estimated rate of 5–20/sec depending on the substrate. Vesicle filling may limit monoamine release from neurons with high rates of firing. Specific amine-binding affinity varies by VMAT isoform; studies indicate that catecholamines dopamine, norepinephrine, and epinephrine have a threefold higher affinity for VMAT2 than VMAT1 binding and uptake. The imidazoleamine histamine has a thirtyfold higher affinity for VMAT2 compared to VMAT1, and is thought to bind to a different site than other monoamines. Unlike catecholamines and histamine, the indoleamine serotonin binds to VMAT1 and VMAT2 with a similar affinity for both transporter isoforms. VMAT1 has a lower turnover number and a lower affinity for most monoamine substrates than VMAT2, which may be because of VMAT2's location in the central nervous system, which demands fast recovery from neurotransmitter release in order to prepare for subsequent releases. The uptake efficiencies of each VMAT substrate can be ranked in order of efficiency as: serotonin, dopamine, epinephrine, and norepinephrine. Methamphetamines decrease Vmax, while cocaine increases Vmax reversibly in rat brain. Inhibition The effects of VMAT inhibition have been studied in-depth in animal models. Mutant homozygous VMAT(-/-) mice move little, feed poorly, and die within a few days of birth. More specifically, inhibition of VMAT2 may cause an increase in cytosolic catecholamine levels, which can result in an increase in efflux of catecholamines through the cell membrane, depleting catecholamine concentrations and causing increased oxidative stress and oxidative damage to the neuron. Heterozygous VMAT mutants display hypersensitivity to amphetamine, cocaine, and MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), the latter being a substance causally linked to Parkinson's disease (PD) in rodents. This suggests a protective role of VMATs against oxidative stress through removal of such substances from the cytosol. VMAT inhibitors include: Reserpine (RES), bietaserpine, and ketanserin (KET) (potent inhibitors of VMAT2 mediated serotonin transport) Tetrabenazine (TBZ) (specific to VMAT2) Phenylethylamine Amphetamine MDMAN-Methyl-4-phenylpyridinium (MPP+) (very potent inhibitors of VMAT2 mediated serotonin transport) Fenfluramine (specific to VMAT1 ) Non-hydrolysable GTP-analogue guanylyllimidodiphosphate GMP-P(NH)P (VMAT2 only) Binding site structures Ligand-binding affinities and structures Two known binding sites for VMAT inhibitors include the RES binding site and the TBZ binding site. Some evidence suggests these two sites may overlap or exist as two separate conformations of the same binding site. VMAT inhibitors tend to fall into two classes: those that interact with the RES binding site and those that interact with the TBZ binding site. RES, methoxytetrabenazine (MTBZ), and the drug amiodarone bind to the