Protein kinase C mediates memory consolidation of taste avoidance conditioning in Lymnaea stagnalis

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.

Combining Factors That Individually Enhance Memory in Lymnaea

When applied individually, thermal stress (1 hour at 30 °C) and (-)epicatechin (a flavonol found in green tea, e.g.) each enhance long-term memory formation following operant conditioning of Lymnaea aerial respiratory behavior. Snails demonstrate enhanced long-term memory formation when trained in epicatechin-treated pond water or when placed in 30 °C pond water for 1 hour, 1 hour prior to training in pond water. We ask here whether the combined application of epicatechin + thermal stress enhances long-term memory retention length beyond the maximal lengths of the individual factors alone. We report that the applied combination of epicatechin + thermal stress has a synergistic memory-enhancing effect; that is, when the two are applied in combination, memory persists longer than when either is applied alone. We then ask whether quercetin, a heat shock protein blocker, will affect the memory enhancement produced by the combined treatment of thermal stress and epicatechin. We report that quercetin does not decrease the memory enhancement of epicatechin, but it does decrease the memory enhancement by thermal stress; and it also decreases the memory persistence of snails exposed to both treatments in combination.

Implication of protein kinase C of the left intermediate medial mesopallium in memory impairments induced by early prenatal morphine exposure in one-day old chicks

Previously we reported that prenatal morphine exposure during embryonic days 5-8 can cause cognitive deficits of one-trial passive avoidance learning (PAL) in one-day old chicks. Because protein kinase C (PKC) has been associated with memory capacity, we investigated the effects of prenatal morphine exposure on PKC isoforms expression in the left intermediate medial mesopallium (IMM) of chick brain at a time when memory tests were performed at 30, 120 and 360min respectively following training in PAL paradigm. We found that the level of PKCα in the membrane fractions in left IMM was decreased but that in the cytosol fractions showed a increased trend in prenatally morphine-exposed chicks with impaired long-term memory (120 and 360min). Moreover, the translocation of PKC δ from cytosol to membrane in left IMM was shown in prenatal morphine group which had significantly impaired long-term memory at 360min after training. Furthermore, there were no statistical differences between the two groups regarding the expressions of PKCα and PKC δ in the membrane fraction, although their levels in the cytosol fraction of prenatal morphine group which showed impaired intermediate-term memory at 30min after training, were quite different from that of prenatal saline group. Taken together, these results indicate that PKCα and PKC δ in the left IMM are differentially involved in the impairments of long-term memory induced by prenatal morphine exposure. Neither PKCα nor PKC δ in left IMM may be associated with the disruption of intermediate-term memory of chicks prenatally exposed to morphine.

Training Lymnaea in the presence of a predator scent results in a long-lasting ability to form enhanced long-term memory

Lymnaea exposed to crayfish effluent (CE) gain an enhanced ability to form long-term memory (LTM). We test the hypothesis that a single CE exposure and operant conditioning training leads to long lasting changes in the capability of snails to form LTM when tested in pond water four weeks later. We trained both juvenile and adult snails with a single 0.5 h training session in CE and show that LTM was present 24 h later. Snails trained in a similar manner in just pond water show no LTM. We then asked if such training in CE conferred enhanced memory forming capabilities on these snails four weeks later. That is, would LTM be formed in these snails four weeks later following a single 0.5 h training session in pond water? We found that both adult and juvenile snails previously trained in CE one month previously had enhanced LTM formation abilities. The injection of a DNA methylation blocker, 5-AZA, prior to training in adult snails blocked enhanced LTM formation four weeks later. Finally, this enhanced LTM forming ability was not passed on to the next generation of snails.

An automated learning apparatus for classical conditioning of Lymnaea stagnalis

BACKGROUND

The pond snail Lymnaea stagnalis is capable of taste avoidance classical conditioning (TAC) with sucrose as the conditional stimulus (CS) and mechanical prodding as the unconditional stimulus (US). After successful training, feeding behavior is significantly suppressed in response to CS presentation.

NEW METHOD

An automated apparatus is described for the training of multiple snails up to 10 snails at the same time. The new apparatus employs an electrical shock obtained from a 9-V dry cell to deliver a consistent and effective current amplitude of 0.4μA (i.e., the US).

RESULTS

Using this apparatus, 10 snails can be conditioned simultaneously. We found that the optimal parameters to result in both short (STM) and long-term memory (LTM) were 15 paired presentations of the CS and US with a 5-min inter-trial interval (ITI) and 0.2-s current duration. However, both STM and LTM were observed with other ITIs tested. Successful TAC with only a single pairing of the CS-US occurred with a CS of 100mM sucrose solution for 60s followed by a US of 9V with 0.4μA for 5s.

COMPARISON WITH EXISTING METHOD

The use of automated training apparatus for TAC will enable us to better examine the relationship between strength of CS and US.

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.

Time-related expression profiles for heat shock protein gene transcripts (HSP40, HSP70) in the central nervous system of Lymnaea stagnalis exposed to thermal stress

Organisms exposed to environmental stressors respond by rapidly synthesising a suite of highly conserved proteins called heat shock proteins (HSPs). Environmental stress can also enhance and/or block memory formation, with long-term memory formation requiring gene activation and protein synthesis. Thermal stress in the pond snail Lymnaea stagnalis can enhance memory formation, and, in this study, the effect of thermal stress on HSP gene expression in the nervous system was investigated. Time-related expression profiles for HSP40 and HSP70 indicated rapid (<30 min) induction for both transcripts. For HSP40, induction was <20 fold relative to control and expression returned to control levels within 8 h, whereas HSP70 induction was >100 fold and expression did not return to control levels within 8 h.

Common mechanisms of Alzheimer's disease and ischemic stroke: the role of protein kinase C in the progression of age-related neurodegeneration

Ischemic stroke and Alzheimer's disease (AD), despite being distinct disease entities, share numerous pathophysiological mechanisms such as those mediated by inflammation, immune exhaustion, and neurovascular unit compromise. An important shared mechanistic link is acute and chronic changes in protein kinase C (PKC) activity. PKC isoforms have widespread functions important for memory, blood-brain barrier maintenance, and injury repair that change as the body ages. Disease states accelerate PKC functional modifications. Mutated forms of PKC can contribute to neurodegeneration and cognitive decline. In some cases the PKC isoforms are still functional but are not successfully translocated to appropriate locations within the cell. The deficits in proper PKC translocation worsen stroke outcome and amyloid-β toxicity. Cross talk between the innate immune system and PKC pathways contribute to the vascular status within the aging brain. Unfortunately, comorbidities such as diabetes, obesity, and hypertension disrupt normal communication between the two systems. The focus of this review is to highlight what is known about PKC function, how isoforms of PKC change with age, and what additional alterations are consequences of stroke and AD. The goal is to highlight future therapeutic targets that can be applied to both the treatment and prevention of neurologic disease. Although the pathology of ischemic stroke and AD are different, the similarity in PKC responses warrants further investigation, especially as PKC-dependent events may serve as an important connection linking age-related brain injury.

Electrophysiological characteristics of feeding-related neurons after taste avoidance Pavlovian conditioning in Lymnaea stagnalis

Taste avoidance conditioning (TAC) was carried out on the pond snail, Lymnaea stagnalis. The conditional stimulus (CS) was sucrose which elicits feeding behavior; while the unconditional stimulus (US) was a tactile stimulus to the head which causes feeding to be suppressed. The neuronal circuit that drives feeding behavior in Lymnaea is well worked out. We therefore compared the physiological characteristics on 3 classes of neurons involved with feeding behavior especially in response to the CS in conditioned vs. control snails. The cerebral giant cell (CGC) modulates feeding behavior, N1 medial neuron (N1M) is one of the central pattern generator neurons that organizes feeding behavior, while B3 is a motor neuron active during the rasp phase of feeding. We found the resting membrane potential in CGC was hyperpolarized significantly in conditioned snails but impulse activity remained the same between conditioned vs. control snails. There was, however, a significant increase in spontaneous activity and a significant depolarization of N1M’s resting membrane potential in conditioned snails. These changes in N1M activity as a result of training are thought to be due to withdrawal interneuron RPeD11 altering the activity of the CGCs. Finally, in B3 there was: 1) a significant decrease in the amplitude and the frequency of the post-synaptic potentials; 2) a significant hyperpolarization of resting membrane potential in conditioned snails; and 3) a disappearance of bursting activity typically initiated by the CS. These neuronal modifications are consistent with the behavioral phenotype elicited by the CS following conditioning.

Zeta inhibitory peptide (ZIP) erases long-term memories in a cockroach

Recent efforts to identify the molecules that are involved in the maintenance of long-term memories in mammals have focused attention on atypical isoforms of protein kinase C (PKC). Inhibition of these kinases by either the general PKC inhibitor, chelerythrine, or the more specific inhibitor, zeta inhibitory peptide (ZIP), can abolish both long-term potentiation in the hippocampus and as well as spatial, fear, appetitive, and sensorimotor memories. These inhibitors can also abolish long-term facilitation and long-term sensitization in the mollusk Aplysia californica. We have extended these results to an insect, the cockroach Leucophaea maderae. We show that systemic injections of either chelerythrine or ZIP erase long-term olfactory memories in the cockroach, but have no effect on memory acquisition during conditioning. We also show that inhibition of either protein kinase A (PKA) or protein synthesis can block memory acquisition but neither has an effect on the memory once it is formed. The results suggest that sustaining memories in insects requires the persistent activity of one or more isoforms of PKC and point to a strong evolutionary conservation of the molecular mechanisms that underlie the persistence of long-term memories in the central nervous system.

An increase in insulin is important for the acquisition conditioned taste aversion in Lymnaea

Conditioned taste aversion (CTA) in Lymnaea is brought about by pairing a sucrose solution (the conditioned stimulus, CS) with an electric shock (the unconditioned stimulus, US). Following repeated CS-US pairings, CTA occurs and it is consolidated into long-term memory (LTM). The best CTA is achieved, if snails are food-deprived for 1 day before training commences. With a longer period of food deprivation (5 days), learning and memory formation does not occur. It has been hypothesized that the levels of insulin in the central nervous system (CNS) are very important for CTA to occur. To test his hypothesis, we injected insulin directly into 5-day food-deprived snails. The injection of insulin, as expected, resulted in a decrease in hemolymph glucose concentration. Consistent with our hypothesis with insulin injection, learning and memory formation of CTA occurred. That is, the 'insulin spike' is more important than an increase in hemolymph glucose concentration for CTA-LTM. If we injected an insulin receptor antibody into the snails before the insulin injection, learning was formed but memory formation was not, which is consistent with our previous study. Therefore, a rise in the insulin concentration (i.e., insulin spike) in the CNS is considered to be a key determining factor in the process of CTA-LTM.