Researchers block morphine’s itchy side effect

Researchers block morphine’s itchy side effect

11:04 PM, 21st October 2011
Researchers block morphine’s itchy side effect
Opioid drugs such as morphine interact with receptors on nerve cells. The drug relieves pain by interacting with one form of the receptor (MOR1) and causes itching when it interacts with a different form of the receptor (MOR1D).

 

ST LOUIS, US: Itching is one of the most prevalent side effects of powerful, pain-killing drugs like morphine, oxycodone and other opioids. The opiate-associated itch is so common that even women who get epidurals for labour pain often complain of itching. For many years, scientists have scratched their own heads about why drugs that so effectively suppress pain also induce itch.

Now in mice, researchers at Washington University School of Medicine in St Louis have shown they can control opioid-induced itching without interfering with a drug’s ability to relieve pain. The discovery raises tantalizing possibilities for new treatments to eliminate itch in cancer and surgery patients as well as others who rely on opioids to relieve chronic and severe pain. The investigators report the findings October 14 in the journal Cell.

By identifying and blocking a specific variant of the opioid receptor in the spinal cord, Zhou-Feng Chen, Director of Washington University’s Centre for the Study of Itch, and his colleagues have demonstrated for the first time that it is possible to inhibit itch without dulling morphine’s pain-killing effects.

“We identified a particular variant of the receptor called MOR1D that mediates itch. When we blocked MOR1D, mice that got morphine no longer needed to scratch and they still received the same level of pain relief,” said Chen, Principal Investigator on the study.

In previous studies, Chen, had identified an itch-specific receptor in the spinal cord called GRPR (gastrin-releasing peptide receptor). His studies also have shown that neurons containing GRPR specifically transmit itch but do not carry pain information. In the new study, his team found that the opioid receptor MOR1D induced itching in the mice on morphine by activating GRPR.

In a surprising twist, first author Xian-Yu Liu, Postdoctoral Researcher in Chen’s lab, found that a major variant of the opioid receptor called MOR1 exclusively mediates morphine’s analgesic effects in the spinal cord. Also other side effects of opioids also have been extremely difficult to separate from the drugs’ analgesic effects.

Chen said, “MOR1 and MOR1D operate like a key that can be used to open a door. Without the key, MOR1 can’t activate GRPR even though the receptor is activated by morphine.”

He said the finding opens up new possibilities for designing novel therapeutic strategies to relieve opioid-induced itching without blocking the analgesic effects of the drugs.

Chen’s team plans to look more closely at other opioid receptors to learn what they do. “There is a similar MOR1D receptor in humans, so we hope to find out whether blocking the same receptor in patients could alleviate itching without interfering with the analgesic effects of pain-killing drugs,” he added.

Funding for this research comes from the National Institute of Arthritis and Musculoskeletal and Skin Diseases and from the National Institute on Drug Abuse of the National Institutes of Health (NIH).

(C) Washington University in St Louis News

 

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