Configural learning memory can be transformed from intermediate-term to long-term in pond snail Lymnaea stagnalis

Focus on cognitive impairment induced by excessive fluoride: An update review

Fluorosis is a global public health concern. Prolonged exposure to excessive fluoride causes fluoride accumulation in the hippocampus, resulting in cognitive dysfunction. Cell death is necessary for maintaining tissue function and morphology, and changes in the external morphology of nerve cells and the function of many internal organelles are typical features of cell death; however, it is also a typical feature of cognitive impairment caused by fluorosis. However, the pathogenesis of cognitive impairment caused by different degrees of fluoride exposure varies. Herein, we provide an overview of cognitive impairment caused by excessive fluoride exposure in different age groups, and the underlying mechanisms for cognitive impairment in various model organisms. The mechanisms underlying these impairments include oxidative stress, synaptic and neurotransmission dysfunction, disruption of mitochondrial and energy metabolism, and calcium channel dysregulation. This study aims to provide potential insights that serve as a reference for subsequent research on the cognitive function caused by excessive fluoride.

How to reduce fear in a snail: Take an aspirin, call me in the morning

Aspirin (Acetylsalicylic acid, ASA), one of the widely used non-steroid anti-inflammatory drugs can easily end up in sewage effluents and thus it becomes necessary to investigate the effects of aspirin on behaviour of aquatic organisms. Previous studies in mammals have shown ASA to alter fear and anxiety-like behaviours. In the great pond snail Lymnaea stagnalis, ASA has been shown to block a 'sickness state' induced by lipopolysaccharide injection which upregulates immune and stress-related genes thus altering behavioural responses. In Lymnaea, eliciting physiological stress may enhance memory formation or block its retrieval depending on the stimulus type and intensity. Here we examine whether ASA will alter two forms of associative-learning memory in crayfish predator-experienced Lymnaea when ASA exposure accompanies predator-cue-induced stress during the learning procedure. The two trainings procedures are: 1) operant conditioning of aerial respiration; and 2) a higher form of learning, called configural learning, which here is dependent on evoking a fear response. We show here that ASA alone does not alter homeostatic aerial respiration, feeding behaviour or long-term memory (LTM) formation of operantly conditioned aerial respiration. However, ASA blocked the enhancement of LTM formation normally elicited by training snails in predator cue. ASA also blocked configural learning, which makes use of the fear response elicited by the predator cue. Thus, ASA alters how Lymnaea responds cognitively to predator detection.

Long-term anti-predator learning and memory differ across populations and sexes in an intertidal snail

Anti-predator behaviours in response to predator cues can be innate, or they can be learned through prior experience and remembered over time. The duration and strength of continued anti-predator behaviour after predator cues are no longer present, and the potential for an enhanced response when re-exposed to predator cues later is less known but could account for the observed variation in anti-predator responses. We measured the carryover effects of past predation exposure and the potential for anti-predator learning and memory in the marine snail Nucella canaliculata from six populations distributed over 1000 km of coastline. We exposed lab-reared snails to cues associated with a common crab predator or seawater control in two serial experiments separated by over seven months. Responses were population- and sex-dependent, with some populations retaining anti-predator behaviours while others showed a capacity for learning and memory. Male snails showed a strong carryover of risk aversion, while females were able to return to normal feeding rates and grow more quickly. These behavioural differences culminated in strong impacts on feeding and growth rates, demonstrating that this variation has implications for the strength of trait-mediated indirect interactions, which can impact entire ecosystems.

Different stressors uniquely affect the expression of endocannabinoid-metabolizing enzymes in the central ring ganglia of Lymnaea stagnalis

The endocannabinoid system (ECS) plays an important role in neuroprotection, neuroplasticity, energy balance, modulation of stress, and inflammatory responses, acting as a critical link between the brain and the body's peripheral regions, while also offering promising potential for novel therapeutic strategies. Unfortunately, in humans, pharmacological inhibitors of different ECS enzymes have led to mixed results in both preclinical and clinical studies. As the ECS has been highly conserved throughout the eukaryotic lineage, the use of invertebrate model organisms like the pond snail Lymnaea stagnalis may provide a flexible tool to unravel unexplored functions of the ECS at the cellular, synaptic, and behavioral levels. In this study, starting from the available genome and transcriptome of L. stagnalis, we first identified putative transcripts of all ECS enzymes containing an open reading frame. Each predicted protein possessed a high degree of sequence conservation to known orthologues of other invertebrate and vertebrate organisms. Sequences were confirmed by qualitative PCR and sequencing. Then, we investigated the transcriptional effects induced by different stress conditions (i.e., bacterial LPS injection, predator scent, food deprivation, and acute heat shock) on the expression levels of the enzymes of the ECS in Lymnaea's central ring ganglia. Our results suggest that in Lymnaea as in rodents, the ECS is involved in mediating inflammatory and anxiety-like responses, promoting energy balance, and responding to acute stressors. To our knowledge, this study offers the most comprehensive analysis so far of the ECS in an invertebrate model organism.

Sensory input from osphradium is involved in fluoride detection that alters feeding and memory phenotype in Lymnaea stagnalis

Fluoride (F-) exposure in organisms remains a significant concern due to its widespread presence and potential health implications. Investigating its detection and subsequent effects on behaviour in aquatic organisms like Lymnaea stagnalis provides valuable insights. Our study focused on elucidating the sensory pathways involved in F- detection and its impact on feeding and memory formation. We explored two potential detection mechanisms: direct flow across the integument onto neurons; and sensory input to the central nervous system (CNS) via the osphradium-osphradial ganglion-osphradial nerve pathway (snails use this system for olfactory sensation of multiple compounds). Injection of F- into snails did not alter feeding behaviour or central neuronal activity, suggesting that internalization might not be the primary detection mode. In contrast, severing the osphradial nerve abolished F-'s suppressive effects on feeding and memory formation, implicating the osphradial pathway in F- sensing and behavioural changes. This finding supports the idea that osphradial nerve signaling mediates the behavioural effects of F-. Our study underscores the importance of sensory pathways in F- detection and behavioural modulation in L. stagnalis. Understanding these mechanisms could provide critical insights into how organisms respond to and adapt to environmental chemical stressors like F-.

The multifaceted effects of flavonoids on neuroplasticity

There has been a significant increase in the incidence of multiple neurodegenerative and terminal diseases in the human population with life expectancy increasing in the current times. This highlights the urgent need for a more comprehensive understanding of how different aspects of lifestyle, in particular diet, may affect neural functioning and consequently cognitive performance as well as in enhancing overall health. Flavonoids, found in a variety of fruits, vegetables, and derived beverages, provide a new avenue of research that shows a promising influence on different aspects of brain function. However, despite the promising evidence, most bioactive compounds lack strong clinical research efficacy. In the current scoping review, we highlight the effects of Flavonoids on cognition and neural plasticity across vertebrates and invertebrates with special emphasis on the studies conducted in the pond snail, Lymnaea stagnalis, which has emerged to be a functionally dynamic model for studies on learning and memory. In conclusion, we suggest future research directions and discuss the social, cultural, and ethnic dependencies of bioactive compounds that influence how these compounds are used and accepted globally. Bridging the gap between preclinical and clinical studies about the effects of bioactive natural compounds on brain health will surely lead to lifestyle choices such as dietary Flavonoids being used complementarily rather than as replacements to classical drugs bringing about a healthier future.

What are olfaction and gustation, and do all animals have them?

Different animals have distinctive anatomical and physiological properties to their chemical senses that enhance detection and discrimination of relevant chemical cues. Humans and other vertebrates are recognized as having 2 main chemical senses, olfaction and gustation, distinguished from each other by their evolutionarily conserved neuroanatomical organization. This distinction between olfaction and gustation in vertebrates is not based on the medium in which they live because the most ancestral and numerous vertebrates, the fishes, live in an aquatic habitat and thus both olfaction and gustation occur in water and both can be of high sensitivity. The terms olfaction and gustation have also often been applied to the invertebrates, though not based on homology. Consequently, any similarities between olfaction and gustation in the vertebrates and invertebrates have resulted from convergent adaptations or shared constraints during evolution. The untidiness of assigning olfaction and gustation to invertebrates has led some to recommend abandoning the use of these terms and instead unifying them and others into a single category-chemical sense. In our essay, we compare the nature of the chemical senses of diverse animal types and consider their designation as olfaction, oral gustation, extra-oral gustation, or simply chemoreception. Properties that we have found useful in categorizing chemical senses of vertebrates and invertebrates include the nature of peripheral sensory cells, organization of the neuropil in the processing centers, molecular receptor specificity, and function.

Agricultural Use of Insecticides Alters Homeostatic Behaviors and Cognitive Ability in Lymnaea stagnalis

Lymnaea stagnalis is an ecologically important, stress-sensitive, freshwater mollusk that is at risk for exposure to insecticides via agricultural practices. We provide insight into the impact insecticides have on L. stagnalis by comparing specific behaviors including feeding, locomotion, shell regeneration, and cognition between snails collected at two different sites: one contaminated by insecticides and one not. We hypothesized that each of the behaviors would be altered in the insecticide-exposed snails and that similar alterations would be induced when control snails were exposed to the contaminated environment. We found no significant differences in locomotion, feeding, and shell regeneration of insecticide-exposed L. stagnalis compared with nonexposed individuals. Significant changes in feeding and shell repair were observed in nonexposed snails inhabiting insecticide-contaminated pond water. Most importantly, snails maintained and trained in insecticide-contaminated pond water did not form configural learning, but this cognitive deficit was reversed when these snails were maintained in insecticide-free pond water. Our findings conclude that insecticides have a primarily negative impact on this higher form of cognition in L. stagnalis. Environ Toxicol Chem 2023;42:2466-2477. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Invertebrates as models of learning and memory: investigating neural and molecular mechanisms

In this Commentary, we shed light on the use of invertebrates as model organisms for understanding the causal and conserved mechanisms of learning and memory. We provide a condensed chronicle of the contribution offered by mollusks to the studies on how and where the nervous system encodes and stores memory and describe the rich cognitive capabilities of some insect species, including attention and concept learning. We also discuss the use of planarians for investigating the dynamics of memory during brain regeneration and highlight the role of stressful stimuli in forming memories. Furthermore, we focus on the increasing evidence that invertebrates display some forms of emotions, which provides new opportunities for unveiling the neural and molecular mechanisms underlying the complex interaction between stress, emotions and cognition. In doing so, we highlight experimental challenges and suggest future directions that we expect the field to take in the coming years, particularly regarding what we, as humans, need to know for preventing and/or delaying memory loss. This article has an associated ECR Spotlight interview with Veronica Rivi.

Aspirin reverts lipopolysaccharide-induced learning and memory impairment: first evidence from an invertebrate model system

By employing a reductionistic (but not simplistic) approach using an established invertebrate model system, the pond snail Lymnaea stagnalis, we investigated whether (1) lipopolysaccharide (LPS)-induced inflammation would cause a sickness state and impair cognitive function, and-if so-(2) would aspirin (acetylsalicylic acid-ASA) restore the impaired cognition. To test our hypotheses, we first determined if the injection of 25 mg (6.25 μg/mL) of Escherichia coli-derived LPS serotype O127:B8 altered homeostatic behavior, aerial respiration, and then determined if LPS altered memory formation when this behavior was operantly conditioned. Next, we determined if ASA altered the LPS-induced changes in both aerial respiration and cognitive functions. LPS induced a sickness state that increased aerial respiration and altered the ability of snails to form or recall long-term memory. ASA reverted the LPS-induced sickness state and thus allowed long-term memory both to be formed and recalled. We confirmed our hypotheses and provided the first evidence in an invertebrate model system that an injection of LPS results in a sickness state that obstructs learning and memory, and this impairment can be prevented by a non-steroidal anti-inflammatory.

Fluoride alters feeding and memory in Lymnaea stagnalis

Fluoride occurs naturally in the terrestrial and aquatic environment and is a major component in tea. Prolonged fluoride exposure alters metabolic activity in several aquatic invertebrates. For the first time, we investigated the effects of fluoride on cognition in the pond snail Lymnaea stagnalis as it is capable of a higher form of associative learning called configural learning. We first showed suppressive effects of black tea and fluoride on feeding (i.e., rasping) behavior. We then investigated how fluoride may alter cognition by introducing fluoride (1.86 mg/L) before, during, after, a day before and a week before the snails underwent the configural learning training procedure. Our results show that any 45-min exposure to fluoride (before, during or after a configural learning training procedure) blocks configural learning memory formation in Lymnaea and these effects are long-lasting. One week after a fluoride exposure, snails are still unable to form a configural learning memory and this result is upheld when the snails are exposed to a lower concentration of fluoride, one which is naturally occurring in ponds that a wild strain of Lymnaea can be found (0.3 mg/L). Thus, fluoride obstructs configural learning memory formation in a fluoride-naïve, inbred strain of Lymnaea.

A flavonoid, quercetin, is capable of enhancing long-term memory formation if encountered at different times in the learning, memory formation, and memory recall continuum

A major extrinsic factor influencing memory and neuro-cognitive performances across taxa is diet. Studies from vertebrates have shown the effects of a flavonoid rich diet on cognitive performance, but the mechanism underlying this action is still poorly understood. A common and abundant flavonoid present in numerous food substances is quercetin (Q). The present study provides the first support for Q-modulated enhancement of cognitive function in an invertebrate model, the pond snail Lymnaea stagnalis, after an operant conditioning procedure. We found that when snails were exposed to Q 3 h before or after a single 0.5 h training session, which typically results in memory lasting ~ 3 h, they formed a long-term memory (LTM) lasting for at least 24 h. Additionally, we assessed the effects of the combined presentation of a single reinforcing stimulus (at 24 h post-training or 24 h before training) and Q-exposure on both LTM formation and reconsolidation. That is, when applied within 3 h of critical periods of memory, Q regulates four different phases: (1) acquisition (i.e., a learning event), (2) consolidation processes after acquisition, (3) memory recall, and (4) memory reconsolidation. In all these phases Q-exposure enhanced LTM persistence.