Impact of scratching on itch and sympathetic reflexes induced by cowhage (Mucuna pruriens) and histamine.

Cowhage and histamine, both applied via spicules, were used to induce itch. The quality and intensity of the sensations, axon reflex flare, sympathetic skin vasoconstrictions and the interference of scratching with itch processing were studied. Axon reflex flare reactions were measured by laser Doppler imaging and reflex vasoconstrictions in the finger were recorded by laser Doppler flowmetry. Magnitude of itch sensations was assessed on an electronic visual analogue scale while the skin was intermittently scratched proximal to the application site. The quality of itch was assessed with a questionnaire. Only histamine produced an axon reflex flare. Histamine itch increased faster, but recovered more slowly after scratching, by which it was more effectively suppressed. Cowhage induced a sharper itch sensation and stronger vasoconstrictor reflexes. These findings support the notion that both agents activate different pathways. The differences in sympathetic reflex induction and in the modulation by scratching indicate differential central nervous processing.