Clinical and behavioral studies have demonstrated that chronic administration of benzodiazepines results in tolerance to many of the therapeutic actions of these drugs. Benzodiazepines are the drugs of first choice for the treatment of status epilepticus and are potential anticonvulsants against a variety of seizure types. However, significant tolerance develops to the anticonvulsant effects of all of the benzodiazepines currently available for clinical use, limiting their usefulness in maintenance therapy. Although the cellular basis for decreasing effectiveness with chronic exposure is not well understood, present knowledge of benzodiazepine function points to involvement of GABAergic processes. We have proposed that tolerance to the benzodiazepines is mediated by changes in the GABA/Bz receptor complex. Work during the present funding period has been directed toward testing this hypothesis. The present proposal will extend these studies, incorporating recent findings that changes in GABA sensitivity seen after chronic diazepam are regionally specific. The hypothesis that differences in GABAergic innervation of the dorsal raphe and substantia nigra pars reticulata underlie differences in GABA sensitivity following chronic diazepam exposure will be tested. Evaluations of GABA sensitivity changes following chronic benzodiazepine exposure to other brain areas using electrophysiological and biochemical (chloride flux) measures will be carried out. Exposure of tolerant animals to a benzodiazepine antagonist has been shown to rapidly restore GABA sensitivity, anticonvulsant efficacy and to diminish physical dependence. This effect will be further investigated and extended to an examination of benzodiazepine ligands of differing efficacy for their tolerance-inducing potency. Possible molecular mechanisms underlying the development of tolerance will be examined by measuring the state of phosphorylation of identified GABA/Bz receptor subunits and by comparing steady state levels of mRNA coding for the receptor complex in naive and tolerant animals. This research will help us understand neural adaptations associated with chronic benzodiazepine exposure and may lead to the development of new chronic therapeutic strategies utilizing these drugs.