Different interaction between tricyclic antidepressants and mecamylamine with the human α3β4 nicotinic acetylcholine receptor ion channel

The interaction of tricyclic antidepressants (TCAs) with the human (h)alpha 3 beta 4 nicotinic acetylcholine receptor (AChR) in different conformational states was compared with that for mecamylamine by using functional and structural approaches including, Ca(2+) influx, radioligand binding, and molecular docking. The results established that: (a) [(3)H]imipramine binds to a single site with relatively high affinity (K(d) = 0.41 +/- 0.04 mu M), (b) imipramine inhibits [(3)H]imipramine binding to the resting/kappa-bungarotoxin-bound AChR = (K(i) = 0.68 +/- 0.08 mu M) with practically the same affinity as to the desensitized/epibatidine-bound AChR (K(i) = 0.83 +/- 0.08 mu M), suggesting that TCAs do not discriminate between these conformational states, and (c) although TCAs (IC(50) similar to 1.8-2.7 mu M) and mecamylamine (IC(50) = 3.3 +/- 0.4 mu M) inhibit (+/-)-epibatidine-induced Ca(2+) influx with potencies in the same concentration range, TCAs (K(i) similar to 1-3.6 mu M), but not mecamylamine (apparent IC(50) similar to 0.2 mM), inhibit [(3)H]imipramine binding to h alpha 3 beta 4 AChRs in different conformational states. This is explained by our docking results where imipramine, in the neutral and protonated states, interacts with the leucine (position 9') and valine/phenylalanine (position 13') rings, whereas protonated mecamylamine (>99% at physiological pH) interacts with the outer ring (position 20'). Our data indicate that TCAs bind to overlapping sites located between the serine and valine/phenylalanine rings in the h alpha 3 beta 4 AChR ion channel, whereas protonated mecamylamine can be attracted to the channel mouth before blocking ion flux by interacting with a luminal site in its neutral state. (C) 2010 Elsevier Ltd. All rights reserved.