Analysis of structure and function of the serotonin type-3 receptor using site directed mutagenesis, structure activity relationship and chimeric constructs

Abstract

The serotonin type-3 receptor (5-HT₃R) is a cation conducting ligand gated ion channel that mediates fast synaptic transmission. The 5-HT₃R belongs to the Cys loop superfamily of ligand gated ion channels that also includes the nicotinic acetylcholine, glycine and GABAa receptors. The 5-HT₃R has been implicated in several processes such as emesis, gastrointestinal motility, drug abuse, alcoholism and nociception. Studies involving the ligand-binding domain will thus aid in development of new drugs that modulate these physiological and pathophysiological processes. The ligand-binding site of this receptor is comprised of six putative loops, viz. loop A-F. The focus of this thesis was to study the interactions of both agonists and antagonists with the 5- HT₃R. Interactions of two agonists, 5-HT and mCPBG, with the loop C region of the receptor were studied employing biochemical and receptor modeling studies. These studies identify novel determinants of 5-HT and mCPBG interactions with the 5-HT3 receptor. Similar studies involving granisetron, a competitive 5-HT₃R antagonist also reveal novel amino acids that interact with this antagonist. In order to further understand antagonist interactions with this receptor, the approach of structure activity relationship (SAR) studies was also employed to study the functional group interactions of lerisetron, a novel 5-HT₃R antagonist. Taken together with data from loops A, B, D and E, these data reveal an emerging picture of ligand interactions with the 5-HT₃R.