Comparison of acetylcholine receptor interactions of the marine toxins, 13-desmethylspirolide C and gymnodimine.

The interaction of 13-desmethylspirolide C (SPX-desMe-C) and gymnodimine with several nicotinic and muscarinic acetylcholine receptors was investigated. Interaction at the muscarinic receptors was minimal. At nicotinic receptors, both SPX-desMe-C and gymnodimine displayed greatest affinity for the α7 receptor. The rank order for binding affinity (Ki) for SPX-desMe-C was α7 > α6β3β4α5 >> rat α3β4, α1βγδ > α4β4, human α3β4 > human α4β2 > rat α4β2 and for gymnodimine was α7, α6β3β4α5 > rat α3β4 > human α3β4, α4β4 > rat α4β2, human α4β2 > α1βγδ. Both molecules antagonized agonist-induced nicotinic responses. The antagonism rank order of potency (IC(50)) for SPX-desMe-C was α7 > low sensitivity (LS) α4β2 > human α3β4 > high sensitivity (HS) α4β2, α1βγδ > α4β4 > rat α3β4 and for gymnodimine was LS α4β2 > human α3β4 > α7 > HS α4β2 > α4β4 > rat α3β4 > α1βγδ. Neither gymnodimine nor SPX-desMe-C antagonism could be surmounted by increasing concentrations of nicotine. To elucidate the nature of this insurmountable blockade, we carried out homology modelling and molecular docking studies of both ligands with α7 nAChR. Their very high binding affinity results from very tight hydrophobic enclosures, in addition to previously reported hydrogen-bond and cation-π interactions. Also, the higher the hydrophilic surface area of the binding site of nAChRs, the weaker the binding affinity of both ligands. Together these results show the targets of action are nicotinic and define these marine toxins as additional tools to advance our understanding regarding interactions between antagonists and the nAChR ligand binding domain.