Abstract : The gene and cDNAs that encode a novel subunit of rodent serotonin 5-HT3 receptors were isolated from mouse and rat tissues. Each of the new rodent subunits shares 40% amino acid identity with the rat 5-HT3A subunit and 73% identity with the human 5-HT3B subunit. Despite a relatively low level of structural conservation, sequence analysis and functional studies suggest that the new rodent subunits are orthologues of the human 5-HT3B subunit. In common with homologous human receptors, rat heteromeric 5-HT3 receptors displayed a substantially larger single-channel conductance than homomeric 5-HT3A receptors. In addition, the rat heteromeric receptors were less sensitive to antagonism by tubocurarine. However, in contrast to human heteromeric receptors, those of the rat displayed pronounced inward rectification of both the whole-cell and single-channel current amplitudes. Transcripts of the mouse 5-HT3A and 5-HT3B subunits are coexpressed in several cell lines that possess endogenous 5-HT3 receptors. In addition, treatment of rat PC12 cells with nerve growth factor induced expression of both subunit mRNAs, with a similar time course for accumulation of each transcript. The combination of functional data and expression patterns is consistent with the existence of heteromeric 5-HT3 receptors in rodent neurons.