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Pain message travels via diffuse signal.

Have you ever experienced pain from an injured finger that slowly spread until your whole hand hurts? You may have "sensed" the diffusion of substance P, a neurotransmitter that helps relay pain messages to the brain, says Allan I. Basbaum.

Basbaum, a neuroscientist at the University of California, San Francisco, studied the distribution of substance P and its receptor molecules in the spinal cord. There, peripheral nerve fibers carrying pain messages release substance P onto receptors on so-called projection neurons, which then forward the unpleasant news along the spinal cord to the brain.

In many neural transmissions, nerve endings lie close to a tiny receptor-containing region of the target cell. Thus, the released transmitter only needs to cross a narrow gap to reach the receptor molecule. Substance P, however, works differently, Basbaum holds.

Labeling substance P receptors on spinal cord neurons with an antibody, Basbaum and his co-workers noticed that the receptors cover most of the surface of the nerve cells -- in contrast to the tiny receptor patches in traditional neural communication. "The antibody reveals in the entire morphology of the substance P neurons," Basbaum explains.

Moreover, the group saw receptor molecules inserted into a neuron membrane without any synapse nearby. Conversely, when labeling substance P itself, they found it in synapses next to membranes devoid of the substance P receptor.

"There is considerable mismatch between the distribution of substance P and this receptor," Basebaum says. "That mismatch is a strong anatomical indication that substance P generally acts by diffusing away from its site of release."

Though the effects of this diffusion remain largely unknown, Basbaum says some evidence suggests that it contributes to an oversensitivity to pain. As for the throbbing hand, he says, the initial pain sensation relates only to the original injury. But as the problem persists, more substance P gets released, diffuses farther away , and excites distant neurons that normally answer to stimuli from areas beyond the injury.
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Title Annotation:substance P contributes to pain oversensitivity
Author:Strobel, Gabrielle
Publication:Science News
Article Type:Brief Article
Date:Nov 27, 1993
Words:322
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