Dopamine vesicular concentration

The pharmacology of amphetamine is considerably more complex. It does not only block monoamine reuptake, but also directly inhibits the vesicular monoamine transporter, causing an increase in cytosolic but not vesicular dopamine concentration. This may lead to reverse transport of the amines via the membrane-bound transporters. Further mechanisms of amphetamine action are direct MAO inhibition and indirect release of both dopamine and serotonin in the striatum. 

Mechanism of action of cocaine and amphetamine on synaptic terminal of dopamine (DA) neurons. Left Cocaine inhibits the dopamine transporter (DAT), decreasing DA clearance from the synaptic cleft and causing an increase in extracellular DA concentration. Right Since amphetamine (Amph) is a substrate of the DAT, it competitively inhibits DA transport. In addition, once in the cell, amphetamine interferes with the vesicular monoamine transporter (VMAT) and impedes the filling of synaptic vesicles. As a consequence, vesicles are depleted and cytoplasmic DA increases. This leads to a reversal of DAT direction, strongly increasing nonvesicular release of DA, and further increasing extracellular DA concentrations. 

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. 

Fig. 3. Ca dependent (vesicular) release of multiple neurotransmitter types from rat hippocampal synaptosomes. To evoke release, the extracellular K concentration was raised to 30 mA/by adding KCl. The basis of the neurotransmitter type categories (Type I, and so forth) is described in Note 19. The measurements are expressed in log[fmol/mg protein]. See Section 3.4, and Note 21 for further discussion. GLU = glutamate, NA = noradrenaline, DA = dopamine, CCK = cholecystokinin, met-Enk = met-enkephalin.

The model allows determination of relative concentrations from individual release events and has been used to examine events at control cells and cells incubated with the dopamine precursor, L-3,4-dihydroxyphenylalanine (l-DOPA) [13]. Exposure to l-DOPA (100 JM for 1 hr) results in 145 detectable exocytosis events for 11 cells compared to 77 events for 29 control cells, again suggesting that the released substance is indeed dopamine and that vesicles can be loaded with the catecholamine. Additionally, each event has a larger half-width (tj/2) in the release of vesicular contents when compared to cells that are not incubated with l-DOPA. This data is significant kinetically, as it suggests a longer release time for the increased quantity of dopamine. The data are sununarized in Figure... 

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