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-.

A translational and multidisciplinary approach to studying the Garcia effect, a higher form of learning with deep evolutionary roots

Animals, including humans, learn and remember to avoid a novel food when its ingestion is followed, hours later, by sickness - a phenomenon initially identified during World War II as a potential means of pest control. In the 1960s, John Garcia (for whom the effect is now named) demonstrated that this form of conditioned taste aversion had broader implications, showing that it is a rapid but long-lasting taste-specific food aversion with a fundamental role in the evolution of behaviour. From the mid-1970s onward, the principles of the Garcia effect were translated to humans, showing its role in different clinical conditions (e.g. side-effects linked to chemotherapy). However, in the last two decades, the number of studies on the Garcia effect has undergone a considerable decline. Since its discovery in rodents, this form of learning was thought to be exclusive to mammals; however, we recently provided the first demonstration that a Garcia effect can be formed in an invertebrate model organism, the pond snail Lymnaea stagnalis. Thus, in this Commentary, after reviewing the experiments that led to the first characterization of the Garcia effect in rodents, we describe the recent evidence for the Garcia effect in L. stagnalis, which may pave the way for future studies in other invertebrates and mammals. This article aims to inspire future translational and ecological studies that characterize the conserved mechanisms underlying this form of learning with deep evolutionary roots, which can be used to address a range of different biological questions.

Better together: isolation impedes memory formation for configural learning in Lymnaea stagnalis

Social interactions play an important role in learning and memory. There is great variability in the literature regarding the effects of social isolation on cognition. Here, we investigated how memory formation was affected when Lymnaea stagnalis, our model system, were socially isolated at three different time periods: before, during or after the configural learning training procedure. Each group of snails underwent configural learning where we recorded and compared their feeding behaviour before and after the pairing of an appetitive food stimulus with predator kairomones (i.e. the training procedure). We found that isolating snails before the training procedure had no effect on their learning and memory. However, when snails were isolated either during the training procedure or immediately after the training procedure, they no longer formed memory. These data provide further insight into how isolation impacts cognitive functioning in the context of higher-order learning.

Fluoride alters feeding in lab-bred pond snails but not in wild snails or their progeny

Increasing concentrations of fluoride in natural bodies of water due to anthropogenic activities can lead to potentially detrimental effects on residing species. Here we investigated the differences in fluoride exposure on feeding behaviour between freshly collected (i.e., wild) and lab-bred Lymnaea stagnalis and if developmental exposure plays a key role in fluoride tolerance. We show that wild snails that reside in naturally fluoridated waters and their fluoride naïve lab-reared progeny have a fluoride tolerance that does not suppress feeding when introduced to a fluoridated food stimulus. These results were also seen in our lab-bred snails who were exposed to artificially fluoridated pond water (at similar concentrations to natural levels) throughout development. However, lab-bred snails that have never been exposed to fluoride, or only exposed during the egg stage demonstrated suppression of feeding in the fluoridated food stimulus condition compared to an unfluoridated food stimulus. Genetic diversity and phenotypic plasticity are suspected to be two key underlying mechanisms for fluoride ion tolerance. These results are critical in understanding how parental and developmental exposure can influence a phenotypic tolerance to a potential chemical pollutant.

A change in taste: The role of MicroRNAs in altering hedonic value

The mechanisms associated with neophobia, and anhedonia remain largely unknown. Neuropsychological disorders such as depression and schizophrenia are associated with excessive fear and anhedonia and have been linked to microRNA 137. We hypothesized that microRNAs (miRNAs) in the snail Lymnaea stagnalis are important for regulating feeding behaviour through either preventing neophobia or establishing hedonic value. To test these hypotheses, we used an injection of Poly-L-Lysine (PLL) to inhibit miRNA biogenesis and observed its effects on feeding behaviour. We repeated these experiments with pre-exposure to novel stimuli capable of eliciting neophobia to disentangle the processes predicted to regulate feeding behaviour. Next, we exposed snails to food stimuli of high hedonic value after PLL injection to reset their hedonic value for that food. Finally, we consolidated our results with previous research by examining the effect of PLL injection on a one trial appetitive classical conditioning procedure (1TT) to induce long term memory (LTM). We found that miRNAs are likely not required for preventing neophobia. Moreover, we discovered that snails experienced anhedonia in response to inhibition of miRNA biogenesis, resulting in diminished feeding behaviour for food stimuli with a previously high hedonic value. Snails showed diminished feeding behaviour for multiple food stimuli of high hedonic value post 1TT with PLL injection. This finding suggested that PLL causes anhedonia rather than an impairment of LTM formation following the 1TT procedure. This is the first evidence suggesting that inhibiting the biogenesis of miRNAs contributes to anhedonia in Lymnaea.

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.

Another Example of Conditioned Taste Aversion: Case of Snails

Simple Summary

It is important to decide what to eat and what not to eat in the life. Children are likely to reject new foods. When eating a new food results in a negative experience, the child will avoid that specific food in the future. This phenomenon is called ‘conditioned taste aversion’ in mammals, and it is considered necessary for survival by preventing subsequent ingestion of sickening foods. Many researchers study the same kind of phenomenon in invertebrates, too. For example, the formation of conditioned taste aversion was found in the pond snail, Lymnaea stagnalis, with the selective associability between a sweet sucrose solution and a bitter KCl solution. A sweet food attracts many kinds of animals, resulting in the feeding response, whereas a KCl solution is an aversive stimulus, inducing a withdrawal response in snails. After repeated temporally-contingent presentations of these two stimuli, the sucrose solution no longer elicits a feeding response, and this phenomenon persists for a long term. In the present review, we first outline the mechanisms of conditioned taste aversion in mammals, then introduce the conditioned taste aversion in snails, and compare them. Furthermore, the molecular events in snails are discussed, suggesting the general mechanism in conditioned taste aversion.

Abstract

Conditioned taste aversion (CTA) in mammals has several specific characteristics: (1) emergence of a negative symptom in subjects due to selective association with a taste-related stimulus, (2) robust long-term memory that is resistant to extinction induced by repeated presentation of the conditioned stimulus (CS), (3) a very-long-delay presentation of the unconditioned stimulus (US), and (4) single-trial learning. The pond snail, Lymnaea stagnalis, can also form a CTA. Although the negative symptoms, like nausea, in humans cannot be easily observed in invertebrate animal models of CTA, all the other characteristics of CTA seem to be present in snails. Selective associability was confirmed using a sweet sucrose solution and a bitter KCl solution. Once snails form a CTA, repeated presentation of the CS does not extinguish the CTA. A long interstimulus interval between the CS and US, like in trace conditioning, still results in the formation of a CTA in snails. Lastly, even single-trial learning has been demonstrated with a certain probability. In the present review, we compare, in detail, CTA in mammals and snails, and discuss the possible molecular events in CTA.

Lymnaea stagnalis as model for translational neuroscience research: From pond to bench

The purpose of this review is to illustrate how a reductionistic, but sophisticated, approach based on the use of a simple model system such as the pond snail Lymnaea stagnalis (L. stagnalis), might be useful to address fundamental questions in learning and memory. L. stagnalis, as a model, provides an interesting platform to investigate the dialog between the synapse and the nucleus and vice versa during memory and learning. More importantly, the "molecular actors" of the memory dialogue are well-conserved both across phylogenetic groups and learning paradigms, involving single- or multi-trials, aversion or reward, operant or classical conditioning. At the same time, this model could help to study how, where and when the memory dialog is impaired in stressful conditions and during aging and neurodegeneration in humans and thus offers new insights and targets in order to develop innovative therapies and technology for the treatment of a range of neurological and neurodegenerative disorders.

Real-Time Analysis of Animal Feeding Behavior With a Low-Calculation-Power CPU

Our goal was to develop an automated system to determine whether animals have learned and changed their behavior in real-time using a low calculation-power central processing unit (CPU). The bottleneck of real-time analysis is the speed of image recognition. For fast image recognition, 99.5% of the image was excluded from image recognition by distinguishing between the subject and the background. We achieved this by applying a binarization and connected-component labeling technique. This task is important for developing a fully automated learning apparatus. The use of such an automated system can improve the efficiency and accuracy of biological studies. The pond snail Lymnaea stagnails can be classically conditioned to avoid food that naturally elicits feeding behavior, and to consolidate this aversion into long-term memory. Determining memory status in the snail requires real-time analysis of the number of bites the snail makes in response to food presentation. The main algorithm for counting bites comprises two parts: extracting the mouth images from the recorded video and measuring the bite rate corresponding to the memory status. Reinforcement-supervised learning and image recognition were used to extract the mouth images. A change in the size of the mouth area was used as the cue for counting the number of bites. The accuracy of the final judgment of whether or not the snail had learned was the same as that determined by human observation. This method to improve the processing speed of image recognition has the potential for broad application beyond biological fields.

Monoamines, Insulin and the Roles They Play in Associative Learning in Pond Snails

Molluscan gastropods have long been used for studying the cellular and molecular mechanisms underlying learning and memory. One such gastropod, the pond snail Lymnaea stagnalis, exhibits long-term memory (LTM) following both classical and operant conditioning. Using Lymnaea, we have successfully elucidated cellular mechanisms of learning and memory utilizing an aversive classical conditioning procedure, conditioned taste aversion (CTA). Here, we present the behavioral changes following CTA training and show that the memory score depends on the duration of food deprivation. Then, we describe the relationship between the memory scores and the monoamine contents of the central nervous system (CNS). A comparison of learning capability in two different strains of Lymnaea, as well as the filial 1 (F1) cross from the two strains, presents how the memory scores are correlated in these populations with monoamine contents. Overall, when the memory scores are better, the monoamine contents of the CNS are lower. We also found that as the insulin content of the CNS decreases so does the monoamine contents which are correlated with higher memory scores. The present review deepens the relationship between monoamine and insulin contents with the memory score.

Configural learning in freshly collected, smart, wild Lymnaea

An inbred laboratory strain (W-strain) of Lymnaea is capable of configural learning (CL). CL a higher form of learning is an association between two stimuli experienced together that is different from the simple sum of their components. In our CL procedure a food substance (carrot, CO) is experienced together with crayfish effluent (CE) (i.e. CO+CE). Following CL, CO now elicits a fear-state rather than increased feeding. We hypothesized that freshly collected wild strains of predator-experiencedLymnaea also possess the ability to form CL; even though they experience crayfish daily in their environment. We therefore subjected freshly collected wild strain Lymnaea to the CL procedure. Following the CL procedure CO became a risk signal and evoked anti-predator behaviours. Thus, CL was demonstrated in wild, freshly collected snails. We believe that CL occurs in the snail's natural environment and is important for their survival.

Configural learning: a higher form of learning in Lymnaea

Events typically occur in a specific context and the ability to assign importance to this occurrence plays a significant role in memory formation and recall. When the scent of a crayfish predator (CE) is encountered in Lymnaea strains known to be predator-experienced (e.g. the W-strain), enhancement of memory formation and depression of feeding occurs, which are part of a suite of anti-predator behaviours. We hypothesized that Lymnaea possess a form of higher-order conditioning, namely configural learning. We tested this by simultaneously exposing W-strain Lymnaea to a carrot food-odour (CO) and predator scent (CE). Two hours later we operantly conditioned these snails with a single 0.5h training session in CO to determine whether training in CO results in long-term memory (LTM). In W-strain snails two 0.5h training sessions are required to cause LTM formation. A series of control experiments followed and demonstrated that only the CO+CE snails trained in CO had acquired enhanced memory forming ability. Additionally, following CE+CO pairing, CO no longer elicited an increased feeding response. Hence, snails have the ability to undergo configural learning. Following configural learning, CO becomes risk-signaling and evokes behavioural responses phenotypically similar to those elicited by exposure to CE.

Green tea and cocoa enhance cognition in Lymnaea

A flavonoid, (-)-epicatechi (Epi), enhances long-term memory (LTM) formation in Lymnaea and reverses memory obstruction caused by stress. Many foods contain substantial amounts of Epi, (e.g. green tea and cocoa). In humans eating such foods may directly or indirectly enhance cognition. We directly test whether operant conditioning training Lymnaea in these natural foods result in the same effects as training snails in pure Epi. We found that exposure to products containing high concentrations of Epi (e.g. green tea and cocoa) during training enhanced memory formation and could even reverse a learning and memory deficit brought about by stress. Epi can be photo-inactivated by exposure to ultraviolet light. We found that following photo-inactivation of Epi, memory enhancement did not occur. Photo-inactivation of foods containing Epi (e,g. green tea) blocked their ability to enhance LTM. Our data are thus consistent with the hypothesis that dietary sources of Epi can have positive benefits on cognitive ability and be able to reverse memory aversive states.

Two Strains of Lymnaea stagnalis and the Progeny from Their Mating Display Differential Memory-Forming Ability on Associative Learning Tasks

The pond snail Lymnaea stagnalis learns and forms long-term memory (LTM) following both operant conditioning of aerial respiratory behavior and classical conditioning of taste aversive behavior. In the present study, we examined whether there are interstrain differences in the ability to form LTM following these two types of conditioning. A strain of Lymnaea (TC1) collected in Alberta, Canada exhibits superior memory-forming ability following aerial respiratory operant conditioning compared to a laboratory-reared strain of Lymnaea from Netherlands known as the Dutch strain. We asked whether the offspring of the Canadian TC1 and Dutch snails (i.e., filial 1 (F₁) cross snails) would have the superior memory ability and found, rather, that their memory ability was average like the Dutch snails. That is, the Canadian TC1 snails have superior ability for LTM formation following aerial respiratory operant conditioning, but the Dutch and the generated F₁ cross have average ability for memory forming. We next examined the Canadian TC1, Dutch and F₁ cross snails for their ability to learn and form memory following conditioned taste aversion (CTA). All three populations showed similar associative CTA responses. However, both LTM formation and the ratio of good-to-poor performers in the memory retention test were much better in the Dutch snails than the Canadian TC1 and F₁ cross snails. The memory abilities of the Canadian TC1 and F₁ cross snails were average. Our present findings, therefore, suggest that snails of different strains have different memory abilities, and the F₁ cross snails do not inherit the memory ability from the smart strain. To our knowledge, there have been a limited number of studies examining differences in memory ability among invertebrate strains, with the exception of studies using mutant flies.

Relationship between the grades of a learned aversive-feeding response and the dopamine contents in Lymnaea

The pond snail Lymnaea learns conditioned taste aversion (CTA) and remembers not to respond to food substances that initially cause a feeding response. The possible relationship between how well snails learn to follow taste-aversion training and brain dopamine contents is not known. We examined this relationship and found the following: first, snails in the act of eating just before the commencement of CTA training were poor learners and had the highest dopamine contents in the brain; second, snails which had an ad libitum access to food, but were not eating just before training, were average learners and had lower dopamine contents; third, snails food-deprived for one day before training were the best learners and had significantly lower contents of dopamine compared to the previous two cohorts. There was a negative correlation between the CTA grades and the brain dopamine contents in these three cohorts. Fourth, snails food-deprived for five days before training were poor learners and had higher dopamine contents. Thus, severe hunger increased the dopamine content in the brain. Because dopamine functions as a reward transmitter, CTA in the severely deprived snails (i.e. the fourth cohort) was thought to be mitigated by a high dopamine content.

An automated learning apparatus for classical conditioning of Lymnaea stagnalis

BACKGROUND

The pond snail Lymnaea stagnalis is capable of taste avoidance classical conditioning (TAC) with sucrose as the conditional stimulus (CS) and mechanical prodding as the unconditional stimulus (US). After successful training, feeding behavior is significantly suppressed in response to CS presentation.

NEW METHOD

An automated apparatus is described for the training of multiple snails up to 10 snails at the same time. The new apparatus employs an electrical shock obtained from a 9-V dry cell to deliver a consistent and effective current amplitude of 0.4μA (i.e., the US).

RESULTS

Using this apparatus, 10 snails can be conditioned simultaneously. We found that the optimal parameters to result in both short (STM) and long-term memory (LTM) were 15 paired presentations of the CS and US with a 5-min inter-trial interval (ITI) and 0.2-s current duration. However, both STM and LTM were observed with other ITIs tested. Successful TAC with only a single pairing of the CS-US occurred with a CS of 100mM sucrose solution for 60s followed by a US of 9V with 0.4μA for 5s.

COMPARISON WITH EXISTING METHOD

The use of automated training apparatus for TAC will enable us to better examine the relationship between strength of CS and US.

Function of insulin in snail brain in associative learning

Insulin is well known as a hormone regulating glucose homeostasis across phyla. Although there are insulin-independent mechanisms for glucose uptake in the mammalian brain, which had contributed to a perception of the brain as an insulin-insensitive organ for decades, the finding of insulin and its receptors in the brain revolutionized the concept of insulin signaling in the brain. However, insulin's role in brain functions, such as cognition, attention, and memory, remains unknown. Studies using invertebrates with their open blood-vascular system have the promise of promoting a better understanding of the role played by insulin in mediating/modulating cognitive functions. In this review, the relationship between insulin and its impact on long-term memory (LTM) is discussed particularly in snails. The pond snail Lymnaea stagnalis has the ability to undergo conditioned taste aversion (CTA), that is, it associatively learns and forms LTM not to respond with a feeding response to a food that normally elicits a robust feeding response. We show that molluscan insulin-related peptides are up-regulated in snails exhibiting CTA-LTM and play a key role in the causal neural basis of CTA-LTM. We also survey the relevant literature of the roles played by insulin in learning and memory in other phyla.

Memory block: a consequence of conflict resolution

Food deprivation for 1 day in the pond snail Lymnaea stagnalis before aversive classical conditioning results in optimal conditioned taste aversion (CTA) and long-term memory (LTM) formation, whereas 5-day food deprivation before training does not. We hypothesize that snails do in fact learn and form LTM when trained after prolonged food deprivation, but that severe food deprivation blocks their ability to express memory. We trained 5-day food-deprived snails under various conditions, and found that memory was indeed formed but is overpowered by severe food deprivation. Moreover, CTA-LTM was context dependent and was observed only when the snails were in a context similar to that in which the training occurred.

Effects of serotonin on the heartbeat of pond snails in a hunger state

Serotonin (5-hydroxytryptamine: 5-HT) is a multimodal transmitter that controls both feeding response and heartbeat in snails. However, the effects of 5-HT on the hunger state are still unknown. We therefore examined the relation among the hunger state, the heartbeat rate and the 5-HT action in food-starved snails. We found that the hunger state was significantly distinguished by the heartbeat rate in snails. The heartbeat rate was high in the food-satiated snails, whereas it was low in the food-starved snails. An increase in 5-HT concentration in the body boosted the heartbeat rate in the food-starved snails, but did not affect the rate in the food-satiated snails. These results suggest that 5-HT application may mimic the change from a starvation to a satiation state normally achieved by direct ingestion of food.

Electrophysiological characteristics of feeding-related neurons after taste avoidance Pavlovian conditioning in Lymnaea stagnalis

Taste avoidance conditioning (TAC) was carried out on the pond snail, Lymnaea stagnalis. The conditional stimulus (CS) was sucrose which elicits feeding behavior; while the unconditional stimulus (US) was a tactile stimulus to the head which causes feeding to be suppressed. The neuronal circuit that drives feeding behavior in Lymnaea is well worked out. We therefore compared the physiological characteristics on 3 classes of neurons involved with feeding behavior especially in response to the CS in conditioned vs. control snails. The cerebral giant cell (CGC) modulates feeding behavior, N1 medial neuron (N1M) is one of the central pattern generator neurons that organizes feeding behavior, while B3 is a motor neuron active during the rasp phase of feeding. We found the resting membrane potential in CGC was hyperpolarized significantly in conditioned snails but impulse activity remained the same between conditioned vs. control snails. There was, however, a significant increase in spontaneous activity and a significant depolarization of N1M’s resting membrane potential in conditioned snails. These changes in N1M activity as a result of training are thought to be due to withdrawal interneuron RPeD11 altering the activity of the CGCs. Finally, in B3 there was: 1) a significant decrease in the amplitude and the frequency of the post-synaptic potentials; 2) a significant hyperpolarization of resting membrane potential in conditioned snails; and 3) a disappearance of bursting activity typically initiated by the CS. These neuronal modifications are consistent with the behavioral phenotype elicited by the CS following conditioning.

An increase in insulin is important for the acquisition conditioned taste aversion in Lymnaea

Conditioned taste aversion (CTA) in Lymnaea is brought about by pairing a sucrose solution (the conditioned stimulus, CS) with an electric shock (the unconditioned stimulus, US). Following repeated CS-US pairings, CTA occurs and it is consolidated into long-term memory (LTM). The best CTA is achieved, if snails are food-deprived for 1 day before training commences. With a longer period of food deprivation (5 days), learning and memory formation does not occur. It has been hypothesized that the levels of insulin in the central nervous system (CNS) are very important for CTA to occur. To test his hypothesis, we injected insulin directly into 5-day food-deprived snails. The injection of insulin, as expected, resulted in a decrease in hemolymph glucose concentration. Consistent with our hypothesis with insulin injection, learning and memory formation of CTA occurred. That is, the 'insulin spike' is more important than an increase in hemolymph glucose concentration for CTA-LTM. If we injected an insulin receptor antibody into the snails before the insulin injection, learning was formed but memory formation was not, which is consistent with our previous study. Therefore, a rise in the insulin concentration (i.e., insulin spike) in the CNS is considered to be a key determining factor in the process of CTA-LTM.

Protein kinase C mediates memory consolidation of taste avoidance conditioning in Lymnaea stagnalis

In Lymnaea stagnalis, in order to obtain a 10 min short-term memory (STM) of taste avoidance conditioning (TAC) at least 10 paired presentations of a conditioned stimulus (CS), sucrose, and an unconditioned stimulus (US), tactile stimulation to the animal's head, are required. Pre-exposure of snails to the protein kinase C (PKC) α and ε activator bryostatin (Bryo) facilitated STM formation in that only 5 paired CS-US trials were required. Typically 20 paired presentations of the CS-US are required for formation of STM and LTM. However, 20 paired presentations do not result in STM or LTM if snails are pre-incubated with a PKC inhibitor, Ro-32-0432. We also found that LTM lasting longer than 48 h was acquired with Bryo incubation for 45 min even after termination of the conditioning paradigm. These data suggest that activation of the α and ε isozymes of PKC is crucially involved in the formation of LTM and provide further support for a mechanism that has been conserved across the evolution of species ranging from invertebrate molluscs to higher mammals.

The Yerkes-Dodson law and appropriate stimuli for conditioned taste aversion in Lymnaea

The pond snail Lymnaea stagnalis can learn conditioned taste aversion and then consolidate it into long-term memory (LTM). A high voltage electric shock was used as the unconditioned stimulus (US), whereas we previously used KCl. We varied both the strength of the conditioned stimulus (CS) and US to determine if the so-called Yerkes-Dodson law prevailed. This is an empirical relationship between the state of arousal and LTM formation, showing that there is an optimal level of arousal leading to memory formation. However, too little or too much arousal results in poorer LTM. We found here that the most appropriate stimuli to use in taste aversion training in Lymnaea were a 10 mmol l-1 sucrose solution as the CS and a 3-s electric shock as the US.

High voltage with little current as an unconditional stimulus for taste avoidance conditioning in Lymnaea stagnalis

A new and better taste avoidance conditioning paradigm for Lymnaea has been developed that replaces the previously used tactile unconditional stimulus (US) with an brief electrical stimulus (1000V, 80μA), while continuing to use a sucrose application to the lips as the conditional stimulus (CS). With 15 paired CS-US presentations on a single day, we were able to elicit both short-term memory (STM) and long-term memory (LTM). The LTM persisted for at least one week. While STM was elicited with 5, 8, or 10 paired presentations of the CS-US on a single day, LTM was not. The new US used here was more consistent than the previously used US, and this stimulus consistency may explain why 15 paired CS-US presentations now result in LTM formation.

Increase in cyclic AMP concentration in a cerebral giant interneuron mimics part of a memory trace for conditioned taste aversion of the pond snail

Conditioned taste aversion (CTA) can be classically conditioned in the pond snail Lymnaea stagnalis and subsequently be consolidated into long-term memory (LTM). The neural trace that subserves CTA-LTM can be summarized as follows: A polysynaptic inhibitory postsynaptic potential recorded in the neuron 1 medial (N1M) cell in the conditioned snails as a result of activation of the cerebral giant cell (CGC) is larger and lasts longer than that in control snails. The N1M cell is ultimately activated by the CGC via the neuron 3 tonic (N3t) cell. That is, the inhibitory monosynaptic inputs from the N3t cell to the N1M cell are facilitated. The N1M and N3t cells are the members of feeding central pattern generator, whereas the CGC is a multimodal interneuron thought to play a key role in feeding behavior. Here we examined the involvement of a second messenger, cAMP, in the establishment of the memory trace. We injected cAMP into the CGC and monitored the potentials of the B3 motor neuron activated by the CGC. B3 activity is used as an index for the synaptic inputs from the N3t cell to the N1M cell. We found that the B3 potentials were transiently enlarged. Thus, when the cAMP concentration is increased in the CGC by taste aversion training, cAMP-induced changes may play a key role in the establishment of a memory trace in the N3t cell.