Hauser_2012_Neuropharmacol_62_2239

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 alpha7 receptor. The rank order for binding affinity (Ki) for SPX-desMe-C was alpha7 > alpha6beta3beta4alpha5 >> rat alpha3beta4, alpha1betagammadelta > alpha4beta4, human alpha3beta4 > human alpha4beta2 > rat alpha4beta2 and for gymnodimine was alpha7, alpha6beta3beta4alpha5 > rat alpha3beta4 > human alpha3beta4, alpha4beta4 > rat alpha4beta2, human alpha4beta2 > alpha1betagammadelta. Both molecules antagonized agonist-induced nicotinic responses. The antagonism rank order of potency (IC(50)) for SPX-desMe-C was alpha7 > low sensitivity (LS) alpha4beta2 > human alpha3beta4 > high sensitivity (HS) alpha4beta2, alpha1betagammadelta > alpha4beta4 > rat alpha3beta4 and for gymnodimine was LS alpha4beta2 > human alpha3beta4 > alpha7 > HS alpha4beta2 > alpha4beta4 > rat alpha3beta4 > alpha1betagammadelta. 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 alpha7 nAChR. Their very high binding affinity results from very tight hydrophobic enclosures, in addition to previously reported hydrogen-bond and cation-pi 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.