EVIDENCE OF AN EXOCYTOTIC-LIKE RELEASE OF [H-3] 5-HYDROXYTRYPTAMINE INDUCED BY D-FENFLURAMINE IN RAT HIPPOCAMPAL SYNAPTOSOMES

The monoamine releasing activity of d-fenfluramine was investigated with an in vitro model consisting of synaptosomes preloaded with the H-3-neurotransmitter and extensively washed in a superfusion apparatus before a 3-min exposure to d-fenfluramine. With this model, the drug-induced release is real and is not confused by inhibition of reuptake by the drug. d-Fenfluramine (0.5 muM) induced only [H-3]5-hydroxytryptamine ([H-3]5-HT) release from hippocampal synaptosomes whereas 10 muM also induced some overflow from hippocampal synaptosomes preloaded with [H-3]noradrenaline or from striatal synaptosomes preloaded with [H-3]dopamine, although the overflow was much lower than from 5-HTergic synaptosomes. We then focused on the [H-3]5-HT release induced by 0.5 muM d-fenfluramine, which was previously shown to be Ca2+ dependent. The same finding was confirmed in the present study with other experimental protocols, indicating the requirement for extracellular Ca2+ ions. By measuring [H-3]5-HT uptake into rat hippocampal synaptosomes we confirmed that Ca2+-ions are not required for the function of the 5-HT uptake carrier or for its interaction with d-fenfluramine. d-Fenfluramine-induced [H-3]5-HT release was not altered by 1 muM nitrendipine (blocking the L-type Ca2+channels) but was slightly decreased (20%) by 0.5 muM omega-conotoxin (blocking the N-type Ca2+ channels). It was also inhibited by 0.5 muM clonidine, interacting with alpha2-adrenergic heteroreceptors, and by 10 nM tetanus toxin, known to affect the exocytosis of different neurotransmitters including 5-HT. These compounds had very similar effects on the Ca2+-dependent, exocytotic release of [H-3]5-HT induced by depolarization, i.e. by 15 mM K+. These data suggest that d-fenfluramine, after its carrier-mediated entry into the synaptosomes, induces an influx of extracellular Ca2+, triggering an exocytotic-like release of 5-HT.