Release of non-neuronal acetylcholine from the human placenta: difference to neuronal acetylcholine

The synthesis and release of non-neuronal acetylcholine, a widely expressed signaling molecule, were investigated in the human placenta. This tissue i s free of cholinergic neurons, i.e. a contamination of neuronal acetylcholi ne can be excluded. The villus showed a choline acetyltransferase (ChAT) ac tivity of 0.65 nmol/mg protein per h and contained 500 nmol acetylcholine/g dry weight. In the absence of cholinesterase inhibitors the release of ace tylcholine from isolated villus pieces amounted to 1.3 nmol/g wet weight pe r 10 min corresponding to a fractional release rate of 0.13% per min. The f ollowing substances did not significantly modify the release of acetylcholi ne: oxotremorine (I muM), scopolamine (1 muM), (+)tubocurarine (30 muM), fo rskolin (30 LM), ouabain (10 muM), 4 alpha -phorbol 12,13-didecanoate (1 mu M) and tetrodotoxin (1 muM). Removal of extracellular calcium, phorbol 12,1 3dibutyrate (1 muM) and colchicine (100 muM) reduced the acetylcholine rele ase between 30% and 50%. High potassium chloride (54 mM and 108 mM) increas ed the acetylcholine release slightly (by about 30%). A concentration of 10 muM nicotine was ineffective, but 100 muM nicotine enhanced acetylcholine release gradually over a 50-min period without desensitization of the respo nse. The facilitatory effect of nicotine was prevented by 30 LM (+)tubocura rine. Inhibitors of cholinesterase (physostigmine, neostigmine; 3 muM) faci litated the efflux of acetylcholine about sixfold, and a combination of bot h (+)-tubocurarine (30 muM) and scopolamine (1 muM) halved the enhancing ef fect. In conclusion, release mechanisms differ between non-neuronal and neu ronal acetylcholine. Facilitatory nicotine receptors are present which are activated by applied nicotine or by blocking cholinesterase. Thus, cholines terase inhibitors increase assayed acetylcholine by two mechanisms, protect ion of hydrolysis and stimulation of facilitatory nicotine receptors.