Pharmacological characterization of a serotonin receptor involved in an early embryonic behavior of Helisoma trivolvis

Role of the Neuroendocrine System of Marine Bivalves in Their Response to Hypoxia

Mollusks comprise one of the largest phylum of marine invertebrates. With their great diversity of species, various degrees of mobility, and specific behavioral strategies, they haveoccupied marine, freshwater, and terrestrial habitats and play key roles in many ecosystems. This success is explained by their exceptional ability to tolerate a wide range of environmental stresses, such as hypoxia. Most marine bivalvemollusksare exposed to frequent short-term variations in oxygen levels in their marine or estuarine habitats. This stressfactor has caused them to develop a wide variety of adaptive strategies during their evolution, enabling to mobilize rapidly a set of behavioral, physiological, biochemical, and molecular defenses that re-establishing oxygen homeostasis. The neuroendocrine system and its related signaling systems play crucial roles in the regulation of various physiological and behavioral processes in mollusks and, hence, can affect hypoxiatolerance. Little effort has been made to identify the neurotransmitters and genes involved in oxygen homeostasis regulation, and the molecular basis of the differences in the regulatory mechanisms of hypoxia resistance in hypoxia-tolerant and hypoxia-sensitive bivalve species. Here, we summarize current knowledge about the involvement of the neuroendocrine system in the hypoxia stress response, and the possible contributions of various signaling molecules to this process. We thusprovide a basis for understanding the molecular mechanisms underlying hypoxic stress in bivalves, also making comparisons with data from related studies on other species.

A thermal stressor, propranolol and long-term memory formation in freshly collected Lymnaea

A heat stressor (1 h at 30°C) in Lymnaea stagnalis before operant conditioning training of aerial respiration is sufficient to enhance long-term memory (LTM) formation in 'average' cognitive ability, laboratory-reared, inbred snails. However, in freshly collected outbred snails, the same heat stressor blocks LTM formation in 'smart' cognitive phenotype but not in average cognitive phenotype strains. Here, we hypothesize that (1) preventing the stress associated with the heat stressor before training allows LTM to form in the smart phenotype strains; and (2) alleviating the stress before a memory recall session allows a formed LTM to be recalled in the smart phenotype strains. We found that an injection of propranolol, which mitigates the stressor, before snails experience the heat stressor enabled two strains of the smart phenotype snails to form LTM, consistent with our first hypothesis. However, the injection of propranolol before a memory test session did not alleviate a memory recall block in the smart phenotype snails. Thus, our second hypothesis was not supported. Therefore, smart cognitive phenotype snails encountering a heat stressor have an inability to form LTM, but this inability can be overcome by the pre-injection of propranolol.

Neuronal coordination of motile cilia in locomotion and feeding

Efficient ciliary locomotion and transport require the coordination of motile cilia. Short-range coordination of ciliary beats can occur by biophysical mechanisms. Long-range coordination across large or disjointed ciliated fields often requires nervous system control and innervation of ciliated cells by ciliomotor neurons. The neuronal control of cilia is best understood in invertebrate ciliated microswimmers, but similar mechanisms may operate in the vertebrate body. Here, we review how the study of aquatic invertebrates contributed to our understanding of the neuronal control of cilia. We summarize the anatomy of ciliomotor systems and the physiological mechanisms that can alter ciliary activity. We also discuss the most well-characterized ciliomotor system, that of the larval annelid Platynereis. Here, pacemaker neurons drive the rhythmic activation of cholinergic and serotonergic ciliomotor neurons to induce ciliary arrests and beating. The Platynereis ciliomotor neurons form a distinct part of the larval nervous system. Similar ciliomotor systems likely operate in other ciliated larvae, such as mollusc veligers. We discuss the possible ancestry and conservation of ciliomotor circuits and highlight how comparative experimental approaches could contribute to a better understanding of the evolution and function of ciliary systems.

This article is part of the Theo Murphy meeting issue ‘Unity and diversity of cilia in locomotion and transport’.

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.

Serotonergic regulation of the buccal (feeding) rhythm of the pond snail, Lymnaea stagnalis. An immunocytochemical, biochemical and pharmacological approach

Hatching is an important phase of the development of pulmonate gastropods followed by the adult-like extracapsular foraging life. Right before hatching the juveniles start to display a rhythmic radula movement, executed by the buccal complex, consisting of the buccal musculature (mass) and a pair of the buccal ganglia. In order to have a detailed insight into this process, we investigated the serotonergic regulation of the buccal (feeding) rhythm in 100% stage embryos of the pond snail, Lymnaea stagnalis, applying quantitative immunohistochemistry combined with the pharmacological manipulation of the serotonin (5-HT) synthesis, by either stimulating (by the 5-HT precursor 5-hydroxytryptophan, 5-HTP) or inhibiting (by the 5-HT synthesis blocker para-chlorophenylalanine, pCPA) it. Corresponding to the direction of the drug effect, significant changes of the fluorescence intensity could be detected both in the cerebral ganglia and the buccal complex. HPLC-MS assay demonstrated that 5-HTP increased meanwhile pCPA decreased the 5-HT content both of the central ganglia and the buccal complex. As to the feeding activity, 5-HTP induced only a slight (20%) increase, whereas the pCPA resulted in a 20% decrease of the radula protrusion frequency. Inhibition of 5-HT re-uptake by clomipramine reduced the frequency by 75%. The results prove the role of both central and peripheral 5-HTergic processes in the regulation of feeding activity. Application of specific receptor agonists and antagonists revealed that activation of a 5-HT1-like receptor depressed the feeding activity, meanwhile activation of a 5-HT6,7-like receptor enhanced it. Saturation binding plot of [3H]-5-HT to receptor and binding experiments performed on membrane pellets prepared from the buccal mass indicated the presence of a 5-HT6-like receptor positively coupled to cAMP. The results suggest that 5-HT influences the buccal (feeding) rhythmic activity in two ways: an inhibitory action is probably exerted via 5-HT1-like receptors, while an excitatory action is realized through 5-HT_6,7-like receptors.

Neuronal Development in the Larvae of the Invasive Biofouler Dreissena polymorpha (Mollusca: Bivalvia), with Special Attention to Sensory Elements and Swimming Behavior

Although understanding of the neuronal development of Trochozoa has progressed recently, little attention has been paid to freshwater bivalves, including species with a strong ecological impact, such as the zebra mussel (Dreissena polymorpha). Therefore, an important question might concern how the developing nervous system is involved in the formation of the rapid and successful invasive behavior of this species. Our aim was to reveal the neuronal development of trochophore and veliger larvae of Dreissena, with special attention to the organization of sensory structures and their possible involvement in detecting environmental cues. After applying serotonin and FMRFamide immunocytochemistry, the first serotonin immunoreactive sensory elements appeared 16-18 hours after fertilization, whereas the first FMRFamide immunoreactive sensory cell was seen only at 32 hours of development (trochophore stage). Later, sensory elements were found in three parts of the larval body, including the apical organ, the posterior region, and the stomach. Although differences in the timing of appearance and the morphology of cells were observed, the two signaling systems showed basic similarity in their organization pattern until the end of the veliger stage. Pharmacological, physiological, and quantitative immunocytochemical investigations were also performed, suggesting the involvement of both the serotoninergic system and the FMRFamidergic system in sensomotor processes. Manipulation of the serotonin synthesis by para-chloroplenylalanine and 5-hydroxytryptophane, as well as application of increased salinity, influenced larval swimming activity, both accompanied by changes in immunofluorescence intensity. We concluded that these two early sensory systems may play an important role in the development of settlement competency of this biofouling invasive bivalve, Dreissena.

Molecular characterization of the apical organ of the anthozoan Nematostella vectensis

Apical organs are sensory structures present in many marine invertebrate larvae where they are considered to be involved in their settlement, metamorphosis and locomotion. In bilaterians they are characterised by a tuft of long cilia and receptor cells and they are associated with groups of neurons, but their relatively low morphological complexity and dispersed phylogenetic distribution have left their evolutionary relationship unresolved. Moreover, since apical organs are not present in the standard model organisms, their development and function are not well understood. To provide a foundation for a better understanding of this structure we have characterised the molecular composition of the apical organ of the sea anemone Nematostella vectensis. In a microarray-based comparison of the gene expression profiles of planulae with either a wildtype or an experimentally expanded apical organ, we identified 78 evolutionarily conserved genes, which are predominantly or specifically expressed in the apical organ of Nematostella. This gene set comprises signalling molecules, transcription factors, structural and metabolic genes. The majority of these genes, including several conserved, but previously uncharacterized ones, are potentially involved in different aspects of the development or function of the long cilia of the apical organ. To demonstrate the utility of this gene set for comparative analyses, we further analysed the expression of a subset of previously uncharacterized putative orthologs in sea urchin larvae and detected expression for twelve out of eighteen of them in the apical domain. Our study provides a molecular characterization of the apical organ of Nematostella and represents an informative tool for future studies addressing the development, function and evolutionary history of apical organ cells.

Mechanisms underlying dual effects of serotonin during development of Helisoma trivolvis (Mollusca)

BACKGROUND

Serotonin (5-HT) is well known as widely distributed modulator of developmental processes in both vertebrates and invertebrates. It is also the earliest neurotransmitter to appear during neuronal development. In aquatic invertebrates, which have larvae in their life cycle, 5-HT is involved in regulation of stages transition including larval metamorphosis and settlement. However, molecular and cellular mechanisms underlying developmental transition in aquatic invertebrate species are yet poorly understood. Earlier we demonstrated that in larvae of freshwater molluscs and marine polychaetes, endogenous 5-HT released from the neurons of the apical sensory organ (ASO) in response to external stimuli retarded larval development at premetamorphic stages, and accelerated it at metamorphic stages. Here we used a freshwater snail Helisoma trivolvis to study molecular mechanisms underlying these dual developmental effects of 5-HT.

RESULTS

Larval development of H. trivolvis includes transition from premetamorphic to metamorphic stages and shares the main features of metamorphosis with free-swimming aquatic larvae. Three types of 5-HT receptors (5-HT1-, 5-HT4- and 5-HT7-like) are functionally active at premetamorphic (trochophore, veliger) and metamorphic (veliconcha) stages, and expression patterns of these receptors and respective G proteins undergo coordinated changes during development. Stimulation of these receptors modulated cAMP-dependent regulation of cell divisions. Expression of 5-HT4- and 5-HT7-like receptors and their downstream Gs protein was down-regulated during the transition of pre- to metamorphic stage, while expression of 5-HT1 -like receptor and its downstream Gi protein was upregulated. In accordance with relative amount of these receptors, stimulation of 5-HTRs at premetamorphic stages induces developmental retardation, while their stimulation at metamorphic stages induces developmental acceleration.

CONCLUSIONS

We present a novel molecular mechanism that underlies stage-specific changes in developmental tempo of H. trivolvis larvae in response to endogenous 5-HT produced by the neurons of the ASO. We suggest that consecutive changes in expression patterns of different receptors and their downstream partners in the course of larval development represent the molecular base of larval transition from premetamorphic (non-competent) to metamorphic (competent) state.

Pharmacological analysis of locomotion and heart contraction during the development of Helisoma (Mollusca: Gastropoda). Short communication

We investigated involvement of different 5-HT receptors in regulation of ciliary rotation, gliding locomotion and heartbeat of Helisoma embryo at pre- and post-metamorphic stages. Pharmacological analysis suggested that activation of 5-HT1 receptor enhance ciliary rotation but do not affect gliding locomotion. Activation of 5-HT4 receptor depresses both types of locomotion. Before metamorphosis heart contraction is depressed by activation of 5-HT4 and enhanced by activation of 5-HT7 receptor. However, the heart became insensitive to all agonists by hatching. We hypothesized that alterations in affinity or expression of particular 5-HT receptors can underlie the well-coordinated character of serotonin-dependent larval behavior.

Identification and evolutionary implications of neurotransmitter-ciliary interactions underlying the behavioral response to hypoxia in Lymnaea stagnalis embryos

Acceleration of embryonic rotation is a common response to hypoxia among pond snails. It was first characterized in Helisoma trivolvis embryos, which have a pair of sensorimotor neurons that detect hypoxia and release serotonin onto postsynaptic ciliary cells. The objective of the present study was to determine how the hypoxia response is mediated in Lymnaea stagnalis, which differ from H. trivolvis by having both serotonergic and dopaminergic neurons, and morphologically distinct ciliated structures at comparative stages of embryonic development. Time-lapse video recordings of the rotational behavior in L. stagnalis revealed similar rotational features to those previously observed in H. trivolvis, including rotational surges and rotational responses to hypoxia. Serotonin and dopamine increased the rate of rotation with similar potency. In contrast, serotonin was more potent than dopamine in stimulating the ciliary beat frequency of isolated pedal cilia. Isolated apical plate cilia displayed an irregular pattern of ciliary beating that precluded the measurement of ciliary beat frequency. A qualitative assessment of ciliary beating revealed that both serotonin and dopamine were able to stimulate apical plate cilia. The ciliary responses to dopamine were reversible in both pedal and apical plate cilia, whereas the responses to serotonin were only reversible at concentrations below 100 μmol l(-1). Mianserin, a serotonin receptor antagonist, and SKF83566, a dopamine receptor antagonist, effectively blocked the rotational responses to serotonin and dopamine, respectively. The rotational response to hypoxia was only partially blocked by mianserin, but was fully blocked by SKF83566. These data suggest that, despite the ability of serotonin to stimulate ciliary beating in L. stagnalis embryos, the rotational response to hypoxia is primarily mediated by the transient apical catecholaminergic neurons that innervate the ciliated apical plate.

Serotonergic cerebral cells control activity of cilia in the foregut of the pteropod mollusk Clione limacina

Bilaterally symmetrical pair of serotonergic cells, named C1 in Clione, has been described in the cerebral ganglia of all gastropod species. Here we describe a new role of C1 cells in gastropod mollusks: control of activity of ciliated epithelium in the foregut. Detailed morphological investigation of C1 neurons in the pteropod mollusk Clione limacina revealed that these cells among other destinations send their neurites into foregut where they produce intense arborization with large varicosities along the processes. Intracellular stimulation of a single C1 induced pronounced activation (often followed by inhibition) of cilia lining the foregut. This activation was substantially reduced by serotonin antagonist mianserin. Bath application of serotonin also induced transient increase in ciliary transport rate, followed by inhibition of ciliary activity up to its full cessation in some areas of isolated foregut. These data suggest that C1 in Clione may use serotonin to influence cilia in the foregut. Taking into account high homology of serotonergic cerebral cells across studied species we can speculate that these cells may be involved in the neural control of cilia in the foregut in other gastropod mollusks.

Serotonin prolongs survival of encapsulated pond snail embryos exposed to long-term anoxia

Embryos of the pond snail, Helisoma trivolvis, develop bilateral serotonergic neurons that innervate ciliary bands and stimulate cilia-driven rotation. This behaviour is postulated to increase oxygen availability during hypoxia by mixing the capsular fluid. We hypothesised that the stimulation of ciliary-driven rotation by serotonin (5-HT) enhances the survival of embryos during prolonged hypoxia. Embryo rotation and survival were monitored in different levels of oxygen for 24-48 h while in the presence or absence of 5-HT (100 micromol l(-1)) or a 5-HT antagonist (50 micromol l(-1)). Long-term hypoxia caused delayed embryonic development that appeared morphologically normal. Hypoxia also induced a transient increase in rotation rate in embryos exposed to artificial pond water (APW) or 5-HT that lasted around 3 h. 5-HT-treated embryos had an elevated rotation rate over embryos in APW throughout the long-term exposure to hypoxia. Long-term anoxia also induced a transient increase in rotation rate in embryos exposed to APW or 5-HT. Rotation ceased in embryos exposed to APW by 13 h but persisted in 5-HT-treated embryos for up to 40 h. Fifty percent mortality was reached at 9 h of anoxia in embryos in APW and at 24 h in 5-HT-treated embryos. The 5-HT antagonist mianserin partially inhibited the 5-HT enhancement of rotation but not the prolongation of survival in anoxia. The ability of 5-HT to prolong survival in anoxia reveals a 5-HT-activated metabolic pathway that liberates an alternative energy source.

Rotational behaviour of encapsulated pond snail embryos in diverse natural environments

Encapsulated freshwater pond snail embryos display a cilia-driven rotation behaviour that is stimulated by artificially induced hypoxia. Previous studies have suggested that the mixing effect of this behaviour causes enhanced oxygen delivery to embryos within their egg capsules. Despite extensive laboratory-based studies describing this behaviour, it is unclear how this behaviour is used to cope with changes in oxygen concentration and other environmental factors in natural water bodies. We made field measurements of embryo rotation rates in laboratory-reared Helisoma trivolvis embryos placed in ponds of different trophic levels that ranged geographically from the southern Alberta prairie to the Rocky Mountains. Abiotic factors including temperature, pH, conductivity and water oxygen concentration were measured to understand how embryonic rotation is influenced by environmental conditions. Results showed that H. trivolvis embryos exhibit differences in rotational behaviour depending on the environmental conditions. Temperature and oxygen concentration were the primary factors significantly affecting rotation rates. The effect of oxygen concentration on rotation rates was not as widespread as observed under laboratory conditions, probably because the measured oxygen concentrations were above the range that influences embryonic rotation in the laboratory. The rotational behaviour of laboratory-reared Lymnaea stagnalis provided confirmation that embryos of other encapsulated pulmonates exhibit a similar rotational response in natural environments. These results suggest that embryo rotation is influenced by a complex interplay of environmental factors.

A 5-HT1A-like receptor is involved in the regulation of the embryonic rotation of Lymnaea stagnalis L

Cilia driven rotation of the pond snail Lymnaea stagnalis embryos is regulated by serotonin (5-HT). In the present study, physiological and biochemical assays were used to identify the 5-HT receptor type involved in rotation. The 5-HTergic agonists applied stimulated the rotation by 180-400% and their rank order potency was as follows: LSD>5-HT>8-OH-DPAT>WB4101>>5-CT. The applied antagonists, spiperone, propranalol and mianserin inhibited the 5-HT or 8-OH-DPAT stimulated rotation of the embryos by 50-70%. (3)H-5-HT was bound specifically to the washed pellet of the embryo homogenates. The specific binding of (3)H-5-HT was saturable and showed a single, high affinity binding site with K(d) 7.36 nM and B(max) 221 fmol/mg pellet values. This is the first report demonstrating the high affinity binding of (3)H-5-HT to the native receptor in molluscs. All of the pharmacons that stimulated the rotation or inhibited the 5-HT or 8-OH-DPAT evoked stimulation displaced effectively the binding of (3)H-5-HT. 5-HT resulted in the inhibition of forskolin stimulated cAMP accumulation, showing that 5-HT is negatively coupled to adenylate cyclase. Our results suggest that in the 5-HTergic regulation of the embryonic rotation in L. stagnalis a 5-HT(1A)-like receptor of the vertebrate type is involved.

Buccal neurons activate ciliary beating in the foregut of the pteropod mollusk Clione limacina

Beating of cilia lining the foregut of gastropods facilitates the swallowing of food and, therefore, plays a role in feeding behavior. Despite the fact that neural control of feeding is well studied in mollusks, no neurons controlling ciliary beating in the foregut have been identified to date. Here we describe for the first time a pair of buccal neurons innervating the foregut of Clione. Intracellular stimulation of these neurons induced vigorous activation of cilia lining the foregut in a semi-intact preparation. Using immunochemistry labeling, buccal foregut cells were found to contain peptides similar to CNP neuropeptides of the terrestrial snail Helix lucorum. Application of DYPRL-amide, a member of the Helix CNP peptide family, mimicked the effect of buccal foregut cell stimulation on ciliary activity. Induction of fictive feeding in an isolated CNS preparation resulted in the activation of buccal foregut cells suggesting that these cells control ciliary beating in the foregut during feeding. Thus, cilia-activating buccal neurons may represent a new intrinsic element of the neural control of feeding in gastropods.

Serotonin increases cilia-driven particle transport via an acetylcholine-independent pathway in the mouse trachea

Background

Mucociliary clearance in the airways is driven by the coordinated beating of ciliated cells. Classical neuromediators such as noradrenalin and acetylcholine increase ciliary beat frequency and thus cilia-driven transport. Despite the fact that the neuromediator serotonin is ciliostimulatory in invertebrates and has been implied in releasing acetylcholine from the airway epithelium, its role in regulating cilia function in vertebrate airways is not established.

Methodology/Principal Findings

We examined the effects of serotonin on ciliary beat frequency and cilia-driven particle transport in the acutely excised submerged mouse trachea and determined the sources of serotonin in this tissue by immunohistochemistry. Serotonin (100 µM) increased cilary beat frequency (8.9±1.2 Hz to 17.0±2.7 Hz) and particle transport speed (38.9±4.6 µm/s to 83.4±8.3 µm/s) to an extent that was comparable to a supramaximal dose of ATP. The increase in particle transport speed was totally prevented by methysergide (100 µM). Blockade of muscarinic receptors by atropine (1 µM) did not reduce the effect of serotonin, although it was effective in preventing the increase in particle transport speed mediated by muscarine (100 µM). Immunohistochemistry demonstrated serotonin in mast cells pointing towards mast cells and platelets as possible endogenous sources of serotonin.

Conclusions/Significance

These results indicate that serotonin is a likely endogenous mediator that can increase cilia-driven transport independent from acetylcholine during activation of mast cells and platelets.

Role of aminergic (serotonin and dopamine) systems in the embryogenesis and different embryonic behaviors of the pond snail, Lymnaea stagnalis

A detailed biochemical and pharmacological analysis of the dopaminergic (DAergic) and serotonergic (5-HTergic) systems was performed during the embryogenesis of Lymnaea stagnalis, to monitor their role in development and different behaviors. The dopamine (DA) level and the synthesizing decarboxylase enzyme activity showed a continuous increase, whereas the serotonin (5-HT) concentration remained low until late postmetamorphic development, when they all showed a rapid and significant increase. Application of monoamine precursors increased, whereas enzyme inhibitors and neurotoxins reduced monoamine levels; all treatments resulting in a prolongation of embryogenesis. Following, p-chlorphenylalanine (pCPA) and 3-hydroxybenzylhydrazine (Nsd-1015) treatments, no 5-HT immunoreactivity could be detected in the embryonic nervous system. These findings suggest that changes of monoamine levels in either (negative or positive) direction cause slowing of embryogenesis. Embryonic rotation and radula protrusion rate was enhanced following both serotonin and dopamine application, whereas frequency of gliding was increased by serotonin treatment. These results clearly indicate the involvement of 5-HT and DA in the regulation of a broad range of embryonic behaviors. Pharmacological characterization of a 5-HT receptor associated with the L. stagnalis embryonic behaviors studied revealed that a mammalian 5-HT(1)-like receptor type is involved in the 5-HTergic regulation of locomotion activity.

Serotonergic, sensory modifications in the apical ganglion during development to metamorphic competence in larvae of the dendronotid nudibranchs Melibe leonina and Tritonia diomedea

The following investigation examines changes in the distance between the right and left dendritic termini arising from the serotonergic sensory neurons found in the apical ganglion of the larval dendronotid nudibranchs, Melibe leonina and Tritonia diomedea. A significant increase in separation, that is different in extent, occurs in both species as they grow from hatching to metamorphic competence. Competent M. leonina larvae exhibit a separation that is about 4.5 times that at hatching, whereas competent larvae of T. diomedea show an increase that is only 1.6 times that at hatching. The increase in separation of the lateral, serotonergic, dendritic termini (particularly in M. leonina) may allow the larva to more effectively assess left versus right differences in an as yet unknown sensory stimulus. The serotonergic innervation that arises from the apical ganglion is known to be associated with the muscles and large ciliated cells of the velum. Better right versus left discrimination of sensory stimuli experienced during the pelagic or settling larval phases may allow the larva to more precisely control swimming activities such that the likelihood of successful feeding or settlement behavior is increased.

Observations of serotonin and FMRFamide-like immunoreactivity in palp sensory structures and the anterior nervous system of spionid polychaetes

Evidence suggests that ciliated sensory structures on the feeding palps of spionid polychaetes may function as chemoreceptors to modulate deposit-feeding activity. To investigate the probable sensory nature of these ciliated cells, we used immunohistochemistry, epi-fluorescence, and confocal laser scanning microscopy to label and image sensory cells, nerves, and their organization relative to the anterior central nervous system in several spionid polychaete species. Antibodies directed against acetylated alphatubulin were used to label the nervous system and detail the innervation of palp sensory cells in all species. In addition, the distribution of serotonin (5-HT) and FMRFamide-like immunoreactivity was compared in the spionid polychaetes Dipolydora quadrilobata and Pygospio elegans. The distribution of serotonin immunoreactivity was also examined in the palps of Polydora cornuta and Streblospio benedicti. Serotonin immunoreactivity was concentrated in cells underlying the food groove of the palps, in the palp nerves, and in the cerebral ganglion. FMRFamide-like immunoreactivity was associated with the cerebral ganglia, nuchal organs and palp nerves, and also with the perikarya of ciliated sensory cells on the palps.

Identification, molecular structure and expression of two cloned serotonin receptors from the pond snail, Helisoma trivolvis

Helisoma trivolvis has served as a model system to study the functions of serotonin (5-HT) from cellular, developmental, physiological and behavioural perspectives. To further explore the serotonin system at the molecular level, and to provide experimental knockout tools for future studies, in this study we identified serotonin receptor genes from the H. trivolvis genome, and characterized the molecular structure and expression profile of the serotonin receptor gene products. Degenerate oligonucleotide primers, based on conserved regions of the Lymnaea stagnalis 5-HT(1Lym) receptor, were used to amplify G protein-coupled biogenic amine receptor sequences from H. trivolvis genomic cDNA, resulting in the cloning of two putative serotonin receptors. The deduced gene products both appear to be G protein-coupled serotonin receptors, with well-conserved structure in the functional domains and high variability in the vestibule entrance of the receptor protein. Phylogenetic analysis placed these receptors in the 5-HT(1) and 5-HT(7) families of serotonin receptors. They are thus named the 5-HT(1Hel) and 5-HT(7Hel) receptors, respectively. In situ hybridization and immunofluorescence studies revealed that these genes and gene products are expressed most heavily in the ciliated pedal and mantle epithelia of H. trivolvis embryos. In adults, widespread expression occurred in all ganglia and connectives of the central nervous system. Expression of both receptor proteins was localized exclusively to neurites when examined in situ. In contrast, when isolated neurons were grown in culture, 5-HT(1Hel) and 5-HT(7Hel) immunoreactivity were located primarily in the cell body. This is the first study to reveal a 5-HT(7) receptor in a molluscan species.

Integrative biology of an embryonic respiratory behaviour in pond snails:the `embryo stir-bar hypothesis'

Embryos of freshwater snails undergo direct development from single cell to juvenile inside egg masses that are deposited on vegetation and other substratum in pond, lake and stream habitats. Helisoma trivolvis, a member of the Planorbidae family of basommatophoran snails, has served as a model for studying the developmental and physiological roles for neurotransmitters during embryogenesis. Early studies revealed that H. trivolvis embryos from stage E15 to E30, the period between gastrulation and the trochophore–juvenile transition, display a cilia-driven behaviour consisting of slow basal rotation and transient periods of rapid rotation. The discovery of a bilateral pair of early serotonergic neurons,named ENC1, which project an apical process to the embryo surface and basal neurites to ciliated cells, prompted the hypothesis that each ENC1 is a dual-function sensory and motor neuron mediating a physiological embryonic response. This article reviews our past and present studies and addresses questions concerning this hypothesis, including the following. (1) What environmental signal regulates ENC1 activity and rotational behaviour? (2)Does ENC1 function as both a primary sensory and motor neuron underlying the rotational behaviour? (3) What are the sensory transduction mechanisms? (4)How does ENC1 regulate ciliary beating? (5) Do other basommatophoran species have similar neural–ciliary pathways and behavioural responses? (6) How is the behaviour manifest in the dynamic natural environment? In this review,we introduce the `embryo stir-bar hypothesis', which proposes that embryonic rotation is a hypoxia-sensitive respiratory behaviour responsible for mixing the egg capsule fluid, thereby enhancing delivery of environmental oxygen to the embryo.

Dopamine modulation of Ca(2+) dependent Cl(-) current regulates ciliary beat frequency controlling locomotion in Tritonia diomedea

The physiological mechanisms controlling ciliary beating remain largely unknown. Evidence exists supporting both hormonal control of ciliary beating and control via direct innervation. In the present study we investigated nervous control of cilia based locomotion in the nudibranch mollusc, Tritonia diomedea. Ciliated pedal epithelial (CPE) cells acting as locomotory effectors may be electrically excitable. To explore this possibility we characterized the cells' electrical properties, and found that CPE cells have large voltage dependent whole cell currents with two components. First, there is a fast activating outward Cl(-) current that is both voltage and Ca(2+) influx dependent (I(Cl(Ca))). I(Cl(Ca)) is sensitive to DIDS and 9-AC, and resembles currents of Ca(2+)-activated Cl(-) channels (CaCC). Ca(2+) dependence also suggests the presence of voltage-gated Ca(2+) channels; however, we were unable to detect these currents. The second current, a voltage dependent proton current (I(H)), activates very slowly and is sensitive to both Zn(2+) and changes in pH. In addition we identify a new cilio-excitatory substance in Tritonia, viz., dopamine. Dopamine, in the 10 mumol l(-1)-1 mmol l(-1) range, significantly increases ciliary beat frequency (CBF). We also found dopamine and Tritonia Pedal Peptide (TPep-NLS) selectively suppress I(Cl(Ca)) in CPE cells, demonstrating a link between CBF excitation and I(Cl(Ca)). It appears that dopamine and TPep-NLS inhibit I(Cl(Ca)) not through changing [Ca(2+)](in), but directly by an unknown mechanism. Coupling of I(Cl(Ca)) and CBF is further supported by our finding that DIDS and zero [Cl(-)](out) both increase CBF, mimicking dopamine and TPep-NLS excitation. These results suggest that dopamine and TPep-NLS act to inhibit I(Cl(Ca)), initiating and prolonging Ca(2+) influx, and activating CBF excitation.

Nervous control of ciliary beating by Cl-, Ca2+ and calmodulin inTritonia diomedea

In vertebrates, motile cilia line airways, oviducts and ventricles. Invertebrate cilia often control feeding, swimming and crawling, or gliding. Yet control and coordination of ciliary beating remains poorly understood. Evidence from the nudibranch mollusc, Tritonia diomedea, suggests that locomotory ciliated epithelial cells may be under direct electrical control. Here we report that depolarization of ciliated pedal epithelial (CPE)cells increases ciliary beating frequency (CBF), and elicits CBF increases similar to those caused by dopamine and the neuropeptide, TPep-NLS. Further,four CBF stimulants (zero external Cl-, depolarization, dopamine and TPep-NLS) depend on a common mode of action, viz. Ca2+influx, possibly through voltage-gated Ca2+ channels, and can be blocked by nifedipine. Ca2+ influx alone, however, does not provide all the internal Ca2+ necessary for CBF change. Ryanodine receptor(RyR) channel-gated internal stores are also necessary for CBF excitation. Caffeine can stimulate CBF and is sensitive to the presence of the RyR blocker dantrolene. Dantrolene also reduces CBF excitation induced by dopamine and TPep-NLS. Finally, W-7 and calmidazolium both block CBF excitation by caffeine and dopamine, and W-7 is effective at blocking TPep-NLS excitation. The effects of calmidazolium and W-7 suggest a role for Ca2+-calmodulin in regulating CBF, either directly or via Ca2+-calmodulin dependent kinases or phosphodiesterases. From these results we hypothesize dopamine and TPep-NLS induce depolarization-driven Ca2+ influx and Ca2+ release from internal stores that activates Ca2+-calmodulin, thereby increasing CBF.

Roles of Ca2+and protein kinase C in the excitatory response to serotonin in embryonic molluscan ciliary cells

We examined the roles of Ca2+and protein kinase C (PKC) in the cilio-excitatory response to serotonin in pedal ciliary cells from Helisoma trivolvis embryos. Serotonin (5-hydroxytryptamine; 5-HT; 100 µmol/L) induced an increase in ciliary beat frequency (CBF) was abolished by microinjected BAPTA (50 mmol/L), but was only partially inhibited by the phospholipase C inhibitor U-73122 (10 µmol/L). The diacylglycerol analogs 1-oleoyl-2-acetyl-sn-glycerol (100 µmol/L) and 1,2-dioctanoyl-sn-glycerol (100 µmol/L) caused increases in [Ca2+]ithat were smaller than those induced by serotonin. In the absence of extracellular Ca2+, 1,2-dioctanoyl-sn-glycerol (100 µmol/L) failed to elicit an increase in both CBF and [Ca2+]i. In contrast, the serotonin-induced increase in CBF persisted in the absence of extracellular Ca2+, although the increase in [Ca2+]iwas abolished. PKC inhibitors bisindolylmaleimide (10 and 100 nmol/L) and calphostin C (10 nmol/L) partially inhibited the serotonin-induced increase in CBF, but didn’t affect the serotonin-induced change in [Ca2+]i. These findings suggest that an intracellular store-dependent increase in [Ca2+]imediates the cilio-excitatory response to serotonin. Furthermore, although PKC is able to cause an increase in [Ca2+]ithrough calcium influx, it contributes to the cilio-excitatory response to 5-HT through a different mechanism.

Apical sensory neurones mediate developmental retardation induced by conspecific environmental stimuli in freshwater pulmonate snails

Freshwater pond snails Helisoma trivolvis and Lymnaea stagnalis undergo larval development and metamorphosis inside egg capsules. We report that their development is permanently under slight tonic inhibitory influence of the anterior sensory monoaminergic neurones, which are the remnants of the apical sensory organ. Conspecific juvenile snails, when reared under conditions of starvation and crowding, release chemical signals that are detected by these neurones in encapsulated larvae and reversibly suppress larval development, thus providing a link between environmental signals and developmental regulation. Induced retardation starts from the trochophore stage and results in up to twofold prolongation of the larval lifespan. Upon stimulation with the signal, the neurones increase synthesis and release of monoamines [serotonin (5-HT) in Helisoma and dopamine in Lymnaea] that inhibit larval development acting via ergometrine-sensitive internal receptors. Thus, the novel regulatory mechanism in larval development of molluscs is suggested and compared with the phenomenon of dauer larvae formation in the nematode Caenorhabditis elegans.

Serotonergic and dopaminergic influence of the duration of embryogenesis and intracapsular locomotion of Lymnaea stagnalis L

The role of the dopaminergic and serotonergic system was studied during the embryonic development of the pond snail Lymnaea stagnalis, with special attention to the effect of dopamine and serotonin as well as their agonists and antagonists on the rotation of the veliger larvae, and to the effect of precursors and inhibitors of the synthetizing enzymes on the duration of the embryonic life. Serotonin, D-lysergic acid diethylamide and N,N-dimethyltryptamine increased at a concentration of 1 microM the rotation by 50%, 90% and 87% respectively, and among them D-Lysergic acid diethylamide was found to be the most potent agonist. Other serotonergic agonists and antagonists enhanced the frequency of the rotation (from 165% to 355%) at higher threshold concentrations in the following rank order: methysergid > tryptamine > 2,5-dimethoxy-4-iodoamphetamine > 5-carboxyamidotryptamine > bromo-lysergic acid diethylamide > 7-methyltryptamine. Application of 1-(2-methoxyphenyl) piperazine decreased the rotation by 76%. The reuptake inhibitor desipramine completely blocked the rotation and killed the embryos. Dopaminergic agonists accelerated the rotation by 62% to 233%, and their effect was ranged as follows: dopamine > apomorphine > m-tyramine approximately equal to p-tyramine. Chlorpromazine at 100 microM concentration killed the embryos. At a concentration of 100 microg/ml, tyrosine, the precursor of DA, slowed down the embryonic development by increasing the duration of the embryonic life from 8 to 10 days. Decarboxylase inhibitors, alpha-methyl-3,4-dihydroxyphenyl-alanine (25 microg/ml) and m-hydroxybenzylhydrazin (5 microg/ml), killed 50% of the embryos, meanwhile the rest hatched ten days later, compared to the control animals. The development was partially blocked by the serotonin precusor L-tryptophane (50 microg/ml). Trytophan hydroxylase blocker, p-chlorphenylalanine (50 microg/ml) resulted in a distortion of the body pattern of the embryos, and prevented the hatching of most (95%) of the animals.

Coordinated development of identified serotonergic neurons and their target ciliary cells in Helisoma trivolvis embryos

Embryonic neuron C1s (ENC1s) are bilateral serotonergic neurons that function as cilioexcitatory motor neurons in embryonic development of the pond snail, Helisoma trivolvis. Recent experiments demonstrated that these neurons stimulate cilia-driven embryo rotation in response to hypoxia. In the present study, a comprehensive anatomic analysis of these cells and their target ciliary structures was done to address the following questions: (1) Does ENC1 have a morphology consistent with an oxygen-sensitive sensory cell; (2) Is the development of ENC1's neurite outgrowth pathway coordinated with the development of its target effectors, the pedal and dorsolateral ciliary bands; and (3) What is the anatomic basis of ENC1-ciliary communication? By using an array of microscopic techniques on live and serotonin-immunostained embryos, we found that each ENC1 possessed an apical dendrite that was capped with an integral dendritic knob penetrating the embryo surface. The dendritic knobs contained both microvilli and nonmotile cilia that suggested a sensory transduction role. Each ENC1 also possessed a descending projection, whose development was characterized by the rapid formation of the primary neurite pathway between stages E13 and E15, with the primary neurite of the right ENC1 developing in advance of its contralateral homologue. Secondary neurite branches formed between stages E15 and E30 in a spatiotemporal pattern that closely matched the development of the dorsolateral and pedal bands of cilia. Both dorsolateral and pedal ciliated cells formed basal processes that contacted ENC1 neurites. Finally, gap junction profiles were observed at neurite-neurite, neurite-ciliary cell, and ciliary cell-ciliary cell apposition sites, whereas putative chemical synaptic profiles were observed at neurite-neurite and neurite-ciliary cell apposition sites.

Regulation of early embryonic behavior by nitric oxide in the pond snail Helisoma trivolvis

Helisoma trivolvis embryos display a cilia-driven rotational behavior that is regulated by a pair of serotonergic neurons named ENC1s. As these cilio-excitatory motor neurons contain an apical dendrite ending in a chemosensory dendritic knob at the embryonic surface, they probably function as sensorimotor neurons. Given that nitric oxide (NO) is often associated with sensory neurons in invertebrates, and has also been implicated in the control of ciliary activity, we examined the expression of NO synthase (NOS) activity and possible function of NO in regulating the rotational behavior in H. trivolvis embryos. NADPH diaphorase histochemistry on stage E25-E30 embryos revealed NOS expression in the protonephridia, buccal mass,dorsolateral ciliary cells and the sensory dendritic knobs of ENC1. At stages E35-40, the pedal ciliary cells and ENC1's soma, apical dendrite and proximal descending axon were also stained. In stage E25 embryos, optimal doses of the NO donors SNAP and SNP increased the rate of embryonic rotation by twofold, in contrast to the fourfold increase caused by 100 μmol l-1serotonin. The NOS inhibitors L-NAME (10 mmol l-1) and 7-NI (100μmol l-1) decreased the rotation rate by approximately 50%,whereas co-addition of L-NAME and SNAP caused a twofold increase. In an analysis of the surge and inter-surge subcomponents of the rotational behavior, the NO donors increased the inter-surge rotation rate and the surge amplitude. In contrast, the NO inhibitors decreased the inter-surge rotation rate and the frequency of surges. These data suggest that the embryonic rotational behavior depends in part on the constitutive excitatory actions of NO on ENC1 and ciliary cells.

Serotonergic sensory-motor neurons mediate a behavioral response to hypoxia in pond snail embryos

Oxygen (O(2)) is one of the most important environmental factors that affects both physiological processes and development of aerobic animals, yet little is known about the neural mechanism of O(2) sensing and adaptive responses to low O(2) (hypoxia) during development. In the pond snail, Helisoma trivolvis, the first embryonic neurons (ENC1s) to develop are a pair of serotonergic sensory-motor cells that regulate a cilia-driven rotational behavior. Here, we report that the ENC1-ciliary cell circuit mediates an adaptive behavioral response to hypoxia. Exposure of egg masses to hypoxia elicited a dose-dependent and reversible acceleration of embryonic rotation that mixed capsular fluid, thereby facilitating O(2) diffusion to the embryo. The O(2) partial pressures (Po(2)) for threshold, half-maximal, and maximal rotational response were 60, 28, and 13 mm Hg, respectively. During hypoxia, embryos relocated to the periphery of the egg masses where higher Po(2) levels occurred. Furthermore, intermittent hypoxia treatments induced a sensitization of the rotational response. In isolated ciliary cells, ciliary beating was unaffected by hypoxia, suggesting that in the embryo, O(2) sensing occurs upstream of the motile cilia. The rotational response of embryos to hypoxia was attenuated by application of the serotonin receptor antagonist, mianserin, correlated to the development of ENC1-ciliary cell circuit, and abolished by laser-ablation of ENC1s. Together, these data suggest that ENC1s are unique oxygen sensors that may provide a good single cell model for the examination of mechanistic, developmental, and evolutionary aspects of O(2) sensing.

Serotonin localization in Phallusia mammillata larvae and effects of 5-HT antagonists during larval development

The neurotransmitter 5-hydroxytryptamine (5-HT, serotonin) plays an important role in a wide range of non-neural processes. Using immunofluorescence with an antiserotonin antibody, 5-HT was localized in the brain and in some neurons of the larval tail of Phallusia mammillata. To test the effect of 5-HT on development, we treated embryos with two different 5-HT receptor subtype antagonists. Treatment at the gastrula stage with 10 microM ondansetron, an antagonist of the 5-HT(3) receptor, induced anterior truncation and a short tail. At 10 microM, ritanserin, a 5-HT(2B) receptor antagonist, induced larval phenotypes characterized by a roundish trunk region with flat papillae. The juveniles developed from these larvae had an abnormal cardiocirculatory system: their heart contractions were ineffective and their blood cells accumulated in the heart cavity. We conclude that an appropriate level of 5-HT is necessary for correct development and morphogenesis. Moreover, a different key role for multiple receptors in modulating the morphogenetic effects of 5-HT is suggested.

Structure and function of invertebrate 5-HT receptors: a review

Over the last decade, knowledge of invertebrate serotonin receptors has expanded greatly. The first 5-HT receptor from Drosophila was cloned 10 years ago, and subsequently, 11 additional receptor genes have been cloned from Drosophila, molluscs (Lymnaea and Aplysia) and nematodes (Caenorhabditis and Ascaris). Information has also accumulated from physiological and biochemical studies that have used vertebrate serotonergic ligands to characterize endogenous invertebrate receptors. Although the endogenous receptors are often classified according to mammalian-based categories, in many cases the pharmacological properties of vertebrate and invertebrate receptors differ significantly and the actual identity of the latter is questionable. By providing information on the gene structure and amino acid sequence, molecular cloning studies offer a more definitive way to identify and classify invertebrate 5-HT receptors. This review summarizes information on the pharmacological and transductional properties of cloned invertebrate 5-HT receptors, and considers recent studies of endogenous receptors in the light of this new data.

Laser ablation reveals regulation of ciliary activity by serotonergic neurons in molluscan embryos

Early in embryonic development, the pond snail Helisoma trivolvis exhibits a rotational behavior that is generated by beating of cilia in the dorsolateral and pedal bands. Although previous anatomical and pharmacological studies provided indirect evidence that a pair of serotonergic neurons, Embryonic Neurons C1 (ENC1s), is involved in regulating embryonic rotation, direct evidence linking ENC1 to ciliary function is still lacking. In the present study, we used laser microbeams to perturb ENC1 in vivo while monitoring ciliary activity in identified ciliary bands. A laser treatment protocol to specifically ablate ENC1 without damaging the surrounding cells was established. Unilateral laser treatment of ENC1 caused transient increases in the activity of the pedal and ipsidorsolateral cilia, lasting 30-50 min. In contrast, activity of cilia that were not anatomically associated with ENC1 was unaffected by laser treatment. Mianserin, an effective serotonin antagonist in Helisoma ciliated cells, decreased the overall CBF of pedal and dorsolateral cilia by reducing the occurrence of spontaneous CBF surges in these cilia. Finally, the cilioexcitatory action of ENC1 laser treatment was mimicked by serotonin and reduced in the presence of mianserin. These results suggest that laser treatment provokes a release of serotonin from ENC1, resulting in a prolonged elevation of activity in the target ciliary cells. We conclude that, in addition to their previously established role in regulating neurodevelopment, ENC1s also function as serotonergic motor neurons to regulate ciliary activity, and therefore the rotational behavior of early embryos.

Comparative study of the apical ganglion in planktotrophic caenogastropod larvae: ultrastructure and immunoreactivity to serotonin

Previous research suggests that a major role of the apical ganglion (also called the apical or cephalic sensory organ) in gastropod larvae is detection and integration of sensory information and relay of motor signals to effectors in the velum. However, the relative impact of ancestry versus velum size and life history on characteristics of the apical ganglion is unresolved. We address this issue by contributing data on the apical ganglion and overlying epidermis in planktotrophic larvae of four caenogastropod species (Euspira [Polinices] lewisii, Lacuna vincta, Trichotropis cancellata, and Amphissa versicolor) derived from light microscopy, scanning and transmission electron microscopy, and immunohistochemical localization of serotonin-like antigenicity. Ultrastructure of the apical ganglion is similar in these caenogastropods, and the basic plan corresponds to previous descriptions of the apical ganglion in planktotrophic opisthobranch larvae (subgroup of Heterobranchia). The only identified structural feature that is unique to all these caenogastropods, relative to opisthobranchs, is modified ciliary axonemes for the ampullary cells, a distinctive type of sensory neuron. Like opisthobranch larvae, caenogastropod larvae have serotonin-immunoreactive neurons within the apical ganglion; the number ranges from three to six, but a lateral pair of serotonergic, nonsensory neurons is common to all species. The pattern of serotonergic neurons in E. lewisii, which develops large, subdivided velar lobes, is the same as that of opisthobranch larvae, which have a relatively small, unelaborated velum. These and other data suggest that common ancestry is a major determinant of overall structural design for the apical ganglion in caenogastropods and heterobranchs, which are sister groups within the Gastropoda. Velum size and life history strategy may account for some, but not all, cases of interspecific differences in the serotonergic component.

Cilia-driven rotational behavior in gastropod (Physa elliptica) embryos induced by serotonin and putative serotonin reuptake inhibitors (SSRIs)

We characterized the serotonin (5-hydroxytryptamine; 5-HT) receptor mediating cilia-driven rotational movement in embryos of the freshwater gastropod Physa elliptica. In addition, putative serotonin reuptake inhibitors (SSRIs), previously shown to induce other 5-HT-mediated processes in molluscs, were tested for their ability to induce rotation. As in previous studies with other freshwater gastropods, 5-HT induced a significant dose-dependent increase in rotation from 10(-6) to 10(-4) M. The 5-HT(1A) agonist 8-OH-DPAT produced a similar dose-dependent increase in rotation. However, the 5-HT(2) agonist alpha-CH3-serotonin evoked a significant rotational response only at the highest concentration of 10(-4) M. The 5-HT(2) receptor antagonist mianserin not only blocked 5-HT-induced rotation, it reduced rotation rate below that of baseline. However, two other antagonists, cyproheptadine (5-HT(2)) and propranolol (5-HT(1)), caused similar responses that consisted of an initial rotational surge followed by reduced rotation. Thus, these drugs appear to act as partial agonists. The putative SSRI fluvoxamine exhibited a significant dose-dependent increase in positive rotation as that seen with 5-HT. The SSRIs paroxetine and fluoxetine both caused an increase in rotation at 10(-6) and 10(-5) M but reduced rotation rate below that of baseline at 10(-4) M. These results agree with other studies on aquatic molluscs, suggest a 5-HT receptor with a mixed 5-HT(1)/5-HT(2) pharmacological profile and add to a now growing body of literature on the pharmacology of molluscan 5-HT receptors. In addition, all the tested putative SSRIs induced cilia-driven rotation in Physa embryos, indicating either the presence of 5-HT reuptake transporters or that these compounds act as 5-HT receptor ligands. J. Exp. Zool. 286:414-421, 2000.

Roles of dopamine and serotonin in larval attachment of the barnacle, Balanus amphitrite

In order to clarify the roles of neurotransmitters including serotonin and dopamine in larval settlement (attachment) and metamorphosis of the barnacle Balanus amphitrite, the effects of lisuride, which acts as both a serotonin agonist/antagonist and a dopamine agonist, were examined. Lisuride did not induce larval attachment and metamorphosis; however, it promoted only larval behavior of searching for attachment sites without actual attachment to substrata which lasted for 5 to 6 days in a dose-dependent manner. Further evidence was obtained with a range of agonists/antagonists; serotonin agonists promoted the attachment, while serotonin antagonists inhibited it. Similarly, dopamine agonists inhibited the attachment. Furthermore, mixtures of serotonin and dopamine showed similar effects to those of lisuride. These results suggested that the promotion effect on larval searching behavior was derived from a combination of activities of serotonin and dopamine. Moreover, both serotonin and dopamine were detected in cyprids by HPLC. Thus, larval attachment process is regulated by both serotonin and dopamine neurons in this species. J. Exp. Zool. 284:746-758, 1999. Copyright 1999 Wiley-Liss, Inc.

Serotonin induces four pharmacologically separable contractile responses in the pharynx of the leech Hirudo medicinalis

Stimulation of the serotoninergic innervation of the leech pharynx or application of serotonin to the isolated pharynx induced four distinct types of contractile activity: an increase in basal tonus, large phasic contractions of 10-15 s in duration, smaller phasic contractions occurring at approximately 1 Hz, and a relaxation after washout of serotonin. Application to the isolated pharynx of the selective serotonin agonists (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin, N-(3-trifluoromethylphenyl)piperazine, 1-(m-chlorophenyl)-piperazine, (+/-)-2,5-dimethoxy-4-iodoamphetamine, 2-methyl-5-hydroxytrypamine, alpha-methyl-5-hydroxytryptamine, and 5-methoxytryptamine induced distinct types of pharyngeal contractile activity. The results of this study suggest that the leech pharynx possesses more than one type of serotonin receptor.

Involvement of protein kinase C in 5-HT-stimulated ciliary activity in Helisoma trivolvis embryos

1. During development, embryos of the pulmonate gastropod, Helisoma trivolvis, undergo a rotation behaviour due to the co-ordinated beating of three bands of ciliated epithelial cells. This behaviour is in part mediated by the neurotransmitter serotonin (5-HT) released from a pair of identified embryonic neurons. Using time-lapse videomicroscopy to measure ciliary beat frequency (CBF) in response to pharmacological manipulations, we determined whether protein kinase C (PKC) is involved in mediating 5-HT-stimulated ciliary beating. 2. Diacylglycerol (DAG) analogues sn-1,2-dioctanoyl glycerol (DiC8; 100 microM) and 1-oleoyl-2-acetyl-sn-glycerol (OAG; 100 microM), partially mimicked the 5-HT-induced increase in CBF. In contrast, application of OAG in the absence of extracellular Ca2+ did not result in an increase in CBF. 3. 5-HT-stimulated CBF was effectively blocked by PKC inhibitors bisindolylmaleimide (10 and 100 nM) and calphostin C (10 nM). In addition, bisindolylmaleimide (100 nM) inhibited DiC8-induced increases in CBF. At a higher concentration (200 nM), bisindolylmaleimide did not significantly reduce 5-HT-stimulated cilio-excitation. 4. Two different phorbol esters, phorbol 12-myristate 13-acetate (TPA; 0.1, 10 or 1000 nM) and phorbol 12beta, 13alpha-dibenzoate (PDBn; 10 microM) did not alter basal CBF. TPA (1 microM) did not alter 5-HT-stimulated CBF. Likewise, the synthetic form of phosphatidylserine, N-(6-phenylhexyl)-5-chloro-1-naphthalenesulphonamide (SC-9; 10 microM), did not increase CBF, whereas a strong increase in CBF was observed upon exposure to 5-HT. 5. The results suggest that a DAG-dependent, phorbol ester-insensitive isoform of PKC mediates 5-HT-stimulated CBF in ciliated epithelial cells from embryos of Helisoma trivolvis.

Changes in the physiological roles of neurotransmitters during individual development

The classical neurotransmitters (acetylcholine and biogenic monoamines) are multifunctional substances involved in intra- and intercellular signaling at all stages of ontogenesis in multicellular animals. A cyclical scheme is proposed to describe age-related changes in neurotransmitter functions at different stages of development from oocyte maturation to neuron formation. This may reflect not only the temporospatial organization of neurotransmitter processes, but also the origin of the functions of acetylcholine and biogenic monoamines from the protosynapses of the cleaved embryo to neuronal synapses.

Regulation of calcium influx and phospholipase C activity by indoleamines in dinoflagellate Crypthecodinium cohnii

Exogenous indoleamines such as melatonin and 5-methoxytryptamine have been shown to induce cyst formation (encystment) in many species of dinoflagellate. Induction of inositol phosphates formation by indoleamine has previously been demonstrated in Crypthecodinium cohnii. In addition, depletion of extracellular Ca2+ blocks the indoleamine-induced encystment. In the present study, 12 indoleamines (including melatonin and related compounds) were examined for their abilities to induce Ca2+ influx, inositol phosphates formation, and encystment in C. cohnii. The results showed that melatonin, 5-methoxytryptamine, and the peptide toxin mastoparan stimulated 45Ca2+ influxes in dose- and time-dependent manners. The EC50 values of 5-methoxytrypramine and mastoparan to stimulate 45Ca2+ uptake were 2 mM and 35 microM, respectively. The 5-methoxytryptamine- and mastoparan-induced 45Ca2+ influx were partially attenuated by the calcium channel blockers, verapamil and ruthenium red. A series of indoleamines were examined for their structure-activity relationship on the induction of encystment and formation of inositol phosphates. Melatonin-induced inositol phosphates formation was completely blocked by U73122, indicating the possible involvement of phospholipase C. Taken together, we conclude that indoleamines may induce encystment of the dinoflagellate C. cohnii via parallel activation of phospholipase C and Ca2+ influx signaling pathways. However, activation of phospholipase C and Ca2+ influx are not always necessary or sufficient for inducing encystment. Also, these data provided the first direct evidence of a Ca2+ influx regulating mechanism in dinoflagellate C. cohnii.

Early development of an identified serotonergic neuron in Helisoma trivolvis embryos: serotonin expression, de-expression, and uptake

In early-stage embryos of Helisoma trivolvis, a bilateral pair of identified neurons (ENC1) express serotonin and project primary descending neurites that ramify in the pedal region of the embryo prior to the formation of central ganglia. Pharmacological studies suggest that serotonin released from ENC1 acts in an autoregulatory pathway to regulate its own neurite branching and in a paracrine or synaptic pathway to regulate the activity of pedal ciliary cells. In the present study, several key features of early ENC1 development were characterized as a necessary foundation for further experimental studies on the mechanisms underlying ENC1 development and its physiological role during embryogenesis. ENC1 morphology was determined by confocal microscopy of serotonin-immunostained embryos and by differential-interference contrast (DIC) microscopy of live embryos. The soma was located at an anteriolateral superficial position and contained several distinguishing features, including a large spherical nucleus with prominent central nucleolus, large granules in the apical cytoplasm, a broad apical dendrite ending in a sensory-like structure at the embryonic surface, and a ventral neurite. ENC1 first expressed serotonin immunoreactivity around stage E13, followed immediately by the appearance of an immunoreactive neurite (stage E14). Both the intensity of immunoreactivity and primary neurite length were consistently greater in the right ENC1 at early stages. Serotonin uptake, as indicated by 5,7-dihydroxytryptamine-induced fluorescence, first occurred between stages E18 and E25. At later stages of embryogenesis (after stage E65), serotonin immunoreactivity disappeared, whereas serotonin uptake and normal cell morphology were retained.

Pharmacology of the serotonergic inhibition of steroid-induced reinitiation of oocyte meiosis in the teleost Fundulus heteroclitus

Serotonin (5-HT) was found to inhibit steroid (17 alpha,20 beta-dihydroxy-4-pregnen-3-one; 17,20 beta P)-induced resumption of oocyte meiosis (oocyte maturation) in vitro in the teleost Fundulus heteroclitus. Serotonin inhibited both follicle-enclosed and denuded oocytes, which indicates the presence of oocyte-associated 5-HT sensitive sites. The response of oocytes to 5-HT was characterized pharmacologically, i.e., the capacity of serotonergic agonists and antagonists to mimic or block the 5-HT inhibition of the steroid-induced oocyte maturation was assessed by the changes in the percentage of oocyte germinal vesicle breakdown (GVBD). Dose-response curves for each compound were drawn and compared. The rank order of potency among the agonists was: 5-HT > 5-methoxytryptamine > tryptamine = 5,6-diHT = 5-carboxidotryptamine > 5,7-diHT = 5-methoxy-dimethyltryptamine > alpha-methyl-5HT > 2-methyl-5HT. Incubation of ovarian follicles with high doses of some antagonists (mianserin and metergoline) induced oocyte GVBD, although this effect was associated with high levels of oocyte atresia during GVBD or shortly after maturation. Consequently, doses of the antagonist too low to induce GVBD were tested for their ability to block the 5-HT inhibitory action; the rank order of potency was: MDL-72222 = metoclopramide > metergoline > propanolol > ketanserin. Dopamine, acetylcholine, epinephrine, and norepinephrine could also inhibit 17,20 beta P-induced GVBD, although at doses much higher than those of 5-HT; melatonin and histamine had no effect on oocyte maturation. These results suggest that specific receptors mediate the inhibitory action of 5-HT on the steroid-triggered meiosis resumption. The pharmacological profile of these 5-HT receptors is different from those of any known mammalian 5-HT receptor, although they showed some similarities to the 5-HT1A, 5-HT2, and 5-HT3 receptors, as well as to 5-HT receptors on oocytes of some bivalve molluscs.