Acetylcholine precursors

Acetylcholine Precursors. Early efforts to treat dementia using cholinomimetics focused on choline [62-49-7] (12) supplement therapy (Fig. 3). This therapy, analogous to L-dopa [59-92-7] therapy for Parkinson s disease, is based on the hypothesis that increasing the levels of choline in the brain bolsters acetylcholine (ACh) synthesis and thereby reverses deficits in cholinergic function. In addition, because choline is a precursor of phosphatidylcholine as well as ACh, its supplementation may be neuroprotective in conditions of choline deficit (104). 

In some cases mania is reduced and depression induced by lecithin, an acetylcholine precursor ... 

Acetylcholine Precursors. Your nerve cells produce acetylcholine from certain dietary precursors (choline and lecithin). Many early studies tried dietary supplements of these precursors. A precedent for this approach was established using the dopamine precursor, L-DOPA, a well-established treatment for Parkinson s disease. Unfortunately, this approach is ineffective in dementia. It appears that the daily doses of these fatty acid precursors needed to have any discernible impact on acetylcholine levels far exceed what an individual can reasonably take in a day. This approach has therefore been abandoned. 

With reference to divided attention, an association has been reported between blood glucose levels and performance on effortful dichotic listening paradigm (Parker Benton, 1995). While similar effects have been attributed to acetylcholine precursor (acetyl CoA) synthesis linked to glucose metabolism (Wenk, 1989), it is also possible that simple provision of metabolic substrates may account for such findings (Kennedy Scholey, 2000). 

Choline has been given in the past as an acetylcholine precursor to raise acetylcholine concentrations in the brain and thus enhance cholinergic neurotransmission. There is evidence that the symptoms of tardive dyskinesia can be reduced by choline or lecithin (1). 

Acetylcholine precursors such as lecithin have been tried, but are of limited effectiveness. 

Several cholinergic strategies, other than cholinesterase inhibition, have been employed with the intention of ameliora ting the symptoms of AD. These include precursor loading acetylcholine release enhancement, and direct activation of both muscarinic and nicotinic receptors. 

Naturally occurring quaternary ammonium compounds have been reviewed (179). Many types of aliphatic, heterocycHc, and aromatic derived quaternary ammonium compounds are produced both in plants and invertebrates. Examples include thiamine (vitamin B ) (4) (see Vitamins) choline (qv) [62-49-7] (5) and acetylcholine (6). These have numerous biochemical functions. Several quaternaries are precursors for active metaboUtes. 

To achieve their different effects NTs are not only released from different neurons to act on different receptors but their biochemistry is different. While the mechanism of their release may be similar (Chapter 4) their turnover varies. Most NTs are synthesised from precursors in the axon terminals, stored in vesicles and released by arriving action potentials. Some are subsequently broken down extracellularly, e.g. acetylcholine by cholinesterase, but many, like the amino acids, are taken back into the nerve where they are incorporated into biochemical pathways that may modify their structure initially but ultimately ensure a maintained NT level. Such processes are ideally suited to the fast transmission effected by the amino acids and acetylcholine in some cases (nicotinic), and complements the anatomical features of their neurons and the recepter mechanisms they activate. Further, to ensure the maintenance of function in vital pathways, glutamate and GABA are stored in very high concentrations (10 pmol/mg) just as ACh is at the neuromuscular junction. 

Acetylcholine is synthesised in nerve terminals from its precursor choline, which is not formed in the CNS but transported there in free form in the blood. It is found in many foods such as egg yolk, liver and vegetables although it is also produced in the liver and its brain concentration rises after meals. Choline is taken up into the cytoplasm by a high-affinity (Am = 1-5 pM), saturable, uptake which is Na+ and ATP dependent and while it does not appear to occur during the depolarisation produced by high concentrations of potassium it is increased by neuronal activity and is specific to cholinergic nerves. A separate low-affinity uptake, or diffusion (Am = 50 pM), which is linearly related to choline concentration and not saturable, is of less interest since it is not specific to cholinergic neurons. 

Acetylcholine synthesis and neurotransmission requires normal functioning of two active transport mechanisms. Choline acetyltransferase (ChAT) is the enzyme responsible for ACh synthesis from the precursor molecules acetyl coenzyme A and choline. ChAT is the neurochemical phenotype used to define cholinergic neurons although ChAT is present in cell bodies, it is concentrated in cholinergic terminals. The ability of ChAT to produce ACh is critically dependent on an adequate level of choline. Cholinergic neurons possess a high-affinity choline uptake mechanism referred to as the choline transporter (ChT in Fig. 5.1). The choline transporter can be blocked by the molecule hemicholinium-3. Blockade of the choline transporter by hemicholinium-3 decreases ACh release,... 

Acetyl coenzyme A formed from glucose is the precursor for acetylcholine 543... 

Citrate has a similar role in proliferating (including tumour) cells and in neurones. Acetyl-CoA is a precursor for many compounds in the former and for formation of acetylcholine in neurones (see Appendix 9.4). 

Medicinal chemistry has many examples of the development of successful therapeutics based on an exploration of endogenous compounds. The treatment of diabetes mellitus, for example, is based upon the administration of insulin, the hormone that is functionally deficient in this disease. The current treatment of Parkinson s disease is based upon the observation that the symptoms of Parkinson s disease arise from a deficiency of dopamine, an endogenous molecule within the human brain. Since dopamine cannot be given as a drug since it fails to cross the blood-brain barrier and enter the brain, its biosynthetic precursor, L-DOPA, has been successfully developed as an anti-Parkinson s drug. Analogously, the symptoms of Alzheimer s disease arise from a relative deficiency of acetylcholine within the brain. Current therapies for Alzheimer s-type dementia are based upon the administration of cholinesterase... 

Enhancing cholinergic function in the brain by increased supply of the biological precursors of acetylcholine, lecithin or choline, similar to the treatment of Parkinson s disease with L-dopa, the precursor of dopamine. This approach, which was tried in dozens of short- and long-term studies, proved to be unsuccessful (Becker and Giacobini, 1988). 

Pharmaceuticals. Lecithin and especially purified phosphatidylcholine can act as excipients in pharmaceutical (drug) formulation to enhance and control the Unavailability of the active component. Moreover, phosphatidylcholine can be utilized as a diedelic source, as it involved in the cholesterol metabolism and the metabolism of fats in the liver also, it can be utilized as a precursor of brain acetylcholine, as neurotransmiticr. 

Acetylcholine synthesis. Acetylcholine (ACh) is a prominent neurotransmitter, which is formed in cholinergic neurons from two precursors, choline and acetyl coenzyme A (AcCoA) (Fig. 12—8). Choline is derived from dietary and intraneuronal sources, and AcCoA is synthesized from glucose in the mitochondria of the neuron. These two substrates interact with the synthetic enzyme choline acetyltransferase to produce the neurotransmitter ACh. 

Noda, M. et al. (1982). primary structure of the a subunit precursor of Torpedo acetylcholine receptor deduced from cDNA sequence. Nature (London) 299, 793-797. 

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