Individual differences in the boldness of female zebrafish are associated with alterations in serotonin function

Abstract

One of the most prevalent axes of behavioral variation in both humans and animals is risk taking, where individuals that are more willing to take risk are characterized as bold while those that are more reserved are regarded as shy. Brain monoamines (i.e. serotonin, dopamine and noradrenaline) have been found to play a role in a variety of behaviors related to risk taking. Using zebrafish, we investigated whether there was a relationship between monoamine function and boldness behavior during exploration of a novel tank. We found a correlation between serotonin metabolism (5-HIAA:5-HT ratio) and boldness during the initial exposure to the tank in female animals. The DOPAC:DA ratio correlated with boldness behavior on the third day in male fish. There was no relationship between boldness and noradrenaline. To probe differences in serotonergic function in bold and shy fish, we administered a selective serotonin reuptake inhibitor, escitalopram, and assessed exploratory behavior. We found that escitalopram had opposing effects on thigmotaxis in bold and shy female animals: the drug caused bold fish to spend more time near the center of the tank and shy fish spent more time near the periphery. Taken together, our findings indicate that variation in serotonergic function has sex-specific contributions to individual differences in risk-taking behavior.

Keywords: 5-HT; Monoamines; Neurotransmitter; Personality; Risk taking; SSRI.

Fig. 1.

Sex differences in behavior and brain chemistry. (A) Experimental design for assessing the relationship between exploration of a novel tank and baseline brain chemistry. Fish were exposed to the novel tank on three consecutive days and brain tissue was collected for testing monoamine and metabolite levels using HPLC 2 days later. Image modified from Rajput et al. (2022). (B) The effect of sex and day on four exploratory behaviors in the novel tank. NTT, novel tank test. Asterisks indicate a main effect of sex from a sex×day ANOVA (*P<0.05, **P<0.001). Data are means±s.e.m. (C) The effect of sex on monoamine and metabolite levels. (D) The effect of sex on the ratio of monoamines to their metabolites. DA, dopamine; DOPAC, 3,4-dihydroxyphenylacetic acid; NA, noradrenaline; 5-HT, serotonin; 5-HIAA, 5-hydroxyindoleacetic acid. Boxplot center line is the median, hinges are interquartile ranges and whiskers are the hinge ±1.5 times the interquartile range. Asterisks indicate significance from independent sample t-tests (*P<0.05, **P<0.01). n=46 females, n=45 males.

Fig. 2.

Correlations between boldness index and brain chemistry. (A) Pearson's correlations between the boldness index on each day of testing in the novel tank and monoamines, their metabolites, or the ratio of metabolites to monoamines. (B) Scatterplots and linear regressions for 5-HIAA:5-HT ratio and boldness index on day 1 of testing in the novel tank. *P<0.05.

Fig. 3.

The effect of escitalopram on the exploratory behavior of bold and shy fish. (A) Fish were separated into bold and shy animals based on a median split of their boldness index during an initial exposure to the tank (day 1). On day 2, fish were given either vehicle (Veh) or 1 mg kg⁻¹ escitalopram (Esc) prior to being placed back into the novel tank. (B) Histogram of boldness index on day 1 of exposure to the novel tank. The dashed line is the median. (C) Center distance on day 2 in female (top) and male (bottom) animals given either vehicle or escitalopram. Boxplot center is the median, hinges are interquartile ranges and whiskers are the hinge ±1.5 times the interquartile range. (D) Scatterplots and regression lines for boldness index on day 1 and center distance on day 2 for female and male fish given vehicle or escitalopram. (E) Pearson's correlations and 95% confidence intervals (error bars) corresponding to scatterplots in D. ^‡P<0.10, *P<0.05 from false discovery rate (FDR)-corrected post hoc t-tests. Shy female: n=19 vehicle, n=12 escitalopram; bold female: n=13 vehicle, n=18 escitalopram; shy male: n=14 vehicle, n=15 escitalopram; bold male: n=15 vehicle, n=13 escitalopram.