A new study has found that a combination of capsaicin, the substance that makes chilli peppers hot, and a drug called QX-314 can obstruct pain-sensing neurons in rats without affecting movement or sense of touch.
Researchers think that the finding is a better way to treat pain from childbirth and surgical procedures. The new combination is also expected to provide relief to many who suffer from chronic pain. The study found that the mixture works on a characteristic unique to pain-sensing neurons, also called nociceptors.
"The Holy Grail in pain science is to eliminate pathologic pain without impairing thinking, alertness, coordination, or other vital functions of the nervous system. This finding shows that a specific combination of two molecules can block only pain-related neurons. It holds the promise of major future breakthroughs for the millions of persons who suffer with disabling pain," Nature quoted said Story C. Landis, Ph.D., director of the National Institute of Neurological Disorders and Stroke (NINDS) at the NIH, as saying.
Lidocaine, the most commonly used local anaesthetic, eases pain by stopping electric currents in all nerve cells. Though QX-314 is a lidocaine derivative, it alone cannot get through cell membranes to block their electrical activity.
Capsaicin opens large pores called TRPV1 channels located in the cell membrane of pain-sensing neurons. When these channels propped open by capsaicin, QX-314 can get through and selectively block the cells' activity.
The research team, led by Clifford J. Woolf, M.D., Ph.D., of Massachusetts General Hospital and Harvard Medical School and Bruce Bean, Ph.D., at Harvard Medical School, tested the combination of capsaicin and QX-314 in neurons in Petri dishes and discovered that it blocked pain-sensing neurons without disturbing nerve cells.
They then administered rats with the drug injected into their paws and found that they could tolerate more heat than usual. It took an estimated half an hour for the drug combination to fully block pain in the rats. However, once it began, the pain relief lasted for several hours.
"Current nerve blocks cause paralysis and total numbness. This new strategy could profoundly change pain treatment in the perioperative setting," Dr. Woolf said.
The treatment tested in this study uses a type of ion channel (TRPV1 channels) as a path to give medication. Ion channels are pores in the cell membrane that regulate electrically charged ions' activity in and out of cells.
"I'm not aware of any other strategy that uses a channel within cells to deliver a drug to a select set of cells," Dr. Woolf said. "This project is a nice illustration of how research trying to understand very basic biological principles can have practical applications," said Dr. Bean.
"This type of treatment has great potential to improve pain treatment during childbirth, dental procedures, and surgery, the researchers say. "Surgical pain is the obvious first application for this type of treatment," Dr. Woolf said.
"However, similar therapies might eventually be useful for treating chronic pain. "While the researchers focused on finding a treatment for pain, this strategy might also be useful for treating itch from eczema, poison ivy rashes, and other conditions.
"Like pain sensations, itch signals come from nociceptors. One problem with the combination treatment is that the capsaicin can cause unpleasant burning sensations until the QX-314 takes effect," Dr. Woolf said.