Structural domains of catecholamine transporter chimeras involved in selective inhibition by antidepressants and psychomotor stimulants

Reuptake systems for monoamines are the initial sites of action for a wide range of therapeutic antidepressants and drugs of abuse, such as cocaine. To delineate structural domains of the dopamine and norepinephrine transporters that contribute to differential interaction with reuptake inhibitors with antidepressant or reinforcing properties, a series of recombinant transporter chimeras were generated and transiently expressed in HeLa cells. The inhibition constants (K(i) values) for cocaine and a variety of selective transport inhibitors were determined for each chimera. Analyses of functional chimeras delineate a segment spanning transmembrane domains 5-7 of the norepinephrine transporter of primary importance for high affinity binding of tricyclic and nontricyclic antidepressants (e.g., K(i) < 20 nM desipramine or nisoxetine). In contrast, all chimeras containing dopamine transporter sequences from this region resemble the dopamine transporter, which demonstrates higher affinity for psychomotor stimulants compared with antidepressants (e.g., K(i) = 391 ± 39 nM cocaine compared with 9365 ± 1260 nM desipramine). A region including transmembrane domains 1-3 of the norepinephrine transporter also contributes to the interaction of desipramine and nisoxetine, whereas the analogous region of the dopamine transporter influences the affinity for piperazine derivatives (e.g., GBR12909 and LR1111) that are selective for the dopamine transporter. These analyses provide a framework for identifying the precise structural determinants of monoamine transporters involved in selective interactions with antidepressant and psychomotor stimulant reuptake inhibitors.