FMRFamide: an endogenous peptide with marked inhibitory effects on opioid-induced feeding behavior

OFP011 Cyclic Peptide as a Multifunctional Agonist for Opioid/Neuropeptide FF Receptors with Improved Blood-Brain Barrier Penetration

Mounting evidence indicates that the neuropeptide FF (NPFF) system is involved in the side effects of opioid usage, including antinociceptive tolerance, hyperalgesia, abuse, constipation, and respiratory depression. Our group recently discovered that the multitarget opioid/NPFF receptor agonist DN-9 exhibits peripheral antinociceptive activity. To improve its metabolic stability, antinociceptive potency, and duration, in this study, we designed and synthesized a novel cyclic disulfide analogue of DN-9, OFP011, and examined its bioactivity through in vitro cyclic adenosine monophosphate (cAMP) functional assays and in vivo behavioral experiments. OFP011 exhibited multifunctional agonistic effects at the μ-opioid and the NPFF₁ and NPFF₂ receptors and partial agonistic effects at the δ- and κ-opioid in vitro, as determined via the cAMP functional assays. Pharmacokinetic and pharmacological experiments revealed improvement in its blood-brain barrier permeability after systemic administration. In addition, subcutaneous OFP011 exhibited potent and long-lasting antinociceptive activity via the central μ- and κ-opioid receptors, as observed in different physiological and pathological pain models. At the highest antinociceptive doses, subcutaneous OFP011 exhibited limited tolerance, gastrointestinal transit, motor coordination, addiction, reward, and respiration depression. Notably, OFP011 exhibited potent oral antinociceptive activities in mouse models of acute, inflammatory, and neuropathic pain. These results suggest that the multifunctional opioid/NPFF receptor agonists with improved blood-brain barrier penetration are a promising strategy for long-term treatment of moderate to severe nociceptive and pathological pain with fewer side effects.

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.

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.

Comparative Distribution of Neurons Containing FLFQPQRFamide-like (morphine-modulating) Peptide and Related Neuropeptides in the Rat Brain

FLFQPQRF-NH2 (F8Famide; morphine-modulating peptide), isolated from bovine brain, is an FMRFamide-like peptide with opioid analgesia modulating effects. In the rat brain, F8Famide is immunohistochemically localized in neurons of the medial hypothalamus and medulla oblongata. Neuropeptide Y (NPY) is structurally related to F8Famide and the mammalian FMRFamide-like immunoreactivity (LI) was once thought to be due to an NPY-like peptide. We compared the anatomical distribution of F8Famide-LI with the localization of enkephalin- and NPY-LI-containing structures in the rat brain to find out if NPY or enkephalins coexist with F8Famide-LI. Cryostat sections of colchicine-treated Wistar rat brains were incubated with specific antisera against F8Famide, NPY, YGGFMRGL (Met-enkephalin-Arg-Gly-Leu), or YGGFMRF (Met-enkephalin-Arg-Phe) raised in rabbits. The immunoreactivity was visualized by the peroxidase - antiperoxidase or immunofluorescence method. The light microscopic mirror method was applied to study the colocalization of F8Famide and NPY. The F8Famide-immunoreactivity was concentrated in smaller areas of medial hypothalamus and nucleus of the solitary tract than that of enkephalins and NPY. In all brain areas, the distributions of F8Famide-, enkephalin- and NPY-immunoreactive neurons were distinct. F8Famide-, NPY- and enkephalin-LI-containing nerve terminals were seen in the nucleus of the solitary tract and in the lateral parabrachial nucleus. These results show that the neuronal systems containing F8Famide-, enkephalin- or NPY-LI are anatomically separate in all brain regions. However, there are terminal areas in which more than one type of these immunoreactivities are detected. These results have anatomical correlation with pharmacological reports, suggesting modulatory functions for these peptides on regulation of blood pressure, feeding behaviour and endocrine functions.

Opioid and anti-opioid peptides

The numerous endogenous opioid peptides (beta-endorphin, enkephalins, dynorphins ... ) and the exogenous opioids (such as morphine) exert their effects through the activation of receptors belonging to four main types, mu, delta, kappa and epsilon. Opioidergic neurones and opioid receptors are largely distributed centrally and peripherally. It is thus not surprising that opioids have numerous pharmacological effects and that endogenous opioids are thought to be involved in the physiological control of various functions, among which nociception is particularly emphasized. Some opioid targets may be components of homeostatic systems tending to reduce the effects of opioids. "Anti-opioid" properties have been attributed to various peptides, especially cholecystokinin (CCK), neuropeptide FF (NPFF) and melanocyte inhibiting factor (MIF)-related peptides. In addition, a particular place should be attributed, paradoxically, to opioid peptides themselves among the anti-opioid peptides. These peptides can oppose some of the acute effects of opioids, and a hyperactivation of anti-opioid peptidergic neurones due to the chronic administration of opioids may be involved in the development of opioid tolerance and/or dependence. In fact, CCK, NPFF and the MIF family of peptides have complex properties and can act as opioid-like as well as anti-opioid peptides. Thus, "opioid modulating peptides" would be a better term to designate these peptides, which probably participate, together with the opioid systems, in multiple feed-back loops for the maintenance of homeostasis. "Opioid modulating peptides" have generally been shown to act through the activation of their own receptors. For example, CCK appears to exert its anti-opioid actions mainly through the activation of CCK-B receptors, whereas its opioid-like effects seem to result from the stimulation of CCK-A receptors. However, the partial agonistic properties at opioid receptors of some MIF-related peptides very likely contribute to their ability to modulate the effects of opioids. CCK- and NPFF-related drugs have potential therapeutic interest as adjuncts to opioids for alleviating pain and/or for the treatment of opioid abuse.

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

FLFQPQRF-NH2 (F8Famide; FMRFamide-like peptide; morphine-modulating peptide) is a peptide isolated from bovine brain. It has some opiate analgesia modulating effects. In an adult rat central nervous system, F8Famide-like immunoreactivity is found in high concentrations in the posterior pituitary, hypothalamus, pons, medulla, and dorsal spinal cord. By using immunohistochemistry and radioimmunoassay, we studied the development of the F8Famide-immunoreactive system in the rat central nervous system during the ontogeny. F8Famide-immoreactive fibers and terminals first appeared in the median eminence on the twentieth embryonal day. Postnatally the F8Famide-immunoreactive system developed rapidly both in the brain and spinal cord, the immunoreactive structures having an adultlike distribution by the age of 1 week. A transient increase of the F8Famide-immunoreactive material was observed during the third and fourth postnatal weeks. By the age of 4 weeks, the F8Famide-containing neuronal system was adultlike both in distribution and quantity. The results suggest that the F8Famide-like peptides may act as neurotransmitters or neuromodulators from the time of their appearance in the brain and spinal cord. The early appearance and the distribution of the F8Famide-like immunoreactivity suggest that these peptides may participate, in addition to nociceptive mechanisms, in the regulation of blood pressure, feeding behaviour, and endocrine functions.

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

Octapeptide FLFQPQRFamide (FMRFamide-like peptide; morphine-modulating peptide), isolated from bovine brain, has some opiate analgesia modulating effects. Octapeptide FLFQPQRFamide-like immunoreactivity is found in high concentrations in the posterior pituitary, hypothalamus, pons-medulla, and dorsal spinal cord. Octapeptide FLFQPQRFamide-immunoreactive neurons of the brain are localized in the medial hypothalamus and in the nucleus of the solitary tract. High densities of octapeptide FLFQPQRFamide-immunoreactive nerve terminals are found in the median eminence, lateral parabrachial nucleus, and nucleus of the solitary tract. By using the retrograde tract tracing method combined with immunohistochemistry, we studied the central pathways interconnecting the octapeptide FLFQPQRFamide-immunoreactive structures. The octapeptide FLFQPQRFamide-immunoreactive neurons of the hypothalamus sent projections bilaterally to the nucleus of the solitary tract. The octapeptide FLFQPQRFamide-immunoreactive neurons of the nucleus of the solitary tract projected to the contralateral side of the same nucleus, to the lateral parabrachial nuclei bilaterally, and to the ipsilateral periambigual region. The results give neuroanatomical evidence of interacerebral pathways containing recently identified FLFQPQRFamide-like peptides, which may belong to a larger family of peptides. These neuroanatomical findings support the previous pharmacological studies, suggesting that the mammalian FMRFamide-like peptides may, in addition to modulatory effects on nociceptive mechanisms, participate in the regulation of blood pressure, feeding behaviour and endocrine functions.

Unexpected responses of the obese "cafeteria" rat to the peptide FMRF-amide

The relationship between the acute effects of FMRF-amide on central monoamines and feeding effects were investigated simultaneously in normophagic and "cafeteria" rats. This tetrapeptide is considered as being representative of an endogenous related peptides family with antagonistic properties on opioid-induced behavioural effects. In normophagic rats, no feeding effect was observed, but there was a decrease in serotonergic metabolism similar to that induced by the classical antagonist, naltrexone. However, in the "cafeteria" rats, FMRF-amide enhanced food intake and increased serotonergic metabolism, exhibiting the classical effects of opiate agonists. Since the effects of FMRF-amide differ according to the ponderal and/or nutritional status, this peptide would appear to act rather as a modulator than a true opiate antagonist on food intake. This raises the question as to the exact role of the recently-discovered endogenous FMRF-amide related family in obesity and/or stimulated feeding patterns.

Endogenous opiates: 1986

This is the ninth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1986. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic processes; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; activity; sex, pregnancy, and development; and some other behaviors.

An octadecaneuropeptide (ODN) derived from diazepam binding inhibitor increases aggressive interactions in mice

The effects of intracerebroventricular administrations of an octadecaneuropeptide (ODN) derived from the polypeptide, diazepam binding inhibitor (DBI), on offensive and defensive aggression were examined in male mice. During the initial period after administration (1-5 min) ODN inhibited social and agonistic behavior. At 30 min after treatment, ODN increased, in a dose-dependent manner, the incidence of and intensity of offensive aggression in dominant resident mice. ODN also increased the number of bites required to obtain defeat in subordinate mice during aggressive interactions, as well as reducing subsequent defeat-induced analgesia. These changes in offensive and defensive aggressive behavior that were induced by ODN were reduced by the benzodiazepine receptor antagonist Ro 15-1788. These results suggest that ODN has significant modulatory effects on aggression.

Food hoarding and ingestion in the deer mouse, Peromyscus maniculatus: selective responses to mu and kappa opiate agonists

The feeding behavior of the deer mouse, Peromyscus maniculatus, includes food hoarding as well as ingestion. Administration of the prototypical mu opiate agonist, morphine sulfate, 1-20 mg/kg, produced over three hours a significant dose-dependent stimulation of hoarding by free feeding deer mice. The specific kappa opiate agonist, U-50,488H, 0.10-10 mg/kg, markedly increased ingestion without having any augmentatory effects on hoarding. The mixed mu and kappa opiate agonist, ketocyclazocine hydrochloride, 1-10 mg/kg, as well as various combinations of morphine sulfate and U-50,488H, augmented both hoarding and ingestion. Food restriction for 24 hr caused a significant, naloxone (1.0 mg/kg) reversible, increase in food intake. Food deprivation also modified the hoarding and ingestion responses of the deer mice to the mu and kappa opiate agonists, reducing the relative amounts of food that were hoarded. These results indicate that mu and kappa opioid systems are differentially involved in the mediation of various aspects of feeding. This also suggests that environmental factors, such as food restriction, can modify the relative roles of mu and kappa opioid systems in the expression of feeding behavior.

Inhibitory influences of FMRFamide and PLG on stress-induced opioid analgesia and activity

The effects of i.c.v. administration of the peptide FMRFamide (Phe-Met-Arg-Phe-NH2), as well as i.p. injections of PLG (Pro-Leu-Gly-NH2) and the opiate antagonist, naloxone, on immobilization-induced analgesia and locomotor activity were examined in CF-1 and C57BL strains of mice. Both naloxone (1.0 mg/kg) and FMRFamide (0.10-1.0 microgram) blocked the experimentally induced analgesia and activity, whereas PLG (0.10-10 mg/kg) suppressed only analgesia. These results indicate that FMRFamide (or FMRFamide-like neuropeptides) and PLG may function as differential antagonists of the behavioral and physiological consequences of endogenous opioid activation.