Lack of involvement of dopamine and serotonin during the orphanin FQ/Nociceptin (OFQ/N)-induced prolactin secretory response

The purpose of these studies was to examine possible mechanisms of Orphanin FQ/Nociceptin (OFQ/N)-induced prolactin release. We investigated the involvement of the dopaminergic neurons by quantifying DOPAC:DA levels in the median eminence and neurointermediate lobe following central administration of OFQ/N to female Sprague-Dawley rats. To specifically determine the involvement of the tuberoinfundibular dopaminergic neurons, immunocytochemical studies were conducted to visualize c-fos protein expression in the arcuate nucleus following central administration of OFQ/N. In addition, the role of serotonergic activation was examined in dose response studies using the selective serotonin antagonist ritansarin and the nonselective antagonist metergoline. Finally, the pharmacological specificity of the prolactin response was examined by pretreating animals with [Nphe1] NC (1-13)NH2, a drug reported to antagonize OFQ/N effects. The results of these studies indicate that the increase in prolactin release following central administration of OFQ/N does not inhibit tuberoinfundibular, tuberohypophyseal or periventricular hypophysial dopaminergic neuronal activity at 10 min after drug administration, a time when prolactin levels were significantly elevated. Furthermore, serotonergic activation is not involved since pharmacological blockade of serotonergic receptors did not alter the prolactin secretory response to OFQ/N. NC (1-13)NH2 did not antagonize the stimulatory effects of OFQ/N on prolactin secretion. The neural effects of OFQ/N on dopaminergic neuronal activity may occur following a different time course than that of the prolactin increase.     

Effects of PKC on inhibition of delayed rectifier potassium currents by N/OFQ

Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the opioid receptor-like-1 (ORL-1) orphan receptor, which is responsible for inhibition of delayed rectifier potassium current (I(K)). But its mechanism of N/OFQ acting on I(K) is not clear and whether PKC is involved in the modulation of this processing is still unknown. Whole-cell patch-clamp recordings were performed in acutely dissociated rat parietal cortical neurons. Bath application of N/OFQ (10 nM-10 microM) resulted in a dose-dependent depression of I(K) with partially recovery on washout. Furthermore, we investigated the role of PKC in the inhibition of I(K) by N/OFQ. Chelerythrine, an inhibitor of PKC, attenuated the inhibition of N/OFQ on I(K). On the contrary, PDBu, an activator of PKC, augmented N/OFQ-evoked responses. The present study suggested that N/OFQ inhibited I(K) and PKC was involved in N/OFQ-evoked response in acutely dissociated rat cerebral parietal cortical neurons.     

Design and synthesis of novel small molecule N/OFQ receptor antagonists

Small molecule N/OFQ receptor antagonists were designed and synthesized to further investigate the therapeutic potential of N/OFQ receptor modulators. The resulting octahydrobenzimidazol-2-ones 14 and 23 show excellent antagonistic activity towards both N/OFQ and mu receptors with high affinity to the human N/OFQ receptor.     

Nociceptin/Orphanin FQ (N/OFQ) Receptors are Involved in Adrenaline-Induced Feeding Behavior in Broiler Cockerels

Nociceptin/orphanin FQ (N/OFQ) is an endogenous ligand of a G protein-coupled receptornamed NOP. This neuropeptide has been identified as an orexigenic stimulus in the brain of birds and mammals. The purpose of the present study was to clarify whether blockade or stimulation of nociceptin receptors affects adrenaline-induced feeding behaviour in broilers. In Experiment 1, birds received intracerebroventricular (ICV) injection of Nociceptin (1–13) NH2 (potent NOP receptor agonist, 16 nmol) followed by adrenaline (80 nmol). In Experiment 2, the birds received UFP-101, (NOP receptor antagonist, 10 nmol) prior to injection of adrenaline (80 nmol). Cumulative food and water intake was measured at 2 h post-injection. When administrated alone, adrenaline significantly increased food and water intake. The ICV injection of Nociceptin (1–13) NH2 significantly increased food intake but not water intake. Pre-injection of Nociceptin (1–13) NH2 significantly increased the adrenaline-induced feeding response. The effect of adrenaline on food intake was transiently blocked by microinjection of UFP-101. UFP-101-induced anorexia was accompanied by a transient increase in water intake. The transient dipsogenic effect of UFP-101 suggests a role of endogenous N/OFQ-NOP receptor pathways in the regulation of water intake in chickens, which is food intake-independent. These results also provide further evidence for a reciprocal interaction between adrenergic receptors and N/OFQ on feeding behavior.     

Differential protection against MPTP or methamphetamine toxicity in dopamine neurons by deletion of ppN/OFQ expression

Nociceptin (N/OFQ) is an endogenous neuropeptide that plays a role in the behavioral deficits associated with Parkinson's disease (PD). The purpose of the present study was to characterize the protective effects of prepro (pp)N/OFQ gene deletion against two dopamine toxins, MPTP and methamphetamine (METH). Results demonstrate that ppN/OFQ gene deletion attenuates the loss of both the number of tyrosine hydroxylase (TH)-positive neurons in the substantia nigra pars compacta (SNpc) and loss of TH and vesicular monoamine transporter-2 (VMAT) immunoreactivity in the caudate putamen (CPu) of MPTP-treated mice. This protection was unaffected by age or gender, although, when loss of TH exceeded 90% in 5-6 month-old mice, the protective effect was greatly diminished. In contrast, METH administration preferentially damaged dopaminergic terminals in the CPu with little effect on dopamine neurons in the SNpc, an effect not reversed by ppN/OFQ gene deletion. To determine if N/OFQ and MPP+ act directly and synergistically on dopamine neurons, differentiated SH-SY5Y cells were incubated with N/OFQ and/or MPP+. N/OFQ did not increase MPP+-mediated cell loss, suggesting an indirect action of N/OFQ. These studies demonstrate that inhibition of the endogenous N/OFQ system may represent a new therapeutic target for prevention of neuronal loss associated with PD.     

The effects of nociceptin peptide (N/OFQ)–receptor (NOP) system activation in the airways

The heptadecapeptide nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the N/OFQ peptide (NOP) receptor. It is cleaved from a larger precursor identified as prepronociceptin (ppN/OFQ). NOP is a member of the seven transmembrane-spanning G-protein coupled receptor (GPCR) family. ppN/OFQ and NOP receptors are widely distributed in different human tissues. Asthma is a complex heterogeneous disease characterized by variable airflow obstruction, bronchial hyper-responsiveness and chronic airway inflammation. Limited therapeutic effectiveness of currently available asthma therapies warrants identification of new drug compounds. Evidence from animal studies suggests that N/OFQ modulates airway contraction and inflammation. Interestingly up regulation of the N/OFQ–NOP system reduces airway hyper-responsiveness. In contrast, inflammatory cells central to the inflammatory response in asthma may be both sources of N/OFQ and respond to NOP activation. Hence paradoxical dysregulation of the N/OFQ–NOP system may potentially play an important role in regulating airway inflammation and airway tone. To date there is no data on N/OFQ–NOP expression in the human airways. Therefore, the potential role of N/OFQ–NOP system in asthma is unknown. This review focuses on its physiological effects within airways and potential value as a novel asthma therapy.     

Chronic high-NaCl intake prolongs the cardiorenal responses to central N/OFQ and produces regional changes in the endogenous brain NOP receptor system

Intracerebroventricular nociceptin/orphanin FQ (N/OFQ) produces cardiovascular depressor, diuretic, and renal sympathoinhibitory responses in conscious rats. These studies examined how a chronic high-NaCl intake alters these peptide-evoked responses and the activity of the endogenous central N/OFQ peptide (NOP) receptor system. In normotensive Sprague-Dawley rats fed a chronic (3-wk) high (8%)-NaCl diet, intracerebroventricular N/OFQ (5.5 nmol) produced prolonged bradycardic, hypotensive, and diuretic responses but failed to suppress renal sympathetic nerve activity. In a separate group of rats maintained on a high-NaCl diet, intracerebroventricular infusion of the NOP receptor antagonist UFP-101 significantly decreased urine output. At the tissue level, high-NaCl treatment of rats significantly increased NOP receptor density, without altering endogenous N/OFQ peptide levels in whole hypothalamus (control, 712 +/- 35 fmol/mg vs. 8% NaCl, 883 +/- 49 fmol/mg, P < 0.05) and paraventricular nucleus. Furthermore, in the hypothalamus, basal GTPgammaS binding was increased without altering the sensitivity of N/OFQ-stimulated G protein coupling. In contrast, in whole medulla and the ventrolateral medulla (VLM), high-NaCl treatment decreased NOP receptor density (medulla: control, 1,473 +/- 131 fmol/mg vs. 8% NaCl, 327 +/- 31 fmol/mg, P < 0.05) and endogenous N/OFQ peptide levels (medulla: control, 35.3 +/- 2 fmol/mg vs. 8% NaCl, 11.9 +/- 3 fmol/mg, P < 0.05), while increasing the sensitivity of G protein signaling pathways to N/OFQ stimulation. Together, these findings suggest that during a chronic high-salt intake, regional changes in the activity of the N/OFQ-NOP system in the brain may contribute to the tonic regulation of cardiovascular function and urine output and to the altered physiological responses to exogenous central N/OFQ.     

GABA(A) signalling is involved in N/OFQ anxiolytic-like effects but not in nocistatin anxiogenic-like action as evaluated in the mouse elevated plus maze

Nociceptin/orphanin FQ (N/OFQ) and nocistatin are two neuropeptides originated from the same precursor prepronociceptin/orphanin FQ (ppN/OFQ). N/OFQ is the endogenous ligand of the NOP receptor, while the target of action of nocistatin is still unknown. N/OFQ modulates various biological functions, including anxiety. Conversely, nocistatin either behaves as a functional N/OFQ antagonist or evokes per se effects opposite to those of N/OFQ. Here we investigated the interaction between the anxiolytic-like effects of N/OFQ and the anxiogenic-like action of nocistatin with those evoked by GABA_A receptor ligands in the mouse elevated plus maze. The anxiogenic-like effects of the GABA_A receptor antagonist pentylenetetrazol (20 mg/kg; intraperitoneal, i.p.) were abolished by the co-treatment with N/OFQ (10 pmol; intracerebroventricular, i.c.v.) while potentiated by the administration of nocistatin (0.01 pmol; i.c.v.). The anxiolytic-like effects of the benzodiazepine receptor agonist diazepam (0.75 mg/kg, i.p.) were reversed by nocistatin (0.1 pmol; i.c.v.), whereas signs of sedation were observed when mice were co-treated with diazepam and N/OFQ (3 pmol). Interesting enough, the i.p. treatment with flumazenil (1 mg/kg) blocked the anxiolytic-like effects of N/OFQ (10 pmol; i.c.v.), but not the anxiogenic effect elicited by nocistatin. Collectively, our findings suggest that the effects on anxiety elicited by pentylenetetrazol and diazepam can be counteracted or potentiated in the presence of N/OFQ and nocistatin. In addition, the effects on anxiety of N/OFQ, but not nocistatin, appear to be dependent on the benzodiazepine site of the GABA_A receptor.     

孤啡肽对大鼠顶叶皮层神经元瞬时外向钾电流的抑制作用

目的：研究孤啡肽（N／OFQ）对大鼠顶叶皮层神经元瞬时外向钾电流（IA）的影响,初步探讨其作用的通道动力学机制。方法：采用全细胞膜片钳技术,观察N／OFQ对急性分离的大鼠顶叶皮层神经元IA的作用。结果：①0．1μmol／L N／OFQ使IA幅值由给药前的（5356．1±361．6）pA下降为（4113．3±312．7）pA,抑制率为23．20％±2．17％（P〈0．01,n=10）。②0．1μmol／L N／OFQ使IA的电流-电压（I-V）曲线降低（P〈0．01,n=10）。③0．1μmol／L N／OFQ使,IA激活曲线的半数激活电压（V1／2）和斜率因子（κ）分别由给药前的（-9．2±2．5）mV和（20．4±2．3）mV变为给药后的（30．6±3．7）mV（P〈0．01,n=8）和（22．6±2．1）mV（P〉0．05,n=8）。④0．1μmol／L N／OFQ使IA失活曲线的半数失活电压（V1／2）和斜率因子（κ）分别由给药前的（-64．1±3．2）mV和（21．5±2．1）mV变为给药后的（-55．9±1．9）mV（P〈0．05,n=5）和（19．6±2．2）mV（P〉0．05,n=5）。结论：N／OFQ可抑制大鼠顶叶皮层神经元IA,使其激活曲线、失活曲线均右移。     

Sodium ions and GTP decrease the potency of [Nphe1]N/OFQ(1-13)NH2 in blocking nociceptin/orphanin FQ receptors coupled to cyclic AMP in N1E-115 neuroblastoma cells and rat olfactory bulb

The pseudopeptide [Nphe(1)]N/OFQ(1-13)NH(2) (Nphe) has been shown to act as a pure, selective and competitive antagonist of nociceptin/orphanin FQ (N/OFQ) receptors in different tissues. However, Nphe displayed a highly variable potency, with pA(2) values ranging from 5.96 to 8.45. In the present study, we show that sodium ions and GTP markedly affect the potency of Nphe in blocking N/OFQ receptors coupled to cyclic AMP inhibition in different cellular systems. In intact N1E-115 neuroblastoma cells, the pA(2) value of Nphe increased from 7.13 to 8.02 when the extracellular sodium concentration was reduced from 138 to 2.5 mM. When N/OFQ inhibition of adenylyl cyclase activity was assayed in cell membranes, 100 mM NaCl decreased the pK(i) value of Nphe from 8.38 to 7.32, but increased that of the nonpeptide N/OFQ receptor antagonist CompB from 8.61 to 8.92. Similar effects of sodium ions on the potencies of Nphe and CompB were observed when the compounds were used to antagonize the N/OFQ inhibition of adenylyl cyclase activity in membranes of the external plexiform layer of the rat olfactory bulb. In the same assay, the increase of GTP concentration from 0.1 to 200 micro M decreased Nphe potency by 8-fold. These data demonstrate that sodium ions and GTP affect the potency of Nphe in a manner similar to that of agonists but not of pure antagonists and suggest that these factors may contribute to the reported variability of Nphe affinity constant.     

Antinociceptive effects of spinally administered nociceptin/orphanin FQ and its N-terminal fragments on capsaicin-induced nociception

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand for the N/OFQ peptide (NOP) receptors, has been shown to be metabolized into some fragments. We examined to determine whether intrathecal (i.t.) N/OFQ (1-13), (1-11) and (1-7) have antinociceptive activity in the pain-related behavior after intraplantar injection of capsaicin. The i.t. administration of N/OFQ (0.3-1.2 nmol) produced an appreciable and dose-dependent inhibition of capsaicin-induced paw-licking/biting response. The N-terminal fragments of N/OFQ, (1-13) and (1-11), were antinociceptive with a potency lower than N/OFQ. Calculated ID₅₀ values (nmol, i.t.) were 0.83 for N/OFQ, 2.5 for N/OFQ (1-13) and 4.75 for N/OFQ (1-11), respectively. The time-course effect revealed that the antinociceptive effects of these N-terminal fragments lasted longer than those of N/OFQ. Removal of amino acids down to N/OFQ (1-7) led to be less potent than N/OFQ and its fragments, (1-13) and (1-11). Antinociception induced by N/OFQ or N/OFQ (1-13) was reversed significantly by i.t. co-injection of [Nphe¹]N/OFQ (1-13)NH₂, a peptidergic antagonist for NOP receptors, whereas i.t. injection of the antagonist did not interfere with the action of N/OFQ (1-11) and (1-7). Pretreatment with the opioid receptor antagonist naloxone hydrochloride did not affect the antinociception induced by N/OFQ and its N-terminal fragments. These results suggest that N-terminal fragments of N/OFQ are active metabolites and may modulate the antinociceptive effect of N/OFQ in the spinal cord. The results also indicate that N/OFQ (1-13) still possess antinociceptive activity through NOP receptors.     

Anxiolytic-like effects of nociceptin/orphanin FQ in the elevated plus maze and in the conditioned defensive burying test in rats

Different reports suggest that nociceptin/orphanin FQ (N/OFQ) may have either anxiolytic- or anxiogenic-like effect in rodents. Since N/OFQ elicits hypolocomotion, which undergoes rapid tolerance, and hypolocomotion may be associated to emotional consequences, the present study was designed to investigate the effect of N/OFQ on anxiety after development of tolerance to its hypolocomotor effect. The effect of single or double intracerebroventricular (i.c.v.) injection of N/OFQ was evaluated on anxiety-related behaviors in rats, in the elevated plus maze (EPM) and conditioned defensive burying (CDB) tests. After single administration, N/OFQ displayed an anxiogenic-like pattern of response on the elevated plus maze but hypolocomotion was also observed. Conversely, in the CDB test, N/OFQ induced a clear-cut anxiolytic pattern. To produce tolerance to N/OFQ-induced hypolocomotion the peptide was administered by two i.c.v. injections separated by 120 min; in these conditions it decreased the expression of anxiety-related behaviors in both tests without affecting locomotor activity. The nociceptin/orphanin FQ peptide (NOP) receptor antagonist UFP-101 significantly reduced the effects of N/OFQ to control values in either tests. Corticosterone levels were significantly increased after a single N/OFQ administration (not in a dose-dependent manner) but this increase did not reach significance after double administration (1 nmol/rat). Our results support the idea that N/OFQ may act as an anxiolytic-like agent in the rat; the apparent anxiogenic-like effect observed following its single administration in the EPM may be consequent to its effect on locomotion.     

Expression of nociceptin/OFQ receptor and prepro-nociceptin/OFQ in lymphoid tissues

This study was undertaken to examine the presence of functional nociceptin/orphanin FQ (N/OFQ) receptors in the immune system. Receptor mRNA signals were detected by RT-PCR in porcine thymus, lymph nodes, spleen and freshly-isolated splenocytes; the distribution of prepro-nociceptin/-orphanin FQ (PP-N/-OFQ) mRNA was similar, with the exception of lymph nodes. However, specific [³H]nociceptin binding sites were not detected in rat or porcine lymphoid tissues, and 0.1–100 nM nociceptin had no effect on forskolin-stimulated cyclic AMP concentrations in porcine splenocytes. Thus, it appears that nociceptin/orphanin FQ receptor mRNA, but not a functional receptor protein is expressed in the immune system.     

The effects of [Arg14, Lys15] nociceptin/orphanin FQ, a highly potent agonist of the NOP receptor, on in vitro and in vivo gastrointestinal functions

Nociceptin/orphanin FQ (N/OFQ) administered into the lateral left cerebral ventricle of rats has been reported to inhibit in vivo gut motor and secretory functions. Recently, a novel N/OFQ analog, [Arg14, Lys15] N/OFQ, was synthesized and demonstrated to behave as a highly potent agonist at the human recombinant N/OFQ peptide (NOP) receptors and to produce long-lasting effects in vivo in mice compared with the natural ligand N/OFQ. In the present study, the pharmacological profile of [Arg14, Lys15] N/OFQ was further evaluated and compared with that of N/OFQ in vitro on guinea pig exocrine pancreas and in vivo on gastric emptying, colonic propulsion and gastric acid secretion in rats. [Arg14, Lys15] N/OFQ and N/OFQ significantly decreased the KCl-evoked amylase secretion from isolated pancreatic lobules of the guinea pig. In in vivo experiments, [Arg14, Lys15] N/OFQ mimicked the effects of N/OFQ, inducing, after intracerebroventricular injection, a delay (up to 70%) in the gastric emptying of a phenol red meal, an increase (about 40 times) of the mean bead colonic expulsion time and a decrease (up to 90%) of gastric acid secretion in water loaded rats after 90 min pylorus ligature. In all these assays, [Arg14, Lys15] N/OFQ was more effective than N/OFQ, and its effective doses were at least 10-fold lower than N/OFQ effective doses. The highly selective NOP receptor antagonist, UFP-101, decreased the efficacy of [Arg14, Lys15] N/OFQ in in vitro and in vivo assays above reported. These findings: (a) show that pancreatic NOP receptors mediate an in vitro inhibitory effect on stimulated guinea pig amylase secretion; (b) confirm that the stimulation of central NOP receptors exerts an inhibitory control on gastric emptying, colonic motility and gastric secretion in rats and (c) put in evidence that [Arg14, Lys15] N/OFQ, being more potent and effective than the natural ligand N/OFQ, represents a new pharmacological tool for the study of the physiological and pharmacological roles mediated by the N/OFQ-NOP receptor system.     

Nociceptin/orphanin FQ and related peptides reduce the increase in plasma corticosterone elicited in mice by an intracerebroventricular injection

Nociceptin/orphanin FQ (=N/OFQ), the endogenous ligand of ORL1 receptor (=NOP), has been reported to induce, in rodents, after intracerebroventricular (i.c.v.) administration, anti-stress and anxiolytic effects. We have observed that the handling of mice followed by an i.c.v. injection of saline, induced a marked increase in the plasma corticosterone level (+250%) measured 30 minutes later. When N/OFQ was injected intracerebroventricularly, using a 1 microg dose, the increase in plasma corticosterone was significantly lower than in saline injected mice. N/OFQ(1-13)NH(2), known as a NOP receptor agonist, at the same 1 microg dose, also induced a lesser increase in plasma corticosterone level than a saline i.c.v. injection. The pseudopeptide [Phe(1)-psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)NH(2), defined either as an agonist or an antagonist of NOP receptor, at the 0.1 microg dose, behaved in a similar manner as N/OFQ, by decreasing the plasma corticosterone level. Finally, [Nphe(1)]N/OFQ(1-13)NH(2), although presumed to be a selective NOP receptor antagonist, also decreased the corticosterone level at the 0.1 microg dose. These observations suggest the implication of N/OFQ in the regulation of response to stress, through an action on the hypothalamo-pituitary-adrenocortical axis. Moreover, they evidence a similar effect of N/OFQ and N/OFQ(1-13)NH(2), but also of two other related peptides displaying antagonist properties on NOP receptors. These data suggest that several subtypes of N/OFQ receptors could exist.     

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

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

侧脑室及下丘脑视前区微量注射孤啡肽对大鼠血压、心率的影响

孤啡肽（OrphaninFQ,OFQ）是1995年底发现的内阿片肽,一级结构与强啡肽A很相似,但生物学作用与其它内阿片肽有所不同。本工作采用侧脑室及核团微量注射的方法,观察了中枢OFQ对大鼠心血管活动的影响。结果表明：侧脑室注射1、10μgONQ可明显降低大鼠平均动脉压（MAP）及心率（HR）;侧脑室预先注射4μg纳洛酮不影响1μgOWQ的降压及减慢心率的效应。下丘脑视前区（POA）微量注射1μgOFQ也可降低血压、减慢心率。结果表明,中枢OFQ与其它内阿片肽相似,可抑制心血管活动,且其抑制作用不是通过μ、δ、κ阿片受体介导。POA为中枢OFQ抑制心血管活动的靶区之一。     

The orphanin FQ/nociceptin gene: structure, tissue distribution of expression and functional implications obtained from knockout mice

Like other neuropeptides, orphanin FQ/nociceptin (OFQ/N) is encoded by a larger precursor protein. The cDNA for the OFQ/N precursor has been cloned from human, rat, mouse and bovine tissue demonstrating that this peptidergic system serves important functions that have been conserved during evolution. The structural organization of the precursor protein is similar to opioid peptide precursors, supporting the view of a common origin for the opioid systems and the OFQ/N system. In addition to OFQ/N, the precursor may encode two other biologically active peptides. Anatomic studies have revealed high levels of expression of the OFQ/N messenger RNA in brain structures involved in sensory, emotional and cognitive processing. In particular, high levels of OFQ/N mRNA were detected in the limbic system, underlining the stress attenuating activities that have been described as an important function of OFQ/N. Recently, mutant mice have been generated that lack the precursor protein of OFQ/N to further define the physiological functions of the OFQ/N system. The OFQ/N-deficient mice are characterized by an increased sensitivity to stressful stimuli and a lack of habituation to chronic and repeated stress. This review will summarize recent findings on the molecular biology of the OFQ/N precursor and relate it to possible physiological functions of this newly discovered neuropeptide system.     

UFP-101 antagonizes the spinal antinociceptive effects of nociceptin/orphanin FQ: behavioral and electrophysiological studies in mice

Nociceptin/orphanin FQ (N/OFQ) modulates various biological functions, including nociception, via selective stimulation of the N/OFQ peptide receptor (NOP). Here we used the NOP selective antagonist UFP-101 to characterize the receptor involved in the spinal antinociceptive effects of N/OFQ evaluated in the mouse tail withdrawal assay and to investigate the mechanism underlying this action by assessing excitatory postsynaptic currents (EPSC) in laminas I and II of the mouse spinal cord dorsal horn with patch-clamp techniques. Intrathecal (i.t.) injection of N/OFQ in the range of 0.1-10 nmol produced a dose dependent antinociceptive effect, which was prevented by UFP-101, but not by naloxone. In contrast the antinociceptive effect of the mu-opioid peptide receptor agonist endomorphin-1 was blocked by naloxone but not by UFP-101. Moreover, N/OFQ and endomorphin-1 induced a significant antinociceptive effect in wild type mice while in mice knockout for the NOP receptor gene only endomorphin-1 was found to be active. In mouse spinal cord slices 1 microM N/OFQ reduced EPSC to 60+/-4% of control values. This inhibitory effect was reversed in a concentration dependent manner by UFP-101 (pA2 value 6.44). The present results demonstrate that N/OFQ-induced spinal antinociception in vivo and inhibition of spinal excitatory transmission in vitro are mediated by receptors of the NOP type.