Spontaneous Behaviors and Wall-Curvature Lead to Apparent Wall Preference in Planarian

Adaptive robustness through incoherent signaling mechanisms in a regenerative brain

Animal behavior emerges from collective dynamics of neurons, making it vulnerable to damage. Paradoxically, many organisms exhibit a remarkable ability to maintain significant behavior even after large-scale neural injury. Molecular underpinnings of this extreme robustness remain largely unknown. Here, we develop a quantitative pipeline to measure long-lasting latent states in planarian flatworm behaviors during whole-brain regeneration. By combining >20,000 animal trials with neural network modeling, we show that long-range volumetric peptidergic signals allow the planarian to rapidly restore coarse behavior output after large perturbations to the nervous system, while slow restoration of small-molecule neuromodulator functions refines precision. This relies on the different time and length scales of neuropeptide and small-molecule transmission to generate incoherent patterns of neural activity that competitively regulate behavior. Controlling behavior through opposing communication mechanisms creates a more robust system than either alone and may serve as a generalizable approach for constructing robust neural networks.

Optimization and calibration of behavioural tests on different species of planaria for ecotoxicological studies

Freshwater planarian are emerging as a valuable in vivo model for (eco) toxicological studies, but the lack of harmonization of procedures between laboratories remains a challenge. This study aimed to optimize, automate and select the best behavioural tests and analyse the potential of different planarian species as models for toxicological assessment. We implemented four tests: exploration, photomotor response, Tapping and Planarian Light Dark Test, on different planaria species using the DanioVision system. We conclude that the exploration assay performed in 24 wellplate at 10 000 lux is the one that is robust and reliable for toxicological studies with planaria. Dugesia japonica and Schmidtea mediterranea have proved to be sensitive models for toxicological screening of organophosphorus pesticides through behavioural analysis. Under necessary experimental conditions, the motility baseline in controls, for both species allowed the detection of behavioural changes, making both good models for behavioural testing in (eco) toxicological context.

Adaptive robustness through incoherent signaling mechanisms in a regenerative brain

Animal behavior emerges from collective dynamics of interconnected neurons, making it vulnerable to connectome damage. Paradoxically, many organisms maintain significant behavioral output after large-scale neural injury. Molecular underpinnings of this extreme robustness remain largely unknown. Here, we develop a quantitative behavioral analysis pipeline to measure previously uncharacterized long-lasting latent memory states in planarian flatworms during whole-brain regeneration. By combining >20,000 animal trials with neural population dynamic modeling, we show that long-range volumetric peptidergic signals allow the planarian to rapidly reestablish latent states and restore coarse behavior after large structural perturbations to the nervous system, while small-molecule neuromodulators gradually refine the precision. The different time and length scales of neuropeptide and small-molecule transmission generate incoherent patterns of neural activity which competitively regulate behavior and memory. Controlling behavior through opposing communication mechanisms creates a more robust system than either alone and may serve as a generic approach to construct robust neural networks.

Differences in neurotoxic outcomes of organophosphorus pesticides revealed via multi-dimensional screening in adult and regenerating planarians

Organophosphorus pesticides (OPs) are a chemically diverse class of commonly used insecticides. Epidemiological studies suggest that low dose chronic prenatal and infant exposures can lead to life-long neurological damage and behavioral disorders. While inhibition of acetylcholinesterase (AChE) is the shared mechanism of acute OP neurotoxicity, OP-induced developmental neurotoxicity (DNT) can occur independently and/or in the absence of significant AChE inhibition, implying that OPs affect alternative targets. Moreover, different OPs can cause different adverse outcomes, suggesting that different OPs act through different mechanisms. These findings emphasize the importance of comparative studies of OP toxicity. Freshwater planarians are an invertebrate system that uniquely allows for automated, rapid and inexpensive testing of adult and developing organisms in parallel to differentiate neurotoxicity from DNT. Effects found only in regenerating planarians would be indicative of DNT, whereas shared effects may represent neurotoxicity. We leverage this unique feature of planarians to investigate potential differential effects of OPs on the adult and developing brain by performing a comparative screen to test 7 OPs (acephate, chlorpyrifos, dichlorvos, diazinon, malathion, parathion and profenofos) across 10 concentrations in quarter-log steps. Neurotoxicity was evaluated using a wide range of quantitative morphological and behavioral readouts. AChE activity was measured using an Ellman assay. The toxicological profiles of the 7 OPs differed across the OPs and between adult and regenerating planarians. Toxicological profiles were not correlated with levels of AChE inhibition. Twenty-two "mechanistic control compounds" known to target pathways suggested in the literature to be affected by OPs (cholinergic neurotransmission, serotonin neurotransmission, endocannabinoid system, cytoskeleton, adenyl cyclase and oxidative stress) and 2 negative controls were also screened. When compared with the mechanistic control compounds, the phenotypic profiles of the different OPs separated into distinct clusters. The phenotypic profiles of adult vs. regenerating planarians exposed to the OPs clustered differently, suggesting some developmental-specific mechanisms. These results further support findings in other systems that OPs cause different adverse outcomes in the (developing) brain and build the foundation for future comparative studies focused on delineating the mechanisms of OP neurotoxicity in planarians.

High-Throughput Phototactic Ecotoxicity Biotests with Nauplii of Artemia franciscana

Analysis of sensorimotor behavioral responses to stimuli such as light can provide an enhanced relevance during rapid prioritisation of chemical risk. Due to technical limitations, there have been, however, only minimal studies on using invertebrate phototactic behaviors in aquatic ecotoxicity testing. In this work, we demonstrate an innovative, purpose-built analytical system for a high-throughput phototactic biotest with nauplii of euryhaline brine shrimp Artemia franciscana. We also, for the first time, present a novel and dedicated bioinformatic approach that facilitates high-throughput analysis of phototactic behaviors at scale with great fidelity. The nauplii exhibited consistent light-seeking behaviors upon extinguishing a brief programmable light stimulus (5500K, 400 lux) without habituation. A proof-of-concept validation involving the short-term exposure of eggs (24 h) and instar I larval stages (6 h) to sub-lethal concentrations of insecticides organophosphate chlorpyrifos (10 µg/L) and neonicotinoid imidacloprid (50 µg/L) showed perturbation in light seeking behaviors in the absence of or minimal alteration in general mobility. Our preliminary data further support the notion that phototactic bioassays can represent an attractive new avenue in behavioral ecotoxicology because of their potential sensitivity, responsiveness, and low cost.

Head removal enhances planarian electrotaxis

Certain animal species utilize electric fields for communication, hunting and spatial orientation. Freshwater planarians move toward the cathode in a static electric field (cathodic electrotaxis). This planarian behavior was first described by Raymond Pearl more than a century ago. However, planarian electrotaxis has received little attention since, and the underlying mechanisms and evolutionary significance remain unknown. To close this knowledge gap, we developed an apparatus and scoring metrics for automated quantitative and mechanistic studies of planarian behavior upon exposure to a static electric field. Using this automated setup, we characterized electrotaxis in the planarian Dugesia japonica and found that this species responds to voltage instead of current, in contrast to results from previous studies using other planarian species. Surprisingly, we found differences in electrotaxis ability between small (shorter) and large (longer) planarians. To determine the cause of these differences, we took advantage of the regenerative abilities of planarians and compared electrotaxis in head, tail and trunk fragments of various lengths. We found that tail and trunk fragments electrotaxed, whereas head fragments did not, regardless of size. Based on these data, we hypothesized that signals from the head may interfere with electrotaxis when the head area/body area reached a critical threshold. In support of this hypothesis, we found that (1) smaller intact planarians that cannot electrotax have a relatively larger head-to-body-ratio than large planarians that can electrotax, and (2) the electrotaxis behavior of cut head fragments was negatively correlated with the head-to-body ratio of the fragments. Moreover, we could restore cathodic electrotaxis in head fragments via decapitation, directly demonstrating inhibition of electrotaxis by the head.

Summary: A new method for quantitative studies of planarian electrotaxis shows that Dugesia japonica move toward the cathode. This behavior is enhanced by removal of the head.

Impact of test chamber design on spontaneous behavioral responses of model crustacean zooplankton Artemia franciscana

The use of small aquatic model organisms to investigate the behavioral effects of chemical exposure is becoming an integral component of aquatic ecotoxicology research and neuroactive drug discovery. Despite the increasing use of invertebrates for behavioral phenotyping in toxicological studies and chemical risk assessments, little is known regarding the potential for environmental factors-such as geometry, size, opacity and depth of test chambers-to modulate common behavioral responses. In this work, we demonstrate that test chamber geometry, size, opacity and depth can affect spontaneous, unstimulated behavioral responses of euryhaline crustacean Artemia franciscana first instar larval stages. We found that in the absence of any obvious directional cues, A. franciscana exhibited a strong innate wall preference behavior. Using different test chamber sizes and geometries, we found both increased wall preference and lowered overall distance traveled by the test shrimp in a smaller chamber with sharper-angled vertices. It was also determined through quantifiable changes in the chambers' color that the A. franciscana early larval stages can perceive, differentiate and react to differences in color or perhaps rather to light transmittance of the test chambers. The interaction between innate edge preference and positive phototaxis could be consistently altered with a novel photic stimulus system. We also observed a strong initial preference for depth in A. franciscana first instar larval stages, which diminished through the acclimatization. We postulate that the impact of test chamber designs on neurobehavioral baseline responses warrants further investigation, in particular considering the increased interest in behavioral eco-neurotoxicology applications.

Quantification of planarian behaviors

Research on individual behaviors can help to reveal the processes and mechanisms that mediate an animal's habits and interactions with the environment. Importantly, individual behaviors arise as outcomes of genetic programs, morphogenesis, physiological processes, and neural functions; thus, behavioral analyses can be used to detect disorders in these processes. Planarians belong to an early branching bilateral group of organisms that possess a simple central nervous system. Furthermore, planarians display various behavioral responses to the environment via their nervous system. Planarians also have remarkable regenerative abilities, including whole-brain regeneration. Therefore, the combination of planarians' phylogenetic position, behavioral properties, regenerative ability, and genetic accessibility provides a unique opportunity to understand the basic mechanisms underlying the anatomical properties of neural morphogenesis and the dynamic physiological processes and neural function. Here, we describe a step-by-step protocol for conducting simple behavioral analyses in planarians with the aim of helping to introduce researchers to the utility of performing behavioral analyses in planarians. Since the conditions of planarians impact experimental results and reproducibility, this protocol begins with a method for maintaining planarians. Next, we introduce the behavioral tests as well as the methods for quantifying them using minimal and cost-effective equipment and materials. Finally, we present a unique RNAi technique that enables conditional silencing of neural activity in the brain of planarians.

Regeneration of Planarian Auricles and Reestablishment of Chemotactic Ability

Detection of chemical stimuli is crucial for living systems and also contributes to quality of life in humans. Since loss of olfaction becomes more prevalent with aging, longer life expectancies have fueled interest in understanding the molecular mechanisms behind the development and maintenance of chemical sensing. Planarian flatworms possess an unsurpassed ability for stem cell-driven regeneration that allows them to restore any damaged or removed part of their bodies. This includes anteriorly-positioned lateral flaps known as auricles, which have long been thought to play a central role in chemotaxis. The contribution of auricles to the detection of positive chemical stimuli was tested in this study using Girardia dorotocephala, a North American planarian species known for its morphologically prominent auricles. Behavioral experiments staged under laboratory conditions revealed that removal of auricles by amputation leads to a significant decrease in the ability of planarians to find food. However, full chemotactic capacity is observed as early as 2 days post-amputation, which is days prior from restoration of auricle morphology, but correlative with accumulation of ciliated cells in the position of auricle regeneration. Planarians subjected to x-ray irradiation prior to auricle amputation were unable to restore auricle morphology, but were still able to restore chemotactic capacity. These results indicate that although regeneration of auricle morphology requires stem cells, some restoration of chemotactic ability can still be achieved in the absence of normal auricle morphology, corroborating with the initial observation that chemotactic success is reestablished 2-days post-amputation in our assays. Transcriptome profiles of excised auricles were obtained to facilitate molecular characterization of these structures, as well as the identification of genes that contribute to chemotaxis and auricle development. A significant overlap was found between genes with preferential expression in auricles of G. dorotocephala and genes with reduced expression upon SoxB1 knockdown in Schmidtea mediterranea, suggesting that SoxB1 has a conserved role in regulating auricle development and function. Models that distinguish between possible contributions to chemotactic behavior obtained from cellular composition, as compared to anatomical morphology of the auricles, are discussed.

Applications of advanced neuro-behavioral analysis strategies in aquatic ecotoxicology

Despite mounting evidence of pleiotropic ecological risks, the understanding of the eco-neurotoxic impact of most industrially relevant chemicals is still very limited. In particularly the acute and chronic exposures to industrial pollutants on nervous systems and thus potential alterations in ecological fitness remain profoundly understudied. Since the behavioral phenotype is the highest-level and functional manifestation of integrated neurological functions, the alterations in neuro-behavioral traits have been postulated as very sensitive and physiologically integrative endpoints to assess eco-neurotoxicological risks associated with industrial pollutants. Due to a considerable backlog of risk assessments of existing and new production chemicals there is a need for a paradigm shift from high cost, low throughput ecotoxicity test models to next generation systems amenable to higher throughput. In this review we concentrate on emerging aspects of laboratory-based neuro-behavioral phenotyping approaches that can be amenable for rapid prioritizing pipelines. We outline the importance of development and applications of innovative neuro-behavioral assays utilizing small aquatic biological indicators and demonstrate emerging concepts of high-throughput chemo-behavioral phenotyping. We also discuss new analytical approaches to effectively and rapidly evaluate the impact of pollutants on higher behavioral functions such as sensory-motor assays, decision-making and cognitive behaviors using innovative model organisms. Finally, we provide a snapshot of most recent analytical approaches that can be applied to elucidate mechanistic rationale that underlie the observed neuro-behavioral alterations upon exposure to pollutants. This review is intended to outline the emerging opportunities for innovative multidisciplinary research and highlight the existing challenges as well barriers to future development.

Calcium ions in the aquatic environment drive planarians to food

BACKGROUND

Even subtle changes in environmental factors can exert behavioral effects on creatures, which may alter interspecific interactions and eventually affect the ecosystem. However, how changes in environmental factors impact complex behaviors regulated by neural processes is largely unknown. The freshwater planarian Dugesia japonica, a free-living flatworm, displays distinct behavioral traits mediated by sensitive perception of environmental cues. Planarians are thus useful organisms for examining interactions between environmental changes and specific behaviors of animals.

RESULTS

Here we found that feeding behavior was suppressed when the concentration of ions in the breeding water was low, while other behaviors were unaffected, resulting in differences in population size. Notably, the decline in feeding behavior was reversed in an ion-concentration-dependent manner soon after the planarians were moved to ion-containing water, which suggests that ions in environmental water rapidly promote feeding behavior in planarians. Moreover, the concentration of ions in the environmental water affected the feeding behavior by modulating the sensitivity of the response to foods. Finally, we found that calcium ions in the aquatic environment were required for the feeding behavior, and exposure to higher levels of calcium ions enhanced the feeding behavior, showing that there was a good correlation between the concentration of calcium ions and the responsiveness of planarians to foods.

CONCLUSIONS

Environmental calcium ions are indispensable for and potentiate the activity level of the feeding behavior of planarians. Our findings suggest that the ions in the aquatic environment profoundly impact the growth and survival of aquatic animals via modulating their neural activities and behaviors.

Behavioral Research with Planaria

This article serves as a brief primer on planaria for behavior scientists. In the 1950s and 1960s, McConnell's planarian laboratory posited that conditioned behavior could transfer after regeneration, and through cannibalization of trained planaria. These studies, the responses, and replications have been collectively referred to as the "planarian controversy." Successful behavioral assays still require refinement with this organism, but they could add valuable insight into our conceptualization of memory and learning. We discuss how the planarian's distinctive biology enables an examination of biobehavioral interaction models, and what behavior scientists must consider if they are to advance behavioral research with this organism. Suggestions for academics interested in building planaria learning laboratories are offered.

Coordination between binocular field and spontaneous self-motion specifies the efficiency of planarians' photo-response orientation behavior

Eyes show remarkable diversity in morphology among creatures. However, little is known about how morphological traits of eyes affect behaviors. Here, we investigate the mechanisms responsible for the establishment of efficient photo-response orientation behavior using the planarian Dugesia japonica as a model. Our behavioral assays reveal the functional angle of the visual field and show that the binocular field formed by paired eyes in D. japonica has an impact on the accurate recognition of the direction of a light source. Furthermore, we find that the binocular field in coordination with spontaneous wigwag self-motion of the head specifies the efficiency of photo-responsive evasive behavior in planarians. Our findings suggest that the linkage between the architecture of the sensory organs and spontaneous self-motion is a platform that serves for efficient and adaptive outcomes of planarian and potentially other animal behaviors.

Yoshitaro Akiyama et al. report the use of innovative behavioral assays in planarian flatworms to investigate the mechanism by which they efficiently respond to light. They find that binocular vision and spontaneous self-motion are key factors for accurately detecting the direction of a light source.

Effects of ethanol on negative phototaxis and motility in brown planarians (Dugesia tigrina)

The behavioral effects of ethanol in brown planarians were studied in four experiments. In the first two experiments, acute administrations of ethanol increased travel time in a dose-dependent fashion in individual planarians moving away from a light source. Orderly results were obtained using both within-subject and between-group designs. In a third experiment, ethanol dose was arranged by time rather than concentration. Ethanol increased travel time overall, but variability between subjects was considerable. In a final experiment, ethanol administration reduced motility and altered movement patterns in planarians in an open-field test. These experiments demonstrated that negative phototaxis by planarians may exhibit sufficient stability to allow for experimental determinations of dose-response curves utilizing both within-subject and between-subject designs.