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

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 hα3β4 AChRs (5.1 ± 1.3) with higher potency (IC50's in µM) 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 for hα4β2. The electrophysiological results indicate that (±)-citalopram competitively inhibits rα9α10 AChRs (7.5 ± 0.9) in a voltage-independent manner, whereas it inhibits a homogeneous population of α3β4* AChRs at MHb (VI) neurons (7.6 ± 1.0) in a voltage-dependent manner. The results indicating that citalopram overlaps the imipramine luminal site and inhibits α3β4* AChRs in a voltage-dependent manner, suggest an ion channel blocking mechanism. Both results were in agreement with the conclusions of automatic molecular docking and molecular dynamics experiments. In conclusion, (±)-citalopram inhibits α3β4 and α9α10 AChRs with higher potency compared to other AChRs but by different mechanisms. (±)-Citalopram also inhibits α3β4*AChRs expressed in MHb (VI) neurons, supporting the notion that these receptors are important endogenous targets related to their anti-addictive activities