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

An examination of the effects of nucleus accumbens core nociceptin on appetitive and consummatory motivation for food

The nucleus accumbens (NAc) is a critical region for regulating the appetitive and consummatory aspects of motivated behavior. Previous work has shown differential effects of NAc µ-, δ-, and κ- receptor stimulation on food intake and for shifting motivation within an effort-based choice (EBC) task. However, the motivational role of the nociceptin opioid peptide (NOP) receptor, a fourth member of the opioid receptor family, is less well understood. These experiments therefore characterized the effect of NAc injections of nociceptin, the endogenous ligand for the NOP receptor, on consummatory and appetitive motivation. Three groups of male Sprague-Dawley rats received nociceptin injections into the NAc core prior to testing in a progressive ratio lever pressing task, an EBC task, or a palatable feeding assay. In the feeding experiment, 10 nmol of nociceptin increased consumption in the first 30 min, but this increase was not sustained through the end of the 2-hr session. Additionally, nociceptin injections did not alter breakpoint in the progressive ratio task. However, in the EBC task, nociceptin significantly decreased breakpoint for sugar pellets without affecting consumption of rat chow. These data suggest that NAc NOP receptor stimulation transiently increases consummatory motivation toward palatable diets and inhibits appetitive motivation when alternate food options are freely available. This pattern of effects contrasts with those obtained following NAc stimulation of other opioid receptors, suggesting that the four opioid receptor classes each serve unique roles in modulating food-directed motivation within the NAc core.

The vital role of arcuate nociceptin/orphanin FQ neurones in mounting an oestradiol-dependent adaptive response to negative energy balance via inhibition of nearby proopiomelanocortin neurones

We tested the hypothesis that N/OFQ neurones in the arcuate nucleus (N/OFQ^ARC ) inhibit proopiomelanocortin (POMC^ARC ) neurones in a diet- and hormone-dependent manner to promote a more extensive rebound hyperphagia upon re-feeding following an 18 h fast. We utilized intact male or ovariectomized (OVX) female mice subjected to ad libitum-feeding or fasting conditions. N/OFQ^ARC neurones under negative energy balance conditions displayed heightened sensitivity as evidenced by a decreased rheobase threshold, increased firing frequency, and increased burst duration and frequency compared to ad libitum-feeding conditions. Stimulation of N/OFQ^ARC neurones more robustly inhibited POMC^ARC neurones under fasting conditions compared to ad libitum-feeding conditions. N/OFQ^ARC inhibition of POMC^ARC neurones is hormone dependent as chemostimulation of N/OFQ^ARC neurones from fasted males and OVX females produced a sizable outward current in POMC^ARC neurones. Oestradiol (E₂ ) markedly attenuated the N/OFQ-induced POMC^ARC outward current. Additionally, N/OFQ tonically inhibits POMC^ARC neurones to a greater degree under fasting conditions than in ad libitum-feeding conditions as evidenced by the abrogation of N/OFQ-nociceptin opioid peptide (NOP) receptor signalling and inhibition of N/OFQ release via chemoinhibition of N/OFQ^ARC neurones. Intra-arcuate nucleus application of N/OFQ further elevated the hyperphagic response and increased meal size during the 6 h re-feed period, and these effects were mimicked by chemostimulation of N/OFQ^ARC neurones in vivo. E₂ attenuated the robust N/OFQ-induced rebound hyperphagia seen in vehicle-treated OVX females. These data demonstrate that N/OFQ^ARC neurones play a vital role in mitigating the impact of negative energy balance by inhibiting the excitability of anorexigenic neural substrates, an effect that is diminished by E₂ in females. KEY POINTS: Nociceptin/orphanin FQ (N/OFQ) promotes increased energy intake and decreased energy expenditure under conditions of positive energy balance in a sex- and hormone-dependent manner. Here it is shown that under conditions of negative energy balance, i.e. fasting, N/OFQ inhibits anorexigenic proopiomelanocortin (POMC) neurones to a greater degree compared to homeostatic conditions due to fasting-induced hyperexcitability of N/OFQ neurones. Additionally, N/OFQ promotes a sustained increase in rebound hyperphagia and increase in meal size during the re-feed period following a fast. These results promote greater understanding of how energy balance influences the anorexigenic circuitry of the hypothalamus, and aid in understanding the neurophysiological pathways implicated in eating disorders promoting cachexia.

The Opioid System in Rainbow Trout Telencephalon Is Probably Involved in the Hedonic Regulation of Food Intake

We hypothesize that opioids are involved in the regulation of food intake in fish through homeostatic and hedonic mechanisms. Therefore, we evaluated in rainbow trout (Oncorhynchus mykiss) hypothalamus and telencephalon changes in precursors, endogenous ligands and receptors of the opioid system under different situations aimed to induce changes in the homeostatic (through fasted/fed/refed fish) and hedonic (through feeding fish a control or a palatable high-fat diet) regulation of food intake. No major changes occurred in parameters assessed related with the nutritional condition of fish (fasted/fed/refed), allowing us to suggest that the opioid system seems not to have an important role in the homeostatic regulation of food intake in rainbow trout. The responses observed in telencephalon of rainbow trout fed the palatable high-fat diet included a decrease in mRNA abundance of the opioid precursor penka, in a way similar to that known in mammals, and increased mRNA abundance of the opioid receptors oprd1 and oprk1 supporting a role for telencephalic opioid system in the hedonic regulation of food intake in fish.

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.

Adaptive Changes in the Central Control of Energy Homeostasis Occur in Response to Variations in Energy Status

Energy homeostasis is regulated in coordinate fashion by the brain-gut axis, the homeostatic energy balance circuitry in the hypothalamus and the hedonic energy balance circuitry comprising the mesolimbcortical A10 dopamine pathway. Collectively, these systems convey and integrate information regarding nutrient status and the rewarding properties of ingested food, and formulate it into a behavioral response that attempts to balance fluctuations in consumption and food-seeking behavior. In this review we start with a functional overview of the homeostatic and hedonic energy balance circuitries; identifying the salient neural, hormonal and humoral components involved. We then delve into how the function of these circuits differs in males and females. Finally, we turn our attention to the ever-emerging roles of nociceptin/orphanin FQ (N/OFQ) and pituitary adenylate cyclase-activating polypeptide (PACAP)-two neuropeptides that have garnered increased recognition for their regulatory impact in energy homeostasis-to further probe how the imposed regulation of energy balance circuitry by these peptides is affected by sex and altered under positive (e.g., obesity) and negative (e.g., fasting) energy balance states. It is hoped that this work will impart a newfound appreciation for the intricate regulatory processes that govern energy homeostasis, as well as how recent insights into the N/OFQ and PACAP systems can be leveraged in the treatment of conditions ranging from obesity to anorexia.

Interaction of the dopaminergic and Nociceptin/Orphanin FQ on central feed intake regulation in chicken

1. The aim of the current study was to determine the effects of the central dopaminergic system on N/OFQ-induced feed intake in 3-h feed-deprived neonatal broilers. 2. In experiment 1, chicken received intracerebroventricular (ICV) injections of a control solution, SCH 23 390 (D1 receptors antagonist, 5 nmol), N/OFQ (16 nmol) or their combination (SCH23 390 + N/OFQ). In experiment 2, a control solution, AMI-193 (D2 receptors antagonist, 5 nmol), N/OFQ (16 nmol) or their combination (AMI-193 + N/OFQ) were ICV injected into chickens. In experiment 3, birds received ICV injections of a control solution, NGB2904 (D3 receptors antagonist, 6.4 nmol), N/OFQ (16 nmol) and co-injection of NGB2904 + N/OFQ. In experiment 4, ICV injections of the control solution, L-741,742 (D4 receptors antagonist, 6 nmol), N/OFQ (16 nmol) or their combination (L-741,742 + N/OFQ) were applied to broilers. In experiment 5, birds were ICV injected with control solution, L-DOPA (dopamine precursor, 125 nmol), N/OFQ (16 nmol) and L-DOPA + N/OFQ. Cumulative feed intake was recorded until 120 min after injection. 3. According to the results, ICV injection of N/OFQ significantly increased feed intake (P < 0.05). Co-injection of N/OFQ and D1 receptor antagonist (SCH 23390) amplified hyperphagic effect of N/OFQ (P < 0.05). The N/OFQ-induced feed intake was increased by the D2 receptor antagonist (P < 0.05). The hyperphagic effect of N/PFQ was weakened by co-injection of L-DOPA + N/OFQ (P < 0.05). 4. These results suggested that an interaction exists between dopamine and N/OFQ via D1 and D2 receptors on central feed intake in neonatal broiler chickens.

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.

Role of nociceptin/orphanin FQ in thermoregulation

Nociceptin/Orphanin FQ (N/OFQ) is a 17-amino acid peptide that binds to the nociceptin receptor (NOP). N/OFQ and NOP receptors are expressed in numerous brain areas. The generation of specific agonists, antagonists and receptor-deficient mice or rats has enabled progress in elucidating the biological functions of N/OFQ. These tools have been employed to identify the biological significance of the N/OFQ system and how it interacts with other endogenous systems to regulate several body functions. The present review focuses on the role of N/OFQ in the regulation of body temperature and its relationship with energy balance. Critical evaluation of the literature data suggests that N/OFQ, acting through the NOP receptor, may cause hypothermia by influencing the complex thermoregulatory system that operates as a federation of independent thermoeffector loops to control body temperature at the hypothalamic level. Furthermore, N/OFQ counteracts hyperthermia elicited by cannabinoids or µ-opioid agonists. N/OFQ-induced hypothermia is prevented by ω-conotoxin GVIA, an N-type calcium channel blocker. Hypothermia induced by N/OFQ is considered within the framework of the complex action that this neuropeptide exerts on energy balance. Energy stores are regulated through the complex neural controls exerted on both food intake and energy expenditure. In laboratory rodents, N/OFQ stimulates consummatory behavior and decreases energy expenditure. Taken together, these studies support the idea that N/OFQ contributes to the regulation of energy balance by acting as an "anabolic" neuropeptide as it elicits effects similar to those produced in the hypothalamus by other neuropeptides such as orexins and neuropeptide Y.

Modulation of opioid-induced feeding behavior by endogenous nitric oxide in neonatal layer-type chicks

The current study was designed to evaluate the effects of central administration of L-arginine (The precursor of nitric oxide), N(G)-nitro-L-arginine methyl ester (L-NAME), a nitric oxide (NO) synthase inhibitor, selective opioid receptor agonists and involvement of central nitrergic/opioidergic systems on feeding behavior in neonatal layer-type chicks. The results of this study showed that the intracerebroventricular (ICV) injection of L-arginine (400 and 800 nmol) significantly decreased food intake (P < 0.001) but the injection of 200 nmol L-arginine had no effect on cumulative food intake in FD3 chickens (P > 0.05). The ICV injection of L-NAME (200 and 400 nmol) increased food intake (P < 0.001) but 100 nmol of L-NAME had no significant effect (P > 0.05). On the other hand, the co-injection of 100 nmol L-NAME significantly attenuated the anorexigenic effect of 800 nmol L-arginine (P < 0.001). Moreover, the food intake of chicks was significantly decreased by ICV injection of DAMGO (μ-opioid receptor agonist, 125 pmol) (P < 0.001) while both DPDPE (δ-opioid receptor agonist, 40 pmol) and U-50488H (κ-opioid receptor agonist, 30 nmol) significantly stimulated food intake (P < 0.001). In addition, the hypophagic effect of DAMGO was significantly amplified by administration of L-arginine (P < 0.001) while the administration of L-NAME attenuated the hypophagic effect of DAMGO (P < 0.001). In contrast, co-injection of L-arginine or L-NAME with DPDPE had no effect on the hyperphagia induced by DPDPE as well as the hyperphagic effect of U-50488H on food intake was not affected by concurrent injection of L-arginine or L-NAME (P > 0.05). These results suggest that nitrergic and opioidergic systems have an important role on feeding behavior in the CNS of neonatal layer-type chicks and it seems that interaction between them is mediated by μ-opioid receptor.

The neuronal circuit between nociceptin/orphanin FQ and hypocretins/orexins coordinately modulates stress-induced analgesia and anxiety-related behavior

The neuropeptide nociceptin/orphanin FQ (N/OFQ), acting on its receptors (NOP), modulates a variety of biological functions and neurobehavior including nociception, stress responses, water and food-intake, locomotor activity, and spatial attention. N/OFQ is conventionally regarded as an "antiopiate" peptide in the brain because central administration of N/OFQ attenuates stress-induced analgesia (SIA) and produces anxiolytic effects. However, naloxone-irreversible SIA and anxiolytic action are unlikely to be mediated by the opiate system. Both N/OFQ and NOP receptors are expressed most abundantly in the hypothalamus, where two other neuropeptides, the hypocretins/orexins (Hcrts), are exclusively synthesized in the lateral hypothalamic area. N/OFQ and Hcrt regulate most cellular physiological responses in opposite directions (e.g., ion channel modulation and second messenger coupling), and produce differential modulations for almost all neurobehavior assessed, including sleep/wake, locomotion, and rewarding behaviors. This chapter focuses on recent studies that provide evidence at a neuroanatomical level showing that a local neuronal circuit linking N/OFQ to Hcrt neurons exists. Functionally, N/OFQ depresses Hcrt neuronal activity at the cellular level, and modulates stress responses, especially SIA and anxiety-related behavior in the whole organism. N/OFQ exerts its attenuation of SIA and anxiolytic action on fear-induced anxiety through direct modulation of Hcrt neuronal activity. The information obtained from these studies has provided insights into how interaction between the Hcrt and N/OFQ systems positively and negatively modulates the complex and integrated stress responses.

Gonadal steroids differentially modulate the actions of orphanin FQ/nociceptin at a physiologically relevant circuit controlling female sexual receptivity

Orphanin FQ/nociceptin (OFQ/N) inhibits the activity of pro-opiomelanocortin (POMC) neurones located in the hypothalamic arcuate nucleus (ARH) that regulate female sexual behaviour and energy balance. We tested the hypothesis that gonadal steroids differentially modulate the ability of OFQ/N to inhibit these cells via presynaptic inhibition of transmitter release and postsynaptic activation of G protein-gated, inwardly-rectifying K(+) (GIRK)-1 channels. Whole-cell patch clamp recordings were performed in hypothalamic slices prepared from ovariectomised rats. OFQ/N (1 μm) decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs), and also caused a robust outward current in the presence of tetrodotoxin, in ARH neurones from vehicle-treated animals. A priming dose of oestradiol benzoate (EB; 2 μg) increased basal mEPSC frequency, markedly diminished both the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents, and potentiated the OFQ/N-induced decrease in mIPSC frequency. Steroid treatment regimens that facilitate sexual receptivity reinstate the basal mEPSC frequency, the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents to levels observed in vehicle-treated controls, and largely abolish the ability of OFQ/N to decrease mIPSC frequency. These effects were observed in an appreciable population of identified POMC neurones, almost one-half of which projected to the medial preoptic nucleus. Taken together, these data reveal that gonadal steroids influence the pleiotropic actions of OFQ/N on ARH neurones, including POMC neurones, in a disparate manner. These temporal changes in OFQ/N responsiveness further implicate this neuropeptide system as a critical mediator of the gonadal steroid regulation of reproductive behaviour.

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.

Estradiol negatively modulates the pleiotropic actions of orphanin FQ/nociceptin at proopiomelanocortin synapses

Orphanin FQ/nociceptin (OFQ/N) inhibits the activity of proopiomelanocortin (POMC) neurons located in the hypothalamic arcuate nucleus (ARH) that regulate female sexual behavior and energy balance. We tested the hypothesis that estradiol modulates the ability of OFQ/N to pre- and postsynaptically decrease the excitability of these cells. To this end, whole-cell patch-clamp recordings were performed in hypothalamic slices prepared from ovariectomized rats, including some that were injected with the retrograde tracer Fluorogold in the medial preoptic nucleus (MPN) to label the POMC neurons regulating sexual receptivity. OFQ/N (1 µM) evoked a robust outward current in ARH neurons from vehicle-treated animals that was blocked by the opioid receptor-like (ORL)1 receptor antagonist UFP-101 (100 nM) and the G protein-gated, inwardly rectifying K⁺ (GIRK-1) channel blocker tertiapin (10 nM). OFQ/N also produced a decrease in the frequency of glutamatergic, miniature excitatory postsynaptic currents (mEPSCs), which was also antagonized by UFP-101. Estradiol benzoate (2 µg) increased basal mEPSC frequency and markedly diminished both the OFQ/N-induced activation of postsynaptic GIRK-1 channel currents and the presynaptic inhibition of glutamatergic neurotransmission. These effects were observed in identified POMC neurons, including eight that projected to the MPN. Taken together, these data reveal that estradiol attenuates the pleiotropic inhibitory actions of OFQ/N on POMC neurons: presynaptically through reducing the OFQ/N inhibition of glutamate release and postsynaptically by reducing ORL1 signaling through GIRK channels. As such, they impart critical insight into a mechanism for estradiol to increase the activity of POMC neurons that inhibit sexual receptivity.

The effect of serotonergic system on nociceptin/orphanin FQ induced food intake in chicken

The present study was designed to examine the effects of intracerebroventricular injection of para-chlorophenylalanine (PCPA) (cerebral serotonin depletive), fluoxetine (selective serotonin reuptake inhibitor), 8-OH-DPAT (5-HT1A autoreceptor agonist) and SB 242084 (5-HT2c receptor antagonist) on nociceptin/orphanin FQ (N/OFQ) induced feeding response in chickens. A guide cannula was surgically implanted into the lateral ventricle of chickens. Before the experiments, 3-h fasting periods had been given to all experimental birds. In experiment 1, chickens were injected with PCPA (1.5 μg) followed by an N/OFQ injection (16 nmol) intracerebroventricularly. In experiment 2, birds received fluoxetine (10 μg) prior to the injection of N/OFQ. In experiment 3, chickens were administered with N/OFQ after the 8-OH-DPAT administration (15.25 nmol). In experiment 4, birds were injected with SB 242084 (1.5 μg) followed by an N/OFQ injection. Cumulative food intake was measured at 3 h post injection. The results of this study show that N/OFQ increases food intake in broiler cockerels (P < 0.05) and that this effect is amplified by pretreatment with PCPA and SB 242084 in an additive manner (P < 0.05). The effect of N/OFQ is not changed by pretreatment with 8-OH-DPAT (P > 0.05). Furthermore, the stimulatory effect of N/OFQ on food intake was significantly attenuated by pretreatment with fluoxetine. These results suggest that N/OFQ induced hyperphagia is mediated by serotonergic mechanisms, and possibly imply an interaction between N/OFQ and the serotonergic system (via 5-HT2C receptors) on food intake in chickens.

Ghrelin, neuropeptide Y, and other feeding-regulatory peptides active in the hippocampus: role in learning and memory

The hippocampus is a brain region of primary importance for neurogenesis, which occurs during early developmental states as well as during adulthood. Increases in neuronal proliferation and in neuronal death with age have been associated with drastic changes in memory and learning. Numerous neurotransmitters are involved in these processes, and some neuropeptides that mediate neurogenesis also modulate feeding behavior. Concomitantly, feeding peptides, which act primarily in the hypothalamus, are also present in the hippocampus. This review aims to ascertain the role of several important feeding peptides in cognitive functions, either through their local synthesis in the hippocampus or through their actions via specific receptors in the hippocampus. A link between neurogenesis and the orexigenic or anorexigenic properties of feeding peptides is discussed.

Food-intake regulation during stress by the hypothalamo-pituitary-adrenal axis

The prevalence of obesity is increasing worldwide with serious consequences such as diabetes mellitus type 2 and cardiovascular diseases. Emotional stress is considered to be one of the main reasons of obesity development in humans. However, there are some contradictory results, which should be addressed. First of all stress induces anorexia, but not overeating in laboratory animals. Glucocorticoids, the effector molecules of the hypothalamo-pituitary-adrenocortical (HPA) axis stimulate and stress inhibits food intake. It is also not clear if stress is diabetogenic or an antidiabetogenic factor. The review will discusses these issues and the involvement of the whole HPA axis and its separate molecules (glucocorticoids, adrenocorticotropin, corticotropin-releasing hormone) in food intake regulation under stress.

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

RATIONALE

Ro 64-6198, the prototypical non-peptide nociceptin/orphanin FQ peptide (NOP) receptor agonist, has potent anxiolytic-like effects in several preclinical models and species. However the effects of Ro 64-6198 on distinctive anxiety-provoking conditions related to unconditioned conflict behavior as well as its role in despair-like behavior remain to be addressed.

OBJECTIVE

Here we examined the effects of Ro 64-6198 on unconditioned conflict anxiety using stimuli with different salience and on regulation of autonomic reactivity and compared these to the effects of benzodiazepine receptor agonists. We also addressed the potential effects of Ro 64-6198 on despair-like behavior.

MATERIALS AND METHODS

Ro 64-6198 (0.1 to 10 mg/kg i.p.) and either diazepam or chlordiazepoxide were tested in the Vogel conflict punished drinking test (VCT) in Sprague Dawley rats, in the social approach-avoidance (SAA) test in Lewis rats, in the novelty-induced hypophagia (NIH) in C57BL/6J mice, and in stress-induced hyperthermia in NMRI mice, as well as in the forced swim test (FST) in Sprague Dawley rats and the tail suspension test (TST) in C57BL/6J mice.

RESULTS

Ro 64-6198 (0.3 to 3 mg/kg) dose-dependently produced anxiolytic-like effects in the VCT, SAA, NIH, and SIH, similar to benzodiazepine receptor agonists. Ro 64-6198 did not alter immobility time in the FST and TST.

CONCLUSIONS

Ro 64-6198 produced marked anxiolytic-like effects in response to a variety of mild to strong anxiogenic stimuli, whereas it did not facilitate depression-related behaviors. This data extend previous literature suggesting that NOP receptors are a viable target for the treatment of anxiety disorders.

Nociceptin/orphanin FQ suppresses the excitability of neurons in the ventromedial nucleus of the hypothalamus

Nociceptin or orphanin FQ (N/OFQ) stimulates food intake when injected into the ventromedial nucleus of the hypothalamus (VMN). The VMN negatively regulates energy balance in part by tonically activating proopiomelanocortin arcuate neurons, thereby suppressing food intake. However, it is not clear how orexigenic neurotransmission within the VMN can stimulate food intake. We tested the hypothesis that the orexigenic action of N/OFQ results from its inhibition of anorexigenic VMN neurons. We studied the effects of N/OFQ on the electrical properties of anorexigenic VMN neurons in acute brain slices. Ionic mechanisms underlying the actions of N/OFQ were studied using whole cell patch-clamp recordings from VMN neurons expressing the anorexigenic leptin receptor (LepRb). Bath application of N/OFQ to LepRb-expressing VMN neurons elicited a robust, reversible membrane hyperpolarization that suppressed neuronal excitability by raising the action potential firing threshold and cell rheobase. N/OFQ activated a postsynaptic, G-protein coupled, inwardly rectifying potassium (GIRK) current that was sensitive to G-protein inactivation, blocked by the GIRK blocker SCH23390, and occluded by the GABAB agonist and potent GIRK activator, baclofen. Application of the selective N/OFQ receptor antagonist SB-612111 blocked the inhibitory effects of N/OFQ. We concluded that N/OFQ directly inhibited VMN neurons by activating a GIRK. These results implicate the site-specific contributions of orexigenic neuropeptides at VMN neurons to suppress anorexigenic output. This study thus advances our understanding regarding the contributions of the VMN to hypothalamic regulation of energy balance.

Energy regulatory signals and food reward

The hormones insulin, leptin, and ghrelin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis, acting at medial hypothalamic sites. Here, we summarize research demonstrating that, in addition to direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and is also a direct and indirect target for the action of these endocrine regulators of energy homeostasis. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, the midbrain dopamine (DA) and opioidergic pathways. Ghrelin can increase food reward behaviors, and support midbrain DA neuronal function. We summarize discussion of behavioral, systems, and cellular evidence in support of the contributions of reward circuitry to the homeostatic roles of these hormones in the CNS. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

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.

Chronic sugar intake dampens feeding-related activity of neurons synthesizing a satiety mediator, oxytocin

Increased tone of orexigens mediating reward occurs upon repeated consumption of sweet foods. Interestingly, some of these reward orexigens, such as opioids, diminish activity of neurons synthesizing oxytocin, a nonapeptide that promotes satiety and feeding termination. It is not known, however, whether consumption-related activity of the central oxytocin system is modified under chronic sugar feeding reward itself. Therefore, we examined how chronic consumption of a rewarding high-sucrose (HS) vs. bland cornstarch (CS) diet affected the activity of oxytocin cells in the hypothalamus at the time of meal termination. Schedule-fed (2h/day) rats received either a HS or CS powdered diet for 20 days. On the 21st day, they were given the same or the opposite diet, and food was removed after the main consummatory activity was completed. Animals were perfused 60 min after feeding termination and brains were immunostained for oxytocin and the marker of neuronal activity, c-Fos. The percentage of c-Fos-positive oxytocin cells in the hypothalamic paraventricular nucleus was significantly lower in rats chronically exposed to the HS than to the CS diet, regardless of which diet they received on the final day. A similar pattern was observed in the supraoptic nucleus. We conclude that the chronic rather than acute sucrose intake reduces activity of the anorexigenic oxytocin system. These findings indicate that chronic consumption of sugar blunts activity of pathways that mediate satiety. We speculate that a reduction in central satiety signaling precipitated by regular intake of foods high in sugar may lead to generalized overeating.

UFP-112 a potent and long-lasting agonist selective for the Nociceptin/Orphanin FQ receptor

Nociceptin/orphanin FQ (N/OFQ) controls several biological functions via selective activation of the N/OFQ peptide receptor (NOP). [(pF)Phe(4) Aib(7) Arg(14) Lys(15) ]N/OFQ-NH(2) (UFP-112) is an NOP receptor ligand designed using a combination of several chemical modifications in the same peptide sequence that increase NOP receptor affinity/potency and/or reduce susceptibility to enzymatic degradation. In the present review article, we summarize data from the literature and present original findings on the in vitro and in vivo pharmacological features of UFP-112. Moreover, important biological actions and possible therapeutic indications of NOP receptor agonists are discussed based on the results obtained with UFP-112 and compared with other peptide and nonpeptide NOP receptor ligands.

Energy regulatory signals and food reward

The hormones insulin, leptin, and ghrelin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis, acting at medial hypothalamic sites. Here, we summarize research demonstrating that, in addition to direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and is also a direct and indirect target for the action of these endocrine regulators of energy homeostasis. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, the midbrain dopamine (DA) and opioidergic pathways. Ghrelin can increase food reward behaviors, and support midbrain DA neuronal function. We summarize discussion of behavioral, systems, and cellular evidence in support of the contributions of reward circuitry to the homeostatic roles of these hormones in the CNS. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

Nociceptin/orphanin FQ suppresses adaptive immune responses in vivo and at picomolar levels in vitro

Nociceptin/orphanin FQ (N/OFQ), added in vitro to murine spleen cells in the picomolar range, suppressed antibody formation to sheep red blood cells in a primary and a secondary plaque-forming cell assay. The activity of the peptide was maximal at 10(-12) M, with an asymmetric U-shaped dose-response curve that extended activity to 10(-14) M. Suppression was not blocked by pretreatment with naloxone. Specificity of the suppressive response was shown using affinity-purified rabbit antibodies against two N/OFQ peptides and with a pharmacological antagonist. Antisera against both peptides were active, in a dose-related manner, in neutralizing N/OFQ-mediated immunosuppression, when the peptide was used at concentrations from 10(-12.3) to 10(-11.6) M. In addition, nociceptin given in vivo by osmotic pump for 48 h suppressed the capacity of spleen cells placed ex vivo to make an anti-sheep red blood cell response. These studies show that nociceptin directly inhibits an adaptive immune response, i.e., antibody formation, both in vitro and in vivo.

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.

Central administration of Orphanin FQ inhibits GnRH secretion by ORL1 receptor in the median eminence of freely moving ovariectomized rats

OBJECTIVE

This study aimed to investigate the possible role of Orphanin FQ (OFQ) in the regulation of hypo-thalamic gonadotropin-releasing hormone (GnRH) secretion.

METHODS

The method of push-pull perfusion and radioimmuno-assay (RIA) were adopted to examine the secretory profile of GnRH in the median eminence (ME) in freely moving ovari-ectomized (OVX) rats after intracerebroventricular (icv) injection of OFQ and/or [Nphe(1)]NC(1-13)NH(2) (NC13), a competitive antagonists of the opioid receptor-like 1 receptor (ORL1 receptor).

RESULTS

GnRH release from ME significantly decreased from 40 min to 80 min after the administration of 20 and 200 nmol OFQ in OVX rats (P < 0.05). This inhibitory effect of 20 nmol OFQ could be abolished by pretreatment with equal dose of NC13. More interestingly, GnRH secretion from ME was increased markedly 60 min after icv injection of 100 and 200 nmol NC13 (P < 0.05).

CONCLUSION

Our results suggested central administration of OFQ could inhibit the release of GnRH in the ME of hypothalamus through ORL1 receptor, providing further in vivo evidence supporting the role of OFQ in the control of GnRH secretion.

Estrogen and progesterone modulate [35S]GTPgammaS binding to nociceptin receptors

Sex steroids modulate reproduction by altering the response of steroid-activated opioid circuits in the hypothalamus and limbic system, by inducing release of endogenous opioids and activation of their cognate receptors. Many studies have concentrated on steroid regulation of exogenous opioid peptides, but steroids also have important actions on opioid receptors inducing receptor trafficking. Opioid receptors are G protein-coupled receptors and their activation catalyzes the exchange of GTP for GDP initiating intracellular signaling cascades. Kinetics of G protein activation were studied using [(35)S]GTPgammaS binding. Catalytic amplification, the number of G proteins activated per occupied receptor, was used as a measure of receptor/transducer amplification. The present study examined whether estrogen and progesterone treatment altered the kinetics of nociceptin opioid receptor (ORL1) in plasma membranes from the medial preoptic area and mediobasal hypothalamus. These hypothalamic regions are important in the gonadal steroid hormone regulation of sexual receptivity. In the mediobasal hypothalamus, estrogen increased ORL1 (B(max)) receptor number 2-fold and maximal GTPgammaS binding (E(max)) 3.9-fold. Subsequent progesterone treatment further increased ORL1 E(max )6.9-fold above baseline, despite a 2-fold decrease in the catalytic amplification factor. In the medial preoptic area, estrogen alone did not increase E(max), but both estrogen and progesterone were able to increase ORL1 B(max) 2.2-fold and E(max) 3-fold, despite having a 3-fold decrease in the catalytic amplification factor. These effects are interesting because they indicate actions of steroids that increase the number of ORL1 but decrease the catalytic amplification suggesting that the steroid effects on opioid receptors are complex and may involve modulation by other signals.

The orphanin FQ/nociceptin (OFQ/N) system

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

Involvement of cocaine-amphetamine regulated transcript in the differential feeding responses to nociceptin/orphanin FQ in dark agouti and Wistar Ottawa Karlsburg W rats

Wistar Ottawa Karlsburg W (WOKW) rats and their controls, dark agouti (DA), present different features: in particular, DA rats are lean, while the WOKW are obese and present symptoms of hypertension, dyslipidemia, hyperinsulinemia, and impaired glucose tolerance. The present study tested the hypothesis that these two strains would demonstrate different sensitivity to nociceptin/orphanin FQ (N/OFQ). N/OFQ was injected into the lateral brain ventricle (LBV) of sated DA and WOKW rats, and corticosterone levels in both strains were measured after LBV injection of N/OFQ. LBV N/OFQ injections dose-dependently produced a significant increase in food intake (4 h) in DA rats, but not in WOKW. However, corticosterone levels were increased by N/OFQ to a greater degree in WOKW than in DA rats. Gene sequencing and gene expression of ORL1 receptor and cocaine-amphetamine regulated transcript (Cart) peptide were evaluated to study the difference in N/OFQ-induced feeding behavior in the two strains. WOKW rats had a different amino acid sequence of Cart peptide and a significantly higher expression of Cart in the hypothalamus. The present data show that DA and WOKW rats demonstrate different sensitivity to N/OFQ, and suggest that Cart peptide might be the underlying mechanism of this difference.

Orphanin FQ: evidence for a role in the control of the reproductive neuroendocrine system

Orphanin FQ (OFQ), also known as nociceptin, is a member of the endogenous opioid peptide family that has been functionally implicated in the control of pain, anxiety, circadian rhythms, and neuroendocrine function. In the reproductive system, endogenous opioid peptides are involved in the steroid feedback control of GnRH pulses and the induction of the GnRH surge. The distribution of OFQ in the preoptic area and hypothalamus overlaps with GnRH, and in vitro evidence suggests that OFQ can inhibit GnRH secretion from hypothalamic fragments. Using the sheep as a model, we examined the potential anatomical colocalization between OFQ and GnRH using dual-label immunocytochemistry. Confocal microscopy revealed that approximately 93% of GnRH neurons, evenly distributed across brain regions, were also immunoreactive for OFQ. In addition, almost all GnRH fibers and terminals in the external zone of the median eminence, the site of neurosecretory release of GnRH, also colocalized OFQ. This high degree of colocalization suggested that OFQ might be functionally important in controlling reproductive endocrine events. We tested this possibility by examining the effects of intracerebroventricular administration of [Arg(14), Lys(15)] OFQ, an agonist to the OFQ receptor, on pulsatile LH secretion. The agonist inhibited LH pulse frequency in both luteal phase and ovariectomized ewes and suppressed pulse amplitude in the latter. The results provide in vivo evidence supporting a role for OFQ in the control of GnRH secretion and raise the possibility that it acts as part of an ultrashort, autocrine feedback loop controlling GnRH pulses.

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

The excellent pharmacological profile displayed by the selective nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor antagonist SB-612111 [(-)-cis-1-methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol] in vitro prompted us to investigate the actions of this compound in vivo. In the mouse tail withdrawal assay, SB-612111 given i.p. up to 3 mg/kg did not modify per se tail withdrawal latencies but was able to prevent the pronociceptive and the antinociceptive action of 1 nmol of N/OFQ given i.c.v. and i.t., respectively. In food intake studies performed in sated mice, SB-612111 (1 mg/kg i.p.) had no effect on food consumption but fully prevented the orexigenic effect of 1 nmol of N/OFQ i.c.v. In 17-h food-deprived mice, the opioid receptor antagonist naltrexone (1 mg/kg s.c.), but not SB-612111 (1 and 10 mg/kg i.p.), produced a statistically significant reduction of food intake. In the mouse forced swimming and tail suspension tests, SB-612111 (1-10 mg/kg) reduced immobility time. The antidepressant-like effect elicited by SB-612111 in the forced swimming test was reversed by the i.c.v. injection of 1 nmol of N/OFQ and no longer evident in mice knockout for the NOP receptor gene. In conclusion, the present findings demonstrate that SB-612111 behaves in vivo as a potent and selective NOP antagonist and suggest that the N/OFQ-NOP receptor endogenous system plays an important role in regulating mood-related behaviors. The use of SB-612111 in future pathophysiological studies will certainly contribute to define the therapeutic potential of selective NOP receptor antagonists in different disease areas.

Chronic intracerebroventricular infusion of nociceptin/orphanin FQ increases food and ethanol intake in alcohol-preferring rats

Central administration of low doses of nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the opioid-like orphan receptor NOP, have been shown to reduce ethanol consumption, ethanol-induced conditioned place preference and stress-induced reinstatement of alcohol-seeking behavior in alcohol preferring rats. The present study evaluated the effect of continuous (7 days) lateral brain ventricle infusions of N/OFQ (0, 0.25, 1, 4, and 8 microg/h), by means of osmotic mini-pumps, on 10% ethanol intake in Marchigian-Sardinian alcohol-preferring (msP) rats provided 2h or 24h access to it. N/OFQ dose-dependently increased food intake in msP rats. On the other hand, in contrast to previous studies with acute injections, continuous lateral brain ventricle infusion of high doses of N/OFQ increased ethanol consumption when the ethanol solution was available for 24h/day or 2h/day. The present study demonstrates that continuous activation of the opioidergic N/OFQ receptor does not blunt the reinforcing effects of ethanol. Moreover, the data suggest that continuous activation of the opioidergic N/OFQ receptor is not a suitable way to reduce alcohol abuse.

Functional interaction between nociceptin/orphanin FQ and alpha-melanocyte-stimulating hormone in the regulation of feeding

Nociceptin/orphanin FQ (N/OFQ), an endogenous agonist of the opioid N/OFQ (NOP) receptor, increases food intake when administered centrally. As N/OFQ is part of a larger neural network that governs consummatory behavior, presumably its orexigenic properties stem from interplay with other neuropeptidergic components of the feeding-related circuitry. One such peptide may be the ligand of the melanocortin-3 and -4 receptors, alpha-melanocyte-stimulating hormone (alpha-MSH), which is known to inhibit food intake. The aim of the present study was to establish whether there is a functional "interaction" between N/OFQ and alpha-MSH in the regulation of feeding. By using double immunostaining for c-Fos and alpha-MSH, we found that intracerebroventricular (i.c.v.) injection of N/OFQ at a 10nmol dose that moderately prolongs deprivation-induced food intake in rats, decreases activation of alpha-MSH neurons involved in feeding termination. However, i.c.v. injections of alpha-MSH at doses previously established to reduce deprivation-induced feeding, do not decrease hyperphagia generated by N/OFQ in ad libitum-fed animals. Our results suggest that while alpha-MSH does not appear to modify the orexigenic response to N/OFQ in sated rats, the NOP receptor ligand promotes a decrease in activation of neurons synthesizing the anorexigenic peptide, alpha-MSH, at the time of re-feeding. Thus, to some degree, the stimulatory effect of N/OFQ on consumption may arise from this peptide's inhibitory influence on activity of anorexigenic pathways containing alpha-MSH.

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.

Cannabinoids, opioids and eating behavior: the molecular face of hedonism?

Obesity represents nowadays one of the most devastating health threats. Published reports even project a decline in life expectancy of US citizens due to the rapidly increasing prevalence of obesity. This alarming increase is intimately linked with recent changes of environment and lifestyle in western countries. In this context, the rewarding or even addictive properties of popular food may represent one of the most serious obstacles to overcome for an effective anti-obesity therapy. Therefore, in addition to molecular networks controlling energy homeostasis, now researchers are starting to define central nervous mechanisms governing hedonic and addictive components of food intake. A recently emerging body of data suggests that the endogenous cannabinoid and opioid systems both represent key circuits responding to the rewarding value of food. This review focuses on the role of these two systems for the homeostatic and hedonic aspects of eating behavior and includes their anatomical and functional interactions. Independent from the degree to which eating can be considered an addiction, cannabinoid and opioid receptor antagonists are promising anti-obesity drugs, since they are targeting both hedonic and homeostatic components of energy balance control.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Involvement of nociceptin/orphanin FQ in release of hypothalamic GnRH mediated by ORL1 receptor in ovariectomized rats

AIM

To investigate effect of the nociceptin/orphanin FQ (OFQ) on hypothalamus gonadotropin-releasing hormone (GnRH) release in ovariectomized (OVX) rats.

METHODS

GnRH radioimmunoassay (RIA) was used to study the effect of OFQ on GnRH release in hypothalamus slices in vitro. Push-pull perfusion and intracerebroventicular (icv) injection were used to examine the effect of OFQ on GnRH release in the hypothalamus medial preoptic area (POA) in vivo. Ovariectomies were performed on female Sprague-Dawley rats, and their plasma luteinizing hormone (LH) levels were measured after icv injection of OFQ with or without [Nphe1]NC(1-13)NH2, a competitive antagonist of opioid receptor-like1 receptor (ORL1 receptor). Reverse transcription-polymerase chain reaction (RT-PCR) was used to investigate the expression of the ORL1 receptor in rat pituitary.

RESULTS

GnRH release from hypothalamus slices was inhibited 90 min after the administration of 2 mmol/L and 20 mmol/L OFQ (P<0.05). Accordingly, GnRH release from hypothalamus POA was also significantly reduced by the injection of 0.2 mmol/L and 2 mmol/L OFQ. Plasma LH levels were also decreased significantly 2 h after icv injection of 20 nmol OFQ in OVX rats (P<0.05) and this effect could be abolished by pretreatment with 20 nmol [Nphe1]NC(1-13)NH2, that is, NC13. More interestingly, plasma LH levels in OVX rats increased markedly 2 h after icv injection of 100 nmol and 200 nmol NC13. RT-PCR analysis further revealed that the ORL1 receptor was not expressed in the pituitary of OVX rats.

CONCLUSION

Central administration of nociceptin/orphanin FQ might inhibit the release of hypothalamic GnRH and decrease the plasma LH levels through ORL1 receptors in OVX rats.

Peripheral mechanisms involved in gastric mucosal protection by intracerebroventricular and intraperitoneal nociceptin in rats

Nociceptin (N/OFQ) exerts multiple effects in the gastrointestinal tract after central or peripheral administration. In the present study, we examined the possible peripheral mechanisms mediating gastric protection by N/OFQ in rats. Gastric mucosal lesions were induced by 50% ethanol (1 ml/rat intragastrically). N/OFQ, administered either intracerebroventricularly (3 microg/rat) or ip (10 microg/kg), significantly reduced macroscopic and histological damage. The protective effect of intracerebroventricular N/OFQ was blocked by atropine, subdiaphragmatic vagotomy, and bretylium. The effect of both central and peripheral N/OFQ was blocked by functional ablation of afferent nerves produced by capsaicin, by the antagonist of calcitonin gene-related peptide, CGRP(8-37), and by the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester. These results indicate that N/OFQ increases gastric mucosal resistance to ethanol by operating both in the central nervous system and in the periphery. Vagal cholinergic and sympathetic pathways mediate the central activity of N/OFQ, whereas vagal nonmuscarinic pathways mediate the peripheral activity of the peptide. The neuronal circuit involving extrinsic sensory neurons, calcitonin gene-related peptide, and nitric oxide is activated by central as well as peripheral N/OFQ. The study provides evidence that N/OFQ contributes to neurally mediated gastric mucosal protection.

Hypothalamic cocaine- and amphetamine-regulated transcript (CART) and agouti-related protein (AgRP) neurons coexpress the NOP1 receptor and nociceptin alters CART and AgRP release

Nociceptin or orphanin FQ (N/OFQ) and its receptor NOP1 are expressed in hypothalamic nuclei involved in energy homeostasis. N/OFQ administered by intracerebroventricular or arcuate nucleus (ARC) injection increases food intake in satiated rats. The mechanisms by which N/OFQ increases food intake are unknown. We hypothesized that N/OFQ may regulate hypothalamic neurons containing peptides involved in the control of food intake such as cocaine- and amphetamine-regulated transcript (CART), alphaMSH, neuropeptide Y (NPY), and agouti-related protein (AgRP). We investigated the ability of N/OFQ to alter the release of CART, alphaMSH, NPY, and AgRP using ex vivo medial basal hypothalamic explants. Incubation of hypothalamic explants with N/OFQ (1, 10, 100 nM) resulted in significant changes in CART and AgRP release. One hundred nanomoles N/OFQ caused a 33% decrease in release of CART (55-102) immunoreactivity (IR) and increased release of AgRP-IR to 163% but produced no change in either alphaMSH-IR or NPY-IR. Double immunocytochemistry/in situ hybridization demonstrated that CART-IR and NOP1 mRNA are colocalized throughout the hypothalamus, in particular in the paraventricular nucleus, lateral hypothalamus, zona incerta, and ARC, providing an anatomical basis for N/OFQ action on CART release. Dual in situ hybridization demonstrated that AgRP neurons in the ARC also express the NOP1 receptor. Our data suggest that nociceptin via the NOP1 receptor may increase food intake by decreasing the release of the anorectic peptide CART and increasing the release of the orexigenic peptide AgRP.

Hepatic vagotomy alters limbic and hypothalamic neuropeptide responses to insulin-dependent diabetes and voluntary lard ingestion

Hypothalamic anorexigenic [corticotropin-releasing factor (CRF) and proopiomelanocortin] peptides decrease and the orexigen, neuropeptide Y, increases with diabetic hyperphagia. However, when diabetic rats are allowed to eat lard (saturated fat) as well as chow, both caloric intake and hypothalamic peptides normalize. These neuropeptide responses to lard require an intact hepatic vagus [la Fleur et al. (2003) Diabetes, 52, 2321-2330]. Here, we delineate temporal interactions after lard consumption +/- hepatic vagotomy (HV) between feeding and brain neuropeptide expression in insulin-dependent diabetic rats. CRF-mRNA was reduced in the paraventricular nuclei (PVN) by 6 h after presentation of lard, before caloric intake increased in HV-diabetic rats, and did not increase at 30 or 36 h, as it did in shamHV-diabetic rats eating lard. CRF-mRNA was increased in the bed nuclei of the stria terminalis of HV-diabetic rats compared with shamHV-diabetic rats only when caloric intake was high at 30 or 36 h. At 36 h, shamHV-diabetic rats eating chow had increased CRF-mRNA in the central amygdala but diabetic rats eating lard had decreased CRF-mRNA, whereas HV-diabetic rats eating chow or lard had normal CRF-mRNA in the central amygdala. We conclude that eating lard restores peptide expression to normal in the hypothalamus of diabetic rats, and because decreased CRF-mRNA in the PVN precedes the increase in caloric intake in HV-diabetic rats eating lard, that the loss of a hepatic vagal signal to PVN may be responsible for increased intake; moreover, CRF-mRNA in limbic structures is also sensitive to both HV and lard ingestion in diabetic rats.