Presence of neuropeptide FF receptors on primary afferent fibres of the rat spinal cord

The new kisspeptin derivative - kissorphin (KSO) - attenuates acute hyperlocomotion and sensitization induced by ethanol and morphine in mice

Kissorphin (KSO) is a new peptide derived from kisspeptin-10. This peptide possesses neuropeptide FF (NPFF)-like biological activity in vitro; NPFF, in many cases, inhibits opioid and ethanol effects in rodents. Therefore, the current study explored the influence of KSO on acute ethanol- and morphine-induced hyperactivity, and on the development and expression of locomotor sensitization induced by these drugs. In the present study, sensitization to locomotor effects was induced by repeated exposure to ethanol (2.4 g/kg, intraperitoneally [i.p.], 1 × 4 days) or morphine (10 mg/kg, subcutaneously [s.c.], 1 × 7 days). We found that KSO (1-10 nmol/300 μL, intravenously [i.v.]) did not have an impact on locomotor activity of naïve mice. However, it reduced both acute ethanol- (10 nmol/300 μL) and morphine-induced hyperactivity (3 and 10 nmol/300 μL). Pretreatment of animals with KSO (10 nmol/300 μL), before every ethanol or morphine injection during development of sensitization or before the ethanol or morphine challenge, attenuated the development, as well as the expression of locomotor sensitization to both substances. Moreover, prior administration of the NPFF receptor antagonist RF9 (10 nmol/300 μL, i.v.) inhibited the ability of KSO (10 nmol/300 μL) to reduce the expression of ethanol and morphine sensitization. KSO given alone, at all used doses, did not influence the motor coordination measured via the rotarod test. The results from this study show that KSO effectively attenuated acute and repeated effects of ethanol and morphine. Thus, KSO possesses NPFF-like anti-opioid activity in these behavioral studies.

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.

Involvement of Mammalian RF-Amide Peptides and Their Receptors in the Modulation of Nociception in Rodents

Mammalian RF-amide peptides, which all share a conserved carboxyl-terminal Arg-Phe-NH2 sequence, constitute a family of five groups of neuropeptides that are encoded by five different genes. They act through five G-protein-coupled receptors and each group of peptide binds to and activates mostly one receptor: RF-amide related peptide group binds to NPFFR1, neuropeptide FF group to NPFFR2, pyroglutamylated RF-amide peptide group to QRFPR, prolactin-releasing peptide group to prolactin-releasing peptide receptor, and kisspeptin group to Kiss1R. These peptides and their receptors have been involved in the modulation of several functions including reproduction, feeding, and cardiovascular regulation. Data from the literature now provide emerging evidence that all RF-amide peptides and their receptors are also involved in the modulation of nociception. This review will present the current knowledge on the involvement in rodents of the different mammalian RF-amide peptides and their receptors in the modulation of nociception in basal and chronic pain conditions as well as their modulatory effects on the analgesic effects of opiates.

Modulatory role of neuropeptide FF system in nociception and opiate analgesia

The tetra-peptide FMRF-NH(2) is a cardioexcitatory peptide in the clam. Using the antibody against this peptide, FMRF-NH(2)-like immunoreactive material was detected in mammalian CNS. Subsequently, mammalian FMRF-NH(2) immunoreactive peptides were isolated from bovine brain and characterized to be FLFQPQRF-NH(2) (NPFF) and AGEGLSSPFWSLAAPQRF-NH(2) (NPAF). The genes encoding NPFF precursor proteins and NPFF receptors 1 and 2 are expressed in all vertebrate species examined to date and are highly conserved. Among many biological roles suggested for the NPFF system, the possible modulatory role of NPFF in nocicetion and opiate analgesia has been most widely investigated. Pharmacologically, NPFF-related peptides were found to exhibit analgesia and also potentiate the analgesic activity of opiates when administered intrathecally but attenuate the opiate induced analgesia when administered intracerebroventricularly. RF-NH(2) peptides including NPFF-related peptides were found to delay the rate of acid sensing ion channels (ASIC) desensitization resulting in enhancing acid gated currents, raising the possibility that NPFF also may have a pain modulatory role through ASIC. The genes for NPFF as well as NPFF-R2, preferred receptor for NPFF, are highly unevenly expressed in the rat CNS with the highest levels localized to the superficial layers of the dorsal spinal cord. These two genes are also present in the dorsal root ganglia (DRG), though at low levels in normal rats. NPFF and NPFF-R2 mRNAs were found to be coordinately up-regulated in spinal cord and DRG of rats with peripheral inflammation. In addition, NPFF-R2 immunoreactivity in the primary afferents was increased by peripheral inflammation. The findings from the early studies on the analgesic and morphine modulating activities suggested a role for NPFF in pain modulation and this possibility is further supported by the distribution of NPFF and its receptor and the regulation of the NPFF system in vivo.

Modulatory roles of the NPFF system in pain mechanisms at the spinal level

The possible roles of the NPFF system in pain processing are summarized from the viewpoints of (1) biological activities of NPFF, (2) anatomical distribution of NPFF and its receptor(s) and (3) the regulation of NPFF and receptor(s) in animal models of pain. NPFF and NPFF analogues were found to have analgesic, pronociceptive and morphine modulating activities. Since the isolation of NPFF, several other RF-NH2 peptides have been identified and some of them were found to have nociceptive or morphine modulating activity. Depending on the pharmacological doses and locations of administration, NPFF may exhibit the biological activities of other structurally related RF-NH2 peptides thus complicating NPFF bioactivity studies and their interpretation. Acid sensing ion channels were found to respond to RF-NH2 peptides including NPFF, raising the possibility that interaction of NPFF and acid sensing ion channels can modulate nociceptive activity. NPFF and NPFF receptor mRNAs are highly expressed and localized in the superficial layers of the dorsal cord, the two genes are also in dorsal root ganglia though at much lower level. The spinal NPFF system is up-regulated by peripheral inflammation in the rat. Furthermore, immunohistochemically, NPFF receptor 2-protein was demonstrated to be increased in the primary afferents in the spinal cord of rats with peripheral inflammation. Regulation and localization of spinal NPFF systems, taken together with the analgesic bioactivity of intrathecally administered NPFF, strongly suggest involvement of spinal NPFF system in pain processing.

A new member of the hypothalamic RF-amide peptide family, LPXRF-amide peptides: structure, localization, and function

Recently, we identified a novel hypothalamic neuropeptide with a C-terminal LPLRF-amide sequence in the quail brain. This avian neuropeptide was shown to inhibit gonadotropin release from the cultured anterior pituitary. This peptide is the first hypothalamic peptide that inhibited gonadotropin release reported in vertebrates. We, therefore, termed it gonadotropin-inhibitory hormone (GnIH). After this finding, we found that GnIH-related peptides were present in the brains of other vertebrates, such as mammals, amphibians, and fish. These GnIH-related peptides possessed a LPXRF-amide (X=L or Q) motif at their C-termini in all investigated animals. Mass spectrometric analyses combined with immunoaffinity chromatography were powerful techniques for the identification of mature endogenous LPXRF-amide peptides. The identified LPXRF-amide peptides were found to be localized in the hypothalamus and brainstem areas, and to regulate pituitary hormone release. Subsequently, cDNAs that encode LPXRF-amide peptides were characterized in vertebrate brains. In this review, we summarize the identification, localization, and function of a new member of the hypothalamic RF-amide peptide family, LPXRF-amide peptides in vertebrates. Recent studies on the receptors for LPXRF-amide peptides will also be reviewed.

Rodent strain differences in the NPFF1 and NPFF2 receptor distribution and density in the central nervous system

The present study evaluates the putative differences between NPFF1 and NPFF2 receptor distribution and density throughout the central nervous system between rat and mouse strains by using in vitro quantitative autoradiography. The binding of [125I]YVP ([125I]YVPNLPQRF-NH2) and [125I]EYF ([125I]EYWSLAAPQRF-NH2), used to label NPFF1 and NPFF2 receptors, respectively, was compared between Sprague-Dawley and Wistar rats and between Swiss and C57BL/6-SV129 mice. In contrast to Wistar, Sprague-Dawley brains contained NPFF1 binding sites in the cortical and spinal cord areas, the accumbens nucleus, the anterodorsal thalamic nucleus, the parafascicular thalamic nucleus, the inferior colliculus and the nucleus of the solitary tract. The distribution of NPFF2 binding sites was also different between the two strains of rats. As compared to Swiss, C57BL/6-SV129 mice showed higher basal NPFF2 receptor levels in cortical areas, telencephalon and some other regions. In contrast, they showed lower amounts in thalamic structures, except the reuniens nucleus, and in mesencephalic and rhombencephalic regions. In the cervical spinal cord the levels of NPFF2 receptors were similar. The NPFF1 binding levels were nearly the same in telencephalic structures while distinct in the forebrain. Differences in amount of NPFF receptor subtypes among these strains of rats or mice could lead to differences in NPFF control of opioid nociception.

Neuropeptide FF receptor 2 (NPFF2) is localized to pain-processing regions in the primate spinal cord and the lower level of the medulla oblongata

Studies have suggested that NPFF-like peptides and their receptors play important roles in physiological and pathological conditions. Here, we show, using multiple expression modalities, that the type 2 NPFF receptor (hNPFF2) is expressed in regions of the primate spinal cord and brainstem mediating pain sensation. In situ hybridization using an NPFF2 riboprobe, and immunohistochemistry using a novel NPFF2 antibody, demonstrated strong NPFF2 expression in the superficial layer of the dorsal horn, and in the spinal trigeminal nucleus of the brainstem of the African green monkey (AGM). In addition, autoradiography using a radiolabeled NPFF analog ([125I]1DMe) revealed dense binding signal in the superficial layer of the dorsal horn in the spinal cord. The distribution pattern of hNPFF2 in the AGM spinal cord and the lower level of the brainstem are consistent with a hypothesized potential role for NPFF peptides in modulation of sensory input, opioid analgesia and morphine tolerance through spinal and supraspinal mechanisms.

Quantitative autoradiographic distribution of NPFF1 neuropeptide FF receptor in the rat brain and comparison with NPFF2 receptor by using [125I]YVP and [(125I]EYF as selective radioligands

The selectivity of two new radioligands, [(125)I]YVP ([(125)I]YVPNLPQRF-NH(2)) and [(125)I]EYF ([(125)I]EYWSLAAPQRF-NH(2)), for neuropeptide FF (NPFF) receptor subtypes was determined using HEK293 cells expressing hNPFF(1) and CHO cells expressing hNPFF(2) receptors. Saturation binding and displacement experiments showed that [(125)I]YVP and [(125)I]EYF bound selectively with a very high affinity, K(D)=0.18 nM and 0.06 nM, to NPFF(1) and NPFF(2) receptors respectively. By using in vitro autoradiography with these radioligands and frog pancreatic polypeptide (PP) as selective unlabelled competitor of NPFF(2) binding sites, NPFF(1) and NPFF(2) receptor distribution was analyzed throughout the rat CNS. The highest densities of [(125)I]EYF binding sites were seen in the most external layers of the dorsal horn of the spinal cord, the parafascicular thalamic nucleus, laterodorsal thalamic nucleus and presubiculum of hippocampus. All specific binding of this radioligand was inhibited by 200 nM frog PP. The density of 0.1 nM [(125)I]YVP binding was much smaller in all brain areas and frog PP-insensitive binding sites (NPFF(1) receptor subtype) were detected in septal, thalamic and hypothalamic areas but were absent in the spinal cord. The restricted distribution of NPFF(1) receptors in the CNS supports its specific role in a limited number of neuronal functions. In contrast to the rat spinal cord where the NPFF(1) system is absent, there is no strict separation between NPFF(1) and NPFF(2) system at the supraspinal level.

A diverse family of GPCRs expressed in specific subsets of nociceptive sensory neurons

In vertebrates, peripheral chemosensory neurons express large families of G protein-coupled receptors (GPCRs), reflecting the diversity and specificity of stimuli they detect. However, somatosensory neurons, which respond to chemical, thermal, or mechanical stimuli, are more broadly tuned. Here we describe a family of approximately 50 GPCRs related to Mas1, called mrgs, a subset of which is expressed in specific subpopulations of sensory neurons that detect painful stimuli. The expression patterns of mrgs thus reveal an unexpected degree of molecular diversity among nociceptive neurons. Some of these receptors can be specifically activated in heterologous cells by RFamide neuropeptides such as NPFF and NPAF, which are analgesic in vivo. Thus, mrgs may regulate nociceptor function and/or development, including the sensation or modulation of pain.

Role of adenosine in the spinal antinociceptive and morphine modulatory actions of neuropeptide FF analogs

The neuropeptide FF (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH(2)) and its synthetic analogs bind to specific receptors in the spinal cord to produce antinociceptive effects that are partially attenuated by opioid antagonists, and at sub-effective doses neuropeptide FF receptor agonists augment spinal opioid antinociception. Since adenosine plays an intermediary role in the production of spinal opioid antinociception, this study investigated whether this purine has a similar role in the expression of spinal effects produced by neuropeptide FF receptor agonists. In rats bearing indwelling spinal catheters, injection of adenosine receptor agonists, N6-cyclohexyladenosine (CHA, 1.72 nmol) and N-ethylcarboxiamidoadenosine (NECA, 1.95 nmol), as well as morphine (13.2 nmol) elicited antinociception in the tail-flick and paw-pressure tests. Pretreatment with intrathecal 8-phenyltheophylline (5.9 and 11.7 nmol), an adenosine receptor antagonist, blocked the effect of all three agents without influencing baseline responses. Administration of two synthetic neuropeptide FF (NPFF) analogs, [D-Tyr(1),(NMe)Phe(3)]NPFF (1DMe, 0. 86 nmol) and [D-Tyr(1),D-leu(2),D-Phe(3)]NPFF (3D, 8.6 nmol) produced sustained thermal and mechanical antinociception. Pretreatment with doses of intrathecal 8-phenyltheophylline (5.9, 11. 7 and 23.5 nmol), producing adenosine receptor blockade, significantly inhibited the antinociceptive effects of 1DMe or 3D. Injection of a sub-antinociceptive dose of 1DMe (0.009 nmol) significantly augmented the antinociceptive effect of intrathecal morphine (13.2 nmol) in the tail-flick and paw-pressure tests. Intrathecal 8-phenyltheophylline (11.7 nmol) reduced the effect of this combination. Administration of low dose of 1DMe (0.009 nmol) or 3D (0.009 nmol) very markedly potentiated the antinociceptive actions of the adenosine receptor agonist, N6-cyclohexyladenosine (0. 43, 0.86 and 1.72 nmol) in the tail-flick and paw-pressure tests 50 min after injection. The results suggest that the antinociceptive and morphine modulatory effects resulting from activation of spinal NPFF receptors could be due to an increase in the actions or availability of adenosine.

Dual localization of neuropeptide FF receptors in the rat dorsal horn

Although neuropeptide FF (NPFF) is generally considered an anti-opioid, its intrathecal administration produces analgesia. In the present study, the stable analog 1DMe ([D.Tyr(1), (NMe)Phe(3)]neuropeptide FF) was used in quantitative autoradiographic experiments in combination with surgical and chemical lesions to precisely localize NPFF receptors in the rat spinal cord. Ligation of lumbar dorsal spinal roots revealed the presence of NPFF receptors in dorsal root fibers and it induced a significant accumulation of [(125)I]1DMe-specific binding on the side peripheral to the ligature, demonstrating that a population of NPFF receptors is synthesized in dorsal root ganglia and migrates anterogradely towards primary afferent nerve endings. Complete mid-thoracic spinal cord transection failed to modify the [(125)I]1DMe labeling density in the dorsal horn, indicating that NPFF receptors are not located on the descending fiber terminals. In contrast, unilateral microinjections of kainic acid into the dorsal horn dramatically reduced [(125)I]1DMe-specific binding in the superficial layers, revealing localization of a population of NPFF receptors on the spinal intrinsic neurons. NPFF receptor binding was not modified during the development of spinal opioid tolerance. The pre- and postsynaptic localization of spinal NPFF receptors provide further support for heterogeneity in the pain modulation by NPFF and related agonists.

The neuropeptide FF analogue, 1DME, enhances in vivo met-enkephalin release from the rat spinal cord

Behavioural studies have suggested that endogenous opioids mediate the antinociceptive action of neuropeptide FF (FLFQPQRF-NH2) at the spinal level in the rat. This hypothesis was directly assessed by investigating the effects of a NPFF analogue, 1DMe ([D-Tyr1,(NMe)Phe3]NPFF), on the spinal outflow of met-enkephalin-like material (MELM) in halothane-anaesthetised rats. Intrathecal infusion (0.1 ml/min) of 1DMe (0.1 microM-0.1 mM, for 45 min) produced a concentration-dependent increase in spinal MELM outflow which persisted for at least 90 min at the highest concentration tested. Intrathecal coadministration of the micro-opioid receptor antagonist CTOP (1 microM) did not significantly affect the spinal MELM overflow due to 0.1 mM 1DMe. In contrast, both naltrindole and nor-binaltorphimine, at concentrations (10 microM) that allow the selective blockade of alpha- and kappa-opioid receptors, respectively, significantly reduced the stimulatory effect of 1DMe on spinal MELM outflow. These data provide the first direct demonstration that met-enkephalin (among other opioid peptides) can mediate the antinociceptive action of NPFF at the spinal level in rats. In addition, they suggest that reciprocal excitatory interactions between opioids and opioid-modulatory factors (such as NPFF) participate in the physiological control of nociception.

Spinal effect of a neuropeptide FF analogue on hyperalgesia and morphine-induced analgesia in mononeuropathic and diabetic rats

1DMe, a neuropeptide FF (NPFF) analogue, has been shown to produce antinociception and to enhance morphine analgesia in rats after intrathecal administration. To determine whether 1DMe could correct hyperalgesia and restore morphine efficacy in mononeuropathic (MN) and diabetic (D) rats we examined the spinal effect of 1DMe in MN and D rats without and after spinal blockade of mu- and delta-opioid receptors with CTOP and naltrindole, respectively. The influence of 1DMe on morphine-induced antinociception was assessed in the two models using isobolographic analysis. Whereas 1DMe intrathecally injected (0.1, 1, 7.5 microg rat(-1)) was ineffective in normal (N) rats, it suppressed mechanical hyperalgesia (decrease in paw pressure-induced vocalisation thresholds) in both MN and D rats. This effect was completely cancelled by CTOP (10 microg rat(-1)) and naltrindole (1 microg rat(-1)) suggesting that it requires the simultaneous availability of mu- and delta-opioid receptors. The combinations of morphine: 1DMe (80.6:19.4% and 99.8:0.2%, in MN and D rats, respectively) followed by isobolographic analysis, showed a superadditive interaction, relative to the antinociceptive effect of single doses, in D rats only. In N rats, the combination of morphine: 1DMe (0.5 mg kg(-1), i.v.: 1 microg rat(-1), i.t., ineffective doses) resulted in a weak short-lasting antinociceptive effect. These results show a different efficacy of 1DMe according to the pain model used, suggesting that the pro-opioid effects of the NPFF in neuropathic pain are only weak, which should contribute to hyperalgesia and to the impaired efficacy of morphine.

Unilateral joint inflammation induces bilateral and time-dependent changes in neuropeptide FF binding in the superficial dorsal horn of the rat spinal cord: implication of supraspinal descending systems

Using quantitative autoradiography, the effects of acute and chronic inflammation on specific 125I-1DMethyl-FLFQPQRFamide binding were investigated in the rat spinal cord dorsal horn superficial layers, at 6 and 24 h and 2, 4, 6 and 12 weeks after induction of monoarthritis produced by injection of killed Mycobacterium butyricum suspended in Freund adjuvant in one tibio-tarsal joint. Six hours after monoarthritis induction, no modification in specific 125I-1DMethyl-FLFQPQRFamide binding was observed, whereas a significant bilateral increase occurred after 24 h and 2 weeks in L4/L5 dorsal horns, with a return to control values at 4, 6 and 12 weeks. Specific 125I-1DMethyl-FLFQPQRFamide binding was also investigated 24 h after monoarthritis induction in rats submitted 4 days before the induction to spinal cord lesions at the thoracic level (T9-T10). Hemisection of the spinal cord contralateral to the affected ankle prevented the transient bilateral increase in specific 125I-1DMethyl-FLFQPQRFamide binding, whereas total spinal cord section induced a significant bilateral decrease. All of these modifications were restricted to the spinal segments receiving afferent input from the arthritic ankle (L4/L5); no modifications were found at the levels L1 or C6-C8. These data suggest that FLFQPQRFamide is involved in spinal nociceptive processing during sustained peripheral nociceptor activation. The effects of spinal cord lesions in monoarthritic rats indicate that the modifications seen in the FLFQPQRFamide system activity, during sustained peripheral inflammation, depend on afferent fiber activation as well as on supraspinal controls.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Evidence for neuropeptide FF (FLFQRFamide) in rat dorsal root ganglia

By using specific antibodies and radioimmunological and immunohistochemical methods, we here show that neuropeptide FF (NPFF) occurs in cervical and lumbar dorsal root ganglia cells. Levels in the ganglia were low because they were detectable only after colchicine treatment or after unilateral dorsal rhizotomy. Similar high-performance liquid chromatography profiles were obtained from dorsal root ganglia and spinal cord extracts, indicating that the NPFF-immunoreactivity in the dorsal root ganglia represented similar molecular forms to that in the spinal cord. Immunocytochemistry localized NPFF-immunoreactivity in small- and medium-sized cells. These data suggest that low levels of NPFF present in fine diameter primary afferent fibers could be involved in the treatment of nociceptive information from fore- or hindlimb.

Neuropeptide FF, pain and analgesia

Neuropeptide FF (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2) and the octadecapeptide neuropeptide AF (Ala-Gly-Glu-Gly-Leu-Ser-Ser-Pro-Phe-Trp-Ser-Leu-Ala-Ala-Pro-Gln-Arg-Phe -NH2) were isolated from bovine brain, and were initially characterized as anti-opioid peptides. They can oppose the acute effects of opioids and an increase in their brain concentrations may be responsible for the development of tolerance and dependence to opioids. Numerous experiments suggest a possible neuromodulatory role for neuropeptide FF. A precursor protein has been identified, in particular in human brain. Neuropeptide FF immunoreactive neurons are present only in the medial hypothalamus, and the nucleus of the solitary tract, and in the spinal cord in the superficial layers of the dorsal horn and areas around the central canal. Depolarization induces a Ca2+-dependent release of neuropeptide FF immunoreactivity from the spinal cord. Neuropeptide FF acts through stimulation of its own receptors and high densities of specific binding sites are found in regions related either to sensory input and visceral functions or to the processing of nociceptive messages. In both isolated dorsal root ganglion neurons and CA1 pyramidal neurons of the hippocampus, neuropeptide FF has little effect of its own but reverses the effects of mu-opioid receptor agonists. In agreement with the hypothesized anti-opioid role of neuropeptide FF, supraspinal injection lowers the nociceptive threshold and reverses morphine-induced analgesia in rats. Furthermore, immunoneutralization of neuropeptide FF increases endogenous and exogenous opioid-induced analgesia. Similarly, microinfusion of neuropeptide FF or neuropeptide FF analogs into the nucleus raphe dorsalis, the parafascicular nucleus, or the ventral tegmental area has no effect on the nociceptive threshold but inhibits the analgesia induced by co-injected morphine. Furthermore, infusion of neuropeptide FF into the parafascicular nucleus or the nucleus raphe dorsalis reverses the analgesic effect of morphine infused into the nucleus raphe dorsalis or the parafascicular nucleus, respectively, demonstrating remote interactions between neuropeptide FF and opioid systems. By contrast, intrathecal administration of neuropeptide FF analogs induces a long lasting, opioid-dependent analgesia and potentiates the analgesic effect of morphine. Analgesic effects of neuropeptide FF after supraspinal injection could also be observed, for example during nighttime. In young mice, (1DMe)Y8Famide (D.Tyr-Leu-(NMe)Phe-Gln-Pro-Gln-Arg-Phe-NH2), a neuropeptide FF analog, increases delta-opioid receptor-mediated analgesia. These findings indicate that neuropeptide FF constitutes a neuromodulatory neuronal system interacting with opioid systems, and should be taken into account as a participant of the homeostatic process controlling the transmission of nociceptive information.

Opiate tolerance and dependence: receptors, G-proteins, and antiopiates

Despite the existence of a large body of information on the subject, the mechanisms of opiate tolerance and dependence are not yet fully understood. Although the traditional mechanisms of receptor down-regulation and desensitization seem to play a role, they cannot entirely explain the phenomena of tolerance and dependence. Therefore, other mechanisms, such as the presence of antiopiate systems and the coupling of opiate receptors to alternative G-proteins, should be considered. A further complication of studies of opiate tolerance and dependence is the multiplicity of endogenous opiate receptors and peptides. This review will focus on the endogenous opioid system--peptides, receptors, and coupling of receptors to intracellular signaling via G-proteins--in the context of their roles in tolerance and dependence. Opioid peptides include the recently discovered endomorphins and those encoded by three known genes--pro-opiomelanocortin, pro-enkephalin, and pro-dynorphin. They bind to three types of receptors--mu, delta, and kappa. Each of the receptor types is further divided into multiple subtypes. These receptors are widely known to be coupled to G-proteins of the Gi and Go subtypes, but an increasing body of results suggests coupling to other G-proteins, such as Gs. The coupling of opiate receptors to Gs, in particular, has implications for tolerance and dependence. Alterations at the receptor and transduction level have been the focus of many studies of opiate tolerance and dependence. In these studies, both receptor down-regulation and desensitization have been demonstrated in vivo and in vitro. Receptor down-regulation has been more easily observed in vitro, especially in response to morphine, a phenomenon which suggests that some factor which is missing in vitro prevents receptors from down-regulating in vivo and may play a critical role in tolerance and dependence. We suggest that antiopiate peptides may operate in vivo in this capacity, and we outline the evidence for the antiopiate properties of three peptides: neuropeptide FF, orphanin FQ/nociceptin, and Tyr-W-MIF-1. In addition, we provide new results suggesting that Tyr-W-MIF-1 may act as an antiopiate at the cellular level by inhibiting basal G-protein activation, in contrast to the activation of G-proteins by opiate agonists.

Autoradiographic characterization of rat spinal neuropeptide FF receptors by using [125I][D.Tyr1, (NMe)Phe3]NPFF

The binding properties of neuropeptide FF (NPFF) receptors were investigated in different laminae of the rat spinal cord by using quantitative autoradiography and [125I][D.Tyr1, (NMe)Phe3]NPFF as radioligand. In the superficial layers, the specific binding of [125I][D.Tyr1, (NMe)Phe3]NPFF was time-dependent, reversible, and saturable (KD = 0.1 nM). Preincubation of spinal sections increased the maximal number of [125I][D.Tyr1, (NMe)Phe3]NPFF binding sites. Bestatin, an inhibitor of aminopeptidases, increased significantly the apparent affinity of NPFF. Optimal binding of [125I][D.Tyr1, (NMe)Phe3]NPFF was observed in the presence of 120 mM NaCl in all laminae of the spinal cord. No significant differences were noted in the salt dependence in laminae I-II, IV-V, and X, and the pharmacological profile of [125I][D.Tyr1, (NMe)Phe3]NPFF binding was similar in each laminae. These results do not support the existence of NPFF receptors subtypes differentially localized in different area of the spinal cord. Our data reveal the effects of tissue treatments on binding characteristics of NPFF receptors and indicate that [125I][D.Tyr1, (NMe)Phe3]NPFF is a useful radioactive probe for the characterization of NPFF receptors in discrete brain areas.