Transcriptional correlates of memory maintenance following long-term sensitization of Aplysia californica

Evidence of Active-Forgetting Mechanisms? Blocking Arachidonic Acid Release May Slow Forgetting of Sensitization in Aplysia

Long-term sensitization in Aplysia is accompanied by a persistent up-regulation of mRNA encoding the peptide neurotransmitter Phe-Met-Arg-Phe-amide (FMRFa), a neuromodulator that opposes the expression of sensitization through activation of the arachidonic acid second-messenger pathway. We completed a preregistered test of the hypothesis that FMRFa plays a critical role in the forgetting of sensitization. Aplysia received long-term sensitization training and were then given whole-body injections of vehicle (N = 27), FMRFa (N = 26), or 4-bromophenacylbromide (4-BPB; N = 31), a phospholipase inhibitor that prevents the release of arachidonic acid. FMRFa produced no changes in forgetting. 4-BPB decreased forgetting measured 6 d after training [d s = 0.55 95% CI(0.01, 1.09)], though the estimated effect size is uncertain. Our results provide preliminary evidence that forgetting of sensitization may be a regulated, active process in Aplysia, but could also indicate a role for arachidonic acid in stabilizing the induction of sensitization.

Snails go on a fast when acetylsalicylic acid comes along with heat stress: A possible effect of HSPs and serotonergic system on the feeding response

A novel food followed by sickness, causes a taste-specific conditioned aversion, known as the 'Garcia effect'. We recently found that both a heat shock stressor (30 °C for 1 h - HS) and the bacterial lipopolysaccharide (LPS) can be used as 'sickness-inducing' stimuli to induce a Garcia effect in the pond snail Lymnaea stagnalis. Additionally, if snails are exposed to acetylsalicylic acid (ASA) present in aspirin tablets before the LPS injection, the formation of the Garcia effect is prevented. Here, we hypothesized that exposing snails to crushed aspirin before the HS (ASA-HS) would prevent the HS-induced 'sickness state' and - therefore -the Garcia effect. Unexpectantly, the ASA-HS procedure induced a generalized and long-lasting feeding suppression. We thus investigate the molecular effects underlying this phenomenon. While the exposure to the HS alone resulted in a significant upregulation of the mRNA levels of the Heat Shock Protein 70 (HSP 70) in snails' central ring ganglia, the ASA-HS procedure induced an even greater upregulation of HSP70, suggesting that the ASA-HS combination causes a severe stress response that inhibits feeding. Additionally, we found that the ASA-HS procedure induced a significant downregulation of the mRNA levels of genes involved with the serotoninergic system which regulates feeding in snails. Finally, the ASA-HS procedure prevented HS-induced upregulation of the mRNA levels of key neuroplasticity genes. Our study indicates that two sickness-inducing stimuli can have different physiological responses even if behavioral outcomes are similar under some learning contexts.

A flavonoid, quercetin, is capable of enhancing long-term memory formation if encountered at different times in the learning, memory formation, and memory recall continuum

A major extrinsic factor influencing memory and neuro-cognitive performances across taxa is diet. Studies from vertebrates have shown the effects of a flavonoid rich diet on cognitive performance, but the mechanism underlying this action is still poorly understood. A common and abundant flavonoid present in numerous food substances is quercetin (Q). The present study provides the first support for Q-modulated enhancement of cognitive function in an invertebrate model, the pond snail Lymnaea stagnalis, after an operant conditioning procedure. We found that when snails were exposed to Q 3 h before or after a single 0.5 h training session, which typically results in memory lasting ~ 3 h, they formed a long-term memory (LTM) lasting for at least 24 h. Additionally, we assessed the effects of the combined presentation of a single reinforcing stimulus (at 24 h post-training or 24 h before training) and Q-exposure on both LTM formation and reconsolidation. That is, when applied within 3 h of critical periods of memory, Q regulates four different phases: (1) acquisition (i.e., a learning event), (2) consolidation processes after acquisition, (3) memory recall, and (4) memory reconsolidation. In all these phases Q-exposure enhanced LTM persistence.

Registered Report: Transcriptional Analysis of Savings Memory Suggests Forgetting is Due to Retrieval Failure

There is fundamental debate about the nature of forgetting: some have argued that it represents the decay of the memory trace, others that the memory trace persists but becomes inaccessible because of retrieval failure. These different accounts of forgetting lead to different predictions about savings memory, the rapid re-learning of seemingly forgotten information. If forgetting is because of decay, then savings requires re-encoding and should thus involve the same mechanisms as initial learning. If forgetting is because of retrieval failure, then savings should be mechanistically distinct from encoding. In this registered report, we conducted a preregistered and rigorous test between these accounts of forgetting. Specifically, we used microarray to characterize the transcriptional correlates of a new memory (1 d after training), a forgotten memory (8 d after training), and a savings memory (8 d after training but with a reminder on day 7 to evoke a long-term savings memory) for sensitization in Aplysia californica (n = 8 samples/group). We found that the reactivation of sensitization during savings does not involve a substantial transcriptional response. Thus, savings is transcriptionally distinct relative to a newer (1-d-old) memory, with no coregulated transcripts, negligible similarity in regulation-ranked ordering of transcripts, and a negligible correlation in training-induced changes in gene expression (r = 0.04 95% confidence interval (CI) [-0.12, 0.20]). Overall, our results suggest that forgetting of sensitization memory represents retrieval failure.

Effect of photoperiod and light intensity on learning ability and memory formation of the pond snail Lymnaea stagnalis

Natural light is regarded as a key regulator of biological systems and typically serves as a Zeitgeber for biological rhythms. As a natural abiotic factor, it is recognized to regulate multiple behavioral and physiological processes in animals. Disruption of the natural light regime due to light pollution may result in significant effects on animal learning and memory development. Here, we investigated whether sensitivity to various photoperiods or light intensities had an impact on intermediate-term memory (ITM) and long-term memory (LTM) formation in the pond snail Lymnaea stagnalis. We also investigated the change in the gene expression level of molluscan insulin-related peptide II (MIP II) is response to the given light treatments. The results show that the best light condition for proper LTM formation is exposure to a short day (8 h light) and low light intensity (1 and 10 lx). Moreover, the more extreme light conditions (16 h and 24 h light) prevent the formation of both ITM and LTM. We found no change in MIP II expression in any of the light treatments, which may indicate that MIP II is not directly involved in the operant conditioning used here, even though it is known to be involved in learning. The finding that snails did not learn in complete darkness indicates that light is a necessary factor for proper learning and memory formation. Furthermore, dim light enhances both ITM and LTM formation, which suggests that there is an optimum since both no light and too bright light prevented learning and memory. Our findings suggest that the upsurge of artificial day length and/or night light intensity may also negatively impact memory consolidation in the wild.

Coordinated Activity of Transcriptional Networks Responding to the Pattern of Action Potential Firing in Neurons

Transcriptional responses to the appropriate temporal pattern of action potential firing are essential for long-term adaption of neuronal properties to the functional activity of neural circuits and environmental experience. However, standard transcriptome analysis methods can be too limited in identifying critical aspects that coordinate temporal coding of action potential firing with transcriptome response. A Pearson correlation analysis was applied to determine how pairs of genes in the mouse dorsal root ganglion (DRG) neurons are coordinately expressed in response to stimulation producing the same number of action potentials by two different temporal patterns. Analysis of 4728 distinct gene-pairs related to calcium signaling, 435,711 pairs of transcription factors, 820 pairs of voltage-gated ion channels, and 86,862 pairs of calcium signaling genes with transcription factors indicated that genes become coordinately activated by distinct action potential firing patterns and this depends on the duration of stimulation. Moreover, a measure of expression variance revealed that the control of transcripts abundances is sensitive to the pattern of stimulation. Thus, action potentials impact intracellular signaling and the transcriptome in dynamic manner that not only alter gene expression levels significantly (as previously reported) but also affects the control of their expression fluctuations and profoundly remodel the transcriptional networks.

The ELAV family of RNA-binding proteins in synaptic plasticity and long-term memory

The mechanisms of de novo gene expression and translation of specific gene transcripts have long been known to support long-lasting changes in synaptic plasticity and behavioral long-term memory. In recent years, it has become increasingly apparent that gene expression is heavily regulated not only on the level of transcription, but also through post-transcriptional gene regulation, which governs the subcellular localization, stability, and likelihood of translation of mRNAs. Specific families of RNA-binding proteins (RBPs) bind transcripts which contain AU-rich elements (AREs) within their 3' UTR and thereby govern their downstream fate. These post-transcriptional gene regulatory mechanisms are coordinated through the same cell signaling pathways that play critical roles in long-term memory formation. In this review, we discuss recent results that demonstrate the roles that these ARE-binding proteins play in LTM formation.

Immediate early genes in social insects: a tool to identify brain regions involved in complex behaviors and molecular processes underlying neuroplasticity

Social insects show complex behaviors and master cognitive tasks. The underlying neuronal mechanisms, however, are in most cases only poorly understood due to challenges in monitoring brain activity in freely moving animals. Immediate early genes (IEGs) that get rapidly and transiently expressed following neuronal stimulation provide a powerful tool for detecting behavior-related neuronal activity in vertebrates. In social insects, like honey bees, and in insects in general, this approach is not yet routinely established, even though these genes are highly conserved. First studies revealed a vast potential of using IEGs as neuronal activity markers to analyze the localization, function, and plasticity of neuronal circuits underlying complex social behaviors. We summarize the current knowledge on IEGs in social insects and provide ideas for future research directions.

Transcriptional changes before and after forgetting of a long-term sensitization memory in Aplysia californica

Most long-term memories are forgotten, becoming progressively less likely to be recalled. Still, some memory fragments may persist, as savings memory (easier relearning) can be detected long after recall has become impossible. What happens to a memory trace during forgetting that makes it inaccessible for recall and yet still effective to spark easier re-learning? We are addressing this question by tracking the transcriptional changes that accompany learning and then forgetting of a long-term sensitization memory in the tail-elicited siphon withdrawal reflex of Aplysia californica. First, we tracked savings memory. We found that even though recall of sensitization fades completely within 1 week of training, savings memory is still detectable at 2 weeks post training. Next, we tracked the time-course of regulation of 11 transcripts we previously identified as potentially being regulated after recall has become impossible. Remarkably, 3 transcripts still show strong regulation 2 weeks after training and an additional 4 are regulated for at least 1 week. These long-lasting changes in gene expression always begin early in the memory process, within 1 day of training. We present a synthesis of our results tracking gene expression changes accompanying sensitization and provide a testable model of how sensitization memory is forgotten.

The New Statistics for Neuroscience Majors: Thinking in Effect Sizes

An ongoing reform in statistical practice is to report and interpret effect sizes. This paper provides a short tutorial on effect sizes and some tips on how to help your students think in terms of effect sizes when analyzing data. An effect size is just a quantitative answer to a research question. Effect sizes should always be accompanied by a confidence interval or some other means of expressing uncertainty in generalizing from the sample to the population. Effect sizes are best interpreted in raw scores, but can also be expressed in standardized terms; several popular standardized effect score measures are explained and compared. Reporting and interpreting effect sizes has several benefits: it focuses on the practical significance of your findings, helps make clear the remaining uncertainty in your findings, fosters better planning for subsequent experiments, fosters meta-analytic thinking, and can help focus efforts on protocol optimization. You can help your students start to think in effect sizes by giving them tools to visualize and translate between different effect size measures, and by tasking them to build a 'library' of effect sizes in a research field of interest.

Memory Synapses Are Defined by Distinct Molecular Complexes: A Proposal

Synapses are diverse in form and function. While there are strong evidential and theoretical reasons for believing that memories are stored at synapses, the concept of a specialized “memory synapse” is rarely discussed. Here, we review the evidence that memories are stored at the synapse and consider the opposing possibilities. We argue that if memories are stored in an active fashion at synapses, then these memory synapses must have distinct molecular complexes that distinguish them from other synapses. In particular, examples from Aplysia sensory-motor neuron synapses and synapses on defined engram neurons in rodent models are discussed. Specific hypotheses for molecular complexes that define memory synapses are presented, including persistently active kinases, transmitter receptor complexes and trans-synaptic adhesion proteins.

Savings memory is accompanied by transcriptional changes that persist beyond the decay of recall

Most long-term memories are forgotten. What happens, then, to the changes in neuronal gene expression that were initially required to encode and maintain the memory? Here we show that the decay of recall for long-term sensitization memory in Aplysia is accompanied both by a form of savings memory (easier relearning) and by persistent transcriptional regulation. A behavioral experiment (N = 14) shows that sensitization training produces a robust long-term sensitization memory, but that recall fades completely within 1 wk. This apparent forgetting, though, is belied by persistent savings memory, as we found that a weak reminder protocol reinstates a long-term sensitization memory only on the previously trained side of the body. Using microarray (N = 8 biological replicates), we found that transcriptional regulation largely decays along with recall. Of the transcripts known to be regulated 1 d after training, 98% (1172/1198) are no longer significantly regulated 7 d after training. Still, there is a small set of transcripts which remain strongly regulated even when recall is absent. Using qPCR (N = 11 additional biological replicates) we confirmed that these include the peptide transmitter FMRFamide, a transcript encoding a putative homolog of spectrin beta chain (Genbank: EB255259) , a transcript encoding a protein with a predicted EF-hand calcium-binding domain (Genbank: EB257711), and eight uncharacterized transcripts. To our knowledge, this is the first work to show that transcriptional changes evoked by learning can outlast recall. The small set of transcriptional changes that persist could mediate the rapid relearning of the memory (savings), or the decay of recall, or both, or neither.

After p Values: The New Statistics for Undergraduate Neuroscience Education

Statistical inference is a methodological cornerstone for neuroscience education. For many years this has meant inculcating neuroscience majors into null hypothesis significance testing with p values. There is increasing concern, however, about the pervasive misuse of p values. It is time to start planning statistics curricula for neuroscience majors that replaces or de-emphasizes p values. One promising alternative approach is what Cumming has dubbed the "New Statistics", an approach that emphasizes effect sizes, confidence intervals, meta-analysis, and open science. I give an example of the New Statistics in action and describe some of the key benefits of adopting this approach in neuroscience education.