Assessing memory in zebrafish using the one-trial test

Duration of aversive memory in zebrafish after a single shock

Studies on memory consolidation and reconsolidation, memory loss, and the associated biochemical mechanisms have garnered interest in the past decades due to knowledge of memory performance-affecting factors such as stress, emotions, sleep, age, several neurological diseases, drugs, and chemical pollutants. Memory research has been using animal models, with increased interest in the zebrafish model. This freshwater fish species shows a wide range of behaviors relevant to memory research such as social behavior, aggression, and predator avoidance; however, few studies have investigated the duration of long-term memory. Hence, we designed an experiment to test memory duration by exposing zebrafish to avoidance conditioning using electroshock as the aversive stimulus. Zebrafish were trained to avoid the black side of a black-and-white tank and subsequently tested for aversive memory at 24 h, 48 h, 72 h, 96 h, 168 h, and 240 h. At the 72 h-interval, another zebrafish group was trained and exposed to MK-801(NMDAr antagonist) and then tested. The fish retained memories of the task and avoided the black side of the tank for up to 7 days. At 10 days post-training, the animals could no longer retrieve the aversive memory. Zebrafish treated with MK-801 did not retrieve memory. Knowledge of memory and of long-term memory duration is crucial for optimizing the zebrafish model for use in research investigating cognitive impairments such as memory loss and its ramifications. Additionally, identifying a long-term aversive memory lasting up to 7 days in zebrafish enables further research into the neuronal changes underlying this persistence. Such in-depth investigation could bring valuable insights into memory mechanisms and facilitate targeted interventions for memory-related conditions.

Zebrafish models for studying cognitive enhancers

Cognitive decline is commonly seen both in normal aging and in neurodegenerative and neuropsychiatric diseases. Various experimental animal models represent a valuable tool to study brain cognitive processes and their deficits. Equally important is the search for novel drugs to treat cognitive deficits and improve cognitions. Complementing rodent and clinical findings, studies utilizing zebrafish (Danio rerio) are rapidly gaining popularity in translational cognitive research and neuroactive drug screening. Here, we discuss the value of zebrafish models and assays for screening nootropic (cognitive enhancer) drugs and the discovery of novel nootropics. We also discuss the existing challenges, and outline future directions of research in this field.

One-trial odour recognition learning and its underlying brain areas in the zebrafish

Distinguishing familiar from novel stimuli is critical in many animals' activities, and procedures based on this ability are among the most exploited in translational research in rodents. However, recognition learning and the underlying brain substrates remain unclear outside a few mammalian species. Here, we investigated one-trial recognition learning for olfactory stimuli in a teleost fish using a behavioural and molecular approach. With our behavioural analysis, we found that zebrafish can learn to recognise a novel odour after a single encounter and then, discriminate between this odour and a different one provided that the molecular structure of the cues is relatively differentiated. Subsequently, by expression analysis of immediate early genes in the main brain areas, we found that the telencephalon was activated when zebrafish encountered a familiar odour, whereas the hypothalamus and the optic tectum were activated in response to the novel odour. Overall, this study provided evidence of single-trial spontaneous learning of novel odours in a teleost fish and the presence of multiple neural substrates involved in the process. These findings are promising for the development of zebrafish models to investigate cognitive functions.

Maze design: size and number of choices impact fish performance in cognitive assays

Although studies on fish cognition are increasing, consideration of how methodological details influence the ability to detect and measure performance is lagging. Here, in two separate experiments the authors compared latency to leave the start position, latency to make a decision, levels of participation and success rates (whether fish entered the rewarded chamber as first choice) across different physical designs. Experiments compared fish performance across (a) two sizes of T-mazes, large and standard, and a plus-maze, and (b) open choice arenas with either two or four doors. Fish in T-mazes with longer arms took longer to leave the start chamber and were less likely to participate in a trial than fish in T-mazes with shorter arms. The number of options, or complexity, in a maze significantly impacted success but did not necessarily impact behavioural measures, and did not impact the number of fish that reached a chamber. Fish in the plus-maze had similar latencies to leave the start box and time to reach any chamber as fish in the same-sized T-maze but exhibited lower overall success. Similarly, in an open choice arena, increasing the number of options - doors to potential reward chambers - resulted in lower probability of success. There was an influence of reward position in the choice arena, with rewarded chambers closest to the sides of the arena resulting in lower latencies to enter and higher probability of decision success. Together the results allow the authors to offer practical suggestions towards optimal maze design for studies of fish cognition.

Teleosts as behaviour test models for social stress

Stress is an important aspect of our everyday life and exposure to it is an unavoidable occurrence. In humans, this can come in the form of social stress or physical stress from an injury. Studies in animal models have helped researchers to understand the body's adaptive response to stress in human. Notably, the use of behavioural tests in animal models plays a pivotal role in understanding the neural, endocrine and behavioural changes induced by social stress. Under socially stressed conditions, behavioural parameters are often measured physiological and molecular parameters as changes in behaviour are direct responses to stress and are easily assessed by behavioural tests. Throughout the past few decades, the rodent model has been used as a well-established animal model for stress and behavioural changes. Recently, more attention has been drawn towards using fish as an animal model. Common fish models such as zebrafish, medaka, and African cichlids have the advantage of a higher rate of reproduction, easier handling techniques, sociability and most importantly, share evolutionary conserved genetic make-up, neural circuitry, neuropeptide molecular structure and function with mammalian species. In fact, some fish species exhibit a clear diurnal or seasonal rhythmicity in their stress response, similar to humans, as opposed to rodents. Various social stress models have been established in fish including but not limited to chronic social defeat stress, social stress avoidance, and social stress-related decision-making. The huge variety of behavioural patterns in teleost also aids in the study of more behavioural phenotypes than the mammalian species. In this review, we focus on the use of fish models as alternative models to study the effects of stress on different types of behaviours. Finally, fish behavioural tests against the typical mammalian model-based behavioural test are compared and discussed for their viability.

Social zebrafish: Danio rerio as an emerging model in social neuroendocrinology

The fitness benefits of social life depend on the ability of animals to affiliate with others and form groups, on dominance hierarchies within groups that determine resource distribution, and on cognitive capacities for recognition, learning and information transfer. The evolution of these phenotypes is coupled with that of neuroendocrine mechanisms, but the causal link between the two remains underexplored. Growing evidence from our research group and others demonstrates that the tools available in zebrafish, Danio rerio, can markedly facilitate progress in this field. Here, we review this evidence and provide a synthesis of the state-of-the-art in this model system. We discuss the involvement of generalized motivation and cognitive components, neuroplasticity and functional connectivity across social decision-making brain areas, and how these are modulated chiefly by the oxytocin-vasopressin neuroendocrine system, but also by reward-pathway monoamine signaling and the effects of sex-hormones and stress physiology.

Melanotan-II reverses memory impairment induced by a short-term HF diet

A high-fat (HF) diet has been shown to increase the risk of neurological impairments and neurodegenerative disorders. The melanotropins used in this study have been associated with diet-related disorders; however, there is an absence of studies on their effect on diet-induced neurobehavioral conditions. Here, we investigated the possible relationship among diet, Melanotan-II (MT-II) targeting melanotropin receptors, and the behavior of zebrafish (Danio rerio). Surprisingly, even a short-term HF diet lasting for ∼ 1 % of the zebrafish's life had a strong developmental effect. Zebrafish fed the HF diet showed an impairment in recognition memory, elevated anxiety levels, and reduced exploratory propensity after just three weeks compared to zebrafish fed the control diet. These HF diet-induced abnormalities were reversed by MT-II. Animals fed a HF diet and treated with MT-II demonstrated recognition memory, anxiety, and exploratory behavior similar to the control group. This study provides evidence that even a short-term HF diet has an impact on memory and emotions and is the first study to show that MT-II reverses these changes.

Disparate behavioral types in wild and reared juveniles of gilthead seabream

Fish differ consistently in behavior within the same species and population, reflecting distinct behavioral types (BTs). Comparing the behavior of wild and reared individuals provides an excellent opportunity to delve into the ecological and evolutionary consequences of BTs. In this work, we evaluated the behavioral variation of wild and reared juvenile gilthead seabreams, Sparus aurata, a highly relevant species for aquaculture and fisheries. We quantified behavioral variation along the five major axes of fish behavioral traits (exploration-avoidance, aggressiveness, sociability, shyness-boldness, and activity) using standardized behavioral tests and a deep learning tracking algorithm for behavioral annotation. Results revealed significant repeatability in all five behavior traits, suggesting high consistency of individual behavioral variation across the different axes in this species. We found reared fish to be more aggressive, social and active compared to their wild conspecifics. Reared individuals also presented less variance in their aggressiveness, lacking very aggressive and very tame individuals. Phenotypic correlation decomposition between behavioral types revealed two different behavioral syndromes: exploration-sociability and exploration-activity. Our work establishes the first baseline of repeatability scores in wild and reared gilthead seabreams, providing novel insight into the behavior of this important commercial species with implications for fisheries and aquaculture.

Acute ethanol exposure leads to long-term effects on memory, behavior, and transcriptional regulation in the zebrafish brain

Alcohol consumption is associated with alterations in memory and learning processes in humans and animals. In this context, research models such as the zebrafish (Danio rerio) arise as key organisms in behavioral and molecular studies that attempt to clarify alterations in the Central Nervous System (CNS), like those related to alcohol use. Accordingly, we used the zebrafish as a model to evaluate the effects of ethanol on the learning and memory process, as well as its relationship with behavior and transcriptional regulation of lrfn2, lrrk2, grin1a, and bdnf genes in the brain. To this end, for the memory and learning evaluation, we conducted the Novel Object Recognition test (NOR); for behavior, the Novel Tank test; and for gene transcription, qPCR, after 2 h, 24 h, and 8 days of ethanol exposure. As a result, we noticed in the NOR that after 8 days of ethanol exposure, the control group spent more time exploring the novel object than when compared to 2 h post-exposure, indicating that naturally zebrafish remember familiar objects. In animals in the Treatment group, however, no object recognition behavior was observed, suggesting that alcohol affected the learning and memory processes of the animals and stimulated an anxiolytic effect in them. Regarding transcriptional regulation, 24 h after alcohol exposure, we found hyper-regulation of bdnf and, after 8 days, a hypo-regulation of lrfn2 and lrrk2. To conclude, we demonstrated that ethanol exposure may have influenced learning ability and memory formation in zebrafish, as well as behavior and regulation of gene transcription. These data are relevant for further understanding the application of zebrafish in research associated with ethanol consumption and behavior.

Learning and memory formation in zebrafish: Protein dynamics and molecular tools

Research on learning and memory formation at the level of neural networks, as well as at the molecular level, is challenging due to the immense complexity of the brain. The zebrafish as a genetically tractable model organism can overcome many of the current challenges of studying molecular mechanisms of learning and memory formation. Zebrafish have a translucent, smaller and more accessible brain than that of mammals, allowing imaging of the entire brain during behavioral manipulations. Recent years have seen an extensive increase in published brain research describing the use of zebrafish for the study of learning and memory. Nevertheless, due to the complexity of the brain comprising many neural cell types that are difficult to isolate, it has been difficult to elucidate neural networks and molecular mechanisms involved in memory formation in an unbiased manner, even in zebrafish larvae. Therefore, data regarding the identity, location, and intensity of nascent proteins during memory formation is still sparse and our understanding of the molecular networks remains limited, indicating a need for new techniques. Here, we review recent progress in establishing learning paradigms for zebrafish and the development of methods to elucidate neural and molecular networks of learning. We describe various types of learning and highlight directions for future studies, focusing on molecular mechanisms of long-term memory formation and promising state-of-the-art techniques such as cell-type-specific metabolic labeling.

Promnesic, Anxiolytic and Antioxidant Effects of Glaucosciadium cordifolium (Boiss.) Burtt & Davis Essential Oil in a Zebrafish Model of Cognitive Impairment

The purpose of this study was to investigate the effect of Glaucosciadium cordifolium essential oil (GCEO, 25 and 150 µL/L) on anxiety and learning and memory impairment induced by scopolamine (SCOP) in zebrafish. The chemical composition was analyzed by GC-MS, and the results showed that the highest content was limonene followed by α- and β-pinene, p-cymene and α-phellandrene. The dementia model was induced by SCOP (100 µM), whereas GCEO and galantamine (GAL, 1 mg/L) were delivered to the SCOP-induced model. It was found that GCEO significantly improved memory impairment and anxiety-like response induced by SCOP through the Y-maze, novel object recognition (NOR) test, and novel tank diving tests (NTT). Biochemical analyses showed that GCEO reduced SCOP-induced oxidative damage. Additionally, the cholinergic system activity was improved in the SCOP-induced model by decreasing the acetylcholinesterase (AChE) activity following the exposure to GCEO. It was clear that as a mixture, GCEO displays positive action in improving memory impairment through restoring cholinergic dysfunction and brain antioxidant status.

A Mini-Review Regarding the Modalities to Study Neurodevelopmental Disorders-Like Impairments in Zebrafish—Focussing on Neurobehavioural and Psychological Responses

Neurodevelopmental disorders (NDDs) are complex disorders which can be associated with many comorbidities and exhibit multifactorial-dependent phenotypes. An important characteristic is represented by the early onset of the symptoms, during childhood or young adulthood, with a great impact on the socio-cognitive functioning of the affected individuals. Thus, the aim of our review is to describe and to argue the necessity of early developmental stages zebrafish models, focusing on NDDs, especially autism spectrum disorders (ASD) and also on schizophrenia. The utility of the animal models in NDDs or schizophrenia research remains quite controversial. Relevant discussions can be opened regarding the specific characteristics of the animal models and the relationship with the etiologies, physiopathology, and development of these disorders. The zebrafish models behaviors displayed as early as during the pre-hatching embryo stage (locomotor activity prone to repetitive behavior), and post-hatching embryo stage, such as memory, perception, affective-like, and social behaviors can be relevant in ASD and schizophrenia research. The neurophysiological processes impaired in both ASD and schizophrenia are generally highly conserved across all vertebrates. However, the relatively late individual development and conscious social behavior exhibited later in the larval stage are some of the most important limitations of these model animal species.

Effects of oxybenzone on zebrafish behavior and cognition

The increased ultraviolet (UV) radiation on the Earth's surface increased the need for UV filters products. One of the most used is oxybenzone, which is indiscriminately released in the environment. Oxybenzone's ecotoxicological effects on physiology have been investigated because of the bioaccumulation and action as an endocrine disruptor. However, little is known about its effects on behavior or cognition. In this study, we approach the effects of short-term oxybenzone exposure on locomotion, anxiety-like, social behavior, and short-term memory in zebrafish (Danio rerio). Adult zebrafish were exposed to oxybenzone 10, 100 and 1000 μg L^-1 for 15 days and then tested (novel tank, shoal preference, mirror test, and T-maze with novelty). Fish exposed to oxybenzone showed reduced locomotion, decreased anxiety-like behavior, less time near/interacting with the shoal, fewer interactions with the mirror image, and decreased exploration of the novel arm in the T-maze test. These results suggest that oxybenzone affects perception, increases risk-taking, impairs proper aggressive response, and jeopardizes the animals' ability to retain information. These results reinforce the risk posed by products discarded into the aquatic ecosystems, especially those with underestimated toxic potential.

The Influence of an Enriched Environment in Enhancing Recognition Memory in Zebrafish (Danio rerio)

Environmental enrichment is used to increase social and physical stimulation for animals in captivity which can lead to enhanced cognition. Fundamental to the positive effect enrichment has on the brain is that it provides opportunities for captive animals to recognize and discriminate between different stimuli in the environment. In the wild, being able to discriminate between novel or familiar stimuli has implications for survival, for example finding food, hiding from predators, or even choosing a mate. The novel object recognition (NOR) test is a cognitive task that is used extensively in the rodent literature to assess object recognition and memory, where the amount of time an animal spends exploring a novel vs. familiar object is quantified. Enrichment has been shown to enhance object recognition in rodents. More recently, the use of the NOR test has been applied to another animal model, zebrafish (Danio rerio), however, the effects of enrichment have not yet been explored. In the current study we looked at the effects of enrichment on object recognition in zebrafish using the NOR test. Adult zebrafish were housed in either enriched conditions (gravel substrate, plastic plants, shelter, heater and a filter) or plain conditions (heater and filter only) for 6 months before behavioral NOR tests were conducted. Enriched fish showed a preference for a novel object over a familiar one at a distance but did not show a preference during close inspection. Control fish did not show a preference at either distance. Our results suggest that enrichment can enhance zebrafish ability to discriminate between novel and familiar objects, but distance from the object may be an important factor. Future research is needed to determine whether any enhancements in object recognition are a result of an increase in sensory stimulation from being reared with enrichment, or whether it is due to a reduction in stress reactivity.

Memory regulation in feeding habit transformation to dead prey fish of Chinese perch (Siniperca chuatsi)

Memory drove a critical process of feeding habit transformation in Chinese perch when they re-trained to eat dead prey fish. To investigate the regulatory mechanism of cAMP-response element-binding protein (CREB) signaling pathway on the memory of Chinese perch during feeding habit transformation, the phosphorylation levels of upstream signal proteins of CREB between the control group (trained once) and the experimental group (trained twice) were measured. The results illustrated that the re-training was correlated to phosphorylation of extracellular regulated protein kinase (ERK1/2) and calcium/calmodulin-dependent protein kinase II (CaMKII), and dephosphorylation of protein kinase A (PKA) of Chinese perch. Inhibition of ERK1/2-CREB pathway decreased the mRNA levels of memory-related genes ((fos-related antigen 2 (fra2), CCAAT enhancer-binding protein delta (c/ebpb), immediate-early gene zif268 (zif268), proto-oncogenes c-fos (c-fox) and synaptotagmin-IV (sytIV)) and mRNA levels of appetite-related genes (agouti-related peptide (agrp) and ghrelin), and activation of PP1-CREB pathway increased the phosphorylated levels of CREB, the mRNA levels of memory-related genes (fra2, c/ebpb, zif268, and c-fox), and the mRNA levels of appetite-related genes (pro-opiomelanocortin (pomc) and leptin) in primary brain cells of Chinese perch. The memory in Chinese perch feeding habit transformation was associated with the ERK1/2-CREB and PP1-CREB pathways, which could regulate the transcription of memory-related genes and appetite-related genes.

Innate visual discrimination abilities of zebrafish larvae

The ability to discriminate between objects visually plays a key role in animals' interactions with their environment because it enables them to recognise companions, prey, and predators. In the zebrafish, Danio rerio, hatching occurs early on during development (48-72 h post fertilisation), and the larvae must forage and evade predators despite their immature sensory and cognitive systems. Using a preference paradigm, we investigated whether larval zebrafish are nonetheless capable of discriminating between visual stimuli. We found that larvae discriminated not only between figures with different colours or different shapes, but also between two identical figures with different orientations and between sets of figures with different numerosities. By manipulating larvae's exposure to objects before the test, we demonstrated that their discrimination abilities are innate and do not depend upon experience. This study highlighted that zebrafish possess relatively sophisticated visual discrimination abilities even at the larval stage. These abilities likely improve larval survival via the recognition of biologically relevant stimuli.

Equal performance but distinct behaviors: sex differences in a novel object recognition task and spatial maze in a highly social cichlid fish

Sex differences in behavior and cognition can be driven by differential selection pressures from the environment and in the underlying neuromolecular mechanisms of decision-making. The highly social cichlid fish Astatotilapia burtoni exhibits dynamic and complex social hierarchies, yet explicit cognitive testing (outside of social contexts) and investigations of sex differences in cognition have yet to be fully explored. Here we assessed male and female A. burtoni in two cognitive tasks: a novel object recognition task and a spatial task. We hypothesized that males outperform females in a spatial learning task and exhibit more neophilic/exploratory behavior across both tasks. In the present study we find that both sexes prefer the familiar object in a novel object recognition task, but the time at which they exhibit this preference differs between the sexes. Females more frequently learned the spatial task, exhibiting longer decision latencies and quicker error correction, suggesting a potential speed-accuracy tradeoff. Furthermore, the sexes differ in space use in both tasks and in a principal component analysis of the spatial task. A model selection analysis finds that preference, approach, and interaction duration in the novel object recognition task reach a threshold of importance averaged across all models. This work highlights the need to explicitly test for sex differences in cognition to better understand how individuals navigate dynamic social environments.

Anxiolytic, Promnesic, Anti-Acetylcholinesterase and Antioxidant Effects of Cotinine and 6-Hydroxy-L-Nicotine in Scopolamine-Induced Zebrafish (Danio rerio) Model of Alzheimer's Disease

Cotinine (COT) and 6-hydroxy-L-nicotine (6HLN) are two nicotinic derivatives that possess cognitive-improving abilities and antioxidant properties in different rodent models of Alzheimer's disease (AD), eluding the side-effects of nicotine (NIC), the parent molecule. In the current study, we evaluated the impact of COT and 6HLN on memory deterioration, anxiety, and oxidative stress in the scopolamine (SCOP)-induced zebrafish model of AD. For this, COT and 6HLN were acutely administered by immersion to zebrafish that were treated with SCOP before testing. The memory performances were assessed in Y-maze and object discrimination (NOR) tasks, while the anxiety-like behavior was evaluated in the novel tank diving test (NTT). The acetylcholinesterase (AChE) activity and oxidative stress were measured from brain samples. The RT-qPCR analysis was used to evaluate the npy, egr1, bdnf, and nrf2a gene expression. Our data indicated that both COT and 6HLN attenuated the SCOP-induced anxiety-like behavior and memory impairment and reduced the oxidative stress and AChE activity in the brain of zebrafish. Finally, RT-qPCR analysis indicated that COT and 6HLN increased the npy, egr1, bdnf, and nrf2a gene expression. Therefore, COT and 6HLN could be used as tools for improving AD conditions.

Inhibitory control in zebrafish, Danio rerio

We assessed whether zebrafish, Danio rerio, display inhibitory control using a simple and rapid behavioural test. Zebrafish were exposed to a prey stimulus placed inside a transparent tube, which initially elicited attack behaviour. However, zebrafish showed a rapid reduction in the number of attacks towards the prey, which indicated the ability to inhibit their foraging behaviour. Zebrafish also exhibited mnemonic retention of foraging inhibition, as indicated by a reduced number of attacks in a subsequent exposure to the unreachable prey. The ability to inhibit the foraging behaviour varied across three genetically separated wild-type strains and across different individuals within strains, suggesting that zebrafish show heritable within-species differences in inhibitory control. Our behavioural test might be suitable for screening large zebrafish populations in mutational studies and assessing the effects of pharmacologically active substances on inhibitory control.

Measuring recognition memory in zebrafish larvae: issues and limitations

Recognition memory is the capacity to recognize previously encountered objects, events or places. This ability is crucial for many fitness-related activities, and it appears very early in the development of several species. In the laboratory, recognition memory is most often investigated using the novel object recognition test (NORt), which exploits the tendency of most vertebrates to explore novel objects over familiar ones. Despite that the use of larval zebrafish is rapidly increasing in research on brain, cognition and neuropathologies, it is unknown whether larvae possess recognition memory and whether the NORt can be used to assess it. Here, we tested a NOR procedure in zebrafish larvae of 7-, 14- and 21-days post-fertilization (dpf) to investigate when recognition memory first appears during ontogeny. Overall, we found that larvae explored a novel stimulus longer than a familiar one. This response was fully significant only for 14-dpf larvae. A control experiment evidenced that larvae become neophobic at 21-dpf, which may explain the poor performance at this age. The preference for the novel stimulus was also affected by the type of stimulus, being significant with tri-dimensional objects varying in shape and bi-dimensional geometrical figures but not with objects differing in colour. Further analyses suggest that lack of effect for objects with different colours was due to spontaneous preference for one colour. This study highlights the presence of recognition memory in zebrafish larvae but also revealed non-cognitive factors that may hinder the application of NORt paradigms in the early developmental stages of zebrafish.

The effect of personality measurement conditions on spontaneous swimming behavior in the pale chub Zacco platypus (Cyprinidae)

Studies on personality have revealed that some personality traits are strongly correlated; thus, researchers may be able to acquire data for variables related to different personality traits from one measurement. Therefore, the aim of the present study was to test whether spontaneous movement traits used in fish personality measurements are correlated or vary among different contexts in a common Chinese cyprinid fish, the pale chub (Zacco platypus, Cyprinidae). The median swimming speed, percent time spent moving and median turning rate were measured in a boldness context (with a shelter available), then in an exploration context (with a novel object nearby) and finally in a control context (i.e., with no shelter or novel object). The median swimming speed, percent time spent moving, and median turning rate all showed positive correlations between the control and the other two contexts, which suggests that future studies might use spontaneous swimming variables measured in exploration or boldness contexts to avoid the need to carry out a separate activity test. Further analysis comparing the distance to and latency to explore the novel object between the exploration context (with the novel object present) and control context (with an imaginary object at the same position) showed that the amount of time it took for the fish to first reach the object for exploration was significantly shorter in an exploration context than in a control context. This suggests that latency to explore might be useful as a variable indicating exploration in the pale chub in the future.

A novel-odour exploration test for measuring anxiety in adult and larval zebrafish

BACKGROUND

Existing methods to assess anxiety in zebrafish are mostly based on visual exploration. However, evidence suggests that zebrafish are more attuned to olfactory than visual stimuli. We developed a novel-odour exploration test (NOEt) for zebrafish.

NEW METHOD

Adult zebrafish were exposed to a stimulus sponge soaked with olfactory cue and a control sponge with no cue, placed at the extremities of a narrow, rectangular tank. We scored time spent close to the two sponges to calculate the attraction towards the olfactory cue. In experiment 1, we tested adult zebrafish in the NOEt using various olfactory cues. In experiment 2, we tested larvae. In experiment 3, we compared the NOEt with the most used method to assess anxiety, the open-field test.

RESULTS

In experiment 1, zebrafish responded markedly to cues, by either approaching or avoiding the soaked sponge according to the type of cue. The temporal pattern of exploration toward the cue was similar to that of novel objects' exploration in this species. In experiment 2, larvae responded to novel odours, but differently from adults. In experiment 3, we found a relationship between the NOEt and the open-field test.

COMPARISON WITH EXISTING METHOD(S)

Compared to existing tests, the NOEt has the advantage of exploiting the preferred sensory modality of zebrafish. Moreover, it can be used in early stages because olfactory receptors develop early in this species.

CONCLUSIONS

The NOEt is a simple, rapid and low-cost test to study anxiety in zebrafish using the spontaneous exploration of novel olfactory cues.

Alarm cue-mediated response and learning in zebrafish larvae

We investigated the behavioural and learning response of zebrafish larvae to chemicals released by injured conspecifics (the alarm cue). Many aquatic vertebrates and invertebrates exhibit an innate antipredator response to alarm cues because in nature, they reliably indicate the presence of predators. Likewise, when an individual simultaneously perceives a novel odour and alarm cue, it learns to recognise the novel odour as a predator odour. Alarm cue-mediated behavioural response and learning have been reported in some fish and amphibians during early ontogeny, but in zebrafish, they have been described only for adults. In this study, we demonstrated that zebrafish at 12 and 24 days post fertilization exhibited reduced activity when exposed to alarm cue obtained by homogenised larvae of the same age, with this response being greater for the older zebrafish. In addition, we showed that 24-dpf zebrafish conditioned with alarm cue plus a novel odour learned to recognise the novel odour as a threat and responded to it with antipredator behaviour. The innate behavioural response and the learned response after conditioning may be used to develop paradigms with which to study anxiety, fear, stress, learning and memory in zebrafish larvae.

High fidelity: Assessing zebrafish (Danio rerio) responses to social stimuli across several levels of realism

Behavioral assays of zebrafish shoaling have recently been employed to investigate social behavior in zebrafish models of psychiatric disease. Many studies have developed simulated models of conspecifics to serve as alternatives to live shoals in order to examine specific cues that contribute to shoaling behavior. However, no studies have investigated the extent to which zebrafish prefer one stimulus over another when given the choice between two conspecific alternatives (live or simulated). In the present study, we employed a new, four-quadrant choice preference task that allowed zebrafish to swim freely between a live shoal and a motorized mobile shoal, a live shoal and playback of a video-recorded shoal, or a motorized mobile shoal and playback of a video-recorded shoal. Behavior tracking software was used to track subjects' movements in upper and lower quadrants on either side of the test arena. Subjects spent more time near the live shoal, especially in the lower quadrant, and exhibited different swim patterns in response to each simulated conspecific alternative, suggesting that zebrafish prefer a live shoal over models of lower fidelity.

Guppies show rapid and lasting inhibition of foraging behaviour

To cope with the variable environment, animals are continuously required to learn novel behaviours or, in certain cases, to inhibit automatic and previously learned behaviours. Traditionally, inhibition has been regarded as cognitively demanding and studied mostly in primates, other mammals and birds, using laboratory tasks, such as the cylinder task. Recent studies have also revealed that fish show high levels of inhibition in the cylinder task. However, conclusions on such results are undermined by evidence that the cylinder task may be inappropriate to compare such phylogenetically distant species. Here, we studied whether a fish, the guppy, Poecilia reticulata, could learn to inhibit behaviour using a different paradigm, which exploited spontaneous foraging behaviour and overcame some drawbacks that characterised the cylinder task. We exposed guppies to live brine shrimp nauplii, Artemia salina, enclosed within a transparent tube. Initially, the guppies attempted to attack the prey but over time showed a rapid decrease of the attacks. Control tests seemed to exclude the possibility that this behavioural trend was due to response to novelty or habituation, and suggested that the guppies were learning to inhibit the foraging behaviour. Memory tests indicated that guppies retained the inhibition of foraging behaviour for at least 24 h. Our study seems to indicate that teleost fish display rapid and durable inhibition of spontaneous foraging behaviour; this may be related to previous evidence, from the cylinder task, supporting efficient behavioural inhibition in this taxon.

Floating novel object recognition in adult zebrafish: a pilot study

The novel object recognition (NOR) tasks can be used to quantify memory function in zebrafish similarly to rodents. The development of zebrafish learning and memory tests provides a means for testing the effects of pharmacological manipulations of memory. Several authors reported on the successful application of different objects in NOR tests placed either at the bottom of test tanks or submerged into the tank water of zebrafish. This pilot study was designed to test the suitability of floating objects in NOR tests using adult zebrafish. Floating objects such as crumpled aluminum balls and pink plastic hollow pearls were found to be suitable for NOR tests when small groups of zebrafish are used as experimental animals. Adult zebrafish of both sexes were capable of distinguishing between the different colors and surface consistencies of certain floating objects. A significantly higher number of mouth-object contacts were recorded when either floating aluminum balls or floating plastic pearls were used as novel object during NOR tests.

Divergent action of fluoxetine in zebrafish according to responsivity to novelty

Here we show that the novel object recognition test can discriminate between high (HRN, neophobic) and low (LRN, neophilic) novelty responders in zebrafish populations. Especially when we observe the latency to the first entry in the novel object zone, zebrafish did not maintain these behavioral phenotypes in sequential tests and only the HRN group returned to their initial responsive behavior when exposed to fluoxetine. Our results have important implications for behavioral data analysis since such behavioral differences can potentially increase individual response variability and interfere with the outcomes obtained from various behavioral tasks. Our data reinforce the validity of personality determination in zebrafish since we show clear differences in behavior in response to fluoxetine.

It's Tea Time: Interference of Ayahuasca Brew on Discriminative Learning in Zebrafish

Ayahuasca is a psychoactive brew traditionally used in shamanistic and vegetalistic rituals and has recently received lot of attention due to potential cognitive benefits. Ayahuasca effects are caused by the synergistic interaction of β-carbolines (harmine, harmaline and tetrahydroarmine) contained in Banisteriopsis caapi stalks combined with the N,N-dimethyltryptamine (DMT) from Psychotria viridis leaves, a potent agonist to serotonin (5-HT) receptors. The present study approaches the effects of chronic and acute exposure to two Ayahuasca concentrations (0.1 and 0.5 ml/L) on the cognitive ability to discriminate objects in a one-trial learning task in zebrafish. Based on the combination of concentrations and exposure regimens, we divided adult zebrafish in five treatment groups: acute 0.1 and 0.5 ml/L, chronic 0.1 and 0.5 ml/L, and control 0.0 (n = 20 for each group). Then we tested them in a memory task of object discrimination. Acute Ayahuasca exposed groups performed similarly to the control group, however chronically treated fish (13 days) presented both impaired discriminative performance and locomotor alterations. Overall, these results indicate that Ayahuasca is a potent psychoactive drug that, in chronic exposure, negatively affects mnemonic parameters in zebrafish. In single exposure it does not affects cognitive performance, but the higher concentration (0.5) affected locomotion. Moreover, we reinforce the importance of the zebrafish for behavioral pharmacological studies of drug screening, in special to psychedelic drug research.

Novel object recognition and object location tasks in zebrafish: Influence of habituation and NMDA receptor antagonism

This study aims to establish a protocol for evaluating the object recognition memory and object location tasks in zebrafish. We evaluated novel the object recognition memory and analyzed the exploration time of the objects during training and testing. Zebrafish explored more the new object in comparison to the familiar object (61% of exploration time during test session). We also tested the object location task and measured the exploration time of each object in the familiar and novel object location. There was a preference to explore the object in the novel location (63% of exploration time during test session). The effect of the non-competitive NMDA receptor antagonist MK-801 was investigated on the object recognition and object location memory. Control (water only) and treated animals (5 μM MK-801) presented a significant preference in exploring the familiar object in comparison to the new object (66 and 68% of exploration time, respectively, during test session); however, 10 μM MK-801-treated animals did not show differences in the exploration time of the objects. In the object location task, the animals treated with the 5 or 10 μM MK-801 did not show a preference for the familiar or novel location whereas the control group had a higher preference in exploring the object in the familiar location (64% of exploration time during test session). Considering the different responses of the control group between original task and in the regimen treatment, we evaluated the impact of habituation on cortisol levels of animals in three different protocols: (1) habituated at the experiment apparatus for 3 days (C1 condition), (2) habituated at the experiment apparatus for 3 days plus treatment tank exposure at fourth day (C2 condition), (3) habituated at the treatment tank and experiment apparatus for 3 days and exposed to treatment tank again at fourth day (C3 condition). The results showed higher levels of cortisol in animals submitted to C2 and C3 conditions compared to animals submitted to C1. When introduced to an acute stressor during C1 condition, we observed an increase in the cortisol levels and an absence of preference for the objects in comparison to control group, which had a preference for novel object and novel location. Fluoxetine treatment induced a decrease in cortisol levels and an absence of preference for the objects in C2 and C3 conditions in comparison to control group, which had a preference for familiar object. However, fluoxetine treatment induced a preference to the novel location in C2 and C3 conditions in comparison to control group, which had a preference for familiar location. These results indicate that treatment tank exposure induced a different performance in object recognition and object location memory due to stress responses. Therefore, these tasks are prone to evaluate memory in physiological and pathological conditions, but its use is limited due to sensitivity to stress caused by manipulation.

First description of behavior and immune system relationship in fish

Considering the intriguing relationship between immune system and behavior recently described in mammals, and the lack of information of this relationship in fish, here we describe for the first time the interaction between the immune system and social and exploratory behavior in zebrafish. Fish high responders to novelty (HRN) presented a proinflammatory profile, with increased IL-1β and reduced IL-10 expression compared to fish low responders to novelty (LRN). Likewise, fish less responsive to social stimuli have a reduced expression of INF-γ. We show that fish with different behavior patterns have differences in the immune response. Our findings indicate that the interplay between immune system and behavior in zebrafish is similar to that found in mammalian models and that zebrafish should be considered as a potential model organism to study the relationship between immune system and behavior.

Good night, sleep tight: The effects of sleep deprivation on spatial associative learning in zebrafish

Learning and memory are vital to an animal's survival, and numerous factors can disrupt cognitive performance. Sleep is an evolutionarily conserved physiological process known to be important for the consolidation of learning and memory. The zebrafish has emerged as a powerful model organism sharing organizational and functional characteristics with other vertebrates, providing great translational relevance. In our study, we used a simple spatial associative learning task to quantify the effects of sleep deprivation (partial vs. total) on learning performance in zebrafish, using an animated conspecific shoal image as a reward. Control animals maintained on a regular light:dark cycle were able to acquire the association between the unconditioned and conditioned stimulus, reinforcing zebrafish as a valid and reliable model for appetitive conditioning tasks. Notably, sleep deprivation did not alter the perception of and response to the conspecific image. In contrast, although partial sleep deprivation did not impair cognitive performance, total sleep deprivation significantly impaired performance on the associative learning task. Our results suggest that sleep is important for learning and memory, and that the effects of sleep deprivation on these processes can be investigated in zebrafish.

Short- and long-term effects of nicotine and the histone deacetylase inhibitor phenylbutyrate on novel object recognition in zebrafish

RATIONALE

Zebrafish have a sophisticated color- and shape-sensitive visual system, so we examined color cue-based novel object recognition in zebrafish. We evaluated preference in the absence or presence of drugs that affect attention and memory retention in rodents: nicotine and the histone deacetylase inhibitor (HDACi) phenylbutyrate (PhB).

OBJECTIVES

The objective of this study was to evaluate whether nicotine and PhB affect innate preferences of zebrafish for familiar and novel objects after short- and long-retention intervals.

METHODS

We developed modified object recognition (OR) tasks using neutral novel and familiar objects in different colors. We also tested objects which differed with respect to the exploratory behavior they elicited from naïve zebrafish.

RESULTS

Zebrafish showed an innate preference for exploring red or green objects rather than yellow or blue objects. Zebrafish were better at discriminating color changes than changes in object shape or size. Nicotine significantly enhanced or changed short-term innate novel object preference whereas PhB had similar effects when preference was assessed 24 h after training. Analysis of other zebrafish behaviors corroborated these results.

CONCLUSIONS

Zebrafish were innately reluctant or prone to explore colored novel objects, so drug effects on innate preference for objects can be evaluated changing the color of objects with a simple geometry. Zebrafish exhibited recognition memory for novel objects with similar innate significance. Interestingly, nicotine and PhB significantly modified innate object preference.

Sleep deprivation effects on object discrimination task in zebrafish (Danio rerio)

The zebrafish is an ideal vertebrate model for neurobehavioral studies with translational relevance to humans. Many aspects of sleep have been studied, but we still do not understand how and why sleep deprivation alters behavioral and physiological processes. A number of hypotheses suggest its role in memory consolidation. In this respect, the aim of this study was to analyze the effects of sleep deprivation on memory in zebrafish (Danio rerio), using an object discrimination paradigm. Four treatments were tested: control, partial sleep deprivation, total sleep deprivation by light pulses, and total sleep deprivation by extended light. The control group explored the new object more than the known object, indicating clear discrimination. The partially sleep-deprived group explored the new object more than the other object in the discrimination phase, suggesting a certain degree of discriminative performance. By contrast, both total sleep deprivation groups equally explored all objects, regardless of their novelty. It seems that only one night of sleep deprivation is enough to affect discriminative response in zebrafish, indicating its negative impact on cognitive processes. We suggest that this study could be a useful screening tool for cognitive dysfunction and a better understanding of the effect of sleep-wake cycles on cognition.

Twelve-Day Reinforcement-Based Memory Retention in African Cichlids (Labidochromis caeruleus)

The formation of long-term memories for food sources is essential for the survival of most animals. Long-term memory formation in mammalian species has been demonstrated through a variety of conditioning tasks, however, the nature of long-term memory in fish is less known. In the current study, we explored whether African cichlids (Labidochromis caeruleus) could form memories for food-reinforced stimuli that last for 12 days. During the training sessions, fish were reinforced for approaching an upward drifting line grating. After a rest period of 12 days, fish demonstrated a significant preference for the upward drifting grating. To determine whether this preference could also be reversed, fish were then reinforced for approaching a downward drifting line grating after a 20-day rest period. When tested 12 days later, there were no significant differences in preference for either stimulus; however, following a second training period for the downward stimulus, there was a significant preference for the downward drifting grating. This suggests that cichlids are able to form reversible discrimination-based memories for food-reinforced stimuli that remain consolidated for at least 12 days.

Episodic-like memory in zebrafish

Episodic-like memory tests often aid in determining an animal's ability to recall the what, where, and which (context) of an event. To date, this type of memory has been demonstrated in humans, wild chacma baboons, corvids (Scrub jays), humming birds, mice, rats, Yucatan minipigs, and cuttlefish. The potential for this type of memory in zebrafish remains unexplored even though they are quickly becoming an essential model organism for the study of a variety of human cognitive and mental disorders. Here we explore the episodic-like capabilities of zebrafish (Danio rerio) in a previously established mammalian memory paradigm. We demonstrate that when zebrafish were presented with a familiar object in a familiar context but a novel location within that context, they spend more time in the novel quadrant. Thus, zebrafish display episodic-like memory as they remember what object they saw, where they saw it (quadrant location), and on which occasion (yellow or blue walls) it was presented.

Guppies Show Behavioural but Not Cognitive Sex Differences in a Novel Object Recognition Test

The novel object recognition (NOR) test is a widely-used paradigm to study learning and memory in rodents. NOR performance is typically measured as the preference to interact with a novel object over a familiar object based on spontaneous exploratory behaviour. In rats and mice, females usually have greater NOR ability than males. The NOR test is now available for a large number of species, including fish, but sex differences have not been properly tested outside of rodents. We compared male and female guppies (Poecilia reticulata) in a NOR test to study whether sex differences exist also for fish. We focused on sex differences in both performance and behaviour of guppies during the test. In our experiment, adult guppies expressed a preference for the novel object as most rodents and other species do. When we looked at sex differences, we found the two sexes showed a similar preference for the novel object over the familiar object, suggesting that male and female guppies have similar NOR performances. Analysis of behaviour revealed that males were more inclined to swim in the proximity of the two objects than females. Further, males explored the novel object at the beginning of the experiment while females did so afterwards. These two behavioural differences are possibly due to sex differences in exploration. Even though NOR performance is not different between male and female guppies, the behavioural sex differences we found could affect the results of the experiments and should be carefully considered when assessing fish memory with the NOR test.

Object recognition memory in zebrafish

The novel object recognition, or novel-object preference (NOP) test is employed to assess recognition memory in a variety of organisms. The subject is exposed to two identical objects, then after a delay, it is placed back in the original environment containing one of the original objects and a novel object. If the subject spends more time exploring one object, this can be interpreted as memory retention. To date, this test has not been fully explored in zebrafish (Danio rerio). Zebrafish possess recognition memory for simple 2- and 3-dimensional geometrical shapes, yet it is unknown if this translates to complex 3-dimensional objects. In this study we evaluated recognition memory in zebrafish using complex objects of different sizes. Contrary to rodents, zebrafish preferentially explored familiar over novel objects. Familiarity preference disappeared after delays of 5 mins. Leopard danios, another strain of D. rerio, also preferred the familiar object after a 1 min delay. Object preference could be re-established in zebra danios by administration of nicotine tartrate salt (50mg/L) prior to stimuli presentation, suggesting a memory-enhancing effect of nicotine. Additionally, exploration biases were present only when the objects were of intermediate size (2 × 5 cm). Our results demonstrate zebra and leopard danios have recognition memory, and that low nicotine doses can improve this memory type in zebra danios. However, exploration biases, from which memory is inferred, depend on object size. These findings suggest zebrafish ecology might influence object preference, as zebrafish neophobia could reflect natural anti-predatory behaviour.

Spontaneous object recognition: a promising approach to the comparative study of memory

Spontaneous recognition of a novel object is a popular measure of exploratory behavior, perception and recognition memory in rodent models. Because of its relative simplicity and speed of testing, the variety of stimuli that can be used, and its ecological validity across species, it is also an attractive task for comparative research. To date, variants of this test have been used with vertebrate and invertebrate species, but the methods have seldom been sufficiently standardized to allow cross-species comparison. Here, we review the methods necessary for the study of novel object recognition in mammalian and non-mammalian models, as well as the results of these experiments. Critical to the use of this test is an understanding of the organism's initial response to a novel object, the modulation of exploration by context, and species differences in object perception and exploratory behaviors. We argue that with appropriate consideration of species differences in perception, object affordances, and natural exploratory behaviors, the spontaneous object recognition test can be a valid and versatile tool for translational research with non-mammalian models.