Feeding behavior of Aplysia: a model system for comparing cellular mechanisms of classical and operant conditioning

Animal models in neuroscience with alternative approaches: Evolutionary, biomedical, and ethical perspectives

Animal models have been a crucial tool in neuroscience research for decades, providing insights into the biomedical and evolutionary mechanisms of the nervous system, disease, and behavior. However, their use has raised concerns on several ethical, clinical, and scientific considerations. The welfare of animals and the 3R principles (replacement, reduction, refinement) are the focus of the ethical concerns, targeting the importance of reducing the stress and suffering of these models. Several laws and guidelines are applied and developed to protect animal rights during experimenting. Concurrently, in the clinic and biomedical fields, discussions on the relevance of animal model findings on human organisms have increased. Latest data suggest that in a considerable amount of time the animal model results are not translatable in humans, costing time and money. Alternative methods, such as in vitro (cell culture, microscopy, organoids, and micro physiological systems) techniques and in silico (computational) modeling, have emerged as potential replacements for animal models, providing more accurate data in a minimized cost. By adopting alternative methods and promoting ethical considerations in research practices, we can achieve the 3R goals while upholding our responsibility to both humans and other animals. Our goal is to present a thorough review of animal models used in neuroscience from the biomedical, evolutionary, and ethical perspectives. The novelty of this research lies in integrating diverse points of views to provide an understanding of the advantages and disadvantages of animal models in neuroscience and in discussing potential alternative methods.

Single-neuron analysis of aging-associated changes in learning reveals impairments in transcriptional plasticity

The molecular mechanisms underlying age-related declines in learning and long-term memory are still not fully understood. To address this gap, our study focused on investigating the transcriptional landscape of a singularly identified motor neuron L7 in Aplysia, which is pivotal in a specific type of nonassociative learning known as sensitization of the siphon-withdraw reflex. Employing total RNAseq analysis on a single isolated L7 motor neuron after short-term or long-term sensitization (LTS) training of Aplysia at 8, 10, and 12 months (representing mature, late mature, and senescent stages), we uncovered aberrant changes in transcriptional plasticity during the aging process. Our findings specifically highlight changes in the expression of messenger RNAs (mRNAs) that encode transcription factors, translation regulators, RNA methylation participants, and contributors to cytoskeletal rearrangements during learning and long noncoding RNAs (lncRNAs). Furthermore, our comparative gene expression analysis identified distinct transcriptional alterations in two other neurons, namely the motor neuron L11 and the giant cholinergic neuron R2, whose roles in LTS are not yet fully elucidated. Taken together, our analyses underscore cell type-specific impairments in the expression of key components related to learning and memory within the transcriptome as organisms age, shedding light on the complex molecular mechanisms driving cognitive decline during aging.

Repeated stimulation of feeding mechanoafferents in Aplysia generates responses consistent with the release of food

How does repeated stimulation of mechanoafferents affect feeding motor neurons? Monosynaptic connections from a mechanoafferent population in the Aplysia buccal ganglia to five motor followers with different functions were examined during repeated stimulus trains. The mechanoafferents produced both fast and slow synaptic outputs, which could be excitatory or inhibitory. In contrast, other Aplysia mechanoafferents produce only fast excitation on their followers. In addition, patterns of synaptic connections were different to the different motor followers. Some followers received both fast excitation and fast inhibition, whereas others received exclusively fast excitation. All followers showed strong decreases in fast postsynaptic potential (PSP) amplitude within a stimulus train. Fast and slow synaptic connections were of net opposite signs in some followers but not in others. For one follower, synaptic contacts were not uniform from all subareas of the mechanoafferent cluster. Differences in properties of the buccal ganglia mechanoafferents and other Aplysia mechanoafferents may arise because the buccal ganglia neurons innervate the interior of the feeding apparatus, rather than an external surface, and connect to motor neurons for muscles with different motor functions. Fast connection patterns suggest that these synapses may be activated when food slips, biasing the musculature to release food. The largest slow inhibitory synaptic PSPs may contribute to a delay in the onset of the next behavior. Additional functions are also possible.

The dopamine receptor antagonist haloperidol disrupts behavioral responses of sea urchins and sea stars

Despite lacking a brain and having an apparent symmetrically pentaradial nervous system, echinoderms are capable of complex, coordinated directional behavioral responses to different sensory stimuli. However, very little is known about the molecular and cellular mechanisms underlying these behaviors. In many animals, dopaminergic systems play key roles in motivating and coordinating behavior, and although the dopamine receptor antagonist haloperidol has been shown to inhibit the righting response of the sea urchin Strongylocentrotus purpuratus, it is not known whether this is specific to this behavior, in this species, or whether dopaminergic systems are needed in general for echinoderm behaviors. We found that haloperidol inhibited multiple different behavioral responses in three different echinoderm species. Haloperidol inhibited the righting response of the sea urchin Lytechinus variegatus and of the sea star Luidia clathrata. It additionally inhibited the lantern reflex of S. purpuratus, the shell covering response of L. variegatus and the immersion response of L. variegatus, but not S. purpuratus or L. clathrata. Our results suggest that dopamine is needed for the neural processing and coordination of multiple different behavioral responses in a variety of different echinoderm species.

Grounding Mental Representations in a Virtual Multi-Level Functional Framework

According to the associative theory of learning, reactive behaviors described by stimulus-response pairs result in the progressive wiring of a plastic brain. In contrast, flexible behaviors are supposedly driven by neurologically grounded mental states that involve computations on informational contents. These theories appear complementary, but are generally opposed to each other. The former is favored by neuro-scientists who explore the low-level biological processes supporting cognition, and the later by cognitive psychologists who look for higher-level structures. This situation can be clarified through an analysis that independently defines abstract neurological and informational functionalities, and then relate them through a virtual interface. This framework is validated through a modeling of the first stage of Piaget's cognitive development theory, whose reported end experiments demonstrate the emergence of mental representations of object displacements. The neural correlates grounding this emergence are given in the isomorphic format of an associative memory. As a child's exploration of the world progresses, his mental models will eventually include representations of space, time and causality. Only then epistemological concepts, such as beliefs, will give rise to higher level mental representations in a possibly richer propositional format. This raises the question of which additional neurological functionalities, if any, would be required in order to include these extensions into a comprehensive grounded model. We relay previously expressed views, which in summary hypothesize that the ability to learn has evolved from associative reflexes and memories, to suggest that the functionality of associative memories could well provide the sufficient means for grounding cognitive capacities.

Memristor Neural Network Circuit Based on Operant Conditioning With Immediacy and Satiety

Most of the operant conditioning only consider the basic theory, but the influencing factors such as immediacy and satiety are ignored. In this paper, a memristor neural network circuit based on operant conditioning with immediacy and satiety is proposed. The designed circuit is mainly composed of voltage control modules, synapse modules and promotion-suppression modules. The various processes of operant conditioning theory are realized. The relationship between positive reward and negative punishment is implemented through promotion-suppression modules and synapse modules. The influence of immediacy on operant conditioning is achieved through voltage control module. The effect of satiety on operant conditioning is discussed using memristive non-volatility. The study of operant conditioning may provide a reference for smarter brain-like nerves.

Sex-specific role of the optic gland in octopus maya: A transcriptomic analysis

The optic glands (OG) of cephalopods are a source of molecules associated with the control of reproductive traits and lifecycle events such as sexual maturation, reproductive behavior, feeding, parental care, and senescence. However, little is known about the role of the optic gland in Octopus maya adults during mating and egg laying. RNA sequencing, de novo transcriptome assembly, ubiquity and differential expression analysis were performed. First, we analyzed the expression patterns of transcripts commonly associated with OG regulatory functions to describe their possible role once the maturation of the gonad is complete. The transcriptomic profiles of the optic gland of both sexes were compared with emphasis on the signaling pathways involved in the dimorphism of reproductive traits. Results suggest that in the OG of males, the reproductive condition (mated or non-mated) did not affect the general expression profile. In contrast, more differentially expressed genes were observed in females. In mated females, the mRNA metabolic process and the response to norepinephrine were enriched, suggesting a high cellular activity in preparation for the laying of the embryos. Whereas in egg-laying females, energetic and metabolic processes were the most represented, including the oxidation-reduction process. Finally, the gene expression patterns in senescence females suggest a physiological response to starvation as well as upregulation of genes involved retrotransposon activity. In conclusion, more substantial fluctuations in gene expression were observed in the optic glands of the fertilized females compared to the males. Such differences might be associated with the regulation of the egg-laying and the onset of senescence.

Specific Plasticity Loci and Their Synergism Mediate Operant Conditioning

Despite numerous studies examining the mechanisms of operant conditioning (OC), the diversity of OC plasticity loci and their synergism have not been examined sufficiently. In the well-characterized feeding neural circuit of Aplysia, in vivo and in vitro appetitive OC increases neuronal excitability and electrical coupling among several neurons leading to an increase in expression of ingestive behavior. Here, we used the in vitro analog of OC to investigate whether OC reduces the excitability of a neuron, B4, whose inhibitory connections decrease expression of ingestive behavior. We found OC decreased the excitability of B4. This change appeared intrinsic to B4 because it could be replicated with an analog of OC in isolated cultures of B4 neurons. In addition to changes in B4 excitability, OC decreased the strength of B4's inhibitory connection to a key decision-making neuron, B51. The OC-induced changes were specific without affecting the excitability of another neuron critical for feeding behavior, B8, or the B4-to-B8 inhibitory connection. A conductance-based circuit model indicated that reducing the B4-to-B51 synapse, or increasing B51 excitability, mediated the OC phenotype more effectively than did decreasing B4 excitability. We combined these modifications to examine whether they could act synergistically. Combinations including B51 synergistically enhanced feeding. Taken together, these results suggest modifications of diverse loci work synergistically to mediate OC and that some neurons are well suited to work synergistically with plasticity in other loci.SIGNIFICANCE STATEMENT The ways in which synergism of diverse plasticity loci mediate the change in motor patterns in operant conditioning (OC) are poorly understood. Here, we found that OC was in part mediated by decreasing the intrinsic excitability of a critical neuron of Aplysia feeding behavior, and specifically reducing the strength of one of its inhibitory connections that targets a key decision-making neuron. A conductance-based computational model indicated that the known plasticity loci showed a surprising level of synergism to mediate the behavioral changes associated with OC. These results highlight the importance of understanding the diversity, specificity and synergy among different types of plasticity that encode memory. Also, because OC in Aplysia is mediated by dopamine (DA), the present study provides insights into specific and synergistic mechanisms of DA-mediated reinforcement of behaviors.

Neuronal population activity dynamics reveal a low-dimensional signature of operant learning in Aplysia

Learning engages a high-dimensional neuronal population space spanning multiple brain regions. However, it remains unknown whether it is possible to identify a low-dimensional signature associated with operant conditioning, a ubiquitous form of learning in which animals learn from the consequences of behavior. Using single-neuron resolution voltage imaging, here we identify two low-dimensional motor modules in the neuronal population underlying Aplysia feeding. Our findings point to a temporal shift in module recruitment as the primary signature of operant learning. Our findings can help guide characterization of learning signatures in systems in which only a smaller fraction of the relevant neuronal population can be monitored.

Costa et al. use single-neuron resolution voltage imaging to identify two low-dimensional motor modules in the neuronal population underlying Aplysia feeding. Their findings point to a temporal shift in module recruitment as the primary signature of operant learning.

An age-related decline in the cholinergic synaptic response may cause the firing pattern in the jaw-closing motor neurons, which resembles the aversive taste response in the feeding behavior of old Aplysia kurodai

Anorexia due to aging is recognized as a syndrome of animal feeding behavior. Age-related functional disorders of the brain often cause behavioral changes. We used Aplysia kurodai to study this neural mechanism, following our previous study on food preference behaviors. The age of each wild animal was defined by a previously described method, and a significant age-related decline in food intake was observed. In this study, we explored the effects of aging on a specific inhibitory synaptic response in jaw-closing (JC) motor neurons produced by cholinergic multiaction (MA) neurons, the size of which determines the delay between MA and JC firings and this delay is reduced during aversive taste responses; in our analyses, we found a significant age-related decline in the synaptic response. Thereafter, we further explored whether such functional decline affects the JC firing pattern during the normal feeding response. During the feeding-like rhythmic responses induced by electrical nerve stimulation, the firing of the JC motor neurons advanced toward that of the MA burst, which typically happens during aversive taste responses. These results suggest that the age-related decline in the cholinergic synaptic response may partly cause the JC firing patterns that resemble the aversive taste response in old animals.

Aplysia Neurons as a Model of Alzheimer's Disease: Shared Genes and Differential Expression

Although Alzheimer’s disease (AD) is the most common form of dementia in the United States, development of therapeutics has proven difficult. Invertebrate alternatives to current mammalian AD models have been successfully employed to study the etiology of the molecular hallmarks of AD. The marine snail Aplysia californica offers a unique and underutilized system in which to study the physiological, behavioral, and molecular impacts of AD. Mapping of the Aplysia proteome to humans and cross-referencing with two databases of genes of interest in AD research identified 898 potential orthologs of interest in Aplysia. Included among these orthologs were alpha, beta and gamma secretases, amyloid-beta, and tau. Comparison of age-associated differential expression in Aplysia sensory neurons with that of late-onset AD in the frontal lobe identified 59 ortholog with concordant differential expression across data sets. The 21 concordantly upregulated genes suggested increased cellular stress and protein dyshomeostasis. The 47 concordantly downregulated genes included important components of diverse neuronal processes, including energy metabolism, mitochondrial homeostasis, synaptic signaling, Ca⁺⁺ regulation, and cellular cargo transport. Compromised functions in these processes are known hallmarks of both human aging and AD, the ramifications of which are suggested to underpin cognitive declines in aging and neurodegenerative disease.

Aversive operant conditioning alters the phototactic orientation of the marbled crayfish

Aversive learning was applied to affect the phototactic behaviour of the marbled crayfish. Animals initially showed negative phototaxis to white light and positive taxis to blue light. Using an aversive learning paradigm, we investigated the plasticity of innate behaviour following operant conditioning. The initial rate of choosing a blue-lit exit was analysed by a dual choice experiment between blue-lit and white-lit exits in pre-test conditions. During training, electrical shocks were applied to the animals when they oriented to the blue-lit exit. Memory tests were given to analyse the orientation rate to the blue-lit exit in trials 1 and 24 h after training and these rates were compared with the pre-test. In general, animals avoided the blue-lit exit in the memory tests. When training was carried out three times, the long-term memory was retained for at least 48 h, although a single bout of training was also enough to form a long-term memory. Cooling animals at 4°C or injection of cycloheximide immediately after training altered the formation of long-term memory, but had no effect on short-term memory formation. Administration of the adenylate cyclase inhibitor SQ22536, the PKA inhibitor H89 or the CREB inhibitor KG-501 immediately after training also blocked the formation of long-term memory, but had no effect on short-term memory formation. Thus, our pharmacological behavioural analyses showed that new protein synthesis was necessary to form long-term memories and that the cAMP/PKA/CREB pathway is the main signal cascade for long-term memory formation in the marbled crayfish.

Dopamine as a Multifunctional Neurotransmitter in Gastropod Molluscs: An Evolutionary Hypothesis

AbstractThe catecholamine 3,4-dihydroxyphenethylamine, or dopamine, acts as a neurotransmitter across a broad phylogenetic spectrum. Functions attributed to dopamine in the mammalian brain include regulation of motor circuits, valuation of sensory stimuli, and mediation of reward or reinforcement signals. Considerable evidence also supports a neurotransmitter role for dopamine in gastropod molluscs, and there is growing appreciation for its potential common functions across phylogeny. This article reviews evidence for dopamine's transmitter role in the nervous systems of gastropods. The functional properties of identified dopaminergic neurons in well-characterized neural circuits suggest a hypothetical incremental sequence by which dopamine accumulated its diverse roles. The successive acquisition of dopamine functions is proposed in the context of gastropod feeding behavior: (1) sensation of potential nutrients, (2) activation of motor circuits, (3) selection of motor patterns from multifunctional circuits, (4) valuation of sensory stimuli with reference to internal state, (5) association of motor programs with their outcomes, and (6) coincidence detection between sensory stimuli and their consequences. At each stage of this sequence, it is proposed that existing functions of dopaminergic neurons favored their recruitment to fulfill additional information processing demands. Common functions of dopamine in other intensively studied groups, ranging from mammals and insects to nematodes, suggest an ancient origin for this progression.

Decreased 14-3-3 expression correlates with age-related regional reductions in CNS dopamine and motor function in the pond snail, Lymnaea

Ageing is associated in many organisms with a reduction in motor movements. We have previously shown that the rate of feeding movements of the pond snail, Lymnaea, decreased with age but the underlying cause is not fully understood. Here, we show that dopamine in the cerebro-buccal complex is an important signalling molecule regulating feeding frequency in Lymnaea and that ageing is associated with a decrease in CNS dopamine. A proteomic screen of young and old CNSs highlighted a group of proteins that regulate stress responses. One of the proteins identified was 14-3-3, which can enhance the synthesis of dopamine. We show that the Lymnaea 14-3-3 family exists as three distinct isoforms. The expression of the 29 kDa isoform (14-3-3Lym3) in the cerebro-buccal complex decreased with age and correlated with feeding rate. Using a 14-3-3 antagonist (R18) we were able to reduce the synthesis of L-DOPA and dopamine in ex vivo cerebro-buccal complexes. Together these data suggest that an age-related reduction in 14-3-3 can decrease CNS dopamine leading to a consequential reduction in feeding rate.

Rapid Adaptation to Changing Mechanical Load by Ordered Recruitment of Identified Motor Neurons

As they interact with their environment and encounter challenges, animals adjust their behavior on a moment-to-moment basis to maintain task fitness. This dynamic process of adaptive motor control occurs in the nervous system, but an understanding of the biomechanics of the body is essential to properly interpret the behavioral outcomes. To study how animals respond to changing task conditions, we used a model system in which the functional roles of identified neurons and the relevant biomechanics are well understood and can be studied in intact behaving animals: feeding in the marine mollusc Aplysia We monitored the motor neuronal output of the feeding circuitry as intact animals fed on uniform food stimuli under unloaded and loaded conditions, and we measured the force of retraction during loaded swallows. We observed a previously undescribed pattern of force generation, which can be explained within the appropriate biomechanical context by the activity of just a few key, identified motor neurons. We show that, when encountering load, animals recruit identified retractor muscle motor neurons for longer and at higher frequency to increase retraction force duration. Our results identify a mode by which animals robustly adjust behavior to their environment, which is experimentally tractable to further mechanistic investigation.

Computational model of the distributed representation of operant reward memory: combinatoric engagement of intrinsic and synaptic plasticity mechanisms

Operant reward learning of feeding behavior in Aplysia increases the frequency and regularity of biting, as well as biases buccal motor patterns (BMPs) toward ingestion-like BMPs (iBMPs). The engram underlying this memory comprises cells that are part of a central pattern generating (CPG) circuit and includes increases in the intrinsic excitability of identified cells B30, B51, B63, and B65, and increases in B63-B30 and B63-B65 electrical synaptic coupling. To examine the ways in which sites of plasticity (individually and in combination) contribute to memory expression, a model of the CPG was developed. The model included conductance-based descriptions of cells CBI-2, B4, B8, B20, B30, B31, B34, B40, B51, B52, B63, B64, and B65, and their synaptic connections. The model generated patterned activity that resembled physiological BMPs, and implementation of the engram reproduced increases in frequency, regularity, and bias. Combined enhancement of B30, B63, and B65 excitabilities increased BMP frequency and regularity, but not bias toward iBMPs. Individually, B30 increased regularity and bias, B51 increased bias, B63 increased frequency, and B65 decreased all three BMP features. Combined synaptic plasticity contributed primarily to regularity, but also to frequency and bias. B63-B30 coupling contributed to regularity and bias, and B63-B65 coupling contributed to all BMP features. Each site of plasticity altered multiple BMP features simultaneously. Moreover, plasticity loci exhibited mutual dependence and synergism. These results indicate that the memory for operant reward learning emerged from the combinatoric engagement of multiple sites of plasticity.

Successful and unsuccessful attempts to swallow in a reduced Aplysia preparation regulate feeding responses and produce memory at different neural sites

Sensory feedback shapes ongoing behavior and may produce learning and memory. Motor responses to edible or inedible food in a reduced Aplysia preparation were examined to test how sensory feedback affects behavior and memory. Feeding patterns were initiated by applying a cholinomimetic onto the cerebral ganglion. Feedback from buccal muscles increased the response variability and response rate. Repeated application of the cholinomimetic caused decreased responses, expressed in part by lengthening protractions. Swallowing strips of "edible" food, which in intact animals induces learning that enhances ingestion, increased the response rate, and shortened the protraction length, reflecting more swallowing. Testing memory by repeating the procedure prevented the decrease in response rate observed with the cholinomimetic alone, and shortened protractions. Training with "inedible" food that in intact animals produces learning expressed by decreased responses caused lengthened protractions. Testing memory by repeating the procedure did not cause decreased responses or lengthened protractions. After training and testing with edible or inedible food, all preparations were exposed to the cholinomimetic alone. Preparations previously trained with edible food displayed memory expressed as decreased protraction length. Preparations previously trained with inedible food showed decreases in many response parameters. Memory for inedible food may arise in part via a postsynaptic decrease in response to acetylcholine released by afferents sensing food. The lack of change in response number, and in the time that responses are maintained during the two training sessions preceding application of the cholinomimetic alone suggests that memory expression may differ from behavioral changes during training.

Monoamines, Insulin and the Roles They Play in Associative Learning in Pond Snails

Molluscan gastropods have long been used for studying the cellular and molecular mechanisms underlying learning and memory. One such gastropod, the pond snail Lymnaea stagnalis, exhibits long-term memory (LTM) following both classical and operant conditioning. Using Lymnaea, we have successfully elucidated cellular mechanisms of learning and memory utilizing an aversive classical conditioning procedure, conditioned taste aversion (CTA). Here, we present the behavioral changes following CTA training and show that the memory score depends on the duration of food deprivation. Then, we describe the relationship between the memory scores and the monoamine contents of the central nervous system (CNS). A comparison of learning capability in two different strains of Lymnaea, as well as the filial 1 (F1) cross from the two strains, presents how the memory scores are correlated in these populations with monoamine contents. Overall, when the memory scores are better, the monoamine contents of the CNS are lower. We also found that as the insulin content of the CNS decreases so does the monoamine contents which are correlated with higher memory scores. The present review deepens the relationship between monoamine and insulin contents with the memory score.

A role for dopamine in the peripheral sensory processing of a gastropod mollusc

Histological evidence points to the presence of dopamine (DA) in the cephalic sensory organs of multiple gastropod molluscs, suggesting a possible sensory role for the neurotransmitter. We investigated the sensory function of DA in the nudipleuran Pleurobranchaea californica, in which the central neural correlates of sensation and foraging behavior have been well characterized. Tyrosine hydroxylase-like immunoreactivity (THli), a signature of the dopamine synthetic pathway, was similar to that found in two other opisthobranchs and two pulmonates previously studied: 1) relatively few (<100) THli neuronal somata were observed in the central ganglia, with those observed found in locations similar to those documented in the other snails but varying in number, and 2) the vast majority of THli somata were located in the peripheral nervous system, were associated with ciliated, putative primary sensory cells, and were highly concentrated in chemotactile sensory organs, giving rise to afferent axons projecting to the central nervous system. We extended these findings by observing that applying a selective D₂/D₃ receptor antagonist to the chemo- and mechanosensory oral veil-tentacle complex of behaving animals significantly delayed feeding behavior in response to an appetitive stimulus. A D₁ blocker had no effect. Recordings of the two major cephalic sensory nerves, the tentacle and large oral veil nerves, in a deganglionated head preparation revealed a decrease of stimulus-evoked activity in the former nerve following application of the same D₂/D₃ antagonist. Broadly, our results implicate DA in sensation and engender speculation regarding the foraging-based decisions the neurotransmitter may serve in the nervous system of Pleurobranchaea and, by extension, other gastropods.

Neural Coding of Appetitive Food Experiences in the Amygdala

Real-life experiences involve the consumption of various foods, yet it is unclear how the brain distinguishes and categorizes such food experiences. Despite the crucial roles of the basolateral amygdala (BLA) in appetitive behavior and emotion, how BLA pyramidal cells and interneurons encode food experiences has not yet been well characterized. Here we employ large-scale tetrode recording techniques to investigate the coding properties of pyramidal neurons vs. fast-spiking interneurons in the BLA as mice freely consumed a variety of foods, such as biscuits, rice, milk and water. We found that putative pyramidal cells conformed to the power-of-two-based permutation logic, as postulated by the Theory of Connectivity, to generate specific-to-general neural clique-coding patterns. Many pyramidal cells exhibited firing increases specific to a given food type, while some other pyramidal cells increased firings to various combinations of multiple foods. In contrast, fast-spiking interneurons can increase or decrease firings to given food types, and were more broadly tuned to various food experiences. We further show that a subset of pyramidal cells exhibited rapid desensitization to repeated eating of the same food, correlated with rapid behavioral habituation. Finally, we provide the intuitive visualization of BLA ensemble activation patterns using the dimensionality-reduction classification method to decode real-time appetitive stimulus identity on a moment-to-moment, single trial basis. Elucidation of the neural coding patterns in the BLA provides a key insight into how the brain's emotion and memory circuits performs the computational operation of pattern discrimination and categorization of natural food experiences.

Unique Configurations of Compression and Truncation of Neuronal Activity Underlie l-DOPA-Induced Selection of Motor Patterns in Aplysia

A key issue in neuroscience is understanding the ways in which neuromodulators such as dopamine modify neuronal activity to mediate selection of distinct motor patterns. We addressed this issue by applying either low or high concentrations of l-DOPA (40 or 250 μM) and then monitoring activity of up to 130 neurons simultaneously in the feeding circuitry of Aplysia using a voltage-sensitive dye (RH-155). l-DOPA selected one of two distinct buccal motor patterns (BMPs): intermediate (low l-DOPA) or bite (high l-DOPA) patterns. The selection of intermediate BMPs was associated with shortening of the second phase of the BMP (retraction), whereas the selection of bite BMPs was associated with shortening of both phases of the BMP (protraction and retraction). Selection of intermediate BMPs was also associated with truncation of individual neuron spike activity (decreased burst duration but no change in spike frequency or burst latency) in neurons active during retraction. In contrast, selection of bite BMPs was associated with compression of spike activity (decreased burst latency and duration and increased spike frequency) in neurons projecting through specific nerves, as well as increased spike frequency of protraction neurons. Finally, large-scale voltage-sensitive dye recordings delineated the spatial distribution of neurons active during BMPs and the modification of that distribution by the two concentrations of l-DOPA.

Relationship between the grades of a learned aversive-feeding response and the dopamine contents in Lymnaea

The pond snail Lymnaea learns conditioned taste aversion (CTA) and remembers not to respond to food substances that initially cause a feeding response. The possible relationship between how well snails learn to follow taste-aversion training and brain dopamine contents is not known. We examined this relationship and found the following: first, snails in the act of eating just before the commencement of CTA training were poor learners and had the highest dopamine contents in the brain; second, snails which had an ad libitum access to food, but were not eating just before training, were average learners and had lower dopamine contents; third, snails food-deprived for one day before training were the best learners and had significantly lower contents of dopamine compared to the previous two cohorts. There was a negative correlation between the CTA grades and the brain dopamine contents in these three cohorts. Fourth, snails food-deprived for five days before training were poor learners and had higher dopamine contents. Thus, severe hunger increased the dopamine content in the brain. Because dopamine functions as a reward transmitter, CTA in the severely deprived snails (i.e. the fourth cohort) was thought to be mitigated by a high dopamine content.

Acute Sleep Deprivation Blocks Short- and Long-Term Operant Memory in Aplysia

STUDY OBJECTIVES

Insufficient sleep in individuals appears increasingly common due to the demands of modern work schedules and technology use. Consequently, there is a growing need to understand the interactions between sleep deprivation and memory. The current study determined the effects of acute sleep deprivation on short and long-term associative memory using the marine mollusk Aplysia californica, a relatively simple model system well known for studies of learning and memory.

METHODS

Aplysia were sleep deprived for 9 hours using context changes and tactile stimulation either prior to or after training for the operant learning paradigm, learning that food is inedible (LFI). The effects of sleep deprivation on short-term (STM) and long-term memory (LTM) were assessed.

RESULTS

Acute sleep deprivation prior to LFI training impaired the induction of STM and LTM with persistent effects lasting at least 24 h. Sleep deprivation immediately after training blocked the consolidation of LTM. However, sleep deprivation following the period of molecular consolidation did not affect memory recall. Memory impairments were independent of handling-induced stress, as daytime handled control animals demonstrated no memory deficits. Additional training immediately after sleep deprivation failed to rescue the induction of memory, but additional training alleviated the persistent impairment in memory induction when training occurred 24 h following sleep deprivation.

CONCLUSIONS

Acute sleep deprivation inhibited the induction and consolidation, but not the recall of memory. These behavioral studies establish Aplysia as an effective model system for studying the interactions between sleep and memory formation.

Deep sequencing of transcriptomes from the nervous systems of two decapod crustaceans to characterize genes important for neural circuit function and modulation

BACKGROUND

Crustaceans have been studied extensively as model systems for nervous system function from single neuron properties to behavior. However, lack of molecular sequence information and tools have slowed the adoption of these physiological systems as molecular model systems. In this study, we sequenced and performed de novo assembly for the nervous system transcriptomes of two decapod crustaceans: the Jonah crab (Cancer borealis) and the American lobster (Homarus americanus).

RESULTS

Forty-two thousand, seven hundred sixty-six and sixty thousand, two hundred seventy-three contigs were assembled from C. borealis and H. americanus respectively, representing 9,489 and 11,061 unique coding sequences. From these transcripts, genes associated with neural function were identified and manually curated to produce a characterization of multiple gene families important for nervous system function. This included genes for 34 distinct ion channel types, 17 biogenic amine and 5 GABA receptors, 28 major transmitter receptor subtypes including glutamate and acetylcholine receptors, and 6 gap junction proteins - the Innexins.

CONCLUSION

With this resource, crustacean model systems are better poised for incorporation of modern genomic and molecular biology technologies to further enhance the interrogation of fundamentals of nervous system function.

Effects of aging on the food intake in the feeding behavior of Aplysia kurodai

In wild Aplysia, the birthdate of animals can typically not be determined. Therefore, we sought a reliable index of old age by taking into consideration the distinguished Japanese seasons. Large amounts of eggs and dead bodies were present on the coast during and after the second half of May (MayS). Body mass decreased after May. We roughly classified animals collected before and after the MayS as mature and old animals. Plots of internalized shell length (S) against body mass (B) gave distinct best-fit curves for mature and old animals. The B/S significantly decreased in the second half of June, suggesting that body mass decreases with age but shell length is maintained in each animal. Therefore, the collected animals were classified into mature and old animals using the best-fit curves for animals classified by the collection period. We examined the amount of food intake every 2 h up to 8 h after providing food. The amounts increased linearly, and the rate was significantly lower in old animals than in mature animals. The amount of 1-day food intake was also significantly lower in old animals. These results suggest that food intake may decline with age and this may cause mass loss in old animals.

Phylogenetic and individual variation in gastropod central pattern generators

Gastropod molluscs provide a unique opportunity to explore the neural basis of rhythmic behaviors because of the accessibility of their nervous systems and the number of species that have been examined. Detailed comparisons of the central pattern generators (CPGs) underlying rhythmic feeding and swimming behaviors highlight the presence and effects of variation in neural circuits both across and within species. The feeding motor pattern of the snail, Lymnaea, is stereotyped, whereas the feeding motor pattern in the sea hare, Aplysia, is variable. However, the Aplysia motor pattern is regularized with operant conditioning or by mimicking learning using the dynamic clamp to change properties of CPG neurons. Swimming evolved repeatedly in marine gastropods. Distinct neural mechanisms underlie dissimilar forms of swimming, with homologous neurons playing different roles. However, even similar swimming behaviors in different species can be produced by distinct neural mechanisms, resulting from different synaptic connectivity of homologous neurons. Within a species, there can be variation in the strength and even valence of synapses, which does not have functional relevance under normal conditions, but can cause some individuals to be more susceptible to lesion of the circuit. This inter- and intra-species variation provides novel insights into CPG function and plasticity.

Change in excitability of a putative decision-making neuron in Aplysia serves as a mechanism in the decision not to feed following food satiation

Although decision making is a ubiquitous function, the understanding of its underlying mechanisms remains limited, particularly at the single-cell level. In this study, we used the decision not to feed that follows satiation in the marine mollusk Aplysia to examine the role of putative decision-making neuron B51 in this process. B51 is a neuron in the feeding neural circuit that exhibits decision-making characteristics in vitro, which bias the circuit toward producing the motor programs responsible for biting behavior. Once satiated, Aplysia decided not to bite for a prolonged period of time (≥24h) when presented with a food stimulus that normally elicits feeding in non-satiated animals. Twenty-four hours after satiation, suppressed feeding was accompanied by a significant decrease of B51 excitability compared to the control group of unfed animals. No differences were measured in B51 resting membrane properties or synaptic input to B51 between the satiated and control groups. When B51 properties were measured at a time point in which feeding had recovered from the suppressive effects of satiation (i.e., 96 h after satiation), no difference in B51 excitability was observed between satiated and control groups. These findings indicate that B51 excitability changes in a manner that is coherent with the modifications in biting resulting from food satiation, thus implicating this neuron as a site of plasticity underlying the decision not to bite following food satiation in Aplysia.

Neuromodulatory adaptive combination of correlation-based learning in cerebellum and reward-based learning in basal ganglia for goal-directed behavior control

Goal-directed decision making in biological systems is broadly based on associations between conditional and unconditional stimuli. This can be further classified as classical conditioning (correlation-based learning) and operant conditioning (reward-based learning). A number of computational and experimental studies have well established the role of the basal ganglia in reward-based learning, where as the cerebellum plays an important role in developing specific conditioned responses. Although viewed as distinct learning systems, recent animal experiments point toward their complementary role in behavioral learning, and also show the existence of substantial two-way communication between these two brain structures. Based on this notion of co-operative learning, in this paper we hypothesize that the basal ganglia and cerebellar learning systems work in parallel and interact with each other. We envision that such an interaction is influenced by reward modulated heterosynaptic plasticity (RMHP) rule at the thalamus, guiding the overall goal directed behavior. Using a recurrent neural network actor-critic model of the basal ganglia and a feed-forward correlation-based learning model of the cerebellum, we demonstrate that the RMHP rule can effectively balance the outcomes of the two learning systems. This is tested using simulated environments of increasing complexity with a four-wheeled robot in a foraging task in both static and dynamic configurations. Although modeled with a simplified level of biological abstraction, we clearly demonstrate that such a RMHP induced combinatorial learning mechanism, leads to stabler and faster learning of goal-directed behaviors, in comparison to the individual systems. Thus, in this paper we provide a computational model for adaptive combination of the basal ganglia and cerebellum learning systems by way of neuromodulated plasticity for goal-directed decision making in biological and bio-mimetic organisms.

The significance of dynamical architecture for adaptive responses to mechanical loads during rhythmic behavior

Many behaviors require reliably generating sequences of motor activity while adapting the activity to incoming sensory information. This process has often been conceptually explained as either fully dependent on sensory input (a chain reflex) or fully independent of sensory input (an idealized central pattern generator, or CPG), although the consensus of the field is that most neural pattern generators lie somewhere between these two extremes. Many mathematical models of neural pattern generators use limit cycles to generate the sequence of behaviors, but other models, such as a heteroclinic channel (an attracting chain of saddle points), have been suggested. To explore the range of intermediate behaviors between CPGs and chain reflexes, in this paper we describe a nominal model of swallowing in Aplysia californica. Depending upon the value of a single parameter, the model can transition from a generic limit cycle regime to a heteroclinic regime (where the trajectory slows as it passes near saddle points). We then study the behavior of the system in these two regimes and compare the behavior of the models with behavior recorded in the animal in vivo and in vitro. We show that while both pattern generators can generate similar behavior, the stable heteroclinic channel can better respond to changes in sensory input induced by load, and that the response matches the changes seen when a load is added in vivo. We then show that the underlying stable heteroclinic channel architecture exhibits dramatic slowing of activity when sensory and endogenous input is reduced, and show that similar slowing with removal of proprioception is seen in vitro. Finally, we show that the distributions of burst lengths seen in vivo are better matched by the distribution expected from a system operating in the heteroclinic regime than that expected from a generic limit cycle. These observations suggest that generic limit cycle models may fail to capture key aspects of Aplysia feeding behavior, and that alternative architectures such as heteroclinic channels may provide better descriptions.

Operant conditioning: a minimal components requirement in artificial spiking neurons designed for bio-inspired robot's controller

In this paper, we investigate the operant conditioning (OC) learning process within a bio-inspired paradigm, using artificial spiking neural networks (ASNN) to act as robot brain controllers. In biological agents, OC results in behavioral changes learned from the consequences of previous actions, based on progressive prediction adjustment from rewarding or punishing signals. In a neurorobotics context, virtual and physical autonomous robots may benefit from a similar learning skill when facing unknown and unsupervised environments. In this work, we demonstrate that a simple invariant micro-circuit can sustain OC in multiple learning scenarios. The motivation for this new OC implementation model stems from the relatively complex alternatives that have been described in the computational literature and recent advances in neurobiology. Our elementary kernel includes only a few crucial neurons, synaptic links and originally from the integration of habituation and spike-timing dependent plasticity as learning rules. Using several tasks of incremental complexity, our results show that a minimal neural component set is sufficient to realize many OC procedures. Hence, with the proposed OC module, designing learning tasks with an ASNN and a bio-inspired robot context leads to simpler neural architectures for achieving complex behaviors.

Behavioral aging is associated with reduced sensory neuron excitability in Aplysia californica

Invertebrate models have advantages for understanding the basis of behavioral aging due to their simple nervous systems and short lifespans. The potential usefulness of Aplysia californica in aging research is apparent from its long history of neurobiological research, but it has been underexploited in this model use. Aging of simple reflexes at both single sensory neuron and neural circuit levels was studied to connect behavioral aging to neurophysiological aging. The tail withdrawal reflex (TWR), righting reflex, and biting response were measured throughout sexual maturity in 3 cohorts of hatchery-reared animals of known age. Reflex times increased and reflex amplitudes decreased significantly during aging. Aging in sensory neurons of animals with deficits in measures of the TWR and biting response resulted in significantly reduced excitability in old animals compared to their younger siblings. The threshold for firing increased while the number of action potentials in response to depolarizing current injection decreased during aging in sensory neurons, but not in tail motoneurons. Glutamate receptor-activated responses in sensory neurons also decreased with aging. In old tail motoneurons, the amplitude of evoked EPSPs following tail shock decreased, presumably due to reduced sensory neuron excitability during aging. The results were used to develop stages of aging relevant to both hatchery-reared and wild-caught Aplysia. Aplysia is a viable aging model in which the contributions of differential aging of components of neural circuits may be assessed.

Pain-relief learning in flies, rats, and man: basic research and applied perspectives

Memories relating to a painful, negative event are adaptive and can be stored for a lifetime to support preemptive avoidance, escape, or attack behavior. However, under unfavorable circumstances such memories can become overwhelmingly powerful. They may trigger excessively negative psychological states and uncontrollable avoidance of locations, objects, or social interactions. It is therefore obvious that any process to counteract such effects will be of value. In this context, we stress from a basic-research perspective that painful, negative events are "Janus-faced" in the sense that there are actually two aspects about them that are worth remembering: What made them happen and what made them cease. We review published findings from fruit flies, rats, and man showing that both aspects, respectively related to the onset and the offset of the negative event, induce distinct and oppositely valenced memories: Stimuli experienced before an electric shock acquire negative valence as they signal upcoming punishment, whereas stimuli experienced after an electric shock acquire positive valence because of their association with the relieving cessation of pain. We discuss how memories for such punishment- and relief-learning are organized, how this organization fits into the threat-imminence model of defensive behavior, and what perspectives these considerations offer for applied psychology in the context of trauma, panic, and nonsuicidal self-injury.

Pathogen intelligence

Different species inhabit different sensory worlds and thus have evolved diverse means of processing information, learning and memory. In the escalated arms race with host defense, each pathogenic bacterium not only has evolved its individual cellular sensing and behavior, but also collective sensing, interbacterial communication, distributed information processing, joint decision making, dissociative behavior, and the phenotypic and genotypic heterogeneity necessary for epidemiologic success. Moreover, pathogenic populations take advantage of dormancy strategies and rapid evolutionary speed, which allow them to save co-generated intelligent traits in a collective genomic memory. This review discusses how these mechanisms add further levels of complexity to bacterial pathogenicity and transmission, and how mining for these mechanisms could help to develop new anti-infective strategies.

The less things change, the more they are different: contributions of long-term synaptic plasticity and homeostasis to memory

An important cellular mechanism contributing to the strength and duration of memories is activity-dependent alterations in the strength of synaptic connections within the neural circuit encoding the memory. Reversal of the memory is typically correlated with a reversal of the cellular changes to levels expressed prior to the stimulation. Thus, for stimulus-induced changes in synapse strength and their reversals to be functionally relevant, cellular mechanisms must regulate and maintain synapse strength both prior to and after the stimuli inducing learning and memory. The strengths of synapses within a neural circuit at any given moment are determined by cellular and molecular processes initiated during development and those subsequently regulated by the history of direct activation of the neural circuit and system-wide stimuli such as stress or motivational state. The cumulative actions of stimuli and other factors on an already modified neural circuit are attenuated by homeostatic mechanisms that prevent changes in overall synaptic inputs and excitability above or below specific set points (synaptic scaling). The mechanisms mediating synaptic scaling prevent potential excitotoxic alterations in the circuit but also may attenuate additional cellular changes required for learning and memory, thereby apparently limiting information storage. What cellular and molecular processes control synaptic strengths before and after experience/activity and its reversals? In this review we will explore the synapse-, whole cell-, and circuit level-specific processes that contribute to an overall zero sum-like set of changes and long-term maintenance of synapse strengths as a consequence of the accommodative interactions between long-term synaptic plasticity and homeostasis.

Implication of dopaminergic modulation in operant reward learning and the induction of compulsive-like feeding behavior in Aplysia

Feeding in Aplysia provides an amenable model system for analyzing the neuronal substrates of motivated behavior and its adaptability by associative reward learning and neuromodulation. Among such learning processes, appetitive operant conditioning that leads to a compulsive-like expression of feeding actions is known to be associated with changes in the membrane properties and electrical coupling of essential action-initiating B63 neurons in the buccal central pattern generator (CPG). Moreover, the food-reward signal for this learning is conveyed in the esophageal nerve (En), an input nerve rich in dopamine-containing fibers. Here, to investigate whether dopamine (DA) is involved in this learning-induced plasticity, we used an in vitro analog of operant conditioning in which electrical stimulation of En substituted the contingent reinforcement of biting movements in vivo. Our data indicate that contingent En stimulation does, indeed, replicate the operant learning-induced changes in CPG output and the underlying membrane and synaptic properties of B63. Significantly, moreover, this network and cellular plasticity was blocked when the input nerve was stimulated in the presence of the DA receptor antagonist cis-flupenthixol. These results therefore suggest that En-derived dopaminergic modulation of CPG circuitry contributes to the operant reward-dependent emergence of a compulsive-like expression of Aplysia's feeding behavior.

Effect of circadian phase on memory acquisition and recall: operant conditioning vs. classical conditioning

There have been several studies on the role of circadian clocks in the regulation of associative learning and memory processes in both vertebrate and invertebrate species. The results have been quite variable and at present it is unclear to what extent the variability observed reflects species differences or differences in methodology. Previous results have shown that following differential classical conditioning in the cockroach, Rhyparobia maderae, in an olfactory discrimination task, formation of the short-term and long-term memory is under strict circadian control. In contrast, there appeared to be no circadian regulation of the ability to recall established memories. In the present study, we show that following operant conditioning of the same species in a very similar olfactory discrimination task, there is no impact of the circadian system on either short-term or long-term memory formation. On the other hand, ability to recall established memories is strongly tied to the circadian phase of training. On the basis of these data and those previously reported for phylogenetically diverse species, it is suggested that there may be fundamental differences in the way the circadian system regulates learning and memory in classical and operant conditioning.

Functional differentiation of a population of electrically coupled heterogeneous elements in a microcircuit

Although electrical coupling is present in many microcircuits, the extent to which it will determine neuronal firing patterns and network activity remains poorly understood. This is particularly true when the coupling is present in a population of heterogeneous, or intrinsically distinct, circuit elements. We examine this question in the Aplysia californica feeding motor network in five electrically coupled identified cells, B64, B4/5, B70, B51, and a newly identified interneuron B71. These neurons exhibit distinct activity patterns during the radula retraction phase of motor programs. In a subset of motor programs, retraction can be flexibly extended by adding a phase of network activity (hyper-retraction). This is manifested most prominently as an additional burst in the radula closure motoneuron B8. Two neurons that excite B8 (B51 and B71) and one that inhibits it (B70) are active during hyper-retraction. Consistent with their near synchronous firing, B51 and B71 showed one of the strongest coupling ratios in this group of neurons. Nonetheless, by manipulating their activity, we found that B51 preferentially acted as a driver of B64/B71 activity, whereas B71 played a larger role in driving B8 activity. In contrast, B70 was weakly coupled to other neurons and its inhibition of B8 counteracted the excitatory drive to B8. Finally, the distinct firing patterns of the electrically coupled neurons were fine-tuned by their intrinsic properties and the largely chemical cross-inhibition between some of them. Thus, the small microcircuit of the Aplysia feeding network is advantageous in understanding how a population of electrically coupled heterogeneous neurons may fulfill specific network functions.

Cognition in insects

A traditional view of cognition is that it involves an internal process that represents, tracks or predicts an external process. This is not a general characteristic of all complex neural processing or feedback control, but rather implies specific forms of processing giving rise to specific behavioural capabilities. In this paper, I will review the evidence for such capabilities in insect navigation and learning. Do insects know where they are, or do they only know what to do? Do they learn what stimuli mean, or do they only learn how to behave?

Effects of aversive stimuli beyond defensive neural circuits: reduced excitability in an identified neuron critical for feeding in Aplysia

In Aplysia, repeated trials of aversive stimuli produce long-term sensitization (LTS) of defensive reflexes and suppression of feeding. Whereas the cellular underpinnings of LTS have been characterized, the mechanisms of feeding suppression remained unknown. Here, we report that LTS training induced a long-term decrease in the excitability of B51 (a decision-making neuron in the feeding circuit) that recovered at a time point in which LTS is no longer observed (72 h post-treatment). These findings indicate B51 as a locus of plasticity underlying feeding suppression. Finally, treatment with serotonin to induce LTS failed to alter feeding and B51 excitability, suggesting that serotonin does not mediate the effects of LTS training on the feeding circuit.

Functional organization and adaptability of a decision-making network in aplysia

Whereas major insights into the neuronal basis of adaptive behavior have been gained from the study of automatic behaviors, including reflexive and rhythmic motor acts, the neural substrates for goal-directed behaviors in which decision-making about action selection and initiation are crucial, remain poorly understood. However, the mollusk Aplysia is proving to be increasingly relevant to redressing this issue. The functional properties of the central circuits that govern this animal’s goal-directed feeding behavior and particularly the neural processes underlying the selection and initiation of specific feeding actions are becoming understood. In addition to relying on the intrinsic operation of central networks, goal-directed behaviors depend on external sensory inputs that through associative learning are able to shape decision-making strategies. Here, we will review recent findings on the functional design of the central network that generates Aplysia’s feeding-related movements and the sensory-derived plasticity that through learning can modify the selection and initiation of appropriate action. The animal’s feeding behavior and the implications of decision-making will be briefly described. The functional design of the underlying buccal network will then be used to illustrate how cellular diversity and the coordination of neuronal burst activity provide substrates for decision-making. The contribution of specific synaptic and neuronal membrane properties within the buccal circuit will also be discussed in terms of their role in motor pattern selection and initiation. The ability of learning to “rigidify” these synaptic and cellular properties so as to regularize network operation and lead to the expression of stereotyped rhythmic behavior will then be described. Finally, these aspects will be drawn into a conceptual framework of how Aplysia’s goal-directed circuitry compares to the central pattern generating networks for invertebrate rhythmic behaviors.

Massed training-induced intermediate-term operant memory in aplysia requires protein synthesis and multiple persistent kinase cascades

The Aplysia feeding system with its high degree of plasticity and well characterized neuronal circuitry is well suited for investigations of memory formation. We used an operant paradigm, learning that food is inedible (LFI), to investigate the signaling pathways underlying intermediate-term memory (ITM) in Aplysia. During a single massed training session, the animal associates a specific seaweed with the failure to swallow, generating short-term (30 min) and long-term (24 h) memory. We investigated whether the same training protocol induced the formation of ITM. We found that massed LFI training resulted in temporally distinct protein synthesis-dependent memory evident 4-6 h after training. Through in vivo experiments, we determined that the formation of ITM required protein kinase A, protein kinase C, and MAPK. Moreover, the maintenance of ITM required PKA, PKM Apl III, and MAPK because inhibition of any of these kinases after training or before testing blocked the expression of memory. In contrast, additional experiments determined that the maintenance of long-term memory appeared independent of PKM Apl III. Using Western blotting, we found that sustained MAPK phosphorylation was dependent upon protein synthesis, but not PKA or PKC activity. Thus, massed training-induced intermediate-term operant memory requires protein synthesis as well as persistent or sustained kinase signaling for PKA, PKC, and MAPK. While short-, intermediate-, and long-term memory are induced by the same training protocol, considerable differences exist in both the combination and timing of signaling cascades that induce the formation and maintenance of these temporally distinct memories.

Nitric oxide as a regulator of behavior: new ideas from Aplysia feeding

Nitric oxide (NO) regulates Aplysia feeding by novel mechanisms, suggesting new roles for NO in controlling the behavior of higher animals. In Aplysia, (1) NO helps maintain arousal when produced by neurons responding to attempts to swallow food; (2) NO biases the motor system to reject and reposition food that resists swallowing; (3) if mechanically resistant food is not successfully swallowed, NO mediates the formation and expression of memories of food inedibility; (4) NO production at rest inhibits feeding, countering the effects of food stimuli exciting feeding. At a cellular level, NO-dependent channels contribute to the resting potential of neurons controlling food finding and food consumption. Increases in L-arginine after animals eat act as a post-feeding inhibitory signal, presumably by modulating NO production at rest. NO also signals non-feeding behaviors that are associated with feeding inhibition. Thus, depending on context, NO may enhance or inhibit feeding behavior. The different functions of NO may reflect the evolution of NO signaling from a response to tissue damage that was then elaborated and used for additional functions. These results suggest that in higher animals (1) elicited and background transmitter release may have similar effects; (2) NO may be produced by neurons without firing, influencing adjacent neurons; (3) background NO production may contribute to a neuron's resting potential; (4) circulating factors affecting background NO production may regulate spatially separated neurons; (5) L-arginine can be used to regulate neural activity; (6) L-arginine may be an effective post-ingestion metabolic signal to regulate feeding.

ARPOP: an appetitive reward-based pseudo-outer-product neural fuzzy inference system inspired from the operant conditioning of feeding behavior in Aplysia

Appetitive operant conditioning in Aplysia for feeding behavior via the electrical stimulation of the esophageal nerve contingently reinforces each spontaneous bite during the feeding process. This results in the acquisition of operant memory by the contingently reinforced animals. Analysis of the cellular and molecular mechanisms of the feeding motor circuitry revealed that activity-dependent neuronal modulation occurs at the interneurons that mediate feeding behaviors. This provides evidence that interneurons are possible loci of plasticity and constitute another mechanism for memory storage in addition to memory storage attributed to activity-dependent synaptic plasticity. In this paper, an associative ambiguity correction-based neuro-fuzzy network, called appetitive reward-based pseudo-outer-product-compositional rule of inference [ARPOP-CRI(S)], is trained based on an appetitive reward-based learning algorithm which is biologically inspired by the appetitive operant conditioning of the feeding behavior in Aplysia. A variant of the Hebbian learning rule called Hebbian concomitant learning is proposed as the building block in the neuro-fuzzy network learning algorithm. The proposed algorithm possesses the distinguishing features of the sequential learning algorithm. In addition, the proposed ARPOP-CRI(S) neuro-fuzzy system encodes fuzzy knowledge in the form of linguistic rules that satisfies the semantic criteria for low-level fuzzy model interpretability. ARPOP-CRI(S) is evaluated and compared against other modeling techniques using benchmark time-series datasets. Experimental results are encouraging and show that ARPOP-CRI(S) is a viable modeling technique for time-variant problem domains.

Unique ionotropic receptors for D-aspartate are a target for serotonin-induced synaptic plasticity in Aplysia californica

The non-L-glutamate (L-Glu) receptor component of D-aspartate (D-Asp) currents in Aplysia californica buccal S cluster (BSC) neurons was studied with whole cell voltage clamp to differentiate it from receptors activated by other well-known agonists of the Aplysia nervous system and investigate modulatory mechanisms of D-Asp currents associated with synaptic plasticity. Acetylcholine (ACh) and serotonin (5-HT) activated whole cell excitatory currents with similar current voltage relationships to D-Asp. These currents, however, were pharmacologically distinct from D-Asp. ACh currents were blocked by hexamethonium (C6) and tubocurarine (D-TC), while D-Asp currents were unaffected. 5-HT currents were blocked by granisetron and methysergide (MES), while D-Asp currents were unaffected. Conversely, while (2S,3R)-1-(Phenanthren-2-carbonyl)piperazine-2,3-dicarboxylic acid(PPDA) blocked D-Asp currents, it had no effect on ACh or 5-HT currents. Comparison of the charge area described by currents induced by ACh or 5-HT separately from, or with, D-Asp suggests activation of distinct receptors by all 3 agonists. Charge area comparisons with L-Glu, however, suggested some overlap between L-Glu and D-Asp receptors. Ten minute exposure to 5-HT induced facilitation of D-Asp-evoked responses in BSC neurons. This effect was mimicked by phorbol ester, suggesting that protein kinase C (PKC) was involved.

Classical conditioning analog enhanced acetylcholine responses but reduced excitability of an identified neuron

Although classical and operant conditioning are operationally distinct, it is unclear whether these two forms of learning are mechanistically distinct or similar. Feeding behavior of Aplysia provides a useful model system for addressing this issue. Both classical and operant appetitive behavioral training enhance feeding, and neuronal correlates have been identified. Behavioral training was replicated by in vitro analogs that use isolated ganglia. Moreover, a single-cell analog of operant conditioning was developed using neuron B51, a cell important for the expression of the conditioned behavior. Here, a single-cell analog of classical conditioning was developed. Acetylcholine (ACh) mediated the conditioned stimulus (CS)-elicited excitation of B51 in ganglia and mimicked the CS in the single-cell analog of classical conditioning. Pairing ACh with dopamine, which mediates the unconditioned stimulus in ganglia, decreased the excitability of B51, and increased the CS-elicited excitation of B51, similar to results following both in vivo and in vitro classical training. Finally, a D1 dopamine receptor (D1R) agonist failed to support classical conditioning in the cellular analog, whereas D1R mediates reinforcement in operant conditioning.

Critical role of the circadian clock in memory formation: lessons from Aplysia

Unraveling the complexities of learning and the formation of memory requires identification of the cellular and molecular processes through which neural plasticity arises as well as recognition of the conditions or factors through which those processes are modulated. With its relatively simple nervous system, the marine mollusk Aplysia californica has proven an outstanding model system for studies of memory formation and identification of the molecular mechanisms underlying learned behaviors, including classical and operant associative learning paradigms and non-associative behaviors. In vivo behavioral studies in Aplysia have significantly furthered our understanding of how the endogenous circadian clock modulates memory formation. Sensitization of the tail-siphon withdrawal reflex represents a defensive non-associative learned behavior for which the circadian clock strongly modulates intermediate and long-term memory formation. Likewise, Aplysia exhibit circadian rhythms in long-term memory, but not short-term memory, for an operant associative learning paradigm. This review focuses on circadian modulation of intermediate and long-term memory and the putative mechanisms through which this modulation occurs. Additionally, potential functions and the adaptive advantages of time of day pressure on memory formation are considered. The influence of the circadian clock on learning and memory crosses distant phylogeny highlighting the evolutionary importance of the circadian clock on metabolic, physiological, and behavioral processes. Thus, studies in a simple invertebrate model system have and will continue to provide critical mechanistic insights to complementary processes in higher organisms.