Inhibition of scratching behaviour caused by contact dermatitis in histidine decarboxylase gene knockout mice

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Abstract
A neuronal system dedicated to itch consists of primary afferent and spinothalamic projection neurons. Histamine is thought to be one of the main mediators for the transmission of itch sensation. However, there are little available information on the role of histamine in scratching behaviour and sensory transmission of atopic dermatitis and chronic eczema. In the present study, the role of histamine in scratching behaviour and neural conduction of sensation in the chronic eczema model was investigated by using l‐histidine decarboxylase (HDC) gene knockout mice lacking histamine. The chronic contact dermatitis was induced with daily application of diphenylcyclopropenone (DCP) on a hind paw of HDC (+/+) and HDC (–/–) mice for 2 months. The observation of scratching behaviour and the hot‐plate test were performed in both mice. Histological studies were performed in the skin and spinal cord tissues. Histological examination revealed that both HDC (+/+) and HDC (–/–) mice displayed the similar extent of inflammatory cell infiltration, hyperplastic epidermis and newly spreading of neuronal processes in the skin tissue. Scratching behaviour was exclusively induced in HDC (+/+) mice, whereas it was barely observed in HDC (–/–) mice. The expression of c‐Fos was specifically upregulated in HDC (+/+) mice in lamina I of the spinal dorsal horn following repeated DCP application. Scratching behaviour in chronic contact dermatitis in mice was thought mainly mediated with histamine. The afferent pathway of sensation in chronic contact dermatitis model may connect with the central nervous system through lamina I of the spinal dorsal horn.