But when Ringkamp and colleagues treated the itches with the compound that makes chili peppers hot--capsaicin, which triggers a pain response--they blocked the cowhage itch, while leaving the histamine itch unaffected.
These results, published in 2007 in the Journal of Neuroscience, showed that while the sensation of the two itches caused by histamine and cowhage felt similar to participants, the mechanisms were undoubtedly different.
Since, like almost all types of chronic, pathological itches, the itch produced by cowhage is impervious to antihistamines, scientists reasoned that if they could figure out exactly which neural fibers were responsible for a cowhage itch, they might understand how pathological itch works.
To test this idea, thin, conductive wires were inserted into the skin of a sedated monkey, and different types of stimuli were applied to the arm: Heat and capsaicin to cause pain, and cowhage and histamine to bring on the itch.
But the majority of the nerve fibers responded strongly to cowhage. The same fibers known to detect painful stimuli like a hard poke or a burn could also detect the cowhage itch.