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.