| Lifetime prevalence of depression in females is two times higher than in males. In females, depression is more common during periods of fluctuating estrogen (E) levels. In vivo data from our laboratory have demonstrated the involvement of E in the regulation of expression of the serotonin transporter (SERT) mRNA. Since dysfunction of the SERT system is involved in the pathogenesis of depression, we focused on investigating the effects of E on SERT. We hypothesized that E regulates SERT through both genomic and non-genomic mechanisms and carried out experiments to investigate effects of E on SERT activity, as well as intracellular signaling pathways involved in E regulation of SERT. Whole animal models are too complex to interpret such mechanisms. Thus, we chose RN46A cells (a line derived from rat embryonal serotonergic neurons) as an in vitro model system to address the effects of E on SERT.; In the present studies, we have shown that 10-9M of E rapidly (within 15 minutes) regulates SERT activity (measured by [ 3H]5-HT uptake). E rapidly increases intracellular Ca2+ levels (measured by intracellular Ca2+ imaging of Fura-2 loaded cells) with a time course and dose consistent with E regulation of serotonin (5-HT) uptake. Additionally, the Ca2+ response in RN46A cells was 17beta-estradiol specific, as other steroids tested did not alter intracellular Ca2+. Estrogen receptor (ER) antagonists blocked the rapid E effects on intracellular Ca2+ levels, indicating that the effects are mediated through the ER. We also found that chronic E treatment resulted in decreased SERT activity and that these effects were mediated through ER. Additionally, the withdrawal of E after chronic E treatment increased 5-HT uptake.; In summary, using RN46A cells as a cell model system of serotonergic neurons, we have shown that E both rapidly and chronically inhibits the serotonin uptake, while withdrawal from chronic E increases the SERT activity. These findings suggest that ambient E levels may play an important role in modulation of mood and other behaviors that are influenced by synaptic 5-HT. |
