A two-neuron system for adaptive goal-directed decision-making in Lymnaea

Functional mapping of the molluscan brain guided by synchrotron X-ray tomography

Molluscan brains are composed of morphologically consistent and functionally interrogable neurons, offering rich opportunities for understanding how neural circuits drive behavior. Nonetheless, detailed component-level CNS maps are often lacking, total neuron numbers are unknown, and organizational principles remain poorly defined, limiting a full and systematic characterization of circuit operation. Here, we establish an accessible, generalizable approach, harnessing synchrotron X-ray tomography, to rapidly determine the three-dimensional structure of the multimillimeter-scale CNS of Lymnaea. Focusing on the feeding ganglia, we generate a full neuron-level reconstruction, revealing key design principles and revising cell count estimates upward threefold. Our atlas uncovers the superficial but also nonsuperficial ganglionic architecture, reveals the cell organization in normally hidden regions-ganglionic "dark sides"-and details features of single-neuron morphology, together guiding targeted follow-up functional investigation based on intracellular recordings. Using this approach, we identify three pivotal neuron classes: a command-like food-signaling cell type, a feeding central pattern generator interneuron, and a unique behavior-specific motoneuron, together significantly advancing understanding of the function of this classical control circuit. Combining our morphological and electrophysiological data, we also establish a functional CNS atlas in Lymnaea as a shared and scalable resource for the research community. Our approach enables the rapid construction of cell atlases in large-scale nervous systems, with key relevance to functional circuit interrogation in a diverse range of model organisms.

Division of labor for defensive retaliation and preemption by the peripheral and central nervous systems in the nudibranch Berghia

Relatively little is known about how peripheral nervous systems (PNSs) contribute to the patterning of behavior in which their role transcends the simple execution of central motor commands or mediation of reflexes. We sought to draw inferences to this end in the aeolid nudibranch Berghia stephanieae, which generates a rapid, dramatic defense behavior, "bristling." This behavior involves the coordinated movement of cerata, dozens of venomous appendages emerging from the animal's mantle. Our investigations revealed that bristling constitutes a stereotyped but non-reflexive two-stage behavior: an initial adduction of proximate cerata to sting the offending stimulus (stage 1) followed by a coordinated radial extension of remaining cerata to create a pincushion-like defensive screen around the animal (stage 2). In decerebrated specimens, stage 1 bristling was preserved, while stage 2 bristling was replaced by slower, uncoordinated ceratal movements. We conclude from these observations that, first, the animal's PNS and central nervous system (CNS) mediate stages 1 and 2 of bristling, respectively; second, the behavior propagates through the body utilizing both peripheral- and central-origin nerve networks that support different signaling kinetics; and third, the former network inhibits the latter in the body region being stimulated. These findings extend our understanding of the PNS' computational capacity and provide insight into a neuroethological scheme in which the CNS and PNS both independently and interactively pattern different aspects of non-reflexive behavior.

Neural division of labor: the gastropod Berghia defends against attack using its PNS to retaliate and its CNS to erect a defensive screen

Relatively little is known about how the peripheral nervous system (PNS) contributes to the patterning of behavior, in which its role transcends the simple execution of central motor commands or mediation of reflexes. We sought to draw inferences to this end in the aeolid nudibranch Berghia stephanieae, which generates a rapid, dramatic defense behavior, "bristling." This behavior involves the coordinated movement of cerata, dozens of venomous appendages emerging from the animal's mantle. Our investigations revealed that bristling constitutes a stereotyped but non-reflexive two-stage behavior: an initial adduction of proximate cerata to sting the offending stimulus (Stage 1), followed by a coordinated radial extension of remaining cerata to create a pincushion-like defensive screen around the animal (Stage 2). In decerebrated specimens, Stage 1 bristling was preserved, while Stage 2 bristling was replaced by slower, uncoordinated, and ultimately maladaptive ceratal movements. We conclude from these observations that 1) the PNS and central nervous system (CNS) mediate Stages 1 and 2 of bristling, respectively; 2) the behavior propagates through the body utilizing both peripheral- and central-origin nerve networks that support different signaling kinetics; and 3) the former network inhibits the latter in the body region being stimulated. These findings extend our understanding of the PNS's computational capacity and provide insight into a neuroethological scheme that may generalize across cephalized animals, in which the CNS and PNS both independently and interactively pattern different aspects of non-reflexive behavior.

A circuit mechanism linking past and future learning through shifts in perception

Long-term memory formation is energetically costly. Neural mechanisms that guide an animal to identify fruitful associations therefore have important survival benefits. Here, we elucidate a circuit mechanism in Lymnaea, which enables past memory to shape new memory formation through changes in perception. Specifically, strong classical conditioning drives a positive shift in perception that facilitates the robust learning of a subsequent and otherwise ineffective weak association. Circuit dissection approaches reveal the neural control network responsible, characterized by a mutual inhibition motif. This both sets perceptual state and acts as the master controller for gating new learning. Pharmacological circuit manipulation in vivo fully substitutes for strong paradigm learning, shifting the network into a more receptive state to enable subsequent weak paradigm learning. Thus, perceptual change provides a conduit to link past and future memory storage. We propose that this mechanism alerts animals to learning-rich periods, lowering the threshold for new memory acquisition.

A combined bioinformatics and LC-MS based approach for the development and benchmarking of a comprehensive database of Lymnaea CNS proteins

Applications of key technologies in biomedical research, such as qRT-PCR or LC-MS based proteomics, are generating large biological (-omics) data sets which are useful for the identification and quantification of biomarkers in any research area of interest. Genome, transcriptome and proteome databases are already available for a number of model organisms including vertebrates and invertebrates. However, there is insufficient information available for protein sequences of certain invertebrates, such as the great pond snail Lymnaea stagnalis, a model organism that has been used highly successfully in elucidating evolutionarily conserved mechanisms of memory function and dysfunction. Here we used a bioinformatics approach to designing and benchmarking a comprehensive CNS proteomics database (LymCNS-PDB) for the identification of proteins from the CNS of Lymnaea by LC-MS based proteomics. LymCNS-PDB was created by using the Trinity TransDecoder bioinformatics tool to translate amino acid sequences from mRNA transcript assemblies obtained from a published Lymnaea transcriptomics database. The blast-style MMSeq2 software was used to match all translated sequences to UniProtKB sequences for molluscan proteins, including Lymnaea and other molluscs. LymCNS-PDB contains 9,628 identified matched proteins that were benchmarked by performing LC-MS based proteomics analysis with proteins isolated from the Lymnaea CNS. MS/MS analysis using the LymCNS-PDB database led to the identification of 3,810 proteins. Only 982 proteins were identified by using a non-specific molluscan database. LymCNS-PDB provides a valuable tool that will enable us to perform quantitative proteomics analysis of protein interactomes involved in several CNS functions in Lymnaea, including learning and memory and age-related memory decline.

Secondary rewards acquire enhanced incentive motivation via increasing anticipatory activity of the lateral orbitofrontal cortex

The motivation to strive for and consume primary rewards such as palatable food is bound by devaluation mechanisms, yet secondary rewards such as money may not be bound by these regulatory mechanisms. The present study therefore aimed at determining diverging devaluation trajectories for primary (chocolate milk) and secondary (money) reinforcers on the behavioral and neural level. Devaluation procedures with repeated exposure to reward combined with a choice (Experiment 1) and an incentive delay (Experiment 2) paradigm consistently revealed decreasing hedonic value for the primary reward as reflected by decreasing hedonic evaluation and choice preference with repeated receipt, while hedonic value and preferences for the secondary reward increased. Concomitantly acquired functional near-infrared spectroscopy (fNIRS) data during the incentive delay paradigm revealed that increasing value of the secondary reward was accompanied by increasing anticipatory activation in the lateral orbitofrontal cortex, while during the consummatory phase the secondary reinforcer associated with higher medial orbitofrontal activity irrespective of devaluation stage. Overall, the findings suggest that-in contrast to primary reinforcers-secondary reinforcers, i.e. money, can acquire progressively enhanced incentive motivation with repeated receipt, suggesting a mechanism which could promote escalating striving to obtain secondary rewards.

To eat or not to eat: a Garcia effect in pond snails (Lymnaea stagnalis)

Taste aversion learning is universal. In animals, a single presentation of a novel food substance followed hours later by visceral illness causes animals to avoid that taste. This is known as bait-shyness or the Garcia effect. Humans demonstrate this by avoiding a certain food following the development of nausea after ingesting that food ('Sauce Bearnaise effect'). Here, we show that the pond snail Lymnaea stagnalis is capable of the Garcia effect. A single 'pairing' of a novel taste, a carrot slurry followed hours later by a heat shock stressor (HS) is sufficient to suppress feeding response elicited by carrot for at least 24 h. Other food tastes are not suppressed. If snails had previously been exposed to carrot as their food source, the Garcia-like effect does not occur when carrot is 'paired' with the HS. The HS up-regulates two heat shock proteins (HSPs), HSP70 and HSP40. Blocking the up-regulation of the HSPs by a flavonoid, quercetin, before the heat shock, prevented the Garcia effect in the snails. Finally, we found that snails exhibit Garcia effect following a period of food deprivation but the long-term memory (LTM) phenotype can be observed only if the animals are tested in a food satiated state.

Report on the First Symposium on Invertebrate Neuroscience held on 13-17th August 2019 at the Balaton Limnological Institute, MTA Centre for Ecological Research, Tihany, Hungary

This meeting report provides an overview of the oral and poster presentations at the first international symposium for invertebrate neuroscience. The contents reflect the contributions of invertebrate neuroscience in addressing fundamental and fascinating challenges in understanding the neural substrates of animal behaviour.

Proactive and retroactive interference with associative memory consolidation in the snail Lymnaea is time and circuit dependent

Interference-based forgetting occurs when new information acquired either before or after a learning event attenuates memory expression (proactive and retroactive interference, respectively). Multiple learning events often occur in rapid succession, leading to competition between consolidating memories. However, it is unknown what factors determine which memory is remembered or forgotten. Here, we challenge the snail, Lymnaea, to acquire two consecutive similar or different memories and identify learning-induced changes in neurons of its well-characterized motor circuits. We show that when new learning takes place during a stable period of the original memory, proactive interference only occurs if the two consolidating memories engage the same circuit mechanisms. If different circuits are used, both memories survive. However, any new learning during a labile period of consolidation promotes retroactive interference and the acquisition of the new memory. Therefore, the effect of interference depends both on the timing of new learning and the underlying neuronal mechanisms.

The importance of identified neurons in gastropod molluscs to neuroscience

Gastropod molluscs have large neurons that are uniquely identifiable across individuals and across species based on neuroanatomical and neurochemical criteria, facilitating research into neural signaling and neural circuits. Novel neuropeptides have been identified through RNA sequencing and mass spectroscopic analysis of single neurons. The roles of peptides and other signaling molecules including second messengers have been placed in the context of small circuits that control simple behaviors. Despite the stereotypy, neurons vary over time in their activity in large ensembles. Furthermore, there is both intra-species and inter-species variation in synaptic properties and gene expression. Research on gastropod identified neurons highlights the features that might be expected to be stable in more complex systems when trying to identify cell types.

A central control circuit for encoding perceived food value

Hunger state can substantially alter the perceived value of a stimulus, even to the extent that the same sensory cue can trigger antagonistic behaviors. How the nervous system uses these graded perceptual shifts to select between opposed motor patterns remains enigmatic. Here, we challenged food-deprived and satiated Lymnaea to choose between two mutually exclusive behaviors, ingestion or egestion, produced by the same feeding central pattern generator. Decoding the underlying neural circuit reveals that the activity of central dopaminergic interneurons defines hunger state and drives network reconfiguration, biasing satiated animals toward the rejection of stimuli deemed palatable by food-deprived ones. By blocking the action of these neurons, satiated animals can be reconfigured to exhibit a hungry animal phenotype. This centralized mechanism occurs in the complete absence of sensory retuning and generalizes across different sensory modalities, allowing food-deprived animals to increase their perception of food value in a stimulus-independent manner to maximize potential calorific intake.

Uncovering the Cellular and Molecular Mechanisms of Synapse Formation and Functional Specificity Using Central Neurons of Lymnaea stagnalis

All functions of the nervous system are contingent upon the precise organization of neuronal connections that are initially patterned during development, and then continually modified throughout life. Determining the mechanisms that specify the formation and functional modulation of synaptic circuitry are critical to advancing both our fundamental understanding of the nervous system as well as the various neurodevelopmental, neurological, neuropsychiatric, and neurodegenerative disorders that are met in clinical practice when these processes go awry. Defining the cellular and molecular mechanisms underlying nervous system development, function, and pathology has proven challenging, due mainly to the complexity of the vertebrate brain. Simple model system approaches with invertebrate preparations, on the other hand, have played pivotal roles in elucidating the fundamental mechanisms underlying the formation and plasticity of individual synapses, and the contributions of individual neurons and their synaptic connections that underlie a variety of behaviors, and learning and memory. In this Review, we discuss the experimental utility of the invertebrate mollusc Lymnaea stagnalis, with a particular emphasis on in vitro cell culture, semi-intact and in vivo preparations, which enable molecular and electrophysiological identification of the cellular and molecular mechanisms governing the formation, plasticity, and specificity of individual synapses at a single-neuron or single-synapse resolution.

Considering aspects of the 3Rs principles within experimental animal biology

The 3Rs - Replacement, Reduction and Refinement - are embedded into the legislation and guidelines governing the ethics of animal use in experiments. Here, we consider the advantages of adopting key aspects of the 3Rs into experimental biology, represented mainly by the fields of animal behaviour, neurobiology, physiology, toxicology and biomechanics. Replacing protected animals with less sentient forms or species, cells, tissues or computer modelling approaches has been broadly successful. However, many studies investigate specific models that exhibit a particular adaptation, or a species that is a target for conservation, such that their replacement is inappropriate. Regardless of the species used, refining procedures to ensure the health and well-being of animals prior to and during experiments is crucial for the integrity of the results and legitimacy of the science. Although the concepts of health and welfare are developed for model organisms, relatively little is known regarding non-traditional species that may be more ecologically relevant. Studies should reduce the number of experimental animals by employing the minimum suitable sample size. This is often calculated using power analyses, which is associated with making statistical inferences based on the P-value, yet P-values often leave scientists on shaky ground. We endorse focusing on effect sizes accompanied by confidence intervals as a more appropriate means of interpreting data; in turn, sample size could be calculated based on effect size precision. Ultimately, the appropriate employment of the 3Rs principles in experimental biology empowers scientists in justifying their research, and results in higher-quality science.

A novel in vitro analog expressing learning-induced cellular correlates in distinct neural circuits

When presented with noxious stimuli, Aplysia exhibits concurrent sensitization of defensive responses, such as the tail-induced siphon withdrawal reflex (TSWR) and suppression of feeding. At the cellular level, sensitization of the TSWR is accompanied by an increase in the excitability of the tail sensory neurons (TSNs) that elicit the reflex, whereas feeding suppression is accompanied by decreased excitability of B51, a decision-making neuron in the feeding neural circuit. The goal of this study was to develop an in vitro analog coexpressing the above cellular correlates. We used a reduced preparation consisting of buccal, cerebral, and pleural-pedal ganglia, which contain the neural circuits controlling feeding and the TSWR, respectively. Sensitizing stimuli were delivered in vitro by electrical stimulation of afferent nerves. When trained with sensitizing stimuli, the in vitro analog expressed concomitant increased excitability in TSNs and decreased excitability in B51, which are consistent with the occurrence of sensitization and feeding suppression induced by in vivo training. This in vitro analog expressed both short-term (15 min) and long-term (24 h) excitability changes in TSNs and B51, depending on the amount of training administered. Finally, in vitro application of serotonin increased TSN excitability without altering B51 excitability, mirroring the in vivo application of the monoamine that induces sensitization, but not feeding suppression.

Use of the Aplysia feeding network to study repetition priming of an episodic behavior

Many central pattern generator (CPG)-mediated behaviors are episodic, meaning that they are not continuously ongoing; instead, there are pauses between bouts of activity. This raises an interesting possibility, that the neural networks that mediate these behaviors are not operating under "steady-state" conditions; i.e., there could be dynamic changes in motor activity as it stops and starts. Research in the feeding system of the mollusk Aplysia californica has demonstrated that this can be the case. After a pause, initial food grasping responses are relatively weak. With repetition, however, responses strengthen. In this review we describe experiments that have characterized cellular/molecular mechanisms that produce these changes in motor activity. In particular, we focus on cumulative effects of modulatory neuropeptides. Furthermore, we relate Aplysia research to work in other systems and species, and develop a hypothesis that postulates that changes in response magnitude are a reflection of an efficient feeding strategy.

Reproductive and metabolic state differences in olfactory responses to amino acids in a mouth brooding African cichlid fish

Olfaction mediates many crucial life-history behaviors such as prey detection, predator avoidance, migration and reproduction. Olfactory function can also be modulated by an animal's internal physiological and metabolic states. While this is relatively well studied in mammals, little is known about how internal state impacts olfaction in fishes, the largest and most diverse group of vertebrates. Here we apply electro-olfactograms (EOGs) in the African cichlid fish Astatotilapia burtoni to test the hypothesis that olfactory responses to food-related cues (i.e. l-amino acids; alanine and arginine) vary with metabolic, social and reproductive state. Dominant males (reproductively active, reduced feeding) had greater EOG magnitudes in response to amino acids at the same tested concentration than subordinate males (reproductively suppressed, greater feeding and growth rates). Mouth brooding females, which are in a period of starvation while they brood fry in their mouths, had greater EOG magnitudes in response to amino acids at the same tested concentration than both recovering and gravid females that are feeding. Discriminant function analysis on EOG magnitudes also grouped the male (subordinate) and female (recovering, gravid) phenotypes with higher food intake together and distinguished them from brooding females and dominant males. The slope of the initial negative phase of the EOG also showed intra-sexual differences in both sexes. Our results demonstrate that the relationship between olfaction and metabolic state observed in other taxa is conserved to fishes. For the first time, we provide evidence for intra-sexual plasticity in the olfactory response to amino acids that is influenced by fish reproductive, social and metabolic state.

The foundations of plant intelligence

Intelligence is defined for wild plants and its role in fitness identified. Intelligent behaviour exhibited by single cells and systems similarity between the interactome and connectome indicates neural systems are not necessary for intelligent capabilities. Plants sense and respond to many environmental signals that are assessed to competitively optimize acquisition of patchily distributed resources. Situations of choice engender motivational states in goal-directed plant behaviour; consequent intelligent decisions enable efficient gain of energy over expenditure. Comparison of swarm intelligence and plant behaviour indicates the origins of plant intelligence lie in complex communication and is exemplified by cambial control of branch function. Error correction in behaviours indicates both awareness and intention as does the ability to count to five. Volatile organic compounds are used as signals in numerous plant interactions. Being complex in composition and often species and individual specific, they may represent the plant language and account for self and alien recognition between individual plants. Game theory has been used to understand competitive and cooperative interactions between plants and microbes. Some unexpected cooperative behaviour between individuals and potential aliens has emerged. Behaviour profiting from experience, another simple definition of intelligence, requires both learning and memory and is indicated in the priming of herbivory, disease and abiotic stresses.

The foundations of plant intelligence

Intelligence is defined for wild plants and its role in fitness identified. Intelligent behaviour exhibited by single cells and systems similarity between the interactome and connectome indicates neural systems are not necessary for intelligent capabilities. Plants sense and respond to many environmental signals that are assessed to competitively optimize acquisition of patchily distributed resources. Situations of choice engender motivational states in goal-directed plant behaviour; consequent intelligent decisions enable efficient gain of energy over expenditure. Comparison of swarm intelligence and plant behaviour indicates the origins of plant intelligence lie in complex communication and is exemplified by cambial control of branch function. Error correction in behaviours indicates both awareness and intention as does the ability to count to five. Volatile organic compounds are used as signals in numerous plant interactions. Being complex in composition and often species and individual specific, they may represent the plant language and account for self and alien recognition between individual plants. Game theory has been used to understand competitive and cooperative interactions between plants and microbes. Some unexpected cooperative behaviour between individuals and potential aliens has emerged. Behaviour profiting from experience, another simple definition of intelligence, requires both learning and memory and is indicated in the priming of herbivory, disease and abiotic stresses.

Satiation level affects anti-predatory decisions in foraging juvenile crayfish

Moving shadows signify imminent threat to foraging juvenile crayfish, and the animals respond with one of two discrete anti-predatory behaviors: They either freeze in place or rapidly flex their tails, which quickly propels them away from the approaching danger signal. Although a freeze might be the more risky choice, it keeps the animal near the expected food reward, while a tail-flip is effective in avoiding the shadow, but puts critical distance between the animal and its next meal. We manipulated the satiation level of juvenile crayfish to determine whether their behavioral choices are affected by internal energy states. When facing the same visual danger signal, animals fed to satiation produced more tail-flips and fewer freezes than unfed animals, indicating that intrinsic physiological conditions shape value-based behavioral decisions. Escape tail-flip latencies, however, were unaffected by satiation level, and an increase in food quality only produced a minor behavioral shift toward more freezing in both fed and unfed animals. Thus, satiation level appears to be the dominant factor in regulating decision making and behavioral choices of crayfish.

Impairment of the serotonergic neurons underlying reinforcement elicits extinction of the repeatedly reactivated context memory

We analyzed changes in the activity of individually identifiable neurons involved in the networks underlying feeding and withdrawal behaviors in snails before, during, and after aversive learning in vitro. Responses to food in the “reinforcing” serotonergic neurons involved in withdrawal changed significantly after training, implying that these serotonergic cells participate in the reactivation of memory and are involved in the reconsolidation process. In behavioral experiments it was shown that impairment of the functioning of the serotonergic system with the selective neurotoxin 5,7-DiHT did not change the memory, when tested once, but resulted in a complete extinction of the contextual memory after repeated reactivation of memory. Conversely, the cued memory to a specific type of food was significantly reduced but still present. Thus, we conclude that it is only for the context memory, that participation of the “reinforcing” serotonergic neurons in memory retrieval may be the gate condition for the choice between extinction/reconsolidation.

Previous motor activity affects transition from uncertainty to decision-making in snails

One of the most commonly accepted benefits of enhanced physical activity is the improvement in the symptoms of depression, including the facilitation of decision-making. Up until now, these effects have been shown in rodents and humans only. Little is known about their evolutionary origin or biological basis, and the underlying cellular mechanisms also remain relatively elusive. Here, we demonstrate for the first time that preceding motor activity accelerates decision-making in an invertebrate, the pond snail Lymnaea stagnalis. To investigate decision-making in a novel environment, snails, which normally live in water, were placed on a flat dry surface to simulate the potentially threatening consequence of being in an arid environment. This stimulus initiated two distinct phases in snail behavior: slow circular movements, followed by intense locomotion in a chosen direction. The first phase was prolonged when the test arena was symmetrically lit, compared to one with an apparent gradient of light. However, forced muscular locomotion for two hours prior to the test promoted the transition from random circular motions to a directional crawl, accompanied by an increase in crawling speed but with no effect on the choice of direction. Two hours of intense locomotion produced also strong excitatory effect on the activity of serotonergic neurons in L. stagnalis. Our results suggest that the beneficial effects of physical exercise on cognitive performance in mammals might have deep roots in evolution, granting the opportunity to unravel the origins of such effects at the single neuron and network levels.