Stress, Memory, Forgetting and What, Lymnaea Can Tell us About a Stressful World

68The ability of animals to learn and remember during their lifetime allows them to successfully adapt to various environmental stressors. Stress modulates (either enhancing or diminishing) the ability to learn and the ability to form memory and the ability to recall memory. We have attempted to use environmentally relevant stressors (e.g., crowding, low levels of calcium, predator detection and thermal shock) to determine how the various stressors change the ability to learn, form memory and to recall that memory. It is difficult, if not impossible, to predict ahead of actually doing the experiment to say with any certainty how a specific stressor will alter memory formation and its recall. It is even more difficult to predict how a combination of stressors alters these cognitive events. Identical stressor stimuli affect different strains of Lymnaea (e.g., smart vs. average) differently. It may be that one of the “costs” of being “smart” is a poor ability to handle stress. The birth of modern neuroscience occurred in the 1950s. A number of seminal events associated with scientists and clinicians from the Montreal area played important but sometimes forgotten roles in establishing what we now call Neuroscience. One event was the brain surgery performed on a patient known as HM that lead to Brenda Milner’s team observations on human memory, which ultimately showed that specific neural circuits were necessary for different forms of declarative and non-declarative memory (Milner et al., 1998). Those observations formed the basis of many experiments that ultimately led to our present understanding of the molecular events occurring in specific neurons, which are necessary for memory formation. However, the techniques and knowledge needed to undertake those studies at the neuronal and circuit level depended in large measure on the realization that molluscs possess large, identifiable neurons, which controlled interesting, tractable behaviors. A second and a third event were the ideas put forward by Donald Hebb in the 1950s (the Hebb synapse (Hebb, 1949) and the inverted U shape function regarding stress and memory formation; Fig. 3.1). Interestingly, this latter idea is most often attributed to a paper by Yerkes and Dodson in 1908; but in reality Hebb (Hebb, 1954) conceived the notion of what is now commandingly referred to as the Yerkes–Dodson law (Lukowiak et al., 2015). Foundational studies that were also necessary for Neuroscience to become a “science” were occurring in France and Monaco led by Tauc (1954) and Arvanitaki and Chalazonitis (1955), respectively, using the central nervous system (CNS) of the sea hare (genus Aplysia). These studies 69laid the groundwork for the use of molluscan model systems to investigate the causal neuronal mechanisms of learning and memory. Building on these earlier studies, Kandel and Tauc’s (1965) discovery of heterosynaptic facilitation in a molluscan preparation laid the experimental groundwork for hypotheses developed subsequently to explain the “Hebb synapse” and the neuronal basis of learning and the subsequent formation of long-lasting memory. Ultimately, the research performed by a multitude of investigators using a wide variety of molluscan preparations culminated in Eric Kandel being awarded the Nobel Prize for Medicine and Physiology in 2000 “for the discoveries concerning signal transduction in the nervous system” (Kandel, 2001). In this chapter, I will tell two main stories. One a compelling (I hope) series of stories of how learning, memory, and forgetting are all altered by environmentally relevant stressors; and, two, why it is important to understand the neuroecology of the model system one works with.

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