Mechanistic Basis of Partial Agonism at 5-HT3A Receptors

Abstract

Partial agonists are unable to elicit full maximal responses. The characterization of a genuine partial agonist is complex because other mechanisms, such as channel block, may also limit maximum open probability. Taking advantage of the high conductance form of the 5-HT3A receptor, we evaluated at the single-channel level its activation by 2-Me-5HT and tryptamine, which have been classically considered as partial agonists of 5-HT3 receptors. For all ligands, activation appears as openings in quick succession grouped in clusters showing high open probability (Popen>0.9), and open time distributions show three components. The slowest open component is 6.5- and 3.5-fold briefer for 2-Me-5HT and tryptamine, respectively, than for 5-HT. The duration of this component decreases as a function of agonist concentration due to open-channel block. For 2-Me-5HT, the forward blocking rate is 10-fold faster than for tryptamine and 5-HT. Single-channel kinetic analysis shows that 2-Me-5HT is actually a full agonist, its maximum response being limited by channel block. In contrast, tryptamine is a genuine partial agonist and its low efficacy is mainly due to a slow transition from the fully-liganded closed state to a pre-open state. After reaching the latter state, activation proceeds similarly as in the presence of 5-HT. Molecular docking shows that interactions at the binding site are similar for 2-Me-5HT and 5-HT. In contrast, the potential to form the cation-Pi interaction with W183 seems to be reduced for tryptamine. The mechanism by which ligands produce non maximal responses has implications for the design of partial agonists for therapeutic use.