Opposite effect of short- versus long-term administration of fluoxetine on the concentrations of neuroactive steroids in rat plasma and brain

Rationale: Recent preclinical and clinical studies have shown that selective serotonin re-uptake inhibitors modulate neurosteroid synthesis in an opposite manner. Objectives: The action of long-term administration of fluoxetine was investigated on the peripheral and central concentrations of 3 alpha ,5 alpha -tetrahydroprogesterone (3 alpha ,5 alpha -TH PROG) and 3 alpha ,5 alpha -tetrahydrodeoxycorticosterone (of 3 alpha ,5 alpha -TH DOC). progesterone, and pregnenolone in rats. We also investigated the effect of chronic treatment with fluoxetine on the foot-shock stress-induced increase in the plasma and brain concentrations of these steroids. Methods: Fluoxetine was administered acutely (20 mg/kg) or chronically (10 mg/kg, once daily for 15 days). Steroids were extracted from plasma and brain. separated and purified by means of high-performance liquid chromatography, and quantified by means of radioimmunoassay. Results: A single dose of fluoxetine (20 mg/kg, i.p.) induced in 20 min significant increases in the cerebral cortical and plasma concentrations of 3 alpha ,5 alpha -TH PROG (+96% and +13%, respectively), 3 alpha ,5 alpha -TH DOC (+129 and +31%, respectively), progesterone (+111 and +58%, respectively), and pregnenolone (+151 and +59%, respectively). In addition. the plasma concentration of corticosterone was also significantly increased (+24%) after acute administration of fluoxetine. In contrast. long-term administration of fluoxetine reduced the basal concentrations of these various steroids (ranging from -22 to -43%), measured 48 h after the last drug injection, in both brain and plasma. A challenge injection of fluoxetine (20 mg/kg, i.p.), however, was still able to increase the concentrations of steroids in both the brain and plasma of rats chronically treated with this drug. Acute foot-shock stress increased the cortical and plasma concentrations of steroids in rats chronically treated with fluoxetine to extents similar to those apparent in control rats. Conclusions: A repetitive increase in the brain concentrations of neuroactive steroids may contribute to the therapeutic action of fluoxetine