Step response analysis of thermotaxis in Caenorhabditis elegans

AWC thermosensory neuron interferes with information processing in a compact circuit regulating temperature-evoked posture dynamics in the nematode Caenorhabditis elegans

Elucidating how individual neurons encode and integrate sensory information to generate a behavior is crucial for understanding neural logic underlying sensory-dependent behavior. In the nematode Caenorhabditis elegans, information flow from sensory input to behavioral output is traceable at single-cell level due to its entirely solved neural connectivity. C. elegans processes the temperature information for regulating behavior consisting of undulatory posture dynamics in a circuit including two thermosensory neurons AFD and AWC, and their postsynaptic interneuron AIY. However, how the information processing in AFD-AWC-AIY circuit generates the posture dynamics remains elusive. To quantitatively evaluate the posture dynamics, we introduce locomotion entropy, which measures bandwidth of the frequency spectrum of the undulatory posture dynamics, and assess how the motor pattern fluctuates. We here found that AWC disorders the information processing in AFD-AWC-AIY circuit for regulating temperature-evoked posture dynamics. Under slow temperature ramp-up, AWC adjusts AFD response, whereby broadening the temperature range in which animals exhibit fluctuating posture undulation. Under rapid temperature ramp-up, AWC increases inter-individual variability in AIY activity and the fluctuating posture undulation. We propose that a compact nervous system recruits a sensory neuron as a fluctuation inducer for regulating sensory-dependent behavior.

Patient-specific variants of NFU1/NFU-1 disrupt cholinergic signaling in a model of multiple mitochondrial dysfunctions syndrome 1

Neuromuscular dysfunction is a common feature of mitochondrial diseases and frequently presents as ataxia, spasticity and/or dystonia, all of which can severely impact individuals with mitochondrial diseases. Dystonia is one of the most common symptoms of multiple mitochondrial dysfunctions syndrome 1 (MMDS1), a disease associated with mutations in the causative gene (NFU1) that impair iron-sulfur cluster biogenesis. We have generated Caenorhabditis elegans strains that recreated patient-specific point variants in the C. elegans ortholog (nfu-1) that result in allele-specific dysfunction. Each of these mutants, Gly147Arg and Gly166Cys, have altered acetylcholine signaling at neuromuscular junctions, but opposite effects on activity and motility. We found that the Gly147Arg variant was hypersensitive to acetylcholine and that knockdown of acetylcholine release rescued nearly all neuromuscular phenotypes of this variant. In contrast, we found that the Gly166Cys variant caused predominantly postsynaptic acetylcholine hypersensitivity due to an unclear mechanism. These results are important for understanding the neuromuscular conditions of MMDS1 patients and potential avenues for therapeutic intervention.

Small flexible automated system for monitoring Caenorhabditis elegans lifespan based on active vision and image processing techniques

Traditionally Caenorhabditis elegans lifespan assays are performed by manually inspecting nematodes with a dissection microscope, which involves daily counting of live/dead worms cultured in Petri plates for 21–25 days. This manual inspection requires the screening of hundreds of worms to ensure statistical robustness, and is therefore a time-consuming approach. In recent years, various automated artificial vision systems have been reported to increase the throughput, however they usually provide less accurate results than manual assays. The main problems identified when using these vision systems are the false positives and false negatives, which occur due to culture media changes, occluded zones, dirtiness or condensation of the Petri plates. In this work, we developed and described a new C. elegans monitoring machine, SiViS, which consists of a flexible and compact platform design to analyse C. elegans cultures using the standard Petri plates seeded with E. coli. Our system uses an active vision illumination technique and different image-processing pipelines for motion detection, both previously reported, providing a fully automated image processing pipeline. In addition, this study validated both these methods and the feasibility of the SiViS machine for lifespan experiments by comparing them with manual lifespan assays. Results demonstrated that the automated system yields consistent replicates (p-value log rank test 0.699), and there are no significant differences between automated system assays and traditionally manual assays (p-value 0.637). Finally, although we have focused on the use of SiViS in longevity assays, the system configuration is flexible and can, thus, be adapted to other C. elegans studies such as toxicity, mobility and behaviour.

Feeding state functionally reconfigures a sensory circuit to drive thermosensory behavioral plasticity

Internal state alters sensory behaviors to optimize survival strategies. The neuronal mechanisms underlying hunger-dependent behavioral plasticity are not fully characterized. Here we show that feeding state alters C. elegans thermotaxis behavior by engaging a modulatory circuit whose activity gates the output of the core thermotaxis network. Feeding state does not alter the activity of the core thermotaxis circuit comprised of AFD thermosensory and AIY interneurons. Instead, prolonged food deprivation potentiates temperature responses in the AWC sensory neurons, which inhibit the postsynaptic AIA interneurons to override and disrupt AFD-driven thermotaxis behavior. Acute inhibition and activation of AWC and AIA, respectively, restores negative thermotaxis in starved animals. We find that state-dependent modulation of AWC-AIA temperature responses requires INS-1 insulin-like peptide signaling from the gut and DAF-16/FOXO function in AWC. Our results describe a mechanism by which functional reconfiguration of a sensory network via gut-brain signaling drives state-dependent behavioral flexibility.

How Caenorhabditis elegans Senses Mechanical Stress, Temperature, and Other Physical Stimuli

Caenorhabditis elegans lives in a complex habitat in which they routinely experience large fluctuations in temperature, and encounter physical obstacles that vary in size and composition. Their habitat is shared by other nematodes, by beneficial and harmful bacteria, and nematode-trapping fungi. Not surprisingly, these nematodes can detect and discriminate among diverse environmental cues, and exhibit sensory-evoked behaviors that are readily quantifiable in the laboratory at high resolution. Their ability to perform these behaviors depends on <100 sensory neurons, and this compact sensory nervous system together with powerful molecular genetic tools has allowed individual neuron types to be linked to specific sensory responses. Here, we describe the sensory neurons and molecules that enable C. elegans to sense and respond to physical stimuli. We focus primarily on the pathways that allow sensation of mechanical and thermal stimuli, and briefly consider this animal’s ability to sense magnetic and electrical fields, light, and relative humidity. As the study of sensory transduction is critically dependent upon the techniques for stimulus delivery, we also include a section on appropriate laboratory methods for such studies. This chapter summarizes current knowledge about the sensitivity and response dynamics of individual classes of C. elegans mechano- and thermosensory neurons from in vivo calcium imaging and whole-cell patch-clamp electrophysiology studies. We also describe the roles of conserved molecules and signaling pathways in mediating the remarkably sensitive responses of these nematodes to mechanical and thermal cues. These studies have shown that the protein partners that form mechanotransduction channels are drawn from multiple superfamilies of ion channel proteins, and that signal transduction pathways responsible for temperature sensing in C. elegans share many features with those responsible for phototransduction in vertebrates.

Microfluidic Devices Developed for and Inspired by Thermotaxis and Chemotaxis

Taxis has been reported in many cells and microorganisms, due to their tendency to migrate toward favorable physical situations and avoid damage and death. Thermotaxis and chemotaxis are two of the major types of taxis that naturally occur on a daily basis. Understanding the details of the thermo- and chemotactic behavioral response of cells and microorganisms is necessary to reveal the body function, diagnosing diseases and developing therapeutic treatments. Considering the length-scale and range of effectiveness of these phenomena, advances in microfluidics have facilitated taxis experiments and enhanced the precision of controlling and capturing microscale samples. Microfabrication of fluidic chips could bridge the gap between in vitro and in situ biological assays, specifically in taxis experiments. Numerous efforts have been made to develop, fabricate and implement novel microchips to conduct taxis experiments and increase the accuracy of the results. The concepts originated from thermo- and chemotaxis, inspired novel ideas applicable to microfluidics as well, more specifically, thermocapillarity and chemocapillarity (or solutocapillarity) for the manipulation of single- and multi-phase fluid flows in microscale and fluidic control elements such as valves, pumps, mixers, traps, etc. This paper starts with a brief biological overview of the concept of thermo- and chemotaxis followed by the most recent developments in microchips used for thermo- and chemotaxis experiments. The last section of this review focuses on the microfluidic devices inspired by the concept of thermo- and chemotaxis. Various microfluidic devices that have either been used for, or inspired by thermo- and chemotaxis are reviewed categorically.

Microfluidic Platform for Analyzing the Thermotaxis of C. elegans in a Linear Temperature Gradient

Caenorhabditis elegans (C. elegans), which shares a considerable amount of characteristics with human genes is one of the important model organisms for the study of behavioral responses. Thermotaxis is a representative behavior response of C. elegans; C. elegans stores the cultivation temperature in thermosensory neurons and moves to the cultivation temperature region in a temperature variation. In this study, we developed a microfluidic system for effective thermotaxis analysis of C. elegans. The microfluidic channel was fabricated using polydimethylsiloxane (PDMS) by soft lithography process. The temperature gradient (15 - 20°C) was generated in the microchannel and controlled by Peltier modules attached to the bottom of the channel. The thermotaxis of wild type (N2), tax-4(p678) and ttx-7(nj50) mutants were effectively analyzed using this microfluidic system. We believe that this system can be employed as a basic platform for studying the neural circuit of C. elegans responding to external stimuli.

C. elegans locomotion analysis using algorithmic information theory

This article investigates the use of algorithmic information theory to analyse C. elegans datasets. The ability of complexity measures to detect similarity in animals' behaviours is demonstrated and their strengths are compared to methods such as histograms. Introduced quantities are illustrated on a couple of real two-dimensional C. elegans datasets to investigate the thermotaxis and chemotaxis behaviours.

Navigational choice between reversal and curve during acidic pH avoidance behavior in Caenorhabditis elegans

BACKGROUND

Under experimental conditions, virtually all behaviors of Caenorhabditis elegans are achieved by combinations of simple locomotion, including forward, reversal movement, turning by deep body bending, and gradual shallow turning. To study how worms regulate these locomotion in response to sensory information, acidic pH avoidance behavior was analyzed by using worm tracking system.

RESULTS

In the acidic pH avoidance, we characterized two types of behavioral maneuvers that have similar behavioral sequences in chemotaxis and thermotaxis. A stereotypic reversal-turn-forward sequence of reversal avoidance caused an abrupt random reorientation, and a shallow gradual turn in curve avoidance caused non-random reorientation in a less acidic direction to avoid the acidic pH. Our results suggest that these two maneuvers were each triggered by a distinct threshold pH. A simulation study using the two-distinct-threshold model reproduced the avoidance behavior of the real worm, supporting the presence of the threshold. Threshold pH for both reversal and curve avoidance was altered in mutants with reduced or enhanced glutamatergic signaling from acid-sensing neurons.

CONCLUSIONS

C. elegans employ two behavioral maneuvers, reversal (klinokinesis) and curve (klinotaxis) to avoid acidic pH. Unlike the chemotaxis in C. elegans, reversal and curve avoidances were triggered by absolute pH rather than temporal derivative of stimulus concentration in this behavior. The pH threshold is different between reversal and curve avoidance. Mutant studies suggested that the difference results from a differential amount of glutamate released from ASH and ASK chemosensory neurons.

Characterization of the crawling activity of Caenorhabditis elegans using a Hidden Markov model

The locomotion behavior of Caenorhabditis elegans has been studied extensively to understand the respective roles of neural control and biomechanics as well as the interaction between them. Constructing a mathematical model is helpful to understand the locomotion behavior in various surrounding conditions that are difficult to realize in experiments. In this study, we built three hidden Markov models (HMMs) for the crawling behavior of C. elegans in a controlled environment with no chemical treatment and in a formaldehyde-treated environment (0.1 and 0.5 ppm). The organism's crawling activity was recorded using a digital camcorder for 20 min at a rate of 24 frames per second. All shape patterns were quantified by branch length similarity (BLS) entropy and classified into four groups using the self-organizing map (SOM). Comparison of the simulated behavior generated by HMMs and the actual crawling behavior demonstrated that the HMM coupled with the SOM was successful in characterizing the crawling behavior. In addition, we briefly discussed the possibility of using the HMM together with BLS entropy to develop bio-monitoring systems to determine water quality.

How and why Caenorhabditis elegans uses distinct escape and avoidance regimes to minimize exposure to noxious heat

Minimizing the exposure to deleterious extremes of temperature is essential for animals to avoid tissue damages. Because their body temperature equilibrates very rapidly with their surroundings, small invertebrates are particularly vulnerable to the deleterious impact of high temperatures, which jeopardizes their growth, fertility, and survival. The present article reviews recent analyses of Caenorhabditis elegans behavior in temperature gradients covering innocuous and noxious temperatures. These analyses have highlighted that worm uses two separate, multi-componential navigational strategies: an avoidance strategy, aiming at staying away from noxious heat, and an escape strategy, aiming at running away after exposure. Here, I explain why efficient escape and avoidance mechanisms are mutually exclusive and why worm needs to switch between distinct behavioral regimes to achieve efficient protective thermoregulation. Collectively, these findings reveal some largely unrecognized strategies improving worm goal-directed navigation and the fascinating level of sophistication of the behavioral responses deployed to minimize the exposure to noxious heat. Because switching between avoidance and escape regimes circumvents constraints that are valid for navigation behaviors in general, similar solutions might be used by worms and also other organisms in response to various environmental parameters covering an innocuous/noxious, non-toxic/toxic range.

Behavioral response of Caenorhabditis elegans to localized thermal stimuli

Background

Nociception evokes a rapid withdrawal behavior designed to protect the animal from potential danger. C. elegans performs a reflexive reversal or forward locomotory response when presented with noxious stimuli at the head or tail, respectively. Here, we have developed an assay with precise spatial and temporal control of an infrared laser stimulus that targets one-fifth of the worm’s body and quantifies multiple aspects of the worm’s escape response.

Results

When stimulated at the head, we found that the escape response can be elicited by changes in temperature as small as a fraction of a degree Celsius, and that aspects of the escape behavior such as the response latency and the escape direction change advantageously as the amplitude of the noxious stimulus increases. We have mapped the behavioral receptive field of thermal nociception along the entire body of the worm, and show a midbody avoidance behavior distinct from the head and tail responses. At the midbody, the worm is sensitive to a change in the stimulus location as small as 80 μm. This midbody response is probabilistic, producing either a backward, forward or pause state after the stimulus. The distribution of these states shifts from reverse-biased to forward-biased as the location of the stimulus moves from the middle towards the anterior or posterior of the worm, respectively. We identified PVD as the thermal nociceptor for the midbody response using calcium imaging, genetic ablation and laser ablation. Analyses of mutants suggest the possibility that TRPV channels and glutamate are involved in facilitating the midbody noxious response.

Conclusion

Through high resolution quantitative behavioral analysis, we have comprehensively characterized the C. elegans escape response to noxious thermal stimuli applied along its body, and found a novel midbody response. We further identified the nociceptor PVD as required to sense noxious heat at the midbody and can spatially differentiate localized thermal stimuli.

Temperature-dependent behaviours are genetically variable in the nematode Caenorhabditis briggsae

Temperature-dependent behaviours in Caenorhabditis elegans, such as thermotaxis and isothermal tracking, are complex behavioural responses that integrate sensation, foraging and learning, and have driven investigations to discover many essential genetic and neural pathways. The ease of manipulation of the Caenorhabditis model system also has encouraged its application to comparative analyses of phenotypic evolution, particularly contrasts of the classic model C. elegans with C. briggsae. And yet few studies have investigated natural genetic variation in behaviour in any nematode. Here we measure thermotaxis and isothermal tracking behaviour in genetically distinct strains of C. briggsae, further motivated by the latitudinal differentiation in C. briggsae that is associated with temperature-dependent fitness differences in this species. We demonstrate that C. briggsae performs thermotaxis and isothermal tracking largely similar to that of C. elegans, with a tendency to prefer its rearing temperature. Comparisons of these behaviours among strains reveal substantial heritable natural variation within each species that corresponds to three general patterns of behavioural response. However, intraspecific genetic differences in thermal behaviour often exceed interspecific differences. These patterns of temperature-dependent behaviour motivate further development of C. briggsae as a model system for dissecting the genetic underpinnings of complex behavioural traits.

Multiparameter behavioral profiling reveals distinct thermal response regimes in Caenorhabditis elegans

BACKGROUND

Responding to noxious stimuli by invoking an appropriate escape response is critical for survival of an organism. The sensations of small and large changes in temperature in most organisms have been studied separately in the context of thermotaxis and nociception, respectively. Here we use the nematode C. elegans to address the neurogenetic basis of responses to thermal stimuli over a broad range of intensities.

RESULTS

C. elegans responds to aversive temperature by eliciting a stereotypical behavioral sequence. Upon sensation of the noxious stimulus, it moves backwards, turns and resumes forward movement in a new direction. In order to study the response of C. elegans to a broad range of noxious thermal stimuli, we developed a novel assay that allows simultaneous characterization of multiple aspects of escape behavior elicited by thermal pulses of increasing amplitudes. We exposed the laboratory strain N2, as well as 47 strains with defects in various aspects of nervous system function, to thermal pulses ranging from ΔT = 0.4°C to 9.1°C and recorded the resulting behavioral profiles.

CONCLUSIONS

Through analysis of the multidimensional behavioral profiles, we found that the combinations of molecules shaping avoidance responses to a given thermal pulse are unique. At different intensities of aversive thermal stimuli, these distinct combinations of molecules converge onto qualitatively similar stereotyped behavioral sequences.

More than the sum of its parts: a complex epistatic network underlies natural variation in thermal preference behavior in Caenorhabditis elegans

Behavior is a complex trait that results from interactions among multiple genes and the environment. Both additive and nonadditive effects are expected to contribute to broad-sense heritability of complex phenotypes, although the relative contribution of each of these mechanisms is unknown. Here, we mapped genetic variation in the correlated phenotypes of thermal preference and isothermal dispersion in the nematode Caenorhabditis elegans. Genetic variation underlying these traits is characterized by a set of linked quantitative trait loci (QTL) that interact in a complex epistatic network. In particular, two loci located on the X chromosome interact with one another to generate extreme thermophilic behavior and are responsible for ∼50% of the total variation observed in a cross between two parental lines, even though these loci individually explain very little of the among-line variation. Our results demonstrate that simultaneously considering the influence of a quantitative trait locus (QTL) on multiple scales of behavior can inform the physiological mechanism of the QTL and show that epistasis can explain significant proportions of otherwise unattributed variance within populations.

Neuronal microcircuits for decision making in C. elegans

The simplicity and genetic tractability of the nervous system of the nematode Caenorhabditis elegans make it an attractive system in which to seek biological mechanisms of decision making. Although work in this area remains at an early stage, four basic types paradigms of behavioral choice, a simple form of decision making, have now been demonstrated in C. elegans. A recent series of pioneering studies, combining genetics and molecular biology with new techniques such as microfluidics and calcium imaging in freely moving animals, has begun to elucidate the neuronal mechanisms underlying behavioral choice. The new research has focussed on choice behaviors in the context of habitat and resource localization, for which the neuronal circuit has been identified. Three main circuit motifs for behavioral choice have been identified. One motif is based mainly on changes in the strength of synaptic connections whereas the other two motifs are based on changes in the basal activity of an interneuron and the sensory neuron to which it is electrically coupled. Peptide signaling seems to play a prominent role in all three motifs, and it may be a general rule that concentrations of various peptides encode the internal states that influence behavioral decisions in C. elegans.

Microfluidic devices for analysis of spatial orientation behaviors in semi-restrained Caenorhabditis elegans

This article describes the fabrication and use of microfluidic devices for investigating spatial orientation behaviors in nematode worms (Caenorhabditis elegans). Until now, spatial orientation has been studied in freely moving nematodes in which the frequency and nature of encounters with the gradient are uncontrolled experimental variables. In the new devices, the nematode is held in place by a restraint that aligns the longitudinal axis of the body with the border between two laminar fluid streams, leaving the animal's head and tail free to move. The content of the fluid streams can be manipulated to deliver step gradients in space or time. We demonstrate the utility of the device by identifying previously uncharacterized aspects of the behavioral mechanisms underlying chemotaxis, osmotic avoidance, and thermotaxis in this organism. The new devices are readily adaptable to behavioral and imaging studies involving fluid borne stimuli in a wide range of sensory modalities.

The computational worm: spatial orientation and its neuronal basis in C. elegans

Spatial orientation behaviors in animals are fundamental for survival but poorly understood at the neuronal level. The nematode Caenorhabditis elegans orients to a wide range of stimuli and has a numerically small and well-described nervous system making it advantageous for investigating the mechanisms of spatial orientation. Recent work by the C. elegans research community has identified essential computational elements of the neural circuits underlying two orientation strategies that operate in five different sensory modalities. Analysis of these circuits reveals novel motifs including simple circuits for computing temporal derivatives of sensory input and for integrating sensory input with behavioral state to generate adaptive behavior. These motifs constitute hypotheses concerning the identity and functionality of circuits controlling spatial orientation in higher organisms.

Does thermoregulatory behavior maximize reproductive fitness of natural isolates of Caenorhabditis elegans?

BACKGROUND

A central premise of physiological ecology is that an animal's preferred body temperature should correspond closely with the temperature maximizing performance and Darwinian fitness. Testing this co-adaptational hypothesis has been problematic for several reasons. First, reproductive fitness is the appropriate measure, but is difficult to measure in most animals. Second, no single fitness measure applies to all demographic situations, complicating interpretations. Here we test the co-adaptation hypothesis by studying an organism (Caenorhabditis elegans) in which both fitness and thermal preference can be reliably measured.

RESULTS

We find that natural isolates of C. elegans display a range of mean thermal preferences and also vary in their thermal sensitivities for fitness. Hot-seeking isolates CB4854 and CB4857 prefer temperatures that favor population growth rate (r), whereas the cold-seeking isolate CB4856 prefers temperatures that favor Lifetime Reproductive Success (LRS).

CONCLUSIONS

Correlations between fitness and thermal preference in natural isolates of C. elegans are driven primarily by isolate-specific differences in thermal preference. If these differences are the result of natural selection, then this suggests that the appropriate measure of fitness for use in evolutionary ecology studies might differ even within species, depending on the unique ecological and evolutionary history of each population.

The shallow turn of a worm

When crawling on a solid surface, the nematode Caenorhabditis elegans (C. elegans) moves forward by propagating sinusoidal dorso-ventral retrograde contraction waves. A uniform propagating wave leads to motion that undulates about a straight line. When C. elegans turns as it forages or navigates its environment, it uses several different strategies of reorientation. These modes include the well-known omega turn, in which the worm makes a sharp angle turn forming an Ω-shape, and the reversal, in which the worm draws itself backwards. In these two modes of reorientation, C. elegans strongly disrupts its propagating sinusoidal wave, either in form or in direction, leading to abrupt directional change. However, a third mode of reorientation, the shallow turn, involves a gentler disruption of the locomotory gait. Analyzing the statistics of locomotion suggests that the shallow turn is by far the most frequent reorienting maneuver in navigation in the absence of food. We show that the worm executes a shallow turn by modulating the amplitude and wavelength of its curvature during forward movement, and provide a minimal description of the process using a three-parameter mathematical model. The results of our study augment the understanding of how these parameters are controlled at the neuromotor circuit level.

Bidirectional regulation of thermotaxis by glutamate transmissions in Caenorhabditis elegans

This paper provides a molecular and genetic analysis of the neural circuitry that regulates the migration of Caenorhabditis elegans towards either warmer or colder temperature and reveals an important role of glutamate signalling in this process.

In complex neural circuits of the brain, massive information is processed with neuronal communication through synaptic transmissions. It is thus fundamental to delineate information flows encoded by various kinds of transmissions. Here, we show that glutamate signals from two distinct sensory neurons bidirectionally affect the same postsynaptic interneuron, thereby producing the opposite behaviours. EAT-4/VGLUT (vesicular glutamate transporter)-dependent glutamate signals from AFD thermosensory neurons inhibit the postsynaptic AIY interneurons through activation of GLC-3/GluCl inhibitory glutamate receptor and behaviourally drive migration towards colder temperature. By contrast, EAT-4-dependent glutamate signals from AWC thermosensory neurons stimulate the AIY neurons to induce migration towards warmer temperature. Alteration of the strength of AFD and AWC signals led to significant changes of AIY activity, resulting in drastic modulation of behaviour. We thus provide an important insight on information processing, in which two glutamate transmissions encoding opposite information flows regulate neural activities to produce a large spectrum of behavioural outputs.

Running hot and cold: behavioral strategies, neural circuits, and the molecular machinery for thermotaxis in C. elegans and Drosophila

Like other ectotherms, the roundworm Caenorhabditis elegans and the fruit fly Drosophila melanogaster rely on behavioral strategies to stabilize their body temperature. Both animals use specialized sensory neurons to detect small changes in temperature, and the activity of these thermosensors governs the neural circuits that control migration and accumulation at preferred temperatures. Despite these similarities, the underlying molecular, neuronal, and computational mechanisms responsible for thermotaxis are distinct in these organisms. Here, we discuss the role of thermosensation in the development and survival of C. elegans and Drosophila, and review the behavioral strategies, neuronal circuits, and molecular networks responsible for thermotaxis behavior.

Caenorhabditis elegans: a model system for systems neuroscience

The nematode Caenorhabditis elegans is an excellent model organism for a systems-level understanding of neural circuits and behavior. Advances in the quantitative analyses of behavior and neuronal activity, and the development of new technologies to precisely control and monitor the workings of interconnected circuits, now allow investigations into the molecular, cellular, and systems-level strategies that transform sensory inputs into precise behavioral outcomes.

Distinct thermal migration behaviors in response to different thermal gradients in Caenorhabditis elegans

The nematode Caenorhabditis elegans exhibits a complex behavior called thermotaxis in response to temperature. This behavior is defined as a form of associative learning, in which temperature pairs with the presence or absence of food. Different interpretations have been drawn from the diverse results obtained by several groups, mainly because of the application of different methodologies for the analysis of thermotaxis. To clarify the discrepancies in behavioral observations and subsequent interpretations by different laboratories, we attempted to systematize several parameters to observe thermotaxis behavior as originally defined by Hedgecock and Russell in 1975. In this study, we show clearly how C. elegans can show a conditioned migration toward colder or warmer areas on a thermal gradient, given certain criteria necessary for the observation of thermotaxis. We thus propose to distinguish thermotaxis from other temperature-related behaviors, such as the warm avoidance response displayed at temperature gradients of 1 degrees C/cm and steeper.

An olfactory neuron responds stochastically to temperature and modulates Caenorhabditis elegans thermotactic behavior

Caenorhabditis elegans navigates thermal gradients by using a behavioral strategy that is regulated by a memory of its cultivation temperature (T(c)). At temperatures above or around the T(c), animals respond to temperature changes by modulating the rate of stochastic reorientation events. The bilateral AFD neurons have been implicated as thermosensory neurons, but additional thermosensory neurons are also predicted to play a role in regulating thermotactic behaviors. Here, we show that the AWC olfactory neurons respond to temperature. Unlike AFD neurons, which respond to thermal stimuli with continuous, graded calcium signals, AWC neurons exhibit stochastic calcium events whose frequency is stimulus-correlated in a T(c)-dependent manner. Animals lacking the AWC neurons or with hyperactive AWC neurons exhibit defects in the regulation of reorientation rate in thermotactic behavior. Our observations suggest that the AFD and AWC neurons encode thermal stimuli via distinct strategies to regulate C. elegans thermotactic behavior.

Sensory mechanisms controlling the timing of larval developmental and behavioral transitions require the Drosophila DEG/ENaC subunit, Pickpocket1

Growth of multicellular organisms proceeds through a series of precisely timed developmental events requiring coordination between gene expression, behavioral changes, and environmental conditions. In Drosophila melanogaster larvae, the essential midthird instar transition from foraging (feeding) to wandering (non-feeding) behavior occurs prior to pupariation and metamorphosis. The timing of this key transition is coordinated with larval growth and size, but physiological mechanisms regulating this process are poorly understood. Results presented here show that Drosophila larvae associate specific environmental conditions, such as temperature, with food in order to enact appropriate foraging strategies. The transition from foraging to wandering behavior is associated with a striking reversal in the behavioral responses to food-associated stimuli that begins early in the third instar, well before food exit. Genetic manipulations disrupting expression of the Degenerin/Epithelial Sodium Channel subunit, Pickpocket1(PPK1) or function of PPK1 peripheral sensory neurons caused defects in the timing of these behavioral transitions. Transient inactivation experiments demonstrated that sensory input from PPK1 neurons is required during a critical period early in the third instar to influence this developmental transition. Results demonstrate a key role for the PPK1 sensory neurons in regulation of important behavioral transitions associated with developmental progression of larvae from foraging to wandering stage.

Memory in Caenorhabditis elegans is mediated by NMDA-type ionotropic glutamate receptors

Learning and memory are essential processes of both vertebrate and invertebrate nervous systems that allow animals to survive and reproduce. The neurotransmitter glutamate signals via ionotropic glutamate receptors (iGluRs) that have been linked to learning and memory formation; however, the signaling pathways that contribute to these behaviors are still not well understood. We therefore undertook a genetic and electrophysiological analysis of learning and memory in the nematode Caenorhabditis elegans. Here, we show that two genes, nmr-1 and nmr-2, are predicted to encode the subunits of an NMDA-type (NMDAR) iGluR that is necessary for memory retention in C. elegans. We cloned nmr-2, generated a deletion mutation in the gene, and showed that like nmr-1, nmr-2 is required for in vivo NMDA-gated currents. Using an associative-learning paradigm that pairs starvation with the attractant NaCl, we also showed that the memory of a learned avoidance response is dependent on NMR-1 and NMR-2 and that expression of NMDARs in a single pair of interneurons is sufficient for normal memory. Our results provide new insights into the molecular and cellular mechanisms underlying the memory of a learned event.

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.

Worm thermotaxis: a model system for analyzing thermosensation and neural plasticity

Elucidation of the principal mechanism for sensory transduction, learning and memory is a fundamental question in neurobiology. The simple nervous system composed of only 302 neurons and the description of neural wiring combined with developed imaging techniques facilitate cellular and circuit level analysis of behavior in the nematode Caenorhabditis elegans. Recent comprehensive analysis of worm thermotaxis, an experience-modulated behavior, has begun to reveal molecular, cellular, and neural circuit basis of thermosensation and neural plasticity.

Thermal preference of Caenorhabditis elegans: a null model and empirical tests

The preferred body temperature of ectotherms is typically inferred from the observed distribution of body temperatures in a laboratory thermal gradient. For very small organisms, however, that observed distribution might misrepresent true thermal preferences. Tiny ectotherms have limited thermal inertia, and so their body temperature and speed of movement will vary with their position along the gradient. In order to separate the direct effects of body temperature on movement from actual preference behaviour on a thermal gradient, we generate a null model (i.e. of non-thermoregulating individuals) of the spatial distribution of ectotherms on a thermal gradient and test the model using parameter values estimated from the movement of nematodes (Caenorhabditis elegans) at fixed temperatures and on a thermal gradient. We show that the standard lab strain N2, which is widely used in thermal gradient studies, avoids high temperature but otherwise does not exhibit a clear thermal preference, whereas the Hawaiian natural isolate CB4856 shows a clear preference for cool temperatures ( approximately 17 degrees C). These differences are not influenced substantially by changes in the starting position of worms in the gradient, the natal temperature of individuals or the presence and physiological state of bacterial food. These results demonstrate the value of an explicit null model of thermal effects and highlight problems in the standard model of C. elegans thermotaxis, showing the value of using natural isolates for tests of complex natural behaviours.

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.

Short-Term Adaptation and Temporal Processing in the Cryophilic Response ofCaenorhabditis elegans

When navigating spatial thermal gradients, the nematode C. elegans migrates toward colder temperatures until it reaches its previous cultivation temperature, exhibiting cryophilic movement. The strategy for effecting cryophilic movement is the biased random walk: C. elegans extends (shortens) periods of forward movement that are directed down (up) spatial thermal gradients by modulating the probability of reorientation. Here, we analyze the temporal sensory processor that enables cryophilic movement by quantifying the movements of individual worms subjected to defined temperature waveforms. We show that step increases in temperature as small as 0.05°C lead to transient increases in the probability of reorientation followed by gradual adaptation to the baseline level; temperature downsteps leads to similar but inverted responses. Short-term adaptation is a general property of sensory systems, allowing organisms to maintain sensitivity to sensory variations over broad operating ranges. During cryophilic movement C. elegans also uses the temporal dynamics of its adaptive response to compute the time derivative of gradual temperature variations with exquisite sensitivity. On the basis of the time derivative, the worm determines how it is oriented in spatial thermal gradients during each period of forward movement. We show that the operating range of the cryophilic response extends to lower temperatures in ttx-3 mutants, which affects the development of the AIY interneurons. We show that the temporal sensory processor for the cryophilic response is affected by mutation in the EAT-4 glutamate vesicular transporter. Regulating the operating range of the cryophilic response and executing the cryophilic response may have separate neural mechanisms.

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.

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.

Quantitative analysis of thermotaxis in the nematode Caenorhabditis elegans

Thermotaxis (TTX) is one of the sophisticated behaviors in the nematode Caenorhabditis elegans. Although the mechanisms of thermotaxis have been deduced from different studies, they are controversial. Previous studies proposed a behavioral model where thermotaxis is regulated by the counterbalance between two opposite driving forces, while recent studies proposed stochastic models. In this study, we analyzed thermotaxis by a novel quantitative population TTX assay using a gentle linear thermal gradient. Analysis of thermotaxis in wild type animals revealed a clear thermal preference to a cultivation temperature with regard to the distribution of animals and the TTX mean expressing temperature preference. A time course assay revealed that the behavioral response to a preferred temperature was initially suppressed for at least 15 min in the animals cultivated at 23 degrees C, but not in those cultivated at 17 degrees C. Our result provides a possible explanation for the inconsistency between the various studies on thermotaxis and is consistent with the early behavioral model, where thermotaxis is regulated by the counterbalance between two driving forces.

Deciphering the neural and molecular mechanisms of C. elegans behavior

Because of its small and well-characterized nervous system and amenability to genetic manipulation, the nematode Caenorhabditis elegans offers the promise of understanding the mechanisms underlying a whole animal's behavior at the molecular and cellular levels. In fact, this goal was a primary motivation behind the development of C. elegans as an experimental organism 40 years ago. Yet it has proven surprisingly difficult to obtain a mechanistic understanding of how the C. elegans nervous system generates behavior, despite the existence of a 'wiring diagram' that contains a degree of information about neural connectivity unparalleled in any organism. This review describes three types of information--molecular data on cellular neurochemistry, temporal information about neural activity patterns, and behavioral data on the consequences of neural ablation and manipulation--that, along with genetic analysis, may ultimately lead to a complete functional map of the C. elegans nervous system.

The Awake Behaving Worm: Simultaneous Imaging of Neuronal Activity and Behavior in Intact Animals at Millimeter Scale

Genetically encoded optical probes of neuronal activity offer the prospect of simultaneous recordings of neuronal activity and behavior in intact animals. A central problem in simultaneous imaging is that the field of view of the high-power objective required for imaging the neuron is often too small to allow the experimenter to assess the overall behavioral state of the animal. Here we present a method that solves this problem using a microscope with two objectives focused on the preparation: a high-power lens dedicated to imaging the neuron and low-power lens dedicated to imaging the behavior. Images of activity and behavior are acquired simultaneously but separately using different wavelengths of light. The new approach was tested using the cameleon calcium sensor expressed in Caenorhabditis elegans sensory neurons. We show that simultaneous recordings of neuronal activity and behavior are practical in C. elegans and, moreover, that such recordings can reveal subtle, transient correlations between calcium levels and behavior that may be missed in nonsimultaneous recordings. The new method is likely to be useful whenever it would be desirable to record simultaneously at two different spatial resolutions from a single location, or from two different locations in space.

Chemosensory behavior of semi-restrained Caenorhabditis elegans

A new behavioral assay is described for studying chemosensation in the nematode Caenorhabditis elegans. This assay presents three main characteristics: (1) the worm is restrained by gluing, preserving correlates of identifiable behaviors; (2) the amplitude and time course of the stimulus are controlled by the experimenter; and (3) the behavior is recorded quantitatively. We show that restrained C. elegans display behaviors comparable to those of freely moving worms. Moreover, the chemosensory response of wild-type glued animals to changes in salt concentration is similar to that of freely moving animals. This glued-worm assay was used to reveal new chemosensory deficits of the potassium channel mutant egl-2. We conclude that the glued worm assay can be used to study the chemosensory regulation of C. elegans behavior and how it is affected by neuronal or genetic manipulations.

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.

Experience-dependent modulation of C. elegans behavior by ambient oxygen

BACKGROUND

Ambient oxygen (O2) influences the behavior of organisms from bacteria to man. In C. elegans, an atypical O2 binding soluble guanylate cyclase (sGC), GCY-35, regulates O2 responses. However, how acute and chronic changes in O2 modify behavior is poorly understood.

RESULTS

Aggregating C. elegans strains can respond to a reduction in ambient O2 by a rapid, reversible, and graded inhibition of roaming behavior. This aerokinetic response is mediated by GCY-35 and GCY-36 sGCs, which appear to become activated as O2 levels drop and to depolarize the AQR, PQR, and URX neurons. Coexpression of GCY-35 and GCY-36 is sufficient to transform olfactory neurons into O2 sensors. Natural variation at the npr-1 neuropeptide receptor alters both food-sensing and O2-sensing circuits to reconfigure the salient features of the C. elegans environment. When cultivated in 1% O2 for a few hours, C. elegans reset their preferred ambient O2, seeking instead of avoiding 0%-5% O2. This plasticity involves reprogramming the AQR, PQR, and URX neurons.

CONCLUSIONS

To navigate O2 gradients, C. elegans can modulate turning rates and speed of movement. Aerotaxis can be reprogrammed by experience or engineered artificially. We propose a model in which prolonged activation of the AQR, PQR, and URX neurons by low O2 switches on previously inactive O2 sensors. This enables aerotaxis to low O2 environments and may encode a "memory" of previous cultivation in low O2.

A circuit for navigation in Caenorhabditis elegans

Caenorhabditis elegans explores its environment by interrupting its forward movement with occasional turns and reversals. Turns and reversals occur at stable frequencies but irregular intervals, producing probabilistic exploratory behaviors. Here we dissect the roles of individual sensory neurons, interneurons, and motor neurons in exploratory behaviors under different conditions. After animals are removed from bacterial food, they initiate a local search behavior consisting of reversals and deep omega-shaped turns triggered by AWC olfactory neurons, ASK gustatory neurons, and AIB interneurons. Over the following 30 min, the animals disperse as reversals and omega turns are suppressed by ASI gustatory neurons and AIY interneurons. Interneurons and motor neurons downstream of AIB and AIY encode specific aspects of reversal and turn frequency, amplitude, and directionality. SMD motor neurons help encode the steep amplitude of omega turns, RIV motor neurons specify the ventral bias of turns that follow a reversal, and SMB motor neurons set the amplitude of sinusoidal movement. Many of these sensory neurons, interneurons, and motor neurons are also implicated in chemotaxis and thermotaxis. Thus, this circuit may represent a common substrate for multiple navigation behaviors.

Genetic control of temperature preference in the nematode Caenorhabditis elegans

Animals modify behavioral outputs in response to environmental changes. C. elegans exhibits thermotaxis, where well-fed animals show attraction to their cultivation temperature on a thermal gradient without food. We show here that feeding-state-dependent modulation of thermotaxis is a powerful behavioral paradigm for elucidating the mechanism underlying neural plasticity, learning, and memory in higher animals. Starved experience alone could induce aversive response to cultivation temperature. Changing both cultivation temperature and feeding state simultaneously evoked transient attraction to or aversion to the previous cultivation temperature: recultivation of starved animals with food immediately induced attraction to the temperature associated with starvation, although the animals eventually exhibited thermotaxis to the new temperature associated with food. These results suggest that the change in feeding state quickly stimulates the switch between attraction and aversion for the temperature in memory and that the acquisition of new temperature memory establishes more slowly. We isolated aho (abnormal hunger orientation) mutants that are defective in starvation-induced cultivation-temperature avoidance. Some aho mutants responded normally to changes in feeding state with respect to locomotory activity, implying that the primary thermosensation followed by temperature memory formation remains normal and the modulatory aspect of thermotaxis is specifically impaired in these mutants.

The C. elegans thermosensory neuron AFD responds to warming

The mechanism of temperature sensation is far less understood than the sensory response to other environmental stimuli such as light, odor, and taste. Thermotaxis behavior in C. elegans requires the ability to discriminate temperature differences as small as approximately 0.05 degrees C and to memorize the previously cultivated temperature. The AFD neuron is the only major thermosensory neuron required for the thermotaxis behavior. Genetic analyses have revealed several signal transduction molecules that are required for the sensation and/or memory of temperature information in the AFD neuron, but its physiological properties, such as its ability to sense absolute temperature or temperature change, have been unclear. We show here that the AFD neuron responds to warming. Calcium concentration in the cell body of AFD neuron is increased transiently in response to warming, but not to absolute temperature or to cooling. The transient response requires the activity of the TAX-4 cGMP-gated cation channel, which plays an essential role in the function of the AFD neuron. Interestingly, the AFD neuron further responds to step-like warming above a threshold that is set by temperature memory. We suggest that C. elegans provides an ideal model to genetically and physiologically reveal the molecular mechanism for sensation and memory of temperature information.

The CMK-1 CaMKI and the TAX-4 Cyclic nucleotide-gated channel regulate thermosensory neuron gene expression and function in C. elegans

The cultivation temperature (T(c)) modulates the thermosensory responses exhibited by C. elegans on thermal gradients. The AFD sensory neurons are essential for thermosensory behaviors, but the molecular mechanisms by which temperature is sensed and the memory of the T(c) is encoded are unknown. Here, we show that the CMK-1 Ca2+/calmodulin-dependent protein kinase I (CaMKI) and the TAX-4 cyclic nucleotide-gated channel regulate gene expression, morphology, and functions of the AFD thermosensory neurons. Mutations in cmk-1 and tax-4 result in temperature-dependent defects in AFD-specific gene expression, and TAX-4 functions are required during larval stages to maintain gene expression in the adult. CMK-1 and TAX-4 act cell autonomously to regulate AFD-mediated thermosensory behaviors. The molecular requirements for CMK-1 activity in the AFD neurons appear to be distinct from those previously described. We propose that the activation of distinct programs of AFD-specific gene expression at different temperatures by CMK-1 and TAX-4 enables C. elegans to sense and/or encode a memory for the T(c).