The SSRI antidepressants, including fluoxetine (Prozac), are the drugs most commonly prescribed for treatment of depressive disorders. The in vivo targets of fluoxetine that are responsible for its antidepressive effects and its side effects are not well established. Systematically establishing such targets in the human or in mammalian models of depression is not possible. We are using the nematode Caenorhabditis elegans to identify mechanisms of fluoxetine action. In preliminary studies we have established that fluoxetine has three distinct effects in C. elegans. One of these effects is probably due to blocking serotonin reuptake, as expected from one known target in mammals. The other two effects appear not to involve serotonin. We have isolated mutations in seven genes that confer resistance to one of the non-serotonergic effects of fluoxetine (Nrf mutants). We have molecularly cloned one of these genes, called nrf-6. nrf-6 encodes the defining member of a family of multi-pass transmembrane proteins. We propose to investigate nrf-6 by completing this initial molecular analysis and by determining its expression pattern, subcellular localization, and site of function in conferring fluoxetine sensitivity. We propose to clone two additional fluoxetine resistance genes that we have identified and to perform similar analyses as for nrf-6. We propose to continue mutagenesis screens to identify other genes that mediate the nose contraction and other fluoxetine responses. Finally, we propose to identify in mammals members of the new nrf-6 family as a first step toward determining whether the nematode-fluoxetine interaction might have direct relevance to humans.