Pain researchers know that, in the immediate aftermath of a severe injury, pain sensitization pathways become active, causing the body to produce opioids—naturally occurring chemicals that inhibit pain by activating receptors. But a mouse study published today in Science reveals that a specific type of opioid receptor found on the surface of nerve cells remains hyperactive months after an injury has healed—a period much longer than previously thought. Moreover, blocking this receptor from binding opioids can produce opioid withdrawal, much like that experienced by people addicted to heroin or codeine. This finding suggests that opioids only serve to mask underlying pain, shedding light on why some chronic nature of pain disorders.  Read more