Variant brain-derived neurotrophic factor (Valine66Methionine) polymorphism contributes to developmental and estrous stage-specific expression of anxiety-like behavior in female mice

Abstract

Background: Most anxiety and depressive disorders are twice as common in women compared with men, and the sex difference in prevalence typically emerges during adolescence. Hormonal changes across the menstrual cycle and during the postpartum and perimenopausal periods are associated with increased risk for anxiety and depression symptoms. In humans and animals, reduced brain-derived neurotrophic factor (BDNF) has been associated with increased expression of affective pathology. Recently, a single nucleotide polymorphism (SNP) in the BDNF gene (BDNF Valine66Methionine [Val66Met]), which reduces BDNF bioavailability, has been identified in humans and associated with a variety of neuropsychiatric disorders. Although BDNF expression can be directly influenced by estrogen and progesterone, the potential impact of the BDNF Val66Met SNP on sensitivity to reproductive hormone changes remains an open question.

Methods: As a predictive model, we used female mice in which the human SNP (BDNF Val66Met) was inserted into the mouse BDNF gene. Using standard behavioral paradigms, we tested the impact of this SNP on age and estrous-cycle-specific expression of anxiety-like behaviors.

Results: Mice homozygous for the BDNF Val66Met SNP begin to exhibit increased anxiety-like behaviors over prepubertal and early adult development, show significant fluctuations in anxiety-like behaviors over the estrous cycle, and, as adults, differ from wild-type mice by showing significant fluctuations in anxiety-like behaviors over the estrous cycle-specifically, more anxiety-like behaviors during the estrus phase.

Conclusions: These findings have implications regarding the potential role of this SNP in contributing to developmental and reproductive hormone-dependent changes in affective disorders in humans.

Figure 1

Histograms depicting measures of anxiety-like or depressive-like behavior in adult (> 6 weeks of age) wild-type (Val/Val) and BDNF Val66Met homozygous (Met/Met) female mice. (A) Met/Met mice (n = 40) spent a significantly lower percentage of their time in the center of the open field relative to Val/Val (n = 29) controls (Student’s t-test; p < 0.005). (B) Met/Met female mice (n = 57) also spent a significantly lower percentage of their time in the open arms of the elevated plus maze relative to Val/Val (n = 68) controls (Student’s t-test; p < 0.005). (C) Met/Met female mice (n = 19) spent a significantly greater percentage of their time immobile in the forced swim task relative to Val/Val (n = 33) control (Student’s t-test; p < 0.04). Error bars represent standard error of the mean (SEM).

Figure 2

Scatterplots of measures of anxiety-like behavior in the elevated plus maze (y-axis) relative to age in postnatal days (x-axis) for wild-type (Val/Val) and BDNF Val66Met homozygous (Met/Met) mice. Shaded areas indicate best-estimates of developmental status based upon previous reports [45, 46]. Trend lines are included along with R² values for correlations.

Figure 3

Histograms of behavior on the elevated plus task and open field task of wild type (Val/Val) and BDNF Val66Met homozygous (Met/Met) mice. Numbers inset in the columns indicate the n for that group. Using analysis of variance (ANOVA) differences in anxiety-like behavior were assessed across the estrous cycle. (A) On the elevated plus maze, Met/Met mice showed significantly elevated levels of anxiety during diestrus and estrus relative to metestrus (# indicates significance p < 0.05). Comparisons were also made between Val/Val and Met/Met mice across the various phases of the estrous cycle. Met/Met mice significantly differed from Val/Val mice primarily during estrus (*). (B) On the open field task, Met/Met mice showed significantly elevated levels of anxiety during estrus relative to metestrus. Met/Met mice also significantly differed from Val/Val mice during the estrus phase of the cycle. (C) Line drawings indicating estimates of changes in circulating estrogen and progesterone levels during the different stages of the estrous cycle. Line drawings were based upon previously published reports in CD-1 mice ([47]). Error bars represent standard error of the mean (SEM).