Mammalian neuronal actions of FMRFamide and the structurally related opioid Met-enkephalin-Arg6-Phe7

ASICs and neuropeptides

The acid sensing ion channels (ASICs) are proton-gated cation channels expressed throughout the nervous system. ASICs are activated during acidic pH fluctuations, and recent work suggests that they are involved in excitatory synaptic transmission. ASICs can also induce neuronal degeneration and death during pathological extracellular acidosis caused by ischemia, autoimmune inflammation, and traumatic injury. Many endogenous neuromodulators target ASICs to affect their biophysical characteristics and contributions to neuronal activity. One of the most unconventional types of modulation occurs with the interaction of ASICs and neuropeptides. Collectively, FMRFamide-related peptides and dynorphins potentiate ASIC activity by decreasing the proton-sensitivity of steady state desensitization independent of G protein-coupled receptor activation. By decreasing the proton-sensitivity of steady state desensitization, the FMRFamide-related peptides and dynorphins permit ASICs to remain active at more acidic basal pH. Unlike the dynorphins, some FMRFamide-related peptides also potentiate ASIC activity by slowing inactivation and increasing the sustained current. Through mechanistic studies, the modulation of ASICs by FMRFamide-related peptides and dynorphins appears to be through distinct interactions with the extracellular domain of ASICs. Dynorphins are expressed throughout the nervous system and can increase neuronal death during prolonged extracellular acidosis, suggesting that the interaction between dynorphins and ASICs may have important consequences for the prevention of neurological injury. The overlap in expression of FMRFamide-related peptides with ASICs in the dorsal horn of the spinal cord suggests that their interaction may have important consequences for the treatment of pain during injury and inflammation. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Acid-sensing ion channels (ASICs): therapeutic targets for neurological diseases and their regulation

Extracellular acidification occurs not only in pathological conditions such as inflammation and brain ischemia, but also in normal physiological conditions such as synaptic transmission. Acid-sensing ion channels (ASICs) can detect a broad range of physiological pH changes during pathological and synaptic cellular activities. ASICs are voltage-independent, proton-gated cation channels widely expressed throughout the central and peripheral nervous system. Activation of ASICs is involved in pain perception, synaptic plasticity, learning and memory, fear, ischemic neuronal injury, seizure termination, neuronal degeneration, and mechanosensation. Therefore, ASICs emerge as potential therapeutic targets for manipulating pain and neurological diseases. The activity of these channels can be regulated by many factors such as lactate, Zn²⁺, and Phe-Met-Arg-Phe amide (FMRFamide)-like neuropeptides by interacting with the channel’s large extracellular loop. ASICs are also modulated by G protein-coupled receptors such as CB₁ cannabinoid receptors and 5-HT₂. This review focuses on the physiological roles of ASICs and the molecular mechanisms by which these channels are regulated. [BMB Reports 2013; 46(6): 295-304]

FMRFamide-related peptide expression in the vestibular-afferent neurons

Vestibular-afferent neurons innervate hair cells from the sensory epithelia of vestibular end-organs and their action-potential discharge dynamics are driven by linear and angular accelerations of the head. The electrical activity of the vestibular-afferent neurons depends on their intrinsic properties and on the synaptic input from hair cells and from the terminals of the efferent system. Here we report that vestibular-afferent neurons of the rat are immunoreactive to RFamide-related peptides, and that the stronger signal comes from calyx-shaped neuron dendrites, with no signal detected in hair cells or supporting cells. The whole-cell voltage clamp recording of isolated afferent neurons showed that they express robust acid-sensing ionic currents (ASICs). Extracellular multiunit recordings of the vestibular nerve in a preparation in vitro of the rat inner ear showed that the perfusion of FMRFamide (a snail ortholog of this family of neuropeptides) exerts an excitatory effect on the afferent-neurons spike-discharge rate. Because the FMRFamide cannot activate the ASIC but reduces its desensitization generating a more robust current, its effect indicates that the ASIC are tonically active in the vestibular-afferent neurons and modulated by RFamide-like peptides.

Recent studies of gonadotropin-inhibitory hormone (GnIH) in the mammalian hypothalamus, pituitary and gonads

The hypothalamo-pituitary-gonadal (HPG) axis integrates internal and external cues via a balance of stimulatory and inhibitory neurochemical systems to time reproductive activity. The cumulative output of these positive and negative modulators drives secretion of gonadotropin-releasing hormone (GnRH), a neuropeptide that causes pituitary gonadotropin synthesis and secretion. Ten years ago, Tsutsui and colleagues discovered a peptide in quail hypothalamus that is capable of inhibiting gonadotropin secretion in cultured quail pituitary cells. Later studies by a variety of researchers examined the presence and functional role for the mammalian ortholog of GnIH. To date, GnIH exhibits a similar distribution and functional role in all mammals investigated, including humans. This overview summarizes the role of GnIH in modulation of mammalian reproductive physiology and suggests avenues for further study by those interested in the neuroendocrine control of reproductive physiology and sexual behavior.

The roles of RFamide-related peptide-3 in mammalian reproductive function and behaviour

To maximise reproductive success, organisms restrict breeding to optimal times of the day or year, when internal physiology and external environmental conditions are suitable for the survival of both parent and offspring. To appropriately coordinate reproductive activity, internal and external standing is communicated to the hypothalamic-pituitary-gonadal axis via a coordinated balance of stimulatory and inhibitory neurochemical systems. The cumulative balance of these mediators ultimately drives the pattern of gonadotrophin-releasing hormone secretion, a neurohormone that stimulates pituitary gonadotrophin secretion. Until 2000, a complementary inhibitor of pituitary gonadotrophin secretion had not been identified. At this time, a novel, avian hypothalamic peptide capable of inhibiting gonadotrophin secretion in cultured quail pituitary cells was uncovered and named gonadotrophin-inhibitory hormone (GnIH). Subsequently, the presence and functional role for the mammalian orthologue of GnIH, RFamide-related peptide, (RFRP-3), was examined, confirming a conserved role for this peptide across several rodent species. To date, a similar distribution and functional role for RFRP-3 have been observed across all mammals investigated, including humans. This overview summarises the role that RFRP-3 plays in mammals and considers the implications and opportunities for further study with respect to reproductive physiology and the neural control of sexual behaviour and motivation.

Endogenous peptide: Met-enkephalin-Arg-Phe, differently regulate expression of opioid receptors on chronic treatment

Endogenous peptide, Met-enkephalin-Arg-Phe (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MERF) induces effects like antinociception, inhibit contraction of guinea pig ileum, mouse vas deferens and anti-tussive action. However, results regarding its functional efficiency and selectivity are controversial. Therefore, present study was undertaken to investigate whether MERF on systemic (intra-peritoneal, i.p.) route of administration induce any antinociception or not; to scrutinize the effect of 6 days chronic i.p. treatment of MERF on expression of mu (MOR1), delta (DOR1) and kappa (KOR1) opioid receptors; and finally, the antinociceptive effect of two synthetic peptides, MERFamide and (D-Ala(2))-MERFamide was compared with MERF on intracerebroventricular administration in order to understand the role of FMRF moiety in analgesic effect of MERF. Pharmacological results revealed that only 68.4 and 91.2 micromol/kg dose induce significant antinociception among various doses. Further, on 6 days chronic treatment, MERF induced significant antinociception in comparison to saline. Differential expression of MOR1 and KOR1 showed continuous up-regulation throughout the treatment whereas DOR1 showed down-regulation in initial 3 days followed by subsequently up-regulation during the latter observable period. Moreover, variation in opioid receptors expression had not affected the MERF antinociception. In conclusion, present study discursively demonstrates that MERF during chronic treatment interacts with all three opioid receptors (mu, delta and kappa) in rats and differently regulates their expression. Further, the interaction was such that the induction was mainly observed at molecular/expression level and not at pharmacological level to affect antinociception.

Peripherally applied neuropeptide SF is equally algogenic in wild type and ASIC3−/− mice

RFa-related peptides play a significant role in the processing of pain in the CNS of mammals. Recently it has been found that, when applied subcutaneously, these peptides elicit a powerful algogenic effect. The question arises whether this peripheral effect can be connected with the ability of RFa-related peptides to decrease the rate of desensitization of acid sensing ionic channels (ASICs) expressed in primary sensory neurons. We have addressed this question by comparing the effects of neuropeptide SF (NPSF), mammalian RFa peptide, in ASIC3-/- and wild-type C57BL/6J mice. Knockout of ASIC3 gene results in the changes in some of the behavioral parameters. However, subcutaneous injections of the NPSF into the n.saphenous innervation area result in a clearly nociceptive behavior in both strains of mice. There is no significant difference in the total time of licking of injected paw in the ASIC3-/- (194+/-22s) and C57BL/6J (227+/-25s) animals. Thus peripheral algogenic effects of NPSF cannot be explained only in terms of their action on the ASIC3 channels and involves some other, still unidentified mechanism.

Pharmacological and functional biochemical properties of d-Ala2-d-Nle5-enkephalin-Arg-Phe

Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (DADN) a synthetic analogue of the endogenous Met-enkephalin-Arg-Phe (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MERF), was investigated in radioligand binding assays, [(35)S]GTPgammaS stimulation experiments as well as in in vivo algesiometric tests. Binding properties of [(3)H]DADN were measured in crude membrane fractions of rat spinal cord tissues and in homogenates of Chinese hamster ovary (CHO) cells selectively expressing delta-, kappa-or micro-opioid receptors. The highest affinity for [(3)H]DADN binding was observed in membranes from CHO cells transfected with micro-opioid receptors confirming the micro-selectivity of the peptide. Unlabeled DADN was also investigated in functional biochemical experiments by measuring opioid receptor-mediated G-protein activation in rat brain membrane fractions. The peptide stimulated the activity of the regulatory G-proteins in a concentration dependent manner, and the stimulation was efficiently inhibited in the presence of micro-receptor specific antagonist ligands further supporting the selectivity profile of DADN. Intrathecally administered DADN produced a dose-related, naloxone-reversible antinociception in rat hot water tail-flick tests. Among the selective opioid antagonists tested, the delta-selective naltrindole (NTI) and the kappa-specific norbinaltorphimine (norBNI) showed only slight blocking effects compared with naloxone. The results obtained in the in vitro agonist-stimulated [(35)S]GTPgammaS binding assays are in good agreement with the opioid agonist effect seen in the in vivo pain test.

Non-opioid actions of opioid peptides

Beside the well known actions of opioid peptides on mu-, delta- and kappa-opioid receptors, increasing amount of pharmacological and biochemical evidence has recently been published about non-opioid actions of various opioid peptides. These effects are not abolished by naloxone treatments. Such non-opioid effects are observed both in nervous tissues and in the cellular elements of the immune system. Peptides exhibiting non-opioid effects include beta-endorphin, dynorphin A, nociceptin/OFQ, endomorphins, hemorphins and a number of Proenkephalin A derived peptides, such as Met-enkephalin, Met-enkephalin-Arg-Phe (MERF) and bovine adrenal medullary peptide (BAM22). Non-opioid actions are exerted through different neuronal receptors, e.g., dynorphin hyperalgesia through NMDA receptor, Met-enkephalin induced regulation of cell growth through zeta receptors, pain modulation by nociceptin through ORL-1 or NOP receptors, while BAM22 acts through sensory neuron specific G protein-coupled receptors (SNSR). We have investigated Met-enkephalin-Arg-Phe (MERF) and its analogues by the means of direct and indirect radioligand binding assays. It has been found that in addition to kappa(2) and delta-opioid receptors, MERF can act also through sigma(2)- or probably via FMRF-NH(2) receptors in rat cerebellum. A role of functionally assembling heterodimer receptors in mediating the non-conventional actions of these peptide ligands can not be excluded as well.

Retrograde Gbb signaling through the Bmp type 2 receptor wishful thinking regulates systemic FMRFa expression in Drosophila

Amidated neuropeptides of the FMRFamide class regulate numerous physiological processes including synaptic efficacy at the Drosophila neuromuscular junction (NMJ). We demonstrate here that mutations in wishful thinking (wit) a gene encoding a Drosophila Bmp type 2 receptor that is required for proper neurotransmitter release at the neuromuscular junction, also eliminates expression of FMRFa in that subset of neuroendocrine cells (Tv neurons) which provide the systemic supply of FMRFa peptides. We show that Gbb, a Bmp ligand expressed in the neurohemal organ provides a retrograde signal that helps specify the peptidergic phenotype of the Tv neurons. Finally, we show that supplying FMRFa in neurosecretory cells partially rescues the wit lethal phenotype without rescuing the primary morphological or electrophysiological defects of wit mutants. We propose that Wit and Gbb globally regulate NMJ function by controlling both the growth and transmitter release properties of the synapse as well as the expression of systemic modulators of NMJ synaptic activity.

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.

Neuropeptide FF and FMRFamide potentiate acid-evoked currents from sensory neurons and proton-gated DEG/ENaC channels

Acidosis is associated with inflammation and ischemia and activates cation channels in sensory neurons. Inflammation also induces expression of FMRFamidelike neuropeptides, which modulate pain. We found that neuropeptide FF (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe amide) and FMRFamide (Phe-Met-Arg-Phe amide) generated no current on their own but potentiated H+-gated currents from cultured sensory neurons and heterologously expressed ASIC and DRASIC channels. The neuropeptides slowed inactivation and induced sustained currents during acidification. The effects were specific; different channels showed distinct responses to the various peptides. These results suggest that acid-sensing ion channels may integrate multiple extracellular signals to modify sensory perception.

Isolation and characterization of a novel bioactive peptide, Carassius RFamide (C-RFa), from the brain of the Japanese crucian carp

A novel bioactive peptide with the C-terminal RFamide was isolated from the brain of the Japanese crucian carp, Carassius auratus langsdorfii, by using the intestine of the fish as the bioassay system. The primary structure of the peptide was determined to be SPEIDPFWYVGRGVRPIGRFamide and it was designated Carassius RFamide (C-RFa). The sequence of C-RFa was found to be significantly homologous to the molluscan neuropeptide termed Achatina cardioexcitatory peptide 1 (ACEP-1). Both peptides have RPXGRFamide structure in their C-terminal moieties. C-RFa was found to have an excitatory effect on visceral muscle tissues of fish, newt, quail, and rat. In the stomach of the fish, Zacco temminckii, IGRFamide (C-RFa 17-20) was shown to be the minimal structure required for the excitatory effect and PIGRFamide (C-RFa 16-20) to be almost equipotent with the parent peptide C-RFa.

Structural requirements and mechanism of the pressor activity of Leu-Val-Val-hemorphin-7, a fragment of hemoglobin beta-chain in rats

A rat blood pressure assay was used to perform a structure-activity relationship study (SAR) of Leu-Val-Val-hemorphin-7 (LVV-H7), a fragment of hemoglobin (Hb) beta-chain, elucidate the mechanisms of its cardiovascular effects, and test its potential involvement in the pressor activity of diaspirin crosslinked Hb (DCLHb), a recently developed Hb-based oxygen carrier. The SAR study revealed that the C-terminal-Arg-Phe-amino acid sequence of LVV-H7 contained the main determinants of the pressor activity of this peptide. Drug interaction studies using various inhibitory drugs (e.g., phentolamine, clonidine, etc.) and LVV-H7 showed that the pressor effect and tachycardia elicited by LVV-H7 involved the activation of the sympathetic nervous system (SNS). Additional studies using phenytoin (sodium channel blocker), [Tic7]H7(5-7)-NH2 (putative antagonist of receptors for LVV-H7) and H7(5-7)-NH2, an amidated C-terminal fragment of LVV-H7, suggested that LVV-H7 activated the SNS by interacting with specific receptors functionally coupled with phenytoin-sensitive sodium channels. The pressor effect and tachycardia caused by LVV-H7 were potentiated by captopril, suggesting that the angiotensin converting enzyme may contribute to the inactivation of LVV-H7 in rats. The pressor activity of DCLHb, in contrast to that elicited by LVV-H7, was not affected by animal pretreatment with LVV-H7 fragments shown to inhibit the pressor effect of LVV-H7. We conclude that: 1) LVV-H7 is unlikely to mediate the pressor activity of DCLHb in rats; 2) the pressor and tachycardic activities of LVV-H7 are mediated by the SNS; 3) the C-terminal-Arg-Phe-amino acid sequence of LVV-H7 contains the chemical groups responsible for the pressor effect of this peptide in rats; 4) LVV-H7 and FMRF amide-related peptides may share the same mechanism of pressor activity in rats.

Met5-enkephalin-Arg6-Phe7, an endogenous neuropeptide, binds to multiple opioid and nonopioid sites in rat brain

Receptor binding properties of the naturally occurring opioid heptapeptide MERF were studied in rat brain membrane preparations using tritium-labeled derivative of the peptide with 40 Ci/mmol specific radioactivity. Binding assays were performed in the presence of broad-spectrum peptidase inhibitors at 0 degree C. Under these conditions, the equilibrium binding was achieved in 30-40 min, and approximately 90% of the applied radioligand remained unchanged as determined by HPLC analysis. The apparent affinity (Kd value) of [3H]Met-enkephalin-Arg6-Phe7, calculated from saturation binding data, was 10.2 +/- 2.5 nM, and the maximal number (Bmax) of the heptapeptide binding sites was found to be 468 +/- 43 fmol/mg protein. About half the sites represent nonopioid sites because the Bmax was only 255 +/- 30 fmol/mg, when the nonspecific binding was measured with 1 microM naloxone. The rank order potencies of the examined compounds revealed that the opioid component of [3H]Met-enkephalin-Arg6-Phe7 recognition site are probably not mu and certainly not kappa 1 sites, whereas these sites are characterized by a kappa 2-like binding profile. Considering the discrepancies between rat and frog brain found in the affinity of some compounds, including naltrindole and norbinaltorphimine, the presence of a novel, MERF-selective "heptapeptide" binding site in rat brain membranes is also suggested. A number of the heterologous competition curves could be described by a high-affinity stereospecific component and a substantially lower-affinity binding element, which could completely be displaced with several peptide ligands such as Met5-enkephalin, dynorphin(1-13), and unlabeled MERF but not by other compounds such as [D-Ala2-(Me)Phe4-Gly5-ol]enkephalin, morphine, or naloxone. [3H]Met-enkephalin-Arg6-Phe7 binding can also be inhibited by FMRF-amide analogs and sigma receptor ligands, such as (+)N-allyl-normetazocine and haloperidol, although with moderate affinity. It is concluded that the stereospecific high-affinity binding is of opioid in character, whereas the residual sites characterized with their lower affinity are naloxone-insensitive nonopioid sites.

Distribution of FMRFamide-like immunoreactivity in the brain of the lungfish Protopterus annectens

The distribution of FMRFamide-like immunoreactive peptides was studied in the brain of the African lungfish, Protopterus annectens, using the indirect immunofluorescence technique. The main populations of FMRFamide-positive cell bodies were detected in the forebrain and in the mesencephalic tegmentum. In the telencephalon, only a small number of FMRFamide-immunoreactive neurons was localized at the level of the subpallium, in the nucleus septi medialis. The diencephalon contained two prominent groups of FMRFamide-positive cell bodies located in the preoptic and periventricular preoptic nuclei. The thalamus exhibited only scattered FMRFamide-immunoreactive perikarya in its ventral part. In the mesencephalon, a group of positive cell bodies was identified in the caudal region of the tegmentum. A strong immunoreaction was also detected in the nervus terminalis. In the pituitary, most of the cells of the intermediate lobe were brightly stained. FMRFamide-like immunoreactive fibers and nerve terminals were widely distributed in the brain. In the telecephalon, numerous fibers were observed in several regions of the pallium and subpallium. A dense plexus of fibers was found in the hypothalamus and the thalamus. Immunoreactive fibers were seen coursing along the ventral wall of the infundibular cavity and terminating in the pars nervosa of the pituitary. The tectum and the ventral mesencephalon were also densely innervated. In contrast, the caudal brainstem only showed scarce immunoreactive processes. Coexistence of FMRFamide- and neuropeptide Y-like immunoreactivity was observed in the preoptic nucleus and in the nervus terminalis. The widespread distribution of FMRFamide-immunoreactive neurons in the brain and pituitary of P. annectens suggests that the peptide may exert both neuromodulator and neuroendocrine functions. The similarity between the distribution patterns of FMRFamide and neuropeptide Y in the brain of lungfish and amphibians supports the concept of a close phylogenetic link between these two groups.

Antitussive effect of [Met5]enkephalin-Arg6-Phe7 in mice

We examined the effect of [Met5]enkephalin-Arg6-Phe7 (MEAP) on the capsaicin-induced cough reflex in mice. Intracerebroventricular administration of MEAP significantly decreased the number of coughs in a dose-dependent manner. The antitussive effect of MEAP was blocked by nor-binaltorphimine, a selective kappa-opioid receptor antagonist. However, beta-funaltrexamine, a mu-opioid receptor antagonist, had no effect on the antitussive effect of MEAP. On the other hand, the antinociceptive effect of MEAP, as determined in the tail-flick test, was blocked by both nor-binaltorphimine and beta-funaltrexamine. Naltrindole, a delta-opioid receptor antagonist, had no effect on either the antitussive effect or the antinociceptive effect of MEAP. These data suggest that MEAP exerts its antitussive effect in mice through the stimulation of kappa-opioid receptors, whereas the antinociceptive effect of MEAP is mediated through the simulation of both kappa- and mu-opioid receptors.

Distribution of FMRFamide-like immunoreactivity in the brain of the lizard Podarcis sicula

The distribution of FMRFamide-like immunoreactive peptides was investigated in the brain of the lizard, Podarcis sicula, using the indirect immunofluorescence technique. The main populations of FMRFamide-immunoreactive cell bodies were located in the forebrain. In the telencephalon, FMRFamide-containing neurons were found both in the pallium and subpallium, namely in the medial cortex, the anterior olfactory nucleus, the nucleus accumbens, the septal nuclei, the nucleus of the medial forebrain bundle, and the nucleus of the diagonal band of Broca. In the diencephalon, a dense accumulation of FMRFamide-immunoreactive neurons was observed in the area preoptica lateralis, the nucleus suprachiasmaticus, the nucleus periventricularis hypothalami, the area lateralis hypothalami, and the dorsal region of the nucleus geniculatus lateralis. In the midbrain, sparse immunoreactive perikarya were found in the tegmentum of the mesencephalon. FMRFamide-immunoreactive fibers were visualized in all regions containing positive cell bodies. In particular, dense bundles of immunoreactive processes were seen in the area preoptica lateralis, in the hypothalamus, and in the median eminence. The tectum and the basal mesencephalon were also densely innervated. Conversely, the caudal brain stem only exhibited scarce immunoreactive processes. The distribution pattern of FMRFamide-immunoreactive neurons in the brain of Podarcis sicula exhibits a number of similarities with that reported in mammals, but significantly differs from that reported in amphibians and fish, suggesting that the neuromodulatory functions of FMRFamide may have diverged during the emergence of terrestrial life.

Colocalization of neuropeptide Y (NPY)-like and FMRFamide-like immunoreactivities in the brain of the Atlantic salmon (Salmo salar)

The colocalization of the peptides neuropeptide Y (NPY) and Phe-Met-Arg-Phe-NH2 (FMRFamide) in the brain of the Atlantic salmon was investigated with double immunofluorescence labeling and peroxidase-antiperoxidase immunocytochemical techniques. Colocalization of NPY-like and FMRF amide-like immunoreactivities was observed in neuronal cell bodies and fibers in four brain regions: in the lateral and commissural nuclei of the area ventralis telencephali, in the nucleus ventromedialis thalami, in the laminar nucleus of the mesencephalic tegmentum, and in a group of small neurons situated among the large catecholaminergic neurons in the isthmal region of the brainstem. All cell bodies in these nuclei were immunoreactive to both NPY and FMRF. We consistently observed larger numbers of FMRF-immunoreactive than NPY-immunoreactive fibers. In the nucleus ventromedialis thalami NPY- and FMRFamide-like immunoreactivities were colocalized in cerebrospinal fluid (CSF)-contacting neurons. NPY-immunoreactive, but not FMRF-immunoreactive, neurons were found in the stratum periventriculare of the optic tectum, and at the ventral border of the nucleus habenularis (adjacent to the nucleus dorsolateralis thalami). Neurons belonging to the nucleus of the nervus terminalis were FMRF-immunoreactive but not NPY-immunoreactive. The differential labeling indicates, as do our cross-absorption experiments, that the NPY and FMRFamide antisera recognize different epitopes. Thus, it is probable that NPY-like and FMRF amide-like substances occur in the same neurons in some brain regions.

Distribution of FMRFamide-like immunoreactivity in the brain of the elasmobranch fish Scyliorhinus canicula

The distribution of FMRFamide-like-immunoreactive peptides was investigated in the brain and pituitary of the elasmobranch fish Scyliorhinus canicula using the indirect immunofluorescence technique. FMRFamide-immunoreactive cells and fibers were mainly observed in the telencephalon and the diencephalon, while other brain structures were almost unstained. In the telencephalon, FMRFamide-like-containing neurons were seen in the caudal part of the area periventricularis pallialis, in the posterior area of the nucleus septi medialis and in the nucleus septi caudoventralis. In the diencephalon, numerous FMRFamide-positive cell bodies were observed in the hypothalamus, ventral thalamus and posterior tuberculum. The highest density of immunofluorescent perikarya was found in the nucleus lobi lateralis hypothalami and in the nucleus periventricularis hypothalami. More caudally, the mesencephalon and the caudal brainstem only contained scattered varicose FMRFamide-immunoreactive fibers. Stained fibers were also identified in the median eminence and several FMRFamide-like-positive cells were detected in the dorsal and rostral parts of the neurointermediate lobe of the pituitary. These data indicate that substances related to the molluscan cardioexcitatory peptide FMRFamide are widely distributed in the brain of S. canicula, suggesting their implication in neuroendocrine and/or neuromodulatory functions.

Immunocytochemical distribution of FMRFamide-like substance in the brain of the cloudy dogfish, Scyliorhinus torazame

The distribution of the molluscan cardioexcitatory tetrapeptide FMRFamide (Phe-Met-Arg-Phe-NH2) in the brain of the cloudy dogfish, Scyliorhinus torazame, was examined by immunocytochemistry. FMRFamide-like immunoreactivity was demonstrated to occur extensively in various regions of the dogfish brain, except for the corpus cerebelli. Immunoreactive neuronal perikarya were located in the ganglion of the nervus terminalis, the preoptic area, and the hypothalamic periventricular gray matter consisting of the nucleus medius hypothalamicus, the nucleus lateralis tuberis, and the nucleus lobi lateralis. Some of the immunoreactive cells in the hypothalamus were identified as cerebrospinal fluid-contacting neurons. The bulk of the immunostained fibers in the nervus terminalis penetrated into the midventral portion of the telencephalon and ran dorsocaudally toward the basal telencephalon and hypothalamus, showing radial projections or ramifications. The labeled fibers were abundant in the midbasal part of the telencephalon and in the hypothalamus, where some fibers were found in loose networks around the cell bodies of the nucleus septi and hypothalamic periventricular nuclei. The fibers demonstrated in the hypothalamus terminated around the vascular wall of the primary capillary plexus of the median eminence or penetrated deeply into the pars intermedia of the hypophysis. These results suggest that, in the dogfish, an FMRFamide-like substance participates in the regulation of adenohypophysial function. This molecule may have a role as a neurotransmitter and/or neuromodulator in the central nervous system.

FMRF-amide and L-Arg-L-Phe increase blood pressure and heart rate in the anaesthetised rat by central stimulation of the sympathetic nervous system

The neuropeptide FMRFamide (L-Phe-L-Met-L-Arg-L-Phe-NH2) increases mean arterial blood pressure (MABP) and heart rate (HR) in the anaesthetised rat at concentrations ranging from 10-1000 micrograms/kg. Here, we demonstrate that peptides containing L-arginyl-L-phenylalanine (L-Arg-L-Phe), the C-terminal sequence of FMRFamide, mimic its haemodynamic effects. L-Arg-L-Phe was approximately 4 fold more potent in increasing MABP and HR than FMRFamide. In 40 different peptides investigated, the following order of potency of the effective compounds was established: L-Arg-L-Phe-L-Ala = L-Arg-L-Phe greater than FMRFamide greater than L-Met-L-Arg-L-Phe = L-Arg-L-Trp greater than L-Arg-L-Tyr greater than D-Arg-L-Phe = L-Arg-L-Phe-OMe greater than L-Arg-L-Leu = L-Arg-L-Ile greater than L-Lys-L-Phe greater than L-Arg-L-Met. L-Arg-L-Phe or FMRFamide did not cause any pressor response in pithed rats, indicating a central mechanism of action. In anaesthetised rats, intravenous injections of FMRFamide or L-Arg-L-Phe (100 micrograms/kg) were associated with a 2-3 fold increase in plasma noradrenaline levels, whereas plasma adrenaline levels remained unchanged. Thus, L-Arg-L-Phe may represent the active principle of FMRFamide acting by a central mechanism involving the release of noradrenaline from sympathetic nerve terminals.

Distribution of FMRFamide-like immunoreactivity in the brain, retina and nervus terminalis of the sockeye salmon parr, Oncorhynchus nerka

Neurons displaying FMRFamide(Phe - Met - Arg - Phe - NH2)-like immunoreactivity have recently been implicated in neural plasticity in salmon. We now extend these findings by describing the extent of the FMRF-like immunoreactive (FMRF-IR) system in the brain, retina and olfactory system of sockeye salmon parr using the indirect peroxidase anti-peroxidase technique. FMRF-IR perikarya were found in the periventricular hypothalamus, mesencephalic laminar nucleus, nucleus nervi terminalis and retina (presumed amacrine cells), and along the olfactory nerves. FMRF-IR fibers were distributed throughout the brain with highest densities in the ventral area of the telencephalon, in the medial forebrain bundle, and at the borders between layers III/IV and IV/V in the optic tectum. High densities of immunoreactive fibers were also observed in the area around the torus semicircularis, in the medial hypothalamus, median raphe, ventromedial tegmentum, and central gray. In the retina, immunopositive fibers were localized to the inner plexiform layer, but several fiber elements were also found in the outer plexiform layer. The olfactory system displayed FMRF-IR fibers in the epithelium and along the olfactory nerves. These findings differ from those reported in other species as follows: (i) FMRF-IR cells in the retina have not previously been reported in teleosts; (ii) the presence of FMRF-IR fibers in the outer plexiform layer of the retina is a new finding for any species; (iii) the occurrence of immunopositive cells in the mesencephalic laminar nucleus has to our knowledge not been demonstrated previously.

Characterization of rat spinal cord receptors to FLFQPQRFamide, a mammalian morphine modulating peptide: a binding study

An in vitro binding assay, using 125I-YLFQPQRFamide, a newly synthetized iodinated analog of FLFQPQRFamide, in which Phe1 (F) has been substituted by a Tyr (Y), was developed to demonstrate and characterize putative binding sites of this brain morphine modulating peptide. This radioligand bound in a time-dependent manner to rat spinal cord membrane preparation. This binding was dose-dependent, saturable and reversible. Both kinetic data and saturation measured at equilibrium lead to the existence of a homogenous population of high affinity binding sites with a Kd value of 0.09-0.1 nM and a maximal capacity Bmax of 14.5 +/- 2 fmol/mg protein. Results of competition experiments show that both FLFQPQRFamide and its analog YLFQPQRFamide had a similar capacity to inhibit the 125I-YLFQPQRFamide binding, suggesting that this radioiodinated analog is a good tool to study binding characteristics of FLFQPQRFamide receptors. The related octadecapeptide AGEGLSSPFWSLAAPQRFamide, another mammalian morphine modulating peptide competes for radioligand binding with similar potency. Our results also show that mu, delta and kappa opiate receptor agonists as well as the antagonist naloxone were not able to affect binding either in presence or in absence of 120 mM NaCl. Together, these data demonstrate that FLFQPQRFamide does not function as an endogenous opiate receptor antagonist and that is capacity to reduce opiate-induced analgesia is supported by specific binding sites.

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.

The molluscan neuropeptide FMRFamide stimulates the release of [14C]acetylcholine from isolated ileal synaptosomal preparations of guinea-pig

FMRFamide stimulated, in a dose-dependent manner, the efflux of [14C]acetylcholine (ACh) from isolated synaptosomes of guinea-pig ileum preloaded with labelled choline. Participation of the cholinergic mechanism and/or substance P was ruled out by the finding that antagonists failed to affect the FMRFamide-induced release of ACh. The ACh-releasing action of FMRFamide was negated by the deletion of calcium ion from the bathing medium and it was also abolished by tetrodotoxin. The results obtained suggest that FMRFamide possesses the ability to induce the release of ACh from enteric synaptosomes of guinea-pig.

Immunohistochemical distribution and partial characterization of FLFQPQRFamidelike peptides in the central nervous system of rats

Two peptides that are structurally related to the molluscan tetrapeptide Phe-Met-Arg-Phe-NH2 (FMRF-NH2) were recently isolated from bovine brain extract (Yang et al.: Proc. Natl. Acad. Sci. USA 82:7757-7761, '85). These peptides have an attenuating effect on morphine-induced analgesia when injected intracerebroventricularly in rats. Antisera against the two peptides--an octapeptide, Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2 (F8F-NH2), and an octadecapeptide, Ala-Gly-Glu-Gly-Leu-Ser-Ser-Pro-Phe-Trp-Ser-Leu-Ala-Ala-Pro-Gln-Arg-Phe- NH2 (A18F-NH2)--were raised in rabbits and characterized with standard radioimmunoassay and immunohistochemical blocking controls. This study was aimed at localizing neurons in the rat brain that contain immunoreactivity for these peptides. Cryostat sections of normal and colchicine-treated Sprague-Dawley rat brains were incubated with the specific antisera and the immunoreactivity was visualized by the PAP or the FITC method. Immunoreactive neurons were detected in the hypothalamus and nucleus of the solitary tract. Relatively dense networks of fibers and terminals were observed in the lateral parabrachial nucleus and in the nucleus of the solitary tract. Fibers and terminals were also seen in the lateral septum, amygdala, hypothalamus, neurohypophysis, thalamus, periaqueductal gray, and several medullary nuclei. In these areas the immunoreactivity was abolished when the antisera were incubated with the corresponding synthetic peptides F8F-NH2 and A18F-NH2. Preincubation of the antisera with neuropeptide Y (NPY) did not affect the staining. The results suggest that there are neurons in the rat brain that contain F8F-NH2- and A18F-NH2-like peptides. The neuronal system described here may have a role in the regulation of autonomic functions, such as hypothalamic control and respiratory functions. The neuronal connections of the cells containing F8F-NH2- and A18F-NH2-like peptides remain to be studied.

IgG from antiserum against endogenous mammalian FMRF-NH2-related peptides augments morphine- and stress-induced analgesia in mice

Two mammalian FMRF-NH2-like peptides have been isolated from bovine brain; an octapeptide with the structure Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2 (F-8-F-NH2) and an octadecapeptide, Ala-Gly-Glu-Gly-Leu-Ser-Ser-Pro-Phe-Trp-Ser-Leu-Ala-Ala-Pro-Gln-Arg-Phe- NH2 (A-18-F-NH2). In the present study determinations were made of the effects of intracerebroventricular administration of IgG prepared from antisera raised against these peptides on nociception and morphine- and immobilization-induced opioid analgesia in mice. Both F-8-F-NH2-IgG and A-18-F-NH2-IgG antisera increased nociception (thermal response latency) and significantly augmented morphine- and immobilization-induced analgesia in a naloxone reversible manner, with F-8-F-NH2-IgG antisera having a greater effect than A-18-F-NH2-IgG antisera. These results provide further evidence that mammalian FMRF-NH2-like peptides function as endogenous opiate antagonists and have a role in the mediation of antinociception.

Distribution of FMRFamide-like immunoreactivity in the central nervous system of the Formosan monkey (Macaca cyclopsis)

The distribution of FMRFamide-like immunoreactivity in the central nervous system of the Formosan monkey (Macaca cyclopsis) was investigated employing immunohistochemical techniques. FMRFamide-containing cells were found to be widely distributed throughout the forebrain. Principal densities of FMRFamide neuronal perikarya were observed in the following areas: the amygdaloid complex, the olfactory tubercle, the cerebral cortex, the basal ganglia, the septum, the caudate-putamen and the arcuate nucleus. A large number of immunoreactive fibers were observed in areas ranging from the cerebral cortex to the spinal cord, and were noted in the following locations: the preoptic area, the tuberal and posterior hypothalamic areas, the bed nucleus of the stria terminalis, the nuclei of the spinal trigeminal nerve, the hypoglossal nucleus, the nucleus of the solitary tract, and the dorsal horn of the spinal cord. The results generally parallel those described in the rat and guinea pig.

Peripheral injection of DNS-RFa, a FMRFa agonist, suppresses morphine-induced analgesia in rats

The present results demonstrate an antagonistic effect of DNS-RFa on morphine-induced analgesia in rats. This confirms previous evidence presented by others on the effects of FMRFa-related peptides when applied centrally. Unlike these peptides, however, it is shown here that DNS-RFa is effective upon peripheral injection. The effects of DNS-RFa on morphine-induced analgesia were dose-dependent (ED50 = 0.5 mg/kg). DNS-RFa alone (5 mg/kg) did not affect the control level of nociception. Peripheral injection of FMRFa (5 mg/kg) did not affect morphine-induced analgesia. DNS-RFa defines the minimal configuration to activate neuronal FMRFa receptors in the pond snail. The present report suggests also that in vertebrates the Arg-Phe-NH2 sequence is essential and that DNS-RFa readily penetrates the blood-brain barrier.

One receptor type mediates two independent effects of FMRFa on neurosecretory cells of Lymnaea

Structure activity relations (SAR) of FMRFa on the transient hyperpolarizing response and long lasting depression of excitability of neurosecretory caudo dorsal cells (CDCs) of the pond snail Lymnaea stagnalis were examined. Although these effects to FMRFa occur independently, the SARs for the induction of both responses were identical suggesting that CDCs possess a single type of FMRFa receptors. Native GDPFLRFa and SDPFLRFa were equipotent to FMRFa receptors. It is concluded that activation of the receptor requires [Arg3-Phe4]-NH2, whereas N-terminal amino acids are involved in binding.

Biochemical characterization of FMRF-NH2-like peptides in spinal cords of various mammalian species using specific radioimmunoassays

Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2 (F-8-F-NH2) and Ala-Gly-Glu-Gly-Leu-Ser-Ser-Pro-Phe-Trp-Ser-Leu-Ala-Ala-Pro-Gln-Arg-Phe-NH2 (A-18-F-NH2), originally detected by FMRF-NH2 antiserum and subsequently isolated from bovine brain, were found to be highly localized in the bovine spinal cord. Using specific radioimmunoassays coupled with HPLC, F-8-F-NH2 and A-18-F-NH2 immunoreactivities in spinal cord of bovine, rat, mouse, guinea pig and human were studied. One major F-8-F-NH2 immunoreactivity was detected in the spinal cord of every species except in human, however, the retention time of F-8-F-NH2 immunoreactivity appears to vary from species to species. In the human spinal cord three major F-8-F-NH2 immunoreactivities are detected and one of them was eluted in the position of F-8-F-NH2. Two major A-18-F-NH2 immunoreactivities were detected in every species except guinea pig; one of these immunoreactivities can be identified as F-8-F-NH2 immunoreactivity due to the high affinity of the A-18-F-NH2 antiserum to F-8-F-NH2. F-8-F-NH2 and A-18-F-NH2 immunoreactivities can also be clearly detected by FMRF-NH2 antiserum, however, the quantities of these peptides can be grossly underestimated by the FMRF-NH2 RIA. These results confirm that there is a novel system of FMRF-NH2-like peptides in mammalian CNS and some of them are more closely related to the bovine peptides, F-8-F-NH2 and A-18-F-NH2 than to FMRF-NH2.

Dual inhibitory action of FMRFamide on neurosecretory cells controlling egg laying behavior in the pond snail

We describe here the electrophysiological characterization of a dual inhibitory action of FMRFamide (FMRFa, Phe-Met-Arg-Phe-NH2) on the caudodorsal cells (CDCs) of the pond snail Lymnaea stagnalis: (i) a transient hyperpolarizing response (H-response) and (ii) a suppression of the excitability of the cells, which lasted as long as the peptide was present. Both effects of FMRFa occurred in silent, excitable cells as well as discharging cells. The effects were reversible and dose-dependent in the range of 10(-9) to 10(-5) M. The H-response was not blocked by any of the antagonists to classical neurotransmitters that were tested. The reversal potential of the H-response was dependent on the [K+]o, which suggests that K+ is the major charge carrier in this response. 4-Aminopyridine (4-AP) blocked the H-response but did not affect the suppression of the excitability by FMRFa. This indicates that the effects of the peptide on these cells are independent. Experiments on the mechanism of the inhibition of the excitability indicated that FMRFa blocks the cAMP-dependent activation of the pacemaking mechanism of the CDCs. In experiments with isolated cells it was demonstrated that the actions of FMRFa are mediated directly through receptors on CDCs (H-response: ED50 = 10(-8) M). Finally, anti-FMRFa-positive varicosities and axons close to the somata, the axons and the neurohaemal endings of the CDCs were demonstrated immunocytochemically. The duality of the action of FMRFa on the neural activity of CDCs indicates its role of high priority in the regulation of egg laying behavior.

Isolation and characterization of a Drosophila gene that encodes multiple neuropeptides related to Phe-Met-Arg-Phe-NH2 (FMRFamide)

A Drosophila gene that encodes neuropeptides related to molluscan Phe-Met-Arg-Phe-NH2 (FMRFamide) was isolated by screening a genomic library with a fragment of an Aplysia Phe-Met-Arg-Phe-NH2 cDNA and with synthetic oligonucleotides. This gene was used to isolate a cDNA from a Drosophila adult head cDNA library. The cDNA was defined by sequence analysis to encode 13 peptides that have Phe-Met-Arg-Phe-NH2 or related sequences at their carboxyl termini. Other putative neuropeptides, including one that has homology to mammalian corticotropin-releasing factor, are present in the deduced approximately equal to 39-kDa precursor. Southern blot analysis suggested the presence of a single Phe-Met-Arg-Phe-NH2-like gene within the haploid genome. RNA blot analysis indicated the expression of at least two transcripts of approximately equal to 1.7 and approximately equal to 0.7 kilobases. Both transcripts are evident throughout larval, pupal, and adult developmental stages. In situ hybridization was used to localize this neuropeptide gene to band 46C on the right arm of the 2nd chromosome. These data provide the basis for utilizing the advanced genetics and molecular techniques of Drosophila to address complex aspects of neuropeptide expression and function.

The effect of Phe-Met-Arg-Phe-NH2 (FMRFamide) on morphine-induced inhibition of colonic propulsive motility in mice

Morphine and the molluscan neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFamide) were administered to mice alone or in combination intracerebroventricularly (i.c.v.) and the effect on colonic propulsive motility was measured. Both morphine (1.0 microgram, i.c.v.) and FMRFamide (10 and 50 micrograms, i.c.v.) delayed expulsion of a 3 mm glass bead placed in the distal colon of mice compared to vehicle-treated controls. The inhibitory effects of morphine and FMRFamide on expulsion time were additive at the doses used and individually blocked by naloxone. These data suggest that FMRFamide does not antagonize this nonanalgesic effect of morphine, but appears to have opioid agonist properties.

Purification and characterization of FMRFamidelike immunoreactive substances from the lobster nervous system: isolation and sequence analysis of two closely related peptides

In the preceding paper (Kobierski et al: J. Comp. Neurol. 266:1-15, '87) FMRFamidelike immunoreactivity (FLI) was localized to specific cells and processes in the nervous system of the lobster Homarus americanus. In an effort to establish a role for this material we have purified and characterized a variety of immunoreactive peptides that can be extracted from the secretory pericardial organs. By using gel-filtration chromatography and three different HPLC systems, it has been established that little or no authentic FMRFamide is present. Of the major immunoreactive components two peptides were purified in sufficient quantity for microsequence analysis and have been tentatively identified as the octapeptides Ser-Asp-Arg-Asn-Phe-Leu-Arg-Phe-amide (FLI 3) and Thr-Asn-Arg-Asn-Phe-Leu-Arg-Phe-amide (FLI 4). Both of these are novel neuropeptides with some sequence homology to the previously described FMRFamide family. The pericardial organs release FLI when depolarized with 100 mM K+ in the presence of calcium. Between 75 and 80% of this release is accounted for by FLI 3 and FLI 4. One of these peptides (FLI 4) has been synthesized and shown to cochromatograph with the endogenous immunoreactive material. Preliminary studies show that this peptide can act as a modulator of exoskeletal and cardiac neuromuscular junctions.

Catch-relaxing peptide isolated from Mytilus pedal ganglia

A peptide that relaxes catch tension of the anterior byssus retractor muscle of Mytilus edulis was purified from pedal ganglion extracts of the mussel. Its primary structure was determined to be H-Ala-Met-Pro-Met-Leu-Arg-Leu-NH2. This peptide was found to have not only catch-relaxing action on the byssus retractor muscle but also modulatory actions on contractions in various molluscan muscles.

Amnesic action of FMRFamide in rats

The effects of FMRFamide on passive avoidance behaviour and electroshock-induced amnesia following intracerebroventricular administration were studied in rats. FMRFamide given immediately after the learning trial, or 20 min before the retention trial, attenuated the avoidance response, thereby impairing the consolidation and retrieval processes. Electroshock induced amnesia when applied immediately after the learning trial. Treatment with FMRFamide facilitated the amnesia of the passive avoidance response. The results indicate that FMRFamide peptide belongs in the class of neuropeptide which are amnesic.

Calcium channel blockers inhibit the antagonistic effects of Phe-Met-Arg-Phe-amide (FMRFamide) on morphine- and stress-induced analgesia in mice

Determinations were made of the effects of the calcium channel blockers, nifedipine and verapamil, on the antagonistic effects of FMRFamide (Phe-Met-Arg-Phe-NH2) and naloxone on morphine- and immobilization-induced opioid analgesia in mice. Intraperitoneal (i.p.) administrations of the calcium channel antagonists significantly reduced the inhibitory effects of intracerebroventricular (i.c.v.) FMRFamide, but had no effects on i.p. or i.c.v. naloxone-mediated inhibition of either morphine- or immobilization-induced analgesia. These results suggest that the antagonistic effects of FMRFamide, (or other endogenous FMRFamide-like peptides) on both opiate- and opioid-mediated analgesia in mice may involve alterations in the functioning of calcium channels.

Distribution and characterization of two putative endogenous opioid antagonist peptides in bovine brain

Highly sensitive radioimmunoassays were developed and used in studies of the distribution and chromatographic properties of two mammalian FMRF-NH2-like peptides recently isolated from bovine brain; an octapeptide with the structure Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2 (F-8-F-NH2) and on octadecapeptide, Ala-Gly-Glu-Gly-Leu-Ser-Ser-Pro-Phe-Trp-Ser-Leu-Ala-Ala-Pro-Gln-Arg-Phe-NH2 (A-18-F-NH2). F-8-F-NH2 and A-18-F-NH2 immunoreactivities are unevenly distributed in bovine brain. The highest concentrations (pmol g-1) of F-8-F-NH2 and A-18-F-NH2 are found in dorsal spinal cord (9.8 and 16.4 respectively), periaqueductal grey (8.6 and 6.8) and pons medulla (7.0 and 8.9); lowest quantities are in cortex, cerebellum and striatum. HPLC analysis coupled with radioimmunoassay reveals that the major immunoreactivities are identical to synthetic F-8-F-NH2 and A-18-F-NH2 while there are additional immunoreactive materials, distinct from NPY, whose structures still remain to be determined. The enrichment of these peptides in dorsal cord and periaqueductal grey, areas important in opioid-mediated pain perception, suggest that they may play a role in mediating antinociception.

Antinociceptive action of cholecystokinin octapeptide (CCK 8) and related peptides in rats and mice: effects of naloxone and peptidase inhibitors

Cholecystokinin octapeptide (CCK 8) produced significant antinociception in the tail immersion test in the rat, paw pressure test in the rat and acetylcholine-induced writhing test in the mouse after subcutaneous (s.c.) administration. In the hot plate test, CCK 8 (s.c.) showed antinociceptive activity if the latency to lick was used as the end point but if the latency to jump was recorded the antinociceptive effects were not evident. Cholecystokinin tetrapeptide (CCK 4) was shown to be antinociceptive in the writhing but not in the hot plate test after subcutaneous administration and appeared to be less potent than CCK 8 when tested under the same conditions. Antinociception induced by CCK 8 in the hot plate test (lick) could also be demonstrated after direct intracerebroventricular (i.c.v.) injection and this observation was also made with the CCK-related peptide FMRF amide. Antinociception induced by CCK 8 (which did not appear to be associated with reduced locomotor activity) was evident 5 min after intraventricular injection and was maximal at 10 min, the effect lasting over a 30-45 min period. The antinociceptive effect of CCK 8 was antagonised by naloxone and was potentiated by simultaneous administration of the peptidase inhibitors bestatin, thiorphan and captopril.

Phe-Met-Arg-Phe-amide (FMRFamide) stimulated growth hormone secretion in conscious OVX rats

The presence of FMRFamide (Phe-Met-Arg-Phe-amide)-like immunoreactivity in neuronal elements in the hypothalamus suggested a role for this in the hypothalamic control of the anterior pituitary function. We report here the action of FMRFamide on growth hormone release following intracerebroventricular administration to rats. The injection of 200 ng (313.8 picomoles) of FMRFamide (in 2 ul) produced a significantly increased plasma GH 15 min after injection. The GH-increasing effect of 400-800 ng (627-1255 picomoles) of FMRFamide was already developed after 5 min and lasted up to 30 min. No change was detected in the plasma FSH, LH and prolactin levels at any time during the experimental period. The intravenous administration of 10, 30 or 100 ug of FMRFamide had no effect on the plasma GH level. We conclude that FMRFamide can act at low doses to increase GH release through the inhibition of somatostatin release or the stimulation of GRF. We could not exclude a direct site of action in the pituitaries.

Antibodies to FMRF amide, and the related pentapeptide LPLRF amide, reveal two groups of immunoreactive peptides in chicken brain

A radioimmunoassay has been developed for the chicken brain peptide, Leu-Pro-Leu-Arg-Phe-amide (LPLRF amide); this peptide was originally discovered because it reacts with antibodies to the molluscan neuropeptide FMRF amide. The present antibody to LPLRF amide reacts about twenty times less well with FMRF amide compared with LPLRF amide. Using radioimmunoassays employing antibodies raised against LPLRF amide and FMRF amide we have separated by gel filtration and HPLC several different immunoreactive peptides in acid alcohol extracts of chicken brain. When LPLRF amide was used as the assay standard one group of peptides reacted similarly with the two types of antibody; the other group, which was represented by a single major component, reacted at least 50 times better with FMRF amide antibodies compared with LPLRF amide antibodies. It seems, therefore, that in the avian central nervous system, and probably other vertebrates, there are several different groups of peptides immunochemically related to FMRF amide.