Dose-dependent modulation of apoptotic processes by fluoxetine in maturing neuronal cells: an in vitro study.

OBJECTIVES: Recent studies indicate that the selective serotonin reuptake inhibitor (SSRI) fluoxetine is not solely effective by the instant inhibition of the serotonin transporter (SERT) but also by its influence on mitotic and/or apoptotic processes. METHODS: To investigate the effects of the compound in vitro, we treated neurons from different brain areas with increasing concentrations of fluoxetine. Additionally, human embryonic kidney (HEK-293) cells and HEK-293 cells stably expressing the SERT were used. Cell viability was quantified by MTT-assay and apoptosis via fluorescence-activated cell-sorting analyses. Fluoxetine's effect on the γ-aminobutyric acid (GABA) receptor was electrophysiologically investigated to test the hypothesis if a GABA-mimetic effect exists that might lead - additionally to the well-known N-methyl-D-aspartate (NMDA)-antagonism - to increased apoptosis in immature neurons. RESULTS: In hippocampal, cortical, and both types of HEK-293 cells, viability decreased and apoptosis increased in a dose-dependent manner (0.5-75 µM). In contrast, in mesencephalic and striatal cells the viability was unchanged or even slightly stimulated up to 20 µM fluoxetine. An anti-apoptotic effect of concentrations below 10 µM was observed in these cells. The GABA(A) receptor was directly activated by fluoxetine. CONCLUSIONS: We conclude that fluoxetine affects apoptotic processes independently from SERT expression. Since especially the combined GABA-mimetic and NMDA-antagonistic effects increase apoptosis in developing neuronal cells, whereas both effects are neuroprotective in adult neurons we hypothesise that these mechanisms explain the discrepancy of in vitro and in vivo studies.