Stoichiometry-Selective Antagonism of α4β2 Nicotinic Acetylcholine Receptors by Fluoroquinolone Antibiotics

Quinolone antibiotics disrupt bacterial DNA synthesis by interacting with DNA gyrase and topoisomerase IV. However, in addition, they have been shown to act as inhibitors of pentameric ligand-gated ion channels such as GABA(A) receptors and the alpha 7 nicotinic acetylcholine receptor (nAChR). In the present study, we have examined the effects of quinolone antibiotics on the human alpha 4 beta 2 nAChR, an important subtype that is widely expressed in the central nervous system. A key feature of alpha 4 beta 2 nAChRs is their ability to coassemble into two distinct stoichiometries, (alpha 4)(2)(beta 2)(3) and (alpha 4)(3)(beta 2)(2), which results in differing affinities for acetylcholine. The effects of nine quinolone antibiotics were examined on both stoichiometries of the alpha 4 beta 2 receptor by two-electrode voltage-clamp recording. All compounds exhibited significant inhibition of alpha 4 beta 2 nAChRs. However, all of the fluoroquinolone antibiotics examined (ciprofloxacin, enoxacin, enrofloxacin, difloxacin, norfloxacin, pefloxacin, and sparfloxacin) were significantly more potent inhibitors of (alpha 4)(2)(beta 2)(3) nAChRs than of (alpha 4)(3)(beta 2)(2) nAChRs. This stoichiometry-selective effect was most pronounced with pefloxacin, which inhibited (alpha 4)(2)(beta 2)(3) nAChRs with an IC50 of 26.4 +/- 3.4 mu M but displayed no significant inhibition of (alpha 4)(3)(beta 2)(2) nAChRs. In contrast, two nonfluorinated quinolone antibiotics (cinoxacin and oxolinic acid) exhibited no selectivity in their inhibition of the two stoichiometries of alpha 4 beta 2. Computational docking studies suggest that pefloxacin interacts selectively with an allosteric transmembrane site at the beta 2(+)/beta 2(-) subunit interface, which is consistent with its selective inhibition of (alpha 4)(2)(beta 2)(3). These findings concerning the antagonist effects of fluoroquinolones provide further evidence that differences in the subunit stoichiometry of heteromeric nAChRs can result in substantial differences in pharmacological properties.

Automated summary

Quinolone antibiotics can disrupt bacterial DNA synthesis and also act as inhibitors of pentameric ligand-gated ion channels, including the alpha 4 beta 2 nicotinic acetylcholine receptor. These antibiotics selectively inhibit different stoichiometries of the alpha 4 beta 2 receptor, which can result in different pharmacological properties.