The function of SLC18A3 (solute carrier family 18 member A3, Ensembl gene identifier ENSG00000187714) is as follows. Electrogenic antiporter that exchanges one cholinergic neurotransmitter, acetylcholine or choline, with two intravesicular protons across the membrane of synaptic vesicles. Uses the electrochemical proton gradient established by the V-type proton-pump ATPase to store neurotransmitters inside the vesicles prior to their release via exocytosis (By similarity) (PubMed:20225888, PubMed:8910293). Determines cholinergic vesicular quantal size at presynaptic nerve terminals in developing neuro-muscular junctions with an impact on motor neuron differentiation and innervation pattern (By similarity). Part of forebrain cholinergic system, regulates hippocampal synapse transmissions that underlie spatial memory formation (By similarity). Can transport serotonin. {ECO:0000250|UniProtKB:O35304, ECO:0000250|UniProtKB:Q62666, ECO:0000269|PubMed:20225888, ECO:0000269|PubMed:25355561, ECO:0000269|PubMed:8910293}.