Acetylcholine uptake into synaptic vesicles

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 events that take place at a synapse, that is synthesis of neurotransmitter, storage in vesicles, release, interaction with receptors and eventual inactivation provide targets for drug action (Chapter 3). Inactivation of a neurotransmitter can be by re-uptake into the neuron it was released from (as with noradrenaline and dopamine), or by the action of enzymes in the synaptic cleft (as with acetylcholine). 

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. 

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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