Estrogen receptor β deficiency impairs BDNF-5-HT signaling in the hippocampus of female brain: A possible mechanism for menopausal depression.

Depression currently affects 350 million people worldwide and 19 million Americans each year. Women are 2.5 times more likely to experience major depression than men, with some women appearing to be at a heightened risk during the menopausal transition. Estrogen signaling has been implicated in the pathophysiology of mood disorders including depression; however, the underlying mechanisms are poorly understood. In this study, the role of estrogen receptor (ER) subtypes, ERα and ERβ, in the regulation of brain-derived neurotrophic factor (BDNF) and serotonin (5-HT) signaling was investigated; two pathways that have been hypothesized to be interrelated in the etiology of depression. The analyses in ERα and ERβ mouse models demonstrated that BDNF was significantly downregulated in ERβ but not ERα mice, and the ERβ-mediated effect was brain-region specific. A 40% reduction in BDNF protein expression was found in the hippocampus of ERβ mice; in contrast, the changes in BDNF were at a much smaller magnitude and insignificant in the cortex and hypothalamus. Further analyses in primary hippocampal neurons indicated that ERβ agonism significantly enhanced BDNF/TrkB signaling and the downtream cascades involved in synaptic plasticity. Subsequent study in ERβ mutant rat models demonstrated that disruption of ERβ was associated with a significantly elevated level of 5-HT but not 5-HT in rat hippocampus, indicating ERβ negatively regulates 5-HT. Additional analyses in primary neuronal cultures revealed a significant association between BDNF and 5-HT pathways, and the data showed that TrkB activation downregulated 5-HT whereas activation of 5-HT had no effect on BDNF, suggesting that BDNF/TrkB is an upstream regulator of the 5-HT pathway. Collectively, these findings implicate that the disruption in estrogen homeostasis during menopause leads to dysregulation of BDNF-5-HT signaling and weakened synaptic plasticity, which together predispose the brain to a vulnerable state for depression. Timely intervention with an ERβ-targeted modulator could potentially attenuate this susceptibility and reduce the risk or ameliorate the clinical manifestation of this brain disorder.