IN-VITRO STUDIES ON THE MECHANISM BY WHICH (-NORFENFLURAMINE INDUCES SEROTONIN AND DOPAMINE RELEASE FROM THE VESICULAR STORAGE POOL())

(+)-Norfenfluramine is the main metabolite of the serotoninergic anore ctic agent (+)-fenfluramine. Both compounds inhibit 5-HT reuptake and stimulate its release, although they induce release from different poo ls, with (+) norfenfluramine acting primarily on the cytoplasmic pool. Moreover, (+)-norfenfluramine was more potent than the parent drug in inducing dopamine release. In order to investigate whether (+)-norfen fluramine induces a Ca2+-dependent vesicular release, like some amphet amine derivatives, in the present study we preloaded synaptosomes with the [H-3]neurotransmitter ([H-3]5-HT or [H-3]dopamine), superfused (w ashed) them for 47 min in the absence of pargyline and then exposed th em to the releasing stimulus. With this protocol, the cytoplasmic pool should be absent and the [H-3] neurotransmitter should mainly be stor ed in synaptic vesicles, where (+)-norfenfluramine should act to induc e release. This was confirmed by a significant decrease of (+)-norfenf luramine-induced [H-3]5 HT and [H-3]dopamine release after reserpine p retreatment. The dose-response curves of (+)-norfenfluramine-induced [ H-3]5-HT release were superimposable in hippocampus and hypothalamus, and also superimposable on the curve for (+)-fenfluramine-induced [H-3 ]5-HT release; the dopamine releasing potency of (+)-norfenfluramine i n the striatum was more than ten times lower. The [H-3]5-HT release in duced by (+)-norfenfluramine was partly (about 50%) but significantly Ca2+-dependent, and it was also markedly (68%) inhibited by Cd2+, a no n-specific blocker of voltage-dependent Ca2+ channels, suggesting that the Ca2+-dependent release is mediated by entry of Ca2+ into the syna ptosomes through these channels. The [H-3]dopamine release induced by 5 mu M (+)-norfenfluramine was completely Ca2+-independent whereas at higher concentrations (10 and 20 mu M) it was only slightly (20%) Ca2-dependent. We have no clear explanation why (+)-norfenfluramine has t hese different effects on serotoninergic and dopaminergic synaptosomes .