BALTIMORE, May 15 (UPI) -- U.S. neuroscientists say they are moving closer to understanding pain sensitivity and specifically why it's variable instead of constant.

"Pain sensitivity increases during inflammation or injury and we want to know what molecules are involved in pain sensation when sensitivity is elevated," said John Hopkins University Assistant Professor Xinzhong Dong.

He said researchers know the ability to sense temperature heat and spice is controlled by the TRPV1 protein channel found on the surface of certain nerve cells. When inactive, TRPV1 channels are closed and there is no pain sensation. However, when heat above 108 degrees Fahrenheit, or capsaicin, the main ingredient in hot peppers, activates a TRPV1 channel, ions create an electrical current that sends pain signals to the brain.

"The interesting thing about this channel is it's not always constant," said Dong, whose team set out to find proteins that modulate TRPV1's action. They found the Pirt protein, phosphoinositide, the interacting regulator of TRP.

But exactly how Pirt regulates the TRPV1 channel isn't yet clear, said Dong. "The goal is to find molecules that specifically affect the pain pathway, but not other nerves," he said.

The research appeared in the May 2 issue of the journal Cell.