Acetylcholine is even present in the placenta and may help control cell proliferation and differentiation (increases in cell number and changes of multiuse cells into dedicated cellular functions) and parturition.Choline uptake into the brain is controlled by a low-affinity transporter located at the blood–brain barrier. Transport occurs when arterial plasma choline concentrations increase above 14 μmol/L, which can occur during a spike in choline concentration after consuming choline-rich foods. Neurons, conversely, acquire choline by both high- and low-affinity transporters. Choline is stored as membrane-bound phosphatidylcholine, which can then be used for acetylcholine neurotransmitter synthesis later. Acetylcholine is formed as needed, travels across the synapse, and transmits the signal to the following neuron. Afterwards, acetylcholinesterase degrades it, and the free choline is taken up by a high-affinity transporter into the neuron again. Uses
Choline chloride and choline bitartrate are used in dietary supplements. Bitartrate is used more often due to its lower hygroscopicity. Certain choline salts are used to supplement chicken, turkey and some other animal feeds. Some salts are also used as industrial chemicals: for example, in photolithography to remove photoresist. Choline theophyllinate and choline salicylate are used as medicines, as well as structural analogs, like methacholine and carbachol. Radiolabeled cholines, like 11C-choline, are used in medical imaging. Other commercially used salts include tricholine citrate and choline bicarbonate. Antagonists and inhibitors
Hundreds of choline antagonists and enzyme inhibitors have been developed for research purposes. Aminomethylpropanol is among the first ones used as a research tool. It inhibits choline and trimethylglycine synthesis.
Daily 8–20 g doses of choline, for example, have been found to cause low blood pressure, nausea, diarrhea and fish-like body odor. The odor is due to trimethylamine (TMA) formed by the gut microbes from the unabsorbed choline (see trimethylaminuria).The liver oxidizes TMA to trimethylamine N-oxide (TMAO). Elevated levels of TMA and TMAO in the body have been linked to increased risk of atherosclerosis and mortality. Thus, excessive choline intake has been hypothetized to increase these risks in addition to carnitine, which also is formed into TMA and TMAO by gut bacteria. However, choline intake has not been shown to increase the risk of dying from cardiovascular diseases. It is plausible that elevated TMA and TMAO levels are just a symptom of other underlying illnesses or genetic factors that predispose individuals for increased mortality. Such factors may have not been properly accounted for in certain studies observing TMA and TMAO level related mortality. Causality may be reverse or confounding and large choline intake might not increase mortality in humans. For example, kidney dysfunction predisposes for cardiovascular diseases, but can also decrease TMA and TMAO excretion. Health effects
Neural tube closure
Some human studies showed low maternal intake of choline to significantly increase the risk of neural tube defects (NTDs) in newborns. Folate deficiency also causes NTDs. Choline and folate, interacting with vitamin B12, act as methyl donors to homocysteine to form methionine, which can then go on to form SAM (S-adenosylmethionine). SAM is the substrate for almost all methylation reactions in mammals.