Altered gene activity correlated with long‐term memory formation of conditioned taste aversion in Lymnaea

Abstract

The pond snail Lymnaea stagnalis is capable of learning conditioned taste aversion (CTA) and then consolidating that learning into long-term memory (LTM) that persists for at least 1 month. LTM requires de novo protein synthesis and altered gene activity. Changes in gene activity in Lymnaea that are correlated with, much less causative, memory formation have not yet been identified. As a first step toward rectifying this situation, we constructed a cDNA microarray with mRNAs extracted from the central nervous system (CNS) of Lymnaea. We then, using this microarray assay, identified genes whose activity either increased or decreased following CTA memory consolidation. We also identified genes whose expression levels were altered after inhibition of the cyclic AMP response element-binding protein (CREB) that is hypothesized to be a key transcription factor for CTA memory. We found that the molluscan insulin-related peptide II (MIP II) was up-regulated during CTA-LTM, whereas the gene encoding pedal peptide preprohormone (Pep) was down-regulated by CREB2 RNA interference. We next examined mRNAs of MIP II and Pep using real-time RT-PCR with SYBR Green. The MIP II mRNA level in the CNS of snails exhibiting "good" memory for CTA was confirmed to be significantly higher than that from the CNS of snails exhibiting "poor" memory. In contrast, there was no significant difference in expression levels of the Pep mRNA between "good" and "poor" performers. These data suggest that in Lymnaea MIP II may play a role in the consolidation process that forms LTM following CTA training.

Involvement of Insulin-Like Peptide in Long-Term Synaptic Plasticity and Long-Term Memory of the Pond SnailLymnaea stagnalis

“…Kojima et al (1997) showed that activity in the CGC caused a large and long-lasting polysynaptic inhibitory effect on the N1M cell, which is one of the feeding central pattern generator neurons, in snails exhibiting CTA-LTM. This can be explained by action of the cAMP-protein kinase A (PKA)-cAMP response element-binding protein (CREB) pathway in the CGC (Nakamura et al, 1999b;Sadamoto et al, 2004Sadamoto et al, , 2010Sadamoto et al, , 2011Wagatsuma et al, 2005Wagatsuma et al, , 2006. These results suggest that CGC-B1 synapses serve as a good model in which to elucidate how changes in synaptic transmission amplitude, for example, serve as a neural correlate of behavioral memory.…”

“…The existence of activator and repressor types of CREB, CREB1 and CREB2, has also been demonstrated in the CGCs and other neurons (Ribeiro et al, 2003;Sadamoto et al, 2004Sadamoto et al, , 2010Sadamoto et al, , 2011Wagatsuma et al, 2005Wagatsuma et al, , 2006. Moreover, prior injection of the CRE oligonucleotide or a PKA inhibitor into the CGC blocked the expression of this long-lasting change in synaptic plasticity at the CGC-B1 synapses (Nakamura et al, 1999b;Sadamoto et al, 2004).…”

“…Azami et al (2006) found that in so-called "good" learners, but not "poor" learners, that some of the upregulated genes found in the CNS were the genes encoding MIPs. Although they referred to snails that showed LTM as good learners, it probably would have been more appropriate to call them snails with good memory, because that was what they were measuring.…”

“…However, whether there is an increase in MIP synthesis or release as a result of the training procedure is not known. We do know that the genes encoding MIPs are upregulated in good learners (i.e., snails that exhibit good memory; Azami et al, 2006). However, it is possible that in the poor learners that in the naive state MIP levels were down.…”

Paired pulse ratio analysis of insulin-induced synaptic plasticity in the snail brain

“…Our group has primarily focused our attention on conditioned taste aversion (CTA) in this model system (Kojima et al, 1996). We previously found that molluscan insulin-related peptides (MIPs) were upregulated in snails exhibiting CTA (Azami et al, 2006). More recently, we have shown at the electrophysiological level that application of mammalian insulin or MIPs to the isolated central nervous system (CNS) evoked long-term synaptic enhancement at the serotonergic excitatory monosynaptic connection between the cerebral giant cell (CGC) and the buccal 1 (B1) motor neuron (Murakami et al, 2013).…”