Acetylcholine, determination neurotransmitter

Measurement of Acetylcholine Levels by pH Changes The concentration of acetylcholine (a neurotransmitter) in a sample can be determined from the pH changes that accompany its hydrolysis. When the sample is incubated with the enzyme acetylcholinesterase, acetylcholine is quantitatively converted into choline and acetic acid, which dissociates to yield acetate and a hydrogen ion ... 

The primary objective of ChE-ChOx bienzyme biosensors was to determine neurotransmitter acetylcholine in biological samples (Chen et al, 1998). The redox potential of acetylcholine (ACh) is too high to be determined directly using electrochemical reaction. For this purpose, acetylcholine esterase (AChE) was employed as ChE. AChE and ChOx were immobilized on the surface of the electrode to attain electrochemical analysis according to reactions (4) and (5) (Scheme 55.3). 

Elucidation of the stmctural requirements for dmg interaction at the recognition site is by the study of stmcture—activity relationships (SAR), in which, according to a specific biologic response, the effects of systematic molecular modification of a parent dmg stmcture are determined. Such studies have permitted the classification of discrete classes of pharmacological receptors. For example, the neurotransmitter acetylcholine acts at both peripheral and central receptors which are of at least three distinct types. The effects of acetylcholine are mimicked in smooth and cardiac muscles and secretory... 

Neurotransmitter transporters There are probably at least five types of transport protein specific for glutamate, acetylcholine, catecholamines, glycine/GABA and ATP. The type of transporter contributes to determining the transmitter specificity of a synapse. 

The concept of a neurotransmitter originated in the 1920s with the acetylcholine molecule. Henry Dale and Otto Loewi originated the concept of chemical transmission of nerve impulses. These scientists shared the 1936 Nobel Prize in Physiology or Medicine for this work. Acetylcholine was also the first neurotransmitter for which the structure was determined. Otto Loewi accomplished that task, also in 1936. 

In addition to their functions as presynaptic autoreceptors, 0C2AR can also modulate release of other neurotransmitters (Figure 3). In the CNS, 0C2A and 0C2C receptors inhibit dopamine release in basal ganglia (Bucheler et al. 2002) as well as serotonin secretion in mouse hippocampus and brain cortex (Scheibner et al. 2001a). In the enteric nervous system, the release of acetylcholine as determined by [3H] -choline overflow from tissue slices was selectively inhibited by (X2aAR (Scheibner et al. 2002). 

Acetylcholine Sensors. The general scheme for determination of the neurotransmitter acetylcholine is outlined in Figure 11. In this scheme, acetylcholine is first converted catalytically to choline by the enzyme acetylcholinesterase. The choline produced reduces the FAD redox centers of choline oxidase, and electron transfer from these centers to the electrode is facilitated by the polymeric relay system. 

NMR spectroscopy is useful for determining the conformations of biological molecules such as neurotransmitters. For example, information about the con formations of acetylcholine has been used to design rigid analogs that are used as drugs to treat Alzheimer s disease. 

The neurotransmitter phenotype, (i.e., what type of neurotransmitter is stored and ultimately will be released from the synaptic bouton) is determined by the identity of the neurotransmitter transporter that resides on the synaptic vesicle membrane. Although some exceptions to the rule may exist all synaptic vesicles of a given neuron normally will express only one transporter type and thus will have a dehned neurotransmitter phenotype (this concept is enveloped in what is known as Dale s principle see also Reference 19). To date, four major vesicular transporter systems have been characterized that support synaptic vesicle uptake of glutamate (VGLUT 1-3), GABA and glycine (VGAT), acetylcholine (VAChT), and monoamines such as dopamine, norepinephrine, and serotonin (VMAT 1 and 2). Vesicles that store and release neuropeptides do not have specific transporters to load and concentrate the peptides but, instead, are formed with the peptides already contained within. 

Acetylcholine is another very important neurotransmitter in the central nervous system, but its detection is usually very difl cult due to its low concentration, which falls in the nanomolar or even subnanomolar range. The enzymatic approach to its determination involves the use of two enzymes, acetylcholinesterase (AChE), which produces choline, and choline oxidase (ChOX) that yields H2O2, which is amperometrically detected. Problems arise from the concomitant presence of high amounts of endogenous choline with... 

The methodology discussed above has been used in the resonance assignment and to determine the backbone conformation of a uniformly N-labeled M2 transmembrane peptide of nicotinic acetylcholine receptor (AChR), one of the major excitatory neurotransmitters in the brain. By comparing with spectra from selectively and specifically N-labeled M2 peptides and using the sequential (z to z-Fl) cross peaks in the spin exchange spectra, all the resonances from the M2 peptide were assigned. 

The very large G protein-coupled receptor family has provided many examples of the definition of residue roles in drug interactions. Thus, for the beta-adrenoceptor, critical interacting residues have been determined to be aspartate-113 on helix III, serine-204 and -207 on helix V and phenylalanine-290 on helix VI. Such studies have defined a homologous binding pocket on this receptor family that is shared by the cationic neurotransmitters, acetylcholine, histamine, norepinephrine etc., and related small ligands. 

One of the structurally and otherwise best known complex receptors is the nicotinic acetylcholine receptor (nAChR). The nAChR functions as a neurotransmitter in higher organisms. It is the most completely characterized neurotransmitter and ion channel. More than 20 acetylcholine receptor subunits have been cloned from different species. The monomeric receptor protein, functioning as a non-specific ion channel, consists of five subunits composed of a, P, y, and 6 chains. The composition of nAChR has been determined as a2Py6 (62). Several detailed structural models have been constructed for this receptor (63-65). Because of the prominence of the nAChR several concepts are illustrated exploiting this receptor (Section 6.2, Appendix 10). 

Accumulating electrophysiological and neuroanatomical analyses indicate that some of the CB, CNR receptor is targeted to the presynaptic terminals of neurons where it acts to inhibit release of classical neurotransmitters as reviewed by Elphick and Egertova, (2001). It is therefore tempting to speculate that the endocannabinoid system may be a major player in the reward pathway, particularly as it is one of the most abundant neurochemical systems in the CNS. Other studies report that THC, the major psychoactive constituent in marijuana, inhibits the synaptosomal uptake of dopamine, serotonin, norepinephrine, acetylcholine, and GABA. This, therefore, warrants pre-clinical evaluations to determine the role(s) of the endocannabinoid system in dmg and alcohol dependence and other neuropsychiatric conditions. 

Neurotransmitters may be determined using the respective hydrolases. Acetylcholine is hydrolyzed by acetylcholinesterase (AChE) according to reaction [VII] ... 

It has been determined that the settlement and metamorphosis of both M edulis and M. galloprovincialis can be influenced by the use of a number of exogenously applied chemical agents, thereby increasing the rate of development over that of untreated larvae. In M edulis, it has been found that L-DOPA, a precursor to the neurotransmitters dopamine, norepinephrine and epinephrine, induces metamorphosis (Cooper, 1981), whilst isobutyl-1-methylxanthine (IBMX) and acetylcholine chloride have been found to induce settlement (Eyster and Pechenik, 1987 Dobretsov and... 

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