Comparison of brain monoamine content in three populations of Lymnaea that correlates with taste-aversive learning ability

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.

Expression Level Changes in Serotonin Transporter are Associated with Food Deprivation in the Pond Snail Lymnaea stagnalis

In the pond snail Lymnaea stagnalis, serotonin (5-HT) plays an important role in feeding behavior and its associated learning (e.g., conditioned taste aversion: CTA). The 5-HT content in the central nervous system (CNS) fluctuates with changes in the nutritional status, but it is also expected to be influenced by changes in the serotonin transporter (SERT) expression level. In the present study, we identified SERT in Lymnaea and observed its localization in 5-HTergic neurons, including the cerebral giant cells (CGCs) in the cerebral ganglia and the pedal A cluster neurons and right and left pedal dorsal 1 neurons in the pedal ganglia by in situ hybridization. Real-time PCR revealed that the SERT mRNA expression level was lower under severe food deprivation than under mild food deprivation in the whole CNS as well as in a single CGC. These results inversely correlated with previous data that the 5-HT content in the CNS was higher in the severely food-deprived state than in the mildly food-deprived state. Furthermore, in single CGCs, we observed that the 5-HT level was significantly increased in the severely food-deprived state compared with the mildly food-deprived state. Our present findings suggest that changes in the SERT expression level associated with food deprivation may affect 5-HT signaling, probably contributing to learning and memory mechanisms in Lymnaea.

Genes Upregulated by Operant Conditioning of Escape Behavior in the Pond Snail Lymnaea stagnalis

The pond snail Lymnaea stagnalis is capable of learning by both classical conditioning and operant conditioning. Although operant conditioning related to escape behavior with punishment has been examined by some research groups, the molecular mechanisms are not known. In the present study, we examined changes in the expression levels of cAMP-response element binding protein 1 (CREB1), CREB2, CREB-binding protein (CBP), and monoamine oxidase (MAO) in the Lymnaea central nervous system (CNS) using real-time PCR following operant conditioning of escape behavior. CREB1 and CREB2 are transcription factors involved in long-term memory in Lymnaea; CBP is a coactivator with CREB1; and MAO is a degrading enzyme for monoamines (e.g., serotonin) with important roles in learning and memory in Lymnaea. In operant conditioning, the punishment cohort, in which snails escaping from the container encountered aversive KCl, exhibited significantly fewer escape attempts than the control cohort, in which snails escaping from the container encountered distilled water, during both the training and memory test periods. After the operant conditioning, CREB1 and CREB2 were upregulated, and the ratio of CREB1/CREB2 was also increased, suggesting that the operant conditioning of escape behavior involves these factors. MAO was also upregulated, suggesting that the content of monoamines such as serotonin in the CNS decreased. The upregulated genes identified in the present study will help to further elucidate learning and memory mechanisms in Lymnaea.

CNS serotonin content mediating food deprivation-enhanced learning is regulated by hemolymph tryptophan concentration and autophagic flux in the pond snail

Nutritional status affects cognitive function in many types of organisms. In the pond snail Lymnaea stagnalis, 1 day of food deprivation enhances taste aversion learning ability by decreasing the serotonin (5-hydroxytryptamin; 5-HT) content in the central nervous system (CNS). On the other hand, after 5 days of food deprivation, learning ability and the CNS 5-HT concentration return to basal levels. How food deprivation leads to alterations of 5-HT levels in the CNS, however, is unknown. Here, we measured the concentration of the 5-HT precursor tryptophan in the hemolymph and CNS, and demonstrated that the CNS tryptophan concentration was higher in 5-day food-deprived snails than in non-food-deprived or 1-day food-deprived snails, whereas the hemolymph tryptophan concentration was not affected by the duration of food deprivation. This finding suggests the existence of a mediator of the CNS tryptophan concentration independent of food deprivation. To identify the mediator, we investigated autophagic flux in the CNS under different food deprivation conditions. We found that autophagic flux was significantly upregulated by inhibition of the tropomyosin receptor kinase (Trk)-Akt-mechanistic target of rapamycin complex 1 (MTORC1) pathway in the CNS of 5-day food-deprived snails. Moreover, when autophagy was inhibited, the CNS 5-HT content was significantly downregulated in 5-day food-deprived snails. Our results suggest that the hemolymph tryptophan concentration and autophagic flux in the CNS cooperatively regulate learning ability affected by different durations of food deprivation. This mechanism may underlie the selection of behaviors appropriate for animal survival depending on the degree of nutrition.

Insulin and Memory in Invertebrates

Insulin and insulin-like peptides (ILP) help to maintain glucose homeostasis, whereas insulin-like growth factor (IGF) promotes the growth and differentiation of cells in both vertebrates and invertebrates. It is sometimes difficult to distinguish between ILP and IGF in invertebrates, however, because in some cases ILP has the same function as IGF. In the present review, therefore, we refer to these peptides as ILP/IGF signaling (IIS) in invertebrates, and discuss the role of IIS in memory formation after classical conditioning in invertebrates. In the arthropod Drosophila melanogaster, IIS is involved in aversive olfactory memory, and in the nematode Caenorhabditis elegans, IIS controls appetitive/aversive response to NaCl depending on the duration of starvation. In the mollusk Lymnaea stagnalis, IIS has a critical role in conditioned taste aversion. Insulin in mammals is also known to play an important role in cognitive function, and many studies in humans have focused on insulin as a potential treatment for Alzheimer’s disease. Although analyses of tissue and cellular levels have progressed in mammals, the molecular mechanisms, such as transcriptional and translational levels, of IIS function in cognition have been far advanced in studies using invertebrates. We anticipate that the present review will help to pave the way for studying the effects of insulin, ILPs, and IGFs in cognitive function across phyla.

Induction of LTM following an Insulin Injection

The pond snail Lymnaea stagnalis learns conditioned taste aversion (CTA) and consolidates it into long-term memory (LTM). One-day food-deprived snails (day 1 snails) show the best CTA learning and memory, whereas more severely food-deprived snails (5 d) do not express good memory. However, previous studies showed that CTA-LTM was indeed formed in 5-d food-deprived snails (day 5 snails), but its recall was prevented by the effects of food deprivation. CTA-LTM recall in day 5 snails was expressed following 7 d of feeding and then 1 d of food deprivation (day 13 snails). In the present study, we thus hypothesized that memory recall occurs because day 13 snails are in an optimal internal state. One day of food deprivation before the memory test in day 13 snails increased the mRNA level of molluscan insulin-related peptide (MIP) in the CNS. Thus, we further hypothesized that an injection of insulin into day 5 snails following seven additional days with access to food (day 12 snails) activates CTA neurons and mimics the food deprivation state before the memory test in day 13 snails. Day 12 snails injected with insulin could recall the memory. In addition, the simultaneous injection of an anti-insulin receptor antibody and insulin into day 12 snails did not allow memory recall. Insulin injection also decreased the hemolymph glucose concentration. Together, the results suggest that an optimal internal state (i.e., a spike in insulin release and specific glucose levels) are necessary for LTM recall following CTA training in snails.

The Role of Serotonin in the Influence of Intense Locomotion on the Behavior Under Uncertainty in the Mollusk Lymnaea stagnalis

The role of serotonin in the immediate and delayed influence of physical exercise on brain functions has been intensively studied in mammals. Recently, immediate effects of intense locomotion on the decision-making under uncertainty were reported in the Great Pond snail, Lymnaea stagnalis (Korshunova et al., 2016). In this animal, serotonergic neurons control locomotion, and serotonin modulates many processes underlying behavior, including cognitive ones (memory and learning). Whether serotonin mediates the behavioral effects of intense locomotion in mollusks, as it does in vertebrates, remains unknown. Here, the delayed facilitating effects of intense locomotion on the decision-making in the novel environment are described in Lymnaea. Past exercise was found to alter the metabolism of serotonin, namely the content of serotonin precursor and its catabolites in the cerebral and pedal ganglia, as measured by high-performance liquid chromatography. The immediate and delayed effects of exercise on serotonin metabolism were different. Moreover, serotonin metabolism was regulated differently in different ganglia. Pharmacological manipulations of the serotonin content and receptor availability suggests that serotonin is likely to be responsible for the locomotor acceleration in the test of decision-making under uncertainty performed after exercise. However, the exercise-induced facilitation of decision-making (manifested in a reduced number of turns during the orienting behavior) cannot be attributed to the effects of serotonin.

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.