Benzodiazepines are the most frequently prescribed psychotropic medications in the U.S. and internationally. Clinical use of these drugs is limited by the development of tolerance and physical dependence. In addition, there is evidence for substantial benzodiazepine abuse, and dependence may contribute to this abuse. The mechanisms underlying benzodiazepine tolerance and dependence remain uncertain. Over the prior phase of this proposal, we have developed a model of benzodiazepine tolerance and withdrawal in the mouse, and have characterized in detail alterations at the GABAA receptor complex in this model. In brief, benzodiazepine agonist administration is associated with behavioral tolerance and receptor downregulation; discontinuation is associated with a transient behavioral withdrawal syndrome coincident with receptor upregulation. Chronic antagonist and inverse agonist administration also are associated with receptor upregulation. Chronic antagonist and inverse agonist administration also are associated with receptor upregulation. In addition, we have developed an in vitro system to assess benzodiazepine effects on cultured cortical neurons. Results in general parallel those obtained in vivo. The present proposal represents an extension of these models in two major areas: 1. Molecular biological methods; and 2. Pharmacological interventions to limit tolerance and dependence. The recent cloning of numerous GABAA receptor subunits opens molecular approaches to studies of tolerance and withdrawal, and the models characterized in our prior studies can be readily extended to these methods. Indeed, our preliminary studies indicate striking decreases in mRNA for several GABAa receptor subunits after prolonged benzodiazepine administration. This finding, which has been confirmed by another laboratory, should be extended to other subunits of the GABAa receptor. In addition, other benzodiazepines will be evaluated during chronic administration and after discontinuation, including in situ hybridization techniques to assess anatomic localization and nuclear run off studies to evaluate effects on transcription. Also, antibodies specific for receptor subunits will be used to assess alterations in subunit protein expression, and to correlate these results with mRNA data. The second major area of study involves pharmacologic interventions which have been proposed to limit benzodiazepine tolerance and dependence. The mouse model we have developed is well-suited to assessing dose-tapering, antagonist and anticonvulsant administration, and partial agonist treatment. The proposed studies will provide a broad picture of effects of chronic benzodiazepines at the molecular level, together with evaluation of interventions that have direct relevance to prevention of benzodiazepine withdrawal.