Topical glucocorticoids are commonly applied for treatment of atopic dermatitis, and are often administered over a long period. However, itching often occurs as a rebound phenomenon after cessation of long-term glucocorticoid application. The present study was an initial trial designed to establish an animal model of glucocorticoid-induced pruritus by topical application of dexamethasone over a long period in mice with contact dermatitis. BALB/c mice with chronic allergic contact dermatitis induced by 5 weeks of repeated application of 2,4,6-trinitro-1-chlorobenzene (TNCB) were treated topically with dexamethasone for 3 weeks from 2 weeks after the elicitation of dermatitis. The effects of dexamethasone on inflammation and pruritus were evaluated by measurement of ear-swelling and scratching behavior, respectively. Significant enhancement of pruritus was confirmed after chronic application of dexamethasone. The increased frequency of scratching behavior was reduced by withdrawal of dexamethasone. On the other hand, ear-swelling was markedly ameliorated by dexamethasone treatment, but rapidly relapsed after dexamethasone withdrawal. The level of interleukin (IL)-4 mRNA in ear skin and that of IgE in serum were increased in the mice with dermatitis and reduced by dexamethasone treatment. On the other hand, the level of nerve growth factor (NGF) mRNA was slightly increased by dexamethasone treatment and remained high even after its discontinuation. It is anticipated that this novel animal model of glucocorticoid-induced pruritus will be useful for clarifying the mechanisms of the rebound phenomenon induced by chronic treatment with topical glucocorticoids, and for developing a new form of therapy.
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