Neuronal control of pedal sole cilia in the pond snail Lymnaea stagnalis appressa

Environmentally relevant concentrations of the tricyclic antidepressant, amitriptyline, affect feeding and reproduction in a freshwater mollusc

Antidepressant drugs (ADDs) are one of the most extensively used pharmaceuticals globally. They act at particularly low therapeutic concentrations to modulate monoamine neurotransmission, which is one of the most evolutionary conserved pathways in both humans and animal species including invertebrates. As ADDs are widely detected in the aquatic environment at low concentrations (ng/L to low µg/L), their potential to exert drug-target mediated effects in aquatic species has raised serious concerns. Amitriptyline (AMI) is the most widely used tricyclic ADD, while monoamines, the target of ADDs, are major bioregulators of multiple key physiological processes including feeding, reproduction and behaviour in molluscs. However, the effects of AMI on feeding, reproduction and mating behaviour are unknown in molluscs despite their ecological importance, diversity and reported sensitivity to ADDs. To address this knowledge gap, we investigated the effects of environmentally relevant concentrations of AMI (0, 10, 100, 500 and 1000 ng/L) on feeding, reproduction and key locomotor behaviours, including mating, in the freshwater gastropod, Biomphalaria glabrata over a period of 28 days. To further provide insight into the sensitivity of molluscs to ADDs, AMI concentrations (exposure water and hemolymph) were determined using a novel extraction method. The Fish Plasma Model (FPM), a critical tool for prioritization assessment of pharmaceuticals with potential to cause drug target-mediated effects in fish, was then evaluated for its applicability to molluscs for the first time. Disruption of food intake (1000 ng/L) and reproductive output (500 and 1000 ng/L) were observed at particularly low hemolymph levels of AMI, whereas locomotor behaviours were unaffected. Importantly, the predicted hemolymph levels of AMI using the FPM agreed closely with the measured levels. The findings suggest that hemolymph levels of AMI may be a useful indicator of feeding and reproductive disruptions in wild population of freshwater gastropods, and confirm the applicability of the FPM to molluscs for comparative pharmaceutical hazard identification.

Surface crawling and pedal surface collecting in aquatic gastropods: A case of scientific amnesia

Aquatic gastropods, both freshwater and marine, often crawl along the water surface with the sole of the foot facing upward. Differential regulation of the activity of cilia on the sole turns the gliding mechanism into pedal surface collecting, by which food particles floating on the water are collected on the sole. Over the last 300 years, surface crawling and pedal surface collecting have been studied in numerous species, but the accumulated knowledge has fallen victim to scientific amnesia. Today, pedal surface collecting usually is considered a unique behaviour that evolved in the family Ampullariidae (Caenogastropoda), which includes the genus Pomacea with some of the globally worst invasive pests. Consequently it may appear feasible to tackle invasive Pomacea species specifically by delivering molluscicides via the water surface. Based on a review of our forgotten literature, I here argue that such an approach would have potentially devastating, unintended effects on native gastropod faunas.

A Review of Pedal Peptide/Orcokinin-type Neuropeptides

Neuropeptides are endogenous active substances that play important roles in a number of physiological processes and are ubiquitous in the nervous tissue in vivo. The gene encoding pedal peptide/orcokinin-type (PP/OK-type) neuropeptide is an important member of the neuropeptide gene family and is ubiquitous in invertebrates of Bilateria; orcokinin (OK) is mainly found in Arthropoda, while pedal peptide (PP) is mainly found in Mollusca. OK and PP are also present in other animals. PP/OK-type neuropeptides are a kind of multifunctional neuropeptides predominantly expressed in the nervous tissue and play important roles in the nerve regulation of movement. Moreover, OK has a number of other physiological functions. This review describes the distribution, expression, function and maturation of PP/OK-type neuropeptides to facilitate investigations of new functions and receptors of PP/OK-type neuropeptides, providing the theoretical foundation for the potential use of PP/OK-type neuropeptides in the prevention and control of agricultural and forestry pests, as an additive for skin care products and in the screening of drugs for the treatment of diabetes.

The Effect of Pedal Peptide-Type Neuropeptide on Locomotor Behavior and Muscle Physiology in the Sea Cucumber Apostichopus japonicus

Neuropeptides are endogenous active substances that are present in nervous tissues and participate in behavioral and physiological processes of the animal system. Locomotor behavior is basic to predation, escape, reproduction in animals, and neuropeptides play an important role in locomotion. In this study, the function of pedal peptide-type neuropeptide (PDP) in the process of locomotor behavior of the sea cucumber Apostichopus japonicus was evaluated. The locomotor behavior of A. japonicus was recorded by infrared camera before and after PDP administration, and muscle physiology was studied by ultra performance liquid chromatography and quadrupole time-off-light mass spectrometry (UPLC-Q-TOF-MS) to clarify the potential physiological mechanisms. The results showed that PDP enhanced the cumulative duration of moving significantly at the 7th h after injection, and reduced the mean and maximum velocity by 16.90 and 14.22% in A. japonicus. The data of muscle metabolomics suggested that some significantly changed metabolites were related to locomotor behavior of sea cucumbers. The decreases of phosphatidylethanolamine (PE) and phosphatidylcholine (PC) might result in the increases of lysophosphatidylcholines (lysoPC) and lysophosphatidylethanolamine (lysoPE), and suggested the change of fluidity and permeability in the muscle cell membrane, which would affect the physiology and function of muscle cells, and finally alter the locomotor behavior. In addition, the increased level of arachidonic acid (ARA) might activate K⁺ ion channels and then affect the signaling of muscle cells, or promote the sensitivity of muscle cells to Ca²⁺ and then result in the contractility of longitudinal muscles in sea cucumbers. ARA was also involved in the linoleic acid metabolism which was the only pathway that disturbed significantly after PDP administration. In conclusion, PDP participated in the regulation of locomotor behavior in the sea cucumber, and the decreased PE and PC, increased lysoPC, lysoPE and ARA might be the potential physiological mechanisms that responsible for behavioral effects of PDP in A. japonicus.

The Role of Serotonin in the Influence of Intense Locomotion on the Behavior Under Uncertainty in the Mollusk Lymnaea stagnalis

The role of serotonin in the immediate and delayed influence of physical exercise on brain functions has been intensively studied in mammals. Recently, immediate effects of intense locomotion on the decision-making under uncertainty were reported in the Great Pond snail, Lymnaea stagnalis (Korshunova et al., 2016). In this animal, serotonergic neurons control locomotion, and serotonin modulates many processes underlying behavior, including cognitive ones (memory and learning). Whether serotonin mediates the behavioral effects of intense locomotion in mollusks, as it does in vertebrates, remains unknown. Here, the delayed facilitating effects of intense locomotion on the decision-making in the novel environment are described in Lymnaea. Past exercise was found to alter the metabolism of serotonin, namely the content of serotonin precursor and its catabolites in the cerebral and pedal ganglia, as measured by high-performance liquid chromatography. The immediate and delayed effects of exercise on serotonin metabolism were different. Moreover, serotonin metabolism was regulated differently in different ganglia. Pharmacological manipulations of the serotonin content and receptor availability suggests that serotonin is likely to be responsible for the locomotor acceleration in the test of decision-making under uncertainty performed after exercise. However, the exercise-induced facilitation of decision-making (manifested in a reduced number of turns during the orienting behavior) cannot be attributed to the effects of serotonin.

Storage and erasure of behavioural experiences at the single neuron level

Although predictions from the past about the future have been of major interest to current neuroscience, how past and present behavioral experience interacts at the level of a single neuron remains largely unknown. Using the pond snail Lymnaea stagnalis we found that recent experience of terrestrial locomotion (exercise) results in a long-term increase in the firing rate of serotonergic pedal (PeA) neurons. Isolation from the CNS preserved the "memory" about previous motor activity in the neurons even after the animals rested for two hours in deep water after the exercise. In contrast, in the CNS, no difference in the firing rate between the control and "exercise-rested" (ER) neurons was seen. ER snails, when placed again on a surface to exercise, nevertheless showed faster locomotor arousal. The difference in the firing rate between the control and ER isolated neurons disappeared when the neurons were placed in the microenvironment of their home ganglia. It is likely that an increased content of dopamine in the CNS masks an increased excitation of PeA neurons after rest: the dopamine receptor antagonist sulpiride produced sustained excitation in PeA neurons from ER snails but not in the control. Therefore, our data suggest the involvement of two mechanisms in the interplay of past and present experiences at the cellular level: intrinsic neuronal changes in the biophysical properties of the cell membrane and extrinsic modulatory environment of the ganglia.

Neuropeptide precursors and neuropeptides in the sea cucumber Apostichopus japonicus: a genomic, transcriptomic and proteomic analysis

The sea cucumber Apostichopus japonicus is a foodstuff with very high economic value in China, Japan and other countries in south-east Asia. It is at the heart of a multibillion-dollar industry and to meet demand for this product, aquaculture methods and facilities have been established. However, there are challenges associated with optimization of reproduction, feeding and growth in non-natural environments. Therefore, we need to learn more about the biology of A. japonicus, including processes such as aestivation, evisceration, regeneration and albinism. One of the major classes of molecules that regulate physiology and behaviour in animals are neuropeptides, and a few bioactive peptides have already been identified in A. japonicus. To facilitate more comprehensive investigations of neuropeptide function in A. japonicus, here we have analysed genomic and transcriptomic sequence data and proteomic data to identify neuropeptide precursors and neuropeptides in this species. We identified 44 transcripts encoding neuropeptide precursors or putative neuropeptide precursors, and in some instances neuropeptides derived from these precursors were confirmed by mass spectrometry. Furthermore, analysis of genomic sequence data enabled identification of the location of neuropeptide precursor genes on genomic scaffolds and linkage groups (chromosomes) and determination of gene structure. Many of the precursors identified contain homologs of neuropeptides that have been identified in other bilaterian animals. Precursors of neuropeptides that have thus far only been identified in echinoderms were identified, including L- and F-type SALMFamides, AN peptides and others. Precursors of several peptides that act as modulators of neuromuscular activity in A. japonicus were also identified. The discovery of a large repertoire of neuropeptide precursors and neuropeptides provides a basis for experimental studies that investigate the physiological roles of neuropeptide signaling systems in A. japonicus. Looking ahead, some of these neuropeptides may have effects that could be harnessed to enable improvements in the aquaculture of this economically important species.

Olfactory navigation in aquatic gastropods

Gastropod diversity is substantial in marine and freshwater habitats, and many aquatic slugs and snails use olfactory cues to guide their navigation behaviour. Examples include finding prey or avoiding predators based on kairomones, or finding potential mates using pheromones. Here, I review the diversity of navigational behaviours studied across the major aquatic taxa of gastropods. I then synthesize evidence for the different theoretical navigation strategies the animals may use. It is likely that gastropods regularly use either chemotaxis or odour-gated rheotaxis (or both) during olfactory-based navigation. Finally, I collate the patchwork of research conducted on relevant proximate mechanisms that could produce navigation behaviours. Although the tractability of several gastropod species for neurophysiological experimentation has generated some valuable insight into how turning behaviour is triggered by contact chemoreception, there remain many substantial gaps in our understanding for how navigation relative to more distant odour sources is controlled in gastropods. These gaps include little information on the chemoreceptors and mechanoreceptors (for detecting flow) found in the peripheral nervous system and the central (or peripheral) processing circuits that integrate that sensory input. In contrast, past studies do provide information on motor neurons that control the effectors that produce crawling (both forward locomotion and turning). Thus, there is plenty of scope for further research on olfactory-based navigation, exploiting the tractability of gastropods for neuroethology to better understand how the nervous system processes chemosensory input to generate movement towards or away from distant odour sources.

The similarity of crawling mechanisms in aquatic and terrestrial gastropods

Crawling gastropods are unique models for studying the functioning of smooth muscles and ciliated epithelia, since they cover the foot sole and are involved in locomotion, allowing for direct investigation. Two types of crawling are known: creeping by muscular waves in terrestrial gastropods such as Helix and сiliary gliding in aquatic gastropods such as Lymnaea. It was found that the smooth muscles that underlie the ciliated epithelium in Lymnaea are involved in gliding and contribute significantly to fast crawling. Thus, the locomotor apparatus is fundamentally the same in both snails and the difference between crawling reflects an adaptation to a habitat. The control of crawling speed is also the same. Tonic contraction, relaxation, and rhythmic contractions are involved in this control. During a locomotor episode, the sole length and crawling speed spontaneously change and directly correlate with each other via the contraction force of the muscle cells in the locomotory waves. Dopamine, unlike ergometrine, decreases the sole length and crawling speed. Serotonin stimulates, increases crawling and determines the number of muscle cells involved in the locomotory waves for each locomotor episode. This control (taking into account heterogeneity) apparently might exist in any other phasic smooth muscle, including vertebrates.

Identification of evolutionarily conserved residues required for the bioactivity of a pedal peptide/orcokinin-type neuropeptide

Pedal peptides and orcokinins are structurally related neuropeptides that were first discovered in protostomian invertebrates - mollusks and arthropods, respectively. Recently, pedal peptide/ocokinin (PP/OK)-type neuropeptides were discovered in a deuterostomian phylum, the echinoderms, indicating that the evolutionary origin of this neuropeptide family can be traced back to the common ancestor of bilaterian animals. Sequences comparison of PP/OK-type neuropeptides reveals several conserved residues, including N- and C-terminally located hydrophobic residues that are important for the bioactivity of orcokinin. Here we report the first comprehensive analysis of the structure-activity relationships of a PP/OK-type neuropeptide - starfish myorelaxant peptide (SMP; FGKGGAYDPLSAGFTD) from the starfish Patiria pectinifera (Phylum Echinodermata). Comparison of the bioactivity of SMP with N-terminally and/or C-terminally truncated and alanine-substituted SMP analogs revealed a core peptide (GAYDPLSAGF; SMP(5-14)) that retains the muscle-relaxing activity of SMP, albeit with reduced potency and efficacy. Within this core peptide, alanine-substitution of several residues resulted in complete or partial loss of bioactivity, whilst loss or substitution of the N-terminal phenylalanine residue of SMP also caused a substantial reduction in bioactivity. Furthermore, analysis of the bioactivity of other SMP-like peptides derived from the same precursor as SMP revealed that none of these were more potent/effective than SMP as muscle relaxants. In conclusion, we have identified key residues required for the bioactivity of a PP/OK-type neuropeptide (SMP), including hydrophobic residues located in the N- and C-terminal regions that are conserved in PP/OK-type peptides from other phyla as well as core residues that are conserved in echinoderm PP/OK-type peptides.

Transcriptomic identification of starfish neuropeptide precursors yields new insights into neuropeptide evolution

Neuropeptides are evolutionarily ancient mediators of neuronal signalling in nervous systems. With recent advances in genomics/transcriptomics, an increasingly wide range of species has become accessible for molecular analysis. The deuterostomian invertebrates are of particular interest in this regard because they occupy an ‘intermediate' position in animal phylogeny, bridging the gap between the well-studied model protostomian invertebrates (e.g. Drosophila melanogaster, Caenorhabditis elegans) and the vertebrates. Here we have identified 40 neuropeptide precursors in the starfish Asterias rubens, a deuterostomian invertebrate from the phylum Echinodermata. Importantly, these include kisspeptin-type and melanin-concentrating hormone-type precursors, which are the first to be discovered in a non-chordate species. Starfish tachykinin-type, somatostatin-type, pigment-dispersing factor-type and corticotropin-releasing hormone-type precursors are the first to be discovered in the echinoderm/ambulacrarian clade of the animal kingdom. Other precursors identified include vasopressin/oxytocin-type, gonadotropin-releasing hormone-type, thyrotropin-releasing hormone-type, calcitonin-type, cholecystokinin/gastrin-type, orexin-type, luqin-type, pedal peptide/orcokinin-type, glycoprotein hormone-type, bursicon-type, relaxin-type and insulin-like growth factor-type precursors. This is the most comprehensive identification of neuropeptide precursor proteins in an echinoderm to date, yielding new insights into the evolution of neuropeptide signalling systems. Furthermore, these data provide a basis for experimental analysis of neuropeptide function in the unique context of the decentralized, pentaradial echinoderm bauplan.

Identification of a novel starfish neuropeptide that acts as a muscle relaxant

Neuropeptides that act as muscle relaxants have been identified in chordates and protostomian invertebrates but little is known about the molecular identity of neuropeptides that act as muscle relaxants in deuterostomian invertebrates (e.g. echinoderms) that are 'evolutionary intermediates' of chordates and protostomes. Here, we have used the apical muscle of the starfish Patiria pectinifera to assay for myorelaxants in extracts of this species. A hexadecapeptide with the amino acid sequence Phe-Gly-Lys-Gly-Gly-Ala-Tyr-Asp-Pro-Leu-Ser-Ala-Gly-Phe-Thr-Asp was identified and designated starfish myorelaxant peptide (SMP). Cloning and sequencing of a cDNA encoding the SMP precursor protein revealed that it comprises 12 copies of SMP as well as 3 peptides (7 copies in total) that are structurally related to SMP. Analysis of the expression of SMP precursor transcripts in P. pectinifera using qPCR revealed the highest expression in the radial nerve cords and lower expression levels in a range of neuromuscular tissues, including the apical muscle, tube feet and cardiac stomach. Consistent with these findings, SMP also caused relaxation of tube foot and cardiac stomach preparations. Furthermore, SMP caused relaxation of apical muscle preparations from another starfish species - Asterias amurensis. Collectively, these data indicate that SMP has a general physiological role as a muscle relaxant in starfish. Interestingly, comparison of the sequence of the SMP precursor with known neuropeptide precursors revealed that SMP belongs to a bilaterian family of neuropeptides that include molluscan pedal peptides (PP) and arthropodan orcokinins (OK). This is the first study to determine the function of a PP/OK-type peptide in a deuterostome. Pedal peptide/orcokinin (PP/OK)-type peptides are a family of structurally related neuropeptides that were first identified and functionally characterised in protostomian invertebrates. Here, we report the discovery of starfish myorelaxant peptide (SMP), a novel member of the PP/OK-type neuropeptide identified in the starfish Patiria pectinifera (phylum Echinodermata). SMP is the first PP/OK-type neuropeptide to be functionally characterised in a deuterostome.

Previous motor activity affects transition from uncertainty to decision-making in snails

One of the most commonly accepted benefits of enhanced physical activity is the improvement in the symptoms of depression, including the facilitation of decision-making. Up until now, these effects have been shown in rodents and humans only. Little is known about their evolutionary origin or biological basis, and the underlying cellular mechanisms also remain relatively elusive. Here, we demonstrate for the first time that preceding motor activity accelerates decision-making in an invertebrate, the pond snail Lymnaea stagnalis. To investigate decision-making in a novel environment, snails, which normally live in water, were placed on a flat dry surface to simulate the potentially threatening consequence of being in an arid environment. This stimulus initiated two distinct phases in snail behavior: slow circular movements, followed by intense locomotion in a chosen direction. The first phase was prolonged when the test arena was symmetrically lit, compared to one with an apparent gradient of light. However, forced muscular locomotion for two hours prior to the test promoted the transition from random circular motions to a directional crawl, accompanied by an increase in crawling speed but with no effect on the choice of direction. Two hours of intense locomotion produced also strong excitatory effect on the activity of serotonergic neurons in L. stagnalis. Our results suggest that the beneficial effects of physical exercise on cognitive performance in mammals might have deep roots in evolution, granting the opportunity to unravel the origins of such effects at the single neuron and network levels.

A novel serotonin-secreting cell type regulates ciliary motility in the mucociliary epidermis of Xenopus tadpoles

The embryonic skin of Xenopus tadpoles serves as an experimental model system for mucociliary epithelia (MCE) such as the human airway epithelium. MCEs are characterized by the presence of mucus-secreting goblet and multiciliated cells (MCCs). A third cell type, ion-secreting cells (ISCs), is present in the larval skin as well. Synchronized beating of MCC cilia is required for directional transport of mucus. Here we describe a novel cell type in the Xenopus laevis larval epidermis, characterized by serotonin synthesis and secretion. It is termed small secretory cell (SSC). SSCs are detectable at early tadpole stages, unlike MCCs and ISCs, which are specified at early neurulation. Subcellularly, serotonin was found in large, apically localized vesicle-like structures, which were entirely shed into the surrounding medium. Pharmacological inhibition of serotonin synthesis decreased the velocity of cilia-driven fluid flow across the skin epithelium. This effect was mediated by serotonin type 3 receptor (Htr3), which was expressed in ciliated cells. Knockdown of Htr3 compromised flow velocity by reducing the ciliary motility of MCCs. SSCs thus represent a distinct and novel entity of the frog tadpole MCE, required for ciliary beating and mucus transport across the larval skin. The identification and characterization of SSCs consolidates the value of the Xenopus embryonic skin as a model system for human MCEs, which have been known for serotonin-dependent regulation of ciliary beat frequency.

Neuropeptides and polypeptide hormones in echinoderms: new insights from analysis of the transcriptome of the sea cucumber Apostichopus japonicus

Echinoderms are of special interest for studies in comparative endocrinology because of their phylogenetic position in the animal kingdom as deuterostomian invertebrates. Furthermore, their pentaradial symmetry as adult animals provides a unique context for analysis of the physiological and behavioral roles of peptide signaling systems. Here we report the first extensive survey of neuropeptide and peptide hormone precursors in a species belonging to the class Holothuroidea. Transcriptome sequence data obtained from the sea cucumber Apostichopus japonicus were analyzed to identify homologs of precursor proteins that have recently been identified in the sea urchin Strongylocentrotus purpuratus (class Echinoidea). A total of 17 precursor proteins have been identified in A. japonicus, including precursors of peptides related to thyrotropin-releasing hormone, pedal peptide/orcokinin-type peptides, AN peptides/tachykinins, luqins, corticotropin-releasing hormone (CRH), GPA2-type glycoprotein hormone subunits and bursicon. In addition, an unusual finding was an A. japonicus calcitonin-type precursor protein (AjCTLPP), the first to be discovered that comprises two calcitonin-like peptides; this contrasts with the products of the alternatively-spliced calcitonin/CGRP gene in vertebrates, which comprise either calcitonin or CGRP. Collectively, the data obtained provide new insights on the evolution and diversity of neuropeptides and polypeptide hormones. Furthermore, because A. japonicus is one of several sea cucumber species that are used for human consumption, our findings may have practical and economic impact by providing a basis for neuroendocrine-based strategies to improve methods of aquaculture.

Pedal sole immunoreactive axons in terrestrial pulmonates: Limax, Arion, and Helix

A century ago histological techniques such as formic acid-gold chloride showed the nerve morphology of the pedal sole in Limax and Helix. There have been no similar descriptions since then of the central nervous system relevant to locomotory pedal waves in the foot of slugs and snails. Topical application of 5-HT affects locomotory waves, but the innervation of the pedal sole with 5-HT axons is not known. Three-dimensional morphology of pedal axons in terrestrial pulmonate embryos is shown herein with modern histological techniques using antibodies and the confocal microscope. In Limax maximus, pedal ganglia are shown with Tritonia pedal peptide (TPep) antibodies. Ladder-like cross bridges in the pedal sole are shown with antibodies to both TPep and 5-HT. In Arion ater, pedal ganglia neurons and their axons that form a plexus in the pedal sole are shown with 5-HT antibodies. In Helix aspersa, 5-HT immunoreactive pedal ganglia neurons and a developing pedal sole axon plexus are seen as in A. ater. Axons in this plexus that grow across the pedal sole can be seen growing into pre-existing nerves. No peripheral 5-HT neurons were identified in these three species. This immunoreactive plexus to 5-HT antibodies in A. ater and H. aspersa spreads over the pedal sole epithelium. Axons immunoreactive to 5-HT antibodies in A. ater and H. aspersa extend the length of the foot, primarily in the rim, so that activity in these axons cannot provide local patterned input to produce locomotory waves, but may provide modulatory input to pedal sole muscles.