Development of serotoninlike immunoreactivity in the embryonic nervous system of the snail Lymnaea stagnalis

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.

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.

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

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

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

The calmodulin-binding, short linear motif, NSCaTE is conserved in L-type channel ancestors of vertebrate Cav1.2 and Cav1.3 channels

NSCaTE is a short linear motif of (xWxxx(I or L)xxxx), composed of residues with a high helix-forming propensity within a mostly disordered N-terminus that is conserved in L-type calcium channels from protostome invertebrates to humans. NSCaTE is an optional, lower affinity and calcium-sensitive binding site for calmodulin (CaM) which competes for CaM binding with a more ancient, C-terminal IQ domain on L-type channels. CaM bound to N- and C- terminal tails serve as dual detectors to changing intracellular Ca²⁺ concentrations, promoting calcium-dependent inactivation of L-type calcium channels. NSCaTE is absent in some arthropod species, and is also lacking in vertebrate L-type isoforms, Ca_v1.1 and Ca_v1.4 channels. The pervasiveness of a methionine just downstream from NSCaTE suggests that L-type channels could generate alternative N-termini lacking NSCaTE through the choice of translational start sites. Long N-terminus with an NSCaTE motif in L-type calcium channel homolog LCa_v1 from pond snail Lymnaea stagnalis has a faster calcium-dependent inactivation than a shortened N-termini lacking NSCaTE. NSCaTE effects are present in low concentrations of internal buffer (0.5 mM EGTA), but disappears in high buffer conditions (10 mM EGTA). Snail and mammalian NSCaTE have an alpha-helical propensity upon binding Ca²⁺-CaM and can saturate both CaM N-terminal and C-terminal domains in the absence of a competing IQ motif. NSCaTE evolved in ancestors of the first animals with internal organs for promoting a more rapid, calcium-sensitive inactivation of L-type channels.

NALCN ion channels have alternative selectivity filters resembling calcium channels or sodium channels

NALCN is a member of the family of ion channels with four homologous, repeat domains that include voltage-gated calcium and sodium channels. NALCN is a highly conserved gene from simple, extant multicellular organisms without nervous systems such as sponges and placozoans and mostly remains a single gene compared to the calcium and sodium channels which diversified into twenty genes in humans. The single NALCN gene has alternatively-spliced exons at exons 15 or exon 31 that splices in novel selectivity filter residues that resemble calcium channels (EEEE) or sodium channels (EKEE or EEKE). NALCN channels with alternative calcium, (EEEE) and sodium, (EKEE or EEKE) -selective pores are conserved in simple bilaterally symmetrical animals like flatworms to non-chordate deuterostomes. The single NALCN gene is limited as a sodium channel with a lysine (K)-containing pore in vertebrates, but originally NALCN was a calcium-like channel, and evolved to operate as both a calcium channel and sodium channel for different roles in many invertebrates. Expression patterns of NALCN-EKEE in pond snail, Lymnaea stagnalis suggest roles for NALCN in secretion, with an abundant expression in brain, and an up-regulation in secretory organs of sexually-mature adults such as albumen gland and prostate. NALCN-EEEE is equally abundant as NALCN-EKEE in snails, but is greater expressed in heart and other muscle tissue, and 50% less expressed in the brain than NALCN-EKEE. Transfected snail NALCN-EEEE and NALCN-EKEE channel isoforms express in HEK-293T cells. We were not able to distinguish potential NALCN currents from background, non-selective leak conductances in HEK293T cells. Native leak currents without expressing NALCN genes in HEK-293T cells are NMDG(+) impermeant and blockable with 10 µM Gd(3+) ions and are indistinguishable from the hallmark currents ascribed to mammalian NALCN currents expressed in vitro by Lu et al. in Cell. 2007 Apr 20;129(2):371-83.

Gene transcription and splicing of T-type channels are evolutionarily-conserved strategies for regulating channel expression and gating

T-type calcium channels operate within tightly regulated biophysical constraints for supporting rhythmic firing in the brain, heart and secretory organs of invertebrates and vertebrates. The snail T-type gene, LCa(v)3 from Lymnaea stagnalis, possesses alternative, tandem donor splice sites enabling a choice of a large exon 8b (201 aa) or a short exon 25c (9 aa) in cytoplasmic linkers, similar to mammalian homologs. Inclusion of optional 25c exons in the III-IV linker of T-type channels speeds up kinetics and causes hyperpolarizing shifts in both activation and steady-state inactivation of macroscopic currents. The abundant variant lacking exon 25c is the workhorse of embryonic Ca(v)3 channels, whose high density and right-shifted activation and availability curves are expected to increase pace-making and allow the channels to contribute more significantly to cellular excitation in prenatal tissue. Presence of brain-enriched, optional exon 8b conserved with mammalian Ca(v)3.1 and encompassing the proximal half of the I-II linker, imparts a ~50% reduction in total and surface-expressed LCa(v)3 channel protein, which accounts for reduced whole-cell calcium currents of +8b variants in HEK cells. Evolutionarily conserved optional exons in cytoplasmic linkers of Ca(v)3 channels regulate expression (exon 8b) and a battery of biophysical properties (exon 25c) for tuning specialized firing patterns in different tissues and throughout development.

Analysis of neurotransmitter distribution in brain development of benthic and pelagic octopod cephalopods

The database on neurotransmitter distribution during central nervous system development of cephalopod mollusks is still scarce. We describe the ontogeny of serotonergic (5-HT-ir) and FMRFamide-like immunoreactive (Fa-lir) neurons in the central nervous system of the benthic Octopus vulgaris and Fa-lir distribution in the pelagic Argonauta hians. Comparing our data to previous studies, we aim at revealing shared immunochemical domains among coleoid cephalopods, i.e., all cephalopods except nautiluses. During development of O. vulgaris, 5-HT-ir and Fa-lir elements occur relatively late, namely during stage XII, when the brain neuropils are already highly differentiated. In stage XII-XX individuals, Fa-lir cell somata are located in the middle and posterior subesophageal mass and in the optic, posterior basal, and superior buccal lobes. 5-HT is predominately expressed in cell somata of the superior buccal, anterior basal, and optic lobes, as well as in the subesophageal mass. The overall population of Fa-lir neurons is larger than the one expressing 5-HT. Fa-lir elements are distributed throughout homologous brain areas of A. hians and O. vulgaris. We identified neuronal subsets with similar cell number and immunochemical phenotype in coleoids. These are located in corresponding brain regions of developmental stages and adults of O. vulgaris, A. hians, and the decapod squid Idiosepius notoides. O. vulgaris and I. notoides exhibit numerous 5-HT-ir cell somata in the superior buccal lobes but none or very few in the inferior buccal lobes. The latter have previously been homologized to the gastropod buccal ganglia, which also lack 5-HT-ir cell somata in euthyneuran gastropods. Among coleoids, 5-HT-ir neuronal subsets, which are located ventrally to the lateral anterior basal lobes and in the anterior middle subesophageal mass, are candidates for homologous subsets. Contrary to I. notoides, octopods exhibit Fa-lir cell somata ventrally to the brachial lobes and 5-HT-ir cell somata close to the stellate ganglia.

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.

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

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

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.

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

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

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.

Localization of glutamate-like immunoreactive neurons in the central and peripheral nervous system of the adult and developing pond snail, Lymnaea stagnalis

We investigated the distribution and projection patterns of central and peripheral glutamate-like immunoreactive (GLU-LIR) neurons in the adult and developing nervous system of Lymnaea. Altogether, 50-60 GLU-LIR neurons are present in the adult central nervous system. GLU-LIR labeling is shown in the interganglionic bundle system and at the varicosities in neuropil of the central ganglia. In the periphery, the foot, lip, and tentacle contain numerous GLU-LIR bipolar sensory neurons. In the juvenile Lymnaea, GLU-LIR elements at the periphery display a pattern of distribution similar to that seen in adults, whereas labeled neurons increase in number in the different ganglia of the central nervous system from juvenile stage P1 up to adulthood. During embryogenesis, GLU-LIR innervation can be detected first at the 50% stage of embryonic development (the E50% stage) in the neuropil of the cerebral and pedal ganglia, followed by the emergence of labeled pedal nerve roots at the E75% stage. Before hatching, at the E90% stage, a few GLU-LIR sensory cells can be found in the caudal foot region. Our findings indicate a wide range of occurrence and a broad role for glutamate in the gastropod nervous system; hence they provide a basis for future studies on glutamatergic events in networks underlying different behaviors.

Neural control of the velum in larvae of the gastropod, Ilyanassa obsoleta

Larval molluscs commonly use ciliated vela to swim and feed. In this study we used immunohistochemistry to demonstrate innervation of velar cilia and muscles by monoaminergic and peptidergic fibres in the caenogastropod, Ilyanassa obsoleta. Photoelectric recordings from pre-oral cilia on isolated pieces of velum revealed that serotonin increased, whereas catecholamines (dopamine and norepinephrine) decreased beat frequency at concentrations of 10(-6) to 10(-9) mol l(-1). Catecholamines also increased the frequency of momentary, isolated arrests of pre-oral cilia, but failed to suppress beating of the post-oral cilia at these concentrations. The neuropeptides, FMRFamide and Leu-enkephalin, did not affect the frequency of ciliary beating or of isolated ciliary arrests, but did induce numerous muscular contractions, which were accompanied by sustained ciliary arrests. In terms of whole animal behaviour, serotonin caused larvae to concentrate toward the top of a water column and to increase feeding, whereas catecholamines caused larvae to concentrate toward the bottom of a water column and decrease feeding. Monoamine analogues which facilitated or opposed the effects of synthetic transmitters on larval behaviour, further suggested that these transmitters are released endogenously to control velar function. Finally, applications of peptides to whole larvae caused increased frequency of locomotory arrests. Together these findings demonstrate several potential roles for the nervous system in controlling larval behaviour in gastropods.

Development of embryonic and larval cells containing serotonin, catecholamines, and FMRFamide-related peptides in the gastropod mollusc Phestilla sibogae

The present immunocytochemical study provides one of the first detailed descriptions of the development of cells containing a variety of neurotransmitters during much of the larval life of a nudibranch gastropod. Throughout much of early development, serotonergic cells were located only in the apical organ; as larvae approached metamorphosis, serotonergic cells were also detected in the cerebropleural and pedal ganglia. Cells exhibiting tyrosine hydroxylase immunoreactivity (indicative of catecholamine synthesis) were first located near the mouth but by late embryonic stages were also located in the apical organ and near the velum and eyes. By late larval stages, numerous catecholaminergic cells were found in the foot, with concentrations in the propodium. Finally, the first cells exhibiting FMRFamide immunoreactivity were detected posterior to the neuropil of the cerebropleural ganglia in the early embryo. Fibers that presumably originated from these cells subsequently invaded the cerebral and pedal ganglia and the apical organ. By early larval stages, a second pair of peptidergic neurons was located near the first pair, and additional peptidergic neurons were located in the apical organ and peripheral positions in the foot and medial and dorsal to the eyes. In addition to providing a unique phyletic perspective to our understanding of gastropod neural development, the present study also sets the stage for future studies into changes in the nervous system as this gastropod undergoes metamorphosis.

Embryogenesis of the serotonergic system in the earthwormEisenia fetida (Annelida, Oligochaeta): Immunohistochemical and biochemical studies

Organization of the serotonergic system and changes of the serotonin (5-HT) content were studied during the embryogenesis of the earthworm Eisenia fetida, using immunocytochemistry and HPLC. A gradual emergence of 5-HT immunoreactive (IR) cells and their axon projections in the several ganglia of the central (CNS) and peripheral nervous system are described in the context of a staged time-scale of development. The first 5-HT-IR neurons appear in the subesophageal ganglion at an early embryonic stage (E2), followed by neurons in some rostrally located ventral ganglia. In the cerebral ganglion, 5-HT-IR cells can be detected only from stage E5. The number of labeled cells in each ganglion of the embryo increases until hatching, when it is still considerably lower than that observed in adults. This shows that the development of the 5-HTergic system is far from complete by the end of embryogenesis. Organization of 5-HT-IR innervation of the body wall starts by stages E3 to E4. In the stomatogastric nervous system the first 5-HT-IR fibers can be detected by stage E5. By stage E9 5-HT immunopositive neurons can be observed in both the stomatogastric ganglia and the enteric plexus. Both 5-HT levels and the numbers of the labeled cells show a significant increase before hatching, which indicate a functional maturation of the 5-HTergic system. Based on the early appearance of 5-HT, we suppose that it may play a regulatory role in both the gangliogenesis and the maturation of peripheral functions necessary during postembryonic life.

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

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

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.

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

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

Embryogenesis of GABAergic elements in the nervous system of Eisenia fetida (Annelida, Oligochaeta)

The appearance and development of the GABA-immunoreactive nervous elements in the central nervous system of Eisenia fetida were studied by immunocytochemistry. The nervous system originates from the neuroectoderm situated on the ventral side of the embryo. The organization of the circumpharyngeal ring starts earlier than that of the ventral cord. In the elementary ring the first GABA-immunopositive neurons can be observed (E1 stage) around the mouth. Later the cell number gradually increases and parallel to this process the elementary ring is separeted into a superficial and a deeper portion. The brain and the subesophageal ganglion will be organized from the superficial ring, while the nervous elements of the deeper ring will give rise for the first GABA-immunoreactive elements of the stomatogastric nervous system. In the early stages of the embryogenesis the immunoreactive cells of the developing brain appear solitary, while from the stage E4 they gradually are observed in groups. According to their position, these cell groups are similar to those observed in the brain of the adult earthworms. During embryogenesis the level of the ventral cord ganglia depends on their position in the ectodermal germ bands. It means, that the more organized ganglia are near the circumpharyngeal ring, mean while less developed ganglia are situated caudally from them. By the end of the embryogenesis all ganglia of the ventral cord will be equally well organized. The nerve tracts of the ganglia are built up from contra- and ipsilateral by projected fibres. From E3 stage the medial tracts, mean while from the E4 stage the lateral tracts begin to be formed. During the next stages, more and more fibres connect to the both tracts. At hatching, the development of the central nervous system of Eisenia fetida is not completed, the process is continued during the postembryonic development.

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.

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

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

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.

Modern views on an ancient chemical: serotonin effects on cell proliferation, maturation, and apoptosis

Evolutionarily, serotonin existed in plants even before the appearance of animals. Indeed, serotonin may be tied to the evolution of life itself, particularly through the role of tryptophan, its precursor molecule. Tryptophan is an indole-based, essential amino acid which is unique in its light-absorbing properties. In plants, tryptophan-based compounds capture light energy for use in metabolism of glucose and the generation of oxygen and reduced cofactors. Tryptophan, oxygen, and reduced cofactors combine to form serotonin. Serotonin-like molecules direct the growth of light-capturing structures towards the source of light. This morphogenic property also occurs in animal cells, in which serotonin alters the cytoskeleton of cells and thus influences the formation of contacts. In addition, serotonin regulates cell proliferation, migration and maturation in a variety of cell types, including lung, kidney, endothelial cells, mast cells, neurons and astrocytes). In brain, serotonin has interactions with seven families of receptors, numbering at least 14 distinct proteins. Of these, two receptors are important for the purposes of this review. These are the 5-HT1A and 5-HT2A receptors, which in fact have opposing functions in a variety of cellular and behavioral processes. The 5-HT1A receptor develops early in the CNS and is associated with secretion of S-100beta from astrocytes and reduction of c-AMP levels in neurons. These actions provide intracellular stability for the cytoskeleton and result in cell differentiation and cessation of proliferation. Clinically, 5-HT1A receptor drugs decrease brain activity and act as anxiolytics. The 5-HT2A receptor develops more slowly and is associated with glycogenolysis in astrocytes and increased Ca(++) availability in neurons. These actions destabilize the internal cytoskeleton and result in cell proliferation, synaptogenesis, and apoptosis. In humans, 5-HT2A receptor drugs produce hallucinations. The dynamic interactions between the 5-HT1A and 5-HT2A receptors and the cytoskeleton may provide important insights into the etiology of brain disorders and provide novel strategies for their treatment.

Early development of the serotonergic and dopaminergic nervous system in Spisula solidissima (surf clam) larvae

We have defined the development of the serotonergic and dopaminergic components of the central nervous system in the early Spisula solidissima (surf clam) embryo using HPLC and immunocytochemistry. HPLC analysis reveals norepinephrine, dopamine, and serotonin are present at 24 h post-fertilization. Immunocytochemistry shows that the serotonergic nervous system emerges during the late trochophore stage with the development of a single serotonergic cell, C/A1, in the cerebral/apical ganglion. After 48 h, a second serotonergic cell forms, C/A2, which is connected to C/A1 by two serotonergic processes, and a single serotonergic cell emerges in the visceral ganglion, V1. At 72 h, a new serotonergic cell body develops in the cerebral/apical ganglion, C/A3. After 96 h, the cerebral/apical ganglion and visceral ganglion are connected by a serotonergic process. Expression of the dopamine receptor, D2, begins by 24 h with a generalized expression in the region of the developing gut. D2 expression in the gut ceases by 48 h. At 48 h, a network of fibers forms dorsolateral to the mouth. By 72 h, D2 expressing projections emerge from this network.

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.

The evolution of the serotonergic nervous system

The pattern of development of the serotonergic nervous system is described from the larvae of ctenophores, platyhelminths, nemerteans, entoprocts, ectoprocts (bryozoans), molluscs, polychaetes, brachiopods, phoronids, echinoderms, enteropneusts and lampreys. The larval brain (apical ganglion) of spiralian protostomes (except nermerteans) generally has three serotonergic neurons and the lateral pair always innervates the ciliary band of the prototroch. In contrast, brachiopods, phoronids, echinoderms and enteropneusts have numerous serotonergic neurons in the apical ganglion from which the ciliary band is innervated. This pattern of development is much like the pattern seen in lamprey embryos and larvae, which leads the author to conclude that the serotonergic raphe system found in vertebrates originated in the larval brain of deuterostome invertebrates. Further, the neural tube of chordates appears to be derived, at least in part, from the ciliary band of deuterostome invertebrate larvae. The evidence shows no sign of a shift in the dorsal ventral orientation within the line leading to the chordates.

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

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

Development of key neurons for learning stimulates learning ability in Lymnaea stagnalis

The pond snails, Lymnaea stagnalis, change their ability of conditioned taste aversion (CTA) during their development, for example, stage 29 embryos can acquire the CTA, whereas immature snails come to use a long-term memory to maintain the conditioned response. We thus examined the relationships between the learning ability and the development of key neurons (cerebral giant cells: CGCs) for this CTA. The immunoreactivity of serotonin, which is a main neurotransmitter employed in the feeding circuitry, was first observed in the CGCs at the stage 29. After hatching, the CGCs developed their neuropile faster than other cells in the buccal and cerebral ganglia, resulting in their early innervation at the immature stage. The present results, therefore, indicate that the development of key neurons for learning stimulates the developmental changes in learning ability.

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.

Does gravity influence the early stages of the development of the nervous system in an amphibian?

As a result of previous studies using hypergravity (centrifuge) or virtual microgravity (clinostat), it was proposed that gravity was involved in embryonic development, i.e., in the establishment of the embryonic polarities and the body plan pattern which subsequently direct morphogenesis and organogenesis of the central nervous system and of sensory organs. Recent experiments were performed in space using sounding rockets and orbiting space-modules to ascertain whether gravity is indeed required for embryogenesis in Invertebrates and Vertebrates. Eggs fertilised in vivo or in vitro in microgravity showed some abnormalities during embryonic development but were able to regulate and produce nearly normal larvae.

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

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

Development of serotonin-like immunoreactivity in the embryos and larvae of nudibranch mollusks with emphasis on the structure and possible function of the apical sensory organ

This investigation provides a light and electron microscopic examination of the development of serotonin-like immunoreactivity and structure of the apical sensory organ (ASO) in embryos and/or larvae of four nudibranch species: Berghia verrucicornis, Phestilla sibogae, Melibe leonina, and Tritonia diomedea. Serotonin-like immunoreactivity is first expressed in somata, dendrites, and axons of a group of five distinct neurons within the ASO. These neurons extend axons into an apical neuropil, a structure that is situated centrally and immediately dorsal to the cerebral commissure. Three of these neurons possess sensory dendrites that extend through the pretrochal epithelium, each supporting two cilia at their distal ends. Later development of serotonin-like immunoreactivity includes 1) axons from the apical neuropil that extend into each of the velar lobes; 2) neuron perikarya in the cerebral and pedal ganglia; 3) axons that extend through the cerebral commissure, cerebral-pedal connectives, pedal commissure, and possibly the visceral loop connective; and 4) axons extending from each pedal ganglion into the larval foot. Ultrastructurally, the ASO can be seen to be composed of three lobes and an apical neuropil that is separately delineated from the cerebral commissure. Four cell types are present within the ASO: ciliary tuft cells, type I and type II parampullary neurons, and ampullary neurons. Immunofluorescence and 3,3' diaminobenzidine tetrahydrochloride (DAB) labeling verify that the serotonergic neurons of the ASO are type I and type II parampullary neurons. The ampullary and type I parampullary neurons possess dendrites that extend through the pretrochal epithelium. These dendrites are partitioned into three bundles, one on either side of the ciliary tuft cells and a third bundle penetrating the pretrochal epithelium centrally between the ciliary tuft cells. One serotonergic type I parampullary neuron is associated with each of these bundles. Two ampullary neurons are associated with each of the lateral dendritic bundles, while the central bundle includes only one. Ultrastructural analyses of serotonergic axonal innervation arising from the ASO agree with those determined from fluorescently labeled material. The structure of the ASO and its associated serotonergic axons suggest that the serotonergic component of this structure senses environmental stimuli affecting velar function, possibly the contractility of muscle fibers in the velar lobes. Similarities and differences among the ASOs of embryos and larvae from various invertebrate phyla may provide useful data that will assist in the reconstruction of phylogenetic relationships.

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

1. In the present study we have investigated the ontogeny of FMRFamide expression in the snail, Lymnaea stagnalis, from its first appearance to its distribution in young adults. 2. The first FMRFamide-like immunoreactive (FaLI) cells within CNS appear by E45 embryonic stage (premetamorphic veliger). The number of FaLI neurons increases throughout both pre- and post-hatching development. 3. Both transient and sustained expression of FMRFamide-like immunoreactivity by specific sets of neurons occurs. Two cells which transiently express immunoreactivity appear outside the future CNS by the stage E45. Other population of transient FaLI neurons includes bilaterally symmetric groups of cells in the cerebral and pedal ganglia during posthatching stages P1 (hatchlings) to P5 (juveniles). All other immunostained cells which appear during development maintain their transmitter phenotype into adulthood. 4. The possible role of FMRFamide-related peptides in the processes of morpho- and neurogenesis is discussed.

Dopamine regulation of neurite outgrowth from identified Lymnaea neurons in culture

1. An identified dopaminergic interneuron (RPeD1) of the snail Lymnaea stagnalis, makes specific synaptic connections with a number of target (VI and VJ) but not non-target (VF and RPB) neurons in vivo. When cultured in vitro with both target and non-target cells, RPeD1 re-establishes synapses with target cells only. 2. To test whether exogenous dopamine exerts effects on the neurite outgrowth of both target and non-target neurons respectively, these cells were cultured in conditioned media (CM) in the presence of dopamine (10(-5) M). The growth of the non-target cells was severely restricted and retarded in the presence of dopamine. These data suggest that dopamine may regulate neurite outgrowth of non-target cells in culture. 3. The growth regulatory effects of dopamine on the non-target cells were blocked in the presence of a dopamine receptor antagonist (R(+) SCH-23390, 10(-4) M). These results indicate that dopamine-induced growth regulation of the non-target cells is mediated via dopamine receptors on these cells. 4. In the absence of conditioned media, dopamine was not sufficient to exert growth promoting effects on either target or non-target cells. 5. Taken together, our data show that dopamine differentially regulates growth of identified Lymnaea neurons in culture. Dopamine alone, however, is not sufficient to initiate and support neurite outgrowth from these cells. Rather, it functions to suppress the neurite outgrowth of the non-target cells, initiated by the conditioned media.