profile of this antagonist compared to NC-797. Overall, the results indicate that these compounds may represent valuable pharmacological tools to investigate the role of the brain N/OFQ system.

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.

Stress enhancement of craving during sobriety: a risk for relapse

This report of the proceedings of a symposium presented at the 2004 Research Society on Alcoholism Meeting provides evidence linking stress during sobriety to craving that increases the risk for relapse. The initial presentation by Rajita Sinha summarized clinical evidence for the hypothesis that there is an increased sensitivity to stress-induced craving in alcoholics. During early abstinence, alcoholics who were confronted with stressful circumstances showed increased susceptibility for relapse. George Breese presented data demonstrating that stress could substitute for repeated withdrawals from chronic ethanol to induce anxiety-like behavior. This persistent adaptive change induced by multiple withdrawals allowed stress to induce an anxiety-like response that was absent in animals that were not previously exposed to chronic ethanol. Subsequently, Amanda Roberts reviewed evidence that increased drinking induced by stress was dependent on corticotropin-releasing factor (CRF). In addition, rats that were stressed during protracted abstinence exhibited anxiety-like behavior that was also dependent on CRF. Christopher Dayas indicated that stress increases the reinstatement of an alcohol-related cue. Moreover, this effect was enhanced by previous alcohol dependence. These interactive effects between stress and alcohol-related environmental stimuli depended on concurrent activation of endogenous opioid and CRF systems. A.D. Lê covered information that indicated that stress facilitated reinstatement to alcohol responding and summarized the influence of multiple deprivations on this interaction. David Overstreet provided evidence that restraint stress during repeated alcohol deprivations increases voluntary drinking in alcohol-preferring (P) rats that results in withdrawal-induced anxiety that is not observed in the absence of stress. Testing of drugs on the stress-induced voluntary drinking implicated serotonin and CRF involvement in the sensitized response. Collectively, the presentations provided convincing support for an involvement of stress in the cause of relapse and continuing alcohol abuse and suggested novel pharmacological approaches for treating relapse induced by stress.

Nociceptin/orphanin FQ acts as a functional antagonist of corticotropin-releasing factor to inhibit its anorectic effect

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the NOP opioid receptor (previously referred to as ORL1 or OP4 receptor), exerts a variety of behavioral effects. N/OFQ as well as the synthetic NOP receptor agonist Ro 64-6198 have been reported to possess antistress properties and to elicit a pronounced hyperphagic effect in freely feeding rats. These findings have raised our interest to investigate possible interactions in the control of ingestive behavior between N/OFQ and corticotropin-releasing factor (CRF), which is well known to be a major mediator of stress and to possess anorectic properties. These studies have shown that intracerebroventricular injections of N/OFQ or of Ro 64-6198 reverse the anorectic action evoked by intracerebroventricular administration of CRF. The anti-anorectic effect of N/OFQ or Ro 64-6198 is antagonized by the selective NOP receptor antagonist [Nphe1]N/OFQ1-13NH2, providing evidence that it is mediated by this receptor. The effect occurs at doses that are not hyperphagic per se and is clearly selective versus the anorectic action of CRF since N/OFQ or Ro 64-6198 do not influence the anorectic effect of Escherichia coli lipopolysaccharide (LPS). Neither N/OFQ nor Ro 64-6198 shows affinity for CRF receptors, suggesting that NOP receptor agonists might act as functional antagonists of CRF with regard to its anorectic action. Microinjection studies have revealed that the bed nucleus of the stria terminalis (BNST) is highly sensitive to the anorectic action of CRF, as well as to the anti-anorectic action of N/OFQ; pretreatment with 0.025-0.25 microg/site of N/OFQ into the BNST blocked the anorectic action of 0.1 microg/site of CRF given in the same area. On the other hand, intra-BNST microinjection of 0.025-0.25 microg/site of N/OFQ did not modify basal food intake. Thus, the BNST may be the site where the functional antagonism between N/OFQ and CRF takes place. These findings raise interest for the N/OFQ-NOP receptor system as a pharmacological target to block the anorectic effect of CRF. In comparison to CRF receptor antagonists, NOP receptor agonists may have the advantage of not inhibiting the hypothalamic-pituitary-adrenal (HPA) axis.

Minireview: Characterization of influence of central nociceptin/orphanin FQ on consummatory behavior

Nociceptin/orphanin FQ (N/OFQ), a peptide closely related to dynorphin A, is the endogenous agonist of the NOP receptor that moderately increases food intake under various conditions. Its orexigenic properties are mediated by the brain circuitry. In the present review, we focus on discussing the nature of hyperphagic effects of N/OFQ with special emphasis on its function within feeding-related neural networks. Although some of N/OFQ's orexigenic effects resemble those induced by opioids, reward-dependent feeding appears to be affected in a different manner by agonists of the NOP and classical opioid receptors. Also, data suggest that N/OFQ may not only promote feeding initiation, but rather its role may be to inhibit signaling responsible for inhibition of consummatory behavior. Central systems involved in termination of feeding that seem to be influenced by N/OFQ encompass oxytocin, alpha-MSH, and CRH.

Endogenous opioids and feeding behavior: a 30-year historical perspective

This invited review, based on the receipt of the Third Gayle A. Olson and Richard D. Olson Prize for the publication of the outstanding behavioral article published in the journal Peptides in 2002, examines the 30-year historical perspective of the role of the endogenous opioid system in feeding behavior. The review focuses on the advances that this field has made over the past 30 years as a result of the timely discoveries that were made concerning this important neuropeptide system, and how these discoveries were quickly applied to the analysis of feeding behavior and attendant homeostatic processes. The discoveries of the opioid receptors and opioid peptides, and the establishment of their relevance to feeding behavior were pivotal in studies performed in the 1970s. The 1980s were characterized by the establishment of opioid receptor subtype agonists and antagonists and their relevance to the modulation of feeding behavior as well as by the use of general opioid antagonists in demonstrating the wide array of ingestive situations and paradigms involving the endogenous opioid system. The more recent work from the 1990s to the present, utilizes the advantages created by the cloning of the opioid receptor genes, the development of knockout and knockdown techniques, the systematic utilization of a systems neuroscience approach, and establishment of the reciprocity of how manipulations of opioid peptides and receptors affect feeding behavior with how feeding states affect levels of opioid peptides and receptors. The role of G-protein effector systems in opioid-mediated feeding responses, which was the subject of the prize-winning article, is then reviewed.

Attenuation of ethanol self-administration and of conditioned reinstatement of alcohol-seeking behaviour by the antiopioid peptide nociceptin/orphanin FQ in alcohol-preferring rats

RATIONALE

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the opioid-like orphan receptor NOP, was shown to reduce home-cage ethanol consumption, ethanol-induced conditioned place preference and stress-induced reinstatement of alcohol-seeking behaviour.

OBJECTIVES

The present study, using genetically selected Marchigian Sardinian alcohol-preferring (msP) rats, was designed to evaluate the effect of this opioid peptide on 10% ethanol and 10% sucrose self-administration, under a fixed-ratio 1 (FR 1) or a progressive-ratio (PR) schedule of reinforcement. Furthermore, using an experimental model of relapse in which rats were trained to lever press for ethanol in the presence of the discriminative stimulus of an orange odour (S(+)) and a 1-s cue light (CS(+)) or for water in the presence of anise odour (S(-)) and 1-s white noise (CS(-)), the effect of N/oFQ on cue-induced reinstatement of extinguished ethanol responding was investigated.

RESULTS

Sub-chronic (6 days) intracerebroventricular (i.c.v.) injection of 0.5 microg or 1.0 microg N/OFQ per rat significantly reduced alcohol self-administration under both the FR 1 and PR schedules of reinforcement. Conversely, i.c.v. administration of 0.5, 1.0 or 4.0 microg of the peptide per rat did not affect sucrose self-administration. In addition, i.c.v. N/OFQ (1.0-2.0 microg per rat) significantly inhibited the reinstatement of extinguished ethanol responding under an S(+)/CS(+) condition, whereas lever pressing under S(-)/CS(-) was not altered.

CONCLUSIONS

The present study demonstrates that the reinforcing effects of ethanol are markedly blunted by activation of the opioidergic N/OFQ receptor system. Moreover, the data provide evidence of the efficacy of N/OFQ to prevent reinstatement of ethanol-seeking behaviour elicited by environmental conditioned stimuli.

The bed nucleus is a neuroanatomical substrate for the anorectic effect of corticotropin-releasing factor and for its reversal by nociceptin/orphanin FQ

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the opioid N/OFQ receptor (NOP), possesses marked functional anti-stress and anti-corticotropin-releasing factor (CRF) actions. We have shown that intracerebroventricular injection of N/OFQ reverses the hypophagic effect induced by stress or by CRF given intracerebroventricularly. To shed new light on the mechanisms involved in the anti-CRF action of N/OFQ, we investigated the ability of N/OFQ to prevent CRF-induced anorexia after microinjection studies into brain areas of potential interest in the control of feeding behavior and coexpressing NOP and CRF receptors. These areas include the bed nucleus of the stria terminalis (BNST), the central amygdala (CeA), the locus ceruleus (LC), the ventromedial hypothalamus (VMH), the paraventricular nucleus (PVN), and the dorsal raphe (DR). The results demonstrated that the anorectic effect of 0.04 nmol of CRF per rat (200 ng per rat) given intracerebroventricularly is reversed by pretreatment with 0.01-0.21 nmol of N/OFQ per rat (25-500 ng per rat) injected into the BNST but not into the CeA, LC, VMH, PVN, or DR. Microinjection of 0.01-0.02 nmol of CRF per site (50-100 ng per site) into the BNST but not into the CeA or the LC induced marked anorexia in food-deprived rats. Pretreatment with 0.01-0.21 nmol of N/OFQ per site (25-500 ng per site) into the BNST also blocked the anorectic action of 0.02 nmol of CRF per site (100 ng per site) given in the same area. Finally, intra-BNST microinjection of 0.01-0.21 nmol of N/OFQ per site (25-500 ng per site) did not modify food intake in either food-sated or food-deprived rats. These data demonstrate that the BNST is involved in the modulation of CRF-induced anorexia, which is prevented by activation of N/OFQ receptors.