A Memory Circuit for Coping with Impending Adversity

Locomotion modulates olfactory learning through proprioception in C. elegans

Locomotor activities can enhance learning, but the underlying circuit and synaptic mechanisms are largely unknown. Here we show that locomotion facilitates aversive olfactory learning in C. elegans by activating mechanoreceptors in motor neurons, and transmitting the proprioceptive information thus generated to locomotion interneurons through antidromic-rectifying gap junctions. The proprioceptive information serves to regulate experience-dependent activities and functional coupling of interneurons that process olfactory sensory information to produce the learning behavior. Genetic destruction of either the mechanoreceptors in motor neurons, the rectifying gap junctions between the motor neurons and locomotion interneurons, or specific inhibitory synapses among the interneurons impairs the aversive olfactory learning. We have thus uncovered an unexpected role of proprioception in a specific learning behavior as well as the circuit, synaptic, and gene bases for this function.

Inheritance of associative memories and acquired cellular changes in C. elegans

Experiences have been shown to modulate behavior and physiology of future generations in some contexts, but there is limited evidence for inheritance of associative memory in different species. Here, we trained C. elegans nematodes to associate an attractive odorant with stressful starvation conditions and revealed that this associative memory was transmitted to the F1 progeny who showed odor-evoked avoidance behavior. Moreover, the F1 and the F2 descendants of trained animals exhibited odor-evoked cellular stress responses, manifested by the translocation of DAF-16/FOXO to cells' nuclei. Sperm, but not oocytes, transmitted these odor-evoked cellular stress responses which involved H3K9 and H3K36 methylations, the small RNA pathway machinery, and intact neuropeptide secretion. Activation of a single chemosensory neuron sufficed to induce a serotonin-mediated systemic stress response in both the parental trained generation and in its progeny. Moreover, inheritance of the cellular stress responses increased survival chances of the progeny as exposure to the training odorant allowed the animals to prepare in advance for an impending adversity. These findings suggest that in C. elegans associative memories and cellular changes may be transferred across generations.

Principles for coding associative memories in a compact neural network

A major goal in neuroscience is to elucidate the principles by which memories are stored in a neural network. Here, we have systematically studied how four types of associative memories (short- and long-term memories, each as positive and negative associations) are encoded within the compact neural network of Caenorhabditis elegans worms. Interestingly, sensory neurons were primarily involved in coding short-term, but not long-term, memories, and individual sensory neurons could be assigned to coding either the conditioned stimulus or the experience valence (or both). Moreover, when considering the collective activity of the sensory neurons, the specific training experiences could be decoded. Interneurons integrated the modulated sensory inputs and a simple linear combination model identified the experience-specific modulated communication routes. The widely distributed memory suggests that integrated network plasticity, rather than changes to individual neurons, underlies the fine behavioral plasticity. This comprehensive study reveals basic memory-coding principles and highlights the central roles of sensory neurons in memory formation.

Why it hurts: with freedom comes the biological need for pain

We argue that pain is not needed to protect the body from damage unless the organism is able to make free choices in action selection. Then pain (including its affective and evaluative aspects) provides a necessary prioritising motivation to select actions expected to avoid it, whilst leaving the possibility of alternative actions to serve potentially higher priorities. Thus, on adaptive grounds, only organisms having free choice over action selection should experience pain. Free choice implies actions must be selected following appraisal of their effects, requiring a predictive model generating estimates of action outcomes. These features give organisms anticipatory behavioural autonomy (ABA), for which we propose a plausible system using an internal predictive model, integrated into a system able to produce the qualitative and affective aspects of pain. Our hypothesis can be tested using behavioural experiments designed to elicit trade-off responses to novel experiences for which algorithmic (automaton) responses might be inappropriate. We discuss the empirical evidence for our hypothesis among taxonomic groups, showing how testing for ABA guides thinking on which groups might experience pain. It is likely that all vertebrates do and plausible that some invertebrates do (decapods, cephalopods and at least some insects).

The synaptic organization in the Caenorhabditis elegans neural network suggests significant local compartmentalized computations

Neurons are characterized by elaborate tree-like dendritic structures that support local computations by integrating multiple inputs from upstream presynaptic neurons. It is less clear whether simple neurons, consisting of a few or even a single neurite, may perform local computations as well. To address this question, we focused on the compact neural network of Caenorhabditis elegans animals for which the full wiring diagram is available, including the coordinates of individual synapses. We find that the positions of the chemical synapses along the neurites are not randomly distributed nor can they be explained by anatomical constraints. Instead, synapses tend to form clusters, an organization that supports local compartmentalized computations. In mutually synapsing neurons, connections of opposite polarity cluster separately, suggesting that positive and negative feedback dynamics may be implemented in discrete compartmentalized regions along neurites. In triple-neuron circuits, the nonrandom synaptic organization may facilitate local functional roles, such as signal integration and coordinated activation of functionally related downstream neurons. These clustered synaptic topologies emerge as a guiding principle in the network, presumably to facilitate distinct parallel functions along a single neurite, which effectively increase the computational capacity of the neural network.

Inhibition of the expression of rgs-3 alleviates propofol-induced decline in learning and memory in Caenorhabditis elegans

BACKGROUND

Exposure to anesthesia leads to extensive neurodegeneration and long-term cognitive deficits in the developing brain. Caenorhabditis elegans also shows persistent behavioral changes during development after exposure to anesthetics. Clinical and rodent studies have confirmed that altered expression of the regulators of G protein signaling (RGS) in the nervous system is a factor contributing to neurodegenerative and psychological diseases. Evidence from preclinical studies has suggested that RGS controls drug-induced plasticity, including morphine tolerance and addiction. This study aimed to observe the effect of propofol exposure in the neurodevelopmental stage on learning and memory in the L4 stage and to study whether this effect is related to changes in rgs-3 expression.

METHODS

Caenorhabditis elegans were exposed to propofol at the L1 stage, and learning and memory abilities were observed at the L4 stage. The expression of rgs-3 and the nuclear distribution of EGL-4 were determined to study the relevant mechanisms. Finally, RNA interference was performed on rgs-3-expressing cells after propofol exposure. Then, we observed their learning and memory abilities.

RESULTS

Propofol time- and dose-dependently impaired the learning capacity. Propofol induced a decline in non-associative and associative long-term memory, rgs-3 upregulation, and a failure of nuclear accumulation of EGL-4/PKG in AWC neurons. Inhibition of rgs-3 could alleviate the propofol-induced changes.

CONCLUSION

Inhibition of the expression of rgs-3 alleviated propofol-induced learning and memory deficits in Caenorhabditis elegans.

C. elegans as a test system to study relevant compounds that contribute to the specific health-related effects of different cannabis varieties

Background

The medicinal effects of cannabis varieties on the market cannot be explained solely by the presence of the major cannabinoids Δ⁹-tetrahydrocannabinol (THC) and cannabidiol (CBD). Evidence for putative entourage effects caused by other compounds present in cannabis is hard to obtain due to the subjective nature of patient experience data. Caenorhabditis elegans (C. elegans) is an objective test system to identify cannabis compounds involved in claimed health and entourage effects.

Methods

From a medicinal cannabis breeding program by MariPharm BV, the Netherlands a set of 12 varieties were selected both THC rich varieties as well as CBD rich varieties. A consecutive extraction process was applied resulting in a non-polar (cannabinoid-rich) and polar (cannabinoid-poor) extract of each variety. The test model C. elegans was exposed to these extracts in a broad set of bioassays for appetite control, body oscillation, motility, and nervous system function.

Results

Exposing C. elegans to extracts with a high concentration of cannabinoids (> 1 μg/mL) reduces the life span of C. elegans dramatically. Exposing the nematodes to the low-cannabinoid (< 0.005 μg/mL) polar extracts, however, resulted in significant effects with respect to appetite control, body oscillation, motility, and nervous system-related functions in a dose-dependent and variety-dependent manner.

Discussion

C. elegans is a small, transparent organism with a complete nervous system, behavior and is due to its genetic robustness and short life cycle highly suitable to unravel entourage effects of Cannabis compounds. Although C. elegans lacks an obvious CB1 and CB2 receptor it has orthologs of Serotonin and Vanilloid receptor which are also involved in (endo)cannabinoid signaling.

Conclusion

By using C. elegans, we were able to objectively distinguish different effects of different varieties despite the cannabinoid content. C. elegans seems a useful test system for studying entourage effects, for targeted medicinal cannabis breeding programs and product development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42238-022-00162-9.

Transcription Factors That Control Behavior-Lessons From C. elegans

Behavior encompasses the physical and chemical response to external and internal stimuli. Neurons, each with their own specific molecular identities, act in concert to perceive and relay these stimuli to drive behavior. Generating behavioral responses requires neurons that have the correct morphological, synaptic, and molecular identities. Transcription factors drive the specific gene expression patterns that define these identities, controlling almost every phenomenon in a cell from development to homeostasis. Therefore, transcription factors play an important role in generating and regulating behavior. Here, we describe the transcription factors, the pathways they regulate, and the neurons that drive chemosensation, mechanosensation, thermosensation, osmolarity sensing, complex, and sex-specific behaviors in the animal model Caenorhabditis elegans. We also discuss the current limitations in our knowledge, particularly our minimal understanding of how transcription factors contribute to the adaptive behavioral responses that are necessary for organismal survival.

Biomolecular Basis of Cellular Consciousness via Subcellular Nanobrains

Cells emerged at the very beginning of life on Earth and, in fact, are coterminous with life. They are enclosed within an excitable plasma membrane, which defines the outside and inside domains via their specific biophysical properties. Unicellular organisms, such as diverse protists and algae, still live a cellular life. However, fungi, plants, and animals evolved a multicellular existence. Recently, we have developed the cellular basis of consciousness (CBC) model, which proposes that all biological awareness, sentience and consciousness are grounded in general cell biology. Here we discuss the biomolecular structures and processes that allow for and maintain this cellular consciousness from an evolutionary perspective.

Toxic stress-specific cytoprotective responses regulate learned behavioral decisions in C. elegans

BACKGROUND

Recognition of stress and mobilization of adequate "fight-or-flight" responses is key for survival and health. Previous studies have shown that exposure of Caenorhabditis elegans to pathogens or toxins simultaneously stimulates cellular stress and detoxification responses and aversive behavior. However, whether a coordinated regulation exists between cytoprotective stress responses and behavioral defenses remains unclear.

RESULTS

Here, we show that exposure of C. elegans to high concentrations of naturally attractive food-derived odors, benzaldehyde and diacetyl, induces toxicity and food avoidance behavior. Benzaldehyde preconditioning activates systemic cytoprotective stress responses involving DAF-16/FOXO, SKN-1/Nrf2, and Hsp90 in non-neuronal cells, which confer both physiological (increased survival) and behavioral tolerance (reduced food avoidance) to benzaldehyde exposure. Benzaldehyde preconditioning also elicits behavioral cross-tolerance to the structurally similar methyl-salicylate, but not to the structurally unrelated diacetyl. In contrast, diacetyl preconditioning augments diacetyl avoidance, weakens physiological diacetyl tolerance, and does not induce apparent molecular defenses. The inter-tissue connection between cellular and behavioral defenses is mediated by JNK-like stress-activated protein kinases and the neuropeptide Y receptor NPR-1. Reinforcement of the stressful experiences using spaced training forms stable stress-specific memories. Memory retrieval by the olfactory cues leads to avoidance of food contaminated by diacetyl and context-dependent behavioral decision to avoid benzaldehyde only if there is an alternative, food-indicative odor.

CONCLUSIONS

Our study reveals a regulatory link between conserved cytoprotective stress responses and behavioral avoidance, which underlies "fight-or-flight" responses and facilitates self-protection in real and anticipated stresses. These findings imply that variations in the efficiency of physiological protection during past episodes of stress might shape current behavioral decisions.

Learning and Memory: Mind over Matter in C. elegans

The capacity to respond to adverse conditions is key for animal survival. Research in the nematode Caenorhabditis elegans demonstrates that retrieval of aversive memories, stored within sensory neurons, is sufficient to induce a protective systemic stress response that improves fitness.

A cellular defense memory imprinted by early life toxic stress

Stress exposure early in life is implicated in various behavioural and somatic diseases. Experiences during the critical perinatal period form permanent, imprinted memories promoting adult survival. Although imprinting is widely recognized to dictate behaviour, whether it actuates specific transcriptional responses at the cellular level is unknown. Here we report that in response to early life stresses, Caenorhabditis elegans nematodes form an imprinted cellular defense memory. We show that exposing newly-born worms to toxic antimycin A and paraquat, respectively, stimulates the expression of toxin-specific cytoprotective reporters. Toxin exposure also induces avoidance of the toxin-containing bacterial lawn. In contrast, adult worms do not exhibit aversive behaviour towards stress-associated bacterial sensory cues. However, the mere re-encounter with the same cues reactivates the previously induced cytoprotective reporters. Learned adult defenses require memory formation during the L1 larval stage and do not appear to confer increased protection against the toxin. Thus, exposure of C. elegans to toxic stresses in the critical period elicits adaptive behavioural and cytoprotective responses, which do not form imprinted aversive behaviour, but imprint a cytoprotective memory. Our findings identify a novel form of imprinting and suggest that imprinted molecular defenses might underlie various pathophysiological alterations related to early life stress.