PET imaging of opioid receptors in pain: progress and new directions.

The endogenous opioid system plays a central role in pain. Recent advances have permitted imaging of opioid receptors by PET in human subjects while experiencing pain and detection of changes in receptor occupancy. The ability to perform these types of studies is dependent on the development of opioid tracer ligands labeled with positron emitting isotopes. This article follows the development and radiochemistry of opioid tracer molecules through their use in human subjects and subsequent application to the study of pain. The role of mu, delta and kappa opioid receptors in pain is reviewed. Occupancy changes in mu receptors have been observed with PET in human subjects subjected to experimental pain paradigms. The implication of this approach to the study of pain and pain syndromes, and possible clinical applications, is also addressed.