Norepinephrine vesicular transporter

Since the enzyme that converts dopamine to norepinephrine (dopamine (3-hydroxylase) is located only within the vesicles, the transport of dopamine into the vesicle is an essential step in the synthesis of norepinephrine. This same transport system is essential for the storage of norepinephrine. There is a tendency for norepinephrine to leak from the vesicles into the cytosol. If norepinephrine remains in the cytosol, much of it will be destroyed by a mitochondrial enzyme, monoamine oxidase MAO). However, most of the norepinephrine that leaks out of the vesicle is rapidly returned to the storage vesicles by the same transport system that carries dopamine into the storage vesicles. It is important for a proper understanding of drug action to remember that this single transport system, called vesicular transport, is an essential element of both synthesis and storage of norepinephrine. 

It is important to make a clear distinction between neuronal and vesicular transport. Neuronal transport occurs from the junctional extracellular fluid (biophase) across the cell membrane of the neuron and into the neuronal cytosol. Vesicular transport is from the neuronal cytosol across the membrane of the vesicle and into the vesicle. Although these two systems readily transport both norepinephrine and epinephrine, certain drugs will selectively inhibit one or the other transport system. 

The effect of released norepinephrine wanes quickly, because -90% is transported back into the axoplasm by a specific transport mechanism (norepinephrine transporter, NAT) and then into storage vesicles by the vesicular transporter (neuronal reuptake). The NAT can be inhibited by tricyclic antidepressants and cocaine. Moreover, norepinephrine is taken up by transporters into the effector cells (extraneuronal monoamine transporter, EMT). Part of the norepinephrine undergoing reuptake is enzymatically inactivated to normetanephrine via catecholamine O-methyltransferase (COMT, present in the cytoplasm of postjunctional cells) and to dihydroxymandelic acid via monoamine oxidase (MAO, present in mitochondria of nerve cells and postjunctional cells). 

Uptake of amine NTs from the neuronal cytosol into synaptic vesicles is achieved by vesicular monoamine transporters (VMAT1 and VMAT2) that sequester dopamine, epinephrine, norepinephrine and serotonin. A similar vesicle transporter (VGAT) sequesters GABA and glycine and a vesicular transporter (VAChT) sequesters acetylcholine into synaptic vesicles. 

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. 

The vesicular monoamine transporters (VMATs) were identified in a screen for genes that confer resistance to the parkinsonian neurotoxin MPP+ [2]. The resistance apparently results from sequestration of the toxin inside vesicles, away from its primary site of action in mitochondria. In addition to recognizing MPP+, the transporter s mediate the uptake of dopamine, ser otonin, epinephrine, and norepinephrine by neurons and endocrine cells. Structurally, the VMATs show no relationship to plasma membrane monoamine transporters. 

The neuronal membrane norepinephrine transporter (NET), the dopamine transporter (DAT) and the vesicular membrane transporter (VMAT-2), which is the same in all catecholamine-containing neurons, have similar numbers of predicted transmembrane segments. They have different numbers of amino acids, pharmacological properties and chromosomal localizations. 

Transport back into the noradrenergic neuron (reup-take), followed by either vesicular storage or by enzymatic inactivation by mitochondrial MAO. The transport of norepinephrine into the neurons is a sodium-facilitated process similar to that for choline transport. 

The norepinephrine transporter (NET) and the vesicular monoamine transporter (VMAT) are presynaptic components of the sympathetic neurons. NET is a Na+ /Cl -dependent transport protein and responsible for the neurotransmitter uptake from the synaptic cleft into the cytoplasm of the neurons. This transport process, called uptake-1, reduces the amount and, thus, the effect of NE released into the synaptic cleft. NE is stored in the cytoplasm of the neurons in specialized vesicles by the H+-dependent transport protein VMAT. Two isoforms VMAT1 and VMAT2, are known. VMAT is localized in the vesicle membranes, and the vesicular storage protects NE from metabolism by monoamine oxidase (MAO), which is localized on the surface membrane of the mitochondria. Vice versa, nerve depolarisation causes NE release from the vesicles into the synaptic cleft by Ca+-mediated exocytose (Fig. 12) [79,132-136],... 

MDR1, multidrug resistance protein-1 MRP1, multidrug resistance-associated protein 1 NET, norepinephrine transporter SERT, serotonin reuptake transporter VMAT, vesicular monoamine transporter. 

Pharmacologic targeting of monoamine transporters. Commonly used drugs such as antidepressants, amphetamines, and cocaine target monoamine (norepinephrine, dopamine and serotonin) transporters with different potencies. A shows the mechanism of reuptake of norepinephrine (NE) back into the noradrenergic neuron via the norepinephrine transporter (NET), where a proportion is sequestered in presynaptic vesicles through the vesicular monoamine transporter (VMAT). and C show the effects of amphetamine and cocaine on these pathways. See text for details. 

HTxR, serotonin receptor CB1R, cannabinoid-1 DAT, dopamine transporter GABA, y-aminobutyric acid Kir3 channels, G protein-coupled inwardly rectifying potassium channels LSD, lysergic acid diethylamide i -OR, H-opioid receptor nAChR, nicotinic acetylcholine receptor NET, norepinephrine transporter NMDAR, N -methyl-D-aspartate receptor SERT, serotonin transporter VMAT, vesicular monoamine transporter indicates data not available. 

Amphetamine s primary effects (increased wakefulness, appetite suppression, and increased locomotor activity) are thought to be mediated by the release of norepinephrine from noradrenergic neurons in the CNS (36). However, research points to the role of plasma transport inhibition of dopamine, norepinephrine, and serotonin as well as inhibition of the vesicular monoamine transporter (138). Wisor et al. (139) summarize evidence that dopamine reuptake inhibition produces a greater alerting effect than norepinephrine transport blockade. 

Indirect sympathomimetics (B) in the narrow sense comprise amphetamine-like substances and cocaine. Cocaine blocks the norepinephrine transporter (NAT), besides acting as a local anesthetic. Amphetamine is taken up into varicosities via NAT, and from there into storage vesicles (via the vesicular monoamine transporter), where it displaces NE into the cytosol. In addition, amphetamine blocks MAO, allowing cytosolic NE concentration to rise unimpeded. This induces the plasmalemmal NAT to transport Luellmann, Color Atlas of Pharmacology All rights reserved. Usage subject to terms... 

Tetrabenazine inhibits the vesicular monoamine transporter, which results in a depletion of stores of norepinephrine (and to a lesser extent dopamine and serotonin) in the central nervous system. 

In addition to terminating the actions of released monoamines, the plasma membrane monoamine transporters present at neuronal locations function in sequence with vesicular monoamme transporters to recycle catecholamines for rerelease (Figure 29-3). Thus most of the norepinephrine released and recaptured by sympathetic nerves is sequestered back into storage vesicles, thereby substantially reducing the requirements for synthesis of new transmitter. 

Vesicular monoamine transporter type 2 (VMAT2) is located on the membrane of the intracellular storage vesicle, and it transports all biogenic amines (e.g. serotonin, norepinephrine, dopamine, acetylcholine, histamine) with practically equivalent affinity. Regional localization of VMAT2 is consistent with the known monoamine nerve terminal density it is highest in the striatum, lateral septum, substantia nigra pars compacta, raphe nucleus, and locus ceruleus. Lower density is evident in the cerebral cortex and in the cerebellum. 

The vesicular carrier in the diagram transports dopamine and norepinephrine into the vesicles for storage. It can be blocked by reserpine. The answer is (E). 

Reserpine blocks the uptake (and storage) of norepinephrine and dopamine into synaptic vesicles by inhibiting the vesicular monoamine transporters. Brodie and colleagues found that reserpine also depleted serotonin from body tissues, including the brain. Their studies included patients with carcinoid tumors that produce serotonin, after there are metastases to the liver. 

Amphetamine and related substances show symphaticomimetic and CNS stimulant activity. Amphetamines are indirect monoamine agonists and interact with the membrane transporters involved in neurotransmitter reuptake and vesicular storage systems. Therefore, they stimulate the release of norepinephrine, dopamine, and serotonin from presynaptic terminals in the CNS and at the peripheral level (De La Torre et al., 2004). Methamphetamine and the methylenedioxy derivatives (MDA, MDMA, MDEA, MBDB) can inhibit the activity of enzymes of dopamine or serotonin biosynthesis (De La Torre et al., 2004). 

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