Transient and sustained expression of FMRFamide-like immunoreactivity in the developing nervous system of Lymnaea stagnalis (Mollusca, Pulmonata)

Expanded expression of pro-neurogenic factor SoxB1 during larval development of gastropod Lymnaea stagnalis suggests preadaptation to prolonged neurogenesis in Mollusca

Introduction

The remarkable diversity observed in the structure and development of the molluscan nervous system raises intriguing questions regarding the molecular mechanisms underlying neurogenesis in Mollusca. The expression of SoxB family transcription factors plays a pivotal role in neuronal development, thereby offering valuable insights into the strategies of neurogenesis.

Methods

In this study, we conducted gene expression analysis focusing on SoxB-family transcription factors during early neurogenesis in the gastropod Lymnaea stagnalis. We employed a combination of hybridization chain reaction in situ hybridization (HCR-ISH), immunocytochemistry, confocal microscopy, and cell proliferation assays to investigate the spatial and temporal expression patterns of LsSoxB1 and LsSoxB2 from the gastrula stage to hatching, with particular attention to the formation of central ring ganglia.

Results

Our investigation reveals that LsSoxB1 demonstrates expanded ectodermal expression from the gastrula to the hatching stage, whereas expression of LsSoxB2 in the ectoderm ceases by the veliger stage. LsSoxB1 is expressed in the ectoderm of the head, foot, and visceral complex, as well as in forming ganglia and sensory cells. Conversely, LsSoxB2 is mostly restricted to the subepithelial layer and forming ganglia cells during metamorphosis. Proliferation assays indicate a uniform distribution of dividing cells in the ectoderm across all developmental stages, suggesting the absence of distinct neurogenic zones with increased proliferation in gastropods.

Discussion

Our findings reveal a spatially and temporally extended pattern of SoxB1 expression in a gastropod representative compared to other lophotrochozoan species. This prolonged and widespread expression of SoxB genes may be interpreted as a form of transcriptional neoteny, representing a preadaptation to prolonged neurogenesis. Consequently, it could contribute to the diversification of nervous systems in gastropods and lead to an increase in the complexity of the central nervous system in Mollusca.

Dinophiliformia early neurogenesis suggests the evolution of conservative neural structures across the Annelida phylogenetic tree

Despite the increasing data concerning the structure of the adult nervous system in various Lophotrochozoa groups, the early events during the neurogenesis of rare and unique groups need clarification. Annelida are a diverse clade of Lophotrochozoa, and their representatives demonstrate a variety of body plans, lifestyles, and life cycles. Comparative data about the early development are available for Errantia, Sedentaria, Sipuncula, and Palaeoannelida; however, our knowledge of Dinophiliformia is currently scarce. Representatives of Dinophiliformia are small interstitial worms combining unique morphological features of different Lophotrochozoan taxa and expressing paedomorphic traits. We describe in detail the early neurogenesis of two related species: Dimorphilus gyrociliatus and Dinophilus vorticoides, from the appearance of first nerve cells until the formation of an adult body plan. In both species, the first cells were detected at the anterior and posterior regions at the early trochophore stage and demonstrated positive reactions with pan-neuronal marker anti-acetylated tubulin only. Long fibers of early cells grow towards each other and form longitudinal bundles along which differentiating neurons later appear and send their processes. We propose that these early cells serve as pioneer neurons, forming a layout of the adult nervous system. The early anterior cell of D. vorticoides is transient and present during the short embryonic period, while early anterior and posterior cells in D. gyrociliatus are maintained throughout the whole lifespan of the species. During development, the growing processes of early cells form compact brain neuropile, paired ventral and lateral longitudinal bundles; unpaired medial longitudinal bundle; and commissures in the ventral hyposphere. Specific 5-HT- and FMRFa-immunopositive neurons differentiate adjacent to the ventral bundles and brain neuropile in the middle trochophore and late trochophore stages, i.e. after the main structures of the nervous system have already been established. Processes of 5-HT- and FMRFa-positive cells constitute a small proportion of the tubulin-immunopositive brain neuropile, ventral cords, and commissures in all developmental stages. No 5-HT- and FMRFa-positive cells similar to apical sensory cells of other Lophotrochozoa were detected. We conclude that: (i) like in Errantia and Sedentaria, Dinophiliformia neurogenesis starts from the peripheral cells, whose processes prefigure the forming adult nervous system, (ii) Dinophiliformia early cells are negative to 5-HT and FMRFa antibodies like Sedentaria pioneer cells.

Peripheral sensory neurons govern development of the nervous system in bivalve larvae

Recent findings regarding early lophotrochozoan development have altered the conventional model of neurogenesis and revealed that peripheral sensory elements play a key role in the initial organization of the larval nervous system. Here, we describe the main neurogenetic events in bivalve mollusks in comparison with other Lophotrochozoa, emphasizing a novel role for early neurons in establishing larval nervous systems and speculating about the morphogenetic function of the apical organ. We demonstrate that during bivalve development, peripheral sensory neurons utilizing various transmitters differentiate before the apical organ emerges. The first neurons and their neurites serve as a scaffold for the development of the nervous system. During veliger stage, cerebral, pleural, and visceral ganglia form along the lateral (visceral) nerve cords in anterior-to-posterior axis. The pedal ganglia and corresponding ventral (pedal) nerve cords develop much later, after larval settlement and metamorphosis. Pharmacological abolishment of the serotonin gradient within the larval body disrupts the navigation of "pioneer" axons resulting in malformation of the whole nervous system architecture. Comparative morphological data on neurogenetic events in bivalve mollusks shed new light on the origin of the nervous system, mechanisms of early axon navigation, and sequence of the tetraneurous nervous system formation. Furthermore, this information improves our understanding of the basic nervous system architecture in larval Bivalvia and Mollusca.

RFamidergic neurons in the olfactory centers of the terrestrial slug Limax

BACKGROUND

The terrestrial slug Limax has long been used as a model for the study of olfactory information processing and odor learning. Olfactory inputs from the olfactory epithelium are processed in the tentacular ganglion and then in the procerebrum. Glutamate and acetylcholine are the major neurotransmitters used in the procerebrum. Phe-Met-Arg-Phe-NH₂ (FMRFamide) has been shown to be involved in the regulation of the network activity of the procerebrum. Although there are thought to be various RFamide family peptides other than FMRFamide that are potentially recognized by anti-FMRFamide antibody in the central nervous system of mollusks, identifying the entire repertoire of RFamide peptides in Limax has yet to be achieved.

METHODS

In the present study, we made a comprehensive search for RFamide peptide-encoding genes from the transcriptome data of Limax, and identified 12 genes. The expression maps of these RFamide genes were constructed by in situ hybridization in the cerebral ganglia including the procerebrum, and in the superior/inferior tentacles.

RESULTS

Ten of 12 genes were expressed in the procerebrum, and nine of 12 genes were expressed in the tentacular ganglia. Gly-Ser-Leu-Phe-Arg-Phe-NH₂ (GSLFRFamide), which is encoded by two different genes, LFRFamide1 (Leu-Phe-Arg-Phe-NH₂-1) and LFRFamide2 (Leu-Phe-Arg-Phe-NH₂-2), decreased the oscillatory frequency of the local field potential oscillation in the procerebrum when exogenously applied in vitro. We also found by immunohistochemistry that the neurons expressing pedal peptide send efferent projections from the procerebrum back to the tentacular ganglion.

CONCLUSION

Our findings suggest the involvement of a far wider variety of RFamide family peptides in the olfactory information processing in Limax than previously thought.

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.

Nervous system development in the Pacific oyster, Crassostrea gigas (Mollusca: Bivalvia)

Background

Bivalves comprise a large, highly diverse taxon of invertebrate species. Developmental studies of neurogenesis among species of Bivalvia are limited. Due to a lack of neurogenesis information, it is difficult to infer a ground pattern for Bivalvia. To provide more comprehensive morphogenetic data on bivalve molluscs and relationships among molluscan clades, we investigated neurogenesis in the Pacific oyster, Crassostrea gigas, from the appearance of the first sensory cells to the formation of the larval ganglionic nervous system by co-immunocytochemistry of the neuronal markers FMRFamide or 5-HT and vesicular acetylcholine transporter (VAChT).

Results

Neurogenesis begins with the emergence of the apical serotonin-immunoreactive (5-HT-ir) sensory cells and paired sensory posttrochal dorsal and ventral FMRFamide-immunoreactive (FMRFamide-ir) cells at the early trochophore stage. Later, at the early veliger stage, the apical organ (AO) includes 5-HT-ir, FMRFamide-ir, and VAChT-ir cells. At the same stage, VAChT-ir cells appear in the posterior region of larvae and send axons towards the AO. Thus, FMRFamide-ir neurites and VAChT-ir processes form scaffolds for longitudinal neurite bundles develop into the paired ventral nerve cords (VNC). Later-appearing axons from the AO/CG neurons join the neurite bundles comprising the VNC. All larval ganglia appear along the VNC as paired or fused (epiathroid) clusters in late veliger and pediveliger larvae. We observed the transformation of the AO into the cerebral ganglia, which abundantly innervated the velum, and the transformation of ventral neurons into the pedal ganglia, innervating the foot, gills, and anterior adductor muscle. The visceral ganglia appear last in the pediveliger oyster and innervate the visceral mass and posterior adductor of premetamorphic larvae. In addition, a local FMRFamide-ir network was detected in the digestive system of pediveliger larvae. We identified VAChT-ir nervous elements in oyster larvae, which have not been observed previously in molluscs. Finally, we performed a morphology-based comparative analysis of neuronal structures among bivalve, conchiferan, and aculiferan species.

Conclusions

We described the development of the nervous system during the larval development in Crassostrea gigas. These data greatly advance the currently limited understanding of neurodevelopment in bivalves and mollusks, which has hampered the generation of a ground pattern reconstruction of the last common ancestor of Mollusca. Our morphological data support phylogenomic data indicating a closer Bivalvia-Gastropoda sister group relationship than the Bivalvia-Scaphopoda (Diasoma) group relationship.

Electronic supplementary material

The online version of this article (10.1186/s12983-018-0259-8) contains supplementary material, which is available to authorized users.

Anatomy and development of the larval nervous system in Echinococcus multilocularis

Background

The metacestode larva of Echinococcus multilocularis (Cestoda: Taeniidae) develops in the liver of intermediate hosts (typically rodents, or accidentally in humans) as a labyrinth of interconnected cysts that infiltrate the host tissue, causing the disease alveolar echinococcosis. Within the cysts, protoscoleces (the infective stage for the definitive canid host) arise by asexual multiplication. These consist of a scolex similar to that of the adult, invaginated within a small posterior body. Despite the importance of alveolar echinococcosis for human health, relatively little is known about the basic biology, anatomy and development of E. multilocularis larvae, particularly with regard to their nervous system.

Results

We describe the existence of a subtegumental nerve net in the metacestode cysts, which is immunoreactive for acetylated tubulin-α and contains small populations of nerve cells that are labeled by antibodies raised against several invertebrate neuropeptides. However, no evidence was found for the existence of cholinergic or serotoninergic elements in the cyst wall. Muscle fibers occur without any specific arrangement in the subtegumental layer, and accumulate during the invaginations of the cyst wall that form brood capsules, where protoscoleces develop. The nervous system of the protoscolex develops independently of that of the metacestode cyst, with an antero-posterior developmental gradient. The combination of antibodies against several nervous system markers resulted in a detailed description of the protoscolex nervous system, which is remarkably complex and already similar to that of the adult worm.

Conclusions

We provide evidence for the first time of the existence of a nervous system in the metacestode cyst wall, which is remarkable given the lack of motility of this larval stage, and the lack of serotoninergic and cholinergic elements. We propose that it could function as a neuroendocrine system, derived from the nervous system present in the bladder tissue of other taeniids. The detailed description of the development and anatomy of the protoscolex neuromuscular system is a necessary first step toward the understanding of the developmental mechanisms operating in these peculiar larval stages.

Phylogenomics meets neuroscience: how many times might complex brains have evolved?

The origin of complex centralized brains is one of the major evolutionary transitions in the history of animals. Monophyly (i.e. presence of a centralized nervous system in urbilateria) vs polyphyly (i.e. multiple origins by parallel centralization of nervous systems within several lineages) are two historically conflicting scenarios to explain such transitions. However, recent phylogenomic and cladistic analysis suggests that complex brains may have independently evolved at least 9 times within different animal lineages. Indeed, even within the phylum Mollusca cephalization might have occurred at least 5 times. Emerging molecular data further suggest that at the genomic level such transitions might have been achieved by changes in expression of just a few transcriptional factors - not surprising since such events might happen multiple times over 700 million years of animal evolution. Both cladistic and genomic analyses also imply that neurons themselves evolved more than once. Ancestral polarized secretory cells were likely involved in coordination of ciliated locomotion in early animals, and these cells can be considered as evolutionary precursors of neurons within different lineages. Under this scenario, the origins of neurons can be linked to adaptations to stress/injury factors in the form of integrated regeneration-type cellular response with secretory signaling peptides as early neurotransmitters. To further reconstruct the parallel evolution of nervous systems genomic approaches are essential to probe enigmatic neurons of basal metazoans, selected lophotrochozoans (e.g. phoronids, brachiopods) and deuterostomes.

Neuromuscular development of Aeolidiella stephanieae Valdéz, 2005 (Mollusca, Gastropoda, Nudibranchia)

Background

Studies on the development of the nervous system and the musculature of invertebrates have become more sophisticated and numerous within the last decade and have proven to provide new insights into the evolutionary history of organisms. In order to provide new morphogenetic data on opisthobranch gastropods we investigated the neuromuscular development in the nudibranch Aeolidiella stephanieae Valdéz, 2005 using immunocytochemistry as well as F-actin labelling in conjunction with confocal laser scanning microscopy (cLSM).

Results

The ontogenetic development of Aeolidiella stephanieae can be subdivided into 8 stages, each recognisable by characteristic morphological and behavioural features as well as specific characters of the nervous system and the muscular system, respectively. The larval nervous system of A. stephanieae includes an apical organ, developing central ganglia, and peripheral neurons associated with the velum, foot and posterior, visceral part of the larva. The first serotonergic and FMRFamidergic neural structures appear in the apical organ that exhibits an array of three sensory, flask-shaped and two non-sensory, round neurons, which altogether disappear prior to metamorphosis. The postmetamorphic central nervous system (CNS) becomes concentrated, and the rhinophoral ganglia develop together with the anlage of the future rhinophores whereas oral tentacle ganglia are not found. The myogenesis in A. stephanieae begins with the larval retractor muscle followed by the accessory larval retractor muscle, the velar or prototroch muscles and the pedal retractors that all together degenerate during metamorphosis, and the adult muscle complex forms de novo.

Conclusions

Aeolidiella stephanieae comprises features of the larval and postmetamorphic nervous as well as muscular system that represent the ground plan of the Mollusca or even the Trochozoa (e. g. presence of the prototrochal or velar muscle ring). On the one hand, A. stephanieae shows some features shared by all nudibranchs like the postmetamorphic condensation of the CNS, the possession of rhinophoral ganglia and the lack of oral tentacle ganglia as well as the de novo formation of the adult muscle complex. On the other hand, the structure and arrangement of the serotonergic apical organ is similar to other caenogastropod and opisthobranch gastropods supporting their sister group relationship.

Early differentiating neuron in larval abalone (Haliotis kamtschatkana) reveals the relationship between ontogenetic torsion and crossing of the pleurovisceral nerve cords

Crossing of the pleurovisceral nerve cords in gastropods has supported the view that gastropods evolved by 180 degrees rotation between the ventral and dorsal body regions. Indeed, a rotation of this type occurs as a dramatic morphogenetic movement ("ontogenetic torsion") during the development of basal gastropods. According to a long-standing hypothesis, ontogenetic torsion in basal gastropods preserves an ancient developmental aberration that generated the contorted gastropod body plan. It follows from this reasoning that crossing of the pleurovisceral nerve cords during gastropod development should be mechanically coupled to ontogenetic torsion. The predicted mechanical coupling can now be examined because of the discovery of an early differentiating neuron in Haliotis kamtschatkana (Vetigastropoda) that expresses 5-hydroxytryptamine-like immunoreactivity. The neuron appeared to delineate the trajectory of the pleurovisceral nerve cords beginning before ontogenetic torsion. Before torsion, the neuronal soma is embedded in mantle epithelium at the ventral midline and two neurites extend anteriorly toward the apical sensory organ. Contrary to expectation, the two neurites of this cell did not cross-over during ontogenetic torsion because the soma of this mantle neuron shifted in the same direction as the rotating head and foot. Full crossing of the pleurovisceral nerve cords occurred gradually during later development as the mantle cavity deepened and expanded leftward. These results are consistent with a generalization emerging from comparative studies indicating a conserved developmental stage for gastropods in which the mantle cavity is localized to one side, despite a fully "post-torsional" orientation for other body components. Developmental morphology before this stage is much more variable among different gastropod clades.

Development of A-type allatostatin immunoreactivity in antennal lobe neurons of the sphinx moth Manduca sexta

The antennal lobe (AL) of the sphinx moth Manduca sexta is a well-established model system for studying mechanisms of neuronal development. To understand whether neuropeptides are suited to playing a role during AL development, we have studied the cellular localization and temporal expression pattern of neuropeptides of the A-type allatostatin family. Based on morphology and developmental appearance, we distinguished four types of AST-A-immunoreactive cell types. The majority of the cells were local interneurons of the AL (type Ia) which acquired AST-A immunostaining in a complex pattern consisting of three rising (RI-RIII) and two declining phases (DI, DII). Type Ib neurons consisted of two local neurons with large cell bodies not appearing before 7/8 days after pupal ecdysis (P7/P8). Types II and III neurons accounted for single centrifugal neurons, with type II neurons present in the larva and disappearing in the early pupa. The type III neuron did not appear before P7/P8. RI and RII coincided with the rises of the ecdysteroid hemolymph titer. Artificially shifting the pupal 20-hydroxyecdysone (20E) peak to an earlier developmental time point resulted in the precocious appearance of AST-A immunostaining in types Ia, Ib, and III neurons. This result supports the hypothesis that the pupal rise in 20E plays a role in AST-A expression during AL development. Because of their early appearance in newly forming glomeruli, AST-A-immunoreactive fibers could be involved in glomerulus formation. Diffuse AST-A labeling during early AL development is discussed as a possible signal providing information for ingrowing olfactory receptor neurons.

Development and steroid regulation of RFamide immunoreactivity in antennal-lobe neurons of the sphinx moth Manduca sexta

During metamorphosis, the insect nervous system undergoes considerable remodeling: new neurons are integrated while larval neurons are remodeled or eliminated. To understand further the mechanisms involved in transforming larval to adult tissue we have mapped the metamorphic changes in a particularly well established brain area, the antennal lobe of the sphinx moth Manduca sexta, using an antiserum recognizing RFamide-related neuropeptides. Five types of RFamide-immunoreactive (ir) neurons could be distinguished in the antennal lobe, based on morphology and developmental appearance. Four cell types (types II-V, each consisting of one or two cells) showed RFamide immunostaining in the larva that persisted into metamorphosis. By contrast, the most prominent group (type I), a mixed population of local and projection neurons consisting of about 60 neurons in the adult antennal lobe, acquired immunostaining in a two-step process during metamorphosis. In a first step, from 5 to 7 days after pupal ecdysis, the number of labeled neurons reached about 25. In a second step, starting about 4 days later, the number of RFamide-ir neurons increased within 6 days to about 60. This two-step process parallels the rise and fall of the developmental hormone 20-hydroxyecdysone (20E) in the hemolymph. Artificially shifting the 20E peak to an earlier developmental time point resulted in the precocious appearance of RFamide immunostaining and led to premature formation of glomeruli. Prolonging high 20E concentrations to stages when the hormone titer starts to decline had no effect on the second increase of immunostained cell numbers. These results support the idea that the rise in 20E, which occurs after pupal ecdysis, plays a role in the first phase of RFamide expression and in glomeruli formation in the developing antennal lobes. The role of 20E in the second phase of RFamide expression is less clear, but increased cell numbers showing RFamide-ir do not appear to be a consequence of the declining levels in 20E that occur during adult development.

Embryogenesis of the histaminergic system in the pond snail, Lymnaea stagnalis L.: an immunocytochemical and biochemical study

Embryogenesis of the histaminergic system in the pond snail, Lymnaea stagnalis, was investigated by means of immunocytochemistry and HPLC assay. From the earliest onset of the of histamine-immunoreactive (HA-IR) elements, the labelled neurons were confined to the pedal, cerebral and buccal ganglia, whereas no IR cells within the pleural, parietal and visceral ganglia were detectable during the embryogenesis. Peripheral projections of the embryonic HA-IR neurons were missing. No transient HA-IR neurons could be found either inside or outside the CNS. The first HA-IR elements appeared at about E55% of embryonic development, at the beginning of metamorphosis, and were represented by three pairs of neurons located in the cerebral ganglia. Following metamorphosis, four pairs of HA-IR neurons were added; two of them occurred in the pedal (E65% stage of development) and two in the buccal (E90% stage of development) ganglia. During embryogenesis, HA-IR fibers were present in the cerebro-pedal connectives and in the cerebral, pedal and buccal commissures, whereas only little arborization could be observed in the neuropil of the ganglia. HPLC measurements revealed a gradual increase of HA content in the embryos during development, corresponding well to the course of the appearance of immunolabeled elements. It is suggested that the developing HAergic system plays a specific role in the process of gangliogenesis and CNS plasticity of embryonic Lymnaea.

Development of the larval nervous system of the gastropod Ilyanassa obsoleta

Gastropods have been well studied in terms of early cell cleavage patterns and the neural basis of adult behaviors; however, much less is known about neural development in this taxon. Here we reveal a relatively sophisticated larval nervous system in a well-studied gastropod, Ilyanassa obsoleta. The present study employed immunocytochemical and histofluorescent techniques combined with confocal microscopy to examine the development of cells containing monoamines (serotonin and catecholamine), neuropeptides (FMRFamide and leu-enkephalin related peptides), and a substance(s) reactive to antibodies raised against dopamine beta-hydroxylase. Neurons were first observed in the apical organ and posterior regions during the embryonic trochophore stage. During later embryonic development neurons appeared in peripheral regions such as the foot, velum, and mantle and in the developing ganglia destined to become the adult central nervous system. In subsequent free-swimming veliger stages the larval nervous system became increasingly elaborate and by late larval stages there existed approximately 26-28 apical cells, 80-100 neurons in the central ganglia, and 200-300 peripherally located neurons. During metamorphosis some populations of neurons in the apical organ and in the periphery disappeared, while others were incorporated into the juvenile nervous system. Comparisons of neural elements in other molluscan larvae reveal several similarities such as comparable arrangements of cells in the apical organ and patterns of peripheral cells. This investigation reveals the most extensive larval nervous system described in any mollusc to date and information from this study will be useful for future experimental studies determining the role of larval neurons and investigations of the cellular and molecular mechanisms governing neural development in this taxon.

Dopamine and histamine in the developing stomatogastric system of the lobster Homarus americanus

Dopamine and histamine are neuromodulators found in the adult stomatogastric nervous system (STNS) of several crustacean species. We used antibodies against tyrosine hydroxylase (TH) and histamine to map the distribution and developmental acquisition of the dopamine and histamine neurons in the STNS of the lobster, Homarus americanus. Embryos, larvae, juvenile and adult animals were studied. TH labeling was present in the STNS as early as E80-85 (80-85% of embryonic development). A subset of preparations in embryos, larvae, juveniles, and adults contained 1-5 labeled somata in the stomatogastric ganglion. Histamine staining appeared in the STNS as early as E50. The distribution of both TH and histamine staining remained relatively constant through development. Electrophysiological recordings demonstrated that receptors for both amines are present in the embryo. Bath application of dopamine increased the frequency of the pyloric rhythm in embryos, and evidence for dopaminergic activation of peripherally initiated spiking in motor axons was seen. In embryos and adults, histamine inhibited the motor patterns produced by the stomatogastric ganglion (STG). These data suggest that the dopaminergic and histaminergic systems in H. americanus appear relatively early in development and that the effects of each are largely maintained through development.

Ultrastructure of neuromuscular contacts in the embryonic pond snail Lymnaea stagnalis L

Ultrastructural characteristics of muscle fibers and neuromuscular contacts were investigated during two stages of embryogenesis of the pulmonate snail Lymnaea stagnalis. The first muscle cells appear as early as during metamorphosis (50-55% of embryonic development), whereas previously, in the trochophore/veliger stages (25-45%), muscular elements cannot be detected at all. The first muscle fibers contain large amounts of free numbers, a well-developed rER system and only a few irregularly arranged contractile elements. The nucleus is densely packed with heterochromatine material. At 75% adult-like postmetamorphic stage, the frequency of muscle fibers increases significantly, but, bundles of muscle fibers cannot yet be observed. Furthermore the muscle cells are characterized by large numbers of free ribosomes and numerous rER elements. Fine axon bundles and single axon processes, both accompanied by glial elements, can already be found at this time. Axon varicosities with different vesicle and/or granule contents form membrane contacts with muscle fibers, but without revealing membrane specialization on the pre- or postsynaptic side. The late development of the muscle system and neuromuscular contacts during Lymnaea embryogenesis correlates well with the maturation of different forms of behavior of adult, free-living life, and also with the peripheral appearance of chemically identified components of the embryonic nervous system of central origin.

Neuronal development in larval chiton Ischnochiton hakodadensis (Mollusca: Polyplacophora)

Chitons are the most primitive molluscs and, thus, a matter of considerable interest for understanding both basic principles of molluscan neurogenesis and phylogeny. The development of the nervous system in trochophores of the chiton Ischnochiton hakodadensis from hatching to metamorphosis is described in detail by using confocal laser scanning microscopy and antibodies raised against serotonin, FMRFamide, and acetylated alpha tubulin. The earliest nervous elements detected were peripheral neurons located in the frontal hemisphere of posthatching trochophores and projecting into the apical organ. Among them, two pairs of unique large lateral cells appear to pioneer the pathways of developing adult nervous system. Chitons possess an apical organ that contains the largest number of neurons among all molluscan larvae investigated so far. Besides, many pretrochal neurons are situated outside the apical organ. The prototroch is not innervated by larval neurons. The first neurons of the developing adult central nervous system (CNS) appear later in the cerebral ganglion and pedal cords. None of the neurons of the larval nervous system are retained in the adult CNS. They cease to express their transmitter content and disintegrate after settlement. Although the adult CNS of chitons resembles that of polychaetes, their general scenario of neuronal development resembles that of advanced molluscs and differs from annelids. Thus, our data demonstrate the conservative pattern of molluscan neurogenesis and suggest independent origin of molluscan and annelid trochophores.

Insights into early molluscan neuronal development through studies of transmitter phenotypes in embryonic pond snails

Pond snails have long been the subject of intense scrutiny by researchers interested in general principles of development and also cellular and molecular neurobiology. Recent work has exploited both these fields of study by examining the ontogeny of the nervous system in these animals. Much of this work has focussed upon the development of specific transmitter phenotypes to provide vignettes of neuronal subpopulations that can be traced from early embryonic life through to adulthood. While such studies have generally confirmed previous explanations of gangliogenesis in gastropods, they have also indicated the presence of several neurons that appear earlier and in positions inconsistent with classical views of gastropods neurogenesis. The earliest of these cells contain FMRFamide-related peptides and have anteriorly projections that mark the future locations of ganglia and interconnecting pathways that will comprise the postembryonic central nervous system. These posterior, peptidergic cells, as well as certain, apical, monoaminergic neurons, disappear and apparently die near the end of embryonic life. Finally, populations of what appear to be peripheral sensory neurons begin to express catecholamines by around midway through embryonic life. Like several of the neurons expressing a variety of transmitters in the developing central ganglia, the catecholaminergic peripheral cells persist into postembryonic life. Transmitter phenotypes, cell shapes and locations, and neuritic morphologies all suggest that many of the neurons observed in early embryonic pond snails have recognizable homologues across the molluscs. Such observations have profoundly altered our views of neurogenesis in gastropods over the last few years. They also suggest the promise for pond snails as fruitful models for studying the roles and mechanisms for pioneering fibres, cues triggering apoptosis, and contrasting origins and mechanisms employed for generating central vs. peripheral neurons within a single organism.

Gene expression and function of FMRFamide-related neuropeptides in the snail Lymnaea

FMRFamide and a large family of related peptides (FaRPs) have been identified in every major metazoan phylum examined, including chordates. In the pulmonate snail Lymnaea this family of neuropeptides is encoded by a five-exon locus that is subject to alternative splicing. The two alternative mRNA transcripts are expressed in the CNS in a mutually exclusive manner at the single cell level, resulting in the differential distribution of the distinct sets of FaRPs that they encode in defined neuronal networks. Biochemical peptide purification, single-cell analysis by mass spectroscopy, and immunocytochemistry have led to an understanding of the post-translational processing patterns of the two alternative precursor proteins and identified at least 12 known and novel peptides contained in neuronal networks involved in cardiorespiration, penial control and withdrawal response. The pharmacological actions of single or co-expressed peptides are beginning to emerge for the cardiorespiratory network and its central and peripheral targets. Peptides derived from protein precursor 1 and contained in the heart excitatory central motoneurons E(he) have distinct functions and also act in concert in cardiac regulation, based on their unique effects on heartbeat and their differential stimulatory effects on second messenger pathways. Precursor-2 derived peptides, contained in the Visceral White Interneuron, a key neuron of the cardiorespiratory network, have mostly inhibitory effects on the VWI's central postsynaptic target neurons but with some of the peptides also exhibiting excitatory effects on the same cells.

Sequential developmental acquisition of neuromodulatory inputs to a central pattern-generating network

The activity of the adult stomatogastric ganglion (STG) depends on a large number of aminergic and peptidergic modulatory inputs. Our aim is to understand the role of these modulatory inputs in the development of the central pattern-generating networks of the STG. Therefore, we analyze the developmental and adult expressions of three neuropeptides in the stomatogastric nervous system of the lobsters Homarus americanus and Homarus gammarus by using wholemount immunocytochemistry and confocal microscopy. In adults, red pigment-concentrating hormone (RPCH)-like, proctolin-like, and a tachykinin-like immunoreactivity are present in axonal projections to the STG. At 50% of embryonic development (E50), all three peptides stain the commissural ganglia and brain, but only RPCH- and proctolin-like immunoreactivities stain axonal arbors in the STG. Tachykinin-like immunoreactivity is not apparent in the STG until larval stage II (LII). The RPCH-immunoreactive projection to the STG consists of two pairs of fibers. One pair stains for RPCH immunoreactivity at E50; the second RPCH-immunoreactive pair does not stain until about LII. One pair of the RPCH fibers double labels for tachykinin-like immunoreactivity. The adult complement of neuromodulatory inputs is not fully expressed until close to the developmental time at which major changes in the STG motor patterns occur, suggesting that neuromodulators play a role in the tuning of the central pattern generators during development.

Regulation of Drosophila FMRFamide neuropeptide gene expression

Physiologically important peptides are often encoded in precursors that contain several gene products; thus, regulation of expression of polypeptide proteins is crucial to transduction pathways. Differential processing of precursors by cell- or tissue-specific proteolytic enzymes can yield messengers with diverse distributions and dissimilar activities. FMRFamide-related peptides (FaRPs) are present throughout the animal kingdom and affect both neural and gastrointestinal functions. Organisms have several genes encoding numerous FaRPs with a common C-terminal structure but different N-terminal amino acid extensions. We have isolated SDNFMRFamide, DPKQDFMRFamide, and TPAEDFMRFamide contained in the Drosophila FMRFamide gene. To investigate the regulation of expression of FMRFamide peptides, we generated antisera to distinguish among the three neuropeptides. We have previously reported the distribution of SDNFMRFamide and DPKQDFMRFamide. In this article, we describe TPAEDFMRFamide expression. TPAEDFMRFamide antisera stain cells in embryonic, larval, pupal, and adult thoracic and abdominal ganglia. In addition, TPAEDFMRFamide-immunoreactive material is present in a lateral protocerebrum cell in adult. Thus, TPAEDFMRFamide antisera staining of neural tissue is different from SDNFMRFamide or DPKQDFMRFamide. In addition, TPAEDFMRFamide antisera stain larval, pupal, and adult gut, while SDNFMRFamide and DPKQDFMRFamide do not. TPAEDFMRFamide immunoreactivity is present in cells stained by FMRFamide antisera. Taken together, these data support the conclusion that TPAEDFMRFamide is differentially processed from the FMRFamide polypeptide protein precursor and may act in both neural and gastrointestinal tissue.

Development of catecholaminergic neurons in the pond snail, Lymnaea stagnalis: I. Embryonic development of dopamine-containing neurons and dopamine-dependent behaviors

The embryonic development of the catecholaminergic system of the pond snail, Lymnaea stagnalis, was investigated by using chromatographic and histochemical methods. High performance liquid chromatography suggested that dopamine was the only catecholamine present in significant concentrations throughout the embryonic development of Lymnaea. Dopamine first became detectable at about embryonic stage (E) 15 (15% of embryonic development) and then increased in amount during early development to reach about 120-140 fmol per animal by around E40. Dopamine content remained stable during mid-embryogenesis (E40-65), increased slowing for the next couple of days, and then increased rapidly to culminate at about 400 fmol per animal by hatching. The detection of aldehyde- and glyoxylate-induced fluorescence and of tyrosine hydroxylaselike immunoreactivity indicated that the first catecholaminergic cells appeared in the late trochophore or early veliger stage of embryonic development (E32-35). The paired perikarya of these transient apical catecholaminergic (TAC) neurons were located beneath the apical plate, remained outside of the central ganglia during embryogenesis, and no longer contained detectable catecholamines close to hatching. TAC neurons bore cilia on the ends of short processes that penetrated the overlying epithelium; their long processes branched repeatedly under the ciliated apical plate. Several smaller catecholaminergic cells first appeared in the anterior margin of the foot at a stage when the embryos began to metamorphose from the veliger form (E55). Similar bipolar cells later appeared in the tentacle and lips. The axons of all of these small peripheral cells projected centrally and terminated within the neuropil of different central ganglia. Central catecholaminergic neurons, including RPeD1, differentiated only after metamorphosis was complete (E75). Development of locomotor, respiratory, and feeding behaviors correlated with maturation of catecholaminergic neurons, as indicated by histology and chromatography.

Development of catecholaminergic neurons in the pond snail, Lymnaea stagnalis: II. Postembryonic development of central and peripheral cells

Catecholamines have long been thought to play important roles in different mollusc neural functions. The present study used glyoxylate- and aldehyde-induced histofluorescence to identify central and peripheral catecholaminergic neurons in the snail Lymnaea stagnalis. The majority of these cells were also found to react to antibodies raised against tyrosine hydroxylase. A minority of the catecholaminergic neurons, however, exhibited no such immunoreactivity. The number of central catecholaminergic neurons nearly doubled (from about 45 to about 80 cells) during the first 2-3 days of postembryonic development. Thereafter, catecholaminergic neurons again doubled in number and generally grew by about 100-200% in soma diameter as the snails grew by 1,000% in overall linear measurements. In contrast to the relatively meager addition of central catecholaminergic neurons, several thousand catecholaminergic somata were added to different peripheral tissues during postembryonic development. These small, centrally projecting neurons were particularly concentrated in the lips, esophagus, anterior margin of the foot, and different regions of the male and female reproductive tracts. Chromatographic analyses indicated that dopamine was the major catecholamine present in the central ganglia, foot, and esophagus, although detectable levels of norepinephrine (approximately 20% of dopamine levels) were also found in the ganglia. The total content but not the concentration of dopamine increased within the tissue samples during postembryonic development. The companion study (Voronezhskaya et al. [1999] J. Comp. Neurol. 404:285-296) and the present study furnish a complete description of central and peripheral catecholaminergic neurons from their first appearance in early embryonic development to adulthood.

Ontogenetic alteration in peptidergic expression within a stable neuronal population in lobster stomatogastric nervous system

In the adult lobster, Homarus gammarus, the stomatogastric ganglion (STG) contains two well-defined motor pattern generating networks that receive numerous modulatory peptidergic inputs from anterior ganglia. We are studying the appearance of extrinsic peptidergic inputs to these networks during ontogenesis. Neuron counts indicate that as early as 20% of development (E20) the STG neuronal population is quantitatively established. By using immunocytochemical detection of 5-bromo-2'-deoxyuridine incorporation, we found no immunopositive cells in the STG by E70. We concluded that the STG neuronal population remains quantitatively stable from mid-embryonic life until adulthood. We then investigated the ontogeny of FLRFamide- and proctolin-like peptides in the stomatogastric nervous system, from their first appearance until adulthood by using whole mount immunocytochemistry. Numerous FLRFamide-like-immunoreactive STG neuropilar ramifications were observable as early as E45 and remain thereafter. From E50 to the first larval stage, one to three STG somata stained, while somatic staining was not observed in larval stage II and subsequent stages. From E50 and thereafter, the STG neuropilar area was immunopositive for proctolin. One to two proctolinergic somata were detected in the STG of the three larval stages but were not seen in embryos, the post-larval stage or in adults. Thus, peptidergic inputs to the STG are present from mid-embryonic life. Moreover, whereas in the adult, STG neurons only contain glutamate or acetylcholine, some neurons transiently express peptidergic phenotypes during development. Although this system expresses an ontogenetic peptidergic plasticity, the STG neurons produce a single stable embryonic-larval motor output (Casasnovas and Meyrand [1995] J. Neurosci. 15:5703-5718).