Near native binding of a fluorescent serotonin conjugate to serotonin type 3 receptors

Described is the synthesis of 5-hydroxytryptamine-tetramethylrhodamine (5HT*); an indole nitrogen linked fluorescent conjugate of serotonin. Through a series fluorescence quenching experiments and experiments in the presence of a known competitive antagonist (Granisetron), it was shown that 5HT* specifically binds to purified homo-pentameric type-3 human serotonin receptors (5HT(3A)). The measured dissociation constant and Hill coefficient are K-d = 83 +/- 3 nM and n = 3.1 +/- 0.3, respectively which is indicative of multi-ligand binding and cooperativity similar to that of unconjugated serotonin. (c) 2012 Elsevier Ltd. All rights reserved.