Long-Term Fluoxetine Treatment Alters Fecal Microbiota and Induces Systemic Disruption in Sprague-Dawley Rats

Major depressive disorder pathophysiology involves complex gut-brain axis interactions, yet the systemic non-neurochemical impacts of selective serotonin reuptake inhibitors re-main poorly defined. This study evaluated the multidimensional effects of chronic fluoxe-tine administration in Sprague-Dawley rats. Male rats received fluoxetine (10 mg/kg/day) or water for 60 days, followed by behavioral phenotyping, fecal microbiological quantifi-cation, serum biochemistry, complete blood count analysis, and immunohistochemical assessment of the immediate-early gene product c-Fos (cellular proto-oncogene protein) in the striatum. Fluoxetine treatment significantly reduced immobility time in the forced swim test, whereas object recognition memory deteriorated without alterations in anxie-ty-like behavior. Pharmacological intervention markedly depleted fecal bacterial popula-tions and significantly decreased serum vitamin D concentrations. Hematologically, treated animals exhibited significant reductions in total leukocytes, red blood cells, and platelets, alongside elevated hematocrit and mean corpuscular volume. Concurrently, c-Fos-positive neuronal activation increased significantly within the striatal nucleus ac-cumbens. In conclusion, chronic fluoxetine exposure induces profound systemic disrup-tion, altering gut microbial homeostasis, depleting vitamin D, and modulating peripheral blood profiles while enhancing central striatal neuronal plasticity. These findings under-score the necessity of monitoring gut-brain axis integrity during prolonged antidepressant therapy.