Selectivity of (±)-citalopram at nicotinic acetylcholine receptors and different inhibitory mechanisms between habenular α3β4* and α9α10 subtypes

Abstract

The inhibitory activity of (±)-citalopram on human (h) α3β4, α4β2, and α7 nicotinic acetylcholine receptors(AChRs) was determined by Ca2+ influx assays, whereas its effect on rat α9α10 and mouse habenular α3β4*AChRs by electrophysiological recordings. The Ca2+ influx results clearly establish that (±)-citalopram inhibits(IC50´s in μM) hα3β4 AChRs (5.1 ± 1.3) with higher potency than that for hα7 (18.8 ± 1.1) and hα4β2(19.1 ± 4.2) AChRs. This is in agreement with the [3H]imipramine competition binding results indicating that(±)-citalopram binds to imipramine sites at desensitized hα3β4 with>2-fold higher affinity than that forhα4β2. The electrophysiological, molecular docking, and in silico mutation results indicate that (±)-citalopramcompetitively inhibits rα9α10 AChRs (7.5 ± 0.9) in a voltage-independent manner by interacting mainly withorthosteric sites, whereas it inhibits a homogeneous population of α3β4* AChRs at MHb (VI) neurons(7.6 ± 1.0) in a voltage-dependent manner by interacting mainly with a luminal site located in the middle ofthe ion channel, overlapping the imipramine site, which suggests an ion channel blocking mechanism. In conclusion,(±)-citalopram inhibits α3β4 and α9α10 AChRs with higher potency compared to other AChRs but bydifferent mechanisms. (±)-Citalopram also inhibits habenular α3β4*AChRs, supporting the notion that thesereceptors are important endogenous targets related to their anti-addictive activities.