Tricyclic antidepressants inhibit hippocampal α7*and α9α10 nicotinic acetylcholine receptors by different mechanisms

The activity of tricyclic antidepressants (TCAs) at alpha 7 and alpha 9 alpha 10 nicotinic acetylcholine receptors (AChRs) as well as at hippocampal alpha 7-containing (i.e., alpha 7*) AChRs is determined by using Ca2+ influx and electrophysiological recordings. To determine the inhibitory mechanisms, additional functional tests and molecular docking experiments are performed. The results established that TCAs (a) inhibit Ca2+ influx in GH3-alpha 7 cells with the following potency (IC50 in mu M) rank: amitriptyline (2.7 +/- 0.3) > doxepin (5.9 +/- 1.1) similar to imipramine (6.6 +/- 1.0). Interestingly, imipramine inhibits hippocampal alpha 7* AChRs (42.2 +/- 8.5 mu M) in a noncompetitive and voltage-dependent manner, whereas it inhibits alpha 9 alpha 10 AChRs (0.53 +/- 0.05 mu M) in a competitive and voltage-independent manner, and (b) inhibit [3H]imipramine binding to resting alpha 7 AChRs with the following affinity rank (IC50 in mu M): imipramine (1.6 +/- 0.2) > amitriptyline (2.4 +/- 0.3) > doxepin (4.9 +/- 0.6), whereas imipramine's affinity was no significantly different to that for the desensitized state. The molecular docking and functional results support the notion that imipramine noncompetitively inhibits alpha 7 AChRs by interacting with two overlapping luminal sites, whereas it competitively inhibits alpha 9 alpha 10 AChRs by interacting with the orthosteric sites. Collectively our data indicate that TCAs inhibit alpha 7, alpha 9 alpha 10, and hippocampal alpha 7* AChRs at clinically relevant concentrations and by different mechanisms of action.