Long term clinical use and recreational abuse of opioid narcotics often lead to the development of tolerance and drug dependence. These conditions limit the usefulness of opioid drugs for the management of chronic pain. Tolerance and dependence can also lead to drug-seeking, often criminal behaviors, overdose, and death in heroin addicts. Thus, researchers have focused on understanding the cellular and molecular changes mediating opioid tolerance and dependence. Initially, these phenomena were thought to result from opioid receptor down-regulation and receptor desensitization. Studies of the three types of opioid receptors, mu, kappa, and delta, indicate they have different mechanisms controlling their down-regulation and desensitization. In particular, the mu receptor seems to differ significantly. Because this receptor is the site of action of the most commonly used and abused opioids, research has focused on elucidating the regulation of this receptor. The carboxyl tail of opioid and related receptors is a common site for coupling of the receptor to effector molecules and for regulating the receptor during chronic agonist exposure. In an attempt to identify novel proteins involved in the regulation of the mu receptor, I screened a brain cDNA expression library with a probe consisting of the C-terminal 62 amino acids of the mu receptor. I identified a novel protein kinase, PKU-alpha, which contains a serine/threonine catalytic domain and a nuclear localization signal. This research proposal will thus focus on characterizing the functional relationship between the mu opioid receptor and this protein kinase. This kinase may potentially interact with the receptor to regulate gene expression and contribute to the cellular and molecular changes necessary for the development of opioid tolerance and dependence.