Forms serotonin (5-hydroxytryptamine/5-HT3)-activated cation- selective channel complexes, which when activated cause fast, depolarizing responses in neurons. {ECO:0000269|PubMed:10521471, ECO:0000269|PubMed:12867984, ECO:0000269|PubMed:17392525, ECO:0000269|PubMed:9950429}. This is the function of Ensembl gene identifier ENSG00000149305 (HTR3B, 5-hydroxytryptamine receptor 3B).