Figure 2.

Receptor/ligand definitions and applications to buprenorphine at the μ-opioid receptor. *Definition of a partial agonist: a compound with an intermediate intrinsic activity that at full receptor saturation produces less than the maximal effect obtainable with full agonists in some specified set of in vitro or clinical circumstances [25]. Buprenorphine is a potent Schedule III opioid with high binding affinity at the μ-opioid receptor that behaves as a partial agonist on the basis of in vitro studies [7,14,26]. Although buprenorphine has less total intrinsic activity (capacity to activate a receptor to induce multiple signaling pathways) than full μ-opioid receptor agonists, it still effectively stimulates the analgesic signaling pathway from the μ-opioid receptor [7,14,25,27,28]. 3D=three-dimensional; OR=opioid receptor.