Chemotaxis by the nematode Caenorhabditis elegans: identification of attractants and analysis of the response by use of mutants

Adverse effects of metal exposure on chemotaxis towards water-soluble attractants regulated mainly by ASE sensory neuron in nematode Caenorhabditis elegans

Chemotaxis to water-soluble attractants is mainly controlled by ASE sensory neuron whose specification is regulated by che-1 in Caenorhabditis elegans. Our data suggested that exposure to high concentrations of metals, such as Pb, Cu, Ag, and Cr, would result in severe defects of chemotaxis to water-soluble attractants of NaCl, cAMP, and biotin. Moreover, the morphology of ASE neuron structures as observed by relative fluorescent intensities and relative size of fluorescent puncta of cell bodies, relative lengths of sensory endings in ASE neurons, and the expression patterns of che-1 were obviously altered in metal exposed animals when they meanwhile exhibited obvious chemotaxis defects to water-soluble attractants. In addition, the dendrite morphology could be noticeably changed in animals exposed to 150 micromol/L of Pb, Cu, and Ag. Furthermore, we observed significant decreases of chemotaxis to water-soluble attractants in Pb exposed che-1 mutant at concentrations more than 2.5 micromol/L, and in Cu, Ag, and Cr exposed che-1 mutant at concentrations more than 50 micromol/L. Therefore, impairment of the ASE neuron structures and functions may largely contribute to the appearance of chemotaxis defects to water-soluble attractants in metal exposed nematodes.

A synaptic DEG/ENaC ion channel mediates learning in C. elegans by facilitating dopamine signalling

An important component of learned behaviour is the ability to forecast positive or negative outcomes based on specific sensory cues. Predictive capacity is typically manifested by appropriate behavioural patterning. However, the molecular mechanisms underlying behavioural plasticity are poorly understood. Caenorhabditis elegans displays experience-dependent behavioural responses by associating distinct environmental signals. We find that ASIC-1, a member of the degenerin/epithelial sodium channel family, which localizes at presynaptic terminals of dopaminergic neurons, is required for associative learning in C. elegans. ASIC-1 functions in these neurons to amplify normal dopaminergic signalling, necessary for associative learning. Our results reveal a novel role of DEG/ENaC ion channels in neuronal communication by enhancing the activity of dopaminergic synapses. Similar mechanisms may facilitate synaptic plasticity in vertebrates.

Phase-dependent preference of thermosensation and chemosensation during simultaneous presentation assay in Caenorhabditis elegans

Background

Multi-sensory integration is necessary for organisms to discriminate different environmental stimuli and thus determine behavior. Caenorhabditis elegans has 12 pairs of amphid sensory neurons, which are involved in generating behaviors such as thermotaxis toward cultivation temperature, and chemotaxis toward chemical stimuli. This arrangement of known sensory neurons and measurable behavioral output makes C. elegans suitable for addressing questions of multi-sensory integration in the nervous system. Previous studies have suggested that C. elegans can process different chemoattractants simultaneously. However, little is known about how these organisms can integrate information from stimuli of different modality, such as thermal and chemical stimuli.

Results

We studied the behavior of a population of C. elegans during simultaneous presentation of thermal and chemical stimuli. First, we examined thermotaxis within the radial temperature gradient produced by a feedback-controlled thermoregulator. Separately, we examined chemotaxis toward sodium chloride or isoamyl alcohol. Then, assays for simultaneous presentations of 15°C (colder temperature than 20°C room temperature) and chemoattractant were performed with 15°C-cultivated wild-type worms. Unlike the sum of behavioral indices for each separate behavior, simultaneous presentation resulted in a biased migration to cold regions in the first 10 min of the assay, and sodium chloride-regions in the last 40 min. However, when sodium chloride was replaced with isoamyl alcohol in the simultaneous presentation, the behavioral index was very similar to the sum of separate single presentation indices. We then recorded tracks of single worms and analyzed their behavior. For behavior toward sodium chloride, frequencies of forward and backward movements in simultaneous presentation were significantly different from those in single presentation. Also, migration toward 15°C in simultaneous presentation was faster than that in 15°C-single presentation.

Conclusion

We conclude that worms preferred temperature to chemoattractant at first, but preferred the chemoattractant sodium chloride thereafter. This preference was not seen for isoamyl alcohol presentation. We attribute this phase-dependent preference to the result of integration of thermosensory and chemosensory signals received by distinct sensory neurons.

Caenorhabditis elegans as an undergraduate educational tool for teaching RNAi

Discovery of RNA-mediated interference (RNAi) is widely recognized as one of the most significant molecular biology breakthroughs in the past 10 years. There is a need for science educators to develop teaching tools and laboratory activities that demonstrate the power of this new technology and help students to better understand the RNAi process. C. elegans is an ideal model organism for the undergraduate laboratory because of the simplicity of worm maintenance, its well-studied genetic background, and the fact that it can be employed as a model organism in laboratory environments where vertebrate research is restricted. Certain unique features of C. elegans make it a very suitable organism for RNAi studies. Specifically, nematode strains highly sensitive to RNAi are readily available from public sources, and RNAi induction by a feeding method is an uncomplicated procedure that lends itself readily as an educational tool. In this article, we provide a detailed depiction of the use of C. elegans as an RNAi educational tool, describing two separate RNAi-based experiments. One is a qualitative experiment where students can examine the effects of knocking down the unc-22 gene involved in the regulation of muscle contraction, which results in a "twitching" phenotype. The other experiment is a quantitative RNAi experiment, where students measure the effect of knocking down the lsy-2 gene involved in neuronal development. Although these experiments are designed for a college-level study, nematode research projects can also be accomplished in secondary school facilities.

Gustatory plasticity in C. elegans involves integration of negative cues and NaCl taste mediated by serotonin, dopamine, and glutamate

While naïve Caenorhabditis elegans individuals are attracted to 0.1-200 mM NaCl, they become strongly repelled by these NaCl concentrations after prolonged exposure to 100 mM NaCl. We call this behavior gustatory plasticity. Here, we show that C. elegans displays avoidance of low NaCl concentrations only when pre-exposure to NaCl is combined with a negative stimulus, e.g., a repellent, or in the absence of food. By testing serotonin and/or dopamine signaling mutants and rescue by exogenously supplying these neurotransmitters, we found that serotonin and dopamine play a role during the plasticity response, while serotonin is also required during development. In addition, we also show that glutamate plays an important role in the response to NaCl, both in chemoattraction to NaCl and in gustatory plasticity. Thus, C. elegans can associate NaCl with negative stimuli using dopaminergic, serotonergic, and glutamatergic neurotransmission. Finally, we show that prolonged starvation enhances gustatory plasticity and can induce avoidance of NaCl in most gustatory plasticity mutants tested. Only mutation of the glutamate-gated Cl(-) channel gene avr-15 affected starvation-enhanced gustatory plasticity. These results suggest that starvation induces avoidance of NaCl largely independent of the normal gustatory plasticity mechanism.

Neuropeptides

The role of neuropeptides in modulating behavior is slowly being elucidated. With the sequencing of the C. elegans genome, the extent of the neuropeptide genes in C. elegans can be determined. To date, 113 neuropeptide genes encoding over 250 distinct neuropeptides have been identified. Of these, 40 genes encode insulin-like peptides, 31 genes encode FMRFamide-related peptides, and 42 genes encode non-insulin, non-FMRFamide-related neuropeptides. As in other systems, C. elegans neuropeptides are derived from precursor molecules that must be post-translationally processed to yield the active peptides. These precursor molecules contain a single peptide, multiple copies of a single peptide, multiple distinct peptides, or any combination thereof. The neuropeptide genes are expressed extensively throughout the nervous system, including in sensory, motor, and interneurons. In addition, some of the genes are also expressed in non-neuronal tissues, such as the somatic gonad, intestine, and vulval hypodermis. To address the effects of neuropeptides on C. elegans behavior, animals in which the different neuropeptide genes are inactivated or overexpressed are being isolated. In a complementary approach the receptors to which the neuropeptides bind are also being identified and examined. Among the knockout animals analyzed thus far, defects in locomotion, dauer formation, egg laying, ethanol response, and social behavior have been reported. These data suggest that neuropeptides have a modulatory role in many, if not all, behaviors in C. elegans.

Light-sensitive neurons and channels mediate phototaxis in C. elegans

Phototaxis behavior is commonly observed in animals with light-sensing organs. C. elegans, however, is generally believed to lack phototaxis, as this animal lives in darkness (soil) and does not possess eyes. Here, we found that light stimuli elicited negative phototaxis in C. elegans and that this behavior is important for survival. We identified a group of ciliary sensory neurons as candidate photoreceptor cells for mediating phototaxis. Furthermore, we found that light excited photoreceptor cells by evoking a depolarizing conductance carried by cyclic guanosine monophosphate (cGMP)-sensitive cyclic nucleotide-gated (CNG) channels, revealing a conservation in phototransduction between worms and vertebrates. These results identify a new sensory modality in C. elegans and suggest that animals living in dark environments without light-sensing organs may not be presumed to be light insensitive. We propose that urbilaterians, the last common ancestor of bilaterians, might have already evolved a visual system that employs CNG channels and the second messenger cGMP for phototransduction.

Ammonium-acetate is sensed by gustatory and olfactory neurons in Caenorhabditis elegans

Background

Caenorhabditis elegans chemosensation has been successfully studied using behavioral assays that treat detection of volatile and water soluble chemicals as separate senses, analogous to smell and taste. However, considerable ambiguity has been associated with the attractive properties of the compound ammonium-acetate (NH₄Ac). NH₄Ac has been used in behavioral assays both as a chemosensory neutral compound and as an attractant.

Methodology/Main Findings

Here we show that over a range of concentrations NH₄Ac can be detected both as a water soluble attractant and as an odorant, and that ammonia and acetic acid individually act as olfactory attractants. We use genetic analysis to show that NaCl and NH₄Ac sensation are mediated by separate pathways and that ammonium sensation depends on the cyclic nucleotide gated ion channel TAX-2/TAX-4, but acetate sensation does not. Furthermore we show that sodium-acetate (NaAc) and ammonium-chloride (NH₄Cl) are not detected as Na⁺ and Cl⁻ specific stimuli, respectively.

Conclusions/Significance

These findings clarify the behavioral response of C. elegans to NH₄Ac. The results should have an impact on the design and interpretation of chemosensory experiments studying detection and adaptation to soluble compounds in the nematode Caenorhabditis elegans.

Neuronal mechanisms of Caenorhabditis elegans and pathogenic bacteria interactions

Individuals interact with environment through different neuronal functions, such as olfaction and mechanosensation; experience shapes these physiological functions. It is not well understood how an individual senses and processes multiple cues of natural stimuli in the environment and how experience modulates these physiological mechanisms. Recent molecular genetics and behavioral studies on the interactions of the genetic model organism Caenorhabditis elegans with pathogenic bacteria have provided insights on the molecular and cellular mechanisms underlying these regulatory processes.

Olfactory Behavior of Swimming C. elegans Analyzed by Measuring Motile Responses to Temporal Variations of Odorants

Caenorhabditis elegans responds to chemical cues using a small number of chemosensory neurons that detect a large variety of molecules in its environment. During chemotaxis, C. elegans biases its migration in spatial chemical gradients by lengthening (/shortening) periods of forward movement when it happens to be moving toward (/away) from preferred locations. In classical assays of chemotactic behavior, a group of crawling worms is placed on an agar plate containing a point source of chemical, the group is allowed to navigate for a period of time, and aggregation of worms near the source is quantified. Here we show that swimming worms exhibit acute motile responses to temporal variations of odor in their surrounding environment, allowing our development of an automated assay of chemotactic behavior with single-animal resolution. By placing individual worms in small microdroplets and quantifying their movements as they respond to the addition and removal of odorized airstreams, we show that the sensorimotor phenotypes of swimming worms (wild-type behavior, the effects of certain mutations, and the effects of laser ablation of specific olfactory neurons) are consistent with aggregation phenotypes previously obtained in crawling assays. The microdroplet swimming assay has certain advantages over crawling assays, including flexibility and precision in defining the stimulus waveform and automated quantification of motor response during stimulus presentation. In this study, we use the microdroplet assay to quantify the temporal dynamics of the olfactory response, the sensitivity to odorant concentration, combinations, and gradients, and the contribution of specific olfactory neurons to overall behavior.

Sensory roles of neuronal cilia: cilia development, morphogenesis, and function in C. elegans

In the free-living nematode Caenorhabditis elegans, cilia are found on the dendritic endings of sensory neurons. C. elegans cilia are classified as 'primary' or 'sensory' according to the '9+0' axonemal ultrastructure (nine doublet outer microtubules with no central microtubule pair) and lack of motility, characteristics of '9+2' cilia. The C. elegans ciliated nervous system allows the animal to perceive environmental stimuli and make appropriate developmental, physiological, and behavioral decisions. In vertebrates, the biological significance of primary cilia had been largely neglected. Recent findings have placed primary/sensory cilia in the center of cellular signaling and developmental processes. Studies using genetic model organisms such as C. elegans identified the link between ciliary dysfunction and human ciliopathies. Future studies in the worm will address important basic questions regarding ciliary development, morphogenesis, specialization, and signaling functions.

Dimensionality and dynamics in the behavior of C. elegans

A major challenge in analyzing animal behavior is to discover some underlying simplicity in complex motor actions. Here, we show that the space of shapes adopted by the nematode Caenorhabditis elegans is low dimensional, with just four dimensions accounting for 95% of the shape variance. These dimensions provide a quantitative description of worm behavior, and we partially reconstruct “equations of motion” for the dynamics in this space. These dynamics have multiple attractors, and we find that the worm visits these in a rapid and almost completely deterministic response to weak thermal stimuli. Stimulus-dependent correlations among the different modes suggest that one can generate more reliable behaviors by synchronizing stimuli to the state of the worm in shape space. We confirm this prediction, effectively “steering” the worm in real time.

A great deal of work has been done in characterizing the genes, proteins, neurons, and circuits that are involved in the biology of behavior, but the techniques used to quantify behavior have lagged behind the advancements made in these areas. Here, we address this imbalance in a domain rich enough to allow complex, natural behavior yet simple enough so that movements can be explored exhaustively: the motions of Caenorhabditis elegans freely crawling on an agar plate. From measurements of the worm's curvature, we show that the space of natural worm postures is low dimensional and can be almost completely described by their projections along four principal “eigenworms.” The dynamics along these eigenworms offer both a quantitative characterization of classical worm movement such as forward crawling, reversals, and Omega-turns, and evidence of more subtle behaviors such as pause states at particular postures. We can partially construct equations of motion for this shape space, and within these dynamics we find a set of attractors that can be used as a rigorous definition of behavioral state. Our observations of C. elegans reveal a precise and complete language of motion and new aspects of worm behavior. We believe this is a lesson with promise for other organisms.

Artificial dirt: microfluidic substrates for nematode neurobiology and behavior

With a nervous system of only 302 neurons, the free-living nematode Caenorhabditis elegans is a powerful experimental organism for neurobiology. However, the laboratory substrate commonly used in C. elegans research, a planar agarose surface, fails to reflect the complexity of this organism's natural environment, complicates stimulus delivery, and is incompatible with high-resolution optophysiology experiments. Here we present a new class of microfluidic devices for C. elegans neurobiology and behavior: agarose-free, micron-scale chambers and channels that allow the animals to crawl as they would on agarose. One such device mimics a moist soil matrix and facilitates rapid delivery of fluid-borne stimuli. A second device consists of sinusoidal channels that can be used to regulate the waveform and trajectory of crawling worms. Both devices are thin and transparent, rendering them compatible with high-resolution microscope objectives for neuronal imaging and optical recording. Together, the new devices are likely to accelerate studies of the neuronal basis of behavior in C. elegans.

The conserved proteins CHE-12 and DYF-11 are required for sensory cilium function in Caenorhabditis elegans

Sensory neuron cilia are evolutionarily conserved dendritic appendages that convert environmental stimuli into neuronal activity. Although several cilia components are known, the functions of many remain uncharacterized. Furthermore, the basis of morphological and functional differences between cilia remains largely unexplored. To understand the molecular basis of cilia morphogenesis and function, we studied the Caenorhabditis elegans mutants che-12 and dyf-11. These mutants fail to concentrate lipophilic dyes from their surroundings in sensory neurons and are chemotaxis defective. In che-12 mutants, sensory neuron cilia lack distal segments, while in dyf-11 animals, medial and distal segments are absent. CHE-12 and DYF-11 are conserved ciliary proteins that function cell-autonomously and are continuously required for maintenance of cilium morphology and function. CHE-12, composed primarily of HEAT repeats, may not be part of the intraflagellar transport (IFT) complex and is not required for the localization of some IFT components. DYF-11 undergoes IFT-like movement and may function at an early stage of IFT-B particle assembly. Intriguingly, while DYF-11 is expressed in all C. elegans ciliated neurons, CHE-12 expression is restricted to some amphid sensory neurons, suggesting a specific role in these neurons. Our results provide insight into general and neuron-specific aspects of cilium development and function.

Modulation of Caenorhabditis elegans chemotaxis by cultivation and assay temperatures

The chemotaxis behaviors of the nematode Caenorhabditis elegans cultivated at various temperatures (15 degrees C, 20 degrees C and 25 degrees C) were examined at various temperatures (10 degrees C, 15 degrees C, 20 degrees C and 25 degrees C) to determine the multi-sensory integration of physical (thermal) and chemical sensory information within its nervous system. Chemotaxis behavior toward sodium acetate and ammonium chloride were differently affected by both assay and cultivation temperatures, suggesting that the temperature effect on chemotaxis is not general, but rather distinctive for each chemosensory pathway. Since thermosensory cues are likely encountered constantly in C. elegans, we supposed that the chemotaxis behaviors of worms are achieved by the integration of chemo- and thermosensory information. To verify the possible contribution of thermosensory function in chemotaxis, we examined the chemotaxis behaviors of ttx-1(p767) mutant worms with defective AFD thermosensory neurons. The chemotaxis behaviors toward sodium acetate or ammonium chloride of mutant worms cultivated at 20 degrees C and 25 degrees C were reduced relative to those of wild-type worms. These results indicate the important role of multi-sensory integration of chemosensory and thermosensory information in chemotaxis behavior of the C. elegans.

The cold war of the social amoebae

When confronted with starvation, the amoebae of Dictyostelium discoideum initiate a developmental process that begins with cell aggregation and ends with a ball of spores supported on a stalk. Spores live and stalk cells die. Because the multicellular organism is produced by cell aggregation and not by growth and division of a single cell, genetically diverse amoebae may enter an aggregate and, if one lineage has a capacity to avoid the stalk cell fate, it may have a selective advantage. Such cheater mutants have been found among wild isolates and created in laboratory strains. The mutants raise a number of questions--how did such a cooperative system evolve in the face of cheating? What is the basis of self recognition? What genes are involved? How is cheating constrained? This review summarizes the results of studies on the social behavior of Dictyostelium and its relatives, including the familiar asexual developmental cycle and the lesser known, but puzzling, sexual cycle.

Pharmacogenetic analysis of lithium-induced delayed aging in Caenorhabditis elegans

Lithium (Li(+)) has been used to treat mood affect disorders, including bipolar, for decades. This drug is neuroprotective and has several identified molecular targets. However, it has a narrow therapeutic range and the one or more underlying mechanisms of its therapeutic action are not understood. Here we describe a pharmacogenetic study of Li(+) in the nematode Caenorhabditis elegans. Exposure to Li(+) at clinically relevant concentrations throughout adulthood increases survival during normal aging (up to 46% median increase). Longevity is extended via a novel mechanism with altered expression of genes encoding nucleosome-associated functions. Li(+) treatment results in reduced expression of the worm ortholog of LSD-1 (T08D10.2), a histone demethylase; knockdown by RNA interference of T08D10.2 is sufficient to extend longevity ( approximately 25% median increase), suggesting Li(+) regulates survival by modulating histone methylation and chromatin structure.

The cellular and genetic basis of olfactory responses in Caenorhabditis elegans

The small soil nematode Caenorhabditis elegans has only 302 neurons in its entire nervous system, so it is possible to analyse the functions of individual neurons in the animal's behaviour. We are using behavioural, cellular and genetic analyses of chemotactic responses to find out how olfactory behaviour patterns are generated and regulated. Single chemosensory neurons in C. elegans can recognize several different attractive odorants that are distinguished by the animal. Distinct sets of chemosensory neurons detect high and low concentrations of a single odorant. Odorant responses adapt after prolonged exposure to an odorant; this adaptation is odorant specific and reversible. Mutants with defects in odorant responses have been identified. Some genes appear to be necessary for the development or function of particular kinds of sensory neurons. Other genes have effects that suggest that they participate in odorant reception or signal transduction.

The molecular signature and cis-regulatory architecture of a C. elegans gustatory neuron

Taste receptor cells constitute a highly specialized cell type that perceives and conveys specific sensory information to the brain. The detailed molecular composition of these cells and the mechanisms that program their fate are, in general, poorly understood. We have generated serial analysis of gene expression (SAGE) libraries from two distinct populations of single, isolated sensory neuron classes, the gustatory neuron class ASE and the thermosensory neuron class AFD, from the nematode Caenorhabditis elegans. By comparing these two libraries, we have identified >1000 genes that define the ASE gustatory neuron class on a molecular level. This set of genes contains determinants of the differentiated state of the ASE neuron, such as a surprisingly complex repertoire of transcription factors (TFs), ion channels, neurotransmitters, and receptors, as well as seven-transmembrane receptor (7TMR)-type putative gustatory receptor genes. Through the in vivo dissection of the cis-regulatory regions of several ASE-expressed genes, we identified a small cis-regulatory motif, the "ASE motif," that is required for the expression of many ASE-expressed genes. We demonstrate that the ASE motif is a binding site for the C2H2 zinc finger TF CHE-1, which is essential for the correct differentiation of the ASE gustatory neuron. Taken together, our results provide a unique view of the molecular landscape of a single neuron type and reveal an important aspect of the regulatory logic for gustatory neuron specification in C. elegans.

Developmental changes in chemotactic response and choice of two attractants, sodium acetate and diacetyl, in the nematode Caenorhabditis elegans

The chemotactic behavior of the nematode Caenorhabditis elegans to chemical attractants, water-soluble sodium acetate and odorant diacetyl, was investigated using nematodes at various developmental stages to examine the effects of postembryonic development on chemotactic response and spontaneous locomotion. The chemotactic responses to attractants increased as development progressed, and the largest responses to either 1.0 M sodium acetate or 0.1% diacetyl were seen at the young adult (YA) or day adult (A1) stage, respectively. Responses to the chemicals declined thereafter in-line with increasing age. The chemotaxis indices for attractants correlated with activity of spontaneous locomotion (p<0.01), suggesting that a change in spontaneous locomotion is one of the factors involved with the change in chemotactic responses during development. We also investigated the effect of aging on attractant choice by the simultaneous presentation of 0.6 M sodium acetate and 0.1% diacetyl. In the presence of both attractants, the fraction of larval animals at the sodium acetate location was greater than that at the diacetyl location (p<0.05). The fractions of YA animals that gathered at either location were almost identical, whereas the fraction of adult animals at the diacetyl location was greater than that at the sodium acetate location (p<0.05). The patterns of attractant choice of the long-lived daf-2 mutants and short lifespan mev-1 mutants showed the same tendency as those of wild type nematodes in the presence of both attractants. These results suggest that a change in the neuronal mechanisms controlling attractant choice and preference occurs during developmental progression.

Circuit motifs for spatial orientation behaviors identified by neural network optimization

Spatial orientation behavior is universal among animals, but its neuronal basis is poorly understood. The main objective of the present study was to identify candidate patterns of neuronal connectivity (motifs) for two widely recognized classes of spatial orientation behaviors: hill climbing, in which the organism seeks the highest point in a spatial gradient, and goal seeking, in which the organism seeks an intermediate point in the gradient. Focusing on simple networks of graded processing neurons characteristic of Caenorhabditis elegans and other nematodes, we used an unbiased optimization algorithm to seek values of neuronal time constants, resting potentials, and synaptic strengths sufficient for each type of behavior. We found many different hill-climbing and goal-seeking networks that performed equally well in the two tasks. Surprisingly, however, each hill-climbing network represented one of just three fundamental circuit motifs, and each goal-seeking network comprised two of these motifs acting in concert. These motifs are likely to inform the search for the real circuits that underlie these behaviors in nematodes and other organisms.

Strongyloides ratti: chemotactic responses of third-stage larvae to selected serum proteins and albumins

Experiments were carried outin vitroto investigate whether the sera of several animals as well as albumins and peptides might act as attractants for larvae ofStrongyloidesratti. Samples of sera from several mammal species were dialysed and the aliquots were further centrifuged using ultrafiltration cartridges to remove any remaining small molecules. Additional test substances included commercially obtained ovalbumin, rat and bovine serum albumins, polypeptides such as peptone, tryptone and tryptose, amino nitrogens, monosaccharides, and reduced glutathione (triaminopeptide). Larvae were strongly attracted to the dialysed mammalian sera, which mainly consisted of serum albumin and globulins. Ov- and serum albumins, and polypeptides also acted as attractants. On the other hand, reduced glutathione, 16 kinds of amino acids and four kinds of monosaccharides did not attract this nematode.

Generation and modulation of chemosensory behaviors in C. elegans

C. elegans recognizes and discriminates among hundreds of chemical cues using a relatively compact chemosensory nervous system. Chemosensory behaviors are also modulated by prior experience and contextual cues. Because of the facile genetics and genomics possible in this organism, C. elegans provides an excellent system in which to explore the generation of chemosensory behaviors from the level of a single gene to the motor output. This review summarizes the current knowledge on the molecular and neuronal substrates of chemosensory behaviors and chemosensory behavioral plasticity in C. elegans.

Left-right asymmetry: making the right decision early

A left-right asymmetry in neuronal function is specified surprisingly early during embryogenesis in Caenorhabditis elegans. Do early cues influence left-right asymmetries in other animals? How are early cues remembered until late in development?

Analysis of the effects of turning bias on chemotaxis in C. elegans

C. elegans advances up a chemical gradient by modulating the probability of occasional large, course-correcting turns called pirouettes. However, it remains uncertain whether C. elegans also uses other behavioral strategies for chemotaxis. Previous observations of the unusual, spiral-shaped chemotaxis tracks made by the bent-head mutant unc-23 point to a different strategy in which the animal continuously makes more subtle course corrections. In the present study we have combined automated tracking of individual animals with computer modeling to test the hypothesis that the pirouette strategy is sufficient on its own to account for the spiral tracks. Tracking experiments showed that the bent-head phenotype causes a strong turning bias and disrupts pirouette execution but does not disrupt pirouette initiation. A computer simulation of disrupted pirouette behavior and turning bias reproduced the spiral tracks of unc-23 chemotaxis behavior, showing that the pirouette strategy is sufficient to account for the mutant phenotype. In addition, the simulation reproduced higher order features of the behavior such as the relationship between the handedness of the spiral and the side to which the head was bent. Our results suggest that the pirouette mechanism is sufficient to account for a diverse range of chemotaxis trajectories.

Effects of sex and insulin/insulin-like growth factor-1 signaling on performance in an associative learning paradigm in Caenorhabditis elegans

Learning is an adaptive change in behavior in response to environmental stimuli. In mammals, there is a distinct female bias to learn skills that is still unprecedented in other animal taxa. Here we have investigated the biological determinants of performance in an associative learning paradigm in the nematode Caenorhabditis elegans. Using an assay of chemotactic reactions associated with food deprivation, wild-type male worms show inferior learning ability relative to hermaphrodites. Sex-based learning difference is therefore an ancient evolutionary feature appearing even in relatively simple animals. C. elegans mutants with reduced insulin/IGF-1 signaling also exhibit a greatly reduced learning ability in this assay. In addition, hyperactivation of insulin/IGF-1 signaling through loss-of-function mutations in the PTEN phosphatase daf-18, a negative regulator of insulin/IGF-1 signaling, enhances learning ability beyond that of wild type. According to our epistasis analysis, the effect of DAF-2 on learning acts via phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) production, but not the DAF-16 FOXO transcription factor. This implies that the signaling pathway from DAF-2 affecting this learning paradigm branches between PIP(3) production and DAF-16. However, learning capacity of nematodes is lowered by loss-of-function mutations in daf-16, suggesting involvement of noninsulin/IGF-1 signaling-dependent DAF-16 activation in learning. Potentially, sex and insulin/IGF-1 signaling affect performance in this learning assay via effects on the neurobiology of learning.

Identifying factors that promote functional aging in Caenorhabditis elegans

A major feature of aging is a reduction in muscle strength from sarcopenia, the loss of muscle mass. Sarcopenia impairs physical ability, reduces quality of life and increases the risk of fall and injury. Since aging is a process of stochastic decline, there may be many factors that impinge on the progression of sarcopenia. Possible factors that may promote muscle decline are contraction-related injury and oxidative stress. However, relatively little is understood about the cellular pathways affecting muscle aging, in part because lifespan studies are difficult to conduct in species with large muscles, such as rodents and primates. For this reason, shorter-lived invertebrate models of aging may be more useful for unraveling causes of sarcopenia and functional declines during aging. Recent studies have examined both physiological and genetic factors that affect aging-related declines in Caenorhabditis elegans nematodes. In C. elegans, aging leads to significant functional declines that correlate with muscle deterioration, similar to those documented for longer-lived vertebrates. This article will examine the current research into aging-related functional declines in this species, focusing on recent studies of locomotory and feeding decline during aging in the nematode, C. elegans.

Chemosensory control of surface antigen switching in the nematode Caenorhabditis elegans

Nematodes change their surface compositions in response to environmental signals, which may allow them to survive attacks from microbial pathogens or host immune systems. In the free-living species Caenorhabditis elegans, wild-type worms are induced to display an L1 (first larval stage) surface epitope at later larval stages when grown on an extract of spent culture medium (Inducible Larval Display or ILD). Before this study, it was not known whether ILD was regulated by the well-characterized, neurologically based chemical senses of C. elegans, which mediate other behavioural and developmental responses to environmental signals such as chemotaxis and formation of the facultatively arrested dauer larva stage. We show here that ILD requires the activities of three genes that are essential for the function of the C. elegans chemosensory neurons. ILD was abolished in chemotaxis-defective che-3, osm-3 and tax-4 mutants. In contrast, chemotaxis-defective mutants altered in a different gene, srf-6, show constitutive display of the L1 epitope on all four larval stages. The ILD-defective che-3, osm-3 and tax-4 mutations blocked the constitutive larval display of an srf-6 mutant. Combining srf-6 and certain dauer-constitutive mutations in double mutants enhanced constitutive dauer formation, consistent with the idea that srf-6 acts in parallel with specific components of the dauer formation pathway. These results taken together are consistent with the hypothesis that ILD is triggered by environmental signals detected by the nematode's chemosensory neurons.

Computer-driven automatic identification of locomotion states in Caenorhabditis elegans

We developed a computer-driven tracking system for the automated analysis of the locomotion of Caenorhabditis elegans. The algorithm for the identification of locomotion states on agar plates (forward movement, backward movement, rest, and curl) includes the identification of the worm's head and tail. The head and tail are first assigned, by using three criteria, based on time-sequential binary images of the worm, and the determination is made based on the majority of the three criteria. By using the majority of the criteria, the robustness was improved. The system allowed us to identify locomotion states and to reconstruct the path of a worm using more than 1h data. Based on 5-min image sequences from a total of 230 individual wild-type worms and 22 mutants, the average error of identification of the head/tail for all strains was 0.20%. The system was used to analyze 70 min of locomotion for wild-type and two mutant strains after a worm was transferred from a seeded plate to a bacteria-free assay plate. The error of identifying the state was less than 1%, which is sufficiently accurate for locomotion studies.

Searching for neuronal left/right asymmetry: genomewide analysis of nematode receptor-type guanylyl cyclases

Functional left/right asymmetry ("laterality") is a fundamental feature of many nervous systems, but only very few molecular correlates to functional laterality are known. At least two classes of chemosensory neurons in the nematode Caenorhabditis elegans are functionally lateralized. The gustatory neurons ASE left (ASEL) and ASE right (ASER) are two bilaterally symmetric neurons that sense distinct chemosensory cues and express a distinct set of four known chemoreceptors of the guanylyl cyclase (gcy) gene family. To examine the extent of lateralization of gcy gene expression patterns in the ASE neurons, we have undertaken a genomewide analysis of all gcy genes. We report the existence of a total of 27 gcy genes encoding receptor-type guanylyl cyclases and of 7 gcy genes encoding soluble guanylyl cyclases in the complete genome sequence of C. elegans. We describe the expression pattern of all previously uncharacterized receptor-type guanylyl cyclases and find them to be highly biased but not exclusively restricted to the nervous system. We find that >41% (11/27) of all receptor-type guanylyl cyclases are expressed in the ASE gustatory neurons and that one-third of all gcy genes (9/27) are expressed in a lateral, left/right asymmetric manner in the ASE neurons. The expression of all laterally expressed gcy genes is under the control of a gene regulatory network composed of several transcription factors and miRNAs. The complement of gcy genes in the related nematode C. briggsae differs from C. elegans as evidenced by differences in chromosomal localization, number of gcy genes, and expression patterns. Differences in gcy expression patterns in the ASE neurons of C. briggsae arise from a difference in cis-regulatory elements and trans-acting factors that control ASE laterality. In sum, our results indicate the existence of a surprising multitude of putative chemoreceptors in the gustatory ASE neurons and suggest the existence of a substantial degree of laterality in gustatory signaling mechanisms in nematodes.

Step-response analysis of chemotaxis in Caenorhabditis elegans

The sensorimotor transformation underlying Caenorhabditis elegans chemotaxis has been difficult to measure directly under normal assay conditions. Thus, key features of this transformation remain obscure, such as its time course and dependence on stimulus amplitude. Here, we present a comprehensive characterization of the transformation as obtained by inducing stepwise temporal changes in attractant concentration within the substrate as the worm crawls across it. We found that the step response is complex, with multiple phases and a nonlinear dependence on the sign and amplitude of the stimulus. Nevertheless, the step response could be reduced to a simple kinetic model that predicted the results of chemotaxis assays. Analysis of the model showed that chemotaxis results from the combined effects of approach and avoidance responses to concentration increases and decreases, respectively. Surprisingly, ablation of the ASE chemosensory neurons, known to be necessary for chemotaxis in chemical gradient assays, eliminated avoidance responses but left approach responses intact. These results indicate that the transformation can be dissected into components to which identified neurons can be assigned.

Functional expression of mammalian bitter taste receptors in Caenorhabditis elegans

Bitter taste has evolved as a central warning signal against the ingestion of potentially toxic substances appearing in the environment. The molecular events in the perception of bitter taste start with the binding of specific water-soluble molecules to G protein-coupled receptors (GPCR) called T2Rs and expressed at the surface of taste receptor cells. The functional characterisation of T2R receptors is far from been completed due to the difficulty to functionally express them in heterologous systems. Taking advantage of the parallelisms between the Caenorhabditis elegans (C. elegans) and mammalian GPCR signalling pathways, we developed a C. elegans-based expression system to express functional human and rodent GPCRs of the T2R family. We generated transgenic worms expressing T2Rs in ASI chemosensory neurons and performed behavioural assays using a variety of bitter tastants. As a proof of the concept, we generated transgenic worms expressing human T2R4 or its mouse ortholog T2R8 receptors, which respond to two bitter tastants previously characterised as their functional ligands, 6-n-propyl-2-thiouracil and denatoniun. As expected, expression of human T2R4 or its mouse ortholog T2R8 in ASI neurons counteracted the water-soluble avoidance to 6-n-propyl-2-thiouracil and denatoniun observed in control wild-type worms. The expression in ASI neurons of human T2R16, the ligand of which, phenyl-beta-d-glucopyranoside, belong to a chemically different group of bitter tastants, also counteracted the water-soluble avoidance to this compound observed in wild-type worms. These results indicate that C. elegans is a suitable heterologous expression system to express functional T2Rs providing a tool to efficiently search for specific taste receptor ligands and to extend our understanding of the molecular basis of gustation.

Antagonistic sensory cues generate gustatory plasticity in Caenorhabditis elegans

Caenorhabditis elegans shows chemoattraction to 0.1-200 mM NaCl, avoidance of higher NaCl concentrations, and avoidance of otherwise attractive NaCl concentrations after prolonged exposure to NaCl (gustatory plasticity). Previous studies have shown that the ASE and ASH sensory neurons primarily mediate attraction and avoidance of NaCl, respectively. Here we show that balances between at least four sensory cell types, ASE, ASI, ASH, ADF and perhaps ADL, modulate the response to NaCl. Our results suggest that two NaCl-attraction signalling pathways exist, one of which uses Ca(2+)/cGMP signalling. In addition, we provide evidence that attraction to NaCl is antagonised by G-protein signalling in the ASH neurons, which is desensitised by the G-protein-coupled receptor kinase GRK-2. Finally, the response to NaCl is modulated by G-protein signalling in the ASI and ADF neurons, a second G-protein pathway in ASH and cGMP signalling in neurons exposed to the body fluid.

Chemotaxis of Caenorhabditis elegans during simultaneous presentation of two water-soluble attractants, l-lysine and chloride ions

Lysine and chloride ions are water-soluble attractants for Caenorhabditis elegans. When chemotaxis behavior to either of these attractants was assayed separately, the radial concentration gradients of 3 M lysine and 0.1 M ammonium chloride had similar potencies for attracting worms. However, when the concentration gradients of lysine and ammonium chloride at these concentrations were presented simultaneously, worms preferred lysine to ammonium chloride more than expected from the results obtained in separate experiments, suggesting the presence of an interaction between these two sensory information pathways within the nervous system. Chemotaxis behavior toward the radial concentration gradient of one of these attractants superimposed on a uniform concentration of the other attractant showed that the chemotaxis was augmented or attenuated by the ammonium chloride background depending on the background concentration, and attenuated by the lysine background, further supporting the interaction between the two sensory information pathways.

Chemotaxis of the nematode Caenorhabditis elegans toward cycloheximide and quinine hydrochloride

We investigated the chemotaxis of the nematode Caenorhabditis elegans toward cycloheximide, denatonium benzoate, 6-n-propyl-2-thiouracil, and quinine hydrochloride. Interestingly, C. elegans was strongly attracted to cycloheximide, while avoiding quinine hydrochloride. This is the first report thus far to describe the chemotactic responses of C. elegans toward bitter tastants for humans.

Humic material induces behavioral and global transcriptional responses in the nematode Caenorhabditis elegans

Humic materials are complex organic molecules constituting the most abundant source of natural organic matter (NOM) in freshwater and soil ecosystems. Recent advances have identified that they interfere with biological systems, via the induction of biotransformation enzymes, the inhibition of photosynthetic oxygen release (in freshwater plants), the production of internal oxidative stress, or through the feminization of fish and amphibians. The nematode model organism Caenorhabditis elegans was chosen to investigate whether a natural and a synthetic humic material induce (i) a behavioral attraction, (ii) the reproduction, and (iii) a response in whole genome transcriptional expression. The phenomenological attractant experiments provided evidence that both humic material sources attract the worm and exert distinct chemical cues. In the reproduction assay, only the highest concentration (32 mg/L DOC of Fuchskuhle NOM, 38 mg/L DOC of HS 1500) resulted in a decrease in brood size, highlighting an overall intrinsic tolerance toward humic material. Finally, oligonucleotide-based whole genome DNA microarray experiments were performed from control and humic material treated worms. Significant transcriptional changes (exceeding a 2-fold increase or decrease) were identified in chemosensors, olfactory receptors, as well as enzymes of the biotransformation system (cytochromes P450, UDP-glucuronosyltransferases, glutathione S-transferases), thereby confirming that humic material is recognized as an environmental signaling chemical.

MPS-1 is a K+ channel β-subunit and a serine/threonine kinase

We report the first example of a K+ channel beta-subunit that is also a serine/threonine kinase. MPS-1 is a single-transmembrane domain protein that coassembles with voltage-gated K+ channel KVS-1 in the nervous system of the nematode Caenorhabditis elegans. Biochemical analysis shows that MPS-1 can phosphorylate KVS-1 and other substrates. Electrophysiological analysis in Chinese hamster ovary (CHO) cells demonstrates that MPS-1 activity leads to a significant decrease in the macroscopic current. Single-channel analysis and biotinylation assays indicate that MPS-1 reduces the macroscopic current by lowering the open probability of the channel. These data are consistent with a model that predicts that the MPS-1-dependent phosphorylation of KVS-1 sustains cell excitability by controlling K+ flux.

Neuronal substrates of complex behaviors in C. elegans

A current challenge in neuroscience is to bridge the gaps between genes, proteins, neurons, neural circuits, and behavior in a single animal model. The nematode Caenorhabditis elegans has unique features that facilitate this synthesis. Its nervous system includes exactly 302 neurons, and their pattern of synaptic connectivity is known. With only five olfactory neurons, C. elegans can dynamically respond to dozens of attractive and repellent odors. Thermosensory neurons enable the nematode to remember its cultivation temperature and to track narrow isotherms. Polymodal sensory neurons detect a wide range of nociceptive cues and signal robust escape responses. Pairing of sensory stimuli leads to long-lived changes in behavior consistent with associative learning. Worms exhibit social behaviors and complex ultradian rhythms driven by Ca(2+) oscillators with clock-like properties. Genetic analysis has identified gene products required for nervous system function and elucidated the molecular and neural bases of behaviors.

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.

A family of K+ channel ancillary subunits regulate taste sensitivity in Caenorhabditis elegans

We have identified a family of ancillary subunits of K(+) channels in Caenorhabditis elegans. MPS-1 and its related members MPS-2, MPS-3, and MPS-4 are detected in the nervous system of the nematode. Electrophysiological analysis in ASE neurons and mammalian cells and epigenetic inactivation by double-stranded RNA interference (RNAi) in vivo show that each MPS can associate with and functionally endow the voltage-gated K(+) channel KVS-1. In the chemosensory neuron ADF, three different MPS subunits combine with KVS-1 to form both binary (MPS-1.KVS-1) and ternary (MPS-2.MPS-3.KVS-1) complexes. RNAi of mps-2, mps-3, or both, enhance the taste of the animal for sodium without altering the susceptibility to other attractants. When sodium is introduced in the test plate as background or as antagonist attractant, the nematode loses the ability to recognize a second attractant. Thus, it appears that the chemosensory apparatus of C. elegans uses sensory thresholds and that a voltage-gated K(+) channel is specifically required for this mechanism.

Behavioral deficits during early stages of aging in Caenorhabditis elegans result from locomotory deficits possibly linked to muscle frailty

Many behavioral responses require the coordination of sensory inputs with motor outputs. Aging is associated with progressive declines in both motor function and muscle structure. However, the consequences of age-related motor deficits on behavior have not been clearly defined. Here, we examined the effects of aging on behavior in the nematode, Caenorhabditis elegans. As animals aged, mild locomotory deficits appeared that were sufficient to impair behavioral responses to sensory cues. In contrast, sensory ability appeared well maintained during aging. Age-related behavioral declines were delayed in animals with mutations in the daf-2/insulin-like pathway governing longevity. A decline in muscle tissue integrity was correlated with the onset of age-related behavioral deficits, although significant muscle deterioration was not. Treatment with a muscarinic agonist significantly improved locomotory behavior in aged animals, indicating that improved neuromuscular signaling may be one strategy for reducing the severity of age-related behavioral impairments.

Maintenance of neuronal positions in organized ganglia by SAX-7, a Caenorhabditis elegans homologue of L1

The L1 family of cell adhesion molecules is predominantly expressed in the nervous system. Mutations in human L1 cause neuronal diseases such as HSAS, MASA, and SPG1. Here we show that sax-7 gene encodes an L1 homologue in Caenorhabditis elegans. In sax-7 mutants, the organization of ganglia and positioning of neurons are abnormal in the adult stage, but these abnormalities are not observed in early larval stage. Misplacement of neurons in sax-7 mutants is triggered by mechanical force linked to body movement. Short and long forms of SAX-7 exhibited strong and weak homophilic adhesion activities in in vitro aggregation assay, respectively, which correlated with their different activities in vivo. SAX-7 was localized on plasma membranes of neurons in vivo. Expression of SAX-7 only in a single neuron in sax-7 mutants cell-autonomously restored its normal neuronal position. Expression of SAX-7 in two different head neurons in sax-7 mutants led to the forced attachment of these neurons. We propose that both homophilic and heterophilic interactions of SAX-7 are essential for maintenance of neuronal positions in organized ganglia.

A neural network model of chemotaxis predicts functions of synaptic connections in the nematode Caenorhabditis elegans

The anatomical connectivity of the nervous system of the nematode Caenorhabditis elegans has been almost completely described, but determination of the neurophysiological basis of behavior in this system is just beginning. Here we used an optimization algorithm to search for patterns of connectivity sufficient to compute the sensorimotor transformation underlying C. elegans chemotaxis, a simple form of spatial orientation behavior in which turning probability is modulated by the rate of change of chemical concentration. Optimization produced differentiator networks capable of simulating chemotaxis. A surprising feature of these networks was inhibitory feedback connections on all neurons. Further analysis showed that feedback regulates the latency between sensory input and behavior. Common patterns of connectivity between the model and biological networks suggest new functions for previously identified connections in the C. elegans nervous system.

The impact of bacterial diet on the migration and navigation of Caenorhabditis elegans

Can diet have a significant impact on the ability of organisms to sense and locate food? Focusing on the bacterial feeder Caenorhabditis elegans, we investigated what effect preconditioning on a range of bacterial substrates had on the subsequent chemotaxis process involved in the nematode locating other bacterial populations. Remarkably, we found that C. elegans, initially fed on a diet of Escherichia coli OP50, was significantly impaired in finding E. coli OP50 populations, compared to other available bacterial populations (P <0.001). We found similar results for another bacterial feeding nematode species, suggesting that a general "substrate legacy" may operate across a wide range of organisms. We discuss this important finding with respect to the variation in response exhibited within a given nematode population, and the impact nematode migration has on bacterial dispersal in the environment.

State-dependency in C. elegans

Memory and the expression of learned behaviors by an organism are often triggered by contextual cues that resemble those that were present when the initial learning occurred. In state-dependent learning, the cue eliciting a learned behavior is a neuroactive drug; behaviors initially learned during exposure to centrally acting compounds such as ethanol are subsequently recalled better if the drug stimulus is again present during testing. Although state-dependent learning is well documented in many vertebrate systems, the molecular mechanisms underlying state-dependent learning and other forms of contextual learning are not understood. Here we demonstrate and present a genetic analysis of state- dependent adaptation in Caenorhabditis elegans. C. elegans normally exhibits adaptation, or reduced behavioral response, to an olfactory stimulus after prior exposure to the stimulus. If the adaptation to the olfactory stimulus is acquired during ethanol administration, the adaptation is subsequently displayed only if the ethanol stimulus is again present. cat-1 and cat-2 mutant animals are defective in dopaminergic neuron signaling and are impaired in state dependency, indicating that dopamine functions in state-dependent adaptation in C. elegans.

RELMbeta/FIZZ2 is a goblet cell-specific immune-effector molecule in the gastrointestinal tract

Gastrointestinal (GI) nematode infections are an important public health and economic concern. Experimental studies have shown that resistance to infection requires CD4(+) T helper type 2 (Th2) cytokine responses characterized by the production of IL-4 and IL-13. However, despite >30 years of research, it is unclear how the immune system mediates the expulsion of worms from the GI tract. Here, we demonstrate that a recently described intestinal goblet cell-specific protein, RELMbeta/FIZZ2, is induced after exposure to three phylogenetically distinct GI nematode pathogens. Maximal expression of RELMbeta was coincident with the production of Th2 cytokines and host protective immunity, whereas production of the Th1 cytokine, IFN-gamma, inhibited RELMbeta expression and led to chronic infection. Furthermore, whereas induction of RELMbeta was equivalent in nematode-infected wild-type and IL-4-deficient mice, IL-4 receptor-deficient mice showed minimal RELMbeta induction and developed persistent infections, demonstrating a direct role for IL-13 in optimal expression of RELMbeta. Finally, we show that RELMbeta binds to components of the nematode chemosensory apparatus and inhibits chemotaxic function of a parasitic nematode in vitro. Together, these results suggest that intestinal goblet cell-derived RELMbeta may be a novel Th2 cytokine-induced immune-effector molecule in resistance to GI nematode infection.

Effect of simultaneous presentation of multiple attractants on chemotactic response of the nematode Caenorhabditis elegans

Chemotactic behaviors of the nematode, Caenorhabditis elegans in response to chemical attractants, such as water-soluble sodium acetate and an odorant diacetyl, which were sensed by different sensory neurons, were investigated using various concentrations of these chemical attractants. In the presence of only sodium acetate attractant, the fraction of animals that were roaming around the outside of the attractant and original locations correlated negatively with the chemotaxis index for sodium acetate (P < 0.01). In contrast, the fraction of animals that remained in the original location correlated negatively with the chemotaxis index in the presence of only diacetyl attractant (P < 0.05). These results indicate that the manner of chemotaxis responses differs between sodium acetate and diacetyl. In order to investigate the effect of multiple attractants on chemotactic behaviors, the chemotactic responses to simultaneous presentation of sodium acetate and diacetyl were examined. The fraction of animals that gathered at the 0.7 M sodium acetate location was greater than that at the 0.1% diacetyl location in the presence of both attractants (P < 0.05), although the chemotaxis indexes for 0.7 M sodium acetate and 0.1% diacetyl were similar in the presence of a single attractant. On the other hand, the fraction of animals that gathered at the 0.02% diacetyl location was greater than that at the 0.1M sodium acetate location in the presence of both attractants (P < 0.05), although the chemotaxis indexes for 0.02% diacetyl and 0.1M sodium acetate were similar in the presence of a single attractant. These results suggest the existence of excitatory and/or inhibitory connections in the neuronal circuit for attractant selection, and that the efficacy of these connections may change according to the concentrations of both attractants.

Dopamine and glutamate control area-restricted search behavior in Caenorhabditis elegans

Area-restricted search (ARS) is a foraging strategy used by many animals to locate resources. The behavior is characterized by a time-dependent reduction in turning frequency after the last resource encounter. This maximizes the time spent in areas in which resources are abundant and extends the search to a larger area when resources become scarce. We demonstrate that dopaminergic and glutamatergic signaling contribute to the neural circuit controlling ARS in the nematode Caenorhabditis elegans. Ablation of dopaminergic neurons eliminated ARS behavior, as did application of the dopamine receptor antagonist raclopride. Furthermore, ARS was affected by mutations in the glutamate receptor subunits GLR-1 and GLR-2 and the EAT-4 glutamate vesicular transporter. Interestingly, preincubation on dopamine restored the behavior in worms with defective dopaminergic signaling, but not in glr-1, glr-2, or eat-4 mutants. This suggests that dopaminergic and glutamatergic signaling function in the same pathway to regulate turn frequency. Both GLR-1 and GLR-2 are expressed in the locomotory control circuit that modulates the direction of locomotion in response to sensory stimuli and the duration of forward movement during foraging. We propose a mechanism for ARS in C. elegans in which dopamine, released in response to food, modulates glutamatergic signaling in the locomotory control circuit, thus resulting in an increased turn frequency.

Worms taste bitter: ASH neurons, QUI-1, GPA-3 and ODR-3 mediate quinine avoidance in Caenorhabditis elegans

An animal's ability to detect and avoid toxic compounds in the environment is crucial for survival. We show that the nematode Caenorhabditis elegans avoids many water-soluble substances that are toxic and that taste bitter to humans. We have used laser ablation and a genetic cell rescue strategy to identify sensory neurons involved in the avoidance of the bitter substance quinine, and found that ASH, a polymodal nociceptive neuron that senses many aversive stimuli, is the principal player in this response. Two G protein alpha subunits GPA-3 and ODR-3, expressed in ASH and in different, nonoverlapping sets of sensory neurons, are necessary for the response to quinine, although the effect of odr-3 can only be appreciated in the absence of gpa-3. We identified and cloned a new gene, qui-1, necessary for quinine and SDS avoidance. qui-1 codes for a novel protein with WD-40 domains and which is expressed in the avoidance sensory neurons ASH and ADL.