State-dependent opioid control of pain

Agonists for the μ-opioid receptor are powerful analgesics and are highly addictive; however, the contribution of the δ- and κ-opioid and opioid receptor-like receptors to motivational states is less clear. Agonists at each receptor modulate neurons in a circuit that selectively controls nociceptive transmission. This circuit can operate in both pain-inhibiting and pain-facilitating states, and the action of opioids contributes to and is...