The benzodiazepines are among the most commonly used and abused of all drugs. They are also known to induce tolerance and physical dependence. A variety of types of information indicates that the benzodiazepines have multiple modes of action and interact with multiple receptor subtypes. Many, probably all, of the benzodiazepines used in clinical medicine are mixed agonists which interact with several receptors. Further, there is evidence that benzodiazepine receptors can exist in more than one configuration and that chronic administration of benzodiazepines may alter the number of benzodiazepine receptors present in the brain. Different benzodiazepines produce physical dependencies whose precipitated and withdrawal abstinence syndromes differ qualitatively. These observations are consistent with the hypothesis that benzodiazepines differ in their modes of action. Further, tolerance and dependence produced by chronic administration of benzodiazepines in rats differs from that produced in the dog. In an attempt to understand the mechanisms underlying these differences between benzodiazepines and between species, both pharmacologic and binding studies will be conducted using graded doses of prototypic benzodiazepines with differing specificities for identified receptors. The effects of these drugs will be studied on diverse physiologic and behavioral parameters (e.g. EKG, autonomic function, sleep, pain, etc.) in nondependent and diazepam- and N-desmethyldiazepam-dependent dogs. In the dependent dogs, withdrawal abstinence will be further studied as well as the ability of several prototypic drugs (Bz1, Bz2 and inverse agonists) to suppress various signs of abstinence. Further, a complete analysis of the degree of tolerance induced by chronic administration of diazepam and N-desmethyldiazepam to the different effects produced by these prototypic drugs will be made. These pharmacologic studies will be related to data obtained on the binding characteristics of prototypic drugs to dog brain benzodiazepine binding sites. Scatchard and Hill analysis of saturation and competition data for prototypic drugs, the determination of association and dissociation kinetics of prototypic drugs and the efficacy of prototypic drugs to alter the barbiturate shift (enhancement of binding) will be used to identify different binding sites or their inverse agonistic properties. Further the distribution of the various binding sites among different brain regions of the dog will be determined and compared to rat data.