Insulin and memory in Lymnaea.

The pond snail, Lymnaea stagnalis, is capable of learning conditioned taste aversion (CTA) and consolidating this CTA into long-term memory (LTM). The DNA microarray experiments showed that some of molluscan insulin-related peptides (MIPs) were up-regulated in snails exhibiting CTA-LTM. On the other hand, the electrophysiological experiments showed that application of secretions from the MIPs-containing cells evoked long-term potentiation (LTP) at the synapses between the cerebral giant cell (a key interneuron for CTA) and the B1 motoneuron (a buccal motoneuron). We thus hypothesized that MIPs and MIP receptors play an important role at the synapses, probably underlying the CTA-LTM consolidation process. To examine this hypothesis, we applied the antibody, which recognizes the binding site of mammalian insulin receptors and is thought to cross-react MIP receptors, to the Lymnaea CNS. Our present data showed that an application of the antibody for insulin receptors to the isolated CNS blocked LTP, and that an injection of the antibody into the Lymnaea abdominal cavity inhibited LTM consolidation, but not CTA formation.