Characterisation of cholinesterases in mucous secretions and their localisation in epidermis of Labeo rohita and Cirrhinus mrigala Ashwini Kumar Nigam & Neeraj Verma & Ayan Srivastava & Usha Kumari & Swati Mittal & Ajay Kumar Mittal Received: 10 July 2018 /Accepted: 24 May 2019 # Springer Nature B.V. 2019 Abstract Cholinesterases are multifunctional enzymes and have been associated with diverse physiological functions in addition to their classical role at synapses. In the present study, cholinesterase (ChE) isozymes have been characterised in mucous secretions and their activity has been localised in the epidermis of Labeo rohita and Cirrhinus mrigala. Zymography using spe- cific substrates and inhibitors revealed the presence of two ChE isozymes—ChE-1 and ChE-2. The isozyme ChE-1 was characterised as an atypical butyrylcholines- terase and ChE-2 as a typical acetylcholinesterase in skin mucous secretions of both the fish species. Enzyme histochemical analysis demonstrated the presence of ChE activity in the epidermis of the fish species inves- tigated. In both the fish species, strong ChE activity was observed in the outer-layer epithelial cells, taste buds and neuromasts. The middle and basal layer epithelial cells showed moderate to weak ChE activity. Club cells and mucous goblet cells showed the absence of ChE activity. Characterisation with specific inhibitors indi- cates that acetylcholinesterase (AChE) was the major cholinesterase type expressed in the epidermis of the two fish species investigated. Immunohistochemical localisation of apoptotic and cell proliferation markers, in addition, revealed high expression of active caspase 3 in the outer-layer epithelial cells, and proliferating cell nuclear antigen (PCNA) in the middle and basal layer epithelial cells. High ChE activity in caspase 3-positive cells in the outer layer of the epidermis and low in PCNA-positive cells in middle and basal layers could point towards the possible involvement of ChEs in cell death and their final extrusion from skin surface. Keywords Fish . Epidermis . Mucus . Epithelial cells, Cholinesterases . Apoptosis Introduction Cholinesterases belong to an important group of B-type serine esterases (Thompson 1999) that are considered multifunctional enzymes involved in a wide spectrum of physiological processes (Karczmar 2010). Cholinester- ases include two crucial enzymes—acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). AChE is the key enzyme at cholinergic synapses and neuromuscular junctions for degradation of acetylcholine to terminate nerve impulse (Walker and Thompson 1991). BChE, a less specific enzyme having similar catalytic activity, is Fish Physiol Biochem https://doi.org/10.1007/s10695-019-00663-0 A. K. Nigam : N. Verma : A. Srivastava : S. Mittal (*) Department of Zoology, Skin Physiology Laboratory, Centre of Advanced Study, Banaras Hindu University, Varanasi 221005, India e-mail: drsmittal73@gmail.com U. Kumari Zoology Section, Mahila Mahavidyalaya, Banaras Hindu University, Varanasi 221005, India Present Address: A. K. Mittal Former Head of Department of Zoology, Banaras Hindu University, 9, Mani Nagar, Kandawa, Near Chitaipur Crossing, Varanasi 221106, India profakmittal@gmail.com