[Isoform 1]: Electrogenic antiporter that exchanges one cationic monoamine with two intravesicular protons across the membrane of secretory and synaptic vesicles. Uses the electrochemical proton gradient established by the V-type proton-pump ATPase to accumulate high concentrations of monoamines inside the vesicles prior to their release via exocytosis. Transports catecholamines and indolamines with higher affinity for serotonin (PubMed:16326835, PubMed:23337945, PubMed:8643547). Regulates the transvesicular monoaminergic gradient that determines the quantal size. Mediates presynaptic monoaminergic vesicle transport in the amygdala and prefrontal brain regions related with emotion processing in response to environmental stimuli (PubMed:23337945). {ECO:0000269|PubMed:16326835, ECO:0000269|PubMed:23337945, ECO:0000269|PubMed:8643547}. [Isoform 2]: Unable to uptake serotonin. . This is the function of Ensembl gene identifier ENSG00000036565 (SLC18A1, solute carrier family 18 member A1).