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) alpha 3 beta 4, alpha 4 beta 2, and alpha 7 nicotinic acetylcholine receptors (AChRs) was determined by Ca2+ influx assays, whereas its effect on rat a9a10 and mouse habenular alpha 3 beta 4* AChRs by electrophysiological recordings. The Ca2+ influx results clearly establish that ( +/- )-citalopram inhibits (IC50's in mu M) h alpha 3 beta 4 AChRs (5.1 +/- 1.3) with higher potency than that for h alpha 7 (18.8 +/- 1.1) and h alpha 4 beta 2 (19.1 +/- 4.2) AChRs. This is in agreement with the [H-3]imipramine competition binding results indicating that ( +/- )-citalopram binds to imipramine sites at desensitized h alpha 3 beta 4 with > 2-fold higher affinity than that for h alpha 4 beta 2. The electrophysiological, molecular docking, and in silico mutation results indicate that ( +/- )-citalopram competitively inhibits r alpha 9 alpha 10 AChRs (7.5 +/- 0.9) in a voltage-independent manner by interacting mainly with orthosteric sites, whereas it inhibits a homogeneous population of alpha 3 beta 4* 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 of the ion channel, overlapping the imipramine site, which suggests an ion channel blocking mechanism. In conclusion, ( +/- )-citalopram inhibits alpha 3 beta 4 and alpha 9 alpha 10 AChRs with higher potency compared to other AChRs but by different mechanisms. ( +/- )-Citalopram also inhibits habenular alpha 3 beta 4*AChRs, supporting the notion that these receptors are important endogenous targets related to their anti-addictive activities.