Simulation of C. elegans thermotactic behavior in a linear thermal gradient using a simple phenomenological motility model

The nematode Caenorhabditis elegans has been reported to exhibit thermotaxis, a sophisticated behavioral response to temperature. However, there appears to be some inconsistency among previous reports. The results of population-level thermotaxis investigations suggest that C. elegans can navigate to the region of its cultivation temperature from nearby regions of higher or lower temperature. However, individual C. elegans nematodes appear to show only cryophilic tendencies above their cultivation temperature. A Monte-Carlo style simulation using a simple individual model of C. elegans provides insight into clarifying apparent inconsistencies among previous findings. The simulation using the thermotaxis model that includes the cryophilic tendencies, isothermal tracking and thermal adaptation was conducted. As a result of the random walk property of locomotion of C. elegans, only cryophilic tendencies above the cultivation temperature result in population-level thermophilic tendencies. Isothermal tracking, a period of active pursuit of an isotherm around regions of temperature near prior cultivation temperature, can strengthen the tendencies of these worms to gather around near-cultivation-temperature regions. A statistical index, the thermotaxis (TTX) L-skewness, was introduced and was useful in analyzing the population-level thermotaxis of model worms.     

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°C/cm and steeper.     

Ca2+ signaling via the neuronal calcium sensor-1 regulates associative learning and memory in C. elegans

On a radial temperature gradient, C. elegans worms migrate, after conditioning with food, toward their cultivation temperature and move along this isotherm. This experience-dependent behavior is called isothermal tracking (IT). Here we show that the neuron-specific calcium sensor-1 (NCS-1) is essential for optimal IT. ncs-1 knockout animals show major defects in IT behavior, although their chemotactic, locomotor, and thermal avoidance behaviors are normal. The knockout phenotype can be rescued by reintroducing wild-type NCS-1 into the AIY interneuron, a key component of the thermotaxis network. A loss-of-function form of NCS-1 incapable of binding calcium does not restore IT, whereas NCS-1 overexpression enhances IT performance levels, accelerates learning (faster acquisition), and produces a memory with slower extinction. Thus, proper calcium signaling via NCS-1 defines a novel pathway essential for associative learning and memory.     

Identification of guanylyl cyclases that function in thermosensory neurons of Caenorhabditis elegans

The nematode Caenorhabditis elegans senses temperature primarily via the AFD thermosensory neurons in the head. The response to temperature can be observed as a behavior called thermotaxis on thermal gradients. It has been shown that a cyclic nucleotide-gated ion channel (CNG channel) plays a critical role in thermosensation in AFD. To further identify the thermosensory mechanisms in AFD, we attempted to identify components that function upstream of the CNG channel by a reverse genetic approach. Genetic and behavioral analyses showed that three members of a subfamily of gcy genes (gcy-8, gcy-18, and gcy-23) encoding guanylyl cyclases were essential for thermotaxis in C. elegans. Promoters of each gene drove reporter gene expression exclusively in the AFD neurons and, moreover, tagged proteins were localized to the sensory endings of AFD. Single mutants of each gcy gene showed almost normal thermotaxis. However, animals carrying double and triple mutations in these genes showed defective thermotaxis behavior. The abnormal phenotype of the gcy triple mutants was rescued by expression of any one of the three GCY proteins in the AFD neurons. These results suggest that three guanylyl cyclases function redundantly in the AFD neurons to mediate thermosensation by C. elegans.     

OLA-1, an Obg-like ATPase,integrates hunger with temperature information in sensory neurons in C. elegans

Animals detect changes in both their environment and their internal state and modify their behavior accordingly. Yet, it remains largely to be clarified how information of environment and internal state is integrated and how such integrated information modifies behavior. Well-fed C. elegans migrates to past cultivation temperature on a thermal gradient, which is disrupted when animals are starved. We recently reported that the neuronal activities synchronize between a thermosensory neuron AFD and an interneuron AIY, which is directly downstream of AFD, in well-fed animals, while this synchrony is disrupted in starved animals. However, it remained to be determined whether the disruption of the synchrony is derived from modulation of the transmitter release from AFD or from the modification of reception or signal transduction in AIY. By performing forward genetics on a transition of thermotaxis behavior along starvation, we revealed that OLA-1, an Obg-like ATPase, functions in AFD to promote disruption of AFD-AIY synchrony and behavioral transition. Our results suggest that the information of hunger is delivered to the AFD thermosensory neuron and gates transmitter release from AFD to disrupt thermotaxis, thereby shedding light onto a mechanism for the integration of environmental and internal state to modulate behavior.     

Mutants of thermotaxis in Dictyostelium discoideum

Amoebae of Dictyostelium discoideum, strain HL50 were mutagenized with N-methyl-N′-nitro-N-nitrosoguanidine, cloned, allowed to form pseudoplasmodia and screened for aberrant positive and negative thermotaxis. Three types of mutants were found. Mutant HO428 exhibits only positive thermotaxis over the entire temperature range (no negative thermotaxis). HO596 and HO813 exhibit weakened positive thermotaxis and normal negative thermotaxis. The weakened positive thermotactic response results in a shift toward warmer temperatures in the transition temperature from negative to positive thermotaxis. Mutant HO209 exhibits weakened positive and negative thermotactic responses and has a transition temperature similar to the ‘wild type’ (HL50). The two types of mutants represented by HO428, HO596 and HO813 support the model that positive and negative thermotaxis have separate pathways for temperature sensing. The type of mutants which contains HO209 suggests that those two pathways converge at some point before the response.     

Environmental-temperature and internal-state dependent thermotaxis plasticity of nematodes

Thermotaxis behavior of Caenorhabditis elegans is robust and highly plastic. A pair of sensory neurons, AFD, memorize environmental/cultivation temperature and communicate with a downstream neural circuit to adjust the temperature preference of the animal. This results in a behavioral bias where worms will move toward their cultivation temperature on a thermal gradient. Thermotaxis of C. elegans is also affected by the internal state and is temporarily abolished when worms are starved. Here I will discuss how C. elegans is able to modulate its behavior based on temperature by integrating environmental and internal information. Recent studies show that some parasitic nematodes have a similar thermosensory mechanism to C. elegans and exhibit cultivation-temperature-dependent thermotaxis. I will also discuss the common neural mechanisms that regulate thermosensation and thermotaxis in C. elegans and Strongyloides stercoralis.     

Age‐dependent changes in response property and morphology of a thermosensory neuron and thermotaxis behavior in Caenorhabditis elegans

Age‐dependent cognitive and behavioral deterioration may arise from defects in different components of the nervous system, including those of neurons, synapses, glial cells, or a combination of them. We find that AFD, the primary thermosensory neuron of Caenorhabditis elegans, in aged animals is characterized by loss of sensory ending integrity, including reduced actin‐based microvilli abundance and aggregation of thermosensory guanylyl cyclases. At the functional level, AFD neurons in aged animals are hypersensitive to high temperatures and show sustained sensory‐evoked calcium dynamics, resulting in a prolonged operating range. At the behavioral level, senescent animals display cryophilic behaviors that remain plastic to acute temperature changes. Excessive cyclase activity of the AFD‐specific guanylyl cyclase, GCY‐8, is associated with developmental defects in AFD sensory ending and cryophilic behavior. Surprisingly, loss of the GCY‐8 cyclase domain reduces these age‐dependent morphological and behavioral changes, while a prolonged AFD operating range still exists in gcy‐8 animals. The lack of apparent correlation between age‐dependent changes in the morphology or stimuli‐evoked response properties of primary sensory neurons and those in related behaviors highlights the importance of quantitative analyses of aging features when interpreting age‐related changes at structural and functional levels. Our work identifies aging hallmarks in AFD receptive ending, temperature‐evoked AFD responses, and experience‐based thermotaxis behavior, which serve as a foundation to further elucidate the neural basis of cognitive aging.     

A Mechanism for Precision-Sensing via a Gradient-Sensing Pathway: A Model of Escherichia coli Thermotaxis

Thermotaxis is the phenomenon where an organism directs its movement toward its preferred temperature. So far, the molecular origin for this precision-sensing behavior remains a puzzle. We propose a model of Escherichia coli thermotaxis and show that the precision-sensing behavior in E. coli thermotaxis can be carried out by the gradient-sensing chemotaxis pathway under two general conditions. First, the thermosensor response to temperature is inverted by its internal adaptation state. For E. coli, chemoreceptor Tar changes from a warm sensor to a cold sensor on increase of its methylation level. Second, temperature directly affects the adaptation kinetics. The adapted activity in E. coli increases with temperature in contrast to the perfect adaptation to chemical stimuli. Given these two conditions, E. coli thermotaxis is achieved by the cryophilic and thermophilic responses for temperature above and below a critical temperature T_c, which is encoded by internal pathway parameters. Our model results are supported by both experiments with adaptation-disabled mutants and the recent temperature impulse response measurements for wild-type cells. T_c is predicted to decrease with the background attractant concentration. This mechanism for precision sensing in an adaptive gradient-sensing system may apply to other organisms, such as Dictyostelium discoideum and Caenorhabditis elegans.     

Serotonin Control of Thermotaxis Memory Behavior in Nematode Caenorhabditis elegans

Caenorhabditis elegans is as an ideal model system for the study of mechanisms underlying learning and memory. In the present study, we employed C. elegans assay system of thermotaxis memory to investigate the possible role of serotonin neurotransmitter in memory control. Our data showed that both mutations of tph-1, bas-1, and cat-4 genes, required for serotonin synthesis, and mutations of mod-5 gene, encoding a serotonin reuptake transporter, resulted in deficits in thermotaxis memory behavior. Exogenous treatment with serotonin effectively recovered the deficits in thermotaxis memory of tph-1 and bas-1 mutants to the level of wild-type N2. Neuron-specific activity assay of TPH-1 suggests that serotonin might regulate the thermotaxis memory behavior by release from the ADF sensory neurons. Ablation of ADF sensory neurons by expressing a cell-death activator gene egl-1 decreased the thermotaxis memory, whereas activation of ADF neurons by expression of a constitutively active protein kinase C homologue (pkc-1(gf)) increased the thermotaxis memory and rescued the deficits in thermotaxis memory in tph-1 mutants. Moreover, serotonin released from the ADF sensory neurons might act through the G-protein-coupled serotonin receptors of SER-4 and SER-7 to regulate the thermotaxis memory behavior. Genetic analysis implies that serotonin might further target the insulin signaling pathway to regulate the thermotaxis memory behavior. Thus, our results suggest the possible crucial role of serotonin and ADF sensory neurons in thermotaxis memory control in C. elegans.     

Discriminative behavior and Pavlovian conditioning in the mollusc Pleurobranchaea

The buccal motor system in the sea slug Pleurobranchaea californica is multifunctional; similar sets of neurons and muscles generate different behaviors through similar electrophysiological motor patterns. Such multifunctional systems compromise the traditional practice of identifying a motor pattern and then using that pattern to indicate the behavior in reduced preparations. We address this issue in a series of experiments leading to the comparison of differential Pavlovian conditioning in whole animals with the conditioned behavior of the same animals during electrophysiological recording. Because differential conditioning requires two conditioned stimuli (CSs), we show here that each of two CSs activated the conditioned response from animals after they received the stimulus (CS+) paired with an unconditioned stimulus (UCS). Conditioning sessions consisted of 5 training trials with a 2-h intertrial interval. In one study, experimental animals received a 60-s CS+, derived from beer (Sbr), paired with a 50-s electrical shock UCS whose onset occurred 10 s after the CS+ onset; control animals received the Sbr and UCS explicitly unpaired. In a second study, animals received similar procedures as in the first but with a CS+ consisting of squid homogenate (Ssq). Tests with both CSs showed that animals did not discriminate between Sbr and Ssq before beginning conditioning, but did so afterward. Experimental animals exhibited robust food aversion (withdrawal and suppressed feeding) to the CS+, but retained strong appetitive responses to the CS they did not receive in training; response thresholds to the CS+ changed as much as 1000-fold by comparison to the preconditioning values. Control animals exhibited similar though significantly smaller behavioral changes as the experimental animals. Both stimuli evoked associatively learned responses, but Sbr produced greater experimental-control differences than Ssq did. Two accompanying papers show the results of using both CSs in differential conditioning, and describe the behavioral/electrophysiological comparisons.     

Behavioral response of manatees to variations in environmental sound levels

Florida manatees (Trichechus manatus latirostris) inhabit coastal regions because they feed on the aquatic vegetation that grows in shallow waters, which are the same areas where human activities are greatest. Noise produced from anthropogenic and natural sources has the potential to affect these animals by eliciting responses ranging from mild behavioral changes to extreme aversion. Sound levels were calculated from recordings made throughout behavioral observation periods. An information theoretic approach was used to investigate the relationship between behavior patterns and sound level. Results indicated that elevated sound levels affect manatee activity and are a function of behavioral state. The proportion of time manatees spent feeding and milling changed in response to sound level. When ambient sound levels were highest, more time was spent in the directed, goal‐oriented behavior of feeding, whereas less time was spent engaged in undirected behavior such as milling. This work illustrates how shifts in activity of individual manatees may be useful parameters for identifying impacts of noise on manatees and might inform population level effects.     

Developmental control of foraging and social behavior by the Drosophila neuropeptide Y-like system

Animals display stereotyped behavioral modifications during development, but little is known about how genes and neural circuits are regulated to turn on/off behaviors. Here we report that Drosophila neuropeptide F (dNPF), a human NPY homolog, coordinates larval behavioral changes during development. The brain expression of npf is high in larvae attracted to food, whereas its downregulation coincides with the onset of behaviors of older larvae, including food aversion, hypermobility, and cooperative burrowing. Loss of dNPF signaling in young transgenic larvae led to the premature display of behavioral phenotypes associated with older larvae. Conversely, dNPF overexpression in older larvae prolonged feeding, and suppressed hypermobility and cooperative burrowing behaviors. The dNPF system provides a new paradigm for studying the central control of cooperative behavior.     

Effect of stimulus strength and adaptation on the thermotactic response of Dictyostelium discoideum pseudoplasmodia

The thermotactic responses of Dictyostelium discoideum strain HL50 and mutants derived from this strain have been characterized by curves of stimulus-strength vs response. With gradient midpoint temperatures of 16 and 24 °C, these curves are typical of those of a single response, i.e., the strength of the response increases with increasing stimulus strength until at some strength the response saturates. However, with a gradient midpoint temperature close to the transition from negative to positive thermotaxis, the sign of the thermotactic response depends on gradient strength. These observations support the hypothesis that the transduction pathways for positive and negative thermotaxis act concurrently and contain separable elements. An investigation of the adaptation of thermotaxis indicated that the stimulus-strength-dependence and midpoint-temperature-dependence of both thermosensory responses was altered by shifting the growth and development temperature.     

Feel the heat: The effect of temperature on development, behavior and central pattern generation in 3rd instar Calliphora vicina larvae

Like in all poikilothermic animals, higher temperatures increase developmental rate and activity in Calliphora vicina larvae. We therefore could expect temperature to have a persistent effect on the output of the feeding and crawling central pattern generators (CPGs). When confronted with a steep temperature gradient, larvae show evasive behavior after touching the substrate with the cephalic sense organs. Beside this reflex behavior the terminal- and dorsal organ might also mediate long term CPG modulation. Both organs were thermally stimulated while their response was recorded from the maxillary- or antennal nerve. The terminal organ showed a tonic response characteristic while the dorsal organ was not sensitive to temperature. Thermal stimulation of the terminal organ did not affect the ongoing patterns of fictive feeding or crawling, recorded from the antennal- or abdominal nerve respectively. A selective increase of the central nervous system (CNS) temperature accelerated the motor patterns of both feeding and crawling. We propose that temperature affects centrally generated behavior via two pathways: short term changes like thermotaxis are mediated by the terminal organ, while long term adaptations like increased feeding rate are caused by temperature sensitive neurons in the CNS which were recently shown to exist in Drosophila larvae.     

Functional organization of a neural network for aversive olfactory learning in Caenorhabditis elegans

Many animals use their olfactory systems to learn to avoid dangers, but how neural circuits encode naive and learned olfactory preferences, and switch between those preferences, is poorly understood. Here, we map an olfactory network, from sensory input to motor output, which regulates the learned olfactory aversion of Caenorhabditis elegans for the smell of pathogenic bacteria. Naive animals prefer smells of pathogens but animals trained with pathogens lose this attraction. We find that two different neural circuits subserve these preferences, with one required for the naive preference and the other specifically for the learned preference. Calcium imaging and behavioral analysis reveal that the naive preference reflects the direct transduction of the activity of olfactory sensory neurons into motor response, whereas the learned preference involves modulations to signal transduction to downstream neurons to alter motor response. Thus, two different neural circuits regulate a behavioral switch between naive and learned olfactory preferences.     

Caenorhabditis elegans as a model system to study aging of learning and memory

The nematode Caenorhabditis elegans is an excellent model organism to study biological processes relevant to a wide variety of human and rodent disease systems. Previous studies have suggested that mutants of the insulin/insulin-like growth factor-1 pathway show life extension and increased stress resistance in various species, including C. elegans, the fruit fly, and the mouse. It has recently been shown that the life-extending mutants, including the age-1 phosphatidylinositol- 3 OH kinase mutants and the daf-2 insulin-like receptor mutants, display improvement in a type of associative learning behavior called thermotaxis learning behavior. The age-1 mutant shows a dramatic threefold extension of the health-span that ensures thermotaxis learning behavior, suggesting strong neuroprotective actions during aging. The age-1 and daf-2 mutants show resistance to multiple forms of stress and upregulates the genes involved in reactive oxygen species scavenging, heat shock, and P450 drug-detoxification. The life-extending mutants may confer resistance to various stress and diseases in neurons. Therefore, C. elegans provides an emerging system for the prevention of age-related deficits in the nervous system and in learning behaviors. This article discusses the aging of learning and memory and the neuroprotection effects of life-extending mutants on learning behaviors.     

Effect of Chronic Caloric Restriction on the Synchronization of Various Physiological Measures in Old Female Fischer 344 Rats

-A variety of physiological and behavioral parameters which relate to metabolism were continuously monitored in 18 month old female Fischer 344 rats which were maintained on either ad libitum or reduced calorie diets. Caloric restriction (CR) stimulated average motor activity per day, the duration of each feeding episode, food consumed per episode, and water consumed per gram lean body mass (LBM). However, CR limited total food consumption, feeding time, number of feeding episodes per day, total eating and drinking time, and the daily ratio of food consumed to water consumed, CR also decreased average body temperature per day, O₂ consumption, CO₂, production, and respiratory quotient. A variety of parameters concerning water consumption were not affected. CR rats ate their food immediately when food was presented during the light span, while ad libitum fed animals ate numerous small meals throughout the entire dark span. An anticipatory response to restricted feeding was also noted. Total motor activity, metabolism, and body temperature increased just prior to scheduled feeding and reached maximum values shortly after feeding, suggesting that these parameters were highly synchronized to feeding. Females and males were found to respond to caloric restriction in a similar fashion. Dramatic changes in respiratory quotient and body temperature suggest rapid shifts between metabolic pathways (glycolysis to giuconeogenesis) to obtain optimal efficiency. Lower body temperature and metabolism may provide protection against DNA damage, thereby increasing the survival potential of restricted rats. These responses may provide insight into the mechanisms by which caloric restriction acts to extend life span.     

Mechanisms of oleoylethanolamide-induced changes in feeding behavior and motor activity

Oleoylethanolamide (OEA), a lipid synthesized in the intestine, reduces food intake and stimulates lipolysis through peroxisome proliferator-activated receptor-alpha. OEA also activates transient receptor potential vanilloid type 1 (TRPV1) in vitro. Because the anorexigenic effect of OEA is associated with delayed feeding onset and reduced locomotion, we examined whether intraperitoneal administration of OEA results in nonspecific behavioral effects that contribute to the anorexia in rats. Moreover, we determined whether circulating levels of other gut hormones are modulated by OEA and whether CCK is involved in OEA-induced anorexia. Our results indicate that OEA reduces food intake without causing a conditioned taste aversion or reducing sodium appetite. It also failed to induce a conditioned place aversion. However, OEA induced changes in posture and reduced spontaneous activity in the open field. This likely underlies the reduced heat expenditure and sodium consumption observed after OEA injection, which disappeared within 1 h. The effects of OEA on motor activity were similar to those of the TRPV1 agonist capsaicin and were also observed with the peroxisome proliferator-activated receptor-alpha agonist Wy-14643. Plasma levels of ghrelin, peptide YY, glucagon-like peptide 1, and apolipoprotein A-IV were not changed by OEA. Finally, antagonism of CCK-1 receptors did not affect OEA-induced anorexia. These results suggest that OEA suppresses feeding without causing visceral illness and that neither ghrelin, peptide YY, glucagon-like peptide 1, apolipoprotein A-IV, nor CCK plays a critical role in this effect. Despite that OEA-induced anorexia is unlikely to be due to impaired motor activity, our data raise a cautionary note in how specific behavioral and metabolic effects of OEA should be interpreted.     

Molecular signaling involved in regulating feeding and other motivated behaviors

The metabolic and nutritional status of an organism influences multiple behaviors in addition to food intake. When an organism is hungry, it employs behaviors that help it locate and ingest food while suppressing behaviors that are not associated with this goal. Alternatively, when an organism is satiated, food-seeking behaviors are repressed so that the animal can direct itself to other goal-oriented tasks such as reproductive behaviors. Studies in both vertebrate and invertebrate model systems have revealed that food-deprived and -satiated behaviors are differentially executed and integrated via common molecular signaling mechanisms. This article discusses cellular and molecular mechanisms for how insulin, neuropeptide Y (NPY), and serotonin utilize common signaling pathways to integrate feeding and metabolic state with other motivated behaviors. Insulin, NPY, and serotonin are three of the most well-studied molecules implicated in regulating such behaviors. Overall, insulin signaling allows an organism to coordinate proper behavioral output with changes in metabolism, NPY activates behaviors required for locating and ingesting food, and serotonin modulates behaviors performed when an organism is satiated. These three molecules work to ensure that the proper behaviors are executed in response to the feeding state of an organism.