Comparative Interaction of Tricyclic Antidepressants and Mecamlyamine with the Human α4β2 Nicotinic Acetylcholine Receptor

Abstract

We compared the interaction of tricyclic antidepressants (TCAs) with that for the noncompetitive antagonist mecamylamine with the human (h) α4β2 nicotinic acetylcholine receptor (AChR) in different conformational states, by using functional and structural methods. The results established that: (a) TCAsinhibit (±)-epibatidine-induced Ca2+ influx in HEK293-hα4β2 cells with potencies that are in the same concentration range (IC50 = 2.2-6.8 μM) as that for mecamylamine (IC50 = 3.0 ± 0.7 μM), (b) [3H]imipramine binds to a single binding site located in the hα4β2 AChR ion channel with relatively high affinity (Kd = 0.83 ± 0.08 μM), (c) TCAsinhibit [3H]imipramine binding to hα4β2 AChRs with affinities (Ki = 1.0-2.1 μM) higher than that for mecamylamine (Ki = 143 ± 31 μM), (d) imipramine and mecamylamine do not differentiate between desensitized and resting AChRs, (e) imipramine interacts with the desensitized AChR mainly by an entropy-driven process, whereas the interactions with the resting AChR are mediated by a combination of enthalpic and entropic components, and (f) neutral imipramine and mecamylamine interact with a domain formed between the leucine (position 9’) and valine (position 13’) rings by van der Waals contacts, whereas protonated mecamylamine interacts electrostatically with the outer ring (position 20’). Our data indicate that although TCAs interact with a binding domain located between the leucine and valine rings, and mecamylamine predominantly acts at the outer ring and by intercalating between two M2 segments, both drugs may efficiently inhibit the ion channel.