Gene expression of two forms of the brain protein synaptotagmin were studied in wild-type and serotonin transporter knockout (KO) mice after receiving 10 mg/kg MDMA. This research seems to follow from detection of altered expression in 11 different genes after MDMA, and after previously determining that 10 mg/kg produced hyperactivity in mice without producing toxicity or death. Analyses of coding DNA (cDNA) were used to detect changes in synaptotagmin gene expression, followed by real time polymerase chain reaction (RT-PCR) and Western blot immunoassay. Most analyses were done in midbrain and frontal areas removed from mice killed 2 hours after MDMA administration; however, time course of alterations in synaptotagmin gene expression were carried out in mice killed up to 7 days after MDMA. Expression for synaptotagmin IV (sytIV) decreased 65% of levels in frontal cortex and midbrain in saline-treated mice 2 hours after MDMA, whereas expression of these genes was not significantly different in serotonin transporter KO mice treated with MDMA. In contrast, MDMA produced an increase in synaptotagmin I (syt1) expression in the midbrain only, with expression four times that recorded after saline. These findings were confirmed using staining with anti-sytIV and anti-sytI antibodies and with Western blot immunoassay. Concentration of sytI protein peaked 1 day after MDMA and declined 2 and 7 days later. Since serotonin transporter KO mice did not show changes in either sytIV or sytI expression after MDMA, it would appear that changes in expression relate to MDMA activity on the serotonin transporter or serotonin neuron vesicles. These proteins may be involved in changes in synaptic trafficking by binding to vesicle proteins in a calcium-dependent manner. An increase in midbrain sytI activity is interpreted as an indicator of direct activity by MDMA in serotonin neurons in the midbrain, whereas decreased sytIV activity is interpreted as an indicator of a global effect of MDMA on widely distributed serotonin terminals throughout the frontal cortex. The authors cite previous research indicating that cocaine, but not amphetamine, produces changes in synaptotagmin gene expression, suggesting that changes in synaptotagmins might be associated with monoamine release or uptake inhibition. The authors propose that changes in synaptotagmin gene expression might relate to MDMA neurotoxicity. However, findings relating to MDMA neurotoxicity made in mice should be applied with care to rats and humans, since differences in the form of MDMA neurotoxicity indicate interspecies differences in MDMA effects or metabolism.