Figure 4.
Schematic time activity curve reflecting changes in regional radioactivity emitted from [11C]carfentanil bound to the μ-opioid receptor when corrected for radioactivity due to unspecifically bound and free compound. Due to the greater availability of endogenous opioids in pain than in the saline control condition with the subject expecting pain but no pain is felt, less radioligand binding occurs because of the increased competition for available binding sites. Sequences are counterbalanced to control for order effects.