Contextual Pavlovian conditioning in the crab Chasmagnathus

Landmark use by ghost crab (Ocypode quadrata) during wayfinding in a complex maze

Species of crab have been shown to spatially track and navigate to consequential locations through different processes, such as path integration and landmark orienting. Few investigations examine their ability to wayfind in complex environments, like mazes, with multiple intersections and how they may utilize specific features to benefit this process. Spatial learning potentially would lend a fitness advantage to animals living in complicated habitats, and ghost crab (Ocypode quadrata) is a semiterrestrial species that typically occupies extensive beach environments, which present many navigational challenges. Despite their potential, there are currently no studies that investigate forms of spatial cognition in these animals. To better diversify our knowledge of this trait, the current research exposed ghost crab to a maze with seven intersections. Animals were given multiple trials to learn the location of a reward destination to a specific criterion proficiency. In one condition several landmarks were distributed throughout the maze, and in another the environment was completely empty. Results showed that ghost crab in the landmark present group were able to learn the maze faster, they required significantly fewer trials to reach the learning criterion than those in the landmark absent group. However, only approximately half of the total sample met the learning criterion, indicating the maze was rather difficult. These findings are interpreted through theories of route learning that suggest animals may navigate by establishing landmark-turn associations. Such processes have implications for the cognitive ability of ghost crab, and spatial learning in this species may support the notion of convergent evolution for this trait.

Molecular insights from the crab Neohelice memory model

Memory acquisition, formation and maintenance depend on synaptic post-translational machinery and regulation of gene expression triggered by several transduction pathways. In turns, these processes lead to stabilization of synaptic modifications in neurons in the activated circuits. In order to study the molecular mechanisms involved in acquisition and memory, we have taken advantage of the context-signal associative learning and, more recently, the place preference task, of the crab Neohelice granulata. In this model organism, we studied several molecular processes, including activation of extracellular signal-regulated kinase (ERK) and the nuclear factor kappa light chain enhancer of activated B cells (NF-κB) transcription factor, involvement of synaptic proteins such as NMDA receptors and neuroepigenetic regulation of gene expression. All these studies allowed description of key plasticity mechanisms involved in memory, including consolidation, reconsolidation and extinction. This article is aimed at review the most salient findings obtained over decades of research in this memory model.

Contextual memory reactivation modulates Ca2+-activity network state in a mushroom body-like center of the crab N. granulata

High-order brain centers play key roles in sensory integration and cognition. In arthropods, much is known about the insect high-order centers that support associative memory processes, the mushroom bodies. The hypothesis that crustaceans possess structures equivalent to the mushroom bodies -traditionally called hemiellipsoid body- has been receiving neuroanatomical endorsement. The recent functional support is limited to the short term: in a structure of the true crab Neohelice granulata that has many insect-like mushroom bodies traits, the plastic learning changes express the context attribute of an associative memory trace. Here, we used in vivo calcium imaging to test whether neuronal activity in this structure is associated with memory reactivation in the long-term (i.e., 24 h after training). Long-term training effects were tested by presenting the training-context alone, a reminder known to trigger memory reconsolidation. We found similar spontaneous activity between trained and naïve animals. However, after training-context presentation, trained animals showed increased calcium events rate, suggesting that memory reactivation induced a change in the underlying physiological state of this center. Reflecting the change in the escape response observed in the paradigm, animals trained with a visual danger stimulus showed significantly lower calcium-evoked transients in the insect-like mushroom body. Protein synthesis inhibitor cycloheximide administered during consolidation prevented calcium mediated changes. Moreover, we found the presence of distinct calcium activity spatial patterns. Results suggest that intrinsic neurons of this crustacean mushroom body-like center are involved in contextual associative long-term memory processes.

Learning a non-neutral conditioned stimulus: place preference in the crab Neohelice granulata

In the wild, being able to recognize and remember specific locations related to food sources and the associated attributes of landmarks is a cognitive trait important for survival. In the present work, we show that the crab Neohelice granulata can be trained to associate a specific environment with an appetitive reward in a conditioned place preference task. After a single training trial, when the crabs were presented with a food pellet in the target quadrant of the training arena, they were able to form a long-term memory related to the event. This memory was evident at least 24 h after training and was protein synthesis dependent. Importantly, the target area of the arena proved to be a non-neutral environment, given that animals initially avoided the target quadrant. In the present work, we introduce for the first time an associative one-trial memory paradigm including a conditioned stimulus with a clear valence performed in a crustacean.

Social context shapes cognitive abilities: associative memories are modulated by fight outcome and social isolation in the crab Neohelice granulata

Cognitive abilities of an animal can be influenced by distinct social experiences. However, the extent of this modulation has not been addressed in different learning scenarios: are all tasks similarly affected by social experiences? In the present study, we analyzed the effect of social dominance in aversive and appetitive memory processes in the crab Neohelice granulata. In addition, we studied the influence of social isolation on memory ability. Social dominance experiments consisted of an agonistic phase immediately followed by a memory phase. During the agonistic phase, matched pairs of male crabs were staged in 10-min encounters and the dominant or subordinate condition of each member of the dyad was determined. During the memory phase, crabs were trained to acquire aversive or appetitive memory and tested 24 h later. Results showed that the agonistic encounter can modulate long-term memory according to the dominance condition in such a way that memory retention of subordinates results higher than their respective dominant. Remarkably, this result was found for both aversive and appetitive memory tasks. In addition, we found that isolated animals showed no memory retention when compared with animals that remained grouped. Altogether this work emphasizes the importance of social context as a modulator of cognitive abilities.

Two spaced training trials induce associative ERK-dependent long term memory in Neohelice granulata

Memory formation depends upon several parametric training conditions. Among them, trial number and inter-trial interval (ITI) are key factors to induce long-term retention. However, it is still unclear how individual training trials contribute to mechanisms underlying memory formation and stabilization. Contextual conditioning in Neohelice granulata has traditionally elicited associative long-term memory (LTM) after 15 spaced (ITI = 3 min) trials. Here, we show that LTM in crabs can be induced after only two training trials by increasing the ITI to 45 min (2t-LTM) and maintaining the same training duration as in traditional protocols. This newly observed LTM was preserved for at least 96 h, exhibiting protein synthesis dependence during consolidation and reconsolidation as well as context-specificity. Moreover, we demonstrate that 2t-LTM depends on inter-trial and post-training ERK activation showing a faster phosphorylation after the second trial compared to the first one. In summary, we present a new training protocol in crabs through a reduced number of trials showing associative features similar to traditional spaced training. This novel protocol allows for intra-training manipulation and the assessment of individual trial contribution to LTM formation.

Identification of a Novel Retrieval-dependent Memory Process in the Crab Neohelice granulata

Fully consolidated associative memories may be altered by alternative retrieval dependent memory processes. While a brief exposure to the conditioned stimulus (CS) can trigger reconsolidation of the original memory, a prolonged CS exposure will trigger memory extinction. The conditioned response is maintained after reconsolidation, but is inhibited after extinction, presumably by the formation of a new inhibitory memory trace. In rats and humans, it has been shown that CS exposure of intermediate duration leave the memory in an insensitive or limbo state. Limbo is characterised by the absence of reconsolidation or extinction. Here we investigated the evolutionary conserved nature of limbo using a contextual Pavlovian conditioning (CPC) memory paradigm in the crab Neohelice granulata. In animals with fully consolidated CPC memory, systemic administration of the protein synthesis inhibitor cycloheximide after 1 CS presentation disrupted the memory, presumably by interfering with memory reconsolidation. The same intervention given after 320 CSs prevented CPC memory extinction. Cycloheximide had no behavioural effect when administered after 80 CS presentations, a protocol that failed to extinguish CPC memory. Also, we observed that a stronger CPC memory engaged reconsolidation after 80 CS instead of limbo, indicating that memory strength affects the parametrical conditions to engage either reconsolidation or limbo. Altogether, these results indicate that limbo is an evolutionary conserved memory process segregating reconsolidation from extinction in the number of CSs space. Limbo appears as an intrinsic component of retrieval dependent memory processing, with a key function in the transition from memory maintenance to inhibition.

A crabs' high-order brain center resolved as a mushroom body-like structure

The hypothesis of a common origin for high-order memory centers in bilateral animals presents the question of how different brain structures, such as the vertebrate hippocampus and the arthropod mushroom bodies, are both structurally and functionally comparable. Obtaining evidence to support the hypothesis that crustaceans possess structures equivalent to the mushroom bodies that play a role in associative memories has proved challenging. Structural evidence supports that the hemiellipsoid bodies of hermit crabs, crayfish and lobsters, spiny lobsters, and shrimps are homologous to insect mushroom bodies. Although a preliminary description and functional evidence supporting such homology in true crabs (Brachyura) has recently been shown, other authors consider the identification of a possible mushroom body homolog in Brachyura as problematic. Here we present morphological and immunohistochemical data in Neohelice granulata supporting that crabs possess well-developed hemiellipsoid bodies that are resolved as mushroom bodies-like structures. Neohelice exhibits a peduncle-like tract, from which processes project into proximal and distal domains with different neuronal specializations. The proximal domains exhibit spines and en passant-like processes and are proposed here as regions mainly receiving inputs. The distal domains exhibit a "trauben"-like compartmentalized structure with bulky terminal specializations and are proposed here as output regions. In addition, we found microglomeruli-like complexes, adult neurogenesis, aminergic innervation, and elevated expression of proteins necessary for memory processes. Finally, in vivo calcium imaging suggests that, as in insect mushroom bodies, the output regions exhibit stimulus-specific activity. Our results support the shared organization of memory centers across crustaceans and insects.

Memory built in conjunction with a stressor is privileged: Reconsolidation-resistant memories in the crab Neohelice

The dynamics of memory processes are conserved throughout evolution, a feature based on the hypothesis of a common origin of the high-order memory centers in bilateral animals. Reconsolidation is just one example. The possibility to interfere with long-term memory expression during reconsolidation has been proposed as potentially useful in clinical application to treat traumatic memories. However, several pieces of evidence in rodents show that either robust fear memories or stressful events applied before acquisition promote reconsolidation-resistant memories, i.e., memories that are resistant to the interfering effect of drugs on memory reconsolidation. Conceivably, the generation of these reconsolidation-resistant fear memories also occurs in humans. Is the induction of reconsolidation-resistant memories part of the dynamics of memory processes conserved throughout evolution? In the semiterrestrial crab Neohelice granulata, memory reconsolidation is triggered by a short reminder without reinforcement. Here, we show that an increase in the salience of the aversive stimulus augmented the memory strength; nonetheless, the protein synthesis inhibitor cycloheximide still disrupted the reconsolidation process. However, crabs stressed by a water-deprivation episode before a strong training session built up a memory that was now reconsolidation-resistant. We tested whether these reconsolidation-resistant effects can be challenged by changing parametric conditions of memory-reminder sessions; multiple memory reactivations without reinforcement were not able to trigger the labilization-reconsolidation of this resistant memory. Overall, the present findings suggest that generation of reconsolidation-resistant memories can be another part of the dynamics of memory processes conserved throughout evolution that protects privileged information from change.

Context-dependent memory traces in the crab's mushroom bodies: Functional support for a common origin of high-order memory centers

The hypothesis of a common origin for the high-order memory centers in bilateral animals is based on the evidence that several key features, including gene expression and neuronal network patterns, are shared across several phyla. Central to this hypothesis is the assumption that the arthropods' higher order neuropils of the forebrain [the mushroom bodies (MBs) of insects and the hemiellipsoid bodies (HBs) of crustaceans] are homologous structures. However, even though involvement in memory processes has been repeatedly demonstrated for the MBs, direct proof of such a role in HBs is lacking. Here, through neuroanatomical and immunohistochemical analysis, we identified, in the crab Neohelice granulata, HBs that resemble the calyxless MBs found in several insects. Using in vivo calcium imaging, we revealed training-dependent changes in neuronal responses of vertical and medial lobes of the HBs. These changes were stimulus-specific, and, like in the hippocampus and MBs, the changes reflected the context attribute of the memory trace, which has been envisioned as an essential feature for the HBs. The present study constitutes functional evidence in favor of a role for the HBs in memory processes, and provides key physiological evidence supporting a common origin of the arthropods' high-order memory centers.

Different dimensions of the prediction error as a decisive factor for the triggering of the reconsolidation process

The reconsolidation process is the mechanism by which strength and/or content of consolidated memories are updated. Prediction error (PE) is the difference between the prediction made and current events. It is proposed as a necessary condition to trigger the reconsolidation process. Here we analyzed deeply the role of the PE in the associative memory reconsolidation in the crab Neohelice granulata. An incongruence between the learned temporal relationship between conditioned and unconditioned stimuli (CS-US) was enough to trigger the reconsolidation process. Moreover, after a partial reinforced training, a PE of 50% opened the possibility to labilize the consolidated memory with a reminder which included or not the US. Further, during an extinction training a small PE in the first interval between CSs was enough to trigger reconsolidation. Overall, we highlighted the relation between training history and different reactivation possibilities to recruit the process responsible of memory updating.

Neural correlates of expression-independent memories in the crab Neohelice

The neural correlates of memory have been usually examined considering that memory retrieval and memory expression are interchangeable concepts. However, our studies in the crab Neohelice (Chasmagnathus) granulata and in other memory models have shown that memory expression is not necessary for memory to be re-activated and become labile. In order to examine putative neural correlates of memory in the crab Neohelice, we contrast changes induced by training in both animal's behavior and neuronal responses in the medulla terminalis using in vivo Ca(2+) imaging. Disruption of long-term memory by the amnesic agents MK-801 or scopolamine (5μg/g) blocks the learning-induced changes in the Ca(2+) responses in the medulla terminalis. Conversely, treatments that lead to an unexpressed but persistent memory (weak training protocol or scopolamine 0.1μg/g) do not block these learning-induced neural changes. The present results reveal a set of changes in the neural activity induced by training that correlates with memory persistence but not with the probability of this memory to be expressed in the long-term. In addition, the study constitutes the first in vivo evidence in favor of a role of the medulla terminalis in learning and memory in crustaceans, and provides a physiological evidence indicating that memory persistence and the probability of memory to be expressed might involve separate components of memory traces.

Protein degradation by ubiquitin-proteasome system in formation and labilization of contextual conditioning memory

The ubiquitin–proteasome system (UPS) of protein degradation has been evaluated in different forms of neural plasticity and memory. The role of UPS in such processes is controversial. Several results support the idea that the activation of this system in memory consolidation is necessary to overcome negative constrains for plasticity. In this case, the inhibition of the UPS during consolidation impairs memory. Similar results were reported for memory reconsolidation. However, in other cases, the inhibition of UPS had no effect on memory consolidation and reconsolidation but impedes the amnesic action of protein synthesis inhibition after retrieval. The last finding suggests a specific action of the UPS inhibitor on memory labilization. However, another interpretation is possible in terms of the synthesis/degradation balance of positive and negative elements in neural plasticity, as was found in the case of long-term potentiation. To evaluate these alternative interpretations, other reconsolidation-interfering drugs than translation inhibitors should be tested. Here we analyzed initially the UPS inhibitor effect in contextual conditioning in crabs. We found that UPS inhibition during consolidation impaired long-term memory. In contrast, UPS inhibition did not affect memory reconsolidation after contextual retrieval but, in fact, impeded memory labilization, blocking the action of drugs that does not affect directly the protein synthesis. To extend these finding to vertebrates, we performed similar experiments in contextual fear memory in mice. We found that the UPS inhibitor in hippocampus affected memory consolidation and blocked memory labilization after retrieval. These findings exclude alternative interpretations to the requirement of UPS in memory labilization and give evidence of this mechanism in both vertebrates and invertebrates.

Memory beyond expression

The idea that memories are not invariable after the consolidation process has led to new perspectives about several mnemonic processes. In this framework, we review our studies on the modulation of memory expression during reconsolidation. We propose that during both memory consolidation and reconsolidation, neuromodulators can determine the probability of the memory trace to guide behavior, i.e. they can either increase or decrease its behavioral expressibility without affecting the potential of persistent memories to be activated and become labile. Our hypothesis is based on the findings that positive modulation of memory expression during reconsolidation occurs even if memories are behaviorally unexpressed. This review discusses the original approach taken in the studies of the crab Neohelice (Chasmagnathus) granulata, which was then successfully applied to test the hypothesis in rodent fear memory. Data presented offers a new way of thinking about both weak trainings and experimental amnesia: memory retrieval can be dissociated from memory expression. Furthermore, the strategy presented here allowed us to show in human declarative memory that the periods in which long-term memory can be activated and become labile during reconsolidation exceeds the periods in which that memory is expressed, providing direct evidence that conscious access to memory is not needed for reconsolidation. Specific controls based on the constraints of reminders to trigger reconsolidation allow us to distinguish between obliterated and unexpressed but activated long-term memories after amnesic treatments, weak trainings and forgetting. In the hypothesis discussed, memory expressibility--the outcome of experience-dependent changes in the potential to behave--is considered as a flexible and modulable attribute of long-term memories. Expression seems to be just one of the possible fates of re-activated memories.

Invertebrate learning and cognition: relating phenomena to neural substrate

Diverse invertebrate species have been used for studies of learning and comparative cognition. Although we have gained invaluable information from this, in this study we argue that our approach to comparative learning research is rather deficient. Generally invertebrate learning research has focused mainly on arthropods, and most of that within the Hymenoptera and Diptera. Any true comparative analysis of the distribution of comparative cognitive abilities across phyla is hampered by this bias, and more fundamentally by a reporting bias toward positive results. To understand the limits of learning and cognition for a species, knowing what animals cannot do is at least as important as reporting what they can. Finally, much more effort needs to be focused on the neurobiological analysis of different types of learning to truly understand the differences and similarities of learning types. In this review, we first give a brief overview of the various forms of learning in invertebrates. We also suggest areas where further study is needed for a more comparative understanding of learning. Finally, using what is known of learning in honeybees and the well-studied honeybee brain, we present a model of how various complex forms of learning may be accounted for with the same neural circuitry required for so-called simple learning types. At the neurobiological level, different learning phenomena are unlikely to be independent, and without considering this it is very difficult to correctly interpret the phylogenetic distribution of learning and cognitive abilities. WIREs Cogn Sci 2013, 4:561-582. doi: 10.1002/wcs.1248 For further resources related to this article, please visit the WIREs website.

The involvement of the GABAergic system in the formation and expression of the extinction memory in the crab Neohelice granulata

There is growing interest in the neurobiological mechanisms involved in the extinction of aversive memory. This cognitive process usually occurs after repeated or prolonged presentation of a conditioned stimulus that was previously associated with an unconditioned stimulus. If extinction is considered to be a new memory, the role of the γ-aminobutyric acid system (GABAergic system) during extinction memory consolidation should be similar to that described for the original trace. It is also accepted that negative modulation of the GABAergic system before testing can impair extinction memory expression. However, it seems possible to speculate that inhibitory mechanisms may be required in order to acquire a memory that is inhibitory in nature. Using a combination of behavioral protocols, such as weak and robust extinction training procedures, and pharmacological treatments, such as the systemic administration of GABAA agonist (muscimol) and antagonist (bicuculline), we investigated the role of the GABAergic system in the different phases of the extinction memory in the crab Neohelice granulata. We show that the stimulation of the GABAergic system impairs and its inactivation facilitates the extinction memory consolidation. Moreover, fine variations in the GABAergic tone affect its expression at testing. Finally, an active GABAergic system is necessary for the acquisition of the extinction memory. This detailed description may contribute to the understanding of the role of the GABAergic system in diverse aspects of the extinction memory.