ACh is made up of choline and acetate. These precursors must be readily available to the neuron terminal at all times so that ACh can be synthesized whenever it is needed.
Choline is a compound that can be obtained from foods such as egg yolks, kidney, liver, seeds, legumes, and various vegetables. It is also naturally produced by the liver. Free choline circulating in blood plasma readily crosses the blood-brain barrier and is taken up by cholinergic nerve terminals, for the most part, by a high-affinity choline uptake (HACU) system that is temperature-, energy- and sodium-dependent. This system is the primary means by which choline needed for the synthesis of ACh is transported into the neuron and is thus thought to act as a rate-limiting step in the production of the neurotransmitter.
It has been found, however, that when metabolic demands for ACh are exceptionally high, a low-affinity choline uptake (LACU) system steps in to assist in transporting additional choline into the nerve terminal. This system is less energy- and sodium-dependent than the HACU system. The LACU system is also present in a much wider range of tissue-types than the HACU and is not exclusively associated with the synthesis of ACh. The LACU system provides many non-neuronal cells with the choline they require to carry out processes that are unrelated to the synthesis of ACh (e.g. the synthesis of various choline-containing phospholipids).
The nature of the high affinity uptake system guarantees that cholinergic cells will always receive priority access to all available choline. Because of this, choline will be transported into non-cholinergic cells for use in processes other than the synthesis of ACh only after the ACh requirements of the cholinergic neurons have been met. Though choline is ultimately obtained through dietary intake and synthesis in the liver, the immediate source for choline comes from the products of the hydrolysis of ACh in the synaptic cleft. In fact, it is estimated that 35-50% of the choline released from the breakdown of ACh in the synaptic cleft is taken up again by the presynaptic neuron.