Neurotransmitters inactivation

Kennedy, C, Todorov, LD, Mihaylova-Todorova, S and Sneddon, P (1997) Release of soluble nucleotidases a novel mechanism for neurotransmitter inactivation. Trends Pharmacol. Sci. 18 263-266. 

Meanwhile, after interacting with the receptor, the actions of the neurotransmitter must be terminated by means of its reabsorption (8) back into the neuron that originally released it. This is called reuptake. A secondary method of neurotransmitter inactivation is by enzymatic conversion (9) into a chemical that can no longer interact with your brain. Once the neurotransmitter is enzymatically inactivated, it is removed from the brain into the bloodstream (10). Such byproducts of the ordinary hustle and bustle of the brain can be easily monitored in many of our body fluids, and this information can be used to determine whether our brains are functioning normally. 

Neurotransmitter inactivation. A number of animal and plant agents act as ANTICHOLINESTERASES, so interfering with synaptic trasmission. These include fasciculins (Fasl, Fas2, FasS), which are peptide snake venoms, huperzine from a plant, onchidal from molluscs and physostigmine (eserine) from the calabar bean. 

Johnston GAR (1981) Glutamate uptake and its possible role in neurotransmitter inactivation. In Roberts PJ. Storm-Mathisen J, Johnston GAR (Ed), Glutamate Transmitter in the Central Nervous System. Chichester Wiley, pp 77-87. 

Monoamine Oxidase Inhibitors. MAOIs inactivate the enzyme MAO, which is responsible for the oxidative deamination of a variety of endogenous and exogenous substances. Among the endogenous substances are the neurotransmitters, norepinephrine, dopamine, and serotonin. The prototype MAOI is iproniazid [54-92-2] (25), originally tested as an antitubercular dmg and a close chemical relative of the effective antitubercular, isoniazid [54-85-3] (26). Tubercular patients exhibited mood elevation, although no reHef of their tuberculosis, following chronic administration of iproniazid. In... 

Monoamine oxidase (MAO) inactivates serotonergic and catecholaimnergic neurotransmitters MAO (A and B) inhibitors exhibit mood elevatmg properties 5-Fluoro-Ot-methyltryptamine 19) is an important MAO A-seleUive inhibitor In the treatment of certam depressive illnesses, 4-fluorotranylcypromine (20b) is 10 tunes more potent than the parent tranylcypromme (TCP, 20a) The enhanced m vivo activity may be due to increased lipophihcity at20b and/or to blockade of metabohc para hydroxylation [52]... 

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. 

The transporters for 5HT, noradrenaline and dopamine, biogenic monoamines, are genetically related, exist as single isoforms and are expressed on the surface of nerve cells, which use monoamines as (or convert them into) their cognate neurotransmitter. The single-isoform monoamine transporters fulfil all three fundamental functions (reuptake, limiting synaptic transmission, and control of the extracellular neurotransmitter concentration). Inactivation of DAT, NET, or SERT results in an increased extracellular lifetime and level of monoamine neurotransmitter, but decreased intracellular storage and evoked release (Fig. 3). 

Synaptic Transmission. Figure 1 Synaptic transmission. The presynaptic terminal contains voltage-dependent Na Superscript and Ca2+ channels, vesicles with a vesicular neurotransmitter transporter VNT, a plasmalemmal neurotransmitter transporter PNT, and a presynaptic G protein-coupled receptor GPCR with its G protein and its effector E the inset also shows the vesicular H+ pump. The postsynaptic cell contains two ligand-gated ion channels LGIC, one for Na+ and K+ and one for Cl-, a postsynaptic GPRC, and a PNT. In this synapse, released transmitter is inactivated by uptake into cells. 

Figure 20.1 Schematic diagram illustrating how antidepressants increase the concentration of extraneuronal neurotransmitter (noradrenaline and/or 5-HT). In the absence of drug (b), monoamine oxidase on the outer membrane of mitochondria metabolises cytoplasmic neurotransmitter and limits its concentration. Also, transmitter released by exocytosis is sequestered from the extracellular space by the membrane-bound transporters which limit the concentration of extraneuronal transmitter. In the presence of a MAO inhibitor (a), the concentration of cytoplasmic transmitter increases, causing a secondary increase in the vesicular pool of transmitter (illustrated by the increase in the size of the vesicle core). As a consequence, exocytotic release of transmitter is increased. Blocking the inhibitory presynaptic autoreceptors would also increase transmitter release, as shown by the absence of this receptor in the figure. In the presence of a neuronal reuptake inhibitor (c), the membrane-bound transporter is inactivated and the clearance of transmitter from the synapse is diminished...

Anandamide is inactivated in two steps, first by transport inside the cell and subsequently by intracellular enzymatic hydrolysis. The transport of anandamide inside the cell is a carrier-mediated activity, having been shown to be a saturable, time- and temperature-dependent process that involves some protein with high affinity and specificity for anandamide (Beltramo, 1997). This transport process, unlike that of classical neurotransmitters, is Na+-independent and driven only by the concentration gradient of anandamide (Piomelli, 1998). Although the anandamide transporter protein has not been cloned yet, its well characterized activity is known to be inhibited by specific transporter inhibitors. Reuptake of 2-AG is probably mediated by the same facilitating mechanism (Di Marzo, 1999a,b Piomelli, 1999). 

Almost invariably, a neuron is genetically programmed to synthesize and release only a single type of neurotransmitter. Therefore, a given synapse is either always excitatory or always inhibitory. Once a neurotransmitter has bound to its receptor on the postsynaptic neuron and has caused its effect, it is important to inactivate or remove it from the synapse in order to prevent its continuing activity indefinitely. Several mechanisms to carry this out have been identified ... 

For any substance to serve effectively as a neurotransmitter, it must be rapidly removed or inactivated from the synapse or, in this case, the neuroeffector junction. This is necessary in order to allow new signals to get through and influence effector tissue function. Neurotransmitter activity may be terminated by three mechanisms ... 

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. 

Because duration of activity of the catecholamines is significantly longer than that of neuronally released norepinephrine, the effects on tissues are more prolonged. This difference has to do with the mechanism of inactivation of these substances. Norepinephrine is immediately removed from the neuroeffector synapse by way of reuptake into the postganglionic neuron. This rapid removal limits duration of the effect of this neurotransmitter. In... 

G -protein-coupled receptors are often located on the presynaptic plasma membrane where they inhibit neurotransmitter release by reducing the opening of Ca2+ channels like inactivation and breakdown of the neurotransmitter by enzymes, this contributes to the neuron s ability to produce a sharply timed signal. An a2 receptor located on the presynaptic membrane of a noradrenaline-containing neuron is called an autoreceptor but, if located on any other type of presynaptic neuronal membrane (e.g., a 5-HT neuron), then it is referred to as a heteroreceptor (Langer, 1997). Autoreceptors are also located on the soma (cell body) and dendrites of the neuron for example, somatodendritic 5-HTia receptors reduce the electrical activity of 5-HT neurons. 

Psychoactive drugs can influence neurotransmission at its five different stages (Chapter 2). First, they may modify the biosynthesis of a neurotransmitter. Second, they can increase or decrease their storage within the presynaptic neuron. Third, they may stimulate or inhibit neurotransmitter release from the synaptic bouton. Fourth, they may affect the binding of the neurotransmitters to its receptor. Finally, they can retard the neurotransmitter s inactivation. Some examples of each of these stages will be given below, but it should be noted that many drugs affect several of these processes. 

D-amphetamine stimulates the release of monoamines independently of the electrical activity of the neuron. This release leads to an increased concentration of monoamines in the synaptic cleft and, thus, an increase in receptor stimulation. Drugs, such as amphetamine, that do not bind to receptors, but increase the availability of the neurotransmitter by stimulating its release or inhibiting its inactivation, are called indirect agonists. 

Many neurotransmitters are inactivated by a combination of enzymic and non-enzymic methods. The monoamines - dopamine, noradrenaline and serotonin (5-HT) - are actively transported back from the synaptic cleft into the cytoplasm of the presynaptic neuron. This process utilises specialised proteins called transporters, or carriers. The monoamine binds to the transporter and is then carried across the plasma membrane it is thus transported back into the cellular cytoplasm. A number of psychotropic drugs selectively or non-selectively inhibit this reuptake process. They compete with the monoamines for the available binding sites on the transporter, so slowing the removal of the neurotransmitter from the synaptic cleft. The overall result is prolonged stimulation of the receptor. The tricyclic antidepressant imipramine inhibits the transport of both noradrenaline and 5-HT. While the selective noradrenaline reuptake inhibitor reboxetine and the selective serotonin reuptake inhibitor fluoxetine block the noradrenaline transporter (NAT) and serotonin transporter (SERT), respectively. Cocaine non-selectively blocks both the NAT and dopamine transporter (DAT) whereas the smoking cessation facilitator and antidepressant bupropion is a more selective DAT inhibitor. 

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. 

Calmodulin, a calcium binding protein, is involved in Ca2+-dependent regulation of several synaptic functions of the brain synthesis, uptake and release of neurotransmitters, protein phosphorylation and Ca+2 transport. It reacts with TET, TMT and TBT which then inactivates enzymes like Ca+2-ATPase and phosphodiesterase. In vitro studies indicated TBT was greater at inhibiting calmodulin activity than TET and TMT, whereas in vivo the order was TET > TMT > TBT. This may be due to the greater detoxification of TBT (66%) in the liver before moving to other organs30,31. 

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