Modification of acetylcholine release by nociceptin in conscious rat striatum

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

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

Nociceptin/orphanin-FQ modulation of learning and memory

In the past decades, a large number of neuropeptides with unknown functions have been identified in the brain. Among the newly discovered peptides, nociceptin or orphanin-FQ (N/OFQ) peptide has attracted considerable attention because of its sequence homology with the opioid peptide family. N/OFQ and its cognate receptor (NOP receptor) are distributed widely in the mammalian central nervous system, though particularly intense expression is found in corticolimbic structures. Such distinctive pattern of expression suggests a key role of N/OFQ system in higher brain functions, such as cognition and emotion. In this chapter, we will outline the findings supporting the role played by N/OFQ and NOP receptors in learning and memory and discuss the underlying mechanisms.

Interactions of orphanin FQ/nociceptin (OFQ/N) system with immune system factors and hypothalamic-pituitary-adrenal (HPA) axis

BACKGROUND

Brain-immune system interactions and neurohormonal changes which are induced by psychophysiological factors are growing areas of scientific interest. Central (CNS) and autonomic nervous-endocrine-immune system pathways are connected with a number of behavioral and physiological factors which may be linked to disease susceptibility and progression.

METHODS

In this paper, influence of orphanin FQ/nociceptin receptor (OFQ/N) on the hypothalamic-pituitary-adrenal (HPA) axis and their influence on the immunological system was reviewed.

CONCLUSIONS

The neuroendocrine system, in particular the hypothalamic-pituitary-adrenal (HPA) axis, is closely connected with the cytokines. HPA axis activation by cytokines, via the release of glucocorticoids has, in turn, been found to play a critical role in restraining and shaping immune responses. Investigation of the OFQ/N system and G-proteins suggests a role for this receptor as a down-regulator of cytokine, chemokine and chemokine receptor expression.

The role of orphanin FQ/nociceptin in neuroplasticity: relationship to stress, anxiety and neuroinflammation

The neuropeptide, orphanin FQ/nociceptin (OFQ/N or simply, nociceptin), is expressed in both neuronal and non-neuronal tissue, including the immune system. In the brain, OFQ/N has been investigated in relation to stress, anxiety, learning and memory, and addiction. More recently, it has also been found that OFQ/N influences glial cell functions, including oligodendrocytes, astrocytes, and microglial cells. However, this latter research is relatively small, but potentially important, when observations regarding the relationship of OFQ/N to stress and emotional functions is taken into consideration and integrated with the growing evidence for its involvement in cells that mediate inflammatory events. This review will first provide an overview and understanding of how OFQ/N has been implicated in the HPA axis response to stress, followed by an understanding of its influence on natural and learned anxiety-like behavior. What emerges from an examination of the literature is a neuropeptide that appears to counteract anxiogenic influences, but paradoxically, without attenuating HPA axis responses generated in response to stress. Studies utilized both central administration of OFQ/N, which was shown to activate the HPA axis, as well as antagonism of NOP-R, the OFQ/N receptor. In contrast, antagonist or transgenic OFQ/N or NOP-R knockout studies, showed augmentation of HPA axis responses to stress, suggesting that OFQ/N may be needed to control the magnitude of the HPA axis response to stress. Investigations of behavior in standard exploratory tests of anxiogenic behavior (eg., elevated plus maze) or learned fear responses have suggested that OFQ/N is needed to attenuate fear or anxiety-like behavior. However, some discrepant observations, in particular, those that involve appetitive behaviors, suggest a failure of NOP-R deletion to increase anxiety. However, it is also suggested that OFQ/N may operate in an anxiolytic manner when initial anxiogenic triggers (eg., the neuropeptide CRH) are initiated. Finally, the regulatory functions of OFQ/N in relation to emotion-related behaviors may serve to counteract potential neuroinflammatory events in the brain. This appears to be evident within the glial cell environment of the brain, since OFQ/N has been shown to reduce the production of proinflammatory cellular and cytokine events. Given that both OFQ/N and glial cells are activated in response to stress, it is possible that there is a possible convergence of these two systems that has important repercussions for behavior and neuroplasticity.

Genetic dissection of medial habenula-interpeduncular nucleus pathway function in mice

The habenular complex linking forebrain and midbrain structures is subdivided into the medial (mHb) and the lateral nuclei (lHb). The mHb is characterized by the expression of specific nicotinic acetylcholine receptor isoforms and the release of acetylcholine to the interpeduncular nucleus (IPN), the sole output region of the mHb. The specific function of this circuit, however, is poorly understood. Here we generated transgenic mice in which mHb cells were selectively ablated postnatally. These lesions led to large reductions in acetylcholine levels within the IPN. The mutant mice exhibited abnormalities in a wide range of behavioral domains. They tended to be hyperactive during the early night period and were maladapted when repeatedly exposed to new environments. Mutant mice also showed a high rate of premature responses in the 5-choice serial reaction time task (5-CSRTT), indicating impulsive and compulsive behavior. Additionally, mice also exhibited delay and effort aversion in a decision-making test, deficits in spatial memory, a subtle increase in anxiety levels, and attenuated sensorimotor gating. IntelliCage studies under social housing conditions confirmed hyperactivity, environmental maladaptation, and impulsive/compulsive behavior, delay discounting, deficits in long-term spatial memory, and reduced flexibility in complex learning paradigms. In 5-CSRTT and adaptation tasks, systemic administration of nicotine slowed down nose-poke reaction and enhanced adaptation in control but not mutant mice. These findings demonstrate that the mHb–IPN pathway plays a crucial role in inhibitory control and cognition-dependent executive functions.

Nociceptin/orphanin FQ reverses mecamylamine-induced learning and memory impairment as well as decrease in hippocampal acetylcholine release in the rat

Nociceptin/orphanin FQ is an endogenous neuropeptide that plays important roles in several physiological functions including pain, anxiety, locomotion, learning, and memory. We previously reported that low doses of nociceptin improved the scopolamine-induced impairment of learning and memory in the passive avoidance test and the spontaneous Y-maze alternation task in mice. In the present study, the effects of nociceptin on learning and memory impairment as well as the decrease in acetylcholine release induced by mecamylamine were investigated in rats. Mecamylamine (49 micromol/kg, s.c.), a nicotinic acetylcholine receptor antagonist, impaired learning and memory in the step-through type passive avoidance test and decreased acetylcholine release in the hippocampus, as determined by in vivo microdialysis. The administration of nociceptin (10 fmol/rat, i.c.v.) reversed the impairment of learning and memory and blocked the decrease in acetylcholine release induced by mecamylamine. This ameliorating effect on the mecamylamine-induced impairment of learning and memory was not blocked by [NPhe(1)]nociceptin(1-13)NH(2) (1 nmol/rat, i.c.v.), an opioid receptor-like 1 (NOP) receptor antagonist. These results suggest that nociceptin improves the impairment of learning and memory as well as decrease in acetylcholine release induced by mecamylamine, and that these effects may not be mediated by NOP receptors.

Enhanced hippocampal acetylcholine release in nociceptin-receptor knockout mice

Nociceptin (NOC), an endogenous ligand of the opioid receptor-like 1 receptor, is thought to be involved in learning and memory processes. Since acetylcholine (ACh) is involved in hippocampal function, and the hippocampus plays a critical role on the learning and memory function, hippocampal ACh release in NOC-receptor knockout mice was examined using an in vivo microdialysis method. The release of hippocampal ACh was largely increased in the knockout mice. Furthermore, in the knockout mice, an enhanced hippocampal theta rhythm, which is known to be linked to hippocampal memory function, was also observed. Immunohistochemically, in septum, co-existence of NOC receptor with cholinergic, but not with GABAergic neurons, was verified. The findings demonstrate that the NOC receptor is involved in hippocampal cholinergic function.

Nociceptin/orphanin FQ modulates spatial learning via ORL-1 receptors in the dorsal hippocampus of the rat

The endogenous peptide nociceptin (orphanin FQ) plays a role in several important physiological functions in the CNS such as pain, anxiety and locomotion. It has previously been found that injection of 10 nmol nociceptin into the CA3 region of the hippocampus markedly impairs spatial learning and memory in the rat. The present study examined the effects of lower doses of nociceptin (3.3, 1, 0.33 and 0.1 nmol/rat) on spatial learning. The 3.3 nmol dose impaired spatial learning over the 5 days of training although the effect was not as strong as with 10 nmol. In contrast, the two lower doses, 1 and 0.33 nmol/rat, improved spatial learning whereas the lowest dose, 0.1 nmol/rat, had no significant effect. Both the impairing and facilitating effect of nociceptin could be blocked by an ORL-1 receptor antagonist, [Phe1Psi(CH(2)-NH)Gly2]NC(1-13)NH2 (10 nmol/rat), indicating that both effects are ORL-1 receptor-mediated. The 3.3 nmol dose of nociceptin did not impair the performance in the visual platform task and did not alter swim speed or motor activity indicating no effects on motivation or motor performance. Taken together, these results show that nociceptin has a biphasic dose-effect curve and provide further evidence for a role of this neuropeptide in cognitive processes in the hippocampus.

Nociceptin inhibition of acetylcholine efflux from different brain areas

The effects of nociceptin on [3H]choline [3H](Ch) efflux from electrically-stimulated rat cortical, hippocampal and caudatal slices as well as from KCl-depolarized synaptosomes and tetrodotoxin-pretreated slices have been studied. The inhibition of electrically evoked [3H]Ch efflux by nociceptin (0.03-3 microM) was moderate (max -33%), more evident in the neocortex than in the hippocampus and was prevented by [Nphe1]NC(1-13)NH(2) 10 microM. This effect was absent in the caudate nucleus, in cortical synaptosomes and in tetrodotoxin-pretreated cortical slices. These data point to a distinct localization of NOP receptors in the different brain areas and to a prevailing inhibitory control by nociceptin on the cortical cholinergic input at pre-terminal level. However, the reported impairment of neocortical and hippocampal function by nociceptin may be referred to the inhibition not only of the cholinergic signal but also of other transmitters such as glutamate.

Utilizing functional genomics to identify new pain treatments : the example of nociceptin

Nociceptin/orphanin FQ (noc/oFQ) is the first novel bioactive substance to have been discovered by the implementation of a functional genomics/reverse pharmacology approach. The neuropeptide was indeed identified in brain extracts as the natural ligand of a previously cloned orphan G protein-coupled receptor, the opioid receptor-like 1 (ORL1) receptor. Since its discovery in 1995, noc/oFQ has been the subject of intensive study to establish its role in normal brain function and its possible involvement in neurophysiopathology. Although the neuropeptide, an inhibitor of neuronal activity, has been found to have a wide spectrum of pharmacological effects in vivo, none has been as intensively investigated as its action on nociception and nociceptive processing. There is now substantial evidence that noc/oFQ has a modulatory role in nociception. However, dependent on the dose and site of injection, and possibly the animal's genetic background and even psychological status, the peptide has been variously reported to cause allodynia, hyperalgesia, analgesia, and even pain, in rodents. Overall, noc/oFQ tends to facilitate pain when administered supraspinally, and to inhibit it when administered spinally. These opposing effects beg the obvious, yet still unanswered, question as to what would be the net effect on nociception of an ORL1 receptor ligand, agonist or antagonist, able to target supraspinal and spinal sites simultaneously. Owing to the research effort of several drug companies, such ligands, i.e. nonpeptidic, brain-penetrating agonists and antagonists, have recently been produced whose systematic screening in animal models of acute and inflammatory pain may help validate the ORL1 receptor as the target for novel, non-opioid analgesics.

Nociceptin/orphanin FQ receptor ligands

Nociceptin (NC), alias Orphanin FQ (OFQ) is a heptadecapeptide structurally related to opioid peptides, especially Dynorphin A, which, however, does not interact with classic opioid receptors. NC selectively activates its own receptor (OP(4)), which has been shown to be insensitive to the naturally occurring opioid peptides as well as to a large number of non-peptide opioid receptor ligands, including naloxone. Thus, the NC/OP(4) system represents a new peptide-based signaling pathway, which is pharmacologically distinct from the opioid systems. The pharmacological tools available for investigating NC actions are at present rather limited and include: 1) peptide ligands obtained from structure activity studies performed using NC(1-13)NH(2) as a template or discovered by screening peptide combinatorial libraries; 2) nonpeptide ligands that are either molecules already known to interact with classic opioid receptors or novel molecules designed and synthesized as selective ligands of the OP(4) receptor. In the present paper the functional data obtained from both in vitro and in vivo studies with each relevant OP(4) receptor ligand will be analyzed and discussed comparing the advantages and disadvantages of each molecule. We hope that the present work will aid investigators, working in the NC/OP(4) field, in the choice of the pharmacological tools suitable for their experiments.

Nociceptin/orphanin FQ and neurotransmitter release in the central nervous system

In this article, the effect of nociceptin (orphanin FQ) on transmitter release in the central nervous system in vitro and in vivo is reviewed. Nociceptin inhibits the electrically or K(+)-evoked noradrenaline, dopamine, serotonin, and glutamate release in brain slices from guinea-pig, rat, and mouse. This effect is usually naloxone-resistant but antagonized by OP(4) receptor antagonists like [Phe(1)psi(CH(2)-NH)Gly(2)]-nociceptin(1-13)NH(2). In the rat in vivo, nociceptin diminishes acetylcholine release in the striatum, reduces dopamine release, and prevents the stimulatory effect of morphine on this transmitter in the nucleus accumbens and also elevates extracellular glutamate and gamma-aminobutyric acid levels in mesencephalic dopaminergic areas. The effect of nociceptin on the mesencephalic dopaminergic system might explain its actions on motor behavior.

Nociceptin/orphanin FQ metabolism and bioactive metabolites

The endogenous ligand for the orphan NOR receptor (earlier named ORL1) was recently discovered. This ligand, nociceptin/orphanin FQ is involved in a number of pharmacological actions in the CNS, including modulation of pain and cognition. However, its specific physiological role remains to be determined. Two major pathways of metabolism have been identified; the action of aminopeptidase(s) that prominently occurs in plasma, and endopeptidase activity that successively generates the N-terminal 1-13 and 1-9 fragments. Both pathways result in fragments that are inactive at the NOR receptor. However, short N-terminal fragments appear to be active in blocking the release of substance P from primary afferent C-fiber terminals in the dorsal spinal cord. The same endopeptidase(s) may also be involved in the fragmentation of dynorphin A since the inhibitor profile is similar. Enzyme activity is upregulated by morphine using either peptide as substrate that may lead to pharmacological interactions.

Improvement by low doses of nociceptin on scopolamine-induced impairment of learning and/or memory

The effects of fmol doses of nociceptin/orphanin FQ on scopolamine-induced impairment of learning and/or memory were examined using spontaneous alternation of Y-maze and step-down type passive avoidance tasks. While fmol doses of nociceptin alone had no effect on spontaneous alternation or passive avoidance behavior in normal mice, administration of nociceptin (10 and/or 100 fmol/mouse) 30 min before spontaneous alternation performance or the training session of the passive avoidance task, significantly improved the scopolamine-induced impairment of spontaneous alternation and passive avoidance behavior. This ameliorating effect was not antagonized by nocistatin (0.5 and 5.0 nmol/mouse, i.c.v.), naloxone benzoylhydrazone (2.3, 11.2, and 56.1 micromol/kg, s.c.) or nor-binaltorphimine (4.9 nmol/mouse, i.c.v.). These results indicated that very low doses of nociceptin ameliorate impairments of spontaneous alternation and passive avoidance induced by scopolamine, and suggested that this peptide has bidirectional modulatory effects on learning and memory; impairment at high doses and amelioration at low doses.