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

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.

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.

5-HT2B, 5-HT7, and DA2 Receptors Mediate the Effects of 5-HT and DA on Agonistic Behavior of the Chinese Mitten Crab (Eriocheir sinensis)

The Chinese mitten crab (Eriocheir sinensis) is a commercially important crab in China and is usually managed at high stocking densities. Agonistic behavior directly impacts crab integrity, survival, and growth and results in economic losses. In the present study, we evaluated the modulatory effects of serotonin (5-HT) and dopamine (DA) though the 5-HT2 and DA2 receptor-cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA) pathway on agonistic behavior. The results showed that injection of either 10^-6 mol/crab 5-HT or DA reduced the agonistic behavior of E. sinensis (P < 0.05), as did 10^-10 mol/crab DA and 10^-8 mol/crab 5-HT and DA (P < 0.05); however, a dose of 10^-10 mol/crab 5-HT promoted agonistic behavior. 5-HT significantly increased the mRNA expression level of 5-HT7 receptor and reduced that of the DA2 receptor in the cerebral ganglion (P < 0.05). In contrast to 5-HT, DA significantly decreased 5-HT2B mRNA levels and increased 5-HT7 and DA2 receptor levels in the thoracic ganglia (P < 0.05). In addition, injections of either 5-HT or DA increased the cAMP and PKA levels in hemolymph (P < 0.05). By using in vitro culture of the thoracic ganglia, the current study showed that ketanserin (5-HT2 antagonist) and [R(-)-TNPA] (DA2 agonist) had obvious effects on the expression levels of the two receptors (P < 0.05). In vivo experiments further demonstrated that ketanserin and [R(-)-TNPA] could both significantly reduce the agonistic behavior of the crabs (P < 0.05). Furthermore, both ketanserin and [R(-)-TNPA] promoted the cAMP and PKA levels (P < 0.05). The injection of CPT-cAMP (cAMP analogue) elevated the PKA levels and inhibited agonistic behavior. In summary, this study showed that 5HT-2B and DA2 receptors were involved in the agonistic behavior that 5-HT/DA induced through the cAMP-PKA pathway in E. sinensis.

Epigenetic mechanisms and memory strength: a comparative study

Memory consolidation requires de novo mRNA and protein synthesis. Transcriptional activation is controlled by transcription factors, their cofactors and repressors. Cofactors and repressors regulate gene expression by interacting with basal transcription machinery, remodeling chromatin structure and/or chemically modifying histones. Acetylation is the most studied epigenetic mechanism of histones modifications related to gene expression. This process is regulated by histone acetylases (HATs) and histone deacetylases (HDACs). More than 5 years ago, we began a line of research about the role of histone acetylation during memory consolidation. Here we review our work, presenting evidence about the critical role of this epigenetic mechanism during consolidation of context-signal memory in the crab Neohelice granulata, as well as during consolidation of novel object recognition memory in the mouse Mus musculus. Our evidence demonstrates that histone acetylation is a key mechanism in memory consolidation, functioning as a distinctive molecular feature of strong memories. Furthermore, we found that the strength of a memory can be characterized by its persistence or its resistance to extinction. Besides, we found that the role of this epigenetic mechanism regulating gene expression only in the formation of strongest memories is evolutionarily conserved.

PKA increases in the olfactory bulb act as unconditioned stimuli and provide evidence for parallel memory systems: pairing odor with increased PKA creates intermediate- and long-term, but not short-term, memories

Neonatal odor-preference memory in rat pups is a well-defined associative mammalian memory model dependent on cAMP. Previous work from this laboratory demonstrates three phases of neonatal odor-preference memory: short-term (translation-independent), intermediate-term (translation-dependent), and long-term (transcription- and translation-dependent). Here, we use neonatal odor-preference learning to explore the role of olfactory bulb PKA in these three phases of mammalian memory. PKA activity increased normally in learning animals 10 min after a single training trial. Inhibition of PKA by Rp-cAMPs blocked intermediate-term and long-term memory, with no effect on short-term memory. PKA inhibition also prevented learning-associated CREB phosphorylation, a transcription factor implicated in long-term memory. When long-term memory was rescued through increased β-adrenoceptor activation, CREB phosphorylation was restored. Intermediate-term and long-term, but not short-term odor-preference memories were generated by pairing odor with direct PKA activation using intrabulbar Sp-cAMPs, which bypasses β-adrenoceptor activation. Higher levels of Sp-cAMPs enhanced memory by extending normal 24-h retention to 48-72 h. These results suggest that increased bulbar PKA is necessary and sufficient for the induction of intermediate-term and long-term odor-preference memory, and suggest that PKA activation levels also modulate memory duration. However, short-term memory appears to use molecular mechanisms other than the PKA/CREB pathway. These mechanisms, which are also recruited by β-adrenoceptor activation, must operate in parallel with PKA activation.

Opposite actions of dopamine on aversive and appetitive memories in the crab

The understanding of how the reinforcement is represented in the central nervous system during memory formation is a current issue in neurobiology. Several studies in insects provide evidence of the instructive role of biogenic amines during the learning and memory process. In insects it was widely accepted that dopamine (DA) mediates aversive reinforcements. However, the idea of DA being exclusively involved in aversive memory has been challenged in recent studies. Here, we study the involvement of DA during aversive and appetitive memories in the crab Chasmagnathus. We found that DA-receptor antagonists impair aversive memory consolidation, in agreement with previous reports in insects, while administration of DA facilitates memory formation after a weak training protocol. In contrast, DA treatment during appetitive training was found to impair formation of long-term appetitive memory. In addition, as a first step in elucidating the neuroanatomical correlates of DA action on memory, we mapped dopaminergic neurons in the central nervous system of the crab. Results of the current study, together with those obtained in a previous work about the role of octopamine (OA), suggest that both amines (DA and OA) play a dual action in memory processes. On the one hand, DA and OA mediate the aversive and the appetitive signals, respectively, throughout training, while on the other hand, they interfere with the formation of memory of the opposite sign (DA in appetitive and OA in aversive). Our results support a new understanding about the way appetitive and aversive stimuli are processed during memory formation to ensure adaptive behavior.

Food odor, visual danger stimulus, and retrieval of an aversive memory trigger heat shock protein HSP70 expression in the olfactory lobe of the crab Chasmagnathus granulatus

Although some of the neuronal substrates that support memory process have been shown in optic ganglia, the brain areas activated by memory process are still unknown in crustaceans. Heat shock proteins (HSPs) are synthesized in the CNS not only in response to traumas but also after changes in metabolic activity triggered by the processing of different types of sensory information. Indeed, the expression of citosolic/nuclear forms of HSP70 (HSC/HSP70) has been repeatedly used as a marker for increases in neural metabolic activity in several processes, including psychophysiological stress, fear conditioning, and spatial learning in vertebrates. Previously, we have shown that, in the crab Chasmagnathus, two different environmental challenges, water deprivation and heat shock, trigger a rise in the number of glomeruli of the olfactory lobes (OLs) expressing HSC/HSP70. In this study, we initially performed a morphometric analysis and identified a total of 154 glomeruli in each OL of Chasmagnathus. Here, we found that crabs exposed to food odor stimuli also showed a significant rise in the number of olfactory glomeruli expressing HSC/HSP70. In the crab Chasmagnathus, a powerful memory paradigm based on a change in its defensive strategy against a visual danger stimulus (VDS) has been extensively studied. Remarkably, the iterative presentation of a VDS caused an increase as well. This increase was triggered in animals visually stimulated using protocols that either build up a long-term memory or generate only short-term habituation. Besides, memory reactivation was sufficient to trigger the increase in HSC/HSP70 expression in the OL. Present and previous results strongly suggest that, directly or indirectly, an increase in arousal is a sufficient condition to bring about an increase in HSC/HSP70 expression in the OL of Chasmagnathus.

Characterization of the beta amyloid precursor protein-like gene in the central nervous system of the crab Chasmagnathus. Expression during memory consolidation

Background

Human β-amyloid, the main component in the neuritic plaques found in patients with Alzheimer's disease, is generated by cleavage of the β-amyloid precursor protein. Beyond the role in pathology, members of this protein family are synaptic proteins and have been associated with synaptogenesis, neuronal plasticity and memory, both in vertebrates and in invertebrates. Consolidation is necessary to convert a short-term labile memory to a long-term and stable form. During consolidation, gene expression and de novo protein synthesis are regulated in order to produce key proteins for the maintenance of plastic changes produced during the acquisition of new information.

Results

Here we partially cloned and sequenced the beta-amyloid precursor protein like gene homologue in the crab Chasmagnathus (cappl), showing a 37% of identity with the fruit fly Drosophila melanogaster homologue and 23% with Homo sapiens but with much higher degree of sequence similarity in certain regions. We observed a wide distribution of cappl mRNA in the nervous system as well as in muscle and gills. The protein localized in all tissues analyzed with the exception of muscle. Immunofluorescence revealed localization of cAPPL in associative and sensory brain areas. We studied gene and protein expression during long-term memory consolidation using a well characterized memory model: the context-signal associative memory in this crab species. mRNA levels varied at different time points during long-term memory consolidation and correlated with cAPPL protein levels

Conclusions

cAPPL mRNA and protein is widely distributed in the central nervous system of the crab and the time course of expression suggests a role of cAPPL during long-term memory formation.

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.

Evolutionarily-conserved role of the NF-kappaB transcription factor in neural plasticity and memory

NF-kappaB is an evolutionarily conserved family of transcription factors (TFs) critically involved in basic cellular mechanisms of the immune response, inflammation, development and apoptosis. In spite of the fact that it is expressed in the central nervous system, particularly in areas involved in memory processing, and is activated by signals such as glutamate and Ca2+, its role in neural plasticity and memory has only recently become apparent. A surprising feature of this molecule is its presence within the synapse. An increasing number of reports have called attention to the role of this TF in processes that require long-term regulation of the synaptic function underlying memory and neural plasticity. Here we review the evidence regarding a dual role for NF-kappaB, as both a signalling molecule after its activation at the synapse and a transcriptional regulator upon reaching the nucleus. The specific role of this signal, as well as the general transcriptional mechanism, in the process of memory formation is discussed. Converging lines of evidence summarized here point to a pivotal role for the NF-kappaB transcription factor as a direct signalling mechanism in the regulation of gene expression involved in long-term memory.

Lessons from a crab: molecular mechanisms in different memory phases of Chasmagnathus

Consolidation of long-term memory requires the activation of several transduction pathways that lead to post-translational modifications of synaptic proteins and to regulation of gene expression, both of which promote stabilization of specific changes in the activated circuits. In search of the molecular mechanisms involved in such processes, we used the context-signal associative learning paradigm of the crab Chasmagnathus. In this model, we studied the role of some molecular mechanisms, namely cAMP-dependent protein kinase (PKA), extracellular-signal-regulated kinase (ERK), the nuclear factor kappa B (NF-kappaB) transcription factor, and the role of synaptic proteins such as amyloid beta precursor protein, with the object of describing key mechanisms involved in memory processing. In this article we review the most salient results obtained over a decade of research in this memory model.

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.

Boosting intermediate-term into long-term memory

Aerial respiration in the pond snail Lymnaea stagnalis can be operantly conditioned. Depending on the specific training procedure used (i.e. a 0.5 h vs a 1.0 h interval between training sessions) either intermediate (ITM) or long-term memory (LTM) is formed. ITM, which persists for 2–3 h, is dependent only on de novo protein synthesis,whilst LTM persists for up to 4 weeks and is dependent on both transcription and de novo protein synthesis. We found that although the behavioural phenotype of ITM was not apparent 24 h after the last training session, a residual memory trace was present that serves as a foundation upon which a subsequent ITM-training-procedure builds on to form LTM (i.e. a `changed memory'). This residual memory trace could be perturbed by cooling, the behavioural process of context-specific extinction and by increasing the interval between the training procedures. Furthermore in preparations where the somata of RPeD1 (one of three interneurons in the central pattern generator required for aerial respiratory behavior) had been ablated before training, LTM could not be observed following a second bout of ITM-training. These data support the concept that a molecular memory trace is established as a consequence of ITM-training, which serves as a `permissive substrate', when the ITM memory is made active, sufficient to permit the necessary transcription and translation processes that are causal for LTM formation.

Dopamine stimulates snail albumen gland glycoprotein secretion through the activation of a D1-like receptor

The catecholamine dopamine is present in both the central nervous system and in the peripheral tissues of molluscs, where it is involved in regulating reproduction. Application of exogenous dopamine to the isolated albumen gland of the freshwater pulmonate snail Helisoma duryi (Wetherby) induces the secretion (release) of perivitelline fluid. The major protein component of the perivitelline fluid of Helisoma duryi is a native 288 kDa glycoprotein that is secreted around individual eggs and serves as an important source of nutrients for the developing embryos. The secretion of glycoprotein by the albumen gland is a highly regulated event that must be coordinated with the arrival of the fertilized ovum at the carrefour (the region where the eggs receive albumen gland secretory products). In order to elucidate the intracellular signalling pathway(s) mediating dopamine-induced glycoprotein secretion, albumen gland cAMP production and glycoprotein secretion were measured in the presence/absence of selected dopamine receptor agonists and antagonists. Dopamine D1-selective agonists dihydrexidine, 6,7-ADTN and SKF81297 stimulated cAMP production and glycoprotein secretion from isolated albumen glands whereas D1-selective antagonists SCH23390 and SKF83566 suppressed dopamine-stimulated cAMP production. Dopamine D2-selective agonists and antagonists generally had no effect on cAMP production or protein secretion. Based on the effects of these compounds, a pharmacological profile was obtained that strongly suggests the presence of a dopamine D1-like receptor in the albumen gland of Helisoma duryi. In addition, secretion of albumen gland glycoprotein was not inhibited by protein kinase A inhibitors, suggesting that dopamine-stimulated protein secretion might occur through a protein kinase A-independent pathway.

Mismatch between what is expected and what actually occurs triggers memory reconsolidation or extinction

In previous experiments on contextual memory, we proposed that the unreinforced re-exposure to the learning context (conditioned stimulus, CS) acts as a switch guiding the memory course toward reconsolidation or extinction, depending on reminder duration. This proposal implies that the system computes the total exposure time to the context, from CS onset to CS offset, and therefore, that the reminder presentation must be terminated for the switching mechanism to become operative. Here we investigated to what extent this requirement is necessary, and we explored the relation between diverse phases in the reconsolidation and extinction processes. We used the contextual memory model of the crab Chasmagnathus which involves an association between the learning context (CS) and a visual danger stimulus (unconditioned stimulus, US). Administration of cycloheximide was used to test the lability state of memory at different time points. The results show that two factors, no-reinforcement during the reminder (i.e., CS re-exposure) and CS offset are the necessary conditions for both processes to occur. Regardless of the reminder duration, memory retrieved by unreinforced CS re-exposure emerges intact and consolidated when tested before CS offset, suggesting that neither reconsolidation nor extinction is concomitant with CS re-exposure. Either process could only be triggered once the definitive mismatch between CS and US is confirmed by CS termination without the expected reinforcement.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Protein synthesis subserves reconsolidation or extinction depending on reminder duration

When learned associations are recalled from long-term memory stores by presentation of an unreinforced conditioned stimulus (CS), two processes are initiated. One, termed reconsolidation, re-activates the association between the conditioned and unconditioned stimuli and transfers it from a stable protein synthesis-independent form of storage to a more labile protein-dependent state. The other is an extinction process in which presentation of the CS alone degrades the association between CS and US. To address the mechanistic relationship between reconsolidation and extinction, we have used an invertebrate model of contextual memory, which involves an association between the learning context and a visual danger stimulus. Here, we show that re-exposure duration to the learning context acts as a switch guiding the memory course toward reconsolidation or extinction, each depending on protein synthesis. Manipulation of this variable allows findings of impaired extinction to be discriminated from those of disrupted reconsolidation.

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.

Reactivation and reconsolidation of long-term memory in the crab Chasmagnathus: protein synthesis requirement and mediation by NMDA-type glutamatergic receptors

Experiments with invertebrates support the view that intracellular events subserving the consolidation phase of memory are preserved across evolution. Here, we investigate whether such evolutionary persistence extends to reconsolidation mechanisms, which have recently received special attention in vertebrate studies. For this purpose, the memory model of the crab Chasmagnathus is used. A visual danger stimulus (VDS) elicits crab escaping, which declines after a few stimulus presentations. The long-lasting retention of this decrement, called context-signal memory (CSM), is mediated by an association between contextual cues of the training site and the VDS. The present results show amnesia for CSM in crabs re-exposed at 24 hr (day 2) for 5 min to the learning context, 24 hr after training, and injected with one of two amnesic agents, then tested 24 hr later. Agents and timing were either 15 microg of cycloheximide given between 1 hr before and 4 hr after re-exposure or 1 microg/gm (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine given between 1 hr before and 2 hr after re-exposure. The amnesic effects are specific to behavior that occurs a long time after reactivation but not a short time after. No CSM deficit is produced by such agents when crabs are exposed to a context different from that of training. Findings are consistent with those reported for vertebrates, with both showing that reactivation induces a recapitulation of the postacquisition cascade of intracellular events. The agreement between results from such phylogenetically disparate animals suggests that evolution may have adopted a given molecular cascade as the preferred means of encoding experiences in the nervous system.

The IkappaB kinase inhibitor sulfasalazine impairs long-term memory in the crab Chasmagnathus

Evidence for the participation of Rel/NF-kappaB transcription factors in long-term memory has recently been reported in the context-signal learning paradigm of the crab Chasmagnathus, in which a high correlation between long-term memory formation and NF-kappaB activation was observed. Two components of the NF-kappaB pathway in the crab brain have now been identified by cross-immunoreactivity using mammalian antibodies for IkappaB-alpha and IkappaB kinase alpha. Furthermore, IkappaB kinase-like phosphotransferase activity, which was inhibited by the IkappaB kinase inhibitor sulfasalazine, was detected in brain extracts. We have evaluated the effect of sulfasalazine administration on long-term memory tested at 48 h. Amnesia was found when sulfasalazine was administered pre-training and 5 h after training but not at 0 or 24 h after training. Thus, two periods for sulfasalazine-induced amnesia were found in coincidence with the two phases of NF-kappaB activation previously described (immediately and 6 h after training). The cyclooxygenase inhibitor indomethacin did not induce amnesia when administered pre-training. Thus, the possibility that sulfasalazine induces amnesia by means of cyclooxygenase inhibition is unlikely to be tenable. In vivo sulfasalazine inhibition of basal NF-kappaB activity was found between 30 and 45 min after injection, as assessed by electrophoretic mobility shift assay. On the other hand, in vivo sulfasalazine administration 6 h after training inhibited the second phase of training-induced NF-kappaB activation, providing evidence that the sulfasalazine effect on memory is due to a direct effect of the drug on the NF-kappaB pathway. These results provide the first evidence that IkappaB kinase and NF-kappaB activation are necessary for memory formation.

Molecular pharmacological dissection of short- and long-term memory

1. It has been discussed for over 100 years whether short-term memory (STM) is separate from, or just an early phase of, long-term memory (LTM). The only way to solve this dilemma is to find out at least one treatment that blocks STM while keeping LTM intact for the same task in the same animal. 2. The effect of a large number of treatments infused into the hippocampus, amygdala, and entorhinal, posterior parietal or prefrontal cortex on STM and LTM of a one-trial step-down inhibitory avoidance task was studied. The animals were tested at 1.5 h for STM, and again at 24 h for LTM. The treatments were given after training. 3. Eleven different treatments blocked STM without affecting LTM. Eighteen treatments affected the two memory types differentially, either blocking or enhancing LTM alone. Thus, STM is separate from, and parallel to the first hours of processing of, LTM of that task. 4. The mechanisms of STM are different from those of LTM. The former do not include gene expression or protein synthesis; the latter include a double peak of cAMP-dependent protein kinase activity, accompanied by the phosphorylation of CREB, and both gene expression and protein synthesis. 5. Possible cellular and molecular events that do not require mRNA or protein synthesis should account for STM. These might include a hyperactivation of glutamate AMPA receptors, ribosome changes, or the exocytosis of glycoproteins that participate in cell addition.