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There is increasing evidence that neurotransmitters play a crucial role in skin physiology and pathology. The expression and production of proopiomelanocortin molecules such as beta-endorphin in human epidermis suggest that an opiate receptor is present in keratinocytes. In this paper we show that human epidermal keratinocytes express a mu-opiate receptor on both the mRNA level and the protein level. Performing polymerase chain reaction with cDNA libraries from human epidermal keratinocytes gave the polymerase chain reaction products of the expected length, which were confirmed as mu-opiate receptors by Southern blot analysis. Using in situ hybridization techniques with a specific probe for mu-opiate receptors we detected the receptor in human epidermis. There was a cytoplasmic expression in all layers of the epidermis, which was more distinct in the suprabasal layers. Immunohistochemistry using the mu-opiate receptor-specific antibody indicates that epidermis expresses protein as well, and that the protein level is more elevated in the basal layer. The correlation between the locations of both mRNA and protein expression in skin indicates that the mu-opiate receptor has not only been transcribed but also has a specific function. To prove a function of the receptor we performed a functional assay using skin organ cultures from human skin transplants. After 48 h incubation with Naloxone or beta-endorphin the expression of the mu-opiate receptor in epidermis was significantly downregulated compared with the control. These results show that a functional receptor indeed exists in human epidermis.

We have previously shown that human epidermal keratinocytes express a functionally active µ-opiate receptor, which adds a new dimension to the recently developed research in neuroimmunodermatology and neurogenic inflammation in skin diseases. Human keratinocytes specifically bind and also produce β-endorphin, the endogenous µ-opiate receptor ligand. Using confocal imaging microscopy, we could now demonstrate that µ-opiate receptors are not only expressed in keratinocytes, but also on unmyelinated peripheral nerve fibers in the dermis and epidermis. Some of the peripheral nerve fibers also express the ligand β-endorphin. The keratinocytes positive for β-endorphin staining are clustered around the terminal ends of the unmyelinated nerve fibers. Therefore the opiate receptor system seems to be crucial in the direct communication between nerves and skin. The keratinocytes can influence the unmyelinated nerve fibers in the epidermis directly via secreting β-endorphin. On the other hand, nerve fibers can also secrete β-endorphin and influence the migration, differentiation and probably also the cytokine production pattern of keratinocytes.

Skin biopsy specimens from 4 patients with typical acne rosacea lesions were examined for the presence of T-cell subsets using monoclonal antibodies. The infiltrates consisted chiefly of LEU-1 reactive T cells with a predominance of LEU-3a antibody positive helper-inducer T cells, while LEU-2a staining suppressor-cytotoxic T cells were scarce. These cells penetrated regularly into the follicular wall and the epidermis. The study showed that most T cells in the dermal granulomatous infiltrates around Demodex parts, which were displaced extrafoUicularly, were helper-inducer T cells. The predominance of helper-inducer T-cell subsets in the dermal infiltrates of acne rosacea lesions in frequent association with Demodex supports the hypothesis that a cell-mediated immune response plays an important role in the pathogenesis of rosacea.

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