Collared mice: a model to assess the effects of scratching.

Background: There is no current method to precisely assess pruritus despite its importance as a major symptom in many skin diseases. Pruritus induces scratching that worsens various inflammatory skin diseases.

Objective: The purpose of this study was to determine the effects of scratching on allergic skin reactions using murine contact hypersensitivity (CH) as a model and to assess classical "anti-pruritic" agents using this model.

Methods: We utilized plastic collars which were placed around the necks of mice to prevent them from scratching their ears during the development of CH. This allowed us to assess ear swelling as an index of CH, obviating the effects of scratching that occurs during the development of CH.

Results: Following elicitation, the ear swelling of these "collared" mice was decreased by approximately 50%, compared to control mice in which collars were not used, suggesting that scratching contributes to the ear swelling that is measured as an index of CH. Using this model, we assessed the anti-pruritic effects of antihistamines, corticosteroids, non-steroidal antiinflammatory and sedative agents. All agents decreased CH when collars were not used. When collars were used, all agents, other than the sedatives, appeared to suppress CH, indicating their antiinflammatory effects. Sedative agents did not decrease CH in collared mice, indicating that their inhibitory effects in CH may be entirely due to their sedative effects.

Conclusions: This model enables the dissection of the various elements assessed when measuring CH in mice and may provide a simple tool to assess or screen potential anti-pruritic agents.