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The neural circuitry comprising the central pattern generator (CPG) that drives feeding behavior in the great pond snail (Lymnaea stagnalis (L., 1758)) has been worked out. Because the feeding behavior undergoes associative learning and long-term memory (LTM) formation, it provides an excellent opportunity to study the causal neuronal mechanisms of these two processes. In this review, we explore some of the possible causal neuronal mechanisms of associative learning of conditioned taste aversion (CTA) and its subsequent consolidation processes into LTM in L. stagnalis. In the CTA training procedure, a sucrose solution, which evokes a feeding response, is used as the conditioned stimulus (CS) and a potassium chloride solution, which causes a withdrawal response, is used as the unconditioned stimulus (US). The pairing of the CS-US alters both the feeding response of the snail and the function of a pair of higher order interneurons in the cerebral ganglia. Following the acquisition of CTA, the polysynaptic inhibitory synaptic input from the higher order interneurons onto the feeding CPG neurons is enhanced, resulting in suppression of the feeding response. These changes in synaptic efficacy are thought to constitute a 'memory trace' for CTA in L. stagnalis. Key words: conditioned taste aversion, feeding, long-term memory, Lymnaea stagnalis, withdrawal. Les circuits neuronaux constituant le generateur central de patrons (CPG) qui regit le comportement d'alimentation de la grande limnee (Lymnaea stagnalis (L., 1758)) ont ete decrits. Etant donne que le comportement d'alimentation est assujetti a l'apprentissage associatif et a la formation de la memoire a long terme (LTM), il presente une excellente occasion d'etudier les mecanismes neuronaux causaux de ces deux processus. Dans la presente synthese, nous examinons certains de mecanismes neuronaux causaux possibles de l'apprentissage associatif de l'aversion gustative conditionnee (CTA) et les processus de consolidation subsequente associes dans la LTM de L. stagnalis. Dans la procedure d'apprentissage de la CTA, une solution de sucrose, qui provoque une reaction d'alimentation, est utilisee comme stimulus conditionne (CS), et une solution de chlorure de potassium, qui provoque une reaction de retrait, est utilisee comme stimulus non conditionne (US). Le jumelage de CS-US modifie la reaction d'alimentation de l'escargot et la fonction d'une paire d'interneurones d'ordre superieur dans le ganglion cerebral. Apres l'acquisition de la CTA, le signal synaptique inhibiteur polysynaptique des interneurones d'ordre superieur vers les neurones associes a l'alimentation du CPG est rehausse, entrainant la suppression de la reaction d'alimentation. Ces changements de l'efficacite synaptique constitueraient une << trace de la memoire >> pour la CTA chez les L. stagnalis. [Traduit par la Redaction] Mots-cles : aversion gustative conditionnee, alimentation, memoire a long terme, Lymnaea stagnalis, retrait., Introduction In many respects, the birth of modern neuroscience occurred in the 1950s. In our view, two seminal events happened. The first was the brain surgery performed on a patient [...]

Bryostatin, a potent agonist of protein kinase C (PKC), when administered to Hermissenda was found to affect acquisition of an associative learning paradigm. Low bryostatin concentrations (0.1 to 0.5 ng/ml) enhanced memory acquisition, while concentrations higher than 1.0 ng/ml down-regulated the pathway and no recall of the associative training was exhibited. The extent of enhancement depended upon the conditioning regime used and the memory stage normally fostered by that regime. The effects of two training events (TEs) with paired conditioned and unconditioned stimuli, which standardly evoked only short-term memory (STM) lasting 7 min, were--when bryostatin was added concurrently--enhanced to a long-term memory (LTM) that lasted about 20 h. The effects of both 4- and 6-paired TEs (which by themselves did not generate LTM), were also enhanced by bryostatin to induce a consolidated memory (CM) that lasted at least 5 days. The standard positive 9-TE regime typically produced a CM lasting at least 6 days. Low concentrations of bryostatin ( Sharp-electrode intracellular recordings of type-B photoreceptors in the eyes from animals conditioned in vivo with bryostatin revealed changes in input resistance and an enhanced long-lasting depolarization (LLD) in response to light. Likewise, quantitative immunocytochemical measurements using an antibody specific for the PKC-activated [Ca.sup.2+]/GTP-binding protein calexcitin showed enhanced antibody labeling with bryostatin. Animals exposed to the PKC inhibitor bisindolylmaleimide-XI (Ro-32-0432) administered by immersion prior to 9-TE conditioning showed no training-induced changes with or without bryostatin exposure. However, if animals received bryostatin before Ro-32, the enhanced acquisition and demonstrated recall still occurred. Therefore, pathways responsible for the enhancement effects induced by bryostatin were putatively mediated by PKC. Overall, the data indicated that PKC activation occurred and calexcitin levels were raised during the acquisition phases of associative conditioning and memory initiation, and subsequently returned to baseline levels within 24 and 48 h, respectively. Therefore, the protracted recall measured by the testing regime used was probably due to bryostatin-induced changes during the acquisition and facilitated storage of memory, and not necessarily to enhanced recall of the stored memory when tested many days after training., Introduction Many aspects of memory and recall can be studied especially well in the nudibranch Hermissenda because the temporal specificity of experimental effects can be determined within 2 or 3 [...]