Another new study in an animal model offers an intriguing look at the way nerve cells in the spinal cord transmit pain signals to the brain. Researchers Ping Li, PhD, and Min Zhuo, PhD, of Washington University in St. Louis showed that painful events in the body can "turn on" pain circuits in the spinal cord. Unfortunately, these pain pathways do not appear to "turn off" once the painful stimulus ends. They may even continue to transmit pain signals in response to non-noxious stimuli. (See Li and Zhuo, 1998.)

According to the researchers, when humans experience a painful event, receptors on the skin, muscle, skeleton, or internal organs trigger an electrical impulse that travels along a nerve fiber to the dorsal horn of the spinal cord. This fiber connects with nerve cells, which transmit the pain signal up the spinal cord to the brain.

Along the way, the pain signals cross junctions or synapses, where the pain signals can be short-circuited or modified en route. Li and Zhuo point out that the brain can block pain by preventing signals from crossing synapses. This may be why soldiers can continue to function painlessly despite grave injuries.

But the brain can also enhance pain by activating silent synapses, according to the new experiment. Li and Zhuo performed experiments on pain signal transmission in Sprague-Dawley rats. They employed whole-cell patch-clamp recording techniques to monitor electrical responses of individual neurons. They note that certain cells in the dorsal horn of the lumbar spinal cord normally have silent synapses. They do not play a role in sensory transmission until stimulated in a certain way.

Unfortunately, when these silent synapses were "turned on" by the researchers, they continued to remain active and transmit pain signals even when noxious stimuli ended. These previously silent synapses may play a role in persistent pain, these researchers assert.

Li and Zhuo believe the synapses can be turned on in two ways: by strong pain signals and by messages from the rostroventral medulla of the brain. Just as humans can't forget an old phone number, says Zhuo, previously silent synapses may not be able to forget how to transmit pain.

These researchers hope that further work will demonstrate ways to render these synapses silent. "Understanding the cellular and molecular mechanisms by which transformation of silent receptors occurs in the spinal cord will be helpful for designing drugs to stop persistent pain," they suggest.