Stress before training alters memory retrieval of a non-declarative memory in Lymnaea

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.

A thermal stressor, propranolol and long-term memory formation in freshly collected Lymnaea

A heat stressor (1 h at 30°C) in Lymnaea stagnalis before operant conditioning training of aerial respiration is sufficient to enhance long-term memory (LTM) formation in 'average' cognitive ability, laboratory-reared, inbred snails. However, in freshly collected outbred snails, the same heat stressor blocks LTM formation in 'smart' cognitive phenotype but not in average cognitive phenotype strains. Here, we hypothesize that (1) preventing the stress associated with the heat stressor before training allows LTM to form in the smart phenotype strains; and (2) alleviating the stress before a memory recall session allows a formed LTM to be recalled in the smart phenotype strains. We found that an injection of propranolol, which mitigates the stressor, before snails experience the heat stressor enabled two strains of the smart phenotype snails to form LTM, consistent with our first hypothesis. However, the injection of propranolol before a memory test session did not alleviate a memory recall block in the smart phenotype snails. Thus, our second hypothesis was not supported. Therefore, smart cognitive phenotype snails encountering a heat stressor have an inability to form LTM, but this inability can be overcome by the pre-injection of propranolol.

In the great pond snail (Lymnaea stagnalis), two stressors that individually enhance memory in combination block memory formation

Stress plays an important role in memory formation in the great pond snail (Lymnaea stagnalis (Linnaeus, 1758)). Individual stressors have been shown to enhance or to perturb long-term memory (LTM) formation. However, when snails perceive a combination of two stressors, it is unclear the outcome with regards to LTM formation. Here we first show that when L. stagnalis are exposed individually to either a predator stressor (crayfish effluent (CE), which is a kairomone) or a thermal stressor (30 °C), LTM formation is enhanced. In their natural environment, L. stagnalis may experience temperatures approaching 30 °C and they may encounter crayfish at the same time. How such a combination of stressors alters adaptive behaviour is unknown. Here we show that when these two stressors are combined, LTM formation is blocked. Since boiling CE inactivates the kairomone, we used previously boiled CE that we combined with the thermal stressor and found that LTM formation is again enhanced. These data show that (i) it cannot accurately be predicted how a combination of stressors when combined interact to alter LTM formation and (ii) there is a difference between hot CE and room temperature CE.