Antagonistic Serotonergic and Octopaminergic Neural Circuits Mediate Food-Dependent Locomotory Behavior in Caenorhabditis elegans

Fmo induction as a tool to screen for pro-longevity drugs

Dietary restriction (DR) and hypoxia (low oxygen) extend lifespan in Caenorhabditis elegans through the induction of a convergent downstream longevity gene, fmo-2. Flavin-containing monooxygenases (FMOs) are highly conserved xenobiotic-metabolizing enzymes with a clear role in promoting longevity in nematodes and a plausible similar role in mammals. This makes them an attractive potential target of small molecule drugs to stimulate the health-promoting effects of longevity pathways. Here, we utilize an fmo-2 fluorescent transcriptional reporter in C. elegans to screen a set of 80 compounds previously shown to improve stress resistance in mouse fibroblasts. Our data show that 19 compounds significantly induce fmo-2, and 10 of the compounds induce fmo-2 more than twofold. Interestingly, 9 of the 10 high fmo-2 inducers also extend lifespan in C. elegans. Two of these drugs, mitochondrial respiration chain complex inhibitors, interact with the hypoxia pathway to induce fmo-2, whereas two dopamine receptor type 2 (DRD2) antagonists interact with the DR pathway to induce fmo-2, indicating that dopamine signaling is involved in DR-mediated fmo-2 induction. Together, our data identify nine drugs that each (1) increase stress resistance in mouse fibroblasts, (2) induce fmo-2 in C. elegans, and (3) extend nematode lifespan, some through known longevity pathways. These results define fmo-2 induction as a viable approach to identifying and understanding mechanisms of putative longevity compounds.

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.

Evaluating non-monotonic dose-response relationships in ecotoxicological risk assessment: A case study based on a systematic review of data on fluoxetine

The environmental presence of pharmaceuticals, including the antidepressant fluoxetine, has become a subject of concern. Numerous studies have revealed effects of fluoxetine at environmental concentrations. Some of these studies have reported non-monotonic dose-response curves (NMDRs), leading to discussion because of the inconsistent detection of subtle effects and lack of mechanistic understanding. Nevertheless, investigating NMDRs in risk assessment is important, because neglecting them could underestimate potential risks of chemicals at low levels of exposure. Identification and quantification of NMDRs in risk assessment remains challenging, particularly given the prevalence of single outliers and the lack of sound statistical analyses. In response, the European Food Safety Authority (Beausoleil et al., 2016) presented a framework delineating six checkpoints for the evaluation of NMDR datasets, offering a systematic method for their assessment. The present study applies this framework to the case study of fluoxetine, aiming to assess the weight-of-evidence for the reported NMDR relationships. Through a systematic literature search, 53 datasets were selected for analysis against the six checkpoints. The results reveal that while a minority of these datasets meet all checkpoints, a significant proportion (27%) fulfilled at least five. Notably, many studies did not meet checkpoint 3, which requires NMDRs to be based on more than a single outlier. Overall, the current study points out a number of studies with considerable evidence supporting the presence of NMDRs for fluoxetine, while the majority of studies lacks strong evidence. The suggested framework proved useful for analysing NMDRs in ecotoxicological studies, but it is still imperative to develop further understanding of their biological plausibility.

Automated multimodal imaging of Caenorhabditis elegans behavior in multi-well plates

Assays of behavior in model organisms play an important role in genetic screens, drug testing, and the elucidation of gene-behavior relationships. We have developed an automated, high-throughput imaging and analysis method for assaying behaviors of the nematode C. elegans . We use high-resolution optical imaging to longitudinally record the behaviors of 96 animals at a time in multi-well plates, and computer vision software to quantify the animals' locomotor activity, behavioral states, and egg laying events. To demonstrate the capabilities of our system we used it to examine the role of serotonin in C. elegans behavior. We found that egg-laying events are preceded by a period of reduced locomotion, and that this decline in movement requires serotonin signaling. In addition, we identified novel roles of serotonin receptors SER-1 and SER-7 in regulating the effects of serotonin on egg laying across roaming, dwelling, and quiescent locomotor states. Our system will be useful for performing genetic or chemical screens for modulators of behavior.

Functional analysis of conserved C. elegans bHLH family members uncovers lifespan control by a peptidergic hub neuron

Throughout the animal kingdom, several members of the basic helix-loop-helix (bHLH) family act as proneural genes during early steps of nervous system development. Roles of bHLH genes in specifying terminal differentiation of postmitotic neurons have been less extensively studied. We analyze here the function of five C. elegans bHLH genes, falling into three phylogenetically conserved subfamilies, which are continuously expressed in a very small number of postmitotic neurons in the central nervous system. We show (a) that two orthologs of the vertebrate bHLHb4/b5 genes, called hlh-17 and hlh-32, function redundantly to specify the identity of a single head interneuron (AUA), as well as an individual motor neuron (VB2), (b) that the PTF1a ortholog hlh-13 acts as a terminal selector to control terminal differentiation and function of the sole octopaminergic neuron class in C. elegans , RIC, and (c) that the NHLH1/2 ortholog hlh-15 controls terminal differentiation and function of the peptidergic AVK head interneuron class, a known neuropeptidergic signaling hub in the animal. Strikingly, through null mutant analysis and cell-specific rescue experiments, we find that loss of hlh-15/NHLH in the peptidergic AVK neurons and the resulting abrogation of neuropeptide secretion causes a substantially expanded lifespan of the animal, revealing an unanticipated impact of a central, peptidergic hub neuron in regulating lifespan, which we propose to be akin to hypothalamic control of lifespan in vertebrates. Taken together, our functional analysis reveals themes of bHLH gene function during terminal differentiation that are complementary to the earlier lineage specification roles of other bHLH family members. However, such late functions are much more sparsely employed by members of the bHLH transcription factor family, compared to the function of the much more broadly employed homeodomain transcription factor family.

Segmentation-free measurement of locomotor frequency in Caenorhabditis elegans using image invariants

An animal's locomotor rate is an important indicator of its motility. In studies of the nematode C. elegans, assays of the frequency of body bending waves have often been used to discern the effects of mutations, drugs, or aging. Traditional manual methods for measuring locomotor frequency are low in throughput and subject to human error. Most current automated methods depend on image segmentation, which requires high image quality and is prone to errors. Here, we describe an algorithm for automated estimation of C. elegans locomotor frequency using image invariants, i.e., shape-based parameters that are independent of object translation, rotation, and scaling. For each video frame, the method calculates a combination of 8 Hu's moment invariants and a set of Maximally Stable Extremal Regions (MSER) invariants. The algorithm then calculates the locomotor frequency by computing the autocorrelation of the time sequence of the invariant ensemble. Results of our method show excellent agreement with manual or segmentation-based results over a wide range of frequencies. We show that compared to a segmentation-based method that analyzes a worm's shape and a method based on video covariance, our technique is more robust to low image quality and background noise. We demonstrate the system's capabilities by testing the effects of serotonin and serotonin pathway mutations on C. elegans locomotor frequency.

Evolutionary conservation of putative suicidality-related risk genes that produce diminished motivation corrected by clozapine, lithium and antidepressants

Background

Genome wide association studies (GWAS) and candidate gene analyses have identified genetic variants and genes that may increase the risk for suicidal thoughts and behaviors (STBs). Important unresolved issues surround these tentative risk variants such as the characteristics of the associated genes and how they might elicit STBs.

Methods

Putative suicidality-related risk genes (PSRGs) were identified by comprehensive literature search and were characterized with respect to evolutionary conservation, participation in gene interaction networks and associated phenotypes. Evolutionary conservation was established with database searches and BLASTP queries, whereas gene-gene interactions were ascertained with GeneMANIA. We then examined whether mutations in risk-gene counterparts in C. elegans produced a diminished motivation phenotype previously connected to suicide risk factors.

Results and conclusions

From the analysis, 105 risk-gene candidates were identified and found to be: 1) highly conserved during evolution, 2) enriched for essential genes, 3) involved in significant gene-gene interactions, and 4) associated with psychiatric disorders, metabolic disturbances and asthma/allergy. Evaluation of 17 mutant strains with loss-of-function/deletion mutations in PSRG orthologs revealed that 11 mutants showed significant evidence of diminished motivation that manifested as immobility in a foraging assay. Immobility was corrected in some or all of the mutants with clozapine, lithium and tricyclic antidepressant drugs. In addition, 5-HT2 receptor and muscarinic receptor antagonists restored goal-directed behavior in most or all of the mutants. These studies increase confidence in the validity of the PSRGs and provide initial clues about possible mechanisms that mediate STBs.

Neuroendocrine gene expression coupling of interoceptive bacterial food cues to foraging behavior of C. elegans

Animal internal state is modulated by nutrient intake, resulting in behavioral responses to changing food conditions. The neural mechanisms by which internal states are generated and maintained are not well understood. Here, we show that in the nematode Caenorhabditis elegans, distinct cues from bacterial food - interoceptive signals from the ingestion of bacteria and gustatory molecules sensed from nearby bacteria - act antagonistically on the expression of the neuroendocrine TGF-beta ligand DAF-7 from the ASJ pair of sensory neurons to modulate foraging behavior. A positive-feedback loop dependent on the expression of daf-7 from the ASJ neurons acts to promote transitions between roaming and dwelling foraging states and influence the persistence of roaming states. SCD-2, the C. elegans ortholog of mammalian anaplastic lymphoma kinase (ALK), which has been implicated in the central control of metabolism of mammals, functions in the AIA interneurons to regulate foraging behavior and cell-non-autonomously control the expression of DAF-7 from the ASJ neurons. Our data establish how a dynamic neuroendocrine daf-7 expression feedback loop regulated by SCD-2 functions to couple sensing and ingestion of bacterial food to foraging behavior. We further suggest that this neuroendocrine feedback loop underlies previously characterized exploratory behaviors in C. elegans. Our data suggest that the expression of daf-7 from the ASJ neurons contributes to and is correlated with an internal state of 'unmet need' that regulates exploratory foraging behavior in response to bacterial cues in diverse physiological contexts.

Distinct neurexin isoforms cooperate to initiate and maintain foraging activity

Neurexins are synaptic adhesion molecules that play diverse roles in synaptic development, function, maintenance, and plasticity. Neurexin genes have been associated with changes in human behavior, where variants in NRXN1 are associated with autism, schizophrenia, and Tourette syndrome. While NRXN1, NRXN2, and NRXN3 all encode major α and β isoforms, NRXN1 uniquely encodes a γ isoform, for which mechanistic roles in behavior have yet to be defined. Here, we show that both α and γ isoforms of neurexin/nrx-1 are required for the C. elegans behavioral response to food deprivation, a sustained period of hyperactivity upon food loss. We find that the γ isoform regulates initiation and the α isoform regulates maintenance of the behavioral response to food deprivation, demonstrating cooperative function of multiple nrx-1 isoforms in regulating a sustained behavior. The γ isoform alters monoamine signaling via octopamine, relies on specific expression of NRX-1 isoforms throughout the relevant circuit, and is independent of neuroligin/nlg-1, the canonical trans-synaptic partner of nrx-1. The α isoform regulates the pre-synaptic structure of the octopamine producing RIC neuron and its maintenance role is conditional on neuroligin/nlg-1. Collectively, these results demonstrate that neurexin isoforms can have separate behavioral roles and act cooperatively across neuronal circuits to modify behavior, highlighting the need to directly analyze and consider all isoforms when defining the contribution of neurexins to behavior.

Neuroendocrine Gene Expression Coupling of Interoceptive Bacterial Food Cues to Foraging Behavior of C. elegans

Animal internal state is modulated by nutrient intake, resulting in behavioral responses to changing food conditions. The neural mechanisms by which internal states are generated and maintained are not well understood. Here, we show that in the nematode Caenorhabditis elegans, distinct cues from bacterial food - interoceptive signals from the ingestion of bacteria and gustatory molecules sensed from nearby bacteria - act antagonistically on the expression of the neuroendocrine TGF-beta ligand DAF-7 from the ASJ pair of sensory neurons to modulate foraging behavior. A positive-feedback loop dependent on the expression of daf-7 from the ASJ neurons acts to promote transitions between roaming and dwelling foraging states and influence the persistence of roaming states. SCD-2, the C. elegans ortholog of mammalian Anaplastic Lymphoma Kinase (ALK), which has been implicated in the central control of metabolism of mammals, functions in the AIA interneurons to regulate foraging behavior and cell-non-autonomously control the expression of DAF-7 from the ASJ neurons. Our data establish how a dynamic neuroendocrine daf-7 expression feedback loop regulated by SCD-2 functions to couple sensing and ingestion of bacterial food to foraging behavior. We further suggest that this neuroendocrine feedback loop underlies previously characterized exploratory behaviors in C. elegans. Our data suggest that the expression of daf-7 from the ASJ neurons contributes to and is correlated with an internal state of "unmet need" that regulates exploratory foraging behavior in response to bacterial cues in diverse physiological contexts.

Using neurons to maintain autonomy: Learning from C. elegans

Understanding how biological organisms are autonomous-maintain themselves far from equilibrium through their own activities-requires understanding how they regulate those activities. In multicellular animals, such control can be exercised either via endocrine signaling through the vasculature or via neurons. In C. elegans this control is exercised by a well-delineated relatively small but distributed nervous system that relies on both chemical and electric transmission of signals. This system provides resources to integrate information from multiple sources as needed to maintain the organism. Especially important for the exercise of neural control are neuromodulators, which we present as setting agendas for control through more traditional electrical signaling. To illustrate how the C. elegans nervous system integrates multiple sources of information in controlling activities important for autonomy, we focus on feeding behavior and responses to adverse conditions. We conclude by considering how a distributed nervous system without a centralized controller is nonetheless adequate for autonomy.

The diverse family of Cys-loop receptors in Caenorhabditis elegans: insights from electrophysiological studies

Cys-loop receptors integrate a large family of pentameric ligand-gated ion channels that mediate fast ionotropic responses in vertebrates and invertebrates. Their vital role in converting neurotransmitter recognition into an electrical impulse makes these receptors essential for a great variety of physiological processes. In vertebrates, the Cys-loop receptor family includes the cation-selective channels, nicotinic acetylcholine and 5-hydroxytryptamine type 3 receptors, and the anion-selective channels, GABAA and glycine receptors, whereas in invertebrates, the repertoire is significantly larger. The free-living nematode Caenorhabditis elegans has the largest known Cys-loop receptor family as well as unique receptors that are absent in vertebrates and constitute attractive targets for anthelmintic drugs. Given the large number and variety of Cys-loop receptor subunits and the multiple possible ways of subunit assembly, C. elegans offers a large diversity of receptors although only a limited number of them have been characterized to date. C. elegans has emerged as a powerful model for the study of the nervous system and human diseases as well as a model for antiparasitic drug discovery. This nematode has also shown promise in the pharmaceutical industry search for new therapeutic compounds. C. elegans is therefore a powerful model organism to explore the biology and pharmacology of Cys-loop receptors and their potential as targets for novel therapeutic interventions. In this review, we provide a comprehensive overview of what is known about the function of C. elegans Cys-loop receptors from an electrophysiological perspective.

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.

Multisite regulation integrates multimodal context in sensory circuits to control persistent behavioral states in C. elegans

Maintaining or shifting between behavioral states according to context is essential for animals to implement fitness-promoting strategies. How the integration of internal state, past experience and sensory inputs orchestrates persistent multidimensional behavioral changes remains poorly understood. Here, we show that C. elegans integrates environmental temperature and food availability over different timescales to engage in persistent dwelling, scanning, global or glocal search strategies matching thermoregulatory and feeding needs. Transition between states, in each case, involves regulating multiple processes including AFD or FLP tonic sensory neurons activity, neuropeptide expression and downstream circuit responsiveness. State-specific FLP-6 or FLP-5 neuropeptide signaling acts on a distributed set of inhibitory GPCR(s) to promote scanning or glocal search, respectively, bypassing dopamine and glutamate-dependent behavioral state control. Integration of multimodal context via multisite regulation in sensory circuits might represent a conserved regulatory logic for a flexible prioritization on the valence of multiple inputs when operating persistent behavioral state transitions.

Contrasting dose response relationships of neuroactive antidepressants on the behavior of C. elegans

Antidepressant prescriptions are on a rise worldwide and this increases the concerns for the impacts of these pharmaceuticals on nontarget organisms. Antidepressants are neuroactive compounds that can affect organism's behavior. Behavior is a sensitive endpoint that may also propagate effects at a population level. Another interesting aspect of antidepressants is that they have shown to induce non-monotonic dose-response (NMDR) curves. While such NMDR relationships may have clear implications for the environmental risk, the resolution of current studies is often too coarse to be able to detect relevant NMDR. Therefore, the current study was performed into the behavioral effects (activity, feeding and chemotaxis) in Caenorhabditis elegans as the model organism of the selective serotonin reuptake inhibitors fluoxetine and sertraline and the acetylcholinesterase inhibiting pesticide chlorpyrifos, using a wide range of concentrations (ng/l to mg/l). In order to statistically examine the non-monotonicity, nonlinear regression models were applied to the results. The results showed a triphasic dose-response relationship for activity and chemotaxis after exposure to fluoxetine, but not to sertraline or chlorpyrifos. Effects of fluoxetine already occurred at low concentrations in the range of ng/l while sertraline only showed effects at concentrations in the μg/l range, similar to chlorpyrifos. The different responses between fluoxetine and sertraline, both SSRIs, indicate that response patterns may not always be extrapolated from chemicals with the same primary mode of action. The effects of fluoxetine at low concentrations, in a non-monotonic manner, confirm the relevance of examining such responses at low concentrations.

Neurophysiological basis of stress-induced aversive memory in the nematode Caenorhabditis elegans

Physiological stress induces aversive memory formation and profoundly impacts animal behavior. In C. elegans, concurrent mitochondrial disruption induces aversion to the bacteria that the animal inherently prefers, offering an experimental paradigm for studying the neural basis of aversive memory. We find that, under mitochondrial stress, octopamine secreted from the RIC modulatory neuron targets the AIY interneuron through the SER-6 receptor to trigger learned bacterial aversion. RIC responds to systemic mitochondrial stress by increasing octopamine synthesis and acts in the formation of aversive memory. AIY integrates sensory information, acts downstream of RIC, and is important for the retrieval of aversive memory. Systemic mitochondrial dysfunction induces RIC responses to bacterial cues that parallel stress induction, suggesting that physiological stress activates latent communication between RIC and the sensory neurons. These findings provide insights into the circuit and neuromodulatory mechanisms underlying stress-induced aversive memory.

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.

The emergence and influence of internal states

Animal behavior is shaped by a variety of "internal states"-partially hidden variables that profoundly shape perception, cognition, and action. The neural basis of internal states, such as fear, arousal, hunger, motivation, aggression, and many others, is a prominent focus of research efforts across animal phyla. Internal states can be inferred from changes in behavior, physiology, and neural dynamics and are characterized by properties such as pleiotropy, persistence, scalability, generalizability, and valence. To date, it remains unclear how internal states and their properties are generated by nervous systems. Here, we review recent progress, which has been driven by advances in behavioral quantification, cellular manipulations, and neural population recordings. We synthesize research implicating defined subsets of state-inducing cell types, widespread changes in neural activity, and neuromodulation in the formation and updating of internal states. In addition to highlighting the significance of these findings, our review advocates for new approaches to clarify the underpinnings of internal brain states across the animal kingdom.

Serotonin and dopamine modulate aging in response to food odor and availability

An organism's ability to perceive and respond to changes in its environment is crucial for its health and survival. Here we reveal how the most well-studied longevity intervention, dietary restriction, acts in-part through a cell non-autonomous signaling pathway that is inhibited by the presence of attractive smells. Using an intestinal reporter for a key gene induced by dietary restriction but suppressed by attractive smells, we identify three compounds that block food odor effects in C. elegans, thereby increasing longevity as dietary restriction mimetics. These compounds clearly implicate serotonin and dopamine in limiting lifespan in response to food odor. We further identify a chemosensory neuron that likely perceives food odor, an enteric neuron that signals through the serotonin receptor 5-HT1A/SER-4, and a dopaminergic neuron that signals through the dopamine receptor DRD2/DOP-3. Aspects of this pathway are conserved in D. melanogaster. Thus, blocking food odor signaling through antagonism of serotonin or dopamine receptors is a plausible approach to mimic the benefits of dietary restriction.

Tyraminergic corollary discharge filters reafferent perception in a chemosensory neuron

Interpreting sensory information requires its integration with the current behavior of the animal. However, how motor-related circuits influence sensory information processing is incompletely understood. Here, we report that current locomotor state directly modulates the activity of BAG CO₂ sensory neurons in Caenorhabditis elegans. By recording neuronal activity in animals freely navigating CO₂ landscapes, we found that during reverse crawling states, BAG activity is suppressed by tyraminergic corollary discharge signaling. We provide genetic evidence that tyramine released from the RIM reversal interneurons extrasynaptically activates the inhibitory chloride channel LGC-55 in BAG. Disrupting this pathway genetically leads to excessive behavioral responses to CO₂ stimuli. Moreover, we find that LGC-55 signaling cancels out perception of self-produced CO₂ and O₂ stimuli when animals reverse into their own gas plume in ethologically relevant aqueous environments. Our results show that sensorimotor integration involves corollary discharge signals directly modulating chemosensory neurons.

Dynamic functional connectivity in the static connectome of Caenorhabditis elegans

A hallmark of adaptive behavior is the ability to flexibly respond to sensory cues. To understand how neural circuits implement this flexibility, it is critical to resolve how a static anatomical connectome can be modulated such that functional connectivity in the network can be dynamically regulated. Here, we review recent work in the roundworm Caenorhabditis elegans on this topic. EM studies have mapped anatomical connectomes of many C. elegans animals, highlighting the level of stereotypy in the anatomical network. Brain-wide calcium imaging and studies of specified neural circuits have uncovered striking flexibility in the functional coupling of neurons. The coupling between neurons is controlled by neuromodulators that act over long timescales. This gives rise to persistent behavioral states that animals switch between, allowing them to generate adaptive behavioral responses across environmental conditions. Thus, the dynamic coupling of neurons enables multiple behavioral states to be encoded in a physically stereotyped connectome.

An Imaging System for Monitoring C. elegans Behavior and Aging

Many experiments in C. elegans neurobiology rely on imaging its behavior. Here we describe procedures for building a flexible and inexpensive imaging system using standard optical and mechanical components.

Five Breakthroughs: A First Approximation of Brain Evolution From Early Bilaterians to Humans

Retracing the evolutionary steps by which human brains evolved can offer insights into the underlying mechanisms of human brain function as well as the phylogenetic origin of various features of human behavior. To this end, this article presents a model for interpreting the physical and behavioral modifications throughout major milestones in human brain evolution. This model introduces the concept of a "breakthrough" as a useful tool for interpreting suites of brain modifications and the various adaptive behaviors these modifications enabled. This offers a unique view into the ordered steps by which human brains evolved and suggests several unique hypotheses on the mechanisms of human brain function.

Acetylcholine signaling genes are required for cocaine-stimulated egg laying in Caenorhabditis elegans

The toxicity and addictive liability associated with cocaine abuse are well-known. However, its mode of action is not completely understood, and effective pharmacotherapeutic interventions remain elusive. The cholinergic effects of cocaine on acetylcholine receptors, synthetic enzymes, and degradative enzymes have been the focus of relatively little empirical investigation. Due to its genetic tractability and anatomical simplicity, the egg laying circuit of the hermaphroditic nematode, Caenorhabditis elegans, is a powerful model system to precisely examine the genetic and molecular targets of cocaine in vivo. Here, we report a novel cocaine-induced behavioral phenotype in C. elegans, cocaine-stimulated egg laying. In addition, we present the results of an in vivo candidate suppression screen of synthetic enzymes, receptors, degradative enzymes, and downstream components of the intracellular signaling cascades of the main neurotransmitter systems that control C. elegans egg laying. Our results show that cocaine-stimulated egg laying is dependent on acetylcholine synthesis and synaptic release, functional nicotinic acetylcholine receptors, and the C. elegans acetylcholinesterases.

Inferring temporal organization of postembryonic development from high-content behavioral tracking

Understanding temporal regulation of development remains an important challenge. Whereas average, species-typical timing of many developmental processes has been established, less is known about inter-individual variability and correlations in timing of specific events. We addressed these questions in the context of postembryonic development in Caenorhabditis elegans. Based on patterns of locomotor activity of freely moving animals, we inferred durations of four larval stages (L1-L4) in over 100 individuals. Analysis of these data supports several conclusions. Individuals have consistently faster or slower rates of development because durations of L1 through L3 stages are positively correlated. The last larval stage, the L4, is less variable than the earlier stages and its duration is largely independent of the rate of early larval development, implying existence of two distinct larval epochs. We describe characteristic patterns of variation and correlation, as well as the fact that stage durations tend to scale relative to total developmental time. This scaling relationship suggests that each larval stage is not limited by an absolute duration, but is instead terminated when a subset of events that must occur prior to adulthood have been completed. The approach described here offers a scalable platform that will facilitate the study of temporal regulation of postembryonic development.

Nonlinear Control in the Nematode C. elegans

Recent whole-brain calcium imaging recordings of the nematode C. elegans have demonstrated that the neural activity associated with behavior is dominated by dynamics on a low-dimensional manifold that can be clustered according to behavioral states. Previous models of C. elegans dynamics have either been linear models, which cannot support the existence of multiple fixed points in the system, or Markov-switching models, which do not describe how control signals in C. elegans neural dynamics can produce switches between stable states. It remains unclear how a network of neurons can produce fast and slow timescale dynamics that control transitions between stable states in a single model. We propose a global, nonlinear control model which is minimally parameterized and captures the state transitions described by Markov-switching models with a single dynamical system. The model is fit by reproducing the timeseries of the dominant PCA mode in the calcium imaging data. Long and short time-scale changes in transition statistics can be characterized via changes in a single parameter in the control model. Some of these macro-scale transitions have experimental correlates to single neuro-modulators that seem to act as biological controls, allowing this model to generate testable hypotheses about the effect of these neuro-modulators on the global dynamics. The theory provides an elegant characterization of control in the neuron population dynamics in C. elegans. Moreover, the mathematical structure of the nonlinear control framework provides a paradigm that can be generalized to more complex systems with an arbitrary number of behavioral states.

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.

Behavioral States

Caenorhabditis elegans' behavioral states, like those of other animals, are shaped by its immediate environment, its past experiences, and by internal factors. We here review the literature on C. elegans behavioral states and their regulation. We discuss dwelling and roaming, local and global search, mate finding, sleep, and the interaction between internal metabolic states and behavior.

Neuromodulators: an essential part of survival

The coordination between the animal's external environment and internal state requires constant modulation by chemicals known as neuromodulators. Neuromodulators, such as biogenic amines, neuropeptides and cytokines, promote organismal homeostasis. Over the past several decades, Caenorhabditiselegans has grown into a powerful model organism that allows the elucidation of the mechanisms of action of neuromodulators that are conserved across species. In this perspective, we highlight a collection of articles in this issue that describe how neuromodulators optimize C. elegans survival.

The control of metabolic traits by octopamine and tyramine in invertebrates

Octopamine (OA) and tyramine (TA) are closely related biogenic monoamines that act as signalling compounds in invertebrates, where they fulfil the roles played by adrenaline and noradrenaline in vertebrates. Just like adrenaline and noradrenaline, OA and TA are extremely pleiotropic substances that regulate a wide variety of processes, including metabolic pathways. However, the role of OA and TA in metabolism has been largely neglected. The principal aim of this Review is to discuss the roles of OA and TA in the control of metabolic processes in invertebrate species. OA and TA regulate essential aspects of invertebrate energy homeostasis by having substantial effects on both energy uptake and energy expenditure. These two monoamines regulate several different factors, such as metabolic rate, physical activity, feeding rate or food choice that have a considerable influence on effective energy intake and all the principal contributors to energy consumption. Thereby, OA and TA regulate both metabolic rate and physical activity. These effects should not be seen as isolated actions of these neuroactive compounds but as part of a comprehensive regulatory system that allows the organism to switch from one physiological state to another.

A microfluidic-induced C. elegans sleep state

An important feature of animal behavior is the ability to switch rapidly between activity states, however, how the brain regulates these spontaneous transitions based on the animal's perceived environment is not well understood. Here we show a C. elegans sleep-like state on a scalable platform that enables simultaneous control of multiple environmental factors including temperature, mechanical stress, and food availability. This brief quiescent state, which we refer to as microfluidic-induced sleep, occurs spontaneously in microfluidic chambers, which allows us to track animal movement and perform whole-brain imaging. With these capabilities, we establish that microfluidic-induced sleep meets the behavioral requirements of C. elegans sleep and depends on multiple factors, such as satiety and temperature. Additionally, we show that C. elegans sleep can be induced through mechanosensory pathways. Together, these results establish a model system for studying how animals process multiple sensory pathways to regulate behavioral states.

Co-option of neurotransmitter signaling for inter-organismal communication in C. elegans

Biogenic amine neurotransmitters play a central role in metazoan biology, and both their chemical structures and cognate receptors are evolutionarily conserved. Their primary roles are in cell-to-cell signaling, as biogenic amines are not normally recruited for communication between separate individuals. Here, we show that in the nematode C. elegans, a neurotransmitter-sensing G protein-coupled receptor, TYRA-2, is required for avoidance responses to osas#9, an ascaroside pheromone that incorporates the neurotransmitter, octopamine. Neuronal ablation, cell-specific genetic rescue, and calcium imaging show that tyra-2 expression in the nociceptive neuron, ASH, is necessary and sufficient to induce osas#9 avoidance. Ectopic expression in the AWA neuron, which is generally associated with attractive responses, reverses the response to osas#9, resulting in attraction instead of avoidance behavior, confirming that TYRA-2 partakes in the sensing of osas#9. The TYRA-2/osas#9 signaling system represents an inter-organismal communication channel that evolved via co-option of a neurotransmitter and its cognate receptor.

Digging deeper: methodologies for high-content phenotyping in Caenorhabditis elegans

Deep phenotyping is an emerging conceptual paradigm and experimental approach aimed at measuring and linking many aspects of a phenotype to understand its underlying biology. To date, deep phenotyping has been applied mostly in cultured cells and used less in multicellular organisms. However, in the past decade, it has increasingly been recognized that deep phenotyping could lead to a better understanding of how genetics, environment and stochasticity affect the development, physiology and behavior of an organism. The nematode Caenorhabditis elegans is an invaluable model system for studying how genes affect a phenotypic trait, and new technologies have taken advantage of the worm's physical attributes to increase the throughput and informational content of experiments. Coupling of these technical advancements with computational and analytical tools has enabled a boom in deep-phenotyping studies of C. elegans. In this Review, we highlight how these new technologies and tools are digging into the biological origins of complex, multidimensional phenotypes.

Sexually Dimorphic Regulation of Behavioral States by Dopamine in Caenorhabditis elegans

Sex differences in behavior allow animals to effectively mate and reproduce. However, the mechanism by which biological sex regulates behavioral states, which underlie the regulation of sex-shared behaviors, such as locomotion, is largely unknown. In this study, we studied sex differences in the behavioral states of Caenorhabditis elegans and found that males spend less time in a low locomotor activity state than hermaphrodites and that dopamine generates this sex difference. In males, dopamine reduces the low activity state by acting in the same pathway as polycystic kidney disease-related genes that function in male-specific neurons. In hermaphrodites, dopamine increases the low activity state by suppression of octopamine signaling in the sex-shared SIA neurons, which have reduced responsiveness to octopamine in males. Furthermore, dopamine promotes exploration both inside and outside of bacterial lawn (the food source) in males and suppresses it in hermaphrodites. These results demonstrate that sexually dimorphic signaling allows the same neuromodulator to promote adaptive behavior for each sex.SIGNIFICANCE STATEMENT The mechanisms that generate sex differences in sex-shared behaviors, including locomotion, are not well understood. We show that there are sex differences in the regulation of behavioral states in the model animal Caenorhabditis elegans Dopamine promotes the high locomotor activity state in males, which must search for mates to reproduce, and suppresses it in self-fertilizing hermaphrodites through distinct molecular mechanisms. This study demonstrates that sex-specific signaling generates sex differences in the regulation of behavioral states, which in turn modulates the locomotor activity to suit reproduction for each sex.

Quantitative imaging of sleep behavior in Caenorhabditis elegans and larval Drosophila melanogaster

Sleep is nearly universal among animals, yet remains poorly understood. Recent work has leveraged simple model organisms, such as Caenorhabditis elegans and Drosophila melanogaster larvae, to investigate the genetic and neural bases of sleep. However, manual methods of recording sleep behavior in these systems are labor intensive and low in throughput. To address these limitations, we developed methods for quantitative imaging of individual animals cultivated in custom microfabricated multiwell substrates, and used them to elucidate molecular mechanisms underlying sleep. Here, we describe the steps necessary to design, produce, and image these plates, as well as analyze the resulting behavioral data. We also describe approaches for experimentally manipulating sleep. Following these procedures, after ~2 h of experimental preparation, we are able to simultaneously image 24 C. elegans from the second larval stage to adult stages or 20 Drosophila larvae during the second instar life stage at a spatial resolution of 10 or 27 µm, respectively. Although this system has been optimized to measure activity and quiescence in Caenorhabditis larvae and adults and in Drosophila larvae, it can also be used to assess other behaviors over short or long periods. Moreover, with minor modifications, it can be adapted for the behavioral monitoring of a wide range of small animals.

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.

Clues to basis of exploratory behaviour of the C. elegans snout from head somatotropy

Wave propagation during locomotory movements of Caenorhabditis elegans is constrained to a single dorso/ventral plane. By contrast, the tip of the head (snout) can make rapid exploratory movements in all directions relative to the body axis. These extra degrees of freedom are probably important for animals to seek and identify desirable passages in the interstices of the three-dimensional matrix of soil particles, their usual habitat. The differences in degrees of freedom of movement between snout and body are reflected in the innervation of the musculature. Along the length of the body, the two quadrants of dorsal muscle receive common innervation as do the two quadrants of ventral muscle. By contrast, muscles in the snout have an octagonal arrangement of innervation. It is likely that the exploratory behaviour of the snout is mediated by octant-specific motor and sensory neurons, together with their associated interneurons. The well-defined anatomical structure and neural circuitry of the snout together with behavioural observations should facilitate the implementation of models of the neural basis of exploratory movements, which could lead to an understanding of the basis of this relatively complex behaviour, a behaviour that has similarities to foraging in some vertebrates.This article is part of a discussion meeting issue 'Connectome to behaviour: modelling C. elegans at cellular resolution'.

Past experience shapes sexually dimorphic neuronal wiring through monoaminergic signalling

Differences between female and male brains exist across the animal kingdom and extend from molecular to anatomical features. Here we show that sexually dimorphic anatomy, gene expression and function in the nervous system can be modulated by past experiences. In the nematode Caenorhabditis elegans, sexual differentiation entails the sex-specific pruning of synaptic connections between neurons that are shared by both sexes, giving rise to sexually dimorphic circuits in adult animals1. We discovered that starvation during juvenile stages is memorized in males to suppress the emergence of sexually dimorphic synaptic connectivity. These circuit changes result in increased chemosensory responsiveness in adult males following juvenile starvation. We find that an octopamine-mediated starvation signal dampens the production of serotonin (5-HT) to convey the memory of starvation. Serotonin production is monitored by a 5-HT1A serotonin receptor homologue that acts cell-autonomously to promote the pruning of sexually dimorphic synaptic connectivity under well-fed conditions. Our studies demonstrate how life history shapes neurotransmitter production, synaptic connectivity and behavioural output in a sexually dimorphic circuit.

Comparison of brain monoamine content in three populations of Lymnaea that correlates with taste-aversive learning ability

To find a causal mechanism of learning and memory is a heuristically important topic in neuroscience. In the pond snail Lymnaea stagnalis, the following experimental facts have accrued regarding a classical conditioning procedure known as conditioned taste aversion (CTA): (1) one-day food-deprived Dutch snails have superior CTA memory formation; (2) the one-day food-deprived snails have a low monoamine content (e.g., serotonin, dopamine, octopamine) in their central nervous system (CNS); (3) fed or five-day food-deprived snails have poorer CTA memory and a higher monoamine content; (4) the Dutch snails form better CTA memory than the Canadian TC1 strain; and, (5) the F₁ cross snails between the Dutch and Canadian TC1 strains also form poor CTA memory. Here, in one-day food-deprived snails, we measured the monoamine content in the CNSs of the 3 populations. In most instances, the monoamine content of the Dutch strain was lower than in the other two populations. The F₁ cross snails had the highest monoamine content. A lower monoamine content is correlated with the better CTA memory formation.

Mass spectrometric evidence for neuropeptide-amidating enzymes in

Neuropeptides constitute a vast and functionally diverse family of neurochemical signaling molecules and are widely involved in the regulation of various physiological processes. The nematode Caenorhabditis elegans is well-suited for the study of neuropeptide biochemistry and function, as neuropeptide biosynthesis enzymes are not essential for C. elegans viability. This permits the study of neuropeptide biosynthesis in mutants lacking certain neuropeptide-processing enzymes. Mass spectrometry has been used to study the effects of proprotein convertase and carboxypeptidase mutations on proteolytic processing of neuropeptide precursors and on the peptidome in C. elegans However, the enzymes required for the last step in the production of many bioactive peptides, the carboxyl-terminal amidation reaction, have not been characterized in this manner. Here, we describe three genes that encode homologs of neuropeptide amidation enzymes in C. elegans and used tandem LC-MS to compare neuropeptides in WT animals with those in newly generated mutants for these putative amidation enzymes. We report that mutants lacking both a functional peptidylglycine α-hydroxylating monooxygenase and a peptidylglycine α-amidating monooxygenase had a severely altered neuropeptide profile and also a decreased number of offspring. Interestingly, single mutants of the amidation enzymes still expressed some fully processed amidated neuropeptides, indicating the existence of a redundant amidation mechanism in C. elegans All MS data are available via ProteomeXchange with the identifier PXD008942. In summary, the key steps in neuropeptide processing in C. elegans seem to be executed by redundant enzymes, and loss of these enzymes severely affects brood size, supporting the need of amidated peptides for C. elegans reproduction.

An automated method for the analysis of food intake behaviour in Caenorhabditis elegans

The study of mechanisms that govern feeding behaviour and its related disorders is a matter of global health interest. The roundworm Caenorhabditis elegans is becoming a model organism of choice to study these conserved pathways. C. elegans feeding depends on the contraction of the pharynx (pumping). Thanks to the worm transparency, pumping can be directly observed under a stereoscope. Therefore, C. elegans feeding has been historically investigated by counting pharyngeal pumping or by other indirect approaches. However, those methods are short-term, time-consuming and unsuitable for independent measurements of sizable numbers of individuals. Although some particular devices and long-term methods have been lately reported, they fail in the automated, scalable and/or continuous aspects. Here we present an automated bioluminescence-based method for the analysis and continuous monitoring of worm feeding in a multi-well format. We validate the method using genetic, environmental and pharmacological modulators of pharyngeal pumping. This flexible methodology allows studying food intake at specific time-points or during longer periods of time, in single worms or in populations at any developmental stage. Additionally, changes in feeding rates in response to differential metabolic status or external environmental cues can be monitored in real time, allowing accurate kinetic measurements.