Spicy Combo Blocks Pain Without Numbness
May 3rd, 2007 Posted by: Dr. Cox
BOSTON, Oct. 3 — Adding capsaicin to an unusual anesthetic resulted in a combination that blocked pain but didn’t cause numbness or paralysis, researchers here said. Action Points
Explain to interested patients that current anesthetics block pain, but also cause numbness and inability to move, because they affect a variety of nerve cells, not just those that sense pain.
Note that this research, carried out in animals, suggests a way to block the activity of only the pain neurons, leaving other nerve cells unaffected.
Caution that more research is needed.
The anesthetic in question is a lidocaine derivative called QX-314, which isn’t used clinically because it doesn’t ordinarily affect nerve cells in the same way other pain-blocking drugs do, according to Bruce Bean, Ph.D., of Harvard Medical School, and colleagues.
When the drug was combined with capsaicin, the potent ingredient in chili peppers, it gained the ability to enter nerve cells — but only the pain-sensing nociceptors, the researchers reported in the Oct. 2 issue of Nature.
The finding — in experimental rats — could lead to new painkillers that specifically target pain-sensing neurons, eliminating the numbness, paralysis, and blockage of autonomic nerves associated with current anesthetics, the researchers said.
“Eventually this method could completely transform surgical and post-surgical analgesia, allowing patients to remain fully alert without experiencing pain or paralysis,” Clifford Woolf, M.D., of Massachusetts General Hospital, the study’s senior author.
Most anesthetics are neutral compounds that diffuse though the cell walls of neurons and, once inside, block sodium ion channel activity, thereby halting electrical signaling in the cell, the researchers said.
But QX-314 is electrically charged and is usually unable to get through the cell walls of nerve cells, although if placed in a cell it also blocks sodium channels.
That’s where capsaicin comes in. Dr. Bean and colleagues took advantage of the fact that only pain-sensing neurons have the so-called TRPV1 receptor — which opens when stimulated either by excessive heat or by the spicy compound.
In vitro experiments showed that the combination of capsaicin and QX-314 was able to block electrical activity in pain-sensing neurons, but had no effect on other nerve cells, the researchers reported.
In rats, injecting QX-314 alone into the animals’ hindpaws had little or no effect on their sensitivity to being pricked with stiff fibers, while injecting capsaicin alone made the animals significantly more sensitive to the painful stimulus.
But a combination of the two immediately eliminated the increased sensitivity caused by the capsaicin and, within two hours, the rats were half as sensitive as they had been at the start — a difference that was significant at P