Foreword

Itching is a sensation that causes a strong desire to scratch. Some allergic dermatitis, especially atopic dermatitis, is accompanied by potent itching and the induced scratching results in a worsening of the dermatitis. Itching is the most important and an intolerable symptom for atopic dermatitis patients, and not only significantly impairs their quality of life but also aggravates the dermatitis through the induced scratching. Scratching the skin is the most important worsening factor in this condition. Therefore, it is reasonable to believe that inhibiting itching and/or preventing scratching could be expected to improve not only the quality of life but also the skin lesions of this disease.

Itching has many similarities to pain, and the discrimination between them has long been unknown, although their behavioral responses are quite different. Pain produces a withdrawal reflex while itching induces a scratching reflex. At present, however, they are considered to be different senses with different physiological properties. Pain appears to be an alerting signal to induce behavior, which avoids harmful stimuli and defends injured sites. These processes seem to be passive. In contrast to pain, scratching in response to an itch seems to be an active process that potentiates an allergic inflammation in the skin. The allergic response is an expression of an immunologic response that aims to eliminate an invaded foreign antigen. It may therefore be reasonable to consider that itching associated with allergic dermatitis is a sense aimed at potentiating host protective immune response in the skin by inducing scratching. At the same time, the sense of pain at the skin lesion may be lessened, and scratching is accompanied by a comfortable sensation. Together, these characteristic features seem to facilitate scratching, namely potentiate host defense. However, it is important to note that potentiation of immune response in the skin causes a worsening of the dermatitis. Furthermore, scratching the skin destroys the skin barrier and facilitates the penetration of foreign antigens through this barrier, resulting in worsening the dermatitis condition. The physiological role of itching has still not been explained clearly.

The comfortable sensation experienced by scratching is very interesting, and in some patients with atopic dermatitis, scratching is induced habitually without an apparent itching sensation. In the central nervous system, there is a “reward” system that produces a comfortable sensation when a desire is satisfied. However, it is not certain at present whether or not this reward system plays a role in the induction of the comfortable sensation after scratching. Furthermore, the sense of pain seems to be reduced in the skin lesion, suggesting that the balance between pain and itch may be closely regulated. Morphine, an analgesic and a μ-opioid receptor antagonist, is well known to cause itching and elevated levels of β-endorphin in atopic dermatitis patients have been reported, suggesting the involvement of endogenous opioids in the induction of itching in these patients. However, the precise role of endogenous opioids in the induction of itching remains to be elucidated.

Although there are many unanswered questions about itching and its induction mechanisms in atopic dermatitis, remarkable progress in our knowledge on itching has been made in the past two decades. In the present “Current Topics,” five distinguished researchers have summarized the current understanding of itching associated with atopic dermatitis. At the same time, remaining problems are delineated for future progress in the study of this subject.