Evidence of an exocytotic-like release of [3H]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 ³H-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 μM) induced only [³H]5-hydroxytryptamine ([³H]5-HT) release from hippocampal synaptosomes whereas 10 μM also induced some overflow from hippocampal synaptosomes preloaded with [³H]noradrenaline or from striatal synaptosomes preloaded with [³H]dopamine, although the overflow was much lower than from 5-HTergic synaptosomes. We then focused on the [³H]5-HT release induced by 0.5 μM 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 [³H]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 [³H]5-HT release was not altered by 1 μM nitrendipine (blocking the L-type Ca²⁺ channels) but was slightly decreased (20%)_by 0.5 μM ω-conotoxin (blocking the N-type Ca²⁺ channels). It was also inhibited by 0.5 μM clonidine, interacting with α₂-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 [³H]5-HT induced by depolarization, i.e. by 15 mMK⁺. 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.