A single channel mutation alters agonist efficacy at 5-HT(3)A and 5-HT(3)AB receptors

BACKGROUND AND PURPOSE: 5-HT(3) receptors are composed of 5-HT(3)A subunits (homomeric receptors), or combinations of 5-HT(3)A and other 5-HT(3) receptor subunits (heteromeric receptors, the best studied of which are 5-HT(3)AB receptors). Here we explore the effects of partial agonists at 5-HT(3)A and 5-HT(3)AB receptors, and the importance of a channel-lining residue in determining the efficacy of activation. EXPERIMENTAL APPROACH: Wild type and mutant 5-HT(3)A and 5-HT(3)AB receptors were expressed in Xenopus oocytes and examined using two-electrode voltage-clamp, or expressed in HEK293 cells and examined using [(3)H]granisetron binding. KEY RESULTS: Dopamine, quipazine and VUF10166 were partial agonists at wild type 5-HT(3)A and 5-HT(3)AB receptors, with quipazine and VUF10166 causing a long-lived (>20 min) inhibition of subsequent agonist responses. At 5-HT(3)A receptors, mCPBG was a partial agonist, but was a superagonist at 5-HT(3)AB receptors, as it produced a response 2.6× greater than that of 5-HT. A T6'S substitution in the 5-HT(3)A subunit decreased EC(50) and increased R(max) of dopamine and quipazine at both homomeric and heteromeric receptors. The greatest changes were seen with VUF10166 at 5-HT(3)A(T6'S)B receptors, where it became a full agonist (EC(50) = 7 nM) with an EC(50) 58-fold less than 5-HT (EC(50) = 0.4 μM) and no longer caused inhibition of subsequent agonist responses. CONCLUSIONS AND IMPLICATIONS: These results indicate that a mutation in the pore lining domain in both 5-HT(3)A and 5-HT(3)AB receptors alters the relative efficacy of a series of agonists, changing some (e.g. quipazine) from apparent antagonists to potent and efficacious agonists.