Effect of fenfluramine on 5-hydroxytryptamine uptake and release by rat blood platelets

1. (+)-Flenfluramine reduces the central stores of 5-hydroxytryptamine (5-HT) by a poorly understood mechanism. 2. Rat blood platelets have been used in this study as a simple model for serotoninergic nerve endings. 3. (+)-Fenfluramine shows a dual effect: it inhibits the uptake of (14C)-5-HT by platelets and it releases newly absorbed (14C)-5-HT from platelets. 4. The inhibition of (14C)-5-HT uptake induced by (+)-fenfluramine appears very rapidly, is concentration-dependent and seems not to be competitive. (+)-Fenfluramine is ten times less effective than chloroimipramine but tem times more effective than (+)-amphetamine; (+)-fenfluramine is more active than its (-)isomer or its metabolite norfenfluramine ((+)- or (-)-form). 5. The release of (14C)-5-HT from platelets induced by (+)-fenfluramine is concentration-dependent but increases wtih increased incubation time. Both chloroimipramine and (+)-amphetamine are in comparison very poor release inducers; (+)-fenfluramine is more active than its (-)-isomer or its metabolites. 6. The effect on (14C)-5-HT uptake exerted by (+)-fenfluramine and chloroimipramine in vitro could not be observed in vivo. 7. The observed effect on fenfluramine on the uptake and release of 5-HT may explain the lowering action of fenfluramine on the brain 5-HT level, an effect considered of importance for the anoretic effect on this drug.