The role of the cutaneous cholinergic system in guttate psoriasis

In previous studies, high levels of acetylcholine (ACh) have been reported in psoriasis lesions. In addition, patients with guttate psoriasis respond to oral treatment with atropine. We wanted to know how the cutaneous cholinergic system could be involved in this process. Since mast cells (MC) are characteristic components of the inflammatory infiltrate of guttate psoriasis, we compared ACh receptor (AChR) composition and ACh production in both epidermis and mast cells of 10 patients with guttate psoriasis in involved and uninvolved skin on protein level using immunofluorescence and in a MC line (HMC‐1) using PCR. We could confirm the presence of numerous MC in guttate psoriasis lesion. Both in vivo and in vitro, MC lacked expression of cholinacetyltransferase (ChAT), vesicular acetylcholintransorter (VAChT) and cholintransporter‐1 (ChT‐1) but contained high levels of acetylcholinesterase (AChE). In mast cells of both involved and uninvolved skin we found both nicotinic (α3, α5, α7, α9, α10, β2 and β4 subunits) and muscarinic (M1, M3, M4, M5) AChR. In HMC‐1 cells all AChR subunits found in skin where present on mRNA level, except α7 and β2. In lesional epidermis both ACh production and AChR expression was shifted from the basal to the suprabasal layers especially the nicotinic α3, α5, α9, β2 and β4 and the muscarinic M3 and M5 AChR subunits. Our results exclude a role of the cholinergic system in the initiation of keratinocyte proliferation in the basal epidermal layer but point towards a role of epidermal AChR in suprabasal processes, most likely terminal differentiation and barrier formation as has been shown in other systems. Most importantly, mast cells are targets of paracrine and endocrine effects mediated by ACh and choline thus modulating inflammatory processes like guttate psoriasis and explaining the clinical efficacity of anticholinergic drugs like atropine.