Acetylcholine has also been discovered in some primitive plants. All of this suggests that acetylcholine has played a vital role in the evolutionary history of life on earth, going back an astounding 3 billion years or so, and that this role, far from originating for purposes of neurotransmission, was much more basic than that, having to do with fundamental aspects of cellular development and function. A particularly important role of ACh in humans is in the function of epithelial cells. These are smooth, tightly packed cells that constitute the membranous lining, inside and outside, of most organs. They form the primary protective barrier against assault by potentially harmful substances, such as bacteria, viruses, and fungi, in their immediate environment.
Epithelial cells are thus extremely important in maintaining homeostasis.
They appear to be directly involved in regulation of the immune response, such as the release of antibodies to attack and destroy microbial invaders. Acetylcholine activity has been detected in epithelial cells of the human airway, the alimentary tract, the kidneys, the urogenital tract, the eyes, the placenta, and the skin, as well as in glandular tissue of the female breast. And it has been observed that the vast majority of epithelial cells contain both nicotinic and muscarinic receptors, the same kinds of specialized molecular receptors that are activated by ACh in the nervous system. That cannot be a coincidence. Because ACh deficiency is characteristic of Alzheimer's disease and other age-related cognitive impairments, the primary treatment is administration of agents that inhibit the action of AChE. One such agent is galantamine, a compound extracted from the snowdrop and daffodil, among other plants.