Simulation of amphetamine-induced serotonin release and partial release. Top: Schematic rendering of the steps involved in facilitated exchange diffusion. The releaser (R) binds to the outward-facing conformation, leading to isomerization of the transporter to the inward-facing conformation. Upon dissociation of the releaser, the internal substrate binds and can be released after the return of the transporter to the outward-facing conformation. Gray, red, and yellow reactions illustrate the mechanisms of partial release. Gray: A partial releaser may bind in two distinct modes, either as inhibitor (I, in gray) or releaser (R, in blue; see B). Red versus green: A partial releaser may display high affinity to the transporter; this results in a longer dwell time of the releaser in the binding site and thus less exchange between releaser and intracellular monoamine (see C). Yellow: Experimental manipulations may reduce the transition rate between the substrate-loaded outward- and inward-facing conformations; this results in reduced release (see G and H). (A–C) Simulation of 5-HT release by increasing concentrations of p-chloroamphetamine (A), MDDMA (B), or PAL-1045 (C) at different intracellular Na⁺ concentrations. The solid lines were generated by fitting the synthetic data to a rectangular hyperbola to extract EC₅₀ values and maximum release (V_max). (D and E) EC₅₀ values (D) and V_max values (E) of p-chloroamphetamine, MDDMA, and PAL-1045 in inducing 5-HT release as a function of the intracellular Na⁺ concentration. (F) Voltage dependence of 5-HT release upon application of 3 µM p-chloroamphetamine (approximately the EC₅₀). The release simulations were performed as in A–E, but at voltages ranging from −150 to 150 mV. (G) Simulation of 5-HT release by p-chloroamphetamine via the N-terminal mutant hSERT-ΔN32. Simulation conditions were the same as in A–F. (H) Turnover rate of N-terminal mutant hSERT-ΔN32 compared with wild-type hSERT. The time course of recovery after substrate application was modeled as the time course of return to ToClNa after 5-HT application.