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 anorectic agent (+)-fenfluramine. Both compounds inhibit 5-HT reuptake and stimulate its release, although they induce release from different pools, 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 (+)-norfenfluramine induces a Ca2+-dependent vesicular release, like some amphetamine derivatives, in the present study we preloaded synaptosomes with the [3H]neurotransmitter ([3H]5-HT or [3H]dopamine), superfused (washed) them for 47 min in the absence of pargyline and then exposed them to the releasing stimulus. With this protocol, the cytoplasmic pool should be absent and the [3H]neurotransmitter should mainly be stored in synaptic vesicles, where (+)-norfenfluramine should act to induce release. This was confirmed by a significant decrease of (+)-norfenfluramine-induced [3H]5-HT and [3H]dopamine release after reserpine pretreatment. The dose-response curves of (+)-norfenfluramine-induced [3H]5-HT release were superimposable in hippocampus and hypothalamus, and also superimposable on the curve for (+)-fenfluramine-induced [3H]5-HT release; the dopamine releasing potency of (+)-norfenfluramine in the striatum was more than ten times lower. The [3H]5-HT release induced by (+)-norfenfluramine was partly (about 50%) but significantly Ca2+-dependent, and it was also markedly (68%) inhibited by Cd2+, a non-specific blocker of voltage-dependent Ca2+ channels, suggesting that the Ca2+-dependent release is mediated by entry of Ca2+ into the synaptosomes through these channels. The [3H]dopamine release induced by 5 microM (+)-norfenfluramine was completely Ca2+-independent whereas at higher concentrations (10 and 20 microM) it was only slightly (20%) Ca2+-dependent. We have no clear explanation why (+)-norfenfluramine has these different effects on serotoninergic and dopaminergic synaptosomes.
Gobbi
M
M
Istituto di Ricerche Farmacologie Mario Negri, Milan, Italy.
Parazzoli
A
A
Mennini
T
T
D018759:Adrenergic Uptake Inhibitors / Q000494:pharmacology; D000818:Animals; D001921:Brain / Q000187:drug effects* / Q000378:metabolism; D002118:Calcium / Q000494:pharmacology; D003029:Coagulants / Q000494:pharmacology; D004298:Dopamine / Q000378:metabolism*; D004347:Drug Interactions; D008297:Male; D009642:Norfenfluramine / Q000494:pharmacology*; D051381:Rats; D012110:Reserpine / Q000494:pharmacology; D012701:Serotonin / Q000378:metabolism*; D017367:Selective Serotonin Reuptake Inhibitors / Q000494:pharmacology*; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism
D018759:Adrenergic Uptake Inhibitors; D002118:Calcium; D003029:Coagulants; D004298:Dopamine; D009642:Norfenfluramine; D012110:Reserpine; D012701:Serotonin; D017367:Selective Serotonin Reuptake Inhibitors