Expression and distribution of transcription factor CCAAT/enhancer-binding protein in the central nervous system of Lymnaea stagnalis

Improvements in operant memory of Aplysia are correlated with age and specific gene expression

The transcription factor Aplysia CCAAT/enhancer binding protein (ApC/EBP) is expressed as an immediate early gene in the cAMP responsive element binding protein (CREB) mediated gene cascade, and it has essential functions in the synaptic consolidation of memory following a learning event. Synaptic consolidation primarily involves morphological changes at neuronal synapses, which are facilitated through the reorganization of the actin and microtubular cytoarchitecture of the cell. During early nervous system development, the transmembrane synaptic protein teneurin acts directly upon neuronal presynaptic microtubules and postsynaptic spectrin-based cytoskeletons to facilitate the creation of new synapses. It is reasonable to hypothesize that teneurin may also be linked to learning-induced synaptic changes and is a potential candidate to be a later gene expressed in the CREB-mediated gene cascade downstream of ApC/EBP. To assess the role of ApC/EBP and teneurin in learning and memory in the marine snail Aplysia californica, young (age 7-8 months) and aged (age 13-15 months; aging stage AII) siblings of Aplysia were trained in an operant conditioning paradigm-learning food is inedible (LFI)-over 2 days, during which they learned to modify the feeding reflex. Aged Aplysia had enhanced performance of the LFI task on the second day than younger siblings although far more aged animals were excluded from the analysis because of the initial failure in learning to recognize the inedible probe. After 2 days of training, ApC/EBP isoform X1 mRNA and teneurin mRNA were quantified in selected neurons of the buccal ganglia, the locus of neural circuits in LFI. Teneurin expression was elevated in aged Aplysia compared to young siblings regardless of training. ApC/EBP isoform X1 expression was significantly higher in untrained aged animals than in untrained young siblings but decreased in trained aged animals compared to untrained aged animals. Elevated levels of ApC/EBP isoform X1 and teneurin mRNA before training may have contributed to the enhancement of LFI performance in the aged animals that successfully learned.

Expression Level Changes in Serotonin Transporter are Associated with Food Deprivation in the Pond Snail Lymnaea stagnalis

In the pond snail Lymnaea stagnalis, serotonin (5-HT) plays an important role in feeding behavior and its associated learning (e.g., conditioned taste aversion: CTA). The 5-HT content in the central nervous system (CNS) fluctuates with changes in the nutritional status, but it is also expected to be influenced by changes in the serotonin transporter (SERT) expression level. In the present study, we identified SERT in Lymnaea and observed its localization in 5-HTergic neurons, including the cerebral giant cells (CGCs) in the cerebral ganglia and the pedal A cluster neurons and right and left pedal dorsal 1 neurons in the pedal ganglia by in situ hybridization. Real-time PCR revealed that the SERT mRNA expression level was lower under severe food deprivation than under mild food deprivation in the whole CNS as well as in a single CGC. These results inversely correlated with previous data that the 5-HT content in the CNS was higher in the severely food-deprived state than in the mildly food-deprived state. Furthermore, in single CGCs, we observed that the 5-HT level was significantly increased in the severely food-deprived state compared with the mildly food-deprived state. Our present findings suggest that changes in the SERT expression level associated with food deprivation may affect 5-HT signaling, probably contributing to learning and memory mechanisms in Lymnaea.

Identification of Putative Molecules for Adiponectin and Adiponectin Receptor and Their Roles in Learning and Memory in Lymnaea stagnalis

Adiponectin enhances insulin sensitivity, which improves cognition in mammals. How adiponectin affects the mechanism’s underlying cognition, however, remains unknown. We hypothesized that experiments using the pond snail Lymnaea stagnalis, which has long been used in learning and memory studies and in which the function of insulin-like peptides affect learning and memory, could clarify the basic mechanisms by which adiponectin affects cognition. We first identified putative molecules of adiponectin and its receptor in Lymnaea. We then examined their distribution in the central nervous system and changes in their expression levels when hemolymph glucose concentrations were intentionally decreased by food deprivation. We also applied an operant conditioning protocol of escape behavior to Lymnaea and examined how the expression levels of adiponectin and its receptor changed after the conditioned behavior was established. The results demonstrate that adiponectin and adiponectin’s receptor expression levels were increased in association with a reduced concentration of hemolymph glucose and that expression levels of both adiponectin and insulin-like peptide receptors were increased after the conditioning behavior was established. Thus, the involvement of the adiponectin-signaling cascade in learning and memory in Lymnaea was suggested to occur via changes in the glucose concentrations and the activation of insulin.

Molecular and functional characterization of an evolutionarily conserved CREB-binding protein in the Lymnaea CNS

In eukaryotes, CREB-binding protein (CBP), a coactivator of CREB, functions both as a platform for recruiting other components of the transcriptional machinery and as a histone acetyltransferase (HAT) that alters chromatin structure. We previously showed that the transcriptional activity of cAMP-responsive element binding protein (CREB) plays a crucial role in neuronal plasticity in the pond snail Lymnaea stagnalis. However, there is no information on the molecular structure and HAT activity of CBP in the Lymnaea central nervous system (CNS), hindering an investigation of its postulated role in long-term memory (LTM). Here, we characterize the Lymnaea CBP (LymCBP) gene and identify a conserved domain of LymCBP as a functional HAT. Like CBPs of other species, LymCBP possesses functional domains, such as the KIX domain, which is essential for interaction with CREB and was shown to regulate LTM. In-situ hybridization showed that the staining patterns of LymCBP mRNA in CNS are very similar to those of Lymnaea CREB1. A particularly strong LymCBP mRNA signal was observed in the cerebral giant cell (CGC), an identified extrinsic modulatory interneuron of the feeding circuit, the key to both appetitive and aversive LTM for taste. Biochemical experiments using the recombinant protein of the LymCBP HAT domain showed that its enzymatic activity was blocked by classical HAT inhibitors. Preincubation of the CNS with such inhibitors blocked cAMP-induced synaptic facilitation between the CGC and an identified follower motoneuron of the feeding system. Taken together, our findings suggest a role for the HAT activity of LymCBP in synaptic plasticity in the feeding circuitry.

Immediate-Early Genes Detection in the CNS of Terrestrial Snail

In the present work, using in situ hybridization, we studied the expression patterns of three molluscan homologs of vertebrate immediate-early genes C/EBP, c-Fos, and c-Jun in the central nervous system (CNS) of terrestrial gastropod snail Helix. The molluscan C/EBP gene was described in literature, while c-Fos and c-Jun were studied in terrestrial snails for the first time. Localization of the expression was traced in normal conditions, and in preparations physiologically activated using stimulation of suboesophageal ganglia nerves. No expression was detected constitutively. In stimulated preparations, all three genes had individual expression patterns in Helix CNS, and the level of expression was stimulus-dependent. The number of cells expressing the gene of interest was different from the number of cells projecting to the stimulated nerve, and thus activated retrogradely. This difference depended on the ganglia studied. At the subcellular level, the labeled RNA was observed as dots (probably small clusters of RNA molecules) and shapeless mass of RNA, often seen as a circle at the internal border of the cell nuclei. The data provide a basis for further study of behavioral role of these putative immediate-early genes in snail behavior and learning.

Lymnaea stagnalis as model for translational neuroscience research: From pond to bench

The purpose of this review is to illustrate how a reductionistic, but sophisticated, approach based on the use of a simple model system such as the pond snail Lymnaea stagnalis (L. stagnalis), might be useful to address fundamental questions in learning and memory. L. stagnalis, as a model, provides an interesting platform to investigate the dialog between the synapse and the nucleus and vice versa during memory and learning. More importantly, the "molecular actors" of the memory dialogue are well-conserved both across phylogenetic groups and learning paradigms, involving single- or multi-trials, aversion or reward, operant or classical conditioning. At the same time, this model could help to study how, where and when the memory dialog is impaired in stressful conditions and during aging and neurodegeneration in humans and thus offers new insights and targets in order to develop innovative therapies and technology for the treatment of a range of neurological and neurodegenerative disorders.

Identification of Immediate Early Genes in the Nervous System of Snail Helix lucorum

Immediate early genes (IEGs) are useful markers of neuronal activation and essential components of neuronal response. While studies of gastropods have provided many insights into the basic learning and memory mechanisms, the genome-wide assessment of IEGs has been mainly restricted to vertebrates. In this study, we identified IEGs in the terrestrial snail Helix lucorum In the absence of the genome, we conducted de novo transcriptome assembly using reads with short and intermediate lengths cumulatively covering more than 98 billion nucleotides. Based on this assembly, we identified 37 proteins corresponding to contigs differentially expressed (DE) in either the parietal ganglia (PaG) or two giant interneurons located within the PaG of the snail in response to the neuronal stimulation. These proteins included homologues of well-known mammalian IEGs, such as c-jun/jund, C/EBP, c-fos/fosl2, and Egr1, as well as homologues of genes not yet implicated in the neuronal response.

Monoamines, Insulin and the Roles They Play in Associative Learning in Pond Snails

Molluscan gastropods have long been used for studying the cellular and molecular mechanisms underlying learning and memory. One such gastropod, the pond snail Lymnaea stagnalis, exhibits long-term memory (LTM) following both classical and operant conditioning. Using Lymnaea, we have successfully elucidated cellular mechanisms of learning and memory utilizing an aversive classical conditioning procedure, conditioned taste aversion (CTA). Here, we present the behavioral changes following CTA training and show that the memory score depends on the duration of food deprivation. Then, we describe the relationship between the memory scores and the monoamine contents of the central nervous system (CNS). A comparison of learning capability in two different strains of Lymnaea, as well as the filial 1 (F1) cross from the two strains, presents how the memory scores are correlated in these populations with monoamine contents. Overall, when the memory scores are better, the monoamine contents of the CNS are lower. We also found that as the insulin content of the CNS decreases so does the monoamine contents which are correlated with higher memory scores. The present review deepens the relationship between monoamine and insulin contents with the memory score.

Function of insulin in snail brain in associative learning

Insulin is well known as a hormone regulating glucose homeostasis across phyla. Although there are insulin-independent mechanisms for glucose uptake in the mammalian brain, which had contributed to a perception of the brain as an insulin-insensitive organ for decades, the finding of insulin and its receptors in the brain revolutionized the concept of insulin signaling in the brain. However, insulin's role in brain functions, such as cognition, attention, and memory, remains unknown. Studies using invertebrates with their open blood-vascular system have the promise of promoting a better understanding of the role played by insulin in mediating/modulating cognitive functions. In this review, the relationship between insulin and its impact on long-term memory (LTM) is discussed particularly in snails. The pond snail Lymnaea stagnalis has the ability to undergo conditioned taste aversion (CTA), that is, it associatively learns and forms LTM not to respond with a feeding response to a food that normally elicits a robust feeding response. We show that molluscan insulin-related peptides are up-regulated in snails exhibiting CTA-LTM and play a key role in the causal neural basis of CTA-LTM. We also survey the relevant literature of the roles played by insulin in learning and memory in other phyla.

From likes to dislikes: conditioned taste aversion in the great pond snail (Lymnaea stagnalis)

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.

Increase in cyclic AMP concentration in a cerebral giant interneuron mimics part of a memory trace for conditioned taste aversion of the pond snail

Conditioned taste aversion (CTA) can be classically conditioned in the pond snail Lymnaea stagnalis and subsequently be consolidated into long-term memory (LTM). The neural trace that subserves CTA-LTM can be summarized as follows: A polysynaptic inhibitory postsynaptic potential recorded in the neuron 1 medial (N1M) cell in the conditioned snails as a result of activation of the cerebral giant cell (CGC) is larger and lasts longer than that in control snails. The N1M cell is ultimately activated by the CGC via the neuron 3 tonic (N3t) cell. That is, the inhibitory monosynaptic inputs from the N3t cell to the N1M cell are facilitated. The N1M and N3t cells are the members of feeding central pattern generator, whereas the CGC is a multimodal interneuron thought to play a key role in feeding behavior. Here we examined the involvement of a second messenger, cAMP, in the establishment of the memory trace. We injected cAMP into the CGC and monitored the potentials of the B3 motor neuron activated by the CGC. B3 activity is used as an index for the synaptic inputs from the N3t cell to the N1M cell. We found that the B3 potentials were transiently enlarged. Thus, when the cAMP concentration is increased in the CGC by taste aversion training, cAMP-induced changes may play a key role in the establishment of a memory trace in the N3t cell.

Identification of the role of C/EBP in neurite regeneration following microarray analysis of a L. stagnalis CNS injury model

Background

Neuronal regeneration in the adult mammalian central nervous system (CNS) is severely compromised due to the presence of extrinsic inhibitory signals and a reduced intrinsic regenerative capacity. In contrast, the CNS of adult Lymnaea stagnalis (L. stagnalis), a freshwater pond snail, is capable of spontaneous regeneration following neuronal injury. Thus, L. stagnalis has served as an animal model to study the cellular mechanisms underlying neuronal regeneration. However, the usage of this model has been limited due to insufficient molecular tools. We have recently conducted a partial neuronal transcriptome sequencing project and reported over 10,000 EST sequences which allowed us to develop and perform a large-scale high throughput microarray analysis.

Results

To identify genes that are involved in the robust regenerative capacity observed in L. stagnalis, we designed the first gene chip covering ~15, 000 L. stagnalis CNS EST sequences. We conducted microarray analysis to compare the gene expression profiles of sham-operated (control) and crush-operated (regenerative model) central ganglia of adult L. stagnalis. The expression levels of 348 genes were found to be significantly altered (p < 0.05) following nerve injury. From this pool, 67 sequences showed a greater than 2-fold change: 42 of which were up-regulated and 25 down-regulated. Our qPCR analysis confirmed that CCAAT enhancer binding protein (C/EBP) was up-regulated following nerve injury in a time-dependent manner. In order to test the role of C/EBP in regeneration, C/EBP siRNA was applied following axotomy of cultured Lymnaea PeA neurons. Knockdown of C/EBP following axotomy prevented extension of the distal, proximal and intact neurites. In vivo knockdown of C/EBP postponed recovery of locomotory activity following nerve crush. Taken together, our data suggest both somatic and local effects of C/EBP are involved in neuronal regeneration.

Conclusions

This is the first high-throughput microarray study in L. stagnalis, a model of axonal regeneration following CNS injury. We reported that 348 genes were regulated following central nerve injury in adult L. stagnalis and provided the first evidence for the involvement of local C/EBP in neuronal regeneration. Our study demonstrates the usefulness of the large-scale gene profiling approach in this invertebrate model to study the molecular mechanisms underlying the intrinsic regenerative capacity of adult CNS neurons.

A homolog of the vertebrate pituitary adenylate cyclase-activating polypeptide is both necessary and instructive for the rapid formation of associative memory in an invertebrate

Similar to other invertebrate and vertebrate animals, cAMP-dependent signaling cascades are key components of long-term memory (LTM) formation in the snail Lymnaea stagnalis, an established experimental model for studying evolutionarily conserved molecular mechanisms of long-term associative memory. Although a great deal is already known about the signaling cascades activated by cAMP, the molecules involved in the learning-induced activation of adenylate cyclase (AC) in Lymnaea remained unknown. Using matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy in combination with biochemical and immunohistochemical methods, recently we have obtained evidence for the existence of a Lymnaea homolog of the vertebrate pituitary adenylate cyclase-activating polypeptide (PACAP) and for the AC-activating effect of PACAP in the Lymnaea nervous system. Here we first tested the hypothesis that PACAP plays an important role in the formation of robust LTM after single-trial classical food-reward conditioning. Application of the PACAP receptor antagonist PACAP6-38 around the time of single-trial training with amyl acetate and sucrose blocked associative LTM, suggesting that in this "strong" food-reward conditioning paradigm the activation of AC by PACAP was necessary for LTM to form. We found that in a "weak" multitrial food-reward conditioning paradigm, lip touch paired with sucrose, memory formation was also dependent on PACAP. Significantly, systemic application of PACAP at the beginning of multitrial tactile conditioning accelerated the formation of transcription-dependent memory. Our findings provide the first evidence to show that in the same nervous system PACAP is both necessary and instructive for fast and robust memory formation after reward classical conditioning.

Glutamate transporters in the central nervous system of a pond snail

Previous studies on glutamate (GLU) and its receptors in the pond snail Lymnaea stagnalis have suggested that GLU functions as a neurotransmitter in various behaviors, particularly for generation of feeding rhythm. The uptake mechanism of GLU is not yet known in Lymnaea. In the present study, we characterized the GLU transporters and examined their functions in the feeding circuits of the central nervous system (CNS) in Lymnaea. First, measurement of the accumulation of (3)H-labeled GLU revealed the presence of GLU transport systems in the Lymnaea CNS. The highest accumulation rate was observed in the buccal ganglia, supporting the involvement of GLU transport systems in feeding behavior. Second, we cloned two types of GLU transporters from the Lymnaea CNS, the excitatory amino acid transporter (LymEAAT) and the vesicular GLU transporter (LymVGLUT). When we compared their amino acid sequences with those of mammalian EAATs and VGLUTs, we found that the functional domains of both types are well conserved. Third, in situ hybridization revealed that the mRNAs of LymEAAT and LymVGLUT are localized in large populations of nerve cells, including the major feeding motoneurons in the buccal ganglia. Finally, we inhibited LymEAAT and found that changes in the firing patterns of the feeding motoneurons that have GLUergic input were similar to those obtained following stimulation with GLU. Our results confirmed the presence of GLU uptake systems in the Lymnaea CNS and showed that LymEAAT is required for proper rhythm generation, particularly for generation of the feeding rhythm.

De novo protein synthesis of syntaxin-1 and dynamin-1 in long-term memory formation requires CREB1 gene transcription in Lymnaea stagnalis

Consolidation of aversive operant conditioning into long-term memory (LTM) requires CREB-dependent de novo protein synthesis. The newly synthesized proteins are distributed to the synapses in neurons that are involved in memory formation and storage. Accumulating evidence indicates that the presynaptic release mechanisms also play a role in long-term synaptic plasticity. Our understanding of whether the presynaptic proteins undergo de novo synthesis during long-term memory formation is limited. In this study, we investigated the involvement of syntaxin-1, a presynaptic exocytotic protein, and dynamin-1, an endocytotic protein, in the formation of long-term memory. We took advantage of a well-established aversive operant conditioning model of aerial respiratory behavior in the fresh water pond snail Lymnaea stagnalis, and demonstrated that the LTM formation is associated with increased expression of syntaxin-1 and dynamin-1, coincident with elevated levels of CREB1. Partial knockdown of CREB1 gene by double stranded RNA inhibition (dsRNAi) prior to operant conditioning prevented snails from memory consolidation, and reduced the expression of syntaxin-1 and dynamin-1 at both mRNA and protein levels. These findings suggest that CREB1-mediated gene expression is required for the LTM-induced up-regulation of synaptic proteins, syntaxin-1 and dynamin-1, in L. stagnalis. Our study thus offers new insights into the molecular mechanisms that mediate CREB1-dependent long-term memory formation.

One-trial conditioned taste aversion in Lymnaea: good and poor performers in long-term memory acquisition

In the majority of studies designed to elucidate the causal mechanisms of memory formation, certain members of the experimental cohort, even though subjected to exactly the same conditioning procedures, remember significantly better than others, whereas others show little or no long-term memory (LTM) formation. To begin to address the question of why this phenomenon occurs and thereby help clarify the causal mechanism of LTM formation, we used a conditioned taste aversion (CTA) procedure on individuals of the pond snail Lymnaea stagnalis and analyzed their subsequent behavior. Using sucrose as an appetitive stimulus and KCl as an aversive stimulus, we obtained a constant ratio of ;poor' to ;good' performers for CTA-LTM. We found that approximately 40% of trained snails possessed LTM following a one-trial conditioning procedure. When we examined the time-window necessary for the memory consolidation, we found that if we cooled snails to 4 degrees C for 30 min within 10 min after the one-trial conditioning, LTM was blocked. However, with delayed cooling (i.e. longer than 10 min), LTM was present. We could further interfere with LTM formation by inducing inhibitory learning (i.e. backward conditioning) after the one-trial conditioning. Finally, we examined whether we could motivate snails to acquire LTM by depriving them of food for 5 days before the one-trial conditioning. Food-deprived snails, however, failed to exhibit LTM following the one-trial conditioning. These results will help us begin to clarify why some individuals are better at learning and forming memory for specific tasks at the neuronal level.

Gaseous neuromodulator-related genes expressed in the brain of honeybee Apis mellifera

Nitric oxide (NO), hydrogen sulfide (H2S), and carbon monoxide (CO) are thought to act as gaseous neuromodulators in the brain across species. For example, in the brain of honeybee Apis mellifera, NO plays important roles in olfactory learning and discrimination, but the existence of H2S- and CO-mediated signaling pathways remains unknown. In the present study, we identified the genes of nitric oxide synthase (NOS), soluble guanylyl cyclase (sGC), cystathionine beta-synthase (CBS), and heme oxygenase (HO) from the honeybee brain. The honeybee brain contains at least one gene for each of NOS, CBS, and HO. The deduced proteins for NOS, CBS, and HO are thought to contain domains to generate NO, H2S, and CO, respectively, and to contain putative Ca2+/calmodulin-binding domains. On the other hand, the honeybee brain contains three subunits of sGC: sGCalpha1, sGCbeta1, and sGCbeta3. Phylogenetic analysis of sGC revealed that Apis sGCalpha1 and sGCbeta1 are closely related to NO- and CO-sensitive sGC subunits, whereas Apis sGCbeta3 is closely related to insect O2-sensitive sGC subunits. In addition, we performed in situ hybridization for Apis NOS mRNA and NADPH-diaphorase histochemistry in the honeybee brain. The NOS gene was strongly expressed in the optic lobes and in the Kenyon cells of the mushroom bodies. NOS activity was detected in the optic lobes, the mushroom bodies, the central body complex, the lateral protocerebral lobes, and the antennal lobes. These findings suggest that NO is involved in various brain functions and that H2S and CO can be endogenously produced in the honeybee brain.

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

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.

Comparative study of visuo-vestibular conditioning in Lymnaea stagnalis

In this review, we compare the current understanding of visuo-vestibular conditioning in Hermissenda crassicornis and Lymnaea stagnalis on the basis of behavioral, electrophysiologic, and morphologic studies. Paired presentation of a photic conditioned stimulus (CS) and an orbital rotation unconditioned stimulus (US) results in conditioned escape behavior in both species. In Hermissenda, changes in excitability of type B photoreceptors and morphologic modifications at the axon terminals follow conditioning. Caudal hair cells, which detect mechanical turbulence, have reciprocal inhibition with type B photoreceptors. In Lymnaea, the interaction between photoreceptors and hair cells is dependent on statocyst location. Furthermore, the organization of the Lymnaea eye is complex, with more than 100 photoreceptors distributed in a uniquely folded retina. Although the optimal conditions to produce long-term memory (memory persistent for >1 week) are almost identical in Hermissenda and Lymnaea, physiologic and morphologic differences suggest that the neuronal mechanisms underlying learning and memory are distinct.

Taste discrimination in conditioned taste aversion of the pond snail Lymnaea stagnalis

Conditioned taste aversion (CTA) in the pond snail Lymnaea stagnalis has been widely used as a model for gaining an understanding of the molecular and behavioral mechanisms underlying learning and memory. At the behavioral level, however, it is still unclear how taste discrimination and CTA interact. We thus examined how CTA to one taste affected the feeding response induced by another appetitive food stimulus. We first demonstrated that snails have the capacity to recognize sucrose and carrot juice as distinct appetitive stimuli. We then found that snails can become conditioned(i.e. CTA) to avoid one of the stimuli and not the other. These results show that snails can distinguish between appetitive stimuli during CTA, suggesting that taste discrimination is processed upstream of the site where memory consolidation in the snail brain occurs. Moreover, we examined second-order conditioning with two appetitive stimuli and one aversive stimulus. Snails acquired second-order conditioning and were still able to distinguish between the different stimuli. Finally, we repeatedly presented the conditional stimulus alone to the conditioned snails, but this procedure did not extinguish the long-term memory of CTA in the snails. Taken together, our data suggest that CTA causes specific, irreversible and rigid changes from appetitive stimuli to aversive ones in the conditioning procedure.

De Novo synthesis of CREB in a presynaptic neuron is required for synaptic enhancement involved in memory consolidation

Interaction between the activator type of cyclic AMP response element binding protein (CREB1) and the repressor type (CREB2) results in determining the emergence of long-lasting synaptic enhancement involved in memory consolidation. However, we still do not know whether the constitutively expressed forms of CREB are enough or the newly synthesized forms are required for the synaptic enhancement. In addition, if the newly synthesized forms are needed, we must determine the time for translation of CREB from its mRNA. We applied the methods of RNA interference and real-time polymerase chain reaction (PCR) to CREB in the cerebral giant cells of Lymnaea. The cerebral giant cells play an important role in associative learning and employ a CREB cascade for the synaptic enhancement to neurons such as the B1 motoneurons. We injected the small interfering RNA (siRNA) of CREB1 or CREB2 into the cerebral giant cells and examined the changes in amplitude of excitatory postsynaptic potential (EPSP) recorded in the B1 motoneurons. The changes in the amounts of CREB1 and CREB2 mRNAs were also examined in the cerebral giant cells. The EPSP amplitude was suppressed 15 min after injection of CREB1 siRNA, whereas that was augmented 60 min after injection of CREB2 siRNA. In the latter case, the decrease in the amount of CREB2 mRNA was confirmed by real-time PCR. Our results showed that the de novo synthesized forms of CREB are required within tens of minutes for the synaptic enhancement in memory consolidation.

Requirement of new protein synthesis of a transcription factor for memory consolidation: paradoxical changes in mRNA and protein levels of C/EBP

Some specific transcription factors are essential for memory consolidation across species. However, it is still unclear whether only the activation of constitutively expressed forms of these conserved transcription factors is involved in memory consolidation or their de novo synthesis also occurs after learning. This question has remained unanswered partly because of the lack of an efficient method for the determination of copy numbers of particular mRNAs in single neurons, which allows the detection of new transcription at the cellular level. Here we applied a newly developed protocol of single-cell quantitative real-time polymerase chain reaction (qRT-PCR) to single neurons playing an important role in associative learning. Specifically, we examined the changes in the mRNA and protein expression levels of a highly conserved transcription factor, CCAAT/enhancer binding protein (C/EBP), in the paired B2 motoneurons of the pond snail Lymnaea stagnalis. These buccal neurons are involved in the motor control of feeding behavior, with a potentially important role in conditioned taste aversion (CTA). Single-cell qRT-PCR revealed a significant decrease in LymC/EBP mRNA copy numbers in the B2 motoneurons during memory consolidation after CTA training. By contrast, isoelectric focusing and immunoblotting of extracts of the buccal ganglia showed that translation and phosphorylation levels of LymC/EBP significantly increased during memory consolidation. The C/EBP-like immunoreactivity in the B2 motoneurons, which are the major immunopositive component in the buccal ganglia, also significantly increased during memory consolidation, suggesting that the main source of increase in the level of protein in the buccal ganglia are the B2 motoneurons. Thus, early memory consolidation after CTA learning in L.stagnalis involves both the rapid synthesis and phosphorylation of LymC/EBP as well as the rapid breakdown of LymC/EBP mRNA in the neural network controlling feeding, suggesting that all of these processes play a role in the function of C/EBP in memory consolidation.

Determination of the exact copy numbers of particular mRNAs in a single cell by quantitative real-time RT-PCR

Gene expression is differently regulated in every cell even though the cells are included in the same tissue. For this reason, we need to measure the amount of mRNAs in a single cell to understand transcription mechanism better. However, there are no accurate, rapid and appropriate methods to determine the exact copy numbers of particular mRNAs in a single cell. We therefore developed a procedure for isolating a single, identifiable cell and determining the exact copy numbers of mRNAs within it. We first isolated the cerebral giant cell of the pond snail Lymnaea stagnalis as this neuron plays a key role in the process of memory consolidation of a learned behavior brought about by associative learning of feeding behavior. We then determined the copy numbers of mRNAs for the cyclic AMP-responsive element binding proteins (CREBs). These transcription factors play an important role in memory formation across animal species. The protocol uses two techniques in concert with each other: a technique for isolating a single neuron with newly developed micromanipulators coupled to an assay of mRNAs by quantitative real-time reverse transcription-polymerase chain reaction (qRT-PCR). The molecular assay determined the mRNA copy numbers, each of which was compared with a standard curve prepared from cDNA solutions corresponding to the serially diluted solutions of Lymnaea CREB mRNA. The standard curves were linear within a range of 10 to 10(5) copies, and the intra-assay variation was within 15%. Each neuron removed from the ganglia was punctured to extract the total RNA directly and was used for the assay without further purification. Using this two-step procedure, we found that the mRNA copy number of CREB repressor (CREB2) was 30-240 in a single cerebral giant cell, whereas that of CREB activator (CREB1) was below the detection limits of the assay (< 25). These results suggest that the CREB cascade is regulated by an excess amount of CREB2 in the cerebral giant cells. Our procedure is the only quantitative analysis for elucidation of the dynamics of gene transcription in a single cell.

Novel kruppel-like factor is induced by neuronal activity and by sensory input in the central nervous system of the terrestrial slugLimax valentianus

In the Limax central nervous system, the procerebrum is thought to be the locus of odor information processing and odor memory retention, but little is known about the input pathway of the noxious stimuli used in this learning protocol. To gain insight into the sensory information processing of the noxious stimuli involved in memory formation, we screened genes induced by artificial neuronal activity, and identified one kruppel-like factor (KLF) family transcription factor gene (Limax KLF; limKLF), which is up-regulated 30 min after depolarization. The limKLF protein fused to GFP was localized to the nucleus in COS-7 cells. We also cloned an immediate early gene, CCAAT enhancer binding protein (C/EBP), of Limax by reverse transcription-polymerase chain reaction (RT-PCR). Both genes were up-regulated by dissection of the central nervous system (CNS) out of the slug in a protein synthesis-independent manner, and also by various noxious stimuli to the slug's body. These genes may be useful as neuronal activity markers in Limax to visualize activated sensory nervous systems.