TY - JOUR T1 - Functional Selectivity at the μ-Opioid Receptor: Implications for Understanding Opioid Analgesia and Tolerance JF - Pharmacological Reviews JO - Pharmacol Rev SP - 1001 LP - 1019 DO - 10.1124/pr.111.004598 VL - 63 IS - 4 AU - Kirsten M. Raehal AU - Cullen L. Schmid AU - Chad E. Groer AU - Laura M. Bohn A2 - Sibley, David R. Y1 - 2011/12/01 UR - http://pharmrev.aspetjournals.org/content/63/4/1001.abstract N2 - Opioids are the most effective analgesic drugs for the management of moderate or severe pain, yet their clinical use is often limited because of the onset of adverse side effects. Drugs in this class produce most of their physiological effects through activation of the μ opioid receptor; however, an increasing number of studies demonstrate that different opioids, while presumably acting at this single receptor, can activate distinct downstream responses, a phenomenon termed functional selectivity. Functional selectivity of receptor-mediated events can manifest as a function of the drug used, the cellular or neuronal environment examined, or the signaling or behavioral measure recorded. This review summarizes both in vitro and in vivo work demonstrating functional selectivity at the μ opioid receptor in terms of G protein coupling, receptor phosphorylation, interactions with β-arrestins, receptor desensitization, internalization and signaling, and details on how these differences may relate to the progression of analgesic tolerance after their extended use. ER -