Behavioral and neurophysiological effects of buspirone in healthy and depression-like state juvenile salmon

Abstract

A proportion of farmed salmon in seawater show a behaviorally inhibited, growth stunted profile known as a depression-like state (DLS). These DLS fish are characterized by chronically elevated serotonergic signaling and blood plasma cortisol levels and the inability to react further to acute stress, which is suggestive of chronic stress. In this study, we characterize the neuroendocrine profile of growth stunted freshwater parr and confirm that they show a DLS-like neuroendocrine profile with a blunted cortisol response and no serotonergic increase in response to acute stress. Furthermore, we attempted to reverse this DLS-like profile through pharmacological manipulation of the serotonin (5-HT) system with buspirone, an anxiolytic medication that acts as a serotonin receptor agonist (i.e., decreases serotonergic signaling). We found that while buspirone decreases anxiolytic-type behavior in healthy fish, no quantifiable behavioral change was found in DLS-like fish. However, there was a physiological effect of diminished basal serotonergic signaling. This suggests that at the physiological level, buspirone appears to reverse the neuroendocrine DLS profile. With a deeper understanding of what causes DLS profiles and growth stunting in juvenile fish, steps can be taken in terms of husbandry to prevent repeated stressors and the formation of the DLS profile, potentially reducing losses in aquaculture due to chronic stress.

Keywords: behavioral inhibition; chronic stress; cortisol; depression models; serotonergic activity.

Figure 1

Schematic representation of the experimental timeline for the dose response experiment, with the part 1 highlighted in red and the second part in green.

Figure 2

Schematic representation of the experimental timeline for the main buspirone experiment. Days indicated with a star (*) are days in which video analysis of behavioral parameters was performed.

Figure 3

Atlantic salmon brain regions. Transverse view of the five regions of interest in the telencephalon (A,B) and brain stem (C). Illustrations have been adapted from several stereotaxic atlases (Carruth et al., 2000; Pérez et al., 2000; Folgueira et al., 2004; Meek and Nieuwenhuys, 2014).

Figure 4

Dose response experiment part 1 (n = 4 fish per treatment). Mean ± SEM of locomotion (i.e., time spent moving more than one body length; (A), time spent at the top half of the water column (B), the number of times fish crossed between top and bottom halves (C), and overall group cohesion (measured as average distance between all fish within the group; (D) of buspirone-treated (3 and 5 mg/L) and control fish. Measurements were taken 10 min before, 10 min after, and at the last 10 min of the buspirone/sham bath which lasted for 1 h. Linear mixed effect model statistics are given within each panel and small letters symbolize Tukey post-hoc differences.

Figure 5

Mean ± SEM plasma cortisol levels of depression-like state (DLS) Atlantic salmon at both basal and post-acute stress conditions for timepoint 0 (not treated in any way), buspirone-treated (i.e., fish were treated twice with 3 mg/L and twice with 5 mg/L buspirone concentrations throughout the course of the experiment) and control (sham-treated) fish. Linear mixed effect model statistics are given within the figure and small letters symbolize Tukey post-hoc differences.

Figure 6

Mean ± SEM serotonin (5-HT) levels (A), its main catabolite 5-hydroxy indole acetic acid (5-HIAA; (B) levels, and the 5-HIAA/5-HT ratio (C) of growth stunted juvenile Atlantic salmon parr at both basal and post-acute stress conditions at timepoint 0 (i.e., fish were taken from their rearing tank 75 days after sorting). The t test statistics are given within each panel.

Figure 7

Mean ± SEM serotonin (5-HT) levels (A), its main catabolite 5-hydroxy indole acetic acid (5-HIAA) (B) levels, and the 5-HIAA/5-HT ratio (C) of depression-like state (DLS) Atlantic salmon at both basal and post-acute stress conditions for buspirone-treated (i.e., fish were treated twice with 3 mg/L and twice with 5 mg/L buspirone concentrations throughout the course of the experiment) and control (sham-treated) fish, in the dorsomedial pallium (Dm; left panel) and the raphe nuclei (RN, right panel). Linear mixed effect model statistics are given within each panel, and lowercase letters symbolize Tukey post-hoc differences.

Figure 8

Mean ± SEM relative mRNA abundance (relative to the reference gene, s20 and normalized to controls) of the serotonin receptor 1Aα (5-HT1Aα) in the raphe nuclei (RN) of depression-like state (DLS) Atlantic salmon at both basal and post-acute stress conditions for buspirone-treated (i.e., fish were treated twice with 3 mg/L and twice with 5 mg/L buspirone concentrations throughout the course of the experiment) and control (sham-treated) fish. Linear mixed effect model statistics are given within the figure and lowercase letters symbolize Tukey post hoc differences.