Figure 1.

Changes in the binding potential (BP) of mu-opioid receptors could indicate either an altered density of mu-opioid receptors (A) or changes in the magnitude of endogenous opioids release (B), which directly correlates to the activity of the mu-opioid system (C). Presynaptic and postsynaptic membranes are represented in green under different conditions. Mu-opioid receptors are represented in blue. Endogenous opioids released in the synaptic cleft are depicted in green. Mu-opioid receptor agonists (eg, carfentanil) are showed in red.