Effect of novelty stress on behavioral responses of Danio rerio and assessment of dose-dependent effects of anxiolytics of benzodiazepine structure with phenazepam as an example

In search of new brain biomarkers of stress

The aim: of the study was to investigate the level of ghrelin in various brain structures during a stress response in Zebrafish to a predator, to evaluate this indicator as a potential biomarker of stress, and the effect of a benzodiazepine tranquilizer (phenazepam) on stress-induced changes

Materials and methods: The object of the study was Zebrafish, or Danio rerio wild type, which was subjected to stress by exposure to a predator Hypsophrys nicaraguensis from the cichlid family. In the tail tissue, the level of cortisol was determined, in the brain – the level of total (acylated and non-acylated) ghrelin by the method of enzyme-linked immunosorbent assay. The benzodiazepine anxiolytic phenazepam (1 mg/L), a ghrelin antagonist [D-Lys3]-GHRP-6 (0.333 mg/l) and corticotropin-releasing hormone (CRF; 0.4 mg/L) were used as the pharmacological agents.

Results and discussion: Exposure to a predator, just as administering CRF, more than doubled the level of cortisol in the tail tissue. [D-Lys3]-GHRP-6 and phenazepam prevented an increase in a tissue cortisol level. Simultaneously, in the medulla oblongata and cerebellum, the phylogenetically most ancient structures, rather than in the forebrain (telencephalon) or in the midbrain (corpora bigemia), the level of ghrelin was recorded about 500 pg/g of total protein. In response to exposure to a predator, the level of ghrelin increased in the forebrain and midbrain to nanogram concentrations and moderately decreased in the cerebellum. The effect was prevented by phenazepam and [D-Lys3]-GHRP-6.

Conclusion: Increases in ghrelin in the brain in response to stressful situations can be seen as a functional brain biomarker of stress, along with increased levels of tissue cortisol levels. Both of these effects are prevented by both the ghrelin antagonist and the benzodiazepine tranquilizer. The mechanism of action of the tranquilizer is a functional antagonism between the GABAergic system of the brain and the ghrelin system.

The zebrafish tail immobilization (ZTI) test as a new tool to assess stress-related behavior and a potential screen for drugs affecting despair-like states

BACKGROUND

Affective disorders, especially depression and anxiety, are highly prevalent, debilitating mental illnesses. Animal experimental models are a valuable tool in translational affective neuroscience research. A hallmark phenotype of clinical and experimental depression, the learned helplessness, has become a key target for 'behavioral despair'-based animal models of depression. The zebrafish (Danio rerio) has recently emerged as a promising novel organism for affective disease modeling and CNS drug screening. Despite being widely used to assess stress and anxiety-like behaviors, there are presently no clear-cut despair-like models in zebrafish.

NEW METHOD

Here, we introduce a novel behavioral paradigm, the zebrafish tail immobilization (ZTI) test, as a potential tool to assess zebrafish despair-like behavior. Conceptually similar to rodent 'despair' models, the ZTI protocol involves immobilizing the caudal half of the fish body for 5 min, leaving the cranial part to move freely, suspended vertically in a small beaker with water.

RESULTS

To validate this model, we used exposure to low-voltage electric shock, alarm pheromone, selected antidepressants (sertraline and amitriptyline) and an anxiolytic drug benzodiazepine (phenazepam), assessing the number of mobility episodes, time spent 'moving', total distance moved and other activity measures of the cranial part of the body, using video-tracking. Both electric shock and alarm pheromone decreased zebrafish activity in this test, antidepressants increased it, and phenazepam was inactive. Furthermore, a 5-min ZTI exposure increased serotonin turnover, elevating the 5-hydroxyindoleacetic acid/serotonin ratio in zebrafish brain, while electric shock prior to ZTI elevated both this and the 3,4-dihydroxyphenylacetic acid/dopamine ratios. In contrast, preexposure to antidepressants sertraline and amitriptyline lowered both ratios, compared to the ZTI test-exposed fish.

COMPARISON WITH EXISTINGMETHOD(S)

The ZTI test is the first despair-like experimental model in zebrafish.

CONCLUSIONS

Collectively, this study suggests the ZTI test as a potentially useful protocol to assess stress-/despair-related behaviors, potentially relevant to CNS drug screening and behavioral phenotyping of zebrafish.