High concentrations of MDMA ('ecstasy') and its metabolite MDA inhibit calcium influx and depolarization-evoked vesicular dopamine release in PC12 cells

Abstract

The popular synthetic drug of abuse 3,4-methylenedioxymethampetamine (MDMA) and its metabolite 3,4-methylenedioxyamphetamine (MDA) act mainly on the serotonergic system, though they also increase the amount of extracellular dopamine (DA) in the brain, presumably via reversal of the membrane dopamine transporter (DAT). As the involvement of exocytotic DA release is debated, we investigated if these drugs alter the intracellular calcium concentration ([Ca(2+)](i)) and subsequent DA exocytosis in single PC12 cells using respectively Fura-2 imaging and amperometry. MDMA and MDA did not change basal [Ca(2+)](i) or exocytosis, but inhibited depolarization-evoked increases in [Ca(2+)](i) and exocytosis following 15 min exposure to high concentrations of drugs (1 mM). Surprisingly, MDA was more potent in inhibiting exocytosis than MDMA and already inhibited exocytosis at concentrations that did not inhibit depolarization-evoked Ca(2+) influx (10-100 μM). Without 15 min pre-exposure, both drugs failed to inhibit depolarization-evoked Ca(2+) influx. These results indicate that at high concentrations both MDMA and MDA inhibit exocytosis via indirect inhibition of Ca(2+) influx, whereas at lower concentrations MDA may also reduce vesicle cycling. Our data suggest that the DAT-independent increase in extracellular DA in vivo is not due to direct stimulation of exocytosis, but rather to effects of these drugs on other neurotransmitter systems that innervate the dopaminergic system.