Shell damage leads to enhanced memory formation in Lymnaea

Ecologically relevant stressors alter the ability of the pond snail, Lymnaea stagnalis, to form long-term memory (LTM). Here, we show that an environmentally relevant stressor, shell damage, has a dramatic effect on the enhancement of LTM formation. Damage in the form of a shell clip 24 h before operant conditioning training resulted in long-term memory (LTM) formation following a single 0.5 h training session (TS). Typically, in these snails, two 0.5 h TSs with a 1 h interval between the sessions are required to cause LTM formation. We show here that even with a 72 h interval between shell clip and training, memory enhancement still occurred. The stress associated with shell clip could be mitigated by an ongoing high-Ca^{2 +} pond water environment, an injection of propranolol and a DNA methylation blocker. However, use of an anaesthetic (MgCl₂) during the clip or intermittent exposure to the high-Ca^{2 +} pond water environment did not mitigate the stress associated with the shell clip. Shell clip was also sufficient to cause juvenile snails, which neither learn nor form memory, to gain the capacity to form LTM. Together, the experiments demonstrate that shell clipping is an environmentally relevant stressor that can cause enhancement of LTM formation.

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.

Strain transformation: Enhancement of invertebrate memory in a new rearing environment

Memory formation is influenced by a variety of factors, including the environmental conditions in which an organism is reared. Here, we studied the memory-forming ability of the lab-bred B-strain of Lymnaea following a change in their rearing environment from Brock University to the University of Calgary. We have previously demonstrated that this move enhances memory-forming ability and here we studied the magnitude of this phenotypic change. Once reared to adulthood at the University of Calgary, the B-strain animals were first tested to determine how many training sessions were required for the formation of long-term memory (LTM) to occur. Following this change in environment, the B-strain transformed into a ‘smart’ lab-bred strain requiring only a single 0.5 h session to form LTM. Next, we tested whether exposure to physiologically relevant stressors would block the formation of LTM in this ‘transformed’ B-strain, as this obstruction has previously been observed in ‘smart’ snails collected from the wild. Interestingly, neither stressor tested in this study perturbed memory formation in this ‘transformed’ lab-bred strain. Additionally, both the ‘smart’ memory phenotype, as well as the increased stress resiliency, were observed in the second generation of ‘transformed’ B-strain at both the juvenile and adult stages. This suggests that a change in rearing environment can contribute to the memory-forming ability of lab-bred Lymnaea.

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.

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

To find a causal mechanism of learning and memory is a heuristically important topic in neuroscience. In the pond snail Lymnaea stagnalis, the following experimental facts have accrued regarding a classical conditioning procedure known as conditioned taste aversion (CTA): (1) one-day food-deprived Dutch snails have superior CTA memory formation; (2) the one-day food-deprived snails have a low monoamine content (e.g., serotonin, dopamine, octopamine) in their central nervous system (CNS); (3) fed or five-day food-deprived snails have poorer CTA memory and a higher monoamine content; (4) the Dutch snails form better CTA memory than the Canadian TC1 strain; and, (5) the F₁ cross snails between the Dutch and Canadian TC1 strains also form poor CTA memory. Here, in one-day food-deprived snails, we measured the monoamine content in the CNSs of the 3 populations. In most instances, the monoamine content of the Dutch strain was lower than in the other two populations. The F₁ cross snails had the highest monoamine content. A lower monoamine content is correlated with the better CTA memory formation.

Combining Factors That Individually Enhance Memory in Lymnaea

When applied individually, thermal stress (1 hour at 30 °C) and (-)epicatechin (a flavonol found in green tea, e.g.) each enhance long-term memory formation following operant conditioning of Lymnaea aerial respiratory behavior. Snails demonstrate enhanced long-term memory formation when trained in epicatechin-treated pond water or when placed in 30 °C pond water for 1 hour, 1 hour prior to training in pond water. We ask here whether the combined application of epicatechin + thermal stress enhances long-term memory retention length beyond the maximal lengths of the individual factors alone. We report that the applied combination of epicatechin + thermal stress has a synergistic memory-enhancing effect; that is, when the two are applied in combination, memory persists longer than when either is applied alone. We then ask whether quercetin, a heat shock protein blocker, will affect the memory enhancement produced by the combined treatment of thermal stress and epicatechin. We report that quercetin does not decrease the memory enhancement of epicatechin, but it does decrease the memory enhancement by thermal stress; and it also decreases the memory persistence of snails exposed to both treatments in combination.

The effect of rearing environment on memory formation

Lymnaea stagnalis is a well-studied model system for determining how changes in the environment influence associative learning and memory formation. For example, some wild strains of Lymnaea, collected from separate geographic locations show superior memory-forming abilities compared to others. Here, we have studied memory formation in two laboratory-bred Lymnaea strains, derived from the same original population in The Netherlands. The two strains were reared in two different laboratories at the University of Calgary (C-strain) and at Brock University (B-strain) for many years and were found to differ in their memory-forming ability. Specifically, the C-strain required only two training sessions to form long-term memory (LTM) whereas the B-strain required four sessions to form LTM. Additionally, the LTM formed by the B-strain persisted for a shorter amount of time than the memory formed by the C-strain. Thus, despite being derived from the same original population, the C- and B-strains have developed different memory-forming abilities. Next, we raised the two strains from embryos ‘Away’ from home (i.e. in the other laboratory) over two generations and assessed their memory-forming abilities. The B-strain reared and maintained at the University of Calgary demonstrated improved memory-forming ability within a single generation, while the C-strain reared at Brock University retained their normal LTM forming ability across two subsequent generations. This suggests that local environmental factors may contribute to the behavioural divergence observed between these two laboratory-bred strains.

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.