Biphasic mechanisms of amphetamine action at the dopamine terminal

In light of recent studies suggesting that amphetamine (AMPH) increases electrically evoked dopamine release ([DA]_o), we examined discrepancies between these findings and literature that has demonstrated AMPH-induced decreases in [DA]_o. The current study has expanded the inventory of AMPH actions by defining two separate mechanisms of AMPH effects on [DA]_o at high and low doses, one dopamine transporter (DAT) independent and one DAT dependent, respectively. AMPH concentrations were measured via microdialysis in rat nucleus accumbens after intraperitoneal injections of 1 and 10 mg/kg and yielded values of 10 and 200 nM, respectively. Subsequently, voltammetry in brain slices was used to examine the effects of low (10 nM), moderate (100 nM), and high (10 μM) concentrations of AMPH across a range of frequency stimulations (one pulse; five pulses, 20 Hz; 24 pulses, 60 Hz). We discovered biphasic, concentration-dependent effects in WT mice, in which AMPH increased [DA]_o at low concentrations and decreased [DA]_o at high concentrations across all stimulation types. However, in slices from DAT-KO mice, [DA]_o was decreased by all concentrations of AMPH, demonstrating that AMPH-induced increases in [DA]_o are DAT dependent, whereas the decreases at high concentrations are DAT independent. We propose that low AMPH concentrations are insufficient to disrupt vesicular sequestration, and therefore AMPH acts solely as a DAT inhibitor to increase [DA]_o. When AMPH concentrations are high, the added mechanism of vesicular depletion leads to reduced [DA]_o. The biphasic mechanisms observed here confirm and extend the traditional actions of AMPH, but do not support mechanisms involving increased exocytotic release.