Nociceptin Research Articles

Nocistatin, a peptide that blocks nociceptin action in pain transmission.

Prolonged tissue damage or injury often leads to chronic pain states such that noxious stimuli evoke hyperalgesia and innocuous tactile stimuli evoke pain (allodynia). The neuropeptide nociceptin, also known as orphanin FQ, is an endogenous ligand for the orphan opioid-like receptor which induces both hyperalgesia and allodynia when administered by injection through the theca of the spinal cord into the subarachnoid space (that is, intrathecally). Here we show that the nociceptin precursor contains another biologically active peptide which we call nocistatin. Nocistatin blocks nociceptin-induced allodynia and hyperalgesia, and attenuates pain evoked by prostaglandin E2. It is the carboxy-terminal hexapeptide of nocistatin (Glu-Gln-Lys-Gln-Leu-Gln), which is conserved in bovine, human and murine species, that possesses allodynia-blocking activity. We have also isolated endogenous nocistatin from bovine brain. Furthermore, intrathecal pretreatment with anti-nocistatin antibody decreases the threshold for nociceptin-induced allodynia. Although nocistatin does not bind to the nociceptin receptor, it binds to the membrane of mouse brain and of spinal cord with high affinity. Our results show that nocistatin is a new biologically active peptide produced from the same precursor as nociceptin and indicate that these two peptides may play opposite roles in pain transmission.

Endomorphin 1 and 2, endogenous μ-opioid agonists, decrease systemic arterial pressure in the rat

The endogenous opioid peptides, endomorphin 1 and 2, are newly isolated, potent, and selective μ-opioid receptor agonists. In the present study, responses to endomorphin 1 and 2 were investigated in the systemic vascular bed of the rat. Endomorphin 1 and 2 induced dose-related decreases in systemic arterial pressure when injected in doses of 1–30 nmol/kg i.v. In terms of relative vasodepressor activity, endomorphin 1 and 2 were approximately equipotent with each other and with the ORL1 ligand, nociceptin (orphanin FQ), and were about 10-fold more potent than met-enkephalin in decreasing systemic arterial pressure. Vasodepressor responses to endomorphin 1 and 2 and met-enkephalin, but not to nociceptin, were inhibited by the opioid receptor antagonist, naloxone. These results demonstrate that endomorphin 1 and 2 produce significant naloxone-sensitive decreases in systemic arterial pressure.

Inhibition of rat oxytocin and vasopressin supraoptic nucleus neurons by nociceptin in vitro

The effects of nociceptin (orphanin FQ) on the excitability of electrophysiologically-identified oxytocin and vasopressin neurons were investigated in rat hypothalamic supraoptic nucleus slices in vitro, using whole-cell patch-clamp recording techniques. Nociceptin inhibited the spontaneous discharge of 9/20 (45%) of supraoptic nucleus neurons tested, while in the remaining 11/20 neurons it inhibited firing rate and induced repetitive burst-firing. There were no differences between the effects of nociceptin on oxytocin and vasopressin neurons. When recordings were made using EGTA-containing patch pipettes, nociceptin caused inhibition in all 30 supraoptic nucleus neurons tested, and burst-firing was not seen. The inhibitory effects of nociceptin persisted in low Ca, Co medium, and were not antagonized by naloxone at concentrations sufficient to antagonize the inhibitory actions of morphine and U50 488. The actions of nociceptin on supraoptic nucleus neurons are therefore likely to be mediated by postsynaptic opioid receptor-like (ORL 1) receptors that are distinct from known opioid receptors. The inhibitory responses to nociceptin were also insensitive to naloxone benzoylhydrazone, which itself had no effect on the spontaneous discharge of the supraoptic nucleus neurons. Our findings demonstrate that endogenous nociceptin may have a functional role in regulating oxytocin and vasopressin secretion through its actions on hypothalamic supraoptic nucleus neurons.

Enhancement of spatial attention in nociceptin/orphanin FQ receptor-knockout mice

We isolated genes for the opioid receptor homologue MOR-C, namely nociceptin receptor (designated alternatively as orphanin FQ receptor) and generated nociceptin receptor-knockout mice. Previously, we have reported that the nociceptin system appears to participate in the regulation of the auditory system. However, the behavior of the nociceptin receptor-knockout mice has yet to be fully characterized. In the present study, we investigated changes in several behavioral performances in mice which lack nociceptin receptor. Nociceptive thresholds of nociceptin receptor-knockout mice were unchanged in the hot-plate and electric foot-shock tests as well as tail-flick and acetic acid-induced writhing tests compared to those of wild-type mice. The nociceptin receptor-knockout mice did not show any behavioral changes in the elevated plus-maze task. Surprisingly, in the water-finding test, the nociceptin receptor-knockout mice showed an enhanced retention of spatial attention (latent learning) compared to wild-type mice. In a biochemical study, dopamine content in the frontal cortex was lower in nociceptin receptor-knockout mice than wild-type mice. These results suggest that nociceptin receptor plays an important role in spatial attention by regulating the dopaminergic system in the brain.

Modulation of orphanin FQ or electroacupuncture (EA) on immune function of traumatic rats.

Orphanin FQ(OFQ) is a recently discovered 17-amino acid neuropeptide[1-2]. In present paper, influence of intracerebroventricular(ICV) administration of OFQ or electroacupuncture(EA) on the surgical trauma-induced inhibition of the splenic natural killer(NK) cell activity in rat was observed. The results showed that administration of 0.1 microgram(0.0055 nmol) and 1 microgram (0.055 nmol) OFQ had no effect on the NK cell activity, while 5 micrograms(2.75 nmol) OFQ reduced the NK cell activity in normal rats. However, 0.1 microgram, 1 microgram or 5 micrograms OFQ were found to antagonise the immune function depression caused by surgical trauma. The NK cell activity was reduced in normal rats after repeated ICV treatment with antisense oligonucleotide (ASO) complementary to bases of translated region of rat OFQ receptor mRNA to block the translation of OFQ receptor mRNA into protein. EA stimulation of Zusanli(St. 36) and Lanwei(Extra. 37) points also obviously improved the immunosuppression produced by trauma. OFQ combined with EA showed antagonism on the suppression, but there was no significant differences compared with OFQ(ICV) or EA alone. When blocking the translation of OFQ receptor mRNA with the ASO, the OFQ induced anti-immunosuppression effect was completely reversed, but EA still improved the inhibition on NK cell activity. The results suggested that the OFQ played a role in the regulation of immunosuppression. EA could modulate the suppression of NK cell activity induced by surgical trauma. The mechanisms of the modulation of OFQ or EA on the immunosuppression induced by surgical trauma need further study.

The Peptide Orphanin FQ Inhibits β-Endorphin Neurons and Neurosecretory Cells in the Hypothalamic Arcuate Nucleus by Activating an Inwardly-Rectifying K+ Conductance

Orphanin FQ (OFQ) is a novel heptadecapeptide whose structure resembles that of dynorphin A_1–17. Its receptor shares appreciable homology with µ-, δ- and ĸ-opioid receptors, and is highly expressed in the hypothalamus. The present study examined the effects of OFQ on neurons within the arcuate nucleus (ARC) of the mediobasal hypothalamus, using intracellular recordings from coronal slices. In current clamp, OFQ produced a hyperpolarization of ARC neurons, including those immunopositive for β-endorphin, tyrosine hydroxylase and gonadotropin-releasing hormone. This hyperpolarization was dose-dependent, insensitive to antagonism by naloxone and was associated with a decrease in input resistance. In voltage clamp, OFQ produced an outward current associated with an increase in conductance. Varying the extracellular K⁺ concentration shifted the reversal potential for the OFQ response to the degree predicted by the Nernst equation. Furthermore, barium chloride markedly attenuated both the OFQ-induced hyperpolarization and decrease in input resistance. Administration of maximally effective concentrations of OFQ, followed by coadministration of maximal concentrations of either OFQ and the µ-opioid receptor agonist DAMGO or OFQ and the GABA_B receptor agonist baclofen produced additive hyperpolarizations and outward currents. If DAMGO was applied first, followed by the coadministration of DAMGO and OFQ, then the responses were occluded. Taken together, these results indicate that OFQ inhibits β-endorphin neurons, as well as A₁₂ dopamine and GnRH neurosecretory cells, within the ARC by activating a subset of inwardly-rectifying K⁺ channels. This suggests that OFQ is not only an antiopioid peptide, but that it also modulates the hypothalamo-pituitary axis and, ultimately, reproductive behavior.

Structures That Delineate Orphanin FQ and Dynorphin A Pharmacological Selectivities

Strict pharmacological selectivity in families of structurally related ligands and receptors may result from a key process in evolution aiming at increasing diversity in neurotransmission. An intriguing example of such exclusive specificity can be found in the newly discovered orphanin FQ (OFQ) system when it is compared with the opioid system. Both OFQ and its receptor share a high degree of sequence similarity to the opioid peptides and their corresponding receptors, respectively. However, OFQ does not activate opioid receptors, nor do the opioid peptides elicit biological activity at the OFQ receptor. We have therefore investigated the basis for the inherent selectivity of the primary structures of OFQ and dynorphin A, its closest counterpart. A series of truncated and/or chimeric peptides led to the conclusion that both peptides contain domains which establish their pharmacological selectivity. In the OFQ molecule we could delineate a domain that prevents its ability to activate the kappa-opioid receptor by apparently repelling its binding. In both peptides the selectivity-generating domains are composed of single residues in key positions together with short stretches of amino acids which do not overlap. To prove this concept, we designed a universal agonist and found it active at both the OFQ receptor and the kappa-opioid receptor. Our observations suggest that a coordinated mechanism of evolution has separated the orphanin FQ system from the opioid system.

Pharmacological studies on the "orphan" opioid receptor in central and peripheral sites

We have exploited the availability of the "orphan" opioid receptor (referred to here as ORL1) in its "natural state" to investigate the effect of nociceptin (orphanin FQ), the endogenous agonist for the ORL1 receptor in the brain, vas deferens, and myenteric plexus of the small intestine. Nociceptin was a potent agonist in electrically stimulated preparations of vasa deferentia (rat and rabbit) and myenteric plexus (guinea-pig) (IC50 ranging from 18 to 31 nM) and susceptible to enzymic cleavage as addition of a cocktail of peptidase inhibitors to the organ bath produced a leftward shift in concentration-response curves (IC50 ranging from 2.1 to 4.9 nM). In radioligand binding experiments using brain membranes from rat, rabbit, and guinea-pig, [3H]nociceptin bound a single population of binding sites with high affinity (KD values ranging from 0.049 to 0.124 nM) and capacity (Bmax ranging from 143 to 254 fmol ·mg-1 protein). However, the response to nociceptin in functional studies and in radioligand binding inhibitory assays was resistant to antagonism/displacement by naloxone and a range of other opioid receptor antagonists, thus displaying a very different pharmacological profile from that of the "classical" opioids. Therefore, we conclude that the effect of nociceptin in these studies is not via an action at µ, delta , or kappa opioid receptors but rather at an orphan opioid receptor, ORL1.Key words: nociceptin, orphanin FQ, vas deferens, myenteric plexus, "orphan" opioid receptor.

Orphanin FQ is the Major OFQ 1–17-Containing Peptide Produced in the Rodent and Monkey Hypothalamus

Quigley, D. I., J. M cDougall, T. Darland, G. Zhzng, O. Ronnekliev, D. K. Grandy and R. G. Allen. Orphanin FQ is the major OFQ 1–17-containing peptide produced in the rodent and monkey hypothalamus. Peptides 19(1) 133–139, 1998.—In order to investigate the processing of OFQ containing peptides in the hypothalamus we have developed a sensitive and quantitative radioimmunoassay for OFQ. We fractionated rodent and monkey hypothalamic extracts by reversed-phase high performance liquid chromatography and found that the extracts contained multiple peaks of OFQ immunoreactivity with the major peak co-eluting with synthetic OFQ 1–17. Mouse hypothalamic extracts were also fractionated by SDS-PAGE to determine the apparent molecular weights of molecules containing the OFQ peptide. Multiple peaks of OFQ immunoreactivity, ranging in size from approximately 1 to 30 kilodaltons, were detected by this method. These results suggest that OFQ 1–17 is processed to smaller peptides in mouse and monkey hypothalamic neurons.

O-175 - Effects of nociceptin / orphanin FQ on release of acetylcholine from striatum of the rat brain in vivo

O-175 - Effects of nociceptin / orphanin FQ on release of acetylcholine from striatum of the rat brain in vivo

Actions of leptin and NPY on the activity of arcuate neurons in rats

The effects of nociceptin (orphanin FQ) on medial vestibular nucleus (MVN) neurons in vitro, and on vestibulo-ocular reflex (VOR) function in vivo, were investigated in order to determine the role of `opioid-like orphan' (ORL1) receptors in modulating vestibular reflex function in the rat. Nociceptin (100 nM–1 μM) potently inhibited the spontaneous discharge of the majority (86%) of MVN neurons tested in the rat dorsal brainstem slice preparation in vitro. This inhibition was dose-dependent and persisted after blockade of synaptic transmission in low Ca2+/Co2+ medium. The inhibitory effects were insensitive to the opioid antagonist naloxone, but were effectively antagonised by the selective ORL1 receptor antagonist, [Phe1Ψ(CH2–NH)Gly2]Nociceptin(1–13)NH2. The majority of MVN neurons (∼70%) were inhibited by both nociceptin and the δ-opioid receptor agonist, [d-ala2, d-leu5]-enkephalin (DADLE), while a minority of cells (∼30%) were selectively responsive either to DADLE or to nociceptin, but not both. Co-application of nociceptin and DADLE to neurons that were responsive to both agonists, resulted in an inhibitory response that was the same as or less than the inhibition evoked by either agonist alone. Intracellular whole-cell patch clamp recordings from identified Type A and Type B MVN cells showed that both these cell types are responsive to nociceptin, which induced membrane hyperpolarisation and decrease in input resistance consistent with its known effects on membrane K currents in other cell types. In alert rats, i.c.v. injection of nociceptin caused a significant decrease in the gain of the hVOR and resulted in a prolongation of post-rotatory nystagmus in darkness. The decrease in VOR gain and the increase in the VOR time-constant was significant even at low doses of nociceptin which did not cause other observable behavioural effects. These findings demonstrate that endogenously released nociceptin may have a hitherto unexplored role in the functional modulation of the neural pathways that mediate vestibular reflexes in vivo.

Inhibitory actions of nociceptin (orphanin FQ) on rat supraoptic nucleus oxytocin and vasopressin neurones in vitro.

In the cloning of genes for the mu, kappa and delta opioid receptors, a previously unsuspected fourth member of this seven trans-membrane-helix G-protein coupled receptor family was identified1. This orphan receptor, now named opioid receptor-like 1, ORL-12 weakly binds opioids and the opioid antagonist naloxone1; it is expressed in brain, notably in the supraoptic nucleus (SON;3,4). Soon after identification of the ORL-1 receptor, an endogenous heptadecapeptide ligand was isolated4,5; this is homologous to dynorphin 1–17, but is the product of a distinct gene6. Initial behavioural studies after central administration in mice indicated a pronociceptive, anti-opioid, action hence the peptide was named nociceptin, and alternatively, because of its selective high affinity for the orphan ORL-1 receptor, orphanin FQ (OFQ,-FQ describing aminoacids 1 and 17, whereas the classical opioid peptides have Y at position 1; OFQ and the opioid peptides share GGF at positions 2,3,4 in the amino terminal ‘message’4,5). The nociceptin gene is expressed in the SON7, and because previous studies have shown inhibitory actions of opioids within the SON on both oxytocin and vasopressin neurones8, we have investigated whether nociceptin could act to oppose opioids on SON neurones. We used coronal 350 µm brain slices, cut on a Vibraslice, from 100–200 g female Sprague-Dawley rats for in vitro whole cell patch recording with current-clamp in an interface chamber with continous superfusion measuring effects on spontaneous firing-rate. Oxytocin and vasopressin neurones were distinguished by their different responses to hyperpolarising pulses9. Briefly, oxytocin neurones show slow depolarisation during a sustained (1.2 s) hyperpolarising (100–300 pA) pulse, with a burst of action potentials at the end of the pulse, due to outward rectification; vasopressin neurones do not show these responses.KeywordsOpioid ReceptorDorsal Root Ganglion NeuroneArcuate NucleusSupraoptic NucleusDelta Opioid ReceptorThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Nitric oxide release mediates vasodilator responses to endomorphin 1 but not nociceptin/OFQ in the hindquarters vascular bed of the rat

We have recently shown that endomorphin1, an endogenous ligand for the μ-opioid receptor, and nociceptin (Orphanin FQ; OFQ), an endogenous ligand for the ORL 1 receptor, have substantial vasodilator activity in the hindquarters vascular bed of the rat. In the present study, the role of nitric oxide, vasodilator prostaglandins, and the opening of K + ATP channels in mediating vasodilator responses to endomorphin 1, PL017, and DAMGO was investigated in the regional vascular bed in the rat. Under constant-flow conditions, injections of the μ-selective agonists endomorphin 1, PL017 ([ N-MePhe 3, d-Pro 4]-morphiceptin), and DAMGO, and the ORL 1 receptor agonist nociceptin/OFQ produced dose-dependent decreases in hindquarters perfusion pressure. Vasodilator responses to endomorphin 1, PL017, and DAMGO, and the endothelium-dependent vasodilators acetylcholine and adrenomedullin were attenuated by the nitric oxide synthase inhibitor l-NAME (50 mg/kg IV) at a time when vasodilator responses to nociceptin/OFQ were not altered. Vasodilator responses to isoproterenol and prostaglandin E 1, agents known to increase cAMP levels, and the nitric oxide donor DEA/NO were not altered by the nitric oxide synthase inhibitor. Responses to endomorphin 1, PL017, DAMGO, and nociceptin/OFQ were not altered by sodium meclofenamate at a time when vasodilator responses to arachidonic acid were reduced significantly or after administration of U-37883A at a time when vasodilator responses to levcromakalim were reduced significantly. The results of these studies indicate that responses to endomorphin 1, PL017, and DAMGO are mediated in large part by the release of nitric oxide, while responses to nociceptin/OFQ are mediated by an l-NAME-insensitive mechanism. Moreover, these results demonstrate that responses to these peptides are not mediated by the release of vasodilator prostaglandins or the opening of K + ATP channels the hindquarters vascular bed.

Reverse physiology: discovery of the novel neuropeptide, orphanin FQ/nociceptin.

The search for novel neurotransmitters and neuropeptides has been recently revolutionized by the development of a purification strategy based on orphan G protein-coupled receptors, cloned receptors for which no natural ligands are known. This strategy uses the orphan receptor as bait to identify its natural ligand. This article will review the discovery of the first natural ligand isolated following this strategy. This ligand is a peptide that shares some striking sequence similarity to the opioid peptides and has been named Orphanin FQ or Nociceptin (OFQ/NOC). The discovery of OFQ/NOC will be described as one example of the use of orphan receptors in identifying novel neurotransmitters and neuropeptides, an example that has already been followed in the identification of other novel neuropeptides. After reviewing the conceptual and technological basis of the strategy and its successful first application, we discuss the criteria used to validate OFQ/NOC as the natural ligand of the orphan receptor and as a genuine neuropeptide. We also discuss the importance and implications of discovering OFQ/NOC mode of synthesis, which is synthesized as expected in the form of a larger polypeptide precursor, which in turn raises the question of the existence of other OFQ/NOC-related peptides. We then present an overview of the numerous studies that have blossomed after the OFQ/NOC discovery and describe the numerous physiological roles that have already been attributed to OFQ/NOC, and in particular the controversy regarding its involvement in pain perception. Because of the similarities between the OFQ/NOC and opioid systems, we also discuss overlaps between these systems and present evidence favoring a pharmacological separation between these systems. We finish by outlining the power of the orphan receptor strategy and by discussing some of its pitfalls.

Antinociceptive and morphine modulatory actions of spinal orphanin FQ

Orphanin FQ (nociceptin, OFQ), a hepatdecapeptide peptide, has been designated as an endogenous ligand at the orphan receptor ORL1, which lacks affinity for opioid receptor ligands. OFQ-like immunoreactivity has been localized in spinal cord areas that are involved in the processing of nociceptive signals. In this study, the effects of spinally administered OFQ on thermal and mechanical nociceptive stimuli were investigated following intrathecal (i.t.) injection in unanesthetized rats bearing chronic indwelling catheters in the subarachnoid space. Intrathecal OFQ produced two distinct acute actions: an opioid-like antinociceptive effect, and antagonism of morphine-induced antinociception. Chronic administration produces tolerance. Acute actions were as follows. First, injection of OFQ alone in the dose range 10-100 nmol produced sustained antinociceptive effects in the tail-flick (baseline latency 1.5-2 s) and paw-pressure tests. These effects peaked at 45-60 min post-injection, were fully reversible, and were observed in absence of discernable motor impairments. Intrathecal naloxone (5 nmol) significantly attenuated the antinociceptive effects of OFQ (50 nmol i.t.) in tail-flick and paw-pressure tests. A 24-h pretreatment with the irreversible opioid receptor antagonist, beta-flunatrexamine (2 nmol i.t.), also attenuated the antinociceptive effects of OFQ (50, 100 nmol i.t.) and of morphine (7.5 nmol i.t.) in both tests. Low doses (1, 5 nmol) of OFQ, which failed to produce antinociception in the tail-flick test involving a baseline latency of 1.5-2 s, produced a strong antinociceptive response when the baseline latency was increased to 5-6 s. Second, intrathecal OFQ, at doses (0.5, 1.0, and 5.0 nmol) that had no antinociceptive activity in the tail-flick (baseline latency 1.5-2 s) and paw-pressure tests, attenuated the antinociceptive effect of morphine (7.5 nmol i.t.) in these tests. However, a threshold OFQ dose (10 nmol) significantly extended the duration of antinociception induced by morphine (7.5 nmol i.t.) or deltorphin (20 nmol i.t.). Chronic actions were as follows. In rats that were rendered tolerant to spinal morphine, by a continuous intrathecal infusion (7.5 nmol/h) of the agonist for 5 days, the OFQ dose-response curves for its antinociceptive effect in the tail-flick and paw-pressure tests were significantly shifted to the right. In separate experiments, repeated intrathecal injection of OFQ (50 nmol) or morphine (7.5 nmol) produced a significant decline in their antinociceptive effects. Thus, intrathecally administered OFQ produces both development of tolerance to its antinociceptive actions and cross-tolerance to the action of morphine.

Orphanin FQ acts as an anxiolytic to attenuate behavioral responses to stress.

Orphanin FQ (OFQ, Nociceptin) is a recently discovered 17-amino acid neuropeptide that is structurally related to the opioid peptides but does not bind opioid receptors. OFQ has been proposed to act as an anti-opioid peptide, but its widespread sites of action in the brain suggest that it may have more general functions. Here we show that OFQ plays an important role in higher brain functions because it can act as an anxiolytic to attenuate the behavioral inhibition of animals acutely exposed to stressful/anxiogenic environmental conditions. OFQ anxiolytic-like effects were consistent across several behavioral paradigms generating different types of anxiety states in animals (light-dark preference, elevated plus-maze, exploratory behavior of an unfamiliar environment, pharmacological anxiogenesis, operant conflict) and were observed at low nonsedating doses (0.1-3 nmol, intracerebroventricular). Like conventional anxiolytics, OFQ interfered with regular sensorimotor function at high doses (>3 nmol). Our results show that an important role of OFQ is to act as an endogenous regulator of acute anxiety responses. OFQ, probably in concert with other major neuropeptides, exerts a modulatory role on the central integration of stressful stimuli and, thereby, may modulate anxiety states generated by acute stress.

Circuitry underlying antiopioid actions of orphanin FQ in the rostral ventromedial medulla.

Several laboratories recently identified a 17 amino-acid peptide, termed "nociceptin" or "orphanin FQ (OFQ)", as the endogenous ligand for the LC132 (or "opioid receptor-like1") receptor. Taken together with the fact that the cellular effects of OFQ are to a large extent opioid-like, the close relationship between the LC132 receptor and known opioid receptors raised expectations that the behavioral effects of this peptide would resemble those of opioids. However studies of the role of OFQ in nociception have not provided a unified view. The aim of the present study was to use a combination of electrophysiological and pharmacological techniques to characterize the actions of OFQ in a brain region in which the circuitry mediating the analgesic actions of opioids has been relatively well characterized, the rostral ventromedial medulla (RVM). Single-cell recording was combined with opioid administration and local infusion of OFQ in the RVM of rats lightlyanesthetized with barbiturates. The tail flick reflex was used as a behavioral index of nociceptive responsiveness. Two classes of physiologically identifiable RVM neurons with distinct responses to opioids have been characterized. -cells are activated, although indirectly, by opioids, and there is strong evidence that this activation is crucial to opioid antinociception. -cells, thought to enable nociception, are directly inhibited by opioids. Cells of a third class, cells, do not respond to opioids and whether or not they have any role in nociceptive modulation remains an open question. OFQ infused within the RVM profoundly suppressed the firing of all classes of RVM neurons, blocking opioid-induced activation of -cells. The antinociceptive effects of a micro-opioid agonist infused at the same site were significantly attenuated in these animals. Those of systemically administered morphine, which can produce its antinociceptive effects by acting at a number of CNS sites, were not blocked by RVM OFQ. Inasmuch as activation of -cells can account for the antinociceptive action of opioids within the RVM, these results demonstrate that, at least within the medulla, OFQ can exert a functional "antiopioid" effect by suppressing firing of this cell class. However to the extent that antinociceptive and pronociceptive outflows from various brain regions involved in both transmission and modulation of nociception are active under different conditions, focal application of OFQ in different regions could potentially produce either hypalgesia or hyperalgesia.

Nociceptin Inhibits T-Type Ca2+Channel Current in Rat Sensory Neurons by a G-Protein-Independent Mechanism

Nociceptin (orphanin FQ) is a novel, opioid-like, heptadecapeptide that is an endogenous ligand for the opioid receptor-like (ORL1) receptor. Unlike classical opioids, nociceptin can produce hyperalgesia when injected intracerebroventricularly into mice. Despite this, nociceptin has been reported to decrease transmitter release, activate an inwardly rectifying K+ conductance, and suppress high-voltage-activated Ca2+ channel conductances (HVA gCa) in much the same way as micro-, delta-, and kappa-opioids. We report an action of nociceptin that is not shared by morphine: the suppression of low-voltage-activated, transient calcium (barium) current (IBa,T) in acutely dissociated rat dorsal root ganglion (DRG) neurons (EC50 = 100 nM). This effect was reflected as inhibition of bursts of action potentials that can be evoked in "medium-sized" DRG neurons. Experiments with GTP-gamma-S (100 microM), GDP-beta-S (2 mM), or aluminum fluoride (AlF3) (100 microM) in the patch pipette failed to provide evidence for G-protein involvement in nociceptin-induced IBa,T suppression. By contrast, both morphine and nociceptin suppressed HVA gCa, and the latter response was affected by intracellular GTP-gamma-S, GDP-beta-S, and AlF3 in ways that confirmed G-protein involvement. The selective effect of nociceptin on IBa,T may therefore be relevant to understanding why its behavioral actions differ from those of other opioids. This G-protein-independent effect of the action of nociceptin may reflect a new general mechanism of action for opioid peptides within the nervous system.

Presence of nociceptin (orphanin FQ) receptors in rat retina: Comparison with receptors in striatum

Nociceptin (orphanin FQ), a heptadecapeptide with some sequence homology to dynorphin A, has been proposed as an endogenous ligand for a previously cloned orphan receptor with significant homology to opioid receptors. Utilizing [ 125 I ][Tyr 14]nociceptin as ligand, saturable and high affinity nociceptin binding sites were detected and characterized in rat retina and striatum. For retina, B max=44.0±4.5 fmol/mg and K d=32.4±2.7 pM; for striatum, B max=51.6±7.7 fmol/mg and K d=98.6±11.3 pM. In competition studies, nociceptin bound with picomolar affinity, dynorphin A with nanomolar affinity, naloxone and dynorphan A-(1–8) with micromolar affinity, while [des-Tyr 1]dynorphin (dynorphin A-(2–17)), several other opioids, morphine and benzomorphans failed to compete for binding at 1–10 μM. Gpp(NH)p plus NaCl markedly decreased binding, consistent with involvement of a G protein-linked receptor. It is concluded that rat retina contains nociceptin receptors similar in concentration to those present in striatum. Properties of both the retinal and the striatal receptors are similar to those previously found for rat hypothalamus.

Antinociception mediated by the periaqueductal gray is attenuated by orphanin FQ.

Orphanin FQ or nociceptin (OFQ/N(1-17)) is a recently discovered peptide which, upon intracerebroventricular administration, reverses opioid-mediated analgesias. OFQ/N(1-17) terminals are located in the periaqueductal gray (PAG), a structure known to be involved in pain modulation, suggesting that the functional anti-opioid effects of OFQ/N(1-17) are mediated by PAG neurons. To test this, subsequent microinjections of morphine or kainic acid and OFQ/N(1-17) were made into the PAG of awake rats. Administration of OFQ/N(1-17) attenuated the tail flick inhibition produced by both morphine and kainic acid microinjection. OFQ/N(1-17) attenuation of antinociception produced by a neuroexcitant indicates that OFQ/N(1-17) reverses opioid antinociception by inhibiting PAG output neurons.