Fluoxetine Disrupts Ovarian Serotonin Signaling and Oocyte Competence in Mice

<b>Background:</b> Selective serotonin reuptake inhibitors (SSRIs) are widely prescribed, yet their direct impact on ovarian function remains poorly understood. While serotonin signaling is known to occur within the ovarian follicle, the specific molecular consequences of its disruption by SSRIs are unclear. This study aimed to elucidate the direct, intra-ovarian mechanisms by which fluoxetine, a common SSRI, affects follicular development and oocyte competence. <b>Methods:</b> We administered fluoxetine (20 mg/kg) or vehicle daily for seven days to both prepubertal and adult female mice to model short-term therapeutic exposure. <b>Results:</b> Fluoxetine treatment successfully blocked peripheral serotonin uptake, reducing serum levels by over 90%. Crucially, this occurred without altering circulating levels of estradiol, FSH, or LH and without disrupting the estrous cycle, indicating a mechanism independent of the central hypothalamic-pituitary-gonadal axis. Instead, we pinpoint a direct ovarian effect: fluoxetine inhibited serotonin transport activity in oocytes and significantly downregulated the expression of the pivotal oocyte-derived growth factor <i>Gdf9</i>. This was accompanied by reduced expression of genes crucial for granulosa cell function (<i>Lhr</i>, <i>Fshr</i>) and steroidogenesis (<i>Cyp19a1</i>). Functionally, these molecular changes manifested as a decline in oocyte quality and a significant reduction in ovulation rates in adult mice. Notably, these detrimental effects were more pronounced in prepubertal mice, indicating a heightened vulnerability during early follicular development. <b>Conclusions:</b> Our findings reveal a direct, intra-ovarian mechanism of fluoxetine-induced disruption. By inhibiting oocyte serotonin transport and downregulating GDF9, fluoxetine impairs critical oocyte-granulosa cell communication, thereby compromising oocyte competence and reducing fertility outcomes. This work identifies follicular development as a critical window of susceptibility to SSRI exposure, holding significant clinical implications for reproductive-aged and adolescent populations.