Acetylcholine transport into synaptic vesicle

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. 

Choline is taken into the nerve terminal from the synaptic cleft by a sodium-dependent, high-affinity active transport process, which is the rate-limiting step in the biosynthesis of acetylcholine in the nerve terminal (1). In the nerve terminal, choline reacts with acetylcoenzyme A in a process catalyzed by choline acetyltransferase. The acetylcholine thus synthesized is sequestered in the synaptic storage vesicles in the nerve terminal for future use as a neurotransmitter. The active transport of acetylcholine into the storage vesicles has been reviewed (2). Vesamicol (2) at micromolar concentrations blocks transport of acetylcholine into the vesicles (3, 4). 

Whereas acetylcholine is degraded by a membrane-anchored acetylcholine esterase (ACE) in the synaptic cleft (choline is afterwards taken up presynaptically), the biogenic amines adrenaline, noradrenaline, serotonin, and dopamine are taken up by the presynaptic membrane by transporters (Fig. 3) or by extraneuronal cells in which they are degraded by a catecholamine O-methyltransferase (COMT). These transporter have similar structure and contain 12 transmembrane regions. Once in the presynapse, the neurotransmitters are either degraded by monoamine oxidase (MAO) or taken up by synaptic vesicles. A proton pumping ATPase of the vesicle membrane (V-type ATPase as in plant vacuoles) causes an increase of hydrogen ion concentrations in the vesicles. Uptake of the neurotransmitter serotonin, adrenaline and noradrenaline could be partly achieved either via a diffusion of the free base into the vesicles where they become protonated and concentrated by an "ion trap" mechanism and via specific proton-coupled antiporters. The excitatory amino acids, acetylcholine and ATP cannot diffuse and enter the vesicles via specific transporters. 

To ready the synapse for another impulse (which must occur about 1000 times per second), the empty synaptic vesicles, which are returned to the axonal terminal bulb by endocytosis, must be refilled with acetylcholine. This task is accomplished by an acetylcholine transporter protein, which brings newly synthesized acetylcholine into the vesicles by exchanging it for protons. As the protons are returned to the cytosol from the vesicles, acetylcholine is transported in the opposite direction. 

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).

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