EFFECTS OF SHORT-TERM AND LONG-TERM ADMINISTRATION OF FLUOXETINE ON THE MONOAMINE CONTENT OF RAT-BRAIN

The effects of repeated doses of fluoxetine over time and dose-responses of the content of indoles and catecholamines and metabolism, were examined in rats in relation to the concentrations of the parent compound and its active metabolite norfluxetine in brain. Brains were removed for assays of the regional content of monoamines and concentrations of drugs 24 hr after the last dose on days 1, 7 and 21 of a twice-daily schedule of fluoxetine (15 mg/kg, i.p.). Measurements were also taken 1 week after the last dose (7.5 and 15 mg/kg, b.i.d.) of the 21-day regimen. On day 1 fluoxetine did not change the content of serotonin (5-HT) but reduced the concentrations of 5-hydroxyindolacetic acid (5-HIAA) in the hippocampus and cortex, compatible with the action of a blocker of the uptake of 5-HT. Continued injections of fluoxetine, however, significant reduced 5-HT in the brain of rat, the depletion being significant on days 7 and 21 in the hippocampus and cortex, respectively. The content of indoles remained significantly decreased for at least a week after the last dose of fluoxetine in the 21-day regimen, although the concentrations of 5-HIAA (but not 5-HT) totally recovered at the smaller dose (7.5 mg/kg) in all regions of the brain (cortex, hippocampus and striatum). In spite of slight changes in the concentrations and metabolism of dopamine (DA) in the striatum, 24 hr after the last dose (15 mg/kg), treatment with drug had no significant long-term effects on the content of catecholamines in these regions of the brain. There was marked accumulation of the drug and its metabolite in plasma and brain. The monoamine-depleting effect of fluoxetine therefore depended on prolonged exposure of the brain of the rat to large concentrations of the parent compound and its active metabolite, possibly due to saturation of the clearance mechanisms with chronic administration. Although further studies are required to clarify the mechanism(s) of the monoamine-depleting effect of fluoxetine, these results are consistent with the theory that mechanisms other than uptake of 5-HT, are involved in the neurochemical action of repeated administration of fluoxetine in rats.