Effect of naloxone pretreatment on habituation in the crab Chasmagnathus granulatus

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.

Surface expression of NMDA receptor changes during memory consolidation in the crab Neohelice granulata

The aim of the present study was to analyze the surface expression of the NMDA-like receptors during the consolidation of contextual learning in the crab Neohelice granulata Memory storage is based on alterations in the strength of synaptic connections between neurons. The glutamatergic synapses undergo various forms of N-methyl-D aspartate receptor (NMDAR)-dependent changes in strength, a process that affects the abundance of other receptors at the synapse and underlies some forms of learning and memory. Here we propose a direct regulation of the NMDAR. Changes in NMDAR's functionality might be induced by the modification of the subunit's expression or cellular trafficking. This trafficking does not only include NMDAR's movement between synaptic and extra-synaptic localizations but also the cycling between intracellular compartments and the plasma membrane, a process called surface expression. Consolidation of contextual learning affects the surface expression of the receptor without affecting its general expression. The surface expression of the GluN1 subunit of the NMDAR is down-regulated immediately after training, up-regulated 3 h after training and returns to naïve and control levels 24 h after training. The changes in NMDAR surface expression observed in the central brain are not seen in the thoracic ganglion. A similar increment in surface expression of GluN1 in the central brain is observed 3 h after administration of the competitive GABAA receptor antagonist, bicuculline. These consolidation changes are part of a plasticity event that first, during the down-regulation, stabilizes the trace and later, at 3-h post-training, changes the threshold for synapse activation.

Reconsolidation involves histone acetylation depending on the strength of the memory

Gene expression is a necessary step for memory re-stabilization after retrieval, a process known as reconsolidation. Histone acetylation is a fundamental mechanism involved in epigenetic regulation of gene expression and has been implicated in memory consolidation. However, few studies are available in reconsolidation, all of them in vertebrate models. Additionally, the recruitment of histone acetylation as a function of different memory strengths has not been systematically analyzed before. Here we studied the role of histone acetylation in reconsolidation using a well-characterized memory model in invertebrate, the context-signal memory in the crab Chasmagnathus. Firstly, we found an increase in histone H3 acetylation 1h after memory reactivation returning to basal levels at 3 h. Strikingly, this increment was only detected during reconsolidation of a long-term memory induced by a strong training of 30 trials, but not for a short-term memory formed by a weak training of five trials or for a long-term memory induced by a standard training of 15 trials. Furthermore, we showed that a weak memory which was enhanced during consolidation by histone deacetylases inhibition, also recruited histone H3 acetylation in reconsolidation as the strong training does. Accordingly, we found the first evidence that the administration of a histone acetyl transferase inhibitor during memory reconsolidation impairs long-term memory re-stabilization. Finally, we found that strong training memory, at variance with the standard training memory, was resistant to extinction, indicating that such strong training induced in fact a stronger memory. In conclusion, the results presented here support that the participation of histone acetylation during reconsolidation is an evolutionary conserved feature and constitutes a specific molecular characteristic of strong memories.

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.

The effect of opioids and their antagonists on the nocifensive response of Caenorhabditis elegans to noxious thermal stimuli

Opiates modulate nociception in vertebrates. This has also been demonstrated in a number of invertebrate models. Herein, the effect of the opiate morphine and opioid neuropeptides Endomorphin 1 and 2 on the thermal avoidance (Tav) behavior of Caenorhabditis elegans is explored. Adult wild-type C. elegans N2 were collected from NGM plates using M9 buffer and exposed to morphine and endomorphine 1 and 2 in concentrations between 10(-8) and 10(-4) M (2.5 pmol/mg to 25 nmol/mg) for 30 min and tested for Tav. The opioid receptor antagonists Naloxone and CTOP were tested in combination with the drugs. Forty-seven percentage of the morphine exposed worms exhibited a class I response versus 76% of the control group (P < 0.001). Endomorphin 1 and 2 also caused a statistically significant reduction in class I responses, 36 and 39%, respectively. These effects were reversed with Naloxone and CTOP. Thermonocifensive behavior in C. elegans is modulated by opioids.

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.

Memory extinction entails the inhibition of the transcription factor NF-kappaB

In contextual memories, an association between a positive or negative reinforcement and the contextual cues where the reinforcement occurs is formed. The re-exposure to the context without reinforcement can lead to memory extinction or reconsolidation, depending on the number of events or duration of a single event of context re-exposure. Extinction involves the temporary waning of the previously acquired conditioned response. The molecular processes underlying extinction and the mechanisms which determine if memory will reconsolidate or extinguish after retrieval are not well characterized, particularly the role of transcription factors and gene expression. Here we studied the participation of a transcription factor, NF-kappaB, in memory extinction. In the crab context-signal memory, the activation of NF-kappaB plays a critical role in consolidation and reconsolidation, memory processes that are well characterized in this model. The administration of a NF-kappaB inhibitor, sulfasalazine prior to extinction session impeded spontaneous recovery. Moreover, reinstatement experiments showed that the original memory was not affected and that NF-kappaB inhibition by sulfasalazine impaired spontaneous recovery strengthening the ongoing memory extinction process. Interestingly, in animals with fully consolidated memory, a brief re-exposure to the training context induced neuronal NF-kappaB activation and reconsolidation, while prolonged re-exposure induced NF-kappaB inhibition and memory extinction. These data constitutes a novel insight into the molecular mechanisms involved in the switch between memory reconsolidation and extinction. Moreover, we propose the inhibition of NF-kappaB as the engaged mechanism underlying extinction, supporting a novel approach for the pharmacological enhancement of this memory process. The accurate description of the molecular mechanisms that support memory extinction is potentially useful for developing new strategies and drug candidates for therapeutic treatments of the maladaptive memory disorders such as post-traumatic stress, phobias, and drug addiction.

Long-term memory consolidation depends on proteasome activity in the crab Chasmagnathus

Long-term memory formation depends on protein and mRNA synthesis that subserves synaptic reorganization. The removal of pre-existing inhibitory proteins by the ubiquitin-proteasome system (UPS) is proposed as a crucial step to support these modifications. The activation of the constitutive transcription factor nuclear factor kappaB (NF-kappaB) depends on the degradation of the inhibitor of NF-kappaB (IkappaB) by the UPS. Here we study the effect of a UPS inhibitor, MG132, on long-term memory consolidation and NF-kappaB activation in the learning paradigm of the crab Chasmagnathus, a model in which this transcription factor plays a key role. Here we found that administration of MG132 interferes with long-term memory but not with short-term memory, and no facilitatory effects were found. Then we studied the effect of the UPS inhibitor on NF-kappaB pathway, finding that MG132 blocks the activation of NF-kappaB induced by training. These results suggest that the UPS is necessary for long-term memory consolidation, allowing for the activation of NF-kappaB as one of the target molecular pathways.

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.

Activation of the transcription factor NF-kappaB by retrieval is required for long-term memory reconsolidation

Several studies support that stored memories undergo a new period of consolidation after retrieval. It is not known whether this process, termed reconsolidation, requires the same transcriptional mechanisms involved in consolidation. Increasing evidence supports the participation of the transcription factor NF-kappaB in memory. This was initially demonstrated in the crab Chasmagnathus model of associative contextual memory, in which re-exposure to the training context induces a well characterized reconsolidation process. Here we studied the role of NF-kappaB in reconsolidation. NF-kappaB was specifically activated in trained animals re-exposed to the training context but not to a different context. NF-kappaB was not activated when animals were re-exposed to the context after a weak training protocol insufficient to induce long-term memory. A specific inhibitor of the NF-kappaB pathway, sulfasalazine, impaired reconsolidation when administered 20 min before re-exposure to the training context but was not effective when a different context was used. These findings indicate for the first time that NF-kappaB is activated specifically by retrieval and that this activation is required for memory reconsolidation, supporting the view that this molecular mechanism is required in both consolidation and reconsolidation.

Neuronal fibrillogenesis: amyloid fibrils from primary neuronal cultures impair long-term memory in the crab Chasmagnathus

Amyloid beta protein (Abeta) fibrillogenesis is considered one of the crucial steps of Alzheimer's disease (AD) pathogenesis. The effect of endogenous neuronal amyloid fibrils on memory processes is unknown. To investigate this issue, we first characterised the Abeta fibrillar aggregates secreted by cerebellar granule cells and then we evaluated the effect of neuronal fibrils on an invertebrate model of memory. An increase of fibril formation, assessed by Thioflavin T (ThT) fluorescence, was observed in the conditioned medium of apoptotic neurons during 48 h of the apoptotic process. Moreover, the depolarisation-stimulated secretion of cerebellar granule cells contains monomers of endogenous Abeta, which undergo cell-free fibrillogenesis over several days of incubation. The pattern of single endogenous fibrils, examined by electron microscopy, was similar to that of synthetic Abeta while a tighter and more complex interfibrillar organization was observed in endogenous fibrils. The biological effect of neuronal fibrils was studied in a long-term memory (LTM) paradigm, namely the context-signal learning of the crab Chasmagnathus. Pre-training injection of neuronal fibril extract (protein concentration, 1 microg/ml) induced amnesia in a dose-dependent manner. On the contrary, no effect on retention was observed with the administration of two orders higher doses (100 microg/ml) of synthetic Abeta1-40. These results indicate that only naturally secreted fibrils, but not synthetic Abeta, clearly interfere with memory process.

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.

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.

Participation of Rel/NF-kappaB transcription factors in long-term memory in the crab Chasmagnathus

The induction of gene expression has been correlated with long-lasting neuronal plasticity and long-term memory (LTM) formation. The fast activation of constitutive transcription factors by signaling mechanisms is thought to be the link between synaptic events and gene expression. However, only one constitutive transcription factor, CREB, has been shown to play a key role in several memory paradigms, both in vertebrates and invertebrates. Here, we report evidences for Rel/NFkappa-B constitutive transcription factors participation in memory. Using the LTM paradigm in the crab Chasmagnathus, an enhancement of NFkappa-B DNA-binding activity was found after spaced training, which induces LTM, but not after massed training which yields an intermediate-term memory (ITM). Such finding is correlated with the requirement of protein synthesis for LTM consolidation but not for ITM. Furthermore, NFkappa-B activation was observed after 15 or 30 training trials, which are sufficient to induce LTM, but not after 5 or 10 trials, a number of trials insufficient to induce LTM. The kinetics of activation was studied and two waves of DNA-binding activity were found, similar to the time course described in other systems. NFkappa-B activation after training was also found in synaptosomal extracts. The latter result supports the hypothesis of a novel synapse-to-nucleus signaling system, in which the transcription factor is locally activated by synaptic events and then transported to the nucleus.

Kappa-B like DNA-binding activity is enhanced after spaced training that induces long-term memory in the crab Chasmagnathus

Regulation of gene expression has been involved in long-term memory consolidation. Present results support the role of Rel/ NFkappa-B like activation in this process. In the crab Chasmagnathus, the spaced presentation of 15 or more danger stimuli induces long-term habituation (LTH), while no LTH is observed after a massed training of 600 trials. When a group trained with 30 spaced trials was compared with a passive control group and massed trained groups, a higher level of specific Rel/kappa-B like DNA-binding activity was found in brain nuclear extracts. These results strongly suggest that the enhancement of Rel/kappa-B like DNA-binding activity in the brain is specifically related to LTH formation.

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.

Effects of activation and inhibition of cAMP-dependent protein kinase on long-term habituation in the crab Chasmagnathus

On sudden presentation of a danger stimulus, the crab Chasmagnathus elicits an escape response that habituates promptly and for a long period. We have previously reported that administration of a cAMP-permeable analog (CPT-cAMP) along with a phosphodiesterase inhibitor (IBMX) improves long-term habituation (LTH). In present experiments we studied the effect of systemic administration of the protein kinase A (PKA) activator Sp-5,6-DCl-cBIMPS and that of the PKA inhibitor Rp-8-Cl-cAMPS on LTH tested 24 h after a weak training protocol (5 trials of danger stimulus presentation) or a strong training protocol (15-30 trials), respectively. A 50 microliters pre-training injection of 75 microM Sp-5,6-DCl-cBIMPS, and to a lesser degree of 25 microM, improved retention of the habituated response but not affect short-term habituation (STH). Like pre-training injection, post-training administration of Sp-5,6-DCl-cBIMPS proved to exert a facilitatory action on retention though with 75 microM dose only. Conversely, both pre- and post-training injection of 25 microM Rp-8-Cl-cAMPS impaired LTH without affecting STH. Thus, the PKA activator Sp-5,6-DCl-cBIMPS enables a weak training to produce LTH while the PKA inhibitor Rp-8-Cl-cAMPS impairs LTH when a strong training is given. Activation of crab PKA by Sp-5,6-DCl-cBIMPS and its inhibition by Rp-8-Cl-cAMPS were assessed using an in vitro PKA activity assay. These results provide independent evidences supporting the view that PKA plays a key role in long-term memory storage in this learning paradigm.

Serotonin-induced short- and long-term sensitization in the crab Chasmagnathus

On sudden presentation of a passing shadow (a visual danger stimulus), the crab Chasmagnathus granulatus responds with a running reaction that we term an escape response (ER). A single administration of 5-HT in a dose range between 0.005 and 0.1 microgram/g enhances the ER at 30 min, and a dose of 10-15 micrograms/g has a similar effect for at least 24 h. The classical 5-HT antagonist cyproheptadine (CYP) given within a dose range 0.01-2.0 micrograms/g has no effect on ER at 30 min, but 0.5 microgram/g blocks the 5-HT-induced short-term sensitization. An enhancing effect of CYP on the ER is shown at 24 h with 1.0-2.0 micrograms/g doses. An explanation of this results in terms of a model similar to that proposed for Aplysia, that is two sensitizing processes linked in parallel and mediated by two different types of 5-HT receptors is discussed. Results indicate that the sensitizing effect of 5-HT is confined to high responders, thus suggesting that crabs of a same population have different degrees of sensitivity to the drug according to their different degrees of reactivity to the visual danger stimulus.

Acute administration of a permeant analog of cAMP and a phosphodiesterase inhibitor improve long-term habituation in the crab Chasmagnathus

A shadow passing overhead acts as a danger stimulus and elicits an escape response in the crab Chasmagnathus that habituates promptly and for a long period. Robust retention is shown at 24 h after 15 trials of shadow presentation or at 120 h after 30 trials, but no significant retention is disclosed at 24 h after 5 trials or at 72 h after 15. A cocktail of the cAMP membrane permeable analog 8-(4-chlorophenylthio)-cAMP (CPT-cAMP), plus the phosphodiesterase inhibitor isobutyl methylxanthine (IBMX), was given by systemic administration. Pretraining injection of the cocktail (25 or 50 microM, 15 min before a 5-trial session) failed to affect short-term habituation, but induced significant retention when tested at 24 h. This facilitatory effect was not shown when a lower dose (5 microM) was used. A post-training injection of 25 microM, immediately after a 5- or 15-trial session, induced retention when tested at 24 or 72 h, respectively. Thus, the administration of CPT-cAMP + IBMX during acquisition of a habituated response or immediately after, improves long-term habituation, a result supporting the view that an increase in the cAMP level is one of the steps in long-term memory consolidation.

Training-to-testing intervals different from 24 h impair habituation in the crab Chasmagnathus

A shadow moving overhead elicits an escape response in the crab Chasmagnathus that habituates promptly and for a long period. Experiments were done to test the effects of time of day (light-phase vs. dark-phase) on the acquisition and retention of the habituated response. The short-term habituation produced by the repetitive presentation of the stimulus does not differ between the two phases of the day though their reactivity during training seems to be higher during the dark phase than during the light, in agreement with the peak of circadian locomotor activity. The magnitude of the long-term habituated response, tested 24 or 72 h after training, does not appear to depend either on the time of day of training or on that of testing, but the retention is impaired when testing is conducted at a time of day that differs from that of the original training. Thus, results indicate a) that habituation of a response to a stimulus presented during the dark phase is not generalized to the same stimulus presented during the light phase, and vice versa; and b) that during training not only information regarding the target stimulus is stored but also information about the phase of the day.

Cycloheximide inhibits context memory and long-term habituation in the crab Chasmagnathus

A shadow moving over head elicits an escape response in the crab Chasmagnathus that habituates promptly and for a long period. The effect of the protein synthesis inhibitor cycloheximide (CY) on this long-term memory was analyzed. Two hours after injection, 10 micrograms CY inhibited [14C]-amino amino acid incorporation into cerebral plus thoracic ganglia by 88% and 20 micrograms by 92%, but no inhibition was found at 24 h. A single injection of 10-20 micrograms CY given 30 min before training, failed to affect the short-term habituation. Similar doses impaired both context memory (CM) and long-term habituation (LTH) when tested at 72 and 120 h but only CM at 24 h. Such a disparity was explained by an unspecific depressing effect upon the response, attributed to an interaction between CY and training. The hypothesis was confirmed, because CY injected immediately after training disclosed amnestic effect at 24 h on both CM and LTH. A similar effect was proven when animals were injected at 2 h but not at 6 h after training. Results from experiments with pretraining and pretesting injections put aside a state-dependence or retrieval deficit effects of the drug. Taken together, findings of this article argue strongly for de novo protein synthesis as a mechanism of LTH and for the close relation between CM and LTH.

Acute administration of angiotensin II improves long-term habituation in the crab Chasmagnathus

A shadow moving overhead acts as a danger stimulus and elicits an escape response in the crab Chasmagnathus granulatus that habituates after 15 trials and for a long period. A shorter training of ten trials fails to induce long-term habituation; however, a good retention of the habituated response is manifest after a 24-h interval when angiotensin II (AII) (10(-6) M, 3 ng/g) is injected post-training. By contrast, no amnestic effect of AII was found even though high doses were administered. The facilitatory effect of AII is suppressed by saralasin (10(-7) M, 0.3 ng/g), a specific angiotensin II receptor antagonist. Results are considered as suggesting that angiotensin on memory processes might have emerged early in evolution.

Ethanol affects context memory and long-term habituation in the crab Chasmagnathus

A shadow moving overhead acts as a danger stimulus and elicits an escape response in the crab Chasmagnathus granulatus that habituates promptly and for a long period. The effect of acute ethanol treatment on this long-term memory was analyzed. A single injection of 0.01, 0.05, or 0.1 micrograms ethanol (ET)/g given 30 min before iterated presentation of a visual danger stimulus failed to affect short-term habituation. Posttraining ethanol (0.01 to 0.1 microgram/g) produces a dose-dependent impairment of long-term habituation, but pretraining ethanol had no amnestic effect. However, a retention deficit confined to context memory was disclosed with both pre- and posttraining ethanol. Results from experiments with double injection (posttraining and pretesting injections) account for the retention impairment in terms of true amnesia (failure to acquire memory) but not due to state-dependence or retrieval deficit. The nonamnestic effect of pretraining ethanol upon long-term habituation is explained by a nonespecific depressing effect caused by interaction between iterative presentation of the danger stimulus and drug-induced internal state during training.

Multitrial inhibitory avoidance learning in the crab Chasmagnathus

Chasmagnathus crabs placed in the dark compartment (DC) of a double-chamber device and given electrical shocks whenever they entered or remained in the light compartment (LC), showed an LC-avoidance behavior when tested 24 h after a training session of three 30-min periods with 60-min intervals. The avoidance behavior depended neither on the shock number nor on the distribution at training but only on exposure to the LC-shock contingency, thus suggesting that crabs learn to associate the LC with an aversive situation. The learning outcome disclosed a higher degree of refraining from entering the LC rather than a faster escaping to the DC. Distributed practice proved more effective on crab avoidance learning than massive practice. Retention of the learned behavior occurred after a 24-h rest interval in an environment different from that of the training apparatus. Experimental devices previously used in avoidance learning studies with crabs were improved here by automating both the computation of latency values and the event recording.

Conditioned facilitatory modulation of the response to an aversive stimulus in the crab Chasmagnathus

A mild electrical shock delivered to the walking legs of the crab Chasmagnathus granulatus through a fine layer of sea water, induces a running response that declines after repeated stimulation. Herein, a first series of experiments was aimed at conditioning the response to a light or a dark pulse, but no detectable CR was disclosed during training nor in a later test with the CS alone, even though several stimulus parameters were assayed. In a second series, crabs were repeatedly exposed during training to a light pulse (CS) immediately followed by shock (UCS), and after a 6-h rest interval, tested with either CS-UCS or UCS. The CS-UCS presentation at testing elicited a higher recovery (potentiated recovery, circa 70%) than that evoked by the UCS alone (unconditioned recovery, circa 35%) (Experiment 3). An experiment including the explicitly unpaired control procedure (Experiment 4), confirmed that the enhanced response (CR) to the shock (UCS) (measured as potentiated recovery) is conditioned to an illumination signal (CS). An alternative explanation of the potentiated recovery in terms of retardation of habituation proved hardly tenable (Experiment 5). In Experiment 6, crabs were exposed to a variety of light pulse shock and dark pulse shock temporal relationships, and results were found consistent with a delay of reinforcement gradient when light termination was considered as CS. Thus a decrease in luminance is able to become associated with shock and to condition an enhanced response regardless of light termination being the offset of a light pulse or the onset of a dark pulse.(ABSTRACT TRUNCATED AT 250 WORDS)

Habituation and sensitization to an electrical shock in the crab Chasmagnathus. Effect of background illumination

The escape response of the crab Chasmagnathus granulatus, elicited by an electrical leg-shock, wanes as a consequence of repeated stimulation, and the decrease persists after a 24-h rest interval. Results concerning stimulus specificity in within- and between-sessions habituation strongly indicate that neither motor fatigue nor sensory adaptation nor damage can account for the response waning, which thus meets the major criteria of habituation. A comparison between the escape response habituation to leg-shock and that to a shadow passing overhead discloses clearcut differences. The shock curve shows an initial hump positively related to stimulus intensity, suggesting that a shock-induced sensitization along with habituation subserves the response curve. The curve asymptote is remarkably high and, unlike the initial waning portion, not greatly affected by treatment changes, hinting that the response may be the combination of an escape response and a basal one (an undirected burst of activity). The waning portion is higher when shocks are given in dark than when given in light, but this effect is not considered as an extra eliciting-stimulus-induced sensitization, since the reactivity enhancement depends necessarily on the darkness-shock concomitance, regardless of prior exposure to darkness. Accordingly, the hypothesis that crab habituates not only to a shock but to a complex stimulus (shock + background illumination) is put forward and discussed.

Morphine and GABA: effects on perception, escape response and long-term habituation to a danger stimulus in the crab Chasmagnathus

Prior results (37) showed that morphine pretreatment reduces the escape response of the crab Chasmagnathus to a shadow passing overhead and prevents the acquisition of a long-term habituation. These results were explained by a reduction in the danger signalled by the stimulus, and to test this hypothesis methods other than morphine injection were used herein to abolish response during training. GABA pretreatment induced a dose-dependent reduction in responsiveness to the danger stimulus, and instances of autotomy were shown with doses larger than 12 micrograms/g. A response was rarely displayed with a 9 micrograms GABA/g dose given 5 min before training, but long-term memory was acquired. In one experiment, both morphine and GABA pretreatment produced similar mild response inhibition during training, but morphine, not GABA impaired long-term habituation. Morphine administered immediately after training had no amnesic effect. These results support the hypothesis that morphine effects may be explained by transient disruption between the stimulus and its danger meaning, ruling out alternative explanations such as response inhibition or amnesia due to either storage or retrieval failure.

Central effect of morphine pretreatment on short- and long-term habituation to a danger stimulus in the crab Chasmagnathus

Morphine is believed to inhibit the crab's escape response to a danger stimulus due to central drug action. To test alternative explanations of such an effect in terms of afferent and/or efferent impairment, experiments were conducted using the crab's optokinetic response as indicator. Doses of morphine with maximal detrimental effect on the escape response (75-100 micrograms/g) showed no effect on the optokinetic response, both by measuring the crab's eyestalk displacement and by recording its body rotation, supporting the hypothesis of a morphine central action on the danger-induced escape response. As regards the effect on habituation, a 75 micrograms morphine/g injection administered 30 min before the first trial produced a parallel shift of the short-term (within-session) habituation curve, suggesting a modulatory central drug action that would mimic a putative endogenous opioid action. A 100 morphine micrograms/g dose injected 30 min before training sharply reduced reactivity during training and impaired the acquisition of long-term (between-session) habituation. It may be speculated that the decrease in the danger meaning of the stimulus due to morphine explains both effects in terms of a stimulation impairment during training.

Long-term habituation to a danger stimulus in the crab Chasmagnathus granulatus

A shadow moving overhead acts as a danger stimulus and elicits an escape response in the crab Chasmagnathus granulatus, that habituates after 15 stimulus presentations at 3-min intervals. Here, the habituation was induced by 3 diverse shadow stimuli and the habituated response persisted after 24 hr in every case. This long-lasting effect failed to appear if the testing stimulus did not keep the same spatial specificity as that of the training one. A partial stimulus generalization was detected, when training and testing stimuli differed in size. The adaptive value of a high danger stimulus specificity in long-term habituation is discussed. Extinction of the long-term habituated response failed to occur although crabs were exposed to the context in the absence of danger stimulation.

Danger stimulus-induced analgesia in the crab Chasmagnathus granulatus

When a danger stimulus (a passing shadow) is presented to the crab Chasmagnathus granulatus, an escape response is elicited that habituates after repeated stimulation. Results from previous work suggest that this habituation might be mediated by endogenous opiates and, thus, that after an habituation session an analgesic effect on the response to a painful stimulus should be found. The purpose here was to test this contention. Each crab was lodged in an actometer where two 2-trial sessions with electrical shock stimulation were given (SS1, SS2), the intersession interval being 60 or 105 min, and the response activity recorded. In some experiments, repeated shadow stimulation was inserted during the interval, and the response activity also recorded. In Expt. 1, shadow sessions of 15 and 30 trials were tested and a significant decrease of the reactivity to the shock was only shown with 30 trials. In Expt. 2, crabs were immediately injected after SS1 with distilled water (d.w.) or naloxone (NX) (3.2 micrograms/g), and a shadow session of 30 trials was given to half of the d.w. animals and to half of the NX ones. NX injected crabs showed an increased responsiveness to the danger stimulus during the shadow session but no inhibitory effect on pain reactivity to the subsequent painful stimulus. Issues concerning dependence of the analgesia on danger stimulus iteration and on stimulus controllability, as well as the opioid nature of the analgesia, are discussed.