Central modulation system of itching and pain in the human brain using positron emission tomography

It was reported that cold pain restricted itching sensation when they applied simultaneously. However, its neural system in the brain has been unclear. Pain and itching are transmitted by the same peripheral fibers (C fibers). As well as the peripheral, the central mechanism of itching is also similar to that of pain such as the anterior cingulate, the parietal and the prefrontal cortices. Our hypothesis was that itching modulation system was also similar to that of pain. So, in the present study, we investigated the modulating system of the itching sensation in the human brain using positron emission tomography (PET). Activations related to itching and subjective evaluation of the itching sensation were decreased by cold pain stimulus simultaneously given to the opposite side of the itching stimulus, as compared to itching alone. Interestingly, midbrain including periaqueductal gray matter (PAG) was activated only during the simultaneous stimulation of itching and cold pain. PAG is known to be modulating noxious stimulus. It was suggested that PAG also worked as itching modulation. That was supported by the animal study that electrical stimulation of PAG decreased itching-related spinal neural responses. These findings indicate that the modified brain activities in the PAG and the itching-related brain regions might be associated with the itch modulation in the brain. Pain and itching modulation systems were partly overlapped. [Jpn J Physiol 55 Suppl:S23 (2005)]