Chronic fluoxetine treatment of juvenile zebrafish (Danio rerio) does not elicit changes in basal cortisol levels and anxiety-like behavior in adulthood

Abstract

Exposure to selective serotonin reuptake inhibitors (SSRIs) during development may elicit long-term neuroadaptive changes that could alter the basal regulation of stress-associated physiological and behavioral processes later in life. Currently, the effects of juvenile fluoxetine exposure in rodent models appear to be dependent on the developmental window targeted as well as the duration of drug exposure. The zebrafish (Danio rerio) model is rapidly becoming a useful tool in pharmacological research and can be used to help elucidate some of the long-term effects of fluoxetine exposure prior to sexual maturation on neuroendocrine and behavioral stress markers. In the current study, juvenile zebrafish were chronically exposed to fluoxetine hydrochloride (0 or 100 μg/L) for 14 days (31-44 days post-fertilization (dpf)), then were left untreated until young adulthood. Starting at 90 dpf, basal neuroendocrine stress and behavioral responses of zebrafish were assessed. Cortisol was extracted from the young adult zebrafish body (trunk) and quantified via enzyme-linked immunosorbent assay (ELISA). Anxiety-like behaviors were assessed in response to introduction to the novel tank test. It was expected that juvenile exposure to fluoxetine would (1) reduce basal cortisol levels and (2) elicit anxiolytic effects in the novel tank test in adulthood. However, fluoxetine exposure during the juvenile period was not associated with alterations in basal levels of cortisol nor were there any significant changes in anxiety-like behavior in the young adult zebrafish. Thus, in zebrafish, it does not appear that SSRI exposure during the juvenile period has a long-term adverse or maladaptive impact on the basal expression of cortisol and anxiety-like behavior in adulthood. Further studies are needed to determine if SSRI exposure during this developmental window influences neuroendocrine and behavioral responses to acute stress.

Keywords: Anxiety; Cortisol; Development; Fluoxetine; HPA; HPI; Juvenile; SSRI; Stress; Zebrafish.

Figure 1. Trunk cortisol levels of young adult zebrafish treated during the juvenile period with and without fluoxetine.

Chronic fluoxetine treatment during the juvenile period (31–44 dpf) did not alter adult levels of trunk cortisol compared to control-treated fish (p = 0.979; independent samples t-test). Values are mean ± SEM of 15–19 fish per group.

Figure 2. Motor activity measures of young adult zebrafish treated during the juvenile period with and without fluoxetine.

Chronic fluoxetine treatment during the juvenile period (31–44 dpf) did not alter the (A) total distance (p = 0.303; independent samples t-test) or (B) mean speed (p = 0.594; independent samples t-test) of adult fish in the novel tank test compared to control-treated fish. Values are mean ± SEM of 12–13 fish per group.

Figure 3. Freezing behaviors of young adult zebrafish treated during the juvenile period with and without fluoxetine.

Chronic fluoxetine treatment during the juvenile period (31–44 dpf) did not alter the (A) number of times immobile (p = 0.634; independent samples t-test) or (B) total time immobile (p = 0.595; independent samples t-test) of adult fish in the novel tank test compared to control-treated fish. Values are mean ± SEM of 12–13 fish per group.

Figure 4. Anxiety-like behaviors of young adult zebrafish treated during the juvenile period with and without fluoxetine.

Chronic fluoxetine treatment during the juvenile period (31–44 dpf) did not alter the (A) distance in top (p = 0.710; independent samples t-test), (B) number of entries to top (p = 0.847; independent samples t-test), (C) time in top (p = 0.622; independent samples t-test), or (D) latency to top (p = 0.984; independent samples t-test) of adult fish in the novel tank test compared to control-treated fish. Values are mean ± SEM of 12–13 fish per group.