MDMA and TAAR1‐mediated RhoA Activation in Serotonin Neurons

Abstract

We have recently described the biochemical cascade through which the psychostimulant amphetamine leads to internalization of the dopamine and norepinephrine transporters, (DAT and NET). Amphetamine is a relatively high affinity substrate for DAT and NET and they both serve as efficient conduits for amphetamine entry into the cell. Once inside the cell, amphetamine activates the trace amine associated receptor 1 (TAAR1), a GPCR that we have shown couples to both GS and G13 signaling pathways. TAAR1 signaling through G13 increases the activity of RhoA, a small GTPase that regulates trafficking of the catecholamine carriers and increases their internalization. We wanted to investigate if similar G‐protein coupled signaling events also occur in serotonergic neurons. Although amphetamine does not have a particularly high affinity for the serotonin transporter (SERT), other TAAR1 ligands do, including 3,4‐methylenedioxymethamphetamine (MDMA).We developed primary cultures from the raphe nuclei of E15 mice. 3H‐serotonin uptake mediated by these cultures was largely citalopram sensitive, indicating SERT‐mediated serotonin transport. Pre‐treatment of these cultures with MDMA resulted in a decrease of SERT activity that could be blocked by an inhibitor of RhoA. Similarly, by total internal reflection fluorescence microscopy in HEK293 cells we found internalization of GFP‐tagged SERT in response to MDMA was blocked by a RhoA inhibitor. In HEK293 cells in which the TAAR1 gene has been deleted, MDMA did not trigger SERT internalization. In noradrenergic neurons cultured from the locus coeruleus or dopamine neurons from the midbrain, only higher doses of MDMA pretreatment could decrease transporter activity at the cell surface, likely reflecting the differences in affinity of the drug for the different monoamine transporters.These data indicate that pharmacological agents can regulate TAAR1 signaling in serotonergic, as well as catecholaminergic neurons and that the selectivity of TAAR1 agonists depends not only on affinity for the GPCR but also on their transport efficiency. Moreover, these findings suggest that the activation of TAAR1 in serotonin neurons and the subsequent internalization of surface SERT likely contribute to the modulation of serotonin signaling observed in response to MDMA.Support or Funding InformationThis work was supported by Intramural Research Program at the NIH, NIMH (ZIAMH002946).