Further evidence of Ca(2+)-dependent, exocytotic-like serotonin release induced by D-fenfluramine

The effect of polymyxin B and KN-62 on the [3H]5-HT release induced by depolarization and by 0.5 microM D-fenfluramine (dF) from superfused rat hippocampal synaptosomes was examined. Polymyxin B and KN-62 were initially characterized as inhibitors, respectively, of calmodulin (CaM) and Ca2+/CaM-dependent protein kinase II (Ca/CaM-KII), although both compounds were subsequently described as inhibitors of the depolarization-induced Ca2+ influx through voltage-operated Ca2+ channels, at concentrations similar to those interacting with the CaM systems. Three micromolar KN-62 significantly inhibited the dF- and the depolarization-induced [3H]5-HT release, by 25% and 33%. Polymyxin B, tested at concentrations from 30 to 1000 IU ml-1, dose-dependently inhibited both the dF- and the depolarization-induced [3H]5-HT release with similar potency, with complete inhibition at the highest concentration tested. These compounds did not significantly alter 5-HT transporter function. Moreover dF had no direct effect on Ca/CaM-KII activity. These results further support the suggestion that the [3H]5-HT release induced by low concentrations of dF (0.5 microM), previously found to be Ca(2+)-dependent, actually involves a dF-induced Ca2+ influx into the nerve terminal and the subsequent exocytosis.