The neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFamide) directly gates two ion channels in an identified Helix neurone

New techniques, applications and perspectives in neuropeptide research

Neuropeptides are one of the most diverse classes of signaling molecules and have attracted great interest over the years owing to their roles in regulation of a wide range of physiological processes. However, there are unique challenges associated with neuropeptide studies stemming from the highly variable molecular sizes of the peptides, low in vivo concentrations, high degree of structural diversity and large number of isoforms. As a result, much effort has been focused on developing new techniques for studying neuropeptides, as well as novel applications directed towards learning more about these endogenous peptides. The areas of importance for neuropeptide studies include structure, localization within tissues, interaction with their receptors, including ion channels, and physiological function. Here, we discuss these aspects and the associated techniques, focusing on technologies that have demonstrated potential in advancing the field in recent years. Most identification and structural information has been gained by mass spectrometry, either alone or with confirmations from other techniques, such as nuclear magnetic resonance spectroscopy and other spectroscopic tools. While mass spectrometry and bioinformatic tools have proven to be the most powerful for large-scale analyses, they still rely heavily on complementary methods for confirmation. Localization within tissues, for example, can be probed by mass spectrometry imaging, immunohistochemistry and radioimmunoassays. Functional information has been gained primarily from behavioral studies coupled with tissue-specific assays, electrophysiology, mass spectrometry and optogenetic tools. Concerning the receptors for neuropeptides, the discovery of ion channels that are directly gated by neuropeptides opens up the possibility of developing a new generation of tools for neuroscience, which could be used to monitor neuropeptide release or to specifically change the membrane potential of neurons. It is expected that future neuropeptide research will involve the integration of complementary bioanalytical technologies and functional assays.

Physiology, signaling, and pharmacology of galanin peptides and receptors: three decades of emerging diversity

Galanin was first identified 30 years ago as a "classic neuropeptide," with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides-galanin-like peptide and alarin-with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein-coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor-specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.

The comprehensive analysis of DEG/ENaC subunits in Hydra reveals a large variety of peptide-gated channels, potentially involved in neuromuscular transmission

Background

It is generally the case that fast transmission at neural synapses is mediated by small molecule neurotransmitters. The simple nervous system of the cnidarian Hydra, however, contains a large repertoire of neuropeptides and it has been suggested that neuropeptides are the principal transmitters of Hydra. An ion channel directly gated by Hydra-RFamide neuropeptides has indeed been identified in Hydra – the Hydra Na⁺ channel (HyNaC) 2/3/5, which is expressed at the oral side of the tentacle base. Hydra-RFamides are more widely expressed, however, being found in neurons of the head and peduncle region. Here, we explore whether further peptide-gated HyNaCs exist, where in the animal they are expressed, and whether they are all gated by Hydra-RFamides.

Results

We report molecular cloning of seven new HyNaC subunits – HyNaC6 to HyNaC12, all of which are members of the DEG/ENaC gene family. In Xenopus oocytes, these subunits assemble together with the four already known subunits into thirteen different ion channels that are directly gated by Hydra-RFamide neuropeptides with high affinity (up to 40 nM). In situ hybridization suggests that HyNaCs are expressed in epitheliomuscular cells at the oral and the aboral side of the tentacle base and at the peduncle. Moreover, diminazene, an inhibitor of HyNaCs, delayed tentacle movement in live Hydra.

Conclusions

Our results show that Hydra has a large variety of peptide-gated ion channels that are activated by a restricted number of related neuropeptides. The existence and expression pattern of these channels, and behavioral effects induced by channel blockers, suggests that Hydra co-opted neuropeptides for fast neuromuscular transmission.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-014-0084-2) contains supplementary material, which is available to authorized users.

The genetic architecture of degenerin/epithelial sodium channels in Drosophila

Degenerin/epithelial sodium channels (DEG/ENaC) represent a large family of animal-specific membrane proteins. Although the physiological functions of most family members are not known, some have been shown to act as nonvoltage gated, amiloride-sensitive sodium channels. The DEG/ENaC family is exceptionally large in genomes of Drosophila species relative to vertebrates and other insects. To elucidate the evolutionary history of the DEG/ENaC family in Drosophila, we took advantage of the genomic and genetic information available for 12 Drosophila species that represent all the major species groups in the Drosophila clade. We have identified 31 family members (termed pickpocket genes) in Drosophila melanogaster, which can be divided into six subfamilies, which are represented in all 12 species. Structure prediction analyses suggested that some subunits evolved unique structural features in the large extracellular domain, possibly supporting mechanosensory functions. This finding is further supported by experimental data that show that both ppk1 and ppk26 are expressed in multidendritic neurons, which can sense mechanical nociceptive stimuli in larvae. We also identified representative genes from five of the six DEG/ENaC subfamilies in a mosquito genome, suggesting that the core DEG/ENaC subfamilies were already present early in the dipteran radiation. Spatial and temporal analyses of expression patterns of the various pickpocket genes indicated that paralogous genes often show very different expression patterns, possibly indicating that gene duplication events have led to new physiological or cellular functions rather than redundancy. In summary, our analyses support a rapid early diversification of the DEG/ENaC family in Diptera followed by physiological and/or cellular specialization. Some members of the family may have diversified to support the physiological functions of a yet unknown class of ligands.

Sensory functions for degenerin/epithelial sodium channels (DEG/ENaC)

All animals use a sophisticated array of receptor proteins to sense their external and internal environments. Major advances have been made in recent years in understanding the molecular and genetic bases for sensory transduction in diverse modalities, indicating that both metabotropic and ionotropic pathways are important in sensory functions. Here, I review the historical background and recent advances in understanding the roles of a relatively newly discovered family of receptors, the degenerin/epithelial sodium channels (DEG/ENaC). These animal-specific cation channels show a remarkable sequence and functional diversity in different species and seem to exert their functions in diverse sensory modalities. Functions for DEG/ENaC channels have been implicated in mechanosensation as well as chemosensory transduction pathways. In spite of overall sequence diversity, all family members share a unique protein topology that includes just two transmembrane domains and an unusually large and highly structured extracellular domain, that seem to be essential for both their mechanical and chemical sensory functions. This review will discuss many of the recent discoveries and controversies associated with sensory function of DEG/ENaC channels in both vertebrate and invertebrate model systems, covering the role of family members in taste, mechanosensation, and pain.

FMRFamide and related peptides in the phylum mollusca

FMRFamide is one of the well-known peptides studied within the phylum Mollusca. It was first isolated from the clam Macrocallista nimbosa during the end of the 1960s. Since then, a number of reports related to FMRFamide have been published from different experimental approaches, revealing that it and its related peptides (FaRPs) are implicated in a variety of physiological processes. As this year is the 30th anniversary since its discovery, this review focuses on diverse findings related to both FMRFamide and FaRPs in the phylum Mollusca.

Ion-channels on parasite muscle: pharmacology and physiology

Ion-channels are essential components of excitable cells. This fact has been exploited in the development of anthelmintic agents; the majority of which act on nematode ion channels. The purpose of this review is to describe the site of action of some frequently used anthelmintic compounds: nAChRs and levamisole/pyrantel; Glu-Cls and avermectins/mylbemycins; GABA receptors and piperazine. Also described is some of the physiological and pharmacological data on other nematode muscle ion-channels which may prove attractive targets for future anthelmintic development: Ca2+ activated Cl(-) channels; peptide gated chloride Cl(-) channels; Ca2+ channels and potassium channels. Emphasis is placed on the pharmacological and physiological data from parasite tissue. Information on the genes involved in ion-channel formation and modulation are reviewed in detail elsewhere in this issue.

FMRFamide-gated sodium channel and ASIC channels: a new class of ionotropic receptors for FMRFamide and related peptides

FMRFamide and related peptides typically exert their action through G-protein coupled receptors. However, two ionotropic receptors for these peptides have recently been identified. They are both members of the epithelial amiloride-sensitive Na+ channel and degenerin (ENaC/DEG) family of ion channels. The invertebrate FMRFamide-gated Na+ channel (FaNaC) is a neuronal Na+-selective channel which is directly gated by micromolar concentrations of FMRFamide and related tetrapeptides. Its response is fast and partially desensitizing, and FaNaC has been proposed to participate in peptidergic neurotransmission. On the other hand, mammalian acid-sensing ion channels (ASICs) are not gated but are directly modulated by FMRFamide and related mammalian peptides like NPFF and NPSF. ASICs are activated by external protons and are therefore extracellular pH sensors. They are expressed both in the central and peripheral nervous system and appear to be involved in many physiological and pathophysiological processes such as hippocampal long-term potentiation and defects in learning and memory, acquired fear-related behavior, retinal function, brain ischemia, pain sensation in ischemia and inflammation, taste perception, hearing functions, and mechanoperception. The potentiation of ASIC activity by endogenous RFamide neuropeptides probably participates in the response to noxious acidosis in sensory and central neurons. Available data also raises the possibility of the existence of still unknown FMRFamide related endogenous peptides acting as direct agonists for ASICs.

Molecular cloning and functional characterization of the Aplysia FMRFamide-gated Na+ channel

FMRFamide-gated Na+ channel (FaNaC) is the only known peptide-gated ion channel, which belongs to the epithelial Na+ channel/degenerin (ENaC/DEG) family. We have cloned a putative FaNaC from the Aplysia kurodai CNS library using PCR, and examined its characteristics in Xenopus oocytes. A. kurodai FaNaC (AkFaNaC) comprised with 653 amino acids, and the sequence predicts two putative membrane domains and a large extracellular domain as in other members of the ENaC/DEG family. In oocytes expressing AkFaNaC, FMRFamide evoked amiloride-sensitive Na+ current. Different from the known FaNaCs (Helix and Helisoma FaNaCs), AkFaNaC was blocked by external Ca2+ but not by Mg2+. Also, desensitization of the current was enhanced by Mg2+ but not by Ca2+. The FMRFamide-gated current was depressed in both low and high pH. These results indicate that AkFaNaC is an FaNaC of Aplysia, and that the channel has Aplysia specific functional domains.

Domain near TM1 influences agonist and antagonist responses of peptide-gated Na+ channels

A molecular biological approach was used to analyse the importance of different amino acids for ligand activation and for determining the action of amiloride on peptide- (Phe-Met-Arg-Phe-NH(2), FMRFamide)-gated Na(+) channels, members of the degenerin/epithelial Na channel (DEG/ENaC) family. Amiloride is a low-affinity blocker of most DEG/ENa channels, but has an unusual enhancing effect on responses of some of them. Chimeras were expressed in Xenopus oocytes and studied electrophysiologically. Differences in properties of channels from Helix aspersa and Helisoma trivolvis highlighted a sequence of 50 residues of the extracellular domain, near the first transmembrane segment (TM1), that affected sensitivity to FMRFamide, and whether amiloride blocked or enhanced the response to FMRFamide. Comparisons of chimeras prepared from H. aspersa and the extracellular domains of two other species, Aplysia californica and Lymnaea stagnalis and the preparation of further constructs, showed that amino acids 128-134 in the H. aspersa sequence are important in determining the predominant effect of amiloride and influencing the EC(50) of FMRFamide. The results also showed that amino acids in this region are influenced by amino acids in other regions of the extracellular domain so as to affect not only the magnitude of responses, but also their time course and desensitisation.

Insect Neuropeptide and Peptide Hormone Receptors: Current Knowledge and Future Directions

Peptides form a very versatile class of extracellular messenger molecules that function as chemical communication signals between the cells of an organism. Molecular diversity is created at different levels of the peptide synthesis scheme. Peptide messengers exert their biological functions via specific signal-transducing membrane receptors. The evolutionary origin of several peptide precursor and receptor gene families precedes the divergence of the important animal Phyla. In this chapter, current knowledge is reviewed with respect to the analysis of peptide receptors from insects, incorporating many recent data that result from the sequencing of different insect genomes. Therefore, detailed information is provided on six different peptide receptor families belonging to two distinct receptor categories (i.e., the heptahelical and the single transmembrane receptors). In addition, the remaining problems, the emerging concepts, and the future prospects in this area of research are discussed.

Effects of neuropeptide SF and related peptides on acid sensing ion channel 3 and sensory neuron excitability

Acid sensing ion channel 3 (ASIC3) is a cation channel gated by extracellular protons. It is highly expressed in sensory neurons, including small nociceptive neurons and has been proposed to participate in pain perception associated with tissue acidosis and in mechanoperception. Neuropeptide FF (NPFF) and FMRFamide have been shown to potentiate proton-gated currents from cultured sensory neurons and acid sensing ion channel (ASIC) cDNA transfected cells. In this study, we report that another mammalian peptide neuropeptide SF (NPSF), derived from the same precursor, also considerably increases the amplitude of the sustained current of heterologously expressed ASIC3 (12-fold vs. 19- and nine-fold for FMRFamide and NPFF, respectively) with an EC(50) of approximately 50 microM. Similar effects were also observed on endogenous ASIC3-like sustained current recorded from DRG neurons although of smaller amplitudes (two-, three- and seven-fold increase for NPSF, NPFF and FMRFamide, respectively), and essentially related to a slowing down of the inactivation rate. Importantly, this modulation induced changes in neuronal excitability in response to an electrical stimulus applied during extracellular acidification. ASIC3-mediated sustained depolarisation, and its regulation by neuropeptides, could thus be important in regulating polymodal neuron excitability particularly under inflammatory conditions where the expression levels of both NPFF precursor and ASIC3 are increased.

Modulation of spontaneous and evoked EPSCs and IPSCs in optic lobe neurons of cuttlefish Sepia officinalis by the neuropeptide FMRF-amide

The effects of the neuropeptide FMRFa on spontaneous excitatory postsynaptic currents (sEPSCs) and inhibitory postsynaptic currents (sIPSCs), as well as on evoked EPSCs and IPSCs, in two types of neurons within the central optic lobe of cuttlefish were examined using the whole-cell voltage-clamp technique. FMRFa (1-10 micro m) did not affect cell membrane resting potentials, but reversibly reduced both the frequency and amplitude of sEPSCs in neurons within the medulla region of the optic lobe while increasing the frequency and amplitude of their sIPSCs. For centrifugal neurons in the inner granule cell layer of the optic lobe, FMRFa (1-10 micro m) decreased both the frequency and amplitude of sEPSCs. In the presence of tetrodotoxin (0.5 micro m), neither the interevent interval, nor amplitude distributions of the miniature EPSCs or the miniature IPSCs, were affected by FMRFa, implying a presynaptic action of FMRFa on the optic lobe neurons. Bath application of the neuropeptide also abolished or reduced in amplitude the evoked EPSCs and increased the amplitude of evoked IPSCs in optic lobe neurons, showing that FMRFa induced similar effects on evoked as on spontaneous postsynaptic currents. These results demonstrate the complex range of modulatory effects FMRFa can have within central nervous system circuits.

Localization of a FMRFamide-related peptide in efferent neurons and analysis of neuromuscular effects of DRNFLRFamide (DF2) in the crustacean Idotea emarginata

In the ventral nerve cord of the isopod Idotea emarginata, FMRFamide-immunoreactive efferent neurons are confined to pereion ganglion 5 where a single pair of these neurons was identified. Each neuron projects an axon into the ipsilateral ventral and dorsal lateral nerves, which run through the entire animal. The immunoreactive axons form numerous varicosities on the ventral flexor and dorsal extensor muscle fibres, and in the pericardial organs. To analyse the neuromuscular effects of a FMRFamide, we used the DRNFLRFamide (DF2). DF2 acted both pre- and postsynaptically. On the presynaptic side, DF2 increased transmitter release from neuromuscular endings. Postsynaptically, DF2 depolarized muscle fibres by approximately 10 mV. This effect was not observed in leg muscles of a crab. The depolarization required Ca2+, was blocked by substituting Ca2+ with Co2+, but not affected by nifedipine or amiloride. In Idotea, DF2 also potentiated evoked extensor muscle contractions. The amplitude of high K+ contractures was increased in a dose dependent manner with an EC50 value of 40 nm. In current-clamped fibres, DF2 strongly potentiated contractions evoked by current pulses exceeding excitation-contraction threshold. In voltage-clamped fibres, the inward current through l-type Ca2+ channels was increased by the peptide. The observed physiological effects together with the localization of FMRFamide-immunoreactive efferent neurons suggest a role for this type of peptidergic modulation for the neuromuscular performance in Idotea. The pre- and postsynaptic effects of DF2 act synergistically and, in vivo, all should increase the efficacy of motor input to muscles resulting in potentiation of contractions.

PF4, a FMRFamide-related peptide, gates low-conductance Cl(-) channels in Ascaris suum

Here we describe the actions of the peptide Lys-Pro-Asn-Phe-Ile-Arg-Phe-NH(2), or PF4, on inside-out membrane patches (n=164), recorded from vesicles derived from Ascaris suum somatic muscle cells. We observed numerous, small-amplitude Cl(-) channels in the membrane patches. The conductance of the Cl(-) channels ranged from 1.09 to 7.07 pS, the open probability (P(open)) ranged from 0.047+/-0.015 (mean+/-S.E.M.) at 0 microM PF4 to 0.156+/-0.026 at 0.1 microM PF4. The channel mean open time was more variable and prolonged at negative potentials than when the membrane patch was clamped at positive potentials: at 0.03 microM PF4, the mean open time (+/-S.E.M) at -80 mV was 522+/-333 ms; at+80 mV, it was 25+/-7 ms. When patches were isolated from the parent vesicle, there were no changes in channel characteristics, suggesting that the channels function without the involvement of cytoplasmic components. Similarly, the channel characteristics were not affected by the G-protein inhibitor, guanosine-5'-O-(2-thiodiphosphate), indicating that the ion channels do not require a G-protein to function. These data indicate that the PF4-activated Cl(-) channels function independently of intracellular signal transducers and are, therefore, directly gated by PF4.

Identification in Drosophila melanogaster of the invertebrate G protein-coupled FMRFamide receptor

We here describe the cloning and characterization of the functionally active Drosophila melanogaster (Drm) FMRFamide receptor, which we designated as DrmFMRFa-R. The full-length ORF of a D. melanogaster orphan receptor, CG 2114 (Berkeley Drosophila Genome Project), was cloned from genomic DNA. This receptor is distantly related to mammalian thyroid-stimulating hormone-releasing hormone receptors and to a set of Caenorhabditis elegans orphan receptors. An extract of 5,000 central nervous systems from the related but bigger flesh fly, Neobellieria bullata (Neb), was used to screen cells expressing the orphan receptor. Successive purification steps, followed by MS, revealed the sequence of two previously uncharacterized endogenous peptides, APPQPSDNFIRFamide (Neb-FIRFamide) and pQPSQDFMRFamide (Neb-FMRFamide). These are reminiscent of other insect FMRFamide peptides, having neurohormonal as well as neurotransmitter functions. Nanomolar concentrations of the Drm FMRFamides (DPKQDFMRFamide, TPAEDFMRFamide, SDNFMRFamide, SPKQDFMRFamide, and PDNFMRFamide) activated the cognate receptor in a dose-dependent manner. To our knowledge, the cloned DrmFMRFa-R is the first functionally active FMRFamide G protein-coupled receptor described in invertebrates to date.

Epithelial sodium channel/degenerin family of ion channels: a variety of functions for a shared structure

The recently discovered epithelial sodium channel (ENaC)/degenerin (DEG) gene family encodes sodium channels involved in various cell functions in metazoans. Subfamilies found in invertebrates or mammals are functionally distinct. The degenerins in Caenorhabditis elegans participate in mechanotransduction in neuronal cells, FaNaC in snails is a ligand-gated channel activated by neuropeptides, and the Drosophila subfamily is expressed in gonads and neurons. In mammals, ENaC mediates Na+ transport in epithelia and is essential for sodium homeostasis. The ASIC genes encode proton-gated cation channels in both the central and peripheral nervous system that could be involved in pain transduction. This review summarizes the physiological roles of the different channels belonging to this family, their biophysical and pharmacological characteristics, and the emerging knowledge of their molecular structure. Although functionally different, the ENaC/DEG family members share functional domains that are involved in the control of channel activity and in the formation of the pore. The functional heterogeneity among the members of the ENaC/DEG channel family provides a unique opportunity to address the molecular basis of basic channel functions such as activation by ligands, mechanotransduction, ionic selectivity, or block by pharmacological ligands.

The brain and salt-sensitive hypertension

Genetically salt-sensitive rats, such as Dahl S and spontaneously hypertensive rats, show clear hypertensive responses to a high salt diet. Neural mechanisms play an essential role in salt-induced hypertension, and recent studies indicate that centrally induced sympathetic hyperactivity actually causes the hypertension. This review discusses the view that the renal genotype is not the only determinant of salt-induced sympathetic hyperactivity and hypertension, and that changes in genetic control of neuronal responses to cerebrospinal fluid Na(+) may play a primary role.

Brain amiloride-sensitive Phe-Met-Arg-Phe-NH(2)--gated Na(+) channels and Na(+)-induced sympathoexcitation and hypertension

Dietary and cerebrospinal fluid (CSF) Na(+) may act through brain amiloride-sensitive, Phe-Met-Arg-Phe-NH(2) (FMRFamide)-gated Na(+) channels (FaNaChs) to cause sympathoexcitation and hypertension. We hypothesized that FaNaChs cause sympathoexcitation via the activation of brain "ouabain" and the brain renin-angiotensin system. In conscious Wistar rats, intracerebroventricular (ICV) infusion of Na(+)-rich (0.3 mol/L) artificial CSF (aCSF) and ICV injection of angiotensin II or ouabain increase renal sympathetic nerve activity (RSNA), blood pressure (BP), and heart rate (HR). ICV benzamil, an amiloride analogue, did not affect baseline values and blocked the responses to ICV infusions of Na(+)-rich aCSF but not ICV angiotensin II or ouabain. ICV FMRFamide also increased RSNA, BP, and HR. Blocking brain "ouabain" with ICV antibody Fab fragments abolished the responses to both ICV FMRFamide and Na(+)-rich aCSF. In conscious spontaneously hypertensive rats (SHR) on a high salt intake for 6 weeks, prolonged ICV but not intravenous infusion of benzamil at 10 to 20 microg/h significantly decreased RSNA, BP, and HR in a dose-related manner. The extent of these responses was significantly smaller in SHR on regular salt intake. These findings suggest that benzamil-blockable brain FaNaChs represent the major mechanism through which a small increase in CSF Na(+) by ICV Na(+)-rich aCSF in Wistar rats or high salt intake in SHR initiates sympathoexcitation and hypertension. Enhanced Na(+) entry through FaNaChs appears to activate brain "ouabain" and the brain renin-angiotensin system and, thereby, increases the sympathetic outflow. Brain FaNaChs appear to contribute to the worsening of hypertension in SHR on a high salt diet and, to a small extent, to the maintenance of hypertension in SHR on a regular salt diet.

Exploration of the pore structure of a peptide-gated Na+ channel

The FMRF-amide-activated sodium channel (FaNaC), a member of the ENaC/Degenerin family, is a homotetramer, each subunit containing two transmembrane segments. We changed independently every residue of the first transmembrane segment (TM1) into a cysteine and tested each position's accessibility to the cysteine covalent reagents MTSET and MTSES. Eleven mutants were accessible to the cationic MTSET, showing that TM1 faces the ion translocation pathway. This was confirmed by the accessibility of cysteines present in the acid-sensing ion channels and other mutations introduced in FaNaC TM1. Modification of accessibilities for positions 69, 71 and 72 in the open state shows that the gating mechanism consists of the opening of a constriction close to the intracellular side. The anionic MTSES did not penetrate into the channel, indicating the presence of a charge selectivity filter in the outer vestibule. Furthermore, amiloride inhibition resulted in the channel occlusion in the middle of the pore. Summarizing, the ionic pore of FaNaC includes a large aqueous cavity, with a charge selectivity filter in the outer vestibule and the gate close to the interior.

A novel technique that measures peptide secretion on a millisecond timescale reveals rapid changes in release

Neuropeptides are ubiquitous transmitters that have been implicated in a wide variety of physiological and pathological conditions, and it is important to understand the processes that control their secretion. We have developed a technique that measures neuropeptide secretion with high temporal resolution. This method involves placing an electrophysiological "tag" in a neuropeptide prohormone. The tagged prohormone is subsequently expressed together with an ionotropic receptor that binds the tag. Because the neuropeptide of interest and the tag enter the same population of dense core granules, neuropeptide secretion gives rise to fast, synaptic-like currents. Using this method, we show that peptide secretion can be modulated on a millisecond time scale. This technique could be readily adapted to measure the secretion of any neuropeptide.

Structure-activity relationships of 18 endogenous neuropeptides on the motor nervous system of the nematode Ascaris suum

Neuropeptides play an important role in all nervous systems and structure-activity studies of related peptides is one approach to understanding this role. This study of the motor nervous system of the parasitic nematode Ascaris suum describes the physiological effects of a family of 18 endogenous Ascaris FMRFamide-like peptides (AF peptides) on the membrane potential and input resistance of the dorsal excitatory type 2 (DE2) and dorsal inhibitory (DI) motor neurons. These motor neurons are part of the final common output pathway from the motor nervous system to the somatic muscle cells responsible for locomotion. AF peptide effects on the frequency of excitatory postsynaptic potentials (EPSPs) in DE2 motor neurons were also measured to infer peptide effects on central presynaptic spiking neurons. AF peptide injections into intact worms were made to assess their qualitative effects on behavior, providing a context for interpreting motor neuron data. One category of AF peptides, N-terminally extended -FIRFa peptides (AF5, AF7 and AF1), has pronounced behavioral effects and qualitatively similar, but quantitatively different effects on DE2 and DI motor neurons. A second category of AF peptides (AF2, AF9, and AF8) also produces dramatic behavioral effects and strong electrophysiological effects on DE2 and/or DI motor neurons. A third category of AF peptides, consisting of six members of the -PGVLRFa group (which are encoded by the same gene and have closely related sequences) and peptide AF11, have pronounced behavioral effects, but relatively weak or negligible effects on DE2 and DI motor neurons. A fourth category of AF peptides, also consisting of structurally unrelated members, has pronounced behavioral effects and, as individual peptides, similar effects on both DE2 and DI motor neurons; AF15 is excitatory, while AF17 and AF19 are inhibitory, on both motor neuron types. Finally, two AF peptides (AF6, AF16) are relatively weak or inactive in producing behavioral or motor neuronal effects. Based on comparisons of the effects of AF peptides on DE2 and DI motor neurons, a tentative list of 5 major response-types is proposed as a working hypothesis to guide the search for AF peptide receptors. The findings attest to the potential complexity of neurosignaling in this comparatively simple nervous system.

Cloning and expression of a FMRFamide-gated Na(+) channel from Helisoma trivolvis and comparison with the native neuronal channel

We have cloned a cDNA encoding a Phe-Met-Arg-Phe-NH(2) (FMRFamide)-gated Na(+) channel from nervous tissue of the pond snail Helisoma trivolvis (HtFaNaC) and expressed the channel in Xenopus oocytes. The deduced amino acid sequence of the protein expressed by HtFaNaC is 65 % identical to that of the FMRFamide-gated channel cloned from Helix aspersa (HaFaNaC). HtFaNaC expressed in oocytes was less sensitive to FMRFamide (EC(50) = 70 microM) than HaFaNaC (EC(50) = 2 microM). The two had a similar selectivity for Na+. The amplitude of the FMRFamide response of HtFaNaC was increased by reducing the extracellular concentration of divalent cations. The conductance of the two channels was similar, but the mean open time of unitary events was shorter for expressed HtFaNaC compared to expressed HaFaNaC. Each channel was susceptible to peptide block by high agonist concentrations. In marked contrast to HaFaNaC and other amiloride-sensitive Na(+) channels, amiloride, and the related drugs benzamil and 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), enhanced the FMRFamide response in oocytes expressing HtFaNaC cRNA. The potentiating effects of EIPA and benzamil were greater than those of amiloride. Unitary current analysis showed that with such drugs, there was channel blockade as well as an increased probability of channel opening. The similar permeability of the oocyte-expressed HtFaNaC and the Helisoma neuronal channel, and the susceptibility of both to agonist blockade and blockade by divalent cations, suggest that the channels are the same. However, neuronal channels were less susceptible to enhancement by amiloride analogues and in some patches were more sensitive to FMRFamide than expressed HtFaNaC.

Functional domains within the degenerin/epithelial sodium channel (Deg/ENaC) superfamily of ion channels

Application of recombinant DNA technology and electrophysiology to the study of amiloride-sensitive Na+ channels has resulted in an enormous increase in the understanding of the structure-function relationships of these channels. Moreover, this knowledge has permitted the elucidation of the physiological roles of these ion channels in cellular processes as diverse as transepithelial salt and water movement, taste perception, volume regulation, nociception, neuronal function, mechanosensation, and even defaecation. Although members of this ever-growing superfamily of ion channels (the Deg/ENaC superfamily) share little amino acid identity, they are all organized similarly, namely, two short N- and C-termini, two short membrane-spanning segments, and a very large extracellular loop domain. In this brief Topical Review, we discuss the structural features of each domain of this Deg/ENaC superfamily and, using ENaC as a model, show how each domain relates to overall channel function.

Cyclic AMP is involved in cardioregulation by multiple neuropeptides encoded on the FMRFamide gene

We have used a combination of biochemical and pharmacological techniques to investigate the role of the cyclic nucleotides, 3′, 5′-cyclic adenosine monophosphate (cyclic AMP) and 3′,5′-cyclic guanosine monophosphate (cyclic GMP), in mediating the cardioregulatory effects of FMRFamide and other neuropeptides encoded on exon II of the FMRFamide gene of Lymnaea stagnalis. The ‘isoleucine’ peptides (EFLRIamide and pQFYRIamide) produced complex biphasic effects on the frequency, force of contraction and tonus of the isolated heart of L. stagnalis, which were dependent on adenylate cyclase (AC) activity of the heart tissue. At a control rate of cyclic AMP production of less than or equal to 10 pmoles min(−)(1)mg(−)(1) protein, the ‘isoleucine’ peptides produced a significant increase in AC activity in heart membrane preparations. This suggested that the enhanced AC activity is responsible for the stimulatory effects of the ‘isoleucine’ peptides on frequency and force of contraction of heart beat. This excitation sometimes followed an initial ‘inhibitory phase’ where the frequency of beat, force of contraction and tonus of the heart were reduced by the ‘isoleucine’ peptides. Hearts that showed the inhibitory phase of the ‘isoleucine’ response, but characteristically lacked the delayed excitatory phase, were found to have high levels of membrane AC activity (breve)10 pmoles min(−)(1)mg(−)(1) protein in controls. Application of the ‘isoleucine’ peptides to membrane homogenate preparation from these hearts failed to increase AC activity. The addition of FMRFamide produced significant increases in the rate of cyclic AMP production in the heart membrane preparations, which could account, at least in part, for the cardioexcitatory effects of this peptide in the isolated whole heart. A membrane-permeable cyclic AMP analogue (8-bromo-cyclic AMP) and an AC activator (forskolin) were also cardioexcitatory. The peptide SEEPLY had no effects on the beat properties of the isolated heart and did not alter AC activity. The activity of the membrane-bound (particulate) guanylate cyclase (GC) was not significantly affected by any of the peptides.

Conformational ensembles: the role of neuropeptide structures in receptor binding

Conformational properties of several similar FMRFamide-like neuropeptides from mollusks were investigated by nuclear magnetic resonance (NMR) spectroscopy. It was found that amino acid substitutions in the N-terminal variable regions of the peptides had dramatic effects on the populations of reverse turns in solution. The populations of turns, as measured by two independent NMR parameters, were found to be highly correlated (r(2) = 0.93 and 0. 82) with IC(50) values using receptor membrane preparations from Helix aspersa (Payza, 1987; Payza et al., 1989). These results suggest that the amount of turn in the free peptide can influence the receptor binding affinities of that peptide. On the basis of these observations, a model was developed in which only a single species from a conformational ensemble of an unbound peptide will bind to a particular receptor. Thus, the conformational ensemble reduces the effective concentration of a particular peptide with respect to a particular receptor.

Block of the helix FMRFamide-gated Na+ channel by FMRFamide and its analogues

1. In Helix neurones high doses of Phe-Met-Arg-Phe-NH2 (FMRFamide) often evoke biphasic inward whole-cell currents with brief application, and suppression of the current with prolonged application. With outside-out patches, a transient early suppression of the unitary current amplitude was seen following application of high doses of FMRFamide. 2. Continuous application of a concentration of FMRFamide from 30 microM to 1 mM resulted in a reduction in the amplitude of the unitary currents and an increase in open state noise. There was also an increase in the occurrence of smaller, 'subconductance' currents with the higher concentrations of FMRFamide. Similar effects were seen with FMRFamide on FaNaC expressed in oocytes. The FMRFamide analogues FLRFamide and WnLRFamide were more effective in evoking the lower conductance state. These effects of agonist at high concentrations were voltage dependent suggesting channel block. 3. A similar effect was seen when one of the related peptides FKRFamide, FM(D)RFamide, nLRFamide or N-AcFnLRFamide was co-applied with a low FMRFamide concentration. However, the non-amidated peptides FKRF, FLRF and nLRF and also WMDFamide did not have this effect. 4. The inhibition of unitary currents induced by amiloride was qualitatively different from that produced by FMRFamide analogues with no obvious occurrence of subconductance levels. FMRFamide-gated channels were also blocked by guanidinium, but only at very high concentrations. 5. The results strongly suggest a partial inhibition of current flow through the FMRFamide- gated channel by some part of the agonist or the related antagonist peptide molecules.

Ionic mechanism mediating Mytilus inhibitory peptides elicited membrane currents in identified Helix neurons

Effects of seven, pressure applied MIP (Mytilus inhibitory peptides) had been studied on D-neurons of the CNS of Helix pomatia in voltage-clamp experiments. In physiological saline, the peptides produced a hyperpolarization usually coupled with the cessation of any spontaneous spiking activity. Clamped at the resting potential ( approximately -60 mV), peptide applications elicited an outward current, which increased its amplitude by shifting the holding potential towards depolarisation. The response was concentration-dependent and accompanied by an increased membrane conductance. Reversal potentials obtained at different [K+]o were plotted with a slope of 52 mV per ten-fold change in [K+]o showing that the peptide-elicited current was mainly due to the increased K+-conductance(s). The peptide-induced outward current could partially be blocked by Ba2+ (5 mM), CdCl2 (1 mM), TEACl (10 mM) or apamin (2.5x10(-5) M) or furosemide (10 mg/ml) and decreased either in Na+-free or Cl--free solutions. 4-Aminopyridine at 5 mM concentration completely blocked the peptide-induced current. In the presence of high [K+]o, the peptide(s) was still found to induce an outward current at membrane potentials beyond K+-reversal potential. This component was not present in Cl--free saline, suggesting that the current was due to the inward flow of Cl- ions. Our results show that the MIPs have at least two (three) independent actions, each associated with different voltage-, concentration-dependence and ionic mechanisms. It is suggested, that the peptide-induced currents are carried by K+, and Cl- ions. According to our present finding, the observed effects are mediated by the same receptor, activating different second messenger systems, inducing multiple conductance changes in the membrane of neurons of the snail ganglia.

Single-channel currents of a peptide-gated sodium channel expressed in Xenopus oocytes

1. Single-channel recordings were made from outside-out membrane patches of Xenopus oocytes injected with the cDNA clone FaNaCh, which encodes a peptide-gated Na+ channel from Helix aspersa. 2. The natural peptides FMRFamide and FLRFamide only activated unitary currents in oocytes injected with FaNaCh; the EC50 values were 1.8 and 11.7 microM, respectively. 3. The slope conductance of the channel was 9.2 pS for both peptides. 4. With FMRFamide, the open probability (Po) of the channel was 0.06 at 0.3 microM and 0.76 at 30 microM, whereas for FLRFamide the open probability increased from 0.04 at 1.8 microM to 0.49 at 50 microM. The Hill coefficient was greater than 1 for both peptides. 5. High concentrations of each peptide evoked very fast flickering between open and closed states which led to decreased unitary current amplitude. 6. At low doses, brief single openings and bursts of longer openings occurred. With higher doses, the occurrence of the brief openings declined and the number of longer openings increased; the duration of the longer openings was shorter with FLRFamide than with FMRFamide. 7. For each peptide, frequency distribution histograms of open events were best fitted by the sum of two exponential components, suggesting the existence of two open states of the channel. Closed events were fitted by the sum of three components, suggesting the existence of three closed states. 8. The data were analysed according to a five-state model in which the brief openings correspond to a single liganded open form of the channel and the longer openings to a doubly liganded open form. According to this interpretation, the greater whole-cell response observed with FMRFamide than with FLRFamide results mostly from a slower closing rate constant for the longer (doubly liganded) channel openings.

A new member of the amiloride-sensitive sodium channel family in Drosophila melanogaster peripheral nervous system

Amiloride sensitivity is a common characteristic of structurally related cationic channels that are associated with a wide range of physiological functions. In Caenorhabditis elegans, neuronal and muscular degenerins are involved in mechanoperception. In animal epithelia, a Na(+)-selective channel participates in vectorial Na+ transport. In the snail nervous system, an ionotropic receptor for the peptide FMRFamide forms a Na(+)-selective channel. In mammalian brain and/or in sensory neurons, ASIC channels form H(+)-activated cation channels involved in nociception linked to acidosis. We have now cloned a new member of this family from Drosophila melanogaster. The corresponding protein displays low sequence identity with the previously cloned members of the super-family but it has the same structural organization. Its mRNA was detected from late embryogenesis (14-17 hours) and was present in the dendritic arbor subtype of the Drosophila peripheral nervous system multiple dendritic (md) sensory neurons. While the origin and specification of md neurons are well documented, their roles are still poorly understood. They could function as stretch or touch receptors, raising the possibility that this Drosophila gene product, called dmdNaC1, could also be involved in mechanotransduction.

The Phe-Met-Arg-Phe-amide-activated sodium channel is a tetramer

The Helix aspersa Phe-Met-Arg-Phe-amide (FMRFamide)-gated sodium channel is formed by homomultimerization of several FMRFamide-activated Na+ channel (FaNaCh) proteins. FaNaCh is homologous to the subunits that compose the amiloride-sensitive epithelial sodium channel, to Caenorhabditis elegans degenerins, and to acid-sensing ionic channels. FaNaCh properties were analyzed in stably transfected human embryonic kidney cells (HEK-293). The channel was functional with an EC50 for FMRFamide of 1 microM and an IC50 (25 degreesC) for amiloride of 6.5 microM as assessed by 22Na+ uptake measurements. The channel activity was associated with the presence of a protein at the cell surface with an apparent molecular mass of 82 kDa. The 82-kDa form was derived from an incompletely glycosylated form of 74 kDa found in the endoplasmic reticulum. Formation of covalent bonds between subunits of the same complex were observed either after formation of intersubunit disulfide bonds following cell homogenization and solubilization with Triton X-100 or after use of bifunctional cross-linkers. This resulted in the formation of covalent multimers that contained up to four subunits. Hydrodynamic properties of the solubilized FaNaCh complex also indicated a maximal stoichiometry of four subunits per complex. It is likely that epithelial Na+ channels, acid-sensing ionic channels, degenerins, and the other proteins belonging to the same ion channel superfamily also associate within tetrameric complexes.

The amiloride-sensitive Na+ channel: from primary structure to function

Three homologous subunits of the amiloride-sensitive Na+ channel, entitled alpha, beta, and gamma, have been cloned either from distal colon of a steroid-treated rat or from human lung. The alpha, beta, and gamma subunits have similarities with degenerins, a family of proteins found in the mechanosensory neurons of the nematode Caenorhabditis elegans. All these proteins are characterized by the presence of a large extracellular domain, located between two transmembrane alpha-helices, and by short NH2 and COOH terminal cytoplasmic segments. They constitute the first members of a new gene super-family of ionic channels. The epithelial Na+ channel is specifically expressed at the apical membrane of Na(+)-reabsorbing epithelial cells. Its activity is controlled by several distinct hormones, especially by corticosteroids. These hormones act either transcriptionally (such as aldosterone in distal colon, or glucocorticoids in lung) and/or post-transcriptionally (such as aldosterone in kidney). Recent works have provided new insights in the function of that important osmoregulatory system.

Families of ion channels with two hydrophobic segments

The family of epithelial sodium channels and nematode degenerins has expanded recently to include a member found only in brain, and another that functions in molluscs as a ligand-gated channel. A new gene family of mammalian ATP-gated channels has been discovered; one of its seven members plays a role in the lysis of macrophages. The mechanosensitive channel of bacteria has emerged as the simplest form of a channel protein subunit with two hydrophobic domains.

The anthozoan neuropeptide Antho-RWamide I modulates Ca2+ current in sea anemone myoepithelial cells

The anthozoan neuropeptide Antho-RWamide I (< Glu-Ser-Leu-Arg-Trp-NH2) excites contraction of endodermal muscles in sea anemones. Single electrode voltage clamp recordings from semi-intact preparations of endodermal myoepithelial cells reveal that Antho-RWamide I increases an inward Ca2+ current. Evidence for the involvement of a Ca2+ current in contraction was supported by the observation that Cd2+ abolished spontaneous contractions and reduced inward current. Contractions and inward currents induced by Antho-RWamide I were not, however, completely abolished in the presence of Cd2+. We conclude that Antho-RWamide, a putative neurotransmitter at sea anemone smooth muscle, acts by opening, either directly or indirectly, Ca2+ channels in the muscle membrane.

Dopamine directly activates a ligand-gated channel in snail neurones

Synaptically released dopamine is known to evoke fast as well as slow synaptic potentials in neurones of gastropod molluscs. Here evidence is presented that the fast excitatory response to dopamine is mediated by the direct activation of a ligand-gated channel: unitary currents were observed in outside-out patches of neurones exposed to dopamine, and the response persisted in the presence of intracellular guanosine 5'-o-(2-thiobiphosphate), GDP[beta-S], a condition known to block G-protein-coupled responses to dopamine and other agents. In whole-cell recordings, the fast response desensitized very rapidly; it was less desensitized in outside-out patches, suggesting dependence of desensitization on an intracellular factor. The response to dopamine was blocked by D-tubocurarine and strychnine (both probably by channel blockade), by apomorphine, chlorpromazine and relatively high doses of (+/-)-sulpiride and spiperone. The channel conducts predominantly monovalent cations. Unexpectedly, the fast response to dopamine was also observed in an identified dopaminergic neurone when maintained in isolation in culture. The receptors on the dopaminergic neurone were unevenly distributed, being more abundant on the axon-hillock, axon and neurite terminals.

Cloning of the amiloride-sensitive FMRFamide peptide-gated sodium channel

The peptide Phe-Met-Arg-Phe-NH2 (FMRFamide) and structurally related peptides are present both in invertebrate and vertebrate nervous systems. Although they constitute a major class of invertebrate peptide neurotransmitters, the molecular structure of their receptors has not yet been identified. In neurons of the snail Helix aspersa, as well as in Aplysia bursting and motor neurons, FMRFamide induces a fast excitatory depolarizing response due to direct activation of an amiloride-sensitive Na+ channel. We have now isolated a complementary DNA from Helix nervous tissue; when expressed in Xenopus oocytes, it encodes an FMRFamide-activated Na+ channel (FaNaCh) that can be blocked by amiloride. The corresponding protein shares a very low sequence identity with the previously cloned epithelial Na+ channel subunits and Caenorhabditis elegans degenerins, but it displays the same overall structural organization. To our knowledge, this is the first characterization of a peptide-gated ionotropic receptor.