Cell 137, 1148–1159 (2009)

Delta and mu opioid receptors regulate pain transmission in the nervous system. Morphine and some endogenous opioids act on mu receptors to relieve pain, for example. But the role of delta receptors has been less clear.

The two types of receptor have long been thought to occupy the same pain-sensing neurons and to work in a cooperative fashion. But by tracking the positions of delta opioid receptors fused to green fluorescent protein in mice, Allan Basbaum of the University of California, San Francisco, and his colleagues suggest that several assumptions made about this receptor are untrue. Mu and delta opioid receptors inhabit different nerve fibres, and have separate roles in controlling pain: mu receptors for heat and delta receptors for mechanical stimuli.