Molecular Pharmacology of the Vesicular Monoamine Transporter

Abstract

The chromaffin granule vesicular monoamine transporter (VMAT), which has a broad specificity toward not only catecholamines but also serotonin and histamine, has been the matter of interest, specially because chromaffin granules are easy to purify in large quantities. However, another interesting aspect of VMAT is the existence of an attractive pharmacology comprising substrates, such as metaiodobenzyl-guanidine or methyl-4-phenylpyridinium (MPP+), and inhibitors, such as reserpine (RES), tetrabenazine (TBZ), or ketanserin (KET). The two substrates are also substrates of the monoamine plasma membrane transporters DAT, NET, and SERT, whereas the inhibitors are more specific, with the exception of KET, which is also an antagonist of HT2 receptors. The most powerful inhibitor is reserpine (RES) (Ki in the subnanomolar concentration range); TBZ and KET inhibit adenosine triphosphate (ATP)-dependent noradrenaline uptake. The high-affinity RES binding site is considered a substrate charge site directed toward the cytoplasmic compartment, whereas the TBZ binding site would be a discharge site oriented toward the vesicle matrix. Each site would be associated with a conformation of the protein, and the conformational change would require the proton electrochemical gradient. Two related but distinct genes encoding VMAT have been identified by the cloning of the corresponding cDNA—VMAT1 and VMAT2. Analysis of the pharmacology of VMAT2 expressed in COS cells indicated a reasonable agreement with the data obtained on bovine chromaffin granule vesicles, for the inhibition of ATP-dependent noradrenaline uptake and for [3H]TBZOH binding. The pharmacology of bovine VMAT1 was tested on COS cells transfected in the same conditions. Consistent with results obtained on rat and human VMAT1, inhibition of [3H]5HT uptake indicated a threefold decrease of the relative affinity of the substrates (5HT, dopamine, noradrenaline, and adrenaline) for VMAT1 compared with VMAT2.