Our lab has produced and analyzed mutations in all murine opioid system genes and receptors and maintain many of these mutations on two strain backgrounds that is allowing us to explore the genetic basis for strain-specific behavioral effects of these mutations. Current work is exploring in detail the role of endogenous opioid peptide enkephalin in depression, which derives from our recent paper demonstrating that the mu opioid receptor is the required target for the exogenous anti-depressant tianeptine. Several additional studies are using our mouse strains to establish the opioid receptor specificity of novel compounds being produced by collaborators throughout the world.