The serotonin transporter (SERT) is the molecular target for most antidepressant drugs as well as many drugs of abuse. Antidepressants such as paroxetine, fluoxetine, and imipramine bind to the transporter and inhibit serotonin uptake, thereby prolonging the extracellular lifespan of the neurotransmitter. The abused psychostimulants cocaine and amphetamine also bind to the transporter, but have distinct pharmacologic effects leading to abuse potential and possible neurotoxicity. Despite the cloning of SERT in the early 1990's, very little information is available regarding the molecular and cellular neurobiology surrounding the function of this transport protein. This project uses a multidisciplinary approach aimed at significantly advancing knowledge regarding the molecular pharmacology of psychostimulants at SERTs. The studies will use multiple techniques including expression and characterization of recombinant transporters in mammalian cells, electrophysiology, immunoblotting, the formation of chimeric proteins, and site- directed mutagenesis to identify amino acids involved with specific transporter functions including antagonist and substrate binding. In addition, computer-assisted structure-activity studies of cocaine and amphetamine derivatives will be coupled with mutagenesis in an attempt to identify direct ligand- transporter interactions. Therefore, the specific aims of this project are 1) to identify transporter domains and residues involved in recognition of SERT antagonists such as cocaine and 2) to determine the contributions of specific SERT domains to substrate properties such as translocation, efflux, and channel- like activity. The proposed strategies will provide critical new information linking protein structure to functional properties of the transporter and an enhanced understanding of the molecular mechanisms of action for psychotherapeutic and abused drugs.