It is now recognized that co-localized opioid, adrenergic and other GPCRs may be associated as dimers and oligomers in addition to the traditionally envisaged monomeric species. These developments have resulted in considerable re-evaluation of our models of GPCR structure and function. Although many reports of GPCR oligomerization now exist, the vast majority of studies have been conducted in transfected cells in vitro. In fact, there are very few reports investigating the existence of GPCR heteromers in vivo. The significance and role of oligomerization in GPCR function in CNS tissue therefore remains largely unexplored. Agonists acting at spinal delta-opioid and alpha-2A-adrenergic receptors inhibit pain transmission and interact synergistically when co-administered in the spinal cord and are highly co-localized in primary afferent neurons (an obvious prerequisite for oligomerization). We propose to utilize these receptor subtypes as a physiologically relevant model to explore the functional implications and importance of GPCR oligomerization in the CNS. We propose a series of studies designed to directly identify heteromeric species in CNS tissue. In addition, we will explore the nature of these interaction(s) at the molecular and cellular levels to identify a potential role for GPCR heteromerization in analgesic synergy. Improving our understanding of the existence of and functional implications of oligomerization in opioid receptor biology could lead to insights into the cellular and molecular mechanisms of analgesic synergy, tolerance and dependency. These studies could therefore have a significant impact in future drug discovery and treatment approaches to narcotic addiction and the treatment of severe acute and chronic pain.