Acetylcholine inactivation

Unlike classical neurotransmitters, adenosine does not have a rapid synaptic uptake system (as for the biogenic amines), and its chemical inactivation system is not as rapid as for the transmitter acetylcholine, for example. Adenosine may be metabolized extracellularly and inactivated with respect to the ARs in a more general fashion by the widespread enzymes adenosine kinase (AK, to produce AMP) and adenosine deaminase (AD, to produce inosine). Both AMP and inosine are only weakly active at ARs, depending on the subtype. 

In the following, the cardiac action potential is explained (Fig. 1) An action potential is initiated by depolarization of the plasma membrane due to the pacemaker current (If) (carried by K+ and Na+, which can be modulated by acetylcholine and by adenosine) modulated by effects of sympathetic innervation and (3-adrenergic activation of Ca2+-influx as well as by acetylcholine- or adenosine-dependent K+-channels [in sinus nodal and atrioventricular nodal cells] or to dqjolarization of the neighbouring cell. Depolarization opens the fast Na+ channel resulting in a fast depolarization (phase 0 ofthe action potential). These channels then inactivate and can only be activated if the membrane is hyperpolarized... 

There are numerous transmitter substances. They include the amino acids glutamate, GABA and glycine acetylcholine the monoamines dopamine, noradrenaline and serotonin the neuropeptides ATP and NO. Many neurones use not a single transmitter but two or even more, a phenomenon called cotransmission. Chemical synaptic transmission hence is diversified. The basic steps, however, are similar across all neurones, irrespective of their transmitter, with the exception of NO transmitter production and vesicular storage transmitter release postsynaptic receptor activation and transmitter inactivation. Figure 1 shows an overview. Nitrergic transmission, i.e. transmission by NO, differs from transmission by other transmitters and is not covered in this essay. 

Chin et al. (1992) have su ested that oxidized LDL and high-density lipoprotein (HDL) inactivate endothelial cell-derived NO. NO inactivation was due to the oxidized lipids within the lipoprotein particles and was thought to be explained by a chemical reaction between the lipoproteins and NO. Other investigators have shown that relaxation of vascular smooth muscle by acetylcholine or bradykinin (endothelium-dependent vasodilators) is inhibited by LDL (Andrews etal., 1987). The role of NO in the modification of LDL is discussed in full detail in Chapter 2. 

The primary mechanism used by cholinergic synapses is enzymatic degradation. Acetylcholinesterase hydrolyzes acetylcholine to its components choline and acetate it is one of the fastest acting enzymes in the body and acetylcholine removal occurs in less than 1 msec. The most important mechanism for removal of norepinephrine from the neuroeffector junction is the reuptake of this neurotransmitter into the sympathetic neuron that released it. Norepinephrine may then be metabolized intraneuronally by monoamine oxidase (MAO). The circulating catecholamines — epinephrine and norepinephrine — are inactivated by catechol-O-methyltransferase (COMT) in the liver. 

Finally, some neurotransmitters, like acetylcholine, are inactivated solely by a catabolic enzyme. Acetylcholinesterase rapidly breaks down the neurotransmitter to acetate and choline, and the choline is then actively transported into the presynaptic... 

Anticholinesterase A drug that inhibits the enzyme acetylcholinesterase, which normally inactivates acetylcholine at the synapse. The effect of an anticholinesterase (or cholinesterase inhibitor) is thus to prolong the duration of action of the neurotransmitter. An example is rivastigmine, used in the treatment of Alzheimer s disease. 

Catabolism The breakdown of complex molecules to simpler ones to yield energy (e.g., triacylglycerols to fatty acids) and the inactivation of physiologically active molecules (e.g., acetylcholine to choline and acetic acid). 

The cholinesterase-inhibiting activity of the phosphorofluoridates was compared quantitatively with that of eserine sulphate thus. To 0-2 ml. of heparinized human plasma was added 05 ml. of a solution containing either eserine or the phosphorofluoridate in varying concentrations then the mixture was kept at room temperature for 10 min. before 1 /tg. of acetylcholine in 1 c.c. saline solution was added. After 5 min. at room temperature, the mixture was made up to 10 ml. with frog saline containing eserine 1/100,000, which at once stopped the action of any cholinesterase not yet inactivated. The solution was then assayed for acetylcholine on the frog rectus-muscle preparation. 

All botulin neurotoxins act in a similar way. They only differ in the amino-acid sequence of some protein parts (Prabakaran et al., 2001). Botulism symptoms are provoked both by oral ingestion and parenteral injection. Botulin toxin is not inactivated by enzymes present in the gastrointestinal tracts. Foodborne BoNT penetrates the intestinal barrier, presumably due to transcytosis. It is then transported to neuromuscular junctions within the bloodstream and blocks the secretion of the neurotransmitter acetylcholine. This results in muscle limpness and palsy caused by selective hydrolysis of soluble A-ethylmalemide-sensitive factor activating (SNARE) proteins which participate in fusion of synaptic vesicles with presynaptic plasma membrane. SNARE proteins include vesicle-associated membrane protein (VAMP), synaptobrevin, syntaxin, and synaptosomal associated protein of 25 kDa (SNAP-25). Their degradation is responsible for neuromuscular palsy due to blocks in acetylcholine transmission from synaptic terminals. In humans, palsy caused by BoNT/A lasts four to six months. 

Kulak JM, Nguyen TA, Olivera BM, McIntosh JM (1997) Alpha-conotoxin Mil blocks nicotine-stimulated dopamine release in rat striatal synaptosomes. J Neurosci 17 5263-5270 Kuryatov A, Gerzanich V, Nelson M, Olale F, Lindstrom J (1997) Mutation causing autosomal dominant nocturnal frontal lobe epilepsy alters Ca + permeabihty, conductance, and gating of human a4 32 nicotinic acetylcholine receptors, J Neurosci 17 9035-9047 Kuryatov A, Olale FA, Choi C, Lindstrom J (2000) Acetylchohne receptor extracellular domain determines sensitivity to nicotine-induced inactivation, Eur J Pharmacol 393 11-21 Langley JN (1880) On the antagonism of poisons. J Physiol 3 11-21... 

Kauer JA, Malenka RC (2007) Synaptic plasticity and addiction. Nat Rev Neurosci 8 844-858 Kawai H, Berg DK (2001) Nicotinic acetylcholine receptors containing alpha subunits on rat cortical neurons do not undergo long-lasting inactivation even when up-regulated by chronic nicotine exposure. J Neurochem 78 1367-1378... 

There are three ways to increase acetylcholine activity (1) increase the supply of acetylcholine, (2) directly stimulate acetylcholine receptors (muscarinic agonists), and (3) block the enzyme that inactivates acetylcholine (cholinesterase inhibitors). Let s take a look at each of these approaches. 

Figure 14.9 Axonal transport of enzymes, neurotransmitter synthesis, storage in vesicles, release and uptake by presynaptic neurone or enzymic degradation. The neurotransmitter in the synaptic cleft may be removed by the presynaptic neurone (i.e. recycling), by the postsynaptic neurone or by glial cells (not shown). Alternatively, the neurotransmitter may be degraded, and therefore inactivated, by enzyme action. For example, acetylcholine is degraded by acetylcholinesterase in the synaptic cleft (Chapter 3). One of the products, choline, is transported back into the neurone to be reacted with acetyl-CoA to re-form acetylcholine. The vesicle, once empty, may also be recycled for re-packaging (Figure 14.8).

The enzyme becomes inactivated, and a toxic level of acetylcholine builds up. Organophosphorus compounds provide a range of insecticides and nerve gases. 

The special case of the endogenous transmitter acetylcholine illustrates well the high velocity of ester hydrolysis. Acetylcholine is broken down at its sites of release and action by acetylcholinesterase (pp. 100,102) so rapidly as to negate its therapeutic use. Hydrolysis of other esters catalyzed by various esterases is slower, though relatively fast in comparison with other biotransformations. The local anesthetic, procaine, is a case in point it exerts its action at the site of application while being largely devoid of undesirable effects at other locations because it is inactivated by hydrolysis during absorption from its site of application. 

Acetylcholine (ACh) is too rapidly hydrolyzed and inactivated by acetylcholinesterase (AChE) to be of any therapeutic use however, its action can be mimicked by other substances, namely direct or indirect parasympathomimetics. 

Selected entries from Methods in Enzymology [vol, page(s)] Sulfonylation reaction, 11, 706 reaction kinetics, 11, 707 second-order rate constants for inactivation of chymotrypsin, trypsin, and acetylcholine esterase by PMSE and related sulfonylat-ing agents, 11, 707 reactivation of PMS-chymotrypsin, 11, 710 as inhibitor [of calcium-activated factor, 80, 674 of cathepsin G, 80, 565 of crayfish trypsin, 80, 639 of elastase, 80, 587 of pro-lylcarboxypeptidase, 80, 465 of protease Re, 80, 691 of protease So, 80, 695 of protein C, 80, 329] proteolysis, 76, 7. 

The clinical picture of carbaryl intoxication results from inactivation of cholinesterase, resulting in the accumulation of acetylcholine at synapses in the nervous system, skeletal and smooth muscle, and secretory glands. Signs and symptoms of overexposure may include (1) muscarinic manifestations such as miosis, blurred vision, lacrimation, excessive nasal discharge or salivation, sweating, abdominal cramps, nausea, vomiting, and diarrhea (2) nicotinic manifestations including fasiculation of fine muscles and tachycardia and (3) central nervous system manifestations characterized by headache, dizziness, mental confusion, convulsions, coma, and depression of the respiratory center. 

At least four fatal, several severe nonfatal, and a number of mild cases of demeton intoxication have been reported. Both animal experiments and human exposures suggest that the toxicity and potency of demeton is similar to that of parathion. Signs and symptoms of overexposure are caused by the inactivation of the enzyme cholinesterase, which results in the accumulation of acetylcholine at synapses in the nervous system, skeletal and smooth muscle, and secretory glands.The sequence of the development of systemic effects varies with the route of entry. The onset of signs and symptoms is usually prompt but may be... 

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