A protein complex which is capable of serotonin-activated cation-selective channel activity. Mainly found in pre- and postsynaptic membranes of the brain and gastrointestinal tract. Depending on its location it transports Ca2+, Mg2+, Na+ or K+. It is always a pentamer, containing at least the 5HT3A subunit forming 5HT3A homopentamers or 5HT3A/B heteropentamers. In human, 5HT3A/C, A/D and A/E heteropentamers also exist. [ http://www.ncbi.nlm.nih.gov/pubmed/16116092 http://purl.obolibrary.org/obo/go/references/0000088 GOC:TermGenie GOC:bhm ]