Endogenous orphanin FQ: evidence for a role in the modulation of electroacupuncture analgesia and the development of tolerance to analgesia produced by morphine and electroacupuncture

1. Our previous work has demonstrated that exogenously administered orphanin FQ (OFQ) antagonizes morphine analgesia and electroacupuncture analgesia (EAA) in the brain and potentiates morphine analgesia and EAA in the spinal cord of the rat. In the present study we evaluated the role of endogenously released OFQ in the development of tolerance to morphine and electroacupuncture (EA) and the analgesia produced by electroacupuncture, by use of the IgG fraction of an anti-OFQ antibody (OFQ-Ab) microinjected into the rat central nervous system (CNS).
2. EAA was produced by stimulating rats at a frequency of 100 Hz. Rats were classified as either high responders (HR) or low responders (LR) based on the analgesic effects of EA. LRs could be converted into HRs by the intracerebroventricular (i.c.v.) microinjection of OFQ-Ab at both 1 : 1 and 1 : 10 dilutions but not 1 : 100. HRs could be changed into LRs by the intrathecal (i.t.) injection of OFQ-Ab at both 1 : 1 and 1 : 10 dilutions, but not 1 : 100.
3. Acute morphine tolerance was induced in rats by repeated subcutaneous (s.c.) injections of morphine (5 mg  kg, every 2 h) for 16 h. When injected i.c.v. the OFQ-Ab (1 : 1 dilution) had no effect on the development of acute morphine tolerance.
4. Chronic morphine tolerance was produced in rats by repeated injection of morphine (5–60 mg  kg, s.c., 3× a day) for 6 days. I.c.v. injection of OFQ-Ab (1 : 1 dilution) reversed this type of morphine tolerance in rats by 50% (P<0.01).
5. Acute tolerance to the analgesia produced by EA developed after 6 h of continuous (100 Hz, 3mA) stimulation. This tolerance was almost completely reversed by the i.c.v. injection of OFQ-Ab (1 : 1 dilution) (P<0.05).
6. Chronic tolerance to the analgesic effect of EA was produced by repeatedly administering increasing current (1, 2 and 3 mA, each lasting for 10 min, for a total of 30 min) at a frequency of 100 Hz once a day for 6 days. I.c.v. injection of OFQ-Ab (1 : 1 dilution) reversed this kind of tolerance by 50% (P<0.01).
7. Together these results suggest that 100 Hz EA may enhance the release of endogenous OFQ in the CNS of the rat, which in turn may act to antagonize EA-produced analgesia in the brain but potentiate EA produced analgesia in the spinal cord. Therefore, OFQ appears to play an important role in the development of tolerance to the analgesic effects produced by EA.
8. The mechanisms underlying the development of acute morphine tolerance and chronic morphine tolerance appear to be different. Central OFQ may play an important role in the development of tolerance after chronic morphine administration.

     

Antagonistic effect of orphanin FQ on opioid analgesia in rat

目的：研究孤啡肽（ＯＦＱ）对痛与阿片镇痛的影响．方法：脑室（ｉｃｖ）与鞘内（ｉｔｈ）给药，以大鼠甩尾模型测痛．结果：小剂量ＯＦＱ（０１μｇ）ｉｃｖ及ｉｔｈ给药对痛反应均无影响；较大剂量ＯＦＱ（０５－１０μｇ）可使痛反应增强．ＯＦＱ１－１０（ＯＦＱ的一个片段）ｉｃｖ对痛反应无影响．μ受体激动剂芬太尼（１μｇ）、δ激动剂ＤＳＬＥＴ（５μｇ）ｉｃｖ或ｉｔｈ给药，以及κ激动剂Ｕ５０４８８Ｈ（１μｇ）ｉｔｈ给药，可使痛阈明显增加．０１μｇ或１μｇＯＦＱ与上述药物合用后，痛阈增加明显减少（除鞘内与ＤＳＬＥＴ合用外）．结论：ＯＦＱ可增强大鼠的痛反应，在脑内对抗由μ和δ受体介导的阿片镇痛，在脊髓对抗由κ和μ但不是由δ受体介导的镇痛．     

Nociceptin/orphanin FQ peptide receptor antagonist JTC-801 reverses pain and anxiety symptoms in a rat model of post-traumatic stress disorder

BACKGROUND AND PURPOSE

Single-prolonged stress (SPS), a rat model of post-traumatic stress disorder (PTSD), also induces long-lasting hyperalgesia associated with hypocortisolism and elevated nociceptin/orphanin FQ (N/OFQ) levels in serum and CSF. Here, we determined the effect of JTC-801 (N-(4-amino-2-methylquinolin-6-yl)-2-(4-ethylphenoxymethyl) benzamide monohydrochloride), a nociceptin/orphanin FQ peptide (NOP) receptor antagonist, on symptoms of pain and anxiety in rats after SPS exposure, and examined N/OFQ-NOP receptor system changes.

EXPERIMENTAL APPROACH

Male Sprague Dawley rats received JTC-801 (6 mg kg⁻¹ i.p., once daily) during days 7–21 of SPS. The ability of JTC-801 to inhibit N/OFQ-stimulated [³⁵S]-GTPγS binding was confirmed in rat brain membranes. Anxiety-like behaviour and pain sensitivity were monitored by changes in elevated plus maze performance and withdrawal responses to thermal and mechanical stimuli. Serum corticosterone and N/OFQ content in CSF, serum and brain tissues were determined by radioimmunoassay; NOP receptor protein and gene expression in amygdala, hippocampus and periaqueductal grey (PAG) were examined by immunoblotting and real-time PCR respectively.

KEY RESULTS

JTC-801 treatment reversed SPS-induced mechanical allodynia, thermal hyperalgesia, anxiety-like behaviour and hypocortisolism. Elevated N/OFQ levels in serum, CSF, PAG and hippocampus at day 21 of SPS were blocked by JTC-801; daily JTC-801 treatment also reversed NOP receptor protein and mRNA up-regulation in amygdala and PAG.

CONCLUSION AND IMPLICATIONS

JTC-801 reversed SPS-induced anxiety- and pain-like behaviours, and NOP receptor system up-regulation. These findings suggest that N/OFQ plays an important role in hyperalgesia and allodynia maintenance after SPS. NOP receptor antagonists may provide effective treatment for co-morbid PTSD and pain.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2     

Involvement of the GABA/benzodiazepine receptor in the axiolytic-like effect of nociceptin/orphanin FQ

We investigated the mechanism underlying the anxiolytic actions of the neuropeptide nociceptin/orphanin FQ (N/OFQ) with an elevated plus-maze test. In mice, intracerebroventricular (i.c.v.) infusions of N/OFQ (0.1 and 0.32 nmol) led to an increase in time spent in the open arms (anxiolytic-like effects). A non-peptidyl N/OFQ receptor (NOP) antagonist, J-113397(1-{(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl}-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one), (1.0 and 3.2 mg/kg, s.c.) blocked the increase induced by N/OFQ. On the other hand, a benzodiazepine receptor antagonist, flumazenil, (10 mg/kg, i.p.) and a GABAA receptor antagonist, (+)-bicuculline, (5.6 mg/kg, i.p.) also inhibited the increase induced by N/OFQ. In rats, microinfusions of N/OFQ (10 and 32 pmol) into the amygdala led to an increase in time spent in the open arms. However, intracranial infusions of N/OFQ (10-100 pmol) into the dorsal hippocampus did not affect the time spent in the open arms. These findings suggest that the anxiolytic-like effects of N/OFQ may be related to the GABA/benzodiazepine system in the amygdala.     

孤啡肽对HL-60细胞增殖及凋亡的影响

目的研究孤啡肽对白血病细胞株HL-60细胞增殖及凋亡的影响。方法选择人急性早幼粒细胞白血病细胞株HL-60为实验模型,应用MTT比色法、形态学观察及流式细胞术(FCM)等方法对孤啡肽诱导其凋亡情况进行研究。结果孤啡肽能显著抑制HL-60细胞增殖,使细胞周期阻滞于G_0/G_1期,细胞凋亡率增加,FCM DNA直方图上G_0/G_1峰前有明显的亚二倍体凋亡峰,表明孤啡肽能明显抑制HL-60细胞的生长,诱导其发生凋亡,且存在时间依赖性,而剂量依赖性不明显。结论孤啡肽对HL-60细胞有增殖抑制和诱导凋亡作用。     

Sympathoinhibitory action of nociceptin in the rat spinal cord

1. Whole-cell patch recordings were made from antidromically identified sympathetic preganglionic neurons (SPN) of immature rat spinal cord slices. Bath application of nociceptin (0.1-1 micromol/L) suppressed excitatory postsynaptic potentials (EPSP) and hyperpolarized a population of SPN; these effects were naloxone (1 micromol/L) insensitive. 2. Nociceptin suppressed the amplitude of EPSP without causing a concomitant change in glutamate-induced depolarizations, suggesting a presynaptic inhibitory action. 3. Analysis of current-voltage relationships showed that nociceptin hyperpolarized SPN by increasing an inwardly rectifying K+ current. 4. Intrathecal injection of nociceptin (3, 10 and 30 nmol) to urethane-anaesthetized rats dose-dependently reduced the mean arterial pressure and heart rate; these effects were not prevented by prior intravenous injection of naloxone (1 mg/kg). 5. Results from our in vitro and in vivo experiments suggest that nociceptin suppresses spinal sympathetic outflow either by attenuating excitatory synaptic responses or hyperpolarizing SPN.     

Nonpeptide/peptide chimeric ligands for the nociceptin/orphanin FQ receptor: design,synthesis and in vitro pharmacological activity

Abstract: Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the G‐protein coupled receptor referred to as N/OFQ peptide (NOP) receptor. NOP receptor activation by N/OFQ modulates several biological functions both at central and peripheral level. Structure activity relationship (SAR) studies demonstrated that the N/OFQ sequence can be divided into a N‐terminal tetrapeptide ‘message’ crucial for receptor activation and a C‐terminal ‘address’ important for receptor binding. On the basis of this message/address concept we synthesized some chimeric compounds in which we substituted the natural message domain with the nonselective nonpeptide NOP ligand (8‐Naphthalen‐1‐yl‐methyl‐4‐oxo‐1‐phenyl‐1,3,8‐triaza‐spiro[4,5]dec‐3‐yl)‐aceticacid methyl ester (NNC 63‐0532) and used as address domain the peptide sequences Thr‐NH₂, N/OFQ(5‐9)‐NH₂, N/OFQ(5‐13)‐NH₂ and N/OFQ(5‐17)‐NH₂. All the compounds were pharmacologically evaluated in the electrically stimulated guinea‐pig ileum. NNC 63‐0532 produced a concentration‐dependent inhibition of the electrically induced twitches showing, in comparison with N/OFQ, lower potency and higher maximal effects. In addition, contrary to N/OFQ, the effects of NNC 63‐0532 were insensitive to the NOP selective antagonist [Nphe¹, Arg¹⁴, Lys¹⁵]N/OFQ‐NH₂ (UFP‐101) while prevented by naloxone. Similar results were obtained with NNC 63‐0532/Thr‐NH₂ and NNC 63‐0532/N/OFQ(1‐9)‐NH₂. On the contrary, the inhibitory effects of NNC 63‐0532/N/OFQ(5‐13)‐NH₂ and NNC 63‐0532/N/OFQ(5‐17)‐NH₂ were slightly antagonized by UFP‐101 while naloxone prevented the effects of the high but not of the low concentrations of the two ligands. These data indicate that it is possible to functionalize with the N/OFQ address sequence a nonpeptide NOP ligand for increasing its binding to the NOP receptor. Moreover, these results corroborate the idea that the 5–13 sequence represents the crucial core of the N/OFQ address domain.     

Signal pathways coupled to activation of neuronal nitric oxide synthase in the spinal cord by nociceptin/orphanin FQ

Nociceptin/orphanin FQ (N/OFQ) was earlier shown to be involved in the maintenance of neuropathic pain by activating neuronal nitric oxide synthase (nNOS). We recently established an ex vivo system to elucidate biochemical and molecular mechanisms for nNOS activation by the use of a combination of isolated intact spinal cord preparations and NADPH-diaphorase histochemistry. Here we examined the N/OFQ signal pathways coupled to nNOS activation in the spinal cord by using this ex vivo system. N/OFQ enhanced nNOS activity in the superficial layer of the spinal cord, as assessed by NADPH-diaphorase histochemistry, in a time- and dose-dependent manner. The maximum effect was observed at 3-10 nM. The N/OFQ-stimulated nNOS activity was inhibited by NMDA receptor antagonists MK-801 and D-AP5, but not by the NR2B-selective antagonist CP-101,606; and the stimulated activity was observed in NR2D(-/-) mice, but not in NR2A(-/-) or NR2A(-/-)/NR2D(-/-) mice. N/OFQ receptor antagonists attenuated the nNOS activity stimulated by N/OFQ, but not that by NMDA. Furthermore, the potentiation of nNOS by N/OFQ was inhibited by calphostin C and Ro 31-8220, PP2, and KN-62, but not by H-89. These results suggest that N/OFQ stimulated nNOS activity by a biochemical cascade initiated by activation of NMDA receptors containing NR2A.     

Reversal of stress- and CRF-induced anorexia in rats by the synthetic nociceptin/orphanin FQ receptor agonist, Ro 64-6198

RATIONALE: (1S,3aS)-8-(2,3,3,4,5,6-hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triazaspiro[4.5]decan-4-one (Ro 64-6198), a non-peptidic agonist for the opioid receptor-like1 (ORL1) receptor, exhibits anxiolytic properties in stressful conditions. OBJECTIVE: The present study was aimed at evaluating whether activation of ORL1 receptors by Ro 64-6198 may reverse the anorectic effect of restraint stress or intracerebroventricular (ICV) CRF injection. METHODS: In body restraint experiments, 20-h food deprived rats were treated with intraperitoneal (IP) injection of Ro 64-6198 or vehicle. Ten minutes later, they were confined in cylindrical Plexiglas tubes for 60 min and then returned to their cage with food. In CRF experiments, 20-h food deprived rats were IP injected with Ro 64-6198 or vehicle. Ten minutes later, they received ICV CRF, 200 ng/rat or vehicle; food was offered after 20 min. RESULTS: Intraperitoneal (IP) pretreatment with Ro 64-6198 reversed the hypophagic effect induced by both restraint or CRF; the effect was statistically significant at the three doses tested (0.3, 1.0 or 2.5 mg/kg). ICV administration of the selective ORL1 receptor antagonist [Nphe(1)]NC(1-13)NH(2)(two injections of 33 or 66 microg/rat) abolished the effect of Ro 64-6198 on CRF-induced anorexia. In freely feeding rats, Ro 64-6198 significantly increased feeding at 2.5, but not at 0.3 or 1.0 mg/kg; in food deprived rats, Ro 64-6198 (0.3 or 1.0 mg/kg) did not modify food intake. Thus, reversal of stress- and CRF-induced anorexia by Ro 64-6198 can be evoked at doses lower than those that are hyperphagic. Ro 64-6198 (1 or 2.5 mg/kg) did not modify the anorectic effect of E. coli lipopolysaccharide, suggesting that its effect is selective for stress- or CRF-induced anorexia. Lastly, the benzodiazepine diazepam was unable to reduce the anorectic effect of CRF at the anxiolytic dose of 0.3 mg/kg, and partially reduced it at the hyperphagic dose of 1 mg/kg. CONCLUSIONS: The results of this study show that the non-peptidic ORL1 receptor agonist Ro 64-6198 markedly and selectively inhibits the anorectic effect of stress and CRF, and provide evidence that this effect is mediated by ORL1 receptors. Thus, Ro 64-6198 may represent an interesting tool for treatment of stress-induced anorexia.     

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

The effects of nociceptin/orphanin FQ on putative serotonin (5HT) neurons of the dorsal raphe nucleus (DRN), known to modulate the behavioral responses to stress, were investigated in vivo and in vitro. In DRN slices from unstressed rats, nociceptin/orphanin FQ concentration-dependently inhibited the firing rate of putative 5HT neurons (EC₅₀ = 21.6 ± 1.21 nM) and the selective NOP receptor antagonist UFP-101 shifted the concentration-response curve to the right (estimated pA₂ 6.86). Nociceptin/orphanin FQ potency was enhanced in slices prepared from rats previously subjected to a 15 min swim stress (EC₅₀ = 1.98 ± 0.11 nM). Swim stress did not change the number or affinity of NOP receptors in DRN. Stress-elicited potentiation involved corticotropin-releasing factor (CRF)₁ receptors, GABA signaling and protein synthesis, being attenuated by pre-treatment with antalarmin (20 mg/kg, i.p.), diazepam (2.4 mg/kg, i.p.) and cycloheximide (2.5 mg/kg, i.p.), respectively. In anesthetized unstressed rats, locally applied nociceptin/orphanin FQ (0.03 and 0.1 ng/30 nl) inhibited the firing rate of DRN neurons (to 80 ± 7 and 54 ± 10% of baseline, respectively). Nociceptin/orphanin FQ inhibition was potentiated both 24 h after swim stress and 1 h after CRF (30 ng/30 nl intra-DRN). Stress-induced potentiation was prevented by the selective CRF₁ receptor antagonist, NBI 30755 (20 mg/kg, i.p.). In contrast, the inhibitory response of DRN neurons to the 5HT_1A agonist, 8OH-DPAT (1μg/1 μl, intra-DRN) was not potentiated by swim stress, ruling out a non-specific enhanced permeability of GIRK channel. Together, these findings suggest that CRF and the nociceptin/orphanin FQ/NOP system interact in the DRN during stress to control 5HT transmission; this may play a role in stress-related neuropsychopathologies.     

Single housing during early adolescence causes time-, area- and peptide-specific alterations in endogenous opioids of rat brain

BACKGROUND AND PURPOSE

A number of experimental procedures require single housing to assess individual behaviour and physiological responses to pharmacological treatments. The endogenous opioids are closely linked to social interaction, especially early in life, and disturbance in the social environment may affect opioid peptides and thereby confound experimental outcome. The aim of the present study was to examine time-dependent effects of single housing on opioid peptides in rats.

EXPERIMENTAL APPROACH

Early adolescent Sprague Dawley rats (post-natal day 22) were subjected to either prolonged (7 days) or short (30 min) single housing. Several brain regions were dissected and immunoreactive levels of Met-enkephalin-Arg⁶Phe⁷ (MEAP), dynorphin B and nociception/orphanin FQ, as well as serum corticosterone were measured using RIA.

KEY RESULTS

Prolonged single housing reduced immunoreactive MEAP in hypothalamus, cortical regions, amygdala, substantia nigra and periaqueductal grey. Short single housing resulted in an acute stress response as indicated by high levels of corticosterone, accompanied by elevated immunoreactive nociceptin/orphanin FQ in medial prefrontal cortex, nucleus accumbens and amygdala. Neither short nor prolonged single housing affected dynorphin B.

CONCLUSIONS AND IMPLICATIONS

Disruption in social environmental conditions of rats, through single housing during early adolescence, resulted in time-, area- and peptide-specific alterations in endogenous opioids in the brain. These results provide further evidence for an association between early life social environment and opioids. Furthermore, the results have implications for experimental design; in any pharmacological study involving opioid peptides, it is important to distinguish between effects induced by housing and treatment.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2     

Anti-nociceptive responses produced by human putative counterpart of nocistatin

b-nocistatin is a heptadecapeptide produced from bovine prepronociceptin and blocks the induction of hyperalgesia and touch-evoked pain (allodynia) by intrathecal administration of nociceptin or prostaglandin E₂ (PGE₂). Human prepronociceptin may generate a 30-amino acid peptide different in length from b-nocistatin. Here, we examine whether the human putative counterpart of nocistatin (h-nocistatin) possessed the same biological activities as b-nocistatin. Simultaneous intrathecal injection of h-nocistatin in mice blocked the induction of allodynia by nociceptin and PGE₂ in a dose-dependent manner with ID₅₀ values of 329 pg kg⁻¹ and 16.6 ng kg⁻¹, respectively. h-nocistatin was about 10 times less potent than b-nocistatin. h-nocistatin also attenuated the nociceptin- and PGE₂-induced hyperalgesia. These results demonstrate that h-nocistatin is biologically active and may be involved in the processing of pain at the spinal level in humans.     

Effects of nociceptin/orphanin FQ and endomorphin-1 on glutamate and GABA release, intracellular [Ca2+] and cell excitability in primary cultures of rat cortical neurons

The effects of nociceptin/orphanin FQ (N/OFQ) and endomorphin-1 (EM-1) on glutamate and GABA release, intracellular calcium, neuronal excitability and glutamate current were investigated in rat primary cortical neuronal cultures. Through their specific receptors N/OFQ and EM-1 (0.02-1 microM) inhibited the electrically evoked outflow of [3H]D-aspartate at most to -50% and that of [3H]GABA to -30%. In addition, at 1 microM, both peptides induced a decrease of the firing rate caused by electrical depolarization. N/OFQ 1-10 microM did not influence either the electrically evoked calcium influx or the glutamate-evoked currents, whereas EM-1 1 microM significantly inhibited them. Thus, in cortical neurons in culture, both N/OFQ and EM-1 inhibited the secretory process and neuronal excitability but EM-1 also affected calcium influx and cell body responsiveness to glutamate. Consequently, EM-1 appeared to dampen this excitatory signal more then N/OFQ did.     

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

Rationale Nociceptin/orphanin FQ (N/OFQ) has been proposed to be a functional antagonist of corticotropin-releasing factor (CRF) in relation to its anti-stress action and its ability to antagonize the anorectic effect of CRF in rats without exhibiting affinity for CRF receptors. The bed nucleus of the stria terminalis (BST) is highly sensitive to the inhibitory effect of N/OFQ on CRF-induced anorexia. Objective The present study was aimed at further evaluating the role of the BST in the functional antagonism between N/OFQ and CRF by examining it at molecular level and in the context of CRF-induced anxiety in the rat. Materials and methods First, in situ hybridization experiments investigated the expression of the pro-N/OFQ precursor and of NOP receptors in several brain areas 6 h after injection of CRF (0.2 and 1 μg/rat) into the lateral cerebroventricle (LV). Second, the elevated plus maze test was used to evaluate whether N/OFQ, injected into the BST (0.05 and 0.5 μg/rat) or into the LV (0.5, 1.8, and 2.4 μg/rat), inhibits the anxiogenic-like effect evoked by LV injection of CRF (1 μg/rat) in rats. Results The in situ hybridization study showed that LV injection of CRF 1 μg/rat increases NOP receptor expression in the BST, while no change of the N/OFQ precursor was observed. On the other hand, N/OFQ injection into the BST blocks the anxiogenic effect of CRF at doses lower than those required by LV injection (0.5 vs 1.8 μg/rat, respectively). Conclusion These data provide further support for the hypothesis that N/OFQ may behave as functional antagonist of CRF and suggest that this antagonism may occur within the BST.     

The ORL-1 (NOP1) receptor ligand nociceptin/orphanin FQ (N/OFQ) inhibits neurogenic dural vasodilatation in the rat

The effects of the ORL-1 (NOP(1)) receptor ligand nociceptin (N/OFQ) and the nociceptin antagonists [Nphe(1)]N/OFQ-(1-13)-NH(2) (Nphe) and nocistatin (NST) on neurogenic dural vasodilatation (NDV) in the rat dura mater evoked by electrical stimulation of a closed cranial window were studied. The middle meningeal artery was visualised using intravital microscopy, and the vessel diameter analysed using a video dimension analyser. N/OFQ (1, 10, 100 nmol kg(-1); i.v., n=10) significantly and dose-dependently suppressed NDV maximally by 65% (P<0.01). Neither Nphe (100 nmol kg(-1); n=5) nor NST (100 nmol kg(-1); n=4) alone had an effect on NDV (P>0.05). Baseline vessel diameter was not significantly affected by application of N/OFQ, NST or Nphe. Application of the selective N/OFQ antagonist Nphe (10, 100 nmol kg(-1) i.v., n=8) dose-dependently and significantly (P<0.01) reversed the inhibition of NDV induced by application of N/OFQ (10 nmol kg(-1)). NST (10, 100 nmol kg(-1); n=7) failed to reverse the effects elicited by N/OFQ. Application of N/OFQ elicited a dose-dependent transient decrease in arterial blood pressure (P<0.01). Nphe dose-dependently reversed the cardiovascular effects induced by application of N/OFQ (10 nmol kg(-1)) (P<0.01),while NST did not alter the blood pressure reaction elicited by N/OFQ. The results show that N/OFQ inhibits NDV, an effect which is antagonised by Nphe, but not by NST. ORL-1 (NOP(1)) receptors located on trigeminal sensory fibres may be involved in the regulation of dural vessel diameter and hence may play a role in migraine pathophysiology.     

Nphe(1)]N/OFQ-(1-13)-NH(2) is a competitive and selective antagonist at nociceptin/orphanin FQ receptors mediating K(+) channel activation in rat periaqueductal gray slices.

A novel member of the opioid related receptor family, the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor was identified and demonstrated to be involved in many physiological functions including pain regulation. [Nphe(1)]N/OFQ-(1-13)-NH(2) (Nphe) is a novel peptide antagonist of NOP receptors, developed using peripheral preparations. We have quantitatively investigated the interaction of Nphe with N/OFQ, the endogenous ligand of NOP receptors, in the midbrain ventrolateral periaqueductal gray (PAG), a crucial brain region for N/OFQ-induced reversal of opioid analgesia, using the patch-clamp recording technique in brain slices. N/OFQ concentration-dependently activated an inwardly rectifying K(+) current in response to hyperpolarization ramps from -60 to -140 mV. Nphe concentration-dependently attenuated the K(+) current activated by N/OFQ without changing its reversal potential. The presence of Nphe right-shifted the concentration-response curve to N/OFQ in a parallel manner. The Schild plot analysis yielded a slope of 1.16 and a pA(2) value of 6.64 that is similar to those obtained in peripheral preparations. At concentrations up to 3 microM, Nphe affected neither the membrane current per se, nor the inwardly rectifying K(+) current activated by [D-Ala(2), N-Me-Phe(4),Gly-ol(5)]-enkephalin or baclofen, a mu-opioid and GABA(B) receptor agonist, respectively. It is concluded that Nphe acts as a pure, selective and competitive antagonist at native NOP receptors of ventrolateral PAG neurons.     

Nociceptin/orphanin FQ modulates the cardiovascular, but not renal, responses to stress in spontaneously hypertensive rats

1. The central administration of the endogenous opioid-like peptide nociceptin/orphanin FQ (N/OFQ) produces marked cardiovascular depressor and renal sympathoinhibitory responses in conscious animals. These findings are evidence that central N/OFQ may modulate the cardiovascular and renal responses to acute environmental stress. 2. The changes in cardiovascular and renal function produced by intracerebroventricular (i.c.v.) N/OFQ were measured in conscious spontaneously hypertensive rats (SHR) under basal conditions and during the acute environmental stimulus of air jet stress. 3. In SHR, central N/OFQ produced profound hypotensive, bradycardic, renal sympathoinhibitory (delayed) and water-diuretic effects by a pathway that does not involve activation of central alpha2-adrenoceptors or classical opioid receptors. 4. Intracerebroventricular injection of N/OFQ prevented the pressor response and blunted the tachycardia to air jet stress. A similar renal sympathoexcitatory and antinatriuretic response was observed in conscious SHR during air stress, before and after i.c.v. N/OFQ. 5. These findings are evidence that, in conscious SHR, i.c.v. N/OFQ selectively inhibited the neural responses to air jet stress by attenuating sympathetic outflow to the heart and, potentially, vasculature, but not to the kidneys. Central endogenous N/OFQ systems may be activated and contribute to regional changes in sympathetic outflow during acute stress.     

Antidepressant-like effects of the nociceptin/orphanin FQ receptor antagonist UFP-101: new evidence from rats and mice

Receptor antagonist and knockout studies have demonstrated that blockade of signalling via nociceptin/orphanin FQ (N/OFQ) and its receptor (NOP) has antidepressant-like effects in mice submitted to the forced swimming test (FST). The aim of the present study was to explore further the antidepressant-like properties of the NOP antagonist UFP-101 in different species (mouse and rat) and using different assays [FST and tail suspension test (TST)], and to investigate the mechanism(s) involved in its actions.UFP-101 (10 nmol i.c.v.) reduced immobility time of Swiss mice in the TST (mean±SEM) from 179±11 to 111±10 s. N/OFQ (1 nmol i.c.v.) was without effect per se, but fully prevented the effect of UFP-101. The spontaneous immobility time of NOP^–/– CD1-C57BL/6J-129 mice in the TST was much lower than that of wild-type (NOP^+/+) littermates (75±11 vs. 144±17 s) or of Swiss mice. UFP-101 (10 nmol i.c.v.) decreased immobility time (–65%) and increased climbing time (71%) in rats submitted to the FST. In rat brain slices, N/OFQ (100 nM) triggered robust K⁺-dependent hyperpolarizing currents in locus coeruleus and dorsal raphe neurons. UFP-101 (3 µM) fully prevented N/OFQ-induced currents, but was inactive per se. Fluoxetine, desipramine (both 30 mg/kg i.p.) and UFP-101 (10 nmol i.c.v.) reduced immobility time of mice in the FST. The serotonin synthesis inhibitor p-chlorophenylalanine methylester (PCPA, 4×100 mg/kg per day i.p.) prevented the antidepressant-like effects of fluoxetine and UFP-101 (but not desipramine), whereas N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4, neurotoxic for noradrenergic neurons; 50 mg/kg i.p., 7 days beforehand), suppressed only the effect of desipramine. Neither pretreatment affected spontaneous immobility time per se.Thus, UFP-101 exhibits pronounced antidepressant-like effects in different species and animal models, possibly by preventing the inhibitory effects of endogenous N/OFQ on brain monoaminergic (in particular serotonergic) neurotransmission. Participation of the N/OFQ-NOP receptor system in mood modulation sets new potential targets for antidepressant drug development.     

Binding of the novel radioligand [3H]UFP-101 to recombinant human and native rat nociceptin/orphanin FQ receptors

Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the N/OFQ peptide receptor (NOP). Binding studies have relied on [leucyl-(3)H]N/OFQ, but as this is an agonist G-protein coupling will affect affinity. In this paper, we describe a new [(3)H]labeled NOP antagonist, [Nphe(1),4'-(3)H-Phe(4),Arg(14),Lys(15)]N/OFQ-NH(2) ([(3)H]UFP-101). We have characterized [(3)H]UFP-101 at recombinant human NOP expressed in Chinese hamster ovary cells (CHO(hNOP)) and native rat NOP in cerebrocortex. Radioligand saturation and competition studies were performed on membranes, and [(3)H]UFP-101 (antagonist) was compared with [(3)H]N/OFQ (agonist). The effects of GTPgammaS (10 microM) and Na(+) were investigated alone and in combination in competition experiments with both radioligands. In CHO(hNOP), B (max), and pK (D), values were 561 and 580 fmol mg protein(-1) and 9.97 and 10.19 for [(3)H]UFP-101 and [leucyl-(3)H]N/OFQ, respectively. In rat cerebrocortex B (max) and pK (D), values were 65 and 88 fmol mg protein(-1) and 10.12 and 10.34 for [(3)H]UFP-101 and [leucyl-(3)H]N/OFQ. The binding of both radioligands was displaced by a range of peptide and non-peptide NOP ligands at both isoforms with good correlation (r (2) 0.92 in Rat and 0.97 in CHO(hNOP)). Naloxone was inactive. The binding of both radioligands was Na(+)-dependent with [(3)H]UFP-101 being more sensitive (IC(50) approximately20 mM). Unlike the agonist [leucyl-(3)H]N/OFQ, the antagonist [(3)H]UFP-101 was unaffected by GTPgammaS. [(3)H]UFP-101 binds to human and rat NOP with high affinity and good agreement with standard [leucyl-(3)H]N/OFQ in competition experiments. In addition, the binding of [(3)H]UFP-101 is unaffected by GTPgammaS. This radioligand will be useful to further characterize NOP in a range of binding paradigms.     

Nociceptin receptor binding in mouse forebrain membranes: thermodynamic characteristics and structure activity relationships

1. The present study describes the labelling of the nociceptin (NC) receptor, ORL₁, in mouse forebrain membranes with a new ligand partially protected from metabolic degradation at the C-terminal; the ligand, [³H]-NC-NH₂, has a specific activity of 24.5 Ci mmol⁻¹
2. Saturation experiments revealed a single class of binding sites with a K_D value of 0.55 nM and B_max of 94 fmol mg⁻¹ of protein. Non specific binding was 30% of total binding. Kinetic binding studies yielded the following rate constants: K_obs=0.104 min⁻¹; K₁=0.034 min⁻¹; T_1/2=20 min; K₊₁=0.07 min nM⁻¹.
3. Thermodynamic analyses indicated that [³H]-NC-NH₂ binding to the mouse ORL₁ is totally entropy driven, similar to what has been observed for the labelled agonists to the opioid receptors OP1(δ), OP2(κ) and OP3(μ).
4. Receptor affinities of several NC fragments and analogues, including the newly discovered ORL-1 receptor antagonist [Phe¹ψ(CH₂-NH)Gly²]NC(1–13)-NH₂ ([F/G]NC(1–13)-NH₂), were also evaluated in displacement experiments. The competition curves for these compounds were found to be parallel to that of NC and the following order of potency was determined for NC fragments: NC-OH=NC-NH₂=NC(1–13)-NH₂ >> NC(1–12)-NH₂ > NC(1–13)-OH >> NC(1–11)-NH₂, and for NC and NC(1–13)-NH₂ analogues: [Tyr¹]NC-NH₂ ⩾ [Leu¹]NC(1–13)-NH₂ ⩾ [Tyr¹]NC(1–13)-NH₂ ⩾ [F/G]NC(1–13)-NH₂ >> [Phe³]NC(1–13)-NH₂ > [DF/G]NC(1–13)-NH².
5. Standard opioid receptor ligands (either agonists or antagonists) were unable to displace [³H]-NC-NH₂ binding when applied at concentrations up to 10 μM indicating that this new radioligand interacts with a non opioid site, probably the ORL₁ receptor.