| Research Abstract |
When an appetitive stimulus (sucrose) is presented with an aversive stimulus (potassium chloride) to the pond snails Lymnaea stagnahs, associative learning, which is referred to as conditioned taste aversion (CTA), is quickly acquired and persisted for up to a month in snails. Previous results obtained by our cellular analyses showed that the cerebral giant cell is the key neuron for the formation of learning and the consolidation of long-term memory for CTA in L.stagnalis. In the present study, the molecular mechanism of CTA was analyzed with paying attention to a transcription factor. The transcription factor, cyclic AMP responsive element binding proteins (CREB), is considered to play major roles in the consolidation of long-term memory in various animals. We therefore cloned the cDNAs in L.stagnalis and analyzed their mRNA localization in identifiable neurons in the central nervous system. We found that the cerebral giant cells contain these mRNAs. Further, the splicing variants of these genes were studied, and 7 isoforms were found for CREB1 (so called "activator"). However, three of these CREB1 isoforms do not have any potential sites of phosphorylation by protein kinase A in their predicted amino acid sequences. This result suggests that the functions of these 3 isoforms as transcription factors differ from the other 4 isoforms and that all the isoforms affect to each other, resulting in the change in their DNA binding ability. Finally, we succeeded in measuring the changes in copy numbers of the CREB mRNAs in the cerebral giant cells during CTA by single-cell real-time quantitative RT-PCR. The reduction of CREB2 (represser) was found to be essential for the consolidation of long-term memory.
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