Opposing interplay between Neuropeptide FF and nitric oxide in antinociception and hypothermia

RF-amide neuropeptides and their receptors in Mammals: Pharmacological properties, drug development and main physiological functions

RF-amide neuropeptides, with their typical Arg-Phe-NH2 signature at their carboxyl C-termini, belong to a lineage of peptides that spans almost the entire life tree. Throughout evolution, RF-amide peptides and their receptors preserved fundamental roles in reproduction and feeding, both in Vertebrates and Invertebrates. The scope of this review is to summarize the current knowledge on the RF-amide systems in Mammals from historical aspects to therapeutic opportunities. Taking advantage of the most recent findings in the field, special focus will be given on molecular and pharmacological properties of RF-amide peptides and their receptors as well as on their implication in the control of different physiological functions including feeding, reproduction and pain. Recent progress on the development of drugs that target RF-amide receptors will also be addressed.

The anti-inflammatory potential of neuropeptide FF in vitro and in vivo

Neuropeptide FF (NPFF) has many functions in regulating various biological processes. However, little attention has been focused on the anti-inflammatory effect of this peptide. In the present study, the in vitro anti-inflammatory activity of NPFF in both primary peritoneal macrophages and RAW 264.7 macrophages was investigated. Our data showed that NPFF suppressed the nitric oxide (NO) production of macrophages in the inflammation process. RF9, a reported antagonist of NPFF receptors, completely blocked the NPFF-induced NO suppression, suggesting a NPFF receptors-mediated pathway is mainly involved. Down-regulation of the nitric oxide synthases significantly inhibited the NPFF-induced NO reduction, indicating the involvement of nitric oxide synthases. However, the nitric oxide synthases were not the only route by which NPFF modulated the NO levels of macrophages. Pharmacological antagonists of the NF-κB signal pathway also completely suppressed the NPFF-induced NO decline. Moreover, we also observed that NPFF is capable of blocking the LPS-induced nuclear translocation of p65 in macrophages, implying the involvement of the NF-κB signal pathway. Finally, we observed that NPFF markedly attenuated the carrageenan-induced mouse paw edema, indicating that NPFF is capable of exerting anti-inflammatory potency in vivo. Collectively, our findings reveal the potential role of NPFF in the anti-inflammatory field both in vitro and in vivo, which will be helpful for the further exploitation of NPFF utility therapeutically.

Neuropeptide FF activates ERK and NF kappa B signal pathways in differentiated SH-SY5Y cells

Neuropeptide FF (NPFF) has been reported to play important roles in regulating diverse biological processes. However, little attention has been focused on the downstream signal transduction pathway of NPFF. Here, we used the differentiated neuroblastoma cell line, dSH-SY5Y, which endogenously expresses hNPFF2 receptor, to investigate the signal transduction downstream of NPFF. In particular we investigated the regulation of the extracellular signal-regulated protein kinase (ERK) and the nuclear factor kappa B (NF-κB) pathways by NPFF in these cells. NPFF rapidly and transiently stimulated ERK. H89, a selective inhibitor of cyclic AMP-dependent protein kinase A (PKA), inhibited the NPFF-activated ERK pathway, indicating the involvement of PKA in the NPFF-induced ERK activation. Down-regulation of nitric oxide synthases also attenuated NPFF-induced ERK activation, suggesting that a nitric oxide synthase-dependent pathway is involved. Moreover, the core upstream components of the NF-κB pathway were also significantly activated in response to NPFF, suggesting that the NF-κB pathway is involved in the signal transduction pathway of NPFF. Collectively, these data demonstrate that nitric oxide synthases are involved in the signal transduction pathway of NPFF, and provide the first evidence for the interaction between NPFF and the NF-κB pathway. These advances in our interpretation of the NPFF pathway mechanism will aid the comprehensive understanding of its function and provide novel molecular insight for further study of the NPFF system.

Neuron-astrocyte signaling network in spinal cord dorsal horn mediates painful neuropathy of type 2 diabetes

Activation of the neuronal-glial network in the spinal cord dorsal horn (SCDH) mediates various chronic painful conditions. We studied spinal neuronal-astrocyte signaling interactions involved in the maintenance of painful diabetic neuropathy (PDN) in type 2 diabetes. We used the db/db mouse, an animal model for PDN of type 2 diabetes, which develops mechanical allodynia from 6 to 12 wk of age. In this study, enhanced substance P expression was detected in the presynaptic sensory fibers innervating lamina I-III in the lumbar SCDH (LSCDH) of the db/db mouse at 10 wk of age. This phenomenon is associated with enhanced spinal ERK1/2 phosphorylation in projection sensory neurons and regional astrocyte activation. In addition, peak phosphorylation of the NR1 subunit of N-methyl-D-aspartate receptor (NMDAR), along with upregulation of neuronal and inducible nitric oxide synthase (nNOS and iNOS) expression were detected in diabetic mice. Expression of nNOS and iNOS was detected in both interneurons and astrocytes in lamina I-III of the LSCDH. Treatment with MK801, an NMDAR inhibitor, inhibited mechanical allodynia, ERK1/2 phosphorylation, and nNOS and iNOS upregulation in diabetic mice. MK801 also reduced astrocytosis and glial acidic fibrillary protein upregulation in db/db mice. In addition, N(G)-nitro-L-arginine methyl ester (L-NAME), a nonspecific NOS inhibitor, had similar effects on NMDAR signaling and NOS expression. These results suggest that nitric oxide from surrounding interneurons and astrocytes interacts with NMDAR-dependent signaling in the projection neurons of the SCDH during the maintenance of PDN.

Neuropeptide FF receptor antagonist, RF9, attenuates the fever induced by central injection of LPS in mice

The endogenous opioid system has been found to be involved in the fever caused by lipopolysaccharide (LPS). Neuropeptide FF (NPFF, FLFQPQRF-NH(2)) is an endogenous peptide known to modulate opioid activity, mainly in the central nervous system. Therefore, those data suggested a link between LPS-induced fever and NPFF systems. Using a model of acute neuroinflammation, we sought to determine the effects of NPFF systems on the fever induced by i.c.v. injection of LPS. Coinjected with different doses of NPFF (10 and 30 nmol), the fever of LPS (125 ng) was not modified. Interestingly, the selective NPFF receptors antagonist RF9 (30 nmol) injected into the third ventricle failed to induce significant effect, but it decreased the fever of LPS (125 ng) after cerebral administration in mice. These results suggest that NPFF receptors activation is required for LPS to produce fever. This interaction is the first evidence that NPFF systems participate in the control of acute neuroinflammation in conscious animals.

Opposite control of body temperature by NPFF1 and NPFF2 receptors in mice

Neuropeptide FF (NPFF) is a neurotransmitter known to modulate opioid functions. This study investigates the effects of RF9, a new antagonist of NPFF receptors, on the roles of NPFF1 and NPFF2 receptors in thermoregulation in mice. RF9 (10 nmol) injected into the third ventricle did not modify the body temperature as compared to saline, but it completely antagonized the hypothermic effects of 10 nmol NPVF, a NPFF1 selective agonist, as well as the hyperthermic actions of dNPA (5 nmol), a NPFF2 selective agonist. The use of a specific antagonist demonstrates here that central NPFF1 and NPFF2 receptors control in an opposite manner the body temperature in mice.

Neuropeptide FF receptors antagonist, RF9, attenuates opioid-evoked hypothermia in mice

The present study used the endpoint of hypothermia to investigate opioid and neuropeptide FF (NPFF) interactions in conscious animals. Both opioid and NPFF systems played important roles in thermoregulation, which suggested a link between opioid receptors and NPFF receptors in the production of hypothermia. Therefore, we designed a study to investigate the relationship between opioid and NPFF in control of thermoregulation in mice. The selective NPFF receptors antagonist RF9 (30nmol) injected into the third ventricle failed to induce significant effect, but it completely antagonized the hypothermia of NPFF (45 nmol) after cerebral administration in mice. In addition, RF9 (30 nmol) co-injected i.c.v. in the third ventricle reduced the hypothermia induced by morphine (5nmol,) or nociceptin/orphanin FQ (N/OFQ) (2 nmol). Neither the classical opioid receptors antagonist naloxone (10 nmol) nor NOP receptor antagonist [Nphe(1)]NC(1-13)NH(2) (7.5 nmol) reduced the hypothermia induced by the central injection of NPFF at dose of 45 nmol. Co-injected with a low dose of NPFF (5 nmol), the hypothermia of morphine (5 nmol) or N/OFQ (2 nmol) was not modified. These results suggest that NPFF receptors activation is required for opioid to produce hypothermia. In contrast, NPFF-induced hypothermia is mainly mediated by its own receptors, independent of opioid receptors in the mouse brain. This interaction, quantitated in the present study, is the first evidence that NPFF receptors mediate opioid-induced hypothermia in conscious animals.

Genetic knockout and pharmacologic inhibition of neuronal nitric oxide synthase attenuate nerve injury-induced mechanical hypersensitivity in mice

Neuronal nitric oxide synthase (nNOS) is a key enzyme for nitric oxide production in neuronal tissues and contributes to the spinal central sensitization in inflammatory pain. However, the role of nNOS in neuropathic pain remains unclear. The present study combined a genetic strategy with a pharmacologic approach to examine the effects of genetic knockout and pharmacologic inhibition of nNOS on neuropathic pain induced by unilateral fifth lumbar spinal nerve injury in mice. In contrast to wildtype mice, nNOS knockout mice failed to display nerve injury-induced mechanical hypersensitivity. Furthermore, either intraperitoneal (100 mg/kg) or intrathecal (30 microg/5 microl) administration of L-NG-nitro-arginine methyl ester, a nonspecific NOS inhibitor, significantly reversed nerve injury-induced mechanical hypersensitivity on day 7 post-nerve injury in wildtype mice. Intrathecal injection of 7-nitroindazole (8.15 microg/5 microl), a selective nNOS inhibitor, also dramatically attenuated nerve injury-induced mechanical hypersensitivity. Western blot analysis showed that the expression of nNOS protein was significantly increased in ipsilateral L5 dorsal root ganglion but not in ipsilateral L5 lumbar spinal cord on day 7 post-nerve injury. The expression of inducible NOS and endothelial NOS proteins was not markedly altered after nerve injury in either the dorsal root ganglion or spinal cord. Our findings suggest that nNOS, especially in the dorsal root ganglion, may participate in the development and/or maintenance of mechanical hypersensitivity after nerve injury.

Effect of genetic knockout or pharmacologic inhibition of neuronal nitric oxide synthase on complete Freund's adjuvant-induced persistent pain

Nitric oxide (NO) acts as a neurotransmitter or neuromodulator involving in the modulation of thermal and/or inflammatory hyperalgesia. The neuronal nitric oxide synthase (nNOS) is a key enzyme for NO production in normal neuronal tissues, but its functional role in chronic pain remains unclear. The present study combined a genetic strategy with a pharmacologic approach to address the role of nNOS in the central mechanism of complete Freund's adjuvant (CFA)-induced chronic inflammatory pain. Targeted disruption of the nNOS gene significantly reduced CFA-induced mechanical pain hypersensitivity during the maintenance (but not the development) of inflammatory pain, while it failed to attenuate either development or maintenance of CFA-induced thermal pain hypersensitivity. Intraperitoneal administration of L-N(G)-nitro-arginine methyl ester (L-NAME), a non-specific NOS inhibitor, blocked CFA-evoked thermal and mechanical pain hypersensitivity at both development (2h) and maintenance (24h) phase in wild type mice, but had no effect in the knockout mice. Furthermore, intrathecal injection of either L-NAME or 7-nitroindazole, a selective nNOS inhibitor, markedly attenuated mechanical pain hypersensitivity at both 2 and 24h after CFA injection. Finally, spinal cord nNOS (but not endothelial NOS or inducible NOS) expression was up-regulated at 24h after CFA injection, occurring mainly in the ipsilateral superficial dorsal horn. Together, these data indicate that spinal cord nNOS may be essential for the maintenance of mechanical pain hypersensitivity and that it may also be sufficient for the development of mechanical pain hypersensitivity and for the development and maintenance of thermal pain hypersensitivity after chronic inflammation. Our findings suggest that spinal cord nNOS might play a critical role in central mechanisms of the development and/or maintenance of chronic inflammatory pain.

Neuropeptide FF receptors exert contractile activity via inhibition of nitric oxide release in the mouse distal colon

Neuropeptide FF (NPFF) and NPVF, two closely NPFF related peptides, have different affinities for the two NPFF receptors (NPFF1 and NPFF2). To assess the peripheral effects of NPFF receptors in the gastrointestinal tract motility, NPFF and NPVF were tested in the mouse isolated distal colon. Both NPFF (1-15 microM) and NPVF (1-15 microM) dose-dependently caused significant colonic contractions. Pre-treatment with the putative NPFF antagonist, BIBP3226 (30 microM) abolished the contractile responses to the two neuropeptides (3 microM). They had no additional contractile activities in colonic preparations contracted by Nomega-nitro-L-arginine (30 microM). Moreover, the contractions of these two neuropeptides were weakened by L-arginine (2 mM). The responses to NPFF (5 microM) and NPVF (5 microM) were not modified by atropine or naloxone (1 microM). Furthermore, NPFF (1 microM) and NPVF (1 microM) did not influence the contractive responses to acetylcholine (0.1-10 microM), morphine (1 microM) or nociceptin (0.1 microM). These data suggest that NPFF and NPVF cause contractions of the mouse distal colon via their NPFF receptors and this effect is mediated by NO but not by cholinergic pathways, independently from opioid system. In addition, the isolated bioassay may be applied as a simple parameter to characterize the potential NPFF agonists and antagonists.

Effects of peptides, with emphasis on feeding, pain, and behavior A 5-year (1999-2003) review of publications in Peptides

Novel effects of naturally occurring peptides are continuing to be discovered, and their mechanisms of actions as well as interactions with other substances, organs, and systems have been elucidated. Synthetic analogs may have actions similar or antagonistic to the endogenous peptides, and both the native peptides and analogs have potential as drugs or drug targets. The journal Peptides publishes many leading articles on the structure-activity relationship of peptides as well as outstanding reviews on some families of peptides. Complementary to the reviews, here we extract information from the original papers published during the past five years in Peptides (1999-2003) to summarize the effects of different classes of peptides, their modulation by other chemicals and various pathophysiological states, and the mechanisms by which the effects are exerted. Special attention is given to peptides related to feeding, pain, and other behaviors. By presenting in condensed form the effects of peptides which are essential for systems biology, we hope that this summary of existing knowledge will encourage additional novel research to be presented in Peptides.

Peptide nucleic acids targeted to the mouse proNPFF(A) reveal an endogenous opioid tonus

Pharmacological studies have implicated the anti-opioid neuropeptide FF (NPFF) in the modulation of pain transmission. Since its physiological role has not yet been fully elucidated, the present study examined whether antisense peptide nucleic acid (PNA) complementary to the NPFF precursor (proNPFF(A)) modified pain sensitivity. Mice received three intraperitoneal (i.p.) injections (10mg/kg) of antisense PNA (As-proNPFF(A)) over a period of 24h. As-proNPFF(A) treatment significantly increased the basal tail withdrawal latency in the tail-flick test. This analgesia persisted during 2 days and was completely reversed by naloxone. Thus, antisense PNAs, by decreasing anti-opioid effects, revealed a basal endogenous opioid activity. Our results evidence a physiological interplay between NPFF and opioid systems and further support the use of PNA as effective antisense agents, for studying gene function in vivo.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.