Baseline Depression-Like Behaviors in Wild-Type Adolescent Mice Are Strain and Age but Not Sex Dependent

Abstract

Depression is a major neuropsychiatric disorder, decreasing the ability of hundreds of millions of individuals worldwide to function in social, academic, and employment settings. Beyond the alarming public health problem, depression leads to morbidity across the entire age including adolescence and adulthood. Modeling depression in rodents has been used to understand the pathophysiological mechanisms behind this disorder and create new therapeutics. Although women are two times more likely to be diagnosed with depression compared to men, behavioral experiments on rodent models of depression are mainly performed in males based on the assumption that the estrous cycles in females may affect the behavioral outcome and cause an increase in the intrinsic variability compared to males. Still, the inclusion of female rodents in the behavioral analysis is mandatory to establish the origin of sex bias in depression. Here, we investigated the baseline depression-like behaviors in male and female mice of three adolescent wild-type inbred strains, C57BL/6N, DBA/2, and FVB/N, that are typically used as background strains for mouse models of neuropsychiatric disorders. Our experiments, performed at two different developmental stages during adolescence (P22-P26 and P32-P36), revealed strain but no sex differences in a set of depression-related tests, including tail suspension, sucrose preference and forced swim tests. Additionally, the 10-day interval during this sensitive period uncovered a strong impact on the behavioral outcome of C57BL/6N and FVB/N mice, highlighting a significant effect of maturation on behavioral patterns. Since anxiety-related behavioral tests are often performed together with depression tests in mouse models of neuropsychiatric disorders, we extended our study and included hyponeophagia as an anxiety test. Consistent with a previous study revealing sex differences in other anxiety tests in adolescent mice, male and females mice behaved differently in the hyponeophagia test at P27. Our study gives insight into the behavioral experiments assessing depression and stresses the importance of considering strain, age and sex when evaluating neuropsychiatric-like traits in rodent models.

Keywords: depression; forced swim test; hyponeophagia test; sex differences; sucrose preference test; tail suspension test.

FIGURE 1

Baseline depression-like behaviors in P22–26 mouse cohort. (A) In the tail suspension test, C57BL/6N mice showed a significant increase in the baseline immobility duration compared to both DBA/2 and FVB/N mice. Additionally, FVB/N mice showed a significant increase in the latency to first immobility compared to both C57BL/6N and DBA/2 mice. (B) In the sucrose preference test, DBA/2 mice showed a lower sucrose preference index than both C57BL/6N and FVB/N mice. (C) In the forced swim test, FVB/N mice showed lower immobility duration and increased latency to first immobility than C57BL/6N and DBA/2 mice. C57BL/6N showed a decreased total traveled distance compared to DBA/2 and FVB/N mice. In (A–C), no sex difference was revealed within any of the aforementioned strains. Blue and red dots represent males and females, respectively. Two-way ANOVA followed by Tukey post hoc test,*p ≤ 0.05, **p ≤ 0.01, and ***p ≤ 0.001. Error bars indicate the standard error of the mean (SEM).

FIGURE 2

Baseline depression-like behaviors in P32–36 mouse cohort. (A) In the tail suspension test, C57BL/6N mice showed a significant increase in the baseline immobility duration compared to both DBA/2 and FVB/N mice. C57BL/6N mice showed a significantly decreased latency to first immobility compared to DBA/2 and borderline decreased latency compared to FVB/N mice. (B) In the sucrose preference test, DBA/2 mice showed an increased baseline anhedonia-like behaviors by having a significantly decreased sucrose preference index compared to both C57BL/6N and FVB/N mice. Moreover, FVB/N mice showed an increased sucrose preference index compared to C57BL/6N mice. (C) In the forced swim test, C57BL/6N mice showed an increased immobility duration, a decreased latency to first immobility and a decreased total traveled distance compared to both DBA/2 and FVB/N mice. In (A–C), no sex difference was revealed within any of the aforementioned strains. Blue and red dots represent males and females, respectively. Two-way ANOVA followed by Tukey post hoc test, **p ≤ 0.01, ***p ≤ 0.001. Error bars indicate the standard error of the mean (SEM).

FIGURE 3

Comparison between the P22–26 and P32–36 mouse cohorts within C57BL/6N, DBA/2, and FVB/N strains. (A) In the tail suspension test, P32 C57BL/6N and P32 FVB/N mice showed a significantly increased immobility duration and decreased latency to first immobility compared to P22 C57BL/6N and P22 FVB/N mice, respectively. (B) In the sucrose preference test, FVB/N mice showed an increased sucrose preference index in older compared to younger mice with no effect of age on C57BL/6N and DBA/2 strains. (C) In the forced swim test, P36 C57BL/6N and P36 FVB/N mice showed a significantly increased immobility duration compared to P26 C57BL/6N and P26 FVB/N mice, respectively. For the latency to the first immobility, only P36 FVB/N mice showed a significantly decreased latency to the first immobility compared to P26 mice. For the total traveled distance, all three strains showed a decreased traveled distance in older compared to a younger age. Two-way ANOVA followed by Tukey post hoc test,*p ≤ 0.05, **p ≤ 0.01, and ***p ≤ 0.001. Error bars indicate the standard error of the mean (SEM).

FIGURE 4

Anxiety-related hyponeophagia test at P27 and P37. Female C57BL/6N and DBA/2 mice showed an increased latency to drink the diluted milk compared to male mice in both trials 1 and 2 at P27 (A) but not at P37 (B). At both P27 and P37, no strain difference was found in the latency to drink the diluted milk. The green asterisk indicates a significant difference between males and females. Two-way ANOVA followed by Tukey post hoc test,*p ≤ 0.05, **p ≤ 0.01. Blue and red dots represent males and females, respectively. Error bars indicate the standard error of the mean (SEM).