Ultrastructural localization of RFamide-like peptides in neuronal dense-cored vesicles in the peduncle of Hydra

Roots and early routes of neuroendocrinology

Carl C. Speidel (1919) and Ernst Scharrer (1928) were privileged witnesses of the encounter between neurons and hormones, a biological phenomenon that had been occurring in nature during millions of years of evolution, as Berta Scharrer started to unfold since 1935 on. The story of neurosecretion is intimately associated to that of the hypothalamus, such a "marvellous region", as Wolfgang Bargmann (1975) called it. This story started more than two millennia ago. We have made an effort to trace the roots of the discoveries that gave rise to a medical discipline, neuroendocrinology. Our trip to the roots covers a period from the fourth century BC, when an extraordinary Medical School was founded in Alexandria, and extends into the late 1970s of the twentieth century, when neuroendocrine research had started to grow exponentially. An effort has been made to track back the origin of each piece of knowledge that was constructing, brick upon brick, the building of this new medical science, hoping that it would help neuroendocrinologists of the new era to find their own roots, to meet their ancestors. Tracking the roots of a particular phenomenon provides the opportunity to have an overview of the whole phenomenon, allowing comprehension rather than merely knowledge. An important purpose pursued throughout this article was to pay a tribute to all those who, in the early days, contributed to the brain-endocrine encounter. We have tried our best to bring back the achievements of most of them.

In vivo neuromodulation of animal behavior with organic semiconducting oligomers

Modulating neural activity with electrical or chemical stimulus can be used for fundamental and applied research. Typically, neuronal stimulation is performed with intracellular and extracellular electrodes that deliver brief electrical pulses to neurons. However, alternative wireless methodologies based on functional materials may allow clinical translation of technologies to modulate neuronal function. Here, we show that the organic semiconducting oligomer 4-[2-{2,5-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)thiophen-3-yl}ethoxy]butane-1-sulfonate (ETE-S) induces precise behaviors in the small invertebrate Hydra, which were dissected through pharmacological and electrophysiological approaches. ETE-S-induced behavioral response relies on the presence of head neurons and calcium ions and is prevented by drugs targeting ionotropic channels and muscle contraction. Moreover, ETE-S affects Hydra's electrical activity enhancing the contraction burst frequency. The unexpected neuromodulatory function played by this conjugated oligomer on a simple nerve net opens intriguing research possibilities on fundamental chemical and physical phenomena behind organic bioelectronic interfaces for neuromodulation and on alternative methods that could catalyze a wide expansion of this rising technology for clinical applications.

Ageing and the Autonomic Nervous System

The vertebrate nervous system is divided into central (CNS) and peripheral (PNS) components. In turn, the PNS is divided into the autonomic (ANS) and enteric (ENS) nervous systems. Ageing implicates time-related changes to anatomy and physiology in reducing organismal fitness. In the case of the CNS, there exists substantial experimental evidence of the effects of age on individual neuronal and glial function. Although many such changes have yet to be experimentally observed in the PNS, there is considerable evidence of the role of ageing in the decline of ANS function over time. As such, this chapter will argue that the ANS constitutes a paradigm for the physiological consequences of ageing, as well as for their clinical implications.

The premetazoan ancestry of the synaptic toolkit and appearance of first neurons

Neurons, especially when coupled with muscles, allow animals to interact with and navigate through their environment in ways unique to life on earth. Found in all major animal lineages except sponges and placozoans, nervous systems range widely in organization and complexity, with neurons possibly representing the most diverse cell-type. This diversity has led to much debate over the evolutionary origin of neurons as well as synapses, which allow for the directed transmission of information. The broad phylogenetic distribution of neurons and presence of many of the defining components outside of animals suggests an early origin of this cell type, potentially in the time between the first animal and the last common ancestor of extant animals. Here, we highlight the occurrence and function of key aspects of neurons outside of animals as well as recent findings from non-bilaterian animals in order to make predictions about when and how the first neuron(s) arose during animal evolution and their relationship to those found in extant lineages. With advancing technologies in single cell transcriptomics and proteomics as well as expanding functional techniques in non-bilaterian animals and the close relatives of animals, it is an exciting time to begin unraveling the complex evolutionary history of this fascinating animal cell type.

A single neuron subset governs a single coactive neuron circuit in Hydra vulgaris, representing a possible ancestral feature of neural evolution

The last common ancestor of Bilateria and Cnidaria is believed to be one of the first animals to develop a nervous system over 500 million years ago. Many of the genes involved in the neural function of the advanced nervous system in Bilateria are well conserved in Cnidaria. Thus, the cnidarian Hydra vulgaris is a good model organism for the study of the putative primitive nervous system in its last common ancestor. The diffuse nervous system of Hydra consists of several peptidergic neuron subsets. However, the specific functions of these subsets remain unclear. Using calcium imaging, here we show that the neuron subsets that express neuropeptide, Hym-176, function as motor circuits to evoke longitudinal contraction. We found that all neurons in a subset defined by the Hym-176 gene (Hym-176A) or its paralogs (Hym-176B) expression are excited simultaneously, followed by longitudinal contraction. This indicates not only that these neuron subsets have a motor function but also that a single molecularly defined neuron subset forms a single coactive circuit. This is in contrast with the bilaterian nervous system, where a single molecularly defined neuron subset harbors multiple coactive circuits, showing a mixture of neurons firing with different timings. Furthermore, we found that the two motor circuits, one expressing Hym-176B in the body column and the other expressing Hym-176A in the foot, are coordinately regulated to exert region-specific contraction. Our results demonstrate that one neuron subset is likely to form a monofunctional circuit as a minimum functional unit to build a more complex behavior in Hydra. This simple feature (one subset, one circuit, one function) found in Hydra may represent the simple ancestral condition of neural evolution.

The chemical brain hypothesis for the origin of nervous systems

In nervous systems, there are two main modes of transmission for the propagation of activity between cells. Synaptic transmission relies on close contact at chemical or electrical synapses while volume transmission is mediated by diffusible chemical signals and does not require direct contact. It is possible to wire complex neuronal networks by both chemical and synaptic transmission. Both types of networks are ubiquitous in nervous systems, leading to the question which of the two appeared first in evolution. This paper explores a scenario where chemically organized cellular networks appeared before synapses in evolution, a possibility supported by the presence of complex peptidergic signalling in all animals except sponges. Small peptides are ideally suited to link up cells into chemical networks. They have unlimited diversity, high diffusivity and high copy numbers derived from repetitive precursors. But chemical signalling is diffusion limited and becomes inefficient in larger bodies. To overcome this, peptidergic cells may have developed projections and formed synaptically connected networks tiling body surfaces and displaying synchronized activity with pulsatile peptide release. The advent of circulatory systems and neurohemal organs further reduced the constraint imposed on chemical signalling by diffusion. This could have contributed to the explosive radiation of peptidergic signalling systems in stem bilaterians. Neurosecretory centres in extant nervous systems are still predominantly chemically wired and coexist with the synaptic brain. This article is part of the theme issue 'Basal cognition: multicellularity, neurons and the cognitive lens'.

A secreted antibacterial neuropeptide shapes the microbiome of Hydra

Colonization of body epithelial surfaces with a highly specific microbial community is a fundamental feature of all animals, yet the underlying mechanisms by which these communities are selected and maintained are not well understood. Here, we show that sensory and ganglion neurons in the ectodermal epithelium of the model organism hydra (a member of the animal phylum Cnidaria) secrete neuropeptides with antibacterial activity that may shape the microbiome on the body surface. In particular, a specific neuropeptide, which we call NDA-1, contributes to the reduction of Gram-positive bacteria during early development and thus to a spatial distribution of the main colonizer, the Gram-negative Curvibacter sp., along the body axis. Our findings warrant further research to test whether neuropeptides secreted by nerve cells contribute to the spatial structure of microbial communities in other organisms.

Certain neuropeptides, in addition to their neuromodulatory functions, display antibacterial activities of unclear significance. Here, the authors show that a secreted neuropeptide modulates the distribution of bacterial communities on the body surface during development of the model organism Hydra.

Back to the Basics: Cnidarians Start to Fire

The nervous systems of cnidarians, pre-bilaterian animals that diverged close to the base of the metazoan radiation, are structurally simple and thus have great potential to reveal fundamental principles of neural circuits. Unfortunately, cnidarians have thus far been relatively intractable to electrophysiological and genetic techniques and consequently have been largely passed over by neurobiologists. However, recent advances in molecular and imaging methods are fueling a renaissance of interest in and research into cnidarians nervous systems. Here, we review current knowledge on the nervous systems of cnidarian species and propose that researchers should seize this opportunity and undertake the study of members of this phylum as strategic experimental systems with great basic and translational relevance for neuroscience.

A serial multiplex immunogold labeling method for identifying peptidergic neurons in connectomes

Electron microscopy-based connectomics aims to comprehensively map synaptic connections in neural tissue. However, current approaches are limited in their capacity to directly assign molecular identities to neurons. Here, we use serial multiplex immunogold labeling (siGOLD) and serial-section transmission electron microscopy (ssTEM) to identify multiple peptidergic neurons in a connectome. The high immunogenicity of neuropeptides and their broad distribution along axons, allowed us to identify distinct neurons by immunolabeling small subsets of sections within larger series. We demonstrate the scalability of siGOLD by using 11 neuropeptide antibodies on a full-body larval ssTEM dataset of the annelid Platynereis. We also reconstruct a peptidergic circuitry comprising the sensory nuchal organs, found by siGOLD to express pigment-dispersing factor, a circadian neuropeptide. Our approach enables the direct overlaying of chemical neuromodulatory maps onto synaptic connectomic maps in the study of nervous systems.

DOI:http://dx.doi.org/10.7554/eLife.11147.001

In the nervous system, cells called neurons connect to each other to form large “neural” networks. The most powerful method that is currently available for tracing neurons and mapping the connections between them is called electron microscopy. This requires slicing brain tissue into ultrathin sections, which are then imaged one by one. However, while electron microscopy provides highly detailed information about the structure of the connections between neurons, it does not reveal which molecules the neurons use to communicate with each other.

To address this question, Shahidi et al. have developed a new approach called ‘siGOLD’. Unlike previous approaches, siGOLD allows signal molecules inside cells to be labeled with protein tags called antibodies without compromising the ability to examine the tissue with electron microscopy. The technique was developed using the larvae of a marine worm called Platynereis. A single larva was sliced into 5000 sections thin enough to view under an electron microscope, and 150 of these were selected to represent the entire body. Because neurons are typically long and thin, individual neurons usually spanned multiple slices.

To identify the neurons, Shahidi et al. then applied an antibody that recognizes a specific signal molecule to a subset of the slices. The antibodies were labeled with gold particles, which show up as black dots under the electron microscope. Because the molecules recognized by the antibodies are present all along the neuron, and because individual neurons extend over multiple slices, it was possible to trace single neurons by labeling only a small number of slices. Repeating this process in different subsets of slices with antibodies that bind to different signal molecules allowed entire neural circuits to be mapped.

In the future, Shahidi et al.’s approach could be adapted to study neural networks in other organisms such as flies, fish and mice.

DOI:http://dx.doi.org/10.7554/eLife.11147.002

Peptide-gated ion channels and the simple nervous system of Hydra

Neurons either use electrical or chemical synapses to communicate with each other. Transmitters at chemical synapses are either small molecules or neuropeptides. After binding to their receptors, transmitters elicit postsynaptic potentials, which can either be fast and transient or slow and longer lasting, depending on the type of receptor. Fast transient potentials are mediated by ionotropic receptors and slow long-lasting potentials by metabotropic receptors. Transmitters and receptors are well studied for animals with a complex nervous system such as vertebrates and insects, but much less is known for animals with a simple nervous system like Cnidaria. As cnidarians arose early in animal evolution, nervous systems might have first evolved within this group and the study of neurotransmission in cnidarians might reveal an ancient mechanism of neuronal communication. The simple nervous system of the cnidarian Hydra extensively uses neuropeptides and, recently, we cloned and functionally characterized an ion channel that is directly activated by neuropeptides of the Hydra nervous system. These results demonstrate the existence of peptide-gated ion channels in Hydra, suggesting they mediate fast transmission in its nervous system. As related channels are also present in the genomes of the cnidarian Nematostella, of placozoans and of ctenophores, it should be considered that the early nervous systems of cnidarians and ctenophores have co-opted neuropeptides for fast transmission at chemical synapses.

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.

Neuropeptides trigger oocyte maturation and subsequent spawning in the hydrozoan jellyfish Cytaeis uchidae

Oocyte maturation and subsequent spawning in hydrozoan jellyfish are generally triggered by light-dark cycles. To examine if the initiation of the maturation process after light stimulus is mediated by neurotransmitters, neuropeptides isolated originally from Hydra magnipapillata were applied to sexually mature female medusae of the hydrozoan jellyfish Cytaeis uchidae. Among the Hydra neuropeptides tested, Hym-53 (NPYPGLW-NH2 ), as well as a nonphysiological peptide, CGLWamide (CGLW-NH2 ), were most effective in inducing oocyte maturation and spawning. Hym-355 (FPQSFLPRG-NH2 ) also triggered these events, but the stimulatory effect was weaker. Since Hym-53-OH (NPYPGLW) and Hym-355-OH (FPQSFLPRG) had no effect, amidation at the C-terminus may be critical for the stimulatory activities of the peptides. Exposure to Hym-53 for 2 min was sufficient to trigger of oocyte maturation, and the spawned eggs were able to be fertilized and to develop normally. Transmission electron microscopy confirmed that bundles of axon-like structures that contain dense-core synaptic vesicles and microtubules are present in the ovarian ectodermal epithelium overlying the oocytes. In addition, immunohistological analyses revealed that some of the neurons in the ectodermal epithelium are GLWamide- and PRGamide-positive. These results suggest that a neuropeptide signal transduction pathway is involved in mediating the induction of oocyte maturation and spawning in this jellyfish.

Subcellular localization of the epitheliopeptide, Hym-301, in Hydra

Peptides, as signaling molecules, play a number of roles in cell activities. An epitheliopeptide, Hym-301, has been described as a peptide involved in morphogenesis in hydra. However, little is known about the intracellular location of the peptide or its specific functions. To investigate the mechanism of morphogenesis that involves peptidic molecules, we have examined the intracellular localization of Hym-301 in hydra by using immunohistochemical and immunogold electron-microscopic analyses. We have found that the pattern of distribution of mature peptide is slightly different from that of its mRNA, and that the peptide is stored in vesicles located adjacent to the cell membrane. We have also found that the peptide is released both extracellularly and internally to the cytoplasm of the cells. Based upon these observations, we have constructed a possible model mechanism of homeostatic regulation of the distribution of the Hym-301 peptide in a dynamic tissue context.

GABA and glutamate immunoreactivity in tentacles of the sea anemone Phymactis papillosa (LESSON 1830)

Sea anemones have a structurally simple nervous system that controls behaviors like feeding, locomotion, aggression, and defense. Specific chemical and tactile stimuli are transduced by ectodermal sensory cells and transmitted via a neural network to cnidocytes and epithelio-muscular cells, but the nature of the neurotransmitters operating in these processes is still under discussion. Previous studies demonstrated an important role of peptidergic transmission in cnidarians, but during the last decade the contribution of conventional neurotransmitters became increasingly evident. Here, we used immunohistochemistry on light and electron microscopical preparations to investigate the localization of glutamate and GABA in tentacle cross-sections of the sea anemone Phymactis papillosa. Our results demonstrate strong glutamate immunoreactivity in the nerve plexus, while GABA labeling was most prominent in the underlying epithelio-muscular layer. Immunoreactivity for both molecules was also found in glandular epithelial cells, and putative sensory cells were GABA positive. Under electron microscopy, both glutamate and GABA immunogold labeling was found in putative neural processes within the neural plexus. These data support a function of glutamate and GABA as signaling molecules in the nervous system of sea anemones.

A review of FMRFamide- and RFamide-like peptides in metazoa

Neuropeptides are a diverse class of signalling molecules that are widely employed as neurotransmitters and neuromodulators in animals, both invertebrate and vertebrate. However, despite their fundamental importance to animal physiology and behaviour, they are much less well understood than the small molecule neurotransmitters. The neuropeptides are classified into families according to similarities in their peptide sequence; and on this basis, the FMRFamide and RFamide-like peptides, first discovered in molluscs, are an example of a family that is conserved throughout the animal phyla. In this review, the literature on these neuropeptides has been consolidated with a particular emphasis on allowing a comparison between data sets in phyla as diverse as coelenterates and mammals. The intention is that this focus on the structure and functional aspects of FMRFamide and RFamide-like neuropeptides will inform understanding of conserved principles and distinct properties of signalling across the animal phyla.

Peptidomic approaches to the identification and characterization of functional peptides in Hydra

Little is known about peptides that control developmental processes such as cell differentiation and pattern formation in metazoans. The cnidarian Hydra is one of the most basal metazoans and is a key model system for studying the peptides involved in these processes. We developed a novel peptidomic approach to the isolation and identification of functional signalling peptides from Hydra (the Hydra peptide project). First, peptides extracted from the tissue of Hydra magnipapillata are purified to homogeneity using high-performance liquid chromatography (HPLC). The isolated peptides are then tested for their ability to alter gene expression in Hydra using differential display-PCR (DD-PCR). If gene expression is altered, the peptide is considered as a putative signalling peptide and is subjected to amino acid sequencing. Following the sequencing, synthetic peptides are produced and compared to their native counterparts by HPLC and/or mass spectrometry (MS). The synthetic peptides, which are available in larger quantities than their native analogues, are then tested in a variety of biological assays in Hydra to determine their functions. Here we present our strategies and a systematic approach to the identification and characterization of novel signalling peptides in Hydra. We also describe our high-throughput reverse-phase nano-flow liquid chromatography matrix-assisted laser desorption ionization time-of-flight mass spectrometry (LC-MALDI-TOF-MS/MS) approach, which was proved to be a powerful tool in the discovery of novel signalling peptides.

Cnidarians and the evolutionary origin of the nervous system

Cnidarians are widely regarded as one of the first organisms in animal evolution possessing a nervous system. Conventional histological and electrophysiological studies have revealed a considerable degree of complexity of the cnidarian nervous system. Thanks to expressed sequence tags and genome projects and the availability of functional assay systems in cnidarians, this simple nervous system is now genetically accessible and becomes particularly valuable for understanding the origin and evolution of the genetic control mechanisms underlying its development. In the present review, the anatomical and physiological features of the cnidarian nervous system and the interesting parallels in neurodevelopmental mechanisms between Cnidaria and Bilateria are discussed.

A peptide-gated ion channel from the freshwater polyp Hydra

Chemical transmitters are either low molecular weight molecules or neuropeptides. As a general rule, neuropeptides activate only slow metabotropic receptors. To date, only one exception to this rule is known, the FMRFamide-activated Na(+) channel (FaNaC) from snails. Until now FaNaC has been regarded as a curiosity, and it was not known whether peptide-gated ionotropic receptors are also present in other animal groups. Nervous systems first evolved in cnidarians, which extensively use neuropeptides. Here we report cloning from the freshwater cnidarian Hydra of a novel ion channel (Hydra sodium channel, HyNaC) that is directly gated by the neuropeptides Hydra-RFamides I and II and is related to FaNaC. The cells expressing HyNaC localize to the base of the tentacles, adjacent to the neurons producing the Hydra-RFamides, suggesting that the peptides are the natural ligands for this channel. Our results suggest that neuropeptides were already used for fast transmission in ancient nervous systems.

Cnidarian chemical neurotransmission, an updated overview

The ultrastructural, histochemical, immunocytochemical, biochemical, molecular, behavioral and physiological evidence for non-peptidergic and peptidergic chemical neurotransmission in the Anthozoa, Hydrozoa, Scyphozoa and Cubozoa is surveyed. With the possible exception of data for the catecholamines and peptides in some animals, the set of cumulative data - the evidence from all methodologies - is incomplete. Taken together, the evidence from all experimental approaches suggests that both classical fast (acetylcholine, glutamate, GABA, glycine) and slow (catecholamines and serotonin) transmitters, as well as neuropeptides, are involved in cnidarian neurotransmission. Ultrastructural evidence for peptidergic, serotonergic, and catecholaminergic synaptic localization is available, but the presence of clear and dense-cored synaptic vesicles also suggests both fast and slow classical transmission. Immunocytochemical studies, in general, reveal a continuous, non-localized distribution of neuropeptides, suggesting a neuromodulatory role for them. Immunocytochemical and biochemical studies indicate the presence of glutamate, GABA, serotonin, catecholamines (and/or their receptors), RFamides, nitric oxide and eicosanoids in cnidarian neurons and tissues. Gene sequences for peptidergic preprohormones have been reported; putative gene homologies to receptor proteins for vertebrate transmitters have been found in Hydra. Behavioral and physiological studies implicate classical transmitters, neuropeptides, eicosanoids and nitric oxide in the coordination of the neuroeffector systems.

GABA and glutamate receptors are involved in modulating pacemaker activity in hydra

The effects of gamma-amino butyric acid (GABA) and glutamate, their ionotropic agonists and antagonists on hydra's ectodermal and endodermal pacemaker systems were studied. GABA decreased ectodermal body contraction bursts (CBs) and the number of pulses in a burst (P/CB) and endodermal rhythmic potentials (RPs); tentacle pulses (TPs) were not affected. The GABA(A) agonist, muscimol, and the benzodiazepine receptor agonist, diazepam, mimicked the effects of GABA on the endodermal system. The GABA(A) antagonist bicuculline counteracted GABA's effects. Low concentrations of glutamate increased CBs and RPs. Higher concentrations required concanavalin A (Con A) to produce the same effect on CBs and P/CB. TPs were increased by high concentrations of glutamate and kainate. The ionotropic glutamate agonist, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) also required Con A to increase CBs and RPs. The effects of AMPA were antagonized by 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2,3-dione (NBQX), which, per se, decreased CBs. The results indicate that GABA and glutamate, acting on their ionotropic receptors, modify the impulses of hydra's pacemaker systems. On the whole GABA decreased the outputs of both ectodermal and endodermal impulse generating systems, while glutamate increased them.

Neuropeptides in cnidarians

Cnidarians are the lowest animal group having a nervous system. In the primitive nervous systems of cnidarians, peptides play important roles as neurotransmitters or neurohormones. So far, we have isolated and sequenced about 35 neuropeptides from different cnidarian classes (Hydrozoa, Scyphozoa, Anthozoa). All these neuropeptides have a C-terminal amide group, which protects against C-terminal degradation, but which also is important for receptor recognition. Also the N-termini of the cnidarian neuropeptides often contain different kinds of protecting groups (such as <Glu residues, L-3-phenyllactyl groups, and X-Pro or X-Pro-Pro sequences). Cnidarian neuropeptides are located in neuronal dense-core vesicles and are synthesized as preprohormones, which can contain up to 41 copies of a neuro peptide sequence. From Hydra, six different neuropeptide genes have been cloned so far. Each gene is expressed by a specific population of neurons, but in two instances coexpression of neuropeptide genes has been found. We have also cloned some of the cnidarian prohormone processing enzymes, among them the enzymes necessary for C-terminal amidation. These enzymes are closely related to their mammalian counterparts. All these data show that the primitive nervous systems of cnidarians have already acquired some of the sophisticated principles that we know from higher animals.

Expression and developmental regulation of the Hydra-RFamide and Hydra-LWamide preprohormone genes in Hydra: evidence for transient phases of head formation

Hydra magnipapillata has three distinct genes coding for preprohormones A, B, and C, each yielding a characteristic set of Hydra-RFamide (Arg-Phe-NH2) neuropeptides, and a fourth gene coding for a preprohormone that yields various Hydra-LWamide (Leu-Trp-NH2) neuropeptides. Using a whole-mount double-labeling in situ hybridization technique, we found that each of the four genes is specifically expressed in a different subset of neurons in the ectoderm of adult Hydra. The preprohormone A gene is expressed in neurons of the tentacles, hypostome (a region between tentacles and mouth opening), upper gastric region, and peduncle (an area just above the foot). The preprohormone B gene is exclusively expressed in neurons of the hypostome, whereas the preprohormone C gene is exclusively expressed in neurons of the tentacles. The Hydra-LWamide preprohormone gene is expressed in neurons located in all parts of Hydra with maxima in tentacles, hypostome, and basal disk (foot). Studies on animals regenerating a head showed that the prepro-Hydra-LWamide gene is expressed first, followed by the preprohormone A and subsequently the preprohormone C and the preprohormone B genes. This sequence of events could be explained by a model based on positional values in a morphogen gradient. Our head-regeneration experiments also give support for transient phases of head formation: first tentacle-specific preprohormone C neurons (frequently associated with a small tentacle bud) appear at the center of the regenerating tip, which they are then replaced by hypostome-specific preprohormone B neurons. Thus, the regenerating tip first attains a tentacle-like appearance and only later this tip develops into a hypostome. In a developing bud of Hydra, tentacle-specific preprohormone C neurons and hypostome-specific preprohormone B neurons appear about simultaneously in their correct positions, but during a later phase of head development, additional tentacle-specific preprohormone C neurons appear as a ring at the center of the hypostome and then disappear again. Nerve-free Hydra consisting of only epithelial cells do not express the preprohormone A, B, or C or the LWamide preprohormone genes. These animals, however, have a normal phenotype, showing that the preprohormone A, B, and C and the LWamide genes are not essential for the basic pattern formation of Hydra.

Three different prohormones yield a variety of Hydra-RFamide (Arg-Phe-NH2) neuropeptides in Hydra magnipapillata

The freshwater polyp Hydra is the most frequently used model for the study of development in cnidarians. Recently we isolated four novel Arg-Phe-NH2 (RFamide) neuropeptides, the Hydra-RFamides I-IV, from Hydra magnipapillata. Here we describe the molecular cloning of three different preprohormones from H. magnipapillata, each of which gives rise to a variety of RFamide neuropeptides. Preprohormone A contains one copy of unprocessed Hydra-RFamide I (QWLGGRFG), II (QWFNGRFG), III/IV [(KP)HLRGRFG] and two putative neuropeptide sequences (QLMSGRFG and QLMRGRFG). Preprohormone B has the same general organization as preprohormone A, but instead of unprocessed Hydra-RFamide III/IV it contains a slightly different neuropeptide sequence [(KP)HYRGRFG]. Preprohormone C contains one copy of unprocessed Hydra-RFamide I and seven additional putative neuropeptide sequences (with the common N-terminal sequence QWF/LSGRFGL). The two Hydra-RFamide II copies (in preprohormones A and B) are preceded by Thr residues, and the single Hydra-RFamide III/IV copy (in preprohormone A) is preceded by an Asn residue, confirming that cnidarians use unconventional processing signals to generate neuropeptides from their precursor proteins. Southern blot analyses suggest that preprohormones A and B are each coded for by a single gene, whereas one or possibly two closely related genes code for preprohormone C. Northern blot analyses and in situ hybridizations show that the gene coding for preprohormone A is expressed in neurons of both the head and foot regions of Hydra, whereas the genes coding for preprohormones B and C are specifically expressed in neurons of different regions of the head. All of this shows that neuropeptide biosynthesis in the primitive metazoan Hydra is already rather complex.

Isolation of three novel neuropeptides, the Cyanea-RFamides I-III, from Scyphomedusae

Cnidarians are the lowest animal group having a nervous system. Using a radioimmunoassay for the C-terminal sequence Arg-Phe-NH2 (RFamide), we have isolated three novel neuropeptides from the jellyfish Cyanea lamarckii (belonging to the class Scyphozoa): (Glu-Trp-Leu-Arg-Gly-Arg-Phe-NH2 (Cyanea-RFamide I), (Glu-Pro-Leu-Trp-Ser-Gly-Arg-Phe-NH2 (Cyanea-RFamide II) and Gly-Arg-Phe-NH2 (Cyanea-RFamide III). The Cyanea-RFamides are neuropeptides and form a peptide family with other known neuropeptides isolated from Hydra and hydromedusae (belonging to the class Hydrozoa), and various sea anemones and sea pansies (belonging to the class Anthozoa). The presence of RFamide neuropeptides in all major cnidarian classes suggests that this type of substance was among the first neurotransmitters used in evolution.

Ultrastructure of synapses in the first-evolved nervous systems

The phylum Cnidaria represents the first group of animals to evolve a recognizable nervous system. A comparison of the ultrastructural features of synaptic loci in animals representing all four classes of the cnidaria has provided an overview of the first-evolved synapses that can be compared morphologically to synapses in higher forms. Synapses in these watery jellylike animals with unmyelinated axons are sparse and difficult to fix well. However, we now have sufficient evidence to define an early synapse as one with paired electron dense plasma membranes separated by a 13-25 nm gap containing intracleft filaments and with vesicles on one or both sides of the synaptic cleft. The vesicles are of three types: dense-cored, clear, and opaque. Neuromuscular synapses resemble neuronal synapses and lack the postsynaptic specializations of higher animals. However, some coelenterates, such as the jellyfish Chrysaora, have a postsynaptic cisterna in the muscle. Neuromuscular and neuronematocyte synapses can have either clear or dense-cored vesicles. Opaque vesicles at two-way interneuronal synapses and at neuromuscular synapses in the oral sphincter muscle of sea anemones can be labelled with antisera to the neuropeptides Antho-RFamide (Antho-Arg-Phe-NH2) and Antho-RWamides (Antho-Arg-Trp-NH2) I and II, respectively. That suggests that neuropeptides evolved as neurotransmitters early in the animal kingdom. The basic differences between first evolved synapses and synapses of higher animals are the lack of postjunctional folds at neuromuscular synapses and the presence of fewer and somewhat larger synaptic vesicles, generally containing granular cores, in the more primitive animals.

Peptides in the nervous systems of cnidarians: structure, function, and biosynthesis

Cnidarians are the lowest animal group having a nervous system and it was probably within this phylum or in a related ancestor group that nervous systems first evolved. The primitive nervous systems of cnidarians are strongly peptidergic. From a single sea anemone species, Anthopleura elegantissima, 17 different neuropeptides have been isolated so far, and we expect that many more neuropeptides (more than 30) must be present. All peptides are localized in neurons of cnidarians and we have demonstrated the presence of some of the peptides in neurosecretory dense-cored vesicles. Most neuropeptides have an excitatory or inhibitory action on whole cnidarians, muscle preparations, and isolated muscle cells, suggesting that these peptides are neurotransmitters or neuromodulators. One neuropeptide induces metamorphosis in planula larvae to become a polyp. This shows that cnidarian neuropeptides also are involved in developmental processes, such as cell differentiation and pattern formation. We have cloned the preprohormones for most of the cnidarian neuropeptides. These preprohormones have a high copy number of the immature neuropeptide sequence, which can be up to 37 neuropeptide copies per precursor molecule. In addition to well-known, "classical" processing enzymes, novel prohormone processing enzymes must be present in cnidarian neurons. These include a processing enzyme hydrolyzing at the C-terminal sides of acidic (Asp and Glu) residues and a dipeptidyl aminopeptidase digesting at the C-terminal sides of N-terminally located X-Pro and X-Ala sequences. All this shows that the primitive nervous systems of cnidarians are already quite complex, and that neuropeptides play a central role in the physiology of these animals.

Biochemical and functional identification of GABA receptors in Hydra vulgaris

GABA binding sites labelled in vitro with [3H]GABA are present in crude membrane preparations from Hydra vulgaris. [3H]GABA binding is specific (70%), saturable and it is completely displaced by the GABA mimetic muscimol but not by bicuculline or baclofen. Scatchard analysis of saturation data indicates the presence of only one population of binding sites with a Bmax of 4, 75 pmol/mg of protein and a KD of 76 nM. In the living animal GABA and benzodiazepines increase the duration of mouth opening during the glutathione-induced feeding response. Bicuculline prevents the GABA-induced increase of the feeding response. Diazepam activity is enhanced by simultaneous GABA administration and it is suppressed by the specific antagonist flumazenil. In contrast, no [3H]-flunitrazepam binding is detected in membrane preparations. We conclude that a population of GABA receptors is present in Hydra tissues, where they are involved in the modulation of response to chemical stimulation.

The nervous systems of cnidarians

Cnidarians have simple nervous systems and it was probably within this group or a closely-related ancestor that nervous systems first evolved. The basic plan of the cnidarian nervous system is that of a nerve net which, at some locations, has condensed to form nerve plexuses, or circular or longitudinal nerve tracts which may be syncytia. At the ultrastructural level, many cnidarian neurons have the combined characteristics of sensory, motor, inter- and neurosecretory neurons and thus appear to be multifunctional. We propose that these multifunctional neurons resemble the ancestors of the more specialized neurons that we find in higher animals today. The primitive nervous system of cnidarians is strongly peptidergic: from a single sea anemone species Anthopleura elegantissima, we have now isolated 16 different novel neuropeptides. These peptides are biologically active and cause inhibitions or contractions in muscle preparations or isolated muscle cells from sea anemones. The various peptides are located in at least six distinct sets of neurons showing that sea anemone neurons have already specialized with respect to their peptide content. Using immuno-electronmicroscopy, we have found that the peptides are located in neuronal dense-cored vesicles associated with both synaptic and non-synaptic release sites. All these data indicate that evolutionarily "old" nervous systems use peptides as transmitters. We have also investigated the biosynthesis of the cnidarian neuropeptides. These neuropeptides are made as large precursor proteins which contain multiple (up to 36) copies of immature neuropeptides. Thus, the biosynthesis of neuropeptides in cnidarians is very efficient and comparable to that of higher invertebrates, such as molluscs and insects, and vertebrates.

The presence and distribution of Antho-RFamide-like material in scyphomedusae

The nervous systems of the scyphomedusae Chrysaora hysoscella, Cyanea capillata and Cyanea lamarckii (Phylum Cnidaria) were stained using an anti-serum against the anthozoan neuropeptide Antho-RFamide. Staining was widespread in all three species. In Chrysaora, the antiserum revealed nerve nets in the subumbrella and exumbrella ectoderm, in both faces of the oral lobes, and in the endoderm lining the subumbrella and exumbrella surfaces of the gastric cavity. The most prominent staining occurred in a dense plexus of neurons in the ectoderm at the base of the tentacles. This nerve net sent projections into the subumbrella ectoderm. For the most part, staining in the two species of Cyanea was similar to that in Chrysaora, with a few exceptions. These include the presence, in Cyanea, of an obvious tentacular nerve tract and nerve nets associated with clusters of cnidocytes in the tentacles. Radioimmunoassays of extracts from Chrysaora and Cyanea lamarkii revealed that both species contain large amounts of Antho-RFamide-like material (up to 55 nmol/animal). The results indicate that Antho-RFamide-like neuropeptides are widespread in scyphomedusae.

Multifunctional features of a gastrodermal sensory cell in Hydra: three-dimensional study

Computer-assisted, three-dimensional reconstructions of two gastrodermal sensory cells from transmission electron micrographs of serial sections of Hydra revealed a unipolar morphology with the nucleus near an apical cilium and a simple unbranched axon with a widened terminal. The sensory cells were similar in size and shape to a unipolar sensory cell isolated from macerated gastrodermis and examined with scanning electron microscopy. In thin sections, the cells were characterized by the presence of numerous dense-cored vesicles in the axon and its terminal. A few dense-cored vesicles were aligned at electron-dense synaptic foci in the axon terminal of the sensory cell, which formed an axo-axonal synapse with a nearby centrally located ganglion cell and a neuromuscular synapse with the basal myoneme of a digestive cell. The ganglion cell possessed a perikaryal cilium and a slender axon that extended adjacent to the sensory cell terminal, where it formed an en passant axo-axonal synapse in reciprocal arrangement with that of the sensory cell. In addition, the ganglion cell axon formed a neuromuscular synapse in sequence with the sensory cell axo-axonal synapse. The presence of a large number of neurosecretory-like granules, apical cilium and reciprocal interneuronal and neuromuscular synaptic loci suggests that this gastrodermal sensory cell, characterized ultrastructurally for the first time, represents a third type of multifunctional neuron in Hydra. Thus, Hydra may contain primitive stem-like neurons, which are sensory-motor and also function in both neurosecretion and neurotransmission.

Isolation of L-3-phenyllactyl-Leu-Arg-Asn-NH2 (Antho-RNamide), a sea anemone neuropeptide containing an unusual amino-terminal blocking group

Using a radioimmunoassay for the carboxyl-terminal sequence Arg-Asn-NH2, we have purified a peptide from acetic acid extracts of the sea anemone Anthopleura elegantissima. By classical amino acid analyses, mass spectrometry, and 1H NMR spectroscopy, the structure of this peptide was determined as 3-phenyllactyl-Leu-Arg-Asn-NH2. By using reversed-phase HPLC and a chiral mobile phase, it was shown that the 3-phenyllactyl group had the L configuration. Immunocytochemical staining with antiserum against Arg-Asn-NH2 showed that L-3-phenyllactyl-Leu-Arg-Asn-NH2 (Antho-RNamide) was localized in neurons of sea anemones. The L-3-phenyllactyl group has not been found earlier in neuropeptides of vertebrates or higher invertebrates. We propose that this residue renders Antho-RNamide resistant to nonspecific aminopeptidases, thereby increasing the stability of the peptide after neuronal release.