Identification of acetylcholine and impact of cholinomimetic drugs on cell differentiation and growth in the unicellular green alga Micrasterias denticulata

Acetylcholine (ACh), one of the best-studied neurotransmitters, has been reported in animals as well as in multicellular plants. In higher plants, ACh affects phytochrome-dependent growth and differentiation. In the present study on the green alga Micrasterias denticulata which has been used as a cell biological model system since several decades, we identified ACh for the first time in a unicellular alga, demonstrated light as a regulatory factor of ACh production and showed that cholinergic agonists and antagonists suppress growth and differentiation. ACh was detected in Micrasterias cells grown under light–dark conditions of 14/10 h, but not in dark-grown algae, by high-performance liquid chromatography coupled with mass spectrometry. To quantify cholinergic effects on cell differentiation, we exposed young developmental stages of Micrasterias to cholinergic antagonists (d-tubocurarine, hexamethonium) as well as agonists (carbachol, nicotine). We found that the cholinergic antagonists and, surprisingly, the agonist nicotine significantly suppressed cell growth and differentiation in a dose-dependent manner. Moreover, we demonstrated that secondary wall formation is specifically inhibited in the presence of nicotine.