The reverse operation of Na(+)/Cl(-)-coupled neurotransmitter transporters--why amphetamines take two to tango

Abstract

Sodium-chloride coupled neurotransmitter transporters achieve reuptake of their physiological substrate by exploiting the pre-existing sodium-gradient across the cellular membrane. This terminates the action of previously released substrate in the synaptic cleft. However, a change of the transmembrane ionic gradients or specific binding of some psychostimulant drugs to these proteins, like amphetamine and its derivatives, induce reverse operation of neurotransmitter:sodium symporters. This effect eventually leads to an increase in the synaptic concentration of non-exocytotically released neurotransmitters [and - in the case of the norepinephrine transporters, underlies the well-known indirect sympathomimetic activity]. While this action has long been appreciated, the underlying mechanistic details have been surprisingly difficult to understand. Some aspects can be resolved by incorporating insights into the oligomeric nature of transporters, into the nature of the accompanying ion fluxes, and changes in protein kinase activities.

Fig. 1

A kinetic scheme of substrate (S) and/or releaser (R) interaction with either the outward (o) or inward (i) facing transporter (T; adapted from Jones et al. 1999).

Fig. 2

Schematic illustration of the oligomer-based counter-transport model (described in more detail in Seidel et al. 2005). The left side illustrates the effect of low concentrations of amphetamine (in this figure, pCA stands for para-chloroamphetamine); only a fraction of SERT moieties is occupied by pCA in the oligomeric complex (shown here for the sake of simplicity as a dimer). Occupancy by pCA precludes phosphorylation by PKC (Ramamoorthy and Blakely 1999). The other SERT moieties in the oligomeric complex that has not been occupied by pCA are subject to phosphorylation and thereby primed for outward transport of substrate. Right, at high pCA concentrations, all transporter subunits are occupied by pCA, which impairs the action of PKC and thus prevents the accumulation of inward-facing conformations: efflux of substrate is impaired.

Fig. 3

Hypothetical representation of a possible cooperative interplay within a transporter oligomer (for the sake of simplicity, the transporter oligomer is shown as a dimer only). The amino-termini drawn with 11 helical turns suffice to reach the centre of the putative oligomeric partner to locate the latch by which for amphetamine-induced substrate efflux is triggered. The scheme is based on the published coordinates of LeuT_Aa (Yamashita et al. 2005) and the artwork created by using UCSF Chimera (http://www.cgl.ucsf.edu/chimera; Pettersen et al. 2004).