Functional and structural interaction of (-)-lobeline with human α4β2 and α4β4 nicotinic acetylcholine receptor subtypes

To determine the pharmacologic activity of (-)-lobeline between human (h)α4β2 and hα4β4 nicotinic acetylcholine receptors (AChRs), functional and structural experiments were performed. The Ca²⁺ influx results established that (-)-lobeline neither actives nor enhances the function of the studied AChR subtypes, but competitively inhibits hα4β4 AChRs with potency ∼10-fold higher than that for hα4β2 AChRs. This difference is due to a higher binding affinity for the [³H]cytisine sites at hα4β4 compared to hα4β2 AChRs, which, in turn, can be explained by our molecular dynamics (MD) results: (1) higher stability of (-)-lobeline and its hydrogen bonds within the α4β4 pocket compared to the α4β2 pocket, (2) (-)-lobeline promotes Loop C to cap the binding site at the α4β4 pocket, but forces Loop C to get apart from the α4β2 pocket, precluding the gating process elicited by agonists, and (3) the orientation of (-)-lobeline within the α4β4, but not the α4β2, subpocket, promoted by the t-(or t+) rotameric state of α4-Tyr98, remains unchanged during the whole MD simulation. This study gives a detailed view of the molecular and dynamics events evoked by (-)-lobeline supporting the differential binding affinity and subsequent inhibitory potency between hα4β2 and hα4β4 AChRs, and supports the possibility that the latter subtype is also involved in its activity.