PROJECT SUMMARY/ABSTRACT Although benzodiazepines (BZs) are considered to be among the safest prescription drugs in modern medicine, their utility is constrained by a number of side effects, including the liability for tolerance and dependence. The overall goal of this application is to investigate the extent to which GABAA receptor subtypes are differentially involved in tolerance and physical dependence associated with acute and long-term BZ exposure. This renewal application builds on research implicating unique roles for α1, α2, α3, and α5 subunit- containing GABAA receptors (α1GABAA, α2GABAA, α3GABAA α5GABAA receptors, respectively) in BZ- induced tolerance and dependence. During our previous Project Period, we obtained several important findings: (1) tolerance develops rapidly to behavioral effects associated with α1GABAA receptors, but not α2/3GABAA receptors; (2) the ability of BZ-type drugs to induce physical dependence involves α1GABAA receptors; but (3) new evidence has suggested a unique inhibitory role for other GABAA subtype(s) in physical dependence. Together, these findings form the framework for our working hypothesis that α1GABAA receptors are key mediators of both tolerance and physical dependence associated with exposure to BZ-type drugs, whereas physical dependence may be mitigated by α2GABAA and/or α3GABAA subtypes. In Specific Aim 1, we will evaluate the hypothesis that α1GABAA subtypes are key mediators of acute and chronic physical dependence, as well as tolerance, induced by BZs. Specific Aim 2 will evaluate the hypotheses that α2/3GABAA receptors play a unique “protective” role of preventing withdrawal, with no tolerance to the effects mediated by these subtypes. We also will explore a novel potential role for α5GABAA subtypes as facilitators of tolerance. The research in this renewal application addresses a key gap in knowledge by systematically exploring the role that GABAA receptor subtypes play in tolerance and dependence associated with BZ-type drugs, an area of research that has received relatively little attention. Because tolerance and physical dependence are significant barriers to use of BZs as effective therapeutics, the research in this application will inform drug discovery strategies for developing improved therapeutics with reduced dependence/abuse potential.