It has long been believed by boffins that an itch is a lower level manifestation of the human body's response to pain. Quite surprisingly now, it has been shown by researchers at Washington University School of Medicine in St. Louis that these two responses to stimuli are indeed two separate sensations as they are rooted in different molecular mechanisms.
According to the researchers, the findings could have important implications for treating both pain and chronic itching.
Last year, the research team, led by Zhou-Feng Chen, Ph.D., an investigator at Washington University's Pain Center, was the first to identify an itch gene - called GRPR ie gastrin-releasing peptide receptor.
GRPR makes a receptor found in a very small population of nerve cells in the spinal cord. That region of the spinal cord transmits pain and itch signals, as well as temperature sensation, from the skin to the brain. When exposed to itchy stimuli, mice without the gene scratched less than their normal littermates.
Many patients with chronic pain receive spinal injections of opioid drugs, such as morphine, to control their pain. One of the well-known side effects of that treatment is itchy skin.
"Most scientists believed that the itching could not be separated from the drug's pain-sedating effects. This type of itching cannot be relieved by anti-histamine treatment. We hypothesized that GRPR may be responsible for the itching but not involved in the pain response," Chen said.
Therefore, the researchers went back to the mice bred with and without GRPR and compared both scratching behaviors and pain-sedating effects following spinal injections of morphine.
They found all of the mice got relief from a mildly painful stimulus, but those without the GRPR gene did not scratch.
Next, the researchers studied normal mice treated with a small peptide that interferes with GRPR function. When injected with the GRPR blocker, mice still got morphine's pain-numbing benefits, but they did not itch.
"If we inject a GRPR inhibitor and morphine into the mouse spinal cord, the drug still has its normal analgesic effect, but the mice don't scratch. This is very interesting because it demonstrates that analgesia and itching can be separated. There may be itch-specific genetic pathways in the spinal cord that are not related to pain sensation," Chen said.
The researchers reported their findings at Neuroscience 2008, the annual meeting of the Society for Neuroscience.