The magnitude and duration of itch produced by intracutaneous injections of histamine.

The magnitude and duration of itch sensation produced by intracutaneous injection of histamine were determined for humans with the procedure of magnitude estimation scaling. Thirteen subjects received a 10-microliter intracutaneous injection of histamine at doses of 0.0001, 0.001, 0.01, 0.1, 1, and 10 micrograms into the volar forearm; eight of these subjects also received a 100-microgram dose. One subject received multiple injections over several weeks to determine the reliability of the magnitude estimates of itch. Following each injection, the area of flare and duration of itch were also determined. Intracutaneous injection of histamine produced a pure sensation of itch, without pain. The magnitude of itch increased in a dose-dependent fashion. The lowest histamine dose that produced itch greater than the itch produced by vehicle was 0.01 micrograms. The greatest itch was produced by the 100-microgram dose. A power function fitted to the mean magnitude estimates had an exponent of 0.17, indicating a negatively accelerating relation between the magnitude of itch and histamine dose. The one subject who received histamine over several weeks gave fairly reproducible estimates of itch magnitude. The duration of itch and the area of flare also increased in a dose-dependent fashion. The lowest dose of histamine that produced a duration of itch longer than the itch produced by the vehicle was 0.1 microgram, while the 100-microgram dose produced the longest duration of itch. Although the area of flare increased with each increase in dose from 0.1 to 10 micrograms, the areas of flare produced by 10 and 100 micrograms of histamine did not differ. These results indicate that humans can scale the magnitude of itch produced by histamine in a dose-dependent manner. In addition, the duration of itch and the area of flare produced by histamine are dose-dependent, confirming results of previous investigators. Intracutaneous histamine is easily quantifiable and may thus be a useful stimulus in neurophysiological studies of the peripheral neural mechanisms of itch.