This manuscript describes a simple method to measure stress behaviorally in adult zebrafish. The approach takes advantage of the innate tendency that zebrafish prefer the bottom half of a tank when in a stressful state. We also describe methods for coupling the assay with pharmacology.
Responding appropriately to stressful stimuli is essential for survival of an organism. Extensive research has been done on a wide spectrum of stress-related diseases and psychiatric disorders, yet further studies into the genetic and neuronal regulation of stress are still required to develop better therapeutics. The zebrafish provides a powerful genetic model to investigate the neural underpinnings of stress, as there exists a large collection of mutant and transgenic lines. Moreover, pharmacology can easily be applied to zebrafish, as most drugs can be added directly to water. We describe here the use of the ‘novel tank test’ as a method to study innate stress responses in zebrafish, and demonstrate how potential anxiolytic drugs can be validated using the assay. The method can easily be coupled with zebrafish lines harboring genetic mutations, or those in which transgenic approaches for manipulating precise neural circuits are used. The assay can also be used in other fish models. Together, the described protocol should facilitate the adoption of this simple assay to other laboratories.
Stress responses are altered behavioral and physiological states resulting from potentially harmful or aversive stimuli. Stress responses are conserved throughout the animal kingdom, and are critical for the survival of an organism1. Decades of research have greatly expanded our knowledge of some of the genetic and neuronal mechanisms underlying stress states. Today, areas of the brain such as the amygdala and the striatum2, and genetic factors such as corticotropin releasing hormone (crh), and the glucocorticoid (gr) and mineralocorticoid receptors (mr) have been studied extensively3,4,5,6. Despite these critical findings, much remains unknown about genetic and neuronal regulation of stress. As such, many stress related disorders suffer from a lack of therapeutics.
Genetically amendable model organisms provide a useful tool in the study of genetic and neuronal control of behavior. Fish models, in particular, are extremely powerful: they are small organisms with short generation times, their use in a laboratory setting is facile, their genomes are easily modified, and, as a vertebrate, they share not only genetic, but also neuroanatomical homology with their mammalian counterparts7,8. Standard assays for measuring stress can be paired with zebrafish lines harboring genetic mutations, or those in which manipulation of precise neuronal subsets is possible, and the effects of single genes or defined neurons can be assessed rapidly and efficiently.
Behaviorally, stress responses can be characterized in fish as periods of hyper-activity or prolonged periods of inactivity (akin to 'freezing')9, reduced exploration10, rapid breathing, reduced food intake11, and a place-preference for the bottom of a tank12. For example, when placed into an unfamiliar tank, adult zebrafish and other small fish models show an initial preference for the bottom half of the tank, yet, over time, the fish begin exploring top and bottom halves with near-equal frequency12. Treatment of adults with drugs known to reduce anxiety cause fish to explore immediately the top half10,13. Conversely, drugs that increase anxiety cause fish to show strong preference for the bottom half of the tank12,14,15. Thus, reduced exploration and preference for the bottom half of the tank are simple and reliable indicators of stress.
Like most vertebrates, stress responses in fish are driven by activation of hypothalamic-pituitary-inter-renal axis (HPI; analogous to the hypothalamic-pituitary-adrenal [HPA] axis in mammals)14,16. Hypothalamic neurons expressing the hormone corticotropin-releasing hormone (CRH) signal to the pituitary, which in turn releases adrenocorticotropic releasing hormone (ACTH). ACTH then signals to the inter-renal gland to produce and secrete cortisol, which has a number of downstream targets16, one of them being negative feedback of the crh-producing hypothalamic neurons3,17,18,19.
Here, we describe a method to assess behavioral measures of innate stress. For the behavior, we detail protocols using the novel tank diving test12,14. We then demonstrate, as an example, that a known anxiolytic drug, buspirone, reduces behavioral measures of stress.
The protocol has been approved by the Institutional Animal Care and Use Committeeat Florida Atlantic University .
1. Preparation
2. Setup
NOTE: The steps in this section describe setting up the novel tank assay. A diagram of the end product is given in Figure 1B.
3. Novel tank test setup
4. Pretreatment with drug
NOTE: The aim of the following steps is to compare the behavior of an individual before and after the use of drugs. This comparison is achieved by first performing a novel tank test as in step 3.4 to 3.6, followed by drug treatment, and then a second novel tank test (Figure 3A).
5. Video analysis
6. Testing for normality
Examining stress in zebrafish
To examine stress behavior over time in wild-type zebrafish, we tested adult fish from the AB strain24 in the novel tank test. AB adults were subjected to the protocol as described above. Briefly, fish were given a 1-h acclimation period in a tank in the behavior room. An individual was placed in a beaker for 10-min, and then placed gently in an unfamiliar tank (novel tank) filled with fresh system water. Locomotor activity was recorded for 10-min, and tracking was performed offline using commercially available software. Comparison of locomotor activity between the first and last minute showed dramatic differences (Figure 2A,B). When first introduced into the tank, fish spent the majority of the time in the bottom (Figure 2B), yet over time, adults had a gradual increase in the amount of time spent in the top (Figure 2B,C). Total duration spent in the top in the first minute compared to the last minute revealed significant differences (6.29 ± 8.21 s vs. 23.23 ± 9.02 s; paired t- test, p < 0.05) (Figure 2C). By contrast, total distance traveled between first and last minute revealed no significant differences (440.4 ± 110.3 cm vs. 405.5 ± 49.71 cm; paired t- test, p = 0.375) (Figure 2D). Because innate preference was different between the first and last minute, and not distance traveled, we believe the change in behavior represents a stress response, and not merely a change in locomotor activity. These results demonstrate that zebrafish exhibit an easily measurable innate stress response. This approach also establishes a foundation to compare stress differences between different groups of animals, and assess the differences in stress between them.
Effects of anxiolytic drugs on stress behavior in zebrafish
Zebrafish are a powerful system for screening drugs, since application of drug can be applied in non-invasive ways by simply adding to the water25,26,27. To validate that bottom dwelling in zebrafish represents an innate stress response, we compared behavior in groups of adult zebrafish tested before and after exposure to an anxiolytic drug; as a control, we handled a separate of adults similarly, yet applied only vehicle (system water) instead of drug. We used the 5HT1A receptor agonist buspirone, which is not a controlled substance and is prescribed to human patients suffering from generalized anxiety disorder28. Buspirone has been validated to cause reduction in behavioral stress responses in various fish and mammalian models10,13,21,29,30,31,32,33,34 . As described in the protocol, zebrafish were first recorded in the novel tank test, then retrieved and placed in a beaker of drug or vehicle for 10 min. Fish were then given a 'wash-out' period, where they were placed in a new beaker for 10 min, and then re-recorded in the novel tank test (Figure 3A).
Analysis of locomotor paths revealed little difference before and after treatment for groups of adults exposed to vehicle alone (Figure 3B). By contrast, adults exposed to buspirone spent a large amount of time in the top compared to the locomotor paths of the same fish before drug exposure (Figure 3B,C). Quantification of duration of time spent in the top revealed no significant differences between pre- and post-treatment in control animals (183.9 ± 90.46 s before vs. 113.8 ± 81.88 s after; one-way ANOVA followed by Sidak's multiple comparisons test, p = 0.4254), yet animals exposed to buspirone spent significantly more time in the top relative to pre-treatment, and control individuals after treatment (Buspirone: 201.4 ± 49.95 s before vs. 552.2 ± 42.97 s after; one-way ANOVA followed by Sidak's multiple comparisons test, p < 0.0001; Control vs. Buspirone post-treatment: p < 0.0001.) (Figure 3C). To examine whether the differences were due to less locomotion in general, we measured total distance traveled. These data revealed no significant differences for any of the groups (4134 ± 601.9 cm before vs. 3471 ± 766 cm after for control; Kruskal-Wallis test, p > 0.05; 3904 ± 301.3 cm before vs. 3644 ± 566.3 cm after for buspirone; Kruskal-Wallis test, p > 0.05) (Figure 3D). Taken together, these data demonstrate that bottom dwelling in adult zebrafish is a measure of innate stress, and establish a foundation for further pharmacological experiments in adult zebrafish.
Figure 1. Diagram of the novel tank setup. (A) Dimensions of the 1.8 L trapezoidal novel tank as seen from the recording side of the tank. (B) Diagram of the setup including positions of the infrared lights, camera, and barriers used to minimize human interference. Please click here to view a larger version of this figure.
Figure 2. Examining innate stress responses in wild-type zebrafish. (A) Representative swim paths of an individual adult in the novel tank test in the first minute (left) and last minute (right) of a 10-min recording. Imaginary lines defining the top, middle, and bottom zones of the tank are indicated with dotted lines. (B) Quantification of total time spend in the top zone for each minute of the 10-min recording. (C & D) Comparisons of total duration spent in the top zone (C) and total distance traveled (D) in the first and last minute of all trials. Paired t – tests were used for analysis since the data passed the Shapiro-Wilk test for normality. n = 5; *: p = 0.028. Error bars indicate standard error of the mean. Please click here to view a larger version of this figure.
Figure 3. Examining the effects of anxiolytic drugs on stress behavior. (A) Schematic of experimental flow. (B) Representative swim paths pre- and post- treatment of an individual from a control individual treated with system water only, and another individual treated with buspirone in the novel tank test. Dotted lines define separation of top, middle, and bottom zones of the tank. Grey tracks represent the control individual, and blue tracks represent the buspirone-treated individual. (C & D) Comparisons of total duration spent in the top zone (C) and total distance traveled (D) between control (Ctrl) and buspirone-treated (Busp) trials. A test for normality using the Shapiro-Wilk test was first done. Where the test for normality failed, Kruskal-Wallis test followed by Dunn's multiple comparisons test was used (C); and if the data passed normality, one-way ANOVA followed by Sidak's multiple comparisons test was used for analysis (D). n= 5 for each condition; ***: p = 0.001. Error bars indicate standard error of the mean. Please click here to view a larger version of this figure.
Zebrafish exhibit a robust stress response in a novel tank
Here, we describe a simple behavioral approach for examining stress responses in adult zebrafish, and validate the approach as a simple measure of stress using pharmacology.
The novel tank test is a widely used test for examining innate stress in zebrafish and other species of fish12,14,21,35,36, and zebrafish has been shown to be a powerful model to examine the pharmacological effects of anxiety-related drugs. Similar to humans, these studies have demonstrated that drugs such as buspirone, nicotine, fluoxetine, and scopolamine have anxiolytic effects in zebrafish12,13,14,37. Moreover, drugs such as scopolamine that are typically not used to treat anxiety in humans can have additional anxiolytic effects37. Drug screens demonstrating these anxiolytic effects in zebrafish can facilitate the study of side effects and their pharmacological mechanisms. Further, the zebrafish has a comparable stress response pathway to humans, hence pairing the assay with quantification of the release of cortisol after a stressor or drug treatment can be used to validate the behavioral responses14. Finally, we wish to point out that this assay is not specific to zebrafish, and has also been extended to other fish species such as the Mexican blind cavefish, Astyanax mexicanus21. It is likely that the assay can be extended to cichlids38, mosquitofish39, killifish40, and other piscine systems.
An important advantage of the novel tank test is its ecological relevance; as the assay measures innate preference for the bottom half of the tank, the response is likely one that would occur in the wild. In addition to the novel tank test, other behavioral assays that have been used in other model organisms can be used to further validate the behavioral stress response, such as an open field test or a light/dark assay41,42. These assays are based on the tendency for an animal to follow the sides of the arena (thigmotaxis), and preference for exploration in the dark (scototaxis) after being exposed to a stressful cue42,43. In addition, electric shock has been used to measure either innate or conditioned fear responses9,44,45, though the ecological relevance of this approach is unclear.
When one is considering an assay for his/her study, it is important to take into account innate bias within strains or species. In addition to maintaining and reducing environmental fluctuations in behavioral assays, keeping the genetic background of the test adults consistent will be vital since research has shown variability within and between individuals of the same genotype41,46. A comprehensive review of the advantages, disadvantages, validity of each common behavioral assay used to study anxiety, and also variations in behavior within common wildtype lines can be found elsewhere41.
Zebrafish as a model for examining stress
Zebrafish are becoming a popular model for examining genetic and neuronal pathways that modulate precise behaviors47,48, and recently developed brain atlases allow for mapping neurons regulating behavior with precision49,50,51,52,53. The approach we describe here to measuring innate anxiety is able to harness powerful genetic and neural circuits tools in zebrafish. Two foreseeable approaches rely on the large collection of mutant lines and transgenic driver lines. Mutant lines, for example, will facilitate investigators to examine the role that precise genes have in modulating stress. Additionally, transgenic Gal4/UAS and QF/QUAS system have been extensively applied to zebrafish54,55, and when crossed to UAS or QUAS effector lines, the function of precise neuronal circuits can be manipulated and behavior assessed. These approaches provide a complement to genetic mutant lines, and permit investigation of how precise neural circuits contribute to stress.
Novel techniques for examining neural activity can be fully integrated with this assay. Quantification of c-fos mRNA or protein are widely used to examine neuronal activity56. This gene is an immediate early gene, whose transcription is activated by neuronal activity. Newer approaches based on similar methodology have been developed. For example, the extracellular-signal-regulated kinase (ERK) was recently developed for examining neuronal activity in zebrafish50. The ERK protein exists in nearly all cells of the central nervous system. Upon neuronal activation, the ERK peptide become phosphorylated. Moreover, reliable antibodies for both un-phosphorylated ERK (total ERK, tERK) and phosphorylated ERK (pERK) have been developed and work well in zebrafish. Thus, by co-labeling with antibodies specific to tERK and pERK, neuronal activity can be reliably measured. Using this approach, adults that significantly display more bottom dwelling in the novel tank test can be removed after recording, stained for either c-fos or tERK/pERK, and resulting brain sections imaged.
Taken together, these approaches should facilitate a facile approach for dissecting the genetic and neuronal mechanisms underlying stress in zebrafish. Moreover, due to the high conservation of genetic and neuronal pathways in zebrafish and mammals, we expect these methods to reveal conserved mechanisms underlying stress behavior.
The authors have nothing to disclose.
This work was supported by funding from the Jupiter Life Science Initiative at Florida Atlantic University to ERD and ACK. This work was also supported by grants R21NS105071 (awarded to ACK and ERD) and R15MH118625 (awarded to ERD) from the National Institutes of Health.
Camera | We use Point Grey Grasshopper3 USB camera with lens from Edmund Optics. | ||
Infrared filter | Edmund Optics | ||
Video Acquisition Program | Use programs such as Virtualdub or FlyCapture because the acquisition framerate can be set. | ||
Infrared LED lights | |||
Assay tank | Aquaneering | Part number ZT180 | Size: M3 1.8 liter |
Stand and clamp, or standard tripod for camera | |||
250mL beaker | |||
Tracking software | We use Ethovision XT 13 from Noldus Information Technology | ||
Buspirone chloride | Sigma-Aldrich | B7148 | |
Randomized trial generator | We use the RANDBETWEEN function in Microsoft Excel |