Zebrafish Preference for Light or Dark Is Dependent on Ambient Light Levels and Olfactory Stimulation

Chemical and Visual Cues as Modulators of the Stress Response to Social Isolation in the Marine Medaka, Oryzias melastigma

The marine medaka is emerging as a potential behavioral model organism for ocean studies, namely on marine ecotoxicology. However, not much is known on the behavior of the species and behavioral assays lack standardization. This study assesses the marine medaka as a potential model for chemical communication. We investigated how short exposure to visual and chemical cues mediated the stress response to social isolation with the light/dark preference test (LDPT) and the open field test (OFT). After a 5-day isolation period, and 1 h before testing, isolated fish were randomly assigned to one of four groups: (1) placed in visual contact with conspecifics; (2) exposed to a flow of holding water from a group of conspecifics; (3) exposed to both visual and chemical cues from conspecifics; or (4) not exposed to any stimuli (controls). During the LDPT, the distance traveled and transitions between zones were more pronounced in animals exposed to the conspecific's chemical stimuli. The time spent in each area did not differ between the groups, but a clear preference for the bright area in all animals indicates robust phototaxis. During the OFT, animals exposed only to chemical cues initially traveled more than those exposed to visual or both stimuli, and displayed lower thigmotaxis. Taken together, results show that chemical cues play a significant role in exploratory behavior in this species and confirm the LDPT and OFT as suitable tests for investigating chemical communication in this species.

Assessment of alcohol-induced aggressive behavior in zebrafish (Danio rerio): A practical class

Ethanol (EtOH) is among the most consumed drugs in the world. The behavior of humans after ingestion of this drug is characteristic: At low doses it may be excitatory and at higher doses, it may induce depressant/sedative effects. Similar effects are observed in the zebrafish experimental model (Danio rerio), which has about 70% genetic similarity with humans and has been widely used in numerous research. With the objective of improving the learning of biochemistry students, this work aimed to develop a practical exercise in the laboratory for students to observe the behavioral repertoire of zebrafish under the effects of exposure to ethanol. Through this practical class, the students were able to observe the similarity of the behavior of the animal model with that of humans, showing its importance for the consolidation of knowledge, awakening in the students an interest in science and its applications in everyday life.

What do male and female zebrafish prefer? Directional and color preference in maze tasks

Studies regarding the animals' innate preferences help elucidate and avoid probable sources of bias and serve as a reference to improve and develop new behavioral tasks. In zebrafish research, data obtained in behavioral assessments are often not replicated between research groups or even inside the same laboratory raising huge concerns about replicability and reproducibility. Among the potential causes that are not well considered, sexual differences can be a probable source of bias. Thus, this study aimed to investigate the male and female zebrafish directional and color preferences in the plus-maze and T-maze behavioral tasks. Experiment 1 evaluated directional preference and experiment 2 evaluated color preference in a plus-maze task; experiment 3 evaluated preference between black or white in a T-maze task. Individual preferences were expressed as the percentage of time spent in each zone. Our results showed that male and female zebrafish demonstrated no difference in directional preference in the plus-maze task. Surprisingly, male and female zebrafish showed color preference differences in the plus-maze task; males did not show any color preference, while female zebrafish demonstrated a red preference compared to white, blue, and yellow colors. Moreover, both male and female zebrafish demonstrated a strong black color preference compared to the white color in the T-maze task. Our findings characterized the spontaneous preference of male and female zebrafish for direction and color, identifying possible biases, and providing insights that contribute to the standardization of future protocols.

Color as an important biological variable in zebrafish models: Implications for translational neurobehavioral research

Color is an important environmental factor that in multiple ways affects human and animal behavior and physiology. Widely used in neuroscience research, various experimental (animal) models may help improve our understanding of how different colors impact brain and behavioral processes. Complementing laboratory rodents, the zebrafish (Danio rerio) is rapidly emerging as an important novel model species to explore complex neurobehavioral processes. The growing utility of zebrafish in biomedicine makes it timely to consider the role of colors in their behavioral and physiological responses. Here, we summarize mounting evidence implicating colors as a critical variable in zebrafish models and neurobehavioral traits, with a particular relevance to CNS disease modeling, genetic and pharmacological modulation, as well as environmental enrichment and animal welfare. We also discuss the growing value of zebrafish models to study color neurobiology and color-related neurobehavioral phenomics, and outline future directions of research in this field.

High-glucose/high-cholesterol diet in zebrafish evokes diabetic and affective pathogenesis: The role of peripheral and central inflammation, microglia and apoptosis

Neuroinflammation and metabolic deficits contribute to the etiology of human affective disorders, such as anxiety and depression. The zebrafish (Danio rerio) has recently emerged as a powerful new model organism in CNS disease modeling. Here, we exposed zebrafish to 2% glucose and 10% cholesterol for 19 days to experimentally induce type 2 diabetes (DM) and to assess stress responses, microglia, inflammation and apoptosis. We analyzed zebrafish anxiety-like behavior in the novel tank and light-dark box (Days 15-16) tests, as well as examined their biochemical and genomic biomarkers (Day 19). Confirming DM-like state in zebrafish, we found higher whole-body glucose, triglyceride, total cholesterol, low-density lipoprotein levels and glucagon mRNA expression, and lower high-density lipoprotein levels. DM zebrafish also showed anxiety-like behavior, elevated whole-body cortisol and cytokines IFN-γ and IL-4, as well as higher brain mRNA expression of the glucocorticoid receptor, CD11b (a microglial biomarker), pro-inflammatory cytokines IL-6 and TNF-α (but not IL-1β or anti-inflammatory cytokines IL-4 and IL-10), GFAP (an astrocytal biomarker), neurotrophin BDNF, its receptors p75 and TrkB, as well as apoptotic Bax and Caspase-3 (but not BCl-2) genes. Collectively, this supports the overlapping nature of DM-related affective pathogenesis and emphasizes the role of peripheral and central inflammation and apoptosis in DM-related affective and neuroendocrine deficits in zebrafish.

The Use of Larval Zebrafish (Danio rerio) Model for Identifying New Anxiolytic Drugs from Herbal Medicine

Anxiety is a widespread psychiatric disorder. The search for a cure is still continuing since many of the synthetic drugs were inefficient in completely treating anxiety, yet caused some dangerous side effects until many of the drugs were withdrawn from the market. One promising source of new anxiolytics could be herbal medicines. The challenge is to screen plant extracts. Rodent models can be used for this purpose but are expensive. Moreover, rodent tests are costly and consume relatively large quantities of sample. For this reason, alternative animal models may be useful. Zebrafish larvae have many advantages for screening natural products. The main advantage is that they can be produced cheaply and in large numbers. Several studies have shown that the zebrafish is a good model for studying drugs that affect anxiety. This review focuses on the use of animal models, including zebrafish larvae, for studying anxiety and screening for herbal medicines that modulate anxiety. Finally, future prospects of the zebrafish larva as an alternative model in this field are also discussed.

Behavioral, morphometric, and gene expression effects in adult zebrafish (Danio rerio) embryonically exposed to PFOA, PFOS, and PFNA

Perfluoroalkylated substances (PFAS) are a class of persistent anthropogenic chemicals that have been detected worldwide. PFASs consist of fluorinated carbon chains of varying length, terminal groups, and have a number of industrial uses. A previous zebrafish study from our laboratory showed that acute (3-120h post fertilization, 0.02-2.0μM), waterborne embryonic exposure to these chemicals resulted in chemical specific alterations at 5days post fertilization (dpf), and some effects persisted up to 14 dpf. Using a gene battery consisting of 100 transcripts identified several genes that were up or down regulated. This current study looks at the long-term impacts of PFASs in adult zebrafish using the same exposure regimen. It was hypothesized that sub-lethal exposure of perfluorooctane sulfonate (PFOS), perfluorononanoic acid (PFNA), or perfluorooctane sulfonate (PFOA) in embryonic zebrafish (3-120 hpf) would result in permanent morphometric, gene expression, and behavioral changes in adult fish similar to those observed at 5 and 14 dpf. Zebrafish were exposed to PFOS, PFOA, and PFNA (Control 0μM, 2.0μM) for the first five days post fertilization. At six months post fertilization, no PFAS treatment resulted in a significant change in total body length or weight. In terms of behavior, PFNA males showed a reduction in total distance traveled and time of immobility, and an increase in thigmotaxis behavior, aggressive attacks, and preference for the bright section of the tank. PFOS treated males had a reduced aggression behavior, and PFOA females preferred the dark section of the tank. Gene expression of slco2b1, slco1d1, and tgfb1a were analyzed because these transcripts were previously found to be affected by PFAS exposure in 5dpf and 14 dpf zebrafish and resulted in: significant decrease in expression of slco2b1 for both sexes in PFNA and PFOS treated groups, significant decrease of slco1d1 in all treatment groups for females and PFOS and PFOA exposed males, significant increase of tgfb1a in males treated with PFOS and PFNA, and a significant increase of bdnf in all PFAS male groups. This study demonstrates that acute, embryonic exposure (5days) to individual PFASs result in significant biochemical and behavioral changes in young adult zebrafish 6 months after exposure. These three PFASs have long term and persistent impacts following short term embryonic exposure that persists into adulthood.

Behavioural plasticity across social contexts is regulated by the directionality of inter-individual differences

An individual's behavioural phenotype is a combination of its unique behavioural propensities and its responsiveness to environmental variation, also known as behavioural plasticity. In social species, we must not only explore how individuals respond to variations in the physical environment but also how they react to changes in their social environment. A growing body of work has demonstrated that the behavioural heterogeneity of a group can alter its responsiveness, decision making, and fitness. Whether an individual is more or less extreme than a partner - what we term its 'relative personality' - may also alter individual behavioural responses. We determined exploratory tendencies of individual zebrafish (Danio rerio) and then constructed pairs with varying differences in 'relative personality' to determine the effect of differences between partners on behavioural plasticity. We find that relative personality, but not the magnitude of the difference between partners, is the most important determinant of behavioural plasticity across social treatments. Despite this overall effect, pairs of fish exhibited no predictable leader-follower interactions, suggesting that details of the experimental paradigm may be important in shaping social dynamics.

Son et lumière: Sound and light effects on spatial distribution and swimming behavior in captive zebrafish

Aquatic and terrestrial habitats are heterogeneous by nature with respect to sound and light conditions. Fish may extract signals and exploit cues from both ambient modalities and they may also select their sound and light level of preference in free-ranging conditions. In recent decades, human activities in or near water have altered natural soundscapes and caused nocturnal light pollution to become more widespread. Artificial sound and light may cause anxiety, deterrence, disturbance or masking, but few studies have addressed in any detail how fishes respond to spatial variation in these two modalities. Here we investigated whether sound and light affected spatial distribution and swimming behavior of individual zebrafish that had a choice between two fish tanks: a treatment tank and a quiet and light escape tank. The treatments concerned a 2 × 2 design with noisy or quiet conditions and dim or bright light. Sound and light treatments did not induce spatial preferences for the treatment or escape tank, but caused various behavioral changes in both spatial distribution and swimming behavior within the treatment tank. Sound exposure led to more freezing and less time spent near the active speaker. Dim light conditions led to a lower number of crossings, more time spent in the upper layer and less time spent close to the tube for crossing. No interactions were found between sound and light conditions. This study highlights the potential relevance for studying multiple modalities when investigating fish behavior and further studies are needed to investigate whether similar patterns can be found for fish behavior in free-ranging conditions.

Keeping eyes peeled: guppies exposed to chemical alarm cue are more responsive to ambiguous visual cues

Abstract

Information received from the visual and chemical senses is qualitatively different. For prey species in aquatic environments, visual cues are spatially and temporally reliable but risky as the prey and predator must often be in close proximity. Chemical cues, by contrast, can be distorted by currents or linger and thus provide less reliable spatial and temporal information, but can be detected from a safe distance. Chemical cues are therefore often the first detected and may provide a context in which prey respond to subsequent ambiguous cues (“context hypothesis”). Depending on this context, early chemical cues may also alert prey to attend to imminent cues in other sensory modalities (“alerting hypothesis”). In the context of predation risk, for example, it is intuitive that individuals become more responsive to subsequent ambiguous cues across sensory modalities. Consistent with the context hypothesis, guppies, Poecilia reticulata, exposed to conspecific alarm cue reduced activity, a classic fright response among fish, in response to a water disturbance more than those exposed to cues of unharmed conspecifics or a water control. Despite this reduction in activity, guppies exposed to alarm cue were more attentive to visual cues than those exposed to the other chemical cues, as predicted by the alerting hypothesis. These responses contrasted with those of guppies exposed to chemical cues of undisturbed, unharmed conspecifics, which were relatively unaffected by the disturbance. This is the first study indicating that unambiguous cues detected by one sensory modality affect animal responses to subsequent ambiguous multimodal cues.

Significance statement

In moving water, chemical cues can be detected over longer distances than visual cues; they may therefore be detected first and alert animals to imminent visual cues. This effect is likely to be particularly important if these chemical cues are indicative of predation. I investigated how different chemical cues affect (1) guppy response to an ambiguous water disturbance and (2) their responsiveness to subsequent ambiguous visual cues. Guppies based their responses to ambiguous cues on the context implied by chemical cues: those exposed to chemical cues indicative of predation reduced activity, a classic fright response, but increased responsiveness to visual cues, relative to those exposed to control chemical cues. This is the first study to show that unambiguous cues detected by one sense affect animal responses to ambiguous cues detected by other senses.

Electronic supplementary material

The online version of this article (doi:10.1007/s00265-016-2076-4) contains supplementary material, which is available to authorized users.

Activation and inhibition of tph2 serotonergic neurons operate in tandem to influence larval zebrafish preference for light over darkness

Serotonergic neurons have been implicated in a broad range of processes, but the principles underlying their effects remain a puzzle. Here, we ask how these neurons influence the tendency of larval zebrafish to swim in the light and avoid regions of darkness. Pharmacological inhibition of serotonin synthesis reduces dark avoidance, indicating an involvement of this neuromodulator. Calcium imaging of tph2-expressing cells demonstrates that a rostral subset of dorsal raphe serotonergic neurons fire continuously while the animal is in darkness, but are inhibited in the light. Optogenetic manipulation of tph2 neurons by channelrhodopsin or halorhodopsin expression modifies preference, confirming a role for these neurons. In particular, these results suggest that fish prefer swimming in conditions that elicits lower activity in tph2 serotonergic neurons in the rostral raphe.

Differences in inhibitory avoidance, cortisol and brain gene expression in TL and AB zebrafish

Recently, we established an inhibitory avoidance paradigm in Tupfel Long-Fin (TL) zebrafish. Here, we compared task performance of TL fish and fish from the AB strain; another widely used strain and shown to differ genetically and behaviourally from TL fish. Whole-body cortisol and telencephalic gene expression related to stress, anxiety and fear were measured before and 2 h post-task. Inhibitory avoidance was assessed in a 3-day paradigm: fish learn to avoid swimming from a white to a black compartment where a 3V-shock is given: day 1 (first shock), day 2 (second shock) and day 3 (no shock, sampling). Tupfel Long-Fin fish rapidly learned to avoid the black compartment and showed an increase in avoidance-related spatial behaviour in the white compartment across days. In contrast, AB fish showed no inhibitory avoidance learning. AB fish had higher basal cortisol levels and expression levels of stress-axis related genes than TL fish. Tupfel Long-Fin fish showed post-task learning-related changes in cortisol and gene expression levels, but these responses were not seen in AB fish. We conclude that AB fish show higher cortisol levels and no inhibitory avoidance than TL fish. The differential learning responses of these Danio strains may unmask genetically defined risks for stress-related disorders.

The effects of environmental enrichment and age-related differences on inhibitory avoidance in zebrafish (Danio rerio Hamilton)

The inhibitory avoidance paradigm allows the study of mechanisms underlying learning and memory formation in zebrafish (Danio rerio Hamilton). For zebrafish, the physiology and behavior associated with this paradigm are as yet poorly understood. We therefore assessed the effects of environmental enrichment and fish age on inhibitory avoidance learning. Fish raised in an environmentally enriched tank showed decreased anxiety-like behavior and increased exploration. Enrichment greatly reduced inhibitory avoidance in 6-month (6M)- and 12-month (12 M)-old fish. Following inhibitory avoidance, telencephalic mRNA levels of proliferating cell nuclear antigen (pcna), neurogenic differentiation (neurod), cocaine- and amphetamine-regulated transcript 4 (cart4), and cannabinoid receptor 1 (cnr1) were lower in enriched-housed fish, while the ratios of mineralocorticoid receptor (nr3c2)/glucocorticoid receptor α [nr3c1(α)] and glucocorticoid receptor β [nr3c1(β)]/glucocorticoid receptor α [nr3c1(α)] were higher. This was observed for 6M-old fish only, not for 24-month (24 M) old fish. Instead, 24 M-old fish showed delayed inhibitory avoidance, no effects of enrichment, and reduced expression of neuroplasticity genes. Overall, our data show strong differences in inhibitory avoidance behavior between zebrafish of different ages and a clear reduction in avoidance behavior following housing under environmental enrichment.

Anxiety, hyperactivity and stereotypy in a zebrafish model of fragile X syndrome and autism spectrum disorder

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and is caused by a loss of function of the fragile X mental retardation (fmr1) gene. Animal fmr1-knockout (KO) models are not only of interest for the study of FXS, but have also important implications for our understanding of autism spectrum disorder (ASD). Here we report the behavioral changes in fmr1-knockout zebrafish in an open field with two white and two transparent walls. The neophobic responses that in wild-type (WT) zebrafish normally occur during the first 5-10 min in an unfamiliar environment (such as freezing, hypo-activity and preferences for the bottom and opaque walls of the tank), were weakened in fmr1 mutants, suggesting a reduction of novelty-induced anxiety. The fmr1-KO zebrafish showed somewhat increased vertical activity beyond the 'neophobic phase', but no overall hyperactivity. The mutants demonstrated a clear habituation-independent preference for the transparent walls. Whether this was attributable to altered spatial information processing or to reduced avoidance of open spaces is discussed. Finally, since restrictive repetitive (or stereotypical) behaviors are frequently present in FXS and ASD patients, we analyzed relative turning angles, directional and preferential turning ratios and performed frequency-domain analysis. However, no indications of abnormal movement patterning were detected. The possible reasons for the absence of stereotypical behaviors are discussed in terms of behavioral endpoint selection and of eliciting conditions. Overall, our findings are consistent with those reported in fmr1-KO mice and suggest that further analysis of the fmr1-KO zebrafish model has potential to deepen our understanding of FXS and ASD.

Unpredictable chronic stress decreases inhibitory avoidance learning in Tuebingen long-fin zebrafish: stronger effects in the resting phase than in the active phase

Zebrafish (Danio rerio Hamilton) are increasingly used as a model to study the effects of chronic stress on brain and behaviour. In rodents, unpredictable chronic stress (UCS) has a stronger effect on physiology and behaviour during the active phase than during the resting phase. Here, we applied UCS during the daytime (active phase) for 7 and 14 days or during the night-time (resting phase) for 7 nights in an in-house-reared Tuebingen long-fin (TLF) zebrafish strain. Following UCS, inhibitory avoidance learning was assessed using a 3 day protocol where fish learn to avoid swimming from a white to a black compartment where they will receive a 3 V shock. Latencies of entering the black compartment were recorded before training (day 1; first shock) and after training on day 2 (second shock) and day 3 (no shock, tissue sampling). Fish whole-body cortisol content and expression levels of genes related to stress, fear and anxiety in the telencephalon were quantified. Following 14 days of UCS during the day, inhibitory avoidance learning decreased (lower latencies on days 2 and 3); minor effects were found following 7 days of UCS. Following 7 nights of UCS, inhibitory avoidance learning decreased (lower latency on day 3). Whole-body cortisol levels showed a steady increase compared with controls (100%) from 7 days of UCS (139%), to 14 days of UCS (174%) to 7 nights of UCS (231%), suggestive of an increasing stress load. Only in the 7 nights of UCS group did expression levels of corticoid receptor genes (mr, grα, grβ) and of bdnf increase. These changes are discussed as adaptive mechanisms to maintain neuronal integrity and prevent overload, and as being indicative of a state of high stress load. Overall, our data suggest that stressors during the resting phase have a stronger impact than during the active phase. Our data warrant further studies on the effect of UCS on stress axis-related genes, especially grβ; in mammals this receptor has been implicated in glucocorticoid resistance and depression.

Inhibitory avoidance learning in zebrafish (Danio rerio): effects of shock intensity and unraveling differences in task performance

The zebrafish (Danio rerio) is increasingly used as a model in neurobehavioral and neuroendocrine studies. The inhibitory avoidance paradigm has been proposed as tool to study mechanisms underlying learning and memory in zebrafish. In this paradigm subjects receive a shock after entering the black compartment of a black-white box. On the next day, latency to enter the black compartment is assessed; higher latencies are indicative of increased avoidance learning. Here, we aimed to understand the effects of different shock intensities (0, 1, 3, and 9 V) and to unravel variation in inhibitory avoidance learning in an in-house reared Tuebingen Long-Fin zebrafish (D. rerio) strain. While median latencies had increased in the 1, 3, and 9 V groups, no increase in median latency was found in the 0 V group. In addition, higher shock intensities resulted in a higher number of avoiders (latency ≥180 s) over nonavoiders (latency <60 s). Both changes are indicative of increased avoidance learning. We assessed whole-body cortisol content and the expression levels of genes relevant to stress, anxiety, fear, and learning 2 h after testing. Shock intensity was associated with whole-body cortisol content and the expression of glucocorticoid receptor alpha [nr3c1(alpha)], cocaine- and amphetamine-regulated transcript (cart4), and mineralocorticoid receptor (nr3c2), while avoidance behavior was associated with whole-body cortisol content only. The inhibitory avoidance paradigm in combination with measuring whole-body cortisol content and gene expression is suitable to unravel (genetic) mechanisms of fear avoidance learning. Our data further show differences in brain-behavior relationships underlying fear avoidance learning and memory in zebrafish. These findings serve as starting point for further unraveling differences in brain-behavior relationships underlying (fear avoidance) learning and memory in zebrafish.

Olfactory stimulation selectively modulates the OFF pathway in the retina of zebrafish

Cross-modal regulation of visual performance by olfactory stimuli begins in the retina, where dopaminergic interneurons receive projections from the olfactory bulb. However, we do not understand how olfactory stimuli alter the processing of visual signals within the retina. We investigated this question by in vivo imaging activity in transgenic zebrafish expressing SyGCaMP2 in bipolar cell terminals and GCaMP3.5 in ganglion cells. The food-related amino acid methionine reduced the gain and increased sensitivity of responses to luminance and contrast transmitted through OFF bipolar cells but not ON. The effects of olfactory stimulus were blocked by inhibiting dopamine uptake and release. Activation of dopamine receptors increased the gain of synaptic transmission in vivo and potentiated synaptic calcium currents in isolated bipolar cells. These results indicate that olfactory stimuli alter the sensitivity of the retina through the dopaminergic regulation of presynaptic calcium channels that control the gain of synaptic transmission through OFF bipolar cells.

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Olfactory stimuli regulate transmission of signals through retinal bipolar cells

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Modulation of synaptic gain and sensitivity occur in OFF bipolar cells but not ON

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An inhibitor of dopamine uptake blocks odor-induced changes in synaptic gain

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Dopamine potentiates presynaptic calcium channels in isolated bipolar cells

Dissociating the effects of habituation, black walls, buspirone and ethanol on anxiety-like behavioral responses in shoaling zebrafish. A 3D approach to social behavior

Understanding the different patterns of anxiety-like behavioral responses is of great interest for pharmacological and genetic research. Here we report the effects of 3.5-hr habituation, buspirone and ethanol on those responses in shoaling zebrafish (Danio rerio). Since in these experiments we used a container with white walls, the effects of black-vs.-white walls were tested in a separate experiment. An important objective was to determine whether factors unrelated to anxiety played a role in modulating the responses. The anxiety-like behavioral responses studied here are social cohesion, distance from bottom and bottom-dwell time, radial distribution (to study thigmotaxis), transparent-wall preference (to study escape responses), locomotion and freezing. The experimental conditions yielded distinctly different response patterns. Thigmotaxis was the most obvious response to white walls and it was significantly reduced after 3.5-hr habituation. It was not affected by any of the drugs. The reduction of social cohesion after 3.5-hr habituation and in the 0.5% ethanol group was probably the most interesting effect seen in this study. A role of anxiety herein was suggested but could not be established with certainty. Other hypotheses were also discussed. The large increase of distance-from-bottom resulting in swimming close to the water surface, which occurred in both buspirone groups and in the 0.5%-ethanol group, is most likely not an anxiolytic response, because of the discrepancy with the in the literature well-established time-course and the absence of any effect of 3.5-hr habituation or black walls on vertical measures. Finally, locomotion and duration freezing could not be specifically taken as indicators for the state of anxiety and the results concerning transparent-wall preference were not sufficient clear. We conclude that the neuronal and ethological mechanisms underlying the effects of habituation, white-aversion, buspirone and ethanol on anxiety-like behavioral responses are complex and need further exploration.

Measures of anxiety in zebrafish (Danio rerio): dissociation of black/white preference and novel tank test

The effects of wall color stimuli on diving, and the effects of depth stimuli on scototaxis, were assessed in zebrafish. Three groups of fish were confined to a black, a white, or a transparent tank, and tested for depth preference. Two groups of fish were confined to a deep or a shallow tank, and tested for black-white preference. As predicted, fish preferred the deep half of a split-tank over the shallow half, and preferred the black half of a black/white tank over the white half. Results indicated that the tank wall color significantly affected depth preference, with the transparent tank producing the strongest depth preference and the black tank producing the weakest preference. Tank depth, however, did not significantly affect color preference. Additionally, wall color significantly affected shuttling and immobility, while depth significantly affected shuttling and thigmotaxis. These results are consistent with previous indications that the diving response and scototaxis may reflect dissociable mechanisms of behavior. We conclude that the two tests are complementary rather than interchangeable, and that further research on the motivational systems underlying behavior in each of the two tests is needed.

Visual social preferences of lone zebrafish in a novel environment: strain and anxiolytic effects

Zebrafish (Danio rerio) have an innate tendency to join shoals. Based on this, we refined visual choice tests to focus on social interaction and novelty preference. Our design follows mouse three-chamber sociability protocols, except testing is conducted under 940 Lux fluorescent lighting. Initially, we compared performance among zebrafish strains: inbred (AB) or wild-crossbred (WIK) from Zebrafish International Resource Center, to golden and short-fin from Petco stores. AB fish exhibited a preference for shoaling; they dwelled longest near transparent boxes containing zebrafish, while short fin favored blue boxes without fish. AB and golden exhibited a strong preference for social novelty, not evident in short-fin or WIK fish. Serotonin and cannabinoids shape mammalian social behavior, and equivalents of both receptor types are expressed in the zebrafish brain. We examined the effects of the cannabinoid receptor agonist WIN 55,212-2 (1 mg/l), or serotonin 5-HT(1A) receptor agonist buspirone (10 mg/l) on Petco short-fin social choice. Fish were bath exposed to test compounds for 10 min, under these conditions [(3) H]CP55,940 (4 nm) bound to brain with a concentration of 1.9-6.4 fmol/mg 5-30 min afterward. Social approach was measured 20 min after acclimation to the test arena. WIN 55,212-2 and buspirone increased dwelling near boxed zebrafish. In zebrafish whole-brain homogenates, buspirone displaced [(3) H] 8-hydroxy-N,N-dipropylaminotetralin (dissociation constant, K(D) = 16 ± 1.2 nm) with an inhibition constant (K(i) ) of 1.8 ± 1.0 nm lower than that of WAY 100,635 (K(i) ∼1000 nm). These fish social choice tests may enhance social behavior research, and are useful for studying the effects of genetic manipulations, pharmaceuticals or environmental toxins.

Animated bird silhouette above the tank: acute alcohol diminishes fear responses in zebrafish

Alcohol dependence and alcohol abuse represent major unmet medical needs. The zebrafish is considered to be a promising vertebrate species with which the effects of alcohol on brain function and behavior and the mechanisms underlying these effects may be studied. Alcohol is known to induce alterations in motor function as well as fear and anxiety. Here we utilize a recently developed fear paradigm in which we employ an animated (moving) image of a bird silhouette. We measure the effect of acute alcohol administration (dose range employed: 0.00-0.75 vol/vol percentage, bath exposure for 60 min) on the behavioral responses of zebrafish. We test these responses during a pre-stimulus, stimulus and post-stimulus period of the task using both a video-tracking and an observation based quantification method. The fear inducing stimulus was found to decrease the distance of the zebrafish from the bottom of the tank, to increase number of erratic movements, and to increase the number of jumps in alcohol exposed fish (versus control fish). Alcohol attenuated these fear responses in a dose dependent manner. In addition, alcohol decreased general activity at the highest dose, an effect that was independent of the presentation of the stimulus. We discuss the similarities and differences between observation and video-tracking based results and conclude that fear paradigms will be useful in revealing alcohol induced functional changes in the brain of zebrafish.

Zebrafish embryos and larvae: a new generation of disease models and drug screens

Technological innovation has helped the zebrafish embryo gain ground as a disease model and an assay system for drug screening. Here, we review the use of zebrafish embryos and early larvae in applied biomedical research, using selected cases. We look at the use of zebrafish embryos as disease models, taking fetal alcohol syndrome and tuberculosis as examples. We discuss advances in imaging, in culture techniques (including microfluidics), and in drug delivery (including new techniques for the robotic injection of compounds into the egg). The use of zebrafish embryos in early stages of drug safety-screening is discussed. So too are the new behavioral assays that are being adapted from rodent research for use in zebrafish embryos, and which may become relevant in validating the effects of neuroactive compounds such as anxiolytics and antidepressants. Readouts, such as morphological screening and cardiac function, are examined. There are several drawbacks in the zebrafish model. One is its very rapid development, which means that screening with zebrafish is analogous to "screening on a run-away train." Therefore, we argue that zebrafish embryos need to be precisely staged when used in acute assays, so as to ensure a consistent window of developmental exposure. We believe that zebrafish embryo screens can be used in the pre-regulatory phases of drug development, although more validation studies are needed to overcome industry scepticism. Finally, the zebrafish poses no challenge to the position of rodent models: it is complementary to them, especially in early stages of drug research.

Stimuli affecting zebrafish (Danio rerio) behavior in the light/dark preference test

Ethanol has been suggested to have an anxiolytic effect on zebrafish, primarily based on its disruption of the novel tank diving response and of some social behaviors. The light/dark preference test offers a complementary measure of anxiety-like behavior in fish, and the purpose of the current study was to determine the effects of acute ethanol exposure on behavior in the light/dark task. In Experiment 1, the stimuli used to induce light/dark preference in zebrafish were varied in order to determine how best to measure the behavior. Subjects exhibited phototaxis (preference for light) when illumination was manipulated, but scototaxis (preference for dark) when wall and substrate color were manipulated. There was a clear interaction between locomotor activity and color preference, with animals preferentially freezing in darker locations. Because of ambiguity in interpreting behavior in the open/covered version of the test, the black/white version was used in Experiment 2. In Experiment 2, zebrafish were exposed to ethanol (0.25%, 0.5%, or 1.0%) or water for 30 minutes, and then placed in a black/white preference tank containing either ethanol (same doses) or water for a 30-minute test. Ethanol exposure increased locomotor activity and reduced freezing. Additionally, there was a significant interaction between ethanol treatment and locomotor activity on side preference. Low doses of ethanol increased white avoidance in normally swimming fish, while high doses did not.