Héctor Maldonado

Memory strengthening by a real‐life episode during reconsolidation: an outcome of water deprivation via brain angiotensin II

A considerable body of evidence reveals that consolidated memories, recalled by a reminder, enter into a new vulnerability phase during which they are susceptible to disruption again. Consistently, reconsolidation was shown by the amnesic effects induced by administration of consolidation blockers after memory labilization. To shed light on the functional value of reconsolidation, we explored whether an endogenous process activated during a concurrent real‐life experience improved this memory phase. Reconsolidation of long‐term contextual memory has been well documented in the crab Chasmagnathus. Previously we showed that angiotensin II facilitates memory consolidation. Moreover, water deprivation increases brain angiotensin and improves memory consolidation and retrieval through angiotensin II receptors. Here, we tested whether concurrent water deprivation improves reconsolidation via endogenous angiotensin and therefore strengthens memory. We show that memory reconsolidation, induced by training context re‐exposure, is facilitated by a concurrent episode of water deprivation, which induces a raise in endogenous brain angiotensin II. Positive modulation is expressed by full memory retention, despite a weak training, 24 or 72 but not 4u2003h after memory reactivation. This is the first evidence that memory can be positively modulated during reconsolidation through an identified endogenous process triggered during a real‐life episode. We propose that the functional value for reconsolidation would be to make possible a change in memory strength by the influence of a concurrent experience. Reconsolidation improvement would lead to memory re‐evaluation, not by altering memory content but by modifying the behaviour as an outcome of changing the hierarchy of the memories that control it.

Context-us association as a determinant of long-term habituation in the crabChasmagnathus

An opaque screen moving overhead elicits an escape response in the crabChasmagnathus that habituates for a long period after just a few presentations. A series of experiments was performed to determine whether the crab’s long-term habituation (LTH) is mediated by an association between contextual cues and the eliciting stimulus.Chasmagnathus failed to exhibit LTH when a visual cue in the experimental environment was changed between training and testing. In addition, long-term exposure to the context in the absence of the eliciting stimulus impaired LTH, both when the exposure preceded the habituation training (latent inhibition) and when the exposure came after the training (extinction). Long exposure to the context alone prior to training also produced a decrease in responsiveness to the eliciting stimulus, which confirmed previous results. However, both effects of long exposure were only manifested when the crabs spent a period of time between exposure and testing out of the experimental context. The results of this paper are interpreted as supporting the view thatChasmagnathus LTH can be understood largely by Wagner’s associative theory of habituation.

Crustaceans as Models to Investigate Memory Illustrated by Extensive Behavioral and Physiological Studies in Chasmagnathus

In this and the following chapter we intend to illustrate the suitability of crustaceans for investigating the learning process through our own research with the crab Chasmagnathus. A visual danger stimulus elicits an escape response in the crab that declines after repeated presentations. When a long rest interval is interspersed between presentations (spaced training), an associative, long-lasting memory is formed, termed context-signal memory (CSM). When a short interval is interspersed (massed training), a non-associative, intermediate memory is formed, termed signal memory (SM). Such waning is retained as context-signal memory (CSM) or signal-memory (SM). The CSM, but not the SM, is cycloheximide-sensitive, mediated by the cAMP signal pathway, regulated by a muscarinic-cholinergic mechanism, and entails an increase in the Rel/NF-KB like-DNA-binding activity. A model standing for two putative memory mechanisms is discussed.

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

A decade of studies on long-term habituation (LTH) in the crab Chasmagnathus is reviewed. Upon sudden presentation of a passing object overhead, the crab reacts with an escape response that habituates promptly and for at least five days. LTH proved to be an instance of associative memory and showed context, stimulus frequency and circadian phase specificity. A strong training protocol (STP) (> or = 15 trials, intertrial interval (ITI) of 171 s) invariably yielded LTH, while a weak training protocol (WTP) (< or = 10 trials, ITI = 171 s) invariably failed. STP was used with a presumably amnestic agent and WTP with a presumably hypermnestic agent. Remarkably, systemic administration of low doses was effective, which is likely to be due to the lack of an endothelial blood-brain barrier. LTH was blocked by inhibitors of protein and RNA synthesis, enhanced by protein kinase A (PKA) activators and reduced by PKA inhibitors, facilitated by angiotensin II and IV and disrupted by saralasin. The presence of angiotensins and related compounds in the crab brain was demonstrated. Diverse results suggest that LTH includes two components: an initial memory produced by spaced training and mainly expressed at an initial phase of testing, and a retraining memory produced by massed training and expressed at a later phase of testing (retraining). The initial memory would be associative, context specific and sensitive to cycloheximide, while the retraining memory would be nonassociative, context independent and insensitive to cycloheximide.

Behavioral and mechanistic bases of long-term habituation in the crab Chasmagnathus.

The exact relationship between findings at the cellular level and the behavioral phenomenon of learning remain complex and obscure in mammals. With invertebrate preparations, however, relationships between cellular and behavioral phenomena are much more readily clarified (Krasne and Glanzman, 1995).

Extinction memory in the crab Chasmagnathus: recovery protocols and effects of multi-trial extinction training

A decline in the frequency or intensity of a conditioned behavior following the withdrawal of the reinforcement is called experimental extinction. However, the experimental manipulation necessary to trigger memory reconsolidation or extinction is to expose the animal to the conditioned stimulus in the absence of reinforcement. Recovery protocols were used to reveal which of these two processes was developed. By using the crab contextual memory model (a visual danger stimulus associated with the training context), we investigated the dynamics of extinction memory in Chasmagnathus. Here, we reveal the presence of three recovery protocols that restore the original memory: the old memory comes back 4xa0days after the extinction training, or when a weak training is administered later, or once the VDS is presented in a novel context 24xa0h after the extinction session. Another objective was to evaluate whether the administration of multi-trial extinction training could trigger an extinction memory in Chasmagnathus. The results evince that the extinction memory appears only when the total re-exposure time is around 90xa0min independently of the number of trials employed to accumulate it. Thus, it is feasible that the mechanisms described for the case of the extinction memory acquired through a single training trial are valid for multi-trial extinction protocols. Finally, these results are in agreement with those reports obtained with models phylogenetically far apart from the crab. Behind this attempt is the idea that in the domain of studies on memory, some principles of behavior organization and basic mechanisms have universal validity.

A field model of learning: 2. Long-term memory in the crab Chasmagnathus granulatus

In the previous companion paper, the possibility of learning by Chasmagnathus in field conditions was demonstrated. Here, we study long-term memory inquiring to what extent an internal representation could be maintained in a complex environment. Two 45-min training sessions, each of 15 visual danger stimulus presentations with 3-min intertrials, were given at a 24-h interval. Throughout the first training session and during the first 22.5xa0min of re-training on day 2, crabs kept the same hiding response level but then, during the second phase of re-training, the re-emerging increased up to the point that 32% of surface crabs ignored the stimulus. Each session was followed by a 22.5-min testing without stimulation. At testing on day 2 after re-training, crabs showed a change in the usual exploring strategy. Results reveal long persistency in responding despite a rest interval of 24xa0h followed by a gradual decrease in response until it vanishes. The statistical analysis was performed by comparing performances at day 2 (Rescorla in Am Psychol 43:151–160, 1988) and then confirmed through comparisons between day 1 and day 2. However, it is not possible to identify separate and taxonomically well-defined learnings but rather a tangled collection of processes that influence each other blurring some of the diagnostic features of each learning.

Cardiovascular component of the context signal memory in the crab Chasmagnathus

Research on diverse models of memory in vertebrates demonstrates that behavioral, autonomic and endocrine responses occur together during fear conditioning. With invertebrates, no similar studies have been performed despite the extensive study of fear memory paradigms, as the context signal memory (CSM) of the crab Chasmagnathus granulatus, usually assessed by a behavioral parameter. Here, we study the crab’s CSM, considering both the behavioral response and the concomitant neuroautonomic adjustments resulting in a heart rate alteration. Results show that upon the first presentation of the visual danger stimulus, a heart arrest followed by bradycardia is triggered together with a conspicuous escape response. The latter declines throughout training, while heart arrests become sporadic and bradycardia tends to deepen along the session. At test, 24xa0h after training, the outcome clearly contrasts with that shown at training, namely, stimulus presentation in the same context induces lower escape, no heart arrests and quick suppression of bradycardia. These results support the view that the same memory process brings about the changes in both responses. High escape, heart arrest and bradycardia are considered three parameters of the unconditioned response while minor escape, no heart arrests and bradycardia attenuation are three parameters of the learned response.

Age-related deficits of long-term memory in the crab Chasmagnathus

Short and long-term memory in adult crabs Chasmagnathus granulatus of different age are evaluated in two learning paradigms: habituation to a visual danger stimulus and appetitive conditioning. No difference between young, middle-aged and aged animals is found in short-term habituation with 15 training trials. A good level of retention of the habituated response at 24 h is exhibited by young and middle-aged crabs but a poor one by aged crabs. When the training-to-testing interval is lengthened to 48 h or the training session reduced to 7 trials, young and middle-aged crabs continue to show long-term habituation but aged individuals exhibit no retention at all. As regards appetitive conditioning, young, middle-aged and aged crabs present similar short-term memory with 5 training trials and similar long-term memory when tested at 24 h, but an age-related deficit in long-term retention is exhibited when the intersession interval is lengthened to 48 h or the training reduced to 3 trials. Thus, a reduction of long-term memory related to age is demonstrated in the crab Chasmagnathus. Since it is shown in two different learning paradigms, the possibility of explaining the deficit in terms of a failure in memory mechanisms due to aging rather than as a consequence of ontogenetic shift in the crabs behavior is discussed.

Angiotensin modulates long-term memory expression but not long-term memory storage in the crab Chasmagnathus

Memory reconsolidation is a dynamic process in which a previously consolidated memory becomes labile following reactivation by a reminder. In a previous study in the crab Chasmagnathus memory model, we showed that a water-shortage episode, via angiotensin modulation during reconsolidation, could reveal a memory that otherwise remains unexpressed: weakly trained animals cannot reveal long-term memory (LTM) except when an episode of noticeable ethological meaning, water deprivation, is contingent upon reconsolidation. However, these results are at variance with two of our previous interpretations: weak training protocols do not build LTM and angiotensin II modulates the strength of the information storing process. A parsimonious hypothesis is that in Chasmagnathus angiotensins regulate LTM expression, but not LTM storage. Here, we tested three predictions of this hypothesis. First, the well-known retrograde amnesic effect of the angiotensin II antagonist saralasin is not due to interference on memory storage, but to modulation of memory expression. Second, the recovery of the LTM memory expression of the apparently amnesic retrograde effect produced by saralasin, through the water-shortage episode contingent upon reconsolidation, must be reconsolidation specific. Consequently, summation-like effects and retrieval deficits cannot explain these results because of the parametric conditions of reconsolidation. Third, weak training protocols build an unexpressed LTM that requires mRNA transcription and translation, a diagnostic characteristic of LTM. Results show that angiotensin modulates LTM expression but not LTM memory storage in the crab Chasmagnathus. The results lead us to suggest that, in Chasmagnathus, LTM expression - the process of gaining appreciable control over behavior of the reactivated trace in the retrieval session - may be considered a distinct attribute of its long-term storage. This strategy, a positive modulation during reconsolidation, is proposed to distinguish between memories that can be reactivated, labilized and are not expressed, and memories that are not stored long term, obliterated or altered in other retrieval mechanisms.