Ontogeny of the F8Famide-like (morphine-modulating) peptides in the central nervous system of rats

An Inhibitory Circuit From Brainstem to GnRH Neurons in Male Mice: A New Role for the RFRP Receptor

RFamide-related peptides (RFRPs, mammalian orthologs of gonadotropin-inhibitory hormone) convey circadian, seasonal, and social cues to the reproductive system. They regulate gonadotropin secretion by modulating gonadotropin-releasing hormone (GnRH) neurons via the RFRP receptor. Mice lacking this receptor are fertile but exhibit abnormal gonadotropin responses during metabolic challenges, such as acute fasting, when the normal drop in gonadotropin levels is delayed. Although it is known that these food intake signals to the reproductive circuit originate in the nucleus tractus solitarius (NTS) in the brainstem, the phenotype of the neurons conveying the signal remains unknown. Given that neuropeptide FF (NPFF), another RFamide peptide, resides in the NTS and can bind to the RFRP receptor, we hypothesized that NPFF may regulate GnRH neurons. To address this question, we used a combination of techniques: cell-attached electrophysiology on GnRH-driven green fluorescent protein-tagged neurons in acute brain slices; calcium imaging on cultured GnRH neurons; and immunostaining on adult brain tissue. We found (1) NPFF inhibits GnRH neuron excitability via the RFRP receptor and its canonical signaling pathway (Gi/o protein and G protein-coupled inwardly rectifying potassium channels), (2) NPFF-like fibers in the vicinity of GnRH neurons coexpress neuropeptide Y, (3) the majority of NPFF-like cell bodies in the NTS also coexpress neuropeptide Y, and (4) acute fasting increased NPFF-like immunoreactivity in the NTS. Together these data indicate that NPFF neurons within the NTS inhibit GnRH neurons, and thus reproduction, during fasting but prior to the energy deficit.

RFamide Peptides in Early Vertebrate Development

RFamides (RFa) are neuropeptides involved in many different physiological processes in vertebrates, such as reproductive behavior, pubertal activation of the reproductive endocrine axis, control of feeding behavior, and pain modulation. As research has focused mostly on their role in adult vertebrates, the possible roles of these peptides during development are poorly understood. However, the few studies that exist show that RFa are expressed early in development in different vertebrate classes, perhaps mostly associated with the central nervous system. Interestingly, the related peptide family of FMRFa has been shown to be important for brain development in invertebrates. In a teleost, the Japanese medaka, knockdown of genes in the Kiss system indicates that Kiss ligands and receptors are vital for brain development, but few other functional studies exist. Here, we review the literature of RFa in early vertebrate development, including the possible functional roles these peptides may play.

Neuropeptide AF differentially affects anorexia in lines of chickens selected for high or low body weight

The recently discovered anorectic effect of neuropeptide AF (NPAF) has not been studied in hypo-and hyperphagia animal models. The present study was designed to examine possible differences in appetite-related effects after central NPAF administration in lines of chickens that had undergone long-term divergent selection for low (LWS) or high (HWS) body weight and exhibit hypo- and hyperphagia, respectively. LWS chicks responded at a similar magnitude of food intake reduction to all doses of NPAF tested at all observation times. HWS chicks had an increased latency (150 min versus 30 min post injection) and an increased dose threshold of response (8 nmol versus 2 nmol) than LWS chicks. Water intake of LWS chicks was reduced in all doses tested at all observation times, whereas HWS chicks responded to the three doses of NPAF tested up to 60 min post injection, after which the decrease was sustained only at a dose of 8 nmol. In a comprehensive behaviour analysis, exploratory pecks, food pecks and locomotion were significantly reduced in both lines by NPAF, whereas sit time was increased in both lines. Other behaviours, including stand time, deep rest time and escape attempts, were not affected. These data suggest that the threshold of NPAF-induced anorexia is lower in LWS than HWS chicks and that NPAF-induced anorexia is a primary effect in both lines, and also support the hypothesis that differences exist in the central NPAF system between hypo- and hyperphagic individuals.

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.

Spatiotemporal sequence of appearance of NPFF-immunoreactive structures in the developing central nervous system of Xenopus laevis

Neuropeptide FF-like immunoreactive (NPFFir) cells and fibers were analyzed through development of Xenopus laevis. The first NPFFir cells appeared in the embryonic hypothalamus, which projected to the intermediate lobe of the hypophysis, the brainstem and spinal cord. Slightly later, scattered NPFFir cells were present in the olfactory bulbs and ventral telencephalon. In the caudal medulla, NPFFir cells were observed in the nucleus of the solitary tract only at embryonic and early larval stages. Abundant NPFFir cells and fibers were demonstrated in the spinal cord. The sequence of appearance observed in Xenopus shares many developmental features with mammals although notable differences were observed in the telencephalon and hypothalamus. In general, NPFF immunoreactivity developed earlier in amphibians than in mammals.

Distribution of neuropeptide FF-like immunoreactivity in the brain of the lizardGekko gecko and its relation to catecholaminergic structures

The present study provides a detailed description of the distribution of neuropeptide FF (NPFF)-like immunoreactivity in the brain of the lizard Gekko gecko. NPFF is found to be involved in nociception, cardiovascular regulation, and endocrine function. Because of its known relationship with catecholamines in mammals, double staining with tyrosine hydroxylase (TH) antibodies was used to corroborate this for reptiles. The present study revealed that NPFF-like-immunoreactive (NPFF-ir) cells and fibers were widely distributed throughout the brain. Major NPFF-ir cell groups were observed in the diagonal band nucleus of Broca, hypothalamus, and dorsal horn of the spinal cord. Additional cells were found in the anterior olfactory nucleus, lateral and dorsal cortices, dorsolateral septum, and diencephalic intergeniculate leaflet formation. Dense plexuses of NPFF-ir fibers were identified in the diagonal band nucleus of Broca, septum, preoptic and hypothalamic areas, isthmic region, ventrolateral tegmentum, solitary tract nucleus, and dorsolateral funiculus of the spinal cord. Extensive fiber staining also occurred in the nucleus accumbens and the midbrain tectum. Although an intimate relationship between NPFF-ir and TH-ir structures was obvious at many places in the brain, colocalization of these two substances was not observed. In conclusion, the distribution of NPFF in the brain of Gekko shares more features with anamniotes in terms of number of cell groups, more elaborate networks of fibers, and lack of colocalization with catecholamines than with mammals, suggesting a decrease in the distribution of this peptide in the latter vertebrate group.

Evidence that morphine tolerance may be regulated by endothelin in the neonatal rat

BACKGROUND

Opioids are widely used in the neonatal intensive care units for analgesia and sedation. Management of tolerance and withdrawal symptoms in neonates remains a major challenge.

OBJECTIVES

The present study investigates the involvement of a central endothelin (ET) mechanism in the development of tolerance to morphine in neonatal rats.

METHODS

Pregnant female rats were rendered tolerant to morphine and rat pups were delivered at term by cesarean section. The affinity (Kd) and density (Bmax) of ET receptors was determined by [125I]ET-1 binding in the brains of neonatal rats. Changes in G-protein stimulation were determined in placebo and morphine-tolerant neonatal rats by [35S]-guanosine-5'-o-(3-thio)triphosphate ([35S]GTPgammaS)-binding assay.

RESULTS

Morphine tolerance did not affect the characteristics (affinity and density) of the ET receptors in the neonatal rat brains. Morphine as well as ET-1 produced significantly lower (p < 0.05) maximal stimulation of [35S]GTPgammaS binding in morphine-tolerant neonatal rats compared to the placebo group. The ETA receptor antagonist, BMS182874, produced significantly higher stimulation of G proteins in the morphine-tolerant compared to the placebo group. The ETB receptor agonist, IRL1620, produced a similar effect in both placebo and morphine-tolerant rats.

CONCLUSIONS

This is the first report indicating the involvement of the G-protein-coupled ETA receptor in neonatal morphine tolerance.

Neuropeptide ff, but not prolactin-releasing peptide, mRNA is differentially regulated in the hypothalamic and medullary neurons after salt loading

Hypothalamic paraventricular and supraoptic nuclei are involved in the body fluid homeostasis. Especially vasopressin peptide and mRNA levels are regulated by hypo- and hyperosmolar stimuli. Other neuropeptides such as dynorphin, galanin and neuropeptide FF are coregulated with vasopressin. In this study neuropeptide FF and another RF-amide peptide, the prolactin-releasing peptide mRNA levels were studied by quantitative in situ hybridization after chronic salt loading, a laboratory model of chronic dehydration. The neuropeptide FF mRNA expressing cells virtually disappeared from the hypothalamic supraoptic and paraventricular nuclei after salt loading, suggesting that hyperosmolar stress downregulated the NPFF gene transcription. The neuropeptide FF mRNA signal levels were returned to control levels after the rehydration period of 7 days. No changes were observed in those medullary nuclei that express neuropeptide FF mRNA. No significant changes were observed in the hypothalamic or medullary prolactin-releasing peptide mRNA levels. Neuropeptide FF mRNA is drastically downregulated in the hypothalamic magnocellular neurons after salt loading. Other neuropeptides studied in this model are concomitantly coregulated with vasopressin: i.e. their peptide levels are downregulated and mRNA levels are upregulated which is in contrast to neuropeptide FF regulation. It can thus be concluded that neuropeptide FF is not regulated through the vasopressin regulatory system but via an independent pathway. The detailed mechanisms underlying the downregulation of neuropeptide FF mRNA in neurons remain to be clarified.

Expression of neuropeptide FF, prolactin-releasing peptide, and the receptor UHR1/GPR10 genes during embryogenesis in the rat

Recently, several RF-amide peptides have been identified in mammals. These peptides have a similar C-terminal RF-motif and share some G-protein coupled receptors. Neuropeptide FF (NPFF) and prolactin-releasing peptide (PrRP) are expressed in the same brain areas in the adult rat and act both in prolactin release and cardiovascular regulation. Here, we characterized the embryonal expression from embryonal day 14 to postnatal day 0 of both peptide mRNAs and the mRNA distribution of UHR1/GPR10-like receptor by using in situ hybridization (ISH) and quantitative reverse transcriptase-polymerase chain reaction. NPFF mRNA was found in the spinal cord, caudal solitary tract nucleus, and surprisingly, in the medullary reticular formation. The only peripheral organs displaying NPFF mRNA expression were the lungs and the spleen. PrRP gene expression was seen in the caudal solitary tract nucleus, medullary reticular formation, pontine isthmus and liver, kidney, and testis. The receptor UHR1/GPR10 gene was expressed consistently in the medullary reticular formation and the adrenal gland but also transiently in several locations. All three genes showed weak but even ISH signal in the pituitary. These findings suggest different roles for the peptides during development and indicate that UHR1/GPR10-like receptor could also bind other ligands in addition to PrRP.

Role of the delta-opioid receptor in (1DMe)NPYF mediated antinociception

A selective delta-opioid antagonist, naltrindole, was used to study the role of the delta-opioid receptor in the antinociceptive actions of a synthetic NPFF analog, (1DMe)NPYF. I.t. (1DMe)NPYF (5 nmol) produced antinociception in the tail flick test and (1DMe)NPYF (0.5 nmol) potentiated the antinociceptive effect of i.t. morphine 7.8 nmol. (1DMe)NPYF (5 nmol) had an antihyperalgesic effect in carrageenan inflammation and it significantly reduced mechanical allodynia in the spinal nerve ligation model. All these effects were prevented or significantly reduced by pretreatment with naltrindole (28 nmol) (P < 0.01-0.001). These data suggest that activation of spinal delta-opioid receptors plays an important role in mediating the spinal antinociceptive effects of (1DMe)NPYF.

Effects of (1DMe)NPYF, a synthetic neuropeptide FF analogue, in different pain models

The antinociceptive effects of intrathecal (IT) (1DMe)NPYF were studied in adult Sprague-Dawley rats. (1DMe)NPYF produced dose-dependent antinociception that was reduced by subcutaneous injection of naloxone. (1DMe)NPYF (0.5 nmol) also potentiated the antinociceptive effects of intrathecal morphine 7.8 nmol. This suggests that the antinociceptive effects of (1DMe)NPYF are partially mediated by opioid receptor activation. In carrageenan inflammation, 5-10 nmol of (1DMe)NPYF was effective against both thermal hyperalgesia and mechanical allodynia. In the neuropathic pain model, the lowest dose tested (0.5 nmol) showed antiallodynic effects against cold allodynia. The results suggest a potential role for (1DMe)NPYF in the treatment of pain including neuropathic pain.

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.

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.

Effects of neuropeptide FF analogs on morphine analgesia in the nucleus raphe dorsalis

The effect of microinfusion into the nucleus raphe dorsalis (DR) of neuropeptide FF (NPFF) analogs on the antinociceptive effects of morphine was evaluated in rats, using the tail-immersion test. infusion of morphine into the DR induced a dose-dependent analgesia significantly reversed by co-infusion of 2.5 nmol opioid antagonist, naloxone. Similarly, 2.5 nmol NPFF and (1DMe)Y8Fa(D-Tyr-Leu-(NMe)Phe-Gln-Pro-Gln-Arg-Phe-NH2) or (3D)Y8Fa(D-Tyr-D-Leu-D-Phe-Gln-Pro-Gln-Arg-Phe-NH2), two neuropeptide FF analogs, inhibited morphine analgesia, although these peptides had no effect on nociceptive thresholds. This anti-opioid effect is indirect since NPFF analogs displayed no significant affinity towards mu and delta opioid binding sites in the DR. After intracerebroventricular infusion, morphine produced the same degree of analgesia as that measured after infusion into the nucleus raphe dorsalis and both NPFF analogs reversed morphine antinociception. This result is the first direct evidence that neuropeptide FF may act on opioid system at the DR and that several nuclei are involved in the suppression of morphine-induced antinociception.

Ontogeny of neuropeptide FF pharmacology and receptors in mouse brain

The postnatal ontogeny of neuropeptide FF (NPFF) receptors in mouse brain was mapped by quantitative autoradiography using [125I](1DME)Y8Famide as selective radioligand. By day 14, the adult distribution of NPFF sites was established in almost all brain areas. The density of NPFF receptors reached adult levels between 14 and 28 days after birth, earlier than did mu or delta opioid receptors. During ontogeny, morphine analgesia was measured in hindpaw, forepaw and tail immersion tests (57 degrees C). An antinociceptive effect of morphine in the forepaw and tail immersion test was evident in 14-day-old but not in 7-day-old mice. In 14 and 21 day-old mice, the influence of (1DME)Y8Famide on morphine analgesia depended on the dose and on the particular test. A low dose of (1DME)Y8Famide inhibited morphine analgesia in the tail or hindpaw test, whereas a higher dose had either no effect (tail test) or increased morphine activity (hindpaw test). In adult mice, only the inhibition of morphine antinociception by (1DME)Y8Famide was observed. These observations revealed the establishment of an equilibrium between NPFF and opioid functions during ontogeny.

Distribution of neuropeptide FF in porcine spinal cord in comparison with other neuropeptides and serotonin

A large number of neurotransmitters and neuropeptides are concentrated in the dorsal horn of the spinal cord, where they interact in a complex manner and modulate sensory mechanisms. Most studies are carried out in the rat, and little is known of other species. It is relevant to study mammals with a more complex central nervous system, because pain mechanisms are central in both human and veterinary medicine. Immunoreactivity for neuropeptide FF, an amidated octapeptide originally isolated from bovine brain, was found immunocytochemically at all levels of porcine spinal cord. In contrast to other species studied so far, the peptide immunoreactivity in porcine spinal cord was confined to the intermediolateral gray matter, especially to the intermediolateral cell column and lamina X of the gray matter. This distribution was remarkably different from that of substance P, proenkephalin A-derived peptides, thyrotropin-releasing hormone, serotonin, and neuropeptide Y. Pharmacologic administration of neuropeptide FF alters behavior in assays for analgesia. The distribution of neuropeptide FF immunoreactivity as revealed by this study suggests that there may be marked species differences in the distribution and function of the peptide.

Immunocytochemical localization of neuropeptide FF (FMRF amide-like peptide) in the hypothalamo-neurohypophyseal system of Wistar and Brattleboro rats by light and electron microscopy

Neuropeptide FF (F8Famide, FMRFamide-like, or morphine modulating peptide) immunoreactivity was localized by light and electron microscopy in the hypothalamo-neurohypophyseal system of Wistar and Brattleboro rats. In Wistar rats neuropeptide FF was present in part of the magnocellular neurones of the paraventricular and supraoptic nuclei in which it was coexpressed with vasopressin. Neuropeptide FF containing fibres were present in the paraventricular and the supraoptic nuclei, and in the central part of the neural lobe. At the electron microscopic level, neuropeptide FF containing nerve terminals in the neural lobe formed synaptoid contacts exclusively with pituicytes. No neuropeptide FF containing neurovascular contacts or contacts with other neuronal structures were observed. In contrast with Wistar rats, neuropeptide FF was almost completely absent in cell bodies of the paraventricular and supraoptic nuclei, and in fibres of the neural lobe in Brattleboro rats. Only a few solitary cells could be observed in these structures. The present results demonstrate that neuropeptide FF coexists with vasopressin within the hypothalamo-neurohypophyseal system. As we did not observe neuropeptide FF containing neurovascular contacts, neuropeptide FF containing nerve terminals probably have a local function within the neural lobe. Neuropeptide FF may be involved in the modulation of oxytocin and vasopressin release, with the pituicyte as an intermediate cell.

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.

Functional development of central nervous system in the rat: ontogeny of nociceptive thresholds

The functional development of central nervous system (CNS) in the rat has been studied from the 10th to the 45th postnatal day, through a sensory function: nociception. Baseline pain responsiveness has been assessed with the tail-flick procedure. The mean tail-flick latency clearly decreases from the 10th to the 25th day, and remains stationary from this age to 45 days. The maturational processes underlying these reductions of tail-flick latencies in developing rats are discussed.

Analog of neuropeptide FF attenuates morphine tolerance

Previous studies suggest that neuropeptide FF (NPFF) plays a role in opiate dependence and subsequent abstinence syndrome. Endogenous NPFF also appears to play a role in opiate tolerance since third ventricle injection of IgG from NPFF antiserum selectively restores morphine sensitivity in morphine-tolerant rats. The NPFF analog, desamino YFLFQPQRamide (daY8Ra) has previously antagonized behavioral effects of NPFF and has attenuated morphine dependence. The present study assessed whether daY8Ra could similarly attenuate morphine tolerance. Third ventricle (i.c.v.) injection of daY8Ra restored the analgesic response to i.c.v. morphine in morphine-tolerant rats (radiant heat tail flick test). Saline injection failed to produce this effect. In opiate-naive rats, however, the same treatment with daY8Ra did not affect the analgesic response to i.c.v. morphine. Thus, daY8Ra appears to selectively restore morphine sensitivity in opiate-tolerant animals. These results further support the hypothesis that endogenous NPFF contributes to opiate tolerance.

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.

IgG from neuropeptide FF antiserum reverses morphine tolerance in the rat

Previous studies suggest that neuropeptide FF (NPFF) plays a role in opiate dependence and subsequent abstinence syndrome. The present study assessed the role of NPFF in opiate tolerance. Third ventricular injection of IgG from NPFF antiserum restored the analgesic response to i.c.v. morphine in morphine-tolerant rats (radiant heat tail flick test). IgG from control serum failed to produce this effect. In opiate-naive rats, however, the same treatment with IgG from NPFF antiserum did not affect the analgesic response to i.c.v. morphine. Thus, immunoneutralization of NPFF appears to selectively restore morphine sensitivity in opiate-tolerant animals. These results support the hypothesis that endogenous NPFF contributes to opiate tolerance.

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.