Structure and physiological activity of the motoneurons of the nematode Ascaris

Forward and backward locomotion patterns in C. elegans generated by a connectome-based model simulation

Caenorhabditis elegans (C. elegans) can produce various motion patterns despite having only 69 motor neurons and 95 muscle cells. Previous studies successfully elucidate the connectome and role of the respective motor neuron classes related to movement. However, these models have not analyzed the distribution of the synaptic and gap connection weights. In this study, we examined whether a motor neuron and muscle network can generate oscillations for both forward and backward movement and analyzed the distribution of the trained synaptic and gap connection weights through a machine learning approach. This paper presents a connectome-based neural network model consisting of motor neurons of classes A, B, D, AS, and muscle, considering both synaptic and gap connections. A supervised learning method called backpropagation through time was adapted to train the connection parameters by feeding teacher data composed of the command neuron input and muscle cell activation. Simulation results confirmed that the motor neuron circuit could generate oscillations with different phase patterns corresponding to forward and backward movement, and could be switched at arbitrary times according to the binary inputs simulating the output of command neurons. Subsequently, we confirmed that the trained synaptic and gap connection weights followed a Boltzmann-type distribution. It should be noted that the proposed model can be trained to reproduce the activity patterns measured for an animal (HRB4 strain). Therefore, the supervised learning approach adopted in this study may allow further analysis of complex activity patterns associated with movements.

My life with Sydney, 1961-1971

During the 1961-1971 decade, Sydney Brenner made several significant contributions to molecular biology-showing that the genetic code is a triplet code; discovery of messenger RNA; colinearity of gene and protein; decoding of chain terminating codons; and then an important transition: the development of the nematode Caenorhabditis elegans into the model eucaryote genetic system that has permeated the whole of recent biology.

The anatomy of the third-stage larva of Toxocara canis and Toxocara cati

This review describes the morphology and ultrastructure of the third-stage larva of Toxocara canis, the larval stage that hatches from the egg and persists in the tissues of various paratenic hosts including infected humans. This larva remains unchanged as it passes from host to host and lives for extended periods in this same body form until it makes its way to the gastro-intestinal tract of its final host. There has been less work on the anatomy and ultrastructure of the larva of Toxocara cati, but overall, it appears very similar to that of T. canis with the only apparent difference being that the larva is a few microns thinner in diameter. The paper describes the anatomy and ultrastructure of the larva and the various major structures of the various body systems including the cuticle, hypodermis, the nervous tissue, oesophagus and the lumen-free intestine, and the large excretory gland cell that fills much of the pseudocoel and which appears to be the source of the components that are recognized immunologically by the infected host. It is suggested that further detailed studies on the larva could provide significant information that would allow a greater understanding of how the larva persists within these hosts and may provide information that would make the larva an excellent link to work on nematode anatomy that would link the work being done with this model system to that of Caenorhabditis elegans and the large amount of historical work already performed on adult ascaridoid nematodes.

C. elegans neurons have functional dendritic spines

Dendritic spines are specialized postsynaptic structures that transduce presynaptic signals, are regulated by neural activity and correlated with learning and memory. Most studies of spine function have focused on the mammalian nervous system. However, spine-like protrusions have been reported in C. elegans (Philbrook et al., 2018), suggesting that the experimental advantages of smaller model organisms could be exploited to study the biology of dendritic spines. Here, we used super-resolution microscopy, electron microscopy, live-cell imaging and genetics to show that C. elegans motor neurons have functional dendritic spines that: (1) are structurally defined by a dynamic actin cytoskeleton; (2) appose presynaptic dense projections; (3) localize ER and ribosomes; (4) display calcium transients triggered by presynaptic activity and propagated by internal Ca++ stores; (5) respond to activity-dependent signals that regulate spine density. These studies provide a solid foundation for a new experimental paradigm that exploits the power of C. elegans genetics and live-cell imaging for fundamental studies of dendritic spine morphogenesis and function.

Different Bioactive Neuropeptides are Expressed in Two Sub-Classes of GABAergic RME Nerve Ring Motorneurons in Ascaris suum

Neuropeptides can have significant effects on neurons and synapses, but among the ∼250 predicted peptides in nematodes, few have been characterized functionally. Here, we report new neuropeptides in the 4 RME nerve ring motorneurons of the nematode Ascaris suum. These GABAergic neurons are involved in three-dimensional head movement. Mass spectrometry (MS) of single dissected RMEs detected a total of 12 neuropeptides (encoded by five genes), nine of which are novel. None of these are expressed in the DI/VI inhibitory GABAergic motorneurons that synapse onto body wall muscle. Using peptide sequences obtained by tandem MS, we cloned the peptide-encoding transcripts and synthesized riboprobes for in situ hybridization (ISH). This complementary technique corroborated the results from single-cell MS, showing that the dissections were not contaminated with adhering tissue from other cells. We also synthesized a multiple antigenic peptide to raise a highly specific antibody against one of the endogenous peptides, which labeled the same cells detected by MS and ISH. Our results show that the RMEs can be divided into two subsets: RMED/V (expressing afp-2, afp-15, Asu-nlp-58, and high levels of afp-16) and RMEL/R (expressing afp-15 and low levels of afp-4 and afp-16). Almost all of these peptides are bioactive in A. suum.

Immobility in the sedentary plant-parasitic nematode H. glycines is associated with remodeling of neuromuscular tissue

The sedentary plant-parasitic nematodes are considered among the most economically damaging pathogens of plants. Following infection and the establishment of a feeding site, sedentary nematodes become immobile. Loss of mobility is reversed in adult males while females never regain mobility. The structural basis for this change in mobility is unknown. We used a combination of light and transmission electron microscopy to demonstrate cell-specific muscle atrophy and sex-specific renewal of neuromuscular tissue in the sedentary nematode Heterodera glycines. We found that both females and males undergo body wall muscle atrophy and loss of attachment to the underlying cuticle during immobile developmental stages. Male H. glycines undergo somatic muscle renewal prior to molting into a mobile adult. In addition, we found developmental changes to the organization and number of motor neurons in the ventral nerve cord correlated with changes in mobility. To further examine neuronal changes associated with immobility, we used a combination of immunohistochemistry and molecular biology to characterize the GABAergic nervous system of H. glycines during mobile and immobile stages. We cloned and confirmed the function of the putative H. glycines GABA synthesis-encoding gene hg-unc-25 using heterologous rescue in C. elegans. We found a reduction in gene expression of hg-unc-25 as well as a reduction in the number of GABA-immunoreactive neurons during immobile developmental stages. Finally, we found evidence of similar muscle atrophy in the phylogenetically diverged plant-parasitic nematode Meloidogyne incognita. Together, our data demonstrate remodeling of neuromuscular structure and function during sedentary plant-parasitic nematode development.

Unexpected Variation in Neuroanatomy among Diverse Nematode Species

Nematodes are considered excellent models for understanding fundamental aspects of neuron function. However, nematodes are less frequently used as models for examining the evolution of nervous systems. While the habitats and behaviors of nematodes are diverse, the neuroanatomy of nematodes is often considered highly conserved. A small number of nematode species greatly influences our understanding of nematode neurobiology. The free-living species Caenorhabditis elegans and, to a lesser extent, the mammalian gastrointestinal parasite Ascaris suum are, historically, the primary sources of knowledge regarding nematode neurobiology. Despite differences in size and habitat, C. elegans and A. suum share a surprisingly similar neuroanatomy. Here, we examined species across several clades in the phylum Nematoda and show that there is a surprising degree of neuroanatomical variation both within and among nematode clades when compared to C. elegans and Ascaris. We found variation in the numbers of neurons in the ventral nerve cord and dye-filling pattern of sensory neurons. For example, we found that Pristionchus pacificus, a bacterial feeding species used for comparative developmental research had 20% fewer ventral cord neurons compared to C. elegans. Steinernema carpocapsae, an insect-parasitic nematode capable of jumping behavior, had 40% more ventral cord neurons than C. elegans. Interestingly, the non-jumping congeneric nematode, S. glaseri showed an identical number of ventral cord neurons as S. carpocapsae. There was also variability in the timing of neurodevelopment of the ventral cord with two of five species that hatch as second-stage juveniles showing delayed neurodevelopment. We also found unexpected variation in the dye-filling of sensory neurons among examined species. Again, sensory neuron dye-filling pattern did not strictly correlate with phylogeny. Our results demonstrate that variation in nematode neuroanatomy is more prevalent than previously assumed and recommend this diverse phylum for future "evo-devo-neuro" studies.

Mass Spectrometry of Single GABAergic Somatic Motorneurons Identifies a Novel Inhibitory Peptide, As-NLP-22, in the Nematode Ascaris suum

Neuromodulators have become an increasingly important component of functional circuits, dramatically changing the properties of both neurons and synapses to affect behavior. To explore the role of neuropeptides in Ascaris suum behavior, we devised an improved method for cleanly dissecting single motorneuronal cell bodies from the many other cell processes and hypodermal tissue in the ventral nerve cord. We determined their peptide content using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS). The reduced complexity of the peptide mixture greatly aided the detection of peptides; peptide levels were sufficient to permit sequencing by tandem MS from single cells. Inhibitory motorneurons, known to be GABAergic, contain a novel neuropeptide, As-NLP-22 (SLASGRWGLRPamide). From this sequence and information from the A. suum expressed sequence tag (EST) database, we cloned the transcript (As-nlp-22) and synthesized a riboprobe for in situ hybridization, which labeled the inhibitory motorneurons; this validates the integrity of the dissection method, showing that the peptides detected originate from the cells themselves and not from adhering processes from other cells (e.g., synaptic terminals). Synthetic As-NLP-22 has potent inhibitory activity on acetylcholine-induced muscle contraction as well as on basal muscle tone. Both of these effects are dose-dependent: the inhibitory effect on ACh contraction has an IC50 of 8.3 × 10(-9) M. When injected into whole worms, As-NLP-22 produces a dose-dependent inhibition of locomotory movements and, at higher levels, complete paralysis. These experiments demonstrate the utility of MALDI TOF/TOF MS in identifying novel neuromodulators at the single-cell level. Graphical Abstract ᅟ.

Presynaptic and postsynaptic regulation of muscle contractions in the ascarid nematode Ascaris suum: a target for drug action

The aim of this study was to determine the role in contractions of postsynaptic nicotinic acetylcholine (nACh) and γ-aminobutyric acid (GABA) receptors, in the bag region of Ascaris suum muscle cells, as well as the role of synaptic receptors between interneurons and motor neurons in the dorsal and ventral nerve cord. We have measured the isometric contractions of isolated segments of A. suum, with or without the nerve cord (dorsal or ventral). Contractions were caused by increasing concentrations of ACh or by electrical field stimulation (EFS). Based on our results, the presence of the nerve cord is essential for the contractile effects of ACh. The EC50 value of ACh for innervated muscle strips was 10.88 μm. Unlike intact (innervated) preparations, there was no contraction of the muscle flaps when the nerve cord was mechanically removed. Furthermore, continuous EFS produced stable contractions of innervated muscle strips, but they are not sensitive to mecamylamine (100 μm). However, GABA (30 μm) significantly inhibited the EFS-induced contractions. EFS with the same characteristics did not cause muscle contractions of denervated muscle strips, but EFS with a wider pulse induced the increasing of tone and irregular contractions. These contractions were completely insensitive to the effect of GABA. The EC50 for ACh did not differ between the dorsal and ventral segments (9.83 μm and 9.45 μm), while GABA exhibited features of competitive and non-competitive antagonists, regardless of whether it acted on the dorsal or ventral segments of A. suum. It is obvious that drugs will be more effective if they act on both the synaptic and extrasynaptic nACh and GABA receptors.

The Validation of Nematode-Specific Acetylcholine-Gated Chloride Channels as Potential Anthelmintic Drug Targets

New compounds are needed to treat parasitic nematode infections in humans, livestock and plants. Small molecule anthelmintics are the primary means of nematode parasite control in animals; however, widespread resistance to the currently available drug classes means control will be impossible without the introduction of new compounds. Adverse environmental effects associated with nematocides used to control plant parasitic species are also motivating the search for safer, more effective compounds. Discovery of new anthelmintic drugs in particular has been a serious challenge due to the difficulty of obtaining and culturing target parasites for high-throughput screens and the lack of functional genomic techniques to validate potential drug targets in these pathogens. We present here a novel strategy for target validation that employs the free-living nematode Caenorhabditis elegans to demonstrate the value of new ligand-gated ion channels as targets for anthelmintic discovery. Many successful anthelmintics, including ivermectin, levamisole and monepantel, are agonists of pentameric ligand-gated ion channels, suggesting that the unexploited pentameric ion channels encoded in parasite genomes may be suitable drug targets. We validated five members of the nematode-specific family of acetylcholine-gated chloride channels as targets of agonists with anthelmintic properties by ectopically expressing an ivermectin-gated chloride channel, AVR-15, in tissues that endogenously express the acetylcholine-gated chloride channels and using the effects of ivermectin to predict the effects of an acetylcholine-gated chloride channel agonist. In principle, our strategy can be applied to validate any ion channel as a putative anti-parasitic drug target.

Different neuropeptides are expressed in different functional subsets of cholinergic excitatory motorneurons in the nematode Ascaris suum

Neuropeptides are known to have dramatic effects on neurons and synapses; however, despite extensive studies of the motorneurons in the parasitic nematode Ascaris suum, their peptide content had not yet been described. We determined the peptide content of single excitatory motorneurons by mass spectrometry and tandem mass spectrometry. There are two subsets of ventral cord excitatory motorneurons, each with neuromuscular output either anterior or posterior to their cell body, mediating forward or backward locomotion, respectively. Strikingly, the two sets of neurons contain different neuropeptides, with AF9 and six novel peptides (As-NLP-21.1-6) in anterior projectors, and the six afp-1 peptides in addition to AF2 in posterior projectors. In situ hybridization confirmed the expression of these peptides, validating the integrity of the dissection technique. This work identifies new components of the functional behavioral circuit, as well as potential targets for antiparasitic drug development.

The beginning of connectomics: a commentary on White et al. (1986) 'The structure of the nervous system of the nematode Caenorhabditis elegans'

The article 'Structure of the nervous system of the nematode Caenorhabditis elegans' (aka 'The mind of a worm') by White et al., published for the first time the complete set of synaptic connections in the nervous system of an animal. The work was carried out as part of a programme to begin to understand how genes determine the structure of a nervous system and how a nervous system creates behaviour. It became a major stimulus to the field of C. elegans research, which has since contributed insights into all areas of biology. Twenty-six years elapsed before developments, notably more powerful computers, made new studies of this kind possible. It is hoped that one day knowledge of synaptic structure, the connectome, together with results of many other investigations, will lead to an understanding of the human brain. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.

Family of FLP Peptides in Caenorhabditis elegans and Related Nematodes

Neuropeptides regulate all aspects of behavior in multicellular organisms. Because of their ability to act at long distances, neuropeptides can exert their effects beyond the conventional synaptic connections, thereby adding an intricate layer of complexity to the activity of neural networks. In the nematode Caenorhabditis elegans, a large number of neuropeptide genes that are expressed throughout the nervous system have been identified. The actions of these peptides supplement the synaptic connections of the 302 neurons, allowing for fine tuning of neural networks and increasing the ways in which behaviors can be regulated. In this review, we focus on a large family of genes encoding FMRFamide-related peptides (FaRPs). These genes, the flp genes, have been used as a starting point to identifying flp genes throughout Nematoda. Nematodes have the largest family of FaRPs described thus far. The challenges in the future are the elucidation of their functions and the identification of the receptors and signaling pathways through which they function.

The structure of the nervous system of the nematode Caenorhabditis elegans

The structure and connectivity of the nervous system of the nematode Caenorhabditis elegans has been deduced from reconstructions of electron micrographs of serial sections. The hermaphrodite nervous system has a total complement of 302 neurons, which are arranged in an essentially invariant structure. Neurons with similar morphologies and connectivities have been grouped together into classes; there are 118 such classes. Neurons have simple morphologies with few, if any, branches. Processes from neurons run in defined positions within bundles of parallel processes, synaptic connections being made en passant. Process bundles are arranged longitudinally and circumferentially and are often adjacent to ridges of hypodermis. Neurons are generally highly locally connected, making synaptic connections with many of their neighbours. Muscle cells have arms that run out to process bundles containing motoneuron axons. Here they receive their synaptic input in defined regions along the surface of the bundles, where motoneuron axons reside. Most of the morphologically identifiable synaptic connections in a typical animal are described. These consist of about 5000 chemical synapses, 2000 neuromuscular junctions and 600 gap junctions.

A perimotor framework reveals functional segmentation in the motoneuronal network controlling locomotion in Caenorhabditis elegans

The neuronal connectivity dataset of the nematode Caenorhabditis elegans attracts wide attention from computational neuroscientists and experimentalists. However, the dataset is incomplete. The ventral and dorsal nerve cords of a single nematode were reconstructed halfway along the body and the posterior data are missing, leaving 21 of 75 motoneurons of the locomotor network with partial or no connectivity data. Using a new framework for network analysis, the perimotor space, we identified rules of connectivity that allowed us to approximate the missing data by extrapolation. Motoneurons were mapped into perimotor space in which each motoneuron is located according to the muscle cells it innervates. In this framework, a pattern of iterative connections emerges which includes most (0.90) of the connections. We identified a repeating unit consisting of 12 motoneurons and 12 muscle cells. The cell bodies of the motoneurons of such a unit are not necessarily anatomical neighbors and there is no obvious anatomical segmentation. A connectivity model, composed of six repeating units, is a description of the network that is both simplified (modular and without noniterative connections) and more complete (includes the posterior part) than the original dataset. The perimotor framework of observed connectivity and the segmented connectivity model give insights and advance the study of the neuronal infrastructure underlying locomotion in C. elegans. Furthermore, we suggest that the tools used herein may be useful to interpret, simplify, and represent connectivity data of other motor systems.

A specific antibody to neuropeptide AF1 (KNEFIRFamide) recognizes a small subset of neurons in Ascaris suum: differences from Caenorhabditis elegans

A monoclonal antibody, AF1-003, highly specific to the Ascaris suum neuropeptide AF1 (KNEFIRFamide), was generated. This antibody binds strongly to AF1 and extremely weakly to other peptides with C-terminal FIRFamide: AF5 (SGKPTFIRFamide), AF6 (FIRFamide), and AF7 (AGPRFIRFamide). It does not recognize 35 other AF (A. suum FMRFamide-like) peptides at the highest concentration tested, nor does it recognize FMRFamide. When crude peptide extracts of A. suum are fractionated by two-step HPLC, the only fractions recognized by AF1-003 are those comigrating with synthetic AF1. By immunocytochemistry, antibody AF1-003 recognizes a small subset of the 298 neurons of A. suum: these include the paired URX and RIP neurons, two pairs of lateral ganglion neurons in the head, and the unpaired PQR and PDA or -B tail neurons that send processes to the head along the dorsal and ventral nerve cords, respectively. AF1 immunoreactivity is also seen in three pairs of pharyngeal neurons. Mass spectroscopy (MS) shows the presence of AF1 in the head, pharynx, and dorsal and ventral nerve cords. In A. suum, the neurons that contain AF1 show little overlap with neurons that express green fluorescent protein constructs targeting the flp-8 gene, which encodes AF1 in Caenorhabditis elegans (Kim and Li [2004] J. Comp. Neurol. 475:540-550); the URX neurons express AF1 in both species, but, in C. elegans, flp-8 expression was not detected in RIP, PQR, and PDA or -B or in the pharynx. Other, less specific monoclonal antibodies recognize AF1, as well as other peptides to differing degrees; these antibodies are useful reagents for determination of neuronal morphology.

Neuropeptide physiology in helminths

Parasitic worms come from two distinct, distant phyla, Nematoda (roundworms) and Platyhelminthes (flatworms). The nervous systems of worms from both phyla are replete with neuropeptides and there is ample physiological evidence that these neuropeptides control vital aspects of worm biology. In each phyla, the physiological evidence for critical roles for helminth neuropeptides is derived from both parasitic and free-living members. In the nematodes, the intestinal parasite Ascaris suum and the free-living Caenorhabditis elegans have yielded most of the data; in the platyhelminths, the most physiological data has come from the blood fluke Schistosoma mansoni. FMRFamide-like peptides (FLPs) have many varied effects (excitation, relaxation, or a combination) on somatic musculature, reproductive musculature, the pharynx and motor neurons in nematodes. Insulin-like peptides (INSs) play an essential role in nematode dauer formation and other developmental processes. There is also some evidence for a role in somatic muscle control for the somewhat heterogeneous grouping ofpeptides known as neuropeptide-like proteins (NLPs). In platyhelminths, as in nematodes, FLPs have a central role in somatic muscle function. Reports of FLP physiological action in platyhelminths are limited to a potent excitation of the somatic musculature. Platyhelminths are also abundantly endowed with neuropeptide Fs (NPFs), which appear absent from nematodes. There is not yet any data linking platyhelminth NPF to any particular physiological outcome, but this neuropeptide does potently and specifically inhibit cAMP accumulation in schistosomes. In nematodes and platyhelminths, there is an abundance of physiological evidence demonstrating that neuropeptides play critical roles in the biology of both free-living and parasitic helminths. While it is certainly true that there remains a great deal to learn about the biology of neuropeptides in both phyla, physiological evidence presently available points to neuropeptidergic signaling as a very promising field from which to harvest future drug targets.

Mapping neuropeptide expression by mass spectrometry in single dissected identified neurons from the dorsal ganglion of the nematode Ascaris suum

We have developed a method for dissecting single neurons from the nematode Ascaris suum, in order to determine their peptide content by mass spectrometry (MS). In this paper, we use MALDI-TOF MS and tandem MS to enumerate and sequence the peptides present in the two neurons, ALA and RID, that comprise the dorsal ganglion. We compare the peptide content determined by MS with the results of immunocytochemistry and in situ hybridization of previously isolated peptides AF2, AF8 and 6 peptides encoded by the afp-1 transcript. We find complete agreement between the three techniques, which validates single neuron MS as a method for peptide localization. We also discovered and sequenced 6 novel peptides in the ALA neuron. Cloning of cDNAs and database searching of Genomic Survey Sequences showed that transcript afp-12 encodes peptide AF36 (VPSAADMMIRFamide), and afp-13 encodes AF19 (AEGLSSPLIRFamide), AF34 (DSKLMDPLIRFamide), AF35 (DPQQRIVTDETVLRFamide), and 3 non-amidated peptides (PepTT, PepTL, and PepGE). We have found no similarities with reported peptide expression in the nematode Caenorhabditis elegans. This method promises to be ideally suited for determining the peptide content of each of the 298 neurons in the nervous system of this nematode.

In situ hybridization of neuropeptide-encoding transcripts afp-1, afp-3, and afp-4 in neurons of the nematode Ascaris suum

The gene transcripts encoding both the AF8 and AF2 neuropeptides of the nematode Ascaris suum have been identified, cloned, and sequenced. The AF8 transcript (afp-3) encodes five identical copies of AF8; each peptide-encoding region is flanked by the appropriate dibasic or monobasic cleavage processing sites. The AF2 transcript (afp-4) encodes three identical copies of AF2 along with the appropriate cleavage sites. In contrast, the afp-1 transcript (Edison et al. [1997] Peptides 18:929-935) encodes six different AF peptides (AF3, 4, 10, 13, 14, 20) which all share a -PGVLRFamide C-terminus but have different N-terminal sequences. By using in situ hybridization, gene transcript expression patterns of afp-1, afp-3, and afp-4 (As-flp-18, As-flp-6, and As-flp-14, respectively, in the naming convention proposed by Blaxter et al. [1997] Parasitol Today 13:416-417) were determined in the adult A. suum anterior nervous system. Each gene transcript can be localized to a different subset of neurons. These subsets of neurons are different from the subsets of Caenorhabditis elegans neurons that were shown to express identical or similar peptides by the use of promoter GFP constructs (Kim and Li [2004] J Comp Neurol 475:540-550).

Functional morphology of the platyhelminth nervous system

As the most primitive metazoan phylum, the Platyhelminthes occupies a unique position in nervous system evolution. Centrally, their nervous system consists of an archaic brain from which emanate one or more pairs of longitudinal nerve cords connected by commissures; peripherally, a diverse arrangement of nerve plexuses of varying complexity innervate the subsurface epithelial and muscle layers, and in the parasitic taxa they are most prominent in the musculature of the attachment organs and egg-forming apparatus. There is a range of neuronal-cell types, the majority being multi- and bipolar. The flatworm neuron is highly secretory and contains a heterogeneity of vesicular inclusions, dominated by densecored vesicles, whose contents may be released synaptically or by paracrine secretion for presumed delivery to target cells via the extracellular matrix. A wide range of sense organ types is present in flatworms, irrespective of life-styles. The repertoire of neuronal substances identified cytochemically includes all of the major candidate transmitters known in vertebrates. Two groups of native flatworm neuropeptides have been sequenced, neuropeptide F and FMRFamide-related peptides (FaRPs), and immunoreactivities for these have been localised in dense-cored neuronal vesicles in representatives of all major fiatworm groups. There is evidence of co-localisation of peptidergic and cholinergic elements; serotoninergic components generally occupy a separate set of neurons. The actions of neuronal substances in flatworms are largely undetermined, but FaRPs and 5-HT are known to be myoactive in all of the major groups, and there is immuno-cytochemical evidence that they have a role in the mechanism of egg assembly.

Peptide products of the afp-6 gene of the nematode Ascaris suum have different biological actions

Matrix-assisted laser desorption/ionization time-of-flight and tandem time-of-flight (MALDI-TOF and MALDI-TOF/TOF) mass spectrometry were used to sequence and localize three novel, related neuropeptides in the nervous system of the nematode Ascaris suum, AMRNALVRFamide (AF21), NGAPQPFVRFamide (AF22), and SGMRNALVRFamide (AF23). The amino acid sequences were used to clone a novel neuropeptide gene (afp-6) that encodes a precursor bearing a single copy of each of the peptides. In situ hybridization and immunocytochemistry revealed that both the transcript and the peptides are expressed in a single cell in the ventral ganglion. Pharmacological studies of intact nematodes injected with these peptides, as well as physiological studies of responses to them in muscle tissue, motor neurons, and the pharynx, reveal that these peptides have potent bioactivity in the locomotory and feeding systems. Further exploration of their effects may contribute to our understanding of neuropeptide modulation of behavior and also to the development of compounds with anthelmintic relevance.

Evolution of neuronal patterning in free-living rhabditid nematodes I: Sex-specific serotonin-containing neurons

As a first step toward understanding the evolution of neuronal patterning and function in a group of simple animals, we have examined serotonin-containing neurons in 17 species of free-living rhabditid nematodes and compared them with identified neurons of Caenorhabditis elegans. We found many serotonin-immunoreactive (serotonin-IR) neurons that are likely homologs of those in C. elegans; this paper focuses on sex-specific neurons such as the egg laying hermaphrodite-specific neurons (HSNs), VCs, and male CAs, CPs, and ray sensory neurons known to function in mating. These cells vary in number and position in the species examined but are consistent with a current molecularly based phylogeny. Two groups (Oscheius and Pristionchus) appear independently to have lost a serotonin-IR HSN. Oscheius furthermore has no serotonin-IR innervation of the vulval region, in contrast to every other species we examined. We also saw variation in the location of somas of putative HSN, consistent with evolutionary changes in HSN migration. In C. elegans, the HSN soma migrates during embryogenesis from the tail to the central body, where it innervates its major postsynaptic targets, the vulval muscles. For other species, we observed putative HSN homologs along the anterior-posterior axis from the head to the tail, but typically HSNs were located near the vulva, which also varies in anterior-posterior position among the species we examined. The varying positions of the HSN somas in other species are reminiscent of phenotypes seen in various C. elegans mutants with altered HSN migration, suggesting possible mechanisms for the evolutionary differences we observed.

Evolution of pharyngeal behaviors and neuronal functions in free-living soil nematodes

To explore the use of Caenorhabditis elegans and related nematodes for studying behavioral evolution, we conducted a comparative study of pharyngeal behaviors and neuronal regulation in free-living soil nematodes. The pharynx is divided into three parts: corpus, isthmus and terminal bulb,and pharyngeal behaviors consist of stereotyped patterns of two motions:pumping and peristalsis. Based on an outgroup species, Teratocephalus lirellus, the ancestral pattern of pharyngeal behaviors consisted of corpus pumping, isthmus peristalsis and terminal bulb pumping, each occurring independently. Whereas corpus pumping remained largely conserved, isthmus and terminal bulb behaviors evolved extensively from the ancestral pattern in the four major free-living soil nematode families. In the Rhabditidae family,which includes Caenorhabditis elegans, the anterior isthmus switched from peristalsis to pumping, and anterior isthmus and terminal bulb pumping became coupled to corpus pumping. In the Diplogasteridae family, the terminal bulb switched from pumping to peristalsis, and isthmus and terminal bulb became coupled for peristalsis. In the Cephalobidae family, isthmus peristalsis and terminal bulb pumping became coupled. And in the Panagrolaimidae family, the posterior isthmus switched from peristalsis to pumping. Along with these behavioral changes, we also found differences in the neuronal regulation of isthmus and terminal bulb behaviors. M2, a neuron that has no detectable function in C. elegans, stimulated anterior isthmus peristalsis in the Panagrolaimidae. Further, M4 was an important excitatory neuron in each family, but its exact downstream function varied between stimulation of posterior isthmus peristalsis in the Rhabditidae,isthmus/terminal bulb peristalsis in the Diplogasteridae, isthmus peristalsis and terminal bulb pumping in the Cephalobidae, and posterior isthmus/terminal bulb pumping in the Panagrolaimidae. In the Rhabditidae family, although M4 normally has no effect on the terminal bulb, we found that M4 can stimulate the terminal bulb in C. elegans if the Ca2+-activated K+ channel SLO-1 is inactivated. C. elegans slo-1 mutants have generally increased neurotransmission, and in slo-1 mutants we found novel electropharyngeogram signals and increased pumping rates that suggested activation of M4-terminal bulb synapses. Thus, we suggest that the lack of M4-terminal bulb stimulations in C. elegans and the Rhabditidae family evolved by changes in synaptic transmission. Altogether, we found behavioral and neuronal differences in the isthmus and terminal bulb of free-living soil nematodes, and we examined potential underlying mechanisms of one aspect of M4 evolution. Our results suggest the utility of Caenorhabditis elegans and related nematodes for studying behavioral evolution.

The ever-expanding neuropeptide gene families in the nematode Caenorhabditis elegans

Neuropeptides act as chemical signals in the nervous system to modulate behaviour. With the ongoing EST projects and DNA sequence determination of different genomes, the identification of neuropeptide genes has been made easier. Despite the relatively 'simple' repertoire of behaviours in the nematode Caenorhabditis elegans, this worm contains a surprisingly large and diverse set of neuropeptide genes. At least 109 genes encoding over 250 potential neuropeptides have been identified in C. elegans; all genes are likely to be expressed and many, if not all, of the predicted peptides are produced. The predicted peptides include: 38 insulin-like peptides, several of which are involved in development and reproductive growth, and over 70 FMRFamide-related peptides, some of which are involved in locomotion, reproduction, and social behaviour. Many of the C. elegans peptides are identical or highly similar to those isolated or predicted in parasitic nematodes, such as Ascaris suum, Haemonchus contortus, Ancylostoma caninum, Heterodera glycines and Meloidogyne arenaria, suggesting that the function of these peptides is similar across species. The challenge for the future is to determine the function of all the genes and individual peptides and to identify the receptors through which the peptides signal.

Mass spectrometric map of neuropeptide expression in Ascaris suum

A mass spectrometric method was used for the localization and sequence characterization of peptides in the nervous system of the parasitic nematode Ascaris suum. Mass spectrometric techniques utilizing MALDI-TOF, MALDI-TOF/TOF, and MALDI-FT instruments were combined with in situ chemical derivatization to examine the expression of known and putative neuropeptides in the A. suum nervous system. This first attempt at peptidomic characterization in A. suum mapped the expression of 39 neuropeptides, 17 of which are considered to be novel and whose expression has not been previously reported. These analyses also revealed that the peptide expression profile is unique to each nervous structure and that the majority of peptides observed belong to the RFamide family of neuropeptides. In addition, four new peptide sequences with a shared C-terminal PNFLRFamide motif are proposed based on in situ sequencing with mass spectrometry.

The Caenorhabditis elegans lev-8 gene encodes a novel type of nicotinic acetylcholine receptor alpha subunit

We have cloned Caenorhabditis elegans lev-8 and demonstrated that it encodes a novel nicotinic acetylcholine receptor (nAChR) subunit (previously designated ACR-13), which has functional roles in body wall and uterine muscles as part of a levamisole-sensitive receptor. LEV-8 is an alpha subunit and is the first to be described from the ACR-8-like group, a new class of nAChR with atypical acetylcholine-binding site (loop C) and channel-lining motifs. A single base pair change in the first intron of lev-8 in lev-8(x15) mutants leads to alternative splicing and the introduction of a premature stop codon. lev-8(x15) worms are partially resistant to levamisole-induced egg laying and paralysis, phenotypes rescued by expression of the wild-type gene. lev-8(x15) worms also show reduced rates of pharyngeal pumping. Electrophysiological recordings from body wall muscle show that currents recorded in response to levamisole have reduced amplitude in lev-8(x15) compared with wild-type animals. Consistent with these phenotypic observations, green fluorescent protein fused to LEV-8 is expressed in body wall and uterine muscle, motor neurons and epithelial-derived socket cells. Thus, LEV-8 is a levamisole receptor subunit and exhibits the most diverse expression pattern of any invertebrate nAChR subunit studied to date.

Genetic Models of Mechanotransduction: The NematodeCaenorhabditis elegans

Mechanotransduction, the conversion of a mechanical stimulus into a biological response, constitutes the basis for a plethora of fundamental biological processes such as the senses of touch, balance, and hearing and contributes critically to development and homeostasis in all organisms. Despite this profound importance in biology, we know remarkably little about how mechanical input forces delivered to a cell are interpreted to an extensive repertoire of output physiological responses. Recent, elegant genetic and electrophysiological studies have shown that specialized macromolecular complexes, encompassing mechanically gated ion channels, play a central role in the transformation of mechanical forces into a cellular signal, which takes place in mechanosensory organs of diverse organisms. These complexes are highly efficient sensors, closely entangled with their surrounding environment. Such association appears essential for proper channel gating and provides proximity of the mechanosensory apparatus to the source of triggering mechanical energy. Genetic and molecular evidence collected in model organisms such as the nematode worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse highlight two distinct classes of mechanically gated ion channels: the degenerin (DEG)/epithelial Na+channel (ENaC) family and the transient receptor potential (TRP) family of ion channels. In addition to the core channel proteins, several other potentially interacting molecules have in some cases been identified, which are likely parts of the mechanotransducing apparatus. Based on cumulative data, a model of the sensory mechanotransducer has emerged that encompasses our current understanding of the process and fulfills the structural requirements dictated by its dedicated function. It remains to be seen how general this model is and whether it will withstand the impiteous test of time.

Microscopy and the helminth parasite

Microscopy has a long and distinguished history in the study of helminth parasites and has made a singularly outstanding contribution to understanding how these complex animals organise their lives and relate to their hosts. Increasingly, the microscope has been used as a powerful investigative tool in multidisciplinary approaches to parasitological problems, placing emphasis on functional correlates rather than anatomical detail. In doing so, microscopy has also uncovered a number of attributes of parasites that are of wider significance in the field of biology. Parasite surfaces have understandably demanded most of the attention of microscopists, largely as a result of the pioneering studies using transmission electron microscopy. Their findings focused the attention of physiologists and immunologists on the tegument and cuticle of helminths and in doing so helped unravel the complex molecular exchanges that are fundamental to understanding host-parasite interactions. Scanning electron microscopy succeeded in augmenting these data by revealing novel microtopographical features of the host-parasite relationship, as well as proving invaluable in helminth taxonomy and in assessing the efficacy of test substances in drug screens. Control of helminth parasites has never been more critical: problems of drug resistance demand urgent action to identify exploitable targets for new generation anthelmintics. In this regard, the neuropeptide signalling system of helminths is envisioned as central to nerve-muscle function, and thereby a crucial regulatory influence on their motility, alimentation and reproduction. The use of immunocytochemistry interfaced with confocal scanning laser microscopy has not only been instrumental in discovering the peptidergic system of helminths and its potential for chemotherapeutic exploitation, but through increasingly sophisticated bio-imaging technologies has continued to help dissect and analyse the molecular dynamics of this and other cellular systems within these important parasites.

Localisation of Globodera pallida FMRFamide-related peptide encoding genes using in situ hybridisation

The present study employed an in situ hybridisation technique to detect the expression of a number of FMRFamide-like peptide encoding (flp) genes, previously identified from Globodera pallida, in whole-mount preparations of the J(2) stage of this worm. gpflp-1, encoding the FMRFamide-related peptide (FaRP) KSAYMRFamide, was expressed in neurones associated with the circumpharyngeal nerve ring and specifically in a number of cell bodies in the lumbar ganglia of the perianal nerve ring. The lumbar ganglia and pre-anal ganglia along with the BDU neurones and a number of cells in the retrovesicular ganglion were observed to express gpflp-2, encoding KNKFEFIRFamide. gpflp-3 (encoding KHEYLRFamide) expression was localised to the anterior ganglion and a number of paired cells posterior to the circumpharyngeal nerve ring whilst expression of gpflp-4, encoding a number of -P(G/Q)VLRFamides, was localised to the retrovesicular ganglion. No expression of gpflp-5 was observed. Identification of the reactive cells has implicated distinct roles for the FaRPs encoded on these genes in regulation of both dorsal and ventral body wall muscles, the musculature of the vulva and in the function of a number of sensory structures in both the head and tail of G. pallida. Comparison with the expression patterns of analogous genes in Caenorhabditis elegans suggests that, whilst some of the encoded peptides are conserved between nematode species, their functions therein are distinct. Furthermore, the expression of some of these genes in a number of interneurones supports the idea that FaRPs fulfil neuromodulatory as well as neurotransmitter roles.

The number of morphological synapses between neurons does not predict the strength of their physiological synaptic interactions: a study of dendrites in the nematode Ascaris suum

Previous electrophysiological and anatomical studies of Ascaris suum motor neurons demonstrated a strong correlation between functional interactions and the presence of anatomically defined synapses. However, one example of a physiologically robust synaptic connection was encountered for which no anatomical evidence of direct chemical synapses was found. This involved synaptic transmission from an identified excitatory motor neuron to its inhibitory partner. In this study, pressure injection of horseradish peroxidase or nickel lysine into inhibitory motor neurons revealed numerous spines projecting from the main neuronal process toward the neuromuscular surface that then branched and extended fine, longitudinal processes up to 130 microm in length. Subsequent examination of nickel lysine-injected spines by electron microscopy revealed numerous chemical synapses, including for the first time direct inputs from the excitatory neuron. However, the numbers of synapses from this motor neuron were very small relative to inputs from other identified cells. Thus, direct synapses are unlikely to explain the robust nature of this physiological interaction.

Structure-activity relationships of 18 endogenous neuropeptides on the motor nervous system of the nematode Ascaris suum

Neuropeptides play an important role in all nervous systems and structure-activity studies of related peptides is one approach to understanding this role. This study of the motor nervous system of the parasitic nematode Ascaris suum describes the physiological effects of a family of 18 endogenous Ascaris FMRFamide-like peptides (AF peptides) on the membrane potential and input resistance of the dorsal excitatory type 2 (DE2) and dorsal inhibitory (DI) motor neurons. These motor neurons are part of the final common output pathway from the motor nervous system to the somatic muscle cells responsible for locomotion. AF peptide effects on the frequency of excitatory postsynaptic potentials (EPSPs) in DE2 motor neurons were also measured to infer peptide effects on central presynaptic spiking neurons. AF peptide injections into intact worms were made to assess their qualitative effects on behavior, providing a context for interpreting motor neuron data. One category of AF peptides, N-terminally extended -FIRFa peptides (AF5, AF7 and AF1), has pronounced behavioral effects and qualitatively similar, but quantitatively different effects on DE2 and DI motor neurons. A second category of AF peptides (AF2, AF9, and AF8) also produces dramatic behavioral effects and strong electrophysiological effects on DE2 and/or DI motor neurons. A third category of AF peptides, consisting of six members of the -PGVLRFa group (which are encoded by the same gene and have closely related sequences) and peptide AF11, have pronounced behavioral effects, but relatively weak or negligible effects on DE2 and DI motor neurons. A fourth category of AF peptides, also consisting of structurally unrelated members, has pronounced behavioral effects and, as individual peptides, similar effects on both DE2 and DI motor neurons; AF15 is excitatory, while AF17 and AF19 are inhibitory, on both motor neuron types. Finally, two AF peptides (AF6, AF16) are relatively weak or inactive in producing behavioral or motor neuronal effects. Based on comparisons of the effects of AF peptides on DE2 and DI motor neurons, a tentative list of 5 major response-types is proposed as a working hypothesis to guide the search for AF peptide receptors. The findings attest to the potential complexity of neurosignaling in this comparatively simple nervous system.

Changes in locomotory behavior and cAMP produced in Ascaris suum by neuropeptides from Ascaris suum or Caenorhabditis elegans

Injection of Ascaris FMRFamide-like (AF) peptides and peptides encoded by genes in Caenorhabditis elegans were analyzed for effects on locomotion, body waveforms, and cAMP concentrations in adult female Ascaris suum. Injection of AF1 (KNEFIRFamide) or AF2 (KHEYLRFamide) inhibited the propagation of locomotory waves and reduced the number of waveforms, decreased the body length, and caused a large, long-lasting increase in cAMP. Muscle tissue was identified as a major source of the cAMP response induced by AF1. The AF1 analog AF1R6A did not affect cAMP levels by itself, but inhibited the cAMP response produced by AF1. AF8 (KSAYMRFamide) produced ventral coiling in the behavioral assay, and AF10 (GFGDEMSMPGVLRFamide) decreased the body length and increased the number of body waveforms. In dorsal muscle strips, AF10 produced a long-lasting contraction. Neither AF8 nor AF10 changed cAMP concentrations. AF17 (FDRDFMHFamide) increased body length and decreased cAMP. The neuropeptides encoded by C. elegans genes flp-4, flp-7, flp-9, and flp-13 produced paralysis and loss of waveforms, increased body length and, like AF17, decreased cAMP. Three new predicted peptides from C. elegans genome sequences were synthesized and tested. One produced ventral coiling but no change in cAMP; the other two gave no detectable responses. The fact that C. elegans neuropeptides produce behavioral and physiological effects in A. suum suggests that structurally related peptides may exist in A. suum. The profound changes in cAMP produced by some neuropeptides has important implications for understanding cAMP signaling and shows that neuropeptide-mediated signal transduction pathways are potential targets for anthelmintic drug development.

Fluoxetine-resistant mutants in C. elegans define a novel family of transmembrane proteins

Fluoxetine (Prozac) is an antidepressant that is thought to act by blocking presynaptic reuptake of the neurotransmitter serotonin. Despite widespread clinical use of fluoxetine, direct evidence for this mechanism has been difficult to obtain in vivo. We have determined that fluoxetine has an additional neuromuscular effect on C. elegans that is distinct from inhibition of serotonin reuptake. By screening for mutants resistant to this effect, we have identified seven genes. We report that two of these genes are homologous to each other and define a novel gene family that encodes over a dozen multipass transmembrane proteins. Our findings may have clinical implications for the mechanism of action of fluoxetine.

Behavioral and cellular effects of serotonin on locomotion and male mating posture in Ascaris suum (Nematoda)

The site and mode of action of serotonin on locomotion were investigated in the parasitic nematode Ascaris suum. Injection of serotonin into Ascaris immediately caused paralysis in animals that were generating locomotory waveforms. Injected serotonin also increased body length and decreased the number of propagating body waves. Similar injections into the male tail produced a ventral tail curl. Injection of N-acetyl-serotonin had no effect on the generation of locomotory waveforms, but increased the body length and decreased the number of body waves in the waveform. Other biogenic amines were also tested but were much less potent. Serotonin decreased the amplitude of a submaximal acetylcholine-induced muscle contraction and increased the time to attain this contraction. The time course of this effect on the response to ACh was much slower than the action of injected serotonin on locomotory waveforms, suggesting that additional elements are involved in the action of serotonin on locomotory behavior. Serotonin abolished spontaneous slow potentials in VI motor neurons and decreased the frequency of EPSPs in DE2 motor neurons, probably by a pre-synaptic mechanism. In the male tail, serotonin depolarized the male-specific transverse ventral muscle cells, but did not affect either dorsal or ventral longitudinal muscle cells.

Neuronal localization of serotonin in the nematode Ascaris suum

We have used immunocytochemical techniques to investigate the distribution of serotonin-like immunoreactivity in the nematode Ascaris suum. Antisera raised against serotonin (5-hydroxytryptamine, 5-HT) conjugated to bovine serum albumin (BSA) labelled a pair of neurons in the pharynx of both sexes and five cells in the ventral cord of the male tail. The labelling was blocked by 5-HT or by 5-HT conjugated to BSA. The 5-HT-immunoreactive cells in the pharynx resemble neurosecretory cells and are probably homologous to the neurosecretory motor neurons (NSM) in Caenorhabditis elegans; the cells in the male tail appear to be motor neurons that are homologous to CP neurons in C. elegans. Other cells that stain with 5-HT antisera have been observed in C. elegans but are not seen in Ascaris.

The motornervous system of Ascaris: electrophysiology and anatomy of the neurons and their control by neuromodulators

Analysis of the electrical properties of neurons in the motornervous system of Ascaris sutom suggests that it is largely an analogue system. The motorneurons do not conduct action potentials and they release transmitter tonically at their normal resting potential; transmitter release is increased or decreased as a continuous function of membrane potential. Despite extensive physiological descriptions of the electrical properties of the neurons and their synapses, as well as morphological descriptions of the synaptic circuitry of the system, the predicted activities of the neurons in the circuit differ from those observed by direct recording in semi-intact behaving animals. We conclude that the description of the circuit is incomplete. There are several possibilities for the missing elements, including chemical, proprioceptive, and additional neuronal components. Recently, attention has been focussed most heavily on the intercellular chemical signalling systems; in addition to those mediated by classical neurotransmitters, a surprisingly complex array of neuropeptides has been identified. One family of these peptides, the AF peptides, has been analyzed in detail. It comprises at least 20 peptides, and they fall into sequence-related subfamilies. One of these subfamilies, containing 6 peptides, is encoded by a single transcript, suggesting that the AF peptides are under multiple genetic control. All AF peptides tested have potent activity on the motornervous system and/or on muscle. There are multiple physiological activities, and cellular localization studies show multiple patterns of cellular expression. Studies on Panagrellus and Caenorhabditis emphasize the diversity of this family and its genetic control.

Electrophysiology of Ascaris muscle and anti-nematodal drug action

Three groups of anthelmintic drugs act directly and selectively on muscle membrane receptors of parasitic nematodes. These groups of anthelmintics are: (1) The Nicotinic Agonists (levamisole, pyrantel, morantel and oxantel) that act on acetylcholine receptors of nematode somatic muscle; (2) The GABA Agonist, piperazine, that acts on nematode muscle GABA receptors; and (3) The Avermectins that open glutamate gated Cl- channels on nematode pharyngeal muscle. The electrophysiology and pharmacology of muscle and neuromuscular transmission the nematode parasite, Ascaris suum, is outlined and effects of anthelmintics that interfere with transmission described. Resistance to anthelmintics has appeared in some parasitic nematodes but the mechanisms of this resistance remain to be determined.

Localization and differential expression of FMRFamide-like immunoreactivity in the nematode Ascaris suum

By immunocytochemical and immunohistochemical methods, FMRFamide-like immunoreactivity (FLI) was localized to many neurons and processes in the Ascaris nervous system, including the head, tail, and lateral lines. Some of these cells were identified; they included sensory neurons, interneurons, and motor neurons. FLI was also present in the pharyngeal neurons and in their varicosities near the surface of the pharynx. By HPLC analysis of extracts, only a subset of the FMRFamide-like peptides (FLPs) expressed in Ascaris heads, and heads from which the pharynx had been removed, were expressed in the pharynx. Furthermore, FLPs appeared to be differentially expressed in female heads and tails and male heads and tails. Acetone and acid methanol differentially extracted subforms of FLI from Ascaris heads and from C. elegans.

Actions of cholinergic drugs in the nematode Ascaris suum. Complex pharmacology of muscle and motorneurons

The cholinergic agonists acetylcholine (ACh), nicotine, and pilocarpine produced depolarizations and contractions of muscle of the nematode Ascaris suum. Dose-dependent depolarization and contraction by ACh were suppressed by about two orders of magnitude by 100 microM d-tubocurarine (dTC), a nicotinic antagonist, but only about fivefold by 100 microM N-methyl-scopolamine (NMS), a muscarinic antagonist. NMS itself depolarized both normal and synaptically isolated muscle cells. The muscle depolarizing action of pilocarpine was not consistently antagonized by either NMS or dTC. ACh receptors were detected on motorneuron classes DE1, DE2, DI, and VI as ACh-induced reductions in input resistance. These input resistance changes were reversed by washing in drug-free saline or by application of dTC. NMS applied alone lowered input resistance in DE1, but not in DE2, DI, or VI motorneurons. In contrast to the effect of ACh, the action of NMS in DE1 was not reversed by dTC, suggesting that NMS-sensitive sites may not respond to ACh. Excitatory synaptic responses in muscle evoked by depolarizing current injections into DE1 and DE2 motorneurons were antagonized by dTC; however, NMS antagonized the synaptic output of only the DE1 and DE3 classes of motorneurons, an effect that was more likely to have been produced by motorneuron conduction failure than by pharmacological blockade of receptor. The concentration of NMS required to produce these changes in muscle polarization and contraction, ACh antagonism, input resistance reduction, and synaptic antagonism was 100 microM, or more than five orders of magnitude higher than the binding affinity for [3H]NMS in larval Ascaris homogenates and adult Caenorhabditis elegans (Segerberg, M. A. 1989. Ph.D. thesis. University of Wisconsin-Madison, Madison, WI). These results describe a nicotinic-like pharmacology, but muscle and motorneurons also have unusual responses to muscarinic agents.

Extracellular recordings from the motor nervous system of the nematode, Ascaris suum

1. The close association of muscle and neurons in Ascaris suum makes it difficult to determine whether spikes recorded from nerve cords originate in muscle or neurons. We have developed criteria that distinguish muscle and neuronal activity. There are two categories of extracellular spikes. 2. The first category consists of spikes with a wide range of amplitudes, marked by large spikes. These spikes, which can be recorded over lateral muscle and over the dorsal and ventral nerve cords, are abolished when muscle is disrupted or removed, or when curare is applied. Large spikes are relatively infrequent, are correlated with intracellularly recorded muscle events, and respond to polarizations of motor neurons, implying that they originate in muscle. 3. The second spike category, small amplitude spikes, is exclusive to the ventral nerve cord, occurs more frequently than large spikes and displays patterned firing. Small spikes are not affected by muscle removal or by curare, and are correlated with motor neuronal post-synaptic potentials, but not with intracellularly recorded muscle events. We infer that they originate in neurons. 4. Low level activity recorded extracellularly over nerve cords may represent muscle activity due to tonic motor neuronal synaptic transmission. It responds to motor neuronal polarization and is suppressed by curare or muscle removal.

Distribution of a fluorescent ivermectin probe, bodipy ivermectin, in tissues of the nematode parasite Ascaris suum

A fluorescent derivative of the anthelmintic ivermectin (4''-5,7-dimethyl bodipy proprionylivermectin, referred to hereafter as bodipy ivermectin) was synthesized for an investigation of the distribution of avermectins. Injected into adult Ascaris suum at doses greater than 0.1 micron per worm, it produced a dose-dependent immobilization. Fluorescent microscopy of frozen sections revealed the distribution of the probe in the whole nematode. Staining of collagenase-isolated muscle cells was studied using bath application of bodipy ivermectin. The trypan-blue quenching technique showed that the ivermectin probe was located in the outer monolayer of the muscle membrane. The cytoplasm was not stained. The interpretation of these observations is discussed in view of the known lipophilic nature of avermectins. Staining of the muscle membrane and nerve cord is consistent with the view that avermectins act at these sites. The significance of the hypodermal and lateral line staining is also discussed.

GABA-immunoreactive neurons in the nematode Ascaris

gamma-Aminobutyric acid (GABA) immunoreactive neurons in the cephalic, somatic, and caudal regions of the Ascaris nervous system were visualized with serial section and whole-mount GABA immunocytochemistry. In the ventral and dorsal nerve cords, GABA-like immunoreactivity (GLIR) is localized to the neurites and cell bodies of identified inhibitory motor neurons and to two fibers, one in each cord, that arise from neurons in the nerve ring. GLIR is absent from identified excitatory motor neurons and from ventral cord interneurons. In neurons containing GLIR, immunoreactivity was present throughout the cell, which argues against an exclusive localization of GABA at conventional synapses. In whole mounts, ten GABA-immunoreactive neurons were present in the cephalic region. These include four nerve ring-associated cells (the RME-like cells), two bilaterally symmetrical pairs of lateral ganglia neurons (the amphid-GABA and deirid-GABA cells) and one bilaterally symmetrical pair of ventral ganglion cells (the VG-GABA cells). In sections, the RME-like cells and the VG-GABA cells were consistently stained through the cephalic region. However, anti-GABA staining of the lateral ganglia cells in sections was light, thus suggesting that they contain less GLIR than the other more intensely stained GABA-immunoreactive neurons. In the caudal region, a single GABA-immunoreactive neuron was present in the dorsal rectal ganglion. Our data suggest that these ten cephalic neurons, and a single dorsal rectal ganglion neuron, use GABA as a neurotransmitter.

Neuropeptides in the nematode Ascaris suum

Most of the successful anti-nematode drugs currently available affect the nematode locomotory system. Their success is due to their interactions with molecules associated with the main neuro-transmitters of the motor nervous system, acetylcholine and GABA. These drugs tend to have a relatively broad spectrum of action, affecting a wide variety of nematodes, presumably because nematode motor nervous systems are conservative in their use of these transmitters.

The physiology and pharmacology of neuromuscular transmission in the nematode parasite, Ascaris suum

The organization of Ascaris motoneurones and nervous system is summarized. There is an anterior nerve ring and associated ganglia, main dorsal and ventral nerve cords which run longitudinally, and a small set of posterior ganglia. Cell bodies of motoneurones are found in the ventral nerve cord and occur in 5 repeating 'segments'; each contains 11 motoneurones. Seven morphological types of excitatory or inhibitory motoneurone are recognized. Each Ascaris somatic muscle cell is composed of the contractile spindle; the bag region, containing the nucleus; the arm; and the syncytial region, the location of neuromuscular junctions. The resting membrane potential of muscle is approximately -30 mV and shows regular depolarizing, Ca-dependent 'spike potentials' superimposed on smaller Na(+)- and Ca2(+)-dependent 'slow waves' and even slower 'modulation waves'. The membrane shows high Cl- permeability. Adjacent cells are electrically coupled so that electrical activity in the cells is synchronized. Acetylcholine (ACh) and gamma-aminobutyric acid (GABA) affect the electrical activity. Bath-applied ACh increases membrane cation conductance, depolarizes the cells, alters the frequency and amplitude of spike potentials and produces contraction. Bath-applied GABA increases Cl- conductance, decreases spike activity and causes hyperpolarization and muscle relaxation. The extra-synaptic ACh receptors on the bag region of Ascaris muscle can be regarded as a separate subtype of nicotinic receptor. ACh and anthelmintic agonists (pyrantel, morantel, levamisole) produce a dose-dependent increase in cation conductance and membrane depolarization which is blocked by tubocurarine, mecamylamine but not by hexamethonium. The potency of GABA agonists, with the exception of sulphonic acid derivatives, correlates with the vertebrate GABAa receptor. The potency of antagonists does not. Thus, bicuculline, securinine, pitrazepine, SR95531 and RU5135 are potent vertebrate GABAa antagonists but have little effect on GABA receptors. The potency order of the arylaminopyridazine GABA antagonists: SR95103, SR95132, SR42666, SR95133, SR95531, SR42627 and SR42640 at the Ascaris GABA receptors contrasts with that at vertebrate GABAa receptors. It has been suggested that the receptor is referred to as a GABAn receptor. Patch-clamp studies show that ACh activates a non-selective cation channel which has a main conductance of 40-50pS and apparent mean open time of 1.3 ms; a smaller channel of 20-30 pS with a similar open-time is also activated. Pyrantel and levamisole also produce openings with similar conductances and open-times.(ABSTRACT TRUNCATED AT 400 WORDS)

A promising indicator of neurobehavioral toxicity using the nematode Caenorhabditis elegans and computer tracking

A promising screening test for neurotoxicity has been developed using a computer tracking system and a species of nematode, Caenorhabditis elegans. The animals are viewed in dark-field illumination by a video camera interfaced directly to a microcomputer. Several hundred nematodes are tracked simultaneously and rates of locomotion and frequency of change of direction are reported in real time. This system can rapidly obtain reliable data on a variety of behavioral parameters relating to locomotion and response to sensory stimulation. Initial testing has examined the effects of six chemicals on locomotion. Four metals (copper, beryllium, mercury, and lead) and two organophosphate pesticides (malathion and vapona) have been studied. Copper and beryllium were chosen as chemicals that have not been shown to be neurotoxins and the other four chemicals were chosen as substances known to be neurotoxins. Our findings indicate that the rate of movement of exposed nematodes compared to the rate of movement of vehicle controls may prove to be useful as an indicator of neurotoxicity.