Opioid Pain Relievers May Prolong Pain

NIH | JUNE 25, 2016

Pain can help protect the body after an injury or infection. It serves as a warning system to rest or avoid certain activities. But sometimes, pain may become chronic. This can happen when nerve cells that detect something harmful alter the electrical or molecular signals they send to the spinal cord. This can trigger the production of chemicals in spinal cord neurons that make pain persist beyond the healing period.

Research into the cellular and molecular effects of opioid use may lead to better ways to treat chronic pain. Photo: Statewide Public Health Project/Works Progress Administration, United States.

Opioids, such as morphine, are often used to treat pain from a nervous system injury. These drugs activate opioid receptors and mimic the effects of natural chemicals produced by the body that both inhibit pain signals and produce a euphoric feeling.

After a nerve injury, a non-painful stimulus, such as a light touch, can cause pain or an unpleasant sensation. The researchers tested rats for increased sensitivity to non-painful stimulation following a sciatic nerve injury or sham surgery. Those with a nerve injury were more sensitive to touch on their hindpaws than the sham controls.

Injured rats treated with morphine were sensitive for longer and to a higher degree; lighter touches caused them to withdraw their paws from the stimulus more often than saline-treated animals.

Experiments showed that the morphine-induced sensitivity wasn’t mediated through opioid receptors. Morphine is also known to activate inflammation-inducing molecules, called cytokines. Blocking cytokine IL-1β or associated cytokines prevented the prolonged sensitivity when given with the morphine treatment. When delivered after morphine treatment, cytokine inhibition dampened the prolonged sensitivity.

Cytokine IL-1β’s activity is regulated by a molecular complex called the inflammasome, which helps trigger the inflammatory process in immune cells. Rats with the morphine-induced sensitivity showed increased expression of inflammasomes within specialized central nervous system immune cells called microglia.

Experiments revealed that the morphine-induced pain sensitivity depended on the activation of an inflammasome protein called NLRP3 in microglia. These results suggest that interfering with the cytokine pathway could prevent opioid-induced prolonged chronic pain without hindering the painkilling properties.

YOU MAY ALSO LIKE

What makes a mammal a mammal? Our spine, say scientists

Mammals are unique in many ways. We're warm-blooded and agile in comparison with our reptilian relatives. But a new study suggests we're unique in one more way -- the makeup of our spines.

Seismic study reveals huge amount of water dragged into Earth’s interior

Slow-motion collisions of tectonic plates under the ocean drag about three times more water down into the deep Earth than previously estimated, according to a first-of-its-kind seismic study that spans the Mariana Trench, a crescent-shaped trench in the Western Pacific that measures 1,500 miles long and is the deepest ocean trench in the world.

Ceres Spots Continue to Mystify in Latest Dawn Images

New images of Ceres from NASA's Dawn spacecraft provide more clues about its mysterious bright spots, and also reveal a pyramid-shaped peak towering over a relatively flat landscape.

A Fresh Look at Older Data Yields a Surprise Near the Martian Equator

Scientists taking a new look at older data from NASA's longest-operating Mars orbiter have discovered evidence of significant hydration near the Martian equator -- a mysterious signature in a region of the Red Planet where planetary scientists figure ice shouldn't exist.

Structure of receptor involved in brain disorders

Scientists determined the structure of the intact NMDA receptor, an ion channel involved in many brain-related illnesses including Alzheimer’s disease, depression, schizophrenia, and autism. The finding may aid in the development of future therapies.

Researchers Discovered Proteins Essential to Development of Skeletal Muscle

An international team of researchers discovered two proteins essential to the development of skeletal muscle. The research, could lead to a better understanding of rare muscular diseases and the development of new treatments.