Chronically relapsing pruritic dermatitis in the rats treated as neonate with capsaicin; a potential rat model of human atopic dermatitis

Background: The pathophysiological mechanisms underlying chronic pruritic skin diseases, e.g. atopic dermatitis (AD), and effective therapies remain elusive due to the paucity of animal models. Recently, we rediscovered that injection of capsaicin into rat pups resulted in vigorous scratching behavior and chronically relapsing AD-like cutaneous lesions well into adulthood. Objectives: To characterize the chronic pruritic dermatitis induced by neonatal capsaicin treatment. Methods: Capsaicin (50. mg/kg) was given to rat pups subcutaneously within 48. h after birth, and then scratching behavior, dermatitis and pathophysiological changes of rat skin were investigated chronologically. Results: Neonatal capsaicin treatment led to not only severe scratching and cutaneous lesions but also a large number of pathophysiological changes in the skin, such as histopathological changes including the deficiency of epidermal filaggrin expression, increases in the number of mast cells, levels of tissue NGF and Th2 cytokine mRNA, impaired skin barrier function and colonization with . S. aureus. In addition, we observed the hyperproduction of serum IgE, which is clinically similar to the pathophysiology seen in the patients with atopic dermatitis. During the follow-up observation, the rats showed the alternative periods of relapsing and remitting skin lesions. Conclusion: Injection of capsaicin into rat pups results in chronically relapsing pruritic dermatitis, similar to human AD. Therefore, we think neonatal capsaicin treatment could be a useful model for studying human AD and for the development of novel therapeutic drugs.