Serotonin promotes exploitation in complex environments by accelerating decision-making

A specific folate activates serotonergic neurons to control C. elegans behavior

Folates are B-group vitamins that function in one-carbon metabolism. Here we show that a specific folate can activate serotonergic neurons in C. elegans to modulate behavior through a pathway that requires the folate receptor FOLR-1 and the GON-2 calcium channel. FOLR-1 and GON-2 physically interact in a heterologous system, and both are expressed in the HSN and NSM serotonergic neurons. Both the folate 10-formyl-THF and a non-metabolic pteroate induce increases in the number of Ca2+ transients in the HSN neurons and egg laying in an FOLR-1- and GON-2-dependent manner. FOLR-1 and GON-2 are required for the activation of the NSM neurons in response to 10-formyl-THF, and for full NSM-mediated stoppage of movement when starved animals encounter bacteria. Our results demonstrate that FOLR-1 acts independently of one-carbon metabolism and suggest that 10-formyl-THF acts as a dietary signal that activates serotonergic neurons to impact behavior through a pathway that involves calcium entry.

Behavioral plasticity

Behavioral plasticity allows animals to modulate their behavior based on experience and environmental conditions. Caenorhabditis elegans exhibits experience-dependent changes in its behavioral responses to various modalities of sensory cues, including odorants, salts, temperature, and mechanical stimulations. Most of these forms of behavioral plasticity, such as adaptation, habituation, associative learning, and imprinting, are shared with other animals. The C. elegans nervous system is considerably tractable for experimental studies-its function can be characterized and manipulated with molecular genetic methods, its activity can be visualized and analyzed with imaging approaches, and the connectivity of its relatively small number of neurons are well described. Therefore, C. elegans provides an opportunity to study molecular, neuronal, and circuit mechanisms underlying behavioral plasticity that are either conserved in other animals or unique to this species. These findings reveal insights into how the nervous system interacts with the environmental cues to generate behavioral changes with adaptive values.

C. elegans foraging as a model for understanding the neuronal basis of decision-making

Animals have evolved to seek, select, and exploit food sources in their environment. Collectively termed foraging, these ubiquitous behaviors are necessary for animal survival. As a foundation for understanding foraging, behavioral ecologists established early theoretical and mathematical frameworks which have been subsequently refined and supported by field and laboratory studies of foraging animals. These simple models sought to explain how animals decide which strategies to employ when locating food, what food items to consume, and when to explore the environment for new food sources. These foraging decisions involve integration of prior experience with multimodal sensory information about the animal's current environment and internal state. We suggest that the nematode Caenorhabditis elegans is well-suited for a high-resolution analysis of complex goal-oriented behaviors such as foraging. We focus our discussion on behavioral studies highlighting C. elegans foraging on bacteria and summarize what is known about the underlying neuronal and molecular pathways. Broadly, we suggest that this simple model system can provide a mechanistic understanding of decision-making and present additional avenues for advancing our understanding of complex behavioral processes.

Genetic regulators of a resource polyphenism interact to couple predatory morphology and behaviour

Phenotypic plasticity often requires the coordinated response of multiple traits observed individually as morphological, physiological or behavioural. The integration, and hence functionality, of this response may be influenced by whether and how these component traits share a genetic basis. In the case of polyphenism, or discrete plasticity, at least part of the environmental response is categorical, offering a simple readout for determining whether and to what degree individual components of a plastic response can be decoupled. Here, we use the nematode Pristionchus pacificus, which has a resource polyphenism allowing it to be a facultative predator of other nematodes, to understand the genetic integration of polyphenism. The behavioural and morphological consequences of perturbations to the polyphenism's genetic regulatory network show that both predatory activity and ability are strongly influenced by morphology, different axes of morphological variation are associated with different aspects of predatory behaviour, and rearing environment can decouple predatory morphology from behaviour. Further, we found that interactions between some polyphenism-modifying genes synergistically affect predatory behaviour. Our results show that the component traits of an integrated polyphenic response can be decoupled and, in principle, selected upon individually, and they suggest that multiple routes to functionally comparable phenotypes are possible.

Mitochondrial energy state controls AMPK-mediated foraging behavior in C. elegans

Organisms surveil and respond to their environment using behaviors entrained by metabolic cues that reflect food availability. Mitochondria act as metabolic hubs and at the center of mitochondrial energy production is the protonmotive force (PMF), an electrochemical gradient generated by metabolite consumption. The PMF serves as a central integrator of mitochondrial status, but its role in governing metabolic signaling is poorly understood. We used optogenetics to dissipate the PMF in Caenorhabditis elegans tissues to test its role in food-related behaviors. Our data demonstrate that PMF reduction in the intestine is sufficient to initiate locomotor responses to acute food deprivation. This behavioral adaptation requires the cellular energy regulator AMP-activated protein kinase (AMPK) in neurons, not in the intestine, and relies on mitochondrial dynamics and axonal trafficking. Our results highlight a role for intestinal PMF as an internal metabolic cue, and we identify a bottom-up signaling axis through which changes in the PMF trigger AMPK activity in neurons to promote foraging behavior.

Bed nucleus of the stria terminalis GABA neurons are necessary for changes in foraging behaviour following an innate threat

Foraging is a universal behaviour that has co-evolved with predation pressure. We investigated the role of the bed nucleus of the stria terminalis (BNST) GABA neurons in robotic and live predator threat processing and their consequences in post-threat encounter foraging. Both robotic and live predator interactions increased BNST GABA neuron activity. Mice were trained to procure food in a laboratory-based foraging apparatus in which food pellets were placed at incrementally greater distances from a nest zone. After mice learned to forage, they were exposed to a robotic or live predator threat, while BNST GABA neurons were chemogenetically inhibited. Post-robotic threat encounter, mice spent more time in the nest zone, but other foraging parameters were unchanged compared with pre-encounter behaviour. Inhibition of BNST GABA neurons had no effect on foraging behaviour post-robotic threat encounter. Following live predator exposure, control mice spent significantly more time in the nest zone, increased their latency to successfully forage, and significantly altered their overall foraging performance. Inhibition of BNST GABA neurons during live predator exposure prevented changes in foraging behaviour from developing after a live predator threat. BNST GABA neuron inhibition did not alter foraging behaviour during robotic or live predator threats. We conclude that these results demonstrate that while both robotic and live predator encounters effectively intrude on foraging behaviour, the perceived risk and behavioural consequences of the threat are distinguishable. Additionally, BNST GABA neurons may play a role in the integration of prior innate predator threat experience that results in hypervigilance during post-encounter foraging behaviour.

Caenorhabditis elegans foraging patterns follow a simple rule of thumb

Rules of thumb are behavioral algorithms that approximate optimal behavior while lowering cognitive and sensory costs. One way to reduce these costs is by simplifying the representation of the environment: While the theoretically optimal behavior may depend on many environmental variables, a rule of thumb may use a smaller set of variables that performs reasonably well. Experimental proof of this simplification requires an exhaustive mapping of all relevant combinations of several environmental parameters, which we performed for Caenorhabditis elegans foraging by covering systematically combinations of food density (across 4 orders of magnitude) and food type (across 12 bacterial strains). We found that worms' response is dominated by a single environmental variable: food density measured as number of bacteria per unit surface. They disregard other factors such as biomass content or bacterial strain. We also measured experimentally the impact on fitness of each type of food, determining that the rule is near-optimal and therefore constitutes a rule of thumb that leverages the most informative environmental variable. These results set the stage for further investigations into the underlying genetic and neural mechanisms governing this simplification process, and into its role in the evolution of decision-making strategies.

Dissecting the functional organization of the C. elegans serotonergic system at whole-brain scale

Serotonin influences many aspects of animal behavior. But how serotonin acts on its diverse receptors across the brain to modulate global activity and behavior is unknown. Here, we examine how serotonin release in C. elegans alters brain-wide activity to induce foraging behaviors, like slow locomotion and increased feeding. Comprehensive genetic analyses identify three core serotonin receptors (MOD-1, SER-4, and LGC-50) that induce slow locomotion upon serotonin release and others (SER-1, SER-5, and SER-7) that interact with them to modulate this behavior. SER-4 induces behavioral responses to sudden increases in serotonin release, whereas MOD-1 induces responses to persistent release. Whole-brain imaging reveals widespread serotonin-associated brain dynamics, spanning many behavioral networks. We map all sites of serotonin receptor expression in the connectome, which, together with synaptic connectivity, helps predict which neurons show serotonin-associated activity. These results reveal how serotonin acts at defined sites across a connectome to modulate brain-wide activity and behavior.

Bed Nucleus of the Stria Terminalis GABA neurons are necessary for changes in foraging behavior following an innate threat

Foraging is a universal behavior that has co-evolved with predation pressure. We investigated the role of bed nucleus of the stria terminalis (BNST) GABA neurons in robotic and live predator threat processing and their consequences in post-threat encounter foraging. Mice were trained to procure food in a laboratory-based foraging apparatus in which food pellets were placed at discrete and incrementally greater distances from a nest zone. After mice learned to forage, they were exposed to either a robotic or live predator threat, while BNST GABA neurons were chemogenetically inhibited. Post-robotic threat encounter, mice spent more time in the nest zone, but other foraging parameters were unchanged compared to pre-encounter behavior. Inhibition of BNST GABA neurons had no effect on foraging behavior post-robotic threat encounter. Following live predator exposure, control mice spent significantly more time in the nest zone, increased their latency to successfully forage, and their overall foraging performance was significantly a ltered. I nhibition o f BNST GABA neurons during live predator exposure prevented changes in foraging behavior from developing after live predator threat. BNST GABA neuron inhibition did not alter foraging behavior during robotic or live predator threat. We conclude that while both robotic and live predator encounter effectively intrude on foraging behavior, the perceived risk and behavioral consequence of the threats are distinguishable. Additionally, BNST GABA neurons may play a role in the integration of prior innate predator threat experience that results in hypervigilance during post-encounter foraging behavior.

Dissecting the Functional Organization of the C. elegans Serotonergic System at Whole-Brain Scale

Serotonin controls many aspects of animal behavior and cognition. But how serotonin acts on its diverse receptor types in neurons across the brain to modulate global activity and behavior is unknown. Here, we examine how serotonin release from a feeding-responsive neuron in C. elegans alters brain-wide activity to induce foraging behaviors, like slow locomotion and increased feeding. A comprehensive genetic analysis identifies three core serotonin receptors that collectively induce slow locomotion upon serotonin release and three others that interact with them to further modulate this behavior. The core receptors have different functional roles: some induce behavioral responses to sudden increases in serotonin release, whereas others induce responses to persistent release. Whole-brain calcium imaging reveals widespread serotonin-associated brain dynamics, impacting different behavioral networks in different ways. We map out all sites of serotonin receptor expression in the connectome, which, together with synaptic connectivity, helps predict serotonin-associated brain-wide activity changes. These results provide a global view of how serotonin acts at defined sites across a connectome to modulate brain-wide activity and behavior.

Automatically tracking feeding behavior in populations of foraging C. elegans

Caenorhabditis elegans feeds on bacteria and other small microorganisms which it ingests using its pharynx, a neuromuscular pump. Currently, measuring feeding behavior requires tracking a single animal, indirectly estimating food intake from population-level metrics, or using restrained animals. To enable large throughput feeding measurements of unrestrained, crawling worms on agarose plates at a single worm resolution, we developed an imaging protocol and a complementary image analysis tool called PharaGlow. We image up to 50 unrestrained crawling worms simultaneously and extract locomotion and feeding behaviors. We demonstrate the tool's robustness and high-throughput capabilities by measuring feeding in different use-case scenarios, such as through development, with genetic and chemical perturbations that result in faster and slower pumping, and in the presence or absence of food. Finally, we demonstrate that our tool is capable of long-term imaging by showing behavioral dynamics of mating animals and worms with different genetic backgrounds. The low-resolution fluorescence microscopes required are readily available in C. elegans laboratories, and in combination with our python-based analysis workflow makes this methodology easily accessible. PharaGlow therefore enables the observation and analysis of the temporal dynamics of feeding and locomotory behaviors with high-throughput and precision in a user-friendly system.

The nematode serotonin-gated chloride channel MOD-1: A novel target for anthelmintic therapy

Anthelmintics are used to treat human and veterinary parasitic diseases and to reduce crop and livestock production loss associated with parasitosis. The free-living nematode Caenorhabditis elegans, a model system for anthelmintic drug discovery, has a serotonin (5-HT)-gated chloride channel, MOD-1, which belongs to the Cys-loop receptor family and modulates locomotory and behavioral functions. Since MOD-1 is unique to nematodes, it is emerging as an attractive anthelmintic drug target, but details of MOD-1 function are unclear. Here, we revealed novel aspects of MOD-1 function from the molecular level to the organism level and identified compounds targeting this receptor, which may provide new directions for anthelmintic drug discovery. We used whole-cell current recordings from heterologously expressed MOD-1 to show that tryptamine (Tryp), a weak partial agonist of vertebrate serotonin type 3 (5-HT3) receptors, efficaciously activates MOD-1. A screen for modulators revealed that GABAergic ligands piperazine (PZE) and muscimol reduce 5-HT-elicited currents, thus identifying novel MOD-1 allosteric inhibitors. Next, we performed locomotor activity assays, and we found 5-HT and Tryp rapidly decrease worm motility, which is reversible only at low 5-HT concentrations. Mutants lacking MOD-1 are partially resistant to both drugs, demonstrating its role in locomotion. Acting as an antagonist of MOD-1, we showed PZE reduces the locomotor effects of exogenous 5-HT. Therefore, Tryp- and PZE-derived compounds, acting at MOD-1 through different molecular mechanisms, emerge as promising anthelmintic agents. This study enhances our knowledge of the function and drug selectivity of Cys-loop receptors and postulates MOD-1 as a potential target for anthelmintic therapy.

Crosstalk between neurons and glia through G-protein coupled receptors: Insights from Caenorhabditis elegans

The past decades have witnessed a dogmatic shift from glia as supporting cells in the nervous system to their active roles in neurocentric functions. Neurons and glia communicate and show bidirectional responses through tripartite synapses. Studies across species indicate that neurotransmitters released by neurons are perceived by glial receptors, which allow for gliotransmitter release. These gliotransmitters can result in activation of neurons via neuronal GPCR receptors. However, studies of these molecular interactions are in their infancy. Caenorhabditis elegans has a conserved neuron-glia architectural repertoire with molecular and functional resemblance to mammals. Further, glia in C. elegans can be manipulated through ablation and mutations allowing for deciphering of glial dependent processes in vivo at single glial resolutions. Here, we will review recent findings from vertebrate and invertebrate organisms with a focus on how C. elegans can be used to advance our understanding of neuron-glia interactions through GPCRs.

Aripiprazole Offsets Mutant ATXN3-Induced Motor Dysfunction by Targeting Dopamine D2 and Serotonin 1A and 2A Receptors in C. elegans

The atypical antipsychotic aripiprazole is a Food and Drug Administration-approved drug for the treatment of psychotic, mood, and other psychiatric disorders. Previous drug discovery efforts pinpointed aripiprazole as an effective suppressor of Machado–Joseph disease (MJD) pathogenesis, as its administration resulted in a reduced abundance and aggregation of mutant Ataxin-3 (ATXN3) proteins. Dopamine partial agonism and functional selectivity have been proposed as the main pharmacological mechanism of action of aripiprazole in the treatment of psychosis; however, this mechanism remains to be determined in the context of MJD. Here, we focus on confirming the efficacy of aripiprazole to reduce motor dysfunction in vivo, using a Caenorhabditis elegans (C. elegans) model of MJD, and on unveiling the drug targets required for its positive action against mutant ATXN3 pathogenesis. We employed pharmacogenetics and pharmacological approaches to identify which dopamine and serotonin receptors are critical for aripiprazole-mediated improvements in motor function. We demonstrated that dopamine D₂-like and serotonin 5-HT_1A and 5-HT_2A receptors play important roles in this process. Our findings strengthen the relevance of dopaminergic and serotoninergic signaling modulation against mutant ATXN3-mediated pathogenesis. The identification of aripiprazole’s cellular targets, relevant for MJD and perhaps other neurodegenerative diseases, may pave the way for prospective drug discovery and development campaigns aiming to improve the features of this prototypical compound and reduce side effects not negligible in the case of aripiprazole.

The role of food odor in invertebrate foraging

Foraging for food is an integral part of animal survival. In small insects and invertebrates, multisensory information and optimized locomotion strategies are used to effectively forage in patchy and complex environments. Here, the importance of olfactory cues for effective invertebrate foraging is discussed in detail. We review how odors are used by foragers to move toward a likely food source and the recent models that describe this sensory-driven behavior. We argue that smell serves a second function by priming an organism for the efficient exploitation of food. By appraising food odors, invertebrates can establish preferences and better adapt to their ecological niches, thereby promoting survival. The smell of food pre-prepares the gastrointestinal system and primes feeding motor programs for more effective ingestion as well. Optimizing resource utilization affects longevity and reproduction as a result, leading to drastic changes in survival. We propose that models of foraging behavior should include odor priming, and illustrate this with a simple toy model based on the marginal value theorem. Lastly, we discuss the novel techniques and assays in invertebrate research that could investigate the interactions between odor sensing and food intake. Overall, the sense of smell is indispensable for efficient foraging and influences not only locomotion, but also organismal physiology, which should be reflected in behavioral modeling.

Modulation of sensory perception by hydrogen peroxide enables Caenorhabditis elegans to find a niche that provides both food and protection from hydrogen peroxide

Hydrogen peroxide (H2O2) is the most common chemical threat that organisms face. Here, we show that H2O2 alters the bacterial food preference of Caenorhabditis elegans, enabling the nematodes to find a safe environment with food. H2O2 induces the nematodes to leave food patches of laboratory and microbiome bacteria when those bacterial communities have insufficient H2O2-degrading capacity. The nematode's behavior is directed by H2O2-sensing neurons that promote escape from H2O2 and by bacteria-sensing neurons that promote attraction to bacteria. However, the input for H2O2-sensing neurons is removed by bacterial H2O2-degrading enzymes and the bacteria-sensing neurons' perception of bacteria is prevented by H2O2. The resulting cross-attenuation provides a general mechanism that ensures the nematode's behavior is faithful to the lethal threat of hydrogen peroxide, increasing the nematode's chances of finding a niche that provides both food and protection from hydrogen peroxide.

A neural circuit for flexible control of persistent behavioral states

To adapt to their environments, animals must generate behaviors that are closely aligned to a rapidly changing sensory world. However, behavioral states such as foraging or courtship typically persist over long time scales to ensure proper execution. It remains unclear how neural circuits generate persistent behavioral states while maintaining the flexibility to select among alternative states when the sensory context changes. Here, we elucidate the functional architecture of a neural circuit controlling the choice between roaming and dwelling states, which underlie exploration and exploitation during foraging in C. elegans. By imaging ensemble-level neural activity in freely moving animals, we identify stereotyped changes in circuit activity corresponding to each behavioral state. Combining circuit-wide imaging with genetic analysis, we find that mutual inhibition between two antagonistic neuromodulatory systems underlies the persistence and mutual exclusivity of the neural activity patterns observed in each state. Through machine learning analysis and circuit perturbations, we identify a sensory processing neuron that can transmit information about food odors to both the roaming and dwelling circuits and bias the animal towards different states in different sensory contexts, giving rise to context-appropriate state transitions. Our findings reveal a potentially general circuit architecture that enables flexible, sensory-driven control of persistent behavioral states.

The transcription factor LAG-1/CSL plays a Notch-independent role in controlling terminal differentiation, fate maintenance, and plasticity of serotonergic chemosensory neurons

During development, signal-regulated transcription factors (TFs) act as basal repressors and upon signalling through morphogens or cell-to-cell signalling shift to activators, mediating precise and transient responses. Conversely, at the final steps of neuron specification, terminal selector TFs directly initiate and maintain neuron-type specific gene expression through enduring functions as activators. C. elegans contains 3 types of serotonin synthesising neurons that share the expression of the serotonin biosynthesis pathway genes but not of other effector genes. Here, we find an unconventional role for LAG-1, the signal-regulated TF mediator of the Notch pathway, as terminal selector for the ADF serotonergic chemosensory neuron, but not for other serotonergic neuron types. Regulatory regions of ADF effector genes contain functional LAG-1 binding sites that mediate activation but not basal repression. lag-1 mutants show broad defects in ADF effector genes activation, and LAG-1 is required to maintain ADF cell fate and functions throughout life. Unexpectedly, contrary to reported basal repression state for LAG-1 prior to Notch receptor activation, gene expression activation in the ADF neuron by LAG-1 does not require Notch signalling, demonstrating a default activator state for LAG-1 independent of Notch. We hypothesise that the enduring activity of terminal selectors on target genes required uncoupling LAG-1 activating role from receiving the transient Notch signalling.

Serotonergic modulation across sensory modalities

The serotonergic system has been widely studied across animal taxa and different functional networks. This modulatory system is therefore well positioned to compare the consequences of neuromodulation for sensory processing across species and modalities at multiple levels of sensory organization. Serotonergic neurons that innervate sensory networks often bidirectionally exchange information with these networks but also receive input representative of motor events or motivational state. This convergence of information supports serotonin's capacity for contextualizing sensory information according to the animal's physiological state and external events. At the level of sensory circuitry, serotonin can have variable effects due to differential projections across specific sensory subregions, as well as differential serotonin receptor type expression within those subregions. Functionally, this infrastructure may gate or filter sensory inputs to emphasize specific stimulus features or select among different streams of information. The near-ubiquitous presence of serotonin and other neuromodulators within sensory regions, coupled with their strong effects on stimulus representation, suggests that these signaling pathways should be considered integral components of sensory systems.

Serotonin signaling by maternal neurons upon stress ensures progeny survival

Germ cells are vulnerable to stress. Therefore, how organisms protect their future progeny from damage in a fluctuating environment is a fundamental question in biology. We show that in Caenorhabditis elegans, serotonin released by maternal neurons during stress ensures the viability and stress resilience of future offspring. Serotonin acts through a signal transduction pathway conserved between C. elegans and mammalian cells to enable the transcription factor HSF1 to alter chromatin in soon-to-be fertilized germ cells by recruiting the histone chaperone FACT, displacing histones, and initiating protective gene expression. Without serotonin release by maternal neurons, FACT is not recruited by HSF1 in germ cells, transcription occurs but is delayed, and progeny of stressed C. elegans mothers fail to complete development. These studies uncover a novel mechanism by which stress sensing by neurons is coupled to transcription response times of germ cells to protect future offspring.

Global Transcriptome Changes That Accompany Alterations in Serotonin Levels in Caenorhabditis elegans

Serotonin (5-hydroxytryptamine, 5-HT), is a phylogenetically ancient molecule best characterized as a neurotransmitter that modulates multiple aspects of mood and social cognition. The roles that 5-HT plays in normal and abnormal behavior are not fully understood but have been posited to be due to its common function as a 'defense signal'. However, 5-HT levels also systemically impact cell physiology, modulating cell division, migration, apoptosis, mitochondrial biogenesis, cellular metabolism and differentiation. Whether these diverse cellular effects of 5-HT also share a common basis is unclear. C. elegans provides an ideal system to interrogate the systemic effects of 5-HT, since lacking a blood-brain barrier, 5-HT synthesized and released by neurons permeates the organism to modulate neuronal as well as non-neuronal cells throughout the body. Here we used RNA-Seq to characterize the systemic changes in gene expression that occur in C. elegans upon altering 5-HT levels, and compared the transcriptomes to published datasets. We find that an acute increase in 5-HT is accompanied by a global decrease in gene expression levels, upregulation of genes involved in stress pathways, changes that significantly correlate with the published transcriptomes of animals that have activated defense and immune responses, and an increase in levels of phosphorylated eukaryotic initiation factor, eIF2α. In 5-HT deficient animals lacking tryptophan hydroxylase (tph-1(mg280)II) there is a net increase in gene expression, with an overrepresentation of genes related to development and chromatin. Surprisingly, the transcriptomes of animals with acute increases in 5-HT levels, and 5-HT deficiency do not overlap with transcriptomes of mutants with whom they share striking physiological resemblance. These studies are the first to catalog systemic transcriptome changes that occur upon alterations in 5-HT levels. They further show that in C. elegans changes in gene expression upon altering 5-HT levels, and changes in physiology, are not directly correlated.

Environmental Programming of Adult Foraging Behavior in C. elegans

Foraging strategies should be tuned to the expected distribution of resources in the environment. Tuning can occur over generations and lead to genetic differences in innate foraging behavior or over shorter timescales within an individual's lifespan. Both genetically encoded and experience-based strategies are implemented by neural circuits that respond to environmental cues and track internal states. Caenorhabditis elegans exhibit both between-strain genetic differences and within-strain plasticity in foraging. In individuals, changes in foraging are usually short term and based on recent experience. Here, we tested whether developmental experience could permanently alter foraging. We found that, in most wild strains, early-life starvation led to "cautious" foraging strategies, in which exploration is reduced, and these behavioral changes are associated with altered dynamics in a locomotory circuit. Possessing either the derived (domestication-associated) or ancestral allele of the neuroglobin glb-5 determines foraging plasticity. Overall, we show that C. elegans exhibit adaptive developmental plasticity that affects multiple aspects of foraging behavior and leads to changes in a core navigation circuit and that innate foraging traits and plasticity in those traits are genetically separable. VIDEO ABSTRACT.

A Memory Circuit for Coping with Impending Adversity

Organisms’ capacity to anticipate future conditions is key for survival. Associative memories are instrumental for learning from past experiences, yet little is known about the processes that follow memory retrieval and their potential advantage in preparing for impending developments. Here, using C. elegans nematodes, we demonstrate that odor-evoked retrieval of aversive memories induces rapid and protective stress responses, which increase animal survival prospects when facing imminent adversities. The underlying mechanism relies on two sensory neurons: one is necessary during the learning period, and the other is necessary and sufficient for memory retrieval. Downstream of memory reactivation, serotonin secreted from two head neurons mediates the systemic stress response. Thus, evoking stressful memories, stored within individual sensory neurons, allows animals to anticipate upcoming dire conditions and provides a head start to initiate rapid and protective responses that ultimately increase animal fitness.

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Reactivation of an aversive memory induces a fast protective stress response

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The fast response provides the animals with a fitness advantage

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One neuron is necessary for memory formation

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Another neuron is necessary and sufficient for memory reactivation

Neuroligin tuning of pharyngeal pumping reveals extrapharyngeal modulation of feeding in Caenorhabditis elegans

The integration of distinct sensory modalities is essential for behavioural decision making. In C aenorhabditis elegans, this process is coordinated by neural circuits that integrate sensory cues from the environment to generate an appropriate behaviour at the appropriate output muscles. Food is a multimodal cue that impacts the microcircuits to modulate feeding and foraging drivers at the level of the pharyngeal and body wall muscle, respectively. When food triggers an upregulation in pharyngeal pumping, it allows the effective ingestion of food. Here, we show that a C elegans mutant in the single gene orthologous to human neuroligins, nlg-1, is defective in food-induced pumping. This was not due to an inability to sense food, as nlg-1 mutants were not defective in chemotaxis towards bacteria. In addition, we found that neuroligin is widely expressed in the nervous system, including AIY, ADE, ALA, URX and HSN neurons. Interestingly, despite the deficit in pharyngeal pumping, neuroligin was not expressed within the pharyngeal neuromuscular network, which suggests an extrapharyngeal regulation of this circuit. We resolved electrophysiologically the neuroligin contribution to the pharyngeal circuit by mimicking food-dependent pumping and found that the nlg-1 phenotype is similar to mutants impaired in GABAergic and/or glutamatergic signalling. We suggest that neuroligin organizes extrapharyngeal circuits that regulate the pharynx. These observations based on the molecular and cellular determinants of feeding are consistent with the emerging role of neuroligin in discretely impacting functional circuits underpinning complex behaviours.

ASICs Mediate Food Responses in an Enteric Serotonergic Neuron that Controls Foraging Behaviors

Animals must respond to the ingestion of food by generating adaptive behaviors, but the role of gut-brain signaling in behavioral regulation is poorly understood. Here, we identify conserved ion channels in an enteric serotonergic neuron that mediate its responses to food ingestion and decipher how these responses drive changes in foraging behavior. We show that the C. elegans serotonergic neuron NSM acts as an enteric sensory neuron that acutely detects food ingestion. We identify the novel and conserved acid-sensing ion channels (ASICs) DEL-7 and DEL-3 as NSM-enriched channels required for feeding-dependent NSM activity, which in turn drives slow locomotion while animals feed. Point mutations that alter the DEL-7 channel change NSM dynamics and associated behavioral dynamics of the organism. This study provides causal links between food ingestion, molecular and physiological properties of an enteric serotonergic neuron, and adaptive feeding behaviors, yielding a new view of how enteric neurons control behavior.

Invertebrate serotonin receptors: a molecular perspective on classification and pharmacology

Invertebrate receptors for the neurotransmitter serotonin (5-HT) have been identified in numerous species from diverse phyla, including Arthropoda, Mollusca, Nematoda and Platyhelminthes. For many receptors, cloning and characterization in heterologous systems have contributed data on molecular structure and function across both closely and distantly related species. This article provides an overview of heterologously expressed receptors, and considers evolutionary relationships among them, classification based on these relationships and nomenclature that reflects classification. In addition, transduction pathways and pharmacological profiles are compared across receptor subtypes and species. Previous work has shown that transduction mechanisms are well conserved within receptor subtypes, but responses to drugs are complex. A few ligands display specificity for different receptors within a single species; however, none acts with high specificity in receptors across different species. Two non-selective vertebrate ligands, the agonist 5-methoxytryptamine and antagonist methiothepin, are active in most receptor subtypes in multiple species and hence bind very generally to invertebrate 5-HT receptors. Future challenges for the field include determining how pharmacological profiles are affected by differences in species and receptor subtype, and how function in heterologous receptors can be used to better understand 5-HT activity in intact organisms.

Serotonergic neuron ADF modulates avoidance behaviors by inhibiting sensory neurons in C. elegans

Serotonin plays an essential role in both the invertebrate and vertebrate nervous systems. ADF, an amphid neuron with dual ciliated sensory endings, is considered to be the only serotonergic sensory neuron in the hermaphroditic Caenorhabditis elegans. This neuron is known to be involved in a range of behaviors including pharyngeal pumping, dauer formation, sensory transduction, and memory. However, whether ADF neuron is directly activated by environmental cues and how it processes these information remains unknown. In this study, we found that ADF neuron responds reliably to noxious stimuli such as repulsive odors, copper, sodium dodecyl sulfonate (SDS), and mechanical perturbation. This response is mediated by cell-autonomous and non-cell autonomous mechanisms. Furthermore, we show that ADF can modulate avoidance behaviors by inhibiting ASH, an amphid neuron with single ciliated ending. This work greatly furthers our understanding of 5-HT's contributions to sensory information perception, processing, and the resulting behavioral responses.

Concerted pulsatile and graded neural dynamics enables efficient chemotaxis in C. elegans

The ability of animals to effectively locate and navigate toward food sources is central for survival. Here, using C. elegans nematodes, we reveal the neural mechanism underlying efficient navigation in chemical gradients. This mechanism relies on the activity of two types of chemosensory neurons: one (AWA) coding gradients via stochastic pulsatile dynamics, and the second (AWC^ON) coding the gradients deterministically in a graded manner. The pulsatile dynamics of the AWA neuron adapts to the magnitude of the gradient derivative, allowing animals to take trajectories better oriented toward the target. The robust response of AWC^ON to negative derivatives promotes immediate turns, thus alleviating the costs incurred by erroneous turns dictated by the AWA neuron. This mechanism empowers an efficient navigation strategy that outperforms the classical biased-random walk strategy. This general mechanism thus may be applicable to other sensory modalities for efficient gradient-based navigation.

Finding one’s way to a food source along a complex gradient is central to survival for many animals. Here, the authors report that in C. elegans, the distinct response dynamics of two sensory neurons to odor gradients can support a navigation model more efficient than the biased-random walk.

A Single-Neuron Chemosensory Switch Determines the Valence of a Sexually Dimorphic Sensory Behavior

Biological sex, a fundamental dimension of internal state, can modulate neural circuits to generate behavioral variation. Understanding how and why circuits are tuned by sex can provide important insights into neural and behavioral plasticity. Here we find that sexually dimorphic behavioral responses to C. elegans ascaroside sex pheromones are implemented by the functional modulation of shared chemosensory circuitry. In particular, the sexual state of a single sensory neuron pair, ADF, determines the nature of an animal's behavioral response regardless of the sex of the rest of the body. Genetic feminization of ADF causes males to be repelled by, rather than attracted to, ascarosides, whereas masculinization of ADF has the opposite effect in hermaphrodites. When ADF is ablated, both sexes are weakly repelled by ascarosides. Genetic sex modulates ADF function by tuning chemosensation: although ADF is functional in both sexes, it detects the ascaroside ascr#3 only in males, a consequence of cell-autonomous action of the master sexual regulator tra-1. This occurs in part through the conserved DM-domain gene mab-3, which promotes the male state of ADF. The sexual modulation of ADF has a key role in reproductive fitness, as feminization or ablation of ADF renders males unable to use ascarosides to locate mates. Our results reveal an economical mechanism in which sex-specific behavioral valence arises through the cell-autonomous regulation of a chemosensory switch by genetic sex, allowing a social cue with salience for both sexes to elicit navigational responses commensurate with the differing needs of each.

Toward a multiscale modeling framework for understanding serotonergic function

Despite its importance in regulating emotion and mental wellbeing, the complex structure and function of the serotonergic system present formidable challenges toward understanding its mechanisms. In this paper, we review studies investigating the interactions between serotonergic and related brain systems and their behavior at multiple scales, with a focus on biologically-based computational modeling. We first discuss serotonergic intracellular signaling and neuronal excitability, followed by neuronal circuit and systems levels. At each level of organization, we will discuss the experimental work accompanied by related computational modeling work. We then suggest that a multiscale modeling approach that integrates the various levels of neurobiological organization could potentially transform the way we understand the complex functions associated with serotonin.

Sensory mutations in Drosophila melanogaster influence associational effects between resources during oviposition

Neighboring resources can affect insect oviposition behavior when the complexity of sensory information obscures information about host resource availability in heterogeneous resource patches. These effects are referred to as associational effects and are hypothesized to occur through constraints in the sensory processing of the insect during host search, resulting into suboptimal resource use. Because the possibilities to study these constraints on naturally occurring animals are limited, we instead used sensory mutants of Drosophila melanogaster to determine the importance of sensory information in the occurrence of associational effects. We found that oviposition was mainly governed by non-volatile chemical cues and less by volatile cues. Moreover, the loss of gustatory sensilla resulted in random resource selection and eliminated associational effects. In conclusion, our study shows that associational effects do not necessarily depend on constraints in the sensory evaluation of resource quality, but may instead be a direct consequence of distinctive selection behavior between different resources at small scales.

Stochastic feeding dynamics arise from the need for information and energy

Animals regulate their food intake in response to the available level of food. Recent observations of feeding dynamics in small animals showed feeding patterns of bursts and pauses, but their function is unknown. Here, we present a data-driven decision-theoretical model of feeding in Caenorhabditis elegans Our central assumption is that food intake serves a dual purpose: to gather information about the external food level and to ingest food when the conditions are good. The model recapitulates experimentally observed feeding patterns. It naturally implements trade-offs between speed versus accuracy and exploration versus exploitation in responding to a dynamic environment. We find that the model predicts three distinct regimes in responding to a dynamical environment, with a transition region where animals respond stochastically to periodic signals. This stochastic response accounts for previously unexplained experimental data.

Food responsiveness regulates episodic behavioral states in Caenorhabditis elegans

Animals optimize survival and reproduction in part through control of behavioral states, which depend on an organism's internal and external environments. In the nematode Caenorhabditis elegans a variety of behavioral states have been described, including roaming, dwelling, quiescence, and episodic swimming. These states have been considered in isolation under varied experimental conditions, making it difficult to establish a unified picture of how they are regulated. Using long-term imaging, we examined C. elegans episodic behavioral states under varied mechanical and nutritional environments. We found that animals alternate between high-activity (active) and low-activity (sedentary) episodes in any mechanical environment, while the incidence of episodes and their behavioral composition depend on food levels. During active episodes, worms primarily roam, as characterized by continuous whole body movement. During sedentary episodes, animals exhibit dwelling (slower movements confined to the anterior half of the body) and quiescence (a complete lack of movement). Roaming, dwelling, and quiescent states are manifest not only through locomotory characteristics but also in pharyngeal pumping (feeding) and in egg-laying behaviors. Next, we analyzed the genetic basis of behavioral states. We found that modulation of behavioral states depends on neuropeptides and insulin-like signaling in the nervous system. Sensory neurons and the Foraging homolog EGL-4 regulate behavior through control of active/sedentary episodes. Optogenetic stimulation of dopaminergic and serotonergic neurons induced dwelling, implicating dopamine as a dwell-promoting neurotransmitter. Our findings provide a more unified description of behavioral states and suggest that perception of nutrition is a conserved mechanism for regulating animal behavior.NEW & NOTEWORTHY One strategy by which animals adapt to their internal states and external environments is by adopting behavioral states. The roundworm Caenorhabditis elegans is an attractive model for investigating how behavioral states are genetically and neuronally controlled. Here we describe the hierarchical organization of behavioral states characterized by locomotory activity, feeding, and egg-laying. We show that decisions to engage in these behaviors are controlled by the nervous system through insulin-like signaling and the perception of food.

Serotonin-dependent kinetics of feeding bursts underlie a graded response to food availability in C. elegans

Animals integrate physiological and environmental signals to modulate their food uptake. The nematode C. elegans, whose food uptake consists of pumping bacteria from the environment into the gut, provides excellent opportunities for discovering principles of conserved regulatory mechanisms. Here we show that worms implement a graded feeding response to the concentration of environmental bacteria by modulating a commitment to bursts of fast pumping. Using long-term, high-resolution, longitudinal recordings of feeding dynamics under defined conditions, we find that the frequency and duration of pumping bursts increase and the duration of long pauses diminishes in environments richer in bacteria. The bioamine serotonin is required for food-dependent induction of bursts as well as for maintaining their high rate of pumping through two distinct mechanisms. We identify the differential roles of distinct families of serotonin receptors in this process and propose that regulation of bursts is a conserved mechanism of behaviour and motor control.