Long-term habituation (LTH) in the crab Chasmagnathus: a model for behavioral and mechanistic studies of memory

Annual changes of Neohelice granulata cognitive abilities indicate opposition between short- and long-term memory retention

Neohelice is a long-standing model for memory studies for its strong retention of a reduced escape response when trained to iterative presentations of a visual danger stimulus (VDS). Here we present year-round changes that are related to the memory acquisition, storage, and expression. First, we evaluated exploratory activity and response to the VDS, as necessary for memory acquisition and expression. Both parameters change year-round. Second, short-term memory (STM) and two types of long-term memory (LTM) were assessed throughout the year. STM and long-term context-dependent signal memory (CSM) change between periods of the year, whereas signal memory (SM) does not, indicating that the cognitive abilities of the crab display circannual rhythms. Third, during the reproductive period, STM retention is higher than both CSM and SM, indicating a trade-off between STM and LTM. This is the first report of memory retention abilities changing seasonally as a trade-off between short- and long-term memories.

Nanoencapsulated Melaleuca alternifolia essential oil exerts anesthetic effects in the brachyuran crab using Neohelice granulate

The aim of this study was to evaluate the efficacy and safety of several anesthetics in the brachyuran crab Neohelice granulata, an emergent experimental model. The essential oils (EOs) of Lippia alba, Aloysia tryphilla, and Melaleuca alternifolia (tea tree oil; TTO), the isolated compounds eugenol, menthol, terpinen-4-ol, and the nanoencapsulated form of TTO, were administered in one or more of the following ways: added to the water (immersion), through an arthrodial membrane (injected), or by oral gavage. Unexpectedly, most EOs did not produce an anesthetic effect after immersion. Only TTO and eugenol induced anesthesia by immersion, with very long induction and recovery times compared to anesthesia of other crustaceans. However, a good anesthetic effect was observed with the injection of terpinen-4-ol and nanoencapsulated TTO in N. granulata; both demonstrated ideal induction and recovery times. These substances appear to be promising anesthetic alternatives for crustaceans.

Parallel memory traces are built after an experience containing aversive and appetitive components in the crab Neohelice

The neurobiology of learning and memory has been mainly studied by focusing on pure aversive or appetitive experiences. Here, we challenged this approach considering that real-life stimuli come normally associated with competing aversive and appetitive consequences and that interaction between conflicting information must be intrinsic part of the memory processes. We used Neohelice crabs, taking advantage of two well-described appetitive and aversive learning paradigms and combining them in a single training session to evaluate how this affects memory. We found that crabs build separate appetitive and aversive memories that compete during retrieval but not during acquisition. Which memory prevails depends on the balance between the strength of the unconditioned stimuli and on the motivational state of the animals. The results indicate that after a mix experience with appetitive and aversive consequences, parallel memories are established in a way that appetitive and aversive information is stored to be retrieved in an opportunistic manner.

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.

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.

High stimulus specificity characterizes anti-predator habituation under natural conditions

Habituation is one of the most fundamental learning processes that allow animals to adapt to dynamic environments. It is ubiquitous and often thought of as a simple form of non-associative learning. Very little is known, though, about the rules that govern habituation and their significance under natural conditions. Questions about how animals incorporate habituation into their daily behaviour and how they can assure only to habituate to non-relevant stimuli are still unanswered. Animals under threat of predation should be particularly selective about which stimuli they habituate to, since ignoring a real threat could be fatal. In this study, we tested the response of fiddler crabs, Uca vomeris, to repeatedly approaching dummy predators to find out whether these animals habituate to potential predators and to test the selectivity of the habituation process. The crabs habituated to model predators, even though they were confronted with real predators during the same habituation process. They showed remarkable selectivity towards the stimulus: a simple change in the approach distance of the stimulus led to a recovery in their responses. The results strongly indicate that in the context of predator avoidance, habituation under natural conditions is highly selective and a stimulus is not defined just by its current sensory signature, but also its spatio-temporal history.

Contrasting role of octopamine in appetitive and aversive learning in the crab Chasmagnathus

Background

Biogenic amines are implicated in reinforcing associative learning. Octopamine (OA) is considered the invertebrate counterpart of noradrenaline and several studies in insects converge on the idea that OA mediates the reward in appetitive conditioning. However, it is possible to assume that OA could have a different role in an aversive conditioning.

Methodology/Principal Findings

Here we pharmacologically studied the participation of OA in two learning processes in the crab Chasmagnathus granulatus, one appetitive and one aversive. It is shown that the aversive memory is impaired by an OA injection applied immediately or 30 minutes after the last training trial. By contrast, the appetitive memory is blocked by OA antagonists epinastine and mianserine, but enhanced by OA when injected together with the supply of a minimum amount of reinforcement. Finally, double-learning experiments in which crabs are given the aversive and the appetitive learning either successively or simultaneously allow us to study the interaction between both types of learning and analyze the presumed action of OA. We found that the appetitive training offered immediately, but not one hour, after an aversive training has an amnesic effect on the aversive memory, mimicking the effect and the kinetic of an OA injection.

Conclusions/Significance

Our results demonstrate that the role of OA is divergent in two memory processes of opposite signs: on the one hand it would mediate the reinforcement in appetitive learning, and on the other hand it has a deleterious effect over aversive memory consolidation.

Behavioral and neuronal attributes of short- and long-term habituation in the crab Chasmagnathus

Investigations using invertebrate species have led to a considerable progress in our understanding of the mechanisms underlying learning and memory. In this review we describe the main behavioral and neuronal findings obtained by studying the habituation of the escape response to a visual danger stimulus in the crab Chasmagnathus granulatus. Massed training with brief intertrial intervals lead to a rapid reduction of the escape response that recovers after a short term. Conversely, few trials of spaced training renders a slower escape reduction that endures for many days. As predicted by Wagner's associative theory of habituation, long-term habituation in the crab proved to be determined by an association between the contextual environment of the training and the unconditioned stimulus. By performing intracellular recordings in the brain of the intact animal at the same time it was learning, we identified a group of neurons that remarkably reflects the short- and long-term behavioral changes. Thus, the visual memory abilities of crabs, their relatively simple and accessible nervous system, and the recording stability that can be achieved with their neurons provide an opportunity for uncovering neurophysiological and molecular events that occur in identifiable neurons during learning.

Evolution of motor memory during the seconds after observation of motor error

When a movement results in error, the nervous system amends the motor commands that generate the subsequent movement. Here we show that this adaptation depends not just on error, but also on passage of time between the two movements. We observed that subjects learned a reaching task faster, i.e., with fewer trials, when the intertrial time intervals (ITIs) were lengthened. We hypothesized two computational mechanisms that could have accounted for this. First, learning could have been driven by a Bayesian process where the learner assumed that errors are the result of perturbations that have multiple timescales. In theory, longer ITIs can produce faster learning because passage of time might increase uncertainty, which in turn increases sensitivity to error. Second, error in a trial may result in a trace that decays with time. If the learner continued to sample from the trace during the ITI, then adaptation would increase with increased ITIs. The two models made separate predictions: The Bayesian model predicted that when movements are separated by random ITIs, the learner would learn most from a trial that followed a long time interval. In contrast, the trace model predicted that the learner would learn most from a trial that preceded a long time interval. We performed two experiments to test for these predictions and in both experiments found evidence for the trace model. We suggest that motor error produces an error memory trace that decays with a time constant of about 4 s, continuously promoting adaptation until the next movement.

Aggressiveness and memory: subordinate crabs present higher memory ability than dominants after an agonistic experience

A relationship between aggressiveness and memory has been proposed in several studies with different animal species. Here, we study this possibility in the crab Chasmagnathus granulatus, using the context-signal memory model (CSM) that involves an association between the learning context and a visual danger stimulus. Each experiment consisted of an agonistic phase and a memory one. During the former, matched pairs of male crabs were staged in two 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 CSM and tested 24 h later. Results showed that the agonistic encounter, staged 48 h before the acquisition of CSM, can modulate memory according to the dominance condition of the fighter; in such a way that memory retention of subordinates results higher than that of dominants. By contrast, when the memory phase preceded the agonist one, forthcoming dominants and subordinates did not differ in their memory ability. The memory modulation would not be linked to a dominance status but to a persistent dominance relationship fully reconstructed in each encounter between the same opponents. Therefore, the crab's CSM would not depend directly on predetermined intrinsic properties, but on the outcome of the fight, which would be determined in turn by the relative aggressiveness of the fighters. The finding that the agonistic episode modulates memory opens the possibility of using this episodic interference to probe the function of diverse phases of CSM.

Differential activity profile of cAMP-dependent protein kinase isoforms during long-term memory consolidation in the crab Chasmagnathus

The isoforms of cAMP-dependent protein kinase (PKA) show distinct biochemical properties and subcellular localization, suggesting different physiological functions, and conferring the fine-tuning between the activation of cAMP-PKA cascade and the cellular response. The critical role of PKA in memory and synaptic plasticity has been extensively demonstrated both in vertebrates and invertebrates, but the role of PKA isoforms is a matter of debate. Here we present experimental data showing differential PKA activation profiles after two different experiences: an instance of associative contextual learning (context-signal learning) and a single exposure to a novel context, both in the learning and memory model of the crab Chasmagnathus. Differences were found in the temporal course of activation and in the involvement of PKA isoforms. We found increased PKA activity immediately and 6 h after context-signal training correlating with the critical periods during which pharmacological inhibition of PKA disrupts memory formation. In contrast, PKA activity increased immediately but not 6 h after single exposure to a novel context. The amounts of PKA I and PKA II holoenzymes were analyzed to determine changes in holoenzyme levels and/or differential activation induced by both experiences. Results indicate that context-induced PKA activation is at least in part due to PKA II, and that PKA activation 6 h after context-signal learning coincides with an increase in the total level of PKA I. Considering the higher sensitivity of PKA I to cAMP, its increment can account for the PKA activation found 6 h after training and is proposed as a novel mechanism providing the prolonged PKA activation during memory consolidation.

Transcription factor NF-kappaB activation after in vivo perforant path LTP in mouse hippocampus

There is increasing evidence that transcription factors (TFs) play a critical role in maintaining later phases of hippocampal long-term potentiation (LTP). We have been led to study the role in synaptic plasticity of the powerful, yet generally unheralded, NF-kappaB TF because it may serve as both a signaling molecule after its activation at the synapse and then a transcription initiator upon reaching the nucleus. In the present study, we show that LTP activates NF-kappaB in the intact mouse hippocampus. Mice were sacrificed 15 min after one of three treatments: tetanization (high-frequency stimulation [HFS]), low-frequency stimulation (LFS), or no stimulated control animals (CT). In a first study, nuclear NF-kappaB activity from hippocampus was estimated by electrophoretic mobility shift assays (EMSAs). A higher level of hippocampal TF binding to the NF-kappaB recognition element was found in the HFS group compared with LFS or CT. In a second study, NF-kappaB activity was evaluated by immunohistochemistry with a specific antibody that recognizes the activated form of NF-kappaB. This antibody binds to the exposed nuclear location sequence on the p65 subunit of NF-kappaB consequent to its dissociation from the inhibitory IkappaB molecule. In the four subfields of hippocampus examined--granule cell layer, hilus of the dentate gyrus, CA3 and CA1 pyramidal fields of the hippocampal gyrus--the highest levels of activated NF-kappaB, statistically significant in all cases were found after HFS. In certain comparisons, LFS animals also showed significant elevation with respect to CT. These results support the role of NF-kappaB as part of the synaptic signaling and transcriptional regulation mechanism required in long-term plasticity, emphasizing the combinatorial nature of TF function.

Angiotensin II and the transcription factor Rel/NF-kappaB link environmental water shortage with memory improvement

One of the essential requirements even in the most ancient life forms is to be able to preserve body fluid medium. In line with such requirement, animals need to perform different behaviors to cope with water shortages. As angiotensin II (ANGII) is involved on a widespread range of functions in vertebrates, including memory modulation, an integrative role, in response to an environmental water shortage, has been envisioned. Previous work on the semi-terrestrial and brackish-water crab Chasmagnathus granulatus showed that endogenous ANGII enhanced an associative long-term memory and, in addition, that high salinity environment induces both an increase of brain ANGII levels and memory improvement. Here, we show that in the crab Chasmagnathus air exposure transiently increases blood sodium concentration, significantly increases brain ANGII immunoreactivity, and has a facilitatory effect on memory that is abolished by a non-selective ANGII receptor antagonist, saralasin. Furthermore, Rel/NF-kappaB, a transcription factor activated by ANGII in mammals and during memory consolidation in Chasmagnathus brain, is induced in the crab's brain by air exposure. Moreover, nuclear brain NF-kappaB is activated by ANGII, and this effect is reversed by saralasin. Our results constitute the first demonstration in an invertebrate that cognitive functions are modulated by an environmental stimulus through a neuropeptide and give evolutionary support to the role of angiotensins in memory processes. Moreover, these results suggest that angiotensinergic system is preserved across evolution not only in its structure and molecular mechanisms, but also in its capability of coordinating specific adaptative responses.

CS-US delay does not impair appetitive conditioning in Chasmagnathus

Habituation and appetitive conditioning have been already described in the crab Chasmagnathus. The purpose of this work is to study whether associative learning can be obtained despite a long conditioned stimulus-unconditioned stimulus interval. Results of the first experiment show that the weakening of temporal contiguity does not prevent appetitive conditioning to occur while after a long 4-h delay, conditioning wanes completely. A second experiment was conducted, after one and three days of training respectively, confirming the above results. Though initially neutral the context trace may be still available immediately after training and for the period of two but not after 4:00 h, demonstrating a forward limit for the conditioning window. After 3 days of training, a further decrease in the exploratory activity suggested that a longer training could increase the relative weight of habituation. Conditioning and habituation seem to work as opponent processes in the crab CHASMAGNATHUS GRANULATUS: if habituation training in the box is followed by the administration of reinforcement after a short period of time, appetitive conditioning will take place. However, as this interval is increased, habituation prevails. A persistent effect of the exposure to a given environment that may underlie trace conditioning in this crab is discussed in adaptive terms.

Two related forms of memory in the crab Chasmagnathus are differentially affected by NMDA receptor antagonists

A visual danger stimulus (VDS) elicits an escape response in the crab Chasmagnathus that declines after a few iterative presentations. Long-lasting retention of such decrement, termed context-signal memory (CSM), is mediated by an association between danger stimulus and environmental cues, cycloheximide sensitive, correlated with PKA activity and NFkappa-B activation, positively modulated by angiotensins, and selectively regulated by a muscarinic-cholinergic mechanism. The present research was aimed at studying the possible involvement of NMDA-like receptors in CSM, given the role attributed to these receptors in vertebrate memory and their occurrence in invertebrates including crustaceans. Vertebrate antagonists (+/-)-2-amino-5-phosphonopentanoic acid (AP5) and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) were used. Memory retention impairment was shown with MK-801 10(-3) M (1 microg/g) injected immediately before training or after training, or delayed 1 or 4 h, but not 6 h, posttraining. An AP5 10(-3) M dose (0.6 microg/g) impairs retention when given before but not after training. Neither antagonist produced retrieval deficit. A memory process similar to CSM but nonassociative in nature and induced by massed training (termed signal memory, SM), proved entirely insensitive to AP5 or MK-801, confirming the view that distinct mechanisms subserve these different types of memory in the crab.

Two critical periods for cAMP-dependent protein kinase activity during long-term memory consolidation in the crab Chasmagnathus

Activation of the cAMP pathway was found to be implicated in the memory process. In the context-signal learning paradigm of the crab Chasmagnathus, the protein kinase (PKA) activator Sp-5,6-DCl-cBIMPS facilitated long-term memory (LTM) induced by spaced training while the PKA inhibitor 8-chloroadenosine-3', 5'-monophosphorothioate, Rp-isomer (Rp-8-Cl-cAMPS) produced amnesia. In the present report the effect of the PKA inhibitor on long-term retention was assessed when administered (systemic injection of 2 microg/animal) at various times after training. According to previous results obtained with a lower dose, retention is impaired when the drug is administered immediately pretraining. An effect on acquisition was ruled out considering that the drug did not affect the performance during training. On the contrary, no effect of the PKA inhibitor was found with an immediately posttraining injection and amnesia was observed only when training was shortened from 15 to 12 trials (training duration from 45 to 36 min). At 2 and 12 h posttraining Rp-8-Cl-cAMPS injection failed to impair retention, but amnesia was found when the drug was injected at 4 and 8 h after training. In order to assess a possible effect of the drug in retrieval, the PKA inhibitor was administered 15 min before testing, and no amnestic effect was observed. These results suggest that two phases of PKA activity are required during consolidation of LTM, one during training and the other between 4 and 8 h after training. The link between these two periods of PKA activation and the two phases of the transcription factor NF-kappaB activation previously found in this model, as well as the similar time course found in rodents, is discussed. An amnestic effect of the drug was not found when administered immediately before a massed training protocol that yielded an intermediate-term memory, suggesting that in this type of memory PKA activation is not required.

High environmental salinity induces memory enhancement and increases levels of brain angiotensin-like peptides in the crab Chasmagnathus granulatus

Previous work on the brackish-water crab Chasmagnathus granulatus demonstrated that an endogenous peptide similar to angiotensin II plays a significant role in enhancing long-term memory that involves an association between context and an iterative danger stimulus (context-signal memory). The present results show that this memory enhancement could be produced by moving crabs from brackish water to sea water (33.0%) and keeping them there for at least 4 days. The possibility that such a facilitatory effect is due to osmotic stress is ruled out. Coincidentally, the level of angiotensin-II-like peptides in crab brain, measured by radioimmunoassay, increases with the length of exposure to sea water, reaching a significantly different level at the fourth day. The presence of angiotensin-II-like immunoreactive material in neural structures of the supraoesophageal and eyestalk ganglia was confirmed by immunohistochemical analysis. The results are interpreted as supporting the hypothesis that exposure to water of high salinity is an external cue triggering a process mediated by angiotensins that leads to enhanced memory in these crabs.

Long-lasting and context-specific freezing preference is acquired after spaced repeated presentations of a danger stimulus in the crab Chasmagnathus

A visual danger stimulus elicits an escape response in the crab Chasmagnathus that declines after repeated presentations. Previous results report that such waning may be retained as context-signal memory (CSM) or signal memory (SM): CSM is long lasting, associative, and produced by spaced training, while SM is an intermediate memory, nonassociative, and produced by massed training. The performances of both spaced and massed trained crabs are here examined, using video analysis to determine topographic changes in the behavioral response during and after training. During spaced training, escape vanishes and is mainly replaced by freezing, while during massed training, escape decreases over trials without being replaced by any defensive response. After 24 h, the marked proclivity to freezing persists in spaced trained crabs, while a high level of escaping is shown by massed trained crabs. The long-lasting freezing preference of spaced trained crabs proves to be context-specific and apparent from the very first presentation of the danger stimulus at testing, though freezing is not triggered by the sole exposure to the context. We conclude (a) that freezing preference is the acquired response of the CSM process; (b) that CSM can be properly categorized as an instance of contextual conditioning and SM of classical habituation; (c) that CSM and SM are not two phases of a memory processing but two distinctly types of memory; and (d) that therefore, the temporal distribution of training trials has a drastic effect on crab's memory, more dramatic than that previously described. The possibility that massed and spaced presentations of the same stimulus may represent two different stimulus types is discussed.

Two related forms of long-term habituation in the crab Chasmagnathus are differentially affected by scopolamine

An opaque screen moving overhead elicits an escape response in the crab Chasmagnathus, which, after a few presentations, habituates for a long period (long-term habituation, LTH). Previous results distinguished two types of LTH: the (context-signal)-LTH yielded by spaced training, determined by an association between context and habituating stimulus, and cycloheximide sensitive: and the (signal)-LTH produced by massed training, context independent, and cycloheximide insensitive. Present experiments were aimed at studying the possible involvement of cholinergic mechanisms in one or both types of LTH, using the muscarinic antagonist, scopolamine (SCP). Results indicate that LTH acquired by spaced training (30 trials separated by 85 s) is blocked in a dose-dependent manner by posttraining SCP. Amnesia is shown with 100 ng SCP/g injected immediately before or after spaced training but not delayed 1-h posttraining. No effect of SCP on LTH acquired by massed training (300 trials separated by 4 s) is detected. Pretraining SCP induces a decrease in the response level at the initial trials of either a spaced or a massed training. It is concluded that the storage of (context-signal)-LTH may be selectively regulated by a muscarinic-cholinergic mechanism. However, the possibility that other cholinergic receptors would be involved in the consolidation of the (signal)-LTH is discussed.

Memory enhancement by the angiotensinergic system in the crab Chasmagnathus is mediated by endogenous angiotensin II

In previous work with the crab Chasmagnathus, it was reported that either exogenous angiotensin II (ANGII) or angiotensin IV (ANGIV), have an enhancing effect on long-term memory, which involves an association between context and an iterative danger stimulus (context-signal memory, CSM). Present results indicate that Dival, an ANGIV antagonist, reverts the facilitatory effect of ANGIV but not that of ANGII, whereas saralasin but not Dival, disrupts CSM. These findings suggest that ANGII is the endogenous angiotensin that plays a significant role in long-term memory, while the ANGIV receptor would not be encompassed in the cascade of events related to crab's CSM.