Modulation of Itch by Conditioning Itch and Pain Stimulation in Healthy Humans

Little is known about endogenous descending control of itch. In chronic pain, descending pain inhibition is reduced as signified by lowered conditioned pain modulation. There are indications that patients with chronic itch may also exhibit reduced endogenous descending inhibition of itch and pain. This study aimed to investigate whether and the extent to which itch can be modulated by conditioning itch and pain stimuli. Twenty-six healthy volunteers participated. The study consisted of 5 conditions designed to systematically assess endogenous modulation of itch or pain: 1) itch-induced modulation of contralateral itch, 2) pain-induced modulation of contralateral itch, 3) pain-induced modulation of ipsilateral itch, 4) pain-induced modulation of contralateral pain, and 5) itch-induced modulation of contralateral pain. Conditioning stimuli were cold pressor-induced pain and histamine-evoked itch, whereas the test stimuli were electrical stimulation paradigms designed to evoke itch or pain. Pain was significantly reduced (conditioned pain modulation-effect) by the conditioning pain stimulus (P < .001), but not by the conditioning itch stimulus (negative control condition). Itch was significantly reduced (conditioned itch modulation-effect) by contra- as well as ipsilateral applied conditioning pain (both P < .001), whereas conditioning itch stimulation only marginally reduced itch. Endogenous descending itch inhibition through mechanisms that are independent of segmental gating can be readily evoked by heterotopic conditioning pain stimulation. However, robust descending inhibition of itch cannot be evoked with conditioning itch stimulation.

The study showed a hierarchical prioritization favoring pain-induced central descending modulation of itch as well as pain in humans. Future studies addressing potential aberrations in pain-evoked descending modulation of itch in chronic itch patients are warranted.