Serotonin-immunoreactivity in peripheral tissues of the opisthobranch molluscs Pleurobranchaea californica and Tritonia diomedea

Division of labor for defensive retaliation and preemption by the peripheral and central nervous systems in the nudibranch Berghia

Relatively little is known about how peripheral nervous systems (PNSs) contribute to the patterning of behavior in which their role transcends the simple execution of central motor commands or mediation of reflexes. We sought to draw inferences to this end in the aeolid nudibranch Berghia stephanieae, which generates a rapid, dramatic defense behavior, "bristling." This behavior involves the coordinated movement of cerata, dozens of venomous appendages emerging from the animal's mantle. Our investigations revealed that bristling constitutes a stereotyped but non-reflexive two-stage behavior: an initial adduction of proximate cerata to sting the offending stimulus (stage 1) followed by a coordinated radial extension of remaining cerata to create a pincushion-like defensive screen around the animal (stage 2). In decerebrated specimens, stage 1 bristling was preserved, while stage 2 bristling was replaced by slower, uncoordinated ceratal movements. We conclude from these observations that, first, the animal's PNS and central nervous system (CNS) mediate stages 1 and 2 of bristling, respectively; second, the behavior propagates through the body utilizing both peripheral- and central-origin nerve networks that support different signaling kinetics; and third, the former network inhibits the latter in the body region being stimulated. These findings extend our understanding of the PNS' computational capacity and provide insight into a neuroethological scheme in which the CNS and PNS both independently and interactively pattern different aspects of non-reflexive behavior.

A peripheral subepithelial network for chemotactile processing in the predatory sea slug Pleurobranchaea californica

Many soft-bodied animals have extensive peripheral nervous systems (PNS) with significant sensory roles. One such, the sea slug Pleurobranchaea californica, uses PNS computations in its chemotactile oral veil (OV) in prey tracking, averaging olfactory stimuli across the OV to target likely source direction, or "stimulus place". This suggests a peripheral subepithelial network (SeN) interconnecting sensory sites to compute the directional average. We pursued anatomy and connectivity of previously described ciliated putative sensory cells on OV papillae. Scanning electron microscopy (SEM) confirmed paddle-shaped cilia in clusters. Anti-tubulin and phalloidin staining showed connections to branching nervelets and muscle fibers for contraction and expansion of papillae. Ciliary cell processes could not be traced into nerves, consistent with sensory transmission to CNS via secondary afferents. Anti-tyrosine hydroxylase-stained ciliated cells in clusters and revealed an at least partially dopaminergic subepithelial network interconnecting clusters near and distant, connections consistent with PNS averaging of multiple stimulated loci. Other, unidentified, SeN neurotransmitters are likely. Confirming chemotactile functions, perfusible suction electrodes recorded ciliary spiking excited by both mechanical and appetitive chemical stimuli. Stimuli induced sensory nerve spiking like that encoding stimulus place. Sensory nerve spikes and cilia cluster spikes were not identifiable as generated by the same neurons. Ciliary clusters likely drive the sensory nerve spikes via SeN, mediating appetitive and stimulus place codes to CNS. These observations may facilitate future analyses of the PNS in odor discrimination and memory, and also suggest such SeNs as potential evolutionary precursors of CNS place-coding circuitry in the segmented, skeletonized protostomes and deuterostomes.

Neural division of labor: the gastropod Berghia defends against attack using its PNS to retaliate and its CNS to erect a defensive screen

Relatively little is known about how the peripheral nervous system (PNS) contributes to the patterning of behavior, in which its role transcends the simple execution of central motor commands or mediation of reflexes. We sought to draw inferences to this end in the aeolid nudibranch Berghia stephanieae, which generates a rapid, dramatic defense behavior, "bristling." This behavior involves the coordinated movement of cerata, dozens of venomous appendages emerging from the animal's mantle. Our investigations revealed that bristling constitutes a stereotyped but non-reflexive two-stage behavior: an initial adduction of proximate cerata to sting the offending stimulus (Stage 1), followed by a coordinated radial extension of remaining cerata to create a pincushion-like defensive screen around the animal (Stage 2). In decerebrated specimens, Stage 1 bristling was preserved, while Stage 2 bristling was replaced by slower, uncoordinated, and ultimately maladaptive ceratal movements. We conclude from these observations that 1) the PNS and central nervous system (CNS) mediate Stages 1 and 2 of bristling, respectively; 2) the behavior propagates through the body utilizing both peripheral- and central-origin nerve networks that support different signaling kinetics; and 3) the former network inhibits the latter in the body region being stimulated. These findings extend our understanding of the PNS's computational capacity and provide insight into a neuroethological scheme that may generalize across cephalized animals, in which the CNS and PNS both independently and interactively pattern different aspects of non-reflexive behavior.

Neural versus alternative integrative systems: molecular insights into origins of neurotransmitters

Transmitter signalling is the universal chemical language of any nervous system, but little is known about its early evolution. Here, we summarize data about the distribution and functions of neurotransmitter systems in basal metazoans as well as outline hypotheses of their origins. We explore the scenario that neurons arose from genetically different populations of secretory cells capable of volume chemical transmission and integration of behaviours without canonical synapses. The closest representation of this primordial organization is currently found in Placozoa, disk-like animals with the simplest known cell composition but complex behaviours. We propose that injury-related signalling was the evolutionary predecessor for integrative functions of early transmitters such as nitric oxide, ATP, protons, glutamate and small peptides. By contrast, acetylcholine, dopamine, noradrenaline, octopamine, serotonin and histamine were recruited as canonical neurotransmitters relatively later in animal evolution, only in bilaterians. Ligand-gated ion channels often preceded the establishment of novel neurotransmitter systems. Moreover, lineage-specific diversification of neurotransmitter receptors occurred in parallel within Cnidaria and several bilaterian lineages, including acoels. In summary, ancestral diversification of secretory signal molecules provides unique chemical microenvironments for behaviour-driven innovations that pave the way to complex brain functions and elementary cognition. This article is part of the theme issue 'Basal cognition: multicellularity, neurons and the cognitive lens'.

ATP signaling in the integrative neural center of Aplysia californica

ATP and its ionotropic P2X receptors are components of the most ancient signaling system. However, little is known about the distribution and function of purinergic transmission in invertebrates. Here, we cloned, expressed, and pharmacologically characterized the P2X receptors in the sea slug Aplysia californica—a prominent neuroscience model. AcP2X receptors were successfully expressed in Xenopus oocytes and displayed activation by ATP with two-phased kinetics and Na⁺-dependence. Pharmacologically, they were different from other P2X receptors. The ATP analog, Bz-ATP, was a less effective agonist than ATP, and PPADS was a more potent inhibitor of the AcP2X receptors than the suramin. AcP2X were uniquely expressed within the cerebral F-cluster, the multifunctional integrative neurosecretory center. AcP2X receptors were also detected in the chemosensory structures and the early cleavage stages. Therefore, in molluscs, rapid ATP-dependent signaling can be implicated both in development and diverse homeostatic functions. Furthermore, this study illuminates novel cellular and systemic features of P2X-type ligand-gated ion channels for deciphering the evolution of neurotransmitters.

Chemical-neuroanatomical organization of peripheral sensory-efferent systems in the pond snail (Lymnaea stagnalis)

Perception and processing of chemical cues are crucial for aquatic gastropods, for proper elaboration of adaptive behavior. The pond snail, Lymnaea stagnalis, is a model species of invertebrate neurobiology, in which peripheral sensory neurons with different morphology and transmitter content have partly been described, but we have little knowledge regarding their functional morphological organization, including their possible peripheral intercellular connections and networks. Therefore the aim of our study was to characterize the sensory system of the tentacles and the lip, as primary sensory regions, and the anterior foot of Lymnaea with special attention to the transmitter content of the sensory neurons, and their relationship to extrinsic elements of the central nervous system. Numerous bipolar sensory cells were demonstrated in the epithelial layer of the peripheral organs, displaying immunoreactivity to antibodies raised against tyrosine hydroxylase, histamine, glutamate and two molluscan type oligopeptides, FMRFamide and Mytilus inhibitory peptide. A subepithelial plexus was formed by extrinsic serotonin and FMRFamide immunoreactive fibers, whereas in deeper regions axon processess of different origin with various immunoreactivities formed networks, too. HPLC-MS assay confirmed the presence of the low molecular weight signal molecules in the three examined areas. Following double-labeling immunohistochemistry, close arrangements were observed, formed by sensory neurons and extrinsic serotonergic (and FMRFamidergic) fibers at axo-dendritic, axo-somatic and axo-axonic levels. Our results suggest the involvement of a much wider repertoire of signal molecules in peripheral sensory processes of Lymnaea, which can locally be modified by central input, hence influencing directly the responses to environmental cues.

Atlas of the neuromuscular system in the Trachymedusa Aglantha digitale: Insights from the advanced hydrozoan

Cnidaria is the sister taxon to bilaterian animals, and therefore, represents a key reference lineage to understand early origins and evolution of the neural systems. The hydromedusa Aglantha digitale is arguably the best electrophysiologically studied jellyfish because of its system of giant axons and unique fast swimming/escape behaviors. Here, using a combination of scanning electron microscopy and immunohistochemistry together with phalloidin labeling, we systematically characterize both neural and muscular systems in Aglantha, summarizing and expanding further the previous knowledge on the microscopic neuroanatomy of this crucial reference species. We found that the majority, if not all (~2,500) neurons, that are labeled by FMRFamide antibody are different from those revealed by anti-α-tubulin immunostaining, making these two neuronal markers complementary to each other and, therefore, expanding the diversity of neural elements in Aglantha with two distinct neural subsystems. Our data uncovered the complex organization of neural networks forming a functional "annulus-type" central nervous system with three subsets of giant axons, dozen subtypes of neurons, muscles, and a variety of receptors fully integrated with epithelial conductive pathways supporting swimming, escape and feeding behaviors. The observed unique adaptations within the Aglantha lineage (including giant axons innervating striated muscles) strongly support an extensive and wide-spread parallel evolution of integrative and effector systems across Metazoa.

A role for dopamine in the peripheral sensory processing of a gastropod mollusc

Histological evidence points to the presence of dopamine (DA) in the cephalic sensory organs of multiple gastropod molluscs, suggesting a possible sensory role for the neurotransmitter. We investigated the sensory function of DA in the nudipleuran Pleurobranchaea californica, in which the central neural correlates of sensation and foraging behavior have been well characterized. Tyrosine hydroxylase-like immunoreactivity (THli), a signature of the dopamine synthetic pathway, was similar to that found in two other opisthobranchs and two pulmonates previously studied: 1) relatively few (<100) THli neuronal somata were observed in the central ganglia, with those observed found in locations similar to those documented in the other snails but varying in number, and 2) the vast majority of THli somata were located in the peripheral nervous system, were associated with ciliated, putative primary sensory cells, and were highly concentrated in chemotactile sensory organs, giving rise to afferent axons projecting to the central nervous system. We extended these findings by observing that applying a selective D₂/D₃ receptor antagonist to the chemo- and mechanosensory oral veil-tentacle complex of behaving animals significantly delayed feeding behavior in response to an appetitive stimulus. A D₁ blocker had no effect. Recordings of the two major cephalic sensory nerves, the tentacle and large oral veil nerves, in a deganglionated head preparation revealed a decrease of stimulus-evoked activity in the former nerve following application of the same D₂/D₃ antagonist. Broadly, our results implicate DA in sensation and engender speculation regarding the foraging-based decisions the neurotransmitter may serve in the nervous system of Pleurobranchaea and, by extension, other gastropods.

Immunohistochemical Approach to Understanding the Organization of the Olfactory System in the Cuttlefish, Sepia officinalis

Cephalopods are nontraditional but captivating models of invertebrate neurobiology, particularly in evolutionary comparisons. Cephalopod olfactory systems have striking similarities and fundamental differences with vertebrates, arthropods, and gastropods, raising questions about the ancestral origins of those systems. We describe here the organization and development of the olfactory system of the common cuttlefish, Sepia officinalis, using immunohistochemistry and in situ hybridization. FMRFamide and/or related peptides and histamine are putative neurotransmitters in olfactory sensory neurons. Other neurotransmitters, including serotonin and APGWamide within the olfactory and other brain lobes, suggest efferent control of olfactory input and/or roles in the processing of olfactory information. The distributions of neurotransmitters, along with staining patterns of phalloidin, anti-acetylated α-tubulin, and a synaptotagmin riboprobe, help to clarify the structure of the olfactory lobe. We discuss a key difference, the lack of identifiable olfactory glomeruli, in cuttlefish in comparison to other models, and suggest its implications for the evolution of olfaction.

NeuroSystematics and Periodic System of Neurons: Model vs Reference Species at Single-Cell Resolution

There is more than one way to develop neuronal complexity, and animals frequently use different molecular toolkits to achieve similar functional outcomes (=convergent evolution). Neurons are different not only because they have different functions, but also because neurons and circuits have different genealogies, and perhaps independent origins at the broadest scale from ctenophores and cnidarians to cephalopods and primates. By combining modern phylogenomics, single-neuron sequencing (scRNA-seq), machine learning, single-cell proteomics, and metabolomic across Metazoa, it is possible to reconstruct the evolutionary histories of neurons tracing them to ancestral secretory cells. Comparative data suggest that neurons, and perhaps synapses, evolved at least 2-3 times (in ctenophore, cnidarian and bilateral lineages) during ∼600 million years of animal evolution. There were also several independent events of the nervous system centralization either from a common bilateral/cnidarian ancestor without the bona fide neurons or from the urbilaterian with diffuse, nerve-net type nervous system. From the evolutionary standpoint, (i) a neuron should be viewed as a functional rather than a genetic character, and (ii) any given neural system might be chimeric and composed of different cell lineages with distinct origins and evolutionary histories. The identification of distant neural homologies or examples of convergent evolution among 34 phyla will not only allow the reconstruction of neural systems' evolution but together with single-cell "omic" approaches the proposed synthesis would lead to the "Periodic System of Neurons" with predictive power for neuronal phenotypes and plasticity. Such a phylogenetic classification framework of Neuronal Systematics (NeuroSystematics) might be a conceptual analog of the Periodic System of Chemical Elements. scRNA-seq profiling of all neurons in an entire brain or Brain-seq is now fully achievable in many nontraditional reference species across the entire animal kingdom. Arguably, marine animals are the most suitable for the proposed tasks because the world oceans represent the greatest taxonomic and body-plan diversity.

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.

The Sea Slug, Pleurobranchaea californica: A Signpost Species in the Evolution of Complex Nervous Systems and Behavior

How and why did complex brain and behavior evolve? Clues emerge from comparative studies of animals with simpler morphology, nervous system, and behavioral economics. The brains of vertebrates, arthropods, and some annelids have highly derived executive structures and function that control downstream, central pattern generators (CPGs) for locomotion, behavioral choice, and reproduction. For the vertebrates, these structures-cortex, basal ganglia, and hypothalamus-integrate topographically mapped sensory inputs with motivation and memory to transmit complex motor commands to relay stations controlling CPG outputs. Similar computations occur in the central complex and mushroom bodies of the arthropods, and in mammals these interactions structure subjective thought and socially based valuations. The simplest model systems available for comparison are opisthobranch molluscs, which have avoided selective pressure for complex bodies, brain, and behavior through potent chemical defenses. In particular, in the sea-slug Pleurobranchaea californica the functions of vertebrates' olfactory bulb and pallium are performed in the peripheral nervous system (PNS) of the chemotactile oral veil. Functions of hypothalamus and basal ganglia are combined in Pleurobranchaea's feeding motor network. The actions of basal ganglia on downstream locomotor regions and spinal CPGs are analogous to Pleurobranchaea's feeding network actions on CPGs for agonist and antagonist behaviors. The nervous systems of opisthobranch and pulmonate gastropods may conserve or reflect relations of the ancestral urbilaterian. Parallels and contrasts in neuronal circuits for action selection in Pleurobranchaea and vertebrates suggest how a basic set of decision circuitry was built upon in evolving segmentation, articulated skeletons, sociality, and highly invested reproductive strategies. They suggest (1) an origin of olfactory bulb and pallium from head-region PNS; (2) modularization of an ancestral feeding network into discrete but interacting executive modules for incentive comparison and decision (basal ganglia), and homeostatic functions (hypothalamus); (3) modification of a multifunctional premotor network for turns and locomotion, and its downstream targets for mid-brain and hind-brain motor areas and spinal CPGs; (4) condensation of a distributed serotonergic network for arousal into the raphe nuclei, with superimposed control by a peptidergic hypothalamic network mediating appetite and arousal; (5) centralization and condensation of the dopaminergic sensory afferents of the PNS, and/or the disperse dopaminergic elements of central CPGs, into the brain nuclei mediating valuation, reward, and motor arousal; and (6) the urbilaterian possessed the basic circuit relations integrating sensation, internal state, and learning for cost-benefit approach-avoidance decisions.

Development of the nervous system in Solenogastres (Mollusca) reveals putative ancestral spiralian features

Background

The Solenogastres (or Neomeniomorpha) are a taxon of aplacophoran molluscs with contentious phylogenetic placement. Since available developmental data on non-conchiferan (that is, aculiferan) molluscs mainly stem from polyplacophorans, data on aplacophorans are needed to clarify evolutionary questions concerning the morphological features of the last common ancestor (LCA) of the Aculifera and the entire Mollusca. We therefore investigated the development of the nervous system in two solenogasters, Wirenia argentea and Gymnomenia pellucida, using immunocytochemistry and electron microscopy.

Results

Nervous system formation starts simultaneously from the apical and abapical pole of the larva with the development of a few cells of the apical organ and a posterior neurogenic domain. A pair of neurite bundles grows out from both the neuropil of the apical organ and the posterior neurogenic domain. After their fusion in the region of the prototroch, which is innervated by an underlying serotonin-like immunoreactive (−LIR) plexus, the larva exhibits two longitudinal neurite bundles - the future lateral nerve cords. The apical organ in its fully developed state exhibits approximately 8 to 10 flask-shaped cells but no peripheral cells. The entire ventral nervous system, which includes a pair of longitudinal neurite bundles (the future ventral nerve cords) and a serotonin-LIR ventromedian nerve plexus, appears simultaneously and is established after the lateral nervous system. During metamorphosis the apical organ and the prototrochal nerve plexus are lost.

Conclusions

The development of the nervous system in early solenogaster larvae shows striking similarities to other spiralians, especially polychaetes, in exhibiting an apical organ with flask-shaped cells, a single pair of longitudinal neurite bundles, a serotonin-LIR innervation of the prototroch, and formation of these structures from an anterior and a posterior neurogenic domain. This provides evidence for an ancestral spiralian pattern of early nervous system development and a LCA of the Spiralia with a single pair of nerve cords. In later nervous system development, however, the annelids deviate from all other spiralians including solenogasters in forming a posterior growth zone, which initiates teloblastic growth. Since this mode of organogenesis is confined to annelids, we conclude that the LCA of both molluscs and spiralians was unsegmented.

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.

Use of axonal projection patterns for the homologisation of cerebral nerves in Opisthobranchia, Mollusca and Gastropoda

Introduction

Gastropoda are guided by several sensory organs in the head region, referred to as cephalic sensory organs (CSOs). These CSOs are innervated by distinct nerves. This study proposes a unified terminology for the cerebral nerves and the categories of CSOs and then investigates the neuroanatomy and cellular innervation patterns of these cerebral nerves, in order to homologise them. The homologisation of the cerebral nerves in conjunction with other data, e.g. ontogenetic development or functional morphology, may then provide insights into the homology of the CSOs themselves.

Results

Nickel-lysine axonal tracing (“backfilling”) was used to stain the somata projecting into specific nerves in representatives of opisthobranch Gastropoda. Tracing patterns revealed the occurrence, size and relative position of somata and their axons and enabled these somata to be mapped to specific cell clusters. Assignment of cells to clusters followed a conservative approach based primarily on relative location of the cells. Each of the four investigated cerebral nerves could be uniquely identified due to a characteristic set of soma clusters projecting into the respective nerves via their axonal pathways.

Conclusions

As the described tracing patterns are highly conserved morphological characters, they can be used to homologise nerves within the investigated group of gastropods. The combination of adequate number of replicates and a comparative approach allows us to provide preliminary hypotheses on homologies for the cerebral nerves. Based on the hypotheses regarding cerebral nerve homology together with further data on ultrastructure and immunohistochemistry of CSOs published elsewhere, we can propose preliminary hypotheses regarding homology for the CSOs of the Opisthobranchia themselves.

Localization of serotonin in the nervous system of Biomphalaria glabrata, an intermediate host for schistosomiasis

The digenetic trematode Schistosoma mansoni that causes the form of schistosomiasis found in the Western Hemisphere requires the freshwater snail Biomphalaria glabrata as its primary intermediate host. It has been proposed that the transition from the free-living S. mansoni miracidium to parasitic mother sporocyst depends on uptake of biogenic amines, e.g. serotonin, from the snail host. However, little is known about potential sources of serotonin in B. glabrata tissues. This investigation examined the localization of serotonin-like immunoreactivity (5HTli) in the central nervous system (CNS) and peripheral tissues of B. glabrata. Emphasis was placed on the cephalic and anterior pedal regions that are commonly the sites of S. mansoni miracidium penetration. The anterior foot and body wall were densely innervated by 5HTli fibers but no peripheral immunoreactive neuronal somata were detected. Within the CNS, clusters of 5HTli neurons were observed in the cerebral, pedal, left parietal, and visceral ganglia, suggesting that the peripheral serotonergic fibers originate from the CNS. Double-labeling experiments (biocytin backfill × serotonin immunoreactivity) of the tentacular nerve and the three major pedal nerves (Pd n. 10, Pd n. 11, and Pd n. 12) disclosed central neurons that project to the cephalopedal periphery. Overall, the central distribution of 5HTli neurons suggests that, as in other gastropods, serotonin regulates the locomotion, reproductive, and feeding systems of Biomphalaria. The projections to the foot and body wall indicate that serotonin may also participate in defensive, nociceptive, or inflammation responses. These observations identify potential sources of host-derived serotonin in this parasite-host system. Inc.

Antidepressants cause foot detachment from substrate in five species of marine snail

Active Pharmaceutical Ingredients (APIs) are released into aquatic ecosystems through discharged sewage wastewater. Antidepressants are among those APIs often detected in wastewater effluent and have been recently reported to cause foot detachment from the substrate in freshwater snails. We tested the effects of four commonly prescribed antidepressants {fluoxetine ("Prozac"), fluvoxamine ("Luvox"), venlafaxine ("Effexor"), and citalopram ("Celexa") on adhesion to the substrate in five species of marine snails, three from the Pacific coast (Chlorostoma funebralis, Nucella ostrina, Urosalpinx cinerea) and two species from the Atlantic coast (Tegula fasciatus and Lithopoma americanum) of North America representing three different gastropod families. All antidepressants tested induced foot detachment from the substrate in all snail species in a mainly dose-dependent manner (p < 0.04-0.00000001). The lowest LOECs (lowest observed effect concentration) for antidepressants and snails were recorded for Lithopoma in 43.4 μg/L (100 nM) fluvoxamine and Chlorostoma in 157 μg/L (500 nM) venlafaxine and 217 μg/L (500 nM) fluvoxamine. The trochids and turbinids were 2-10× more sensitive to the antidepressants than the muricids. Latency to detachment was also dose dependent, with the fastest average times to detach seen in Chlorostoma and Lithopoma (7.33 and 13.16 min respectively in 3.13 mg/L venlafaxine). The possible physiological mechanisms regulating antidepressant-induced foot detachment in marine snails and the possible ecological consequences are discussed.

A core circuit module for cost/benefit decision

A simple circuit for cost-benefit decision derived from behavioral and neural studies of the predatory sea-slug Pleurobranchaea may closely resemble that upon which the more complex valuation and decision processes of the social vertebrates are built. The neuronal natures of the pathways in the connectionist model comprise classic central pattern generators, bipolar switch mechanisms, and neuromodulatory state regulation. Marked potential exists for exploring more complex neuroeconomic behavior by appending appropriate circuitry in simulo.

Organization of the serotonergic innervation of the feeding (buccal) musculature during the maturation of the pond snail Lymnaea stagnalis: a morphological and biochemical study

The serotonergic innervation of the buccal musculature responsible for feeding (radula protraction) was investigated during the maturation of the pond snail, Lymnaea stagnalis L., applying light and electron microscopic immunohistochemistry and biochemical approaches. According to epifluorescence and laser confocal microscopy, the first 5-HT-like-immunoreactive (5-HTLIR) processes appeared on the surface of the musculature at the postmetamorphic E80% embryonic stage. Until hatching, the innervation continued to increase in density, showing axon arborizations with projections into the deeper muscle levels. An adult-like pattern of 5-HTLIR innervation appeared at P2-P3 juvenile stages. At the ultrastructural level, close (16-20 nm) but mostly unspecialized neuromuscular contacts were formed by both unlabeled and 5-HTLIR axon profiles from the E80% embryonic stage. Labeled processes were also found located relatively far from the muscle cells. An HPLC assay showed a gradual increase of the 5-HT level in the buccal mass during development. The buccal mass was characterized by a single-component high-affinity 5-HT uptake system, and 5-HT release could be evoked by 100 mM K(+) and blocked in Ca(2+) -free medium. It is suggested that 5-HT plays a wide modulatory role in the peripheral feeding system and is also involved in the functional maturation of the muscle system.

Toward locating the source of serotonergic axons in the tail nerve of Aplysia

Stimulation of the tail nerve (pedal nerve 9, p9) of the mollusk, Aplysia californica, causes release of serotonin (5-HT), which mediates sensitization of withdrawal responses. There are about 35 serotonin-immunoreactive (5-HT-ir) axons in p9, yet the cell bodies of these axons have not been located. Backfills of p9 were combined with 5-HT immunohistochemistry to locate the cell bodies of 5-HT-ir neurons with axons in p9. About 100 neurons had axons in p9. Only about ten neurons, however, were both backfilled and 5-HT-ir. These double-labeled neurons were all located in the pedal ganglion associated with p9, which had a total of approximately 42 5-HT-ir somata. The discrepancy between the number of 5-HT-ir axons and double-labeled cell bodies is not likely due to neurons having multiple axons in the nerve; intracellular fills suggest that these neurons do not branch before entering p9. Additionally, no evidence was found for peripheral 5-HT-ir cell bodies that project axons centrally through p9. Thus, approximately 70% of the neurons that give rise to the 5-HT-ir axons in tail nerve are unaccounted for, but likely to reside in the pedal ganglion.

Serotonin influences locomotion in the nudibranch mollusc Melibe leonina

Serotonin (5-HT) influences locomotion in many animals, from flatworms to mammals. This study examined the effects of 5-HT on locomotion in the nudibranch mollusc Melibe leonina (Gould, 1852). M. leonina exhibits two modes of locomotion, crawling and swimming. Animals were bath-immersed in a range of concentrations of 5-HT or injected with various 5-HT solutions into the hemolymph and then monitored for locomotor activity. In contrast to other gastropods studied, M. leonina showed no significant effect of 5-HT on the distance crawled or the speed of crawling. However, the highest concentration (10(-3) mol l(-1) for bath immersion and 10(-5) mol l(-1) for injection) significantly increased the time spent swimming and the swimming speed. The 5-HT receptor antagonist methysergide inhibited the influence of 5-HT on the overall amount of swimming but not on swimming speed. These results suggest that 5-HT influences locomotion at the behavioral level in M. leonina. In conjunction with previous studies on the neural basis of locomotion in M. leonina, these results also suggest that this species is an excellent model system for investigating the 5-HT modulation of locomotion.

Putative lateral inhibition in sensory processing for directional turns

Computing targeted responses is a general problem in goal-directed behaviors. We sought the sensory template for directional turning in the predatory sea slug Pleurobranchaea californica, which calculates precise turn angles by averaging multiple stimulus sites on its chemotactile oral veil (Yafremava LS, Anthony CW, Lane L, Campbell JK, Gillette R. J Exp Biol 210: 561-569, 2007). Spiking responses to appetitive chemotactile stimulation were recorded in the two bilateral pairs of oral veil nerves, the large oral veil nerve (LOVN) and the tentacle nerve (TN). The integrative abilities of the peripheral nervous system were significant. Nerve spiking responses to punctate, one-site stimulation of the oral veil followed sigmoid relations as stimuli moved between lateral tentacle and the midline. Receptive fields of LOVN and TN were unilateral, overlapping, and oppositely weighted for responsiveness across the length of oral veil. Simultaneous two-site stimulation caused responses of amplitudes markedly smaller than the sum of corresponding one-site responses. Plots of two-site nerve responses against the summed approximate distances from midline of each site were markedly linear. Thus the sensory paths in the peripheral nervous system show reciprocal occlusion similar to lateral inhibition. This outcome suggests a novel neural function for lateral inhibitory mechanisms, distinct from simple contrast enhancement, in computation of both sensory maps and targeted motor actions.

Evolving concepts of arousal: insights from simple model systems

Arousal states strongly influence behavioral decisions. In general, arousal promotes activity and enhances responsiveness to sensory stimuli. Earlier work has emphasized general, or nonspecific, effects of arousal on multiple classes of behaviors. However, contemporary work indicates that arousal has quite specific effects on behavior. Here we review studies of arousal-related circuitry in molluscan model systems. Neural substrates for both general and specific effects of arousal have been identified. Based on the scope of their actions, we can distinguish two major classes of arousal elements: localized versus general. Actions of localized arousal elements are often limited to one class of behavior, and may thereby mediate specific effects of arousal. In contrast, general arousal elements may influence multiple classes of behaviors, and mediate both specific and nonspecific effects of arousal. One common way in which general arousal elements influence multiple behaviors is by acting on localized arousal elements of distinct networks. Often, effects on distinct networks have different time courses that may facilitate formation of specific behavioral sequences. This review highlights prominent roles of serotonergic systems in arousal that are conserved in gastropod molluscs despite extreme diversification of body forms, diet and ecological niches. The studies also indicate that the serotonergic elements can act as either localized or general arousal elements. We discuss the implications of these findings across animals.

Localization of biogenic amines in the foregut of Aplysia californica: catecholaminergic and serotonergic innervation

This study examined the catecholaminergic and serotonergic innervation of the foregut of Aplysia californica, a model system in which the control of feeding behaviors can be investigated at the cellular level. Similar numbers (15-25) of serotonin-like-immunoreactive (5HTli) and tyrosine hydroxylase-like-immunoreactive (THli) fibers were present in each (bilateral) esophageal nerve (En), the major source of pregastric neural innervation in this system. The majority of En 5HTli and THli fibers originated from the anterior branch (En(2)), which innervates the pharynx and the anterior esophagus. Fewer fibers were present in the posterior branch (En(1)), which innervates the majority of the esophagus and the crop. Backfills of the two En branches toward the central nervous system (CNS) labeled a single, centrifugally projecting serotonergic fiber, originating from the metacerebral cell (MCC). The MCC fiber projected only to En(2). No central THli neurons were found to project to the En. Surveys of the pharynx and esophagus revealed major differences between their patterns of catecholaminergic (CA) and serotonergic innervation. Whereas THli fibers and cell bodies were distributed throughout the foregut, 5HTli fibers were present in restricted plexi, and no 5HTli somata were detected. Double-labeling experiments in the periphery revealed THli neurons projecting toward the buccal ganglion via En(2). Other afferents received dense perisomatic serotonergic innervation. Finally, qualitative and quantitative differences were observed between the buccal motor programs (BMPs) produced by stimulation of the two En branches. These observations increase our understanding of aminergic contributions to the pregastric regulation of Aplysia feeding behaviors.

Development of the 5-HT-like immunoreactive pedal plexus in the pond snail Lymnaea stagnalis appressa

In many gastropods, a serotonin-like immunoreactive axon plexus lies over ciliary cells on the pedal sole. The origin and function of axons in this plexus is uncertain. By using serotonin antibodies in the direct-developing embryo of the pond snail, the axons that initially form this plexus were traced from seven large neurons in each pedal ganglion. Soon after metamorphosis begins, the first immunoreactive pedal ganglion neuron sends multiple branched neurites to lie directly over pedal sole ciliary cells. By 70% of the 11 days required for hatching, axons from the seven neuron pairs form a plexus over ciliary cells in the whole sole. The axon from each of the seven neurons is guided to a specific area of the pedal plexus where ciliary cells are developing. Axons from two pairs of these neurons, which form the pedal plexus in the posterior part of the foot, are in the unpaired nerve that comes from the pedal ganglia ventral commissure. It is likely that these two developing neuron pairs are homologs of the two neuron pairs in Lymnaea stagnalis that have axons in this ventral commissure nerve. Identification of these neurons and the other five neuron pairs with axons in the pedal plexus will provide a basis for future studies of the relation between the plexus and pedal ciliary locomotion.

5-HT and 5-HT-SO4, but not tryptophan or 5-HIAA levels in single feeding neurons track animal hunger state

Serotonin (5-HT) is an intrinsic modulator of neural network excitation states in gastropod molluscs. 5-HT and related indole metabolites were measured in single, well-characterized serotonergic neurons of the feeding motor network of the predatory sea-slug Pleurobranchaea californica. Indole amounts were compared between paired hungry and satiated animals. Levels of 5-HT and its metabolite 5-HT-SO4 in the metacerebral giant neurons were observed in amounts approximately four-fold and two-fold, respectively, below unfed partners 24 h after a satiating meal. Intracellular levels of 5-hydroxyindole acetic acid and of free tryptophan did not differ significantly with hunger state. These data demonstrate that neurotransmitter levels and their metabolites can vary in goal-directed neural networks in a manner that follows internal state.

Neurogenesis of cephalic sensory organs of Aplysia californica

The opisthobranch gastropod Aplysia californica serves as a model organism in experimental neurobiology because of its simple and well-known nervous system. However, its nervous periphery has been less intensely studied. We have reconstructed the ontogeny of the cephalic sensory organs (labial tentacles, rhinophores, and lip) of planktonic, metamorphic, and juvenile developmental stages. FMRFamide and serotonergic expression patterns have been examined by immunocytochemistry in conjunction with epifluorescence and confocal laser scanning microscopy. We have also applied scanning electron microscopy to analyze the ciliary distribution of these sensory epithelia. Labial tentacles and the lip develop during metamorphosis, whereas rhinophores appear significantly later, in stage 10 juveniles. Our study has revealed immunoreactivity against FMRFamides and serotonin in all major nerves. The common labial nerve develops first, followed by the labial tentacle base nerve, oral nerve, and rhinophoral nerve. We have also identified previously undescribed neuronal pathways and other FMRFamide-like-immunoreactive neuronal elements, such as peripheral ganglia and glomerulus-like structures, and two groups of conspicuous transient FMRFamide-like cell somata. We have further found two distinct populations of FMRFamide-positive cell somata located both subepidermally and in the inner regions of the cephalic sensory organs in juveniles. The latter population partly consists of sensory cells, suggesting an involvement of FMRFamide-like peptides in the modulation of peripheral sensory processes. This study is the first concerning the neurogenesis of cephalic sensory organs in A. californica and may serve as a basis for future studies of neuronal elements in gastropod molluscs.

Serotonin-like immunoreactivity in the central and peripheral nervous systems of the interstitial acochlidean Asperspina sp. (Opisthobranchia)

Species of Acochlidea are common members of the marine interstitial environment and defined in part by their minuscule size and highly divergent morphology relative to other benthic opisthobranchs. Despite these differences, acochlideans such as species of Asperspina display many plesiomorphic characteristics, including an unfused condition of their neural ganglia. To gain insight into the distribution of specific neural subsets within acochlidean ganglia, a species of Asperspina was studied by using anti-serotonin immunohistochemistry and epifluorescence and confocal laser scanning microscopy. Results reveal similarities between Asperspina and larger opisthobranchs in the general distribution of serotonergic perikarya in the central nervous system. Specifically, the arrangement of perikarya into regional clusters within the cerebral and pedal ganglia and the absence of immunoreactive perikarya in the pleural ganglia are similar to the model species of Aplysia californica, Pleurobranchaea californica, and Tritonia diomedea. Moreover, serotonergic innervation of the rhinophores in all opisthobranchs, including Asperspina sp., originates from the cerebral ganglion instead of directly from the rhinophoral ganglion. Serotonergic innervation of the body wall, including the epithelium, muscles, and pedal sole, appears to arise exclusively from pedal and accessory ganglia. These observations indicate a general conservation of serotonin-like immunoreactivity in the central and peripheral nervous systems of acochlidean and other benthic opisthobranchs.

Functional neuroanatomy of the rhinophore of Aplysia punctata

Background

For marine snails, olfaction represents a crucial sensory modality for long-distance reception, as auditory and visual information is limited. The posterior tentacle of Aplysia, the rhinophore, is a chemosensory organ and several behavioural studies showed that the rhinophores can detect pheromones, initiate orientation and locomotion toward food. However the functional neuroanatomy of the rhinophore is not yet clear. Here we apply serotonin-immunohistochemistry and fluorescent markers in combination with confocal microscopy as well as optical recording techniques to elucidate the structure and function of the rhinophore of the sea slug Aplysia punctata.

Results

With anatomical techniques an overview of the neuroanatomical organization of the rhinophore is presented. Labelling with propidium iodide revealed one layer of cell nuclei in the sensory epithelium and densely packed cell nuclei beneath the groove of the rhinophore, which extends to about two third of the total length of the rhinophore. Serotonin immunoreactivity was found within the olfactory glomeruli underneath the epithelium as well as in the rhinophore ganglion. Retrograde tracing from the rhinophore ganglion with 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA) demonstrated the connection of glomeruli with the ganglion. Around 36 glomeruli (mean diameter 49 μm) were counted in a single rhinophore. Fluorimetric measurements of intracellular Ca²⁺ levels using Fura-2 AM loading revealed Ca²⁺-responses within the rhinophore ganglion to stimulation with amino acids. Bath application of different amino acids revealed differential responses at different positions within the rhinophore ganglion.

Conclusion

Our neuroanatomical study revealed the number and position of glomeruli in the rhinophore and the rhinophore ganglion as processing stage of sensory information. Serotonin-immunoreactive processes were found extensively within the rhinophore, but was not detected within any peripheral cell body. Amino acids were used as olfactory stimuli in optical recordings and induced sensory responses in the rhinophore ganglion. The complexity of changes in intracellular Ca²⁺-levels indicates, that processing of odour information takes place within the rhinophore ganglion. Our neuroanatomical and functional studies of the rhinophore open up a new avenue to analyze the olfactory system in Aplysia.

Monoamines in the pedal plexus of the land snail Megalobulimus oblongus (Gastropoda, Pulmonata)

In molluscs, the number of peripheral neurons far exceeds those found in the central nervous system. Although previous studies on the morphology of the peripheral nervous system exist, details of its organization remain unknown. Moreover, the foot of the terrestrial species has been studied less than that of the aquatic species. As this knowledge is essential for our experimental model, the pulmonate gastropod Megalobulimus oblongus, the aim of the present study was to investigate monoamines in the pedal plexus of this snail using two procedures: glyoxylic acid histofluorescence to identify monoaminergic structures, and the unlabeled antibody peroxidase anti-peroxidase method using antiserum to detect the serotonergic component of the plexus. Adult land snails weighing 48-80 g, obtained from the counties of Barra do Ribeiro and Charqueadas (RS, Brazil), were utilized. Monoaminergic fibers were detected throughout the pedal musculature. Blue fluorescence (catecholamines, probably dopamine) was observed in nerve branches, pedal and subepithelial plexuses, and in the pedal muscle cells. Yellow fluorescence (serotonin) was only observed in thick nerves and in muscle cells. However, when immunohistochemical methods were used, serotonergic fibers were detected in the pedal nerve branches, the pedal and subepithelial plexuses, the basal and lateral zones of the ventral integument epithelial cells, in the pedal ganglion neurons and beneath the ventral epithelium. These findings suggest catecholaminergic and serotonergic involvement in locomotion and modulation of both the pedal ganglion interneurons and sensory information. Knowledge of monoaminergic distribution in this snail s foot is important for understanding the pharmacological control of reflexive responses and locomotive behavior.

Monoamines in the albumen gland, plasma, and central nervous system of the snail Biomphalaria glabrata during egg-laying

The potential role of selected biogenic monoamines and related compounds in the reproductive physiology of the freshwater snail Biomphalaria glabrata was investigated. Extracts of the albumen gland (AG), plasma, and central nervous system (CNS) were subjected to high pressure liquid chromatography with electrochemical detection (HPLC-ED), and under the extraction and separation conditions employed the following amines were detected: tyrosine, dihydroxyphenylalanine (DOPA), dopamine, and tryptophan in the AG; DOPA, tyrosine, and tryptophan in the plasma; DOPA, tyrosine, dopamine and 5-hydroxytryptamine in the CNS. These compounds were then quantified in individual samples taken from snails known to be in a particular stage of the egg-laying process. AG dopamine levels were highest in snails in the first stage of the reproductive process, when the AG is secreting perivitelline fluid around each fertilized ovum. Significant decreases in AG protein content during the later stages of the egg-laying process were also evident. Plasma tyrosine and DOPA levels were lowest in snails that contained a fully packaged egg mass, while no changes in monoamine content were observed in the CNS. These data provide insights into the role(s) that monoamines, especially catecholamine-related compounds, may play in B. glabrata reproductive physiology.

Escape swim network interneurons have diverse roles in behavioral switching and putative arousal in Pleurobranchaea

Escape swimming in the predatory sea slug Pleurobranchaea is a dominant behavior that overrides feeding, a behavioral switch caused by swim-induced inhibition of feeding command neurons. We have now found distinct roles for the different swim interneurons in acute suppression of feeding during the swim and in a longer-term stimulation of excitability in the feeding network. The identified pattern-generating swim neurons A1, A3, A10, and their follower interneuron A-ci1, suppress feeding motor output partly by excitation of the I1 feeding interneurons, which monosynaptically inhibit both the feeding command neurons, PC(P), PSE, and other major interneurons, the I2s. This mechanism exerts broad inhibition of the feeding network suitable to an escape response; broader than feeding suppression in learned and satiation-induced food avoidance and acting through a different presynaptic pathway. Four intrinsic neuromodulatory neurons of the swim network, the serotonergic As1-4, add little to direct suppression of feeding. Rather, they monosynaptically excite the serotonergic metacerebral giant (MCG) neurons of the feeding network, themselves intrinsic neuromodulators of feeding, as well as a cluster of adjacent serotonergic feeding neurons, with both fast and slow EPSPs. They also provide mild neuromodulatory excitation of the PC(P)/PSE feeding command neurons, and I1 and I2 feeding interneurons, which is masked by inhibition during the swim. As1-4 also excite the serotonergic pedal ganglion G neurons for creeping locomotion. These observations further delineate the nature of the putative serotonergic arousal system of gastropods and suggest a central coordinating role to As1-4.

Sources of magnetic sensory input to identified neurons active during crawling in the marine mollusc Tritonia diomedea

Although the nudibranch mollusc Tritonia diomedea orients to the geomagnetic field, the anatomical site and the mechanism of the geomagnetic transducer are not known. Previous work on semi-intact preparations of Tritonia diomedea in which the brain is intact and nerve connections to the periphery are maintained showed that identifiable pedal ganglion neurons Pd5 fired an increased number of action potentials when the horizontal component of the ambient magnetic field was rotated. This response disappeared when all nerves emerging from the brain were cut, suggesting a peripheral locus for the geomagnetic transducer. In the present work, we recorded intracellularly from Pd5 in preparations in which all peripheral nerves were cut except those containing the axons of neurons Pd5 (pedal nerves 2 and 3). These uncut, mixed, sensory-motor trunks innervate the locomotory epithelium of the foot upon which the animal crawls. In this further-reduced preparation, Pd5 again responded to magnetic field rotations with action potentials. To determine the direction of this action potential transmission in response to magnetic field rotations, we analyzed extracellular recordings from nerves containing the Pd5 axons and found that action potentials elicited in Pd5 by magnetic stimuli originate centrally and are transmitted peripherally. In addition, we have explored the behavioral function of Pd5 neurons by simultaneously recording intracellular electrical activity and crawling rate of the semi-intact animal. A significant correlation was found between crawling rate and Pd5 action potential rate. We also found that action potentials in dorsal swim interneurons depolarized both Pd5 and the established locomotion motoneuron Pd21.

Single neuron analysis by capillary electrophoresis with fluorescence spectroscopy

A technique to identify and quantitate simultaneously more than 30 compounds in individual neurons is described. The method uses nanoliter volume sampling, capillary electrophoresis separation, and wavelength-resolved native fluorescence detection. Limits of detection (LODs) range from the low attomole to the femtomole range, with 5-hydroxytryptamine (or serotonin [5-HT]) LODs being approximately 20 attomoles. Although the cellular sample matrix is chemically complex, the combination of electrophoretic migration time and fluorescence spectral information allows positive identification of aromatic monoamines, aromatic amino acids and peptides containing them, flavins, adenosine- and guanosine-nucleotide analogs, and other fluorescent compounds. Individual identified neurons from Aplysia californica and Pleurobranchaea californica are used to demonstrate the applicability and figures of merit of this technique.