Central neuronal pathways containing FLFQPQRFamide-like (morphine-modulating) peptides in the rat brain

Anorexigenic neuropeptides as anti-obesity and neuroprotective agents: exploring the neuroprotective effects of anorexigenic neuropeptides

Since 1975, the incidence of obesity has increased to epidemic proportions, and the number of patients with obesity has quadrupled. Obesity is a major risk factor for developing other serious diseases, such as type 2 diabetes mellitus, hypertension, and cardiovascular diseases. Recent epidemiologic studies have defined obesity as a risk factor for the development of neurodegenerative diseases, such as Alzheimer's disease (AD) and other types of dementia. Despite all these serious comorbidities associated with obesity, there is still a lack of effective antiobesity treatment. Promising candidates for the treatment of obesity are anorexigenic neuropeptides, which are peptides produced by neurons in brain areas implicated in food intake regulation, such as the hypothalamus or the brainstem. These peptides efficiently reduce food intake and body weight. Moreover, because of the proven interconnection between obesity and the risk of developing AD, the potential neuroprotective effects of these two agents in animal models of neurodegeneration have been examined. The objective of this review was to explore anorexigenic neuropeptides produced and acting within the brain, emphasizing their potential not only for the treatment of obesity but also for the treatment of neurodegenerative disorders.

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.

Anoretic effects of neuropeptide FF are mediated via central mu and kappa subtypes of opioid receptors and receptor ligands

Recently, we demonstrated that neuropeptide FF (NPFF) causes anorexigenic effects in chicks that were associated with the hypothalamus. The present study was designed to better understand some of the central mechanisms that mediate these effects. Co-injection of NPFF and beta-funaltrexamine (FNA, a mu opioid receptor antagonist) did not suppress food intake more than when NPFF and FNA were injected alone. However, co-injection of NPFF and ICI-174,864 (ICI, a delta opioid receptor antagonist) caused a greater reduction in food intake than when NPFF and ICI were injected alone. Co-injection of NPFF and nor-binaltorphimine (BNI, a kappa opioid receptor antagonist) did not cause a greater suppression of food intake than when NPFF and BNI were injected alone. Hyperphagia induced by neuropeptide Y and beta-endorphin (both ligands of opioid receptors) was reversed by NPFF. These results suggest that NPFF-induced satiety has a relationship with mu and kappa but not delta subtypes of opioid receptors, and since NPFF does not bind opioid receptors itself NPFF-associated satiety is likely mediated by effects on opioid receptor ligands such as NPY and beta-endorphin. Thus, NPFF induced satiety may be mediated via modulation of the chick's innate opioid-associated orexigenic system.

Short-term anorexigenic effects of central neuropeptide VF are associated with hypothalamic changes in chicks

The present study was designed to measure food and water intake, changes in hypothalamic chemistry, and other behaviour modifications after central injection of neuropeptide (NP) VF in broiler type chicks. In Experiment 1, chicks responded to central NPVF with a reduction in food intake for up to 90 min post injection. Water intake was unaffected. In Experiment 2, NPVF exerted a less potent and shorter duration of attenuated food intake than did the structurally related NPFF. In Experiment 3, 16.0 nmol NPVF reversed the prolactin-releasing peptide induced orexigenic effect. In Experiment 4, central NPVF treatment was associated with decreased c-Fos immunoreactivity in the lateral hypothalamus, whereas c-Fos immunoreactivity in the dorsomedial nucleus, infundibular nucleus (homologue to the mammalian arcuate nucleus) and ventromedial nucleus was increased. In Experiment 5, behaviours unrelated to ingestion including sit, stand, deep rest and locomotion were affected by central NPVF injection. Some of these behaviours are incompatible with ingestion and may contribute to hypothalamic associated perception of satiety after central NPVF. In conclusion, NVPF is a short-term regulator of appetite and its effects are associated with hypothalamic and behaviour changes in chicks.

Central neuropeptide FF reduces feed consumption and affects hypothalamic chemistry in chicks

Information on the physiological functions of neuropeptide FF; NPFF, a morphine modulating octapeptide in avians is lacking. Thus, we designed a study to investigate the effects of central NPFF with particular emphasis on appetite-related processes. Cobb-500 chicks were intracerebroventricularly (ICV) injected with 0, 4.16, 8.32 or 16.6nmol NPFF, and feed and water intake were quantified. Feed intake was linearly decreased as NPFF dose increased, and this effect decayed over time and was not significant by 120min post-injection. Water intake was not affected by ICV NPFF. In a second exp, we observed that naloxone completely reversed the NPFF-induced decrease in feed intake. The amount of time a visible marker took to travel through the total length of the alimentary canal linearly increased as NPFF dose increased. We measured neuronal activation in the lateral hypothalamus (LH), paraventricular nucleus (PVN) dorsomedial nucleus (DMN) and ventromedial hypothalamus (VMN) of the hypothalamus, and nucleus dorsomedialis posterior thalami (DMP) of the thalamus. The DMN, DMP, PVN and VMH were all activated by ICV NPFF while the LH was not affected. Finally, we determined that the anorexigenic effect of ICV NPFF is primarily behavior specific, since behaviors unrelated to ingestion were not increased the same duration of time as was consumatory pecking. We conclude that NPFF causes anorexigenic effects in chicks that are primarily behavior specific.

Distribution of neuropeptide FF-like immunoreactive structures in the lamprey central nervous system and its relation to catecholaminergic neuronal structures

The neuropeptide FF (NPFF) is an octapeptide of the RFamide-related peptides (FaRPs) that was primarily isolated from the bovine brain. Its distribution in the CNS has been reported in several mammalian species, as well as in some amphibians. Therefore, in order to gain insight in the evolution on the expression pattern of this neuropeptide in vertebrates, we carried out an immunohistochemical study in the sea lamprey, Petromyzon marinus. The distribution of NPFF-like-immunoreactive (NPFF-ir) structures in the lamprey brain is, in general, comparable to that previously described in other vertebrate species. In lamprey, most of the NPFF-ir cells were found in the hypothalamus, particularly in two large populations, the bed nucleus of the tract of the postoptic commissure and the tuberomammillary area. Numerous NPFF-ir cells were also observed in the rostral rhombencephalon, including a population in the dorsal isthmic gray and the reticular formation. Additional labeled neurons were found inside the preoptic region, the parapineal vesicle, the periventricular mesencephalic tegmentum, the descending trigeminal tract, the nucleus of the solitary tract, as well as in the gray matter of the spinal cord. The NPFF-ir fibers were widely distributed in the brain and the spinal cord, being, in general, more concentrated throughout the basal plate. The presence of NPFF-ir fibers in the lamprey neurohypophysis suggests that the involvement of NPFF-like substances in the hypothalamo-hypophyseal system had emerged early during evolution.

Changes in neuropeptide FF and NPY immunohistochemical patterns in rat brain under heroin treatment

Immunohistochemical distribution patterns of neuropeptide FF (NPFF) and neuropeptide tyrosine (NPY) were studied in the brain of rats submitted to two different protocols of heroin treatment. In drug-naive rats, acutely injected heroin significantly depleted NPFF-immunoreactive material within the neurons of the nucleus of solitary tract (NTS), significantly decreased the density of NPFF-immunoreactive nerve fibers within the median eminence, pituitary stalk, and neurohypophysis, and markedly increased NPY-immunoreactive neurons and nerve fibers in the thalamic paraventricular nucleus and bed nucleus of stria terminalis. In drug-sensitized rats, heroin significantly increased the number and immunostaining intensity of the NPFF-immunoreactive neurons within the NTS and induced minor changes in the NPFF-immunoreactive nerve fiber network of the median eminence, pituitary stalk, and neurohypophysis and a relatively minor increase in NPY neurons in the thalamic paraventricular nucleus and bed nucleus of stria terminalis. These heroin-induced changes suggest that NPFF is involved in regulating the effects of the heroin injection and in the mechanisms underlying behavioral sensitization. They also add further support to the key role of NPY in any conditions tending to change the animal homeostasis.

RFamide-related peptides signal through the neuropeptide FF receptor and regulate pain-related responses in the rat

The mammalian RFamide-related peptide RFRP1 was found to signal through the neuropeptide FF 2 receptor expressed in Xenopus oocytes. The peptide induced a dose-dependent outward current, which was dependent on the simultaneous expression of GIRK1 and GIRK4 potassium channels. In neuropathic rats, RFRP1 administered intrathecally induced tactile antiallodynia and thermal antinociception, whereas in the solitary tract nucleus it produced only mechanical antihyperalgesia. Expression of the RFamide-related peptide mRNA in the rat CNS was distinctly different from that of neuropeptide FF. Most notably, the gene was not expressed in the hindbrain or spinal cord at detectable levels. However, there was a prominent group of RFamide-related peptide mRNA-expressing neurons in the central hypothalamus, in the area in and between the dorsomedial and ventromedial nuclei. The results suggest that RFamide-related peptides are potentially involved in pain regulation through a hypothalamo-medullary projection system, and possibly via action on neuropeptide FF 2 receptors. In neuropathic animals, the pain suppressive effect of RFamide-related peptide varies depending on the submodality of noxious test stimulation and the site of RFamide-related peptide administration.

Detailed distribution of neuropeptide FF receptors (NPFF1 and NPFF2) in the rat, mouse, octodon, rabbit, guinea pig, and marmoset monkey brains: a comparative autoradiographic study

The distribution of neuropeptide FF receptors (NPFF(1) and NPFF(2)) was analyzed throughout the central nervous system of rodents (rat, mouse, Octodon degus, and guinea pig), rabbit, and marmoset monkey brains, representing three orders of mammals. Quantitative in vitro receptor autoradiography with [(125)I]EYF ([(125)I]EYWSLAAPQRF-NH(2)) and [(125)I]YVP ([(125)I]YVPNLPQRF-NH(2)) as specific radioligands for NPFF(2) and NPFF(1) receptors, respectively, was used. The NPFF(2) receptor is predominantly expressed in all species, except in the central nervous system of Octodon degus, in which it is undetectable. The density of the NPFF(1) subtype is low in rat and mice, moderate in octodon, rabbit, and monkey, and relatively high in the guinea pig. The present study reveals prominent species differences in the NPFF receptors expression in the brain. The distribution pattern of NPFF(2) receptors in the diencephalon and the superficial layers of the spinal cord is consistent with a hypothesized potential role for NPFF in the modulation of sensory input and opioid analgesia. In contrast, the constant presence of NPFF(1) receptors in the septum, the nucleus of the tractus solitarius, and the hypothalamus suggest its participation in neuroendocrine functions.

Prolactin-releasing peptide affects pain, allodynia and autonomic reflexes through medullary mechanisms

Prolactin-releasing peptide (PrRP) and neuropeptide FF (NPFF) are RF-amide peptides expressed in brain areas involved in pain modulation. NPFF displays multiple effects on acute, inflammatory and neuropathic pain. The potential role of PrRP in pain was addressed by intrathecal and intracerebral injections of PrRP on pain-related responses in both neuropathic and normal rats. Particularly in the dorsal medulla, PrRP produced significant antinociception in normal rats and an antiallodynic effect in neuropathic rats. To understand the basis of PrRP-induced pain modulation, distributions of PrRP, PrRP receptor, and NPFF were compared in the rat central nervous system. PrRP and NPFF mRNA were expressed in different parts of the nucleus of the solitary tract. In the medulla, PrRP receptor mRNA expression was abundant only in area postrema. Of the peptides studied, only NPFF mRNA was found in the dorsal horn of the spinal cord and spinal nucleus of the trigeminal nerve. PrRP-immunoreactivity corresponded to the mRNA distribution. Even if the neuronal groups producing NPFF and PrRP were distinct, the fiber networks immunoreactive for PrRP and NPFF overlapped. The results show that PrRP modulates nociception due to supraspinal rather than spinal action, and that its antinociceptive mechanism differs from that previously characterized for NPFF.

Neuropeptide FF exerts pro- and anti-opioid actions in the parabrachial nucleus to modulate food intake

Neurons that synthesize the morphine modulatory peptide neuropeptide FF (NPFF; Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2) densely innervate the parabrachial nucleus (PBN), an area implicated in regulating food intake. We analyzed opioid-related actions of NPFF in feeding in adult male Sprague-Dawley rats. Unilateral infusion of 2 nmol/0.5 microl of the mu-opioid receptor agonist [d-Ala2,NMe-Phe4,glycinol5]enkephalin (DAMGO) into the lateral PBN increased 4-h food intake from 0.7 +/- 0.1 to 3.3 +/- 0.3 g. NPFF (1.25-5.0 nmol) prevented this hyperphagic mu-opioidergic action. In rats fed after 4-h deprivation (baseline = 12.3 +/- 0.3 g/2 h), 5 nmol of NPFF did not alter and larger doses (10 and 20 nmol) actually increased food intake (+36, 54%). Twenty nanomoles also elevated intake of freely feeding rats (from 0.7 +/- 0.1 to 5.1 +/- 1.0 g/4 h). The opioid receptor blocker naloxone (10 nmol) antagonized this increase. These data reveal both pro- and anti-opioid actions of NPFF in the PBN to modulate feeding. The mechanisms for the opposite actions of low and high concentrations of this neuropeptide in parabrachial regulation of food intake remain to be determined.

Endothelin receptor antagonists restore morphine analgesia in morphine tolerant rats

Several neurotransmitter mechanisms have been proposed to play a role in the development of morphine tolerance. The present study provides evidence for the first time that endothelin (ET) antagonists can restore morphine analgesia in morphine tolerant rats. Tolerance to morphine was induced by subcutaneous implantation of six morphine pellets during a 7-day period. The degree of tolerance to morphine was measured by determining analgesic response (tail-flick latency) and hyperthermic response to morphine sulfate (8 mg/kg, subcutaneously (s.c.)) in placebo and morphine pellet implanted rats. The maximal tail-flick latency in morphine pellet-vehicle treated rats (7.54 s) was significantly lower (P<0.05) when compared to placebo pellet-vehicle treated rats (10s), indicating that tolerance developed to the analgesic effect of morphine. In separate sets of experiments, ET antagonists, BQ123 (10 microg, intracerebroventricularly (i.c.v.)) and BMS182874 (50 microg, i.c.v.) were administered in placebo and morphine tolerant rats. BQ123 was injected twice daily for 7 days and once on day 8. BMS182874 was administered only on day 8. Morphine (8 mg/kg, s.c.) was administered 30min after BQ123 or BMS182874 administration. It was found that both BQ123 and BMS182874 potentiated morphine analgesia in placebo and morphine tolerant rats. BQ123 potentiated tail-flick latency by 30.0% in placebo tolerant rats and 94.5% in morphine tolerant rats compared to respective controls. BMS182874 potentiated tail-flick latency by 30.2% in placebo tolerant rats and 66.7% in morphine tolerant rats. Morphine-induced hyperthermic effect was also potentiated by BQ123 and BMS182874. The duration of analgesic action was also prolonged by BQ123 and BMS182874. The effect of BMS182874 was less as compared to BQ123. BQ123 and BMS182874 are selective ET(A) receptor antagonists. Therefore, it is concluded that ET(A) receptor antagonists restore morphine analgesia in morphine tolerant rats.

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.

Central administration of neuropeptide FF causes activation of oxytocin paraventricular hypothalamic neurones that project to the brainstem

Neuropeptide FF (NPFF), a morphine modulatory peptide, is emerging as an important neuromodulator in the context of central autonomic and neuroendocrine regulation. NPFF immunoreactivity and receptors have been identified in discrete autonomic regions within the brain and spinal cord, including the hypothalamic paraventricular nucleus (PVN). In this study, we examined the effects of intracerebroventricular (i.c.v.) administration of NPFF on activation of chemically identified PVN neurones that project to the brainstem nucleus of the solitary tract (NTS). In conscious rats, i.c.v. NPFF at a dose of 10 micro g, but not 8 micro g, caused an increase in arterial blood pressure. Immunohistochemical analysis revealed a dose-dependent increase in activated (Fos positive) PVN neurones following i.c.v. NPFF administration compared to controls receiving i.c.v. saline. Activated PVN neurones were located predominantly in the parvocellular compartment of the nucleus with relatively few Fos positive cells in the magnocellular subdivision. Chemical identification of activated neurones revealed significant number of activated cells to be oxytocin positive, whereas only few vasopressin, tyrosine hydroxylase (TH) or corticotrophin-releasing factor (CRF) neurones were double-labelled. Injection of the retrograde tracer fluorogold into the NTS resulted in labelling of significant numbers of parvocellular oxytocin, but not vasopressin, TH or CRF, PVN neurones. We conclude that centrally administered NPFF stimulates brainstem-projecting oxytocin PVN neurones. Oxytocin released from terminals within the NTS oxytocin thus modulate the activity of ascending visceral autonomic pathways that synapse initially within the NTS.

Isolation, characterization, and distribution of a novel neuropeptide, Rana RFamide (R-RFa), in the brain of the European green frog Rana esculenta

A novel neuropeptide of the RFamide peptide family was isolated in pure form from a frog (Rana esculenta) brain extract by using reversed-phase high performance liquid chromatography in combination with a radioimmunoassay for mammalian neuropeptide FF (NPFF). The primary structure of the peptide was established as Ser-Leu-Lys- Pro-Ala-Ala-Asn-Leu-Pro-Leu- Arg-Phe-NH(2). The sequence of this neuropeptide, designated Rana RFamide (R-RFa), exhibits substantial similarities with those of avian LPLRFamide, gonadotropin-inhibitory hormone, and human RFRP-1. The distribution of R-RFa was investigated in the frog central nervous system by using an antiserum directed against bovine NPFF. In the brain, immunoreactive cell bodies were primarily located in the hypothalamus, i.e., the anterior preoptic area, the suprachiasmatic nucleus, and the dorsal and ventral hypothalamic nuclei. The most abundant population of R-RFa-containing neurons was found in the periependymal region of the suprachiasmatic nucleus. R-RFa- containing fibers were widely distributed throughout the brain from the olfactory bulb to the brainstem, and were particularly abundant in the external layer of the median eminence. In the spinal cord, scattered immunoreactive neurons were found in the gray matter. R-RFa-positive processes were found in all regions of the spinal cord, but they were more abundant in the dorsal horn. This study provides the first characterization of a member of the RFamide peptide family in amphibians. The occurrence of this novel neuropeptide in the hypothalamus and median eminence and in the dorsal region of the spinal cord suggests that, in frog, R-RFa may exert neuroendocrine activities and/or may be involved in the transmission of nociceptive stimuli.

Central administration of neuropeptide FF (NPFF) causes increased neuronal activation and up-regulation of NPFF gene expression in the rat brainstem

Neuropeptide FF (NPFF) is a morphine modulatory peptide that plays an important role in a wide variety of physiological functions, including those related to nociception and central autonomic regulation. NPFF fibers and cells have been shown to be discretely localized in key autonomic centers within the brain, including the brainstem nucleus of the solitary tract (NTS). Central applications of NPFF evoke a number of important biological effects through activation of central neuronal circuits whose identities remain unknown at present. NPFF administered in this manner may also be capable of up- or down-regulating its own gene expression. In this study, we investigated the effects of intracerebroventricular (i.c.v.) administration of NPFF on the activation and the gene expression of NPFF in NTS neurons. Conscious rats received saline or NPFF (8 or 10 microg i.c.v.), with concomitant monitoring of arterial blood pressure. Brains were prepared for Fos immunohistochemistry to identify neuronal activation and NPFF in situ hybridization to determine cells expressing NPFF mRNA in the NTS. At a dose of 8 microg, i.c.v., NPFF did not evoke alterations in blood pressure, but, at 10 microg, there was an increase in arterial blood pressure of 30-40 mmHg. Image analysis showed a dose-dependent increase in number of NPFF neurons that were activated in rats receiving i.c.v. NPFF compared with saline controls. NPFF gene expression in the NTS showed a similar dose-dependent increase following i.c.v. administration of either 8 or 10 microg of NPFF. Significantly greater numbers of activated neurons expressing the NPFF gene (double labeled) were observed in the NTS at the level of the area postrema in animals receiving i.c.v. NPFF compared with saline controls. These data indicate that centrally administered NPFF is capable of up-regulating its own gene expression in the NTS and that this effect appears in part to be independent of elevations in arterial blood pressure that this peptide can evoke when administered i.c.v. at the higher dose. The up-regulation of NPFF may play a homeostatic role in response to specific cardiovascular challenges, such as hypotension.

Intracerebroventricular injection of neuropeptide FF, an opioid modulating neuropeptide, acutely reduces food intake and stimulates water intake in the rat

Neuropeptide FF (NPFF) is a mammalian peptide that is found in high concentrations in the central nervous system (CNS) and has also been detected in plasma. Various functions have been attributed to this peptide although its main action in the CNS remains unclear. In this study we observed that intracerebroventricular (ICV) injection of human NPFF, at early light phase in fasted rats, acutely reduced food intake and caused a large increase in water intake compared with saline injected controls. This effect was independently observed in two separate studies yielding similar results. Thus the central effects of NPFF to decrease food intake may be largely attributable to increased water intake.

Distribution and development of FMRFamide-like immunoreactive neuronal systems in the brain of the brown trout, Salmo trutta fario

The distribution of Phe-Met-Arg-Phe-amide (FMRFamide) peptide-immunoreactive (FMRF-ir) cells and fibers in the terminal nerve and central nervous system was investigated in developing stages and adults of the brown trout, Salmo trutta fario. The first FMRF-ir neurons appeared in the terminal nerve system of 8-mm embryos in and below the olfactory placode. In the brain, FMRF-ir neurons were first observed in the rostral hypothalamus, primordial hypothalamic lobe, mesencephalic laminar nucleus, and locus coeruleus of 12- to 13 -m embryos. After hatching, FMRF-ir cells appeared in the lateral part of the ventral telencephalic area and the anterior tuberal nucleus. In adult trout, FMRF-ir cells were observed in all these areas. The number of FMRF-ir neurons increased markedly in some of these populations during development. Dense innervation by FMRF-ir fibers was observed in the dorsal and lateral parts of the dorsal telencephalic area, and in the ventral telencephalic area, the lateral preoptic area, the medial hypothalamic and posterior tubercle regions, midbrain tegmentum and rhombencephalic reticular areas, the central gray, the superior raphe nucleus, the secondary visceral nucleus, the vagal nuclei, and the area postrema. Fairly rich FMRF-ir innervation was also observed in the optic tectum and some parts of the torus semicircularis. The saccus vasculosus and hypophysis received a moderate amount of FMRF-ir fibers. Innervation of most of these regions appeared either in late alevins or fry, although FMRF-ir fibers in the preoptic area, hypothalamus, and reticular areas appeared in embryos. Comparative analysis of the complex innervation pattern observed in the brain of trout suggests that FMRF is involved in a variety of functions, like the FMRF family of peptides in mammals.

Identification of neuropeptide FF-related peptides in rodent spinal cord

Peptides which should be generated from the neuropeptide FF (NPFF) precursor were identified in mouse and rat spinal cord, by using reverse phase high pressure liquid chromatography with radioimmunoassay and electrospray mass spectrometry detection. In both species, two octapeptides, NPFF (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-amide) and NPSF (Ser-Leu-Ala-Ala-Pro-Gln-Arg-Phe-amide) were identified but a longer peptide NPA-NPFF (Asn-Pro-Ala-Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-amide) was present at the highest concentration in rat spinal cord. In mouse, the homologous peptide, SPA-NPFF (Ser-Pro-Ala-Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-amide) was not detected. Both peptides NPFF and NPSF reverse morphine-induced analgesia in the tail flick test. Our data reveal species differences in the maturation of NPFF precursor.

New central projections of neuropeptide FF: colateral branching pathways in the brainstem and hypothalamus in the rat

Neuropeptide FF (NPFF), a morphine modulatory peptide, has been identified within discrete autonomic regions in the brainstem and hypothalamus. Triple fluorescence labelling was employed to identify collateral branching projections of NPFF neurons located within the nucleus tractus solitarius (NTS) and in the region of the hypothalamus between the dorsomedial and ventromedial hypothalamus. Injections of two retrograde tracers, rhodamine- and fluorescein-labelled latex microspheres into the pontine parabrachial nucleus (PBN) and the ventrolateral medulla resulted in labelling of NPFF neurons in the NTS that contained one (double-labelled) or both (triple-labelled) tracers. Within the NTS, most double- and triple-labelled NPFF neurons were localized at the level of the area postrema or just rostral to it and within the medial and dorsomedial subdivisions of the nucleus. Injections of tracers into the PBN and hypothalamic paraventricular nucleus revealed double- and triple-labelled NPFF neurons, a majority of which were located in a zone between the dorsomedial and ventromedial hypothalamus. These results indicate that NPFF neurons in the brainstem and hypothalamus may simultaneously transmit signals to their target nuclei in the brainstem and forebrain. This coordinated signalling may lead to synchronized responses of NPFF target sites and provide insights into the role of this peptide in cardiovascular and nociceptive responses.

Synaptic actions of neuropeptide FF in the rat parabrachial nucleus: interactions with opioid receptors

The pontine parabrachial nucleus (PBN) receives both opioid and Neuropeptide FF (NPFF) projections from the lower brain stem and/or the spinal cord. Because of this anatomical convergence and previous evidence that NPFF displays both pro- and anti-opioid activities, this study examined the synaptic effects of NPFF in the PBN and the mechanisms underlying these effects using an in vitro brain slice preparation and the nystatin-perforated patch-clamp recording technique. Under voltage-clamp conditions, NPFF reversibly reduced the evoked excitatory postsynaptic currents (EPSCs) in a dose-dependent fashion. This effect was not accompanied by apparent changes in the holding current, the current-voltage relationship or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced inward currents in the PBN cells. When a paired-pulse protocol was used, NPFF increased the ratio of these synaptic currents. Analysis of miniature EPSCs showed that NPFF caused a rightward shift in the frequency-distribution curve, whereas the amplitude-distribution curve remained unchanged. Collectively, these experiments indicate that NPFF reduces the evoked EPSCs through a presynaptic mechanism of action. The synaptic effects induced by NPFF (5 microM) could not be blocked by the specific mu-opioid receptor antagonist, D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (1 microM), but application of delta-opioid receptor antagonist Tyr-Tic-Phe-Phe (5 microM) almost completely prevented effects of NPFF. Moreover, the delta-opioid receptor agonist, Deltorphin (1 microM), mimicked the effects as NPFF and also occluded NPFF's actions on synaptic currents. These results indicate that NPFF modulates excitatory synaptic transmission in the PBN through an interaction with presynaptic delta-opioid receptors. These observations provide a cellular basis for NPFF enhancement of the antinociceptive effects consequent to central activation of delta-opioid receptors.

Pain facilitatory circuits in the mammalian central nervous system: their behavioral significance and role in morphine analgesic tolerance

Sensitivity to noxious stimulation is not invariant; rather, it is modulated by discrete pain inhibitory and facilitatory circuits. This paper reviews the neural circuits for pain facilitation, describes the conditions governing their environmental activation, and examines their role in an animal's behavioral repertoire. Mechanisms for pain facilitation are contrasted at both the neural and behavioral level with mechanisms for pain inhibition. In addition, the involvement of mechanisms for pain facilitation in morphine analgesic tolerance is discussed, and the implications of this involvement for accounts of the role of associative processes in analgesic tolerance are highlighted.

Neuropeptide FF and modulation of pain

Neuropeptide FF (NPFF) and the related longer peptide neuropeptide AF (NPAF) derive from a single gene in several mammalian species. The gene product is expressed mainly in the CNS, where the posterior pituitary and dorsal spinal cord contain the highest concentrations. Evidence from biochemical and immunohistochemical studies combined with in situ hybridization using NPFF gene-specific probes suggest that all NPFF-like peptides may not derive from the characterized NPFF gene, but that other genes can exist which give rise to related peptides. Intraventricular NPFF exerts antiopioid effects, but intrathecal NPFF potentiates the analgesic effects of morphine. NPFF mRNA expression is upregulated in the dorsal horn of the spinal cord after carrageenan-induced inflammation in the hind paw of the rat, but not in the neuropathic pain model induced by ligation of the spinal roots. NPFF produces a submodality-selective potentiation of the antinociceptive effect induced by brain stem stimulation in the spinal cord during inflammation, and this effect is independent of naloxone-sensitive opioid receptors. In neuropathic animals, NPFF injected into the periaqueductal grey produces a significant attenuation of tactile allodynia, which is not modulated by naloxone. NPFF thus modulates pain sensation and morphine analgesia under normal and pathological conditions through both spinal and brain mechanisms.

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.

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.

Melanocortins and cardiovascular regulation

The melanocortins form a family of pro-opiomelanocortin-derived peptides that have the melanocyte-stimulating hormone (MSH) core sequence, His-Phe-Arg-Trp, in common. Melanocortins have been described as having a variety of cardiovascular effects. We review here what is known about the sites and mechanisms of action of the melanocortins with respect to their effects on cardiovascular function, with special attention to the effects of the gamma-melanocyte-stimulating hormones (gamma-MSHs). This is done in the context of present knowledge about agonist selectivity and localisation of the five melanocortin receptor subtypes cloned so far. gamma2-MSH, its des-Gly12 analog (= gamma1-MSH) and Lys-gamma2-MSH are 5-10 times more potent than adrenocorticotropic hormone-(4-10)(ACTH-(4-10)) to induce a pressor and tachycardiac effect following intravenous administration. The Arg-Phe sequence near the C-terminal seems to be important for full in vivo intrinsic activity. Related peptides with a C-terminal extension with (gamma3-MSH) or without the Arg-Phe sequence (alpha-MSH, as well as the potent alpha-MSH analog, [Nle4,D-Phe7]alpha-MSH), are, however, devoid of these effects. In contrast, ACTH-(1-24) has a depressor effect combined with a tachycardiac effect, effects which are not dependent on the presence of the adrenals. Although the melanocortin MC3 receptor is the only melanocortin receptor subtype for which gamma2-MSH is selective, in vivo and in vitro structure-activity data indicate that it is not via this receptor that this peptide and related peptides exert either their pressor and tachycardiac effects or their extra- and intracranial blood flow increasing effect. We review evidence that the pressor and tachycardiac effects of the gamma-MSHs are due to an increase of sympathetic outflow to the vasculature and the heart, secondary to activation of centrally located receptors. These receptors are most likely localised in the anteroventral third ventricle (AV3V) region, a brain region situated outside the blood-brain barrier, and to which circulating peptides have access. These receptors might be melanocortin receptors of a subtype yet to be identified. Alternatively, they might be related to other receptors for which peptides with a C-terminal Arg-Phe sequence have affinity, such as the neuropeptide FF receptor and the recently discovered FMRFamide receptor. Melanocortin MC4 receptors and still unidentified receptors are part of the circuitry in the medulla oblongata which is involved in the depressor and bradycardiac effect of the melanocortins, probably via interference with autonomic outflow. Regarding the effects of the gamma-MSHs on cortical cerebral blood flow, it is not yet clear whether they involve activation of the sympathetic nervous system or activation of melanocortin receptors located on the cerebral vasculature. The depressor effect observed following intravenous administration of ACTH-(1-24) is thought to be due to activation of melanocortin MC2 receptors whose location may be within the peripheral vasculature. Melanocortins have been observed to improve cardiovascular function and survival time in experimental hemorrhagic shock in various species. Though ACTH-(1-24) is the most potent melanocortin in this model, alpha-MSH and [Nle4,D-Phe7]alpha-MSH and ACTH-(4-10) are quite effective as well. As ACTH-(4-10) is a rather weak agonist of all melanocortin receptors, it is difficult to determine via which of the melanocortin receptors the melanocortins bring about this effect. Research into the nature of the receptors involved in the various cardiovascular effects of the melanocortins would greatly benefit from the availability of selective melanocortin receptor antagonists.

Anti-opioid efficacy of neuropeptide FF in morphine-tolerant mice

The modulatory effects of 1DMe (d-Tyr-Leu-(NMe)Phe-Gln-Pro-Gln-Arg-Phe-NH2), an agonist of Neuropeptide FF (NPFF) receptors, on opioid antinociceptive activity have been compared in naive and tolerant mice in the tail-flick and the hot-plate tests. In naive mice, 1DMe alone had no effect on pain threshold but decreased dose-dependently (3-22 nmol) the analgesic activity of morphine in both tests. In tolerant mice, injections of 60-fold lower doses of 1DMe (0.05-0.5 nmol) reverse morphine-induced analgesia in the tail-flick test but this anti-opioid effect was no longer observed with the highest doses of 1DMe tested (3-22 nmol). In the hot-plate test, the anti-opioid action of 1DMe was not detected, whatever doses tested. Neither the NPFF-like immunoreactivity content of spinal cord and of olfactory bulbs, nor the density of NPFF receptors in olfactory bulbs, were altered. These results indicate that a chronic morphine treatment modifies the pharmacological properties of NPFF but the type of pain test is crucial in determining NPFF effects.

A differential modulation of allodynia, hyperalgesia and nociception by neuropeptide FF in the periaqueductal gray of neuropathic rats: interactions with morphine and naloxone

The effect of neuropeptide FF in the periaqueductal gray on pain behaviour was studied in rats with a chronic neuropathy induced by unilateral ligation of two spinal nerves. Neuropeptide FF produced in a non-monotonic fashion a significant attenuation of tactile allodynia. The antiallodynic effect was not significantly modulated by naloxone administered systemically or intracerebrally. The dose of neuropeptide FF producing a significant antiallodynic effect was not antinociceptive in a test of mechanical or thermal nociception. The thermal antinociceptive effect induced by morphine administered in the periaqueductal gray was significantly attenuated by neuropeptide FF, whereas that induced by systemically administered morphine was not. The interaction of neuropeptide FF with intracerebrally or systemically administered morphine in a test of tactile allodynia was not significant. The results indicate that neuropeptide FF in the periaqueductal gray may produce a selective attenuation of tactile allodynia in neuropathic rats. This antiallodynic effect is at least partly independent of naloxone-sensitive opioid receptors. Furthermore, neuropeptide FF in the periaqueductal gray attenuates antinociception induced by intracerebrally but not systemically administered morphine.

Neuropeptide FF in the rat adrenal gland: presence, distribution and pharmacological effects

Neuropeptide FF (NPFF, FLFQPQRFamide) is an FMRFamide-like octapeptide exhibiting antiopiate activity. The presence of both NPFF-immunoreactivity (NPFF-IR) and NPFF-specific receptors has been described in the mammalian central nervous system (CNS). The peripheral effects of NPFF indicate that NPFF-IR material is present outside the CNS. Biochemical and immunohistochemical methods enabled us to determine the presence and distribution of NPFF-IR in the rat adrenal gland. The amount of NPFF-IR material in whole gland was estimated by radioimmunoassay to be 19.00 +/- 4.00 fmol/gland. High performance liquid chromatography analysis of adrenal extracts revealed a single molecular form which coeluted with authentic NPFF. Demedullation decreased adrenal NPFF-IR content, indicating that NPFF-IR was present in both cortex and medulla. Light microscopy revealed NPFF-IR in beaded fibers confined in the outer part of the cortex and in medullary cells. Double-labeling with antityrosine-hydroxylase and anti-NPFF antibodies showed NPFF-IR in cortical catecholaminergic postganglionic fibers restricted to the subcapsular and glomerulosa zonae. NPFF-IR was also located in medullary chromaffin cells and in rays and islets of chromaffin cells dispersed throughout the cortex. Insulin-induced hypoglycemia did not alter NPFF-IR content. Denervation lowered adrenal NPFF-IR content. These data indicate that this peptide is present in nerve fibers of extrinsic origin. In vitro approaches using adrenal slices have shown that NPFF inhibited aldosterone release in a dose-dependent manner. Taken together, these data suggest that NPFF may participate in the control of aldosterone production and adrenal blood supply.

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.

Regulation of mu binding sites after chronic administration of antibodies directed against specific anti-opiate peptides

There is some indication that anti-opiate peptides (AOP) modulate opioid receptor systems by altering mu-receptor density. To further characterize this phenomenon, we investigated the effects of continuous infusion of anti-AOP IgG on mu binding sites in the brains of rats. Specifically, male Sprague-Dawley rats received intracerebroventricular (i.c.v.) infusions for 13 days of either control (rabbit) IgG or test IgGs: anti-dynorphin A IgG, anti-dynorphin A1-8 IgG, anti-alpha-MSH IgG, or the monoclonal anti-NPFF IgG. Administration of anti-NPFF IgG or the anti-dynorphin1-8 IgG significantly increased mu labeling by 40-70% in several brain regions at the caudate level. Contrary to these findings, anti-alpha-MSH IgG decreased (19-32%) [125I]-DAMGO labeling in several thalamic nuclei. The results suggest that the density of mu-opioid receptors is regulated in part by anti-opiate peptides in the extracellular fluid of the brain.

Peptide GEGLSS-like immunoreactivity in the rat central nervous system

A rabbit antiserum was raised against the N-terminal fragment peptide, GEGLSS (Gly-Glu-Gly-Leu-Ser-Ser) of bovine neuropeptide AF (NPAF, A18Famide). NPAF is an octadecapeptide isolated from the bovine brain together with neuropeptide FF (NPFF). GEGLSS-like immunoreactivity was localized with immunofluorescence technique in colchicine-treated rats in neuronal cell bodies of the supraoptic (SON) and paraventricular (PVN) hypothalamic nuclei. A few neurons were also observed in the retrochiasmatic part of the SON. GEGLSS-like immunoreactivity was also localized to nerve terminals of the posterior pituitary. No GEGLSS-ir neuronal cell bodies were observed in the medial hypothalamus, in an area that contains NPFF-ir neurons. GEGLSS immunoreactivity was also seen in the fibers and terminals of nucleus of the solitary tract. We injected a retrograde tracer, fluorogold, to the posterior pituitary gland and visualized GEGLSS-ir neuronal cell bodies double-labeled with the tracer in SON, PVN, and SOR. The pituitary stalk transsection totally abolished the GEGLSS-ir structures from the posterior pituitary. Our results suggest that GEGLSS immunoreactivity in the rat brain has a more limited distribution than NPFF immunoreactivity. GEGLSS immunoreactivity was partially colocalized with arginine-vasopressin and oxytocin in neuronal cell bodies in the SON and PVN. Considering the fact that the known rat NPFF-NPAF precursor does not contain GEGLSS structure, the detected GEGLSS immunoreactivity may be derived from a previously unknown precursor.

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.

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

A radioiodinated analogue of neuropeptide FF, [125I][D. Tyr1,(NMe) Phe3]neuropeptide FF, was used as a selective probe to label neuropeptide FF receptors in the rat spinal cord. Following neonatal capsaicin treatment, dorsal rhizotomy or sciatic nerve section, the distribution and possible alterations of spinal cord specific [125I][D.Tyr1,(NMe)Phe3]neuropeptide FF binding sites were evaluated using in vitro quantitative receptor autoradiography. In normal rats, the highest densities of sites were observed in the superficial layers of the dorsal horn (laminae I-II) whereas moderate to low amounts of labelling were seen in the deeper (III-VI) laminae, around the central canal, and in the ventral horn. Capsaicin-treated rats showed a bilateral decrease (47%) in [125I][D.Tyr1,(NMe)Phe3]neuropeptide FF binding in all spinal areas. Unilateral sciatic nerve section and unilateral dorsal rhizotomy induced significant depletions (15-27%) in [125I][D.Tyr1,(NMe)Phe3]neuropeptide FF labelling in the ipsilateral dorsal horn. These results suggest that a proportion of neuropeptide FF receptors is located on primary afferent terminals of the dorsal horn and could thus play a role in the modulation of nociceptive transmission.

Cardiovascular effects induced by the stimulation of neuropeptide FF receptors in the dorsal vagal complex: an autoradiographic and pharmacological study in the rat

Previous studies have suggested that central administration of neuropeptide FF-related peptides may modulate cardiovascular parameters in the rat. In the present study, we investigated the role of dorsal vagal complex neuropeptide FF receptors in the central regulation of cardiovascular parameters. The fate of neuropeptide FF receptors in normal and nodose ganglionectomized rats was investigated using an autoradiographic approach with 125I-[DTyr1, (NMe)Phe3]NPFF as ligand for these receptors. We showed that neuropeptide FF binding sites are preferentially located postsynaptically with respect to the vagal afferent fibers in the nucleus tractus solitarius. Thus, ganglionectomy reduced by only 30% and 17% the density of peptide binding sites in the rostral and caudal regions of this nucleus, respectively. Bilateral microinjection of neuropeptide FF (1 nmol) into the commissural nucleus tractus solitarius produced an increase in blood pressure (+13.8 +/- 0.8 mmHg, n = 6), bradycardia (-29.0 +/- 3.2 bpm) and a significant inhibition (-47.6 +/- 3.1%) of the cardiac component of the baroreceptor reflex. Further studies with doses below 1 nmol indicate that NTS microinjections of the neuropeptide produced a dose-dependent decrease in heart rate. Similar cardiovascular effects were observed after bilateral NTS microinjections of one analog neuropeptide FF receptor agonist, [DTyr1, (NMe)Phe3]NPFF (1 nmol). Pretreatment with prazosin (100 micrograms/kg), an alpha 1-adrenoreceptor antagonist, inhibited the neuropeptide FF-evoked blood pressure effect. In addition, the neuropeptide FF-induced heart rate decrease was abolished by pretreatment with atropine (30 micrograms/kg), a muscarinic receptor antagonist. Taken together, these anatomical and pharmacological data suggest that neuropeptide FF receptors within the nucleus tractus solitarius, preferentially located on the postsynaptic component, are involved in the central reflex regulation of cardiovascular parameters.

Role of opioid receptors in the spinal antinociceptive effects of neuropeptide FF analogues

1. Neuropeptide FF (NPFF) has been shown to produce antinociceptive effects and enhance morphine-induced antinociception after intrathecal (i.t.) injection. In this study, the spinal effects of two NPFF analogues, -D-Tyr1,(NMe)Phe3-NPFF (1DMe) and [D-Tyr1,D-Leu2,D-Phe3]NPFF (3D), which are resistant to degradation and exhibit a high affinity for NPFF binding sites, were examined in tests of thermal and mechanical nociception. 2. 1DMe and 3D produced potent dose-dependent spinal antinociception in the tail-flick test. On a molar basis, 1DMe was 20 and 50 times more potent than 3D and morphine, respectively, and high doses of 1DMe and 3D produced a sustained antinociceptive effect without visible signs of motor impairment. 3. Spinal antinociceptive effects produced by 1DMe (0.86 nmol) or 3D (8.6 nmol) were significantly reduced by i.t. co-administration of naloxone (11 nmol) or i.t. pre-administration of D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP, 9.25 nmol) or beta-funaltrexamine (beta-FNA, 2 nmol) or naltrindole (2.2 nmol). The doses of the mu-antagonists (CTOP and beta-FNA) or the delta-antagonist (naltrindole) used in 1DMe and 3D experiments blocked the antinociceptive effects of mu- or delta-receptor-selective agonists. 4. When administered in combination with antinociceptive doses of the mu-receptor agonist, morphine (13.2 nmol) or the delta-receptor agonist, [D-Ala2]deltorphin I (20 nmol), sub-effective dose of 1DMe or 3D (0.009 nmol) enhanced and prolonged the spinal effects of these opioid agonists. 5. The results of this study show that spinal mu- and delta-opioid receptors play a role in antinociception produced by NPFF analogues. These results also suggest a role for NPFF in modulation of nociceptive signals at the spinal level.

Downregulation of mu-opioid binding sites following chronic administration of neuropeptide FF (NPFF) and morphine

The effect of continuous ICV infusion of NPFF (10 micrograms/microliter) and morphine (40 micrograms/microliter) on mu-opioid binding sites was examined in rats using the in vitro radiolabeled techniques of whole-brain homogenate receptor binding and quantitative autoradiography. Mu receptors were labeled with [3H][D-Ala2-MePhe4,Glyol5] enkephalin in the homogenate binding experiments and with [125I][D-Ala2-MePhe4,Gly-ol5]enkephalin in autoradiographic studies. In homogenate binding studies, chronic administration of NPFF or morphine significantly downregulated mu receptors by 20% and 44%, respectively. Quantitative autoradiographic experiments demonstrated downregulation of mu opioid receptors in specific brain nuclei for both NPFF- and morphine-treated animals. Within the striatum and several nuclei of the thalamus, the mu receptors of the NPFF- and morphine-treated animals were decreased by 20-30% and 38-73%, respectively. These results suggest that NPFF may modulate opioid-mediated responses in part by altering the density of mu-opioid receptors.

Neuropeptide FF-containing efferent projections from the medial hypothalamus of rat: a Phaseolus vulgaris leucoagglutinin study

Neuropeptide FF (FMRFamide-like peptide, morphine-modulating peptide) is an octapeptide isolated from the bovine brain. There is evidence that neuropeptide FF participates in the modulation of nociceptive mechanisms. Neuropeptide FF acts through its own receptors which are distinct from the opiate receptors. In the rat brain neuropeptide FF is found in two major cell populations. We have studied the efferent connections of the hypothalamic neuropeptide FF-containing cell group, which is located in the medial hypothalamus between the dorsomedial, ventromedial and periventricular hypothalamic nuclei. By using an anterograde tracing method (Phaseolus vulgaris leucoagglutinin) combined with double-staining immunohistochemistry we characterized the connections of this cell group with the limbic system, certain hypothalamic nuclei, periaqueductal gray and with the solitary tract nucleus. In the limbic system, the major targets were the lateral septal nucleus, bed nucleus of stria terminalis and certain subnuclei in the amygdala. These connections suggest that neuropeptide FF may act, in addition to its well-characterized action in the sensory system, in limbic functions. Efferent connections to the periaqueductal gray suggest that neuropeptide FF may modulate the opiate mediated analgesia at this site. Good correlation between our results and receptor autoradiography support the idea that the terminal areas which our results show are target areas of the neuropeptide FF-containing system.

Distribution and characterization of neuropeptide FF-like immunoreactivity in the rat nervous system with a monoclonal antibody

Monoclonal antibodies against neuropeptide FF were produced and characterized. The antibodies are directed and highly specific to neuropeptide FF, and reactivity requires the C-terminal dipeptide of neuropeptide FF (Arg-Phe-NH2). Tissue extracts from bovine spinal cord, rat spinal cord and hypothalamus were analysed by high-pressure liquid chromatography coupled with radioimmunoassay using the characterized monoclonal antibody. Only one immunoreactive peptide was detected and it coeluted with authentic neuropeptide FF. Using this highly specific monoclonal antibody, the distribution of neuropeptide FF-like immunoreactivity was further studied by indirect immunohistochemistry. Immunoreactivity was seen in two major cell groups in the rat brain. The largest cell group was located in the medial hypothalamus between the dorsomedial and ventromedial nuclei. The other one was found in the nucleus of the solitary tract. Fibres immunoreactive for neuropeptide FF were located in the lateral septal nucleus, amygdala, different hypothalamic areas, nucleus of the solitary tract, ventral medulla, trigeminal complex and the dorsal horn of the spinal cord. Spinal and sympathetic ganglia were non-reactive. No neuropeptide FF immunoreactivity was seen in the gut autonomic nervous system or endocrine cells. The results show that neuropeptide FF-like immunoreactivity has a clearly more limited distribution in the nervous system than typical brain-gut peptides.

Neuropeptide FF (FLQPQRFamide) and IgG from neuropeptide FF antiserum affect spatial learning in mice

There is evidence indicating that the mammalian octapeptide FLQPQRFamide (F8Fa or neuropeptide FF, NPFF) is an endogenous modulator ('anti-opioid') of opioid systems. There is also substantial evidence implicating opioid systems in the mediation of spatial learning and memory. In the present study determinations were made of the effects of NPFF and IgG from antiserum against NPFF on the spatial performance of male mice in a water maze task, whereby over one day in six blocks of four trials the animals had to acquire the location of a submerged hidden platform using distal visual cues. Pretraining intracerebroventricular (i.c.v.) injection of NPFF-IgG, impaired spatial acquisition and retention, while i.c.v. administration of 1.0 microgram of NPFF marginally improved, and 10 micrograms of NPFF significantly reduced spatial acquisition. These results suggest that NPFF may have a modulatory influence on spatial acquisition.

Antinociceptive effects of intrathecally administered F8Famide and FMRFamide in the rat

The effects of intrathecal injections of F8Famide (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2, 0.05-17.5 nmol) and FMRF-amide (Phe-Met-Arg-Phe-NH2, 0.002-25 nmol), known as anti-opioid agents, were investigated by using noxious thermal (tail flick) and mechanical (paw pressure) tests in the rat. Both peptides produced significant long-lasting (24-48 h) analgesia in both tests without causing detectable motor dysfunction. Pretreatment with systemic naloxone (5.5 mumol/kg i.p.) attenuated the initial antinociceptive effects (first hour) induced by both peptides (8.8 nmol) in the tail flick test and only by FMRFamide in the paw pressure test. A subeffective dose of F8Famide (0.05 nmol) enhanced both the intensity and the duration of spinal morphine (6.6 nmol) analgesia in both tests. In contrast, a subanalgesic dose of FMRFamide (0.002 nmol) decreased the intensity and enhanced the duration of the effect of morphine. These results show that, besides acting as antinociceptive agents in the spinal cord, F8Famide and FMRFamide could differentially modulate spinal opioid functions.

Morphine physical dependence in the hibernator: central nervous system mechanisms underlying the development of dependence remain functional during depression induced by pentobarbital anesthesia

Earlier work suggested that adaptive mechanisms of the hibernating brain may also block the development of morphine physical dependence. To test an alternate view that CNS depression itself might be the major factor in the failure of dependence to develop, we compared the strength of dependence produced by morphine (two 75-mg pellets, s.c.) given for 12 hr in the presence, versus in the absence, of continuous pentobarbital anesthesia in nonhibernating ground squirrels (Citellus lateralis) and, in addition, in rats. Dependence was measured by the naloxone (5 mg/kg, s.c.) evoked abstinence syndrome in the awake state. The results demonstrated that pentobarbital-induced general anesthesia does not significantly reduce the development of morphine dependence in either species. We conclude that CNS depression alone does not account for the hibernation-related reduction in morphine physical dependence.

Neuropeptide FF is colocalized with catecholamine-synthesizing enzymes in neurons of the nucleus of the solitary tract

Neuropeptide FF (NFF) is an amidated octapeptide of bovine origin. It has some antiopioid-like effects and it elevates blood pressure when injected intravenously in rats. NFF-immunoreactive nerve cells and terminals are localized in large numbers in the dorsomedial caudal brainstem which is a region involved in central regulation of blood pressure. We compared the localization of NFF-immunoreactive neurons with medullary catecholamine-synthesizing neurons by using immunohistochemical double-labeling and light microscopic mirror methods. NFF and tyrosine hydroxylase coexisted in a minor portion of the NFF neurons in the nucleus of the solitary tract and occasional cell bodies were stained with both NFF and PNMT (phenylethanolamine N-methyltransferase) antisera. The results have anatomical correlation with previous pharmacological reports, suggesting that NFF is present in neuronal systems involved in cardiovascular reflex arcs.

Autoradiographic distribution of receptors to FLFQPQRFamide, a morphine-modulating peptide, in rat central nervous system

The neuropeptide FLFQPQRFamide is a structure related to FMRFamide which is able to inhibit the effects of both endogenous and exogenous opiates. This morphine-modulating activity is mediated via the stimulation of specific FLFQPQRFamide receptors, different from opiate receptors. In vitro quantitative receptor autoradiography was performed on frozen sections of rat central nervous system to characterize binding properties and visualize FLFQPQRFamide receptors using the specific ligand [125I]YLFQPQRFamide, a radio-iodinated analogue of FLFQPQRFamide. [125I]YLFQPQRFamide appeared to interact reversibly with a single class of binding sites (KD = 0.2 nM). The specific binding represented 80% of the total binding at 0.05 nM, the FLFQPQRFamide concentration used in this mapping study. Sites labelled with [125I]YLFQPQRFamide were distributed heterogeneously within the brain and spinal cord. A high density of FLFQPQRFamide binding sites was detected in the most external layers of the dorsal horn of spinal cord and various nuclei of pons and medulla including trigeminal, dorsal tegmental and reticular nuclei. Nucleus of solitary tract, parabrachial, ambiguous and facial nuclei are also intensively labelled. Some structures of mesencephalon and diencephalon exhibited a high density of FLFQPQRFamide binding sites: central gray, raphe nuclei and thalamic nuclei such as parafascicular, laterodorsal, central median, paratenial and paraventricular nuclei. Suprachiasmatic and mammillary nuclei, lateral, posterior and anterior areas of hypothalamus and medial preoptic area exhibited high labelling. FLFQPQRFamide binding sites were also seen in some structures of the dopaminergic meso-cortico-limbic system including ventral tegmental area, cingulate cortex, lateral septum and the head of the caudate-putamen. Dense labelling appeared in the presubiculum of hippocampus. The dissimilar mapping of FLFQPQRFamide and opiate brain receptors confirms our previous pharmacological findings in FLFQPQRFamide binding studies on rat spinal cord membranes, showing that FLFQPQRFamide receptors are different from opiate receptors. There was a good correspondence between localization of binding sites and that of the putative endogenous peptide. Both occur in brain areas previously associated with analgesic action of opiates. However, the mapping of FLFQPQRFamide receptors in the central nervous system suggests that the FLFQPQRFamide system could be implicated in other physiological functions.

Sex differences in the effects of neuropeptide FF and IgG from neuropeptide FF on morphine- and stress-induced analgesia

There is evidence suggesting that the endogenous mammalian octapeptide FLFQPQRFamide (F8Fa or neuropeptide FF, NPFF) has modulatory effects on opioid-mediated analgesia in rodents. There is also substantial evidence for sex differences in opioid analgesia, whereby male rats and mice display greater levels of opioid-mediated analgesia than females. In the present study, determinations were made of the effects of NPFF and IgG from antiserum against NPFF on morphine- and restraint stress-induced opioid analgesia in male and female deer mice. Intracerebroventricular (ICV) administrations of NPFF (0.10-10 micrograms) reduced in a dose-dependent manner morphine- and stress-induced analgesia in both male and female mice, with NPFF having markedly greater antagonistic effects in the male than female mice. Additionally, ICV administrations of NPFF-IgG increased the levels of morphine- and stress-induced analgesia and significantly reduced basal nociceptive sensitivity in male mice, whereas, in female mice, NPFF-IgG had no significant effects on either opioid-mediated analgesia or nociceptive sensitivity. These results indicate that there are sex differences in the modulatory effects of NPFF on opioid-mediated analgesia.

Ultrastructural localization of neuropeptide FF, a new neuropeptide in the brain and pituitary of rats

The octapeptide FLFQPQRF-NH2 or neuropeptide FF ('F8Famide'; FMRFamide-like peptide'; 'morphine-modulating peptide') has been isolated from the bovine brain. In this study, the ultrastructural localization of neuropeptide FF-like immunoreactivity was examined with pre-embedding immuno-electron microscopy in the nucleus of the solitary tract and in the posterior lobe of the pituitary gland of an adult rat. Neuropeptide FF-like immunoreactivity was detected only in neuronal structures of the medial and commissural nuclei of the solitary tract and in the neurohypophysis. In the medulla, the peroxidase-antiperoxidase reaction product was localized in large (100 nm) dense-cored vesicles and in the cytoplasm of the neuronal perikarya, dendrites and axon terminals. In the labeled terminals, small (50 nm) clear vesicles rimmed with the peroxidase-antiperoxidase reaction product were seen. Synaptic contacts of labeled perikarya and dendrites with unlabeled axon terminals were observed. Labeled axon terminals formed contacts with unlabeled dendrites and perikarya. In the posterior lobe of the pituitary gland, neuropeptide FF-like immunoreactivity was localized in nerve terminals frequently associated with blood vessels. The results suggest that neuropeptide FF-like peptides are localized exclusively in neuronal structures and that they are synthesized in cell somata and released from axon terminals. In the brain, neuropeptide FF-like peptides may act as neuromodulators involved in the regulation of autonomic functions. The localization of neuropeptide FF-like immunoreactivity in the neurohypophysis suggests endocrine regulatory functions of these peptides.