Evolutional study on acetylcholine expression.
Acetylcholine (ACh) is a well-known neurotransmitter in the cholinergic nervous systems of vertebrates and insects; however, there is only indirect evidence for its presence in lower invertebrates, such as plants and fungi. We therefore investigated the expression of ACh in invertebrates (sea squirt, sea urchin, trepang, squid, abalone, nereis, sea anemone, coral and sponge), plants (arabidopsis, eggplant, bamboo shoot, cedar, hinoki, pine, podcarp, fern, horsetail and moss), fungi (yeast and mushroom) and bacteria by assaying ACh content and synthesis, focusing on the presence of two synthetic enzymes, choline acetyltransferase (ChAT) and carnitine acetyltransferase (CarAT). Using a specific radioimmunoassay, ACh was detected in all samples tested. The levels varied considerably, however, with the upper portion of bamboo shoots having the highest content (2.9 micromol/g). ACh synthesis was also detected in all samples tested; moreover, the activity in most samples from the animal kingdom, as well as bamboo shoots and the stem of the shiitake mushroom, were sensitive to both ChAT and CarAT inhibitors. Levels of ACh synthesis were lower in samples from other plants, fungi and bacteria and were insensitive to ChAT and CarAT inhibitors. These findings demonstrate the presence of ACh and ACh-synthesizing activity in evolutionally primitive life as well as in more complex multicellular organisms. In the context of the recent discovery of non-neuronal ACh in various mammalian species, these findings suggest that ACh been expressed in organisms from the beginning of life, functioning as a local mediator as well as a neurotransmitter.