Mefloquine - antagonists of 5-HT3

This one of the reasons, I am opposed to anti-depressants. Although, this paper was published in 2007, it is still very relevant!!

The antimalarial drugs quinine, chloroquine and mefloquine are antagonists at 5-HT3 receptors
A J Thompson,1 M Lochner,1 and S C R Lummis1*
1Department of Biochemistry, University of Cambridge, Cambridge, UK
*Author for correspondence:
Received November 1, 2006; Revised January 3, 2007; Accepted January 5, 2007.

Abstract
Background and Purpose:
The antimalarial compounds quinine, chloroquine and mefloquine affect the electrophysiological properties of Cys-loop receptors and have structural similarities to 5-HT3 receptor antagonists. They may therefore act at 5-HT3 receptors.

Experimental Approach:
The effects of quinine, chloroquine and mefloquine on electrophysiological and ligand binding properties of 5-HT3A receptors expressed in HEK 293 cells and Xenopus oocytes were examined. The compounds were also docked into models of the binding site.

Key Results:
5-HT3 responses were blocked with IC 50 values of 13.4 μM, 11.8 μM and 9.36 μM for quinine, chloroquine and mefloquine. Schild plots indicated quinine and chloroquine behaved competitively with pA 2 values of 4.92 (K B=12.0 μM) and 4.97 (K B=16.4 μM). Mefloquine displayed weakly voltage-dependent, non-competitive inhibition consistent with channel block. On and off rates for quinine and chloroquine indicated a simple bimolecular reaction scheme.

Quinine, chloroquine and mefloquine displaced [3H]granisetron with K i values of 15.0, 24.2 and 35.7 μ M. Docking of quinine into a homology model of the 5-HT3 receptor binding site located the tertiary ammonium between W183 and Y234, and the quinoline ring towards the membrane, stabilised by a hydrogen bond with E129. For chloroquine, the quinoline ring was positioned between W183 and Y234 and the tertiary ammonium stabilised by interactions with F226.

Conclusions and Implications:
This study shows that quinine and chloroquine competitively inhibit 5-HT3 receptors, while mefloquine inhibits predominantly non-competitively. Both quinine and chloroquine can be docked into a receptor binding site model, consistent with their structural homology to 5-HT3 receptor antagonists.

Keywords: 5-HT3 receptor, Cys-loop receptor, binding site, ligand docking, malaria, quinine, chloroquine, mefloquine, antagonist.....

Conclusion

Cont/.....

In summary, we have used a combination of electrophysiology, ligand binding, homology modelling and simulated docking to define the mechanisms by which quinine, chloroquine and mefloquine inhibit the 5-HT3 receptor response. Our observations further extend the number of receptors known to be affected by these compounds and the growing diversity of targets may account for the broad spectrum of side effects that have been reported by patients receiving them (Luzzi and Peto, 1993; Palmer et al., 1993; Taylor and White, 2004). Inhibition of the 5-HT3-mediated current could have wide-ranging effects in the nervous system, as 5-HT3 receptors can modulate a variety of neurotransmitter responses such as those to GABA, dopamine and cholecystokinin (Thompson et al., 2006b).http://tinyurl.com/ny2zqv