The researchers knew this because they had introduced a radioactive tracer that selectively binds to the same type of receptor in the brain, the mu-opioid variety, as the endorphins.

ECONOMIST: Pain perception

They combined conventional PET imaging, which requires a radioactive tracer to highlight the brain's metabolic activity, with other recent advances in the genetic engineering of brain cells that allows them to target specific brain circuits in lab animals.

WSJ: Brain Science: Mysterious Neural Circuitry Becomes Viewable