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 microM) induced only [3H]5-hydroxytryptamine ([3H]5-HT) release from hippocampal synaptosomes whereas 10 microM 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 microM 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 Ca(2+)-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 microM nitrendipine (blocking the L-type Ca2+ channels) but was slightly decreased (20%) by 0.5 microM omega-conotoxin (blocking the N-type Ca2+ channels). It was also inhibited by 0.5 microM clonidine, interacting with alpha 2-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 Ca(2+)-dependent, exocytotic release of [3H]5-HT induced by depolarization, i.e. by 15 mM K+.(ABSTRACT TRUNCATED AT 250 WORDS)
Gobbi
M
M
Istituto di Ricerche Farmacologiche Mario Negri, Milan, Italy.
Frittoli
E
E
Uslenghi
A
A
Mennini
T
T
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D004298:Dopamine / Q000494:pharmacology; D005277:Fenfluramine / Q000008:administration & dosage / Q000494:pharmacology*; D006624:Hippocampus / Q000187:drug effects* / Q000378:metabolism; D066298:In Vitro Techniques; D008297:Male; D009638:Norepinephrine / Q000494:pharmacology; D051381:Rats; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000378:metabolism; D012701:Serotonin / Q000378:metabolism*; D013574:Synaptosomes / Q000187:drug effects* / Q000378:metabolism; D013744:Tetanus Toxin / Q000494:pharmacology
D002121:Calcium Channel Blockers; D011942:Receptors, Adrenergic, alpha; D013744:Tetanus Toxin; D005277:Fenfluramine; D012701:Serotonin; D002118:Calcium; D004298:Dopamine; D009638:Norepinephrine