A Systems Approach to the Cellular Analysis of Associative Learning in the Pond Snail Lymnaea

Abstract

We show that appetitive and aversive conditioning can be analyzed at the cellular level in the well-described neural circuitries underlying rhythmic feeding and respiration in the pond snail, Lymnaea stagnalis. To relate electrical changes directly to behavior, the snails were first trained and the neural changes recorded at multiple sites in reduced preparations made from the same animals. Changes in neural activity following conditioning could be recorded at the level of motoneurons, central pattern generator interneurons and modulatory neurons. Of significant interest was recent work showing that neural correlates of long-term memory could be recorded in the feeding network following single-trial appetitive chemical conditioning. Available information on the synaptic connectivity and transmitter content of identified neurons within the Lymnaea circuits will allow further work on the synaptic and molecular mechanisms of learning and memory.A large body of work using the very successful cellular and molecular approach to learning and memory has provided important models of plasticity in both invertebrates and vertebrates (Byrne 1987;Milner et al. 1998). These studies emphasize changes occurring at a single locus. However, there is increasing evidence to suggest that most forms of learning involve changes at several different sites in the brain (Wolpaw 1997;Lisberger 1998), suggesting that more complex models will be required. The identification of multiple sites of plasticity requires a systems approach to the analysis of learning and memory, which makes it important to first identify the electrical changes that result from conditioning throughout a neural network and then attempt to relate these to behavioral plasticity. Given the complexity of the brain, systems-level analyses of learning and memory have often been inevitable in vertebrate studies striving to link learning-related behavioral and cellular changes. In our opinion there is a clear requirement for a similar approach in numerically more simple invertebrate systems, where 100-1000s of neurons form circuits, especially because of the evidence that the invertebrate central nervous system (CNS) can also show changes resulting from conditioning in several different structures (e.g., antennal lobes, mushroom bodies, and lateral protocerebral lobes of the honeybee; Hammer 1997). A systems approach to invertebrate learning is particularly useful if we have no prior information about the numbers and locations of sites of plasticity in the various elements of the nervous system. Despite the recent developments in in vivo single-cell recording and imaging techniques in vertebrates (Fetcho and O'Malley 1997;Svoboda et al. 1997), studies using invertebrates still have the added advantage that more specific learning-related changes in electrical activity at the single identified cell or single identified synapse level can be examined throughout a specific behavioral neural network.

Context Learning and the Effect of Context on Memory Retrieval in Lymnaea

“…The data obtained here allow a number of conclusions to be drawn about learning, memory formation, and memory retrieval in Lymnaea and strengthen the Lymnaea model (Benjamin et al 2000) for the study of the neuronal mechanisms of associative learning and memory.…”

“…We chose to study the neuronal mechanisms that underlie associative learning in the freshwater pond snail, Lymnaea stagnalis (for review, see Benjamin et al 2000). An important homeostatic behavior of this organism, aerial respiration, can be operantly conditioned .…”

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.…”

Multiple Types of Control by Identified Interneurons in a Sensory-Activated Rhythmic Motor Pattern

“…1 A) while applying a food stimulus to the lips and /or manipulating interneuronal firing in reduced preparations. All the interneuron and motoneuron types recorded in these experiments were identified by their characteristic position, size, and color as well as by their firing patterns and connections to other cells in the feeding network (for recent overviews of the feeding system, see Brierley et al, 1997;Kemenes, 1997;Staras et al, 1998a;Benjamin et al, 2000). The rhythmic neuronal activity known to underlie feeding in intact animals is called fictive feeding, and it is generated by a set of premotor C PG interneurons (Rose and Benjamin 1981b;Elliott and Benjamin 1985a).…”

“…First, semi-intact preparations already have been developed in which electrophysiological fictive feeding rhythms can be evoked by chemosensory stimuli applied to the lips (Kemenes et al, 1986;Staras et al, 1998b). Second, the feeding system is known in cellular detail (Benjamin et al, 2000), allowing the effect of sensory inputs to be studied simultaneously on modulatory neurons, CPG neurons, and motoneurons.…”