Stressful stimuli modulate memory formation in Lymnaea stagnalis

The genome of the simultaneously hermaphroditic snail Lymnaea stagnalis reveals an evolutionary expansion of FMRFamide-like receptors

The great pond snail Lymnaea stagnalis has served as a model organism for over a century in diverse disciplines such as neurophysiology, evolution, ecotoxicology and developmental biology. To support both established uses and newly emerging research interests we have performed whole genome sequencing (avg.176 × depth), assembly and annotation of a single individual derived from an inbred line. These efforts resulted in a final assembly of 943 Mb (L50 = 257; N50 = 957,215) with a total of 22,499 predicted gene models. The mitogenome was found to be 13,834 bp long and similarly organized as in other lymnaeid species, with minor differences in location of tRNA genes. As a first step towards understanding the hermaphroditic reproductive biology of L. stagnalis, we identified molecular receptors, specifically nuclear receptors (including newly discovered 2xDNA binding domain-NRs), G protein-coupled receptors, and receptor tyrosine kinases, that may be involved in the cellular specification and maintenance of simultaneously active male and female reproductive systems. A phylogenetic analysis of one particular family of GPCRs (Rhodopsin neuropeptide FMRFamide-receptor-like genes) shows a remarkable expansion that coincides with the occurrence of simultaneous hermaphroditism in the Euthyneura gastropods. As some GPCRs and NRs also showed qualitative differences in expression in female (albumen gland) and male (prostate gland) organs, it is possible that separate regulation of male and female reproductive processes may in part have been enabled by an increased abundance of receptors in the transition from a separate-sexed state to a hermaphroditic condition. These findings will support efforts to pair receptors with their activating ligands, and more generally stimulate deeper insight into the mechanisms that underlie the modes of action of compounds involved in neuroendocrine regulation of reproduction, induced toxicity, and development in L. stagnalis, and molluscs in general.

Better together: isolation impedes memory formation for configural learning in Lymnaea stagnalis

Social interactions play an important role in learning and memory. There is great variability in the literature regarding the effects of social isolation on cognition. Here, we investigated how memory formation was affected when Lymnaea stagnalis, our model system, were socially isolated at three different time periods: before, during or after the configural learning training procedure. Each group of snails underwent configural learning where we recorded and compared their feeding behaviour before and after the pairing of an appetitive food stimulus with predator kairomones (i.e. the training procedure). We found that isolating snails before the training procedure had no effect on their learning and memory. However, when snails were isolated either during the training procedure or immediately after the training procedure, they no longer formed memory. These data provide further insight into how isolation impacts cognitive functioning in the context of higher-order learning.

A flavonoid, quercetin, is capable of enhancing long-term memory formation if encountered at different times in the learning, memory formation, and memory recall continuum

A major extrinsic factor influencing memory and neuro-cognitive performances across taxa is diet. Studies from vertebrates have shown the effects of a flavonoid rich diet on cognitive performance, but the mechanism underlying this action is still poorly understood. A common and abundant flavonoid present in numerous food substances is quercetin (Q). The present study provides the first support for Q-modulated enhancement of cognitive function in an invertebrate model, the pond snail Lymnaea stagnalis, after an operant conditioning procedure. We found that when snails were exposed to Q 3 h before or after a single 0.5 h training session, which typically results in memory lasting ~ 3 h, they formed a long-term memory (LTM) lasting for at least 24 h. Additionally, we assessed the effects of the combined presentation of a single reinforcing stimulus (at 24 h post-training or 24 h before training) and Q-exposure on both LTM formation and reconsolidation. That is, when applied within 3 h of critical periods of memory, Q regulates four different phases: (1) acquisition (i.e., a learning event), (2) consolidation processes after acquisition, (3) memory recall, and (4) memory reconsolidation. In all these phases Q-exposure enhanced LTM persistence.

A thermal stressor, propranolol and long-term memory formation in freshly collected Lymnaea

A heat stressor (1 h at 30°C) in Lymnaea stagnalis before operant conditioning training of aerial respiration is sufficient to enhance long-term memory (LTM) formation in 'average' cognitive ability, laboratory-reared, inbred snails. However, in freshly collected outbred snails, the same heat stressor blocks LTM formation in 'smart' cognitive phenotype but not in average cognitive phenotype strains. Here, we hypothesize that (1) preventing the stress associated with the heat stressor before training allows LTM to form in the smart phenotype strains; and (2) alleviating the stress before a memory recall session allows a formed LTM to be recalled in the smart phenotype strains. We found that an injection of propranolol, which mitigates the stressor, before snails experience the heat stressor enabled two strains of the smart phenotype snails to form LTM, consistent with our first hypothesis. However, the injection of propranolol before a memory test session did not alleviate a memory recall block in the smart phenotype snails. Thus, our second hypothesis was not supported. Therefore, smart cognitive phenotype snails encountering a heat stressor have an inability to form LTM, but this inability can be overcome by the pre-injection of propranolol.

Stress before training alters memory retrieval of a non-declarative memory in Lymnaea

Stress alters both memory formation and its retrieval. Here, we show that a combination of stressors before an associative learning event alters memory retrieval of a non-declarative memory in an invertebrate model system. Previously, two combinations of stressors were purported to prevent long-term memory (LTM) formation in 'smart' Lymnaea and this inability to form LTM was considered to be a cost of being smart. Here, we show that is not the case. The specific combinations of stressors used here cause emotional memory formation. Previously, it was shown that propranolol, a synthetic beta-blocker, altered emotional memory in Lymnaea. We show here that when propranolol but not saline is injected into smart snails before they perceive the combination of stressors, these snails form LTM. We then show that the injection of propranolol but not saline before a memory activation session allowed the memory to be recalled. That is, LTM formed but was not retrievable unless propranolol was injected pre-retrieval. Thus, the smart snails formed LTM in the face of the stressors but could not retrieve it.

Configural learning: a higher form of learning in Lymnaea

Events typically occur in a specific context and the ability to assign importance to this occurrence plays a significant role in memory formation and recall. When the scent of a crayfish predator (CE) is encountered in Lymnaea strains known to be predator-experienced (e.g. the W-strain), enhancement of memory formation and depression of feeding occurs, which are part of a suite of anti-predator behaviours. We hypothesized that Lymnaea possess a form of higher-order conditioning, namely configural learning. We tested this by simultaneously exposing W-strain Lymnaea to a carrot food-odour (CO) and predator scent (CE). Two hours later we operantly conditioned these snails with a single 0.5h training session in CO to determine whether training in CO results in long-term memory (LTM). In W-strain snails two 0.5h training sessions are required to cause LTM formation. A series of control experiments followed and demonstrated that only the CO+CE snails trained in CO had acquired enhanced memory forming ability. Additionally, following CE+CO pairing, CO no longer elicited an increased feeding response. Hence, snails have the ability to undergo configural learning. Following configural learning, CO becomes risk-signaling and evokes behavioural responses phenotypically similar to those elicited by exposure to CE.

Green tea and cocoa enhance cognition in Lymnaea

A flavonoid, (-)-epicatechi (Epi), enhances long-term memory (LTM) formation in Lymnaea and reverses memory obstruction caused by stress. Many foods contain substantial amounts of Epi, (e.g. green tea and cocoa). In humans eating such foods may directly or indirectly enhance cognition. We directly test whether operant conditioning training Lymnaea in these natural foods result in the same effects as training snails in pure Epi. We found that exposure to products containing high concentrations of Epi (e.g. green tea and cocoa) during training enhanced memory formation and could even reverse a learning and memory deficit brought about by stress. Epi can be photo-inactivated by exposure to ultraviolet light. We found that following photo-inactivation of Epi, memory enhancement did not occur. Photo-inactivation of foods containing Epi (e,g. green tea) blocked their ability to enhance LTM. Our data are thus consistent with the hypothesis that dietary sources of Epi can have positive benefits on cognitive ability and be able to reverse memory aversive states.

Place avoidance learning and memory in a jumping spider

Using a conditioned passive place avoidance paradigm, we investigated the relative importance of three experimental parameters on learning and memory in a salticid, Servaea incana. Spiders encountered an aversive electric shock stimulus paired with one side of a two-sided arena. Our three parameters were the ecological relevance of the visual stimulus, the time interval between trials and the time interval before test. We paired electric shock with either a black or white visual stimulus, as prior studies in our laboratory have demonstrated that S. incana prefer dark 'safe' regions to light ones. We additionally evaluated the influence of two temporal features (time interval between trials and time interval before test) on learning and memory. Spiders exposed to the shock stimulus learned to associate shock with the visual background cue, but the extent to which they did so was dependent on which visual stimulus was present and the time interval between trials. Spiders trained with a long interval between trials (24 h) maintained performance throughout training, whereas spiders trained with a short interval (10 min) maintained performance only when the safe side was black. When the safe side was white, performance worsened steadily over time. There was no difference between spiders tested after a short (10 min) or long (24 h) interval before test. These results suggest that the ecological relevance of the stimuli used and the duration of the interval between trials can influence learning and memory in jumping spiders.

Environmentally relevant stressors alter memory formation in the pond snail Lymnaea

Stress alters adaptive behaviours such as learning and memory. Stressors can either enhance or diminish learning, memory formation and/or memory recall. We focus attention here on how environmentally relevant stressors alter learning, memory and forgetting in the pond snail, Lymnaea stagnalis. Operant conditioning of aerial respiration causes associative learning that may lead to long-term memory (LTM) formation. However, individual ecologically relevant stressors, combinations of stressors, and bio-active substances can alter whether or not learning occurs or memory forms. While the behavioural memory phenotype may be similar as a result of exposure to different stressors, how each stressor alters memory formation may occur differently. In addition, when a combination of stressors are presented it is difficult to predict ahead of time what the outcome will be regarding memory formation. Thus, how combinations of stressors act is an emergent property of how the snail perceives the stressors.

Control of breathing in invertebrate model systems

The invertebrates have adopted a myriad of breathing strategies to facilitate the extraction of adequate quantities of oxygen from their surrounding environments. Their respiratory structures can take a wide variety of forms, including integumentary surfaces, lungs, gills, tracheal systems, and even parallel combinations of these same gas exchange structures. Like their vertebrate counterparts, the invertebrates have evolved elaborate control strategies to regulate their breathing activity. Our goal in this article is to present the reader with a description of what is known regarding the control of breathing in some of the specific invertebrate species that have been used as model systems to study different mechanistic aspects of the control of breathing. We will examine how several species have been used to study fundamental principles of respiratory rhythm generation, central and peripheral chemosensory modulation of breathing, and plasticity in the control of breathing. We will also present the reader with an overview of some of the behavioral and neuronal adaptability that has been extensively documented in these animals. By presenting explicit invertebrate species as model organisms, we will illustrate mechanistic principles that form the neuronal foundation of respiratory control, and moreover appear likely to be conserved across not only invertebrates, but vertebrate species as well.

A flavonol present in cocoa [(-)epicatechin] enhances snail memory

Dietary consumption of flavonoids (plant phytochemicals) may improve memory and neuro-cognitive performance, though the mechanism is poorly understood. Previous work has assessed cognitive effects in vertebrates; here we assess the suitability of Lymnaea stagnalis as an invertebrate model to elucidate the effects of flavonoids on cognition. (-)Epicatechin (epi) is a flavonoid present in cocoa, green tea and red wine. We studied its effects on basic snail behaviours (aerial respiration and locomotion), long-term memory (LTM) formation and memory extinction of operantly conditioned aerial respiratory behaviour. We found no significant effect of epi exposure (15 mg l(-1)) on either locomotion or aerial respiration. However, when snails were operantly conditioned in epi for a single 0.5 h training session, which typically results in memory lasting ~3 h, they formed LTM lasting at least 24 h. Snails exposed to epi also showed significantly increased resistance to extinction, consistent with the hypothesis that epi induces a more persistent LTM. Thus training in epi facilitates LTM formation and results in a more persistent and stronger memory. Previous work has indicated that memory-enhancing stressors (predator kairomones and KCl) act via sensory input from the osphradium and are dependent on a serotonergic (5-HT) signalling pathway. Here we found that the effects of epi on LTM were independent of osphradial input and 5-HT, demonstrating that an alternative mechanism of memory enhancement exists in L. stagnalis. Our data are consistent with the notion that dietary sources of epi can improve cognitive abilities, and that L. stagnalis is a suitable model with which to elucidate neuronal mechanisms.

Social snails: the effect of social isolation on cognition is dependent on environmental context

Social isolation is often considered to have negative effects on cognitive function in a wide range of species. Here we assess how environmental context alters the effect of isolation on long-term memory formation (24 h) in the pond snail Lymnaea stagnalis. We operantly trained snails to reduce aerial respiration in hypoxia following exposure to one of three social conditions: (1) maintained and trained in groups; (2) maintained in groups, trained in isolation; or (3) maintained and trained in isolation. In addition, snails also experienced four stress exposure levels: control, exposure to low calcium availability, predator kairomone exposure during training or a combination of low calcium and predator kairomones. Snails isolated during training alone demonstrated no difference in memory formation compared with the snails trained in groups. Maintaining snails in social isolation for 8 days prior to training had a neutral effect on memory in control conditions or in the presence of predator kairomones alone. However, social isolation enhanced long-term memory formation in snails exposed to low calcium conditions, a stress that blocks memory formation in snails maintained in groups. Conversely, when exposed to low calcium and predator kairomones combined, grouped snails normally demonstrate long-term memory, but following maintenance in isolation long-term memory was blocked. Therefore, the effect of social isolation on cognitive function is highly dependent on the environmental context in which it is experienced.

Sensory input from the osphradium modulates the response to memory-enhancing stressors in Lymnaea stagnalis

In the freshwater environment species often rely on chemosensory information to modulate behavior. The pond snail, Lymnaea stagnalis, is a model species used to characterize the causal mechanisms of long-term memory (LTM) formation. Chemical stressors including crayfish kairomones and KCl enhance LTM formation (≥24 h) in Lymnaea; however, how these stressors are sensed and the mechanism by which they affect the electrophysiological properties of neurons necessary for memory formation are poorly understood. Here, we assessed whether the osphradium, a primary chemosensory organ in Lymnaea, modulates LTM enhancement. To test this we severed the osphradial nerve proximal to the osphradium, using sham-operated animals as controls, and assessed the behavioral and electrophysiological response to crayfish kairomones and KCl. We operantly conditioned aerial respiratory behavior in intact, sham and osphradially cut animals, and tested for enhanced memory formation after exposure to the chemical stressors. Sham-operated animals displayed the same memory enhancement as intact animals but snails with a severed osphradial nerve did not show LTM enhancement. Extracellular recordings made from the osphradial nerve demonstrate that these stressors evoked afferent sensory activity. Intracellular recordings from right pedal dorsal 1 (RPeD1), a neuron necessary for LTM formation, demonstrate that its electrophysiological activity is altered by input from the osphradium following exposure to crayfish kairomones or KCl in sham and intact animals but no response is seen in RPeD1 in osphradially cut animals. Therefore, sensory input from the osphradium is necessary for LTM enhancement following exposure to these chemical stressors.

What's hot: the enhancing effects of thermal stress on long-term memory formation in Lymnaea stagnalis

The pond snail, Lymnaea stagnalis, naturally inhabits slow flowing, shallow and stagnant environments in the northern temperate zone. Consequently, it will experience wide temperature fluctuations dependent on prevailing weather conditions. We hypothesize that periods of warming act as a thermal stressor to alter memory formation. Snails were exposed to an acute 1h period of 30°C pond water and we determined how memory formation following operant conditioning of aerial respiration was affected. In snails used here (the Dutch strain), a single 0.5h training session (TS) results in intermediate-term (3h) but not long-term memory (LTM). Applying the thermal stressor during training caused memory enhancement (i.e. LTM lasting 24 h). However, the breathing rate also increased in warm water, which might explain the enhanced memory. Therefore, we applied the thermal stressor (1h at 30°C) up to 4h before or 1h after training. This did not alter baseline breathing rate during the period when snails would experience training. However, the thermal stressor weather experienced prior to or following the single TS, resulted in an enhanced memory that persisted up to 48h (i.e. LTM). We conclude that memory enhancement is due to the stress associated with the thermal stimulus.

The role of serotonin in the enhancement of long-term memory resulting from predator detection in Lymnaea

Serotonergic systems play important roles in modulating stress-induced arousal and vigilance behaviours. The pond snail, Lymnaea, shows multiple defensive vigilance behaviours in response to the stress associated with predator detection. Predator detection elicited by crayfish effluent (CE), increases the time to re-emerge from the shell and enhances the shadow withdrawal response. More importantly, in Lymnaea, CE enhances the ability to form long-term memory (LTM). We investigated the role of the serotonergic system in these anti-predator responses in Lymnaea. Using a serotonin-receptor antagonist, mianserin, we found that two defensive vigilance behaviours (e.g. increasing the time to re-emerge from their shell and shadow response) elicited by CE were not observed when the serotonergic system was disrupted. Also, methysergide, another serotonin antagonist, blocked the enhanced LTM formation after training in CE. Importantly, mianserin did not alter LTM formation in pond water (PW). These data suggest that a serotonergic system is activated only when Lymnaea detect a predator. When snails were trained in CE using a training procedure that in PW produces a 24-h LTM, a more persistent form of LTM (5 days) occurred. This more persistent form of LTM was abolished after mianserin treatment. Increasing 5-HT levels in the snail by the injection of 5-HT was also associated with enhanced LTM formation. Lastly, we tested whether the osphradium is implicated in CE detection and subsequent enhanced formation of LTM. Cutting the osphradial nerve to the CNS resulted in the loss of the ability to form enhanced LTM in CE. Together, these findings support the hypothesis that the serotonergic system plays a key role in modulating the predator-induced stress responses in Lymnaea.

The shadow-induced withdrawal response, dermal photoreceptors, and their input to the higher-order interneuron RPeD11 in the pond snailLymnaea stagnalis

The shadow-induced withdrawal response in Lymnaea stagnalis is mediated by dermal photoreceptors located on the foot, mantle cavity, and skin around the pneumostome area. Here, we determined whether we could obtain a neural correlate of the withdrawal response elicited by a shadow in a higher-order central neuron that mediates withdrawal behavior. We measured the electrophysiological properties of the higher-order interneuron Right Pedal Dorsal 11 (RPeD11), which has a major role in Lymnaea withdrawal behavior. In semi-intact preparations comprising the circumesophageal ganglia, the mantle cavity and the pneumostome, but not the foot and eyes, a light-on stimulus elicited a small short-lasting hyperpolarization and a light-off stimulus elicited a depolarization of RPeD11. We also determined that dermal photoreceptors make a monosynaptic contact with RPeD11. The dermal photoreceptor afferents course to the circumesophageal ganglia via the anal and genital nerves to the visceral ganglion, and/or via the right internal and external parietal nerves to the parietal ganglion. Finally, in addition to responding to photic stimuli, RPeD11 responds to both mechanical and chemical stimuli delivered to the pneumostome.

The participation of NMDA receptors, PKC, and MAPK in the formation of memory following operant conditioning in Lymnaea

Background

Memory is the ability to store, retain, and later retrieve information that has been learned. Intermediate term memory (ITM) that persists for up to 3 h requires new protein synthesis. Long term memory (LTM) that persists for at least 24 h requires: DNA transcription, RNA translation, and the trafficking of newly synthesized proteins. It has been shown in a number of different model systems that NMDA receptors, protein kinase C (PKC) and mitogen activated protein kinase (MAPK) are all involved in the memory formation process.

Results

Here we show that snails trained in control conditions are capable of forming, depending on the training procedure used, either ITM or LTM. However, blockage of NMDA receptors (MK 801), inhibition of PKC (GF109203X hydrochloride) and MAPK activity (UO126) prevent the formation of both ITM and LTM.

Conclusions

The injection of either U0126 or GF109203X, which inhibit MAPK and PKC activity respectively, 1 hour prior to training results in the inhibition of both ITM and LTM formation. We further found that NMDA receptor activity was necessary in order for both ITM and LTM formation.

Increase in excitability of RPeD11 results in memory enhancement of juvenile and adult Lymnaea stagnalis by predator-induced stress

Memory consolidation following learning is a dynamic process. Thus, long-term memory (LTM) formation can be modulated by many factors, including stress. We examined how predator-induced stress enhances LTM formation in the pond snail Lymnaea stagnalis at both the behavioral and electrophysiological levels. Training snails in crayfish effluent (CE; i.e., water from an aquarium containing crayfish) significantly enhanced LTM. That is, while memory persists for only 3h in adult control experiments following a single 0.5-h training session in pond water in which the pneumostome receives a contingent tactile stimulus to the pneumostome; when the snails are trained in CE, the memory persists for at least 24h. In juveniles, the data are more dramatic. Juveniles are unable to form LTM in pond water, but form LTM when trained in CE. Here we examined whether juvenile snails form LTM following a one-trial training procedure (1TT). Following the 1TT procedure (a single-trial aversive operant conditioning training procedure), juveniles do not form LTM, unless trained in CE. Concomitantly, we observe changes in the excitability of RPeD11, a key neuron mediating the whole snail withdrawal response, which may be a neural correlate of enhanced memory formation.

Predator detection enables juvenile Lymnaea to form long-term memory

Learning and memory provide the flexibility an organism requires to respond to changing social and ecological conditions. Juvenile Lymnaea have previously been shown to have a diminished capacity to form long-term memory (LTM) following operant conditioning of aerial respiratory behavior. Juvenile Lymnaea, however, can form LTM following classical conditioning of appetitive behaviors. Here, we demonstrate that laboratory-reared juvenile Lymnaea have the ability to detect the presence of a sympatric predator (i.e. crayfish) and respond to the predator by altering their aerial respiratory behavior. In addition to increasing their total breathing time, predator detection confers on juvenile Lymnaea an enhanced capability to form LTM following operant conditioning of aerial respiratory behavior. That is, these juveniles now have the ability to form long-lasting memory. These data support the hypothesis that biologically relevant levels of stress associated with predator detection induce behavioral phenotypic alterations (i.e. enhanced LTM formation) in juveniles, which may increase their fitness. These data also support the notion that learning and memory formation in conjunction with predator detection is a form of inducible defense.

Differences in LTM-forming capability between geographically different strains of Alberta Lymnaea stagnalis are maintained whether they are trained in the lab or in the wild

We found strain differences in the ability of wild Alberta Lymnaea stagnalis to form long-term memory (LTM) following operant conditioning when L. stagnalis were collected from the wild and trained in the laboratory. Lymnaea stagnalis obtained from the Belly River watershed had an enhanced ability to form LTM compared with those from an isolated pond (referred to as Jackson snails). We therefore asked whether the differences in cognitive ability were an epiphenomenon as a result of training in the laboratory. To answer this question we trained each specific strain (Belly and Jackson) in both the laboratory and the field (i.e. in their home pond and in the pond where the other strain resided - referred to as the visitor pond). We found that within each strain there was no difference in the LTM phenotype whether they were trained in the lab or in either their home or visitor pond. That is, the strain differences in the ability to form LTM were still present. Interestingly, we found no strain differences in the ability to learn or the ability to form intermediate-term memory (ITM).

Contrary effects of octopamine receptor ligands on behavioral and neuronal changes in locomotion of lymnaea

The pond snail Lymnaea stagnalis moves along the sides and bottom of an aquarium, but it can also glide upside down on its back below the water's surface. We have termed these two forms of locomotion "standard locomotion" and "upside-down gliding," respectively. Previous studies showed that standard locomotion is produced by both cilia activity on the foot and peristaltic contraction of the foot muscles, whereas upside-down gliding is mainly caused by cilia activity. The pedal A neurons are thought to receive excitatory octopaminergic input, which ultimately results in increased cilia beating. However, the relationship between locomotory speed and the responses of these neurons to octopamine is not known. We thus examined the effects of both an agonist and an antagonist of octopamine receptors on locomotory speed and the firing rate of the pedal A neurons. We also examined, at the electron and light-microscopic levels, whether structural changes occur in cilia following the application of either an agonist or an antagonist of octopamine receptors to the central nervous system (CNS). We found that the application of an octopamine antagonist to the CNS increased the speed of both forms of locomotion, whereas application of octopamine increased only the firing rate of the pedal A neurons. Microscopic examination of the cilia proved that there were no changes in their morphology after application of octopamine ligands. These data suggest that there is an unidentified octopaminergic neuronal network in the CNS whose activation reduces cilia movement and thus locomotory speed.

Melatonin preserves longevity protein (sirtuin 1) expression in the hippocampus of total sleep-deprived rats

Sleep disorders cause cognitive dysfunction in which impaired neuronal plasticity in the hippocampus may underline the molecular mechanisms of this deficiency. As sirtuin 1 (SIRT1) plays an important role in maintaining metabolic homeostasis and neuronal plasticity, this study is aimed to determine whether melatonin exerts beneficial effects on preserving SIRT1 activation following total sleep deprivation (TSD). TSD was performed by disc on water method for five consecutive days. During this period, animals daily received melatonin at doses of 5, 25, 50 or 100 mg/kg. The cytochrome oxidase (COX) histochemistry, SIRT1 immunohistochemistry together with Morris water maze learning test were performed to examine the metabolic, neurochemical, as well as the behavioral changes in neuronal plasticity, respectively. The results indicate that in normal rats, numerous COX and SIRT1 positive-labeled neurons with strong staining intensities were found in hippocampal pyramidal and granular cell layers. Following TSD, both COX and SIRT1 reactivities were drastically decreased as revealed by reduced staining pattern and labeling frequency. Behavioral data corresponded well with morphological findings in which spatial memory test in water maze was significantly impaired after TSD. However, in rats receiving different doses of melatonin, both COX and SIRT1 expressions were successfully preserved. Considerably better performance on behavioral testing further strengthened the beneficial effects of melatonin. These findings suggest that melatonin may serve as a novel therapeutic strategy directed for preventing the memory deficits resulting from TSD, possibly by effectively preserving the metabolic function and neuronal plasticity engaged in maintaining cognitive activity.

The perception of stress alters adaptive behaviours in Lymnaea stagnalis

Stress can alter adaptive behaviours, and as well either enhance or diminish learning, memory formation and/or memory recall. We show here that two different stressors have the ability to alter such behaviours in our model system, Lymnaea stagnalis. One, a naturally occurring stressor - the scent of a predator (crayfish) - and the other an artificially controlled one - 25 mmol l(-1) KCl - significantly alter adaptive behaviours. Both the KCl stressor and predator detection enhance long-term memory (LTM) formation; additionally predator detection alters vigilance behaviours. The predator-induced changes in behaviour are also accompanied by specific and significant alterations in the electrophysiological properties of RPeD1 - a key neuron in mediating both vigilance behaviours and memory formation. Naive lab-bred snails exposed to crayfish effluent (CE; i.e. the scent of the predator) prior to recording from RPeD1 demonstrated both a significantly reduced spontaneous firing rate and fewer bouts of bursting activity compared with non-exposed snails. Importantly, in the CE experiments we used laboratory-reared snails that have not been exposed to a naturally occurring predator for over 250 generations. These data open a new avenue of research, which may allow a direct investigation from the behavioral to the neuronal level as to how relevant stressful stimuli alter adaptive behaviours, including memory formation and recall.

Comparing memory-forming capabilities between laboratory-reared and wildLymnaea: learning in the wild, a heritable component of snail memory

We set out to determine whether the ability to form long-term memory (LTM)is influenced by laboratory rearing. We investigated the ability of four populations of Lymnaea stagnalis to form LTM following operant conditioning both in the freely behaving animal and at the electrophysiological level in a neuron, RPeD1, which is a necessary site for LTM. We hypothesized that laboratory rearing results in a decreased ability to form LTM because rearing does not occur in an `enriched environment'. Of the four populations examined, two were collected in the wild and two were reared in the laboratory – specifically, (1) wild Dutch snails; (2) their laboratory-reared offspring; (3) wild Southern Alberta snails (Belly); and (4)their laboratory-reared offspring. We found that Belly snails had an enhanced capability of forming LTM compared with Dutch laboratory-reared snails. That is, the Belly snails, which are much darker in colour than laboratory-reared snails (i.e. blonds), were `smarter'. However, when we tested the offspring of Belly snails reared in the laboratory we found that these snails still had the enhanced ability to form LTM, even though they were now just as `blond' as their laboratory-reared Dutch cousins. Finally, we collected wild Dutch snails, which are also dark, and found that their ability to form LTM was not different to that of their laboratory-reared offspring. Thus, our hypothesis was not proved. Rather, we now hypothesize that there are strain differences between the Belly and Dutch snails, irrespective of whether they are reared in the wild or in the laboratory.

Crowding, an environmental stressor, blocks long-term memory formation inLymnaea

Crowding is an environmental stressor. We found that this stressor altered(i.e. prevented) the ability of Lymnaea to form long-term memory(LTM) following operant conditioning of aerial respiratory behaviour. The ability to form LTM was compared between snails that had been crowded (20 snails per 100 ml of pond water) and those maintained in uncrowded conditions(two snails per 100 ml of pond water). Crowding either immediately before or after two different operant conditioning procedures – the traditional training procedure and the memory augmentation procedure – blocked LTM formation. However, if crowding is delayed by more than 1h following training or if crowding stops 1h before training, LTM results. If memory is already formed, crowding does not block memory recall. Pond water from a crowded aquarium or crowding with clean shells from dead snails, or a combination of both, is insufficient to block LTM formation. Finally, crowding does not block intermediate-term memory (ITM) formation. Since ITM is dependent on new protein synthesis whereas LTM is dependent on both new protein synthesis and altered gene activity, we hypothesize that crowding alters the genomic activity in neurons necessary for LTM formation.

Electrophysiological and behavioral evidence demonstrating that predator detection alters adaptive behaviors in the snail Lymnaea

Stress has been shown to both impair and enhance learning, long-term memory (LTM) formation, and/or its recall. The pond snail, Lymnaea stagnalis, both detects and responds to the scent of a crayfish predator with multiple stress-related behavioral responses. Using both behavioral and electrophysiological evidence, this investigation is a first attempt to characterize how an environmentally relevant stressor (scent of a predator) enhances LTM formation in Lymnaea. Using a training procedure that, in "standard" pond water (PW), results in an intermediate-term memory that persists for only 3 h, we found that training snails in "crayfish effluent" (CE) induces a memory that persists for 48 h (i.e., its now an LTM). In addition, if we use a training procedure that in PW produces an LTM that persists for 1 d, we find that snails trained in CE have an LTM that persists for at least 8 d. Furthermore, we describe how a single neuron (RPeD1), which has been shown to be a necessary site for LTM formation, reflects the behavioral changes in its firing properties that persist for the duration of the LTM. Finally, Lymnaea exhibit context-specific memory, that is, when a memory is formed in a specific context (food odorant), it is only recalled in that context. Here, we found that snails trained in CE demonstrate context generalization, that is, memory is recalled in multiple contexts. All data are consistent with the hypothesis that learning in a stressful, yet biologically relevant, environment enhances LTM and prolongs its retention.

One-trial conditioning of aerial respiratory behaviour in Lymnaea stagnalis

Repeated spaced training sessions of contingent tactile stimulation to the pneumostome as it opens are required to cause long-term memory (LTM) formation of aerial respiratory behaviour making if difficult to determine exactly when memory forms. We have devised a single-trial aversive operant conditioning training procedure in Lymnaea to be better able to elucidate the causal mechanisms of LTM formation. Observations of baseline breathing behaviour in hypoxia were first made. Twenty-four hours later the snails were trained using the single trial procedure, by placing them in a small Petri dish containing 4 ml of 25 mM KCl for 30-35s as soon as the first pneumostome opening in hypoxia was attempted. LTM was present if (1) breathing behaviour following training was significantly less than before; and (2) breathing behaviour post-training was significantly less in experimental groups than in yoked control groups. LTM persisted for 24 h but not 48 h. Yoked controls that received an aversive stimulus not contingent with pneumostome opening had no evidence of memory. Cooling directly after, but not at any other time, blocks LTM formation. LTM formation was also prevented by removal of the cell body of the neuron RPeD1 before training.