Environmentally relevant stressors alter memory formation in the pond snailLymnaea

Abstract

Stress alters adaptive behaviours such as learning and memory. Stressors can either enhance or diminish learning, memory formation and/or memory recall. We focus attention here on how environmentally relevant stressors alter learning, memory and forgetting in the pond snail, Lymnaea stagnalis. Operant conditioning of aerial respiration causes associative learning that may lead to long-term memory (LTM) formation. However, individual ecologically relevant stressors, combinations of stressors, and bio-active substances can alter whether or not learning occurs or memory forms. While the behavioural memory phenotype may be similar as a result of exposure to different stressors, how each stressor alters memory formation may occur differently. In addition, when a combination of stressors are presented it is difficult to predict ahead of time what the outcome will be regarding memory formation. Thus, how combinations of stressors act is an emergent property of how the snail perceives the stressors. KEY WORDS: Operant conditioning, Environmental impact, Memory formation, Long-term memory, Social snails IntroductionThe ability of animals to learn and remember enables them to adapt to environmental changes, and directly affects 'fitness'. Its common knowledge that stress affects learning and memory. This has been known in the literature since Bacon (Bacon, 1620) and is exemplified in the 'Yerkes-Dodson Law' or as we call it the 'Goldilocks Rule' (Yerkes and Dodson, 1908) (Fig. 1). Too much or too little stress impedes long-term memory (LTM) formation, while 'just the right amount' enhances LTM. Organisms are thought to 'decide' to only expend the 'neuronal cost' (e.g. altered gene activity and new protein synthesis) necessary to form LTM to 'relevant' events. Relevancy is in part determined by the level of stress perceived at the time of learning. Because memory is dynamic, stress can modify memory (e.g. false memory and post-traumatic stress syndrome) (Shors, 2004; Kim and Diamond, 2002;Lukowiak et al., 2003;Lukowiak et al., 2008;Lukowiak et al., 2010). Here we will focus on environmentally relevant stressors that alter memory formation in Lymnaea.Hans Selye, the 'father of stress research' stated: 'Everyone knows what stress is and nobody knows what it is ' (Selye, 1973). Stress is defined here as a state that requires a physiological, psychological or behavioural readjustment or modification in order to maintain the well being of the organism. However, the same stimulus may be perceived as a stressor for one organism but not for another, or may be perceived as a stressor only at certain times and not at others in the same organism. The effects of stress on learning and memory sometimes yield contradictory results (Shors, 2004). Because of the complexities of the brain and the multitude of behaviours tested, in addition to the different ways in which stressors act, disagreement in the literature on the role that stress plays in learning and memory formation is not too surprising. Using a simpler model system and...

Memory block: A consequence of conflict resolution

“…Snails are able to consolidate the associative learning into long-term memory (LTM) (Benjamin et al, 2000;Otsuka et al, 2013;Takahashi et al, 2013;Lukowiak et al, 2014;Sunada et al, 2014). Here, we look at the ability of Lymnaea to learn and remember for many days not to respond to a food substance that normally elicits a feeding response.…”

A flavonol, epicatechin, reverses the suppressive effects of a stressor on LTM formation

“…We defined stress here as a condition that significantly alters the physiological or psychological homeostasis of an organism (Lukowiak et al, 2014). The idea that exposure to a low-calcium environment, even for a short period of time (e.g.…”

“…For example, detection of predator smells such as crayfish effluent (CE) enhanced both vigilance behaviours and LTM formation (Orr et al, 2007;Orr et al, 2009;Orr and Lukowiak, 2008); while crowding, low environmental calcium, and certain heavy metals (Zn and Cd) and H 2 S in pond water block LTM formation (Rosenegger et al, 2004;De Caigny and Lukowiak, 2008;Byzitter et al, 2012). A comprehensive summary of which ecologically relevant stressors significantly alter LTM formation can be found in two recent reviews (Lukowiak et al, 2014). It has also been shown that a flavonoid, epicatechin, found in dark chocolate, green tea and red wine (Kühnau, 1976;Matsuoka et al, 1995;Galliet al, 2002;Youdim et al, 2002) enhances LTM formation in Lymnaea (Fruson et al, 2012).…”

Black tea differs from green tea: it suppresses long-term memory formation inLymnaea

“…Memory formation is susceptible to alteration by environmental factors, such as stress and lifestyle choices [1]. One of these key lifestyle choices is diet.…”

“…That is, the number of attempted pneumostome openings in the memory test session (MT) was not significantly less than the number in TS1. In the W-strain snails used here, this training procedure results in LTM formation when the training and testing sessions take place in pond water [1,18]. …”