Antimalarial drugs inhibit human 5-HT(3) and GABA(A) but not GABA(C) receptors

Abstract

Background and purpose: Antimalarial compounds have been previously shown to inhibit rodent nicotinic acetylcholine (nACh) and 5-HT(3) receptors. Here, we extend these studies to include human 5-HT(3A), 5-HT(3AB), GABA(A) alpha1beta2, GABA(A) alpha1beta2gamma2 and GABA(C) rho1 receptors.

Experimental approach: We examined the effects of quinine, chloroquine and mefloquine on the electrophysiological properties of receptors expressed in Xenopus oocytes.

Key results: 5-HT(3A) receptor responses were inhibited by mefloquine, quinine and chloroquine with IC(50) values of 0.66, 1.06 and 24.3 microM. At 5-HT(3AB) receptors, the potencies of mefloquine (IC(50)=2.7 microM) and quinine (15.8 microM), but not chloroquine (23.6 microM), were reduced. Mefloquine, quinine and chloroquine had higher IC(50) values at GABA(A) alpha1beta2 (98.7, 0.40 and 0.46 mM, respectively) and GABA(A) alpha1beta2gamma2 receptors (0.38, 1.69 and 0.67 mM, respectively). No effect was observed at GABA(C) rho1 receptors. At all 5-HT(3) and GABA(A) receptors, chloroquine displayed competitive behaviour and mefloquine was non-competitive. Quinine was competitive at 5-HT(3A) and GABA(A) receptors, but non-competitive at 5-HT(3AB) receptors. Homology modelling in combination with automated docking suggested orientations of quinine and chloroquine at the GABA(A) receptor binding site.

Conclusions and implications: The effects of mefloquine, quinine and chloroquine are distinct at GABA(A) and GABA(C) receptors, whereas their effects on 5-HT(3AB) receptors are broadly similar to those at 5-HT(3A) receptors. IC(50) values for chloroquine and mefloquine at 5-HT(3) receptors are close to therapeutic blood concentrations required for malarial treatment, suggesting that their therapeutic use could be extended to include the treatment of 5-HT(3) receptor-related disorders.

Figure 1

Concentration–response curves for 5-HT₃ and GABA receptors expressed in Xenopus oocytes (a). Parameters derived from these curves are shown in Table 1. Typical agonist EC₅₀ responses (b) and inhibition of these responses by mefloquine (c). Agonist application is indicated by a grey line above the current traces. Mefloquine (MEF) was pre-applied for 20 s and then co-applied at the concentrations shown.

Figure 2

Concentration-dependent inhibition of homomeric (a) and heteromeric (b) 5-HT₃ responses. Inhibition was measured in the presence of EC₅₀ concentrations of 5-HT. Values are shown as mean±s.e.mean, and parameters derived from these curves can be seen in Table 2. Inset: Inhibition of the EC₅₀ response versus pre-application time: 5-HT_3A by 3 μM mefloquine, 5-HT_3AB by 30 μM quinine and 30 μM mefloquine (n=4 for each curve).

Figure 3

Concentration-dependent inhibition of GABA responses. Inhibition was measured in the presence of EC₅₀ concentrations of GABA for each receptor. Values are shown as mean±s.e.mean, and parameters derived from these curves can be seen in Table 3.

Figure 4

Inhibition by quinine, chloroquine and mefloquine at 5-HT_3A and GABA_A receptors. Concentration–response curves were performed in the presence or absence of antagonist. Antagonist concentrations are shown on each graph; Q=quinine, C=chloroquine, M=mefloquine. Values are shown as mean±s.e.mean. n values are shown in parentheses.

Figure 5

Chloroquine and quinine docked ligands in the GABA_A receptor β2/α1 binding interface showing the orientation of the main residues that define these models. These residues are highlighted in Figure 6. Chloroquine (a, b) and quinine (c–e) are shown in white at the centre of each image.

Figure 6

Sequence alignment of ligand-gated ion channel receptor subunits that have been examined for their susceptibility to antimalarial compounds. Residues with similar chemical properties are shown in grey and the approximate positions of the binding loops (A–F) are indicated with a bar. Residues that are shown in Figure 5 are marked with an asterisk (GABA_A β2: Y157, S201, Y205; GABA_A α1: F65, R67, L128, R120, T130). Accession numbers for the subunits are GABA_A , GABA_A , GABA_C , 5-HT_3A , 5-HT_3B and .