Hemicholinium

The reaction of choline with mitochondrial bound acetylcoenzyme A is catalysed by the cytoplasmic enzyme choline acetyltransferase (ChAT) (see Fig. 6.1). ChAT itelf is synthesised in the rough endoplasmic reticulum of the cell body and transported to the axon terminal. Although the precise location of the synthesis of ACh is uncertain most of that formed is stored in vesicles. It appears that while ChAT is not saturated with either acetyl-CoA or choline its synthesising activity is limited by the actual availability of choline, i.e. its uptake into the nerve terminal. No inhibitors of ChAT itself have been developed but the rate of synthesis of ACh can, however, be inhibited by drugs like hemicholinium or triethylcholine, which compete for choline uptake into the nerve. 

Apart from inhibiting the uptake of choline and hence its availability for ACh synthesis, with hemicholinium (see above), there are no drugs that directly affect the actual storage or release of ACh. Some experimental tools have, however, been used such as vesamicol, which appears to block the packaging of ACh into its vesicles and thus initiates the slow rundown of releasable vesicular ACh. Some toxins also inhibit ACh release. 

ACh is widely distributed throughout the brain and parts of the spinal cord (ventral horn and dorsal columns). Whole brain concentrations of lOnmolg" tissue have been reported with highest concentrations in the interpeduncular, caudate and dorsal raphe nuclei. Turnover figures of 0.15-2.0 nmol g min vary with the area studied and the method of measurement, e.g. synthesis of labelled ACh from [ " C]-choline uptake or rundown of ACh after inhibition of choline uptake by hemicholinium. They are all sufficiently high, however, to suggest that in the absence of synthesis depletion could occur within minutes. 

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,... 

Ferguson, S. M., Bazalakova, M., Savchenko, V. et al. (2004). Lethal impairment of cholinergic neurotransmission in hemicholinium-3-sensitive transporter knockout mice. Proc. Natl Acad. Sci. USA 101, 8762-7. 

Crilly, K.S., Tomono, M. and Kiss, Z. (1998) The choline kinase inhibitor hemicholinium-3 can inhibit mitogen-induced DNA synthesis independent of its effect on phosphocholine formation. Archives of Biochemistry and Biophysics 352, 137-143. 

Hamza, M., Lloveras,J., Ribbes, G., Soula, G. and Douste-Blazy, L. (1983) An in vitro study of hemicholinium-3 on phospholipid metabolism of Krebs II ascites cells. Biochemical Pharmacology 32, 1893—1897. 

There is also evidence for cholinergic involvement in caffeine analgesia (Ghelardini et al. 1997). The muscarinic antagonists atropine and pirenzepine, and the choline uptake inhibitor hemicholinium-3 prevent caffeine analgesia. In contrast, it was unaffected by an opioid antagonist (naloxone) or a tyrosine hydroxylase inhibitor (o-methyl-p-tyrosine). 

The answers are 333-c, 334-a, 333-d. (Katzung, pp 77-80. Hardman, pp 116, 132, 147—148.) Acetylcholine is synthesized from acetyl-CoA and choline. Choline is taken up into the neurons by an active transport system. Hemicholinium blocks this uptake, depleting cellular choline, so that synthesis of ACh no longer occurs. 

Schematic illustration of a generalized cholinergic junction (not to scale). Choline is transported into the presynaptic nerve terminal by a sodium-dependent choline transporter (CHT). This transporter can be inhibited by hemicholinium drugs. In the cytoplasm, acetylcholine is synthesized from choline and acetyl -A (AcCoA) by the enzyme choline acetyltransferase (ChAT). Acetylcholine is then transported into the storage vesicle by a second carrier, the vesicle-associated transporter (VAT), which can be inhibited by vesamicol. Peptides (P), adenosine triphosphate (ATP), and proteoglycan are also stored in the vesicle. Release of transmitter occurs when voltage-sensitive calcium channels in the terminal membrane are opened, allowing an influx of calcium. The resulting increase in intracellular calcium causes fusion of vesicles with the surface membrane and exocytotic expulsion of acetylcholine and cotransmitters into the junctional cleft (see text). This step can he blocked by botulinum toxin. Acetylcholine s action is terminated by metabolism by the enzyme acetylcholinesterase. Receptors on the presynaptic nerve ending modulate transmitter release. SNAPs, synaptosome-associated proteins VAMPs, vesicle-associated membrane proteins.

Transmitter synthesis Hemicholinium Cholinergic nerve terminals membrane Blocks uptake of choline and slows synthesis... 

Hemicholinium blocks the transport of choline into synaptosomes, whereas botulinum toxin blocks the calcium-mediated release of acetylcholine. The released acetylcholine is hydrolyzed rapidly by acetylcholinesterase to choline and acetate. 

Synthesis of acetylcholine Choline is transported from the extracellular fluid into the cytoplasm of the cholinergic neuron by a carrier system that cotransports sodium and can be inhibited by the drug hemicholinium. Choline acetyltransferase (CAT) catalyzes the reaction of choline with acetyl CoA to form acetylcholine in the cytosol. 

Muscle relaxation is caused by various mechanisms, such as depression of ACh synthesis (e.g, hemicholinium), storage (vesamicol) and release (magnesium, botulinum toxin) and depression of muscle activity by drugs such as dantrolene and baclofen,... 

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