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In this study we investigated the regulation of the activity of the vesicular monoamine transporters VMAT1 and VMAT2 by heterotrimeric G-proteins. In the human neuroendocrine cell line BON both transporters are expressed. They colocalize in these cells with the a-subunit of the heterotrimeric G-protein Go2 predominantely on Large Dense Core Vesicles (LDCVs). The activity of both VMAT1 and VMAT2 is regulated by Gao2. G-protein activation results in a down-regulation of vesicular monoamine uptake. VMAT2 appears to be more sensitive towards the observed G-protein regulation than VMAT1. Serotonergic raphe neurons in primary culture express VMAT2 as the neuronal form of the transporter. In these neurons VMAT2 predominantely localizes to Small Synaptic Vesicles (SSVs). Here, VMAT2 colocalizes with Gao2 on SSVs. In these neurons Gao2-dependent down-regulation of VMAT2 activity was observed, too. Immunoelectron microscopic analysis confirmed a localization of VMAT2 and Gao2 on SSVs from serotonergic terminals in the rat prefrontal cortex. In addition, Gao2-dependent regulation of VMAT2 activity could also be demonstrated when using a crude synaptic vesicle preparation of this brain area. Even in platelets obtained from mice the vesicular serotonin uptake is down-regulated by heterotrimeric G-proteins. In serotonin-depleted platelets from peripheral tryptophane-hydroxylase knockout mice no G-protein-dependent down-regulation of monoamine uptake was observed. After preincubation of the platelets with serotonin, the G-protein regulation was restored. Therefore, the vesicular transmitter content appears to be a likely factor of G-protein activation in platelets