An investigation into the effects of allosteric modulators of the human 5-Ht\(_3\)\(_A\) receptor

Palandri, Josephine C. (2015). An investigation into the effects of allosteric modulators of the human 5-Ht\(_3\)\(_A\) receptor. University of Birmingham. M.Sc.

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Abstract

The 5-Ht\(_3\)\(_A\) receptor is a cys-loop ligand gated ion channel re-emerging as an attractive target in irritable bowel syndrome (IBS), which currently affects 5-10% of the global population. At present, IBS therapies involve the complete blockade of the 5-Ht\(_3\)\(_A\) receptor by orthosteric antagonists, which leads to complications such as severe constipation and ischaemic colitis. Allosteric modulation could bypass such side effects, as receptor function relies upon the endogenous neurotransmitter to retain physiological control. Here, we investigate the structure and function of the 5-Ht\(_3\)\(_A\) receptor allosteric binding site by the identification of novel allosteric compounds and the generation of an α7/5-HT\(_3\)\(_A\) chimeric receptor. Intracellular calcium assays and competitive radioligand binding experiments indicated that the halogenated indole derivatives, 5-chloroindole (5-Cl) (Newman et al., 2013) and 5- (trifluoromethyl)indole (5-TFMI) are positive allosteric modulators (PAM) of the 5-Ht\(_3\)\(_A\) receptor; however 5-TFMI also displayed a degree of orthosteric binding. The structural analogues, 5-bromoindazole (5-BI) and 5-bromo-benzimidazole (5-BBI) exhibited contrasting effects, by potentiating and decreasing 5-HT-evoked responses, respectively. Further studies suggested some orthosteric binding by 5-BI and 5-BBI at high concentrations. The allosteric binding site for 5-Cl was previously located in the N-terminus of the mouse 5-Ht\(_3\)\(_A\) receptor. To identify the site in the human receptor, we attempted to construct a human chimeric α7V\(_2\)\(_0\)\(_1\)5-HT\(_3\)\(_A\) receptor to allow further investigation into this question. Our data suggest these halogenated indoles are allosteric compounds and, when studied in combination with the α7V\(_2\)\(_0\)\(_1\)5-HT\(_3\)\(_A\) chimera, could identify the required core structure of high affinity compounds needed for negative allosteric modulation in the treatment of IBS.

Type of Work:

Thesis (Masters by Research > M.Sc.)

Award Type:

Masters by Research > M.Sc.

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