The extraneuronal cholinergic system of the skin. Basic facts and clinical relevance

Acetylcholine (ACh) is a prototypical neurotransmitter that has recently been recognized to occur extraneuronally in a large variety of cells. ACh and its nicotinic and muscarinic receptors are produced in the epidermis and in the adnexal structures of the skin in highly complicated pattern. They are also produced in melanocytes, fibroblasts, endothelial cells and immune cells. Through autocrine, paracrine and endocrine mechanisms,,the cholinergic system is involved in the basic functions of the skin, such as keratinocyte differentiation, epidermal barrier formation, sweating, sebum production, blood circulation, angiogenesis and variety of immune reactions. Hence diseases like acne vulgaris, vitiligo, psoriasis, pemphigus vulgaris and atopic dermatitis may be influenced. The exploration of the extraneuronal cholinergic system of the skin has only just begun.