Background: Investigations of pruritogenic substances in humans have involved intradermal injections in normal skin; itching of inflamed skin has been little studied.
Objectives: To develop an itch model with provocation of itch in experimentally inflamed skin as well as in normal skin, using subjects as self-controls.
Methods: In 32 non-atopic volunteers aged 21-30 years, the skin of five selected test sites on one volar forearm was pretreated for 24 h with large Finn chambers containing 1% sodium lauryl sulphate (SLS) used as a standard contact irritant to induce inflammation. Twenty microlitres of different pruritogenic substances [histamine, substance P, neurokinin A, neurokinin B, trypsin, platelet-activating factor (PAF) and serotonin] and saline as control were injected intradermally into the inflamed test sites and in corresponding non-treated sites on the opposite forearm. The test individuals scored itch intensity on a visual analogue scale for 20 min, and weal area was then measured. :
Results: Histamine and substance P induced itch in both normal and inflamed skin compared with a saline reference. Neurokinin A, trypsin, PAF and serotonin only elicited itch in normal skin, and neurokinin B neither elicited itch in normal skin nor in inflamed skin. Itch was induced in normal and SLS-inflamed skin to a similar magnitude. However, weal area after histamine was significantly (P < 0.001) larger in inflamed skin when compared with normal skin.
Conclusions: Histamine and substance P elicited itch to the same degree in normal skin and inflamed skin pretreated with SLS despite a stronger weal response in inflamed skin. Mediators present in inflamed skin did not potentiate itch, a c-fibre-mediated neuronal response. The weal reaction is based on enhanced vascular permeability (protein extravasation). A greater skin perfusion in inflamed skin may therefore have increased the weal size. We propose an experimental model in humans for testing of itch involving both normal and inflamed skin. The model has the potential for use in evaluating new topical and systemic treatments of itch.